FREEZE-THAW RESISTANCE OF CONCRETE: THE INFLUENCE OF AIR ENTRAINMENT, WATER TO CEMENT RATIO, AND SATURATION

Today, we generally accept that air entrainment is a nearly essential part of concrete mixture design as a method to improve freeze-thaw durability. But what role does the air-entrainment play to accomplish this task? Air entraining admixtures are surfactants that help to stabilize air-filled, spherical voids in the concrete matrix. Air entrainment reduces the degree of saturation by increasing the proportion of voids filled with air in the concrete relative to the total volume of voids, which in addition to air include gel pores, capillary pores, and chemical shrinkage pores (all of which are generally water-filled). This paper discusses two specific levels of saturation, the saturation at the nick point, which is transition point between initial and secondary sorption, and the critical degree of saturation, beyond which mixtures become susceptible to freeze-thaw damage. This paper presents a model to predict the saturation at the nick point for a concrete element, considering the role of water to cement ratio, air content, and paste volume. This model can be used in conjunction with the rate of absorption and the critical saturation theory which estimates the critical degree of saturation to be approximately 86% [1]-[3]. The effects of air entrainment on sealed degree of saturation, the degree of saturation at the nick point, and the theoretical time to reach the critical degree of saturation are also discussed.