Predicting masonry brick-veneer and cavity brick wall-tie corrosion

Corrosion of metal wall-ties over extended periods of exposure is a well know problem, however efforts over many years have continued to rely on short-term laboratory tests. Available atmospheric corrosion models for longer term exposure could be used to predict loss of structural capacity of wall-ties however these models consider deterioration for the part(s) of the ties exposed directly to the atmosphere. Yet, observations have shown that pitting corrosion in the portion of the tie embedded in the mortar can be more severe than corrosion elsewhere on the wall-ties and rapidly reduce structural reliability of the brick construction. Moreover, the existing atmospheric models are sensitive to parameters changes and bound by theoretical assumptions which in turn can result in increased prediction uncertainty. To this date, there is limited information on the forms of corrosion or development of pitting on the surface of metallic wall-ties. This thesis reports investigations on the long-term mass loss and pitting corrosion of Z600 and Z950 galvanized and SS304 and SS316 stainless ties embedded in three different mortar mixes exposed in two different environments: coastline and artificial salt-spray chamber. Test couplets were recovered periodically after exposure periods of 6 months, 12 months, 18 months and 24 months exposed in coastline exposure environment and after 1 month, 3 months, 4 months, 6 months in the salt-spray chamber. The corrosion properties, such as mass loss and pit depth for each material in each condition were analysed carefully and reported for each exposure period. The results varied significantly in different mortar mixes in the coastline environment. It was discovered that pitting initiated on the metal surface even in the mortar with a pH > 12 and the deepest pits in the steel wall-ties were found for those ties embedded in mortar with highest cement content. Also, in the lime and sand mortar with no cement content there were almost no pits even after 2-year exposure. Factors influencing the pitting corrosion and mass loss and its implications are discussed.