A study of reactive silica dissolution for determining potential alkali silica reactivity in concrete
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Alkali silica reaction (ASR) has been recently identified as an issue relating to problematic performance characteristics of aggregates sourced in western Washington State. An abundance of literature on ASR attributes minerals and rocks containing amorphous silica as initiating the chemical and physical mechanisms that damages concrete. The geological source, composition, and quantity of these reactive aggregates contained in local aggregate resources is unknown. Therefore, an understanding of the structure and properties of amorphous silica in reactive aggregates is essential to the assessment of ASR and consequent expansion in concrete structures. To better discern this problem, a series of experiments were conducted using two different sources of aggregate (local sand ìBHî, and a control sand ìMTî) to observe the behavior of reactive silica in concrete. First, I conducted a point count of grains to determine the composition of each aggregate. Then, under guidance of accelerated mortar bar test ASTM C1260, I mixed 34 mortar bars using 0%, 2%, 4%, 6%, 10%, and 20% chert in the aggregates and measured the resultant expansion. Using the remaining product of alkali solution from the mortar bar tests, I measured the concentrations of NaOH in 10 samples and recorded the consumption of alkali as a result of silica dissolution. I then cut the mortar bars into thin sections and observed reaction within chert grains. From five thin sections, I modeled reaction rate of silica dissolution in chert. The maximum expansion, or pessimum limit, was recorded at ~0.24% using BH sands containing 2% chert, and at ~0.25% using MT sands containing 10% chert. BH sands, containing 28% volcanic lithics, expanded ~0.20% without added chert. MT sands, containing no volcanic lithics, expanded below deleterious threshold limits, at ~0.032%. From the titration trials, a simple linear regression was inferred between alkali concentration and mortar bar expansion. A maximum of 0.068 moles of alkali was consumed by the highest expansive mortar bar, at ~0.25%. This indicated that remaining free alkali was in excess and therefore, not the limiting reactant in observed maximum expansion limit of mortar bars. Finally, measurements of reaction depth were approximated for 25 chert grains from thin sections of mortar bars containing BH sand and 10% chert. Using values of these measurements, the diffusivity of silica dissolution was found to be between 9.5E-16µm2/s and 8.0E-15µm2/s. I propose that the concentration of NaOH solution from ASTM C1260 tests, and the diffusivity of silica dissolution are applicable in further assessing aggregate susceptibility to ASR. While this project confirms that chert causes ASR in concrete, it was not found in the point count survey of the local aggregate (BH sands). Instead, volcanic lithics that are predominantly andesitic are suspected to be the primary source of reactive aggregates in these sands. It is recommended that similar experiments presented in this report are conducted to determine the reactive properties of volcanic lithics that are used as concrete aggregates.