Sources: Concrete Sustainability Hub, Cambridge, Mass.; CP staff
In their current Research Brief, “Simulating the Formation of ASR Gels,” staff at the Massachusetts Institute of Technology-hosted Concrete Sustainability Hub (CSHub) shed new light on conditions, mechanics and protracted timelines surrounding alkali-silica reactivity in concrete.
“Although it has received much research attention, an essential question remains—how does a soft gel, as is formed by this reaction, induce the critical level of stress to exceed the strength of, and crack the concrete matrix?” researchers explain. “One possibility is that the ASR gel increases in viscosity as it imbibes calcium with age and a concomitant expansion causes the concrete failure. Another is that the gel’s rate of flow into the porous microstructure is slower than the rate at which it forms, which builds up stresses that cannot be relieved.”
CSHub researchers opted to simulate gel formation at the atomic level, noting, “This new method enables us to evaluate the impact of the water to silicon ratio. Modeled gels respond to chemical changes, such as water content, and relative composition of alkali and calcium. More importantly, [the method] demonstrates that drying the gel strongly affects its structure. This drying effect has not been considered in past research and provides an opportunity to reconsider [previously] proposed mechanisms … Additionally, we observed a reduction in the volume of gel as it calcifies, which offers further insight into reasons for concrete failure.”
Brief authors and MIT Research Scientists Romain Dupuis and Rolland Pellenq conclude: Better control of ASR requires knowledge of gel’s molecular structure; a simulation approach at the atomic level is an efficient way of examining gel structure; and, progress in simulation techniques could help characterize gels, advancing the industry’s approach to ASR. Their work continues with oversight of CSHub sponsors, the Ready Mixed Concrete Research & Education Foundation and Portland Cement Association.