MULTI-DISCIPLINARY APPLICATION OF GEOLOGY AND ENGINEERING TO UNDERSTANDING AND MAPPING AGGREGATE QUALITY IN IDAHO

The Idaho Geological Survey, in conjunction with the Idaho Department of Transportation (ITD), recently completed a project that combined lithologic characterization of aggregate in Idaho, petrographic observations, ArcMap analysis of geologic maps, and civil engineering techniques. The applied nature of the technical problem and the diverse methods used are reminiscent of the wide-ranging approach championed by Eric Cheney in his long and honorable career studying geology of the Pacific Northwest.

Alkali-silica reactivity (ASR) is a deleterious reaction that causes premature cracking in concrete surfaces and structures worldwide. It develops when amorphous silica in aggregate reacts with alkalis in cement to form an expansive gel that cracks the concrete prematurely. ASR has long been known to be a problem in Idaho; thus, the ITD uses numerous standard engineering practices and tests to avoid ASR-affected concrete. Only certain lithologies of aggregate are likely to cause ASR, and our study was instigated to better understand the geographic patterns and causes of high ASR potential within Idaho. We sampled 40 aggregate sources qualified to produce concrete for ITD. Clast lithologies were identified quantitatively, and the composition of each source compared to geologic maps. ArcMap was used to delineate watersheds and the corresponding geology for the aggregate sources, which were primarily of fluvial origin.

ASR potential was determined from results of a standard commercial engineering test, the AASHTO T 303, for the 40 unmitigated sources. Reactive lithologies were identified within the sources from petrographic examination of mortar bars from the AASHTO T 303 test and from affected concrete pavement. Experimental mortar bars were also made with specific lithologies. For the Idaho aggregate, lithologies with the highest ASR potential were chalcedony and opal (veins and Pedogenic coatings), Miocene-Pleistocene rhyolites and obsidian from the Snake River Plain, as well as some siliceous quartzites and impure sandstones in south-central Idaho. Identification of lithologies with higher or lower ASR risk and their geographic distribution is useful information for transportation planners, engineers, and geologists, as well as material source operators.