Macrocell corrosion mechanisms of prestressing strands in various concretes

This paper investigates the mechanisms of corrosion of seven-wire strands and the reasons for failures in detecting corrosion at early stages. As per ASTM G109 and ACI 222 R-01, a 1-year study on chloride-induced corrosion of seven-wire prestressing strands embedded in various concretes was conducted. The type of corrosion current circuits in the prismatic specimens with one top and two bottom strands was investigated. Unlike solid rebars, the concrete resistivity had a limited role on the type of corrosion circuits in strand systems, with a high probability of corrosion occurring between different wires in the top strand itself. This may be due to the flower-like geometry and the complex corrosion mechanisms within the top strand. It is recommended to perform other electrochemical measurements on the top strand - prior to making conclusions. Also, until about 6% corrosion loss is reached, the rust will fill the interstitial space between the seven wires and will not exert expansive stresses on the surrounding concrete. Hence, no staining/cracking will be visible on the concrete surface - a serious structural safety issue - necessitating frequent chloride/corrosion condition assessment of prestressed-concrete structures.