14 DAY TRIAL // Concrete is, arguably, the most widespread construction material in the world and, as such, any vulnerabilities it may have are of direct interest to governments anywhere. It makes up most of the current global infrastructure, including most tall buildings, large bridges and roads. Still, after decades in use, the concrete inevitably becomes useless, as it deforms. Finally, researchers at the Massachusetts Institute of Technology have managed to figure out why this happens, and to hint at possible solutions for this global problem. They say the most pertinent observations were made at the nanoscale.
In a paper published online during the week of June 15th, in the Early Edition of the journal Proceedings of the National Academy of Sciences (PNAS), civil engineers at MIT, led by Professor Franz-Josef Ulm, report that concrete creep is directly caused by the fact that particles making up the strong mix rearrange themselves at the nanoscale. Creep is the technical term used to describe the deformations that concrete experiences when subjected to constant loads.
“Finally, we can explain how creep occurs. We can't prevent creep from happening, but if we slow the rate at which it occurs, this will increase concrete's durability and prolong the life of the structures. Our research lays the foundation for rethinking concrete engineering from a nanoscopic perspective,” Ulm, who is also a co-author of the PNAS paper, says. With an estimated five to eight percent of the world's carbon emissions coming from the concrete industry, this research is suddenly placed in a new perspective. Materials that could last hundreds of times longer could mean massive savings, both as far as money and the damage caused on the environment go.
The new investigation revealed that calcium-silicate-hydrate (C-S-H) particles, the basic assembly blocks of cement paste, the “active ingredient” in concrete, rearrange at the nanoscale, either aggregating in clumps, or moving apart from each other. This causes the deformations in the concrete over the years. However, C-S-H do not only exist in the two states currently available, but also in a third state, which can be obtained with the help of a waste material of the aluminum industry, silica fumes. In fact, the engineers say, a host of other mineral materials work just as well.
When applied inside experimental concrete, the minerals increased the density of C-S-H with about 87 percent, and gradually hindered the movement of the particles. “There is a search by industry to find an optimal method for creating such ultra-high-density materials based on packing considerations in confined spaces, a method that is also environmentally sustainable. The addition of silica fumes is one known method in use for changing the density of concrete; we now know from the nanoscale packing why the addition of fumes reduces the creep of concrete. From a nanoscale perspective, other means now exist to achieve such highly packed, slow-creeping materials,” Ulm explains.
“The insight gained into the nanostructure puts concrete on equal footing with high-tech materials, whose microstructure can be nanoengineered to meet specific performance criteria: strength, durability and a reduced environmental footprint,” the other co-author of the paper, Matthieu Vandamme, PhD, from the Ecole des Ponts ParisTech, Université Paris-Est, in France, adds. He got his PhD in 2008, from the MIT Department of Civil and Environmental Engineering.