Using Miniature Concrete Prism Test (MCPT) and Accelerated Mortar bar Test (AMBT) to Determine Alkali Threshold of Aggregates

Although the alkali content of cement required to initiate and sustain alkali-silica reaction (ASR) was thought to be >0.60% Na₂O_eq, the data from exposure sites show that concrete blocks made with low-alkali portland cements can still show deleterious expansion. The more important parameter is the total available alkali content in kg/m³ which will lead to ASR. The alkali threshold of an aggregate can be defined as the alkali loading of a mixture below which ASR will not initiate and/or cause deleterious expansion. Obtaining information about the alkali threshold of different aggregates has been largely ignored but now merits significant attention as a key to developing effective long-term ASR preventive measures. Most ASR performance tests use a fixed cement content and alkali level; therefore they cannot be used to determine alkali threshold. In this paper, several features of the miniature concrete prism test (MCPT) and the accelerated mortar bar test (AMBT) were modified to determine the alkali threshold of several different known ASR aggregates. Low- and high-alkali portland cements, and a combination of both, were used to obtain a spectrum of alkali loadings to determine the alkali level at which ASR expansion would occur for different aggregates. The alkalinity of the soak solution was matched to that of the pore solution for each mixture. The results showed that MCPT can differentiate between mixtures with different alkali loadings. The AMBT results show that there is potential for this method, but further interpretation of results is needed since this is a mortar bar method and the cement content (and thus alkali loading) needs to be scaled appropriately to concrete.