The grade and quality of mortar has a crucial part in the performance of a brick wall particularly in relation to its durability, weather fastness and resistance to any potential cracking. Mortar consists of over 17% of the wall area and the correct choice is crucial. Under the old Standards, mortar was specified by volume eg. 1:1:6, that is one part of cement, one part of lime and six parts of sand. This traditionally gave a mortar strength of around 4N/mm2 . Mortars now are designated by strength category, eg. M4, which in theory is equivalent to 4N/mm2 . In practice, because that is a minimum strength requirement, pre-mixed mortar suppliers have to take account of strength variations and hence average strengths maybe 6N/mm2 so that there are no results less than 4N/mm2 . The same analogy applies to Edenhall bricks. In order to claim a minimum strength of 22.5N/mm2 the actual brick average strength may run at 28-32N/mm2 . The strength of mortar therefore is particularly important in relation to a risk of cracking within a wall.
As well as constituting 17% of the overall area, mortar differs from bricks in three ways:
• Mortar has a higher mix water content than bricks
• Mortar has a higher cement content than bricks
• Mortar is made with finer aggregate than bricks
Consequently mortar has a higher shrinkage and moisture movement value than Edenhall bricks. The stronger the mortar the greater the risk of cracking.
Lime based mortars are more flexible and accommodate movement better than pure sand:cement mixes. In addition they have the capacity for autogenous healing whereby any micro cracks self seal themselves over a period of time.
Experience from sites has shown therefore that if properties are built with no facility for accommodating movement in the form of joints or bed joint reinforcement in the appropriate locations and strong mortars are used then there is a risk of cracking around the openings