Pullout behavior of studs in ultra-high performance concrete with steel fibers and novel structural fibers

Abstract

To study the pull-out behavior of studs in Ultra-High Performance Concrete(UHPC) in steel–concrete composite structures, 12 scenarios are designed, including the studs in UHPC without fiber(BFUHPC), UHPC with steel fibers alone(DFUHPC) and UHPC with hybrid steel fibers and structural fibers(HFUHPC) under different embedment depths (hef = 35 mm, 50 mm and 65 mm) and stud sizes (d = 13 mm and 16 mm). Failure modes, ultimate tensile strength (Nu), load–displacement curve, initial stiffness (K0.3), displacement at Nu (SN) and ductility coefficient(μ) of test results are analyzed. The failure modes of BFUHPC specimens are all brittle concrete cone (CC) failure, while DFUHPC and HFUHPC specimens show both CC and steel failures with ductile characteristics, and the critical hef/d is 3.5–4.0. For the HFUHPC specimen, increasing the stud size is beneficial in increasing Nu and K0.3, but leads to reductions in SN and μ. For the specimens with CC failure, both K0.3 and Nu increase as hef increases. At the same hef, compared to BFUHPC specimens, Nu, K0.3, and μ of DFUHPC specimens with CC failure increase significantly due to the improved elastic modulus and post-cracking properties of UHPC by adding steel fibers. Nu, K0.3, and μ of HFUHPC specimens are further improved because structural fibers take additional effects in bridging macroscopic cracks. Based on the concrete capacity design (CCD) method and considering the influences of tensile strength of UHPC, a modified prediction model for Nu of studs in UHPC is proposed and shows a good agreement with test results.