BRE Scottish Laboratory

The use of lightweight materials is one way of achieving the thermal insulation requirements of walls and autoclaved aerated concrete (AAC) products have several advantages. However, some cases of cracking of lightweight AAC blockwork have led to a reduced level of confidence in its use. Often the cause of cracking is perceived as a materials problem, such as the use of a mortar that is too strong and will not accommodate movement. Designation (iii) mortars described in BS 5628 are often recommended for AA C blockwork. BRE’s General Purpose Mortar (GPM) falls within this designation, although it is also air-entrained. This paper reports some initial results on the flexural and compressive strength of GPM to assess the potential for cracking in AAC blockwork. A number of designation (iii) mortars were investigated for strengths versus w/c ratios, cement content and curing conditions. The results indicate that mortar without air-entrainment could be too strong for AAC blockwork. Air-entrainment reduces the strength to nearer the minimum strengths of AAC blocks. The results are discussed and other testing is suggested.