Bending damage analysis and multi-objective optimization of CA-UHPC based on acoustic emission and digital image correlation

This study investigated the effects of coarse aggregate (CA) content and steel fiber (SF) volume fraction on the mechanical properties, flexural damage evolution, and crack propagation behavior of ultra-high-performance concrete (UHPC) and coarse aggregate ultra-high-performance concrete (CA-UHPC). Acoustic emission (AE), digital image correlation (DIC), cross-sectional observations, and SEM characterization were combined to interpret the fracture evolution mechanism, and the entropy weight–TOPSIS method was used for mechanical–environmental–economic multi-objective optimization. The results show that CA incorporation improved compressive strength but reduced flexural strength and toughness compared with the UHPC reference mixture without CA. Increasing SF content improved flexural performance, whereas excessive SF provided limited additional improvement. CA20S2 exhibited balanced mechanical performance, with compressive strength, flexural strength, and flexural toughness of 137.5 MPa, 18.94 MPa, and 115.517 N·m, respectively. AE parameters characterized the transition from stable damage development to accelerated crack propagation, and cumulative AE energy showed better early-warning potential than ring count. DIC quantified strain evolution, CMOD, main crack length, and crack propagation rate, indicating that CA20S2 maintained a longer multiple-cracking stage and more stable crack propagation. The AE–DIC analysis showed good spatial consistency between high-energy AE events and DIC-identified high-strain regions. An exponential relationship was established between cumulative AE energy and main crack length, with ( R ² >0.94), enabling quantitative coupling between AE-based damage information and DIC-based crack propagation information. Considering the TOPSIS ranking, sensitivity analysis, normalization-method comparison, and AE–DIC results, CA20S2 was recommended as a balanced mixture, while CA30S2 may be preferred when compressive strength and low-carbon economy are prioritized.