Synergistic modification of concrete with waterborne epoxy resin and steel fibers: Experimental and numerical investigation of compressive mechanical properties

The construction of deep underground engineering requires concrete with high strength and toughness, yet the multi-scale synergistic effects and damage evolution mechanisms underlying their simultaneous improvement remain poorly understood. In this study, waterborne epoxy resin (WER) and steel fiber (SF) are used as modifiers, with a combined experimental-numerical approach including uniaxial/triaxial tests, LS-DYNA simulations, and SEM analysis, to investigate their synergistic toughening under ordinary stress states and the influence of confining pressure. Results show that WER and SF dosages regulate this synergy. The optimal formulation (SF 1.0%, WER 8%) yields a peak stress of 119.13 MPa at 15 MPa confining pressure. Mechanistically, WER not only densifies the matrix by pore filling but also forms an interfacial coating on SF, which improves stress transfer efficiency and enhances energy dissipation during fiber pull-out, overcoming single-fiber limitations. The reinforcing effect of SF exhibits a clear confining-pressure dependence: at low pressures (≤5 MPa), fiber bridging and pull-out dominate, leading to a maximum strength increase of 38.7%; at 15 MPa, compaction gradually supersedes bridging, and the strength increment drops below 3.8%, indicating a transition from fiber-dominated to matrix-dominated dissipation. This transition is validated by experimental data at various pressures. LS-DYNA simulations successfully reproduce the stress-strain responses and failure modes under both uniaxial and triaxial loading, matching well with experiments. This work provides both fundamental understanding and practical guidance for designing high-toughness, high-strength concrete under ordinary stress conditions.