Design and behavior of traditional lime-based plasters and renders. Review and critical appraisal of strengths and weaknesses

Abstract

Traditional lime-based plasters and renders are complex multilayer systems composed of various mortars; they exhibit behavior affected by their own properties, mutual interaction, interaction with the substrate and by external conditions (e.g. environment). A full understanding of the many mechanisms which impact on system performance is essential to improve the success of interventions on ancient renders. This paper reviews the main aspects that influence the mix design and behavior of traditional lime-based multilayer plasters/renders. Special attention is paid to the differences between fine mortars (rendering mortars frequently used in the outer layers) and coarse mortars (rendering mortars used in the inner layers). The diverse pore structure of these mortar types is analyzed and related, first, to the mix design and, second, to the mechanical and hygric behavior. Finally, the interaction of each mortar layer within the multilayer system is analyzed. The functional and technical requirements of plasters/renders as regards their wall-covering role are also presented and considered in the discussion. The analysis enables a critical appraisal of the strengths and weaknesses of traditional lime-based plasters/renders and provides lines of reasoning to enable professionals working in the field to build specific solutions to real problems.