Two types of damage—numerous cracks in the main girder and excessive midspan deflections—are sometimes observed in conventional long-span prestressed concrete box-girder bridges. To this end, a new box-girder bridge structure is proposed based on the superior properties of the reactive powder concrete (RPC). Diaphragms are densely distributed in the proposed structure to reduce the torsional warping stresses of the box girder and the local stresses of the deck slabs under wheel loads, as well as to improve the shear performance of the webs and the compression stability of the bottom slabs. Moreover, compared to the conventional box-girder bridge, the proposed structure is characterized by unidirectional (longitudinal) and partial external prestressing, as well as thinner slabs. According to the aforementioned characteristics of the proposed structure, the experimental model of the RPC box girder was designed and fabricated to explore its feasibility and mechanical performance. Tests on the deck slabs of the experimental model were first carried out to investigate the load path in deck slabs and evaluate the effect of diaphragm spacing. Results show that it is feasible to remove the transverse prestressing tendons in the deck slabs of the proposed box-girder structure. Subsequently, the shear failure test on the new box-girder structure was conducted. It was found that the postcracking shear performance of the proposed structure is improved due to the presence of the ribbed structures consisting of diaphragms and webs. Combined with the finite-element (FE) analyses, the safety factor of the shear cracking resistance of the web was calculated, up to 1.844 for a 400-m RPC box-girder bridge. Thus, it is feasible to remove the vertical prestressing tendons in this new structure. In addition, the shear capacity of the tested box girder was predicted by using the related design recommendations. Comparisons were performed among these predicted values and the test results. Although the feasibility of the proposed bridge structure is clearly demonstrated in this paper, further studies prior to its practical applications are being extensively performed to clarify its flexural capacity, shear behavior of webs, torsional behavior, fatigue behavior of deck slabs, seismic behavior, dynamic behavior under wind loads, prestressing optimization, and anchor details.

In order to study the direct shear properties of ultra-high performance concrete (UHPC) structures, 15 Z-shaped monolithic placement specimens (MPSs) and 12 Z-shaped waterjet treated specimens (WJTSs) were tested to study the shear behavior and failure modes. The effects of steel fiber shape, steel fiber volume fraction and interface treatment on the direct shear properties of UHPC were investigated. The test results demonstrate that the MPSs were reinforced with steel fibers and underwent ductile failure. The ultimate load of the MPS is about 166.9% of the initial cracking load. However, the WJTSs failed in a typical brittle mode. Increasing the fiber volume fraction significantly improves the shear strength, which can reach 24.72 MPa. The steel fiber type has little effect on the shear strength and ductility, while increasing the length of steel fibers improves its ductility and slightly reduces the shear strength. The direct shear strength of the WJTSs made from 16 mm hooked-type steel fibers can reach 9.15 MPa, which is 2.47 times the direct shear strength of the specimens without fibers. Finally, an interaction formula for the shear and compressive strength was proposed on the basis of the experimental results, to predict the shear load-carrying capacity of the cast-in-place UHPC structures.