The influence of pressure in the pore system on fire spalling of concrete

Robert Jansson* & Lars Boström

SP Technical Research Institute of Sweden Fire Technology Box 857, SE-501 15 Borås, Sweden Tel: +46 10 516 50 00 E-mail: [email protected] E-mail: [email protected]

Keywords: Fire spalling, concrete, pressure, permeability, polypropylene fibres,

ABSTRACT

Recent results from fire tests on Self Compacting Concrete (SCC) have shown that the connection between pressure in the pore system and fire spalling of concrete not is obvious [1]. This issue has been further investigated by complementing pressure measurement tests on SCC and a new test series performed on concrete designed for tunnel construction. In the test series conducted on tunnelling concrete, pressure measurements were performed in concrete exposed to the standard fire exposure (EN1363-1) as well as the RWS fire exposure. In general the highest pressures in the two test series were measured in the concretes that not exhibit spalling during fire i.e in many cases concrete including PP-fibres. This is a contradiction compared with pressure measurements performed on high strength concrete specimens exposed to slow heating exposure, 5 oC/min [2], where the highest pressure was recorded in the specimens that spalled. This is probably because different mechanisms govern the spalling behaviour at high and low heating rates. At high heating rates the internal destruction/cracking due to thermal gradients seems to releases the pressure which is not the case during slow heating. The experimental evidence of this difference in behaviour has been shown in experiments performed at CSTB [3] were the pressure inside concrete exposed to high heating rates were substantially lower then the pressure measured at low heating rates. To investigate the issue further tests of the residual permeability have been performed on specimens heated with different heating regimes. In the experiments the specimens heated with a thermal shock to 600 C compared with slow heating, 1 C/min the residual permeability was twice compared with the slow heated specimen which is result of larger internal destruction.

In the final paper overview of the pressure measurement results from the two projects will be given as well as results from the residual permeability measurements. The paper will also include a discussion about the role of pressure in the pore system during fire spalling.

References

1 [1] Jansson, R. “Liquid/steam pressure measurement inside concrete exposed to fire”, Structures in Fire 06, Proceedings from the 4th international workshop, Aveiro, Portugal, 2006, pp. 747-756 [2] Phan T. L., “High-Strength Concrete at High Temperature – An Overview” Utilization of High Strength/High Performance Concrete, 6th International Symposium. Proceedings. Volume 1. June 2002, Leipzig, Germany, Konig, G.; Dehn, F.; Faust, T., Editor(s)(s), 501-518 pp, 2002. [3] Mindeguia J.C., Pimienta P., La Borderie C., Carre H, “Experimental study of the influence of polypropylene fibres on material properties and fire spalling of concrete” Presentation and personal communication during the FIB Workshop, Fire Design of Concrete Structures: From Materials Modelling to Structural Performance, Coimbra, Portugal, 2007

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