Corrosion of steel in carbonated (low CO2 emission) concrete

Background

Carbonation of concrete is of concern for the durability of reinforced concrete structures, because the loss of alkalinity may potentially lead to reinforcing steel corrosion. The production of modern cement types (blended cements) generally has lower emissions of greenhouse gases compared to traditional Portland cement. However, concrete produced from these modern cements is known to generally carbonate faster than Portland cement. Thus, in principle, corrosion of the reinforcing steel may start earlier. It is thus important to keep corrosion in check, that is, to limit corrosion rates to levels that do not lead to structural damage within the design life of a structure.
It is well known that the corrosion rate of steel in carbonated concrete is dependent on the concrete moisture state. The higher the moisture content in the concrete, the higher the corrosion rate. Recent research studies have shown, that this is also true for modern binders. In wetting/drying exposure conditions (XC4 according to the European standards), the extent to which the moisture state can increase at the steel surface in the concrete and how this influences the corrosion propagation over time are of interest to ensure the durability and sustainability of modern (low emission) cements.

Aims and objectives

The aim is to understand and quantify the influence of wetting/drying exposure on corrosion of steel in carbonated concrete. This will be studied both by means of experiments as well as numerical modelling. The overall goal is to deliver a model to predict corrosion propagation in carbonated concrete in XC4 exposure.

Methodology

We study the moisture penetration through the carbonated concrete cover exposed to wetting/drying and to characterize the moisture level achieved at the steel surface. Moreover, the corrosion rate and other relevant electrical and electrochemical parameters are experimentally studied.
Mortar samples containing small rebars are carbonated and subsequently exposed to wetting / drying cycles. By measuring the concrete moisture state and the corrosion rate, the influence of the concrete moisture on the corrosion rate can be determined.
A numerical model taking into account moisture transport in cementitious media and corrosion processes is developed and validated against the experiments.