TunnelsUsing shotcrete to make tunnels withstand terrorist attacks
Conflagrations and terrorist attacks are a threat for tunnels and bridges, so engineers are searching for ways to make tunnels and bridges as robust as possible. Construction materials, such as special types of high-performance concrete, which can partly absorb the impact of explosions, already exist, but due to their manufacturing principle, they cannot be made in any other shape than the slab, which cannot be used for cladding surfaces with complex geometries. A new type of shotcrete — which used to be considered impossible to manufacture — was created by scientists to render the structures more robust. Despite its high steel and synthetic-fiber contents, it can be sprayed on easily.
A two-by-one-meter concrete block is lying in the open. “Watch out! Three, two, one,” a voice is counting down. Then, a loud crash, dust swirls through the air. This explosion would have left a crater in traditional concrete. The experiment, however, results in only a handful of scratches. This is because the stone block is coated with shotcrete which was developed by Ruhr-Universität Bochum (RUB) engineers in order to ensure better protection of building structures from large fires and terrorist attacks.
A RUB release reports that for seven and a half years, the team headed by Dr. Götz Vollmann from the Institute for Tunneling and Construction Management has been studying issues surrounding tunnel safety under the umbrella of a project sponsored by the German Federal Ministry of Education and Research (see also “Psychology in the tunnel”).
“When today’s tunnels were built, nobody had foreseen that one day a madman with a bomb may run inside,” says Vollmann. “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.”
This, however, is counterproductive if one wants to protect them from terrorist attacks. Today, engineers are searching for ways to make tunnels and bridges as robust as possible. Construction materials, such as special types of high-performance concrete, which can partly absorb the impact of explosions already exist. Due to their manufacturing principle, however, they cannot be made in any other shape than the slab, which cannot be used for cladding surfaces with complex geometries.
“This has been a problem for years,” explains Vollmann. A problem for which he had a solution in mind: shotcrete. This is because it is easy to apply on surfaces of any shape.
A concrete which is required to be ultra-solid must contain as many steel fibers as possible. Those fibers, however, render it stiff and unsuitable for getting pumped through a hose to be sprayed on a surface. “It used to be said that approx. seventy kilogram steel fibers per cubic meter of concrete are just about as good as it gets. That’s the absolute limit in terms of what is still processable,” explains Vollmann.
