Mechanical behaviour of lime based mortars after surface consolidation

Plasters have been used since ancient times in architectural applications to cover the masonry or to support paintings and decorations. Actually, plasters are subjected to several degradation phenomena which can lead to the loss of material cohesion mostly caused by the binder degradation. To recover the internal cohesion of the material is a very important topic in the conservation of historical architecture because alteration phenomena often produce the detachment of large areas of plasters (lacuna). Surface consolidation, directed to restore cohesion and stability, is based on the use of materials with aggregating properties. How to choose the best treatment with specific requirements is still a controversial issue. Actually, the lack of suitable evaluation protocols makes rather difficult the choice among the available consolidants, in spite of the research improvements which are patenting and presenting to the market new consolidation products. The testing protocols generally neglect the evaluation of mechanical properties (cohesion, adhesion and indentation) which are the base of the consolidating action of a material penetrating in the porous micro-structure of the substrate. In the case of plasters, the focus should be directed to the mechanical resistance of a composite thin material subjected to flexural strain. This study is aimed at determining the suitability of the four-point bending test, a flexural strength test, to evaluate the efficacy of consolidation treatments for plasters, used in the practice of conservation and restoration of ancient buildings.

Research highlights
► Consolidation of artistic and historical plasters is a challenging issue.
► Three consolidants, Paraloid B72, TEOS, Ba(OH)2, have been selected and tested.
► Four-point bend test has been set-up for the evaluation of commercial products.
► The test proved to be satisfactory in assessing mechanical behaviour of mortars.