Experimental validation and fracture mechanics analysis of an innovative UHPC-based material for structural strengthening

This study presents an experimental and fracture mechanics analysis of an innovative composite material engineered for structural strengthening, which is based on ultra-high performance concrete (UHPC) enhanced with a high dosage of steel fibres. The effectiveness of this material is evaluated on reinforced concrete (RC) beams that had been subjected to 24 years of sustained loading, representing a realistic, pre-damaged substrate. Combining four-point bending tests with acoustic emission (AE) monitoring, the research analyzes crack propagation, failure modes, and the interplay between mechanical response and AE parameters across micro-, meso‑, and macro-scales from a fracture mechanics perspective. Key findings include a 18% increase in ductility coefficient for beams with deeper UHPC layers, and AE-based precursors such as the b-value evolution and natural time variance reliably identified macro-fracture initiation. The RA-AF analysis quantified a meso‑scale transition from shear to tensile cracking with increased UHPC depth. The UHPC-RC vertical interface acts as a critical meso‑scale fracture process zone governing failure modes, with deeper UHPC applications enhancing ductility by promoting a tensile-dominated cracking mechanism. These results validate the superior performance of the proposed UHPC-based material in rehabilitating severely aged infrastructure and demonstrate that AE techniques, interpreted through fracture mechanics principles, offer unique insights into real-time multiscale damage progression beyond conventional measurements.