Research On Dynamic Response Of Recycled Aggregate Concrete Beam Under Impact Action

Recycled aggregate concrete(RAC),which is made with recycled aggregates and natural aggregates,has been extensively studied and gradually applied to the construction.During their life time,concrete structures might be threatened by impact actions such as vehicle collisions,industrial accidents and terrorist attacks,besides static loads like dead and live loads.Compared with the static loads,which are applied to the structure slowly,the impact actions transmit a large amount of energy to the structure within a short period;hence the response of structural member under high rate loading would be quite different from the one under static loads.Therefore,it is of important significance to predict the dynamic response of RAC beams subjected to impact actions.Concrete is a strain rate dependent material,and the dynamic strength increases with an increase of strain rate.To gain the dynamic compressive strength of RAC,four recycled coarse aggregate(RCA)replacement percentages,i.e.0,30%,70% and 100%,were used in the material static compressive tests and split Hopkinson pressure bar(SHPB)tests.Based on the dynamic compressive strength of RAC with various RCA replacement percentages,the Continuous Surface Cap Model(CSCM)for ordinary concrete was modified to be applicable to RAC.The modified Continuous Surface Cap Model was applied to the finite element program LS-DYNA and validated by the SHPB tests.Based on the background that the beam is subjected to the emergency landing action caused by helicopter,four RAC beams with various RCA replacement percentages,i.e.0,30%,70% and 100%,were designed for drop weight impact tests.The numerical simulation was conducted by LS-DYNA,in which the modified CSCM was considered,to predict the dynamic behavior,energy transfer and damage pattern of the RAC beams on drop weight impact tests.Consequently,the prediction results were then used to guide the test design for such as the support style,data acquisition system and test procedure.Eight RAC beams with the four RCA replacement percentages,each RCA replacement percentage corresponds with two beams,were tested under quasi static loading and drop weight impact.The outcome of test results was presented and the damage patterns on different tests were compared.The effects of various RCA replacement percentages on the bending resistance and stiffness of the RAC beams were analyzed by the bending resistance versus midspan displacement curve from quasi static loading.The impact force,displacement,stress distribution and bending energy dissipation of the RAC beams were researched by the drop weight impact tests.The dynamic bending resistance was obtained by enhancing the static one according to the strain rate effect,and then the bending energy dissipation of the RAC beams could be gained based on the residual midspan displacement.In addition,the inertial effect on the beams under impact actions was researched.The inertial effect on the beams under impact actions was researched.Because of the inertial effect,only a portion of the total impact force is involved in beam deforming itself.Therefore the notion "Impact Action" was proposed to replace the "Impact Load",for the reason that the notion "Impact Load" means the impact force.The parametric analyses were carried out by considering various impact mass and impact velocity,and the consecutive drop weight impact tests were conducted on four RAC beams with the four RCA replacement percentages in addition.Both of the results indicated that the impact energy would not be rational to describe the impact action,and the same to the impact impulse.Hence the group of impact mass and impact velocity may be suitable to describe the impact action.Moreover,the deformation failure criterion was used to estimate the impact capacity of the beam.A two-degree-of-freedom mass-spring model was developed in the paper.The simplified model was validated by the drop weight impact tests on the RAC beams and the impact tests on the ordinary reinforced concrete beams conducted by another relevant research.The yield velocity was analyzed and the yield status of the beam under impact actions was researched by the simplified model.According to the deformation failure criterion and utilizing the simplified model,the displacement-based design method of RAC beams under impact actions was developed.In the end,an example about a beam subjected to the emergency landing actions caused by helicopter was given to explain the design method.