STUDY OF TWISTED REINFORCING MATERIALS AND 'HELICAL EFFECT'

摘要

Reinforcement with helical shape has excellent performance, including twisted bars and twisted fibers. Under pull-out load, the reinforcement tends to untwist while slipping out from the matrix, thus provides a constant or increasing resistance. It is found that the twisted bars results in the second peak value much larger than the first maximum on "pull-slip" curve under a fixed spiral pitch. They maintain a relatively high pull-out load up to very large slips, which corresponds to about 70%～80% of embedded length. The larger the difference of value between the long side and the short side of section is, the higher the second peak will be, and bond anchorage ductility of the bars will be better, which means this composite will absorb larger energy in service. This phenomenon is called "lack-of-fit effect" by R. Gustavson, "repetitive stick-slip mechanism" by Antoine E. Naaman, and "helical-effect" by us. It has a good structural ductility also. In flexural test, obvious distortion appears before component fails, and multiple cracking can be found, which leads to a decrease in crack width. Usually, after cracking, the working stress of bars will rise up to 95% ultimate strength, Though the experimental study of the structure indicates there is special strengthening function in the base body, but its mechanism has been remained unclear for a long time.