Relative importance of corrosion rate and exposure condition on the practical use of new environmentlally friendly binders

Lowering the clinker content of concrete using SCMs can contribute significantly to reduce the energy consumption and the CO2 emissions of building materials. Uncertainty about durability, especially carbonation induced corrosion, is the main factor limiting the practical use: containing less CaO they have less capacity to neutralize CO2 and thus higher carbonation rates, which may lead to premature corrosion of steel reinforcement. Results in literature concerning corrosion of steel in carbonated concrete are rare and refer mostly to ordinary Portland cement. Generally, a trend to higher corrosion rates at higher relative humidity was found. To estimate service life of concrete structures made with new blended cements corrosion rate data are urgently needed, because the so called “corrosion propagation stage” might significantly contribute to the total service life. Corrosion rate has to be measured for different blended cements, w/c ratios and exposure conditions. To collect data of corrosion propagation rates in a reasonable time, a new experimental set up has been designed. The new test setup consists of small (8 x 8 cm) and thin (6 mm) mortar samples instrumented with a reference electrode, 5 steel wire electrodes and a stainless steel grid counter electrode. The thin sample allows full carbonation within 1 week (4% CO2). Parameters that can be measured are electrical resistivity, corrosion potential and corrosion rate, oxygen diffusion. These results should allow to investigate the mechanism, particularly the kinetics, of carbonation induced corrosion. The first results show that new blended cements could be more susceptible to corrosion in certain exposure conditions. Depending on the environment the steel dissolution rate can vary of a factor of 200, from < 0.1 µm/year at 50% RH, to 20 µm/year in wet conditions. To define the application limits of new binders, the interaction with variable exposure conditions has to be carefully evaluated.