Building Pathology Flashcards
Why do buildings crack?
- Thermal movement (expansion and contraction of the fabric caused by hot and cold) - lack of expansion joints.
- Frost action (freeze-thaw cycle of moisture within the material).
- Ground movement.
- Wall tie failure.
- Chemical reactions (carbonation, sulphate attack, chloride attack).
- Overloading.
- Vibration.
What are the issues of cracking?
- Can cause structural issues/become unsafe/unstable.
* Can lead to water penetration which can cause damp issues and further exacerbate the cracking.
How to measure/monitor a crack?
- Use of tell-tales
* fixing screws either side and using a calliper to measure distance.
Categories of cracking?
BRE Digest 251 table 1 gives 6 categories of cracking from 0-5, 0 being hairline and 7 being 25mm+
What is subsidence?
Subsidence is the downward movement of a building caused by loss of support underneath.
Causes:
• Volumetric changes within the subsoil
• Influence of trees on cohesive soils (take up water and cause cohesive soil to become desiccated)
• Washing away of non-cohesive soils (leaking drain, burst water main)
• Nearby excavations
• Changes in ground water levels
How do you remedy subsidence?
- Alleviate the cause (e.g. repair leaking drain, reduce tree height
- Monitor cracking - if no further movement repoint & patch repair
- If movement still occurs - underpin foundation
What is Heave?
Heave is the upward movement of the building foundation, caused by expansion/swelling of the subsoil
Causes:
• Removal of trees on cohesive soils
• Freezing of ground water in frost susceptible soils
• Excess water in ground - Expands
What is Settlement?
Natural compaction of soil due to load of building.
Only problematic is differential. (happens in different parts of the structure at different times - could be due to variation in ground conditions or different foundation depths.
What is underpinning?
The process of strengthening the buildings foundation by transferring the load to a new level at lower depth.
Traditional method - Concrete below excavation 150mm long
Jack Pile - pre cast concrete pile hydraulically beneath original foundation.
Needles & Pile - Reinforced concrete means connected to small diameter bored piles.
What is cavity wall tie failure?
A cavity wall tie failure is when the cavity wall tie holding the two walls together fails in keeping the two walls connected.
Causes:
• Rusting of metal ties
• Poor mortar - reduced bond
• Not enough ties installed
How would you identify cavity wall tie failure?
- Horizontal cracking every 450mm (6 course)
- Use of borescope to inspect the cavity
- Remove brickwork and inspect
How would you remedy cavity wall tie failure?
- Remove old wall ties
- Drill in new ties (usually a high tensile strength bar)
- Repoint and make good brickwork
BRE 329 - inserting wall ties into existing construction.
How does water enter buildings?
- Condensation (Condensation occurs when moist air cools below its dew point, and water condenses. This can occur as surface condensation, but also within the fabric of a building itself, as interstitial condensation).
- Penetrating dampness
- Rising damp (Rising damp is caused by incorrect placing of, faults to, or the absence of a damp-proof course, and is generally only apparent up to 1 m above ground level because of the limits of capillary action to draw moisture up through porous elements of the building structure).
How do you record damp?
- Protimeter (measure of conductance between two points).
- Carbide meter/testing (Test a weight sample of drilled masonry - mixed with calcium carbide powder to produce gas & indicates moisture level).
- Oven drying (Gravimetric test) sample is weighed then dried then weighed - moss of water lost as a percentage.
What is dry rot?
- Fungus known as Serpula Lacrymans
- Rot starts out as airborne spores which come into contact with damp timber that has a moisture content over 20%
- Most common in poor ventilated/damp air conditions
- The fungus feeds on the cellulose in the timber and reduces strength.
Why is dry rot serious?
Dry rot can spread across other materials such as masonry to reach other timbers.
How would you identify dry rot?
- The timber becomes dry and crumbly.
- The timber may become a dull brown colour
- Cuboidal cracking is evident
- Mycelium look like cotton wool
- Fruiting body is often pancaked shaped brown with white outer edge.
What is the process of rot?
Airborne spores remain inactive until come into contact with timber moisture enough to make them active (typically 20%)
Spores develop into Hyphae (often white fine stringy strands)
Mycelium (collection/mass of hyphae) - can spread across masonry and behind wall plaster in search of timber.
The fruiting body is the final stage of the Dry Rot fungus’s life cycle. The fruiting body takes on a mushroom like appearance and it produces Dry Rot spores that it emits from the surface to be carried along by air currents out into the air.
How would you remedy dry rot?
- Remove cause of water source/ventilate space.
- Remove all affected timber +300-450mm past infected area
- Treat remaining timber with preservative (ACQ alkaline copper quaternary most popular)
- All affected timber replaced with new treated timber.
(Walls can be treated with masonry fluid/irrigated).
What is wet rot?
- Wet rot occurs when there are large amounts of moisture in timber, which then causes the timber itself to naturally decay.
- Thrives in timber with moisture content between 40-60%.
- fungus laches onto dap timber as food source.
How would you identify wet rot?
The common signs of wet rot include:
- Darkened timber – darker than surrounding timber
- Soft and spongy timber
- Spitting or longitudinal cracking along the grain
- Timber that is damp to touch
- Localised fungus growth
- A damp, musty smell
How would you remedy wet rot?
- Eliminate the source of water (this may be a leaking pipe or roof covering).
- Promote rapid drying (heating and ventilation).
- Remove rotten wood (unless historical significance and needs to be retained).
- Apply deep penetrative preservatives.
- Replace any timber as necessary.
Why is wet rot less severe than dry rot?
Wet rot will stay localised whereas dry rot can spread across other materials to get to other timbers.
What is concrete carbonation?
The reaction of carbon dioxide in the environment with the calcium hydroxide in the cement paste. This reaction produces calcium carbonate and lowers the pH to around 9.
At this value the protective oxide layer surrounding the reinforcing steel breaks down and corrosion becomes possible.
