Research On The Flexural Behavior Of RC Beam Strengthened With NSM FRP Bars Considering Effect Of Secondary Loading

FRP bar is a new type of composite material that possesses better mechanics and durability.NSM(near-surface-mounted)FRP reinforcement is one of the latest and most promising strengthening techniques,which has gradually become research focus in the field of concrete structure strengthening.Compared to the traditional strengthening methods,NSM FRP reinforcement can save installation time and features advantage of convenient construction.The bond of FRP and concrete is strengthened,which makes the FRP force more effective.Furthermore,this method is suitable for flexural strengthening in the negative moment regions of bridge decks and concrete beams.It can improve the durability of members.What is more,the FRP bars can be effectively protected into concrete.Currently,a large number of research had been conducted on flexural behavior of RC beam strengthened by NSM FRP bars at home and abroad,but the impact of secondary loading has not been considered.In order to provide more understanding for research and application of NSM reinforcement techniques,the flexural behavior of RC beam strengthened by NSM FRP bars considering effect of secondary loading had been studied.Based on the results of bending test of 12 RC beams strengthened with NSM FPR bars,the failure mode and the ultimate capacity,as well as the deflections of the strengthened beams were investigated,by using the parameters such as preload,unload,the amount of FRP and the bond lengths of FRP.The testing results show that the NSM FRP method is a promising technique to enhance the flexural capacity of RC beams.A remarkable increase in stiffness and cracking resistance also is obtained.The tested RC beams show the different failure modes including bending failure,concrete crushing,bond failure and epoxy adhesive fragmentation.The bearing capacity of the strengthened beams decreases because of the existing of the initial load by analyzing the influence factors.With the increase of the initial load,the failure mode of RC beam changes from bending failure to concrete crushing.The average crack spacing and the maximum crack width of the specimen obviously decrease,and the development of the crack is limited by the NSM FRP reinforcement.The average crack spacing and the maximum crack width increase with the increasing of the initial load.The amount of FRP reinforcement has little effect on the crack width of specimen under condition of larger initial load.For the reinforced concrete beam with low reinforcement ratio,the ultimate bearing capacity of the specimens can be effectively improved by increasing the amount of FRP reinforcement.Based on the different initial load level,post-strain calculation formulate is established,considering actual compression zone of concrete stress state.The flexural bearing capacity calculation formulate is established under different failure mode by analyzing the stress of cross sections.The calculated values are in good agreement with the experiment results.According to the computing theory of reinforced concrete beam crack,the average crack spacing and maximum crack width calculation formulates of RC beam strengthened with NSM FRP bars are established by introducing the secondary loading influence coefficient.