NON-LINEAR BEHAVIOR OF ±55˚ GFRP TUBES FILLED WITH CONCRETE AND DEVELOPMENT OF A NOVEL LOW-CARBON, GYPSUM-MODIFIED CONCRETE FOR SUSTAINABLE STRUCTURAL APPLICATIONS

Abstract

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) with ±55˚ glass-FRP (GFRP) tubes exhibit highly non-linear tensile behavior, unlike the linear response of cross-ply or near-cross-ply configurations. While the confinement effect in axial compression is well understood, the interaction between the tube and concrete core under axial tension remains less explored. This study investigates the non-linear tensile and flexural behavior of ±55˚ GFRP tubes in CFFTs. A novel experimental setup successfully captured an ultimate tensile strain of 0.077 mm/mm, providing key insights into their tensile response. Finite element models accurately simulated tensile and bending behavior, with strong agreement to experimental data. A design-oriented constitutive model was developed for ±55˚ GFRP tubes filled with concrete and integrated into an analytical framework for flexural capacity prediction, achieving 85-100% accuracy in estimating experimental ultimate load and deflection. To mitigate the environmental impact of cement production and drywall waste, this study proposes a low-carbon, gypsum-modified concrete that reduces cement usage by 80% through the incorporation of recycled gypsum, fly ash, and slag. An experimental program with 135 specimens optimized material compositions, achieving a cement content of only 21% of the total binder. The challenge of early-age strength loss, typical in high cement replacement mixes, was mitigated through confinement in ±55˚ GFRP tubes. Structural testing of six ±55˚ CFFT specimens under monotonic and cyclic compression demonstrated confinement-induced strength gains of 2.1-4.8 times in monotonic loading and 2.1-2.5 times in cyclic loading. These findings highlight the potential of integrating sustainable concrete with CFFT technology for enhanced structural and environmental performance.