Experimental Study On Mechanical Properties Of Recycled Concrete And Bond-slip Between Steel Rebars And Concrete After Freeze-thaw Cycles

For the requirement of the sustainable development and construction of civil engineering,as a green building material,recycled concrete is feasible to civil engineering,particularly to structural engineering,and becomes an inevitable trend.Frost resistance is one of the important indexes to evaluate the durability of concrete structures,especially in cold areas.After freeze-thaw cycle,the mechanical properties of concrete degrade,which leads to the bonding property of steel bar and reinforced concrete destroy.In order to promote the application of recycled concrete in engineering,it is of great significance to study the mechanical properties of recycled concrete and its bond with steel bars after freeze-thaw cycles.Therefore,from the respective of durability,basic mechanical properties after freeze-thaw cycles,bond-slip behaviors between recycled concrete and steel bars of recycled concrete are tested in this paper.Constitutive relation and bond-slip mechanism is also discussed on different types of experiments.In this paper,the recycled concrete specimens with 100% recycled coarse aggregate and with the same strength of ordinary concrete were chosen as contrast.The variations of recycled concrete quality,its relative dynamic modulus of elasticity and compressive strength after freeze-thaw cycles were studied by changing times of freeze-thaw cycles and strength grades of concrete,in order to obtain necessary experimental data for bond-slip behavior between recycled concrete and steel bars.Targeting at the situation of almost not considering bond performance after freeze-thaw cycles study on recycled concrete,bond-slip constitutive relation of recycled concrete was suggested considering center pull-out test and beam test.Based on the contrastive test results,this paper emphasized on the effect factors of bond-slip between steel bars and recycled concrete.The main research contents and achievement were summarized as follows:Based on the rapid frozen-thaw method,the variations of recycled concrete with 100% recycled coarse aggregate quality and with the same strength of ordinary concrete quality,their relative dynamic modulus of elasticity and compressive strength after freeze-thaw cycles were discussed.Then the basic mechanical properties of recycled concrete after freeze-thaw cycles were developed.Electron microscope was used to conduct microanalysis on recycled concrete.Through the micro-structure and macro-performance analysis,the relationship between macro-performance and micro-structure of recycled concrete was established.Firstly,7 groups of recycled concrete center pull-out specimens which anchorage length are 3d,5d,8d,10 d and 1 group of ordinary concrete center pull-out specimens with 5d anchorage length(d means diameter)were tested.The effect of freeze-thaw cycles and anchorage length on bond-slip behavior of recycled concrete was analyzed by changing times of freeze-thaw cycles,then bond-slip constitutive relation was established.The results indicate that there were two kinds of failure modes,rebar yield failure and center pullout failure in center pullout test.When the anchorage length was 10 d,the rebar yield failure appeared,and there was smaller relative sliding on the loading end,at the same time the free end was no.There was pull-out failure in the rest of the specimen in loading process when the steel bars were pulled out slowly.In the status of freeze-thaw cycles,with the increase of anchorage length and loading,bond strength decreased,and the bond strength was inversely proportional to the load value.When the anchorage length increased to a certain value(5d),the change curve of bond strength tended to be gentle.The curve can be divided into five stages,the micro slip linear phase,the slip stage,the slip acceleration stage,the nonlinear descent stage and the residual stage based on the characteristics of bond slip curve between steel and recycled concrete.An empirical model of characteristic value of bond and anchorage was developed,by which times of freeze-thaw cycles,anchorage length and the thickness of protective layer should be considered in the model.By means of mathematical statistics and error analysis,it was proved that the model has a good agreement.However,there were no shearing force and bending moment in the center pull-out specimens,and the stress state of structural members was not consistent with the actual engineering.Results of center pull-out tests and laws were different from the actual engineering.So beam test was carried out.38 groups of beam were prepared in the experiment with anchorage lengths ranging from 5d,8d,10d(d=16mm,d=18mm and d=20mm)and strength grades of concrete being C20,C30,C40.The effect of above factors on bond-slip behavior of recycled concrete was analyzed by changing times of freeze-thaw cycles,then bond-slip constitutive relation was established.The results indicated that bond-slip behavior of recycled concrete was greatly affected by the diameter of steel bar,anchorage length,the strength of recycled concrete,and the times of freeze-thaw cycles.The position function established by the distribution of the bonding stress along the anchor length and constitutive relation was better to reflect the bond stress at different anchorage locations and the change law of the bond slip relation of the measuring points during the test.Slip constitutive model based on the position function agreed well with the measured value.Compared the position function and load slip curve of two different specimens,it can be shown that the force condition and thickness of the protection layer were the main factors influencing the accuracy and curve characteristics of the position function.Compared with the center pull-out tests and test results of a beam test,in terms of ultimate bond strength,center pull-out test was better than beam test because of the protective layer with large thickness and uniform distribution.With the increase of freeze-thaw cycles,the ultimate bond strength of the beam specimen was uniform,and the rate of falling out of the specimen was faster.In the terms of slip value,eventually slip value of center pull-out specimen reached the 6 ~ 10 times than that of beam specimen.Based on the constitutive relation,bond properties between recycled concrete and steel bar were analyzed by the finite element software named ANSYS,and the results were compared with the experimental results.So the bond properties of recycled concrete were clarified on condition that section area of reinforcement and times of freeze-thaw cycles change in order to provide theoretical foundation for further bond-slip behavior between recycled concrete and steel bar after freeze-thaw cycles recycled research.