| The bond strength between steel bars and concrete will directly affect the safety of concrete members.In China,the bond strength between steel bars and concrete has been studied since the last century.The central pull-out test of steel bar pulling out of concrete under external force is generally used to study the bond strength between steel bar and concrete.The specimens for determining bond strength between steel bar and concrete by central pull-out test were made by the research group.Since the diameter of the steel bar reaches 40 mm,the steel bar reserved for clamping is too long,and the test is difficult.Therefore,this thesis uses the central pull-out specimens with the diameter of steel bar of 40 mm to carry out the press-out test,so as to determine the bond strength between the large-diameter steel bar and concrete.Press-out means that the steel bar at the free end of the specimen is gradually pressed out of the concrete during the test.Through the analysis of the results of press-out test and the comparison with the results of central pull-out test,the basic requirements of method for determining the bond strength and bond stress distribution between large-diameter steel bar and concrete by press-out test is proposed.The main research work and research results are as follows:(1)The press-out test of six specimens of large-diameter reinforced concrete with the diameter of steel bar of 40 mm were carried out.The failure load,the displacement of the free end were measured.The failure mode,load-slip curve and bond strength of large-diameter reinforced concrete specimens were studied.The results of compression test show that the failure modes of specimens are mainly splitting failure and splitting-pressing failure.The bond-slip curve of specimens can be divided into five stages:micro-slip,slip,split,decline and residual.(2)The press-out test of six specimens of pasting strain gauges in steel bar with steel bar diameter of 40 mm were carried out.The strain along the embedded length of the steel bar in the specimens with strain gauges was measured,and the distribution curves of steel bar stress and bond stress were plotted.The distribution of steel bar stress and bond stress in the bond section of steel bars was studied.The stress of large-diameter steel bars in the specimen with strain gauges attached to the steel bars in the press-out test decreases with the increase of the distance from the loading end.When the load of the test is large,the bond stress distribution curve has two peaks,and the two peaks appear near the loading end and the free end,which are located at 0.25la and 0.75la from the loading end(la=5d).(3)The failure mode,bond strength and bond-slip curve of press-out test and central pull-out test were compared and analyzed.The bond strength between large-diameter steel bars and concrete determined by press-out test is lower than that determined by central pull-out test.(4)The details of specimen,loading requirements and measurement content of the press-out test are proposed.The calculation formula of the measured value of bond strength and the calculation formula of the standard value of bond strength between steel bars and concrete when the number of specimens is 6,the quantile is 5%and the confidence level is 0.9 are established.The research on the determination of bond strength between steel bar and concrete by press-out test is still in the start stage.Therefore,the specimen structure,loading device and evaluation method of press-out test need to be further studied. |
