Why Heated Concrete Can Explode Like Popcorn

Unlike wood, insulation, and other building materials, concrete doesn't burn. Modern civilization has practically built its infrastructure on it, from stairs to floors to office towers. While not bursting into flames is a good thing, concrete does have vulnerability—exposed to high temperatures, it can explode like a bag of microwaved popcorn. And scientists are getting better at understanding why.

A recent paper published in the journal Cement and Concrete Research offered some insight into this unusual phenomenon. Researchers at Empa, the Swiss Federal Laboratories for Materials Science and Technology; the University of Grenoble, and Institut Laue-Langevin conducted an experiment using blocks of concrete, which typically consists of cement, sand, water, and other additives to increase strength or reduce permeability. Heated to 1112°F, portions of the block exploded.

Using neutron tomography, researchers were able to visualize the accumulation of water as the block heats up.

The principle behind it is largely the same as popping popcorn kernels. In both instances, heated water vaporizes and becomes trapped. With nowhere to go, the built-up energy is released, with the concrete becoming pressurized and breaking apart.

Part of the reason the vapor becomes trapped is because water moves away from the heat source—say, a fire in an interior room—and toward the cooler portion of the concrete. In doing so, the water acts as a moisture barrier, preventing vapor from coming through pores. High-performance concrete, which is often used in commercial applications, has few pores, which makes the pressure from the vapor more concentrated. The water in the mix can turn to vapor when temperatures reach 392°F.

Why is understanding this process important? By exploring how and why concrete can burst, additives can be developed to reduce or eliminate the effect. The result will be safer, more fire-resistant buildings.