Damage mechanism investigation on fiber reinforced recycled cement aggregate concrete in a cold region

The microscopic, mechanical, and freeze-thaw cycle experimental programs were conducted to investigate the internal damage mechanism of carbon fiber-reinforced recycled concrete. Three categories of recycled concrete specimens with different recycled concrete admixtures and reinforcement modes were designed in this study. Axial compression and freeze-thaw tests, combined with SEM and fracture surface observation and analysis, were used to study the effects of the ratio of carbon fiber reinforcement and the admixture of recycled aggregates. Research results indicate that carbon fiber reinforcement can improve recycled concrete's mechanical performance and frost resistance. The reinforcement ratio is positively related to mechanical strength and frost resistance. Compared with the unreinforced concrete, the axial strength of semi-reinforced and fully reinforced specimens was enhanced by 25 and 42.5%, respectively. The recycled substitution rate with fully reinforced specimens has compressive strengths similar to unreinforced natural aggregate concrete, and the effect of the various recycled substitution rates was discussed. Furthermore, the damage mechanism of strength loss in recycled concrete was investigated through microstructural analysis. A uniaxial compression numerical model of CFRP-reinforced recycled concrete was presented, and a modified CFRP-reinforced cement-recycled concrete was proposed to accurately determine the ontological relationship for different reinforcement ratios and recycled aggregate admixtures. A BP neural network model was developed by MATLAB to effectively predict the concrete compressive strength with different mix ratios subjected to various freeze-thaw cycles.