Load capacity and failure analysis of alternative repair materials for fibrous plaster wads

Fibrous plaster has been used since 1856 for decorative ceilings in historic buildings. Ceilings are often suspended by fibrous plaster wads – gypsum plaster, hessian scrim, and sometimes a steel wire – in roof spaces. The 2013 Apollo Theatre ceiling collapse emphasised the importance of understanding the tensile capacities of fibrous plaster wads. This study analysed the capacities and failure mechanisms of wads by conducting tests on laboratory-manufactured ≈45 mm dia. wads using both traditional (‘beta’ plaster, hessian scrim) and modern (‘alpha’ plaster, quadaxial glass fabric, RE Aramid Gel™) materials. Beta wads reinforced with quadaxial fabric increased the loading capacity (≈8 kN with a wire and ≈4 kN without) in comparison to hessian (≈3 and ≈1.5 kN). Applying Aramid Gel increased the capacity of beta-hessian-wire wads to ≈4 kN (≈2 kN without a wire). Alpha is stronger than beta, but alpha wads were less ductile, typically breaking at smaller elongations. Used with beta, quadaxial fabric possesses a sustained load capacity of ≈2 kN as plaster spalls – a capacity not observed with alpha, as fibres rupture in the stiff plaster. This study has a major impact in quantifying the properties of wads, providing material understanding and knowledge for conserving culturally significant buildings.