Helitec technical staff discuss why buildings crack and examine some of the causes.
What causes a building to crack? A seemingly simple question which is often asked by frustrated and building owners when cracks appear and sometimes even disappear, at least temporarily.
Unfortunately, there is often no simple answer. There can be a multitude of reasons why buildings crack.
It is easier to understand the complexity of cracking by dividing the possible causes into two main categories; 1. Below Ground and 2. Above Ground, related cracking.
- Below Ground (or Foundation) related cracking.
i). Clay Foundation Shrinkage and Heave
If pore water is removed from a clay soil, the particles tend to be forced closer together, causing shrinkage and;
Additional contact between particle
Reduction in thickness of the water bound clay particle
Reorientation and rearrangement of the clay particles
If pore fluid returns, some expansion (heave) may take place as the particles return to their original volume. But particles which forced into contact or which have been rearranged by rotation, may not return to their original position and therefore some of the volume change can be irreversible.
Changes in soil moisture levels of foundations can often be caused by drainage problems or trees and vegetation in close proximity to buildings.
ii). Foundation Settlement
Some Settlement of soil foundations under buildings will generlly occur over time.
“Differential settlement” of foundations and footings of masonry buildings can result in cracks if not adequately addressed in the original design.
There are generally three stages of foundation settlement;
Immediate settlement – the distortion of the ground caused by the weight or “dead load” of the building or structure.
Consolidation settlement – dissipation of excess pore water pressure from the foundation until equilibrium reached (short to medium term).
Secondary settlement – continued compression or “creep” of the foundation, which can occur over many years.
In clay soils, foundation settlement usually occurs over a longer period, in comparison to granular soils, where settlement is more rapid and often complete by the end of construction.
In compact filled foundations, settlement can be extremely varied depending on the type of fill, depth of fill, moisture content and amount of compaction applied.
iii). Shallow Footings
Buildings on shallow footings are more prone to movement caused by the building loads, moisture changes, settlement, trees & vegetation.
Where shallow footings are used alongside deeper footings in the same building, differential settlement can also occur. Often seen in older buildings where new extensions or additions have been made to adjoin the old.
iv). Concentrated Loading on foundations
Each building has a unique pattern of loading and load paths. Concentrated loads can occur in certain locations in a building due to its design and features which may then result in movement due to the increased loads on the footings.
The building sub-structure and footings are designed to transfer the building loads into the foundation, however excessive concentrated load can sometimes cause failure of footings resulting in differential settlement of the foundation which can lead to cracks in the brickwork.
Internal alterations to the original building may result in changes to load patterns and cause concentrated loading e.g. addition or removal of walls, new or larger openings.
v). Sloping Site Foundations
Buildings on sloping sites can often be more prone to cracking and movement. Particular attention needs to be given to roof and storm water runoff and diversion away from the building and footings. Additional geotechnical advice, underpinning, helical piers or chemical ground injection and stabilisation may also be required to help treat cracking and related problems.
vi). Ground Slope Instability
Where ground slope instability is suspected as the cause of cracking, specific specialist geotechnical advice is generally recommended.
vii). Foundation Erosion and Softening
Water seepage below building and footings can cause erosion and soil softening if left unattended. This can result in major damage to the foundation over time and commonly occurs in sandy and silty soils, where the loss of small particles (fines) under the footings, is sometimes difficult to detect.
Signs to look for;
Groundwater seepage and poor runoff causing solution of minerals (leaching) from the soil
– Leaking pipes, drains, water tanks
Poorly constructed or inadequate ag-drains and groundwater retention pits. Excavation below the water table
– Soak-aways too close to a building
Discharging rainwater gutters and downpipes onto the ground around the building.
viii). Foundations affected by Mining Subsidence
Properties built above mines can suffer from subsidence over long periods. Where underground e.g. “longwall” and other types of mining is carried out, subsidence of building foundations of up to several metres can occur. e.g. Parts of Camden, Picton, Hunter Valley, Newcastle, Wollongong in NSW, Gippsland in Vic and many other regions. Home owners and builders should take extra precautions and seek specialist advice regarding buildings located in a mine subsidence area. Contact Mines Subsidence Board.
ix). Vibration effects on foundations
Granular soils e.g. silts and fine sands, can be prone to consolidation by vibration and subsidence.
Vibration to foundations can be caused by;
Deep mining settlement
Nearby excavations or pile driving
Motor vehicles and traffic
- Above Ground – (Building Movement) related cracking.
i). Thermal Movement in walls
All materials are affected by changes in temperature. Various studies show that the thermal expansion and contraction of brickwork is approximately (8 x 10e-6 /K) or 0.008 mm/m/degC. For example, a wall which is 10m long can typically undergo thermal expansion and contraction of around 3.2mm for a temp. change of 40degC. The extent of change is affected by the material and also external influences, e.g. Friction, fixings, location and orientation of wall etc.
Temperature induced changes are generally reversible, in other words the dimensions tend to go back to normal when the temperature reverts to normal.
ii). Moisture Movement in walls (e.g. Brick Growth & Concrete Shrinkage)
Moisture movement is caused by the chemical reactions between moisture and some constituents of the material. It occurs in all three dimensions and the changes in dimensions may be either reversible (to some extent), or non-reversible.
Fired clay products begin to expand as soon as they have cooled enough from the kiln to enable measurements to be made. This is commonly known as “brick growth” and can be a problem particularly in new buildings and with certain bricks or brickyards.
Brick Growth slows over time and it is said that roughly one third of the total long-term movement occurs in the first six months, the next third within five years, and the remainder over hundreds of years. Some relief gained by storing the bricks for six months before use.
The coefficient of expansion (the e-value), represents the likely permanent unrestrained growth over the first 15 years of the brick’s life. A test method is given in AS 4456.11 and the ranges of the coefficient are classified in the following manner;
- a) Low: up to 0.8 mm/m;
- b) Medium: 0.8 to 1.6 mm/m;
- c) High: 1.6 to 2.4 mm/m;
For example, based on the above classifications a 10.0m long wall, can undergo brick growth of up to 24mm during the first 15years.
The Australian National Construction Code (NCC) 2014 sets out requirements for movement joints in new masonry buildings. Typically 6.0m – 7.0m horizontal spacing.
ii). Freeze/Thaw Action in Brickwork
When water freezes it causes an expansionary force and when it freezes in the pores of bricks, stone or mortar, the expansion is often sufficient to cause crumbling and flaking of the brick surface. Freeze/thaw action is common in under-fired bricks with low frost resistance and buildings in frost prone areas.
Brick retaining walls of porous masonry with no damp proof course or weep holes are particularly vulnerable. Parapets, chimneys and walls that trap snow can also present a significant risk of frost damage.
iii). Chemical Action in Brickwork
Usually sulphates show as harmless albeit, unsightly efflorescence. Under wet conditions however the salts can react with Portland cement and cause a marked volume increase.
The mortar is attacked, not the bricks and the volume increase can cause vertical expansion of brickwork which can be typically as high as 0.2%
The most vulnerable walls tend to be earth retaining walls, sub-floor and basement walls and sometimes in parapet walls which are exposed or affected by moisture seepage from roofs or balconies.
iv). Corrosion of Embedded Steelwork in Brickwork
Iron and steel in brickwork corrodes causing the surrounding brickwork to fracture, lift and loosen. Corroding steel beams, columns, lintels, railings and bolted connections, can all cause significant problems and cracking in brickwork. Corrosion can expand the volume of the parent metal by up to seven times and with very large forces.
In a cavity wall, the corrosion product (rust) can also cause the external leaf to expand independently to the internal leaf and can result in outward bulging of the wall. Walls which are more exposed to the weather and prevailing winds, often suffer more damage and deterioration than more protected walls.
Wall tie corrosion may occasionally show up as horizontal cracks in the brickwork every six to eight courses that correspond with wall tie positions. However, this may be difficult to detect and would only usually occur with the larger ties e.g. fish tail ties which are rare in Australia. (Corrosion of the more common “galvanized wire ties”, can be more difficult to detect and will be covered in detail at a later date).
v). Brickwork Lintel Failure
There are many different types of lintels used in brick construction including timber lintels, in-situ concrete lintels, precast concrete lintels, steel lintels, and reinforced brick lintels and lintel failure can be due to failure of the lintel itself or may sometimes be also due to the walls or foundations.
Wide, shallow and flat arches are often more vulnerable to vertical loads but lintel failure may also occur because of spreading of the brickwork supporting the arch on either side of the opening – e.g. inadequate abutment or buttressing action.
Other causes of lintel failure may also include; deterioration e.g. rotting timber lintels, under capacity of design, cracking within the arch, overloading e.g. ‘Boot’ lintels tend to rotate forwards where insufficient bearing on outer leaf, steel lintels with a thin shelf supporting the external leaf can show similar problems which can also result in bulging or cracking of the arch.
vi). Overloading of Walls
Masonry cracking can occur beneath a concentrated load for one of two reasons:
- Any movement the wall is undergoing, whatever the cause, is likely to also focus at the same point as the concentrated load, because the load can act as a restraint.
- The load causes a bearing failure by over-stressing the masonry.
Consideration must be given to the following, during and after the removal of supporting walls or making larger openings:
– The floors above must be adequately propped whilst the wall is removed
– Before the prop removal walls must be tightly dry-packed to the new supporting beam
– Deflection of the beam once the load is applied that causes cracking
Excess loads on floors due to change of use, may result in overloading of structural walls which can cause leaning or bulging.
Care must be also taken when creating new openings in brickwork and expanding existing doors and windows, which may transfer increased loads to smaller areas of brickwork.
Some very old buildings or poorly detailed buildings may have masonry walls built straight off from timber floors or framing or steel or concrete which may be deteriorating, moving or under-capacity to cope with the loads. These walls may typically exhibit large raking or horizontal cracks due to deflection of the wall and support structure.
vii). Structural Alterations
Whenever contemplating structural alterations, careful consideration needs to be given to whether the building will continue to support its loads adequately and without affecting its stability and serviceability.
Bulging, cracking or collapse of brickwork can result from alterations which have been carried out with poor design or construction techniques.
From this discussion, we can see that there is not always a single and simple answer to what caused the crack. It is often a matter of examining some possible causes as described above and then narrowing down the list through a process of elimination.