ABSTRACT
Visual inspection and field testing were performed to identify the possible sources of reinforcing steel corrosion in a Transit System Tunnel. The testing included electrical continuity measurements, half cell potential measurements, stray current testing and resistivity and chemical analysis of the soil.
The results of the investigation indicated that corrosion activity was occurring on the reinforcing steel mat at the interior face of the tunnel wall. There was also evidence that corrosion was occurring in the concrete pads to which the return rails are fastened. Chloride levels, measured from concrete cores removed from the walls of the tunnel, indicate that threshold levels for corrosion of reinforcing steel in concrete have been exceeded. It is believed that the corrosion activity in the walls is due to a combination of sources including, stray current flowing along the reinforcing steel and returning to the rails, high chloride concentrations in the concrete at the interior wall surface, significant variations in half cell potentials along the wall surface, the availability of oxygen and the lack of sufficient concrete cover. Further, it is likely that similar corrosion activity is taking place at the rail fasteners.
The recommendations for remedial measures included, electrical isolation of the track fasteners from the floor slab and the investigation of the installation of cathodic protection on the internal wall surfaces to provide protection to the reinforcing steel.
