Microscopic and Image Processing Characterization of Aggregates to Predict ASR Expansion Potential of Concrete

Preventing ASR occurrence in new concrete structures requires reliable and quick methods to identify reactive aggregates and to evaluate proper mitigation alternatives. The current accelerated mortar-bar expansion tests (ASTM C 1260 or ASTM C1567) and the concrete prism expansion test (ASTM C 1293) have been reported to have several limitations. Assessment of the extend of ASR damage in existing affected structures requires more understanding on how ASR expansion and damage develop in field conditions such as under confinements and under relatively slow rate of ASR reaction.

The significance of ASR expansion rate and bi-axial restrain on concrete degradation has been investigated. Results show that degradation of mechanical properties was significantly influenced by the rate of ASR expansion and slightly affected by confinement. Specifically, slow rate of ASR expansion was beneficial in limiting concrete degradation while the direction of restraint influenced the crack orientation.

A quantitative petrographic characterization was conducted to predict the ASR reactivity of Tennessee aggregates, mainly granite and limestone aggregates. The reactivity of limestone aggregates is strongly related to the amount of SiO². The silica content of 26 limestones shows a strong correlation with the expansion in the concrete prism test. The early and highly expansive limestones are characterized by a high clay/ mica content (> 5%). The reactivity of limestone aggregates is mainly caused by crypto/microcrystalline quartz, chert, and finely disseminated silica in carbonate and clay/mica matrices.

The reactivity of granite aggregates is related to the microstructural features of the minerals, namely the quartz grain size distribution and quartz grain boundaries. These features can be quantified using image processing of combined polarized microscope images. The grain size of quartz can only provide an indication of aggregate reactivity. The amount and distribution of quartz need to be considered to confirm aggregate reactivity and the potential expansion of an aggregate. Results indicated that an increase in the amount of quartz < 60 µm shows a corresponding increase in the ASR expansion. Granite aggregates with more than 7% of quartz < 60 µm exceed the expansion limits on both ASTM C1293 and ASTM C1260.

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