Bridges

Cast iron was extensively used in bridge piers in early metal bridges in Australia. These cast iron bridge piers have performed well against atmospheric corrosion. However, the portion immersed in water suffers from graphitic corrosion, a form of corrosion wherein iron is leached out from the material matrix resulting in a residual, porous, flaky graphite structure. This visually appears to be similar to the cast iron but, in reality, is significantly weaker. Therefore, structural strength assessment becomes necessary to ensure the structural integrity of such bridges. For structural strength assessment, an investigation into graphitised areas is carried out to determine the extent of graphitisation and loss of section. As the graphitised layer typically retains the original shape of the element and covers the surface of the sound cast iron, a visual inspection alone is not sufficient. NDT methods such as ultrasonic testing, in conjunction with core sampling are often used to provide detailed data on the loss of section due to graphitisation. This paper describes an investigation into graphitic corrosion of cast iron piers on a bridge in NSW, including different approaches used in the estimation of mechanical properties of the material for the purpose of structural strength assessment.