Abstract:By means of a series of pull out tests on the concrete specimens of spirally twisted bars with different cross-sections, plain round bars as well as ribbed bars, the bond anchorage mechanism of spirally twisted bars was studied. It is found that the spirally twisted bars results in the second peak value which is much larger than the first peak value on pull-slip curve under a fixed spiral pitch,and the larger the difference value between the long side and the short side of cross-section is, the higher the second peak will be, and bond anchorage ductility of the bars will be better as well.