Experimental Research On Behavior Of RC Column Strengthened With Recycled Concrete Filled Steel Jacket Under Eccentric Compression Loading

The development of China’s economy has led to the development of the construction industry.After entering the 21 st century,a considerable part of the buildings in China have entered the aging stage,and even worse,some buildings have been close to their design working life.In addition,many buildings are also faced with much problems,such as the use of conditions change,the needs to repair due to disaster,cultural relics protection,which indicates an urgent need to develop a new kind of strengthening method with excellent mechanical behavior.In this paper,a new type of recycled aggregate concrete steel tubular column(RACFST)to strengthen the frame columns under eccentric compression is presented.The features of the strengthening method are that the original frame column is surrounded by steel jacket while the gap between them is filled with recycled aggregate concrete(RAC)so that the three parts can bear the external load simultaneously.The advantages of the new strengthening method include: the steel jacket can play the advantages of its confinement effect on core concrete;can reuse a large number of abandoned concrete as the coarse aggregate of recycled concrete,which embodies the concept of sustainable development;structure can be artistic after strengthening.The following innovative works are presented in the paper:(1)A total of 12 specimens which includes 1 reinforced concrete(RC)column without strengthening,3 circular RACFST column under axial compression column and 8 circular RACFST columns under eccentric compression.were tested respectively to study the mechanical properties with different parameters.The main research parameters include the replacement ratio of recycled coarse aggregate,the initial axial compression ratio(initial stress)and eccentricity of the original column.The results show that the bearing capacity,rigidity and ductility of the RC column are greatly improved by using RACFST.The results also prove that the interface between the column concrete and the recycled concrete coheres and can working together.The mechanical properties of RACFST column and natural aggregate concrete filled concrete column(NACFST)are similar and there is no significant difference in bearing capacity and stiffness between different replacement ratio of recycled coarse aggregate.The eccentricity has a great influence on the bearing capacity of the strengthened specimen.With the increase of eccentricity,the bearing capacity of the specimen decreases,but the influence of recycled concrete strength on the bearing capacity of the specimen is not obvious.The difference of the bearing capacity of the specimen is not significant when the initial axial compression ratio is considered.(2)After a verification of the test results with general finite element software ABAQUS,seven parametrics were performed,including the replacement ratio of coarse aggregate of recycled concrete,the initial axial compression ratio(initial stress),the eccentricity,the recycled concrete strength,the steel jacket thickness,the steel jacket yield strength and the load angle.The results show that the finite element model can simulate the stress state and bearing capacity of the specimen rather well.The eccentricity,the thickness and the yield strength of the steel jacket have the greatest influence on the bearing capacity of the strengthened specimen.With the increase of the eccentricity,the bearing capacity of the specimen decrease.The bearing capacity of the specimen is improved rapidly with the increase of the thickness and the yield strength of the steel jacket.The change of the load angle can affect the bearing capacity of the reinforced specimen to a certain extent,but the influence of the initial axial compression ratio,the strength of the recycled concrete and the ratio of the coarse aggregate on the bearing capacity of the specimen are not obvious.(3)According to the test results and the results of finite element analysis,the bearing capacity of the specimen is calculated by using the formulas of different codes of Europe,the United States,Japan,China and other countries.The results show that the EC4 formula is the best and safest formula to predict the bearing capacity of the RACFST columns,while the results predicted by other formulas have higher discreteness or lower precision.The actual engineering can refer to this conclusion,but a more accurate formula to calculate the bearing capacity of the RACFST columns remains to be determined by further experimental study.