Lime render, shrinkage cracks and craftsmanship in building restoration

Abstract The present project concerns early shrinkage cracks in renders. This problem relates to the type of lime and rendering methods used in Sweden during the 19th and early 20th century as well as in current restoration practice. Results from different render restoration projects have indicated that a high frequency of shrinkage cracks could be related to the slaking procedure and the reworking of the slaked lime putty. The aim of the project was to investigate whether there is a connection between different handling procedures for wet-slaked lime putty and early shrinkage cracks in render. An investigation involving practical application and a laboratory test programme was initiated to examine the problem of early shrinkage cracks in renders based on wet-slaked sub-hydraulic lime. This type of lime was commonly used in Sweden in the 19th century. The laboratory work in the project included 3 different mix proportions based on reworked and non-reworked lime putty. The renders were applied after 1, 8, 15, 32 and 68 days storage of the lime putty. This gave 30 mortar mixes applied as rendered test surfaces. The flow number of the fresh mortars was determined and the workability, mixability and open time were assessed. The frequency of shrinkage cracks in the rendered surfaces was measured. The results showed that the frequency of shrinkage cracks was related to the methods applied in the slaking process and the storage time. Reworking and storage of the lime putty increased the frequency of shrinkage cracks. The properties of the fresh mortar were also influenced.