By Alice C. Waugh
Civil & Environmental Engineering

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 micro-scale.

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, they are interested in how this interface changes when it gets wet. The researchers 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, said Oral Buyukozturk, professor in the Department of Civil and Environmental Engineering (CEE). Buyukozturk, CEE Professor Markus Buehler and graduate students Denvid Lau and Chakrapan Tuakta Ph.D. ’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.

Previous research has shown that the presence of moisture increases the likelihood of delamination between concrete and epoxy in situations where epoxy is used to attach fiber-reinforced polymer to the concrete. Researchers had also found that the presence of water changes the way in which the interface starts to fail: moisture makes it more likely to fail because of separation of the layers of material at the interface rather than from cracking in one of the materials itself.

“When water seeps in, it changes the dynamics of the system on a molecular level, though exactly how that happens is unclear,” Buyukozturk said.

The new study was able to quantify the decrease of adhesive energy in the interface by examining the changes in the physical forces of attraction and repulsion between molecules in the two materials – changes that can lead to failure of the concrete-epoxy interface. When peel and shear forces were measured, the simulation showed that in a “wet” scenario, adhesive energy decreased by approximately 15 percent compared to a “dry” scenario. This reduction at the molecular scale may translate into greater adhesive energy reductions in large-scale structures, because local deterioration caused by moisture at different locations of the structure can lead to stress concentrations that can compromise the overall structure.

“The molecular modeling result validates our hypothesis that the adhesive strength of the interface is weakened due to interaction between epoxy and water, and provides a detailed chemistry-based view on the mechanical properties of the interface,” the authors wrote.

When moisture comes in contact with the epoxy bonded on the silica surface, the water molecules can go into the gaps between epoxy and silica and interfere with their interaction, leading to a weakened interface and the eventual debonding of the epoxy.
Image / Buyukozturk, Buehler, Lau and Tuakta

Molecular dynamics simulation was first developed in the late 1950s to study the dynamics of a system consisting of several hundred particles. The technique has been more widely applied as computational power has increased and is now used in fields including molecular biology and protein modeling, and increasingly as a powerful tool in computational mechanics.

The continuum approach gives information about materials on a scale of millimeters to meters, while the atomistic approach used in molecular simulation is useful for studying distances of 0.1 to 100 nanometers in materials. A “hand-shaking region” where information can be exchanged between the two regions may be somewhere from 1 to 100 micrometers. The next challenge is further study of that hand-shaking region to bridge the emerging understanding of materials at the nano-level with existing macro-level knowledge and improve the accuracy of predictions about the deterioration and life cycles of large civil structures.

“By advancing the understanding of civil structures from the realm of structures and materials into the domains of chemistry, physics and mathematics, molecular simulation may do for engineering analysis what the introduction of the now-standard mathematical technique of finite element analysis did for continuum mechanics four decades ago,” Buyukozturk said

The work was supported by the National Science Foundation.