Decrease of pore solution alkalinity in concrete tested for alkali-silica reaction

Knowledge of alkali concentration in concrete pore solution is key for long-term evaluation of alkali-silica reaction (ASR) expansion. This study aimed at providing experimental data on the alkalinity evolution in concrete tested for ASR in order to determine critical alkalinity values that stop concrete expansion. The pore solution was systematically expressed (i.e. extracted at high pressures) from concrete specimens tested at 38 degrees C over water (Canadian standard). Alkali concentration showed a decreasing trend with time, which can be mainly explained by alkali leaching. After 52 weeks, the decrease in alkalinity was up to 25% of the original Na2Oeq content in the concrete, contributing to underestimate long-term expansion of reactive concrete. Comparisons were also made between pore solution expressed out of cement paste specimens and concrete specimens; differences (from 25% to 43% higher for cement paste) suggest that results obtained from cement paste are not suitable for concrete as they tend to overestimate the concrete pore solution alkalinity. The alkali concentration threshold varied from one mixture to another (205-335 mmol/l).