Full-Scale Precast UHPC–Postcast NC Hollow Composite Slab: Interface Technique Validation

To enhance cracking resistance and long-term durability of bridge superstructure subjected to flooding and seawater environment in the coastal area, ultrahigh-performance concrete (UHPC) is applied to the tensile side of a full-scale normal concrete (NC) hollow slab; to accelerate bridge construction, the UHPC is prefabricated as a formwork before casting NC. When prefabricating the UHPC layer, an interface treatment combination technique is performed, including creating grooves, anchoring rebars, and fixing stirrups. This study aims to confirm the effectiveness of the proposed interface treatment combination technique in a full-scale UHPC-NC hollow composite slab. The failure mode, load–deflection response, load–strain of concrete and reinforcement, load–crack width, and interface slip are discussed. Additionally, the theoretical models for self-restrained UHPC shrinkage stress due to inside reinforcements and more stresses due to NC shrinkage are established, respectively. The results show that the proposed interface technique successfully guarantees the composite working behavior with an interface slip of 0.072 mm at the failure. The NC crack (appearing first), UHPC crack, and reinforcement yielding govern the entire slab flexural response. The UHPC layer is within the strain-hardening stage with effective crack control ability until the composite slab fails. The added shrinkage stresses in the UHPC and NC layers interpret their flexural cracking stress reductions, which provides a new angle into the property mismatch between the full-scale test and material property test.