Push-out tests on perfobond connectors in hybrid FRP-concrete-steel double-skin tubular members (DSTMs)

Hybrid FRP-concrete-steel double-skin tubular members (DSTMs) have been adopted in a number of engineering applications and gained increasing popularity in research community. The bond behavior between the concrete and the steel in DSTMs is the essential scientific issue yet to be investigated. This paper presents push-out tests on perfobond shear connectors (PBLs) in hybrid DSTMs. The test variables include the presence and thickness of GFRP tubes, PBL plate thickness, PBL hole size, and concrete thickness. The results, including failure modes, load-slip curves, and strain developments, are presented and discussed. All specimens experienced concrete splitting cracks, tear-off or shear rupture of the PBL plates, and shear-bending deformation of the PBL rebars. The typical load-slip curve comprises four stages: (1) concrete-PBL bonding, (2) elastic ascending, supported by concrete dowels and concrete end bearings, (3) plastic ascending, where shear forces are transmitted to the PBL rebars, and (4) descending, due to PBL plate rupture. The PBL plate thickness, concrete thickness, and hole size significantly influenced push-out behavior. Increasing PBL plate thickness (from 3 to 8 mm) and concrete thickness (by 15 mm) raised peak shear loads by 35 % and 23 %, respectively, due to enhanced concrete end-bearing resistance. The shear peak loads were positively correlated with PBL plate thickness and concrete thickness, but showed a declining trend with increasing hole sizes on PBL plates. In comparison, the GFRP tube provided only weak confinement to the concrete, resulting in a marginal increase (3–6 %) in shear peak loads. • Push-out tests between PBL shear connectors and concrete in DSTMs are carried out. • The load-slip curves exhibit a four-stage behavior. • The PBL plate thickness, concrete thickness, and hole sizes significantly influence push-out behavior. • The shear peak loads are positively correlated with PBL plate thickness and concrete thickness. • The GFRP tube leads to a marginal increase in shear peak loads.