Experimental and analytical study on existing concrete slabs strengthened with ultra-high performance concrete: From interfacial bond to structural flexural performance

This is the authors' abstract. We don't add key points for this paper.

Strengthening normal concrete (NC) bridge decks with an ultra-high-performance concrete (UHPC) overlay effectively mitigates cracking in negative moment regions, though structural integrity relies heavily on robust interfacial bonding. This study systematically evaluates the NC-UHPC composite behavior through a two-phase investigation. Phase I quantified interfacial shear transfer under combined compression-shear and near-pure shear states using slant and Bi-surface shear tests, examining the effects of surface treatment, moisture conditions, and curing age. Phase II assessed the flexural performance and interfacial slip behavior of UHPC-strengthened NC beams via four-point bending tests. Results demonstrate that substrate pre-wetting localized desiccation and promotes hydration, significantly enhancing shear capacity. When synergized with manual roughening, bond strengths comfortably exceed standard code requirements. Additionally, theoretical cohesion and friction coefficients derived from the Mohr-Coulomb criterion conservatively satisfy the CEB-FIB model code. Structurally, the composite slabs exhibited flexural failure; macroscopic interfacial slip was driven by strain incompatibility following UHPC tensile cracking rather than direct shear bond failure. Finally, adopted analytical models accurately predicted cracking and ultimate moments, yielding mean predicted-to-measured ratios of 1.110 and 0.982, respectively. This research provides a reliable experimental and theoretical foundation for the design of NC-UHPC composite decks.