Chloride threshold for starting corrosion process – relationship between laboratory and field experiments

Resumen del trabajo presentado en el XIII International Congress on the Chemistry of Cement, celebrado en Madrid (España), del 3 al 8 de julio de 2011Concrete reinforcement is susceptible to corrosion when chlorides from de-icing salts, seawater or marine aerosol penetrate into concrete and exceed a certain level of concentration at the rebar surface. This chloride threshold plays an important role on service life analysis of reinforced concrete structures and thus values close to field exposure conditions contribute to a more realistic analysis. Most of experimental work published on this topic was carried out under laboratory conditions and analysed several variables that influence on this critical chloride content. Studies based on field exposure experiments and in real structures are still scarce. Although they take a longer time than laboratory experiments, there is the expectancy that they can help on setting chloride thresholds closer to realistic conditions of concrete structures. The present work aimed to analyse results from both exposure conditions and set relationships between them. Reinforced concrete specimens cast with distinct cements and a large range of water/cement ratios were exposed in marine atmosphere zone of Northeast Brazilian coast. Electrochemical parameters of the rebar embedded in the specimens were monitored along about eight years until identifying reinforcement depassivation. After corrosion has been initiated, the concrete specimens were broken and the total chloride content at the rebar level was measured and assumed as the chloride threshold for starting corrosion. In addition to this, laboratory tests were carried out with the same materials and under weekly wetting and drying cycles. Electrochemical monitoring and chloride content determinations were done in a similar way to field exposure tests. Results show that average total chloride threshold varies in a range between 1.8 and 2.4 (% cement mass) and between 0.8 and 1.6 (% cement mass) for laboratory and field exposure experiments, respectively. A good correlation could be obtained between both experimental conditions, which helps on transporting laboratory results to field condition