Development and experiment of grippers to prevent debonding in H-shaped steel beams strengthened with carbon fiber reinforced polymer plates

External bonding of carbon fiber–reinforced polymer (CFRP) plates is a common method for the flexural strengthening of steel beams. However, the interfacial stress concentration at the CFRP plate end can easily cause premature debonding failure of strengthened beams. Consequently, the high performance of the materials is not fully utilized, and the safety of the strengthened beams may be threatened. To address this issue, the paper proposes a simple C-shaped plate gripper to strengthen H-shaped steel beams, which can improve the anchorage at the CFRP plate end. Four-point bending tests were conducted for strengthened beams to verify gripper reliability. The strengthening effect was examined by changing the ratio of the bond length (measured from load point to the CFRP plate end) to the shear span length and anchorage types. There are three types of anchorage: in the first type, the CFRP plate is bonded only on the tensile face of the beam; in the second type, the CFRP plate is attached to the tensile face of the beam and a gripper is attached to the end of the CFRP plate; in the third type, a combination of the above two types is utilized. These types are called pure bonding, end anchorage, and hybrid strengthening, respectively. The case with an anchorage at the CFRP plate end is considerably different from the case of pure bonding. The former relies on mechanical force transfer, while the latter relies on an epoxy resin for bonding. As long as failure does not occur first in the outer section of the CFRP plate, the strain and utilization ratio of the CFRP plate, along with the related strengthening effect, will increase as the length of the CFRP plate decreases. For a strengthened beam with a CFRP plate length of 600 mm, the ultimate strain and load are 27.3% and 8.1% higher than those of the case with a CFRP plate length of 750 mm, respectively. Notably, the hybrid strengthened beams debonded suddenly and degraded to beams with only end anchorage in the subsequent loading stage. There is no obvious improvement in beams with hybrid strengthening compared with those with only end anchorage. This can be attributed to the poor surface treatment of the beams. Thus, it is necessary to improve the quality of surface treatment in the future. Compared with the cases of pure bonding, the flexural behavior is greatly improved for the cases with end anchorage for hybrid strengthened beams because the grippers can successfully impede the debonding of the CFRP plate. Herein, the debonding process was detected in real time using the piezoresistance method, which exhibits consistent results with the test phenomena.