MIT engineers discover origin of concrete creep

Summer 2009

PROBLEM

Concrete, the solid that forms at room temperature when Portland cement is mixed with water, sand and aggregates, is the oldest engineered building material, as well as the most widely used: 20 billion tons per year worldwide with a 5 percent increase annually. But concrete suffers from time-dependent deformation under load. This “creep” deteriorates the durability of the material and truncates the lifespan of concrete structures. Despite decades of research on the subject, the origin of creep remained unknown, until now.

APPROACH

Professor Franz-Josef 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 the material’s nanostructure. This led to publication of his paper in 2007 that said the basic building block of cement paste at the nanoscale — calcium-silicate-hydrates or C-S-H — is granular in nature. 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). Using a nanoindentation device, Ulm and Matthieu Vandamme were recently able to apply load to the C-S-H and measure in minutes creep properties that are usually measured in year-long creep experiments at the macroscopic scale.

FINDINGS

Ulm and Vandamme show that concrete creep occurs when these nanometer-sized C-S-H particles rearrange into altered densities — some looser and others more tightly packed — and that a third, more dense phase of C-S-H can be induced by carefully engineering the cement mix. This can be achieved, for example, by lowering the water-to-cement ratio; adding other minerals, such as silica fumes, a waste material of the aluminum industry; or applying heat treatment. This ultra-high-density phase is formed when additional smaller particles fill the spaces between the nanogranules of C-S-H, spaces that were formerly filled with water. This has the effect of increasing the density of C-S-H to up to 87 percent, which in turn greatly hinders the movement of the C-S-H granules over time.

The researchers show experimentally that the rate of creep is logarithmic, and demonstrate mathematically that creep can be slowed by a rate of 2.6. That would have a truly remarkable effect on durability. A containment vessel for nuclear waste built to last 100 years with today’s concrete could last up to 16,000 years if made with an ultra-high-density concrete.

IMPACT

This research places concrete on an equal footing with high-tech materials, whose microstructure can be nanoengineered to meet specific performance criteria of strength, durability and reduced environmental footprint. It likely will lead to concrete infrastructure capable of lasting hundreds of years rather than tens and could alter structural designs, as well as have enormous environmental implications. An estimated 5 to 8 percent of all human-generated atmospheric CO2 worldwide comes from the concrete industry.

MORE

An Ulm and Vandamme paper describing this work appeared in the Proceedings of the National Academy of Sciences in the online early edition the week of June 15. Vandamme earned the Ph.D. from MIT’s Department of Civil and Environmental Engineering in 2008 and is now on the faculty of the Ecole des Ponts ParisTech, Université Paris-Est.

The image shows the imprint left by a nanoindenter in a particle of cement paste. The horizontal units are micrometer; vertical units are nanometer. The round blob is actually an extremely fine piece of dust on the surface. Photo / Ulm Lab and Chris Bobko, North Carolina State University

Download this issue as a pdf.