Multi-scale deterioration and microstructure of polypropylene fiber concrete by salt freezing

Salt freeze damage is a significant factor impacting the durability of hydraulic concrete buildings in cold, saline soil areas. To settle the durability problem of hydraulic concrete in a salt freezing environment in cold regions, concrete specimens with different levels of polypropylene fiber dosing under two conditions of freeze–thaw cycles and freeze–thaw–sulfate erosion were prepared based on water diversion projects in cold and saline soil regions of China. The indoor accelerated deterioration test was designed, and the relative dynamic elastic modulus(RDEM) and mass loss rate, which were macroscopic indicators, were measured. The pore structure distribution in concrete with increasing test times was examined using nuclear magnetic resonance (NMR) technology. Combined with scanning electron microscope(SEM) images, the salt frost resistance of polypropylene fiber concrete was studied. The results showed that the mass loss rate of group A4 (1.2 kg/m3 of polypropylene fibers) under freeze–thaw cycles was 2.06% lower than that of group A1 (0 of polypropylene fibers), and the RDEM was 7.92% higher than that of group A1; the mass loss rate of group B4 under freeze–thaw–sulfate attack was 2.1% lower than that of group B1, and the RDEM was 20.79% higher than that of group B1; at the fine level, polypropylene fibers could inhibit the development and expansion of pores inside the concrete, which improved the salt–freezing resistance of the concrete; from the microscopic morphology, expansion products like gypsum and ettringite generated at the internal pores of the concrete are the leading cause of salt freeze damage crack development. The capability of concrete to withstand freeze-thaw cycles and salt freezing was enhanced with the increase of polypropylene fiber dosing. Compared with the concrete with polypropylene fiber content of 0.9 kg/m3, the frost resistance and salt freezing resistance of concrete with 1.2 kg/m3 content are not significantly improved. The study can provide a theoretical reference for applying polypropylene fibers to hydraulic concrete in saline soils in cold regions.