Variation in the chloride threshold level for steel corrosion in concrete arising from different chloride sources

Abstract

In order to compare the role of chlorides from internal and external sources in corroding, the steel bar in concrete/mortar, the chloride threshold level (CTL), corrosion resistance, chloride transport, observation of the steel– concrete/mortar interface and the pH-dependent release of bound chlorides were examined. As binders, ordinary Portland cement (OPC), 30% pulverised fuel ash (pfa), 60% ground granulated blastfurnace slag (ggbs) and 10% silica fume (SF) were used. For external chlorides, the onset of corrosion was detected by a jump of the galvanic current and for internal chloride the corrosion rate was measured using a polarisation technique to determine the CTL. As a result, the variation in the CTL was marginal, despite the different chloride source, but binder type greatly affected the corrosion behaviour and the CTL: OPC concretes/mortars provided the highest resistance to chloride-induced corrosion and the highest CTL, and the polarisation resistance was also the greatest before depassivation, compared to the mixtures with pozzolanic materials. For the rate of chloride transport, 60% ggbs concretes produced the lowest diffusivity, while OPC produced the highest. The visual examination of the steel–concrete interface showed that corrosion always starts in the voids in contact with the steel bar, because of a higher reactive environment. In addition, it was found that acid soluble chlorides indicate only about 90% of total.