What are the Federal guidelines on ASR mitigation?

The Federal Highway Administration recommends two approaches to selecting preventative measures for alkali-silica reaction: a performance approach using the concrete prism test or accelerated mortar bar test and a prescriptive approach using the reactivity of the aggregates, intended service environment and exposure, service life, and availability of alkalis in the system. Both of these approaches are detailed fully in an FHWA report online.

The performance approach to ASR mitigation can be done with ASTM C1293, Standard Test Method for Determination of Length Change of Concrete Due to Alkali-Silica Reaction, ASTM C1260, Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method), or ASTM C1567, Standard Test Method for Determining the Potential Alkali-Silica Reactivity of Combinations of Cementitious Materials and Aggregate (Accelerated Mortar-Bar Method).

The concrete prism test (ASTM C1293) was originally designed to evaluate the reactivity of aggregates after one year, however, the standard allows for assessing the reactivity with regards to an aggregate-slag or aggregate-pozzolan combination after two years. Tests are run with varying levels of SCMs to provide a better understanding of the optimal value. Suitable combinations will exhibit an expansion less than or equal to 0.04% after two years. Aqueous lithium nitrate can also be tested using the two year modification of the concrete prism test. The lithium solution is included as a replacement of mix water and has the same expansion criteria as that for SCMs.

Two years is a long time to wait for a test result and many project schedules simply would not allow for it. A faster method is available using the accelerated mortar bar method (AMBT). This method begins with the one-year assessment of the aggregate using the concrete prism test and a 14-day assessment of the aggregate using the mortar-bar method (ASTM C1260). The results of these tests are compared according to figure 1. If the results fall within the acceptable range, then the use of SCMs can be assessed according to the AMBT method (ASTM C1567). This comparison should be made at least every two years. The limit for expansion using the AMBT method is less than or equal to 0.10% after 14 days. Assessing the effectiveness of lithium using the AMBT method is more complicated and is discussed in detail in the FHWA report.

Figure 1 – Comparison of concrete prism test to accelerated mortar-bar test to determine suitability of accelerated mortar-bar test method for evaluating preventive measures with a specific aggregate. (Thomas, Fournier, and Folliard 2008)