Experimental studies of the behavior of 'pessimum' aggregates in different test procedures used to evaluate the alkali reactivity of aggregates in concrete

Alkali-silica reaction (ASR) is a common deterioration mechanism responsible for numerous concrete durability issues. Since ASR was first discovered in the 1940's, a significant number of investigations have been carried out in order to understand its mechanisms. However, due to the complexity of the reaction and to the numerous factors that affect its development, many aspects still remain unexplained. The research described in this document was funded by the Texas Department of Transportation (TxDOT), and it focused on a specific type of reactive aggregates, known as 'pessimum'; they present an unexpected behavior with respect to the relation between the amount of material present in the mixture and the extent of ASR related damage. The main objective of this investigation was to determine a method for identifying aggregates that exhibit the 'pessimum' behavior by means of a short-term testing regime. Modified versions of the Accelerated Mortar Bar Test (AMBT) and the Concrete Microbar Test (CMBT) were considered for this purpose. In addition, the behavior of a selected group of 'pessimum' aggregates in the Concrete Prism Test (CPT) and the Chemical Method was evaluated. The petrographic characteristics for a reduced number of the aggregates studied were linked to their performance in the ASR tests. The results obtained from the experimental program conducted were combined with results from previous investigations performed at UT Austin to draw conclusions about the overall behavior of ‘pessimum’ aggregates. ‘Pessimum’ aggregates were successfully identified with a modification proposed to the AMBT. As for their behavior, it was found that depending on the amount of reactive constituents present in each test, these aggregates are classified as reactive (for low chert contents) or as non-reactive (for chert contents above the 'pessimum' proportion). Whether these aggregates will generate durability problems depends on the amount of reactive silica in the concrete mixture.