A Probabilistic model to predict the durability of surface treatments

Waterproof or consolidation surface treatments of brick and stone facing walls have usually to be avoided in presence of soluble alkaline salts due to the possible formation of cryptoefflorescence. A research carried out within an EC contract had the aim to establish the maximum salt content, below which the surface treatments does not fail. Crystallisation tests were carried out on treated and non-treated masonry units made with different materials, stones and bricks. Increasing salt percentages were used in order to find the allowable salt content for successful treatments. In order to compare the results a suitable damage parameter describing the material deterioration process has been chosen. The parameter assumed is the loss of surface material at each measurement carried out with a laser device along chosen profiles on the sample surface. The high randomness connected with the material characteristics and decay in a natural environment suggests to assume the deterioration process L() as a stochastic process of the random variable  (where  is the loss of surface material). Following this way, the probability of reaching or exceeding a given damage over time can be described by the corresponding fragility curve. By using this approach the magnitude of the expected damage over time and the occurrence time of it can be predicted.