Substitution ventilated render systems for historic masonry: Salt crystallization tests evaluation

The treatment of rising damp in historic building walls is very complex due to the thickness and heterogeneity of the walls. Some techniques, such as watertight barriers and injection of hydrofuge products, that have been used until now to deal with rising damp, are sometimes ineffective, justifying the need for a new approach. When there is the presence of rising damp with soluble salts the solution to the problem may involve a higher level of complexity. Salt damage is one of the major causes of render decay, not only near the sea but also in continental areas. Salts can appear in the walls from different sources: from the ground due to rising damp, carried by the wind or fog as salt spray, by flooding, or they can originally be present in the building materials themselves as is the case of unwashed beach sand. Salts crystallization depends on several factors including salt transport behavior of the substrate/render and the surrounding ambient environmental severity (temperature and relative humidity). An experimental program was developed using six different small scale specimens with traditional bricks as substrate, rendered on both sides with several optimized render systems. These specimens were submitted to several cycles of dissolution/crystallization with a NaCl solution due to the common presence of this salt in historic buildings. Particular attention is given to the influence of different renders when contaminated with NaCl and to the location of sodium chloride crystallization in the test specimens. The final goal of this study is the development of a replacement render system called “ventilated render” (“emboço ventilado” in Portuguese) for historic constructions with renders damaged due to the presence of high moisture content and soluble salts. The ventilated renders have two layers (base and external layer) which are supposed to act as an accumulating system in which the salt is induced to crystallize in the base layer (executed with vertical grooves) of the render and not in the masonry nor in the external layer.