Optimising compressive strength properties of seawater sea sand concrete-filled hybrid carbon-glass fibre-reinforced tubes: A study on the impact of layer sequencing

Utilising fibre-reinforced polymer (FRP) composites with seawater and sea sand concrete (SWSSC) in coastal regions tackles environmental concerns linked to traditional concrete and corrosion issues of conventional steel reinforcements. While previous studies have explored the mechanical properties of single-material FRP tubes, such as glass fibre-reinforced polymer (GFRP) and carbon fibre-reinforced polymer (CFRP), research on the durability of hybrid fibre tubes has been limited to a single stacking lay-up configuration. This study aims to investigate how layer sequencing affects the compressive strength properties of SWSSC-filled hybrid glass-carbon FRP (HFRP) tubes under seawater exposure. It explores two different fibre orientations, namely cross-ply and hoop. One approach which has been used in this study involves constructing tubes where 50 % of the inner tube thickness comprises CFRP layers and 50 % of the outer tube thickness comprises GFRP layers, while another approach alternates carbon and glass layers throughout the tube thickness. Laboratory accelerated ageing tests involved three temperatures (25°C, 40°C, and 60°C) and three conditioning durations (30, 90, and 150 days). Arrhenius predictive models were used to predict the tubes’ long-term residual compressive strength. The results indicate that the 50 %-50 % layer configuration outperforms the layer-by-layer arrangement in cross-ply tubes, with a service life strength retention of 63.9 % in the 50 %-50 % layer configuration compared to 48.8 % in the layer-by-layer configuration. However, no significant difference was observed between the two configurations in hoop tubes. This suggests that utilising CFRP as inner layers in cross-ply fibre-oriented tubes can potentially slow down damage progression from the harsh SWSSC environment toward the GFRP layers across the tube thickness. However, given that the compressive load acts perpendicular to the orientation of the fibres, the resin contributes more to the load-carrying capacity of the hoop-oriented fibres than cross-ply fibre tubes. This suggests that solely relying on this sequencing strategy may not yield highly effective results in this specific orientation. Implementing an alkaline-resistant coating on the inner surface of the tubes or augmenting the tube thickness are potential approaches to further delay damage progression, especially in hoop-oriented tubes.