*Cracking Flashcards
Why do buildings crack?
- Drying shrinkage (sand-lime bricks, too strong rendering mixes)
- Thermal movement (lack of vertical movement joints)
- Frost action (freeze-thaw cycle)
- Ground movement (settlement, subsidence, heave)
- Wall tie failure
- Chemical reactions (carbonation, chloride attack, sulphate attack, ASR)
- Lack of lateral restraint (‘book-end’ effect)
- Overloading (roof spread, increase of internal imposed loads)
- Vibration
Why is cracking a problem?
Cracking is problematic as not only can it cause the building to be structurally unsafe, it can also lead to water penetration, leading to damp problems and can also exacerbate the cracking in some cases (frost action, carbonation, wall tie failure)
Name some of the causes of stepped cracking in brickwork.
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You see a vertical crack internally to a property’s wall - what may have caused it?
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What would cracking at DPC level indicate?
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How do you monitor cracking and what are the procedures?
- Three studs/screws method - allows for the precise measurement of the sides of the triangle (ideally with a calliper or crack width gauge), indicating the extent and direction of the movement
- Proprietary calibrated tell-tale - measurement grid/scale is placed over the crack, however they are not always clear to read and are more easily affected by weather and vandals
- Glass tell-tale - placed over the crack and breaks if there is movement, therefore the least effective method, as it only indicates movement has happened, nothing else
Categorise the severity of cracking.
- BRE Digest 251, Table 1 gives 6 categories of cracks based on size, with 0 being hairline cracks and 5 being cracks over 25mm
- Anything under 5mm (category 2) are not regarded as severe
How can cracked, worn or spalling bricks be repaired?
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Your client’s Victorian building has solid brick external walls. There are a number of diagonal cracks in different parts of the building. What action would you take?
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Cracking has been identified to the sides of a bay window to a traditionally constructed property circa 1900s. What are the potential causes and how can the problem be rectified?
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What is subsidence?
Downward movement of a building foundation caused by loss of support beneath
What are the common causes of subsidence?
Usually associated with volumetric changes in the subsoil, possibly due to:
- Influence of trees on shrinkable (cohesive) soils
- Washing away of non-cohesive soils (e.g. leaking drains, burst water mains or underground streams)
- Change in ground-water levels (e.g. abstraction or land drainage)
- Mining
- Nearby excavations
How can trees cause subsidence?
Influence of trees on shrinkable (cohesive) soils:
- Trees and shrubs in close proximity to the building can cause the soil to become desiccated and lose its cohesion as a result of water being taken up through their roots
- Worse throughout periods of hot, dry weather (soils can become desiccated even without the presence of trees)
- As a tree grows it will extract increasingly more water from the ground, which can still cause desiccation even without hot weather
- Introducing new trees causes more water to be removed from the ground, thus heightening the problem
How can subsidence be rectified?
- Repair/alleviate the cause (e.g. repair/replace leaking drains, reduce height of trees)
- Consult with an arboriculturalist when removing/gradually reducing the height of trees is concerned
- Monitor the cracks and if no further movement occurs, repoint/patch/replace as necessary
- Where movement is excessive and on-going, underpinning may be required
- Where movement is slight and thought to be seasonal (usually where trees are concerned), it will be necessary to monitor the movement
Who usually pays for damage caused by subsidence?
Most insurance policies cover the cost of repairing the loss and damage caused by ground movement, but not necessarily the cost of preventing further movement