Understanding how masonry materials expand and contract due to changes in moisture levels and temperature can help mitigate structural damage.
As temperatures rise and fall, or moisture levels change, building materials continuously adjust in volume. This movement causes stress within the structure and often results in cracks, plaster crazing and potential integrity issues.
Knowing the factors that influence masonry movement, such as the type of material, climatic zone and rainfall, architects and contractors can minimise damage and reduce maintenance costs over the life of the building. The Clay Brick Association (CBA) of South Africa highlights that SANS 10249: Masonry Walling provides important information on movement in masonry, as well as methods of mitigation.
Changing moisture levels
Although masonry materials expand and contract during wet weather, clay bricks rarely exhibit movements in excess of 1mm per 10m of walling. Because concrete is more porous, cement bricks exhibit reversible expansion and contraction in the range of 3mm to 6mm per 10m of walling.
Clay bricks have a once-off, permanent expansion after manufacture due to the firing process that extracts all moisture from the brick. This expansion is between 0 and 0,2%. The bulk of a clay brick’s expansion takes place in the first six months after manufacture and is typically accommodated during construction with vertical movement joints.
Most areas in South Africa can see daily temperature swings of about 20˚C during both winter and summer, and the country often experiences heat extremes. In winter rainfall regions, the combined factors of low outdoor temperatures, rain and frost require special consideration to prevent water condensation and damage due to damp on interior walls of all types.
• Clay brick:
Thermal expansion and contraction is minimal throughout the year. Depending on the clay mixture and firing process, the coefficient of linear thermal movement is 4 to 8.
• Cement brick:
Moisture movement is exacerbated by a daily cycle of thermal expansion and contraction. Depending on the type of aggregate and proportions, the coefficient of linear thermal movement is 7 to 14.
• Cement block:
The movement coefficient of concrete blocks is similar to cement bricks, but because blocks are larger, the magnitude of movement is greater, therefore large blocks show cracks and plaster crazing more than smaller units.
The CBA advises architects to consider factors beyond the walling material. Movement of the adjoining structural steel frames (linear thermal coefficient of 12), aluminium, timber and concrete floor or roof slabs can all cause distress in either supported or infill masonry of every type.
Masonry movement due to shifts in foundations and soil compaction is serious and requires a professional engineer to assess its impact on structural stability.
In terms of clay brick construction, there is a significant body of knowledge available to contractors and architects during specification, design, construction and maintenance. Free technical guides and notes on how to deal with movement in clay brick masonry walls are available on the Clay Brick Association’s website.
The Clay Brick Association of South Africa
Tel: 011 805 4206
Caption: The Gables, a converted brick garage, won the 2015 New London Architecture Award for the best use of materials with its innovative blend of polished concrete, timber screens and whitewashed original brick.