Flexural performance of transverse wet joint in steel-UHPC composite beam under negative bending moments

To mitigate the issues of concrete cracking in the negative bending moment region of traditional steel-concrete composite beams, the concept of a prefabricated steel-UHPC (ultra-high performance concrete) lightweight composite beam was employed. Taking advantage of UHPC's superior tensile properties, a wet joint for the negative bending moment region of the steel-UHPC composite beam that includes a normal concrete transverse beam and a UHPC bridge deck is proposed in this study. A 1:2 scaled model bending test was conducted to experimentally study the flexural performance of the proposed wet joint and demonstrate its feasibility. The test results revealed that the interface between the transverse concrete beam and the longitudinal steel beam was generally the weak zone where failure occurred, with the fracture of the UHPC deck slab and the yielding of the steel bars in the deck recorded as the major factors contributing to the failure of the proposed joint structure. Through an actual bridge analysis, the flexural capacity of the joint structure, which was 3.7 times higher than the design load of the bridge, was quantitatively verified to meet the design requirements for practical engineering applications. In addition, the calculation method for computing the ultimate bending capacity of the joint structure was also proposed, in which the connection between the transverse concrete beam and longitudinal steel beam is considered as the crucial zone – with the analytical results showing good agreement with the experimental data. Overall, the calculation method provides a worthy reference datum for the development of steel-UHPC lightweight composite beams and their wet-joint designs in the negative moment region.