ABSTRACT
Abstract Moisture uptake at constant sub-freezing temperatures was studied on three different mortars (w/c 0.40, 0.55 and 0.65). The specimens were frozen by submerging them in pre-cooled salt solutions. The main purpose was to test a previously presented hypothesis on the mechanism of salt frost scaling of porous materials, a hypothesis which predicts weight increases due to water absorption caused by the growthof ice crystals formed in the pores. Effects of temperature level, NaCI-concentration (both in the pore solution and in the outer solution), pre-drying, rate of cooling and airentrainment were studied. Only results from the main test, moisture uptake at different temperature levels, are given in this paper. Large weight increases were observed, a fact which supports the hypothesis. Keywords: Absorption, frost, moisture, mortar, salt
1 Introduction In a previous paper [1], a first draft of a hypothesis on the mechanism of saltfrost scaling of concrete and other porous, brittle building materials has been presented. The key idea of the hypothesis is that ice crystals which form inside the specimen are able to feed from the remaining pore solution just as was shown by Powers and Helmuth [2]. They explained this ice lens growth would come to a stop when the pressure difference between ice and remaining pore solution was large enough. In specimens with a low enough degree of saturation, such a pressure difference may be established mainly as a reduced pressure in the pore solution. In the present hypothesis, it is assumed that close to the specimen surface, this reduced pressure cannot be established, as new liquid will be sucked in from the outer salt solution. Thus the only way to establish equilibrium is by applying a positive pressure on the ice. At normal freezing temperatures, the pressure needed will be too large for normal building materials to withstand and thus the surface will be destroyed.
