Evolution of surface roughness and particle breakage with interface shear behaviour of sand-epoxied GFRP against sand under elevated normal stress

Glass fibre-reinforced polymer (GFRP) composites are interestingly adopted in geotechnical applications, especially in harsh marine environments. However, their inherently smooth surface and relatively low hardness may compromise interface performance with soil. The application of a sand-epoxy coating enhances surface roughness, friction, and hardness, making it essential to assess its behaviour under moderate to large normal loads for reliable offshore deployment. A comprehensive investigation of surface roughness evolution in both uncoated and sand-epoxied GFRP composites is conducted through interface tests, emphasising their interface shear performance when sheared against standard silica sand under moderate to high normal stress. The optimal sand-epoxy coating combination is determined through monotonic testing, while long-term performance is evaluated by cyclic testing combined with measurements of roughness variation. Results show that surface roughness evolution and particle breakage intensify with increasing median particle size, cycle number, and normal stress. Cyclic shearing exhibits higher interfacial friction angles compared to monotonic shearing. This trend results from the synergistic interplay of surface degradation, particle breakage, particle interlocking, and stress redistribution at interface under elevated normal stress. Ultimately, design charts are developed to facilitate the practical deployment of optimised sand-epoxied GFRP composites for durable reinforcement in applications such as foundations and marine structures.