Sustainable Flexural Enhancement of Concrete-Filled Steel Tubular Beams Using Internal Curing Agents and Fabricated I-Section Reinforcement

Abstract

This study investigates the flexural performance of Concrete-Filled Steel Tube (CFST) beams incorporating centrally embedded Fiber Glass Composite (FGC) I-sections and self-curing concrete using polyethylene glycol (PEG 400) admixture. Five concrete mixes with varying PEG 400 dosages (0-1.25% by cement weight) were optimized through comprehensive fresh and hardened properties testing, confirming optimum workability (80-100 mm slump, 500-600 mm flow), air content (<4%), and superior compressive (32.75 N/mm²), split tensile (3.69 N/mm²), and flexural (5.41 N/mm²) strengths at 28 days for the 1.25% mix compared to normal concrete. Two CFST beams—B1 (self-curing concrete) and B2 (normal concrete)—were subjected to four-point bending. Both beams exhibited high ultimate load capacity, stiffness, and ductility due to synergistic composite action between the steel tube, concrete core, and internal FGC I-section (88.9 × 38.1 × 4.67 mm). B1 demonstrated superior performance with minimal surface cracking, delayed yielding, better stiffness retention, and gradual localized indentation failure under loading points. The PEG 400 admixture ensured effective internal hydration within the sealed steel tube environment, promoting better microstructure development and steel-concrete bonding. Results confirm that the combination of steel confinement, internal FGC reinforcement, and self-curing concrete creates a structurally efficient flexural system with excellent energy dissipation and post-peak capacity. The study establishes PEG 400 at 1.0-1.25% dosage as optimal for CFST applications where external curing is impractical.