Experimental validation of damaged reinforced concrete beam strengthened by pretensioned prestressed ultra-high-performance concrete layer

Strengthening reinforced concrete (RC) beams with ultra-high-performance concrete (UHPC) can improve me-chanical properties and durability from previous studies. To further improve UHPC strengthening efficiency, a new technology using a pretensioned prestressed UHPC (P-UHPC) layer for flexural strengthening was proposed in this study. The flexural capacity and crack resistance of the damaged RC beams strengthened by the P-UHPC layer were investigated through a four-point bending test in order for the results to be used in the analysis and design of strengthening damaged structures. The flexural performance of two P-UHPC strengthened beams (PU), one reinforced UHPC (R-UHPC) layer strengthened beam (RU), and one unstrengthened RC control beam (CB) was tested. This study also determined the influence of UHPC shrinkage and prestressing force on the cracking load and ultimate flexural capacity of the P-UHPC strengthened beams by proposing a formal theory. Results showed that the failure modes of the PU beams were typically bending failure. The PU beams showed good composite performance, and there was no debonding failure at the UHPC-RC interface during the test. Moreover, the flexural performance of the damaged RC beams strengthened with the P-UHPC layer was significantly improved, with the average cracking load of the PU beams higher than that of the CB and RU beams by 87.4% and 28.1%, respectively. The average ultimate flexural capacity was 85.2% and 31.7% higher than the CB and RU beams, respectively. In addition, the P-UHPC layer had a more substantial inhibitory effect on the crack development of the damaged RC beams, and the re-expansion load of the cracks in the damaged RC of the PU beams was 111.5 % higher than the RU beam. Finally, the calculation model and theoretical formula for pre-dicting the cracking load and ultimate flexural capacity of the P-UHPC strengthened beams were proposed, and the test results verified the applicability of the proposed calculation methods.