Research On Flexural Behavior On The Damaged Reinforced Concrete Beam Strengthened By Steel Plate And Ultra-High Performance Concrete (UHPC) Composite Technique

Steel plate-concrete composite strengthening technology is a new strengthening method derived from steel-concrete composite structure which is able to surmount the intrinsic defects of the traditional strengthening technology.What’s more,it also possesses the superiority of high stiffness,good crack resistance,and small increase in self-weight,which results in an expansive application prospects in bridge strengthening engineering.However,due to the drawbacks of normal concrete such as low tensile strength and weak bond performance,steel plate-concrete composite strengthening technology is still limited in engineering application.Based on the excellent mechanical properties and durability of ultra-high performance concrete(UHPC),this paper proposes a new strengthening method(steel plate-UHPC composite strengthening method)to solve the problems afforementioned.In order to study the flexural performance of damaged RC beams strengthened by steel plate-UHPC,five steel plate-UHPC strengthened beams(SUSB)and one reinforced UHPC strengthened beam(RUSB)were tested.The effects of different strengthening methods,thickness of steel plate and UHPC on the flexural performance of strengthened beams were investigated.The theoretical calculation method and finite element analysis model were developed.The main research work and conclusions are as follows:(1)Seven RC rectangular beams were designed and fabricated,and six of them were strengthened at the bottom after pre-damage loading.In this paper,the flexural performance of RUSB and SUSB beams strengthened with different parameters were studied by four-point bending tests.The test results show that the strengthened beams are all interface stripping damage except for the bending damage for the contrast beam(RCB).In terms of the strengthening effect,the crack resistance,stiffness and ultimate bearing capacity of SUSB and RUSB strengthened beams are increased by 241.4%/43.7%,98.1%/99.6%,61.3%/53.4% in average comparing with RCB,respectively;as for different strengthening methods,the crack resistance and slip resistance of SUSB are increased by 137.6% and 109.2% in average comparing with the RUSB.As for SUSB with different parameters,the flexural stiffness of beams can be improved to some extent by increasing the thickness of steel plate and UHPC,and the internal rebar stress of RC beams can be improved effectively too.However,there is no significant difference in the bearing capacity of all strengthened beams as a result of the interface stripping failure;(2)Under the comprehensive consideration of the post-cracking strain hardening behavior of UHPC and RC residual strain,the calculation formulas for the UHPC cracking load and flexural bearing capacity of the SUSB beams were proposed.Considering the possibility of debonding of interface,the interface stripping criterion and the theoretical formula of interface stripping bearing capacity were proposed,and the theoretical results of UHPC cracking load and the interface stripping agree with the experimental results well.In addition,in order to avoid the interface stripping failure,a design method of interfacial reinforcement was proposed and an optimized scheme of interfacial reinforcement design was proposed according to this specimen;(3)The numerical analysis of the RCB and SUSB beams were carried out by the finite element software ABAQUS,and the numerical results agreed with the test results well,which verifies the accuracy of the finite element model.Meanwhile,on the premise of stable interfacial bonding,the effects of UHPC and steel plate thickness,and RC beam depth on the flexural performance of SUSB beams were further studied.The thickness of UHPC and steel plate has utmost significant improvement the UHPC cracking load and bearing capacity of strengthened respectively.The size effect of the test beam has a significant impact on the stiffness and crack resistance of the strengthened beam,which shows higher depth of the RC beam resulting in worse strengthening effect.Meanwhile,the bearing capacity of numerical and theoretical results with different parameters are all in good agreement,which verifies the accuracy of the theoretical calculation formulas of bearing capacity.