Prescriptive Specification/Performance-Based Formulation
For a proper evaluation of the adequacy to use of applied renders, a performance – based approach must be followed comprising three main steps:
• Definition of the functions to be performed by the render (functional requirements)
• Identification of the most relevant performance characteristics (i. e. performance parameters quantifiable whenever possible and capable of being monitored in service)
• Definition of the methods of quantification and evaluation of the previous parameters, resorting to in situ or laboratory tests
In fact, the application of this approach is easier in non-traditional renders because there is backing from harmonized European standards (requirement standards) or homologation/application documents produced by LNEC (additional requirements) or other national entities. Concerning traditional renders, their specification still puts emphasis on a prescriptive formulation, through the recommendation of volumetric or weight proportions.
According to EN 13914-1:2005, the choice of the render type, its composition, the number of coats and their thickness depends on the intended aspect, the exposure conditions, the nature of the substrate and the requirements. To specify the composition (proportions), it must be stated whether they are determined by weight or by volume and whether dry or damp sand is used. This European standard does not define proportions, due to the technological and climatic differences between the various countries. However, it typifies mortar mixes (used in exterior renders), as a function of the substrate:
• Strong mixes—they yield strong, impermeable, high shrinkage mortars, consequently highly susceptible to cracking; their use should be restricted to stronger substrates.
• Moderately strong mixes—they probably lead to satisfactory results in most renders; they are more permeable and have less drying shrinkage than the former ones; their strength is adequate in most situations; the mixes with lower cement content are less prone to cracking, but may not be strong enough to withstand severe abrasion.
• Weak mixes—adequate for weak substrates under protected conditions and for repair work of less compact lime-based renders.
Table 2.3 presents a synthesis of the volumetric proportions of the base and finishing coats of renders applied on brick masonry substrates , taking into account the type of finish of the rendered surface and the exposure conditions.
It is stressed that prescriptive specification, even though providing relevant information (about the various coats, exposure conditions, types of substrate and types of finish), is clearly insufficient for periodic monitoring of the in-service performance . Therefore, technical documentation has gradually changed from prescriptive to performance based, in which the functional requirements are translated into quantifiable performance characteristics.