Experimental Investigation and Strength Prediction Model for Unsaturated Concrete under the Coupled Action of Freeze–Thaw Cycles and Salt Erosion

To study the changing law of compressive strength for concrete with different saturation degrees, water, sodium chloride (NaCl), and sodium sulfate (Na2SO4) solutions are used to erode concrete specimens. Concrete specimens with 0%, 20%, 40%, 60%, 80%, and 100% saturation are obtained, and freeze–thaw tests with different numbers of freeze–thaw cycles (0, 30, 60, 90, 120, and 150 times) are conducted. Axial stress–axial strain curves are obtained by uniaxial compressive tests. The test results show a significant reduction in uniaxial compressive strength and an increase in axial peak strain with increasing saturation level. Following 150 freeze–thaw cycles, the strength loss rates for the water, NaCl, and Na2SO4 solution groups are 7.9%, 13.2%, and 29.7% for low (20%), 10.6%, 20.1%, and 41.7% for medium (60%), and 10.1%, 22.0%, and 45.9% for full (100%) saturation, respectively. The corresponding increases in axial peak strain are 22.9%, 39.1%, and 41.9% for low saturation, 21.4%, 29.8%, and 54.0% for medium saturation, and 17.0%, 34.4%, and 65.4% for complete saturation. It can be observed that the erosion of NaCl and Na2SO4 solutions results in more serious degradation of concrete strength. Freeze–thaw cycles further exacerbate the degradation of concrete strength, especially at high saturation levels. In this paper, a prediction model based on the binary medium strength criterion is proposed, which can effectively predict the strength of concrete after freeze–thaw cycles.