Concrete observed when exploding

Fine porosity and moisture barrier in the concrete lead to overpressure in the material

In tunnel fires, concrete becomes a major hazard as it can explode in heat. Why, now show pictures from the inside of the concrete. ©
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Concrete does not burn, but can explode when heated - for example, during tunnel fires. Why this is so, researchers have now cleared with the help of real-time images of exploding concrete. Accordingly, a moisture barrier is created in the concrete in the heat, which prevents the water vapor from escaping. As a result, the steam accumulates, until the whole thing bursts under pressure like a steam boiler.

Whether for bridges, buildings or tunnels: Almost all constructions of modern times are made of concrete - a building material that even the Romans used 2, 000 years ago. However, as stable as the tubes of tunnels such as the Gotthard Base Tunnel are - they have a weak point. In tunnel fires, the infernal temperatures of up to 1, 000 degrees bring the concrete to its limits - and depending on the material used, it can explode.

Neutron view into the interior of the concrete

But how can that be? What happens with such a concrete explosion in the material has now been discovered by Dorjan Dauti of the Federal Materials Testing and Research Institute Empa and his team. Using neutron tomography, they were able to take real-time images of heated concrete for the first time, creating a three-dimensional model of the process.

Even before, there was a suspicion that the water bound in the cement plays a decisive role in the concrete explosion. When the temperature in a fire exceeds 200 degrees Celsius, the cement dehydrates in the concrete and the bound water becomes water vapor. The enormous vapor pressure in the fine pores of the concrete and the thermal load of the material can then cause parts to be blown off.

Moisture as a vapor barrier

Now the images reveal that when the concrete gets hot, the water bound in the concrete moves away from the heat source and builds up - creating a moisture barrier. In addition to the fine porosity and low porosity of high-performance concrete, this barrier helps to prevent the resulting water vapor from escaping, as the researchers discovered. As a result, the vapor pressure in the concrete increases so much that the material finally left nothing but to explode. display

Researchers bring concrete to explode. Empa

So advantageous the fine porosity and low permeability of high performance concrete is therefore for its strength, so it is disadvantageous in the case of a tunnel fire. On the basis of their new findings, the Empa researchers have already developed and patented additives that are intended to prevent the formation of such moisture barriers in concrete and thus also the explosions. (Cement and Concrete Research, 2018; doi: 10.1016 / j.cemconres.2018.06.010)

Source: Empa - Confederation of Materials Testing and Research

- Nadja Podbregar