Compatibility issues regarding mortars for repairs and additions in interventions on historic masonry

Abstract

The functions of the materials used for interventions on historic fabric are multiple, and go beyond the plain repairing or completion of damaged parts or in need of strengthening. This is a significant reason (among others) due to which the recourse to original materials, traditional, or with otherwise akin properties, is not always enough. In regard to the composition of mortars, reproducing the original recipes is often difficult. However, the existence of contemporary mortars based on hydraulic lime or trass, the composition and physico-mechanical properties of which are sufficiently known, reliable, and compliant with contemporary standards, allow for compatible interventions with good results in regard to rainwater protection. An effective mortar recipe depends on a clear understanding of its role in masonry, in respect of each of its functions. The technical requirements that a mortar must meet in order to be compatible with the historic fabric are mechanical strength, water absorption (absorption of water while in the state of immersion, capillary absorption and vapour permeability - each with different implications in regard to the physical behaviour of the masonry), adhesion the substrate, contraction, vibration resistance and freeze-thaw cycles. The paper presents some guidelines regarding these compatibility issues and the decision factors that contribute to the design of contemporary compatible recipes of mortars for repairs and additions in interventions on historic masonry.