Technical Note #20 Stopping cracks & crazing in masonry walling
All building materials adjust their volume continuously in response to changes in temperature or moisture in the environment. This movement causes stress within the structure and often results in unsightly cracks and plaster crazing, as well as potential integrity issues.
The amount of movement depends on the material, the climatic zone and rainfall. By knowing the factors that influence masonry movement, architects and contractors can minimise damage and reduce maintenance costs over the life of the building. SANS 10 249: Masonry Walling provides important information on movement in masonry, as well as methods of mitigation.
Movement of Masonry due to Moisture
Masonry materials expand and contract due to changes in moisture levels during wet weather (reversible moisture movement). Clay bricks rarely exhibit movements in excess of 1mm per 10 metres of walling.
Because concrete is more porous, cement bricks exhibit reversible expansion and contraction in the range of 3-6 mm per 10 m 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.
Movement of Masonry due to Temperature
All building materials also undergo daily thermal expansion and contraction and these daily temperature swings can result in stress cracks over time.
South Africa experiences well-defined daily temperature swings of about 20˚C during both winter and summer. We have extremes of heat, compared with the extremes of cold experienced in the northern hemisphere.
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.
Thermal expansion and contraction is minimal throughout the year. Depending on the clay mixture and firing process, the coefficient of linear thermal movement is 4-8.
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 7-14.
The movement coefficient of concrete blocks is similar to cement bricks, but because blocks are larger the magnitude of movement is greater. Large blocks show cracks and plaster crazing more than smaller concrete units.
Architects need 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.
Structural Instability due to Masonry Movement
Masonry movement due to shifts in foundations and soil compaction are serious and require a professional engineer to assess their 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.
Download our free technical books for further information on accommodating movement associated with clay brick masonry walls.
Renovations & Extensions of brickwork
When extending a masonry building, it is important that the new brickwork is bonded, or joined correctly to old masonry as well as new construction materials to prevent structural cracking. If walls are removed, in most cases clay bricks can be reused.