Experimental study on flexural behavior of damaged reinforced concrete (RC) beam strengthened by toughness-improved ultra-high performance concrete (UHPC) layer

The flexural test was conducted to investigate crack resistance, loading bearing capacity, deformation characteristics and failure modes of damaged reinforced concrete (RC) beams strengthened by reinforced ultra-high performance concrete (UHPC) layer (Abbreviated as UC). Also, their mechanical properties were compared with those of the unstrengthened RC beam (Abbreviated as I-C). Additionally, the effects of pre-damage degrees in the RC beams and three strategies for the improvement of toughness of the reinforced UHPC layer on the cracking and flexural performance of UC were discussed. The results showed that UC acted monolithically under flexure without interfacial debonding before typically flexural failure. Compared with I-C, cracking and ultimate loads of UC increased by 1.57-3.32 times and 1.72-2.21 times, respectively. The reinforced UHPC layer effectively suppressed the cracking of the RC beam, making crack width in the RC beam propagate slowly with the load. Moreover, the severer the pre-damage degree of the RC beam, the smaller improvement of the flexural performance of UC. The addition of steel wire mesh, orientation of steel fibers and moderate-temperature steam curing further improved the cracking and flexural performance of UC with the most evident improvement in the addition of steel wire mesh. Finally, the theoretical model and formula were proposed for the calculation of load bearing capacity of UC with consideration of the influence of the pre-damage degrees in the RC beams, and the experimental results verified that the theoretical formula can accurately predict the load bearing capacity of UC.