Exploring Natural Cotton FRP as a Sustainable Method for Confinement of Concrete Made with Natural and Recycled Aggregates

Over recent decades, earthquakes have caused significant damage to buildings worldwide, prompting innovative solutions to enhance the seismic resistance of concrete structures. This study explores the use of natural cotton strips (NCS) as a bio-based and low-cost confinement method for natural aggregate concrete (NAC) and recycled brick aggregate concrete (ReBAC). It contrasts partial confinement (using strip and mid-height) with full wrapping to leverage local materials for cost-effective seismic improvements. Experimental results showcased a sequential failure progression similar to fiber-reinforced polymer (FRP) systems, including cracking, rope rupture, and core crushing. NCS confinement improved ductility without altering elastic stiffness, producing a three-phase stress-strain response in circular specimens, while square specimens demonstrated limited effectiveness due to stress concentrations. For NAC, full wrapping achieved strength and strain ratios of 2.42 and 3.1, respectively. Strip wrapping with 20 mm spacing showed ratios of 2.18 and 2.50. In ReBAC, strength ratios increased from 1.26 to 1.57 for strip wrapping. Notable improvements in square ReBAC specimens occurred with 100 mm strips, achieving 19–22% greater strength than 20 mm strips. Two-layer designs consistently enhanced strength and ductility, confirming NCS as an effective, economical confinement technology.