For more than a century, engineers studying the deterioration of materials used to build bridges, roads and dams have relied on continuum mechanics, an approach that looks at the material as one mass rather than a collection of atoms, and assumes that at any area of a structure, the material will follow certain rules of behavior in response to stress. While useful for studying materials on a large scale, the continuum approach doesn’t reveal much about what’s happening with the material at the microscale.

MIT researchers have used a technique called molecular-dynamics simulation to study how materials interact at the molecular level and recently applied it for the first time to take a close look at the interface between epoxy and silica, one of the primary molecules forming concrete. Epoxy is often used to bond a stretchy supportive fabric or a thin plate made of reinforced polymer composites to concrete structures in order to increase the strength and durability of the structure. Specifically, the researchers are interested in how this interface changes when it gets wet. They hope their work will introduce a new paradigm for structural and design engineers to use when predicting the lifespan of building components and large structures.

Using the classical continuum mechanics model, engineers have learned how epoxy and concrete behave as separate and homogenous materials. “But this is not sufficient to understand the fundamentals of deterioration” where the atoms in epoxy molecules interact with silica and other atoms in concrete — especially when that epoxy-concrete interface is exposed to water, says Oral Buyukozturk, professor in the Department of Civil and Environmental Engineering (CEE). Buyukozturk, CEE Associate Professor Markus Buehler and graduate students Denvid Lau and Chakrapan Tuakta PhD ’11 co-authored a paper published in an April issue of the International Journal of Solids and Structures that describes their use of molecular-dynamics simulation to study an epoxy-silica interface from a fundamental perspective that unifies chemistry and mechanics.

Read a news story: http://cee.mit.edu/news/releases/2011/epoxy-silica-interface

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