Experimental investigation of headed studs in steel-ultra-high performance concrete (UHPC) composite sections

Featuring high compressive and tensile strengths, ultra-high performance concrete (UHPC) is an efficient and durable alternative for conventional concrete in steel–concrete composite sections. However, few experimental studies to date have involved large diameter (≥ 25 mm) headed studs, UHPC with compressive strengths larger than 130 MPa, and relatively low strength but cost-effective UHPC. This study investigates the shear performance of steel headed studs embedded in UHPC through pushout tests with 24 specimens. Specimens included headed studs with diameters of 16 mm, 19 mm, 22 mm, and 25 mm, and either UHPC with compressive strength of 152.5 MPa or a cost-effective UHPC with a strength of 85.7 MPa (named as HPC hereafter). The results showed that smaller-diameter specimens (16 or 19 mm) failed with stud shank fracture, while the larger-diameter specimens (22 or 25 mm) failed at welding lines. Compared to HPC, UHPC did not notably increase the performance of headed shear strength. The load response consisted of a linear stage, a softening stage, and a post-peak stage lasting up to a slip of 10 mm or greater, with high variability in post-peak response among specimens with identical parameters. A global database of 120 push-out test specimens from this study and references was established, covering concrete strengths from 23.5 MPa to 152 MPa. The formulae given by AASHTO LRFD Bridge Design Specification, Eurocode-4, and Chinese code GB50017-2003 overestimated the shear capacity of headed studs by 52 %, 5 % and 53 %, respectively. The equation for the slip modulus slightly overestimated the slip modulus of headed studs. In conclusion, this study broadened the ranges of concrete strength and diameter of headed shear studs, and revealed that the existing equations of predicting shear capacity and slip modulus need improvement.