Low-cycle fatigue bond behaviour between sand-coated hybrid FRP tubes and sustainable seawater sea sand concrete

• Cyclic loading weakens the interface, causing greater sand coating detachment from SWSSC. • Bond strength reduces with more cyclic repetitions due to cumulative damage. • Static tests reach ultimate load at smaller displacements than cyclic tests. This study examines the bond behaviour of sand-coated hybrid Fibre Reinforced Polymer (HFRP) tubes filled with seawater sea sand concrete (SWSSC) under static and cyclic axial loads through push-out tests. The HFRP tubes, manufactured from glass and carbon fibre-reinforced polymer composites, were tested under incrementally increasing cyclic loading. Four distinct displacement-controlled loading regimes were explored, with cycles repeated 1, 3, 5, and 10 times for each specified displacement. The results showed that bond strength and energy dissipation capacity decrease with increased cycle repetitions due to cumulative interface degradation, while static tests indicated higher bond strengths at smaller displacements. ANOVA analysis confirmed that although significant differences in bond strength were observed with varying cycle repetitions, the displacements corresponding to maximum bond strength remained statistically similar. Quantitatively, the bond strength decreased from an average of 2.35 MPa in the single-cycle regime to approximately 1.89 MPa in the 10-cycle regime (a reduction of around 19 %). At lower cyclic repetitions (1 and 3 cycles), failure was primarily characterised by partial debonding at the interfaces between the sand coating and both the SWSSC and the HFRP tube. With increasing cycle repetitions (5 and 10 cycles), the bond damage significantly escalated, and a shift to complete coating detachment occurred. The damage mechanism evolved from interface failure within the SWSSC/HFRP–sand coating layers to complete separation of the composite layers from the SWSSC. These results show how cumulative damage effectively undermines the interface bonding between the components, emphasising the relevance of considering the number of cycles in evaluating composite durability under real-world conditions. These findings can improve the design and durability assessment of HFRP-concrete composite structures, particularly in environments subjected to repetitive loading conditions.