Prediction of steel corrosion in porous building materials by means of a new hygrothermal model

Decay of ancient buildings is often due to corrosion of metallic reinforcements, which are embedded in the construction in order to improve its structural behaviour or to prevent the propagation of cracks. If wet conditions occur on the surface of steel, corrosion can start. Corrosion may weaken reinforcements and produce expanding oxides that lead to the detachment of covering materials, thus being harmful for the durability of the construction and challenging for its restoration. When dealing with heritage buildings, however, the problem becomes even more complex due to the need of non-invasive and reversible techniques. Here the only possibility is often to limit the corrosion rate by controlling moisture and temperature conditions in the assembly or the indoor climate. To this end, a method has been developed which focuses on the corrosion behaviour of steel reinforcements in constructions, considering the complex transient processes of corrosion chemistry and the local hygrothermal conditions. In particular, the model is based on the correlation between the transient hygrothermal conditions (i.e. temperature, relative humidity and water content) and the corrosion rate of steel reinforcements embedded in porous building materials i.e. mortars and bricks. It allows the prediction of the temperature and humidity dependent corrosion rate of the steel over time. The model has been validated through several laboratory tests, showing a good agreement between measured and simulated results. It assesses the corrosion risk depending on parameters which are easy to monitor and allows both preventive conservation/restoration of cultural heritage buildings and safe design of new durable building components. Concerning the former point, the model permits state-of-preservation assessments when material sampling is not possible and enables to define measures to reduce/stop corrosion of steel reinforcements.