Investigation of autoclave methods for determining alkali-silica reactivity of concrete aggregates
Alkali-silica reaction (ASR) is a deleterious reaction in concrete between alkalis in the pore solution and reactive forms of silica found in some aggregates. The product is a gel that absorbs water and exerts an expansive pressure on the concrete, leading to cracking, reductions in the concrete’s mechanical properties, and further damage caused by the ingress of water through the cracks. Since the 1980s, researchers have developed and investigated the use of autoclave test methods to quickly and accurately identify potential reactive aggregates. Autoclave methods involve boosting the alkali content of the concrete or mortar and conditioning the specimens at elevated temperatures in a closed, steam environment. Expansion results are obtained in a matter of days, and some autoclave test methods have shown promise in correctly classifying aggregate reactivities. The objective of this research was to investigate autoclave test methods for determining ASR potential in concrete aggregates. Methods by which autoclave testing were evaluated included expansion measuring, determination of the degree of alkali leaching, pore solution chemical analysis, and petrography of the hardened concrete. Additionally, test parameters such as alkali loading, autoclaving temperature, and autoclaving duration were altered to identify appropriate ranges for testing fine aggregates in mortar bars. Repeatability of the autoclave test methods was also examined. Aggregate reactivity classifications from autoclave expansions generally agreed with classifications from ASTM C1293 and showed less agreement with those from ASTM C1260. Alkali leaching from both mortar and concrete specimens was less than what has been observed for concrete prisms in ASTM C1293. Sulfate concentrations in pore solutions were high and resolved the charge imbalance initially observed between the alkalis and hydroxides. Petrographic examinations of two reactive concrete prisms showed the presence of ASR gel. Appropriate ranges of alkali loading, autoclaving temperature, and autoclaving duration are recommended for testing fine aggregates in mortar bars. Within- and multi-laboratory variability for autoclaved mortar bars and concrete prisms were low and between those of ASTM C1260 and ASTM C1293. Finally, integration of the autoclaved concrete prism test and the 5-hour autoclaved mortar bar test into ASTM C1778 protocol for mitigating ASR is suggested and outlined.