Study On Flexural Behavior Of Unbonded Prestressed Recycled Coarse Aggregate Concrete Composite Beams

With the acceleration of urbanization in China,the original buildings have been demolished,resulting in a large number of construction waste,which directly affects the living environment and sustainable development.It is necessary to reuse these construction wastes by scientific means.The concrete components that have lost the usage function are crushed by a crusher,then cleaned and screened manually.According to the design mix proportion,natural coarse aggregate is replaced by a certain proportion and mixed with other cementitious materials to form recycled coarse aggregate concrete.Applying it to the existing engineering structure can effectively reduce environmental pollution,save resources,and play an effective role in promoting the sustainable development of China.However,whether the recycled concrete can be applied to a complete building structure,that is,the natural concrete can be completely replaced by recycled concrete,which needs more in-depth research and engineering practice of the relevant application and demonstration results.After all,the recycled coarse aggregate has the characteristics of complex mechanical properties,large physical performance differences,and a variety of origins.There are still some limitations in the understanding of recycled aggregate performance in engineering.However,it can be realized to apply recycled concrete to the area where the stress of concrete component is small or the non bearing area of building structure.Reinforced concrete composite members not only have the advantages of good seismic performance and integrity of cast-in-place structure,but also have the advantages of assembling structure to save formwork,green environmental protection and construction period,which can well meet the current domestic and foreign policies proposed to improve the quality of traditional construction industry and shorten the construction period,and accelerate the pace of building industrialization promotion and application have played a promoting role.The research on the physical and mechanical properties of recycled concrete composite beams,combining the research results with engineering practice,is the main research direction of researchers in the next period of time.The main work of this paper includes the following aspects:(1)The bending performance tests of an unbonded prestressed ordinary concrete beam,an unbonded prestressed recycled concrete beam,and six unbonded prestressed recycled concrete composite beams were discussed.Factors such as the position of recycled concrete,the height of the superposed layer,and the reinforcement ratio of ordinary rebars affect the bending performance.The main conclusions are as follows:The working performance of the composite beams is good,and the concrete in the mid-section of test beams can basically meet the assumption of flat section along the beam height direction.With the increasing of bearing capacity,the deflection of unbonded prestressed recycled concrete composite beams is similar to that of unbonded prestressed concrete beams.It is a three-stage curve with concrete cracking and ordinary rebars yielding as the dividing point.That is,the elastic phase,the cracked working phase and the failure phase.(2)The simulated models of 8 unbonded prestressed recycled concrete composite beams were established by ABAQUS.The simulation results are in good agreement with the test results.Based on the test,15 simulated beams were designed with concrete strength grade,span-to-height ratio and prestress degree as parameters.The analysis results show that increasing the concrete strength level can effectively improve the ultimate bearing capacity.It is more obvious to increase the ultimate bearing capacity of the simulated beams by increasing the strength of the precast concrete.With the increase of the span height ratio,the ultimate bearing capacity decrease.The number of prestressed tendons has a great influence on the ultimate bearing capacity,and the more the number of prestressed tendons,the greater the increase of the ultimate bearing capacity.Before the ordinary rebars yielding,the load deflection curves of simulated beams under different tensile control stresses basically coincide with the midspan deflection curves.With the increase of tensile control stress,the ultimate bearing capacity of the simulated beams increase after the ordinary rebars yielding.(3)Based on the above research results and the relevant formulas in the "Technical Code for Unbonded Prestressed Concrete Structures"(JGJ 92-2016),a formula for calculating the stress increment of prestressed tendons suitable for unbonded prestressed recycled concrete composite beams is established.The load characteristic values of unbonded prestressed recycled concrete composite beams are calculated by using the relevant formula in the "Code for Design of Concrete Structures"(GB50010-2010).The calculation results is in good agreement with the test datas.The load characteristic values of unbonded prestressed recycled concrete composite beams is applied in the code.This method provides theoretical and experimental basis for calculation.(4)The calculation formula of the average crack spacing in the code is applicable to the unbonded prestressed recycled concrete composite beams,but the influence of the prefabricated layer height is not considered.Therefore,it is suggested that the effective tensile area of concrete be taken as the cross-section area of the precast layer to calculate the average crack spacing.The corrected calculated value is in good agreement with the measured value.The calculated value of the maximum crack width of the test beams is in good agreement with the measured value.The maximum crack width of unbonded prestressed recycled concrete composite beams can be calculated by the method in the code.