Salt frost attack on concrete: the combined effect of cryogenic suction and chloride binding on ice formation

Abstract

Scaling of concrete due to salt frost attack is an important durability issue in moderate and cold climates. The actual damage mechanism is still not completely understood. Two recent damage theories—the glue spall theory and the cryogenic suction theory—offer plausible, but conflicting explanations for the salt frost scaling mechanism. The present study deals with the cryogenic suction theory, which assumes that freezing concrete can take up unfrozen brine from a partly frozen deicing solution during salt frost attack. According to the model hypothesis, the resulting saturation of the concrete surface layer intensifies the ice formation in this layer and causes salt frost scaling. In this study an experimental technique was developed that makes it possible to quantify to which extent brine uptake can increase ice formation in hardened cement paste (used as a model material for concrete). The experiments were carried out with low temperature differential scanning calorimetry, where specimens were subjected to freeze–thaw cycles while being in contact with NaCl brine. Results showed that the ice content in the specimens increased with subsequent freeze–thaw cycles due to the brine uptake at temperatures below 0 °C. The ability of the hardened cement paste to bind chlorides from the absorbed brine at the same time affected the freezing/melting behavior of the pore solution and the magnitude of the ice content.

The peculiar behaviour of low‐carbon concretes under salt frost exposure

“…The resulting stresses and increased saturation of the surface layer can cause scaling. This effect was verified in various experiments [17,18,22]. In [22] it was also found, that the extent of additional ice formation due to brine uptake is influenced by the capillary porosity and by the chloride binding capacity of the HCP.…”

“…Furthermore, LTDSC was used in a specific test setup with small droplet of 22.4% NaCl solution on top of the HCP specimen to characterize the change of the content in freezable water due to the uptake of brine during salt frost exposure. Details on the test procedures can be found in [22].…”

Salt frost scaling of concrete – new insights regarding the damage mechanism

“…The resulting stresses and increased saturation of the surface layer cause scaling according to the theory. The cryogenic suction process was verified in several experimental setups [4,14,15]. In [15] it was also found, that the extent of additional ice formation due to brine uptake is influenced mainly by the porosity and by the chloride binding capacity of the hardened cement paste.…”

“…Fresh cement paste was produced with w/c-ratio of 0.50 using the same OPC as for the concrete. More information regarding the OPC properties is given in [15,16]. The fresh paste was filled in a 50 ml test tube, which was sealed airtight and then slowly rotated for four hours to avoid sedimentation.…”

“…The necessary heat of fusion for melting the frozen pore solution during the heating phase was used to assess the mass of ice in the samples, which equates the amount of water in the sample, which is freezable in the temperature range to -20 °C. A detailed description of the calculation process is given in [15].…”