What is Alkali Aggregate Reactivity (AAR)?

AAR results in deleterious expansive cracking of concrete occurring at later ages after construction. While mostly inert, some concrete aggregates can react in the highly alkaline environment in concrete resulting in internal expansion that causes deleterious cracking. Alkalis include sodium and potassium that are minor constituents in Portland cement but can be from other concrete ingredients or from external sources. Expansion due to AAR is a slow process and results in visible deterioration 10 to 15 years after the concrete structure has been built. In rare cases deterioration at earlier ages may be observed.

Two forms of alkali aggregate reactions are recognized:

Alkali carbonate reactions (ACR) occur with dolomitic limestone aggregate of a specific mineralogy and microstructure. Sources of these aggregates is relatively rare. ACR is typically a more aggressive reaction and occurs earlier in the life of the structure.

Alkali silica reactions (ASR) occur with certain forms of silica (SiO2) minerals in aggregates that react in a high alkaline (pH) medium in concrete creating an expansive gel. The gel expands by absorbing moisture that causes the expansion of concrete and subsequent damage. Three conditions are required for deleterious ASR to occur:

1. Reactive forms of silica in aggregate
2. High alkali pore solution (pH) in concrete. and
3. Presence of moisture

Why is AAR a concern?

Deterioration to concrete structures due to AAR does not generally result in catastrophic failures. Where dimensional stability is important such as in dams, the expansions can impact the functioning of the structure. In most cases, synergy with other deterioration processes like cycles of freezing and thawing and corrosion of reinforcement exacerbates the rate of deterioration of concrete structures. ASR in concrete pavements and transportation infrastructure can result in spalling of cracked sections. Moisture, additional alkalis from deicing salts, and traffic loading accelerate the process.

How is the potential for AAR determined?

Aggregates with a distinct mineralogy of dolomite crystals embedded in a clay matrix cause ACR. A qualified petrographer can identify this. Quarries in North America where these aggregates occur are known, and their use in hydraulic cement concrete is avoided. Test methods for determining potential for ACR include a rock cylinder expansion test, ASTM C586 and an expansion test of concrete prisms, ASTM Cl 105.

Cases of ASR have been noted in most areas in North America. Existing signs of ASR in concrete structures in a region is the most definitive way of establishing that the problem exists. A petrographic evaluation of an aggregate source, ASTM C295, can identify potentially reactive silica minerals in aggregates but will not definitively establish whether an ASR problem will occur when the aggregate is used in concrete.

The more reliable test method that has been correlated to actual deterioration in field structures or field-exposed test specimens is an expansion test using  concrete prisms, ASTM C1293. This test requires a one-year period and may not be conducive to project schedules if not conducted ahead of time. Aggregates are considered to be potentially reactive when the expansion exceeds 0.04% at 12 months.

A more common test is an accelerated mortar bar expansion test, ASTM C1260. This test provides a result in about 2 weeks. Aggregates are considered potentially reactive when the expansion exceeds 0.20% (ASTM C33). Many agencies, however, use an expansion criterion of 0.10% at 14 days. ASTM C1260 is an aggressive test and aggregates that do not cause deleterious ASR reactions in the field are often characterized as reactive by the test. It is recommended that ASTM C1260 results should be supplemented with other information in determining the potential reactivity of an aggregate source.

Other test methods, like the quick chemical test, ASTM C289 and a longer term mortar bar expansion test, ASTM C227 are not considered to be reliable.

The Appendix of ASTM C33, Specification for Concrete Aggregates, provides guidance on AAR test methods, criteria and mitigation methods.

How is AAR avoided?

There are no recommended methods of preventing deleterious expansion when the available aggregate source has been verified to be ACR reactive. The only recourse is to use an alternative source of aggregate.

For deleterious ASR expansion to occur the three factors discussed before are required: alkalis, reactive silica and exposure to moisture. Concrete that remains dry inside buildings and not in contact with soil will typically not need preventive measures. In other situations various strategies can be used to avoid damage due to ASR.

One option is to avoid the use of aggregate sources that are determined to be reactive. This may not be feasible because alternative non-reactive aggregates may not be economically available or data may not exist as to their potential performance. Another option is the use of a low alkali cement, typically characterized as one with Na20 e4, less than 0.60%. Low alkali cement, however, is not available in many regions. Alternatively, limiting the total alkali content in concrete is often considered a better option. Only the alkali from Portland cement is considered. The total alkali content in concrete is determined by multiplying the cement content by the alkali content. Concrete alkali content is typically limited to a 5.0 lb/yd3 (3.0 kg/m3 or lower for more critical structures, like in concrete dams. With this option – low alkali cement or low concrete alkali content – it should be recognized that alkali concentrations can build up in concrete during service conditions from exposure to external sources like deicing chemicals and sea water, or from migration of alkalis within concrete due to drying.

The more accepted option to mitigate deleterious ASR is to incorporate a supplementary cementitious materials (SCM) in concrete. SCM include fly ash, natural pozzolan, slag cement, or silica fume. SCMs bind alkalis in the hydration products and prevent the deleterious expansion from occurring. One exception is fly ashes that have calcium oxide contents greater than 20%, typically characterized as Class C fly ash. Class C fly ashes typically need higher dosage levels to mitigate the reaction.

The quantity of SCM required will depend on the reactivity of the aggregate, the alkali loading in the concrete, the type of SCM and the exposure of the concrete to external sources of alkalis. In many regions historically established SCM contents required to mitigate ASR are used and work well. Alternatively, the effectiveness of an SCM can be evaluated by testing. The SCM contents evaluated should cover a range typical of those proposed for construction. The more common test methods are ASTM C441, ASTM C1293, and ASTM C1567. These test methods accelerate the reaction either by using highly reactive artificial aggregates, elevating the alkali loading in the test mixture, exposure to highly alkaline solutions, use of elevated temperatures, or some combination thereof. The concrete prism test, ASTM C1293 is performed for a 2-year period at which point the expansion should be less than 0.04%. This tends to be too long for typical project submittals. More commonly SCMs are evaluated using ASTM C1567 with a 14-day expansion criterion of 0.10%. Research supports that these methods provide a conservative estimate of the quantity of SCM needed to mitigate ASR in concrete. Regardless of the process used to establish the minimum SCM content required, the impact on other project requirements for concrete must be considered. These include, but are not limited to setting time, bleeding  characteristics. workability, and early and later age strength development.

Chemical admixtures, primarily lithium nitrate, have been shown to be effective to mitigate deleterious ASR. Manufacturer recommendations should be sought to establish effective dosage levels for specific concrete mixtures. The Corps of Engineers method CRD-C 662 is referenced to evaluate the effectiveness of the lithium admixture dosage. In some cases, combinations of these options such as the use of SCM and lithium admixtures have proven successful. Because test methods accelerate the reaction, none evaluate potential for deleterious ASR of the actual composition of concrete mixtures proposed for projects. No test evaluates the effectiveness of the alkali content of Portland cement. Test methods evaluate single aggregate sources. When the fine and coarse aggregates are determined to be reactive, the dosage of SCM that mitigates the more reactive aggregate should be used.

AASHTO PP65 provides a step by step method for evaluating aggregates and a prescriptive and performance­ based methodology to minimize the potential for damage in field concrete. The methodology requires consideration of the risk level for the occurrence of ASR in structure to establish preventative measures.

References

1. ACI 221.l R, 2008. Report on Akali-Aggregate Reactivity, ACI Manual of Concrete Practice, ACI, Farmington Hills, MI, www.concrete.org
2. Guide and Guide Specification for Concrete subject to Alkali Silica Reaction. Portland Cement Association. Skokie, IL. IS413.02 and IS4l 7.07, www.cement.org.
3. ASTM Standards referenced, Annual Book of ASTM Standards, Volume 4.02, ASTM International. West Conshohocken, Pennsylvania, wwv.astm.org
4. AASHTO PP 65-10 Practice for Determining the Reactivity of Concrete Aggregates and Selecting Appropriate Measures for Preventing Deleterious Expansion in New Concrete Construction, AASHTO, Washington DC, www.aashto.org
5. Resources on ASR from Federal Highway Administration­ www.fhwa.dot.gov/pavement/concrete/asr/resources

What is acceptance of concrete?

Acceptance testing is the process of testing representative samples of concrete furnished to a project. Acceptance test­ing includes tests on plastic concrete for slump, air content, density (unit weight), temperature, and tests on hardened concrete for strength and other durability properties as re­quired in contract documents or project specifications.

Acceptance testing on hardened concrete is conducted in accordance with standardized procedures to determine whether the concrete as delivered has the potential of devel­oping the desired properties intended by the design profes­sional. These test results are not intended to imply the ac­tual properties of concrete in the structure. There are several variables during construction that will impact in-place con­crete properties that are beyond the control of the concrete supplier.

Why conduct acceptance testing?

Acceptance testing is conducted to quantitatively verify that concrete conforms to the requirements of the purchaser. The requirements of the purchaser, relative to the tests and acceptance criteria, are generally stated in writing in project specifications or are invoked by reference to industry standards, such as ACI 301, ACI 318 and ASTM C 94.

Contractors are legally bound to facilitate or to conduct acceptance testing by local jurisdictions which adopt model codes such as the International Building Code. These model codes in turn refer to the ACI 318 Building Code.

It is important for those involved in testing to realize that the results of acceptance testing have significant implications on the project schedule, cost to project participants, and may impact the safety of the structure and its inhabitants.

How should acceptance testing be conducted?

Acceptance testing must be conducted by certified technicians who have demonstrated a written and practical knowledge of performing tests in accordance with the pertinent standards. Certification programs are offered by the American Concrete Institute (ACI) and other organizations for tests conducted in the field and laboratory. Laboratories performing acceptance tests should conform to the requirements of ASTM C 1077. Laboratories should be proficient in testing concrete, should have been through quality system audit by an independent evaluation organization and participate in reference sample testing programs to evaluate their testing proficiency and correct processes if necessary. Laboratory inspections and reference sample programs of the Cement and Concrete Reference Laboratory (CC RL), or equivalent, are established standards.

All acceptance testing of concrete must be conducted in accordance with established standards referenced in Contract documents. Any deviation from standard procedures is adequate reason for invalidating test results so obtained.

It is important that the process of conducting acceptance testing and the responsibilities of all involved parties for proper sampling, specimen storage, handling, transportation to the laboratory, jobsite sample disposition and subsequent laboratory testing are clearly defined prior to the start of a project. In medium to large projects a pre-construction conference is strongly recommended to establish processes, contingencies and responsibilities (CIP 32).

Sampling: Samples of concrete from concrete delivery vehicles for acceptance tests should be obtained in accordance with ASTMC 172. The sample should be obtained at the end of the truck chute. Two or more portions of concrete as discharged from the middle portion of the load are composited to obtain a sample that is representative of the load. When the specification requires additional tests to be conducted at the point of placement in the structure after concrete has been moved through some conveying means (such as a pump, bucket or conveyor) sampling procedures should be conducted such that the means of conveyance is not temporarily shut off or relocated to ease sampling as this can temporarily change the properties measured. ASTM C 94 permits a preliminary sample to be obtained after 0.25 yd3 (0.20 m3)  has been discharged to measure slump and air content and make appropriate adjustments to the load at the jobsite. The preliminary sample should not be used to make specimens for acceptance tests of hardened concrete.

Slump and Air Content: When the slump and air content measured on the preliminary sample are lower than specified jobsite adjustments with water or admixtures followed by adequate mixing are permitted. If slump and air contents are higher then a retest is made immediately and if the retest fails then the concrete is considered to have failed the requirements of the specification.

Slump of concrete is measured in accordance with ASTM C 143. The tolerance on slump varies by slump level as ordered or specified. The slump tolerances of ASTM C 94 are summarized in the table below. There is no established tolerance for the slump flow of self consolidating concrete, that is measured in accordance with  ASTM C 1611.

The air content of concrete is measured in accordance with the pressure method, ASTM C 231 or by the volumetric method, ASTM C 173 for lightweight concrete or for aggregates with high absorptions. For air-entrained concrete, the tolerance on air content as ordered or specified is ±1.5%.

Density and Yield: When samples are obtained for strength tests ASTM C 94 requires measuring the density (unit weight) of the concrete in accordance with ASTM C 138. This can be done by determining the weight of the air meter container after the sample has been prepared. The minimum container size based on the nominal maximum size of the aggregate in the concrete mixture should be followed. Density measurements can also be correlated with air content measurements and can be an indicator of the water content in the mixture. When determining yield, ASTM C 94 requires that the density should be determined on separate samples from three different loads of concrete and compliance with volume of concrete ordered be done on that basis (CIP 8).

Temperature: The temperature of concrete is measured in accordance with ASTM C 1064. Temperature is measured to determine conformance to temperature limits in a specification and is a required test when strength test specimens are prepared. It is permitted to measure the temperature of concrete in place when it is not measured in conjunction with strength tests.

Hardened Concrete Tests: ASTM C 31 describes the procedures for preparing cylinders and beams for compressive and flexural strength tests, respectively. It describes the procedures for storing specimens at the jobsite and transporting specimens to the laboratory. ASTM C 31 requires the test specimens to be maintained in a moist condition in a temperature range of 60 to 80°F (16-27°C) in the field. For high strength concrete with a specified strength greater than 5000 psi (35 MPa), the storage temperature limits are tighter at 68 to 78°F(20-26°C). A record of the temperature conditions during field storage of the specimens should be maintained. A curing box with max/min temperature recording device is generally required to verify conformance to these requirements. The same procedures should be adhered to for test specimens prepared for other tests. Test specimens should not be stored at the jobsite for longer than 48 hours. Specimens should be protected with adequate cushioning when transported to the laboratory. Transportation time should not exceed 4 hours. Specimens delivered to the laboratory should be stripped of molds, logged and placed in moist curing as defined in ASTM C 31 as soon as possible and no later than about 6 hours. More details can be found in CIPs 9 and 34.

While most specifications delegate the contractor with the responsibility for providing adequate facilities for storage of specimens at the jobsite, it is also incumbent on the testing technicians and the individual certifying test results to ensure that standard procedures are followed. Concrete is very sensitive to temperature and moisture at early ages and any deviation from standard procedures is a basis for rejecting results of these acceptance tests as it increases the likelihood of failing test results of acceptable concrete. This has implications to the project cost and schedule. A significant number of low strength results can be attributed to cylinders being subjected to non-standardized initial curing at the job site (CIP 9).

Test reports with data on all tests conducted, as well as other reporting requirements addressed in the standards, should be distributed to the owner or his representative, contractor and concrete producer in a timely manner. This is very important to the ongoing project quality and serves as documentation for the ability of the concrete producer to furnish quality concrete for future projects.

References

1. International Building Code 2006, International Code Council, Inc. Falls Church, Virginia, www.iccsafe.org
2. ACI 301 and 318, American Concrete Institute, Farmington Hills, Michigan, www.concrete.org
3. ASTM C 31, C 94, C 138, C 143, C 172, C 173, C 231, C1064, C1077, C 1611, Annual Book of ASTM Standards, Volume 4.02, ASTM International, West Conshohocken, Pennsylvania.
4. CIP 8, 9, 32, 34, Concrete in Practice Series, NRMCA, Silver Spring, Maryland, www.nrmca.org
5. Technical Bulletins #1, #2 , #3, Virginia Ready-Mix Concrete Association, Charlottesville, Virginia.

What is popout?

A “popout” is a small, generally cone-shaped cavity in a horizontal concrete surface left after a near-surface aggregate particle has expanded and fractured. Gen­erally, part of the fractured aggregate particle will be found at the bottom of the cavity with the other part of the aggregate still adhering to the point of the popout cone. The cavity can range from¼ in. (6 mm) to few inches in diameter.

Why do concrete popouts occur?

The aggregate particle expands and fractures as a re­sult of a physical action or a chemical reaction:

Physical

The origin of a physical popout usually is a near-sur­face aggregate particle having a high absorption and relatively low relative density (specific gravity). As that particle absorbs moisture, or if freezing occurs un­der moist conditions, its swelling creates internal pres­sures sufficient to rupture the particle and the overlying concrete surface. The top portion of the fractured aggregate particle separates from the concrete surface taking a portion of the surface mortar with it. In some cases the aggregate forces water into the surrounding mortar as it freezes thus causing the surface mortar to pop off, exposing an intact aggregate particle. Clay balls, coal, wood or other contaminants can uptake water and swell even without freezing, but the result­ing pressure rarely is great enough to cause popouts. Also, there are reported cases of grain (soybeans, corn) contamination of aggregate shipments that have resulted in surface popouts. Such occurrences are not within the scope of this document.

Popouts as a result of physical action are typically only a problem with exterior flatwork in climates subject to freezing and thawing under moist conditions and re­sulting expansion. Even aggregates which meet the requirements of ASTM C 33 Class SS, for architectural concrete in severe exposure, allow several types of particles which may cause popouts when exposed to freezing and thawing in the saturated condition. The most common type of particles resulting in popouts are low density chert in natural aggregate deposits.

Crushed aggregates are less likely to contain light­weight, absorptive particles which are more suscep­tible to popouts.

Chemical

The cause of a popout due to a chemical reaction is often related to alkali-silica reaction (ASR). Alkalis from cement or other source cause an environment of high pH (high concentrations of OH ions) causing the breakdown of silica and formation of an ASR gel, which absorbs water and expands, removing a small portion of the surface mortar with it. These are called ASR popouts. They are typically small and are often accompanied by a small spot that is discolored and/or appears to be clamp. The aggregate particle does not often fracture and split as is the case of pop outs from physical action. However, the ASR phenomenon can result in micro-fractures within the aggregate particles. Some alkali-silica reaction popouts can occur within a few days after the concrete is placed.

How do I avoid concrete popouts?

Most popouts are aesthetic defects that do not impact the structual performance of the concrete members. A large number of popouts however make it easier for water and other harmful chemicals to enter the con­crete, which can ultimately lead to other forms of dete­rioration such as corrosion of steel reinforcement. The following steps can be taken to avoid concrete popouts.

Physical Popouts

1. Avoid using aggregates which contain particles which may cause popouts, or that have a history of popouts.
2. However, in some parts of the United States, the available natural gravels contain some particles that are likely to result in surface popouts. Due to the unavailability of economical alternate aggregates, the occurrence of popouts on sidewalks and pavements is an accepted, albeit undesirable, likelihood in those locations.
3. If popouts are unacceptable, an alternate source of aggregates must be located. If appropriate, two­ construction can be used, whereby the popout susceptible aggregate is used for the lower course and the pop-out free aggregate that is likely to be more expensive is used for the surface  course.
4. Aggregates can be beneficiated to remove light­weight materials, but the added cost of beneficiation can be prohibitive for most uses.
5. Reduce the water to cementitious materials ratio of the concrete, as this will reduce the likelihood of saturation and will increase the resistance to swell­ing forces. Provide proper curing for exterior flatwork, as this results in improved strength of the cementitious materials, especially on the surface. This will reduce permeability thereby lowering the amount of water migrating to coarse aggregate particles. These steps can reduce the frequency, but will not necessarily, eliminate popouts.
6. Reduce the maximum aggregate size, as smaller aggregates will develop lower stresses due to freez­ing, and fewer popouts will occur. Those that do will be smaller and less objectionable.

Chemical Popouts

1. Use a low-alkali cement or a non-reactive aggregate. This is often not a practical option in many regions
2. Flush the surfaces with water after the concrete has hardened and before applying the final curing. This will remove the alkalis that may have accumulated at the surface as a result of evaporation of bleed water.
3. Permit the use of Class F fly ash or slag cement as a partial cement substitute to reduce the permeabil­ity of the paste and mitigate deleterious reactions due to ASR.

How do I repair concrete popouts?

Prior to undertaking a repair program, it is advisable to confirm the cause of the popouts by obtaining core samples containing one or more typical popouts and having them studied by a qualified petrographer.

Popouts can be repaired by chipping out the remaining portion of the aggregate particle in the surface cavity, cleaning the resulting void, and by filling the void with a proprietary repair material such as a dry pack mortar, epoxy mortar, or other appropriate material following procedures recommended by the manufacturer. It will be difficult to match the color of the existing concrete. If the popouts in a surface are too numerous to patch individually, a thin bonded concrete overlay may be used to restore a uniform surface appearance. Specific recommendations for such overlays are beyond the scope of this publication.

References

1. Popouts: Causes, Prevention, Repair, Concrete Technology Today, PL852, Portland Cement Association, www.cement.org
2. Guide to Residential Cast-in-Place Concrete Construction, ACI 332R, American Concrete Institute, Farmington Hills, MI, www.concrete.org
3. Closing in on ASR Popouts, Concrete Technology Today, CT022, Portland Cement Association, www.cement.org
4. R. Landgren, and D. W. Hadley, Surface Popouts caused by Alkali-Aggregate Reaction, Portland Cement Association, RD121, www.cement.org
5. N. E. Henning, K. L. Johnson, and L. J. Smith , Popouts, Con­struction Bulletin, March 4, 1971, Upper Midwest News Weekly.
6. Richard H. Campbell, Wendell Harding, Edward Misenhimer, and Leo P. Nichelson, Surface Popouts: How are they Affected by Job Conditions?, ACI Journal, American Concrete Institute, June 1974, pp. 284-288.

What is cold weather concreting?

Cold weather is defined as a period when for more than 3 consecutive days the average daily temperature is less than 40°F [5°C] and the air temperature is not more than 50°F [10 °C] for more than one-half of any 24-hr period. These conditions warrant special precautions when placing, finishing, curing and protecting concrete against the effects of cold weather. Since weather conditions can change rapidly in the winter months, good concrete practices and proper planning are critical.

Why consider cold weather?

Successful cold-weather concreting requires an understanding of the various factors that affect concrete properties.

In its fresh state concrete freezes if its temperature falls below about 25°F [-4°C]. The potential strength of frozen concrete can be reduced by more than 50% and it will not be durable. Concrete should be protected from freezing until it attains a compressive strength of 500 psi [3.5 MPa] – about two days after placement.

Concrete at a low temperature has a slower setting and rate of strength gain. A rule of thumb is that a drop in concrete temperature by 20°F [10°C] will approximately double the setting time. These factors should be accounted for when scheduling construction operations, such as form removal.

Concrete that will be in contact with water and exposed to cycles of freezing and thawing should be air-entrained. Newly placed concrete is saturated with water and should be protected from cycles of freezing and thawing until it has attained a compressive strength of at least 3500 psi [24.0 MPa].

The reaction between cement and water, called hydration, generates heat. Insulating concrete retains heat and maintains favorable curing temperatures. Temperature differences between the surface and the interior of concrete should be controlled. Thermal cracking may occur when the difference exceeds about 35°F [20°C]. Insulation or protective measures should be gradually removed to avoid thermal shock.

Recommended concrete temperatures at the time of placement are shown below. The ready mixed concrete producer can control concrete temperature and furnish concrete to comply.

Concrete temperature should not exceed these temperatures by more than 20°F [10°C].  Concrete at a higher temperature requires more mixing water, has a higher rate of slump loss, and is more susceptible to cracking. Concreting in cold weather provides the opportunity for better quality, as cooler initial concrete temperature will typically result in higher ultimate strength and improved durability.

In cold weather, slower setting time and rate of strength gain of concrete can delay finishing operations and form removal. Chemical admixtures and other materials can be used to offset these effects. Accelerating admixtures, conforming to ASTM C 494–Types C (accelerating) and E (water-reducing and accelerating), are commonly used.

Calcium chloride is an effective accelerating admixture, but should not exceed a dosage of 2% by weight of cement. Non-chloride, non-corrosive accelerators should be used for prestressed concrete or when corrosion of steel reinforcement or metal in contact with concrete is a concern. Accelerating admixtures do not prevent concrete from freezing and their use does not preclude the requirements appropriate curing and protection from freezing.

Rate of setting and strength gain increased by increasing Portland cement content or by using a Type III cement (high early strength). The quantity of fly ash or slag cement in concrete may be reduced in cold weather for a similar effect. This may not be possible if a minimum quantity of SCM is required for durability. The selected solution should be economical and not compromise on the required concrete performance.

Concrete should be placed at the lowest practical slump. Adding water to achieve slump can delay setting time and prolong the duration of bleeding, thereby impacting finishing operations.

Adequate preparations should be made prior to concrete placement. Snow and ice should be removed and the temperature of surfaces and metallic embedment in contact with concrete should be above freezing. This might require insulating or heating subgrades and contact surfaces prior to placement.

Materials and equipment should be in place to protect concrete from freezing temperature and for adequate curing, both during and after placement. Insulated blankets and tarps, as well as straw covered with plastic sheets, are commonly used measures. Enclosures and insulated forms may be needed for additional protection depending on ambient conditions. Corners and edges are most susceptible to heat loss. Fossil-fueled heaters in enclosed spaces should be vented for safety reasons and to prevent carbonation of newly placed concrete surfaces, which causes dusting.

The concrete surface should not be allowed to dry before it sets as this can cause plastic shrinkage cracks. Subsequently, concrete should be adequately cured. Water curing is not recommended when freezing temperatures are imminent. Use membrane­ forming curing compounds or impervious paper and plastic sheets for concrete slabs.

Forming materials, except for metals, maintain and evenly distribute heat and provide adequate protection in moderately cold weather. In extremely cold temperatures, insulating blankets or forms should be used, especially for thin sections. Forms should not be stripped for 1 to 7 days depending on rate of strength gain, ambient conditions, and anticipated loading on the structure. Field-cured cylinders or nondestructive methods should be used to estimate in-place concrete strength prior to stripping forms or applying loads. Removal of protective measures and formwork should not cause thermal shock to the concrete.

Concrete test specimens used for acceptance of concrete should be carefully managed. In accordance with ASTM C31, cylinders should be stored in insulated containers, which may need temperature controls, to insure that they are cured at 60°F to 80°F [16°C to 27°C] for the first 24 to 48 hours. A minimum/maximum thermometer should be placed in the curing box to maintain a temperature record of curing test specimens at the jobsite.

Cold Weather Concreting Guidelines

1. Use air-entrained concrete when exposure to moisture and freezing and thawing conditions are expected.
2. Keep surfaces in contact with concrete free of ice and snow and at a temperature above freezing prior to placement.
3. Place and maintain concrete at the recommended temperature.
4. Place concrete at the lowest practical slump.
5. Protect fresh concrete from freezing or drying.
6. Protect concrete from early-age freezing and thawing cycles until it has attained adequate strength.
7. Limit rapid temperature changes when protective measures are removed.

References

1. Cold Weather Concreting, ACI 306R., American Concrete Institute Farmington Hills, MI.
2. Design and Control of Concrete Mixtures, Portland Cement Association, Skokie, IL.
3. ASTM C94 Standard Specification for Ready Mixed Concrete, ASTM, West Conshohocken, PA.
4. ASTM C31 Making and Curing Concrete Test Specimens  in  the Field  AST,M  West Conshohocken PA.
5. Cold-Weather Finishing Concrete Construction, November 1993

What are admixtures?

Admixtures are natural or manufactured chemicals added to the concrete before or during mixing. The most often used chemical admixtures are air­ entraining agents, water reducers, water-reducing retarders, and accelerators.

Why use admixtures?

Admixtures are used to give special properties to fresh or hardened concrete. Admixtures may enhance the workability of fresh concrete and the durability strength of hardened concrete. Admixture are used to overcome difficult construction situations, such as hot or cold weather placements, pumping requirements, early-age strength requirements, or specifications that require low water-cementitious materials ratio. Admixture can be used to optimize the cementitious composition of concrete mixtures for performance and sustainability.

How to use admixtures?

Consult your ready mixed concrete supplier about admixture(s) appropriate for your application. Admixtures are evaluated for compatibility with cementitious materials, construction practices, job specifications and economic benefits before being used. Purchasers of ready mixed concrete should avoid requiring the use of specific brands or using products of their own accord.

Follow This Guide to Use Admixtures

1. AIR-ENTRAINING ADMIXTURES are liquid chemicals added when batching concrete to produce microscopic air bubbles called entrained air, produced by the mixing action. These air bubbles improve the concrete’s resistance to damage caused by exposure to cycles of freezing and thawing and deicing salt application. In fresh concrete entrained air improves workability and reduces bleeding and segregation. For exterior flatwork (parking lots, driveway, sidewalks, pool decks, patios) subject to freezing and thawing cycles, or in areas where deicer salts are used, an air content of 4% to 7% of the concrete volume is used depending on the size of coarse aggregate (see table). Air entrainment is not necessary for interior structural concrete since it is not subject to freezing and thawing. Entrained air should be avoided for concrete flatwork that will have a smooth troweled finish. In concretes with higher cementitious materials content. entrained air will reduce strength by about 5% for each 1% of air added; but in low cement content concretes, adding air has less effect and can reduce segregation and result in a modest increased strength due to the reduced water needed for required slump. Air entraining admixtures for use in concrete should meet the requirements of ASTM C260, Specification for Air-Entraining Admixtures for Concrete.

• WATER REDUCERS are used for two different purposes: (1) to lower the water content in fresh concrete and to increase its strength; (2) to obtain higher slump without adding additional water. Water-reducers reduce the required water content of a concrete mixture for a target slump. These admixtures disperse the cement particles in concrete and make more efficient use of cement. This increases strength or allows the use of less cement to achieve a similar strength. Water-reducers are useful for pumping concrete and in hot weather, to offset the increased water demand. Some water-reducers may cause an increased rate of slump loss with time. Water-reducers should meet the requirements for Type A in ASTM C 494 Specification for Chemical Admixtures for Concrete.Mid-range water reducers are now commonly used and are used for a greater water reduction than typical water reducers. These admixtures are popular as they improve the finish ability of concrete flatwork. Mid-range water reducers must at  least meet the requirements for Type A in ASTM C494. There is separate classification for these products in ASTMC494.

• HIGH RANGE WATER  REDUCERS (HRWR) is a special class of water reducer. Often referred to as superplasticizers, HRWRs reduce the water content of a given concrete mixture between 12 and 40% to maintain the same slump. HRWRs are therefore used to increase strength and reduce permeability of concrete by reducing the water content in the mixture; greatly increase the slump to produce “flowing” concrete or self-consolidating concrete (CIP 37) by using less water. These admixtures are essential for producing high strength and high performance concretes that contain higher contents of cementitious materials and mixtures containing silica fame. Some HRWRs may cause a higher rate of slump loss with time. In some cases, HRWRs may be added at the jobsite in a controlled manner to provide the required slump for placement. HRWRs are covered by ASTM Specification C494. Types F and G, and Types 1 and 2 in ASTM Cl0l7 Specification for Chemical Admixtures for Use in Producing Flowing Concrete.

• RETARDERS are chemicals that delay the initial setting of concrete by an hour or more. Retarders are often used in hot weather to counter the rapid setting caused by high temperatures. For large jobs, or in hot weather, concrete with retarder allows more time for placing and finishing. Retarders are typically a component of water reducers. Retarders should meet the requirements for Type B or D in ASTM C494.

Recommended Air Content in Concrete

• Moderate exposure – concrete in a cold climate will be only occasionally exposed to moisture prior to freezing and not ex­posed to deicing salt application.
• Severe exposure – concrete in cold climate will be continu­ously in contact with water prior to freezing or where deicing salts are used.

• ACCELERATORS reduce the initial setting time of concrete and produces higher strength at early ages. Accelerators do not prevent concrete from freezing; rather, they speed up the setting to permit finishing concrete earlier; and increase the rate of strength gain, thereby making the concrete stronger to resist damage from freezing in cold weather. Accelerators are also used in fast track construction requiring early form removal, opening to traffic, or load application on structures. Liquid accelerators should conform to ASTM C494 Types C and E. There are two kinds of accelerating admixtures: chloride based and non­ chloride based. Calcium chloride is a commonly used effective and economical accelerators, which is available in liquid or flake form. Calcium chloride must meet the requirements of ASTM D98. For non­-reinforced concrete, calcium chloride can be used to a limit of 2% by the weight of the cement. Because of concerns with corrosion of reinforcing steel induced by chloride. lower limits on chlorides apply to reinforced concrete. Prestressed concrete and concrete with embedded aluminum or galvanized metal should not contain any chloride-based materials because of the increased potential for corrosion of the embedded metal. Non-chloride based accelerators are used where there is concern of corrosion of embedded metals or reinforcement in concrete.

Besides these standard types of admixtures, there are products available for enhancing concrete properties for a wide variety of applications. Some of these products include: corrosion inhibitors, shrinkage reducing admixtures, anti-washout admixtures, hydration stabilizing or extended set retarding admixtures, admixtures to reduce potential for alkali aggregate reactivity, pumping aids, permeability reducing admixtures, workability retaining admixtures, rheology and viscosity modifying admixtures and a variety of colors and products that enhance the aesthetics of concrete. Consult with your local ready mixed concrete producer on admixture products that add value to your project.

References

1. ASTM C 260, C 494, C 1017, D 98, ASTM International, West Conshohocken, PA, www.astm.org
2. Chemical and Air-Entraining Admixtures for Concrete, ACI Educational Bulletin, E4, American Concrete Institute, Farmington Hills, MI, www.concrete.org
3. Chemical Admixtures for Concrete, ACI 212.3R, American Concrete Institute, Farmington Hills, MI.
4. Building Code Requirements for Structural Concrete, ACI 318, American Concrete Institute, Farmington Hills, MI.
5. Understanding Chloride Percentages, NRMCA Publication No. 173, NRMCA, Silver Spring, MD.
6. Self Consolidating Concrete, CIP 37, NRMCA Concrete in Practice Series, Silver Spring, MD, www.nrmca.org