Concrete is one of the world’s most popular building materials, with over 20 billion tons produced annually around the world. The production of concrete alone accounts for 5 to 10 percent of the world’s overall atmospheric CO2, so cutting down on its production could have major environmental benefits. Civil engineers at MIT have finally discovered what causes “creep,” the deformation of concrete that occurs over time. This discovery is the first step toward prolonging the life of concrete by slowing the rate at which creep occurs.
The MIT team discovered that creep is caused by tiny nano-scale rearrangements of particles. The discovery means that the engineers can use this new understanding of concrete creep to create incredibly long-lasting buildings and infrastructure objects. Until now, concrete structures have had to be pretty thick in order to remain stable over the years. Thin concrete succumbs to creep quickly, so lightweight concrete buildings have always been impossible.
“Finally, we can explain how creep occurs,” said Professor Franz-Josef Ulm, co-author of the PNAS paper. “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.”
“This research comes at a time when the American Society of Civil Engineers has assigned an aggregate grade of D to U.S. infrastructure, much of which is made of concrete. It likely will lead to concrete infrastructure capable of lasting hundreds of years rather than tens, which will bring enormous cost-savings and decreased concrete-related CO2 emissions. An estimated 5 to 8 percent of all human-generated atmospheric CO2 worldwide comes from the concrete industry.”
“Ulm, who has spent nearly two decades studying the mechanical behavior of concrete and its primary component, cement paste, has in the past several years focused on its nano-structure. This led to his publication of a paper in 2007 that said the basic building block of cement paste at the nano-scale — calcium-silicate-hydrates, or C-S-H — is granular in nature. The paper explained that C-S-H naturally self-assembles at two structurally distinct but chemically similar phases when mixed with water, each with a fixed packing density close to one of the two maximum densities allowed by nature for spherical objects (64 percent for the lower density and 74 percent for high).”
In the future, concrete structures could be lightweight and thin while using far less material than current generations. And because they would be far stronger and longer-lasting than our current concrete structures, they will require fewer repairs and will last longer than any other concrete buildings. In theory, this nano approach to concrete engineering could result in concrete that lasts for thousands of years.