A Solid Understanding

Concrete is the most widely used man-made material on earth. But over time, it deforms and weakens when subjected to sustained load. “Concrete is like chewing gum,” says Franz-Josef Ulm, a professor of civil engineering. “When you hold the ends of a stick of gum in both hands and pull, it weakens in the center. Concrete sags over time like that.” That sagging, known as creep, decreases concrete’s durability and shortens the life span of everything from bridges to containment vessels for nuclear waste.

Now Ulm’s research has pinpointed the cause of the problem: the rearrangement of particles at the nanoscale.

“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,” says Ulm, coauthor of a recent paper in the Proceedings of the National Academy of Sciences. “Our research lays the foundation for rethinking concrete engineering from a nanoscopic perspective.”

Such rethinking is clearly in order, given that much of the U.S. infrastructure is made of concrete–and the American Society of Civil Engineers has assigned it an overall grade of D. If concrete creep can be slowed, new structures–or old ones renovated with new materials–might last hundreds of years instead of tens, saving money and decreasing the carbon dioxide emissions related to concrete production. An estimated 5 to 8 percent of all human-generated atmospheric carbon dioxide worldwide comes from the concrete industry.

Ulm has spent nearly two decades studying the mechanical behavior of concrete and its primary component, cement paste. In a 2007 paper, he showed that the basic building block of cement paste at the nanoscale–calcium silicate hydrate, or C-S-H–is granular and, when mixed with water, naturally self-assembles in two structurally distinct but chemically similar phases. The particles approach the maximum natural packing densities for spherical objects: 64 percent in one phase and 74 percent in the other. But they still have room to shift around when pressure is applied.

In their paper, Ulm and coauthor ­Matthieu Vandamme, PhD ‘08, explain that this shifting is what causes concrete creep. They also explain that a third, denser phase of C-S-H can be induced by carefully manipulating the cement mix with other minerals–such as silica fumes, a waste material of the aluminum industry. This treatment forms additional, smaller particles that fit into the spaces between the nanogranules of C-S‑H, which were formerly filled with water. The C-S-H can thus reach a density of up to 87 percent, so the granules have much less opportunity to move.

To quantify creep, the researchers used a nanoscale device to apply pressure to the C-S-H for several minutes and measured the resulting indentation. They determined that the rate of creep is logarithmic, which means that slowing creep would increase concrete’s durability exponentially: 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 (UHD) concrete such as the silica-treated material.

Since 20 billion tons of concrete per year are currently produced worldwide, Ulm says, optimizing UHD concrete could have enormous environmental implications. Builders could use less concrete, and the resulting structures would be more durable.

“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,” says Ulm. “With this new understanding of concrete, we could produce filigree: light, elegant, strong structures that will require far less material.”