Pull-out behavior of cluster stud-post pouring UHPC connectors in prefabricated steel-concrete composite beam

The use of ultra-high performance concrete (UHPC) as a connection material for cluster studs of prefabricated steel-concrete composite beams can achieve higher connection strength and less space for reserved holes. However, in addition to bearing the shear force, the studs need to bear the pull-out force to counteract the vertical relative separation between the steel and concrete. The pull-out behavior for cluster stud-post pouring UHPC connectors is complex and has not been clarified completely. Thus, a total of 30 cluster stud pull-out tests in 10 groups were carried out in this study, and the interface type, length-to-diameter ratio of the stud, transverse spacing of stud group, dimensions of the reserved holes, and type of post pouring concrete were considered. The results showed that the pull-out behavior of the cluster studs was different from that of studs embedded in castin-place intact UHPC slabs due to the UHPC conical integrity having been reduced significantly by the old-to-new concrete interface. The rough groove interface and UHPC as a post pouring material significantly improved the pull-out capacity and stiffness of the cluster studs. The stud fracture belongs to ductile failure, whereas UHPC and normal concrete (NC) cone failures are brittle. The bearing capacity of the UHPC cone is observed to increase with the effective embedment depth of the stud, the transverse spacing, and the dimensions of the reserved holes. The angle of the UHPC conical failure surface is 70 degrees based on 3D laser scanning. Finally, the theoretical equations for predicting the pull-out capacity of stud-post pouring UHPC connectors with a rough groove interface were proposed, and the critical length-to-diameter ratio of the stud was calculated for different combinations of stud and UHPC strength.