|PDF Printable Version|
Three tests used to assess the susceptibility of an aggregate to alkali-silica reaction.
Guide Specification for Concrete Subject to ASR
Beatrix Kerkoff, Consultant to the Portland Cement AssociationMost aggregates are chemically stable in hydraulic cement concrete without deleterious interaction with other concrete constituent materials. However, this is not the case for aggregates containing certain siliceous minerals that react with soluble alkalies in the concrete, sometimes resulting in detrimental expansion and cracking of concrete structures.
The best way to avoid deleterious alkali-aggregate reactions is to take appropriate precautions before concrete is placed. Reducing the potential for alkali-silica reaction (ASR) requires (1) understanding the ASR mechanism; (2) properly using tests to identify potentially reactive aggregates; and, if needed, (3) taking steps to minimize the potential for expansion and related cracking.
Because different geographic regions have different needs and available materials, the Portland Cement Association has developed a Guide Specification for concrete subject to alkali-silica reactions.
Testing the Aggregates
The Guide Specification provides for a combination of three separate laboratory tests to assess the susceptibility of an aggregate to ASR. The tests may be done in any order; however, petrographic examination (ASTM C295) and the mortar-bar test (ASTM C1260) would generally be performed simultaneously, while the concrete prism test (ASTM C1293) is performed later, if needed.
The aggregate is examined petrographically to identify and quantify the constituents, with maximum limits set for the various minerals that are potentially reactive. In the mortar-bar test, a 14-day expansion exceeding 0.10% indicates that the aggregate is potentially reactive. If either of these tests indicates the aggregate is potentially reactive, it may be further evaluated by the concrete prism test, with a one-year expansion limit of 0.04%.
Materials and Methods to Inhibit ASR
Most concrete is not affected by ASR and special requirements are not needed. However, if historical experience or the aggregate tests mentioned above demonstrate that ASR is a potential concern, then concrete mixtures must be specifically designed to mitigate ASR.
A variety of materials can be used to control ASR. Supplementary cementitious materials (SCMs) such as fly ash, slag cement, or silica fume can be included as a concrete ingredient added at batching, as a component of a blended hydraulic cement, or both. Blended hydraulic cements should conform to ASTM C595 (AASHTO M 240) or ASTM C1157. SCMs added directly to concrete are governed by ASTM C618 or AASHTO M 295 for fly ash and natural pozzolans; ASTM C989 or AASHTO M 302 for slag cement; and ASTM C1240 or AASHTO M 307 for silica fume. Specifiers can invoke the optional physical and chemical ASR requirements in these standards; however limits on expansion are typically not applicable for a particular project as the tests do not use job aggregates, and the limits may be more restrictive than are necessary or achievable. Using locally available materials in appropriate amounts is generally the most efficient solution to mitigate ASR.
When pozzolans, slag cements, or blended cements are used to control ASR expansion, their effectiveness should be determined using the following flowchart.
The accelerated mortar-bar test (ASTM C1567) can be used to evaluate combinations of cementitious materials and aggregates. A mortar-bar expansion at 14 days of less than or equal to 0.10% is considered acceptable to control ASR for a particular job aggregate. Combinations of actual cementitious materials and aggregates that do not meet this limit can be further evaluated by the concrete prism test.
Combinations of materials that exhibit a concrete prism expansion greater than 0.04% at 2 years are considered potentially reactive. Combinations of cementitious materials and aggregate exhibiting expansions less than 0.04% and demonstrating no prior evidence of reactivity in the field are considered nonreactive.
Where possible, different amounts of pozzolan or slag cement should be tested to determine the optimum dosage. Some materials exhibit a “pessimum” effect: dosages that are too low may actually result in higher ASR-related expansions than if no pozzolan or slag cement is used.
The flow chart above shows the sequence of checking the suitability of blended cements or supplementary cementitious materials to mitigate ASR. For the entire guide specification process to determine if potential aggregate reactivity exists and to select materials to control ASR, click here.
If pozzolans, slag cements, and/or blended cements are not available, or if testing or other engineering concerns preclude their use, portland cement and other concrete ingredients can be selected to limit the concrete’s alkali content based on the reactivity level of the aggregate, or based on proven field performance with the potentially reactive aggregate (Farny and Kerkhoff 2007). For service conditions more severe than experienced in the past, such as increased exposure to external alkalies or increased concrete alkali content, relying on proven field performance may not be a valid option. Another solution is the use of chemical inhibitors, such as lithium compounds. The degree to which lithium compounds suppress expansive ASR depends on aggregate reactivity and concrete alkali content. The Federal Highway Administration has published guidance on testing, specifying, and using lithium compounds in new concrete construction (Thomas et al. 2007).
Farny, James A. and Kerkhoff, B., "Diagnosis and Control of Alkali-Aggregate Reactions in Concrete," IS413, Portland Cement Association, 2007, 26 pp.
PCA Durability Subcommittee, "Guide Specification to Control Alkali-Silica Reactions," IS415, Portland Cement Association, 2007, 8 pp.
Portland Cement Association, "Alkali-Aggregate Reaction."
Thomas, M. D. A., Fournier, B., Folliard, K. J., Ideker J. H., and Resendez, Y., "The Use of Lithium to Prevent or Mitigate Alkali-Silica Reaction in Concrete Pavements and Structures," FHWA-HRT-06-133, Federal Highway Administration, McLean, Virginia, 2007, 47 pp.
Thomas, M. D. A., Fournier, B., Folliard, K., Shehata, M., Ideker, J., and Rogers, C., "Performance Limits for Evaluating Supplementary Cementing Materials Using the Accelerated Mortar Bar Test," R&D Serial No. 2892, Portland Cement Association, Skokie, Illinois, USA, 2005, 22 pp.
A summary of the different tests for ASR is given in HPC Bridge Views, Issue No. 36, November/December 2004.
HPC Bridge Views, Issue 51, Sept/Oct 2008