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Disclaimer: This document is a part of my M.Sc. thesis on Use of Superplasticizers for High Strength Concrete, published on 1999, Tribhuwan University, Institute of Engineering, Nepal. Readers' discretion is advised. The author bears no responsibility to any loss or damage by use of this document. ---------------------------------------------------------------------------------------------------------------
Cement Concrete and Superplasticizers Er. Saroj Bhattarai, M.Sc. Structures, Senior Divisional Engineer, Department of Roads, Nepal
What is Cement Concrete?
Structure of concrete Concrete in its basic form is a conglomerate of fine and coarse aggregate bound together by hydrated cement paste. At macroscopic level concrete can be considered as a twophase material, consisting of aggregate particles dispersed in an incoherent mass of the hydrated cement paste (hcp). Large part of concrete is covered by coarse aggregate. The voids between the coarse aggregates are filled with mortar from cement paste and fine aggregates. Further at microscopic level, a third phase – the transition zone, can be distinguished as an interface between the aggregate particles and bulk hydrated cement paste (Fig. 2-1-1).17
Strength of concrete Among the two main phases of concrete, the aggregate phase mostly remains unchanged during the hardening process. But the cement undergoes through several changes once it comes into contact with water. This changing process is called hydration and the paste is known as hydrated cement paste (hcp). The hcp has a very complex structure which is a combination of solids such as calcium silicate hydrate (CS-H), calcium hydroxide (CH), calcium sulfoaluminates (C-A-S-H), unhydrated clinker grains; different types of voids such as interlayer space in C-S-H, capillary voids, air voids and water in different states such as capillary water, absorbed water, interlayer water and chemically combined water.
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Transition zone
Aggregate
C-S-H
CH
Bulk Cement Paste
C-A-S-H (Ettringite)
Figure 2-1-1 Simplified diagrammatic representation of the three phases of concrete C-S-H = Calcium silicate hydrate; CH = Calcium hydroxide; C-A-S-H = Calcium sulfoaluminates
The strength of concrete is mainly due to the van der Walls force of attraction between the products of hydration themselves and between these products and aggregate particles. This force of attraction is dependent upon the volume and size of voids present in the bulk hcp and the transition zone. In the initial stage of hardening water films form around aggregate particles which means a higher water/cement ratio closer to the aggregates. Due to the high w/c ratio the crystalline products of hydration near the aggregates (i.e. at the transition zone) are of relatively larger size and therefore form a more porous structure. That is why the transition zone is considered the weakest part at least at an early age of concrete. However, later the voids in the transition zone start to be filled with products of slow chemical reaction between the cement paste constituents and the aggregates, which increases the strength of the transition zone. Usually at an age of three months or more the strength of the transition zone tends to be equal or sometimes even greater than that of the bulk cement paste 17. But, since the structure is loaded much earlier the microcracks present in the transition zone, which are there even
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before load is applied, tend to propagate further, leading ultimately to failure of the concrete.
Relationships between water/cement ratio, porosity and strength of concrete There are several factors that influence on strength of concrete. As stated by Gilkey*, "For a given cement and acceptable aggregates, the strength that may be developed by a workable, properly placed mixture of cement, aggregate and water (under the same mixing, curing and testing conditions) is influenced by the: (a) ratio of cement to mixing water (b) ratio of cement to aggregate (c) grading, surface texture, shape, strength and stiffness of aggregate particles (d) maximum size of the aggregate"
Generally the factors (b) through (d) are of lesser importance than factor (a) when usual aggregates up to 40 mm maximum size are employed 19. That means the ratio of cement to mixing water or water/cement (w/c) ratio plays a major role in strength of concrete. Based on works of T.C. Powers it has been found that there is a certain relationship between w/c ratio and total volume of capillary porosity. A study with constant volume of cement (100 cm³) but varying water-cement ratios (0.7, 0.6, 0.5 and 0.4) showed that after 100 percent degree of hydration showed that the paste with w/c ratio of 0.7 had 37 percent of capillary voids while the same with w/c ratio of 0.4 had only 11 percent 17. Under the assumption made in this study with a 0.32 w/c ratio paste there would be no capillary porosity after a 100 percent hydration. (Fig. 2-1-2).
*
From Discussion paper by H.J. Gilkey: Water/cement ratio versus strength – another look, J. Amer. Concr. Inst., Part 2, 58, pp. 1851 – 78 (Dec. 1961). Quoted from [19].
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0.3
0.4 25%
W/C Ratio
100% Hydration
0.5
50%
75%
0.6 0.7 0
0.1
0.2
0.3
0.4
0.5
0.6
Capillary Porosity, Vol. Fraction P Figure 2-1-2 Influence of water/cement ratio and degree of hydration on capillary porosity (figure partially extracted from [17], p.33)
For normally hydrated portland cement mortars, Powers* showed that there is an exponential relationship of the type S = kx³, where S is the compressive strength, x is the solid-to-space ratio (which is inversely proportional to porosity) and k is a constant equal to 34,000 psi (234 MPa). Fig. 2-1-3 explains this relationship graphically. Although this relationship can not be directly applied to concrete because presence of microcracks in the transition zone makes it a relatively non-homogeneous complex material, but since the strength of concrete is governed both by strength of cement paste and strength of transition zone, the validity of this relationship is retained.
The effect of porosity is seen not only on strength but on durability also. P.C. Aitcin et al
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observed that when concrete is subjected to external aggression the most effective
way to decrease the intensity of this aggression is to reduce its porosity and permeability. This is why w/c ratio has always been the controlling factor of concrete durability.
*
T.C. Powers, J. Am. Ceram. Soc., Vol. 41, No. 1, 1958. Quoted from [19]
Strength of 5 cm cube mortar (fc), MPa
5
200 175
fc=234 x³
150 125 100 75 50 25
0
0.2
0.4 0.6 0.8 Gel / Space ratio (x)
1.0
Figure 2-1-3 Porosity – Strength relationship of cement mortar as established by T.C. Powers in J. Am. Ceram. Soc., Vol. 41, No. 1, pp. 1-6, 1958. (figure partially extracted and modified from [17], p.45)
From the discussion it is clear that water content in a unit volume of concrete plays a major role in development of strength of concrete. Obviously, greater water/cement ratio causes more porosity and hence lesser strength, provided that other parameters are kept constant. Earlier in 1918 after extensive testing at the Lewis Institute, University of Illinois, Duff Abrams found that a relation existed between water/cement ratio and strength of concrete, which is represented by: fc
k1 k 2w / c
where, fc is the compressive strength of concrete, k1 and k2 are empirical constants and w/c is the water/cement ratio. Many mix design methods are based on this relationship. As an example, the Indian Standard method suggests to use the same relation (Fig. 2-14) for selection of w/c ratio for required target mean strength 18. It should be noted here however that for w/c ratio lower than 0.3, disproportionately high increase in strength can be achieved for very small reduction in w/c ratio. This may be due to significant increase in the strength of the transition zone at very low w/c ratio.17 Therefore the above equation may not hold good for very high strength concrete. But nevertheless, effect of w/c ratio is not found to be inverse in any case.
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28-day compressive strength of concrete, N/mm²
60
50
40
30
20
10
0 0.3
0.35
0.4
0.45
0.5
0.55
0.6
Water / Cement ratio Figure 2-1-4 Generalized relationship between free w/c ratio and compressive strength of concrete suggested in Indian Standard, IS : 10262-1982. (Figure from [18])
Workability of concrete From the previous sections it is clear that for higher strength of concrete the water/cement ratio should be decreased. But for a given amount of cement, and given type and amount of aggregate, reduction of water beyond a certain volume will result in stiff consistency and poor workability. A concrete mix with poor workability is difficult to place and compact by conventional methods. As a result, the desired strength and durability characteristics are very difficult to achieve.
Workability of concrete is defined in ASTM C 125 as the property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity. There is another term – consistency, which describes the state of fresh concrete in terms of its wetness
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. But wetness alone does not make the mix
workable since mixes with same consistency may vary in workability. Properties of aggregate, and presence of admixtures are other factors that control workability. There is yet another term – cohesiveness, which describes the tendency to bleed or segregate17. Thus workability is a composite property which can not be determined by a single factor.
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Workability is measured by several methods. But no single test can measure all the properties that describe complete workability of the mix. Some popular methods are: slump test; Vebe test; compacting factor test; remoulding test; flow test etc. The slump test is the easiest and widely used method for measuring workability. Although slump test, as other tests, does not describe the workability as a whole, but due to its simplicity and portability of the equipment it is the most popular method of controlling consistency of the mix in field conditions. Many codes including the Indian Standards allow slump test as measurement of workability.
Workability and high strength concrete Effect of water on strength of concrete was already discussed in sections 2.1.2 and 2.1.3. From the discussions it is clear that workability (particularly in terms of consistency) and strength are two opposing properties of concrete. With the decreasing water content, fresh concrete becomes more and more difficult to mix, place and consolidate. For the production of high strength concrete, the opposing effects of water cement ratio on consistency and strength of concrete cannot be harmonized without the use of water reducing admixtures 17. There are examples of using zero slump concrete, but such concrete requires special arrangement for placing and compacting, which may not always be possible, particularly in heavily reinforced parts of the structure. As mentioned elsewhere in this text (Section 1.2) workability, especially for high strength concrete, is the basic necessity in the context of this country since most of the concrete works are carried out manually.
Superplasticizer and its role Superplasticizers are high range water reducing admixtures, which are able to reduce the amount of water by up to 1/3 of that required for normal concrete mix. Superplasticizers were first introduced in around 1964/65 in Japan
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and have already
found a wide acceptance in the concrete industry. As classified by ASTM C 494-92, superplasticizers fall in to Types F and G, High-range Water Reducing and High-range Water Reducing with retarding, respectively. These are water-soluble organic polymers which have to be synthesized, using a complex polymerization process, to produce long
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molecules of high molecular mass.
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The main types of these admixtures are based
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on : Lignosulphonates Hydroxycarboxylic acid salts Melamine sulphonates Naphthalene sulphonates
The last two, sulfonated melamine-formaldehyde condensates (SMF) and sulfonated naphthelene-formaldehyde condensates (SNF) are most commonly used. The molecular weight of superplasticizers is in the range of 20,000 to 30,000 mass, within limits, improves its efficiency
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17
; the larger molecular
from the view point of excessive
retardation or air entrainment at higher dose 15.
Superplasticizers do not alter fundamentally the structure of hydrated cement paste, the main effect being a better distruibution of cement particles and consequently, their better hydration19. The main action of the long molecules of superplasticizers is to wrap themselves around the cement particles and give them a highly negative charge so that they repel each other. This results in deflocculation, and better dispersion of the cement particles19. (Fig. 2.1.6).
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Thus, superplasticizers themselves do not enhance the
property of cement or other ingredients of concrete. It is the temporary deflocculation of cement particles that has to be exploited.
Figure 2.1.6 Action of superplasticizer on cement particles.
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Some of the main features of the superplasticizers and their use are as follows:
Capable of reducing water content up to 30 percent for a given workability
Inert organic chemical, action on the cement particles – mostly physical, does not alter fundamental structure of the hydrated cement paste. Although some recent experiments have shown that naphthalene based SP can react particularly with C3A and substantially reduce the initial surface hydration rate 11.
Problems of segregation and bleeding are minimum even with slight overdose.
Suitable for concrete produced at site and placed immediately due to rapid slump loss (within 30 to 60 minutes). But now-a-days there are superplasticizers with retarders available in the market, so this problem is somehow negotiated.
It is necessary to find out its compatibility with the cement being used and optimum dose before using it in mass production. Optimum dose depends upon type of cement, desired strength and w/c ratio, desired workability and type of aggregate.
The Figure 2.1.7 shows the effect of superplasticizer on workability (slump) and compressive strength 14.
Compressive strength
w/c=0.30, admixture 1.2% Mix ratio 1:0.65:1.95 w/c=0.38, No admixture
Time, days
For a constant slump 65 mm, average increase in 28 days strength = 30% Figure 2.1.7. Effect of superplasticizer on workability and strength.
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Why superplasticizer? There are other methods also available for increasing the strength of concrete. Some of them are:
Using higher grade cement
Lowering the slump value (making zero slump concrete)
Increasing cement content to meet the normal w/c ratio
Use of normal water reducing agents
Before the introduction of superplasticizers these methods were used for production of high strength concrete. But there were problems associated with these methods.
Higher grade of cement means more fine particles. Although the fineness is mainly responsible for early development of strength but there is better hydration of all the particles, which leads to some increment in final strength. But cost of grinding to higher fineness is considerable and the finer the cement the more rapid deterioration on exposure to the atmosphere.19
Lowering the slump or making zero slump concrete makes it very difficult to mix, cast and compact which is a serious problem when these procedures are done manually. The problem is more pronounced in reinforced concrete. (Please refer to Section 2.1.4 also)
Increasing the cement content to meet the normal w/c ratio is always associated with the cost. Furthermore the increased heat of hydration may cause cracks and increment in volume particularly in heavy sections.
Normal water reducing admixtures (ASTM Type A, D & E) are relatively cheaper but these admixture are capable to reduce water content only up to 10 – 15 percent. Overdose causes considerable retardation and segregation.
Superplasticizers can reduce the required amount of water by as much as 30 percent without lowering the workability. Superplasticizers are costlier than normal water reducer, but in overall scenario, the total cost can be decreased by taking advantage of
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better workability, early strength development and reduced quantity of concrete work. Although some types of superplasticizers can cause retardation, other adverse effects due to accidental over-dosage are minimum. Since the action of superplasticizer on cement is primarily physical and subsides in due time, there are no major harmful chemical reactions. These are the main reasons for choosing superplasticizer for the study.
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References 1. AITCIN, P.C., The Art and Science of High Performance Concrete, Milburn New Zealand Ltd., in association with NZ Concrete Society and Structural Engineering Groups, 1999. 2. AITCIN, P.C. & BAALBAKI, M., Concrete Admixtures - Key Components of Modern Concrete (proceedings of the International RILEM Workshop, ConTech 94, Barcelona 7-9 Nov. 1994), Concrete Technology: New Trends, Industrial Applications. E&FN Spon London, UK, 1994, pp. 33-47. 3. AL-MANASEER, A.A. & ALBERT, A.J., Measuring the Consistency and Workability of Superplasticized Concrete, ACI Material Journal, vol 92, No. 3, May-June, 1995, pp. 286-290. 4. BAJHENOV, Y.M., Concrete Technology (in Russian "Technologiya Betona"), 2nd edition, Vysha Schola, Moscow, 1987 5. BATRAKOV, V.G., Modified Concrete (in Russian "Modifitsirovannye Betony"), Stroiijdat, Moscow, 1990 6. CETIN, A. & CARRASQUILLO, R.L., High-performance Concrete: Influence of Coarse Aggregates on Mechanical Properties, ACI Material Journal, vol 95, No. 3, MayJune, 1998, pp. 252-261. 7. CHITHARANJAN, N., Workability Agent as a Cement Saver, Indian Concrete Journal, vol 61, No. 12, December, 1987, pp. 325-329. 8. Collection of Abstracts of Articles (Collected by S. Bhattarai), Collection of abstracts of articles on high strength concrete and superplasticizers published in the Internet web sites, U.R.Ls.- http://www.tfhrc.gov, http://www.pubs-asce.org, http://www.aci-int.org, 1999 9. COLLEPARDI, M., Admixtures Used to Enhance Placing Characteristics of Concrete, Cement and Concrete Composites, 20(1998), Elsevier Science Ltd., Great Britain, 1998, pp. 103-112 10. De ALMEIDA, I.R. & GONCALVES, A.F., Properties of Freshly Mixed High Strength Concrete (proceedings of the RILEM colloquium, Hanover Oct 3-5, 1990), Properties of Fresh Concrete, Chapman & Hall, 1990, pp. 227-234. 11. GAGNE, R., BOISVERT, A & PIGEON, M., Effect of Superplasticizer Dosage on Mechanical Properties, Permeability, and Freeze-Thaw Durability of High-Strength Concretes With and Without Silica Fume, ACI Material Journal, vol 93, No.2, March-April, 1996, pp. 111-120. 12. HALL, D.J., Proportioning Concrete Mixtures Containing Chemical Admixtures, ACI Publication, SP - 46, pp. 97-108. 13. Hover, K.C., Concrete Mixture Proportioning with Water-reducing Admixtures to Enhance Durability: A Quantitative Model, Cement and Concrete Composites, 20(1998), Elsevier Science Ltd., Great Britain, 1998, pp. 113-119. 14. KUMAR, V., ROY, B.N, & SAI, A.S.R., Effect of Superplasticizer on Concrete, Indian Concrete Journal, vol 63, No. 1, January, 1989, pp. 31-33, 42. 15. LIMAYE, R.G., Admixtures for High Strength Concrete, Indian Concrete Journal, vol 70, No. 12, December, 1996, pp. 689-691. 16. MANJREKAR, S.K., Use of Superplasticizers: Myths and Reality, Indian Concrete Journal, vol 68, No. 6, June, 1994, pp. 317-320. 17. MEHTA P.K & MONTEIRO, P.J.M, Concrete - Microstructure, Properties, and Materials (Indian edition), Indian Concrete Institute, Chennai, India, 1997 18. N. Krishna Raju, Design of Concrete Mixes, CBS Publishers & Distributors, New Delhi, India, 1997 19. NEVILLE, A.M., Properties of Concrete (4th edition), ELBS/Addison Wesley Longman, England, 1996 20. PENTTALA, V., Possibilities of Increasing the Workability Time of High Strength Concrete (proceedings of the RILEM colloquium, Hanover Oct 3-5, 1990), Properties of Fresh Concrete, Chapman & Hall, 1990, pp. 67-76.
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21. PLANTE, P., PIGEON, M., & SAUCIER, F., Air-Void Stability, Part II: Influence of Superplasticizers and Cement, ACI Material Journal, vol 86, No. 6, November-December, 1989, pp. 581-589. 22. PLANTE, P., PIGEON, M., & SAUCIER, F., Air-Void Stability, Part III: Field Test of Superplasticized Concretes, ACI Material Journal, vol 87, No. 1, January-February, 1990, pp. 3-11. 23. PUNKKI, J., GOLASZEWSKI, J., & GJORV, O.E., Workability Loss of High-Strength Concrete, ACI Material Journal, vol 93, No. 5, September-October, 1996, pp. 427-431. 24. RAMACHANDRAN, V.S., Superplasticizers in Concrete, CBD - 203 (extracted from web site:), IRC Publications, Canadian Building Digest, 1979 25. REDDI, S.A., Use of HSC/HPC for Road Bridges in India, Indian Concrete Journal, vol 70, No. 12, December, 1996, pp. 661-673. 26. SARKAR, S.L., High Strength Concrete: Past, Present and Future, Indian Concrete Journal, vol 68, No. 3, March, 1994, pp. 141-144. 27. SASIADEK, S. & SLIWINSKI, M., Means of Prolongation of Workability of Fresh Concrete in Hot Climate Conditions (proceedings of the RILEM colloquium, Hanover Oct 3-5, 1990), Properties of Fresh Concrete, Chapman & Hall, 1990, pp. 109-115. 28. VARSHNEY, R.S., Tests on Concrete Materials, Oxford & IBH Publishing Co., 1982, pp. 197-241. 29. VARSHNEY, R.S., Tests on Concrete, Oxford & IBH Publishing Co., 1982, pp. 242270. 30. IS:383 - 1979, Specification for Coarse and Fine Aggregates from Natural Sources for Concrete, Indian Standards Institution, New Delhi, India. 31. IS:269 - 1967, Specification for Ordinary, Rapid-hardening and Low-heat Portland Cement, Indian Standards Institution, New Delhi, India. 32. IS:9103 - 1979, Specification for Admixtures for Concrete, Indian Standards Institution, New Delhi, India, 1979 33. IS:456 - 1978, Code of Practice for Plain and Reinforced Concrete, Indian Standards Institution, New Delhi, India, 1979 34. IS:6925 - 1973, Methods of Test for Determination of Water Soluble Chlorides in Concrete Admixtures, Indian Standards Institution, New Delhi, India, 1973 35. IS:516 - 1959, Methods of Tests for Strength of Concrete, Indian Standards Institution, New Delhi, India. 36. IS:4031 - 1968, Methods of Physical Tests for Hydraulic Cement, Indian Standards Institution, New Delhi, India. 37. IS:10262 - 1982, Indian Standard Recommended Guidelines for Concrete Mix Design, Indian Standards Institution, New Delhi, India. 38. Various reports on design of bridges, from the library of the Department of Roads, Babar Mahal, Kathmandu. 39. Interviews with the suppliers of chemical admixtures and technicians involved in the use of such admixtures. (Mr. B.R. Tater, Mr. Prashanta Mathema from H.G. Enterprises; Mr. Rajesh Thapaliya from Indco Trading Co.; Mr. Ramesh Sapkota from Himalayan Engineering Associates).
