ABSTRACT
Abstract The frost resistance of concrete is guaranteed when a sufficient air void system is available. Practical experiences show that in Central Europe concretes without air entrainment can also have a high durability and serviceability. This practical experiences are confirmed by testing frost resistance with CF test. The frost resistance essentially depends on the degree of saturation and porosity. During the first freeze thaw cycles the water absorption increases until a critical degree of saturation is reached. If this critical value is surpassed the water uptake decreases and the internal damage increases. The investigations of water absorption during freeze thaw test confirms that reduction of capillary pore and coarse pore volume can increase the frost resistance. In case of no application of air entrainment in concrete the performance concept is necessary to test the frost resistance. Keywords: Capillary suction, CF test, internal damage, frost resistance, water absorption
1 Introduction
Freezing of free water leads to pressure due to volume expansion. Since water in gel pores generally is unfrozen above -20°C a hydraulic pressure increases. The freezing point depression of the gel pore water results from interaction mechanisms of specific surfaces. These also lead to tension in the gel pore system and hysteresis effects between freezing and thawing. Due to the different coefficient of thermal expansion of water, solid matrix and ice the hysteresis leads to additional pressure. Finally, during ice formation different soluble salts of the pore liquid are crystallized or increase the concentration of unfrozen water. The arising osmotic pressure causes on one hand transport mechanism, e.g. capillary suction and diffusion and on the other hand tensile tension in the matrix and micro cracks. Therefore, the pore system,
2.1.1 Capillary suction at room temperature During the capillary suction and during the freeze-thaw cycles, the specimens are stored with the test surface inverted in stainless steel containers. They are placed on 10 mm high spacers. The level of the test liquid is kept at 15 mm so that the specimens dip 5 mm into the liquid. By this kind of capillary suction, a degree of saturation is reached, as in practice, in a very well defined manner.
