Hot Weather Concreting

Hot Weather Concreting Concrete Technology Home > Concrete Design & Construction >Hot Weather Concreting

Things to Consider about Hot Weather Concreting When the temperature of freshly mixed concrete approaches approximately 25°C (77°F) adverse site conditions can impact the quality of concrete. Ambient temperatures above 32°C (90°F) and the lack of a protected environment for concrete placement and finishing (enclosed building) can contribute to difficulty in i n producing quality concrete . The precautions required to ensure a quality end product will vary depending on the actual conditions during concrete placement and the specific application for which the concrete will be used. In general, if the temperature at the time of concrete placement will exceed 25°C (77°F) a plan should be developed to negate the effects of high temperatures . The precautions may include some or all of the following:

The use of liquid nitrogen is one option to reduce concrete’s temperature during hot-weather concreting.

1. Moisten subgrade, steel reinforcement, and form work prior to concrete placement. 2. Erect temporary wind breaks to limit wind velocities and sunshades sunsha des to reduce concrete surface temperatures. 3. Cool aggregates and mixing water added to the concrete mixture to reduce its initial temperature. The effect of hot cement on concrete temperature is only minimal. 4. Use a concrete consistency that allows rapid placement and consolidation. 5. Protect the concrete surface during placement with plastic sheeting or evaporation retarders to maintain the initial moisture in the concrete mixture. 6. Provide sufficient labor to minimize the time required to place and finish the concrete, as hot weather conditions substantially shorted the times to initial and final set. 7. Consider fogging the area above the concrete placement to raise the

relative humidity and satisfy moisture demand of the ambient air. 8. Provide appropriate curing methods as soon as possible after the concrete finishing processes have been completed. 9. In extreme conditions consider adjusting the time tim e of concrete placement to take advantage of cooler temperatures, such as early morning or night time placement. With proper planning and execution concrete can be successfully placed and finished to produce high quality durable concrete at temperatures of 35°C (95°F) or more. Setting Time The effect of high ambient temperatures and high temperature concrete component materials have on the setting time of concrete mixtures is a topic of concern due to the reduced time in which concrete must be placed, consolidated and finished; increased potential for plastic shrinkage cracking, thermal cracking and cold joints; potential strength reduction due to high water demand and high curing temperatures; difficulty in controlling air content; and increased urgency for applying appropriate curing method at an early age. As a general rule of thumb an increase of 11°C (20°F) will reduce the setting time of a concrete mixture by as much as 50%. As an example a concrete mixture that reaches final set in 3 hours at 16°C (60°F) may reach final set in as little 1 ½ hours at 27°C (80°F). As the concrete temperature increases the setting time is further reduced. The actual temperature of the concrete mixture as delivered is effected by the temperature of the materials used in the mixture, the cementitious content of the mixture, the temperature of the equipment used to batch and transport the concrete, and the ambient temperature and conditions at the project site. Concrete applications may be considered hot weather concrete at temperatures ranging from 25°C to 35°C (77°F to 95°F) depending on the specific application. Precautions should be planned in advance to counter the effects of high temperature well in advance of execution to counter these effects.

relative humidity and satisfy moisture demand of the ambient air. 8. Provide appropriate curing methods as soon as possible after the concrete finishing processes have been completed. 9. In extreme conditions consider adjusting the time tim e of concrete placement to take advantage of cooler temperatures, such as early morning or night time placement. With proper planning and execution concrete can be successfully placed and finished to produce high quality durable concrete at temperatures of 35°C (95°F) or more. Setting Time The effect of high ambient temperatures and high temperature concrete component materials have on the setting time of concrete mixtures is a topic of concern due to the reduced time in which concrete must be placed, consolidated and finished; increased potential for plastic shrinkage cracking, thermal cracking and cold joints; potential strength reduction due to high water demand and high curing temperatures; difficulty in controlling air content; and increased urgency for applying appropriate curing method at an early age. As a general rule of thumb an increase of 11°C (20°F) will reduce the setting time of a concrete mixture by as much as 50%. As an example a concrete mixture that reaches final set in 3 hours at 16°C (60°F) may reach final set in as little 1 ½ hours at 27°C (80°F). As the concrete temperature increases the setting time is further reduced. The actual temperature of the concrete mixture as delivered is effected by the temperature of the materials used in the mixture, the cementitious content of the mixture, the temperature of the equipment used to batch and transport the concrete, and the ambient temperature and conditions at the project site. Concrete applications may be considered hot weather concrete at temperatures ranging from 25°C to 35°C (77°F to 95°F) depending on the specific application. Precautions should be planned in advance to counter the effects of high temperature well in advance of execution to counter these effects.

Precautions may include use of materials with a good performance history in high temperature conditions, cool concrete materials or concrete mixture, provide concrete consistency and placement equipment and crew for rapid placement, reduce time of transport, schedule placement to limit exposure to atmospheric conditions (night time placement or more favorable weather), plan to limit rapid moisture loss (sun screens, wind screens, misting or fogging), and consider the use of an evaporation retarder. Schedule a preconstruction meeting including all of the participants to discuss the plan to control the effects specific to the project and expected conditions.

Additional information is available in the following references: " Hot Hot Weather Concreting," Chapter 13 of  Design and Control of Concrete Mixtures ,EB001.14,  ,EB001.14, Portland Cement Association, 2002, 12 pages. Hot and Cold Weather Concreting, Concreting , CD057, Portland Cement Association, 2005 ACI 305, Hot Weather Concreting

Cold Weather Concreting Concrete Technology Home > Concrete Design & Construction >Cold Weather Concreting

What Happens When Concrete Freezes? Weather conditions at a jobsite – hot or cold, windy or calm, dry or humid – may be vastly different from the optimum conditions assumed at the time a concrete mix is specified, designed, or selected – or from laboratory conditions in which concrete specimens are stored and tested. Concrete can be placed in cold weather conditions provided adequate precautions are taken to alleviate the negative impacts of low ambient temperatures. The current American Concrete Institute definition of coldweather concreting, as stated in ACI 306 is, ―a period

when for more than 3 successive days the average daily air temperature drops below 5°C (40°F) and stays below 10°C (50°F) for more than one-half of any 24 hour period.‖ This definition can potentially lead to problems with freezing of the concrete at an early age. All concrete must be protected from freezing until it has reached a minimum strength of 3.5 MPa (500 psi), which typically happens within the first 24 hours. If concrete freezes while it is still fresh or before it has developed sufficient strength to resist the expansive forces associated with the freezing water, ice formation results in the disruption of the cement paste matrix causing an irreparable loss in strength. Early freezing can result in a reduction of up to 50% in the ultimate strength. Once concrete has attained a compressive strength of around 3.5 MPa (500 psi), it is generally considered to have sufficient strength to resist significant expansion and damage if frozen. Whenever air temperature at the time of concrete placement is below 5°C (40°F) and freezing temperatures within the first 24 hours after placement are expected, the following general issues should be considered: 1. Initial concrete temperature as delivered During cold weather, it may be necessary to heat one or more of the concrete materials (water and/or aggregates) to provide the proper concrete temperature as delivered. Due to the quantities and heat capacity of cement, using hot cement is not an effective method in raising the initial concrete temperature. 2. Protection while the concrete is placed, consolidated, and finished The exposure of concrete to cold weather will extend the time required for it to reach initial

set, which may require finishing crews to be available for a longer period. Depending on the actual ambient temperature, protection of a concrete placement may require the use of windbreaks, enclosures, or supplementary heat. It may also be appropriate to adjust the concrete mixture constituents for the effect of ambient temperature on setting time. This may require an increase in cement content, the use of an accelerating chemical admixture or both. Windbreaks protect the concrete and construction personnel from biting winds that cause temperature drops and excessive evaporation. Typically, a height of 2 m (6 ft) is sufficient. Windbreaks could be taller or shorter depending on anticipated wind velocities, ambient temperatures, relative humidity, and concrete placement temperatures. Heated enclosures are very effective for protecting concrete in cold weather, but are probably the most expensive option. Enclosures can be made of wood, canvas tarpaulins, or polyethylene. Prefabricated rigid-plastic enclosures are also available. Three types of heaters are used in cold-weather concrete construction: direct fired, indirect fired, and hydronic systems. To avoid carbonation of fresh concrete surfaces, indirectfired heaters should be used. If the concrete is not exposed to the heater or exhaust directly, then a direct-fired heater is suitable. Caution should be taken to ensure that workers are not overexposed to carbon monoxide anytime a heater is used inside an enclosure. Hydronic systems transfer heat by

circulating a glycol/water solution in a closed system of pipes or hoses. Typical applications for hydronic systems include thawing and preheating subgrades and heating areas that are too large to be practical for an enclosure. 3. Curing to produce quality concrete Curing not only requires adequate moisture, but also appropriate temperature. The temperature of the concrete as placed should be above 5°C (40°F) using methods described above, however the duration of heating is dependent on the type of service for the concrete, ranging from 1 day for high-early strength concrete that is not exposed to freeze-thaw events during service to 20 days or more for a concrete element that would carry large loads at an early age. In structures that will carry large loads at an early age, concrete must be maintained at a minimum of 10°C (50°F) to accommodate stripping of forms and shoring and to permit loading of the structure. In no case should concrete be allowed to freeze during the first 24 hours after it has been placed. Since cement hydration is an exothermic reaction, the concrete mixture produces some heat on its own. Protecting that heat from escaping the system using polyethylene sheeting or insulating blankets may be all that is required for good concrete quality. More severe temperatures may require supplemental heat. Concrete retained in forms or covered with insulation seldom loses enough moisture at 5°C to 15°C (40°F to 55°F) to impair curing. However, drying from

low wintertime humidities and heaters used in enclosures is a concern. It is good practice to leave forms in place as long as possible, because they help distribute heat more evenly and help prevent drying of the concrete. Live steam exhausted into an enclosure around the concrete is an excellent method of curing because it provides both heat and moisture. Liquid membrane-forming compounds can also be used within heated enclosures for early curing of concrete surfaces. It is also important to prevent rapid cooling of the concrete upon termination of the heating period. Sudden cooling of the concrete surface while the interior is warm may cause thermal cracking. Methods for gradual cooling of concrete include loosening the forms while maintaining cover with plastic sheeting or insulation, gradual decrease in heating inside an enclosure, or turning off the heat and allowing the enclosure to slowly equilibrate to ambient temperature. Massive structures may require several days or even weeks of gradual cooling to mitigate the probability of thermal cracking (see Mass Concrete).

Curing vs. Drying Concrete Concrete Technology Home > Concrete Construction >Curing vs. Drying Concrete

The Difference Between Curing and Drying

The terms curing and drying are frequently used interchangeably with regard to the moisture condition of new concrete slabs. To clarify these terms the following definitions may be useful. Curing: Curing of concrete is defined as providing adequate moisture, temperature and time to allow the concrete to achieve the desired properties for its intended use. This would mean maintaining a relative humidity in the concrete of greater than 80%, a temperature greater than 10°C (50°F), and for a time typically ranging from three to fourteen days depending on the specific application. When these recommendations are properly specified and performed in the field the final properties of the concrete mixture will be achieved. Drying: Drying of concrete is defined as providing the proper conditions to allow the concrete to achieve a moisture condition appropriate for its intended use. The moisture condition of a concrete slab is of significant importance for the application of moisture sensitive floor finishes (vct tile, linoleum, wood flooring, and non-breathable coating such as epoxy). The moisture condition is specified as a maximum relative humidity by percent (%) or a vapor transmission rate in µg/sec•m2(lb/1000 ft2 /24 hr.). A typical value specified for relative humidity may be less than 75% to 80% to assure the successful application of the flooring materials, while a commonly specified value for vapor transmission rate may be 170 µg/sec•m 2 (3 lb/1000 ft 2 /24 hr).

Drying of Concrete Concrete Technology Home > Concrete Construction >Drying of Concrete

How long does it take concrete to dry?

A concrete surface may look dry, but the slab can still contain sufficient moisture to cause problems when covered. The term ―concrete moisture‖ is understood to mean the total water used in the concrete batch, plus curing water, minus the water bound in hardened cement due to hydration. The Moisture in concrete. amount of concrete moisture can be considerable. In practical terms, several pounds of water must evaporate from every square foot of concrete for the slab to be considered adequately dry for floor finishes. An industry rule of thumb for estimating the drying time necessary for concrete floors to reach acceptable moisture content is 1 month of drying for each inch of concrete thickness (1 mm per day). Drying begins when water is no longer available at the exposed surface. If concrete is moist cured by sealing in the original mix water with wet burlap or plastic sheets, drying will begin when these covers are removed. Spray-applied curing membranes are somewhat breathable, and therefore, drying begins shortly after the membrane is applied. However, curing compounds can drastically reduce the drying rate and significantly extend the drying period. It is possible to calculate the drying time for a given concrete. For this calculation, information is needed about the absorption characteristics, diffusion coefficients for water and water vapor, porosity and pore size distribution, and degree of hydration. Since such information usually is not available, practice relies on experimental data combined with measurements of the actual moisture condition of the concrete slab in the field. A method of characterizing the moisture condition of

a concrete slab is to measure the relative humidity of the air in the concrete pore system. This is done by placing a relative humidity probe into a hole drilled in the concrete. The relative humidity achieved within a concrete slab depends on a combination of factors including the initial water-to-cement ratio, drying history, pore structure, and concentration of soluble ions in the pore water solution. The Swedish Concrete Association describes a method to estimate drying times for concrete slabs using relative humidity. Correction factors for thickness, number of drying sides, ambient conditions, and curing conditions make it possible to adjust for deviations from a set w/cm and target relative humidity. The purpose of this calculation is to enable a contractor or construction manager to estimate minimum drying times for concrete slabs during the planning stage of a project. See Concrete Floors and Moisture, 2nd edition, EB119, for the details of this method and to understand moisture in concrete. Careers | Sitemap | Disclaimer | Privacy Policy | © 2012 Portland Cement Association - All Rights Reserved Frequently Asked Questions Cement & Concrete Technology Home > FAQs > Moisture content in concrete

Q: What is the moisture content of concrete? A: The moisture content of concrete must be viewed from the context of total water content of the fresh concrete mixture and the available moisture content of the hardened concrete. The total water content of a fresh concrete mixture is a function of the total cementitious materials and water cement ratio (w/cm). Typical fresh concrete mixtures vary in cementitious material content in a range of 279 kg/m3 to 415 kg/m 3(470 lb/yd3 to 700 lb/yd 3). Water cement ratios typically vary from 0.4 to 0.55. To estimate the available moisture

content of hardened concrete one must start with the total water content of the fresh mixture and define the service condition of the hardened concrete with regard to relative humidity (%). In addition the water that is chemically bound with the cement in the hydration process must be accounted for. The water bound with the cement is in the range of 0.22 to 0.24 of the cement content. As an example, the moisture content of a concrete mixture with 334 kg/m3 (564 lb/yd3) of cement and a w/c of 0.45 and in a service environment with a 50% relative humidity could be estimated as follows:

Total water content: 334 kg cement/m 3 times 0.45 w/c ~ 150 kg water/m 3 (564 lb cement/yd 3 times 0.45 w/c ~ 254 lb water/yd 3 ) Chemically bound water at 0.24 w/c: 334 kg cement/m 3 times 0.24 w/c ~ 80 kg water/m 3 (564 lb cement/yd 3 times 0.24 ~ 135 lb water/yd 3 ) Moisture content: 150 kg water/m 3 - 80 kg water/m3times .50 relative humidity ~ 35 kg water/m3 (254 lb water/yd3 – 135 lb water/yd3 times .50 relative humidity ~ 60 lb water/yd3) In reality the relative humidity of the concrete will only reach 50% at the near surface of the concrete and the moisture gradient with depth will increase toward 100% relative humidity so this method of estimation would typically overstate the quantity of moisture available to leave the concrete due to the initial mixing water in the mixture. This is only an estimate of the moisture available to leave the concrete, but may help to give some perspective to the limited amount of water that the concrete can contribute when considering the drying time of hardened concrete.

Relaltive Humidity Profile

Vapor Retarders Concrete Technology Home > Concrete Design & Construction >Vapor Retarders

Vapor retarders are sheet materials used under concrete slabs on ground to restrict the flow of moisture vapor from the subgrade into and through the slab (Figure 1). Moisture migration through concrete slabs can lead Figure 1. A variety of vapor retarders is available for to microbial growths (mold and mildew) and use under concrete floor slabs. These materials offer failures of adhesives, flooring coverings, and a wide range of resistance coatings. Therefore, all to moisture movement and concrete slab-on-ground mechanical properties, such as puncture and tear floors that will receive resistance. floor coverings or coatings must have a vapor retarder below the slab. Even floors that might not initially receive floor coverings should have subslab vapor retarders to reduce humidity in the conditioned space, to prevent mold and mildew, and to provide for future adaptive reuse. Installation of

floor coverings during reuse or remodeling of industrial or warehouse space often leads to failures when the slab lacks a vapor retarder. Vapor retarders must have a permeance rating less than 0.3 perms according to ASTM E1745. There is no standard specification for vapor barrier compared to vapor retarder. However, ACI 302.1R states,  ―True vapor barriers are products that have a permanence (water-vapor transmission rating) of 0.00 perms when tested in accordance with ASTM E96.‖ It is generally accepted in the construction industry that a material having a permeance rating less than 0.01 perms is considered a vapor barrier. Vapor barriers are commercially available with permeability ratings of less than 0.001 g/m 2 hr. Even so, much published literature mistakenly refers to vapor retarders as vapor barriers. Quality portland cement concrete has extremely low permeability to liquid water, but will permit passage of water vapor. It is essential to keep liquid water from contacting the underside of a concrete slab and at the same time minimize the exposure to water vapor. A vapor retarder installed directly under the slab fulfills this dual purpose. Materials with a variety of properties are available as vapor retarders. Specifications for Vapor Retarders Vapor retarders are produced to meet specifications such as ASTM E1745,Standard Specification for Water Vapor Retarders Used in Contact with Soil or Granular Fill Under Concrete Slabs, or ASTM D4397, Standard Specification for Polyethylene Sheeting for Construction, Industrial, and gricultural Applications.ASTM E1745 defines three classes of membranes with a single moisture vapor permeability rating and three levels of physical strength, Class A has the most resistance to tearing and puncture, and Class C the least, as shown in Table 1:

Table 1. Classification of Vapor Retarders* Class A

Class B Class C

0.3 perms

0.3 perms

0.3 perms

5.3 7.9 kN/m Tensile strength (ASTM kN/m (45.0 D828 or D882) (30.0 lbf/in.) lbf/in.)

2.4 kN/m (13.6 lbf/in.)

Puncture resistance (ASTM D1709)

475 g (1 lb)

Water vapor permeance (ASTM E96)

2200 g (5 1700 g lb) (4 lb)

*ASTM E1745 Construction methods may rule out less robust materials (such as simple polyethylene sheeting) if mechanized ride-on screeds are used to consolidate and strikeoff concrete or if many electrical or plumbing penetrations must be sealed.

Location of Vapor Retarder Construction practice and placement of vapor retarders has been the subject of much debate for many years. Some experts believe that concrete placed directly on a vapor retarder will bleed excessively, warp and crack more frequently, and take longer to dry than a slab placed on a compacted granular subbase. Other experts believe that vapor retarders function best to exclude moisture when directly below the concrete with no intervening material that can act as plenum space for the passage of moisture. This debate has resulted in many articles and letters in construction trade  journals. ASTM E1643, Standard Practice for Installation of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under Concrete Slabs, includes an

Appendix with a detailed discussion of materials that should or should not be used above and below the vapor retarder, along with arguments in favor and opposed to cushions, blotters, and protective courses. The 1996 ACI Committee 302 Guide recognized the importance of maintaining the integrity of the vapor retarder and its resistance against moisture transmission by recommending a minimum vapor retarder thickness of 0.25 mm (10 mils). Additionally, a thin layer of fine material was recommended over any crushed stone subbase to protect the underside of the vapor retarder sheet. In April 2001, after much debate, ACI Committee 302 issued an update to its 1996 report defining recommendations for vapor retarders. The Committee now recommends that any floor that will receive a moisture-sensitive finish should have a vapor retarder directly under the concrete slab with no intervening blotter or cushion layer (Figure 2).

Figure 2. Flowchart to determine how vapor retarder should be installed. Note it permits omitting vapor

retarder for floors without floor coverings where humidity will not be controlled, such as unconditioned warehouse space. However, adaptive reuse and installation of flooring in such spaces often leads to flooring problems due to subslab moisture. Therefore, vapor retarders should be considered for use under all floor slabs (adapted from Figure 3-1 of ACI 302.1R-04). Click for larger table.

See Concrete Floors and Moisture, EB119, and Concrete Slabs on Ground , EB075, for more information on vapor retarders.

Careers | Sitemap | Disclaimer | Privacy Policy | © 2012 Portland Cement Association - All Rights Reserved Identifying and Evaluating Concrete Defects Concrete Technology Home > Concrete Design & Construction >Identifying and Evaluating Concrete Defects

Concrete structures are regularly constructed without complications. However, defects can occur that can be traced to problems related to environmental conditions during construction or with the concreting procedures used. In order to determine a repair method, it is necessary to identify what caused the defect. Evaluation of deficiencies helps ensure that repairs will be effective and the defect will not

extend into the surrounding concrete. Identifying Concrete Defects Many concrete defects are immediately recognized and others are not. Being able to properly identify the defect will aid in choosing the best evaluation methods and subsequently the best repair methods. Concrete defects can be broken down into four broad groups based on visual observation: deformation of the surface, cracking of the surface, disintegration of the surface, and other defects. Deformation of the Surface Three defects cause deformation of the concrete surface, but may not exhibit any other symptoms. The first is curling or warping. This is the deformation of the edges and corners of a slab-onground in the absence of any loads. When caused by moisture, this deformation is called warping; when caused by temperature, it is called curling. The second defect is the delamination of surface mortar from underlying concrete. It is difficult to visually observe a delamination before it becomes dislodged from the surface. However, in small discrete locations, the surface may exhibit convex rising called blistering. Blisters are generally isolated, but may be closely spaced and can combine to form a large blister or delamination. A third defect occurs when isolated low spots on the surface collect water and have no means of drainage. These surface water pools are known as ―birdbaths.‖  Cracking of the Surface Cracks appear in concrete for many reasons. Some cracks can appear as secondary symptoms of other defects, such as a long rounded crack following the structural failure of a warped slab. Discussed here are cracks that are primary symptoms of distress, caused by volume changes and structural failure.

Shrinkage cracks have many different looks and can be difficult to distinguish from cracks caused by other mechanisms. Discreet, parallel cracks that look like tearing of the surface are caused by shrinkage while the concrete is still fresh, called plastic shrinkage. Fine random cracks or fissures that may only be seen when the concrete is drying after being moistened are called crazing. This defect may also become evident when a translucent coating is applied to the concrete surface. Cracking that occurs in a three-point pattern is generally caused by drying shrinkage. Large pattern cracking, called map-cracking, can be caused by alkali-silica reaction within the concrete. Structural failure cracking may look like many other types of cracking; however, in slabs they are often associated with subsequent elevation changes, where one side of the crack is be lower than the other. Disintegration of the Surface Disintegration of the surface is generally caused by three types of distress. When laitance forms on the surface, it is called dusting. This can be caused by a number of reasons, which include carbonation of the surface by unventilated heaters or by applying water during finishing. Raveling or spalling at joints occurs when aggregates or pieces of concrete from the joint edges are dislodged. The last form of disintegration is the breaking of pieces from the surface of the concrete generally caused by delaminations and blistering. Popouts are conical fragments that come off the surface, typically leaving a broken aggregate at the bottom of the hole. Popoffs, or mortar flaking, is similar to popouts, except that the aggregate is not broken and the broken piece is generally smaller.

Flaking of the concrete surface over a widespread area is called scaling. Other Defects Other defects include discoloration of the concrete, which is covered in depth by PCA’s Removing Stains and Cleaning Concrete Surfaces, IS214; bugholes, which are small voids in the surface of vertical concrete placements; and honeycombing, which is the presence of large voids in concrete caused by inadequate consolidation.

Evaluating Concrete Defects Visual examination typically does not provide enough information to determine the cause or causes of a defect. In some cases, it may not provide evidence of a defect at all. In order to narrow the scope of an investigation to probable causes and suitable repair methods, the appropriate information factors and the proper evaluation methods need to be identified. Evaluation Information Factors Information about the structural capacity and loading of a concrete placement may provide important details as to the cause of defect or failure. The thickness and the loading of the slab will allow assessment of the inplace strength versus actual loads in use. Crack properties, such as its width and location of reinforcement, will provide information that can be used to determine activity and origin. Joint characteristics, such as spall dimensions and activity, will provide information on the effectiveness of the joint. Many other factors that improve the quality of an investigation are listed in PCA’s Concrete Floors on Ground , EB075. Evaluation Methods The visual examination is typically the first method of evaluation used as it identifies that a problem exists and that further evaluation is needed. Surface elevation profile measurements taken according to ASTM E1155, Standard Test Method for Determining F F  Floor Flatness and F L Floor Levelness Numbers, can be used to quantify deformations of the surface. Sounding according to ASTM D4580, Standard Practice for Measuring Delaminations in Concrete

Bridge Decks by Sounding, can locate delaminations. Coring can be used for a variety of reasons including strength testing according to ASTM C42, Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete, or petrographic examination according to ASTM C856, Standard Practice for Petrographic Examination of Hardened Concrete. Summary There are many different evaluation methods and each provides specific information that can be used to identify or help identify the cause of a specific defect; thereby allowing for proper design of a repair method to ensure a quality repair and mitigation of further damage. Using visual examination as the initial evaluation method allows for identification of the information factors that are needed to complete the evaluation and repair. More information on concrete defect causes, prevention and repair, see PCA’sConcrete Floors on Ground , EB075, Concrete Finisher’s Guide, EB122, andConcrete Slab Surface Defects: Causes, Prevention, Repair , IS177.

Careers | Sitemap | Disclaimer | Privacy Policy | © 2012 Portland Cement Association - All Rights Reserved Quality Control of Pavements Concrete Technology Home > Concrete Design & Construction >Quality control of pavements

Issues and Test Methods The final quality of concrete pavement may be affected by any one or combination of the following

parameterss: 1. Water-cement ratio (w/c) - accurate control of water-cement ratio is a key component to control of strength properties of the material.

2. Aggregate gradation - changes in aggregate gradation, increased fineness or void content, may increase water demand, workability, and paste requirements. These changes may increase the potential for segregation, bleeding, and shrinkage.

3. Mixture setting characteristics - setting characteristics of the concrete mixture are critical to placement, consolidation and finishing practices.

4. Cement fineness - cement fineness affects heat of hydration, water demand, strength gain characteristics, and workability. Changes in fineness can introduce the possibility of incompatibilities between cement and chemical admixtures, shorten setting times, and affect setting characteristics complicating timing for sawing of joints. 5. Unit weight - unit density is an easy method to track sudden changes or mistakes in mixture proportioning.

6. Heat signature of the mixture - rapid changes in the heat signature of the concrete mixture can be useful to identify variations in cement and admixture chemistry as well as

changes in proportioning.

7. Concrete temperature, subgrade temperature, and weather at the time of concrete placement - initial concrete temperature and external influences from subbase temperatures and weather conditions which add to the rate of evaporation effects setting characteristics, setting times, and plastic shrinkage potential

8. Concrete maturity – may be useful to estimate the strength properties of the concrete at any given time to allow the pavement to be placed in service at the most appropriate time.

9. Compressive and flexural strength – strength properties are used as a indication of concrete quality and commonly impacts the pay factors for the owner and contractor.

10. Air content – careful control of air content is required to assure durability of concrete pavements.

11. Concrete permeability – permeability is another good indicator of concrete quality, and is particularly important where the concrete section contain steel reinforcement or embedment.

12. Coefficient of thermal expansion – this property is of importance as it effects the volume change of the pavement throughout its service life. A lower coefficient of thermal expansion improves joint performance and lowers cracking potential. Given the importance of these issues the following table contains test methods that are recommended for quality control of concrete pavements. Table 1. Quality Control Tests for Concrete Pavements (adapted from Taylor and others 2007) Concrete Property

Test name (standard test method)

Laboratory

Workability

Differential scanning calorimetry (DSC)

Central lab

Blaine fineness (ASTM C204/AASHTO Central lab T 153) Combined grading

Mobile lab

Penetration resistance (false set) Mobile lab (ASTM C359/AASHTO T 185) Cementitious materials temperature profile

Mobile lab

Water/cementitious materials ratio (microwave) (AASHTO T 318)

Mobile lab

Unit weight (ASTM Mobile lab C138/AASHTO T 121)

Heat signature

Mobile lab

Concrete temperature, subgrade temperature, project Mobile lab environmental conditions (weather data) Concrete maturity Strength (ASTM development C1074/AASHTO T 325)

Mobile lab

Flexural strength and compressive strength (ASTM C78/ASTM Mobile lab C39/AASHTO T 97/AASHTO T 22) Air content

Air-void analyzer

Mobile lab

Air content (pressure) (ASTM Mobile lab C231/AASHTO T 152) Air content (hardened concrete) (ASTM C457) Chloride ion penetration (ASTM Permeability C1202/AASHTO T 277) Thermal movement

Central lab

Central lab

Coefficient of thermal expansion (ASTM Central lab C531/AASHTO TP 60)

References: Taylor, Peter C.; Voigt, Gerald; Kosmatka, Steven H.; and Brink, Marcia, ―Test Methods for Quality Control in Concrete Paving,‖  Concrete International , Vol. 33, No. 3, March 2011, pages 57 to 62.

Taylor, Peter C.; Kosmatka, Steven H.; Voigt, Gerald; and others, Integrated Materials and Construction Practices for Concrete Pavement: A State-of-the-Practice Manual , National Concrete Pavement Technology Center/Center for Transportation Research and Education, Iowa State University, Ames, Iowa, October 2007, 350 pages.

Careers | Sitemap | Disclaimer | Privacy Policy | © 2012 Portland Cement Association - All Rights Reserved Early-Age Cracking Concrete Technology Home > Concrete Design and Production > Early-age cracking Early-Age Cracking By Matthew D’Ambrosia1 and Nathaniel Mohler 2 Early-age cracking can be a significant problem in concrete. Volume changes in concrete will drive tensile stress development when they are restrained. Cracks can develop when the tensile stress exceeds the tensile strength, which is generally only 10% of the compressive strength. At early ages, this strength is still developing while stresses are generated by volume changes. Controlling the variables that affect volume change can minimize high stresses and cracking. Mechanisms of Early-Age Volume Change The volume of concrete begins to change shortly after it is cast. Early volume changes, within 24 hours, can influence tensile stress and crack formation in hardened concrete. Chemical Shrinkage Chemical shrinkage occurs due to the reduction in absolute volume of solids and liquids in the hydrating paste. Chemical shrinkage continues to occur as long as cement hydrates. After initial set, the paste resists deformation, causing the

formation of voids in the microstructure.  Autogenous Shrinkage Autogenous shrinkage is the dimensional change of cement paste, mortar, or concrete caused by chemical shrinkage (Figure 1). When internal relative humidity is reduced below a given threshold (i.e., extra water is not available), selfdesiccation of the paste occurs, resulting in a uniform reduction of volume.

Figure 1 – Chemical shrinkage and autogenous shrinkage volume changes of fresh concrete. Not to scale. Creep Creep is the time-dependent deformation of concrete under sustained load. During early age, concrete creep is generally as much as 3-5 times higher than for mature concrete. Early load application due to construction forces or prestressing operations can therefore have a significant impact on total deformation. Furthermore, the magnitude of creep in tension i s greater than in compression, and early tensile creep can be relied upon as a stress relaxation mechanism. Creep is influenced by drying or selfdessication at early age, and this synergy is often referred to as the Pickett Effect, after Gerald

Pickett, a PCA researcher who discovered the phenomena in the 1940s (Pickett 1947). Swelling Concrete, mortar, and cement paste will sometimes swell when sealed or in the presence of external water. Swelling is generally caused by pore pressure, but can be accentuated by the formation of some expansive hydration products. The swelling is not significant, between 50-100 millionths at early ages; therefore, we will not be discussing swelling further. Thermal Expansion As cement hydrates, the reaction provides a significant amount of heat. In large elements, this heat is trapped and can induce significant expansion. When thermal changes are superimposed upon autogenous shrinkage at early age, cracking can occur. In particular, differential thermal stress can occur due to rapid cooling of massive concrete elements.

Testing of Early-Age Volume Changes Chemical shrinkage test  Volume change due to chemical shrinkage can be estimated from the hydrated cement phases and their crystal densities or it can be determined by physical test. The physical test places a measured amount of lime-saturated water in an open pipet over a known amount of cement paste inside a closed container. The change in water level within the pipet indicates the change in volume due to chemical shrinkage (Figure 2).

Figure 2 – Test for chemical shrinkage of cement paste showing flask for cement paste and pipet for absorbed water measurement.  ASTM C157 - Modified for early age  shrinkage The standard drying shrinkage test for concrete can be modified to capture early age volume change by elimination of the curing period (usually 7-28 days) and beginning measurements as early as possible. Prisms may also be sealed after casting to provide an estimate of autogenous shrinkage. Surfaces should be sealed as quickly as possible to eliminate loss of moisture. It should be emphasized that autogenous shrinkage depends on temperature history (maturity) and will be different in a laboratory prism when compared to a larger concrete member in service under variable ambient temperatures. As with drying shrinkage measurements, the test result will not represent the actual shrinkage in the structure.  ASTM C1581  Restrained Ring Shrinkage The restrained ring shrinkage test consists of a –

concrete ring specimen 150 mm (6 in.) tall, 13 mm (0.5 in.) thick and 330 mm (13 in.) diameter that is cast surrounding an instrumented steel ring (Figure 3). The steel ring prevents the concrete from shrinking from the time that the concrete is first cast. Shrinkage stresses continue to grow as the concrete passes from initial set to final set and beyond. Tensile creep relaxation alleviates stress development and is considered beneficial at early age.The instrumented ring uses strain sensors to monitor the development of stress. If the shrinkage in the concrete is significant, the stresses will eventually cause cracking. The strain sensors provide an indicator of the cracking time, which is used to compare the cracking tendency between different concrete mixtures.

Figure 3 – Restrained ring shrinkage test setup. (Courtesy of CTLGroup)  ASTM C512 - Compressive Creep The standard creep test consists of a frame and hydraulic loading system to apply constant stress to 150X300 mm (6x12 in.) cylindrical specimens (Figure 4). Deformation is monitored periodically over time and compard to compansion unloaded specimens to obtain the creep strain of the concrete, which can then be used to calculate the creep compliance, or ―specific creep‖ of the material. Tests are typically started at 7 or 28 days of age, but this test can be modified for early age by starting the test as early as 24 hours. Sealed tests are used to evaluate ―basic‖

creep and unsealed tests incorporate the Pickett Effect, or ―drying‖ creep.

Figure 4 – Standard creep test frames. (Courtesy of CTLGroup)

Mitigating Early-Age Cracking Optimization of aggregates to reduce total cementitious content  Since volume changes are more a function of the cement paste, rather than the more volumestable aggregates, reducing the overall cementitious content is the best way to mitigate early-age volume changes. Typical concrete mixtures have gap-graded aggregates that leave significant void space for cement paste to fill. By optimizing the aggregate gradation across the entire spectrum, as opposed to the coarse and fine aggregates individually, the amount of paste required to surround each aggregate particle and fill the void space is minimized (Figure 5); thereby minimizing the effects of early-age volume change of the paste.

Figure 5 – A comparison of void space with different aggregate gradations. Minimum w/cm ratio Autogenous shrinkage increases with a decrease in water to cementitious materials ratio (w/cm). Concrete mixtures with a w/cm of 0.30 can experience autogenous shrinkage upwards of half of the normal drying shrinkage. Using the highest w/cm that still provides adequate strength and durability can reduce the impact of autogenous shrinkage.  Internal curing Internal curing is a method by which water is encapsulated within a concrete mixture for continued release during the hydration process. Typical internal curing materials include high absorption lightweight aggregate particles and super-absorbent polymers. The self-dessication of the paste draws the water out of these particles to continue the hydration of the cement particles. This is particularly helpful in mitigating autogenous shrinkage of concrete mixtures with very low w/cm (0.30 or less). Shrinkage-reducing admixtures

Shrinkage-reducing admixtures (SRAs) are typically used as mitigation of cracking and curling caused by drying shrinkage; however, SRAs can be utilized to mitigate autogenous shrinkage as well. The SRA, typically propylene glycol or polyoxyalkylene alkyl ether based, alters the surface tension of the pore water and reduces the stresses developed during desiccation, whether self-induced or by evaporation. Concreting procedures Several concreting procedures can be used to minimize early-age volume changes. When autogenous shrinkage is a concern, the use of moist curing methods will help mitigate selfdesiccation near the concrete surface. The use of a well-developed thermal control plan will mitigate the effects of thermal-based volume changes.

References Kosmatka, Steven H.; Wilson, Michelle L.; Design and Control of Concrete Mixtures , EB001.15, 15th edition, Portland Cement Association, Skokie, Illinois, USA, 2011. Pickett, Gerald, The Effect of Change in Moisture Content on the Creep of Concrete Under a Sustained Load , Research Department Bulletin RX020, Portland Cement Association, 1947. Notes 1 Associate, CTLGroup 2 Concrete Engineer, PCA

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Durability Concrete Technology Home > Durability

Durability is the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. Different concretes require different degrees of durability depending on the exposure environment and the properties desired. Concrete ingredients, their proportioning, interactions between them, placing and curing practices, and the service environment determine the ultimate durability and life of the concrete. Exposure Conditions and Deterioration Mechanisms The table below shows important exposure conditions and deterioration mechanisms in concrete co ncrete structures. In practice, several of these deterioration mechanisms can act simultaneously with possible synergistic effects. Durablility Test Aspect/Exposu Mechanism Method Resources re s

AlkaliAggregate Reaction

ASTM C227 ASTM C289 ASTM Alkali-Silica C441 Reaction ASTM C586 AlkaliASTM Carbonate C856 Reaction ASTM C1260 ASTM C1105

Publications Research Reports PowerPoint presentation s/ Images

Links

ASTM C1293 ASTM C1567 Los Alamos Method More.. Sulfates 

Chemical Resistance

DEF

Seawater Acids

ASTM C1012 ASTM D516 ASTM C1582

Publications Links

ASTM C1202 AASHTO T 259 ASTM C1556 AASHTO T 260 Corrosion ASTM C1152 Publications Corrosion ASTM Resistance C1218 ASTM Links Carbonation C1524 Corrosion of AASHTO Reinforcement TP 11 AASHTO TP 22 AASHTO TP 26 AASHTO TP 55

Freezing and Thawing Deicer Scaling Freeze-Thaw

D-Cracking

ASTM C666 AASHTO T 161 Publications AASHTO TP 18 Links ASTM C457 ASTM C672

ASTM C131 ASTM Erosion C535 Publications ASTM Fire C3744 Resistance Links ASTM Miscellaneous C1137 Efflorescenc AASHTO e TP 58 Abrasion

Guide To Durable Concrete Different concretes require different degrees of durability depending on the exposure environment and the properties desired. The Specifer’s Guide for Durable Concrete is intended to provide sufficient information to allow the practitioner to select materials and mix design parameters to achieve durable concrete in a variety of environments.

Addressing Durability with the Prescriptive or Performance Approach Durability of concrete can be addressed by two approaches. The first is called the ―prescriptive‖ approach, where designers specify materials,

proportions, and construction methods based on fundamental principles and practices that exhibit satisfactory performance. The second is called the  ―performance‖ approach, where designers identify functional requirements such as strength, durability, and volume changes, and rely on concrete producers and contractors to develop concrete mixtures to meet those requirements. Performance specifications define performance for a given exposure and life expectancy, and include tests, which are tied to the field performance of concrete. Refer to NRMCA’s Performance-Based Specifications for Concrete for details. Very often a specification will contain prescriptive as well as performance elements. Click here for durability requirements of the 2008 ACI 318 Building Code. Case Studies Confederation Bridge. Concrete used for the Confederation Bridge across the Northumberland Strait between Prince Edward Island and New Brunswick was specifically designed for high durability in a severe environment. The bridge has to resist freezing and thawing, seawater exposure, and abrasion from floating ice. With a design life of 100 years, the use of high performance concrete and careful attention to production and construction practices were imperative. Over 400,000 cubic meters (520,000 cubic yards) of concrete was used for the structure. More. Wacker Drive. Determined to build maximum durability into the heavily traveled Wacker Drive thoroughfare, the City of Chicago set an ambitious goal: a 75- to 100-year design life of the structure. The material of choice? High-performance concrete. According to Paul Krauss, senior consultant with WJE, the mix was designed with

the goal of maximizing durability, not compressive strength. More.

Concrete in the Marine Environment Island Marine Exposure Site

Treat

—

Located on the Bay of Fundy near Eastport, Maine, the Treat Island exposure station was established by the US Army Corps of Engineers (USACE) in 1936 to study concrete durability in long-term programs. The exposure site inherently imposes a unique combination of natural severe environmental conditions ideally representative of severe field exposure conditions. Under the heading of ―Improved Durability of Concrete,‖ the field exposure durability studies at Treat Island have provided data useful in determining concrete’s resistance to frost attack, alkali -aggregate reaction, sulfate attack, and corrosion of steel with variable cementitious types and contents, aggregate types, chemical admixtures, and water-cementitous ratios. More. Preventing Joint Deterioration Concrete provides a durable, attractive paved surface for roads. It provides a longer life than any other leading paving materials and does not rut or shove due to traffic or warm temperatures. Concrete’s higher reflectance keeps surfaces cooler, helps minimize the urban heat island effect, and can lower infrastructure and ongoing lighting costs, while boosting safety for vehicles and pedestrians. Some pavements in northern states have exhibited

 joint deterioration: the premature disintegration of concrete around the joint. This deterioration typically begins to show after several years in place. The mechanism behind joint deterioration is complex and is understood to have basis in freeze-thaw damage and mechanical damage. More. References

Detwiler, R. J., and Taylor, P. C., Specifier’s Guide to Durable Concrete, EB221, Portland Cement Association, Skokie, Illinois, USA, 2005, 68 pages. References related to: 



Alkali-Aggregate Reaction Chemical Resistance (Sulfates/DEF/Seawater/Acid)

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Steel Corrosion



Freeze-Thaw Resistance

Kerkhoff, Beatrix, Effects of Substances on Concrete and Guide to Protective Treatments, IS001, Portland Cement Association, 2001, 36 pages. Miller, F. M.; Detwiler, R.; and Powers, L., Investigation of Deteriorated Concrete in Pavement , Portland Cement Association, Skokie, Illinois, August, 2000. Pavement Durability: A Case Study   , Concrete Technology Today, Vol. 21, No. 2, CT002, Portland Cement Association, Skokie, Illinois, July, 2000 SHRP, Distress Identification Manual for the Long-

Term Pavement Performance Project . SHRP-P-338, Strategic Highway Research Program, Washington, DC, 1993 Sutter, L. L.; Peterson, K. R.; Van Dam, T. J.; Smith, K. D. and Wade, M. J.;Guidelines for Detection, nalysis, and Treatment of Materials-Related Distress (MRD) In Concrete Pavements 



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Guidelines for Detection, Analysis, and Treatment of MRD in Concrete Pavements Vol I – Final Report   , FHWA-RD-01-163, March 2002. Guidelines for Detection, Analysis, and Treatment of MRD in Concrete Pavements Vol II – Guidelines Description and Use, FHWA-RD-01164, March 2002. Guidelines for Detection, Analysis, and Treatment of MRD in Concrete Pavements Vol III – Case Studies, FHWA-RD-01-165, March 2002.

PCA, Types and Causes of Concrete Deterioration, IS536, Portland Cement Association, 2002. PCA, Concrete Slab Surface Defects: Causes, Prevention, Repair , IS177, Portland Cement Association, 2002 PowerPoint Presentations/Images Powerpoints Identification of Alkali-Silica Reactivity in Highway Structures, PT315 Concrete Slab Surface Defects, PT177 Images

To search for related images, click here. Library Search PCA’s library, which include over 22,000 books, reports, standards, journals, electronic documents, and videos. A list of many of the durability-related publications may be found HERE. Although library loans are available only to PCA Members, the Library staff is happy to assist nonmembers in obtaining items through interlibrary loan. In addition, the Library has access to many technical databases and can provide literature searches on particular aspects of durability. The result is a bibliography, often with abstracts, of articles, conference papers, reports, patents, and other literature on that topic. If the requestor then requires copies of the actual papers, the Library can provide that as well. Major journals addressing durability issues include  ACI Materials Journal , Cement and Concrete Research, Cement and Concrete Composites, Concrete International, Concrete Producer, Materials and Structures. For subscription information, contact the Library at [email protected]. Links

Bugholes Concrete Technology Home > Concrete Design & Construction >Bugholes

Bugholes The ever-increasing use of structural concrete as an architectural building material has catapulted concrete’s measure of quality in surface appearance to a prominent position within the concrete construction industry. One of the primary influences affecting the surface aesthetics of concrete is bugholes. Bugholes are surface voids that result from the migration of entrapped air (and to a lesser extent water) to the fresh concrete-form interface. These surface defects manifest themselves mostly in vertically formed surfaces. During consolidation, the densification and subsequent volume shrinkage of the fresh concrete forces entrapped air voids and excess water out of the cementitious matrix. The water tends to migrate upward due to a density differential and become bleed water. The air bubbles, however, seek the nearest route to reach pressure equilibrium. When in a vertical form, the closest distance for the air bubbles’ migration is to the interior form surface. If these bubbles are not directed vertically to the free surface of the setting concrete, after form removal, bugholes will be present if not abundant. Bugholes are found more frequently in the upper portion of the concrete structure or at angled form surfaces as a result of additive accumulation from escaping air voids along the height of the structure. These surface voids are primarily an aesthetic problem for exposed structural concrete. However, problems do arise if the concrete surface is to be painted or if the voids reach a larger diameter (typically greater than 25mm [1 in.]).

Causes Perhaps the most influential cause of bugholes is improper vibration. Consolidation, usually through vibration, sets the air and water bubbles into motion. A proper amount of vibration sends both entrapped air and excess water to the free surface of the concrete – either vertically winding through the matrix or laterally in a direct route to the form wall. Improper vibration will either insufficiently liberate the voids or over-consolidate the concrete resulting in segregation and bleeding. (See ACI 309 or PCA’s Design and Control of Concrete Mixtures for a full description of consolidation using vibration.) Another factor that promotes bughole formation is the form material itself. Nonpermeable forms (i.e. polymer impregnated wood and steel) and the use of form-releasing agents can restrict the movement of the air voids between the concrete-form interface that is necessary for bughole reduction. It is imperative that when form-releasing agents are used, they are used according to the manufacturer’s recommendations and used only with specified form material. Mix design can also be considered a significant contributor to bughole formation. Mix designs vary widely in their use of aggregate type, size, and grading and their use of admixtures and airentrainment. A sticky or stiff mixture that does not respond to consolidation can be directly linked to increased surface void formation. Cure (1) Proper consolidation. Vibration should be completed with each lift of concrete placed. As successive lifts are placed, the vibrator should penetrate the previous lift, working the entrapped air towards the form and then vertically up the sides.

(2) Permeable Forms. When impermeable forms are used, more vibration is necessary to move the air voids to the free surface of the concrete. The use of permeable forms has been shown through research to reduce bugholes significantly by allowing escaping air to move through the form to the ambient air. Choosing the proper form releasing agent in the proper amount can also improve the surface quality. (3) Mix design. Workable, flowing mixtures are easier to place and consolidate and therefore reduce the risk of bughole formation. Concrete with an optimally graded aggregate that avoids excessive quantities of fine aggregate, properly proportioned cement content, and any admixture that provides increased flow, workability, or ease of consolidation contributes to bughole reduction. Self-Consolidating Concrete (SCC) is becoming increasing popular for industries (precast especially) to improve surface quality. Bugholes as surface defects are not detrimental to structural concrete from a durability standpoint. Nevertheless, with the increased use of structural concrete in finished construction, surface quality is increasingly important. Through careful selection of materials, quality workmanship, and dutiful supervision, surface voids can be minimized.

Related Publications: Bugholes in Concrete Surfaces: Annotated Bibliography (LB14) Guide for Surface Finish of Formed Concrete (LT233) Painting Concrete (IS134)

Concrete Slab Surface Defects: Causes, Prevention, Repair (IS177) References: Berger, Dean M., ―Preparing concrete surfaces for painting,‖ Concrete Construction, v. 22, no. 9, Sept. 1977, p. 481-484, 526-527. (ftp://209.143.248.167/woc/C770481.pdf) Identification and Control of Visible Effects of Consolidation on Formed Concrete Surfaces (ACI309.2R-98) Reported by ACI Committee 309. American Concrete Institute, 2003, p. 6, 11, 1819. Price, W.F.; Widdows, S.J., ―The effects of permeable formwork on the surface properties of concrete,‖ Magazine of Concrete Research, v. 43, no. 155, 1991, p. 93-104. Samuelsson, Paul, ―Voids in concrete surfaces,‖  ACI ournal, Proceedings, v. 67, no. 11, Nov. 1970, p. 868-874.

Careers | Sitemap | Disclaimer | Privacy Policy | © 2012 Portland Cement Association - All Rights Reserved Pervious Concrete Concrete Technology Home > Concrete Construction > Pervious Concrete and Durability Placement, Finishing, and Curing of Pervious Concrete Pavement Pervious concrete mixtures are stiff, zero-slump mixtures with placement, finishing, and curing requirements falling outside of normal concrete

flatwork processes. Due to the dry nature of the mixture (w/cm < 0.35) and high surface area, it is important to consider rapid concrete placement methods. As these mixtures are not appropriate for pumping, rapid placement methods may include chute placement directly from the truck mixers, wheel barrows or buggies, conveyors, or dump placement into an asphalt type paving machines. Regardless of the placement method, the quicker the placement is, the better. Strike-off of the pavement may also be accomplished in a number of ways: 1. While strike-off with a simple straight edge is rare it is acceptable as long as the forms are elevated with a removable strip 50 mm to 75 mm (½ in. to ¾ in.) in thickness which can be removed to allow compaction of the pavement using a steel roller. 2. Low frequency vibrating screeds and asphalt pavers may also be used for strike-off. Again it is recommended to strike-off slightly higher than the final elevation and compact with a roller to achieve the final height. 3. Hydraulically powered roller screeds rotate against the direction of travel providing strikeoff and compaction in a single operation.

Finishing of pervious pavement requires a minimum of effort and is simple. After strike-off and compaction with a roller, a finned roller (sometimes referred to as a

pizza cutter) is used to tool joints, and the edges are compacted with an edging tool. Additional decorative treatments can be added such as stamped impressions using braided rope over plastic and an additional pass with the roller to embed the shape of the rope. Other more traditional stamping tools may also be used provided the operation is done in rapid order. The reason rapid placement and finishing techniques are stressed is due to curing requirements for pervious pavements. Since the moisture content of the mixture is initially so low it is particularly susceptible to moisture loss that would inhibit hydration of the cement. For this reason curing is typically specified as a minimum 0.15 mm (6 mil) plastic cover placed and securely anchored within 15 minutes of the pervious mixture being discharged from the mixer truck. Additionally, white plastic is used for hot weather to reduce the solar loading on the pavement. Reference: Tennis, Paul, D.; Leming, Michael, L.; and Akers, David, J., Pervious Concrete Pavements, EB302, Portland Cement Association, Skokie, Illinois, and National Ready Mixed Concrete Association, Silver Spring, Maryland, USA, 2004, 36 pages.

 Air-Entrained Concrete Concrete Technology Home > Concrete Design & Construction >Finishing Air-Entrained Concrete

Finishing Air-Entrained Concrete

Used in many applications, air-entrained concrete uses a chemical admixture (or sometimes, air-entraining cement) to produce a system of small voids during the mixing process. These voids are stabilized by the air-entraining admixture and remain in the hardened concrete paste. The primary use of air-entraining concrete is for freeze-thaw resistance. The air voids provide pressure relief sites during a freeze event, allowing the water inside the concrete to freeze without inducing large internal stresses. Another related use is for deicer-scaling resistance. The air voids again provide relief sites for the buildup of salt concentrations and the pressures that result due to concentration gradients. Other uses of air-entrained concrete include sulfate resistance, resistance to alkali-silica reactivity, and improved workability. Hard troweling is a process by which a finisher uses a steel trowel to densify the surface of the concrete. This finish is optional and produces a hard, smooth surface. Repeated passes of steel troweling will result in a burnished finish, which has a mirror-like appearance. Hard-troweled surfaces are not recommended for exterior concrete slabs, because the smooth finish becomes slippery when wet. Hard troweling is also not recommended for airentrained concrete for several reasons. The primary reason is densification. Densification pushes air out of the surface, leading to a decrease in the air content. This increases the probability of freeze-thaw damage and deicer-scaling damage. Additionally, the densification can push the air down and rupture the voids, leading to a large void just below the surface mortar. This will cause delaminations and may cover a significant portion of the placement. Once the surface has been opened, through scaling or

delamination, the densified surface gained through hard troweling is lost. Many alternatives to hardtroweled surfaces exist for concrete. A simple broom texture is the easiest method. A broom texture provides a slip resistant and durable surface. Other options include decorative finishes, such as stamped or colored concrete.

Careers | Sitemap | Disclaimer | Privacy Policy | © 2012 Portland Cement Association - All Rights Reserved Curing Concrete in Construction By Jerzy Z. Zemajtis, Ph.D., PE (WA)* Curing plays an important role on strength development and durability of concrete. Curing takes place immediately after concrete placing and finishing, and involves maintenance of desired moisture and temperature conditions, both at depth and near the surface, for extended periods of time. Properly cured concrete has an adequate amount of moisture for continued hydration and development of strength, volume stability, resistance to freezing and thawing, and abrasion and scaling resistance. The length of adequate curing time is dependent on the following factors: 

Type of cementitious materials used



Mixture proportions



Specified strength



Size and shape of concrete member



Ambient weather conditions



Future exposure conditions

Slabs on ground (e.g. pavements, sidewalks, parking lots, driveways, floors, canal linings) and structural concrete (e.g. bridge decks, piers, columns, beams, slabs, small footings, cast-in-place walls, retaining walls) require a minimum curing period of seven days for ambient temperatures above 5°C (40°F) 1. American Concrete Institute (ACI) Committee 301 recommends a minimum curing period corresponding to concrete attaining 70% of the specified compressive strength 2. The often specified 7-day curing commonly corresponds to approximately 70% of the specified compressive strengths. The 70% strength level can be reached sooner when concrete cures at higher temperatures or when certain cement/admixture combinations are used. Similarly, longer time may be needed for different material combinations and/or lower curing temperatures. For this reason, ACI Committee 308 recommends the following minimum curing periods 3: 

ASTM C 150 Type I cement 7 days



ASTM C 150 Type II cement 10 days



ASTM C 150 Type III cement 3 days

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ASTM C 150 Type IV or V cement 14 days



ASTM C 595, C 845, C 1157 cements variable

Effect of curing duration on compressive strength development is presented in Figure 1 1.

Figure 1. Moist Curing Time and Compressive Strength Gain Higher curing temperatures promote an early

strength gain in concrete but may decrease its 28day strength. Effect of curing temperature on compressive strength development is presented in Figure 21.

Figure 2. Effect of Curing Temperature on Compressive Strength

There are three main functions of curing: 1) Maintaining mixing water in concrete during the early hardening process a. Ponding and immersion Ponding is typically used to cure flat surfaces on smaller jobs. Care should be taken to maintain curing water temperature at not more than 11°C (20°F) cooler than the concrete to prevent cracking due to thermal stresses. Immersion is mainly used in the laboratory for curing concrete test specimens. b. Spraying and fogging

Spraying and fogging are used when the ambient temperatures are well above freezing and the humidity is low. Fogging can minimize plastic shrinkage cracking until the concrete attains final set. c. Saturated wet coverings Wet coverings saturated with water should be used after concrete has hardened enough to prevent surface damage. They should be kept constantly wet. d. Left in Place Forms Left in place forms usually provide satisfactory protection against moisture loss for formed concrete surfaces. The forms are usually left in place as long as the construction schedule allows. If the forms are made of wood, they should be kept moist, especially during hot, dry weather.

2) Reducing the loss of mixing water from the surface of the concrete a. Covering concrete with impervious paper or plastic sheets Impervious paper and plastic sheets can be applied on thoroughly wetted concrete. The concrete surface should be hard enough to prevent surface damage from placement activities. b. Applying membrane-forming curing compounds Membrane-forming curing compounds are used to retard or reduce evaporation of moisture from concrete. They can be clear or translucent and white pigmented. White-pigmented compounds are recommended for hot and sunny weather conditions to reflect solar radiation. Curing compounds should be applied immediately after final finishing. Curing compound shall comply with ASTM C309 4 or ASTM C13155.

3) Accelerating strength gain using heat and additional moisture a. Live steam Live steam at atmospheric pressure and highpressure steam in autoclaves are the two methods of steam curing. Steam temperature for live steam at atmospheric pressure should be kept at about 60°C (140°F) or less until the desired concrete strength is achieved. b. Heating coils Heating coils are usually used as embedded elements near the surface of concrete elements. Their purpose is to protect concrete from freezing during cold weather concreting. c. Electrical heated forms or pads Electrical heated forms or pads are primarily used by precast concrete producers. d. Concrete blankets Concrete insulation blankets are used to cover and insulate concrete surfaces subjected to freezing temperatures during the curing period. The concrete should be hard enough to prevent surface damage when covering with concrete blankets.

Other forms of curing include internal moist curing with lightweight aggregates or absorbent polymer particles. For mass concrete elements (usually thicker than 3 ft.), a thermal control plan is usually developed to help control thermal stresses. Additional information can be found in ACI Committee 308 report Guide to Curing Concrete 3. For specialty concretes, it is recommended to refer to other ACI reports as follows: 

Refractory concrete ACI 547.1R

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Insulating concrete ACI 523.1R

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Expansive cement concrete ACI 223

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Roller-compacted concrete ACI 207.5R

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Architectural concrete ACI 303R

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Shotcrete ACI 506.2

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Fiber-reinforced concrete ACI 544.3R

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Vertical slipform construction ACI 313

Curing in either cold or hot weather requires additional attention. In cold weather, some of the procedures include heated enclosures, evaporation reducers, curing compounds, and insulating blankets. The temperature of fresh concrete shall be above 10°C (50°F). The curing period for cold weather concrete is longer than the standard period due to reduced rate of strength gain. Compressive strength of concrete cured and maintained at 10°C (50°F) is expected to gain strength half as quickly as concrete cured at 23°C (73°F). In hot weather, curing and protection are critical due to rapid moisture loss from fresh concrete. The curing

actually starts before concrete is placed by wetting substrate surfaces with water. Sunscreens, windscreens, fogging, and evaporation retardants can be used for hot weather concrete placements. Since concrete strength gain in hot weather is faster, curing period may be reduced. Additional information can be found in ACI 306.1, Standard Specification for Cold Weather Concreting, ACI 306R, Cold Weather Concreting, ACI 305.1, Specification for Hot Weather Concreting, and ACI 305R, Hot Weather Concreting.

Curing Concrete Test Specimens Curing of concrete test specimens is usually different from concrete placed during construction. American Society for Testing and Materials (ASTM) has developed two standards for making and curing concrete specimens. ASTM C192 6 is intended for laboratory samples while ASTM C31 7 is intended for field samples. Both documents provide standardized requirements for making, curing, protecting, and transporting concrete test specimens under field or laboratory conditions, respectively. ASTM C192 provides procedures for evaluation of different mixtures in laboratory conditions. It is usually used in the initial stage of the project, or for research purposes. ASTM C31 is used for acceptance testing and can also be used as a decision tool for form or shoring removal. Depending on its intended purpose, the standard defines two curing regimes: standard curing for acceptance testing and field curing for form/shoring removal. Variation in standard curing of test specimens can dramatically affect measured concrete properties. According to the National Ready Mix Concrete Association 8 (NRMCA), strength for concrete air cured for one day followed by 27 days moist cured will be approximately 8% lower than for

concrete moist cured for the entire period. The strength reduction is 11% and 18% for concrete specimens initially cured in air for 3 days and 7 days, respectively. For the same air/moist curing combinations, but 38°C (100°F) air curing temperature, the 28-day strength will be approximately 11%, 22%, and 26% lower, respectively. * Jerzy Z. Zemajtis, Ph.D., PE (WA) Senior Engineer, CTLGroup, Skokie, IL (847) 832-0260, [email protected] References: 1

S. Kosmatka et al, Design and Control of Concrete Mixtures, 14th Edition, PCA Engineering Bulletin EB 001, Portland Cement Association , Skokie, IL 2002 2

Specifications for Structural Concrete, ACI 301 (www.concrete.org) 3

 Guide to Curing Concrete, ACI 308R-01 (www.concrete.org) 4

 ASTM C309, Standard Specification for Liquid Membrane-Forming Compounds for Curing Concrete (www.astm.org) 5

 ASTM C1315, Standard Specification for Liquid Membrane-Forming Compounds Having Special Properties for Curing and Sealing Concrete (www.astm.org) 6

ASTM C192 / C192M, Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory (www.astm.org) 7

ASTM C31 / C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field  (www.astm.org)

8

 David N. Richardson, Review of Variables that Influence Measured Concrete Compressive Strength , NRMCA Publication 179, NRMCA, Silver Spring, MD, 1991.

The Link Between Concrete Sustainability and Curing

Sustainability, according to the Bruntland Report and adopted by many experts, is development that meets the needs of the present without compromising the ability of future generations to meet their own needs. This can be accomplished in one of two ways: either by using recyclable, reusable, or so little resources that future generations have the same access to them; or by producing development that meets our needs as well as the needs of future generations. We can use proper curing of concrete to advance towards the reduction of resource use. A concrete element is expected to last a certain number of years. In order to meet this expected service life, it must be able to withstand structural loading, fatigue, weathering, abrasion, and chemical attack. The duration and type of curing plays a big role in determining the required materials necessary to achieve the high level of quality. Curing is the process in which the concrete is protected from loss of moisture and kept within a reasonable temperature range. The result of this process is increased strength and decreased permeability. Curing is also a key player in mitigating cracks in the concrete, which severely impacts durability. Cracks allow open access for

harmful materials to bypass the low permeability concrete near the surface. Good curing can help mitigate the appearance of unplanned cracking. When smart, suitable, and practical curing is used, the amount of cement required to achieve a given strength and durability can be reduced by either omission or replacement with supplementary cementitious materials. Since the cement is the most expensive and energy intensive portion of a concrete mixture, this leads to a reduction in the cost as well as the absolute carbon footprint of the concrete mixture. Additionally, being practical with curing methods can enhance sustainability by reducing the need for resource intensive conditioning treatments, should the curing method be incompatible with the intended service environment.

Curing Pavements and Bridge Decks

While curing of concrete is an important issue with all concrete applications concrete pavements and bridge decks require careful consideration and have significantly different needs with regard to curing of the concrete of these structures. Both categories have basic requirements for the durability of the structures including strength, abrasion resistance, freezing and thawing and deicer resistance, and, in the case of bridges, low permeability for corrosion protection of the reinforcement of the structure. Typical recommendations for curing of pavements allow the use of sheet curing, moist curing, or application of a film forming curing compound. Due to the large surface areas typical of concrete

paving the application of curing compound to all exposed surfaces is the most common curing method. Moist curing and sheet curing of large surface areas may become cost prohibitive due to the large quantity of materials required to cover the full surface of concrete placed in any single day. In addition moist curing and sheet curing require maintenance to assure the curing method is properly completed for the full time duration chosen for paving (typically 7 days). Moist coverings require rewetting and sheet goods are prone to being disturbed by wind, either of which would reduce the effectiveness of the curing method. Curing compounds should be applied to pavements as soon as possible after bleed water has left the surface of the concrete at a rate of 5 m 2 /L (200 ft2 /gal) for standard mixtures and application, 3.75 m2 /L (150 ft2 /gal) for fast track paving, and 2 1/2 m2 /L (100 ft2 /gal) for slabs thinner than 125 mm (5.0 in.) In contrast concrete bridges require a higher standard of curing to achieve the low permeability required for protection of steel reinforcement. Standard recommendations for curing bridge decks is moist curing for a minimum of 7 days for concrete mixtures containing only portland cement and as long as 14 days when supplementary cementing materials are included in the concrete mixture. Some states also require the application of curing compound upon removal of the moist curing methods. Typical moist curing for bridge decks requires the application of adequate quality water saturated burlap or other approved absorptive material covered with minimum 6 mil plastic covering. The temperature of the saturated materials should be within 11°C (20°F) of the temperature of the inplace concrete. In most cases plastic will be specified

to be white in color to reflect solar radiation, reducing the temperature rise beneath the plastic, while cold temperatures (less than 10°C (50°F)) may allow the use of black plastic to add heat to the system. Proper moist curing will also require uncovering and rewetting the absorptive material to assure that there is a constant supply of water available to satisfy the evaporation rate at the project site. References Design and Control of Concrete Mixtures , 14th Edition, EB001 Construction Specification Guidelines for Concrete Streets and Local Roads, IS119 HPC Bridge Views, Issue No. 45, Fall 2006 Read an FAQ  on "What should be considered when choosing a curing method for slabs that will receive a moisture sensitive floor covering?"

Careers | Sitemap | Disclaimer | Privacy Policy | © 2012 Portland Cement Association - All Rights Reserved Mass Concrete Concrete Technology Home > Concrete Design and Production > Mass Concrete Mass Concrete How Do You Handle the Heat? By John Gajda, PE  —

Mass concrete is a hot topic. Owners desire long service lives so engineers design concrete mixes for low permeability. These mixtures typically have high cementitious material contents, which results in high temperatures within Mass concrete columns the concrete. To avoid and footings for the cracking and other temperature related damage James River Bridge. to the concrete, contractors (Courtesy of Fred Parkinson, PB.) must control the maximum temperature and temperature difference between the interior and the surface of the concrete. This can pit the schedule against the service life. When all involved parties work together, appropriate changes can be made to achieve the desired service life with minimal impacts to the schedule. The key is an understanding of mass concrete. First of all, what is mass concrete?  Mass concrete is defined by the American Concrete Institute (ACI) as: Any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from hydration of the cement and attendant volume change to minimize cracking. While this is a perfect definition, the question is often asked, ―so, is this placement considered mass concrete?‖ As a general rule of thumb, any placement of structural concrete with a minimum dimension equal to or greater than 1 meter (36 in.) should be considered mass concrete. Similar considerations should be given to other concrete placements that do not meet this minimum dimension, but contain ASTM C150 Type III or ASTM C1157 HE cement, accelerating admixtures, or cementitious materials in excess of 355 kg/m3 (600 lb/yd3) of concrete.

Now that we know what placements are considered mass concrete, what makes a mass concrete placement any different than a typical placement? The answer is that temperatures A severe case of in a mass concrete thermal cracking in a placement can get high concrete footing. enough to damage the concrete. All concretes generate heat. Heat is a byproduct of the hydration reactions which gives concrete its strength and durability. In most placements, the heat escapes almost as rapidly as it is generated. In a mass concrete placement, the heat escapes more slowly than it is generated. The result is that temperatures within the concrete can get quite hot. If the internal temperature exceeds 70°C (158°F), the long term durability of some concretes can be affected by delayed ettringite formation (DEF). DEF is rare and only certain concretes can be affected. When DEF occurs, the concrete paste expands and cracks the concrete with detrimental results, which may not be evident for many years. Additionally, while the interior can be quite hot, the surface can be relatively cool. The resulting large temperature difference results in large thermal stresses which can cause cracking of the surface. Historically, limiting the temperature difference between the interior and surface so that it is less than 20°C (35°F) has been found to prevent or minimize thermal cracking. Certain concretes are more tolerant of thermal cracking than others, and these concretes can withstand a higher temperature difference without thermally cracking. How do I prevent high internal temperatures and large temperature differences? The first step is to select an appropriate mix design. This will reduce other efforts to control temperatures and

temperature differences after placement. The temperature rise of concrete is directly related to the types and quantities of cementitious materials in the concrete. An appropriate mix design contains the least amount of cementitious materials needed for strength and durability. Placeability of concrete must also factor into the concrete mix design. This sometimes increases the cementitious content. To reduce heat of hydration, Class F fly ash or slag cement is typically used to replace a portion of the cement. The percentage depends on several factors including environmental exposure and durability requirements. Once I have a reasonable concrete mix design, do I need to do anything else? In most cases, the answer is yes; two items must be considered. First, you must ensure that the maximum temperature in the concrete will not exceed 70°C (158°F). In placements over about 1.80 m (6 ft) thick, the maximum temperature is the sum of the installed concrete temperature plus the temperature rise of the Concrete concrete. The temperature rise insulating can be measured or estimated. If blankets on a the maximum temperature of the column. concrete is predicted to exceed 70°C (158°F), the concrete can be precooled by using chilled batch water, substituting ice for a portion of the batch water, or by liquid nitrogen injection into the fresh concrete. If significant precooling is required, internal cooling pipes can be used to reduce the amount of precooling. Second, the concrete surface will likely also need to be insulated. Insulation is needed to limit the temperature difference between the center and

surface so that thermal cracking is prevented or minimized. One or two layers of concrete insulating blankets are often used. Thermal modeling is sometimes done to optimize the amount of insulation and precooling, so that the most cost-effective measures are used. Do I need to do anything else?  To document the means and methods that are required and will be used, a thermal control plan should be developed. A thermal control plan is similar to a quality control plan, and will allow all involved parties to agree on the measures that will be used, and the expected results. Such measures may include precooling of the concrete, cooling pipe installation and operation, insulation, temperature monitoring equipment and locations. Additional and more detailed information on mass concrete can be found in PCA’s publication Mass Concrete for Buildings and Bridges (EB547). Read a case study on the San Francisco-Oakland Bay Bridge. John Gajda, PE, Principal Engineer CTLGroup, Skokie, Ill. [email protected]

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Building a Better (Cleaner) World in the 21st Century Self-cleaning buildings and pollution-reducing roadways: These may sound like futuristic ideas, but they are realities of some of today’s concrete. Recently introduced formulations of cement are able to neutralize pollution. Harmful smog can be turned into harmless compounds and washed away. Anything made out of concrete is a potential application, because these cements are used in the same manner as regular portland cements. These products provide value through unique architectural and environmental performance capabilities. Proprietary technology (based on particles of titanium dioxide) is what makes this cement special. The technology can be applied to white or gray cement and it works like any other portland cement: it can be used in all varieties of concrete, including plaster. Presumably, applications for mortar might be beneficial, too., although the mortar has a smaller surface area. The only difference is that it is capable of breaking down smog or other pollution that has attached itself to the concrete substrate, in a process known as photocatalysis. As sunlight hits the surface, most organic and some inorganic pollutants are neutralized. They would otherwise lead to discolored concrete surfaces. The titanium-based catalyst is not spent as it breaks down pollution, but continues to work. Typical products are oxygen, water, carbon dioxide, nitrate, and sulfate. Because rain washes away the pollution from the concrete surface, buildings stay cleaner and do not require chemical applications that are potentially harmful to the environment. Maintenance costs are reduced. This is true even for buildings in

highly polluted locations—one noted application is the Air France headquarters at Roissy-Charles de Gaulle International Airport near Paris, a white concrete building that has remained white. Another is the Church of the Year 2000 in Rome. Clean buildings are great: A perhaps even more astounding environmental benefit is the potential for cleaner air. Concrete products that are exposed to sunlight throughout their life, like precast building panels, pavers, and roof tiles, are especially suited to manufacture with photocatalytic cement. For instance, city streets made with special pavers are capable of reducing the pollution at its source — where it comes out of the tailpipe. Photocatalytic Pavements Reduce Air Pollution The need for pavements is ubiquitous. As our population increases, more roads are needed to get people and goods from place to place. Unfortunately, with this added traffic comes a higher concentration of air pollution. To reduce our current and future levels of air pollution, more attention is being given to the use of photocatalytic concrete in pavements. The internal combustion engine is used everywhere in the United States, from cars to trains to jets. These engines produce the power required to travel, but also emit pollution. The primary pollution from fossil fuel combustion is carbon dioxide (CO 2), but other gasses are formed, such as NOx and SOx, that can lead to acid rain, smog, and respiratory issues.According to the EPA, 34% of the national NOx emissions come from vehicles on roads.

Photocatalytic concrete contains titanium dioxide particles that act as the catalyst for the natural breakdown of NOx into nitrates in sunlight. This occurs at the surface of the concrete, where the nitrates can be easily washed away. Without the catalyst, the NOx will breakdown in the atmosphere, creating photochemical smog and ground level ozone. With an abundant surface area and proximity to a major source of air pollution, the use of photocatalytic concrete for pavements is a logical concept. A study conducted in the Netherlands used photocatalytic concrete pavers on a section of a busy roadway and monitored the air quality 0.5 m (19.5 in.) to 1.5 m (58.5 in.) above the pavement in both a control area with normal pavers and the test section. It was found that the NOx levels were reduced by 25 to 45 percent. The Missouri Department of Transportation, with consultation from the CP Tech Center and the FHWA, is currently preparing for a trial section of photocatalytic pavement in the St. Louis area. The St. Louis area is designated a non-attainment area with regards to particulate matter with a moderate concern for ground-level ozone, both being primary components of photochemical smog. The use of photocatalytic pavements or pavement-related structures is being discussed to bring the region down to attainment levels. The photocatalytic cement is being evaluated in comparison to typical local Type I cement for strength, permeability, deicer scaling resistance, air void system, and compatibility with admixtures and curing compounds. So far, the differences between the control mixture and photocatalytic mixture have been statistically negligible. Ultimately, the photocatalytic concrete will be tested on a roadway. The current concept for the design is

a two-lift pavement, with the photocatalytic portion being a 2-inch bonded overlay on-top of an 8-inch non-photocatalytic concrete base. The air quality and, possibly, the run-off water quality will be monitored. The ultimate goal is to assess the effectiveness of photocatalytic concrete for use in pavements, barrier walls, sound walls, or other pavement-related structures.

Click here to buy our recently updated bibliography of photocatalytic concretearticles, reports, conference papers, and patents, identified through searches of online scientific databases as well as the Library’s collection.

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Identifying Material Incompatibilities Concrete Technology Home > Concrete Design and Production > Identifying Material Incompatibilities

Incompatibility

Too Many Good Things?

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The wide variety of materials options and mix proportions possible in concrete allows it to be customized for a wide range of applications and placement and service environments. However, the cementitious materials (cements, fly ashes, slag cements, etc.) and chemical admixtures (accelerators, retarders, water reducers, etc.) are all chemically complex and this complexity can lead to problems when they don’t work together properly. Even when all materials meet and exceed their

specification requirements individually, problems can arise under field conditions. These problems might manifest themselves as: 





Early stiffening or excessive retardation (which can lead to workability, placeability, consolidation, and finishing issues) Early-age cracking (including plastic shrinkage and possibly the ability to attribute the cause of cracking to chemical, physical, and environmental phenomena) Air-void system issues (including nonuniformity, low air contents, coalescence of air voids around aggregate, and excessively large voids)

Although these problems are relatively rare, the resulting construction delays, performance issues, and loss of confidence in concrete as the preferred construction material are unacceptable. FHWA and PCA co-sponsored research into these phenomena, with the goal of minimizing or preventing these problems in the field. The project developed relatively simple protocols for evaluating concrete material combinations both pre-construction and during construction.

Preconstruction The pre-construction protocol is suited for testing of new mix designs and evaluating new materials sources. Tests should be performed over the range of temperatures expected in the field and cover the range of mix proportions expected to be used. Figure 1, from the FHWA TechBrief (see complete reference below), summarizes the pre-construction protocol. The protocol begins with a review of material characteristics that have been implicated in incompatibility issues, for example, cementitious

materials fineness and alkali contents. A review of historical properties of the materials being used is valuable as changes in some characteristics can raise a flag that might cause the concrete producer to be wary. Additional testing is suggested to bracket potential problems, for example, some incompatibility issues only appear with certain materials during hot weather. Potential solutions are also recommended in the pre-construction protocol, and testing can confirm which of these solutions will be effective. This allows the problems to be avoided or permits changes to be quickly implemented if problems arise during construction.

Figure 1. Summary of pre-construction protocol. Source: FHWA TechBrief 

During Construction A separate testing protocol has also been developed for use during construction and it is best suited to identifying issues that arise during construction. The

goal of these tests is to assure that materials used are similar to those used in pre-construction testing. Figure 2 suggests routine tests during construction that can be used to monitor performance, and quickly detect potential problems as they arise, allowing a more rapid response, particularly if various solutions have been evaluated with pre-construction testing.

Figure 2. Summary of protocol for use during construction. Source: FHWA Tech Brief  It is important to note that the protocol offers flexibility because not all of these tests are routinely available. Some are relatively more time consuming or costly to run. The decision about which suite of tests to use will be based on engineering judgment based on the costs/benefits and project-specific risks. A high-profile bridge project will obviously require more tests than a neighborhood sidewalk.

Related resources: 

TechBrief: Protocol to Identify Incompatible Combinations of Concrete Materials, FHWAHRT-06-082, Federal Highway Administration, Turner-Fairbank Highway Research Center,