Interfacial bond strength and failure modes of traditional and modern repair materials for historic fibrous plaster

Many culturally important historic buildings contain fibrous plaster ceilings. The collapse at London’s Apollo Theatre in 2013, which injured 88 people, highlighted the importance of inspecting and restoring ceilings effectively. This study focuses on traditional and modern materials which are applied to the topsides of existing historic fibrous plaster ceiling elements during repair and maintenance. Fibrous plaster ceilings are commonly suspended from primary or secondary structural roof members using fibrous plaster wadding ties or ‘wads’. The application of additional repair material requires the formation of an interface, defining the strength of the repair. Properties of this interface were evaluated through a novel methodology employing pull-off tests’ of approximately 200 specimens consisting of Alpha plaster, Beta plaster, Jesmonite and Aramid gel. Notably, the effect of fibrous reinforcement, and compatibility with historic and degraded material was also investigated. This study has enabled quantification of interfacial properties and evaluated cohesive and adhesive failure modes. Importantly, the extent of redundancy within historic plaster ceiling practice has been demonstrated, with pull-off occurring from 0.5 kN to 2 kN loading, and the ductile behaviour of repair materials evaluated. Results highlight the importance of surface condition, with clean surfaces exhibiting double the tensile loading capacity compared to soiled (dirty) surfaces representative of those encountered on-site. The significance of this study lies in the quantification of repair material performances and consideration of variations in performance, methodology and in-situ environmental factors. Impact stems from the ability of practitioners to make informed decisions relating to adhesion performance when carrying out repairs. A key outcome is more effective preservation of historic elements in heritage buildings, higher levels of safety and serviceability.