SERVICE LIFE MODELLING OF STONE AND BRICK MASONRY WALLS SUBJECT TO SALT DECAY E. Garavaglia, B. Lubelli, L. Binda Polytechnic of Milan, Department of Structural Engineering, Piazza Leonardo da Vinci, 32, 20123 Milan Abstract The high randomness of decay level in a natural environment connected with material characteristics suggests to consider the deterioration process as a stochastic process and to analyse it as a reliability problem. Under this point of view, for the system considered, the failure probability function P f (t) has to be defined. A possible approach to define it is proposed by the authors. This approach is able to give information on the deterioration process of the system over time and to predict its service life. The efficiency of the approach has been tested on laboratory samples and on on-site real wall. The results are presented. 1. INTRODUCTION Salt decay is one of the most frequent causes of damage to masonry walls. Water contained in the wall, due to capillary rise and/or rain penetration, is the vehicle through which soluble salts migrate in the material. The evaporation process takes the salts toward the exposed surfaces of the walls; salts crystallising behind the surface cause delamination and crumbling of the masonry components. On the basis of recorded experimental data, a suitable damage parameter describing the material deterioration process has been chosen. This parameter, when measured, is able to quantify the loss of surface material at each measurement. The surface profile is first measured along chosen lines; the loss of material is then calculated as the variation of the profile depth over time. The high randomness connected with material characteristics and decay in a natural environment suggests to treat the deterioration process as a stochastic process and to analyse it as a reliability problem (or durability problem) (Bekker99). Under this point of view, for the system considered, the failure probability function P f (t) can be defined. The proposed model is described in the following section and its application to laboratory test and real walls is presented. The model can be used to predict the service life of the masonry and of the masonry materials. 2. THEORETICAL APPROACH 2.1 The damage parameter ∆A It is well known that the surface damage caused by salt crystallisation and frost defrost action appears as delamination and/or powdering; the damage proceeds from the exterior to the interior of the surface Fig.1 Profile position on laboratory wallettes Fig.2 Profiles position on on-site wall and an example of a deterioration profile 26 28 3133 35 5 111416192023 2 7 Profile 7 45 50 55 60 65 mm 0 100 200 300 400 500 600 Profile Lenght (mm) 1 2 3 4