| We studied several topics that are important to explain the mechanisms of frost damage to concrete, including the volume change of concrete during freezing, the role of air voids in protecting concrete from frost damage, the pore structure of concrete, and the nucleation and propagation of ice in concrete.;By combining calorimetric measurements with dilatometry, we were able to calculate the contributions of thermal expansion, pore pressure, and crystallization pressure of ice to the strain observed in a mortar during freezing/thawing cycles. Air-entrained mortars contract upon freezing due to the cryo-suction effect, while non-air-entrained mortars expand primarily due to hydraulic pressure. Based on the theory originally proposed by Powers and Helmuth, we show that the poromechanical calculations account quantitatively for the contraction of samples with air entrainment, which is shown to quantitatively account for a reduction of salt scaling damage based on the glue-spall theory. The method of thermoporometry (TPM) that we used to study the pore structure of concrete is also discussed.;In a study of ice propagation inside concrete, we re-examined experiments by Helmuth [Proc. 4th Int. Cong. Chem. Cement, NBS Monog. 43, Vol. II (National Bureau of Standards, Washington, D.C., 1962) pp. 855--869] from which he concluded that ice grows in the pores of cement paste under heat-flow control, and that the internal temperature rises to the melting point given by the Gibbs-Thomson equation. Using experimental and computational methods, we find that his conclusions are correct, but the growth rates he reports are misleading. Our experiment reveals the true growth rate, which is about three times smaller than found by Helmuth. The dendritic morphology explains how fast constant growth rates can occur when the interior temperature of the sample is very near the melting point: the temperature at the tip of the dendrite is a few degrees below the melting point, but the liquid behind the tip (between adjacent dendrites) is at the melting point. The implications of this type of growth for frost damage are discussed. |
