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Spalling of fire exposed concrete

B.B.G. Lottman 1, E.A.B. Koenders 3, C.B.M. Blom 2, 4, J.C. Walraven 2

1 Witteveen+Bos / Tunnel Engineering Consultants (TEC)
2 Delft University of Technology, the Netherlands
3 Technische Universitšt Darmstadt, Germany
4 Municipality of Rotterdam, the Netherlands

Concrete spalling due to fire exposure is a topic that has gained a lot of attention from the scientific research community in the past decades as well as at present in the Netherlands. This destructive phenomenon is commonly characterised by a sudden and in some cases repetitive breaking off of pockets of concrete from the heated surface. Prevailing theories indicate a simultaneous action of both pore pressure and thermal stresses with their respective dominance still not yet fully understood. In this article a new multi-scale FEM-based numerical model is presented, which was derived to enable a fundamental analysis of both actions. A coupled thermal-hygral-mechanical approach is used to examine the temperature and pore pressure developments as well as the associated fracture patterns that develop in a heated concrete structural element. The simulated crack patterns indicate the occurrence of thermal instabilities at the heated concrete surface with pore pressures only adding a minor contribution. Failure is shown to occur by the progressive compression of the heated surface layer while inducing local delaminations. Finally, based on these numerical simulations and on observations from full-scale heated slabs, thermal buckling is proposed to be the main mechanism that drives concrete spalling due to fire exposure.

Key words: Concrete, fire, (explosive) spalling, pore pressure, fracture mechanics, finite element method, thermal buckling mechanism