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.
