Special conditions coated rebars and prestressing

There are particular problems when the reinforcing steel is not of the conventional type of steel bars embedded directly into the concrete. There are problems for galvanized steel bars with respect to reference electrode and corrosion rate measurement because the zinc affects the readings in poorly understood ways. However, when the bar is coated in epoxy or the reinforcement is in the form of wires in ducts the problems are multiplied as described later. 

Epoxy coated rebars present particular problems to determining the corrosion condition of the Steel. In the first place the bars are electrically isolated from the concrete except at areas of damage. The size and locations of the areas of damage are obviously unknown. Attempts to carry out reference electrode potential surveys and linear polarization measurements have therefore been unable to come up with definitive criteria for corroding and non-corroding areas. The other problem is that the bars are isolated from each other, therefore a connection must be made to each bar measured to be sure that there is electrical contact. 

When the steel is covered in other coatings, such as the zinc of galvanizing, then the potentials are created by the zinc, not the steel. This leads to very negative potentials (about 500 mV with respect to a Ag/AgCl or calomel electrode) when the structure is new. This drifts down to less negative values either as the zinc passivates in the alkaline concrete or as it is consumed and the steel becomes active. It can be impossible to distinguish the two 

Corrosion rate measurements should be representative of galvanized steel but the B value in equation (4.1) for zinc may be very different from that of steel in concrete. The author is not aware that any B values for zinc or galvanized steel in concrete have been published. However, comparative results may be useful in showing areas of high and low corrosion. 

Investigations around the world has shown extensive problems with poor grouting of the ducts and consequent leakage of water, with and without chlorides, onto the steel cables with subsequent severe corrosion. In some cases this has lead to failure of bridges and other structures (e.g. Woodward and Williams, 1988). The problem is that once the steel has been placed 50-100 mm or deeper within a structure, surrounded by a steel or polymer conduit, normal non-destructive test techniques are not effective. 

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