Keywords: high-strength concrete, coupling, permeability, elevated temperatures, polypropylene fibres.

It is widely agreed that the behaviour of concrete exposed to high temperatures is greatly dependent on its composite structure and in particular on the physical and chemical composition of the cement paste. When exposed to high temperature, heat is conducted and convected through the material, resulting in changes in the chemical composition, physical structure and fluid content of the cement paste which in turn affect the overall mechanical and other physical properties of the concrete. All of these phenomena and their coupled interactions must be considered when constructing an appropriate mathematical model for concrete subjected to thermal loading.

A thermo-hygral model for concrete is presented, with particular emphasis on the constitutive relationships for intrinsic (Bamforth [1]) and relative permeabilities (Chung [2]). It has been shown that this model gives very reasonable results of the prediction of the peak values of the gas pressure and the model should be able to simulate the spalling of High Strength Concrete. The peak values of gas pressure correspond well with the moisture concentration zone in the specimen, called 'moisture clog'. The values of gas pressure in the high-strength concrete are approaching the tensile strength and the possibility of spalling for this type of concrete is high.

The introduction of polypropylene fibres into the concrete mix has been shown experimentally to reduce the danger of spalling of high-strength concrete under severe heating. The presented simulation results correspond well with the available experimental data (Kalifa et al. [3]), although further work to include mechanical damage is required for a fully predictive model.

To summarise, a new approach to permeability is introduced. The introduction of polypropylene fibres to the model shows a large improvement in the reduction of the gas pressure, what corresponds well with the available experimental data.

1

Bamforth, P.B., "Relationship between permeability coefficients for concrete obtained using liquid and gas", Magazine of Concrete Research, 39(138), 3-11, 1987.

2

Chung, J.H., "Numerical Simulation of Hydro-thermo-mechanical Behavior of Concrete Structures Exposed to Elevated Temperatures", in College of Engineering, University of Florida: Gainesville, 2003.

3

Kalifa, P., Chene, G. and Galle, C. "High-temperature behaviour of HPC with polypropylene fibres: From spalling to microstructure", Cement and Concrete Research, 31(10), 1487-1499, 2001. doi:10.1016/S0008-8846(01)00596-8

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