Experimental investigation into the interfacial shear strength of AGS-FRP tube confined concrete pile

Fiber reinforced polymer (FRP) tube confined concrete piles have been increasingly used in the infrastructure industry. However, debonding is one of the critical issues that need to be adequately addressed before FRP confined concrete pile can reach its full market potential. To improve drivability of conventional FRP tube confined concrete cylinders for pile applications, Li [Li G, Experimental study of hybrid composite cylinders, Compos Struct 2007; 78 (2): 170–181] proposed a novel advanced grid stiffened (AGS) FRP tube encased concrete cylinder. The AGS-FRP tube was made of a lattice of interlaced FRP ribs that was wrapped by a thin layer of FRP skin. The AGS-FRP tube was then filled with concrete. Test results show a considerably increased compressive strength, elastic range, and positive composite action due to the enhanced interfacial bonding strength through mechanical interlocking. The purpose of the current study was to experimentally investigate the effect of the thickness of the FRP ribs and the FRP skin on the interfacial bonding strength through push-out test. Five groups of specimens were manufactured. Groups A and B consisted of regular FRP tube confined concrete cylinders with different wall thicknesses. Groups C, D, and E consisted of the AGS-FRP tube confined concrete cylinders of varying rib thicknesses. The push-out test results indicated that the AGS-FRP tube confined concrete cylinders had stronger interfacial bonding than the regular FRP tube confined concrete cylinders. Skin thickness was positively related to the interfacial bonding in the regular FRP tube confined concrete columns. To the opposite of the common belief that the interfacial bonding strength increases as the rib thickness increases, the interfacial shear strength decreases as the rib thickness increases, due to the lower concrete strength used. It is thus suggested that the rib thickness should match the concrete strength in order to fully display the efficiency of mechanical interlocking.