Avoid Those Unwanted Cracks
Building with masonry has been around for centuries with historic structures still standing today, such as the Egyptian pyramids, Roman amphitheatres and the Great Wall of China. It is no surprise masonry walls are still the most popular building structures used today with benefits like: durability, strength, temperature control, fire protection, noise barrier, ‘green’ material and large range of aesthetics. However, movement in these types of walls are common and provisions must be incorporated in the design to avoid the structure being compromised and unwanted cracking.
Causes of Movement
After construction, buildings are subject to small dimensional changes, which can be caused by one or more of the following factors:
1) Change in temperature
2) Change in moisture content
3) Absorption of water vapour
4) Chemical action, e.g. carbonation
5) Deflection under loads
6) Ground movement and differential settlement.
Masonry is not completely free to expand or contract because restraints are often present, and so compressive or tensile forces can develop and these can lead to bowing or cracking.
Movement characteristics of masonry units made of clay, calcium silicate, concrete and stone are likely to vary within each material type, due to differences in raw materials and/or manufacture, but movement characteristics differ more significantly between units of different types of material.
All materials expand and contract in response to thermal changes. In addition, clay materials undergo an irreversible expansion after their manufacture as moisture is absorbed from the atmosphere. Other masonry materials shrink following manufacture to reach the equilibrium state.
Units made from different types of material should not be bonded together in masonry in a manner that would cause stress to develop as a result of dissimilar characteristic movement. Where anticipated movements are different in magnitude and nature, parts of masonry of different material type should be effectively separated, e.g. by vertical or horizontal movement joints and/or slip planes. Alternatively, they should be suitably reinforced.
Generally, changes in temperature, moisture content or rate of drying-out due to the presence of insulation, do not need to be taken into account when considering movement in masonry. However, for brick masonry of certain clay bricks in applications where restraint is low (e.g. cladding) and high levels of solar heating are anticipated because of orientation, reduced spacing between movement joints is beneficial.
Accommodation for movement in masonry
i) Movement joints should be provided in the masonry to accommodate its expansion and/or contraction due to changes in temperature and the moisture characteristics of the masonry units. Joints should be built-in as work proceeds.
ii) Slip planes should be designed to allow parts of the construction to slide, one in relation to the other, thus reducing shear stresses in the adjacent materials. The design and positioning of movement joints and slip planes should be carefully considered, to ensure that in addition to accommodating movements, such joints or planes do not impair the stability of the wall or any of its functions.
iii) In external walls, movement joints and slip planes should be sealed, protected or otherwise designed to prevent water penetration.
vi) Fixings and services should not interfere with the performance of the joints or slip planes. Finishes should be discontinuous at movement joints and slip planes. Fixings and fittings should not tie across joints.