Alkali-silica reactive (ASR) gels are a distress mechanism in concrete structures. The ASR gels are formed by the reaction of alkalis from the cement with silica in certain aggregates. The gels can absorb water and swell causing the concrete to crack. 

ASR gels were first discovered in the 1930s. Not all aggregates form ASR gels. The first test methods to determine the susceptibility of aggregates to form ASR gels were developed in 1947. There have been many tests since then and almost all rely on the measurement of engineering properties, typically physical expansion. None of the tests work particularly well because they tend to both over and underestimate ASR reactivity. 

The Turner-Fairbank Highway Research Center Chemistry Laboratory developed a new test method that accurately determines ASR reactivity in aggregates and that relies purely on chemical measurements. The test is called T-FAST (Turner-Fairbank Highway Research Center ASR Susceptibility Test). It is simple and takes only 21 days. 

Like other standard tests (typically the American Standard Test Method (ASTM) 1260 and ASTM 1293), the T-FAST method will determine the presence of ASR reactive sites in the aggregate. These are laboratory tests; however, the results do not mean that ASR will be formed in the concrete. The formation of ASR gels depends upon the alkali level of the concrete which is mainly determined by the alkali level of the cement (obtained from the mill report) and the amount of cement in the concrete. Other supplementary cementitious materials (SCMs) can also exert an influence although to a lesser extent. 

There have been discussions in industry over many years on the need to measure the alkali threshold of aggregates. The threshold is the level of alkali which is sufficiently high to trigger the ASR reaction. Until now, no such test has been available.

The TFHRC developed the Alkali Threshold Test (ATT) which can accurately determine whether the level of alkali in the concrete is sufficient to trigger the ASR in the aggregates used (both coarse and fine). The test is like the T-FAST method and is also completed in 21 days. 

In previous tests, like, when an aggregate was shown to be reactive, then it became common practice to add SCMs like fly ash when in fact there may not have been any need. There are other reasons to use fly ash, however, with its dwindling supply, the new test means fly ash might only be used when it is really needed. 

The T-FAST method for coarse aggregates was accepted by American Association of State Highway and Transportation Officials (AASHTO) as provisional specification TP144-21. The methods for fine aggregates and the ATT were also submitted to AASHTO for consideration. 

T-FAST Video

Researchers at the FHWA Chemistry Laboratory demonstrate the T-FAST test process, which accurately predicts the formation of ASR gels, ultimately helping to prevent cracks in concrete. Until the development of T-FAST, cracks in concrete caused by ASR gels that absorb water were unreliably tested. This instructional video demonstrates the steps needed to perform T-FAST in a chemistry laboratory.