Flexural Strengthening of Reinforced Concrete Beams Using Near-Surface Mounted (NSM) Carbon Fiber-Reinforced Polymer (CFRP) Strips with Additional Anchorage

The work presents and examines a fiber anchoring system of NSM CFRP strips proposed for strengthening RC beams. The study included 11 beams: 3 unstrengthened beams, 3 beams strengthened with NSM CFRP strip without anchorage, and 5 beams strengthened with NSM CFRP strips with additional anchorage in two variants (the fiber anchor wrapped around the CFRP strip end and fan-folded on the beam surface; the fiber anchor connected with a 20 cm overlap to the strip). All beams were loaded until failure with two concentrated forces (four-point loading test). The measurements were carried out using digital image correlation (DIC). The obtained ultimate load values reached an average of 43.5 kN for unstrengthened beams, while for strengthened beams, they ranged between 56.6 kN and 60.2 kN. The strengthening efficiency was comparable for all beams regardless of the anchorage used and ranged from 29% to 37%. All strengthened beams failed due to strip debonding. The obtained results did not allow confirmation of the effectiveness of the proposed anchoring system. Detailed analysis showed that the lack of anchoring effectiveness was related to the debonding initiating factor, i.e., vertical crack opening displacement, which has not been described in proper detail by the researchers.