•Naturally bonded composites have the weakest slippage resistance.•Local cracks of flax FRP at interlocking-holes interface limits the bond strength.•Interlocking-keys increase the bond between concrete core and flax FRP confinement.•Strip interlocking keys impeded concrete slippage cause the highest bond strength.•An equation for predicting slippage resistance for different interfaces was proposed. This paper addresses an experimental study on the bond of flax fibre reinforced polymer confined coconut fibre reinforced concrete composite. Four different conditions of the interface between the flax fibre reinforced polymer and coconut fibre reinforced concrete were considered. Twelve cylindrical specimens were fabricated and push-out tests were performed. Test results show that the mechanically bonded interface highly enhanced the bond between the polymer and concrete core. In addition, an equation was developed based on the experimental data to estimate the slippage resistance provided by the mechanically interface. The failure modes of all type of specimens will also be discussed.