Composite Action Assessment of Concrete-Filled FRP Tubes Subjected to Flexural Cyclic Load

Concrete filled fiber-reinforced polymer (FRP) tubes (CFFT) is a principal competitor to replace the conventional reinforced concrete (RC) members in severe environmental conditions. This paper investigates the composite action (interfacial bond) between the FRP tube and its concrete core. Four full-scale CFFT columns associated with RC footings were tested under lateral cyclic load without axial load. Two different diameters of CFFT columns were tested to study the size effect on the bond performance then consequently on the flexural behavior of the CFFT column. The interior surface of two FRP tubes have been covered by sand coating to improve the interfacial bond between the FRP tube and the concrete core. A new approach was proposed to evaluate the composite action between the FRP tube and the concrete core based on measuring strains inside the concrete core by using embedded concrete strain gauges. The assessment of the composite action between the FRP tube and the concrete core was implemented by comparing the interior concrete strains and the corresponding strains on the external tube skin. The experimental results illustrated that the bond significantly influences the flexural strength and stiffness of the CFFT column. Increasing the tube diameter leads to reduce the interfacial bond between the FRP tube and its concrete core. Using sand-coating as a bond enhancer improved the bond between the FRP tube and the concrete core, minimize the adverse effect of increasing the tube diameter, and increased the flexural capacity and stiffness of tested CFFT columns. An analytical model was developed to estimate the flexural capacity of the fully bonded CFFT member.