While concrete it the world's most used construction material, many questions still linger about it's microscopic structure and behavior. In a new paper published by researchers from MIT, Georgetown University, and France’s CNRS (together with other universities in the U.S., France, and U.K.), they claim to have solved the riddle.

While Rolland Pellenq, a senior research scientist in MIT’s department of civil and environmental engineering, director of the MIT-CNRS lab <MSE>2 hosted by the MIT Energy Initiative, and a co-author of the new paper, says that while the first scale model of concrete has been constructed questions still remain. As to the question of its structure it is both a a continuous matrix and an assembly of discrete particles:

“You can always find a smaller grain to fit in between” the larger grains, Pellenq says, and thus “you can see it as a continuous material.” But the grains within the CSH “are not able to get to equilibrium,” or a state of minimum energy, over length scales involving many grains, and this makes the material vulnerable to changes over time, he says. That can lead to “creep” of the solid concrete, and eventually cracking and degradation. “So both views are correct, in some sense,” he explains.

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