Compatibility of rendering systems with salt loaded ordinary masonry walls

Abstract

Salt damage results mainly from the crystallization of soluble salts inside the pores of construction materials. It may cause numerous problems to the built heritage, namely, the degradation of the habitability conditions, the lost of historic material and the high costs associated to the (usually recurrent) repairs. A progressive decrease of the structural resistance of ancient load-bearing walls is another negative, but unfortunately common, consequence. Most of the Portuguese ancient buildings are composed, similarly to what happens in many southern Europe countries, of thick load-bearing ordinary masonry walls, which are made of irregular stones of small size and a week lime-bedding-mortar. Soluble salts are usually very active in this kind of walls, which were built directly over the terrain, are composed of very porous and hydrophilic materials and have often been subjected to periods of constructive deterioration leading to an internal accumulation of moisture and salts. Ordinary masonry walls are also particularly sensitive to salt damage. Sanding of the bedding-mortar and further disaggregation of the masonry easily occur if proper protection is not provided by adequately compatible plasters and renders. Many critical situations occur due to an excessively quick degradation of traditional mortar coverings or to the use of inadequate plasters and renders (many of them, factory-made) which, as a means of increasing their own performance, “push” the degradation front to the masonry. The present research aimed at evaluating the compatibility of eight different rendering systems with salt loaded ordinary masonry walls. Test panels of five traditional mortars and of three industrial systems (specific for salt loaded walls) were made at S. Sebastião Chapel, in Almada, Portugal. The systems execution techniques and overall performance along three years were evaluated on the basis of the observation of these panels. The masonry was characterized in terms of its hydric behaviour and salt / moisture load by means of Karsten pipe measurements (done on the bedding-lime-mortar and on the stone elements), ion chromatography analysis and profiles expressing the variation (in height and in depth) of the walls moisture and hygroscopic moisture content. The results indicated, namely, that the bedding-mortar controls the transport of moisture through the masonry and, therefore, its characteristics should be used, as reference, for the choice of the rendering system. The rendering systems hydric behaviour was evaluated by in-site measurements with the Karsten pipe, as well as by laboratory capillary absorption tests. The systems performance in what concerns the salts action and preferential depth of deposition was evaluated through the determination of the hygroscopic absorption of moisture of samples collected from the tests panels, at different depths. The compatibility of each one of the rendering systems with the masonry is discussed on the basis of all the results obtained.