Investigating the effect of coarse aggregate content on flexural performance of ultra-high performance concrete containing coarse aggregate using acoustic emission technology

This study thoroughly explores the synergistic effect of coarse aggregates and steel fibers on the flexural performance mechanism, opening new avenues for the optimization of ultra-high-performance concrete containing coarse aggregates (CA-UHPC) material design. This study investigated the effects of coarse aggregate volume content (0 %, 10 %, 15 %, 20 %, 25 %) on the workability, compressive strength, and flexural performance of CA-UHPC. Additionally, the damage evolution during various stages of CA-UHPC was analyzed through acoustic emission (AE) parameters, including amplitude, ring count, energy, rise angle/average frequency (RA/AF), and source location. Finally, the synergistic mechanism between coarse aggregate and steel fibers was elucidated. The results revealed that the incorporation of coarse aggregate reduced the workability, compressive strength, and flexural performance of ultra-high-performance concrete (UHPC), while enhancing its elastic toughness. The compressive strength decreased by 10.0 %–23.1 %, and the elastic toughness increased by 9.0 %–40.3 % across different coarse aggregate contents. The use of 20 % coarse aggregate reduced the average spacing between fibers, altered the crack propagation path, enhanced the fiber bridging effect and energy dissipation caused by the altered path, and resulted in excellent workability and mechanical properties of CA20. The slump is 110 mm, the compressive strength is 145.8 MPa, and the flexural strength is 15.54 MPa. The addition of coarse aggregate transformed the crack propagation at the bottom of the specimen from multiple fine cracks to fewer fine cracks, and further to a single-through crack pattern when the content reached 25 %. The increased density of AE amplitude points corresponded to the increase in internal defects within CA-UHPC. The premature occurrence of high ring counts in the later stages indicated earlier steel fiber pull-out behavior, whereas the sporadic appearance of high energy values and the increased proportion of shear cracks during the softening stage highlighted the superior bridging effect of steel fibers in CA20. The AE localization points shifted from a widespread distribution in the pure bending region to a more concentrated distribution toward the center of the specimen, corresponding to the actual crack positions. CA-UHPC demonstrates excellent elastic toughness, offering new possibilities for structural design and sustainable development in bridges, and combined with AE technology, it enables on-site monitoring and early warning of crack propagation and material degradation in engineering structures. However, its durability under different environmental conditions still requires further verification.