Study objectives

To evaluate a wide selection of concrete mix designs to validate the use of the new AASHTO TP-144-21 (T-FAST) and alkali threshold test (ATT) methods in conjunction with mix design data, cement mill reports and SCM properties to determine the likelihood of ASR gel formation in concrete.

Background

The formation of ASR gels in concrete has been a major issue for the industry since they were first discovered in the 1930s. The gels are formed by the reaction of the alkalis from the cement with the silica in certain ASR susceptible aggregates. The gels can absorb water, swell, and cause the concrete to crack.

The first tests to determine the ASR susceptibility of aggregates came out in 1947. There have been many tests since then. They mostly rely on the measurement of engineering properties. Typically, a sample of mortar or aggregate is immersed in sodium hydroxide solution at elevated temperature and the physical expansion of the sample measured after a period. The T-FAST method is a more sensitive and accurate test which predicts ASR expansion using purely chemical measurements. The test is completed in 21 days.

Although these tests can detect ASR reactive sites in aggregates, they cannot determine whether gels will form in the resulting concrete. That depends on the alkali loading of the mix. The idea of measuring the alkali threshold of aggregates has been discussed for many years although no simple or reliable test has been available. A new test, (ATT) has been developed in the chemistry laboratory at the Turner Fairbank Highway Research Center (TFHRC). The test is simple and is completed in 21 days. With the ATT method it is possible to determine the likelihood of ASR formation in concrete.

While research at TFHRC supports this notion there is a need to verify the validity of this technique by evaluating many commonly used aggregates.

Partners in this pooled fund study will select concrete mix designs with known ASR field performance either positive or negative.  They will supply samples of concrete mixtures components including aggregates, cements, and SCMs along with the corresponding mix designs to TFHRC where the chemistry laboratory will use the T-FAST and ATT methods to evaluate their ASR reactivity and the alkali threshold of the aggregate combination. This information will be compared against the alkali loading of the provided mixes to ensure that the T-FAST/ATT combination would have predicted field performance. Further, these results will be used to design an ASR mitigation test like T-FAST.

Funding: Partner agencies who are interested in the topic are requested to contribute a minimum of $5,000 a year for the five years of the TPF study. The scope of the project is scalable based on the level of funding received. FHWA is already contributing significant resources to the topic area and will likely dedicate further resources in the future.