The Influence of Carbon Fiber Content and Strain Rate on the Mechanical Properties and Microscopic Damage Evolution of Recycled Aggregate Concrete

This study systematically investigated the effects of carbon fiber (CF) content (0%, 0.15%, 0.3%) and strain rate (10−5/s, 10−4/s, 10−3/s, 10−2/s) on the mechanical properties, microstructure, and microscopic damage evolution of carbon fiber-modified recycled concrete (CFRRAC) using uniaxial compression testing, scanning electron microscopy (SEM) observation, acoustic emission (AE), and statistical damage theory. The results indicate that the moderate addition of CF can effectively improve the compactness of the microstructure of the specimen, enhance the strain rate effect of CFRRAC, and improve its initial macroscopic mechanical properties. The microstructure characteristics of specimens with different CF contents and the Stefan effect related to strain rate further affect the initiation and propagation morphology, propagation path, and adjustment process of effective stress skeleton of microcracks during uniaxial compression, leading to regular changes in characteristic parameters characterizing microfracture and yield damage evolution with CF content and strain rate. The above factors collectively determine the evolution characteristics of the macroscopic nonlinear stress–strain behavior of CFRRAC, combined with the CF bridging toughening effect, ultimately resulting in an increase in strength with increasing strain rate and maintaining good ductility. Compared with the specimens without CF doping, the peak stress of CFRRAC increased by 37.16% to 41.18% and the peak strain increased by 22.94% to 36.57% in the strain rate range of 10−5 to 10−2/s at a dosage of 0.3%. Taking the CFRRAC specimen with a content of 0.3% as an example, compared with the strain rate of 10−5/s, the peak stress of the specimen increased by 9.31%, 18.66%, and 31.24% at strain rates ranging from 10−4 to 10−2/s, respectively. The research results can provide theoretical support for the promotion and application of CFRRAC in the engineering field.