Frost resistance investigation of fiber reinforced recycled brick aggregate cementitious materials

In order to solve the problem of environmental pollution caused by construction waste, one typical waste of red bricks was selected as raw material in recycled concrete. This study presented recycled concrete by substituting some natural aggregates with treated red brick aggregates to study and analyze the degradation law mechanism of recycled brick aggregates concrete in the cold region. A total of fifteen categories of specimens and three experimental parameters were considered, which included numbers of freeze–thaw cycles (0, 50, and 100), steel fiber admixtures (0, 1, and 2%), and brick aggregate substitution rates (0, 25, 50, 75, and 100%), respectively. The quick freeze–thaw test method was selected to investigate recycled concrete's degradation mass loss rate and relative dynamic elastic modulus under various freeze–thaw cycles. The digital microscope and SEM were used to observe the internal microstructural changes in the specimens under different freeze–thaw times. In addition, the specimens’ microscopic damage morphology and damage mechanism were analyzed. Finally, the flexural strength of the frost-damaged specimens was tested to analyze the mechanical deterioration of the recycled concrete, and the numerical model corresponding to steel fiber dosing and recycled aggregate replacement rate was presented. The gray correlation analysis was used to quantify the influence of each experimental variable on the corresponding experimental indexes under various freeze–thaw cycles. Results showed that the specimen's mass decreased after freeze–thaw cycles, and the highest mass loss was found for the specimens with 50 and 75% brick substitution rates. In addition, the specimens showed the best relative dynamic modulus and the maximum flexural strength when the steel fiber doping was 1%. The numerical model agreed with experimental data and effectively predicted the specimens' mass loss rate, relative dynamic modulus, and flexural strength after freeze–thaw cycles. The gray correlation analysis showed that the steel fiber contents had a maximum correlation with the flexural strength, the brick substitution rates for the relative dynamic modulus, and mass loss controls the freeze–thaw cycles.