Preventing terrorist attacks with shotcrete
New material makes tunnels and bridges more robust
by Julia Weiler
May 4, 2015
A two-by-one-metre 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 (video). This explosion would have left a crater in traditional concrete. However, the experiment results in only a handful of scratches. That is because the stone block is coated with shotcrete that was developed by RUB engineers in order to ensure better protection of building structures from large fires and terrorist attacks. For seven and a half years, the team headed by Dr Götz Vollmann from the Institute for Tunnelling and Construction Management has been studying issues surrounding tunnel safety, for example under the umbrella of a project sponsored by the 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 Götz 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, that can partly absorb the impact of explosions do 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. “This has been a problem for years,” explains Vollmann. A problem for which he had a solution in mind: shotcrete. That is because it is easy to apply on surfaces of any shape.
A concrete that is required to be ultra-solid must contain as many steel fibres as possible. Those fibres, 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. 70 kilogram steel fibres per cubic metre of concrete are just about as good as it gets. That’s the absolute limit in terms of what is still processable,” explains Götz Vollmann. Together with external partners, his team has now realised a shotcrete with 140 kilogram steel fibres per cubic metre, and they added three kilogram of synthetic fibres on top of that. “Everyone with whom we discussed this in the run-up had said: you’ll never pull it off. And indeed, we were going to throw in the towel,” tells us the engineer. About to give up, the group had one last brainwave: air. The researchers foamed up the concrete until the mixture contained approx. 20 per cent air bubbles. “We assume that this process generates a ball-bearing effect of sorts,” says Vollmann. “The fibres presumably roll over the air bubbles, and everything is thus rendered smoother.” Accordingly, the concrete can be pumped through a hose and sprayed through a nozzle despite its high steel-fibre content. However, the air must not remain in the mixture, because it would reduce the strength of the concrete. Therefore, the RUB team built in a defoaming mechanism.
For the spraying step, the concrete is pumped through a nozzle. There, a substance is added by default which accelerates the solidifying process. “Otherwise, the concrete would be much too liquid and, at the required layer thickness, would slide off the wall,” explains Vollmann. “We have simply added a defoaming agent to the accelerating agent.” Small-scale lab experiments had shown that the defoaming agent takes effect instantaneously and extracts air from the concrete within the fraction of a second. This effect was verified on the large scale, following experiments with a shotcrete robot. The Institute for Tunnelling and Construction Management and the Institute for Construction Material Technology operate a shotcrete test rig (video). Its heart is a reprogrammable robot manufactured by the company “Kuka”, which is commonly deployed for welding cars. At the RUB campus, the robot applies shotcrete – layers with a thickness of several centimetres onto traditional concrete. The engineers conveyed a portion of the thus manufactured slabs to Leipzig, for fire tests in the material testing facilities there. Other slabs were tested by project partners from the Ernst-Mach-Institut at Fraunhofer in Freiburg in controlled explosion tests, in order to find out the shotcrete’s resistance capacities. By deploying shotcrete, they were indeed able to maintain up to 60 per cent of the remaining load-bearing capacity of the construction that was to be protected. To compare: in the same experiment setup, the remaining load-bearing capacity of unprotected concrete amounts to a mere 20 per cent (fig. 1).
Fig. 1© Fraunhofer, Ernst-Mach-Institut
Controlled explosion tests: the new shotcrete (right) was found to be much more robust than regular concrete (left).
It is of course not possible to apply the new shotcrete to all structures or to render them safer by another method. The costs would be much too high. Rather, it is necessary to determine which tunnels and bridges in Germany are particularly at risk. In a project managed by the Federal Highway Research Institute, Götz Vollmann’s team investigated this issue together with other partners. The group analysed which structures are crucial for a traffic infrastructure to work and which constructions are particularly vulnerable to fire or explosion damage. They developed a process which can be used to compile such a ranking of critical structures, the results of which cannot, however, be published for security reasons. “Our structures are actually more robust than we’d assumed,” says Vollmann. “Still, with enough explosives, one could theoretically cause any building to collapse.” The ranking is now going to help decide for which structures measures are going to be implemented next to boost security.