THE PRESENTATION OF THE CHLORIDE THRESHOLD LEVEL FOR CORROSION OF STEEL IN CONCRETE

It has been argued that presenting the chloride threshold level as a f ree chloride content or chloride to hydroxyl concentration ratio in th e pore solution of concrete is an improvement over the more commonly u sed total chloride content. In this review the basis for this hypothes is is examined. Contrary to expectations, an analysis of the literatur e suggests that, on balance, bound chloride presents a corrosion risk, an effect which may be due to its contribution to the reservoir of av ailable chloride at the steel concrete interface. Furthermore, the sol uble hydroxyl concentration in the pore solution, which is largely det ermined by the presence of alkali metals, is not an adequate measure o f the inhibitive properties of the cement. Its most important property appears to be its ability to resist a fall in pH to values below that required to sustain a passive film. Thus, in terms of currently used representations, chloride threshold levels are best presented as total chloride contents expressed relative to the weight of cement. This ma y be viewed as the total potential aggressive ion content expressed re lative to the total potential inhibitor content. It may be possible to improve this by, for example, expressing the total chloride content r elative to the alkaline reserves of the concrete, but further work is needed to confirm this hypothesis. (C) 1997 Elsevier Science Ltd.