Static and Cyclic Compressive Properties of Self-Compacting Concrete-Filled Flax Fiber–Reinforced Polymer Tubes

In the present study, natural fiber (i.e., flax fiber) reinforced epoxy (FFRP) plates and tubes were prepared with a filament winding process. FFRP tubes with 150-mm diameter were filled with self-compacting concrete and tested in terms of static and cyclic compressive properties. Basalt fiber–reinforced epoxy (BFRP) plates and tubes were prepared and tested for comparison. The FFRP plate possesses a nominal tensile strength and modulus of 333 MPa and 48.5 GPa, respectively, and exhibits a nonlinear tensile strain-stress curve. Compared with the static tensile modulus of the FFRP plate, the strain modulus–controlled low-frequency cyclic test steadily decreases with the number of load-unload cycles as a result of cumulative damage, and the ultimate stress is also reduced by 36%. According to the static compression results, FFRP tubes exhibit a notable confinement effect on concrete cylinders. The confinement effectiveness coefficient determined by the static compression test ranges from 4 to 7 for the FFRP tubes wound with four to 12 layers of flax fibers. Cyclic compression results show that the compression stress of the concrete-filled FFRP tubes is lower than that of the static compression stress under the same compression strain. This finding is attributed to the cumulative damage of the FFRPs during the cyclic process, similar to the FFRP coupons. On the contrary, the cyclic ultimate compression strain and stress of the concrete-filled BFRP tubes are notably close to the static values.