Effect of time-dependent chloride profile and temperature variation on chloride diffusion in concrete

Reinforcement corrosion due to chlorides diffusion is one of the main problems in reinforced concrete structures exposed to certain environments. The diffusion rate is function of chloride surface concentration, concrete temperature, humidity, composition and microstructure. The intruded chlorides are partially chemically bound by the concrete and it is the unbound or free chloride which upon exceeding a defined threshold initiates corrosion. The time to corrosion initiation depends on the above variables; therefore, it is important to model their spatial and temporal variations in a manner that will yield a realistic estimate of the actual initiation period. In this study, the chloride surface concentration and temperature temporal variations are approximated in several ways to gauge the sensitivity of the chloride diffusion kinetics to them. Temperature profiles with constant 6-hours, daily, monthly, seasonally and yearly variations are used to approximate the actual temperature variation recorded for Toronto, Canada in a typical year. The surface chloride concentration is assumed either constant or allowed to vary monthly according to the reported values for Toronto. It is discovered that due to the limited temperature range encountered even in cold regions like Toronto, the diffusion kinetics is not very sensitive to the temperature approximation method, but it is more sensitive to the way the surface chloride variation is approximated. For structures subjected to deicing salt applications, assuming constant seasonal temperature and monthly chloride variation in the analysis may yield a realistic estimate of the time to corrosion initiation and thus the prediction of the life-time of the structure.