Bond-slip behaviour between FRP tubes and seawater sea sand concrete

In the present study the bond-slip behavior between FRP tubes and seawater sea sand concrete is studied using push-out tests. Pultruded and filament winding FRP tubes with different fibre orientations and diameter-to-thickness ratios (D/t) were constructed to investigate the effect of confinement pressure on the bond behaviour. Moreover, to study the effect of fibre type, three types of FRP (i.e.; GFRP, CFRP and BFRP) were used. In addition, the mechanical and chemical bond strength of concrete filled FRP tubes were quantified and compared. CFRP and BFRP tubes showed the highest and the lowest bond strengths, respectively. Furthermore, it was concluded that the real confinement pressure of the tubes has a significant effect on the bond strength: the greater the real confinement pressure, the larger the bond strength (i.e. the tubes with smaller D/t ratio and fibres orientated in 89° showed greater bond strength compared to the tubes with larger D/t and different fibres orientations). Repeating the push-out test for four times, for each sample, showed that the slip starts earlier by increasing the number of repetition, while the bond strength remained almost the same. This shows that the bond strength of tubes is mostly governed by the friction (confinement pressure) rather than the adhesion (chemical bond).