This study investigates the flexural behavior of pultruded GFRP–concrete composite beams strengthened with GFRP T-section stiffeners. The experimental setup involved 2600 mm long GFRP I-beams connected to concrete slabs (500 mm wide and 80 mm thick) with two rows of shear connectors (bolts) to achieve full shear connection. The concrete slabs were categorized into normal-strength concrete (NSC) and high-strength concrete (HSC). The stiffeners were either bolt-epoxy or bolt-only, and corrugated metal sheets were included in some cases. The results showed that GFRP T-section stiffeners improved the flexural and shear strength of the beams. The failure loads for bolt-epoxy connections were 8.2% and 10.0% higher than for bolt-only connections when the concrete compressive strengths were 20.1 MPa and 52.3 MPa, respectively. The use of HSC increased failure loads by 79.9% and 77.1% for bolt-epoxy and bolt-only connections, respectively. The epoxy adhesive with mechanical connectors provided sufficient composite action and delayed shear failure in the GFRP beam. Strain levels in the concrete slabs were higher for bolt-epoxy connections compared to bolt-only connections, with HSC slabs showing 20.0% and 21.8% higher strains. The enhanced composite interaction between the concrete slabs and GFRP I-beams was attributed to this difference. Numerical models using ABAQUS software were developed to analyze the tested beams and provide a parametric study. The models showed good agreement with experimental results, confirming the accuracy of the simulations. The study concludes that GFRP I-beams combined with concrete offer improved flexural performance, and further research is needed to optimize their use in structural applications.This study investigates the flexural behavior of pultruded GFRP–concrete composite beams strengthened with GFRP T-section stiffeners. The experimental setup involved 2600 mm long GFRP I-beams connected to concrete slabs (500 mm wide and 80 mm thick) with two rows of shear connectors (bolts) to achieve full shear connection. The concrete slabs were categorized into normal-strength concrete (NSC) and high-strength concrete (HSC). The stiffeners were either bolt-epoxy or bolt-only, and corrugated metal sheets were included in some cases. The results showed that GFRP T-section stiffeners improved the flexural and shear strength of the beams. The failure loads for bolt-epoxy connections were 8.2% and 10.0% higher than for bolt-only connections when the concrete compressive strengths were 20.1 MPa and 52.3 MPa, respectively. The use of HSC increased failure loads by 79.9% and 77.1% for bolt-epoxy and bolt-only connections, respectively. The epoxy adhesive with mechanical connectors provided sufficient composite action and delayed shear failure in the GFRP beam. Strain levels in the concrete slabs were higher for bolt-epoxy connections compared to bolt-only connections, with HSC slabs showing 20.0% and 21.8% higher strains. The enhanced composite interaction between the concrete slabs and GFRP I-beams was attributed to this difference. Numerical models using ABAQUS software were developed to analyze the tested beams and provide a parametric study. The models showed good agreement with experimental results, confirming the accuracy of the simulations. The study concludes that GFRP I-beams combined with concrete offer improved flexural performance, and further research is needed to optimize their use in structural applications.