Structural and non-structural cracks we find in Perth - pre-purchase building inspection perspective
Minor cracking and foundation movement will occur in a significant proportion of buildings, particularly those on the Perth reactive clays. In most cases, this cracking that appears on walls is not a structural concern and does not mean that the structural integrity of your home is affected. Crack widths and depths should be monitored where these is a concern. If you have any concerns it is recommended that a professional structural engineer is appointed to carry out an investigation and to provide a structural report.
Shrinkage cracking is common in reinforced concrete structures but can be avoided with good detailing practices. Cracks can arise for many reasons including thermal movements, foundation movement and loads. Reinforcement that rusts will also cause cracking.
Understanding the site conditions and the soil classification is important in assessing cracking in walls and floors. Structural defects affect the mechanical behaviour of the construction element . Structural defects can also affect some or all of the non-structural materials. It is therefore important to understand and consider the interaction between these types of materials to prevent the defects.
Common masonry structural defects in Perth include:
cracking due to the settlement of foundations, excessive loading and deformations and other effects (creep, shrinkage and thermal);
local crushing due to high compressive loads;
corrosion of metallic elements or chemical reactions.
And the most common masonry non-structural defects are:
undesired changes in the physical properties of the materials due to the presence of water/humidity thus affecting the durability, aesthetics and the environmental conditions of the buildings or building elements;
cracking in non-structural elements.
Settlement home cracking
Settlement cracking in Perth homes occurs in buildings when a loaded foundation experience a vertical movement. This may occur due to dead loads, changes in moisture content, undermining or inadequate site preparation and compaction.
Settlement in the foundation can occur particularly if the compaction of the sand pad was lower than the engineering specification. However, where a particularly heavy load is placed on the soil such as a large column or a strip footing, movement may occur as moisture is squeezed out of the soil or the soil readjusts itself. This soil consolidation in sands is time dependent and will generally stop when the soil has finally compacted enough to support the load. Subsequent cracking due to settlement should be minimal.
Moisture related movements
The volume of absorbent materials such as bricks changes with the increase or decrease of moisture and these changes can be reversible or irreversible. Bricks begin to absorb moisture immediately after firing This moisture expansion occurs mostly in the first few weeks or months after production.
Shrinkage and creep
Shrinkage is the amount by which the material gets smaller. For example, the reduction in the dimensions of timber after loss of moisture or for concrete the relative change in dimensions with time. Concrete shrinks over a long period during hardening and if it is restrained tensile stresses can develop.
In simple terms, creep in a material such as concrete or timber is a permanent deformation over time. Creep develops due to gradual increasing strain and deformation of the material under constant load. At low temperatures concrete will creep under load. As an example, a retaining wall holding an embankment may creep if the clay shrinks each year during drought and the cracks fill up with debris and then the clay swell when wet.
Vertical and horizontal movement joints (such as brick control joints) need to be detailed in masonry in order to accommodate the above mentioned movements. The required location and thickness of those joints will depend on the building geometry, masonry material properties, and expected differential movements.
In most cases, vertical expansions joints are not needed in concrete masonry since drying shrinkage usually exceeds thermal expansion. However, in clay brick masonry, vertical and horizontal expansion joints need to be used as the clay masonry walls expand in both directions due to the combination of thermal and moisture expansion.
Concrete slabs supported by masonry walls will generally undergo drying shrinkage and, if they are bonded to the masonry, it can lead to undesirable stresses in the ground floor walls. Cracking of this type can be avoided by incorporating a suitable slip joint between the slab and the wall (for further reading refer Manual 7, Design of Clay Masonry for Serviceability, Think Brick Australia).
Moisture control to the building perimeter
Masonry cracking and structural damage can result from movement in clay soils caused by varying moisture conditions around the perimeter of homes. It is essential that piped services in the ground should be checked regularly by a plumber to ensure that there are no breaks or leaks leading to periodic local wetting of the subsoil. This includes sanitary drainage, stormwater and water supply services. External water taps and downpipes should discharge into drains and not onto the ground.
Water reticulation systems that maintain a reasonably constant moisture content in the soil may be acceptable however those that are used only periodically may exacerbate wetting and drying cycles. Generally, it is preferable not to have planting or irrigation systems immediately adjacent to walls of buildings in reactive clay soils. Flower beds should be separated from the building by paved areas in the order of 2 metres wide, if possible.
The purpose of moisture control devices is to maintain a stable water content in the soil below the footings. Typical systems include:
paving to the perimeter of the building;
cut off walls to the perimeter of the building;
buried impermeable membranes around the building;
drip irrigation systems to maintain moisture content in the soil.
When cracks appear in the walls of your home it is recommended that you contact a professional structural engineer with expertise to undertake a building inspection and determine the causes of visible damage. In general the homeowner should refer to the CSIRO pamphlet, Building Technology File 18, Foundation maintenance and footing performance: A homeowner’s guide, and its recommendations should be followed (this guide covers many of the aspects related to foundation maintenance and the performance of walls).
As with any home purchase, be sure to get a thorough pre-purchase building inspection to see whether the house needs any repairs. If you have any questions, get in touch with one of our structural engineers to discuss your requirements in Perth or the South West and to obtain a free quote.
Rotaru Building Consultants
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