Experimental Study on Shear Strengthening of Reinforced Concrete Deep Beams with Both Embedded Through-Section GFRP Bars and Externally Bonded CFRP Sheets

This study investigated the shear resistance of reinforced concrete (RC) deep beams strengthened with a combination of embedded through-section glass fiber-reinforced polymer (ETS-GFRP) bars and externally bonded carbon fiber-reinforced polymer (EB-CFRP) sheets. Four-point bending tests were conducted on nine rectangular RC deep beams: one control beam with internal steel stirrups, two beams strengthened only with EB-CFRP sheets, and six beams retrofitted with both EB-CFRP sheets and ETS-GFRP bars under various configurations. The main design variables were the ETS-GFRP bar diameter (10 or 13 mm), FRP sheet wrapping scheme (U-wrap or full wrap), number of sheet layers (one or two for U-wrap and one for full wrap), and the shear span-to-effective depth ratio ( a/d = 1.5 or 2.2). Experimental results indicate that RC deep beams with shear strengthening systems (EB-FRP combined with ETS-FRP) presented an increased peak load (i.e., beam capacity) and shear slope of the load‒deflection curve (i.e., beam stiffness) compared with those of unstrengthened beams and those strengthened via only one strengthening method, with the shear capacity increasing by up to 13%. Notably, the use of the hybrid retrofitting method helps delay premature debonding of the EB-FRP element and postpones the final failure, as confirmed by higher debonding loads and extended strain development in the FRP systems. Increasing the ETS bar diameter enhances the shear strength of the beam. By contrast, increasing the a/d ratio reduced the shear strength by 14% approximately. Finally, predictions made by the existing design model aligned well with the test data.