Tunnel fires: preventing explosive spalling of concrete

Published:  23 March, 2015

Engineers at the Technische Universität München (TUM) discover how to measure the action of polypropylene fibres inside concrete.

When fire breaks out in a tunnel the ensuing heat has little possibility of escaping. Temperatures rise rapidly to over 1,000 degrees C. The heat that develops causes the concrete to spall explosively, weakening the structural integrity of the tunnel. And while this can be prevented by adding polypropylene fibers to the concrete, up to now it has been impossible to find out what is happening within the concrete.

The heat produced by the fire vaporises water in the concrete. The ensuing pressure is initially released into the pores of the concrete. But as the pressure continues to rise, small pieces of concrete blast off like popcorn in a process called explosive spalling. The thickness of the concrete is reduced, and likewise its load-bearing capacity. A real danger of the tunnel collapsing arises, putting later reconstruction work at risk.

Fibers melt away

Since 2012 fire protection certification has been required for all new street tunnels in Germany, and a way for ensuring fire protection is to add polypropylene (PP) fibers to the concrete. "Once a temperature of 110 degrees Celsius is reached, the fibres in the concrete start to melt away," explains Prof. Christian Große at the Chair of Non-destructive Testing (ZpF) at TUM. In the process, small pockets are created in the concrete into which the pressure can escape.

Yet how exactly synthetic fibres altered the behaviour of concrete has remained unclear. Now the TUM scientists, in collaboration with the Institute of Construction Materials at Universität Stuttgart and the MFPA Leipzig GmbH, have developed a methodology for peering inside the concrete.

Cracking sounds in concrete

The researchers placed concrete slabs on top of an open test kiln (imagine a lid on a pot). They installed acoustic sensors on the top sides of the slabs. The concrete was then heated from below to 1,300 degrees C.

"When concrete is damaged, cracking sounds are produced," explains Roland Richter, doctoral candidate at the ZpF. The acoustic waves travel through the material and can be measured at the surface. Since multiple sensors are attached to the concrete, the precise source of the sounds can be determined - not unlike the manner in which earthquakes are monitored using seismographs.

For the first time ever, the engineers were able to observe the temporal progression of damage during a simulated tunnel fire. The concrete slabs without PP fibers produced ten times more acoustic events than the slabs with PP fibers.

The scientists hope to further refine and validate their measurement method. The technique could prove useful in comparing – and thus optimising – different concrete mix designs with regard to their behaviour in fire.

The research was carried out in the context of the German Research Foundation (DFG) project "Explosive spalling of concrete in fire" (project partner: Institute of Construction Materials at Universität Stuttgart), as well as an AiF project funded by the German Federal Ministry of Economics and Technology in collaboration with the MFPA Leipzig GmbH.

  • Operation Florian

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