Flax FRP tube and steel spiral dual-confined recycled aggregate concrete: Experimental and analytical studies

Using fibre reinforced polymer (FRP) tube as outer confining material of recycled aggregate concrete (RAC) column improved its compressive strength and deformation capacity although the RAC core still behaved in a brittle failure manner. To achieve a ductile failure mode of RAC core in compression, outer flax FRP tube (FFRP) and inner steel spiral reinforcement (SR) were used in this study to confine the RAC cylinders, i.e., to form an FFRP tube-SR-confined RAC column, which is shortened as FFRP-SR-RAC. To evaluate the effects of spacing of the inner steel spiral (i.e., 25 and 50 mm), strength of RAC (i.e., 26.3 and 33.7 MPa), and the FFRP tube thickness (i.e., 3, 6 and 9 layers of FFRP) on the compressive behaviour of FFRP-SR-RAC, 36 concrete cylinders were constructed and tested in uniaxial compression. The test results indicated that comparing with unconfined RAC or FFRP tube confined RAC cylinders, the FFRP-SR-RAC specimens possessed higher compressive strength and failed in a ductile manner. When the number of FFRP layer increased and the spacing of SR decreased, the FFRP-SR-RAC cylinder gained more pronounced enhancement in the compressive strength. Based on the experimental results, stress and ultimate strain models were developed to predict the axial compressive behaviour of FFRP-SR-RAC in uniaxial compression.