Mechanical performance and crack propagation characteristics of the recycled concrete using fractal gradation

To explore the influence of the aggregate particle size distribution on the mechanical behavior of the recycled concrete, the fractal theory is utilized to describe the relationship of the recycled aggregates gradation composition. The digital image correlation technology, scanning electron microscopy, and particle flow code (PFC) are further employed to establish a multi-scale analytical approach for investigating strength, crack propagation, and particle failure. This approach reveals the underlying mechanisms by which aggregate particle size distribution influences the mechanical behavior and failure modes of recycled concrete at the microscale. The results indicate that the compressive strength of recycled concrete initially increases and then decreases. Specifically, the 28-day compressive strength reaches a maximum of 25.1 MPa at a fractal dimension of 2.5, representing an improvement of approximately 10.8 %-50.8 % compared to other gradations. Fractal analysis of the fracture surface reveals that hardened recycled concrete demonstrates approximately 5 % higher surface fragmentation, which contributes to improved resistance against external forces. Furthermore, PFC simulations suggest that crack initiation in recycled concrete occurs at stress concentrations near the angular edges of aggregates and propagates along the cement–aggregate interface. Recycled concrete characterized by a low fractal dimension in aggregate particle size distribution is more susceptible to localized breakage of particle force chains, leading to increased particle fragmentation and interparticle slippage. Likewise, an overly high fractal gradation of aggregates results in a higher content of fine particles, promoting the widening of the interfacial transition zone and inducing force chain breakage between particles, thereby facilitating particle slippage. • The fractal gradation significantly influences the strength of recycled concrete. • Reasonable aggregate gradation promotes stable failure mode of recycled concrete. • SEM verifies the optimization effect on the microstructure is under 2.5 fractal gradation. • PFC simulations show that 2.5 fractal gradation recycled concrete maintain ideal structural integrity.