Tensile Behaviour of Hollo-bolts Embedded in Concrete

To examine the tensile response of Hollo-bolts considering the bonding effect of concrete, a new device to perform pullout tests was used to investigate the tensile behaviour of Hollo-bolts embedded in concrete. First, the tensile force-displacement relationships for all specimens under different experimental conditions were obtained. Then, the failure modes and the tensile force-displacement relationship curves were compared and analysed to examine the influence of the tested experimental variables, which include the concrete strength grade and tubular sections with or without concrete filling, on the initial stiffness, strength, and ductility of the structure. Finally, the tension loading-bearing mechanism of Hollo-bolts was analysed considering the bonding effect of concrete. In addition, reasonable measures to improve the tensile-behaviour of Hollo-bolts were subsequently suggested. The test results revealed that the bonding effect of concrete changes the tension loading-bearing mechanism of Hollo-bolts, as the load on the Hollo-bolt eventually shifts from the bolt sleeve to focus on the centre screw. Accordingly, the failure model of the Hollo-bolt changes from sleeve failure to centre screw failure. Furthermore, it was determined that the concrete strength grade significantly influences the stiffness of Hollo-bolts. When the concrete strength grade increased from C30 to C50, the initial stiffness increased from 15.1% to 35.7% and the post-yield stiffness improved 1.27 and 1.64 times compared to the specimens for which the bonding effect of concrete was not considered. However, the findings indicate that the yield tensile strength and ductility of Hollo-bolts are primarily determined by the capacity of the sleeve, while the ultimate tensile strength of Hollo-bolts is influenced by the capacity of the centre screw, as the concrete grade shows no significant effect on the tensile strength of Hollo-bolts.