MAPPING IDAHO AGGREGATE QUALITY BY INTEGRATING GEOLOGIC MAPS WITH PETROLOGIC OBSERVATIONS

Alkali-silica reactivity (ASR) is a deleterious reaction that causes premature cracking in concrete surfaces and structures worldwide. ASR develops when amorphous and microcrystalline silica in aggregate reacts with alkalis in cement to form an expansive gel that initiates and promotes cracking in concrete. In Idaho, ASR has been recognized as a significant problem for the Idaho Transportation Department (ITD), and numerous tests and mitigation methods are required to avoid ASR-affected concrete. However, because only certain lithologies of aggregate used in concrete are likely to cause ASR, a geologic research study was instigated to better understand both the causal and geographic patterns of high ASR potential within Idaho.

This project involved statewide sampling of 40 aggregate sources qualified to produce concrete for ITD. Clast lithologies in the aggregate, primarily fluvial gravels, were identified and their proportions determined quantitatively. The lithologic composition of each source and group of sources was then compared to geologic maps, including the new Idaho state geologic map (Lewis et al., 2012) plus more detailed geologic mapping where available. ArcMap was used to delineate watersheds and to clip out the geology of relevant watersheds upstream of sampled sources.

The geologic units were compared spatially with results of a standard commercial test for ASR potential, the AASHTO T 303, for the 40 sources and reactive lithologies were identified. Petrographic observations of the mortar bars from the AASHTO T 303 confirmed lithologies with highest ASR potential: chalcedony and opal (veins and pedogenic coatings), Miocene-Pleistocene rhyolites and obsidian from the Snake River Plain, and certain siliceous quartzites and impure meta-sandstones in south-central Idaho. Spatial analysis and petrography identified other units, such as Eocene rhyolites and dacites, as moderately reactive for ASR. Geographic distribution of the map units which likely correspond to the lithologies of higher or lower ASR risk provides important information for ITD planners and geologists, as well as the material source operators.