Application of ultra-high-performance concrete (UHPC) in joints can improve the impact resistance, crack resistance, and durability of structures. In this paper, the direct shear performance of ultra-high-performance concrete (UHPC) adhesive joints was experimentally studied. Twenty-four direct shear loading tests of UHPC adhesive joints were carried out considering different interface types and constraint states. The failure modes and load-slip curves of different interfaces were studied. Results indicated that passive confinement could enhance the strength and ductility of the interface; the average ultimate bearing capacity of the smooth, rough, grooved, and keyway specimens with passive restraint were, respectively, increased by 11.92%, 8.91%, 11.93%, and 17.766% compared with the unrestrained ones. The passive constraint force changes with the loading and finally tends to be stable. The epoxy adhesive has high reliability as a coating for the UHPC interface. The adhesive layer is not cracked before the failure of the specimen, which is also different from the common failure mode of adhesive joints. Failure of all specimens occurred in the UHPC layer, and the convex part of the groove interface shows the UHPC matrix peeling failure; the keyway interface is the shear damage of the key-tooth root, and the rest of the keyway showed UHPC surface peeling failure. According to the failure mode, the shear capacity of UHPC keyway adhesive joints under passive restraint is mainly provided by the shear resistance of key teeth, the friction force of the joint surface, and the bonding force of the UHPC surface. The friction coefficient was determined based on the test results, and the high-precision fitting formula between the shear strength of the UHPC surface and the passive constraint force was established. According to the Mohr stress circle theory, the proposed formula for direct shear strength of UHPC bonded joints under passive constraint was established. The average ratio of the proposed UHPC adhesive joint calculation formula to the test results was 0.99, and the standard deviation was 0.027.

A precast concrete deck-steel beam-connection concrete (PCSC) connector using ultra-high-performance concrete (UHPC) as post-cast concrete has been proposed to enable rapid on-site construction of assembled composite bridges. This paper aims to optimize the structure of PCSC connectors using the finite element (FE) model to maximize material utilization and economic efficiency. A refined FE model comprising the bond degradation at the steel–UHPC interface was developed based on the push-out experimental results of PCSC connectors. The shear mechanism of the PCSC connectors was analyzed. Subsequently, parametric analyses were performed to investigate the effects of stud diameter, height, spacing, and concrete strength on the mechanical properties of PCSC connectors. The results indicate that the bond at the steel–connection concrete interface positively affects the shear bearing capacity and stiffness of the PCSC connectors. When UHPC was used as connection concrete, it improved the bearing capacity by about 20% and the shear stiffness of the stud by about 16% compared with normal concrete, but the ductility was 38% lower. It was also found that increasing the compressive strength of the connection concrete increased the shear strength of specimens. However, when the compressive strength of UHPC exceeded 130 MPa, the additional UHPC strength did not significantly enhance the shear performance of specimens. In order to ensure the effective restraint of the connection concrete to studs, it is recommended that the minimum width and height of the connection concrete (UHPC) be determined based on the minimum horizontal spacing and height of the studs. Specifically, the length-to-diameter ratio of studs is greater than or equal to 3.18, the horizontal spacing of studs can be at least 2.82 d, and the clear distance between the outer stud shank and the edge of the UHPC cannot be less than 30 mm. The results are expected to provide a reference for the engineering design of PCSC connectors and a reference for conventional stud connectors with UHPC.

Using ultra-high performance concrete (UHPC) as the pavement layer on the orthotropic steel deck (OSD) can greatly enhance the rigidity to solve the fatigue cracking of the steel deck and the resulting damage to the pavement layer. However, the thin UHPC layer limits the extensive use of the short stud connector. In this study, three types of interfaces were designed by introducing different additives into the epoxy matrix, and the shear properties of each interface and short stud connector were compared. Then, the influence of short stud connectors and optimized interface on the composite bridge deck’s bending property was analyzed through the composite bridge deck’s bending test. Moreover, the mechanical behavior of the interface in the composite bridge deck is discussed based on the finite element model. The results show that the ultimate load of the composite bridge deck with the optimized interface is reduced by 21.5% compared with that of the composite bridge deck with the short stud connector. However, the composite bridge deck with the optimized interface has higher bending stiffness before failure and is less affected by cyclic loading. Moreover, the composite bridge deck with the optimized interface is unsuitable for cyclic loading conditions when the defect area reaches 30%. Finally, the numerical simulation of the composite bridge deck with the optimized interface is successfully realized based on the cohesive zone model, and the applicability of the interface in the composite bridge deck is verified.

Assembled steel-composite bridges generally use stud connectors to achieve the connection between the deck slab and the steel main girders. However, the commonly-used cluster studs weaken the integrity of the precast deck slabs and are not conducive to reducing the size of the precast deck slabs. Based on the excellent mechanical performance of UHPC, a precast steel-concrete composite bridge system consisting of precast bridge deck slabs, bonding cavities, and steel girders was proposed in this study. The system was named PSCC (Precast Steel-Concrete Connectors). To verify the applicability of PSCC connectors in engineering, push-out tests and finite element analysis were carried out in this paper to investigate the shear performance and influence parameters of PSCC connectors. The results showed that compared with the full bonding at the steel-UHPC interface, the shear bearing capacity of the specimens with 30% debonded area rate (the ratio of defect area to total interface area) and the shear bearing capacity of the specimens with 60% debonded area rate decreased by 0.35% and 9.74%, the elastic stiffness decreased by 14.86% and 21.72%, and the elastic-plastic stiffness decreased by 1.6% and 12.8%, respectively. When the steel-UHPC percentage of debonded area is less than 30%, the shear resistance of PSCC connectors is affected very little. However, when the steel-UHPC percentage of debonded area is 60%, the shear resistance of PSCC connectors is greatly affected. Therefore, adequate filling of the UHPC connection layer should be ensured in the project. In addition, the PSCC connectors have excellent ductility, their characteristic slip value $S_u$ is much higher than the EC4 specification of 6 mm, and they have better shear performance than conventionally installed stud connectors. According to the results of the parametric analysis, it was found that the failure mode of the PSCC connectors was shear reinforcement fracture when the area ratio of shear reinforcement to stud was less than 1.55, under the premise of the same material strength. On the contrary, the failure mode of PSCC connections was stud fracture. When the transverse spacing of both studs and shear reinforcement is 4d, the PSCC connectors can maintain a high ultimate load capacity while reducing the amount of UHPC in the bonding cavity. Therefore, 4d was chosen as the best spacing for both studs and shear reinforcement.

Stud connectors are widely used to realize the shear connection of steel girders and prefabricated bridge decks in prefabricated steel–concrete composite beams. However, the dense arrangement of stud connectors in the combined bending and shear region makes fabricating prefabricated bridge decks difficult and weakens their integrity. Therefore, a novel prefabricated steel–concrete connector (PSCC) is proposed to accelerate the prefabrication and construction of prefabricated steel–concrete composite bridges. To validate the mechanical properties of PSCC connectors, four groups of 11 static push-out specimens were tested to analyze the effect of interfacial defects and different concrete materials for the shear performance of PSCC connectors. The PSCC connectors have high shear stiffness and ductility. At load levels up to 0.9Pu, the slip values of PSCC connectors range from 6.74 mm to 11.72 mm, which specifies a minimum slip of 6 mm (EC4). The shear capacity of the specimens with 30% and 60% interface defect areas decreased by 0.39% and 8.91%, respectively, compared to the specimens with no interface defects. When the connection layer material was replaced from ordinary concrete to UHPC, the shear capacity of PSCC connectors increased by 39%, and the failure mode shifted from shear failure of studs to shear failure of reinforcements. Moreover, cyclic loading marginally enhanced the shear capacity of PSCC connectors, serving as a safety reserve in practical engineering. Finally, a shear capacity calculation formula for PSCC connectors, derived from finite element analysis and existing formulas, achieved a prediction accuracy within 6.2%.

Conventional stud connectors were proved to be inconvenient for the later disassembly and replacement in ultra-high performance concrete (UHPC) bridge decks. In this study, the headless studs which is a type of studs with their heads removed were employed as a substitute for conventional stud connectors, aiming to enhance the detachability of the bridge decks on the premise of ensuring the anti-lift bearing capacity of the decks. Three push-out specimens containing the headless studs with a diameter of 13 mm were carried out to study the mechanical properties of headless studs. In addition, the finite element model was developed to reveal the failure mechanism of headless studs in UHPC. Results indicated that the load-slip curve of headless stud connector can be divided into elastic stage, plastic stage and failure stage. The failure mode of headless studs was the shear fracture at the root of stud shank. Headless stud connections under cyclic loading can produce greater plastic deformation. The ultimate shear capacity and shear stiffness of headless studs were lower than those of conventional studs. Additionally, the UHPC slabs configured with headless studs have a small quantity of separation at the steel-UHPC interface, so it is suggested to use headless studs and conventional studs together in UHPC slabs to ensure the necessary anti-lift bearing capacity. Finite element analysis indicated that in the elastic stage, the bearing capacity of the push-specimen was dominated by the material properties of the headless stud. After entering the plastic stage, the bearing capacity of the push-specimen was mainly controlled by the UHPC below the headless stud.