During the construction of a prestressed concrete viaduct, several 12.2 mm diam wires ruptured after tensioning but before the channels were grouted. They were made of heat treated prestressed concrete steel St 145/160. While the wire bundles, each containing over 100 wires, were being drawn into the channels they were repeatedly pulled over the sharp edges of square section guide blocks. The fractures were initiated at these chafe zones. It was concluded that the chafing of the wires on the edges of the guide blocks, particularly the resulting martensite formation, caused the wires to rupture.

Microstructure, corrosion, and fracture morphologies of prestressed steel wires that failed in service on concrete siphons at the Central Arizona Project (CAP) are discussed. The CAP conveys water for municipal, industrial, and agricultural use through a system of canals, tunnels, and siphons from Lake Havasu to just south of Tucson, AZ. Six siphons were made from prestressed concrete pipe units 6.4 m (21 ft) in diam and 7.7 m long, making them the largest circular precast structures ever built. The pipe was manufactured on site and consisted of a 495-mm thick concrete core, wrapped with ASTM A648 steel prestressing wire. All of the CAP failures evaluated were attributed to corrosion. Longitudinal splits reduced the service life of the pipe significantly by facilitating corrosion and introducing sharp cracks into the microstructure of the wire. A few failures were attributed to general corrosion, where the cross section of the wire is reduced until the strength of the wire is exceeded. Most of the failures evaluated were attributed to stress-corrosion cracking.

A tool used to stretch reinforcement wires in prestressed concrete failed. All eight individual jaws were broken. Visual examination of the fracture surfaces indicated that about half of the broken parts had a partially dendritic appearance. Further, fracture surfaces near the exteriors of the parts were clean and smooth, and there was evidence of a case. Examination of the flat surfaces of the parts revealed surface cracking where actual failure had not occurred. Chemical analysis showed the material to be a low-alloy carburizing steel. The microstructure was compatible with a steel which is cast, carburized, quenched, and tempered. The structure was generally satisfactory, except for the presence of severe shrinkage porosity. It was concluded that the presence of shrinkage porosity in critical areas was the primary cause of fracture. Extremely high hardness indicating a lack of adequate tempering was the secondary cause.