Calcium silicate hydrate (C-S-H) granules’ tendency to rearrange into altered densities–some looser, others more tightly packed–leads to creep, the deformation concrete experiences under loading
Source: Massachusetts Institute of Technology, Cambridge
Calcium silicate hydrate (C-S-H) granulesÌ tendency to rearrange into altered densities–some looser, others more tightly packed–leads to creep, the deformation concrete experiences under loading. In a National Academy of Sciences paper, MIT engineers contend that their nano-scale observations of C-S-H allow measurement of specimensÌ creep properties in minutes versus the conventional method of gauging deformation trends over a year-long period at macroscopic view.
“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, contends co-author Professor Franz-Josef Ulm. The paper shows experimentally that creep rate is logarithmic, he adds, which means slowing the process increases durability exponentially. Demonstrating mathematically that creep can be slowed by a rate of 2.6, he cites a remarkable potential effect on concrete durability: A nuclear waste containment vessel built to last 100 years with current mix designs could last up to 16,000 years if made with an ultra-high-density concrete.
In a 2007 paper reflecting on his initial nano-scale study of concrete and cement paste, Ulm 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, 64 percent or 74 percent, allowed by nature for spherical objects. In follow-up research, he notes that a third, denser phase of C-S-H can be induced by manipulating the mix design with other minerals, such as silica fume. Its smaller particles fit between the nano-granules of C-S-H, spaces formerly filled with water; effectively increasing the density of C-S-H up to 87 percent, silica fume in turn greatly hinders C-S-H granulesÌ movement over time.
“The thinner the structure, the more sensitive it is to creep, so up until now, we have been unable to build large-scale lightweight, durable concrete structures,” explains Ulm. “With this new understanding of concrete, we could produce filigree: light, elegant, strong structures that will require far less material.”
His co-author on the new paper is Matthieu Vandamme, Ph.D., a 2008 MIT Department of Civil and Environmental Engineering graduate now on the faculty of the Ecole des Ponts ParisTech, Universit» Paris-Est. Paris-based Lafarge Group partly funded the C-S-H nano-scale research.