Concrete-filled GFRP tubes with recycled needle- or granule-shaped GFRP aggregates subjected to axial compression

The disposal of Fiber-Reinforced Polymer (FRP) waste presents significant environmental and economic challenges. Recycling FRP waste into aggregates for concrete (i.e., recycled FRP aggregates, RFAs for short) offers a viable solution, potentially reducing costs and minimizing environmental impact. This study primarily investigates the influence of RFA shape (needle shape vs. granule shape) on axial compression performance of concrete-filled FRP tubes (RFA-CFFTs), while also examining the effects of RFA replacement ratio and FRP tube thickness using thirty-five specimens. The experimental findings reveal that: (1) the axial stress-strain curves of the specimens consisting of needle-shaped RFAs closely resemble those with granule-shaped RFAs, with only minor deviations observed in the terminal portions of these curves; (2) a relatively large amount of needle-shaped RFAs lead to significant mixing and casting challenges, setting a practical limit on their replacement ratio, while granule-shaped RFAs can be used at much higher replacement ratios with better mixture consistency; (3) although needle-shaped RFAs are reported to enhance the splitting strength of unconfined concrete, this benefit does not extend to RFA-CFFTs, where the concrete core is confined and subjected to tri-axial compression; (4) increasing RFA replacement ratio generally decreases the peak axial stress of FRP-confined concrete, with needle-shaped RFAs showing a more pronounced reduction compared to granule-shaped RFAs; (5) increased FRP tube thickness (from 3.3 mm to 5.7 mm) results in a higher peak compressive stress and a larger ultimate strain, with specimens containing needle-shaped RFAs showing smaller increase values compared to those with granule-shaped RFAs. In conclusion, granule-shaped RFAs are found to be more effective in RFA-CFFTs due to better aggregate grading and higher feasible replacement ratios. An accurate prediction of compressive behavior of RFA-CFFTs requires the consideration of the biaxial stress behaviour of filament-wound GFRP tubes.