Cracking Flashcards
What are the common reasons for cracking?
a) Drying/shrinking, like with sand-lime bricks, or with a too strong render mix b) Thermal movement, are there sufficient expansion joints? c) Frost action d) Ground movement, such as settlement and subsidence e) Wall tie failure f) Chemical attack, such as suphate attack, or carbonation g) Lack of lateral restraints h) Overloading i) Vibration
Why is cracking a problem?
Cracking is a problem as it can cause buildings to become structurally unsound. Additionally, cracks can allow water ingress, which can cause other problems separate from the cracking itself
Name the causes of stepped cracking in brickwork
Stepped cracking to brickwork would generally be expected to be caused by movement in the foundations, such as cause by subsidence or settlement
What are the procedures for monitoring cracking?
a) Three stud screws - this method allows for precise measurement, using calipers, of the three sides of a triangle; monitoring the direction and size of the crack. b) Proprietary calibrated tell-tales - this method involves a measurement grid being placed over the crack. These can be difficult to read, and are susceptible to damage from both weather and vandals c) Glass tell-tale - this method results in the glass breaking is there is movement. This does not allow for quantifying the movement
How do you categorise the severity of a crack?
The BRE Digest 251 lays out classifications of cracking:
What are signs of ground movement?
a) Crackings showing internally and externally that correlate, may in some cases go through a DPC to foundations
b) Cracks with tapering that is consistent with differntial foundation movement (see picture)
c) Floor slope, wall tilt, window/door opening distortion
What is subsidence?
Subsidence occurs when the soil beneath a building becomes unstable or shrinks
What are the common causes of subsidence?
a) It almost always is related to a loss of moisture in the soil
b) Trees - their roots absorp the water from the soil
c) Collapse - such as caused by drains/mines
d) Buried organic matter with decomposes and destabalises the foundations
e) Improperly compacted gound
f) Burst water mains - washing soluble matter away
How can trees cause subsidence?
Trees generally only have major influence on shrinkable (cohesive) soils.
Trees require water to grow, and they take this water from the soil around them via their roots. This can cause the surrounding soil to be desiccated due to a lack of water. This effect is exacerbated by periods of hot and/or dry weather. When this desiccation effect takes place in close proximity to a building it can result in the building becoming destablised.
How do you treat subsidence?
As with treatments for anything, the most effective treatment will depend of the cause and severity of the problem:
a) Minor cracking that is not progressive may be able to be monitored and simply re-pointed or decorated
b) A burst water main that has been washing soluble soils away can be repaired, and the problem will not worsen
c) Trees causing subsidence can be pruned, or removed. Alternatively, if it’s protected trees implementing root barrier system might be possible. If this were the issue, I would look to seek advice from an arboliculturalist (tree specialist) before making final recommendaitons
d) If severe and progressing subsidence, it may be necessary to take more drastic action, such as underpinning
Who usually pays for damage cuases by susidence?
Most building insurance policies will cover rectification of damage caused by subsidence, but in some cases they may not cover the cost of preventing further subsidence
What is heave?
Upward movement of soil caused by swelling or expansion of subsoil, common in clay soils when wet
What are common causes of heave?
a) Trees are the most common cause. When they are removed or when the die. The roots stop drawing water from the subsoil, so water accumolates and causes swelling
b) “Stress Relief” - when soil is removed and the pressure is release, such as when excavating for a basement
c) A change in the water table
d) Broken drains that effect the ground drainage
e) Swelling of subsoils with seasonal changes
f) Water expanding as it freezes. Silty and sandy clays are particularly susceptible to this
