Partial composite action in prestressed UHPC-NC composite girders: interface characterisation, prestress gain and a conservative assessment of a bond-based check

Abstract Prestressed composite girders that pair a UHPC tension layer with normal concrete (NC) are designed as fully bonded, so neither interface slip nor tendon slip enters the calculation. We tested 26 specimens to find out what that costs: eight prestressed girders with the UHPC layer between 160 and 250 mm deep, eight L-shaped direct-shear specimens, and ten damaged beams strengthened with a UHPC layer. Twenty-four of the 26 contain an interface. A load cell at the anchorage measured the tendon force throughout loading and gave a prestress gain of 12.74% of the seated force, reached after the strand had yielded. The direct-shear series separates bare and dowelled joints completely: bare joints fail at 0.98–1.11 times their cracking load, dowelled joints at 5.61–6.01 times, a ratio of 5.53 between the means. No member failed by interface slip. Applying a bond-based partial composite action check to these measurements gives a connection degree of unity in all 16 parameter cases and 0.46–0.57 of the measured capacity. The check reduces to one product, which is the interface quantity a design check would need; a bond strength alone will not serve.