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Unlike materials such as wood and insulation, concrete doesn't ignite. It has become the backbone of modern construction, from stairs to floors to towering office buildings. While its resistance to flames is an advantage, concrete does have a weakness—exposed to extreme heat, it can detonate like popcorn in the microwave. Researchers are gaining more insight into why this happens.
A recent study published in the journal Cement and Concrete Research shed light on this unexpected occurrence. Scientists from Empa, the Swiss Federal Laboratories for Materials Science and Technology; the University of Grenoble, and Institut Laue-Langevin conducted experiments with concrete blocks, which are composed of cement, sand, water, and various additives to enhance strength or reduce permeability. When heated to 1112°F, parts of the concrete block exploded.
By employing neutron tomography, the researchers were able to observe how water accumulated as the concrete block was heated.
The underlying principle is quite similar to how popcorn kernels pop. In both cases, water is heated, turns into vapor, and gets trapped. With no way to escape, the built-up pressure is released, causing the concrete to become pressurized and fracture.
One reason the vapor gets trapped is because water tends to move away from the heat source—like a fire in an interior room—and towards the cooler areas of the concrete. This movement creates a moisture barrier, blocking vapor from escaping through the concrete's pores. High-performance concrete, commonly used in commercial structures, has fewer pores, which means the vapor pressure builds up more intensely. The water in the mix can turn into vapor at temperatures as low as 392°F.
Why is it crucial to understand this phenomenon? By studying how and why concrete can explode, new additives can be developed to reduce or eliminate this effect, ultimately leading to safer, more fire-resistant buildings.
