STEPHEN L. GARVIN, MARK C. PHILLIPSON AND IAN H. MURRAY Building Research Establishment, Scottish Laboratory

A programme to assess the suitability of the.BRE general use mortar (GUM) for use in aircrete blockwork is described. GUM is a designation (iii) mortar (cement:lime:sand (1:1:5.5) plus air-entraining agent) which has both frost and sulfate resisting properties. Although satisfactory for use in brickwork there has been concern about it being too strong for use with some lightweight blocks. The results of large panel tests to assess movement, cracking potential and moisture contents are reported. Panels of lightweight aircrete blockwork, consisting of restrained internal and external leaves, were monitored over a 2 year period of natural exposure. Each panel was constructed with a different mortar so that comparison of performance could be made between the GUM and both stronger (1:3) mortars and weaker (1:2:9) mortars. Movement in the external leaves was found to be dependent on climatic conditions, driven by temperature and moisture content changes. The external leaves cracked either during the first summer period (1994) or the second, hotter, summer period (1995). This demonstrated the greater effect of thermal movement of the blockwork in the external leaves compared with the internal leaves, which showed no cracking on any of the panels.