Conservation of historic renders and plasters: from lab to site

Abstract eng:
In an intervention on historic renders and plasters, the first step is to decide upon the strategy: repair or substitution, based on an evaluation of the cultural value of the render and of the building itself and on a careful diagnosis of the typology of defects, their density and reparability. New renders or repaired renders should fulfil the main functions they are required to, especially protection and aesthetic functions. In any case, compatible materials should always be used. Compatibility is needed for durability, not of the render, but of the wall as a whole, and also for preserving the testimonial and symbolic value of the building as well as its appearance. Compatibility is defined in relation with the substrate and the pre-existent mortars, therefore tests are to be carried out on the old materials and on possible solutions, to compare their characteristics and assist the selection of the best one. It is adequate to begin by non-destructive or slightly destructive in situ tests, because with them it is possible to collect useful information without destruction of the historic renders and in a quick way. Simple mechanical and physical tests can be carried out on the old mortars and a few chemical tests can also be performed nowadays, with the appropriate portable equipment. When more rigorous and complete tests are needed, some samples can be collected and tested in laboratory, using methods adapted to non-regular, possibly friable specimens. The characteristics of the mortars to use can be established with a basis on the set of results obtained, in order to fulfil both functionality and compatibility. However, sometimes it is not possible to get enough data concerning the old materials, especially concerning masonry, which is rather more difficult to test than mortars. For that situation, some general requirements have been established, based on previous work carried out on Portuguese masonry historic buildings, which can be used without risk of damaging pre-existent materials. The decision concerning materials to use, especially binder materials, should also take into account the climatic and environmental conditions. Then, appropriate application technique, workmanship and curing conditions are indispensable to achieve good results, from the aesthetic, physical and mechanical points of view; therefore it is important to know which conditions are available in the application phase.

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