Gina Ahlstrom, Federal Highway Administration
In August 2008, a task group was formed under the American Association of State Highway and Transportation Officials (AASHTO) Subcommittee on Materials to review current specifications pertaining to alkali-silica reactivity (ASR). The task group was asked to review existing specifications related to ASR and determine if they were appropriate and adequately addressed the subject. The task group found that the AASHTO ASR Lead States Team, which existed from 1995 to 2000, developed a specification for designing ASR-resistant concrete. However, the specification at that time was never adopted. The task group determined that the lack of an existing specification and the increase in the body of knowledge due to research related to designing concrete mixtures resistant to ASR warranted a new specification.
Recommended Practice
The task group reviewed the Federal Highway Administration (FHWA) publication titled “Report on Determining the Reactivity of Concrete Aggregates and Selecting Appropriate Measures for Preventing Deleterious Expansion in New Concrete Construction.”(1) Based on the review, the task group determined that the current AASHTO specifications for materials did not fully address the prevention of ASR. The FHWA report outlines the testing required and two approaches to preventing ASR. Both a performance-based and a prescriptive-based approach are presented to allow users to determine the best method for designing a concrete mixture resistant to ASR.
A document titled “Recommended Practice for Determining the Reactivity of Concrete Aggregates and Selecting Appropriate Measures for Preventing Deleterious Expansion in New Concrete Construction” was developed for AASHTO and incorporated the concepts and technical information from the FHWA report. The recommended practice was presented to AASHTO at the August 2009 meeting of the Subcommittee on Materials. Late last year, it was balloted and approved. It will be included in the 2010 publication of AASHTO’s “Standard Specifications for Transportation Materials and Methods of Sampling and Testing.”
The recommended practice outlines tests to determine the reactivity of aggregates. A process to evaluate performance history, petrographic examination, concrete prism test (ASTM C1293),(2) and accelerated detection test (AASHTO T 303)(3) is presented. Information is also provided on determining the potential for alkali-carbonate reaction (ACR). The sequence of tests is shown in the flow chart at the end of this article. Agencies are encouraged to develop their own testing plan based on the recommended practice, prior experience with ASR, and the acceptable level of risk the agency is willing to accept for ASR in new construction.
Performance-Based Approach
The performance-based approach provides guidelines on test limits for the concrete prism test (ASTM C1293)(2) and the accelerated mortar bar test for evaluating supplementary cementitious materials (SCMs) or blended cements (ASTM C1567).(4) Specifiers should note that ASTM C1567 and AASHTO T 303 have a duration of 16 days, whereas, ASTM C1293 may take up to 2 years. There is, however, general agreement in the research community that ASTM C1293 is more accurate. Guidance is also provided on modifying the detection test (AASHTO T 303)(3) to determine the dosage of lithium nitrate necessary to suppress ASR for a mixture with a specific aggregate. The appendix of the recommended practice includes a worked example for calculating lithium nitrate additions.
Prescriptive-Based Approach
The prescriptive-based approach prevents ASR in new construction by considering the class, size, and exposure condition of the structure, degree of aggregate reactivity, and the level of alkalis from the portland cement. The specifier uses a series of tables to determine the appropriate preventative measures for a concrete mixture. Using the prescriptive approach, ASR prevention can be achieved by limiting the alkali content of the concrete and/or using supplementary cementitious materials or blended cements. A worked example is provided for determining the appropriate preventative measures using the prescriptive approach.
The adoption of this recommended practice is a big step in designing concrete mixtures resistant to ASR. A commentary section will be presented for adoption at the AASHTO Subcommittee of Materials Meeting in August 2010. It is anticipated that the recommended practice will be updated as additional research is completed and new information on test methods and designing concrete free of ASR becomes available.
Further Information
If you would like further information on the recommended practice to prevent ASR in new construction, contact Gina Ahlstrom at [email protected].
References
1. Thomas, M. D. A., Fournier, B., and Folliard, K. J., “Report on Determining the Reactivity of Concrete Aggregates and Selecting Appropriate Measures for Preventing Deleterious Expansion in New Concrete Construction,” FHWA, U.S. Department of Transportation, Report No. FHWA-HIF-09-001, 2008, 20 pp.
2. Standard Test Method for Determination of Length Change of Concrete Due to Alkali-Silica Reaction, ASTM C1293, ASTM International, West Conshohocken, PA.
3. Standard Method of Test for Accelerated Detection of Potentially Deleterious Expansion of Mortar Bars Due to Alkali-Silica Reaction, AASHTO T 303, American Association of State Highway and Transportation Officials, Washington, DC.
4. Standard Test Method for Determining the Potential Alkali-Silica Reactivity of Combinations of Cementitious Materials and Aggregate (Accelerated Mortar-Bar Method), ASTM C1567, ASTM International, West Conshohocken, PA.
