Experimental investigations of strengthened beam with co-cured carbon FRP and mussel shell-modified epoxy

Traditional strengthening materials, such as carbon fibre reinforced polymer (CFRP) plates bonded with synthetic fillers, often rely on non-renewable resources and may have limited environmental compatibility. Thus, mussel shells with a high content of calcium carbonate particles act as rigid particles and alternatives to synthetic filler counterparts added in epoxy resin, enhancing the mechanical properties of filled epoxy. This paper aims to investigate the improvement of flexural resistance by incorporating mussel shell powder as a bio-filler of epoxy resin (hereafter referred to as mussel shell-modified epoxy, MME), which was then used as the bonding agents with CFRP sheets via co-cured technique as a beam strengthening method. A four-point bending test was conducted to investigate four parametric studies, i.e., CFRP bonded lengths (Series A), mussel shell powder volume fraction (Series B) and different percentages of pre-load applied on concrete beams with different bonding agent types, i.e., MME (Series C) and neat epoxy resin (Series D). A 7.5% volume fraction of MME and the most extended CFRP sheet enhanced the ultimate load with deflection (associated with concrete ductility) of the strengthened plain beams by 108% and 58%, respectively, compared to the control beam. Interestingly, up to 66% improvement was observed in Series C by applying pre-load at 60% of the control beam’s ultimate load, comparing beams strengthened with MME to those using neat epoxy (Series D). In this test, pre-loading was applied by subjecting the concrete beams to 60% of their ultimate capacity using a Universal Testing Machine (UTM) before applying CFRP sheets, ensuring consistent initial stress conditions for reliable comparison. Hence, MME is a viable bio-filler incorporated in epoxy resin to enhance co-cured CFRP as a strengthening material.