Experimental study on frost resistance of hybrid fiber fly ash concrete

Based on the orthogonal test, the optimized ratio of hybrid fiber fly ash concrete prepared with different volume admixtures of polypropylene fiber and polyacrylonitrile fiber, and different lengths, was obtained, and its frost resistance was evaluated in terms of mass change, dynamic elastic modulus, compressive strength, and splitting tensile strength before and after water and salt freezing. Then, the damage mechanism was discussed by combining the pore structure and SEM analysis. The results show that the tensile strength and relative dynamic elastic modulus of specimens after the water freezing cycle are better than those after the salt freezing cycle with the same number of freeze-thaw cycles, and the mass loss rate is less than that of the salt freezing cycle; the compressive strength and the splitting tensile strength of hybrid fiber fly ash concrete increased by 8.5% and 9.5%, and 46.8% and 12.1%, respectively, after 60 times of water freezing and salt freezing cycles compared with the benchmark fly ash concrete. As the number of freeze-thaw cycles increased, the pore air content and the frequency of large pores increased and the frequency of small pores decreased; in terms of the pore distribution of concrete, the distribution of pore structure was more reasonable for water freeze cycle relative to salt freeze cycle and for hybrid fiber fly ash concrete relative to the base fly ash concrete. Thus, it can be found that hybrid fibers incorporated into concrete can reduce the damage to the concrete interior by the freeze-thaw cycle and significantly improve the frost resistance of concrete.