Quantitative assessment of ASR potential of quartz-rich aggregates: comparison of chemical test and accelerated mortar bar test improved by SEM-PIA
Abstrakt
Alkali silica reaction (ASR) originates in highly alkaline conditions in concrete where reactive forms of silica are available causing serious damages on concrete structures. ASR potential of various quartz-rich aggregates (pegmatite quartz, quartzite, quartz meta-greywacke, and chert) was quantified employing 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 evaluated as deleterious, according to the CT; the other aggregates were classified as innocuous. The AMBT indicated different results.
Quartz meta-greywacke, chert, and quartzite exceeded the 0.100% expansion limit after the 14-day test period. The 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 aggregate/cement paste interface.
The medium (resp. low) degree of ASR was confirmed in AMBs containing quartzite (resp. pegmatite quartz). 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 ASR potential of pegmatite quartz. Variable ASR potential of investigated aggregates was explained by significant content of poorly crystalline matrix (in chert and quartz meta-greywacke), and by presence of strained quartz typical with undulose extinction and origin of quartz subgrains (in quartzite and pegmatite quartz).