Fracture reasons of prestressed finish rolled screw-thread steel bars

Several prestressed finish rolled screw-thread steel bars of a cantilever structure of an expressway fractured after the completion of tensioning and anchoring. The causes of steel bar fracture were analyzed by macroscopic observation, chemical composition analysis, micro analysis and mechanical property test. The results show that the microstructure near the surface of the steel bar was tempered martensite, which made the surface of the steel bar more sensitive to stress corrosion, and the steel bar did not take effective protection measures after installation, resulting in corrosion pits on the surface. The steel bar was not neutral, so that the steel bar was subjected to additional shear stress after tension. Under the action of tensile stress and shear stress, the steel bar was finally broken.