Testing a model for the critical degree of saturation at freezing of porous building materials

Frost resistance of porous materials can be characterized by the critical degree of saturation, SCR. An experimental determination of SCR is very laborious and therefore only seldom used when testing frost resistance. A theoretical model for prediction of SCR based on fracture mechanics and phase geometry of two-phase materials has been developed. The degradation is modelled as being caused by different eigenstrains of the pore phase and the solid phase when freezing, leading to stress concentrations and crack propagation. Calculations are based on porosity, pore size distribution, modulus of elasticity, tensile strength, amount of freezable water, thermal expansion coefficients and parameters characterizing the pore structure and its effect on strength, modulus of elasticity and volumetric expansion. For the present, the model assumes non air-entrained homogeneous materials subjected to freeze-thaw without de-icing salts. The model has been tested on various concretes without air-entrainment and on brick tiles with different porosities. Results agree qualitatively with values of the critical degree of saturation determined by measuring resonance frequencies and length change of sealed specimens during freezing. The reliability and usefulness of the model are discussed, e.g. in relation to air-entrained materials and in relation to the description of the pore structure.Keywords: Brick tile, concrete, critical degree of saturation, eigenstrain, fracture mechanics, frost resistance, pore structure, theoretical model.