Research On Dynamic Behavior Of Reinforced Concrete Beam And Frame Retrofitted With CFRP Under Impact Loading

Concrete structures can be subjected to impact loads induced by a variety of sources(e.g.vehicle,barge,rock fall,debris,and terrorist activities).Impact loading is characterized by its large amplitude within a short duration,causing strain-rate ef-fects in the materials(e.g.,concrete and steel)and inertia effects in the structures.These effects can lead to significantly different behavior of concrete structures under impact loading compared with that under static loading.Concrete structural members subjected to impact loading are prone to suffer diagonal shear cracks and unrecoverable deformation,even shear failure.In case of the occurrence of shear failure of load-bearing members,their adjacent structures are vulnerable to be damaged,and if the damage propagates throughout the structural system,the whole structure is likely to collapse with disastrous consequences.Accordingly,more efforts need to be made to enhance the impact resistance of concrete structures,especially for the case of existing concrete structures designed previously without consideration of this type of extreme impact load.Due to the outstanding characteristics of high strength-to-weight ratio,corrosion resistance,and ease of installation,the use of fiber reinforced polymer(FRP)in retrofitting a concrete structure has become increasingly popular in the past three decades.A significant amount of research demonstrates the effectiveness of FRP ret-rofits for increasing the strength,stiffness,and ductility of concrete structural members under static loading conditions.However,the research on dynamic behavior of con-crete structures retrofitted with FRP under impact loading is so far insufficient and in a growing need to expand this application of FRP.In this dissertation,a detailed literature review is first presented,including the study about the dynamic behavior of reinforced concrete(RC)beams,columns,and slabs retrofitted with FRP under impact loading,and the study about the dynamic be-havior of RC frames retrofitted with FRP.The review shows that at the structural mem-ber level,the research on impact behavior of FRP-retrofitted concrete members is still in its initial stage at present,since many key issues have not yet been clearly under-stood,such as damage mechanism,dynamic response,dynamic distribution of internal forces,FRP contribution to impact resistance,and retrofit design method;at the struc-tural system level,the research on the collapse resistance of concrete frame retrofitted with FRP is quite rare.At the structural member level,drop-weight impact tests and finite element(FE)simulation are employed to study the impact behavior of carbon fiber reinforced poly-mer(CFRP)shear-retrofitted RC beams with and without stirrups.The impact tests are conducted by using a well-instrumented drop-weight equipment with the main experi-mental variables including the impact mass,velocity,and energy,FRP width,spacing,and amount,shear span/depth ratio.Only the most basic retrofit method using CFRP-wrapping strips was adopted for shear enhancement of the RC beam specimens.The primary objective is to investigate the dynamic loading effects on the behavior of RC beams retrofitted with CFRP wrapping,in terms of crack patterns and failure modes,as well as dynamic time histories.The test results show that the CFRP retrofit can enhance the impact resistance of RC beams,significantly reduce the deflection,and limit the damage.The dynamic forces including impact force,inertia force,and reac-tion force can well satisfy the dynamic force equilibrium.Based on the force equilib-rium,the distribution of internal forces,such as shear force and bending moment,along the RC beam under impact loading are discussed and analyzed.Some retrofit design suggestions are proposed to apply FRP-wrapping strips for improving the shear re-sistance of RC beams under impact loading.A nonlinear explicit FE analysis is performed by using software program LS-DYNA.To simulate the same test condition,a three-dimensional(3D)FE model of CFRP-retrofitted RC beams is established with some simplifications.The FE method is reasonably validated by the test data obtained from some references and current drop-weight tests in this dissertation.The numerical results match well with the exper-imental results in terms of cracking pattern,impact force,reaction force,and midspan deflection.Based on the reliable FE model,a detailed parametric analysis is carried out to investigate their effects on the impact behavior of FRP-retrofitted RC beams,such as FRP tensile strength,elastic modulus,thickness,and spacing,impact velocity and energy.At the structural system level,a half-scale,three-story,two-bay and three-span reinforced concrete model frame retrofitted with CFRP strip cables is tested under sud-den removal of two side middle columns in the long direction on the ground floor to study dynamic responses,resisting mechanism and CFRP retrofit effect.The CFRP strip cables are anchored to concrete slabs by specially designed anchorage.The testing columns are removed by impact of a projectile fired by a compressed hydrogen gas gun.The experimental results are compared with previous tests of the same frame without CFRP retrofit to validate the effectiveness of CFRP strip cables for preventing the total collapse of the frame after the loss of columns.A simplified method is proposed to calculate the average strain on CFRP strip cables and a combining resisting mechanism is established to compute the ultimate capacity provided by slabs,beams and CFRP strip cables.