Quantitative assessment of alkali silica reaction potential of quartz-rich aggregates: comparison of chemical test and accelerated mortar bar test improved by SEM-PIA

Abstract

The alkali silica reaction (ASR), which originates in highly alkaline conditions in concrete where reactive forms of silica are available, causes serious damage to concrete structures. The ASR potential of various quartz-rich aggregates (pegmatite quartz, quartzite, quartz meta-greywacke, and chert) was quantified using accelerated mortar bar test (AMBT), chemical test (CT), and scanning electron microscopy combined with petrographic image analysis (SEM-PIA). Only two samples (quartz meta-greywacke, and pegmatite quartz) were judged as deleterious according to the CT; the other aggregates were classified as innocuous. AMBT gave different results. Quartz meta-greywacke, chert, and quartzite exceeded the 0.100 % expansion limit after 14 days of testing. Pegmatite quartz indicated a lower value. The results of SEM-PIA confirmed the results of AMBT, indicating the most extensive ASR in those AMBs containing chert and quartz meta-greywacke. Parts of aggregates were leached out and massive deposits of alkali–silica gels were found filling air voids, microcracks, and the aggregate/cement paste interface. The medium or low degree of ASR was confirmed in AMBs containing quartzite or pegmatite quartz, respectively. ASR accentuated pre-existing microcracks and formed new dissolution gaps. In contrast, no correlation was found with the results of CT, which under-evaluated the ASR potential of chert and quartzite and over evaluated the ASR potential of pegmatite quartz. The variable ASR potential of the investigated aggregates was explained by the significant content of poorly crystalline matrix (in chert and quartz meta-greywacke), and by the presence of strained quartz typical with undulose extinction and the origin of quartz subgrains (in quartzite and pegmatite quartz).