Bond performance between sand coated hybrid glass-carbon FRP tubes and seawater sea sand concrete after exposure to elevated temperatures

Employing hybrid glass-carbon FRP tubes in concrete-filled FRP tubes for marine applications offers a balance between durability and cost-effectiveness, making them ideal for harsh, corrosion-prone environments. The investigation aimed to explore the bond-slip characteristics of sand-coated hybrid carbon-glass FRP tubes filled with seawater sea sand concrete (SWSSC) subjected to varying high temperatures. Elevated temperatures can affect the bond by causing thermal expansion and altering material properties, while sand coating enhances the bond by increasing surface roughness and improving mechanical interlocking. Utilising a push-out test, the study addresses this crucial but insufficiently studied aspect of the interface between SWSSC and FRP tubes. Thirty-three sand coated hybrid tubes were tested experimentally, with temperature being the sole variable. The samples underwent testing at eleven different temperature levels (ranging from 25°C to 250°C) to investigate the relationship between temperature fluctuations and bond-slip behaviour. The results highlighted three key factors influencing bond performance. Two factors had a positive influence: firstly, the expansion of the tube due to increased temperatures, promoting better interlocking among the concrete, sand coat, and tube; secondly, resin's mechanical properties as a result of post-curing. Conversely, a negative impact was observed due to sand coating detachment and tube degradation resulting from elevated temperatures. Analysis of the findings revealed a pattern in bond strength retention: a decrease to 77 % renetion up to 80°C, followed by an increase to 108 % up to 220°C, and finally a decrease again to 83 % at 250°C. The findings from this study enhance the progress and utilisation of hybrid FRP tubes together with SWSSC in producing environmentally sustainable construction materials, all the while ensuring they meet the necessary structural standards.