ACI 363r 92 State-of-the-Art Report on High-Strength Concrete

ACI 363R-92 (Reapproved 1997)

State-of-the-Art Report on High-Strength Concrete Reported by ACI Committee 363 Henry G. Russell Chairman Arthur R. Anderson Jack O. Banning Irwin G. Cantor* Ramon L. Carrasquillo* James E. Cook Gregory C. Frantz Weston T. Hester

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Jaime Moreno Secretary Anthony N. Kojundic Brian R. Mastin* William C. Moore Arthur H. Nilson* William F. Perenchio Francis J. Principe

Kenneth L Saucier* Surendra P. Shah* J. Craig Williams* John Wolsiefer, Sr. J. Francis Young Paul Zia

Members responsible for individual chapters

ACI Committee 363 Members Balloting 1992 Revisions William F. Perenchio Secretary

Kenneth L Saucier Chairman Pierre Claude Aitcin F. David Anderson Claude Bedard Roger W. Black Irwin G. Cantor Ramon L. Carrasquillo Judith A. Castello James E. Cook Kingsley D. Drake Gregory C. Frantz Thomas G. Guennewig

Weston T. Hester+ Nathan L Howard Anthony N. Kojundic Mark D. Luther Heshem Marzouk Brian R. Mastin William C. Moore Jaime Moreno Arthur H. Nilson Clifford R. Ohwiler

Currently available information about high-strength concrete is summarized. Topics discussed include selection of materials, concrete mix proportioning, batching miring, transporting placing, control procedures, concrete properties, structural design, economics, and applications. A bibliography is included.

Chapter l-Introduction, pg. 363R-2

1.l-Historical background 1.2-Committee objectives

Keywords: bibliographies; bridges (structures); buildings; conveying; economics; high-strength concretes; mechanical properties; mixing; mix proportioning; placing; quality control; raw materials; reviews; structural design.

ACI Committee Reports, Guides, Standard Practices, and Commentaries are intended for guidance in designing, planning, executing, or inspecting construction and in preparing specifications. Reference to these documents shall not be made in the Project Documents. If items found in these documents are desired to be part of the Project Documents, they should be phrased in mandatory language and incorporated into the Project Documents.

Henry G. Russell Michael T. Russell Surendra P. Shah Bryce P. Simons Ava Szypula Dean J. White, II J. Craig Williams John T. Wolsiefer Francis J. Young Paul Zia

Chapter 2-Selection of materials, pg. 363R-3

2.1-Introduction 2.2-Cements 2.3-Chemical admixtures 2.4-Mineral admixtures and slag cement

c 1992, American Concrete Institute. Copyright O All rights reserved including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by any electronic or mechanical device, printed or written or oral, or recording for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors. 363R-1

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ACI COMMITTEE REPORT

2.5-Aggregates 2.6-Water 2.7-Cited references Chapter 3-Concrete mix proportions, pg. 363R-8

3.1-Introduction 3.2-Strength required 3.3-Test age 3.4-Water-cement ratio or water-cementitious ratio 3.5-Cement content 3.6-Aggregate proportions 3.7-Proportioning with admixtures 3.8-Workability 3.9-Trial batches 3.10-Cited references Chapter 4-Batching, mixing, transporting, placing, curing, and control procedures, pg. 363R-16

4.1-Introduction 4.2-Batching

7.2-Cost studies 7.3-Case histories 7.4-Other studies 7.5-Selection of materials 7.6-Quality control 7.7-Areas of application 7.4-Conclusion 7.9-Cited references Chapter 8-Applications, pg. 363R-44

8.1-Introduction 8.2-Buildings 8.3-Bridges 8.4-Special applications 8.5-Potential applications 8.6-Cited references Chapter 9-Summary, pg. 363R-48 Chapter 10-References, pg. 363R-49

4.3-Mixing

4.4-Transporting 4.5-Placing procedures 4.6-Curing 4.7-Quality assurance 4.8-Quality control procedures 4.9-Strength measurements 4.10-Cited references Chapter 5-Properties of high-strength concrete, pg. 363R-22

5.1-Introduction 5.2-Stress-strain behavior in uniaxial compression 5.3-Modulus of elasticity 5.4-Poisson’s ratio 5.5-Modulus of rupture 5.6-Tensile splitting strength 5.7-Fatigue strength 5.8-Unit weight 5.9-Thermal properties 5.10-Heat evolution due to hydration 5.11-Strength gain with age 5.12-Freeze-thaw resistance 5.13-Shrinkage 5.14-Creep 5.15-Cited references Chapter 6-Structural design considerations, pg. 363R-

29 6.1-Introduction 6.2-Axially-loaded columns 6.3-Beams and slabs 6.4-Eccentric columns 6.5-Summary 6.6-Cited references Chapter 7--Economic considerations, pg. 363R-41 7.1-Introduction

CHAPTER 1-INTRODUCTION 1.1-Historical background

Although high-strength concrete is often considered a relatively new material, its development has been gradual over many years. As the development has continued, the definition of high-strength concrete has changed. In the 1950s, concrete with a compressive strength of 5000 psi (34 MPa) was considered high strength. In the 1960s, concrete with 6000 and 7500 psi (41 and 52 MPa) compressive strengths were used commercially. In the early 1970s, 9000 psi (62 MPa) concrete was being produced. More recently, compressive strengths approaching 20,000 psi (138 MPa) have been used in cast-in-place buildings. For many years, concrete with compressive strength in excess of 6000 psi (41 MPa) was available at only a few locations. However, in recent years, the applications of high-strength concrete have increased, and high-strength concrete has now been used in many parts of the world. The growth has been possible as a result of recent developments in material technology and a demand for higher-strength concrete. The construction of Chicago’s Water Tower Place and 311 South Wacker Drive concrete buildings would not have been possible without the development of high-strength concrete. The use of concrete superstructures in long span cable-stayed bridges such as East Huntington, W.V., bridge over the Ohio River would not have taken place without the availability of high-strength concrete. 1.2-Committee objectives Since the definition of high-strength concrete has changed over the years, the committee needed to define an applicable range of concrete strengths for its activities. The following working definition was adopted: “The immediate concern of Committee 363 shall be concretes

HIGH STRENGTH CONCRETE

have specified compressive strengths for design of 6000 psi (41 MPa) or greater, but for the present time, considerations shall not include concrete made using exotic materials or techniques.” The word exotic was included in the definition so that the committee would not be concerned with concretes such as polymer-impregnated concrete, epoxy concrete, or concrete with artificial normal and heavy-weight aggregates. Although 6000 psi (41 MPa) was selected as the lower limit, it is not intended to imply that there is a drastic change in material properties or in production techniques that occur at this compressive strength. In reality, all changes that take place above 6000 psi (41 MPa) represent a process which starts with the lowerstrength concretes and continues into high-strength concretes. Many empirical equations used to predict properties of concrete or to design structural members are based on tests using concrete with compressive strengths less than about 6000 psi (41 MPa). The availability of data for higher-strength concretes requires a reassessment of the equations to determine their applicability with higher-strength concretes. Consequently, caution should be exercised in extrapolating data from lowerstrength to high-strength concretes. If necessary, tests should then be made to develop data for the materials or applications in question. The committee also recognized that the definition of high-strength concrete varies on a geographical basis. In regions where concrete with a compressive strength of 9000 psi (62 MPa) is already being produced commercially, high-strength concrete might be in the range of 12,000 to 15,000 psi (83 to 103 MPa) compressive strength. However, in regions where the upper limit on commercially available material is currently 5000 psi (34 MPa) concrete, 9000 psi (62 MPa) concrete is considered high strength. The committee recognized that material selection, concrete mix proportioning, batching, mixing, transporting, placing, and control procedures are applicable across a wide range of concrete strengths. However, the committee felt that material properties and structural design considerations given in this report should be concerned with concretes having the highest compressive strengths. The committee has tried to cover both aspects in compiling this state-of-the-art report. CHAPTER 2-SELECTION OF MATERIALS 2.1-Introduction

The production of high-strength concrete that consistently meets requirements for workability and strength development places more stringent requirements on material selection than for lower-strength concretes. Quality materials are needed and specifications require enforcement. High-strength concrete has been produced using a wide range of quality materials based on the results of trial mixtures. This chapter cites the state of I

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knowledge regarding material selection and provides a baseline for the subsequent discussion of mix proportions in Chapter 3.

Fig. 2.1-Effects of cement on concrete compressive strength. 2.2 2.2-Cements The choice of portland cement for high-strength constrength is the objective, such as in prestressed concrete, there is no need to use a Type III cement. Furthermore, within a given cement type, different brands will have different strength development characteristics because of the variations in compound composition and fineness that are permitted by ASTM C 150. Initially, silo test certificates should be obtained from potential suppliers for the previous 6 to 12 months. Not only will this give an indication of strength characteristics from the ASTM C 109 mortar cube test, but also, more importantly, it will provide an indication of cement uniformity. The cement supplier should be required to report uniformity in accordance with ASTM C 917. If the tricalcium silicate content varies by more than 4 percent, the ignition loss by more than 0.5 percent, or the fineness by more than 375 cm2/g (Blaine), then problems in maintaining a uniform high strength may result.2.1Sulfate (SO,) levels should be maintained at optimum with variations limited to ± 0.20 percent. Although mortar cube tests can give a good indication of potential strength, tests should be run on trial batches. These should contain the materials to be used in the job and be prepared at the proposed slump, with strengths determined at 7, 28, 56, and 91 days. The effect of cement characteristics on water demand is more noticeable in high-strength concretes because of the higher cement contents. High cement contents can be expected to result in a high temperature rise within the concrete. For example,

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mixtures with sufficient retarder dosage to give the desirable rate of hardening under the anticipated temperature conditions. Since retarders frequently provide an increase in strength that will be proportional to the dosage rate, mixtures can be designed at different doses if it is expected that significantly different rates will be used. However, there is usually an offsetting effect that minimizes the variations in strengths due to temperature. As temperature increases, later age strengths will decline; however, an increase in retarder dosage to control the rate of hardening will provide some mitigation of the temperature-induced reduction. Conversely, dosages should be decreased as temperatures decline. While providing initial retardation, strengths at 24 hours and later are usually increased by normal dosages. 2.3-Chemical admixtures 2.3.1 General-Admixtures are widely used in the pro- Extended retardation or cool temperatures may affect duction of high-strength concretes. These materials in- early (24-hour) strengths adversely. 2.3.4 Normal-setting water reducers (ASTM C 494, clude air-entraining agents and chemical and mineral admixtures. Air-entraining agents are generally surfac- Type A-Normal setting ASTM C 494 Type A conventants that will develop an air-void system appropriate for tional water-reducing admixtures will provide strength durability enhancement. Chemical admixtures are gener- increases without altering rates of hardening. Their ally produced using lignosulfonates, hydroxylated car- selection should be based on strength performance. Inboxylic acids, carbohydrates, melamine and naphthalene creases in dosage above the normal amounts will genercondensates, and organic and inorganic accelerators in ally increase strengths, but may extend setting times. various formulations. Selection of type, brand, and dos- When admixtures are used in this fashion to provide reage rate of all admixtures should be based on perfor- tardation, a benefit in strength performance sometimes mance with the other materials being considered or results. 2.3.5 High-range water reducers 2.4,2.5 (ASTM C 494, selected for use on the project. Significant increases in compressive strength, control of rate of hardening, ac- Types F and G-High-range water reduction provides celerated strength gain, improved workability, and dur- high-strength performance, particularly at early (24-hour) ability are contributions that can be expected from the ages. Matching the admixture to the cement, both in type admixture or admixtures chosen. Reliable performance and dosage rate, is important. The slump loss characteron previous work should be considered during the selec- istics of a high-range water reducer (HRWR) will detertion process. mine whether it should be added at the plant, at the site, or a combination of each. 2.3.2 Air-entraining admixtures (ASTM C 260)-The Use of a HRWR in high-strength concrete may serve use of air entrainment is recommended to enhance durability when concrete will be subjected to freezing and the purpose of increasing strength at the slump or inthawing while wet. As compressive strengths increase and creasing slump. The method of addition should distribute water-cement ratios decrease, air-void parameters im- the admixture throughout the concrete. Adequate mixing prove and entrained air percentages can be set at the is critical to uniform performance. Supervision is imlower limits of the acceptable range as given in ACI 201. portant to the successful use of a HRWR. The use of 2.6 Entrained air has the effect of reducing strength, parti- superplasticizers is discussed further in ACI SP-68. 2.3.6 Accelerators (ASTM C 494, Types C and E)-Accularly in high-strength mixtures, and for that reason it has been used only where there is a concern for durabili- celerators are not normally used in high-strength concrete unless early form removal is critical. High-strength ty. See also Section 5.12. 2.3.3 Retarders (ASTM C 494, Types B and D)-High- concrete mixtures can provide strengths adequate for verstrength concrete mix designs incorporate high cement tical form removal on walls and columns at an early age. factors that are not common to normal commercial con- Accelerators used to increase the rate of hardening will crete. A retarder is frequently beneficial in controlling normally be counterproductive in long-term strength deearly hydration. The addition of water to retemper the velopment. 2.3.7 Admixture combinations-Combinations of highmixture will result in marked strength reduction. Further, structural design frequently requires heavy reinforcing range water reducers with normal-setting water reducers steel and complicated forming with attendant difficult or retarders have become common to achieve optimum placement of the concrete. A retarder can control the performance at lowest cost. Improvements in strength rate of hardening in the forms to eliminate cold joints gain and control of setting times and workability are and provide more flexibility in placement schedules. Pro- posstble with optimized combinations. In certain cirjects have used retarders successfully by initially designing cumstances, combinations of normal-setting or retarding the temperature in the 4 ft (1.2 m) square columns used in Water Tower Place which contained 846 lb cement/yd 3 (502 kg/m3), rose to 150 F (66 C) from 75 F (24 C) during hydration. 2.2 The heat was dissipated within 6 days without harmful effects. However, when the temperature rise is expected to be a problem, a Type II low-heat-ofhydration cement can be used, provided it meets the strength-producing requirements. A further consideration is the optimization of the cement-admixture system. The exact effect of a waterreducing agent on water requirement, for example, will depend on the cement characteristics. Strength development will depend on both cement characteristics and cement content.


water-reducing admixtures plus an accelerating admixture have also been found to be useful. When using a combination of admixtures, they should be dispensed individually in a manner approved by the manufacturer(s). Air-entraining admixtures should, if used, be dispensed separately from water-reducing admixtures. 2.4-Mineral admixtures and slag cement Finely divided mineral admixtures, consisting mainly of fly ash and silica fume, and slag cement have been widely used in high-strength concrete. 2.4.1 Fly ash-Fly ash for high-strength concrete is classified into two classes. Class F fly ash is normally produced from burning anthracite or bituminous coal and has pozzolanic properties, but little or no cementitious properties. Class C fly ash is normally produced from burning lignite or subbituminous coal, and in addition to having pozzolanic properties, has some autogenous cementitious properties. In general, Class F fly ash is available in the eastern United States and Canada, and Class C fly ash is available in the western United States and Canada. Specifications for fly ash are covered in ASTM C 618. Methods for sampling and testing are found in ASTM C 311. Variations in physical or chemical properties of mineral admixtures, although within the tolerances of these specifications, may cause appreciable variations in properties of high-strength concrete. Such variations can be minimized by appropriate testing of shipments and increasing the frequency of sampling. ACI 212.2R provides guidelines for the use of admixtures in concrete. It is extremely important that mineral admixtures be tested for acceptance and uniformity and carefully investigated for strength-producing properties and compatibility with the other materials in the high-strength concrete mixture before they are used in the work. 2.4.2 Silica fume - Silica fume and admixtures containing silica fume2.8 have been used in high-strength concretes 2.9, 2.10 for structural purposes and for surface applications and as repair materials in situations where abrasion resistance and low permeability are advantageous. Silica fume is a by-product resulting from the reduction of high-purity quartz with coal in electric arc furnaces in the production of silicon and ferrosilicon alloys. The fume, which has a high content of amorphous silicon dioxide and consists of very fine spherical particles, is collected from the gases escaping from the furnaces. Silica fume consists of very fine vitreous particles with a surface area on the order of 20,000 m 2/kg when measured by nitrogen adsorption techniques 2.29 The particle-size distribution of a typical silica fume shows most par-titles to be smaller than one micrometer (1 µm) with an average diameter of about 0.1 µm, which is approximately 100 times smaller than the average cement particle. The specific gravity of silica fume is typically 2.2, but may be as high as 2.5. The bulk density as collected

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is 10 to 20 lb/ft3 (160 to 320 kg/m3); however, it is also available in densified or slurry forms for commercial application. Silica fume, because of its extreme fineness and high silica content, is a highly effective pozzolanic material. The silica fume reacts pozzolanically with the lime during the hydration of cement to form the stable cementitious compound calcium silicate hydrate (CSH). The availability of high-range water-reducing admixtures has facilitated the use of silica fume as part of the cementing material in concrete to produce high-strength concretes.2.29 Normal silica fume content ranges from 5 to 15 percent of portland cement content. The use of silica fume to produce high-strength concrete increased dramatically in the 1980s. Both laboratory and field experience indicate that concrete incorporating silica fume has an increased tendency to develop plastic shrinkage cracks. 2.29 Thus, it is necessary to quickly cover the surfaces of freshly placed silica-fume concrete to prevent rapid water evaporation. Since it is a relatively new material to the concrete industry in the United States, the user is referred to several recent symposia and publications for additional information on 2.4.3 Slag cement - Ground slag cement is produced only in certain areas of the United States and Canada. Specifications for ground granulated blast furnace slag are given in ASTM C 989. The classes of portland blast furnace slag cement are covered in ASTM C 595. Slag appropriate for concrete is a nonmetallic product that is developed in a molten condition simultaneously with iron in a blast furnace. When properly quenched and processed, slag will act hydraulically in concrete as a partial replacement for portland cement. Slag can be interground with cement or used as an additional cement at the batching facility. Blast furnace slag essentially consists of silicates and alumino-silicates of calcium and other bases. Research using ground slag shows much promise for its use in high-strength concrete. 2.4.4 Evaluation and selection - Mineral admixtures and slag cement, like any material in a high-strength concrete mixture, should be evaluated using laboratory trial batches to establish the optimum desirable qualities. Materials representative of those that will be employed later in the actual construction should be used. Particular care should be taken to insure that the mineral admixture comes from bulk supplies and that they are typical. Generally, several trial batches are made using varying cement factors and admixture dosages to establish curves which can be used to select the amount of cement and admixture required to achieve the desired results. When fly ash is to be used, the minimum requirement is that it comply with ASTM C 618. Although this specification permits a higher loss on ignition, an ignition loss 2.11 of 3 percent or less is desirable. High fineness, uniformity or production, high pozzolanic activity, and compatibility with other mixture ingredients are items of primary importance.

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2.5-Aggregates

2.5.1 General - Both fine and coarse aggregates used for high-strength concrete should, as a minimum, meet the requirements of ASTM C 33; however, the following exceptions may be beneficial. 2.5.2 - Grading 2.5.2.1 Fine aggregate - Fine aggregates with a rounded particle shape and smooth texture have been found to require less mixing water in concrete and for this reason are preferable in high-strength concrete2.11’ 2.12 The optimum gradation of fine aggregate for highstrength concrete is determined more by its effect on water requirement than on physical packing. One report 2.10 stated that a sand with a fineness modulus (PM) below 2.5 gave the concrete a sticky consistency, making it difficult to compact. Sand with an FM of about 3.0 gave the best workability and compressive strength. High-strength concretes typically contain such high contents of fine cementitious materials that the grading of the aggregates used is relatively unimportant compared to conventional concrete. However, it is sometimes helpful to increase the fineness modulus. A National Crushed Stone Association report2.13 made several recommendations in the interest of reducing the water requirement. The amounts passing the No. 50 and 100 sieves should be kept low, but still within the requirements of ASTM C 33, and mica or clay contaminants should be avoided. Another investigation2.13 found that the sand gradation had no significant effect on early strengths but that “at later ages and consequently higher levels of strength, the gap-graded sand mixes exhibited lower strengths than the standard mixes.” 2.5.2.2 Coarse aggregate compressive strength with high cement content and low water-cement ratios the maximum size of coarse aggregate should be kept to a minimum, at ½ in. (12.7 mm) or H in. (9.5 mm). Maximum sixes of ¾ in. (19.0 mm) and 1 in. (25.4 mm also have been used successfully. Cordon and Gillespie 2.19 felt that the strength increases were caused by the reduction in average bond stress due to the increased surface area of the individual aggregate. Alexander 2.20 found that the bond to a 3 in. (76 mm) aggregate particle was only about l/10 of that to a ½-in. (13 mm) particle. He also stated that except for very good or very bad aggregates the bond strength was about 50 to 60 percent of the paste strength at 7 days. Smaller aggregate sixes are also considered to produce higher concrete strengths because of less severe concentrations of stress around the particles, which are caused by differences between the elastic moduli of the paste and the aggregate. Many studies have shown that crushed stone produces higher strengths than rounded gravel. The most likely reason for this is the greater mechanical bond which can develop with angular particles. However, accentuated angularity is to be avoided because of the attendant high water requirement and reduced workability. The ideal ag-

gregate should be clean, cubical, angular, 100 percent crushed aggregate with a minimum of flat and elongated particles. 2.13 Because, as stated earlier, bond strength is the limiting factor in the development of high-strength concrete, the mineralogy of the aggregates should be such as to promote chemical bonding. Some work has been done with artificial material such as portland and aluminous cement clinkers and selected slags.2.14,221 The long-term stability of the clinkers is in question, however. Harris2.22 states that Moorehead measured a potential silica-lime bond of at least 28,000 psi (193 M Pa). Presumably many siliceous minerals would prove to have good bonding potential with portland cement. This would appear to be a promising area for further research. 2.5.3 Absorption -Curing is extremely important in the production of high-strength concrete. To produce a cement paste with as high a solids content as possible, the concrete must contain the absolute minimum mix water. However, after the concrete is in place and the paste structure is established, water should be freely available, especially during the early stages of hydration 2.14,2.23 During this period, a great deal of water combines with the cement. All of this water loses approximately ¼ of its volume after the chemical reactions are completed. This creates a small vacuum that is capable of pulling water short distances into the concrete which, at this time, is still relatively permeable. Any extra water which can enter the structure will increase the ultimate amount of hydration and, therefore the percent of solids per unit volume of paste, thereby increasing its strength. If the aggregates are capable of absorbing a moderate amount of water, they can act as tiny curing-water reservoirs distributed throughout the concrete, thereby providing the added curing water which is beneficial to these low water-cement ratio pastes. 2.5.4 Intrinsic aggregate strength-It would seem obvious that high-strength concrete would require highstrength aggregates and, to some extent, this is true. However, several investigators2.24,2.25 have found that, for some aggregates, a point is reached beyond which further increases in cement content produce no increase in the compressive strength of the concrete. This apparently is not due to having fully developed the compressive strength of the concrete but to having reached the limit of the bonding potential of that cement-aggregate combination. 2.6-Water The requirements for water quality for high-strength concrete are no more stringent than those for conventional concrete. Usually, water for concrete is specified to be of potable quality. This is certainly conservative but usually does not constitute a problem since most concrete is produced near a municipal water supply. However, cases may be encountered where water of a lower quality must be used. In such cases, test concrete should be made with the water and compared with concrete made

TH CONCRETE

with distilled water, or it may be more convenient to make ASTM C 109 mortar cubes. In either case, specimens should be tested in compression at 7 and 28 days. If those made with the water in question are at least equal to 90 percent of the compressive strength of the specimens made with distilled water, the water then can be considered acceptable to U.S. Army Corps of Engineers’ requirements2.26 and ASTM C 94. For more detailed information on specific contaminants refer to the literature in References 2.27, 2.28, and 2.29. Test methods for water for special situations are given in AASHTO T26. 2.7-Cited references

(See also Chapter 10-References) 2.1. Hester, Weston, “High Strength Air-Entrained Concrete,” Concrete Construction, V. 22, No. 2, Feb. 1977, pp. 77-82. 2.2. “High Strength Concrete in Chicago High-Rise Buildings,” Task Force Report No. 5, Chicago Committee on High-Rise Buildings, Feb. 1977, 63 pp. 2.3. Freedman, Sydney, “High-Strength Concrete,” Modern Concrete, V. 34, No. 6, Oct. 1970, pp. 29-36; No. 7, Nov. 1970 pp 28-32; No. 8, Dec. 1970, pp. 21-24; No. 9, Jan. 1971, pp. 15-22; and No. 10, Feb. 1971, pp. 16-23. Also, Publication No. IS176T, Portland Cement Association. 2.4. “Superplasticizing Admixtures in Concrete,” Publication No. 45.030, Cement and Concrete Association, Wexham Springs, 1976, 32 pp. 2.5. Eriksen, Kirsten, and Nepper-Christensen, Palle, “Experiences in the Use of Superplasticizers in Some Special Fly Ash Concretes,” Developments in the Use of Superplasticizers, SP-68, American Concrete Institute, Detroit, 1981, pp. 1-20. 2.6. Developments in the Use of Superplasticizers, SP-68, American Concrete Institute, Detroit, 1981, 572 pp. 2.7. Wolsiefer, John, “Ultra High-Strength Field Placeable Concrete with Silica Fume Admixture,” Concrete International Design & Construction, V. 6, No. 4, Apr. 1984, pp. 25-31. 2.8. Malhotra, V.M., and Carette, G.G., “Silica Fume,” Concrete Construction, V. 27, No. 5, May 1982, pp. 443446. 2.9. Fly Ash, Silica Fume, Slag, and other Mineral By-Products in Concrete, SP-79, American Concrete Institute, Detroit, 1983, 1196 pp. 2.10. Blick, Ronald L., “Some Factors Influencing High-Strength Concrete,” Modern Concrete, V. 36, No. 12, Apr. 1973, pp. 38-41. 2.11. Wills, Milton H., Jr., “How Aggregate Particle Shape Influences Concrete Mixing Water Requirement and Strength,” Journal of Materials, V. 2, No. 4, Dec. 1967, pp. 843-865. 2.12. Gaynor, R.D., and Meininger, R.C., “Evaluating Concrete Sands: Five Tests to Estimate Quality,” Concrete International Design & Construction, V. 5, No. 12, Dec. 1983, pp. 53-60.

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2.13. “High Strength Concrete,” Manual of Concrete Materials-Aggregates, National Crashed Stone Association, Washington, D.C. Jan. 1975, 16 pp. 2.14. Perenchio, W.P., “An Evaluation of Some of the Factors Involved in Producing Very High-Strength Concrete,” Research and Development Bulletin No. RD014, Portland Cement Association, Skokie, 1973, 7 pp. 2.15. “Methods of Achieving High Strength Concrete,” ACI JOURNAL , Proceedings V. 64, No. 1, Jan. 1967, pp. 45-48. 2.16. Fowler, Earl W., and Lewis, D.W., “Flexure and Compression Tests of High Strength, Air-Entraining Slag Concrete,” ACI JOURNAL, Proceedings V. 60, No. 1, Jan. 1963, pp. 113-128. 2.17. Harris, A.J., “High-Strength Concrete: Manufacture and Properties,” The Structural Engineer (London), V. 47, No. 11, Nov. 1969, pp. 441-446. 2.18. Walker, Stanton, and Bloem, Delmar L., “Effects of Aggregate Size on Properties of Concrete,” ACI JOURNAL, Proceedings V. 57, No. 3, Sept. 1960, pp. 283298. 2.19. Cordon, William A, and Gillespie, H. Aldridge, “Variables in Concrete Aggregates and Portland Cement Paste Which Influence the Strength of Concrete,” ACI JOURNAL , Proceedings V. 60, No. 8, Aug. 1963, pp. 10291052. 2.20. Alexander, K.M., “Factors Controlling the Strength and Shrinkage of Concrete,” Constructional Review (North Sydney), V. 33, No. 11, Nov. 1960, pp. 19-28. 2.21 “Tentative Interim Report of High Strength Concrete,” ACI JOURNAL , Proceedings V. 64, No. 9, Sept. 1967, pp. 556-557. 2.22. Harris, A.J., “Ultra High Strength Concrete,” Journal, Prestressed Concrete Institute, V. 12, No. 1, Feb. 1967, pp. 53-59. 2.23. Klieger, Paul, “Early High Strength Concrete for Prestressing,” Proceedings, World Conference on Prestressed Concrete, San Francisco, 1957, pp. A5-1-A5-14. 2.24. Burgess. A. James; Ryell, John; and Bunting, John, “High Strength Concrete for the Willows Bridge,” ACI JOURNAL, Proceedings V. 67, No. 8, Aug. 1970, pp. 611-619. 2.25. Gaynor, Richard D., “High Strength Air-Entrained Concrete,” Joint Research Laboratory Publication No. 17, National Sand and Gravel Association/National Ready Mixed Concrete Association, Silver Spring, Mar. 1968, 19 pp. 2.26. “Requirements for Water for Use in Mixing or Curing Concrete,” (CRD-C 400-63), Handbook for Concrete and Cement, U.S. Army Engineer Waterways Experiment Station, Vicksburg, 2 pp. 2.27. Concrete Manual, 8th Edition, U.S. Bureau of Reclamation, Denver, 1975, 627 pp. 2.28. McCoy, W.J., “Mixing and Curing Water for Concrete,” Significance of Tests and Properties of Concrete and Concrete-Making Materials, STP-169A, American Society for Testing and Materials, Philadelphia, 1966, pp.

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515-521. 2.29. “Silica Fume in Concrete,” preliminary report by ACI Committee 226, Materials Journal, American Concrete Institute, Detroit, V. 84, No. 2, Mar.-Apr. 1987. 2.30 Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, SP-91, American Concrete Institute, Detroit, 1986, 1628 pp. 2.31 Proceedings of the International Workshop on Condensed Silica Fume in Concrete, CANMET, Montreal, Canada, May 1987. 2.32 Proceedings of the Third International Conference on Fly Ash, Silica Fume, and Natural Pozzolans in Concrete, SP-114, American Concrete Institute, Detroit, 1989.

tors, inspection agencies, and environmental conditions. All factors which will affect the variability of strengths and strength measurements should be considered when selecting mix proportions and when establishing the standard deviation acceptable for strength results. Materials and proportions used for qualifying the mixture should not be more closely controlled than is planned for the proposed work. Kennedy and Price have identified factors which contribute to the variability of measured compressive strengths of concretes in lower strength ranges. 3.3,3.4 Hester identified sources of measured strength variations in high-strength concretes. 3.5 High-strength concrete is recognized to be more difficult to test accurately than normal strength concretes. Testing difficulties may contribute to lower measured values or higher variability. A high variance in test results will dictate a higher CHAPTER 3 - CONCRETE MIX PROPORTIONS required average strength. If variability is predicted to be relatively low, but proves to be higher, the frequency of test results below the specified strength may be unacceptConcrete mix proportions for high-strength concrete ably high. Therefore, when selecting a target standard have varied widely depending upon many factors. The deviation the concrete producer should submit the most 3.6 A higher required average strength level required, test age, material characteristics, appropriate test record. and type of application have influenced mix proportions. strength may be difficult or impossible to attain when In addition, economics, structural requirements, manufac- producing high-strength concretes because mix proporturing practicality, anticipated curing environment, and tions may already be optimized. ACI 318 recognizes that some test results are likely to even the time of year have affected the selection of mix proportions. Much information on proportioning con- be lower than the specified strength. The most common crete mixtures is available in ACI 211.1 and ACI design approach has been to limit the frequency of tests SP-46. 3.1 Included in ACI publication SP-46 is the paper allowed to fall below the specified strength. The concrete “Proportioning and Controlling High Strength Concrete” has been judged acceptable if the following requirements are met: (SP-46-9). a) The average of all sets of three consecutive strength High-strength concrete mix proportioning is a more critical process than the design of normal strength con- test results shall equal or exceed the required fc'. b) No individual strength test (average of two cylincrete mixtures. Usually, specially selected pozzolanic and chemical admixtures are employed, and the attainment of ders) shall fall below fc' by more than 500 psi (3.4 MPa). However, some designers have specified higher or a low water-cementitious ratio is considered essential. Many trial batches are often required to generate the lower overdesign strengths than called for in ACI 318 data that enables the researcher to identify optimum mix regardless of established performance. Schmidt and Hoffman3.7 report that they do not autoproportions. maticalIy order removal of concrete which is represented by cylinders 500 psi (3.4 MPa) below specified strength 3.2-Strength required but do order adjustment of the mixture and correction of 3.2.1 ACI 318- The ACI Building Code Requirements for Reinforced Concrete (ACI 318) describes concrete the deficiency. This is because the ACI 318 Section 4.7.4 strength requirements. Normally the concrete has been was established for concretes with strengths in the range proportioned in such a manner that the mean average of of 3000 to 5000 psi (21 to 34 MPa). High-strength concompressive strength test results has exceeded the spe- cretes continue to gain considerable strengths above and beyond design requirements with the passage of time, cified strength fc' by an amount sufficiently high to 3.7 minimize the relative frequency of test results below the more than lower-strength concretes. While the percentage gain of compressive strength of high-strength conspecified strength value. An average value can be calculated for any set of cretes from 7 days to 90 days may be equal to or lower measurement data. The amount that individual test val- than concretes in lower strength ranges, the order of ues deviate from the average is usually quantified by magnitude of strength gain expressed in psi is actually calculation of the standard deviation. Calculation of much higher. For example, a mixture which averages standard deviation on concrete test histories can be a 2500 psi (17.2 MPa) in 7 days may average 4200 psi (29 MPa) in 90 days. It would have gained strength equal to valuable aid in predicting future test result variability. Many factors can influence the variability of the test 68 percent of the 7-day strength, or 1700 psi (11.7 MPa) results, including the individual materials, plants, contrac- at the age of 90 days. A mixture averaging 7300 psi (50.3


MPa) in 7 days could average 10,000 psi (69 MPa) in 90 days. That would be an increase of only 37 percent, but it would have gained 2700 psi (18.6 MPa), a full 1000 psi (6.9 MPa) higher total gain than the lower-strength mixture. ACI 318 allows mix designs to be proportioned based on field experience or by laboratory trial batches. When the concrete producer chooses to select high-strength concrete mix proportions based upon laboratory trial batches, confirming tests results from concretes placed in the field should also be established. 3.2.2 ACI 214-- Once sufficient test data have been generated from the job, a reevaluation of mix proportions using “Recommended Practice for Evaluation of Compression Test Results of Concrete (ACI 214)” may be appropriate. Analyses affecting reproportioning of mixtures based upon test histories are described in Sections 4.8.1 and 4.8.2. 3.2.3 Other Strength Requirements--In some situations, considerations other than compressive strength may influence mix proportions. Detailed discussion of material properties including flexural and tensile strengths is given in Chapter 5. 3.3- Test age The selection of mix proportions can be influenced by the testing age. This testing age has varied depending upon the construction requirements. Most often the testing age has been thought to be the age at which the acceptance criteria are established, for example at 28 days. Testing, however, has been conducted prior to the age of acceptance testing, or after that age, depending upon the type of information required. 3.3.1 Early Age-- Prestressed concrete operations may require high strengths in 12 to 24 hours. Special applications for early use of machinery foundations, pavement traffic lanes, or slip formed concrete have required high strengths at early ages. Post-tensioned concrete is often stressed at ages of approximately 3 days and requires relatively high strengths. Generally concretes which develop high later-age strengths will also produce high early-age strengths. However, the optimum materials selected, and therefore the mix proportions, may vary for different test ages. For example, Type III cement and no fly ash have been used in a high early-strength design, compared to Type I or II cement and fly ash for a laterage strength design. Early-age strengths may be more variable due to the influence of curing temperature and the early-age characteristics of the specific cement. Therefore, anticipated mix proportions should be evaluated for a higher required average strength or a later test age. 3.3.2 Twenty-eight days- A very common test age for compressive strength of concrete has been 28 days. Performance of structures has been empirically correlated with moist-cured concrete cylinders, usually 6 x 12 in. (152 x 305 mm) prepared according to ASTM C 31 and C 192. This has produced good results for concretes with-

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in lower strength ranges not requiring early strengths or early evaluation. High-strength concretes gain considerable strengths at later ages and, therefore, are evaluated at later ages when construction requirements allow the concrete more time to develop strengths before loads are imposed. Proportions, notably cementitious components, have usually been adjusted depending upon test age. 3.3.3 Later age-- High-strength concretes are frequently tested at later ages such as 56 or 90 days. High-strength concrete has been placed frequently in columns of highrise buildings. Therefore, it has been desirable to take advantage of long-term strength gains so that efficient use of construction materials can be achieved. This has often been justified in high-rise buildings where full loadings may not occur until later ages. In cases where later-age acceptance criteria have been specified, it may be advantageous for the concrete supplier to develop earl -age or accelerated tests to predict later-age strengths.3.8 The ACI publication SP-56, Accelerated StrengthTesting, provides information on accelerated testing. 3.9 Of course, historical correlation data must be developed relative to the materials and proportions to be used in the work. These tests may not always accurately predict later-age strengths; however, these tests could provide an early identification of lower-strength trends before a long history of non-compliance is realized. Later-age acceptance criteria can leave suspect concrete in question for a long time. Test cylinders have been held for testing at ages later than the specified acceptance age. In cases where the specified compressive strength fc' was not achieved, subsequent testing of later-age or “hold” cylinders has sometimes justified the acceptance of the concrete in question. 3.3.4 Test age in relationship to curing- When selecting mix proportions, the type of curing anticipated should be considered along with the test age, especially when designing for high early strengths. Concretes gain strength as a function of maturity, which is usually defined as a function of time and curing temperature. 3.4- Water-cement ratio or water-cementitious ratio 3.4.1 Nature of water-cement ratio in high-strength con-

crete- The relationship between water-cement ratio and compressive strength, which has been identified in lowstrength concretes, has been found to be valid for higherstrength concretes also. Higher cement contents and lower water contents have produced higher strengths. Proportioning larger amounts of cement into the concrete mixture, however, has also increased the water demand of the mixture. Increases in cement beyond a certain point have not always increased compressive strengths. Other factors which may limit maximum cement contents are discussed in Section 3.5.3. When pozzolanic materials are used in concrete, a water-cement plus pozzolan ratio by weight has been considered in place of the traditional water-cement ratio by weight. Fly ash meeting requirements of ASTM C 618 with a loss on ignition of less than 3.0 percent and ASTM C 494 types


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A, D, F, and G chemical admixtures have usually been used. 3.10 Of course the slump of the concrete is related to the water-cementitious ratio and the total amount of water in the concrete. While 0 to 2 in. slump concrete has been produced in precast operations, special consolidation efforts are required. Specified slumps for cast-in-place concretes not containing high-range water reducers have ranged from 21/2 to 41/4 in. (64 to 114 mm). Field-placed nonplasticized concretes have had measured slumps averaging as high as 43/4 in. (121 mm). 3.10 The use of high-range water reducers has provided lower water-cementitious ratios and higher slumps.3.11 Water-cementitious ratios by weight for high-strength concretes typically have ranged from 0.27 to 0.50. The quantity of liquid admixtures, particularly high-range water reducers, sometimes has been included in the water-cementitious ratio. 3.4.2 Estimating compressive strength-- The compressive strength that a concrete will develop at a given watercementitious ratio has varied widely depending on the cement, aggregates, and admixtures employed. Principal causes of variations in compressive strengths at a given water-cementitious ratio include the strengthproducing capabilities of the cement and potential for pozzolanic reactivity of the fly ash or other pozzolan if used. Different types and brands of portland cement have produced different compressive strengths as shown in Fig. 3.2,3.12 3.1.

particular pozzolan employed has varied and has generally increased with increasing fineness of the pozzolan. Often water requirements for fly ash concrete are lower than for portland cement. This helps to lower the watercementitious ratio of the mixture. Perenchio 3.15 has reported variable compressive strength results at given water-cement ratios in laboratory prepared concretes, depending on the aggregates used. In addition, these results have differed from results achieved in actual production with materials from the same area. A range of typical strengths reported at given watercementitious ratios is represented in Fig. 3.2. Trial batches with materials actually to be used in the work have been found to be necessary. Generally, laboratory trial batches have produced strengths higher than those strengths which are achievable in production, as seen in Fig. 3.33.2

Krxo

Compressive Strength, MPa

Compressive

Strength , psi 800

Compressive Strength , psi

10000

Water - Cementitious

Ratio

Fig. 3.2-- Strength versus water-cement ratios of various

L

mixtures

3.2,3.10,3.15,3.16

3.5-Cement content Fig. 3.1-- Effects of various brands of cement on concrete 3.2,3.12 compressive strength

Specific information pertaining to the range of values of compressive strengths of cements has been published in ASTM C 917 and Peters.3.13 Fly ashes may vary in pozzolanic activity index from 75 percent to 110 percent of the portland cement control, as defined in ASTM C 618. Proprietary pozzolans containing silica fume have been reported to have activity indexes in excess of 200 percent.3.14 The water requirement of the

The cement quantity proportioned into a high-strength mixture has been determined best by the fabrication of trial batches. Common cement contents in high-strength concrete test programs range from 660 to 940 lb per yd 3 3.2,3.16 (392 to 557 kg/m3). In evaluating optimum cement contents, trial mixes usually are proportioned to equal consistencies, allowing the water content to vary according to the water demand of the mixture. 3.5.1 Strength-For any given set of materials in a concrete mixture, there may be a cement content that produces maximum concrete strength. The maximum strength may not always be increased by the use of


r

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50 -

. Agg. No. I . Agg. No. 2 . Agg. No.3

Reduction in Comp. 40 Strength Below Non-A.E. Concrete 30

8000 Compessive Strength, MPo

Compressive Strength, psi 6000

of Some W/C, %

40 Points Represent Avg. of ?-and 28-Day Tests

4000 0

2

4

6

6

IO

Added Air , percent

Fig. 3.4--Strength reduction by air entrainment 3.26

2000

the completeness of the testing programs, but particular attention has been given to evaluation of the brand of 28 56 cement to be used with the class and source of pozzolan, Age , days if a pozzolan is to be used. Prior to 1977, Chicago highstrength experience was based on concretes using Class Fig. 3.3--Laboratory-molded concrete strengths versus F fly ash, while other high-strength work has been done ready-mixed field-molded concrete strengths for 9000 psi (62 in Houston using Class C fly ash. 3.2,3.10 Class C fly ash MPa) concrete. 3.2 has been used in Chicago since 1977. The strength efficiency of cement will vary for differcement added to the mixture beyond this optimum ceent maximum size aggregates at different strength levels. ment content. The strength for any given cement content Higher cement efficiencies are achieved at high strength will vary with the water demand of the mixture and the levels with lower maximum aggregate sixes. -* Fig. 3.5 strength-producing characteristics of that particular illustrates this principle. For example, a maximum agcement as shown in Fig. 3.1. The “Standard Method of gregate size of less than % in. (9.5 mm) yields the highest Evaluation of Cement Strength Uniformity from a Single cement efficiency for a 7000 psi (48.3 MPa) mixture. Source” (ASTM C 917) may prove useful in considering cement mill sources. 3.13 Mortar cube compressive IO strength data of cements at ages of up to 90 days have been evaluated when proportioning cement in highstrength mixtures. The strength of the concrete mixture will depend upon the gel-space ratio, which is defined as the “ratio of the volume of hydrated cement paste to the sum of the volumes of the hydrated cement and of the capillary 6 pores.“ 3.17 This is particularly true when air-entraining Strength Efficiency, admixtures are employed. Higher cement contents in airp i / l b o f c e m e n t / c u Yd entrained concrete have not been found to be useful in r/ / producing strengths equivalent to, or approaching, strengths attainable with non-air-entrained concretes. Incorporation of entrained air may reduce strength at a ratio of 5 to 7 percent for each percent of air in the mix = 3.8 to 5.8 in. = 28 days, Moist as shown in Fig. 3.4. 3.5.2 Optimization-- A principal consideration in establishing the desired cement content will be the identification of combinations of materials which will produce optimum strengths. Ideally, evaluations of each potential No. 4 + $ It 3 6 source of cement, fly ash, liquid admixture, and aggregate Maximum Size Aggregate , in. in varying concentrations would indicate the optimum cement content and optimum combination of materials. Fig. 3.5-- Maximum size aggregate for strength efficiency enTesting costs and time requirements usually have limited velop. 3.2 1

1

-11 90

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ACI COMMlTTEE REPORT

3.5.3 Limiting factors-There are several factors which may limit the maximum quantity of cement which may be desirable in a high-strength mixture. The strength of the concrete may decrease if cement is added above and beyond a given optimum content. The maximum desirable quantity of cement may vary considerably depending upon the efficiency of dispersing agents, such as highrange water reducers, in preventing flocculation of cement particles. Stickiness and loss of workability will be increased as higher amounts of cement are incorporated into the mixture. Combinations of cement, pozzolans, and sand should be evaluated for the effect of cementitious content upon mixture placeability. Incorporation of an airentraining admixture may necessitate reevaluation of the effect of the cement upon mixture workability. The maximum temperature desired in the concrete element may limit the quantity or type of cement in the mixture. 3.2,3.18 Modification of the mixture with ice, set retarders, or pozzolans may be helpful. Cement-rich mixtures frequently have very high water demands. Therefore, it is possible that special precautions may be necessary to provide adequate curing water, so that sufficient hydration can occur. It may be preferable to reduce the amount of cement in the mixture and to rely upon more careful selection of aggregates, aggregate proportions, etc., optimizing the use of other constituents. The amount of slump loss experienced, with attendant increase in retempering water, and the setting time of the concrete has varied depending upon the type, brand, and quantity of cement use. Lower cement contents, within limits, are desirable in order to enhance the placement capabilities of the mixture, provided that adequate strengths can be achieved. 3.6-Aggregate proportions In the proportioning of high-strength concrete, the aggregates have been a very important consideration since they occupy the largest volume of any of the ingredients in the concrete. Usually, high-strength concretes have been produced using normal weight aggregates. Shideler3.19 and Holm 3.20 have reported on light-weight high-strength structural concrete. Mather3.21 has reported on high-strength high-density concrete using heavyweight aggregate. 3.6.1 Fine aggregates-In proportioning a concrete mixture, it is generally agreed that the fine aggregates or sand have considerably more impact on mix proportions than the coarse aggregates. The fine aggregates contain a much higher surface area for a given weight than do the larger coarse aggregates. Since the surface area of all the aggregate particles must be coated with a cementitious paste, the proportion of fine to coarse can have a direct quantitative effect on paste requirements. Furthermore, the shape of these sand particles may be either spherical, subangular, or very angular. This property can alter paste

requirements even though the net volume of the sand remains the same. The gradation of the fine aggregate plays an important role in properties of the plastic as well as the hardened concrete. For example, if the sand has an overabundance of the No. 50 and No. 100 sieve sixes, the plastic workability will be improved but more paste will be needed to compensate for the increased surface area. This could result in a costlier mixture, or if the paste volume is increased by adding water, a serious loss in strength could result. It is sometimes possible, although not always practical economically, to blend sands from different sources to improve their gradation and their capacity to produce higher-strength concrete. Low fine aggregate contents with high coarse aggregate contents have resulted in a reduction in paste requirements and normally have been more economical. Such proportions also have made it possible to produce higher strengths for a given amount of cementitious materials. However, if the proportion of sand is too low, serious problems in workability become apparent. Consolidation by means of mechanical vibrators may help to overcome the effects of an undersanded mixture, and the use of power finishing equipment can help to offset the lack of trowelability. Particle shape and surface texture of fine aggregate can have as great an effect on mixing water requirements as those of coarse aggregate.“” Tests made by Bloem and Gaynor 3.22 show that concrete-mixing water requirements for each cubic yard of concrete change 1 gal. (3.8 L) for each change of 1 percent in the void content of the sand. Following the work by Bloem and Gaynor, the NSGA-NRMCA Joint Research Laboratory has simplified the procedure for conducting the void content test of sand and a modified gradation is now used. The new procedure is described in Reference 2.12. 3.6.2 Coarse aggregates-The optimum amount and size of coarse aggregate for a given sand will depend to a great extent on the characteristics of the sand. Most particularly it depends on the fineness modulus (FM) of the sand. This is brought out specifically in Table 3.1, which is taken from ACI 211.1. One reference 3.23 suggests that the proportion of coarse aggregate shown in Table 3.1 might be increased by up to 4 percent if sands with low void contents are used. If the sand particles are very angular, then it is suggested that the amount of coarse aggregate should be decreased by up to 4 percent from the values in the table. Such adjustments in the proportion of coarse aggregate and sand have been intended to produce concretes of equivalent workability, although such changes will alter the water demand for a given slump. When more or less water is needed in a given volume of concrete, to preserve the same consistency of paste, it is also necessary to adjust the amount of cement or cementitious materials if a given watercement ratio is to be maintained. Another possible expedient in the proportioning of coarse aggregates for high-strength concrete is to alter


Table 3.1-Volume of coarse aggregate per unit of volume of concrete*

I

Volume of dry-rodded coarse aggregate’ per unit volume of concrete for different fineness moduli of sand

*Table 3.1 Taken from ACI 211.1. +Volumes are based on aggregates in dry-rodded condition as described in ASTM C 29 for Unit Weight of Aggregate. These volumes are selected from empirical relationships to produce concrete with a degree of workability suitable for usual reinforced construction. For less workable concrete such as required for concrete pavement construction, they may be increased about 10 percent. For more workable concrete see Section 5.3.6.1.

the amount of these aggregates passing certain sieve sixes from the amounts shown in ASTM C 33. This method is described in Reference 3.24 and 3.25 as a means of avoiding “particle interference,” thus permitting a greater amount of coarse aggregate and less total sand. This has helped to reduce the paste requirements or permit the use of a more viscous paste, resulting in a higher strength. 3.6.3 Proportioning aggregates-The amounts of coarse aggregate suggested in Table 3.1 (which is Table 5.3.6 of ACI 211.1) are recommended for initial proportioning. Considerations should be given to the properties of the sand (FM, angularity, etc.) which may alter the quantity of coarse aggregate. In general, the least sand consistent with necessary workability has given the best strengths for a given paste. Mechanical tools for handling and placing concrete have helped to decrease the proportion of sand needed. As previously stated, the use of the smaller sixes of coarse aggregate are generally beneficial, and crushed aggregates seem to bond best to the cementitious paste. 3.7-Proportioning with admixtures

Nearly all high-strength concretes have contained admixtures. Changes in the quantities and combinations of these admixtures affect the plastic and hardened properties of high-strength concrete. Therefore, special attention has been given to the effects of these admixtures (described in Sections 2.3 and 2.4). Careful adjustments to mix proportions have been made when changes in admixture quantities or combinations have been made. Material characteristics have varied extensively, making experimentation with the candidate materials necessary. Some of the more common adjustments are described in Sections 3.7.1 and 3.7.2. 3.7.1 Pozzolanic admixtures- Pozzolanic admixtures are often used as a cement replacement. In high-strength concretes they have been used to supplement the portland cement from 10 to 40 percent by weight of the ce-

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ment content. In those cases where a net increase in the absolute volume of the cementitious materials was experienced due to the addition of a pozzolan, a corresponding decrease in the absolute volume of the sand was usually made. The use of fly ash has often caused a slight reduction in the water demand of the mixture, and that reduction in the volume of water (if any) has been compensated for by the addition of sand. The opposite relationship has been found to be true for other pozzolans. Silica fume, for example, dramatically increases the water demand of the mixture which has made the use of retarding and superplasticizing admixtures a requirement. Proprietary products containing silica fume include carefully balanced chemical admixtures as wel13.14 3.7.2 Chemical admixtures 3.7.2.1 Conventional water-reducers and retardersThe amount of these admixtures used in high-strength concrete mixtures has varied depending upon the particular admixture and application. Generally speaking, the tendency has been to use larger than normal or maximum quantities of these admixtures. Typical water reductions of 5 to 8 percent may be increased to 10 percent. Corresponding increases in sand content have been made to compensate for the loss of volume due to the reduction of water in the mixture. 3.7.3.2 Superplasticizers or high-range water-reducing admixtures-Adjustments to high-strength concrete made with high-range water reducers have been similar to those adjustments made when conventional water reducers are used. These adjustments have typically been larger due to the larger amount of water reduction, approximately 12 to 25 percent. Corresponding increases in sand content have been made to compensate for the loss of volume from reduction of water in the mixture. Some designers have simply added high-range water reducers to existing mixtures without any adjustments to the mix proportions to improve the workability of that concrete. Sometimes cement or cementitious content has been reduced for reasons of economy or to achieve a reduction of the heat of hydration. Usually, however, in high-strength concretes high-range water reducers are used to lower the water-cementitious ratio. These admixtures have been effective enough to both lower the water-cementitious ratio and increase the slump. Due the relatively large quantity of liquid that has been added to the mixture in the form of superplasticizing admixture, the weight of these admixtures has sometimes been included in the calculation of the water-cementitious ratio. 3.7.2.3 Air-entraining agents- Although sometimes required, air-entraining agents have been found to be very undesirable in high-strength concretes due the dramatic decrease in compressive strength which occurs when these admixtures are used. Modifications to lower the water-cementitious ratio and adjust the yield of the concrete by reduction of sand content have been made. Larger dosage rates of air-entraining admixture have

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been found to be required in high-strength concretes, especially in very rich low-slump mixtures and mixtures containing large quantities of some fly ashes. 3.7.2.4 Combinations- Most but not all highstrength concretes have contained both mineral and chemical admixtures. It has been common for these mixtures to contain combinations of chemical admixtures as well. High-range water reducers have performed better in high-strength concretes when used in combination with conventional water reducers or retarders. This is because of the reduced rate of slump loss experienced. It is not unusual for portland-pozzolan high-strength concretes to contain both a conventional and high-range water reducer. 3.8-Workability

Workability is defined in ACI 116R “Cement and Concrete Terminology” as “that property of freshly mixed concrete . . . which determines the ease and homogeneity with which it can be mixed, placed, compacted, and finished.” 3.8.1 Slump- ASTM C 143 describes a standard test method for the slump of portland cement concrete which has been used to quantify the consistency of plastic, cohesive concretes. This test method has not usually been considered applicable to ultra-low and ultra-high slump concretes. Other test methods such as the Vebe consistometer have been used with very stiff mixes and may be a better aid in proportioning some high-strength concretes. High-strength concrete performance demands a dense, void-free mass with full contact with reinforcing steel. Slumps should reflect this need and provide a workable mixture, easy to vibrate, and mobile enough to pass through closely placed reinforcement. Normally a slump of 4 in. (102 mm) will provide the required workability; however, details of forms and reinforcing bar spacing should be considered prior to development of mix designs. Slumps of less than 3 in. (76 mm) have made special consolidation equipment and procedures a necessity. Without uniform placement, structural integrity may be compromised High-strength mixes tend to lose slump more rapidly than lower-strength concrete. If slump is to be used as a field control, testing should be done at a prescribed time after mixing. Concrete should be discharged before the mixture becomes unworkable. 3.8.2 Placeability-- High-strength concrete, often designed with % in. (12 mm) top size aggregate and with a high cementitious content, is inherently placeable provided attention is given to optimizing the ratio of sand to coarse aggregate. Local material characteristics have a marked effect on proportions. Cement fineness and particle size distribution influence the character of the mixture. Admixtures have been found to improve the placeability of the mixture. Placeability has been evaluated in mock-up forms prior to final approval of the mix proportions. At that

time placement procedures, vibration techniques, and scheduling have been established since they greatly affect the end product and will influence the apparent placeability of the mixture. 3.8.3 Flow properties and stickiness-Slumps needed for almost any flow can be designed for the concrete; however, full attention must be given to aggregate selection and proportioning to achieve the optimum slump. Elongated aggregate particles and poorly graded coarse and fine aggregates are examples of characteristics that have affected flow and caused higher water content for placeability with attendant strength reduction. Stickiness is inherent in high-fineness mixtures required for high strengths. Certain cements or cementpozzolan or cement-admixture combinations have been found to cause undue stickiness that impairs flowability. The cementitious content of the mixture normally has been the minimum quantity required for strength development combined with the maximum quantity of coarse aggregate within the requirements for workability. Mixtures that were designed properly but appear to change in character and become more sticky can be considered suspect and quickly checked for proportions, possible false setting of cement, undesirable air entrainment, or other changes. A change in the character of a highstrength mixture could be a warning sign for quality control and, while a subjective judgment, may sometimes be more important than quantitative parameters. 3.9- Trial batches Frequently the development of a high-strength concrete pro ram has required a large number of trial batches. 3.2,3.10 In addition to laboratory trial batches, field-sized trial batches have been used to simulate typical production conditions. Care should be taken that all material samples are taken from bulk production and are typical of the materials which will be used in the work. To avoid accidental testing bias, some researchers have sequenced trial mixtures in a randomized order. 3.9.1 Laboratory trial batch investigations- Laboratory trial batches have been prepared to achieve several goals. They should be prepared according to “Standard Method of Making and Curing Concrete Test Specimens in the Laboratory” (ASTM C 192). However, whenever possible, timing, handling, and environmental conditions similar to those which are likely to be encountered in the field should be approximated. Selection of material sources has been facilitated by comparative testing, with all variables except the candidate materials being held constant. In nearly every case, particular combinations of materials have proven to be best. By testing for optimum quantities of optimum materials, the investigator is most likely to define the best combination and proportions of materials to be used. Once a promising mixture has been established, further laboratory trial batches may be required to quantify the characteristics of those mixtures. Strength characteristics at various test ages may be defined. Water

TH

demand, rate of slump loss, amount of bleeding, segregation, and setting time can be evaluated. The unit weight of the mixture should be defined and has been used as a valuable quality control tool. Structural considerations such as shrinkage and elasticity may also be determined. While degrees of workability and placeability may be difficult to define, at least a subjective evaluation should be attempted. 3.9.2 Field-production trial batches- Once a desirable mixture has been formulated in the laboratory, field testing with production-sized batches is recommended. Quite often laboratory trial batches have exhibited a strength level significantly higher than that which can be reasonably achieved in production as shown in Fig. 3.3 3.2 Actual field water demand, and therefore concrete yield, has varied from laboratory design significantly. Ambient temperatures and weather conditions have affected the performance of the concrete. Practicality of production and of quality control procedures have been better evaluated when production-sized trial batches were prepared using the equipment and personnel that were to be used in the actual work. 3.10-Cited references (See also Chapter l0-References) 3.1. Proportioning Concrete Mixes, SP-46, American Concrete Institute, Detroit, 1974, 240 pp. 3.2. Blick, Ronald L.; Petersen, Charles F.; and Winter, Michael E., “Proportioning and Controlling High Strength Concrete,” Proportioning Concrete Mixes, SP-46, American Concrete Institute, Detroit, 1974, p. 149. 3.3. Kennedy, T.B., “Making and Curing Concrete Specimens,” Significance of Tests and Properties of Concrete and Concrete-Making Materials, STP-169A, American Society for Testing and Materials, Philadelphia, 1966, pp. 90-101. 3.4. Price, Waller H., “Factors Influencing Concrete Strength,” ACI JOURNAL, Proceedings V. 47, No. 6, Feb. 1951, pp. 417-432. 3.5. Hester, Weston T., “Testing High Strength Concretes: A Critical Review of the State of the Art,” Concrete International Design & Construction, V. 2, No. 12, Dec. 1980, pp. 27-38. 3.6. Gaynor, Richard D., “Mix Design Submission Under ACI 318 and ACI 301--(or Which Test Record Should I Use?),” NRMCA Technical Information Letter No. 372, National Ready Mixed Concrete Association, Silver Spring, May 8, 1980, 7 pp. 3.7. Schmidt, William, and Hoffman, Edward J., “9000 psi Concrete-Why? Why Not?,” Civil EngineeringASCE, V. 45, No. 5, May 1975, pp. 52-55. 3.8. Gaynor, Richard D., “An Outline on High Strength Concrete,” Publication No. 152, National Ready Mixed Concrete Association, Silver Spring, May 1975, pp. 3, 4, and 10. 3.9. Accelerated Strength Testing, SP-56, American Concrete Institute, Detroit, 1978, 328 pp. 3.10. Cook, James E., “A Ready-Mixed Concrete

CONCRETE

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Company’s Experience with Class C Fly Ash,” Publication No. 163, National Ready-Mixed Concrete Association, Silver Spring, Apr. 1981, 11 pp. 3.11. Hester, Weston, T., and Leming, M., “Use of Superplasticizing Admixtures in Precast, Prestressed Concrete Operations.” 3.12. “High Strength Concrete,” National Crushed Stone Association, Washington, D.C., Jan. 1975, 16 pp. 3.13. Peters, Donald J., “Evaluation of Cement Variability-The First Step,” Publication No. 161, National Ready Mixed Concrete Association, Silver Spring, Apr. 1980, 9 pp. 3.14. Wolsiefer, John, “Ultra High-Strength Field Placeable Concrete with Silica Fume Admixture,” Concrete International: Design & Construction, V. 6, No. 4, Apr. 1984, pp. 25-31. 3.15. Perenchio, William F., and Khieger, Paul, “Some Physical Properties of High Strength Concrete,” Research and Development Bulletin No. RD056.01T, Portland Cement Association, Skokie, 1978, 7 pp. 3.16. Freedman, Sydney, “High-Strength Concrete, Modern Concrete, V. 34, No. 6, Oct. 1970, pp. 29-36; No. 7, Nov. 1970, pp. 28-32; No. 8, Dec. 1970, pp. 21-24; No. 9, Jan. 1971, pp. 15-22; and No. 10, Feb. 1971, pp. 16-23. Also, Publication No. IS176T, Portland Cement Association. 3.17. Neville, A.M., Properties of Concrete, 3rd Edition, Pitman Publishing Limited, London, 1981, 779 pp. 3.18. Bickley, John A, and Payne, John C., “High Strength Cast-in-Place Concrete in Major Structures in Ontario,” paper presented at the ACI Annual Convention, Milwaukee, Mar. 1979. 3.19. Shideler, J.J., “Lightweight-Aggregate Concrete for Structural Use, ACI JOURNAL , Proceedings V. 54, No. 4, Oct. 1957, pp. 299-328. 3.20. Holm, T.A., “Physical Properties of High Strength Lightweight Aggregate Concretes,” Proceedings, 2nd International Congress on Lightweight Concrete (London, Apr. 1980), Ci8O, Construction Press, Lancaster, 1980, pp. 187-204. 3.21. Mather, Katharine, “High Strength, High Density Concrete,” ACI JOURNAL , Proceedings V. 62, No. 8, Aug. 1965, pp. 951-962. Also, Technical Report No. 6-635, U.S. Army Engineer Waterways Experiment Station. 3.22. Bloem, Delmar L., and Gaynor, Richard D., “Effects of Aggregate Properties on Strength of Concrete,” ACI JOURNAL , Proceedings V. 60, No. 10, Oct. 1963, pp. 1429-1456. 3.23. Tobin, Robert E., “Flow Cone Sand Tests,” ACI JOURNAL, Proceedings V. 75, No. 1, Jan. 1978, pp. l-12. 3.24. Ehrenburg, D.O., “An Analytical Approach to Gap-Graded Concrete,” Cement, Concrete, and Aggregates, V. 2, No. 1, Summer 1980, pp. 39-42. 3.25. Tuthill, Lewis H., “Better Grading of Concrete Aggregates,” Concrete International Design & Construction, V. 2, No. 12, Dec. 1980, pp. 49-51. 3.26. Gaynor, Richard D., “High Strength Air-Entrained Concrete,” Joint Research Laboratory Publication


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No. 17, National Sand and Gravel Association/National Ready Mixed Concrete Association, Silver Spring, Mar. 1968, 19 pp. CHAPTER 4- BATCHING, MIXING, TRANSPORTING, PLACING, CURING, AND CONTROL PROCEDURES 4.1-Introduction

The batching, mixing, transporting, placing, and control procedures for high-strength concrete are not different in principle from those procedures used for conventional concrete. Thus ACI 304 can be followed. Some changes, some refinements, and some emphasis on critical points are necessary. Maintaining the unit water content as low as possible, consistent with placing requirements, is good practice for all concrete; for high-strength concrete it is critical. Since the production of highstrength concrete will normally involve the use of relatively large unit cement contents with resulting greater heat generation, some of the recommendations given in Chapter 3 on Production and Delivery and Chapter 4 on Placing and Curing in ACI 305R, “Hot Weather Concreting,” may also be applicable. In addition, the production and testing of highstrength concrete requires well-qualified concrete producers and testing laboratories, respectively. 4.2 - Batching 4.2.1 Control, handling and storage of materials- The

control, handling, and storage of materials need not be substantially different from the procedures used for conventional concrete as outlined in ACI 304. Proper stockpiling of aggregates, uniformity of moisture in the batching process, and good sampling practice are essential. It may be prudent to place a maximum limit of 170 F (77 C) on the temperature of the cement as batched in warm weather and 150 F (66 C) in hot weather. Where possible, batching facilities should be located at or near the job site to reduce haul time. The temperature of all ingredients should be kept as low as possible prior to batching. Delivery time should be reduced to a minimum and special attention paid to scheduling and placing to avoid having trucks wait to unload. 4.2.2 Measuring and weighing- Materials for production of high-strength concrete may be batched in manual, semiautomatic, or automatic plants. However, since speed and accuracy are required, ACI 304 recommends that cements and pozzolans be weighed with automatic equipment. Automatic weigh batchers or meters are recommended for water measurement. To maintain the proper water-cement ratios necessary to secure highstrength concrete, accurate moisture determination in the fine aggregate is essential. A combination of warm weather and high cement content often requires the cooling of mixing water. ACI 305R notes that the use of

cold miring water effects a moderate reduction in concrete placing temperature. The use of ice is more effective than cold water; however, this will require ice making or chipping equipment at the batch plant. 4.2.3 Charging of materials-Batching procedures have important effects on the ease of producing thoroughly mixed uniform concrete in both stationary and truck mixers. The uniformity of concrete mixed in central mixers is generally enhanced by ribbon loading the aggregate, cement, and water simultaneously. However, if truck mixers are being used, ribbon loading will prevent delayed miring, which is sometimes used to prevent hydration of the cement during long hauls. This procedure involves stopping the mixer drum after aggregates and three-quarters of the water are charged and before the cement is loaded and not starting the drum again until the job site is reached. Slump loss problems may thus be minimized. High-range water-reducing admixtures are another consideration. These admixtures are very likely to be used in the production of high-strength concrete. According to the guidelines in the Canadian Standards Association’s Preliminary Standard A 266.5-M 1981, tests have shown that high-range water-reducing admixtures are most effective and produce the most consistent results when added at the end of the mixing cycle after all other ingredients have been introduced and thoroughly mired. If there is evidence of improper mixing and nonuniform slump during discharge, procedures used to charge truck and central mixers should be modified to insure uniformity of mixing as required by ASTM C 94. 4.3- Mixing High-strength concrete may be mixed entirely at the batch plant, in a central or truck mixer, or by a combination of the two. In general, mixing follows the recommendations of ACI 304. Experience and tests 4.0,4.2 and standards documents of the Concrete Plant Manufacturers Bureau have indicated that high-strength concrete can be mired in all common types of mixers.4.3,4.4,4.5 It may prove beneficial to reduce the batch size below the rated capacity to insure more efficient mixing. 4.3.2 Mixer performance- The performance of mixers is usually determined by a series of uniformity tests (ASTM C 94) made on samples taken from two to three locations within the concrete batch being mixed for a given time period.4.6 Some work 4.2,4.7 has indicated that due to the relatively low water content and high cement content and the usual absence of large coarse aggregate, the efficient mixing of high-strength concrete is more difficult than conventional concrete. Special precautions or procedures may by required. Thus, it becomes more important for the supplier of high-strength concrete to check mixer performance and efficiency prior to production mixing. 4.3.3 Mixing time- The mixing time required is based upon the ability of the central mixer to produce uniform concrete both within a batch and between batches. Manufacturers’ recommendations, ACI 304, and usual specifi-


cations, such as 1 min for 1 yd 3(0.75 m3) plus ‘/4 min for each additional yd 3of capacity, are used as satisfactory guides for establishing mixing time. Otherwise, mixing times can be based on the results of mixer performance tests. The mixing time is measured from the time all ingredients are in the mixer. Prolonged miring may cause moisture loss and result in lower workability,4.8 which in turn may require retempering to restore slump, thereby reducing strength potential. 4.3.4 Ready-mixed concrete-High strength concrete may be mixed at the job in a truck mixer. However, not all truck mixers can mix high-strength concrete, especially if the concrete has very low slump. Close job control is essential for high-strength ready-mixed concrete operations to avoid causing trucks to wait at the job site due to slow placing operations. (Note Section 4.7.) Retarding admixtures are used to prolong the time the concrete will respond to vibration after it has been placed in the forms. Withholding some of the mixing water until the truck arrives at the job site is sometimes desirable. Then after adding the remaining required water, an additional 30 revolutions at mixing speed are used to incorporate the additional water into the mixture adequately. (See ACI 304.) When loss of slump or workability cannot be offset by these measures, complete batching and miring can be conducted at the job site. If a high-range waterreducer is added at the site, a truck mounted dispenser or an electronic field dispenser is usually required. 4.4-Transporting 4.4.1 General considerations- High-strength concrete can be transported by a variety of methods and equipment, such as truck mixers, stationary truck bodies with and without agitators, pipeline or hose, or conveyor belts. Each type of transportation has specific advantages and disadvantages depending on the conditions of use, mixture ingredients, accessibility and location of placing site, required capacity and time for delivery, and weather conditions. 4.4.2 Truck-mixed concrete- Truck miring is a process in which proportioned concrete materials from a batch plant are transferred into the truck mixer where all mixing is performed. The truck is then used to transport the concrete to the job site. Sometimes dry materials are transported to the job site in the truck drum with the miring water carried in a separate tank mounted on the truck. Water is added and mixing is completed. This method, which evolved as a solution to long hauls and placing delays, is adaptable to the production of highstrength concrete where it is desirable to retain the workability as long as possible. However, free moisture in the aggregates, which is part of the mixing water, may cause some cement hydration. 4.4.3 Stationary truck body with and without agitatorUnits used in this form of transportation usually consist of an open-top body mounted on a truck. The smooth, streamlined metal body is usually designed for discharge of the concrete at the rear when the body is tilted. A

363R-17

discharge gate and vibrators mounted on the body are provided at the point of discharge. An apparatus that ribbons and blends the concrete as it is unloaded is desirable. However, water is not added to the truck body because adequate mixing cannot be obtained with the agitator. 4.4.4 Pumping- High-strength concrete will in many cases be very suitable for pumping. Pumps are available that can handle low-slump mixtures and provide high pumping pressure. High-strength concrete is likely to have a high cement content and small maximum size aggregate--both factors which facilitate concrete pumping. Chapter 9 of ACI 304 provides guidance for the use of pumps for transporting high-strength concrete. In the field, the pump should be located as near to the placing areas as practicable. Pump lines should be laid out with a minimum of bends, firmly supported, using alternate lines and flexible pipe or hose to permit placing over a large area directly into the forms without rehandling. Direct communication is essential between the pump operator and the concrete placing crew. Continuous pumping is desirable because if the pump is stopped, movement of the concrete in the line may be difficult or impossible to start again. 4.4.5 Belt conveyor- Use of belt conveyors to transport concrete has become established in concrete construction. Guidance for use of conveyors is given in ACI 304.4R. The conveyors must be adequately supported to obtain smooth, nonvibrating travel along the belt. The angle of incline or decline must be controlled to eliminate the tendency for coarse aggregate to segregate from the mortar fraction. Since the practical slump range for belt transport of concrete is 1 to 4 in. (25 to 100 mm), belts may be used to move high-strength concrete only for relatively short distances of 200 to 300 ft (60 to 90 m). Over longer distances or extended time lapses, there will be loss of slump and workability.4.9 Enclosures or covers are used for conveyors when protection against rain, wind, sun, or extreme ambient temperatures is needed to prevent significant changes in the slump or temperature of the concrete. As with other methods of transport for high-strength concrete, proper planning, timing, and control are essential. 4.5-Placing procedures 4.5.1 Preparations- Preparations for placing highstrength concrete should include recognition at the start of the work that certain abnormal conditions will exist which will require some items of preparation that cannot be provided readily the last minute before concrete is placed. Since workability time is expected to be reduced, preparation must be made to transport, place, consolidate, and finish the concrete at the fastest possible rate. This means, first, delivery of concrete to the job site must be scheduled so it will be placed promptly on arrival, particularly the first batch. Equipment for placing the concrete must have adequate capacity to perform its functions efficiently so there will be no delays at distance

363R-18


portions of the work. There should be ample vibration equipment and manpower to consolidate the concrete quickly after placement in difficult areas. All equipment should be in the first class operating condition. Breakdowns or delays that stop or slow the placement can seriously affect the quality of the work. Due to more rapid slump loss, the strain on vibrating equipment will be greater. Accordingly, provision should be made for an ample number of standby vibrators, at least one standby for each three vibrators in use. A high-strength concrete placing operation is in serious trouble, especially in hot weather, when vibration equipment fails and the standby equipment is inadequate. 4.5.3 Equipment- A basic requirement for placing equipment is that the quality of the concrete, in terms of water-cement ratio, slump, air content, and homogeneity, must be preserved. Selection of equipment should be based on its capability for efficiently handling concrete of the most advantageous proportions that can be consolidated readily in place with vibration. Concrete should be deposited at or near its final position in the placement. Buggies, chutes, buckets, hoppers, or other means may be used to move the concrete as required. Bottom-dump buckets are particularly useful however, side slopes must be very steep to prevent blockages. High-strength concrete should not be allowed to remain in buckets for extended periods of time, as the delay will cause sticking and difficulty in discharging. 4.5.3 Consolidation- Proper internal vibration is the most effective method of consolidating high-strength concrete. The advantages of vibration in the placement of concrete are well established. The provisions of ACI 309 must be followed. Hi h-strength concrete can be very” sticky" material. 4.1,4.10 indeed, effective consolidation procedures may well start with mix proportioning. Coarse sands have been found to provide the best workability.4.10 The im ortance of full compaction cannot be overstated. Davies 4.11 has shown that up to 5 percent loss in strength may be sustained from each 1 percent void space in concrete. Thus, vibration almost to the point of excess may be required for high-strength concrete to achieve its full potential. 4.5.4 Special considerations- Where different strength concretes are being used within or between different structural members, special placing considerations are required. To avoid confusion and error in concrete placement in columns, it is recommended that, where practical, all columns and shearwalls in any given story be placed with the same strength concrete. For formwork economy, no changes in column size in the typical highrise buildings are recommended.4.12 In areas where two different concretes are being used in column and floor construction, it is important that the high-strength concrete in and around the column be placed before the floor concrete. With this procedure, if an unforeseen cold joint forms between the two concretes, shear strength will still be available at the column interface.4.13

4.6-Curing 4.5.1 Need for curing- Curing is the process of maintaining a satisfactory moisture content and a favorable temperature in concrete during the hydration period of the cementitious materials so that desired properties of the concrete can be developed. Curing is essential in the production of quality concrete; it is critical to the production of high-strength concrete. The potential strength and durability of concrete will be fully developed only if it is properly cured for an adequate period prior to being placed in service. Also, high-strength concrete should be water cured at an early age since partial hydration may make the capillaries discontinuous. On renewal of curing, water would not be able to enter the interior of the concrete and further hydration would be arrested.4.14 4.5.3 Type of curing- Water curing of high-strength concrete is highly recommended4.14 due to the low watercement ratios employed. At water-cement ratios below 0.4, the ultimate degree of hydration is significantly reduced if free water is not provided. Water curing will allow more efficient, although not complete, hydration of the cement. Klieger4.15 reported that for low watercement ratio concretes it is more advantageous to supply additional water during curing than is the case with higher water-cement ratio concretes. For concretes with water-cement ratio of 0.29, the strength of specimens made with saturated aggregates and cured by ponding water on top of the specimen was 850 to 1000 psi (5.9 to 6.9 M Pa) greater at 28 days than that of comparable specimens made with dry aggregates and cured under damp burlap. He also noted that although early strength is increased by elevated temperatures of mixing and curing, later strengths are reduced by such temperatures. However, work by Pfieffer 4.16 has shown that later strengths may have only minor reductions if the heat is not applied until after time of set. Others 4.1,4.17 have reported that moist curing for 28 days and thereafter in air was highly beneficial in securing high-strength concrete at 90 days. 4.5.3 Methods of curing- As pointed out in ACI 308, the most thorough but seldom used method of water curing consists of total immersion of the finished concrete unit in water. “Ponding” or immersion is an excellent method wherever a pond of water can be created by a ridge or dike of impervious earth or other material at the edge of the structure. Fog spraying or sprinkling with nozzles or sprays provides satisfactory curing when immersion is not feasible. Lawn sprinklers are effective where water runoff is of no concern. Intermittent sprinkling is not acceptable if drying of the concrete surface occurs. Soaker hoses are useful, especially on surfaces that are vertical. Burlap, cotton mats, rugs, and other coverings of absorbent materials will hold water on the surface, whether horizontal or vertical. Liquid membrane-forming curing compounds retain the original moisture in the concrete but do not provide additional moisture.


363R-19

4.7-- Quality assurance mum elapsed time of 11/2 hr after the cement has entered 4.7.1 Materials- Once the high-strength concrete mixthe drum until completion of discharge is frequently speture has been proportioned, the concrete supplier or cified (See ASTM C 94). Reduction to 45 minutes may sampling and testing program is recommended to assure be necessary under hot weather conditions or where the physical properties required. The use of ASTM severe slump loss is experienced. For extreme job temStandard Method for Evaluation of Cement Strength peratures, field production trial batches are often made. Uniformity from a Single Source (ASTM C 917) with appropriate limits will provide the proper basis for such 4.8-Quality control procedures uniformity. It is desirable that the aggregates and ad4.8.1 Criteria- The first consideration for selecting mixtures specified in the mixture be uniform and come quality control procedures is determining that the disfrom the same source for the duration of the project. tribution of the compressive strength test results follows a normal distribution curve. It has been suggested4.18 that 4.7.2 Control of operations4.10- Effective coordination and control procedures between the supplier and the a skew distribution may prevail due to the mean apcontractor are critical to the operations. The supplier proaching a limit. This may be the case for very-highnormally has full control of high-strength concrete until strength concrete, 15,000 psi (103 MPa) or higher. Howit is placed in the forms. Control of the slump, time on ever available data4.19,4.20 indicate that in the range of job, mixing, and mixture adjustments is under the juris- 6000; to 10,000 psi (41 to 69 MPa) normal distribution is diction of the supplier. The contractor must be prepared achieved. Thus ACI 214 will normally be a convenient to handle, place, and consolidate the concrete promptly tool for quality control procedures for high-strength as received. Cement hydration, temperature rise, slump concrete. Another point which needs consideration both loss, and aggregate grinding during mixing all increase in the quality control and the design phase is the queswith passage of time; thus it is important that the period tion of the age at the time of testing for acceptance for between initial mixing and delivery be kept to an abso- high-strength concrete. Compressive strength tests show lute minimum. The dispatching of trucks is coordinated that a considerable strength gain may be achieved after with the rate of placement to avoid delays in delivery. 28 days in high-strength concrete. To take advantage of When elapsed time from batching to placement is so long this fact, several investigators 4.10,4.13,4.20 have suggested as to result in significant increases in mixing water that the specification for compressive strength should be demand, or in slump loss, mixing in the trucks is delayed modified from the typical 28-day criterion to either 56 or until only sufficient time remains to accomplish mixing 90 days. This extension of test age would then allow, for before the concrete is placed. example, the use of 7000 psi (48 MPa) concrete at 56 days in lieu of 6000 psi (41 MPa) at 28 days for design 4.7.3 Communication equipment- Equipment for direct communication between the supply and placement loca- purposes. In this case the same mixture could be used to tions for use by the inspection force is essential. The meet this criterion. High-strength concrete is generally need for other equipment such as signaling and identi- used in high-rise structures; therefore, the extension of fying devices depends on the complexity of the project the time for compressive strength test results is reasonand the number of different concrete mixtures employed. able since the lower portion of the structure will not The project engineer will normally advise the contractor attain full dead load for periods up to one year and of the equipment that is necessary and require him to longer. present plans or descriptions of the equipment for review 4.8.2 Method of evaluation - To satisfy strength perforwell in advance of the start of placement. mance requirements, the average strength of concrete 4.7.4 Laboratoy 4.10-- A competent concrete laboratory must be in excess of fc', the design strength. The amount must be available for testing the concrete delivered to the of excess strength depends on the expected variability of job site. This laboratory should be inspected regularly by test results as expressed by a coefficient of variation or the Cement and Concrete Reference Laboratory (CCRL) standard deviation and on the allowable proportion of and conform to the requirements of ASTM E 329. A low tests. Available information4.14,4.19,4.20 indicates that minimum of one set of cylinders is normally made for the standard deviation for high-strength concrete beeach 100 yd 3 (76 m3) of concrete placed, with at least comes uniform in the range of 500 to 700 psi (3.5 to 4.8 two cylinders cast for each test age; that is, 7, 28, 56, and MPa), and therefore, the coefficient of variation will actually decrease as the average strength of the concrete 90 days. increases. This, of course, may be the result of increased 4.7.5 Contingency plans-Plans need to be developed to provide for alternate operations in case difficulty is vigilance and quality assurance on the part of the proexperienced in the basic placing concept. Backup equip- ducer. Thus, the method of quality control is closely rement is essential, especially vibrators. Batch sixes are lated to the factors noted in Section 4.7. Assuming that reduced if placing procedures are slowed. For truck- the producer will devote a reasonable effort to proper mixed concrete, rush hour traffic delays can cause serious quality assurance measures, the standard deviation methproblems. It may be desirable to reduce the elapsed time od of evaluation appears to be a logical quality control between contact of the cement and water (mixing and procedure. Consider, for example, that good quality contransporting), especially during warm weather. A maxi- trol may be expected on a job where an fc' of 10,000 psi

363R-20


(69 MPa) is required. A required average strength fc' of only 11,000 psi (76 MPa) is thus required with a standard deviation of 645 psi (4.4 MPa)

fCI’ = f,’+ 134s

= 10,000 + 1.34 x 645 = 10,864 psi (75 MPa)

(4-la)

or

f,’ = f,’+ 2.33s - 500

= 10,000 + 2.33 x 645 - 500 = 11,000 psi (76 MPa)

(4-lb)

s = standard deviation. Of course, a close check of the field results and maintenance of records in the form of control charts or other means are necessary to maintain the desired control. Early-age control of concrete strength such as the accelerated curing and testing of compression test specimens according to ASTM C 684 is often used, especially where later-age (56 or 90 days) strength tests are the final acceptance criterion. 4.9 - Strength measurements 4.9.1 Conditions-since much of the interest in highstrength concrete is limited to strength only in compression, compressive strength measurements are of primary concern in the testing of high-strength concrete. Standard test methods of the American Society for Testing and Materials (ASTM) are followed except where changes are dictated by the peculiarities of the highstrength concrete. The potential strength and variability of the concrete can be established only by specimens made, used, and tested under standard conditions. Then standard control tests are necessary as a first step in the control and evaluation of the mixture. Curing concrete test specimens at the construction site and under job conditions is sometimes recommended since this is considered more representative of the curing applied to the structure. Tests of job-cured specimens may be highly desirable and are necessary when determining the time of form removal, particularly in cold weather, and when establishing the rate of strength development of structural members. They should never be used for quality control testing. Strength specimens of concrete made or cured under other than standard conditions provide additional information but are analyzed and reported separately. ASTM C 684 requires that a minimum of two cylinders be tested for each age and each test condition. 4.9.2 Specimen size and shape- ASTM standards specify a cylindrical specimen 6 in. (152 mm) in diameter and 12 in. (305 mm) long. This size specimen has evolved over a period of time, apparently from practical considerations. It is about the maximum weight one person can handle with reasonable effort and is large enough to be used for concrete containing 2 in. (50 mm) maximum size aggregate and smaller, which encompasses the

majority of concrete being placed today. Designers generally assume 6 x 12 in. (152 x 305 mm) specimens as the standard for measured strengths. Recently some 4 x 8 in. (102 x 204 mm) cylinders have been used for determining compressive strength. 4.18,4.19 However, 4 x 8 in. (102 x 204 mm) cylinders exhibit a higher strength and an increase in variability compared to the standard 6 x 12 in. (152 x 305 mm) cylinder.4.23,4.24 Regardless of the specimen size, as the compressive stress is transferred through the loading platen-specimen interface, a complex, triaxial distribution of stresses in the specimen end may develop which can radically alter the specimen failure mode and affect results. 4.9.3 Testing apparatus- Testing machine characteristics that may affect the measured compressive strength include calibration accuracy, longitudinal and lateral stiffness, stability, alignment of the machine components, type of platens, and the behavior of the platen spherical seating. Testing machines should meet the requirements of ASTM C 39 when used for testing compressive strength of cylindrical specimens. Overall testing machine design including longitudinal and lateral stiffness and machine stability will affect the behavior of the specimen at its maximum load. The type of platens and behavior of the spherical seating will affect the level of measured compressive strength.4.23 Sigvaldason 4.25 recommended a minimum lateral stiffness of 10 xl0 4 lb/in. (17.5 x l0 6 N/m), and a longitudinal stiffness of 10 x l06 lb/in. (17.5 x l06 N/m). He reported that a longitudinally “flexible” machine would contribute to an explosive failure of the specimen at the maximum stress, but that the actual stress achieved was insensitive to machine flexibility. However, he also noted that a machine which is longitudinally stiff but laterally flexible deleteriously influences the measured compressive strengths. Sigvaldason and Cole4.26 reported that use of proper platen size and design is critical if strengths are to be maximized and variations reduced. The upper platen must have a spherical bearing block seating and be able to rotate and achieve full contact with the specimen under the initial load and perform in a fixed mode when approaching the ultimate load. Cole demonstrated that a testing machine with a spherical bearing block seating (able to rotate under load) measured increasingly erroneous results for higher strength concretes, with reductions as high as 16 percent for 10,000 psi (69 MPa) cubes. The diameters of the platen and spherical bearing socket are critically important.4.23 Ideally, the platen and spherical bearing block diameters should be approximately the same as the bearing surface of the specimen. Bearing surfaces larger than the specimen will be restrained (due to size effects) against lateral expansion will probably not expand as rapidly as the specimen, and will consequently create confining stresses in the specimen end. Bearing surfaces and spherical seating blocks smaller in diameter than those of the specimen may result in portions of the specimens remaining unloaded and bend-


ing of the platen around the socket with a consequent nonuniform distribution of stresses. 4.9.4 Type of mold-- The choice of mold materials, and specify construction of the mold regardless of the types of material used, can have a significant effect on measured compressive strengths. A given consolidation effort is more effective with rigidly constructed molds, and sealed waterproofed molds reduce leakage of mortar paste and inhibit the dehydration of the concrete. Blick 4.15 compared high-strength specimens cast in steel and high-quality paper molds and reported that use of the rigid steel molds increased strengths approximately 13 percent but that use of either mold material did not consistently affect variability of the measured strengths. Hester 4.7 evaluated a number of mold materials used under actual field conditions. Measured compressive strengths achieved with properly prepared specimens were compared. Specimens cast in steel molds achieved approximately 6 percent higher strengths but had a slightly higher coefficient of variation compared to specimens cast in tin molds. Specimens cast in steel molds achieved approximately 16 percent higher strength than specimens cast in plastic molds. 4.9.5 Specimen preparation- For many years concrete technologists have recognized the need to cap or grind the ends of cast concrete test specimens prior to testing for compressive strength. The detrimental effects of nonplaneness, irregulari din and grease, etc., have been well documented.4.23 For high-strength concrete the strength of the cap, if used, is another consideration. Troxell,4.27 Wemer, 4.28 and other have compared the relative merits of sulfur mortars, gypsum plaster, highalumina cements, and other capping materials. If the compressive strength or modulus of elasticity of the capping material is less than that of the specimen, loads applied through the cap will not be transmitted uniformly. Sulfur mortar is the most widely used capping material. Most commercially available sulfur mortar capping compounds are combinations of sulfur with inert minerals and fillers and, when properly prepared, are economical, convenient to apply, and develop a relatively high strength in a short period of time. However, these materials are sensitive to the material formulations and handling practices. Kennedy4.29 and Werner investigated the effect of the thickness of sulfur mortar caps on compressive strengths of moderate strength concretes. Cap thicknesses in the range of 1/16 to ‘/s in. (1.5 to 3 mm) are desirable for use on high-strength concrete. However, caps consistently thinner than Ya in. (3 mm) are difficult to obtain. Kennedy4.26 and Hester4.23 note that the principal problems with thin caps are air voids at the specimen-cap interface and cracking of the specimen cap under load. Caps with a thickness of ‘/ in. (6 mm) are apparently satisfactory. Low-strength thick caps may creep laterally under load and therefore contribute to increased tensile stresses in the specimen ends and consequently substantially reduce measured compressive

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strengths for the concrete specimen. Gaynor 4.30 and Saucier 4.31 indicate that concrete strengths up to 10,000 psi (69 MPa) may be determined using high-strength capping materials, including sulfur mortar, which have strengths in the range of 7000 to 8000 psi (50 to 60 MPa), if the cap thickness is maintained at approximately l/4 in. (6 mm). For expected compressive strengths above 10,000 psi (69 MPa), the ends are usually formed or ground to tolerance.4.29.4.30 4.10- Cited References (See also Chapter l0-- References) 4.1. Saucier, K.L.; Tynes, W.O.; and Smith, E.F., “High-Compressive-Strength Concrete-Report 3, Summary Report,” Miscellaneous Paper No. 6-520, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Sept. 1965, 87 pp. 4.2. Saucier, K.L., “Evaluation of Spiral-Blade Concrete Mixer, Shelbyville Reservoir Project, Shelbyville, Illinois,” Miscellaneous Paper No. 6-975, U.S. Army Engineer, Waterways Experiment Station, Vicksburg, Mar. 1968, 17 pp. 4.3. Strehlow. Robert W., “Concrete Plant Production,” Concrete Plant Manufacturers Bureau, Silver Spring, 1973, 112 pp. 4.4. “Concrete Plant Standards of the Concrete Plant Manufacturers Bureau,” 7th Revision, Concrete Plant Manufacturers Bureau, Silver Spring, Jan. 1, 1983, 11 pp. 4.5. “Concrete Plant Mixer Standards of the Plant Mixer Manufacturers Division, Concrete Plant Manufacturers Bureau,” 5th Revision, Concrete Manufacturers Bureau, Silver Spring, July 18, 1977, 4 pp. 4.6. Concrete Manual, 8th Edition, U.S. Bureau of Reclamation, Denver, 1975. 627 pp. 4.7. Saucier, K.L., “Evaluation of a 16-cu-ft Laboratory Concrete Mixer,” Miscellaneous Paper No. 6-692, U.S. Army Engineer Waterways Experiment Section, Vicksburg, Jan. 1965. 4.8. Bloem, Delmar L., “High-Energy Mixing,” Technical Information Letter No. 169. National Ready Mixed Concrete Association, Silver Spring, Aug. 1961, pp. 3-8. 4.9. Saucier Kenneth L., “Use of Belt Conveyors to Transport Mass Concrete,” Technical Report No. C-74-4, U.S. Army Engineer Waterways Experiment Station, Vicksburg, 1974, 42 pp. 4.10. Blick, Ronald L., “Some Factors Influencing High-Strength concrete,” Modern Concrete, V. 36, No. 12, Apr. 1973, pp. 38-41. 4.11. Davies, R.D., “Some Experiments on the Compaction of Concrete by Vibration,” Magazine of Concrete Research (London), V. 3, No. 8, Dec. 1951, pp. 71-78. 4.12. Schmidt, William, and Hoffman, Edward J., “9000-psi ConcreteWhy?-Why Not?,” Civil Engineering -ASCE, V. 45, No. 5, May 1975, pp. 52-55. 4.13. “High-Strength Concrete in Chicago High-Rise Buildings,” Task Force Report No. 5, Chicago Committee on High-Rise Buildings, 1977, 63 pp. 4.14. Neville, A.M., Properties of Concrete, 2nd Edition,

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John Wiley and Sons, New York, 1973, 686 pp. 4.15. Klieger. Paul, “Early High Strength Concrete for Prestressing,” Proceedings, World Conference on Prestressed Concrete, San Francisco, 1957, pp. A5-l-A5-14. 4.16. Pfieffer, D.W., and Ladgren, J.R., “Energy Efficient Accelerated Curing of Concrete-A Laboratory Study for Plant-Produced Prestressed Concrete,” Technical Report No. 1, Prestressed Concrete Institute, Chicago, Dec. 1981. 4.17. Price, Walter H., “Factor Influencing Concrete Strength,” ACI JOURNAL, Proceedings V. 47, No. 6, Feb 1951, pp. 417-432. 4.18. Mather, Brant, “Stronger Concrete,” Highway Research Record No. 210, Highway Research Board, 1967, pp. l-28. 4.19. Day, K.W., “Quality Control of 55 MPa Concrete for Collins Place Project, Melbourne, Australia,” Concrete International Design & Construction, V. 3, No. 3, Mar. 1981, pp. 17-24. 4.20. Cook, James E., “Research and Application of High-Strength Concrete Using Class C Fly Ash,” Concrete International: Design & Construction, V. 4, No. 7, July 1982, pp. 72-80. 4.21. Fortie, Douglas A, and Schnoreier, P.E., “Fourby-Eight Test Cylinder Are Big Enough,” Concrete Construction, V. 24, No. 11, Nov. 1979, pp. 751-753. 4.22. Wolsiefer, John, private communication with ACI Committee 363, 1982. 4.23. Hester, Weston T., “Field Testing High-Strength Concretes: A Critical Review of the State-of-the-Art,” Concrete International Design & Construction, V. 2, No. 12, Dec. 1980, pp. 27-38. 4.24. Carrasquillo, Ramon L.; Nilson, Arthur H.; and Slate, Floyd O., “Properties of High-Strength Concrete Subject to Short-Term Loads,” ACI JOURNAL, Proceedings V. 78, No. 3, May-June 1981, pp. 171-178. 4.25. Sigvaldason, O.T., “The Influence of Testing Machine Characteristics Upon the Cube and Cylinder Strength of Concrete, Magazine of Concrete Research (London), V. 18, No; 57, Dec. 1966, pp. 197-206. 4.26. Cole, D.G., “Some Mechanical Aspects of Compression Testing Machines,” Magazine of Concrete Research (London), V. 19, No. 61, Dec. 1967, pp. 247251. 4.27. Troxell, G.E., “The Effect of Capping Methods and End Conditions Before Capping Upon the Compressive Strength of Concrete Test Cylinder,” Proceedings, ASTM, V. 41, 1941, pp. 1038-1052. 4.28. Werner, George, “The Effect of Type of Capping Material on the Compressive Strength of Concrete Cylinder,” Proceedings, ASTM V. 58, 1958, pp. 1166-1186. 4.29. Holland, Terrence C., “Testing High Strength Concrete,” Concrete Construction, June 1987, pp. 534-536. 4.30. Godfrey, K.A., Jr.,"Concrete Strength record Jumps 36%,” Civil Engineering, Oct. 1987, pp. 84-88. 4.31. Saucier, K.L., “Effect of Method of Preparation of Ends of Concrete Cylinders for Testing," Miscellaneous Paper No. C-72-12, U.S. Army Engineer Waterways Ex-

periment Station, Vicksburg, Apr. 1972, 51 pp.

CHAPTER 5-PROPERTIES OF HIGH-STRENGTH CONCRETE 5.1-Introduction

Concrete properties such as stress-strain relationship, modulus of elasticity, tensile strength, shear strength, and bond strength are frequently expressed in terms of the uniaxial compressive strength of 6 x 12-m. (152 x 305mm) cylinders. Generally, the expressions have been based on experimental data of concrete with compressive strengths less than 6000 psi (41 MPa). Various properties of high-strength concrete are reviewed in this chapter. The applicability of current and proposed expressions for predicting properties of high-strength concrete are examined. 5.2-Stress-strain behavior in uniaxial compression

Axial-stress versus strain curves for concrete of compressive strength up to 12,000 psi (83 MPa) are shown in Fig. 5.1. The shape of the ascending part of the stressstrain curve is more linear and steeper for high-strength concrete, and the strain at the maximum stress is slightly higher for high-strength concrete.5.5-5.6 The slope of the descending part becomes steeper for high-strength concrete. To obtain the descending part of the stress-strain curve, it is generally necessary to avoid the specimentesting system interaction; this is more difficult to do for high-strength concrete. 5.3,5.5,5.8 A simple method of obtaining a stable descending part of the stress-strain curve is described in References 5.3

Stress, ksi

Strain, percent

Fig. 5.1 -Complete compressive stress-strain curves 5.I

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and 5.7. Concrete cylinders were loaded in parallel with a hardened steel tube with a thickness such that the total load exerted by the testing machine was always increasing. This approach can be employed with most conventional testing machines. An alternate approach is to use a closed-loop testing machine.‘.’ In a closed-loop testing machine, specimens can be loaded so as to maintain a constant rate of strain increase and avoid unstable failure. High-strength concrete exhibits less internal microcracking than lower-strength concrete for a given imposed axial strain. 5.9 As a result, the relative increase in lateral strains is less for high-strength concrete (Fig. 5.2).5.10 The lower relative lateral expansion during the inelastic range may mean that the effects of triaxial stresses will be proportionally different for high-strength concrete. For example, the influence of hoop reinforcement is observed to be different for high-strength concrete.5.11 It was reported that the effectiveness of spiral reinforcement is less for high-strength concrete.5.11

Fig. 5.2-- Axial stress versus axial strain and lateral strain 5.10 for plain normal weight concrete

5.3-Modulus of elasticity In 1934, Thoman and Raede 5.12 reported values for the modulus of elasticity determined as the slope of the tangent to the stress-strain curve in uniaxial compression at 25 percent of maximum stress from 4.2 x l06 to 5.2 x l06psi (29 to 36 GPa) for concretes having compressive strengths ranging from 10,000 (69 MPa to 11,000 psi (76 MPa). Many other investigators5.4,5.13-5.18 have reported values for the modulus of elasticity of high-strength concretes of the order of 4.5 to 6.5 x l0 6 psi (31 to 45 GPa) depending mostly on the method of determining the modulus. A comparison 5-19of experimentally determined values for the modulus of elasticity with those predicted by the expression given in ACI 318, Section 8.5 for lower-strength concrete, based on a dry unit weight of 145 lb/ft 3(2346 kg/m 3) is given in Fig. 5.3. The ACI 318 expression overestimates the modulus of elasticity for concretes with compressive strengths over 6000 psi (41 MPa) for the data given in Fig. 5.3. A correlation between the modulus of elasticity Ec and the compressive strength fc' for normal weight concretes

(see Fig. 5.3) was reported in Reference 5.19 as Ec

= 40,000 K + 1.0 x l06 psi for 3000 psi < f,’< 12,000 psi

(EC = 3320 &’ + 6900 M P a for 21 MPa c
(5-l)

Other empirical equations for predicting elastic modulus have been proposed.5.17,5.18 Deviation from predicted values are highly dependent on the properties and proportions of the coarse aggregate. For example, higher values than predicted by Eq. (5-l) were reported by Russell,5.20 Saucier,5.21 and Pfeiffer. 5.22 5.4-Poisson’s ratio Experimental data on values of Poisson’s ratio for high-strength concrete are very limited. Shideler5.23 and Carrasquillo et al5.2 reported values for Poisson’s ratio of lightweight-aggregate high-strength concrete having uniaxial compressive strengths up to 10,570 psi (73 MPa) at 28 days to be 0.20 regardless of compressive strength, age, and moisture content. Values determined by the dynamic method were slightly higher. On the other hand, Perenchio and Klieger5.24 reported values for Poisson’s ratio of normal weight high-strength concretes with compressive strengths ranging from 8000 to 11,600 psi (55 to 80 MPa) between 0.20 and 0.28. They concluded that Poisson’s ratio tends to decrease with increasing water-cement ratio. Kaplan5.25 found values for Poisson’s ratio of concrete determined using dynamic measurements to be from 0.23 to 0.32 regardless of compressive strength, coarse aggregate, and test age for concretes having compressive strengths ranging from 2500 to 11,500 psi (17 to 79 MPa). Based on the available information, Poisson’s ratio of high-strength concrete in the elastic range seems comparable to the expected range of values for lower-strength concretes. 5.5-Modulus of rupture The values reported by various investigators 5.23,5.26-5.28 for the modulus of rupture of both lightweight and normal weight high-strength concretes fall in the range of 7.5 &r to 12 g where both the modulus of rupture and the compressive strength are expressed in psi. The following equation was recommended 5.2 for the prediction of the tensile strength of normal weight concrete, as measured by the modulus of rupture f,’from the compressive strength as shown in Fig. 5.4

fr ’

= 11.7 &r psi for 3000 psi < f,’< 12,000 psi

c-

= 0.94 &r MPa for 21 MPa <

f,’ < 83 MPa)

(5-2)

C

8’1 - \\

0

\

363R-25


q, MPa 0 I

2 I

4 I

6 I f’,.MPo

8

1200

q,MP* 4 6

2

0 I

1O /

8

t; I MPa 20 40

0 4-x6"

60

80

(102mm r203mml

1000 Splitting Tensile

800

Strength t;p

Modulus

Modulus of

Rupture

Rupture

f; , psi

f’, , MPa

psi

600 400

OO I

I

0

20

I 1 I 2000 6000 10000 Cylinder Strength t; . psi / / I I 40

R.psi

8o 100

I 14000 L 120

Fig. 5.5-Tensile strength based on split cylinder test 5.2 jected to repeated load varied between 66 and 71 percent of the static strength for a minimum stress level of 1250 psi (8.6 MPa). The lower values were found for the higher-strength concretes and for concrete made with the smaller-size coarse aggregate, but the actual magnitude 5.6-Tensile splitting strength Dewar 5.27 studied the relationship between the indirect of the difference was small. To the extent that is known, tensile strength (cylinder splitting strength) and the com- the fatigue strength of high-strength concrete is the same pressive strength of concretes having compressive as that for concretes of lower strengths. strengths of up to 12,105 psi (83.79 MPa) at 28 days. He concluded that at low strengths, the indirect tensile 5.8-Unit Weight The measured values of the unit weight of highstrength may be as high as 10 percent of the compressive strength but at higher strengths it may reduce to 5 per- strength concrete are slightly higher than lower-strength cent. He observed that the tensile splitting strength was concrete made with the same materials. about 8 percent higher for crushed-rock-aggregate concrete than for gravel-aggregate concrete- In addition, he 5.9-Thermal properties The thermal properties of high-strength concretes fall found that the indirect tensile strength was about 70 percent of the flexural strength at 28 days. Carrasquillo, within the a proximate range for lower-strength con5.21,5.26 Quantities that have been measured are Nilson, and Slate 5.2 reported that the splitting strength cretes. did not vary much from the usual range shown in Fig. specific heat, diffusivity, thermal conductivity, and co5.5, although as the compressive strength increases, the efficient of thermal expansion. values for the splitting strength fall in the upper limit of the expected range. The following equation for the pre- 5.10-Heat evolution due to hydration The temperature rise within concrete due to hydration diction of the tensile splitting strength &,’ of normal depends on the cement content, water-cement ratio, size weight concrete was recommended5.2 of the member, ambient temperature, environment, etc. Freedman 5.15 has concluded from data of Saucier et al. = 7.4 K psi f SP' in Fig. 5.6 that the heat rise of high-strength concretes for 3000 psi c f,’ < 12,000 psi will be approximately 11 to 15 F/l00 lb of cement/yd 3 (6 to 8 C per 59 kg/m 3 of cement). Values for temperature (fJp’ = 0.59 g MPa confor 21 MPa < f,’< 83 MPa) (5-3) rise of the order of 100 F (56 C) in high-strength crete members containing 846 lb of cement/yd 3 (502 kg/m3) were measured in a building in Chicago as shown 5.7-Fatigue strength 5.16 The available data on the fatigue behavior of high- in Fig. 5.7. strength concrete is very limited. Bennett and Muir.5.29 studied the fatigue strength in axial compression of high- 5.11-Strength gain with age High-strength concrete shows a higher rate of strength strength concrete with a 4-in. (102-mm) cube compressive strength of up to 11,155 psi (76.9 MPa) and found that gain at earl a es as compared to lower-strength conafter one million cycles, the strength of specimens sub- crete5.1,5.2,5.13,5.15but at later ages the difference is not Fig. 5.4-- Tensile strength based on modulus of rupture test 5.2

ACI

363R-26

COMMITTEE

Temp. , F

40 0 0

I

I

I

I

20

40

60

80

100

Time, hours

5.21

200

C o l u m n c3

150

yeroture.

,oo

i

,1

50

t

I

0

5

10

15

20

Age, days

Fig. 5.7-- Measured concrete temperatures at Water Tower Place. 5.16

@---I

100 -

Compressive Strength Compressive Strength at 95 days

0

7

28

strength concrete and 0.7 to 0.75 for lower-strength c o n crete, while Carrasquillo, Nilson, and Slate5.2,5.9 found typical ratios of 7-day to 95-day strength of 0.60 for low-strength, 0.65 for medium-strength, and 0.73 for high-strength concrete. It seems likely that the higher rate of strength development of high-strength concrete at early ages is caused by (1) an increase in the internal curing temperature in the concrete cylinders due to a higher heat of hydration and (2) shorter distance between hydrated particles in high-strength concrete due to low water-cement ratio. 5.12-Freeze thaw resistance Information about air content requirement for highstrength concrete to produce adequate durability is contradictory. For example, Saucier, Tynes, and Smith5.21 concluded from accelerated laboratory freeze-thaw tests that, if high-strength concrete is to be frozen under wet conditions, air-entrained concrete should be considered despite the loss of strength due to air entrainment. In contrast, Perenchio and Klieger5.24 obtained excellent resistance to freezing and thawing of all of the highstrength concretes used in their study, whether air entrained or non-air-entrained. They attributed this to the greatly reduced freezable water contents and the increased tensile strength of high-strength concrete.

Fig. 5.6-Temperature rise of high-strength field-cast 10 x

20 x 5-ft (3 x 6 x 1.5-m) blocks

REPORT

95

Age, days

Fig. 5.8-- Normalized strength gain with age for moist-cured limestone concretes 5.2 significant (see Fig. 5.8). Parrott 5.26 reported typical ratios of 7-day to 28-day strengths of 0.8 to 0.9 for high-

Little information is available on the shrinkage behavior of high-strength concrete. A relatively high initial rate of shrinkage has been reported, 5.26,5.30 but after drying for 180 days there is little difference between the shrinkage of high-strength and lower-strength concrete made with dolomite or limestone. Reducing the curing period from 28 to 7 days caused a slight increase in the shrinkage. 5.26 Shrinkage was unaffected by changes in water-cement ratio5.15 but is approximately proportional to the percentage of water by volume in the concrete. Other laboratory studies 5.31 and field studies 5.16,5.22,5.28 have shown that shrinkage of high-strength concrete is similar to that of lower-strength concrete. Nagataki and Yonekuras 5.32 reported that the shrinkage of highstrength concrete containing high-range water reducers was less than for lower-strength concrete. 5.14-- Creep Parrott 5.26 reported that the total strain observed in sealed high-strength concrete under a sustained loading of 30 percent of the ultimate strength was the same as that of lower-strength concrete when expressed as a ratio of the short-term strain. Under drying conditions, this ratio was 25 percent lower than that of lower-strength concrete. The total long-term strains of drying and sealed high-strength concrete were 15 and 65 percent higher, respectively, than for a corresponding lower-strength concrete at a similar relative stress level. Ngab et a1.5.31 found little difference between the creep of high-strength concrete under drying and sealed conditions. The creep


of high-strength concrete made with high-range water reducers is reported 5.32 to be decreased significantly. The maximum specific creep was less for high-strength concrete than for lower-strength concrete loaded at the same age. 5.16,5.20,5.31 An example is shown in Fig. 5.9. 5 . 2 8 However, high-strength concretes are subjected to higher stresses. Therefore, the total creep will be about the same for any strength h concrete. No problems due to creep were found 5.22 in columns cast with high-strength concrete. As is found with lower-strength concrete, creep decreases as the age at loading increases,5.31 specific creep increases with increased water-cement ratio,5.24 and there is a linear relationship with the applied stress.5.31 This linearity extends to a higher stress-strain ratio than for lower-strength concrete. Some additional information on properties of highstrength concrete can be obtained from References 5.33 to 5.42.

,-

1.75

Unsealed - Sealed

De-

1.50 yft;=48DD psi 1.25

Creep Coefficient, Cc

I.00

075

0.50 1000 psi = 6.895 M PO = 0.45 in All Cases

0.25

,

0 0

15

Age at Loading = 2 Days After 28 Days Curing I

I

I

30

45

60

75

Time After Loading, days

Fig. 5.9-- Relationship between creep coefficient and time for sealed and unsealed concrete specimens 5.31 5.15-Cited references (See also Chapter 10-References) 5.1 Wischers, Gerd, “Applications and Effects of Compressive Loads on Concrete,” Betontechnische Berichte 1978, Betone Verlag GmbH, Dusseldorf, 1979, pp. 31-56. (in German) 5.2. Carrasquiho, Ramon L.; Nilson, Arthur H.; and Slate, Floyd O., “Properties of High Strength Concrete Subjected to Short-Term Loads,” ACI JOURNAL , Proceedings V. 78, No. 3, May-June 1981, pp. 171-178, and Discussion, Proceedings V. 79, No. 2, Mar.-Apr. 1982, pp. 162-163. 5.3. Wang, P.T.; Shah, S.P.; and Naaman, A.E.,

363R-27

“Stress-Strain Curves of Normal and Lightweight Concrete in Compression,” ACI JOURNAL, Proceedings V. 75, No. 10, Nov. 1978, pp. 603-611. 5.4. Kaar, P.H.; Hanson, N.W.; and Capell, H.T., “Stress-Strain Characteristics of High-Strength Concrete,” Douglas McHenry International Symposium on Concrete and Concrete Structures, SP-55, American Concrete Institute, Detroit, 1978, pp. 161-185. Also, Research and Development Bulletin No. 051.01D, Portland Cement Association. 5.5. Shah, S.P.; Gokoz, U.; and Ansari, F., “An Experimental Technique for Obtaining Complete StressStrain Curves for High Strength Concrete,” Cement, Concrete and Aggregates, V. 3, No. 1, Summer 1981, pp. 21-27. 5.6. Shah, S.P., “High Strength Concrete-A Workshop Summary,” Concrete International Design & Construction, V. 3, No. 5, May 1981, pp. 94-98. 5.7. Shah, S.P.; Naaman, A.E.; and Moreno, J., “Effect of Confinement on the Ductility of Lightweight Concrete,” International Journal of Cement Composites and Lightweight Concrete (Harlow, Essex), V. 5, No. 1, Feb. 1983, pp. 15-25. 5.8. Holm, T.A., “Physical Properties of High Strength Lightweight Aggregate Concrete,” Proceedings, 2nd International Congress on Lightweight Concrete (London, Apr. 1980) Ci80, Construction Press, Lancaster, 1980, pp. 187-204. 5.9. Carrasquillo, Ramon L.; Slate, Floyd 0.; and Nilson, Arthur H., “Microcracking and Behavior of HighStrength Concrete Subject to Short-Term Loading,” ACI JOURNAL, Proceedings V. 78, No. 3, May-June 1981, pp. 179-186. 5.10. Ahmad, Schuaib, and Shah, Surendra P., “Complete Triaxial Stress-Strain Curves for Concrete,” Proceedings, ASCE, V. 108, ST4, Apr. 1982, pp. 728-742. 5.11. Ahmad, S.H., and Shah, S.P., “Stress-Strain Curves Of Concrete Confined by Spiral Reinforcement, ACI JOURNAL , Proceedings V. 79, No. 6, Nov.-Dec. 1982, pp. 484-490. 5.12. Thoman, William H., and Raeder, Warren, “Ultimate Strength and Modulus of Elasticity of High Strength Portland Cement Concrete,” ACI J OURNAL , Proceedings V. 30, No. 3, Jan-Feb. 1934, pp. 231-238. 5.13. Smith, E.F.; Tynes, W.O.; and Saucier, K.L., “High-Compressive-Strength Concrete, Development of Concrete Mixtures,” Technical Documentary Report No. RTD TDR-63-3114, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Feb. 1964, 44 pp. 5.14. Nedderman, Howard, “Flexural Stress Distribution in Very-High-Strength Concrete,” MSc thesis, University of Texas at Arlington, Dec. 1973, 182 pp. 5.15. Freedman, Sydney, High-Strength Concrete,” Modern Concrete, V. 34, No. 6, Oct. 1970, pp. 29-36; No. 7, Nov. 1970, pp. 28-32; No. 8, Dec. 1970, pp. 21-24; No. 9, Jan. 1971, pp. 15-22; and No. 10, Feb. 1971, pp. 16-23. Also, Publication No. lS176T, Portland Cement Association.

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5.16. “High-Strength Concrete in Chicago High-Rise pression,” Magazine of Concrete Research (London), V. 19, No. 59, June 1967, pp. 113-117. Buildings,” Task Force Report No. 5, Chicago Committee 5.30. Swamy, R.N., and Anand, K.L., “Shrinkage and on High-Rise Buildings, Feb. 1977, 63 pp. 5.17. Teychenne, D.C.; Parrott, L.J.; and Pomeroy, Creep of High Strength Concrete,” Civil Engineering and C.D., “The Estimation of the Elastic Modulus of Con- Public Works Review (London), V. 68, No. 807, Oct. 1973, crete for the Design of Structures,” Current Paper No. pp. 859-865, 867-868. CP23/78, Building Research Establishment, Garston, 5.31. Ngab, A.S.; Slate, F.O.; and Nilson, A.H., Watford, 1978, 11 pp. “Behavior of High-Strength Concrete Under Sustained 5.18. Ahmad, S.H., “Properties of Confined Concrete Compressive Stress,” Research Report No. 80-2, DepartSubjected to Static and Dynamic Loading,” PhD thesis, ment of Structural Engineering, Cornell University, University of IIIinois at Chicago Circle, Mar. 1981. Ithaca, Feb. 1980, 201 pp. Also, PhD dissertation, Cornell 5.19. Martinez, S.; NiIson, AH.; and Slate, F.O., University, 1980, and “Shrinkage and Creep of High “Spirally-Reinforced High-Strength Concrete Columns,” Strength Concrete,” ACI JOURNAL , Proceedings V. 78, Research Report No. 82-10, Department of Structural No. 4, July-Aug. 1981, pp. 255-261. 5.32. Nagataki, S., and Yonekura, A., “Studies of the Engineering, Cornell University, Ithaca, Aug. 1982. 5.20. Russell, H.G., and Corley, W.G., “Time- Volume Changes of High Strength Concretes with SuperDependent Behavior of Columns in Water Tower Place, plasticizer,” Journal, Japan Prestressed Concrete Engineering Association (Tokyo), V. 20, 1978, pp. 26-33. Douglas McHenry International Symposium on Concrete and Concrete Structures, SP-55, American Concrete 5.33. Ahmad, S.H. and Shah, S.P. “Behavior of Hoop Institute, Detroit, 1978, pp. 347-373, Also, Research and Confined Concrete Under High Strain Rates,” ACI Development Bulletin No. RD052.01B, Portland Cement JOURNAL , Proceedings V. 82, No. 5. Sept.-Oct. 1985, pp. Association. 634-647. 5.21. Saucier, K.L.; Tynes, W.O.; and Smith, E.F., 5.34. “Research Needs for High-Strength Concrete,” “High Compressive-Strength Concrete-Report 3, Sum- reported by ACI Committee 363, ACI Materials Journal, mary Report,” Miscellaneous Paper No. 6-520, U.S. Army Proceedings V. 84, No. 6, Nov.-Dec. 1987, pp. 559-561. Engineer Waterways Experiment Station, Vicksburg, 5.35. Proceedings of Symposium on Utilization of HighSept. 1965, 87 pp. Strength Concrete, Stavanger, Norway, June 15-18, 1987, 5.22. Pfeifer, Donald W.; Magura, Donald D.; Russell, Tapir, Publishers, N-7034 Trondheim-N7H, Norway, 688 Henry G.;, and Corley, W.G., “Time Dependent Defor- PP. mations in a 70 Story Structure,” Designing for Effects of 5.36. Yogendram, Langan, Hagne and Ward, “Silica Creep, Shrinkage, Temperature in Concrete Structures, Fume in High Strength Concrete,” ACI Materials Journal, SP-37, American Concrete Institute, Detroit, 1971, pp. V. 84, No. 2, Mar.-Apr. 1987, pp. 124-129. 159-185. 5.37. Carrasquillo, P., and Carrasquillo, R., “Current 5.23. Shideler, J J., “Lightweight-Aggregate Concrete Practice in Evaluation of High Strength Concrete,” for Structural Use,” ACI JOURNAL , Proceedings V. 54, ACI Materials Journal, V. 85, No. 1, Jan.-Feb., 1988, pp. No. 4, Oct. 1957, pp. 299-328. 49-54. 5.24. Perenchio, William F., and Khieger, Paul, “Some 5.38. Smadi, M.M., Slate, F.O., and Nilson, A.H., Physical Properties of High Strength Concrete,” Research “Shrinkage and Creep of High, Medium, and Low and Development Bulletin No. RD056.01T, Portland Strength Concretes Including Overloads,” ACI Materials Cement Association, Skokie, 1978, 7 pp. Journal, Proceedings V. 84, No. 3, May-June 1987, pp. 5.25. Kaplan, M.F., “Ultrasonic Pulse Velocity, 224-234. Dynamic Modulus of Elasticity, Poisson’s Ratio and the 5.39. Smadi, M.M., Slate, F.O., and Nilson, AH., Strength of Concrete Made with Thirteen Different “High, Medium, and Low Strength Concrete Subject to Sustained Loads-Strains, Strengths and Failure Coarse Aggregates,” RILEM Bulletin (Paris), New Series Mechanisms,” ACI JOURNAL, Proceedings V. 82, No. 5, No. 1, Mar. 1959, pp. 58-73. 5.26. Parrot, LJ., “The Properties of High-Strength Sept.-Oct. 1985, pp. 657-664. 5.40. Ahmad, S.H., and Shah, S.P., “Structural Concrete,” Technical Report No. 42.417, Cement and Properties of High Strength Concrete and Its ImpliConcrete Association, Wexham Springs, 1969, 12 pp. 5.27. Dewar, J.D., “The Indirect Tensile Strength of cations on Precast and Prestressed Concrete,” Journal of Concretes of High Compressive Strength,” Technical Prestressed Concrete Institute, Nov.-Dec. 1985. Report No. 42.377, Cement and Concrete Association, 5.41. Ahmad, S.H., and Shah, S.P., “High Strength Concrete-A Review,” Proceedings of International SymWexham Springs, Mar. 1964, 12 pp. 5.28. Kaplan, M.F., “Flexural and Compressive posium on Utilization of High Strength Concrete, Strength of Concrete as Affected by the Properties of the Stavanger, Norway, June 15-18, 1987. 5.42. Shirley, T. Scott, Burg, G. Ronald, and Fiorato, Coarse Aggregates, ACI JOURNAL, Proceedings V. 55, E. Anthony, “Fire Endurance of High Strength Concrete No. 11, May 1959, pp. 1193-1208. 5.29. Bennett, E.W., and Muir, S.E. St. J., “Some Slabs,” ACI Materials Journal, Mar.-Apr. 1988, pp. 102Fatigue Tests of High-Strength Concrete in Axial Com- 108.


CHAPTER 6-STRUCTURAL DESIGN CONSIDERATIONS High-strength concretes have some characteristics and engineering properties that are different from those of lower-strength concretes. Internal changes resulting from short-term and sustained loads and environmental factors are known to be different. Directly related to these internal differences are distinctions in mechanical properties that must be recognized by design engineers in predicting the performance and safety of structures. These distinctions are increasingly important as strengths increase. Tests of unreinforced high-strength concrete have shown, for example, that such material in many cases may be closely characterized as linearly elastic up to stress levels approaching the maximum stress. Thereafter, the stressstrain curve of high-strength concrete decreases at a much greater rate than lower-strength concretes.6.1-6.10 Extensive experimentation at several research centers has provided a fundamental understanding of the behavior of high-strength concrete. While substantial information is now available on many aspects, some final recommendations must await the results of current and future work. In this chapter, the emphasis will be placed on design of members and structures.6.11 Where recommendations are made, they are based on the best current experimental information and in most cases must be considered tentative. 6.2-Axially-loaded columns Few columns in practice are subjected to truly axial loads. Bending moments, due to eccentric application of load or associated with rigid frame action, are usually superimposed on axial loads. ACI 318-83 requirements for design and ACI 318R-83 reflect this. However, it is useful to look first at the behavior of columns carrying axial load only. 6.2.1 Strength contribution of steel and concrete-The attribute of main interest is the ultimate strength. Present design practice, in calculating the nominal strength of an axially loaded member, is to assume a direct addition law summing the strength of the concrete and that of the steel. The justification for this is seen in Fig. 6.1, which superimposes typical stress-strain curves in compression for three concretes with that for reinforcing steel having 60,000 psi (414 MPa) yield strength (the last curve is drawn to a different vertical scale for convenience). The usual assumption is made that steel and concrete strains are identical at any load stage. For lower-strength concrete, when the concrete reaches the range of significant nonlinearity (about 0.001 strain), the steel is still in the elastic range and consequently starts to pick up a larger share of the load. When the strain is close to 0.002, the slope of the concrete curve is nearly zero and it can be thought of as deforming plastically, with little or no increase in stress.

363R-29

70 r

1,s -60 “c, M PO

M PO

440

-2-o

O?

0

02 Strain,

04

06

JO

percent

Fig. 6.1-- Concrete and steel stress-strain curves The steel reaches its yield point at about the same strain in this case; thus, concrete is at its maximum stress, steel is at fy, and the strength of the column is predicted by P = 0.85 fc' Ac + fy As where fc' fy Ac As

(6-l)

= cylinder compressive strength of the concrete = yield strength of steel = area of concrete section = area of steel

The factor 0.85 is used to account for the observed difference in strength of concrete in columns compared with concrete of the same mix in standard compression-test cylinders. A similar analysis holds for high-strength concrete columns, except the steel will yield before the concrete reaches its peak strength. However, the steel will continue to yield at essentially constant stress until the concrete is fully stressed. Prediction of strength may therefore still be based on Eq. (6-l). Experimental documentation also supports use of the factor 0.85 for high-strength concrete.6.12-6.13 6.2.2 Effects of confinement steel-Lateral steel in columns, preferably in the form of continuous spirals, has two beneficial effects on column behavior: (a) it greatly increases the strength of the core concrete inside the spiral by confining the core against lateral expansion under load, and (b) it increases the axial strain capacity of the concrete, permitting a more gradual and ductile failure, i.e., a tougher column.6.12-6.16 The basis for design of spiral steel under the 1977 and later versions of ACI 318 is that the strengthening effect of the spiral must be at least equal to the column strength lost when the concrete shell outside of the spiral spalls off under load. The ACI 318 equation for mini-

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mum volumetric ratio of spiral is

f' ps = 0.45 2 - 1 -L i E I4

associated with increasing spacing of the spiral wires. 6.13,6.17 Thus an improved version of Eq. (6-3a) is

P-2)

where ps

= ratio of volume of spiral reinforcement to volume of concrete core = gross area of concrete section Ag = area of concrete core r”: = cylinder compressive strength of concrete 6 = yield strength of spiral steel

The increase in compressive strength of columns provided by spiral steel is based on an experimentally derived relationship for strength gain x - fcl’ = 4.oJy where

(6-3a)

x = compressive strength of spirally reinforced concrete column f,” = compressive strength of unconfined concrete column f.2' = concrete confinement stress produced by spiral

This relationship can be shown to lead directly to Eq. (6-2). The concrete confinement stress produced by spiral f2' is calculated on the basis that the spiral steel has yielded, using the familiar hoop tension equation.

or 2J4 f

f2' = ds c where ASP 4 S

= area of spiral steel = diameter of concrete core = pitch of spiral

and other terms are as already defined. Recent work by Ahmad and Shah 6.14 has shown that spiral reinforcement is less effective for columns of higher-strength concrete and for lightweight concrete columns. They found also that the stress in the steel spiral at peak load for high-strength concrete columns and lightweight concrete columns is often significantly less than the yield strength assumed in the development of Eq. (6-2). These conclusions are consistent with results of experimental research at Cornell University.6.13 In the Cornell research, an “effective” confinement stress f2 (1 - s/dc) was used in evaluating results, where f2 is the confinement stress in the concrete, calculated using the actual stress in the spiral steel, often less than fy . The term (1 s/dc) reflects the reduction in effectiveness of spirals

z - fc,’ = 4.Of, (1 - s/d,)

(6-3b)

Fig. 6.2 shows the results of the Cornell tests on columns using different strength concretes. It is clear that the strength gain predicted by Eq. (6-3b) is valid for normal weight concrete of all strengths for confinement stress up to at least 3000 psi (21 MPa). A similar plot based on Eq. (6-3a) shows a somewhat unconservative prediction for higher confinement stresses, but it can be shown that typical confinement stresses for practical column spirals are seldom more than about 1000 psi (7 MPa). For this range Eq. (6-3a) gives good results. From the strength viewpoint, the present ACI 318 equation for minimum spiral steel ratio can be used safely for highstrength normal weight columns as well as for lowerstrength concrete columns. Fig. 6.2 also shows that a spiral has much less confining effect in lightweight concrete columns. The lightweight concrete tends to crush under the spirals at heavy loads, relieving the confining pressure.6.13 For lightweight spirally reinforced columns, Martinez has suggested that Eq. (6-3a) be replaced by z - fcl’ = 1.8fi’

(6-4a)

and Eq. (6-3b) be replaced by x - fcl’ = 1.8 f2’ (1 - s/d,)

(6-4b)

This important difference in behavior means that Eq. (6-2) found in ACI 318 must be reexamined. It appears that lightweight concrete columns would require about 2.5 times more spiral steel than corresponding normal weight columns to satisfy strength requirements after the cover spalls off, a requirement that is not reflected in ACI 318. Whether or not such heavy spirals are practical may be questioned. There is not yet general agreement on the effectiveness of spiral steel for improving the ductility of highstrength concrete columns, that is, for increasing the strain limit and flattening the negative slope of the stress-strain curve past the point of peak stress. A paper by Ahmad and Shah 6.14 indicates that confining spirals are about as effective in flattening the negative slope of the stress-strain curve for high-strength concrete columns as for lower-strength concrete columns. However, the Cornell work 6.13 showed significant differences. Fig. 6.3 shows experimental stress-strain curves for different strengths of normal weight concrete columns with varying spiral reinforcement. Three groups of curves are identified by the three concrete strength levels studied. Each of these groups consists of three sets of curves corresponding to three different amounts of lateral reinforcement. Indicated in each set of curves with a short horizontal line is the average unconfined column strength corresponding to that particular set of confined columns.

363R-31

HIGH STRENGTH CONCRETE Effective Confinement Stress f,(l- s/d,) , MPa IO 5 I I Normal Weight Concrete 12 A High- Strength . Medium - Strength l Low- Strength 10 Lightweight Concrete A High- Strength a Medium - Strength o Low- Strength 0 t

Strength Incrememt

T,-

20 I

15 I

I - 80

- 60 Strength Increment Tc-f‘ , ) MPa

f’; ,

ksi

6 -

fc= f;t l.Bfe(l-s/d,) - 2 0

4 x 16 - in. (102 x 406 -mm) Cylinder Stroke Rote: 12,000 p-in (0.30 mm)/min. I 2000 3000

0

Effective Confinement Stress 1,(1-s/d,) , psi

Fig. 6.2-Strength increment provided by spiral reinforcement action on 4 x 16-in. columns

Normal Weight Spiral Columns 4 x 16 - in. (102 x 406 -mm) Cylinder Stroke Rate: 12.000 p-in.(O 30mmtlnun 1; = Unconfined Column Strength (2500) = Effective Ccnfinement stress ‘L, t- l/d&

Axial Stress, ksi

Axial Stress , M Pa

Axial

Strain , in. / in.

Fig. 6.3-Experimental stress-strain curves of 4 x 16-in. normal weight spiral columns Referring to Fig. 6.3, the curves for high-strength concrete columns NC167 that had an effective confinement stress of 767 psi (5 MPa) are compared with the curves for lower-strength concrete columns NC163 with an effective confinement stress of 800 psi (6 MPa). Different behavior for comparable confinement stress is evident. Not only is the strain at peak stress much less for high-strength concrete, but the stress falls off sharply just past the peak value. This last fact is seen to be true even for columns NC169 with a very high confinement stress of 2500 psi (17 MPa) (probably not attainable in practical columns). Based on the available evidence, one may conclude that normal density high-strength concrete columns with spiral steel show strength gain due to the spirals that is

predicted well by present equations, but that their properties past peak stress may be deficient compared with lower-strength columns. The design of lightweight concrete columns with spiral steel should be approached very carefully. 6.13,6.17 Another interesting and important observation relating to spirally reinforced columns generally is that the level of confinement stress corresponding to spirals designed by ACI 318 is rather low for all columns. The confinement stress becomes significantly lower for larger diameter columns, assuming that the cover requirements remain constant. This follows directly from Eq. (6-2). For larger columns, the ratio Ag/Ac becomes much smaller; consequently the required spiral steel ratio becomes smaller and the effective confinement stress becomes


363R-32

proportionately smaller. Confinement stress produced by spirals designed to ACI 318 for lower-and high-strength concrete, for 15 and 50 in. column core diameters are compared in Table 6.1. Table 6.1-Confinement stress produced by spirals designed by ACI 318 f’(1 - s/a,

4.

in. (mm)

A, /A

Ps*

psi (MPa)

s,

in. (mm)

strains associated with these stresses have a profound effect on the structural behavior. Such strains are directly related to long-term deflection, losses in prestress force, and cracking. Column strength may be reduced due to sustained loading of high intensity. It may also be increased because of the capability of a concrete structure to adjust itself to local high over-stresses through creep. Creep may be described either in terms of the creep coefficient

f,’ = 3000 psi (21 MPa) (#3 spiral bar) 15(38) 50( 172)

;:;“:

0.0099 0.0028

238 (1.64) 83 (0.57)

2.96 (75) 3.17 (81)

f: = 10,000 psi (69 MPa) (#5 spiral bar) 5 0 ( 1 2 7 )

0.0330 0.0093

825(5.69) 263(1.81)

2.50(64) 2.67(68)

*Ratio of volume of spiral reinforcement to total volume of core (out-to-out of spirals).

Cc =

creep strain initial elastic strain

(6-5)

or by the coefficient of specific creep (unit creep coefficient) 6, = creep strain per unit stress

(6-6)

The two can be related through the modulus of elasticity Tests show that for lower-strength concrete even the reduction in confinement stress from 238 to 83 psi (1.64 to 0.57 MPa) obtained under ACI 318 wilI produce a column with very large strain capacity without significant loss of resistance. For high-strength concrete, the reduction of confinement stress from 825 to 263 psi (5.69 to 1.81 MPa) produces a column with virtually no postpeak strain capacity. Even the higher confinement stress of 825 psi (5.69 MPa) produces a column with the undesirable characteristic of a sharp drop-off of resistance immediately after peak stress.6.13 While some experimental data are available at this time for high-strength concrete columns using lateral ties rather than spirals,6.18,6.19 more work must be done for such members. 6.2.3 Repeated loading-High-strength concrete is relatively free of internal microcracking, even close to ultimate load, when loaded monotonically.6.1 On the other hand, high-strength concrete is reported to be more brittle than lower-strength concrete,6.2 lacking much of the ductility that accompanies progressive crack growth. Some experimental research indicates that fatigue strength is essentially independent of compressive strength.6.20 Recent research indicates that failure of concrete subject to repeated loading can be approximately predicted by the concept of the envelope curve, directly related to the short-term monotonic stress-strain curve. 6.21 For high-strength concrete, each load application causes relatively less incremental damage. However, the number of cycles to failure may not be necessarily larger because of the greater negative slope of the post-peak envelope curve. While important work has been done,6.20,6.22,6.23 it is clear that additional research is needed on all aspects of high-strength concrete, with and without confinement steel, subject to various repeated load regimens, before design recommendations can be made. 6.2.4 Sustained loading-In most structures, concrete is subjected to sustained loads. The time-dependent

cc = E,6,

(6-7)

There is general agreement that creep of high-strength concrete is significantly less than that of a lower-strength concrete6.7,6.24-6.27 The most recent information, for concretes with strength up to about 10,000 psi (69-MPa), indicates that high-strength concrete has a specific creep only about 20 percent that of lower-strength concrete and a creep coefficient about 30 percent as high.6.27 As a result, for axially loaded high-strength concrete columns, creep shortening at a given stress level will be less than that of lower-strength columns, a fact of possible significance in high-rise concrete structures.6.30 In addition, the distribution of load between concrete and steel of high-strength concrete columns will be less subject to change with the passage of time. Elastic distribution of stresses may be more nearly maintained. Loss of stress in a prestressed member due to creep shortening will be much less at a given concrete stress level, but this advantage may be largely canceled if higher sustained load stresses are permitted. 6.3-Beams and slabs The material properties described in Chapter 5 and in Section 6.2 may effect the characteristic behavior of highstrength concrete beams.6.31-6.34 In some cases, improvements are seen; in other cases less satisfactory behavior will result. In many ways, high-strength beams may behave according to essentially the same rules that have been used to describe behavior of beams made of lowerstrength concrete. However, some questions remain to be answered. 6.3.1 Compressive stress distribution-The compressive stress distribution in beams is directly related to the shape of the stress-strain curve in uniaxial compression. Consequently, for high-strength concrete, which displays differences in that shape, as shown in Fig. 6.1, it is reasonable to expect differences in flexural compressive


stress distribution, particularly at loads approaching ultimate. In present U.S. practice as in ACI 318 and ACI 318R, proportioning of beam sections is generally based on conditions at a hypothetical state of incipient collapse at factored loads. Fig. 6.4(a) shows the generally parabolic shape of the compressive stress distribution in a beam made of lower-strength concrete. The nominal resisting moment may be calculated knowing the internal forces T and C and the internal lever arm between them. The actual shape of the compressive stress distribution at incipient failure, always highly variable even within a given range of concrete strengths, may be considered irrelevant if one knows (a) the magnitude of the compressive resultant C, and (b) the level in the beam at which it acts. These may be established in terms of three parameters characteristic of a given stress distribution [see Fig. 6.4(a)]. k 1 = ratio of average to maximum compressive stress in beam k 2 = ratio of depth to compressive resultant to neutral axis depth k 3 = ratio of maximum stress in beam to maximum stress in corresponding axially loaded cylinder

For ordinary design purposes, it is convenient to work with an equivalent rectangular compressive stress distribution, shown in Fig. 6.4(b), with magnitude of compressive resultant and line of action the same as before. Such an equivalent distribution is specifically referenced and permitted in ACI 318 and its Commentary, ACI 318R. With the uniform value of concrete compression assumed equal to 0.85fc', a single parameter PI is sufficient to define both magnitude and line of action. For high-strength concrete, the stress-strain curve is more linear than parabolic. Therefore, there is reason to suspect that the stress block parameters may be different. Experimental research has confirmed that differences do exist, and alternatives to the rectangular stress block have been proposed, such as in Fig. 6.4(c).6.34 However, differences in calculated strength values for beams and eccentric columns depend on steel ratios and other factors. ACI 318R suggests, based on an equivalent rectangular stress block, that the nominal flexural strength of a singly reinforced beam that is under-reinforced can be calculated by

= nominal moment strength at section, in-lb = area of tension reinforcement, in.2 = specified yield strength of reinforcement, ; psi = distance from extreme compression fiber d to centroid of tension reinforcement, in. = ratio of tension reinforcement = specified compressive strength of conif crete, psi The coefficient 0.59 can be shown to be equivalent to k2/k1k3. The experimental variation of k2/k1k3 with concrete compressive strength based on research at several centers is shown in Fig. 6.5. 6.6,6.31-6.35 While a detailed study of the separate k values indicates that significant differences in the separate values exist depending on concrete strength, it is clear from Fig. 6.5 that the differences are compensative and that the combined coefficient is well-represented by the constant value 0.59. This statement is reinforced by the results shown in Fig. 6.6, which compares flexural strength predictions obtained using the usual rectangular stress block, a triangular stress block, and a distribution based on experimentally derived stress-strain curves with test data for beams of varying reinforcement ratios and concrete strengths to 11,000 psi (76 MPa). Test values were best predicted using actual stress-strain curves, but either the rectangular or triangular distributions gave acceptable lower bounds to the experimental and theoretical values.6.36 Based on these and similar studies, it appears that, for under-reinforced beams, the present ACI 318 methods can be used without change, at least for concrete strengths up to 12,000 psi (83 MPa). For over-reinforced where Mn

C=k,k,f;bc

> T = Asfs

C-O.85 f’,ab

drp-

C=$.f;bc

T=A,f,

Fig. 6.4-Concrete stress distributions for rectangular beams

363R-33

363R-34


Concrete Strength ,

Concrete Strength , M Pa 0 0.005 r

20 I

40 I

60 1

M Pa

80 I

l00 I

120 I

.

08 I

0.6 k2 -

k,

&

-

-

-

-

A

.. -

-

- 0.500 Rectangular

-

.

0.4

i t 0001 t

0.2 0 0

I

I

I

I

I

4

8

12

16

20

Concrete Strength , ksi 0

L

0

t

I

1

I

4

8

I2

16

20

Concrete Strength , ksi

Fig. 6.5-- Stress block parameter k2/k1k3 versus concrete strength 140 Test

Dato 0 -4.-

ACI Cods Triangular

Stress Block --*-

Theoretical

I*--

100 Mu, kip-ft 80

60

Fig. 6.7-- Ultimate concrete flexural strain Q, versus concrete compressive strength energy release from the testing equipment. Fig. 6.76.5 shows the variation of concrete strain at failure at the extreme compression face of singly reinforced concrete beams or eccentrically loaded columns without lateral confinement steel. The constant value of strain at extreme concrete compression fiber of 0.003 prescribed by ACI 318 is seen to represent satisfactorily the experimental results for high-strength as well as lower-strength concrete, although it is not as conservative for highstrength concrete. 6.3.3 Influence of confinement steel and compression Steel-Considering the more limited strain capacity of high-strength concrete in compression, it is necessary to evaluate the ductility of beams made of high-strength concrete. Deflection ductility index /1 will be defined here as

4 0

20 0.5

I

2

3

Reinforcement Ratio p ,%

Fig. 6.6-- Comparison of flexural strength Mu , of beams for several compressive stress distributions

beams, which are not allowed by ACI 318, or for members combining axial compression and bending, important differences may occur. 6.33 6.2.3 Limiting compressive strain-While high-strength concrete reaches its peak stress at a compressive strain slightly higher than that for lower-strength concrete, the ultimate strain is lower for high-strength concrete, both in uniaxial compression tests and in beam tests. 6.13,6.34 It has been suggested that this result apparently is due to

where

4

= beam deflection at failure load 4, = beam deflection at the load producing yielding of tensile steel 6.34 Tests by Pastor et al of beams made of relatively highstrength concrete are summarized in Table 6.2 (Series A) and Table 6.3 (Series B). Beams of Series A were singly reinforced with no compression steel and no confinement steel. The series includes beams with concrete strengths from 3700 to 9265 psi (26 to 64 MPa). For the highstrength beams, tensile steel ratio varied from 0.29 &, to 1.11 pb where ,,b = reinforcement ratio for balanced strain conditions. The results show a lower ductility for the beams with

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the higher concrete strengths. Based on these results, the second series summarized in Table 6.3 was performed. These beams included varying amounts of compression steel (50 to 100 percent of tensile steel area) and lateral confinement steel in the form of closed hoops at spacing of 3, 6, and 12 in. (7.6, 15.2, and 30.5 mm). All beams were of high-strength concrete and comparable to Beam A4 of the first series, which had no ties and no compression steel. Table 6.2-Deflection ductility index for Series A beams 6.34 Ductility

index

PmfJl = 7 for fy in psi Y P#&=

Table 6.3-Deflection ductility index for Series B b e a m s 6.34

T

Ductility

Beam

psi

p/p,

index WA.

Bl

8534 8605 8578 8478 8516 8466

0.57 0.55 0.57 0.59 0.56 0.58

2.36 2.64 4.88 8.32 5.61 6.14

i: E B6

From Beams Bl and B2, compared with Beam A4, it can be concluded that ties at 12 in. (30.5 mm) increased the ductility index, but not significantly. Ductility index increased markedly when the tie spacing was reduced to 6 in. (15.2 mm) in Beams B3 and B4, but showed no upward trend when the spacing was further reduced to 3 in. (7.6 mm). A comparison between Beams B3 and B4 indicates a beneficial effect in adding more compression steel, although this trend is not clearly reflected in a comparison of Beams B5 and B6. 6.3.4 Minimum tensile steel ratios-ACI 318 sets an upper limit on the tensile steel ratio for beams at 75 percent of the balanced ratio to insure that failure, should it occur, will be a gradual, yielding type. A lower limit of tensile steel ratio is set to guard against sudden failure of very lightly reinforced beams upon concrete cracking, when the tension formerly carried by the concrete is transferred to the steel reinforcement. The present ACI 318 expression for minimum steel ratio

(6.10)

Y

is derived on the basis that the resisting moment of the cracked section should be at least as great as the moment that caused the member to crack, based on the modulus of rupture. Since the latter is known to be greater for high-strength concrete than for lower-strength concrete, 6.2,6.12 it is evident that the strength of concrete should be included in a revised version of Eq. (6.10). With modulus of rupture taken at 7.5 &r (0.62 q), it can be shown that

Pmi?a *Ratio of tension reinforcement divided by reinforcement ratio producing balanced strain conditions.

1.7 for 4 in MPa

_ i-7 $r

2 m/l’y (1.38/f,)

(6.11)

4

would be an appropriate equation for all concrete strengths from 3000 to 12,000 psi (21 to 83 MPa).6.28 6.3.5 Shear and diagonal tension-In current US. practice, design for shear is based on conditions at factored loads. The total shear resistance is made up of two parts: Vs provided by the stirrups and Vc, nominally the “concrete contribution.” The nominal concrete contribution includes, in an undefined way, the contributions of the still uncracked concrete at the head of a hypothetical diagonal crack, the resistance provided by aggregate interlock along the diagonal crack face, and the dowel resistance provided by the main reinforcing steel. High-strength concrete loaded in uniaxial compression fractures suddenly and, in so doing, may form a failure surface that is smooth and nearly a plane. 6.1-6.3 This is in contrast to the rugged failure surface characteristic of lower-strength concrete. In beams controlled by shear strength, the state of stress is biaxial, combining diagonal compression in the direction from the load point to the support with diagonal tension in the perpendicular direction. Diagonal tension cracks in high-strength concrete beams can be expected to have a smooth surface, likely to be deficient in aggregate interlock. Tests confirm that aggregate interlock decreases as concrete strength increases. Thus, a shear strength deficiency may be produced which is not accounted for by present design equations. Data from Frantz 6.38,6.39 at the University of Connecticut have indicated that the calculated concrete contribution Vc is ade uate for highstrength concrete. Data byNilson 6.40,6.41 at Cornell University indicates that current design methods are not conservative for high strength concrete. Experimental research by Ahmad et al. 6.42,6.43 indicates that the shear strength contribution of the concrete is conservatively predicted by ACI 318-83 Eq. (11-3) for shear-span ratios of 2.5 or less, but for higher ratios, more typical in ordinary construction, and for relatively low steel ratios, the ACI equation may be unconservative. It was further

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shown by their research that the more complex ACI 31883 Eq. (11-6) is unconservative for high-strength concrete beams with low steel ratios. Recent research by Russell and Roller6.37 indicates that, for beams with high flexural steel ratios, the current ACI Code equations are safe. The beneficial effects of high-strength concrete for prestressed beams was demonstrated, using an analysis based on truss models, by Kaufman and Ramirez.6.44 Higher strength concrete increases the strength of the diagonal truss members, resulting in increased efficiency of the web reinforcement through the mobilization of more stirrups as well as increased load-carrying capacity of the struts themselves. Currently, no research data are available regarding the minimum requirement of web reinforcement to prevent brittle failure resulting from the formation of a critical diagonal crack. 6.3.6 Bond, anchorage, and development lengthPresent ACI 318 methods of design for development length and anchorage of tensile steel are based on tests, generally using concretes with compressive strengths not greater than about 4000 psi (28 MPa). Although some information has recently become available for high-strength concrete, not enough data have been obtained to permit recommendations. 6.3.7 Cracking-The modulus of rupture, which is the appropriate measure of concrete tensile strength for use in predicting flexural cracking load, has been reported in Chapter 5 to be 11.7 K for normal weight concretes with strengths in the range from 3000 to 12,000 psi (21 to 83 MPa). It thus appears that the ACI 318 value of 7.5 g is too low. However, for curing conditions such as seven days moist curing followed by air drying, a value of 7.5 K is probably fairly close for the full strength range. It may, therefore, be recommended with no change. The assumption of a modulus of rupture lower than the actual value for a flexural member is neither conservative nor unconservative but simply results in an inaccurate prediction of cracking load. This will result in inaccurate estimation of both elastic and creep deflections. The direct tensile strength is seldom measured but is of interest in studying web-shear cracking in prestressed concrete members, for one example. Both modulus of rupture and tensile splitting strength of high-strength concrete are well above the corresponding values for lower-strength concrete. In this respect, at least, empirically derived equations for flexural shear and torsional shear strength could be used for high-strength concrete calculations based on the lower-strength material. However, other aspects of concern are discussed in Section 6.3.5. 6.3.8 Elastic deflections--The main uncertainties in predicting elastic deflections of reinforced concrete beams are (a) elastic modulus Ec; (b) modulus of rupture fr; and (c) effective moment of inertia, which depends on the extent of cracking of the beam. For the elastic modulus, the following equation of

Chapter 5 may be used unless actual values of modulus are known. Ec = 40,000 K + 1.0 x 106 psi (Ec = 3320 K + 6900 MPa)

(6-12)

Eq. (6.12) should be modified by the correction factor (w/c/145)1.5 [for SI units (wc/2300)1.5 for concrete densities other than 145 lb/ft3 (2320 kg/m 3). 6.2,6.13,6.46 The modulus of rupture has been discussed in Section 6.3.7. For prediction of deflections a value of 7.5 E may be used to calculate the flexural cracking moment of the beam. The eauation for effective moment of inertia Ie included in the ACI 318 is

(6-13) where Mcr Ma Ig Icr

cracking moment maximum moment = gross moment of inertia of section = moment of inertia of cracked transformed section =

=

This provides a basis for beam deflection calculation that appears valid for high-strength concrete as well as normal concrete beams, based on information currently availablee 6.34,6.47,6.48 6.3.9 Time-dependent deflections-Time-dependent deflections of beams due to creep and shrinkage are presently calculated by applying multipliers to o computed elastic deflections. This procedure is generally valid for high-strength concrete members, but experimental data indicates that the multipliers may be significantly less because of the lower creep coefficient typical of highstrength concrete. According to ACI 318, additional longterm deflections are obtained using the following multiplier f 1 + 5Op’

where = reinforcement ratio for nonprestressed compression reinforcement = time-dependent factor The time-dependent factor is given by Fig. 6.8, taken from ACI 318R. Research in progress,6.47,6.48 providing an indication of long-term multipliers and their variation with time, is summarized in Fig. 6.9. Results are currently available up to about 1 year loading age, and clear trends are evident, as follows:


0 1 3 6

12

18

24

30

48

36

60

Duration of Load , months

Fig. 6.8-ACI 318R Commentary multiplier for long-time deflections of beams

Multiplier

f’c=36w psi I

I

I

l00

200

300

0 0

Duration of Load, days

Multiplier

f;=6500 psi I

1

1

100

200

300


Multiplier

/

1

I

l00

200

300


Fig. 6.9-- Multipliers for long-term deflections for different strength concrete beams a. For 3600 psi (2.5 MPa) concrete beams, l-year multipliers of 0.85, 0.60, and 0.50 for beams with p’/p, respectively, equal to 0, 0.5, and 1.0 are less than the ACI 318 l-year values of 1.40, 1.10, and 0.80, which were determined for lower-concrete strengths. b. For high-strength concrete beams, deflection multipliers are still lower than the ACI 318 values. For ex-

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ample, for high-strength beams with no compression steel, the value of 0.55 at 1 year is only 40 percent of the ACI 318 value and 65 percent of the experimental value for lower-strength concrete. c. The influence of compression steel may be less important for high-strength concrete beams than for lowerstrength beams. For beams of lower-strength concrete, addition of compression steel having an area equal to that of the tensile steel reduces l-year deflections by 41 percent. For high-strength concrete, the beam with compression steel shows about 35 percent reduction. This could be expected because the role of compression steel is mainly to reduce the creep of the concrete in the compression zone under sustained loads, the high-strength concrete with lower creep coefficient needs less help in this respect. Deflection measurements are continuing in the research described. Results over a longer period of time will be available as well as information for beams with compressive strengths fc' to 12,000 psi (83 MPa). Concrete strength should appear as a parameter in equations to predict long-term deflections. Concrete strength not only influences long-term deflections directly because of the lower creep coefficient but also influences the effectiveness of compression steel. 6.3.10 Repeated Loading-With reference to Section 6.2.3, it appears that high-strength concrete, because of its relative freedom from internal microcracking at service loads, would be more resistant to repeated loading consisting of a large number of cycles at relatively low stress ranges such as in bridges. If ductility is an important consideration, as is the case in seismic resistant design, it would be important to include lateral confinement steel in the form of closed hoop stirrups as well as compression reinforcement. While the subject has been thoroughly studied for lower-strength concrete,6.20,6.22,6.23 little information on high-strength concrete beams subject to repeated loads is available at this time. 6.3.11 Prestressed concrete beams-Characteristics of high-strength concrete, discussed previously in this chapter in the context of axially loaded members and reinforced concrete beams, affect the behavior of prestressed concrete beams in corresponding ways. Special mention must be made, however, of the effects of a very low creep coefficient. At the same concrete stress levels, time-dependent deflection of high-strength beams will be less. On the other hand, low concrete creep may have little effect on prestressed beam deflections because upward creep deflection due to prestress is, in many cases, canceled by downward creep deflection due to sustained loads. This results in only very small net deflections associated with all sustained loads. For a given level of concrete stress, loss of prestress force due to creep could be expected to be much smaller for prestressed beams using high-strength concrete. Higher sustained concrete stress would negate this advantage.

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6.4-Eccentric columns 6.4.1 Compressive stress distribution-It was pointed out in discussing beams in Section 6.3.1 that the shape of the compressive stress distribution in high-strength concrete beams is apt to be different from that in lower-strength concrete beams, reflecting the different shape of the compressive stress-strain curve as shown in Fig. 6.1. For under-reinforced concrete beams, with strength controlled by the yield strength of the reinforcement, the actual shape of the compressive stress block used in calculation of the nominal flexural strength is of little importance so long as the internal lever arm to the compressive resultant is close to the true value. The conventional rectangular stress block and equations for determining nominal flexural strength based on the rectangular stress block will normally be satisfactory. Overreinforced beams are not permitted according to ACI 318, and so one concludes that present procedures will produce adequate results for all beams designed under provisions of ACI 318, whether lower- or high-strength concrete is used. In the case of combined bending and axial load, i.e., eccentric columns, members failing in flexural compression cannot be avoided. For members with relatively low eccentricity, failure will be initiated by the concrete reaching its limiting strain, while the steel on the far side of the column may be well below tensile yielding or may remain in compression at the failure load. For such cases, a more accurate representation of the concrete compressive stress block could be important. 6.4.2 Interaction diagram for strength of short columnsLimited analytical studies have been made of eccentric columns comparing the predictions of the current ACI 318R Commentary approach based on the equivalent rectangular stress block, with a trapezoidal concrete stress distribution.6.49 The general shape of the trapezoid would vary, ranging from nearly rectangular for lower-strength concrete to nearly triangular for very-high-strength as discussed in Section 6.3.1. Fig. 6.10 shows a comparison of the strength interaction diagram relating axial load capacity Pn and flexural capacity Mn for a 14 x 14 in. column made of 12,000 psi (83 MPa) strength concrete. Reinforcement is provided by four No. 11 comer bars having yield strength fy = 60,000 psi (414 MPa). Strength under combined axial load and bending was computed first using the conventional rectangular stress block (solid line), then using a variable-proportioned trapezoid (dashed line). For relatively large eccentricities, when moment dominates and failure is initiated by tensile reinforcement yielding, the two curves are almost indistinguishable. For intermediate to small eccentricities, ACI 318 results in larger values for both moment and axial force at a given eccentricity at failure than those obtained by the more exact calculation. Differences of up to 15 percent in the interaction diagram relating moment to axial load have been found based on comparative calculations.6.49 ACI 318 procedures in corporate an assumed concrete

3000

“r

2000 Nominal Axial Load Strength at given Eccentricity P n, kips 1000

0 0

200

400

500

Nominal M o m e n t Strength Mu,ff-klp

Fig. 6.10-- Comparative interaction diagrams for highstrength concrete column strain limit in compression of 0.003. It has been shown in Section 6.3.2 that this is less conservative for highstrength concrete than for lower-strength concrete. In the presence of effective lateral confinement, such as provided by continuous spirals in normal weight concrete columns, the effective strain limit is larger than this value, and strain compatibility analysis can be based on 0.003 strain. However, there is no apparent justification for increasing limiting strain assumptions above present values. 6.4.3 Slenderness effects-The moment magnification method for dealing with slenderness effects in reducing the strength of reinforced concrete columns appears to be generally valid for high-strength concrete. An exception may be in the equations for calculating effective flexural rigidity. Two alternative equations are given in ACI 318 for flexural rigidity, both of which include factors to account for the effect of concrete creep in an approximate way. The validity of these equations for high-strength concrete may at least be questioned, recognixing the significantly lower creep coefficient for highstrength concrete. No experimental information is available at this time. In addition, calculations should incorporate estimates of Ec as given by Eq. (6-12). 6.5-Summary 6.5.1 Review-A brief summary has been given of the special characteristics of high-strength concrete as they bear upon the behavior and design of reinforced concrete members and structures. For axially loaded columns, the direct addition of concrete and steel strength contributions is generally valid, as for lower-strength concrete members. Lateral steel plays a particularly important role in that it is necessary to improve ductility and toughness. Of special concern is


the sharp drop-off of load after peak stress and the apparent diminished effectiveness of lateral steel in increasing ductility compared with lower-strength concrete columns. Further studies are needed. High-strength concrete columns will exhibit less shortening under load than lower-strength concrete columns because of the higher elastic modulus and lower creep coefficient. For beams, use of the conventional equivalent rectangular stress block appears to give satisfactory results for under-reinforced members required by ACI 318 procedures. The compressive strain limit is less than found for lower-strength concrete but still may be taken at 0.003. Confinement steel and compressive steel should be used in designing concrete beams where ductility is important as for seismic resistant structures. Changes have been recommended for ACI 318 values for minimum tensile steel ratio to reflect the influence of concrete strength, as well as in the modulus of elasticity to be used in deflection calculations. Significant changes should also be considered in the calculation of long-term beam deflections to reflect the much lower creep coefficient and reduced effectiveness of compression steel in the case of high-strength concrete beams. The calculation of eccentric column strength may be influenced by the shape of the compressive stress block used, particularly for columns with relatively small eccentricity with neutral axis at failure close to an edge. Limited trial calculations comparing rectangular stress block with trapezoidal stress block indicate only small differences. In determining slenderness effects, special consideration should be given to the lower creep coefficient for high-strength concrete, as it affects the effective flexural rigidity used in the calculations, and to improved values of modulus of elasticity. 6.5.2 Research needs-The material of Chapter 6 should be considered to be subject to revision based on future research results. Areas in which information is lacking include shear, diagonal tension, torsion, bond, anchorage, development length, and the effects of repeated loading. Research programs are now in progress in several centers that are aimed at filling some of these gaps. In this way, the research base will be expanded so that the many advantages of high-strength concrete may be used safely and with confidence based on thorough documentation of material properties and behavioral characteristics of members. 6.6-Cited references (See also Chapter l0--References) 6.1. Carrasquillo, Ramon L.; Nilson, Arthur H.; and Slate, Floyd O., “Microcracking and Engineering Properties of High-Strength Concrete,” Research Report No. 80-1, Department of Structural Engineering, Cornell University, Ithaca, Feb. 1980, 254 pp. 6.2. Carrasquillo, Ramon L.; Nilson, Arthur H.; and Slate, Floyd O., “Properties of High Strength Concrete Subject to Short-Term Loads,” ACI JOURNAL , Proceedings V. 78, No. 3, May-June 1981, pp. 171-178.

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6.3. Carrasquillo, Ramon L.; Slate, Floyd 0.; and Nilson, Arthur H., “Microcracking and Behavior of High Strength Concrete Subject to Short-Term Loading,” ACI JOURNAL, Proceedings V. 78, No. 3, May-June 1981, pp. 179-186. 6.4. “High Strength Concrete in Chicago High-Rise Buildings,” Task Force Report No. 5, Chicago Committee on High-Rise Buildings, 1977, 63 pp. 6.5. Kaar, P.H.; Hanson, N.W.; and Capell, H.T., “Stress-Strain Characteristics of High-Strength Concrete,” Douglas McHenry International Symposium on Concrete and Concrete Structures, SP-55, American Concrete Institute, Detroit, 1978, pp. 161-185. Also, Research and Development Bulletin No. RD051.01D, Portland Cement Association. 6.6 Perenchio, William F., and Klieger, Paul, “Some Physical Properties of High Strength Concrete,” Research and Development Bulletin No. RD056.01T, Portland Cement Association, Skokie, 1978, 7 pp. 6.7. Shah, S.P., Editor, Proceedings, National Science Foundation Workshop on High Strength Concrete, University of Illinois at Chicago Circle, Dec. 1979, 226 pp. 6.8. Wang, P.T.; Shah, S.P.; and Naaman, A.E., “Stress-Strain Curves of Normal and Lightweight Concrete in Compression,” ACI JOURNAL , Proceedings V. 75, No. 11, Nov. 1978, pp. 603-611. 6.9. “Research Needs for High-Strength Concrete,” reported by ACI Committee 363, ACI Materials Journal, Proceedings V. 84, No. 6, Nov.-Dec. 1987, pp. 559-561. 6.10. Proceedings of Symposium on Utilization of HighStrength Concrete, Stavanger, Norway, June 15-18, 1987, Tapir Publishers, N-7034 Trondheim-NTH, Norway, 688 pp. 6.11. Nilson, A.H., “Design Implications of Current Research on High-Strength Concrete,” High-Strength Concrete, SP-87, American Concrete Institute, Detroit, 1985, pp. 85-118. 6.12. Martinez, S., Nilson, AH., and Slate, F.O., “Spirally-Reinforced High-Strength Concrete Columns,” Research Report No. 82-10, Department of Structural Engineering, Cornell University, Ithaca, Aug. 1982. 6.13. Martinez, S., Nilson, AH., and Slate, F.O., “Spirally-Reinforced High-Strength Concrete Columns, ACI JOURNAL , Proceedings V. 81, No. 5, Sept.-Oct. 1984, pp. 431-442. 6.14. Ahmad, S.H., and Shah, S.P., “Stress-Strain Curves of Concrete Confined by Spiral Reinforcement,” ACI JOURNAL , Proceedings V. 79, No. 6, Nov.-Dec. 1982, pp. 484-490. 6.15. Fafitis, A., and Shah, S.P., “Lateral Reinforcement for High-Strength Concrete columns,” HighStrength Concrete, SP-87, American Concrete Institute, Detroit, 1985, pp. 213-232. 6.16. Yong, Y.K., Nour, M.G., and Nawy, E.G., “Behavior of Laterally-Confined High-Strength Concrete Under Axial Loads,” Journal of Structural Engineering, V. 114, No. 2, Feb. 1988, pp. 332-351. 6.17. Slate, F.O., Nilson, A.H., and Martinez, S.,

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“Mechanical Properties of High-Strength Lightweight Concrete,” ACI JOURNAL , Proceedings V. 83, No. 4, JulyAug. 1986, pp. 606-613. 6.18. Vallenas, J.; Bertero, V.V.; and Popov, E.P., “Concrete Confined by Rectangular Hoops and Subjected to Axial Loads,” Report No. UCB/EERC-77/13, Earthquake Engineering Research Center, University of California, Berkeley, 1977. 6.19. Sheikh, S.A. and Uzumeri, S.M., “Strength and Ductility of Tied Concrete Columns,” Journal of Structural Division, ASCE, V. 106, No. ST5, May 1980, pp. 10791102. 6.20. Bennett, E.W., and Muir, S.E. St. J., “Some Fatigue Tests on High Strength Concrete in Uniaxial Compression,” Magazine of Concrete Research (London), V. 19, No. 59, June 1967, pp. 113-117. 6.21. Ahmad, S.H., “Properties of Confined Concrete Subject to Static and Dynamic Loading,” PhD thesis, University of Illinois at Chicago Circle, Mar. 1981. 6.22. Bertero, V.V.; Bresler, B.; and Liao, H., “Stiffness Degradation of Reinforced Concrete Members Subject to Cyclic Flexural Moments,” Report No. EERC69/12, University of California, Berkeley, Dec. 1969. 6.23. Bresler, B., and Bertero, V.V., “Influence of High Strain Rate and Cyclic Loading Behavior of Unconfined and Confined Concrete in Compression,” Proceedings, 2nd Canadian Conference on Earthquake Engineering, Department of Civil Engineering, McMaster University, Hamilton, June 1975, pp. 15-1 - 15-38. 6.24. Ngab, AS.; Slate, F.O.; and NiIson, A.H., “Behavior of High-Strength Concrete Under Sustained Compressive Stress,” Research Report No. 80-2, Department of Structural Engineering, Cornell University, Ithaca, Feb. 1980, 201 pp. 6.25. Ngab, Ali S.; NiIson, Arthur H.; and Slate, Floyd O., “Shrinkage and Creep of High Strength Concrete, ACI JOURNAL, Proceedings V. 78, No. 4, July-Aug. 1981, pp. 255-261. 6.26. Ngab, Ali S.; Slate, Floyd 0.; and Nilson, Arthur H., “Microcracking and Time-Dependent Strains in HighStrength Concrete, ACI JOURNAL , Proceedings V. 78, No. 4, JuIy-Aug. 1981, pp. 262-268. 6.27. Smadi, M.M.; Slate, F.O.; and NiIson, A.H., “Time-Dependent Behavior of High-Strength Concrete Under High Sustained Compressive Stresses,” Research Report No. 82-16, Department of Structural Engineering, Cornell University, Ithaca, Nov. 1982. 6.28. Smadi, M.M., Slate, F.O., and Nilson, A.H., “Shrinkage and Creep of High-, Medium-, and LowStrength Concretes, Including Overloads,” ACI Materials Journal Proceedings V. 84, No. 3, May-June 1987, pp. 224-234. 6.29. Smadi, M.M., Slate, F.O., and Nilson, A.H., “High-, Medium-, and Low-Strength Concrete Subject to Sustained Overloads--Strains, Strengths, and Failure Mechanisms,” ACI JOURNAL, Proceedings V. 82, No. 5, Sept.-Oct. 1985, pp. 657-664. 6.30. Russell, H.G., and Corley, W.G., “Time-

Dependent Behavior of Columns in Water Tower Place,” Douglas McHenry International Symposium on Concrete and Concrete Structures, SP-55, American Concrete Institute, Detroit, 1978, pp. 347-373. Also, Research and Development Bulletin No. RD052.01B, Portland Cement Association. 6.31. Leslie, Keith E.; Rajagopalan, K.S.; and Everard, Noel J., “Flexural Behavior of High-Strength Concrete Beams,” ACI JOURNAL, Proceedings V. 73, No. 9, Sept. 1976, pp. 517-521. 6.32. Nedderman, H., “Flexural Stress Distribution in Very-High Strength Concrete,” MSc thesis, University of Texas at Arlington, Dec. 1973, 182 pp. 6.33. Zia, Paul, “Structural Design with High Strength Concrete,” Report No. PZIA-77-01, Civil Engineering Department, North Carolina State University, Raleigh, Mar. 1977, 65 pp. 6.34. Pastor, J.A.; NiIson, A.H.; and Slate, F.O., “Behavior of High Strength Concrete Beams,” Research Report No. 84-3, Department of Structural Engineering, Cornell University, Ithaca, Feb. 1984. 6.35. Wang, Pao-Tsan; Shah, Surendra P.; and Naaman, Antoine E., “High Strength Concrete in Ultimate Strength Design,” Proceedings, ASCE, V. 104, ST11, Nov. 1978, pp. 1761-1773. 6.36. Discussion of “Flexural Behavior of HighStrength Concrete Beams” by Keith E. Leslie, K.S. Rajagopalan, and Noel J. Everard, ACI JOURNAL , Proceedings V. 74, No. 3, Mar. 1977, pp. 140-145. 6.37. Russell, H., and Roller, J.J., “Shear Strength of High-Strength Concrete Beams,” accepted for publication in ACI Structural Journal. 6.38. Mphonde, A.G., and Frantz G.C., “Shear Tests of High and Low Strength Concrete Beams Without Stirrups,” ACI JOURNAL, Proceedings V. 81 No. 4, JulyAug. 1984. 6.39. Mphonde, A.G., and Frantz G.C., “Shear Tests of High- and Low-Strength Concrete Beams with Stirrups,” High-Strength Concrete, SP-87, American Concrete Institute, Detroit, 1985, pp. 179-196. 6.40. El-Zanaty, AH., Nilson, AH., and Slate, F.O., “Shear Capacity of Prestressed Concrete Beams Using High-Strength Concrete,” ACI JOURNAL, Proceedings V. 83, No. 3, May-June 1986, pp. 359-368. 6.41. El-Zanaty, A.H., Nilson, A.H., and Slate, F.O., “Shear Capacity of Reinforced Concrete Beams Using High-Strength Concrete,” ACI JOURNAL, Proceedings V. 83, No. 2, Mar.-Apr. 1986, pp. 290-296. 6.42. Ahmad, S.H., Khaloo, A.R., and Poveda, A., “Shear Capacity of Reinforced High-Strength Concrete Beams,” ACI JOURNAL , Proceedings, V. 83, No. 2, Mar.Apr. 1986, pp. 297-305. 6.43. Ahmad, S.H., and Lue, D.M., “Flexure-Shear Interaction of Reinforced High-Strength Concrete Beams,” ACI Structural Journal, V. 84, No. 4, July-Aug. 1987, pp. 330-341. 6.44. Kaufman, M.K., and Ramirez, J.A., “Re-evaluation of the Ultimate Shear Behavior of High-Strength

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Concrete Prestressed I-Beams,” ACI Structural Journal, Proceedings V. 85, No. 3, May-June 1988, pp. 295-303. 6.45. Treece, R.A., and Jima, J.O., “Bond Strength of Epoxy-Coated Reinforcing Bars,” ACI Materials Journal, V. 86, No. 2, Mar.-Apr. 1989. 6.46. Pauw, Adrian, “Static Modulus of Elasticity of Concrete as Affected by Density,” ACI JOURNAL , Proceedings V. 57, No. 6, Dec. 1960, pp. 679-688. 6.47. Leubkeman, C.H., Nilson, A.H., and Slate, F.O., “Sustained Load Deflection of High-Strength Concrete Beams,” Research Report No. 85-2. Department of Structural Engineering, Cornell University, Ithaca, Feb. 1985, 164 PP. 6.48. Paulson, K.A., and Nilson, A.H., “Deflections of High-Strength Concrete beams Under Sustained Loading,” Research Report (in preparation), Department of Structural Engineering, Cornell University, Ithaca. 6.49. Garcia, D.T., and Nilson, AH., “A Comparative Study of Eccentrically Loaded High-Strength Concrete Columns,” Research Report (in preparation), Department of Structural Engineering, Cornell University, Ithaca.

amount of reinforcing steel equal to 4 percent of the column area for a given load, whereas the same column in 9,000 psi would require only 1 percent steel-the minimum allowed by code."7.3 7.2-Cost studies The Material Service Corporation 7.4 conducted a pricing study that dramatically demonstrated the cost advantage of replacing percentages of steel with highstrength concrete in short tied columns. This 1983 study was made for a column supporting a design load (1.4D + 1.7L) of 1000 kips (4.45 MN) and based on the following prices: $760/ton in place $80/yd3 in place $85/yd3 in place $104/yd 3 in place $129/yd 3 in place $280/yd 3 in place

Reinforcing steel 7000 psi concrete 9000 psi concrete 11,000 psi concrete 14,000 psi concrete Formwork

As Fig. 7.1 shows, using high-strength concrete with a minimum of steel is the most economical solution. CHAPTER 7-- ECONOMIC CONSIDERATIONS

As earlier chapters have demonstrated, high-strength concrete is a state-of-the-art material, and like most state-of-the-art materials, it commands a premium price. In some instances, the benefits are well worth the additional effort and expense; in others they are not. Before the cost/benefit trade-offs in specific applications are discussed, the economic considerations regarding the use of high-strength concrete generally will be examined. In many areas and for many uses, the benefits of highstrength concrete more than compensate for the increased costs of raw materials and quality control. Basically, high-strength concrete will carry a compression load at less cost than any lower-strength concrete.7.1 Chicago-based structural engineers William Schmidt and Edward S. Hoffman compiled charts indicating the cost of supporting 100,000 lb (445 kN) of service load comes to $5.02 per story with 6000 psi (41 MPa) concrete, $4.21 with 7500 psi (52 MPa), and drops to $3.65 with 9000 psi (62 MPa) concrete (which the authors report they had no difficulty obtaining in the Chicago area).7.2 While the figures reflect 1975 costs, the ratio should remain similar. The reason for these economies is that, although the concrete itself is more costly than lowerstrength mixtures, the cost differential is offset by significant reduction in the given member size. This capability is particularly attractive for use in columns. Since column size is so important for architectural and rental reasons, the ability to limit the sixes for taller structures often allows the use of a concrete solution in lieu of one of structural steel. In 1976, Architectural Record noted that ". . . a 30 x 30-in. column of 6,000 psi concrete might require an

30

Compressive Strength , MPa 60 80 I I

Cost of 40 x 40 in. Column per Foot of 20 Length per 1000 kips of Design Load (1.4 D’ +l.7L), $ 10 _

0, 6

1 8

I IO

I 12

14

Compressive Strength , ksi

Fig. 7.1-- Cost of columns 7.3-Case histories Two examples might help translate this savings into actual dollars. 7.3.1 Case history No. 1-In 1968, Philadelphia’s first high-rise office building using 6000 psi (41 MPa) concrete was designed. To meet the span requirements [approximately 30 ft (9m) square bays] while avoiding unacceptable oversized columns on the lower floors, the columns of the first three floors were built of structural steel. However, a comparison study made by the designing engineers for 8000 psi (55 MPa) concrete showed that 7.3.1.1- With the same column sixes as the original unacceptable sixes, a 60 percent reduction in reinforcing steel with the 8000 psi (55 MPa) concrete would have been made. This would also have resulted in 24 fewer splices per column, a side benefit in labor and time cost savings. 7.3.12- With the same amount of reinforcing used

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as in the original column, the column size could have been reduced from 36 x 46 in. (915 x 1170 mm) to 30 x 30 in. (760 x 760 mm). This size would have been acceptable to the architect and owner and would have eliminated the need for an additional trade-structural steel-on the job. Rough calculations show that 8000 psi (55 MPa) concrete for the lower-floor columns with a stepped strength reduction, as the building reached the upper floors, to 3000 psi (21 MPa) concrete at the top would have resulted in a column size that met the demands of the architect, owner, and rental agent. This would have saved close to $530,000 in 1968 dollars. 7.3.2 Case history No. 2-- The economies of highstrength concrete were more dramatically demonstrated in the construction of New York City’s first building using 8000 psi (55 MPa) concrete, The Palace Hotel built in 1979. The building was originally conceived using structural steel for the lower floors, designated for ballroom and restaurant functions, with a reinforced concrete superstructure for the hotel facilities. However, the engineers were able to convert the entire design, except for two columns on the lowest four levels, to reinforced concrete through the use of 8000 psi (55 MPa) concrete. These ballroom and restaurant areas required large column spacing. The common limitations of 6000 psi (41 MPa) concrete would have made the columns prohibitively large and uneconomical. A presentation to the New York City Building Department about the values of high-strength concrete, together with the proposed controls to insure quality, resulted in its acceptance for use in New York. Concrete with compressive strengths of 8000 and 7000 psi (55 and 48 MPa) was used in the columns of the building only. Lightweight concrete with compressive strength of 3500 psi (24 MPa) was used for floor slabs, and 5000 to 6000 psi (34 to 41 MPa) concrete was used in wall construction. On the lower five levels of the hotel, column sixes were reduced by approximately 25 percent. Approximately 10 percent less reinforcing steel was used because of the strength of the concrete. In addition, No. 11 reinforcing bars remained a viable size, avoiding the need for mechanical connections between the reinforcing bars, thus considerably reducing the floor-framing time requirements. Further economies were realized by minimixing changes in column sixes and reducing column reinforcement on the upper floors. The ability to reduce the amount of costly reinforcing steel without sacrificing strength is an attractive benefit to owners, builders, and engineers, but the use of highstrength concrete in building columns has a corollary economic benefit. It enables the lower floors of high-rise buildings to maintain an acceptable column size, while at the same time increasing the number of possible stories7.1 This is a case of a relatively new material meeting the needs of market economics. The Chicago Committee study7. 5 noted “The potential number of stories in

high-rise buildings is limited by the required large columns if they were to be built with ordinary lowstrength concrete. Real estate properties in prime locations had to be developed with maximum rental floor area. Architectural layout of apartment or condominium units demanded flexibility, which is restricted by large columns. High-strength concrete satisfied this condition by allowing column sixes to be reduced to a minimum.” 7.4-Other studies In Ontario, Canada, the Richmond-Adelaide Center’s use of high-strength concrete columns enabled the architect to increase the use of the underground parking garage by approximately 30 percent.7.6 In times when all building construction is difficult to capitalize, a material that both reduces construction costs and substantially increases the amount of revenue-gathering space within a building can be a tremendous factor in the decision to build. 7.5-Selection of materials The economic consequences of requiring fly ash may vary. On The Royal Bank Plaza Project in Ontario, Canada, a 43-story building constructed from 1973-1976 (one of the first to use fly ash in high-strength concrete), all of the various strength concrete mixtures on the project were converted to local fly ash. This resulted in a saving of approximately $100,000 over the contract and produced concretes with extremely good fresh and hardened properties. 7.6 The Scotia Plaza-A 68-story building in Toronto, Canada, constructed in 1988--is one of the first buildings to employ the use of silica fume in concrete as an element in increasing strength. Strengths of up to 10,000 psi have been achieved. Two Union Square in Seattle employs 19,000-psi concrete containing silica fume-the highest strength used to date in a conventional building. 7.6-Quality control While selection of materials will influence costs, another factor, and one more exclusively the result of the use of high-strength concrete, is the cost of the increased testing, quality control, and inspection that the use of high-strength concrete requires. The quality and consistency of the concrete is crucial, and additional steps must be taken to insure that quality and consistency. In the Royal Bank Plaza Project, a number of precautions were necessary. The supplier had to have a quality control person at the site to control both the scheduling of trucks and the consistency of the concrete at the time it was delivered. For this central plant project, the supplier agreed that there would be no water added to the trucks after they had come onto the site and that any minor adjustments would be made prior to sending the truck to the site. Regular visits were made to the batch plant to check batching procedures and to obtain the test samples. Furthermore, a full-time technician was employed to carry out sampling and testing on site. This was


found to be an essential feature of quality control. On a later project, Richmond-Adelaide Center, Phase II, Ontario, Canada, 1977-1979, by the same engineers, not only did the supplier maintain full-time inspection on the site to insure that the delivered material met requirements but the engineers employed by the owner also maintained full-time inspection and regularly inspected the batch plant. Often this type of stringent quality control is required by regulation. For The Palace Hotel, the New York City Building Department stipulated that at least two suppliers of concrete prequalify the concrete mixtures to strengths up to 8000 psi (55 MPa). The prequalification was to be performed by an independent testing laboratory, and a full-time professional engineer would be required to continuously inspect the progress of the work, performing no other work during the construction.7.7 For hot weather concreting, the engineers required mixing water limited to no more than 50 F (10 C) and the truck drums to be hosed down if standing in direct sunlight. Further, all trucks were limited to 10 yd 3 (7.6 m 3) loads, despite capacities of 16 yd3(12.2 m3 ). While the professional inspection does add to cost, the continuing education of the suppliers and concrete subcontractors in the areas of quality control should ultimately create better concretes of all strengths and result in better and more economical use of materials.

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premium material such as high-strength concrete for slabs or beams. Long-span bridges are another area where the qualities of high-strength concrete are proving themselves economically attractive. High-strength concrete’s comparatively greater compressive strength per unit weight and unit volume allows lighter, more slender bridge piers. This provides improved horizontal clearances. In addition, the increased stiffness of high-strength concrete is advantageous when deflections or stability govern the bridge design. Increased tensile strength of high-strength concrete is helpful in service load design in prestressed concrete. 7.9 In bidding to build a cable-stayed bridge across the Ohio River, a concrete deck proposal beat steel by 29 percent-roughly $10 million. The two-lane crossing between Huntington, West Virginia, and Proctorville, Ohio, includes the first major asymmetrical stayed-girder structure in the United States. The bridge has a main span of 900 feet over one pier. The three bids to construct the bridge using concrete ranged from $23.5 million to $29.7 million, all well below the lowest steel bid ($33.3 million). The designer, Arvid Grant Associates, specified box girders only 5 ft (1.5 m) deep cast of 8000 psi (55 MPa) high-strength concrete.7.8 7.8-Conclusion

7.7-Areas of application In general, the economic advantages of high-strength concrete are most readily realized when the concrete is used in the columns of high-rise buildings. In this application, engineers may take full advantage of its increased compressive strength: reducing the amount of steel, reducing column size to increase usable floor space, or allowing additional stories without detracting from lower floors. These benefits overshadow the increased quality control costs and possible higher cost of raw materials discussed earlier. Yet the use of high-strength concrete has also spread to other applications, primarily slabs, beams, and long-span bridges. The economic considerations of these uses should also be examined. Parking garages, bridge decks, and other installations requiring improved density, lower permeability, and increased resistance to freeze-thaw and corrosion have become prime candidates for consideration of the use of high-strength materials. The primary advantage of high-strength concrete in slabs is the resulting reduction in dead load.7.8 However, as Schmidt and Hoffman point out, significant economies can be achieved only by reducing the thickness that is required for stiffness; the additional reinforcement required may offset the concrete savings. Used for rectangular beams, T-beams, and one-way slabs, highstrength concrete yields reduced section width or thickness and allows for longer spans, but (as with slabs) less expensive lightweight concrete continues to perform this job satisfactorily. Presently, there is no economic justification, under normal circumstances, for the use of a

The economic benefits of high-strength concrete are just now becoming fully apparent. Certainly as the use of high-strength concrete increases, additional and possibly even greater benefits will be realized. In any case, those projects that have led the way in the use of high-strength concrete have clearly demonstrated its economic advantages. For now, it allows the profession to engineer most cost effectively and space effectively. In the future, those considerations may tip the balance on whether certain projects are constructed at all. 7.9-Cited references

(See also Chapter 10--References) 7.1. “High Strength Concrete--Costs More in the Truck, Costs Less in the Structure,” PCA Concrete Technology Today, No. 4, Dec. 1980, p. 3. 7.2. Schmidt, William, and Hoffman, Edward S., “9,000-psi Concrete-Why?, Why Not?,” Civil Engineering -ASCE, V. 45, No. 5, May 1975, pp. 52-55. 7.3. “High-Strength Concrete Allows Bigger Loads on Smaller Columns,” Architectural Record, V. 159, No. 7, June 1976, pp. 133-135. 7.4. Private correspondence from J. Moreno of Material Service Corp. to Irwin G. Cantor, May 12, 1983. 7.5. “High-Strength Concrete in Chicago High-Rise Buildings,” Task Force Report No. 5, Chicago Committee on High-Rise Buildings, Feb. 1977, 63 pp. 7.6. Bickley, John A., and Payne, John C., “High Strength Cast-in-Place Concrete in Major Structures in Ontario,” paper presented at the ACI Annual Convention, Milwaukee, Mar. 1979.


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7.7. Private correspondence from J. T. Walsh, Department of Buildings, New York, to Irwin G. Cantor, Aug. 11, 1977. 7.8. “Concrete Beats Steel by 29%,” Engineering News-Record, V. 206, May 14, 1981, p. 16. 7.9. Carpenter, James E., “Applications of High Strength Concrete for Highway Bridges,” Public Roads, V. 44, No. 2, Sept. 1980, pp. 76-83. CHAPTER 8-APPLICATIONS

Some specific applications of high-strength concrete are described in this chapter. Separate sections describe applications in buildings, bridges, and special structures. The applications are not all-inclusive but demonstrate a range of applications of high-strength concrete. Some potential applications for high-strength concrete are also discussed.

8.2-Buildings The largest application of high-strength concrete in buildings has been for columns of high-rise structures. The history of high-strength concrete columns in the Chicago area has been described in Task Force Report No.58.1 of the Chicago Committee on High-Rise Buildings. Since 1972, more than 30 buildings in the Chicago area have been constructed with columns having a design compressive strength of 9000 psi (62 MPa). The development of concrete for use in two buildings in Toronto has been reported by Bickley and Payne. 8.2 Other applications have been reported in New York,8.3 Houston, 8.4,8.5 Minneaplis,8.6 Melbourne, Australia,8.7 Dallas, 8.25 and Seattle.8.26 Information obtained from these and other sources is summarized in Table 8.1. 8.3-Bridges

There have been many applications of high-strength concrete in precast prestressed bridge girders. However, published information on actual structures is limited.

Table 8.1-- Buildings with high-strength concrete

Total Building S.E. Financial center Petrocanada Building Lake Point Tower 1130 S. Michigan Ave. Texas Commerce Tower Helmsley Palace Hotel Trump Tower City Center Project Collins Place Larimer Place Condominium 499 Park Avenue Royal Bank Plaza Richmond-Adelaide Center Midcontinental Plaza Water Tower Place River Plaza Chicago Mercantile Exchange Columbia Center Interfirst Plaza Eugene Terrace 311 S. Wacker Drive 900 N. Michigan Annex Two Union Square 225 W. Wacker Drive Scotia Plaza

Location Miami Calgary Chicago Chicago Houston New York New York Minneapolis Melbourne Denver New York Toronto Toronto Chicago Chicago Chicago Chicago Seattle Dallas Chicago Chicago Chicago Seattle Chicago

Toronto

Year* 1982 1982 1965 1981 1978 1981 1980 1975 1978 1972 1975 1976 1982 1983 1983 1987 1988 1986 1987 1988 1988

* Year in which high-strength concrete was cast.

t ‘Avo experimental columns of 11,000 psi strength were included. $ Two experimental columns of 14,000 psi strength were included. ~9,CN~psiakousedhflowskbroflowerkvek. l * 19,000 psi indirectly specified to achieve a high modulus of elasticity.

53 34 70 75 53 68 52 44 31 27 43 33 50 79 56 40 76 72 44 70 15 62 30 68

Maximum design concrete strength, psi 7000 7250 7500 7500 7500 8000 8000 8000 8000 8000 8500 8800 8800 9000 9000 9Qw g@m 9500 10,000 11,000 12,000 ss 14,000 14,000** 14,000 10,000

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Table 8.2-- Bridges with high-strength concrete

Year

San Diego to Coronado 8.13 Linn Cove Viaduct

Pasco-Kennewick Intercity

Coweman River Bridges 8.14

Huntington to Proctorville Annicis Bridge Nitta Highway Bridge Kaminoshima Highway Bridge Tower Road Fukamitsu Highway Bridge Ootanabe Railway Bridge Akkagawa Railway Bridge

California

I North Carolina Washington Washington W. Va. to Ohio British Columbia Japan

Japan Washington Japan Japan Japan

Maximum span, ft 158 750 140

1967 1981 1969 1979 1978

981

1984

146 900

1986

1968

1970 1987 1974 1973 1976

180

1526 98 282 161 85 79

150

Maximum design concrete strength, psi 6,000 6,000 6,000 L* 6,000 6,000 7,000 8,000 8,000 8,500 8,500 9,000 10,000 11,400 11,400

* Lightweight concrete Metric equivalent: 100 psi = 6.895 MPa

The effect of using high-strength concrete in four different solid-section girders has been described by Carpenter. 8.8 For integral deck bulb tees, span capability for closely spaced girders increased with increase in concrete strength. For wider spaced girders, capability increased when concrete strength was increased up to 8000 psi (55 MPa). Above 8000 psi (55 MPa) compressive strength, span capability did not increase because sufficient prestress forces could not be provided. Similar results were obtained for other cross sections. For post-tensioned box girder bridges, Carpenter reported that high-strength concrete can be used to increase span capability. However, for higher-strength concretes, maximum available prestress force again limited maximum spans. For segmental box girder bridges, he showed that high-strength concrete is feasible in regions where member thickness is controlled by stress. However, where thickness is controlled by other factors, highstrength concrete may not be beneficial. Some actual bridges in which the use of high-strength concrete has been reported are listed in Table 8.2. Perhaps the most significant application in the United States is the Huntington, West Virginia, to Proctorville, Ohio, crossing for which a compressive strength of 8000 psi (55 MPa) was specified.8.9 The bridge consists of an asymmetrical stayed-girder superstructure with a main span of 900 ft (274 m). The use of concrete with compressive strengths up to 11,000 psi (76 MPa) in railway bridges in Japan has also been reported. 8.10,8.11 Nagataki8.11 reports that strengths of 11,400 psi (79 MPa) can be easily obtained in the field in Japan. 8.4-Special applications

In 1948, concrete with a specified compressive strength of 9000 psi (62 MPa) was used for precast panels for a powerhouse at Fort Peck Dam, Montana. High-strength concrete was specified to provide an extremely dense

concrete that would withstand the harsh exposure. Actual compressive strengths of concrete were reported 8.15 to be considerably higher than 9000 psi (62 MPa). The use of 10,000 psi (69 MPa) concrete for prestressed concrete poles produced by spinning has been described by Skrastins8.16 m 1970. High-strength concrete was selected to reduce the size of the poles. Copen 8.17 has indicated that the use of 10,000 psi (69 MPa) concrete in thin arch dams would usually result in greater economy through reduced volume of concrete. High-strength concrete would tend to reduce deflections in a dam and may improve strength of construction joints and permit earlier removal of formwork. Disadvantages of high-strength concrete listed by Copen include development of stress concentrations, particularly in the foundation for the dam; tendency toward more cracking in concrete; increased temperature control problems; and complications involved with openings through the dam and railways over the dam. The application of high-strength concrete in two grandstand roofs has been described by Bobrowski8.18 Lightweight concrete with a density of 118 lb/ft3 (1.89 Mg/m 3) and a minimum cube strength of 7500 psi (52 MPa) at 28 days was used in the roof beams at Doncaster racecourse, England. Roof beams at Leopardstown racecourse, Ireland, had 28 day cube compressive strengths between 7200 and 8850 psi (50 and 61 MPa) and an average density of 115 lb/ft3 (1.84 Mg/m3). Anderson has reported8.19 the use of high-strength concrete in piles for marine foundations in northwestern United States. Measured 28 day compressive strengths ranged between 7900 and 9900 psi (55 to 68 MPa). Highstrength concretes with compressive strengths up to 9400 psi (65 MPa) have also been used for decks of dock structures in the northwestern United States. In 1984, the Glomar Beaufort Sea I8.27 was placed in the arctic, This exploratory drilling structure contains about 12,000 yd3 (9200 m3) of high-strength lightweight

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Table 8.3-- Corrosion resistance data for selected high-strength concrete (data from reference 8.29)

(psi)

Strength (mpa)

5,160 7,360 8,580 9,290 12,120

35.6 50.8 59.2 64.1 83.6

hficrosilicat (wt. %) 0 15 15 8 15

Chloride Permeability* (Coulombs) 3,663 198 98 132 75

Electrical Resistivity (ohm.cm) 8 95 118 74 161

18 month Corrosion Data %orl~ ‘ 4 ( pohm/cm’) (mv) -456 22 -26 4 -53 4 -3 1 +53 4

t By weight of portland cement. * Measured by AASHTO-T-227 rapid chloride permeability tat at 28 days age. $ Corrosion potential measured with respect to a copper sulfate reference electrode. Q F$ is the polarization current; its reciprocal is a measure of the rate of corrosion.

concrete with unit weights of about 112 lb/ft 3 (1794 kg/m3) with 56-day compressive strength of 9000 psi (62 MPa) and about 6500 yd3 (5000 m3) of high-strength normal-weight concrete with unit weights of about 145 lb/ft3 (2323 kg/m3) and 56-day compressive strengths of about 10,000 psi (69 MPa). Field placements of high-strength, low-permeability, and chemical-resistant concretes for industrial manufacturing applications were reported by Wolsiefer.8.20 Special applications have included several modular bank vaults placed at slumps of 9 in. (230 mm) with measured compressive strengths of 12,000 psi (83 MPa) at 45 days. The protection of reinforcing steel from corrosion can be expected to be enhanced when high-strength concrete is used. The resultant low porosity should increase the electrical resistivity and reduce the rate at which oxygen reaches the steel, both of which will reduce corrosion rates. Moreover, the ease with which chloride ions from deicing salts can reach the steel and initiate corrosion is also reduced. Although there are many studies evaluating the corrosion of steel embedded in regular strength concrete, no systematic studies in the influence of concrete strength appear to have been reported. Published data for highstrength concrete can be extracted from studies investigating other factors, particularly the influence of silica fume. The conclusions obtained with regular concretes are also applicable to high-strength concrete: namely, there is an increase in electrical resistivity 8.28,8.29 and a reduction in chloride permeability 8.29,8.30 with increased strength. Data linking these parameters with laboratory corrosion data are given in Table 8.3. The corrosion behavior of a very high-strength mortar has also been reported.8.31 Useful discussions regarding the factors affecting the corrosion of steel in concretes with silica fume are to be found in references.8.31,8.32,8.33 8.5-- Potential applications

Most applications of high-strength concrete have used the strength property of the material. However, highstrength concrete may possess other characteristics that could be used advantageously in concrete structures.

LeMessurier 8.21 proposed the use of high-strength concrete to satisfy the need for a high modulus of elasticity. Similarly, high-strength concrete can be used in slabs to allow early removal of formwork and avoid reshoring.8.22 This takes advantage of both the high modulus of elasticity and lower creep of high-strength concrete. Anderson 8.19 had suggested that the low creep of high-strength concrete should be taken into account when considering prestress losses. Since most of the prestress loss is attributable to creep and shrinkage, prestress losses for high-strength concrete members should be less than for lower-strength concrete members. Rabbat and Russell 8.23 have reported that the maximum span capability of solid-section girders can be increased by 15 percent when the concrete compressive strength is increased from 5000 to 7000 psi (34 to 48 MPa). Finally, Manning 8.24 has suggested that the relationship between high-strength concrete and high-quality concrete may make high-strength concrete attractive not for its strength but for its long-term service performance. More recently, high-strength concrete has been specified for applications in warehouses, foundries, parking garages, bridge deck overlays, dam spillways, and heavy duty industrial floors. In these applications, high-strength concrete is being used to provide a concrete with improved resistance to chemical attack, better abrasion resistance, improved freeze-thaw durability, and reduced permeability. 8.6-Cited references (See also Chapter 10-References) 8.1. “High-Strength Concrete in Chicago High-Rise Buildings,” Task Force Report No. 5, Chicago Committee on High-Rise Buildings, Feb. 1977, 63 pp. 8.2. Bickley, John A., and Payne, John C., “HighStrength Cast-in-Place Concrete in Major Structures in Ontario,” paper presented at the ACI Annual Convention, Milwaukee, Mar. 1979. 8.3. “New York City Gets Its First High-Strength Concrete Tower,” Engineering News-Record, V. 202, Nov. 2, 1978, p. 22. 8.4. Pickard, Scott S., “Ruptured Composite Tube


Design for Houston’s Texas Commerce Tower,” Concrete International: Design & Construction, V. 3, No. 7, July 1981, pp. 13-19. 8.5. Cook, James E., “Research and Application of High-Strength Concrete Using Class C Fly Ash,” Concrete International Design & Construction, V. 4, No. 7, July 1982, pp. 72-80. 8.6. Venema, T.P., and Regnier, H.J., “Placement, Batching, and Tests of High Strength Concrete for Minneapolis City Center Project,” submitted to ACI for publication. 8.7. Day, K.W., “Quality Control of 55 MPa Concrete for Collins Place Project, Melbourne, Australia,” Concrete International Design & Construction, V. 3, No. 3, Mar. 1981, pp. 17-24. 8.8. Carpenter, James E., “Applications of High Strength Concrete for Highway Bridges,” Public Roads, V. 44, No. 2, Sept. 1980, pp. 76-83. 8.9. “Concrete Beats Steel by 29%,” Engineering News-Record, V. 206, May 14, 1981, p. 16. 8.10. “Stronger Concrete,” EngineeringNews-Record, V. 189, June 8, 1982, p. 12. 8.11. Nagataki, Shigeyoshi, “On the Use of Superplasticizers,” Seminar on Special Concretes, 8th FIP Congress (London, 1978), Federation Intenationale de la Precontrainte, Wexham Springs, 1978, 15 pp. 8.12. “Concrete Box Girder Span Establishes U.S. Record,” Engineering News-Record, V. 208, No. 1, Jan. 7, 1982, pp. 22-25. 8.13. Pfeifer, Donald W., “Development of the Concrete Technology for a Precast Prestressed Concrete Segmental Bridge,” Journal, Prestressed Concrete Institute, V. 27, No. 5, Sept.-Oct. 1982, pp. 78-99. 8.14. Hurlbut, Roger, “146-ft Long Precast Prestressed Bridge Girders in Washington State, “Journal, Prestressed Concrete Institute, V. 24, No. 1, Jan.-Feb. 1979, pp. 8892. 8.15. “Unusual Strengths Attained in Precast Slabs Used for Facing Power House Walls,” Concrete, V. 57, No. 5, May 10, 1949, pp. 9-10. 8.16. Skrastins, Janis I., ‘Toward High-Strength Concrete,” Modern Concrete, V. 34, No. 1, May 1970, pp. 4448. 8.17. Copen, Merlin D., “Problems Attending Use of Higher Strength Concrete in Thin Arch Dams,” ACI JOURNAL, Proceedings V. 72, No. 4, Apr. 1975, pp. 138 140. 8.18. Bobrowski, J., and Bardham-Roy, B.K., “Structural Assessment of Lightweight Aggregate Concrete,” Concrete (London), V. 5, No. 7, July 1971, pp. 229-234. 8.19. Anderson, Arthur R., “Research Answers Needed for Greater Utilization of High Strength Concrete,” Journal, Prestressed Concrete Institute, V. 25, No. 4, July-Aug. 1960, pp. 162-164. 8.20. Wolsiefer, John, “Ultra High-Strength Field Placeable Concrete with Silica Fume Admixtures,” Con-

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crete International Design & Construction, V. 6, No. 4, Apr. 1984, pp. 25-31. 8.21. Fischer, R.E., “Round Table--Concrete in Architecture: A Current Assessment,” Architectural Record, Nov. 1982. 8.22. Nilson, AH., “Structural Design Considerations for High Strength Concrete,” Proceedings, National Science Foundation Workshop on High Strength Concrete, University of Illinois at Chicago Circle, Dec. 1979. 8.23. Rabbat, Basile G., and Russell, Henry G., “Optimized Sections for Precast, Prestressed Bridge Girders,” Journal, Prestressed Concrete Institute, V. 27, No. 4, July-Aug. 1982, pp. 88-104. 8.24. Young, F.J., and Russell, H.G., “Session V-Summary of Floor Discussion,” Proceedings, National Science Foundation Workshop on High Strength Concrete, University of Illinois at Chicago Circle, Dec. 1979. 8.25. “Tower Touches Few Bases,” Engineering NewsRecord, V. 210, No. 24, June 16, 1983, pp. 24-25. 8.26 Godfey, K.A., Jr., “Concrete Strength Record Jumps 36%,” Civil Engineering, V. 57, No. 10, Oct. 1987, pp. 84-88. 8.27. Fiorato, A.E., Person, A, and Pfeifer, D.W., “The First Large-Scale Use of High Strength Lightweight Concrete in the Arctic Environment,” Second Symposium on Artic Offshore Drilling Platforms, Houston, Texas, Apr. 1984. 8.28. Vennesland. O., and Gjorv, O.E., “Silica Concrete-- Protection Against Corrosion of Embedded Steel,” Fly Ash, Silica Fume and Other Mineral By-Products in Concrete, ACI SP-79, V. 2, 1983, pp. 719-730. 8.29. Burke, N.S., and Weil, T.G., “Corrosion Protection Through the Use of Concrete Admixtures,” Supplementary Paper, Proceedings, 2nd International Conference on Performance of Concrete in the Marine Environment, St. Andrews-by-the-Sea, New Brunswick, Aug. 1988. 8.30. Preece, C.M., Frolund, T., and Bager, D.H., “Chloride Ion Diffusion in Low Porosity Silica Cement Paste,” Condensed Silica Fume in Concrete, Report BML 82.610, Norwegian Institute of Technology, Trondheim, 1982, pp. 51-58. 8.31. Preece, C.M., Frolund, T., and Bager, D.H., “Electrochemical Behavior of Steel in Dense SilicaCement Mortar,” Fly Ash, Silica Fume and Other Mineral By-Products in Concrete, ACI SP-79, V. 2, 1983, pp. 785796. 8.32. Fidjestol, P., “Reinforcement Corrosion and the Use of CSF-Based Additives,” Concrete Durability, ACI SP-100, V. 2, 1987, pp. 1445-1458. 8.33. Scali, M.J., Chin, D., and Burke, N.S., “Effect of Microsilica and Fly Ash Upon the Microstructure and Permeability of Concrete,” Proceedings, Ninth International Conference on Cement Microscopy, International Cement Microscopy Association, Texas, 1987, pp. 375397.


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CHAPTER 9-- SUMMARY

The objective of this report was to present state-ofthe-art information on concrete with strengths in excess of about 6000 psi (41 MPa) but not including concrete made using exotic materials or techniques. This section of the report presents a summary of the material contained in the previous chapters. All materials for use in high-strength concrete must be carefully selected using all available techniques to insure uniform success. Items to be considered in selecting materials include cement characteristics, aggregate size, aggregate strength, particle shape and texture, and the effects of set-controlling admixtures, water reducers, silica fume, and pozzolans. Trial mixtures are essential to insure that required concrete strengths will be obtained and that all constituent materials are compatible. Mix proportions for high-strength concrete generally have been based on achieving a required compressive strength at a specified age. Depending on the appropriate application, a specified age other than 28 days has been used. Factors included in selecting concrete mix proportions have included availability of materials, desired workability, and effects of temperature rise. All materials must be optimized in concrete mix proportioning to achieve maximum strength. High-strength concrete mixes have usually used high cement contents, low watercement ratios, normal weight aggregate, and chemical and pozzolanic admixtures. Required strength, specified age, material characteristics, and type of application have strongly influenced mix design. High-strength concrete mix proportioning has been found to be a more critical process than the proportioning of lower-strength concrete mixes. Laboratory trial batches have been required in order to generate necessary data on mix design. In many cases, laboratory mixes have been followed by field production trial batches. Batching, mixing, transporting, placing, and control procedures for high-strength concrete are not essentially different from procedures used for lower-strength concretes. However, special attention is required to insure a high-strength uniform material. Special consideration should be given to minimizing the length of time between concrete batching and final placement in the forms. Delay in concrete placement can result in a subsequent loss of long-term strength or difficulties in concrete placement. Special attention should also be paid to the testing of high-strength concrete cylinders since any deficiency will result in an apparent lower strength than that actually achieved by the concrete. Items deserving specific attention include manufacture, curing, and capping of control specimens for compressive strength measurements; characteristics of testing machines; type of mold used to produce specimens; and age of testing. In many instances, strength measurements at early ages have been made even though the compressive strength has not been specified until 56 or 90 days. Some research data have indicated that the modulus

of elasticity of high-strength concrete is lower than would have been predicted from data on lower-strength concretes. However, values of Poisson’s ratio appear to be in the expected range, based on lower-strength concretes. The modulus of rupture for high-strength concretes is higher than would have been anticipated. However, the tensile splitting strength values appear to be consistent with lower-strength concretes. Unit weight, specific heat, diffusivity, thermal conductivity, and coefficient of thermal expansion have been found to fall generally within the usual range for lower-strength concretes. Highstrength concrete has shown a higher rate of strength gain at early ages as compared to lower-strength concrete, but at later ages the difference is not significant. Information on creep and shrinkage of high-strength concrete has indicated that the shrinkage is similar to that for lower-strength concrete. However, specific creep is much less for high-strength concretes than for lowerstrength concretes. In the area of structural design, it has been found that axially loaded columns with high-strength concrete can be designed in the same way as lower-strength columns. It has also been identified that high-strength concrete columns exhibit less shortening under load than lowerstrength columns because of the higher modulus of elasticity and lower creep coefficients. For beams, use of the conventional equivalent rectangular stress block appears to give satisfactory results for under-reinforced concrete members. The compressive strain limit of 0.003 appears to be acceptable. However, changes have been recommended for present code values for minimum tensile steel ratio, modulus of rupture, modulus of elasticity, shear strength, and development length. Changes are also needed in the area of calculating long-term beam deflections. The economic advantages of using high-strength concrete in the columns of high-rise buildings have been clearly demonstrated by applications in many cities. The ability to reduce the amount of reinforcing steel in columns without sacrificing strength and to keep, the columns to an acceptable size has been an economic benefit to owners of high-rise buildings. Consequently, concrete with compressive strengths in excess of 6000 psi (41 MPa) has been used in the columns of high-rise buildings in cities throughout North America. Studies have also indicated advantages in the use of high-strength concrete in long-span concrete bridges. However, this application has yet to be fully implemented. There have also been applications where high-compressive-strength concrete has been needed to satisfy special local requirements. These have included dams, prestressed concrete poles, grandstand roofs, marine foundations, parking garages, bridge deck overlays, heavy duty industrial floors, and industrial manufacturing applications. Although high-strength concrete is often considered a relatively new material, it is becoming accepted in more parts of North America as shown by the many examples of its usage. At the same time, material producers are


responding to the demands for the material and are learning production techniques. As with many developments of new materials, research data supporting the growth has also increased. However, the need for additional research has been documented in ACI 363.1R. Some research projects are underway to satisfy these needs. However, further work is needed to fully use the advantages of high-strength concrete and to affirm its capabilities. This report has documented existing knowledge of high-strength concrete so that the direction for future development may be ascertained.

CHAPTER 10-REFERENCES 10.1-Recommended references

The documents of the various standards-producing organizations referred to in this document are listed below with their serial designation. American Association of State Highway and Transportation Officials Quality of Water to be Used in Concrete T-26 American Concrete Institute 116R Cement and Concrete Terminology 20l.lR Guide for Making a Condition Survey of Concrete in Service 211.1 Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete 212.2R Guide for Use of Admixtures in Concrete 214 Recommended Practice for Evaluation of Strength Test Results of Concrete 304 Guide for Measuring, Mixing, Transporting, and Placing Concrete 304.4R Placing Concrete with Belt Conveyors 305R Hot Weather Concreting 308 Standard Practice for Curing Concrete Guide for Consolidation of Concrete 309 318 Building Code Requirements for Reinforced Concrete 318R Commentary on Building Code Requirements for Reinforced Concrete American Society for Testing and Materials Standard Method of Making and Curing ConC 31 crete Test Specimens in the Field Standard Specification for Concrete Aggregates C 33 C 39 Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens Standard Specification for Ready-Mixed ConC 94 crete C 109 Standard Test Method for Compressive Strength of Hydraulic Cement, Mortars (using 2 in. or 50 mm cube specimens) C 143 Standard Test Method for Slump of Portland Cement Concrete

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Standard Specification for Portland Cement Standard Method of Making and Curing Concrete Test Specimens in the Laboratory C 260 Standard Specification for Air-Entraining Admixtures for Concrete C 311 Standard Methods of Sampling and Testing Fly Ash or Natural Pozzolans for Use as a Mineral Admixture in Portland Cement Concrete C 494 Standard Specification for Chemical Admixtures for Concrete C 595 Standard Specification for Blended Hydraulic Cements C 618 Standard Specification for Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete C 684 Standard Method of Making, Accelerated Curing, and Testing of Concrete Compression Test Specimens C 917 Standard Method for Evaluation of Cement Strength Uniformity from a Single Source C 989 Standard Specification for Ground Iron BlastFurnace Slag for use in Cement and Mortars E 329 Standard Recommended Practice for Inspection and Testing Agencies for Concrete, Steel, and Bituminous Materials as Used in Construction Canadian Standards Association A 266.5-M1981 Guidelines for the Use of Superplasticizing Admixtures in Concrete Concrete Plant Manufacturers Bureau Concrete Plant Manufacturers Standards of the Plant Mixer Manufacturers Division The above publications may be obtained from the following organizations: American Association of State Highway and Transportation Officials 333 N Capitol St. N.W. Suite 225 Washington, D.C. 20001 American Concrete Institute P.O. Box 19150 Detroit, MI 48219 American Society for Testing and Materials 1916 Race Street Philadelphia, PA 19103 Canadian Standards Association 178 Rexdale Blvd. Rexdale, Ont. Canada M9W 1R3

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Concrete Plant Manufacturers Bureau 900 spring St. Silver Spring, Md. 20910 10.2-- Cited references Cited references are provided at the end of each chapter. 10.3-- Bibliography

The purpose of this bibliography is to call attention to literature on high-strength concrete in addition to that listed at the ends of chapters in this report. The entries are organized alphabetically by author. Anonymous references are listed alphabetically according to their titles. 10.1. Abeles Paul W., “Experience with High-Strength Concrete in Combination with High-Strength Steel in Precast Reinforced and Prestressed Concrete,” Materials and Structures, Research and Testing (RILEM Paris) V. 6, No. 36, Nov.-Dec. 1973, pp. 464-472. 10.2. Ahmad, S.H., and Shah, S.P., “Complete StressStrain Curve of Concrete and Nonlinear Design,” Progress Report, National Science Foundation Grant PFR 78 22878, University of Illinois at Chicago Circle, Aug. 1979, 29 pp. Also, Nonlinear Design of Concrete Structures, University of Waterloo Press, 1980, pp. 61-81. 10.3. Aitcin, Pierre-Claude, “How to Produce High Strength Concrete,” Concrete Construction, V. 25, No. 3, Mar. 1980, pp. 222-230. 10.4. Albinger, John, and Moreno, Jaime, “HighStrength Concrete, Chicago Style,” Concrete Construction, V. 26, No. 3, Mar. 1981, pp. 241-245. 10.5. Alexander, K.M.; Bruere, G.M.; and Ivanusec, I., “The Creep and Related Properties of Very HighStrength Superplasticized Concrete,” Cement and Concrete Research, V. 10, No. 2, Mar. 1980, pp. 131-137. 10.6. Anderson, Arthur R., “Some Examples of Energy and Resource Conservation Utilizing High-Strength Concrete,” presented at the ACI Annual Convention, Milwaukee, Mar. 1979. 10.7. Bache, H.H., “Compression Failure in Brittle Materials. Fracture Hardening (Trykbrud I Skore Materialer),” Nordisk Betong (Stockholm), No. 1, 1977, pp. 7-10. (in Swedish) 10.8. Bazant, Z.P., “High Strength Concrete: Discussion on Material Behavior Under Various Types of Loading,” Proceedings, National Science Foundation Workshop on High Strength Concrete, University of Illinois at Chicago Circle, 1979, Report D-l, 13 pp. 10.9 Bennett, E.W., “Fatigue in Concrete,” Concrete (London), May 1974, pp. 43-45. 10.10. Berntsson, L.; Hedberg, B.; and Malinowski, R., “Triaxial Deformations by Uniaxial Load on Heat-Cured and High-Strength Concrete (Triaxiala Deformationer Till Foljd av Enaxlig Tryckbelastning pa Varmerhardad, hoghallfast betong),” Cement-och-Betonginstitutet, V. 45, No. 2, May 1970, pp. 205-224. (in Swedish) 10.11. Berry, E.E., and Malhotra, V.M., “Fly Ash for Use in Concrete-A Critical Review,” ACI J OURNAL ,

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Proceedings V. 77, No. 2, Mar.-Apr. 1980, pp. 59-73. 10.12. Bertero, Vitelmo V., “Inelastic Behavior of Structural Elements and Structures,” Proceedings, National Science Foundation Workshop on High Strength Concrete, University of Illinois at Chicago Circle, 1979, Report E-l, 70 pp. 10.13. Bickley, J.A., “Concrete Optimization,” Concrete International: Design & Construction, V. 4, No. 6, June 1982, pp. 38-41. 10.14. Billington, C.J., “Underwater Repair of Concrete Offshore Structures,” Proceedings, 11th Annual Offshore Technology Conference (Houston, 1979), Offshore Technology Conference, Dallas, 1979, V. 2, pp. 927-937. 10.15. Bloss, D.R.; Hubbard, S.J.; and Gray, B.H., “Development and Evaluation of a High-Strength Polyester Synthetic Concrete,” Technical Report No. M-2, U.S. Army Construction Engineering Research Laboratory, Champaign, Mar. 1970, 70 pp. 10.16. Bremer, F., “Prestressed Concrete Pressure Vessels for Nuclear Reactors,” Technical Session on Design and Construction of Nuclear Power Plants, 7th FIP Congress (New York, 1974), Federation Internationale de la Precontrainte, Wexham Springs, 1975, pp. 34-40. 10.17. Bromham, S.B., “Superplasticizing Admixtures in High Strength Concrete,” Symposium on Concrete in Engineering: Engineering for Concrete (Brisbane, Aug. 1977), National Conference Publication No. 77/8, Institution of Engineers, Australia, Brisbane, 1977, pp. 17-22. 10.18. Brooks, J.J., and Neville, AM., “Predicting Long-Term Creep and Shrinkage from Short-Term Tests,” Magazine of Concrete Research (London), V. 30, No. 103, June 1978, pp. 51-61. 10.19. Brown, Colin B., “A Discussion on the Micromechanics of Achieving High Strength and Other Superior Properties,” National Science Foundation Workshop on High Strength Concrete, University of Illinois at Chicago Circle, Dec. 1979, Report No. B-l, 5 pp. 10.20. Carrasquillo, R.L.; Nilson, A.H.; and Slate, F.O., “High-Strength Concrete: An Annotated Bibliography 1930-1979,” Cement, Concrete, and Aggregates, V. 2, No. 1, Summer 1980, pp. 3-19. 10.21. Carrasquillo, R.L.; Nilson, A.H.; and Slate, F.O., “The Prediction of High-Strength Concrete,” Report No. 78-1, Department of Structural Engineering, Cornell University, Ithaca, May 1978, 91 pp. Also, MSc thesis, Cornell University, Ithaca, May 1978, 90 pp. 10.22. Carrasquillo, R.L.; Nilson, A.H.; and Slate, F.O., “Very High-Strength Concrete-An Annotated Bibliography 1930-1976,” Report No. 367, Department of Structural Engineering, Cornell University, Ithaca, Apr. 1977, 46 pp. 10.23. Carrasquillo, Ramon L., and Slate, Floyd O., “Micro-cracking and Definition of Failure of High- and Normal-Strength Concretes,” Cement, Concrete, and Aggregates, V. 5, No. 1, Summer 1983, pp. 54-61. 10.24. Chernobaev, V.I., “Investigation of the Carrying Capacity of High Strength Concrete Flexible Columns (Issledovanie Nesush-ehei Sposobnosti Gibkikh Kolonn


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Iz Vysokoprochnykh Betonov),” Beton i Zhelezobeton “Consistency, Setting, and Strength Gain Characteristics (Moscow), No. 4, Apr. 1975, pp. 9-11. (in Russian) of a ‘Low Porosity’ Portland Cement Paste,” Cement and 10.25. Chung, H.; Hayashi, S; and Kokusho, S., Concrete Research, V. 8, No. 5, Sept. 1978, pp. 613-621. “Experimental Study on the Shear Strength of High 10.39. Dikeou, J.T.; Kukacka, L.E.; Backstrom, J.E.; Strength Concrete Beams,” Transactions, Japan Concrete and Steinberg, M., “Polymerization Makes Tougher ConInstitute, Tokyo, V. 2, 1980, pp. 233-240. crete,” ACI JOURNAL , Proceedings V. 66, No. 10, Oct. 10.26. Chung, H.; Hayashi, S.; and Kokusho, S., 1969, pp. 829-839. “Reinforced High Strength Concrete Columns Subjected 10.40. Erntroy, H.C., and Shacklock, B.W., “Design of to Axial Forces, Bending Moments and Shear Forces,” High Strength Concrete Mixes,” Reprint No. 32, Cement and Concrete Association, London, 1954. Transactions, Japan Concrete Institute, Tokyo, V. 2, 1980, 10.41. “Federal Complex Strikes Low-Key Note,” pp. 335-342. 10.27. Colaco, Joseph P.; Ames, Jay B.; and Dubinsky, Building Design and Construction, V. 22, No. 11, Nov. Eli, “Concrete Shear Walls and Spandrel Beam Moment 1981, pp. 92-94. Frame Brace New York Office Tower,” Concrete Interna10.42. FIP 7th Congress (New York, 1974), Proceedtional Design & Construction, V. 3, No. 6, June 1981, pp. ings, V. 2, Lectures and General Reports, Federation Internationale de la Prewntrainte, Wexham Springs, 23-28. 10.28. Collepardi, Mario, and Corradi, Mario, “Influ- 1975, 137 pp. ence of Naphthalene-Sulfonated Polymer Based Super10.43. Fintel, Mark, “Creep, Shrinkage and Temperaplasticizers on the Strength of Ordinary and Lightweight ture Effects in Tall Buildings,” Concrete Industry Bulletin, Concretes,” Superplasticizers in Concrete, SP-62, American V. 14, No. 3, Mar. 1974, pp. 4-11. Concrete Institute, Detroit, 1979, pp. 315-336. 10.44. French, P.J.; Montgomery, R.G.J.; and Robson, 10.29. Collins, A.R., “The Principles of Making High- T.D., “High Concrete Strength Within the Hour,” ConStrength Concrete,” Civil Engineers Review, V. 4, Maycrete (London), V. 5, No. 8, Aug. 1971, pp. 253-258. June 1954, pp. 172-176, 203-206. Also, Civil Engineering 10.45. Fukuchi, Toshio, and Ohama, Yoshihiko, ‘Proand Public Works Review (London), V. 45, No. 524, Feb. cess Technology and Properties of 2500 kg/cm2--Strength 1950, pp. 110-112, and No. 525, Mar. 1950, pp. 170-171, Polymer-Impregnated Concrete,” Proceedings, 2nd Inter180. national Congress on Polymers in Concrete (Austin, Oct. 10.30. “Concrete Strength Secret: Dry Mix,” En1978), University of Texas at Austin, 1979, pp. 45-56. gineering News-Record, V. 189, June 15, 1972, p. 3. 10.46. Fukuchi, Toshio, et al., “Effect of Course 10.31. Coppetti, G.; Cambini, F.; and Tognon, G., Aggregate on Compressive Strength of Polymer-Impreg“Use of Very High-Strength Concrete for the Manufac- nated Autoclaved Concrete,” Proceedings, 22nd Japan ture of Centrifuged Piles (Impiego Di Calcestruzzi Ad Congress on Materials Research (Kyoto, Sept. 1978), Altissime Resistenze Per la Producione De Pali Centri- Society of Materials Science, Kyoto, 1979, pp. 373-376. fugati),” Industria Italiana del Cemento (Rome), V. 50, 10.47. Funakoshi, M., and Okamoto, T., “The Shear No. 2, Feb. 1980, pp. 121-130. (in Italian) Strength of Prestressed Beams for which Very High 10.32. Craven, M.A., “High-Strength and Lightweight Strength Concrete is Employed,” Transactions, Japan Concrete Institute, Tokyo, V. 2, 1980, pp. 271-278. Concretes for Prestressing,” New Zealand Concrete Construction (Wellington), V. 11, No. 3, Mar. 1967, pp. 40-41. 10.48. Gallagher, J.E., “Acrylic-Latex Additives Create 10.33. Cross, Hardy, “Design of Reinforced Concrete Extra Strength New Concretes,” Architectural Record, Columns Subject to Flexure,” ACI JOURNAL , Proceedings Mar. 1967, pp. 199-200. 10.49. Galwey, AK., et al., “Relatively High Strength V. 26, No. 2, Dec. 1929, pp. 157-169. 10.34. Crow, L.J., and Bates, R.C., “Strengths of Sul- of a Chalk-Aggregate Concrete,” Journal of Applied fur-Basalt Concretes,” Report Investigations No. 7349, U.S. Chemistry, May 1966, pp. 159-162. Bureau of Mines, Washington, D.C., 1970, 21 pp. 10.50. Garas, F.K., “Research and Development in 10.35. Dawson, P., “Design and Construction of a Pre- Support of the Design of a Prestressed Concrete Pressure stressed Concrete Pressure Vessel for a Working Pres- Vessel for a Working Pressure of 69 N/mm2 (10,000 psi),” International Journal of Pressure Vessels Piping, V. 8, No. sure of 69 N/mm2 (10,000 psi),” Transactions, 4th International Conference on Structural Mechanics in Reactor 3, May-June 1980, pp. 233-244. Technology (San Francisco, Aug. 1977) Committee of the 10.51. Gaynor, R.D., “Producing High Strength AirEuropean Communities, Luxemburg, 1977, Volume H, Entrained Concrete,” unpublished discussion paper. For detailed information of the test data, see Gaynor, Paper H l/5, 13 pp. 10.36. Desov, A.E., “Basic Principles of High Strength Richard D., “High Strength Air-Entrained Concrete,” Joint Research Laboratory Publication No. 17, National Concrete,” Transportation Research Record No. 504, Sand and Gravel Association/National Ready Mixed ConTransportation Research Board, 1974, pp. 37-42. 10.37. “Development of Prestressed Concrete High crete Association, Silver Spring, Mar. 1968, 19 pp. 10.52. Ghosh, R.S., and Malhotra, V.M., “Use of Strength Concrete,” Concrete and Constructional EnSuper-plasticizers as Water Reducers,” Cement, Concrete, gineering (London), V. 57, No. 7, July 1962, p. 268. 10.38. Diamond, Sidney, and Gomez-Toledo, Carlos, and Aggregates, V. 1, No. 2, 1979, pp. 56-63.

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10.53. Givens, J.J., Jr., and Carter, G., “Rehabilitation of Offshore Platforms,” Civil Engineering-- ASCE, V. 40, No. 4, Apr. 1970, pp. 47-49. 10.54. Golikov, A.E., “The Effect of High-Strength Concrete Moulding Technology on the Physico-Mechanical Properties,” Beton i Zhelezobeton (Moscow), No. 9, 1967, pp. 34-35. (in Russian) 10.55. Green, Arthur N., “Low Dosage Super Water Reducer,” presented at the International Symposium on Superplasticizers in Concrete, Ottawa, May 1978. 10.56. Gupchup, V.N.; Jayaram, S.; and Kulkarni, J.A., “Effect of Admixtures on Properties of High-Strength Concrete Mixes,” Indian Concrete Journal (Bombay), V. 53, No. 12, Dec. 1979, pp. 331-335. 10.57. Guy, I.N., Editor, Advances in Concrete (Symposium Proceedings, University of Birmingham, Sept. 1971), The Concrete Society, London, 1972. 10.58. Hanson, J.A, “Shear Strength of Lightweight Reinforced Concrete Beams,” ACI JOURNAL , Proceedings V. 55, No. 3, Sept. 1958, pp. 387-403. 10.59. Harris, Alan, “Optimization of Concrete Hulls,” Proceedings, Conference on Concrete Ships and Floating Structures (Berkeley, Sept. 1975), University of California, Berkeley, 1976, pp. 270-273. 10.60. Hattori, Kenichi, ‘Experiences with Mighty Superplasticizer in Japan,” Superplasticizers in Concrete, SP-62, American Concrete Institute, Detroit, 1979, pp. 37-66. 10.61. Hattori, K., “Properties of Admixtures for High Strength Concrete and Their Water Reducing Meehanism,” Concrete Journal (Tokyo), V. 14, No. 3, 1976, pp. 12-19. (in Japanese) 10.62. Hester, Weston T., “High Strength, Superplasticized Concrete: The Significance of Mix Water-Cement Ratio, Mortar-Aggregate Bond and Cement Efficiency, presented at the ACI Annual Convention, Atlanta, Jan. 1982. 10.63. “High-Strength Concrete,” Building (London), V. 211, No. 6436, 1966, pp. 129-130. 10.64. “High-Strength Concrete-Crushed Stone Aggregate Makes the Difference,” National Crushed Stone Association, Washington, D.C., Nov. 1974, 31 pp. 10.65. Hognestad, E., and Perenchio, W.F., “Developments in High-Strength Concrete,” Proceedings, 7th FIP Congress (New York, 1974), Federation Internationale de la Prewntrainte, Wexham Springs, 1975, V. 2, Lectures and General Reports, pp. 21-24. 10.66. Hollister, S.C., “Urgent Need for Research in High-Strength Concrete,” ACI JOURNAL, Proceedings V. 73, No. 3, Mar. 1976, pp. 136-137. 10.67. “How Super are Superplasticizers?,” Concrete Construction, V. 27, No. 5, May 1982, pp. 409-415. 10.68. Hughes, B.P., “Temperature Rises in Low-Heat Cement Concrete,” Proceedings, ASCE, V. 97, ST12, Dec. 1971, pp. 2807-2823. 10.69. Ikenaga, Hirotake, and Oshima, Hisaji, “Study on the Relation between an Age of Concrete and Shrinkage, Creep, and Strength--Study of Mixing Design of

Site Concrete Preventing from Cracking Due to Shrinkage and Creep,” Transactions, Architectural Institute of Japan (Tokyo), V. 215, Jan. 1974, pp. 13-30. (in Japanese with English abstract) 10.70. “Innovation in Concrete,” Progressive Architecture, V. 59, No. 5, May 1978, pp. 100-109. 10.71. “In Sub-Freezing Weather: Bridge Deck Repaired with Quick-Set Gunned Concrete,” Better Roads, V. 45, No. 5, May 1975, p. 44. 10.72. James, Robert M., “High-Strength Concrete Does Have Its Problems,” ACI JOURNAL , Proceedings, V. 75, No. 2, Feb. 1978, p. N8. 10.73. Johnston, Colin D., “Fifty-Year Developments in High Strength Concrete,” Proceedings, ASCE, V. 101, C04, Dec. 1975, pp. 801-818. 10.74. Kageyama, H.; Nakagawa, K.; and Nagafuchi, T., “High-Strength Concrete Made With Special Cement Admixture,” Zairyo, V. 29, No. 318, Mar. 1980, pp. 220225. Also, abstract in Chemical Abstracts, V. 93, No. 3, Aug. 11, 1980, p. 371. 10.75. Kar, Anil K., “Underwater Structures,” Bulletin, International Association for Shell and Spatial Structures (Madrid), No. 50, Dec. 1972, pp. 49-56. 10.76. Karlsson, Inge, “High-Strength Concrete (Hoghallfast Betong),” Nordisk Betong (Stockholm), No. 4, 1977, pp. 19-22. (in Swedish) 10.77. Kemi, Toroa, et al., ‘Experiment of Grouting by Special Super High Early Strength Cement Paste,” Review of the 31st General Meeting-Technical Session, Cement Association of Japan, Tokyo, May 1977, pp. 218-221. 10.78. Kennedy, Henry, “High Strength Concrete,” Proceedings, 1st United States Conference on Prestressed Concrete, Massachusetts Institute of Technology, Cambridge, 1951, pp. 126-135. 10.79. Klieger, Paul, “High Strength Concrete,” presented at the 3rd Symposium on Modern Concrete Technology, Caracas, Nov. 1976, 29 pp. 10.80. Kobayashi, Masaki, and Tanaka, Hiroshi, “On Frost Resistance of High-Strength Concrete,” Review of the 28th General Meeting- Technical Session, Cement Association of Japan, Tokyo, 1974, pp. 173-174. 10.81. Krishna, Raju N., “Compressibility and Modulus of Rupture of High-Strength Concrete,” Journal of the Institute of Engineering (India), Civil Engineering Division, V. 52, No. 3, Part C12, Nov. 1971, pp. 98-101. 10.82. Law, Sheldon M., and Rasoulian, Masood, “Design and Evaluation of High Strength Concrete for Girders,” Report No. FHWA/LA-80/138, Louisiana Department of Transportation, Baton Rouge, 1980, 50 pp. Also, PB81-151 623, National Technical Information Service. 10.83. Lawrence, C.D., “The Properties of Cement Paste Compacted Under High Pressure,” Research Report No. 19, Cement and Concrete Association, Wexham Springs, June 1969, pp. 1-20. 10.84. Lobanov, A.T., et al., “Practice of Prefabrication of High-Strength Concrete Columns for Buildings (Opyt Izgotovleniya Kolonn Iz Vysokoprochnyky Betonov Dlya Zhilykh Domov),” Beton i Zhelezobeton (MOSCOW ), No.


12, Dec. 1976, pp. 14-15. (in Russian) 10.85. Machida, F.; Nakahara, S.; Hirose, T.; Kumonda, T.; Miyasaka, T.; and Ishikawa, H., “Design and Execution of Prestressed Concrete Girder Using High Strength Concrete,” Journal, Japan Prestressed Concrete Engineering Association (Tokyo), V. 16, No. 4, 1974, pp. 30-36, and No. 5, 1974, pp. 36-45. (in Japanese) 10.86. MacInnis, Cameron, and Kosteniuk, Paul W., “Effectiveness of Revibration and High-Speed Slurry Mixing for Producing High-Strength Concrete,” ACI J OURNAL , Proceedings V. 76, No. 12, Dec. 1979, pp. 1255-1265. 10.87. MacInnis, Cameron, and Thomas, Donald V., “Special Techniques for Producing High Strength Concrete,” ACI JOURNAL , Proceedings V. 67, No. 12, Dec. 1970, pp. 996-1002. 10.88. Malhotra, V.M., “Development of Sulphur-lnfiltrated High-Strength Concrete,” ACI JOURNAL, Proceedings V. 72, No. 9, Sept. 1975, pp. 466-473. 10.89. Malhotra, V.M., “Superplasticizers in Concrete,” Modern Concrete, V. 41, No. 12, Apr. 1978, pp. 38-43. 10.90. Malhotra, V.M.; Painter, K.E.; and Soles, J.A., “Development of High-Strength Concrete at Early States Using a Sulphur Infiltration Technique,” Mines Branch Internal Report No. MPI (A) 74-4, CANMET, Department of Energy, Mines and Resources, Ottawa, July 1974, 13 pp. 10.91. Mather, Bryant, “High-Compressive-Strength Concrete, A Review of the State of the Art,” Technical Documentary Report No. AFSWC-TDR-62-56, Air Force Special Weapons Center, Kirtland Air Force Base, Aug. 1962, 90 pp. 10.92. Mather, Bryant, “High Strength Concrete,” Seminar on Control of Quality of Concrete and Construction Practice, ACI Canadian Capital Chapter, Ottawa, 1968, 56 pp. 10.93. Mather, Bryant, “Tests of High-Range WaterReducing Admixtures,” Superplasticizers in Concrete, SP-62, American Concrete Institute, Detroit, 1979, pp. 157-166. 10.94. Mather, Katharine, “High Strength, High Density Concrete,” ACI JOURNAL, Proceedings V. 62, No. 8, Aug. 1965, pp. 951-962. 10.95. Matsumoto, Y., et al., “Precast Prestressed Concrete Truss Railway Bridge Using Extremely High Strength Concrete,” Final Report, 10th IABSE Congress (Tokyo, 1976), International Association for Bridge and Structural Engineering, Zurich, 1976, pp. 433-438. 10.96. Matsushita, H., “Studies on High-Strength Concrete with Superplasticizer,” Abstract of 31st General Meeting, Cement Association of Japan, Tokyo, 1977, pp. 191-192. (in Japanese) 10.97. Mattison, E.N., and Beresford, F.D., “Studies of the Production of High Strength Concrete,” 4th Symposium on Concrete Research and Development 1970-1973, National Conference Publication No. 73/6, Institution of Engineers, Australia, Sydney, 1973, pp. 5-10. 10.98. Maxson, Orwin G., and Achenbach, Gary D.,

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“Properties of Concrete with Pressured Hydrocarbons and Seawater,” Proceedings, 8th Annual Offshore Technology Conference, Houston, May 1976, paper OTC2662, V. 3, pp. 507-512. 10.99. McBee, William C., and Sullivan, Thomas A., “Development of Specialized Sulphur Concretes,” Report No. 8346, U.S. Bureau of Mines, Washington, D.C., 1979, 21 pp. 10.100. Melnik, R.A., and Patsula, A.Y., “Investigation of the Nonlinear Creep of High Strength Concrete (Issledovanie Nelineinoi Polzuchesti Vysokoprochnykh Betonov),” Beton i Zehelzobeton (Moscow), No. 3, Mar. 1973, pp. 39-40. (in Russian) 10.101. “Mix Design for Pre-Mixed Concrete 50-55 MPa,” Boral Resources (Vic) Pty Limited, Boral Concrete, Abbotsford, Australia, 11 pp. 10.102. Moe, Johannes, “Feasibility Study of Prestressed Concrete Tanker Ships,” ACI JOURNAL , Proceedings V. 71, No. 12, Dec. 1974, pp. 617-626. 10.103. Moreno, Jaime, “Sixteen Years of HighStrength Concrete in the Chicago Area,” presented at the ACI Annual Convention, Atlanta, Jan. 1982. 10.104. Morgan, Austin H., “High-Strength ReadyMixed Concrete,” National Ready Mixed Concrete Association, Silver Spring, Jan. 1971, 18 pp. 10.105. Morin, A.L.; Tkachuk, V.M.; and Korytnyuk, Y.V., “Investigation of the Eccentrically Compressed Structural Components Built by Using High-Strength Concrete (Issledovaniya Vnetsen-trenno Szhatykh Elementov Iz Betonov Vysokikh),” Beton i Zhelezobeton (Moscow), No. 1, Jan. 1974, pp. 39-41. (in Russian) 10.106. Muguruma, Hiroshi, and Tanaka, Shinzo, “Mechanical Properties of High-Strength Concrete,” Review of the 27th General Meeting-Technical Session, Cement Association of Japan, Tokyo, May 1973, pp. 140143. 10.107. Nagataki, S., “The Properties of High-Strength Concrete,” Concrete Journal (Tokyo), V. 14, No. 3, 1976, pp. 38-41. (in Japanese) 10.108. Nagataki, S., and Imai, M., “Some Experiments on High-Strength Concrete,” 27th Annual Meeting, Japan Society of Civil Engineers, Tokyo, 1972, pp. V-187-190. (in Japanese) 10.109. Nasser, George D., “Are We Headed Towards Very High-Strength Concretes?"Concrete Products, V. 70, Oct. 1967, pp. 53-54. 10.110. Nasser, George D., “Bibliography on High Strength Concretes,” ACI JOURNAL , Proceedings V. 64, No. 10, Oct. 1967, pp. 690-691. 10.111. Nilson, A.H., and Slate, F.O., “Properties of High Strength Concrete,” presented at the Session on Inelastic Response of Normal, Lightweight, and High-S trength Concrete, ASCE Fall Convention, Chicago, Oct. 1978. 10.112. Nilson, A.H., and Slate, F.O., “Structural Properties of High-Strength Concrete,” presented at the ACI Annual Convention, Milwaukee, Mar. 1979. 10.113. Nishi, H.: -Ohshio, A.: and Fukuzawa, K.,

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“Autoclave-Cured High Strength Concrete and Piles,” Cement and Concrete, No. 299, Cement Association of Japan, Tokyo, 1972, pp. 23-29. (in Japanese) 10.114. Okada, Kiyoshi, and Azimi, M. Azam, “Strength and Ductility of Reinforced High Strength Concrete Beams,” Memoirs, Faculty of Engineering, Kyoto University, V. 43, Part 2, Apr. 1981, pp. 304-318. 10.115. Okada, Kiyoshi, and Kobayashi, Kazuo, “Effects of Addition of Gypsum and Super Water-Reducing Agent on Mechanical Properties of Blast-Furnace Slag Cement Mortar,” Proceedings, 21st Japan Congress on Materials Research (Tokyo, Oct. 1977), Society of Materials Science, Kyoto, 1978, pp. 209-213. 10.116. Parrot, L.J., “High-Strength Concrete,” Concrete (London), V. 4, No. 2, Feb. 1970, pp. 83-84. 10.117. Parrot, L.J., “The Production and Properties of High-Strength Concrete,” Concrete (London), V. 3, No. 11, Nov. 1969, pp. 443-448. 10.118. Parrott, L.J., “The Selection of Constituents and Proportions for Producing Workable Concrete with a Compressive Cube Strength of 80 to 110 n/mm 2 (11,600 to 15,900 lbf/in2),” Technical Report No. 416, Cement and Concrete Association, Wexham Springs, 1969, 12 pp. 10.119. Pastor, J.A.; Nilson, A.H.; and Slate, F.O., “Strength and Deformation of High Strength Reinforced Concrete Beams,” Research Report, Department of Structural Engineering, Cornell University, Ithaca (in preparation). 10.120. Perenchio, W.F.; Whiting, D.A.; and Kantro, D.L., “Water Reduction, Slump Loss, and Entrained Air Void Systems as Influenced by Superplasticizers,” Superplasticizers in Concrete, SP-62, American Concrete Institute, Detroit, 1979, pp. 137-155. 10.121. Pollet, Henri M., “Attainment of Very High Strength Concrete-eater than 1000 kg/cm 2 (Realisation de Betons a Tres Haute Resistance-Supreiure a 1000 kg/cm2),” Annales, Institut Technique du Batiment et des Travaux Publics (Paris), No. 214, Oct. 1965, pp. 1425-1426. (in French) 10.122. Popovics, Sandor, “Strength Relationships for Fly Ash Concrete,” ACI, JOURNAL, Proceedings V. 79, No. 1, Jan.-Feb. 1982, pp. 43-49. 10.123. “Precasting Efficiency Pays Off on Long Bridge," Construction Equipment, V. 64, No. 1, Aug. 1981. 10.124. P’yachev, V.A.; P’yachev, G.E.; and Kokhaev, N.F., “Raw Materials in Cement Manufacture for Producing High-Strength Concrete (Tsementy Dlya Vysokoprochniykh Betonov),” Tsement (Leningrad), No. 1, Jan. 1974, pp. 21-22. (in Russian) 10.125. Rayner, Johnathan, “Floating Docks in Vancouver Need Continuous Pour of New High-Strength Concrete,” Engineering and Contract Record (Don Mills), V. 89, No. 9, Sept. 1976, pp. 22-24. 10.126. Reigstad, Gordon H., “Energy Conservation in Buildings: A Prestressed Concrete System,” Professional Engineer, V. 47, No. 4, Apr. 1977, pp. 27-28. 10.127. Richart, F.E., “A Study of the Economics of High Strength Concrete in Building Construction,” ACI

JOURNAL , Proceedings V. 32, No. 4, Mar.-Apr. 1936, pp.

459-472. 10.128. Roy, Della M., and Gouda, G.R., “High Strength Generation in Cement Pastes,” Cement and Concrete Research, V. 3, No. 6, Nov.-Dec. 1973, pp. 807-820. 10.129. Roy, D.M.; Gouda, G.R.; and Bobrowsky, A., “Very High Strength Cement Pastes Prepared by Hot Pressing and Other High Pressure Techniques,” Cement and Concrete Research, V. 2, No. 3, May-June 1972, pp. 349-366. 10.130. Ryaboshapko, Y.I.; Vaslavskii, V.F.; and Olginskii, A.G., “Experience in the Application of HighStrength Concrete with Acid FIy Ash Admixture (Opyt Primeneniya Vysokomarochnogo Betona S Prisadkoi Kisloi Zoly-Unosa),” Beton i Zhelezobeton (Moscow), No. 5, May 1974, pp. 12-13. (in Russian) 10.131. Ryell, John, “High Strength Concrete,” Tenth Annual School of Concrete Technology, Ready Mixed Concrete Association of Ontario, Toronto, Apr. 1969, 19 pp.

10.132. Ryell John, “High Strength Concrete,” Canadian Pit and Quarry (Don Mills), Jan. 1970, pp. 16-19, and Feb. 1970, pp. 26-28. 10.133. Saito, T.; Ohshio, A.; Goto, Y.; and Omori, Y., “High Strength Concrete. Part 2, Strength Properties, Durability, and Thermal Characteristics,” Journal of Research, Onoda Cement Co., V. 28, 1976, pp. 12-27. (in Japanese) 10.134. Saucier, Kenneth L., “High-Strength Concrete, Past, Present, Future: Concrete International Design & Construction, V. 2, No. 6, June 1980, pp. 46-50. 10.135. Saucier, Kenneth L., “Determination of Practical Ultimate Strength of Concrete,” Miscellaneous Paper No. C-72-16, U.S. Army Engineer Waterways Experiment Station, Vicksburg, June 1972, 29 pp. 10.136. Savage, E.S., “Deep-Bed Filtration Lengthens Filter Runs, Lowers Backwash Water Needs,” American City, V. 88, No. 1, Jan. 1973, p. 44. 10.137. Schrader, Ernest K., and Munch, Anthony V., “Fibrous Concrete Repair of Cavitation Damage,” Proceedings, V. 102, C02, June 1976, pp. 385-399. 10.138. Shah, S.P., and Ahmad, S.H., “Effective Confinement on High-Strength Concrete,” presented at the ACI Annual Convention, Atlanta, Jan. 1982. 10.139. Shah, S.P.; Gokoz, UIker; and Ansari, Farhad, “Experimental Technique for Obtaining Complete StressStrain Curves for High Strength Concrete,” Cement, Concrete, and Aggregates, V. 3, No. 1, Summer 1981, pp. 21-27. 10.140. Schukla, S.N., and Mittal, M.K., “Short-Term Deflection in Two-Way Reinforced Concrete Slabs After Cracking,” ACI JOURNAL , Proceedings V. 73, No. 7, July 1976, pp. 416-419. 10.141. Slate, F.O., and Nilson, A.H., “High-Strength Concrete-Preliminary Results on Microcracking and Creep,” presented at the ACI Annual Convention, Milwaukee, Mar. 1979.


10.142. Southworth, George, and Scott, Norman L., “Special Concretes,” Concrete Construction, V. 26, No. 3, Mar. 1981, pp. 229-233, 275-279. 10.143. “Soviet Concrete Not So Tough?,” Engineering News-Record, V. 166, No. 22, June 1, 1961, p. 47. 10.144. Stachiw, J.D., ‘Concrete Deep Submergence Hollow Shell Structures,” Proceedings, ASCE, V. 95, ST 12, Dec. 1969, pp. 2931-2954. 10.145. Stamenkovic, Hrista, “Causes, Mechanisms and Control of Surface Voids,” Concrete (London), V. 7, No. 7, July 1973, pp. 45-48. 10.146. “Structural Trends in New York City Buildings,” Civil Engineering-- ASCE, V. 53, No. 1, Jan. 1983, pp. 30-37. 10.147. Superplasticizers in Concrete, SP-62, American Concrete Institute, Detroit, 1979, 436 pp. 10.148. Swamy, R.N., and Anand, K.L., “Shrinkage and Creep Properties of High Strength Structural Concrete,” Civil Engineering and Public Work Review (London), V. 68, No. 807, Oct. 1973, pp. 859-868. 10.149. Swamy, R.N., and Anand, K.L., “Structural Behavior of High Strength Concrete Beams,” Building Science, V. 9, No. 2, June 1974, pp. 131-141. 10.150. Swamy, R.N., and Ibrahim, A.B., “Shrinkage and Creep Properties of High Early Strength Structural Lightweight Concrete, Proceedings, Institution of Civil Engineers (London), V. 55, Sept. 1973, pp. 635-646. 10.151. Swamy, R.N.; Ibrahim, A.B.; and Anand, K.L., “The Strength and Deformation Characteristics of High Early Strength Structural Concrete,” Materials and Structures, Research and Testing (RILEM, Paris), V. 8, No. 48, 1975, pp. 413-423. 10.152. Symposium on Concrete for Engineering, Engineering for Concrete (Brisbane, Aug. 1977), National Conference Publication No. 7718, Institution of Engineers, Australia, Brisbane, 1977. 10.153. “Tapping Concrete’s Potentials,” Building Design and Construction, V. 20, No. 10, Oct. 1979, pp. 63-93. 10.154. ‘Thaulow, Sven, ‘Tensile Splitting Test and High Strength Concrete Test Cylinders,” ACI JOURNAL , Proceedings V. 53, No. 7, Jan. 1957, pp. 699-706. 10.155. “The World’s Tallest Concrete BuildingsToday and Yesterday,” Concrete Construction, V. 28, No. 2, Feb. 1983, pp. 91-100. 10.156. Tognon, G.; Copetti, G.; and Ursella, P., “Very High Strength Concretes for Precasting: Production Technology and Characteristic Properties,” Proceedings, 9th International Congress of the Precast Concrete Industry (Vienna, Oct. 1978), BIBM, Linz/Donau, 1978, pp. 1-39-I-46. 10.157. Tognon, G.; Ursella, P.; and Copetti, G., “Design and Properties of Concretes with Strength over 1500 kg/cm2,” ACI JOURNAL, Proceedings V. 77, No. 3, May-June 1980, pp. 171-178. 10.158. Towles, Thomas T., “Advantages in the Use of High Strength Concretes,” ACI JOURNAL , Proceedings V. 28, No. 9, Apr. 1932, pp. 607-612.

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10.159. “Undersea Restaurant Uses 25,500-psi Concrete,” Engineering News-Record, V. 190, July 13, 1972, p. 17. 10.160. Valore, R.C.; Kudrenski, W.; and Gray, D.E., “Application of High-Range Water Reducing Admixtures in Steam Cured Cement-Fly Ash Concretes,” Superplasticizers in Concrete, SP-62, American Concrete Institute, Detroit, 1979, pp. 337-373. 10.161. Vivesaraya, H.C.; Desay, P.; and Babu, Shri K.H., “High Strength Concrete Mix Design, A Case Study,” Special Publication No. SP-2, Cement Research Institute of India, New Delhi, Mar. 1970, 28 pp. 10.162. Walz, K., “The Production of High Strength Concrete,” Translation Tech. No. 2037/R39, Cement and Marketing Company Limited, London, June 1966, p. 7. 10.163. Wang, Pao-Tsan; Shah, Surendra P.; and Naaman, Antoine, “High-Strength Concrete in Ultimate Strength Design,” Proceedings, ASCE, V. 104, ST11, Nov. 1978, pp. 1761-1773. 10.164. Wantanabe, A., et al., “Fatigue Behavior of High-Strength Concrete,” Abstract of 31st General Meeting, Cement Association of Japan, Tokyo, 1977, pp. 221222. (in Japanese) 10.165. “Water-Tower Place-High-Strength Concrete,” Concrete Construction, V. 21, No. 3, Mar. 1976, pp. 102-104. 10.166. Wittman, F.H., “Micromechanics of Achieving High-Strength and Other Superior Properties,” Proceedings, National Science Foundation Workshop on HighStrength Concrete (Dec. 1979), University of Illinois at Chicago Circle, 1979, Report A-l, 22 pp. 10.167. Woolgar, G., and Oates, D-B., “Fly Ash and the Ready-Mixed Concrete Producer,” Concrete International: Design & Construction, V. 1, NO. 11, NOV. 1979, pp. 34-40. 10.168. Yamamoto, Y., and Kobayashi, M., “Use of Mineral Fines in High Strength Concrete-Water Requirement and Strength,” Concrete International Design & Construction, V. 4, No. 7, July 1982, pp. 33-40. 10.169. Young, J.F.; Berger, R.L.; and Breese, J., “Accelerated Curing of Compacted Calcium Silicate Mortars on Exposure to CO2,” Journal of the American Ceramic Society, V. 57, No. 9, Sept. 1974, pp. 394-397. 10.170. Zaitsev, Y.B., and Wittman, F.H., “Simulation of Crack Propagation and Failure of Concrete,” Materials and Structures, Research and Testing (RILEM, Paris), V. 14, No. 83, Sept.-Oct. 1981, pp. 357-365. 10.171. Zia, Paul, “Review of ACI Code for Design with High-Strength Concrete,” Concrete International Design & Construction, V. 5, No. 8, Aug. 1983, pp. 16-20. 10.172. Zorich, A.S., “Bearing Capacity of Eccentrically Tensioned Reinforcing Elements of Ordinary and High-Strength Concrete Under Transversal Forces (Nesushchaya Sposobnost Vnetsentrenno Rastyanutykh Zahelezobetonnykh Elementov Iz Obychnogo I Vysokoprochnogo Betonov Pri Deistvii Poperechnykh Sil): Beton i Zhelezobetotn, No. 11, Nov. 1976, pp. 34-37. (in Russian)

ACI 363r 92 State-of-the-Art Report on High-Strength Concrete

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