Long-Term Performance of Alkali–Silica Reactive Aggregates in a 63-Year-Old Bridge in Alaska

The microstructure analysis of cores from a decommissioned 63-year-old bridge in Alaska provided an opportunity to study alkali–silica reaction (ASR) products found in the field under low rates of reaction. As part of the microstructural analysis, aspects of the ASR gel, such as the morphology and chemical composition, were correlated with the location and mineralogy of the aggregate particle in which the gel formed. The resulting information was compared with previously published data on ASR products in field samples. The results showed that mineral phases adjacent to the crack, other than alkali–silica reactive ones, often influenced the chemical composition of infilling gels. The ASR gels formed inside quartzite particles had higher Ca/Si and lower (Na+K)/Si values than their counterparts found in rhyolite particles. Lastly, the reactivities and alkali thresholds of the aggregates were evaluated using the American Association of State Highway and Transportation Officials (AASHTO) TP 144-23 (Turner–Fairbank Alkali–Silica Reactivity Susceptibility Test) and the AASHTO T 416-24 (Alkali Threshold Test), respectively. The long-term ASR performance of a similar concrete mixture to the one used in the construction of the bridge was predicted by inputting this information in a newly proposed ASR risk-assessment chart and found to be consistent with field observations.