Initiation of Stress Corrosion Cracking in Cold-Drawn Prestressing Steel in Hardened Cement Mortar Exposed to Chlorides

Abstract

Cold-drawn, high-strength, prestressing (PS) steel strands are widely used in pretensioned concrete (PTC) structures. This paper discusses the stress corrosion cracking (SCC) of PS steel embedded in cement mortar and gradually exposed to chlorides. Various stages of the passive to active (P-to-A) transition, which marks the onset of SCC, were investigated using the electrochemical impedance spectroscopy technique. The key mechanisms were identified and confirmed using scanning electron microscopy/energy dispersive x-ray analysis, x-ray diffarction, and confocal Raman spectroscopy. It was found that the passive film on unstressed PS steel has better electrochemical characteristics than that on conventional steel rebars. However, the residual tensile stress at the surface of PS steels can assist passive film cracking after chloride attack—contrary to the pitting corrosion without cracking of passive film in conventional steels. Further, tests indicated that the concentration of chlorides required to crack the passive film in PS steels can reduce by about 50% when prestressed—as in field structures. Chemical composition, stress state, and microstructural features at the PS steel surface were identified as possible factors influencing the initiation of SCC in PTC structures.