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.

The structural behavior of the anchorage zone is a key issue in prestressed ultra-high performance concrete (UHPC) box-girder bridges. This study investigates the mechanical behavior of a diaphragm–blister integrated anchorage system (DBIAS) used in UHPC box-girder bridges through the full-scale model test. Parametric analyses were also conducted using test-validated finite-element models. Experimental results show that no visible cracks have emerged when the applied load reaches 4,700 kN, which is 1.36 times the design value. Although the strain in some local areas has entered into the tensile strain-hardening domain of UHPC under the prestressing force, the whole structure still works well. The contribution of strain-hardening behavior of UHPC to the loading capacity of anchorage blister is 8.7%, based on nonlinear finite-element analysis. Therefore, the tensile strain capacity of UHPC may be taken into consideration in the design so as to fully make use of the material strength. Compared with the conventional independent blister anchorage system, the local effects on the anchorage zone are significantly reduced by setting anchorage diaphragms and transverse ribs in a DBIAS. The increase in the width and longitudinal spacing of adjacent diaphragms shows effectiveness in relieving the stress concentration in the anchorage zone. However, local effects will not be continuously reduced when the width of the diaphragm is more than six times that of the anchorage blister or the spacing between the diaphragm and the transverse rib exceeds a certain distance (2 m in the present study case).