This paper investigates the flexural behavior of pultruded glass fiber-reinforced plastic (GFRP) I-beams composite with concrete slabs, using both experimental and numerical methods. The study focuses on the use of GFRP I-beams in conjunction with concrete slabs to form composite beams, with the concrete slabs categorized into two groups: those made from normal-strength concrete (NSC) and high-strength concrete (HSC). Various parameters, such as the type of concrete, the type of stiffeners (bolt-epoxy or bolt-only), and the inclusion of corrugated metal sheets, were examined. The experimental design involved 2600 mm long GFRP I-beams connected to 500 mm wide and 80 mm thick concrete slabs. Shear connectors in the form of bolts were used to ensure a full shear connection between the GFRP I-beams and the concrete slabs. The strengthening of the shear webs with GFRP T-section stiffeners enhanced the flexural and shear strength of the composite beams. The failure loads were 8.2% and 10.0% higher for bolt-epoxy connections compared to bolt-only connections, with significant improvements when using HSC. Strain levels in the concrete slabs were consistently higher for bolt-epoxy connections, and the use of HSC increased strain levels by 20.0% and 21.8% compared to NSC. Finite element (FE) models were developed using ABAQUS software to analyze the composite beams and provide a parametric study. The FE models accurately predicted the load-deformation behavior and strain measurements, validating the experimental findings. The study concludes that the use of GFRP I-beams in composite beams with concrete requires further research to establish a rational basis for their effective utilization.This paper investigates the flexural behavior of pultruded glass fiber-reinforced plastic (GFRP) I-beams composite with concrete slabs, using both experimental and numerical methods. The study focuses on the use of GFRP I-beams in conjunction with concrete slabs to form composite beams, with the concrete slabs categorized into two groups: those made from normal-strength concrete (NSC) and high-strength concrete (HSC). Various parameters, such as the type of concrete, the type of stiffeners (bolt-epoxy or bolt-only), and the inclusion of corrugated metal sheets, were examined. The experimental design involved 2600 mm long GFRP I-beams connected to 500 mm wide and 80 mm thick concrete slabs. Shear connectors in the form of bolts were used to ensure a full shear connection between the GFRP I-beams and the concrete slabs. The strengthening of the shear webs with GFRP T-section stiffeners enhanced the flexural and shear strength of the composite beams. The failure loads were 8.2% and 10.0% higher for bolt-epoxy connections compared to bolt-only connections, with significant improvements when using HSC. Strain levels in the concrete slabs were consistently higher for bolt-epoxy connections, and the use of HSC increased strain levels by 20.0% and 21.8% compared to NSC. Finite element (FE) models were developed using ABAQUS software to analyze the composite beams and provide a parametric study. The FE models accurately predicted the load-deformation behavior and strain measurements, validating the experimental findings. The study concludes that the use of GFRP I-beams in composite beams with concrete requires further research to establish a rational basis for their effective utilization.