The researchers of Ruhr-Universität Bochum have developed a method of spraying steel-fiber-infused shotcrete, which makes tunnels and bridges more robust against fire.

The researchers of Ruhr-Universität Bochum have developed a method of spraying steel-fiber-infused shotcrete, which makes tunnels and bridges more robust against fire. Goetz Vollmann, assistant professor at the university's Institute for Tunnelling and Construction Management says "When today's tunnels were built, nobody had foreseen that one day a madman with a bomb may run inside. Bizarrely, in the 1950s and 1960s Europeans were building bridges that were partly fitted out with so-called explosion chambers, having retrospectively learned from the Second World War. Accordingly, the structures have predetermined breaking points, the purpose of which is to ensure that a bridge can be blown up in order to cut off the enemy's supply lines."

The new formula is easy to be applied on surfaces of any shape. "The problem was naturally the workability of the shotcrete with the needed levels of steel and PP [polypropene] fibers." Vollmann added. The research team focused on the workability of the wet mix and they concluded that air-entraining chemicals are essential for the concrete mixture.

Despite the fact that air entrainment is used very commonly in the concrete industry the research team went far beyond the typical levels for this application. "For our test series, we simply ignored the normal levels of air in a concrete mixture and instead tried to go to extremes, looking at how much air we could add and how the mixture would react." Vollmann admits.

Due to the fact that the cost for this method is very high, it is impossible this formula to be applied on every structure.