Building Pathology Flashcards
What is HAC (high alumina cement) and what are the issues with it?
How would you identify issues?
What remedies are there?
- HAC contains calcium aluminates which is found in certain types of clay.
- HAC initially used for maritime application - develops strength rapidly.
- HAC was used for structural concrete during the 70s in particular.
- Undergoes mineralogical change called conversion - increases its’ porosity and susceptibility to chemical attack.
- Conversion is identified by concrete becoming friable and changes to a chocolate brown colour.
There are three main stages in relation to investigating HAC which include:
- Identification - assessing the areas affected.
- Strength assessment - confirm the structural strength of the affected elements e.g precast concrete beam.
- Durability assessment - confirms the long term durability of the concrete and risk of chemical attach to associated reinforcement - involves petrographic analysis.
What are the typical defects associated with Victorian buildings?
- Differential settlement of part basements in terraces.
- Lack of lateral support between terrace houses (known as the book end effect); joists run parallel with part walls.
- Blocked air vents to suspended timber floors from removal or raising of external pavement level.
Defective slates and nail sickness. - Sagging roof timbers; can be from replacement of slates with cheap + heavier cement roof tiles.
- Lead water pipes.
- Rot and creep in timber members.
What are the typical defects associated with Georgian Buildings?
- Water ingress through parapet walls, valley gutters.
- Decayed timber to floor joists: can be poor cross ventilation to floor voids in long narrow terraces.
- Failure of roof trusses when roof ties rot due to past unknown bomb damage.
- Missing or poorly altered load-bearing timber partitions.
- Damp basements.
- Poorly applied / painted stucco plaster preventing moisture escape
What are the typical defects associated with industrial buildings?
- Cut edge corrosion.
- Delaminating plastisol finishes.
- Missing caps to fixings.
- Leaking gutter joints.
- UV damaged roof lights.
- Cracking – subsidence, heave, settlement etc.
- Impact damaged cladding.
- Asbestos.
- Carbonation.
- Cracked floor slabs.
What is wet rot?
How would you identify it?
How would you treat it?
What would happen if it was left (prognosis)?
- Most common type is Ciniophora Puteana (known as cellar fungus).
- Highly vulnerable to fluctuations in moisture and thrives in timber with 50-60% moisture content.
- Identified by
Surface cracks that follow the line of the timber grain. - Thread-like strands of hyphae which are yellowish and become darker brown with age.
- Fruiting body rarely found.
- If fruiting body is present (usually outdoors) it is this olive brown plate irregular in shape with a knobbly texture.
Treatment:
1. Locate and eliminate source of moisture.
2. Promote rapid drying.
3. Determine extent of damage.
4. Remove timber 500mm beyond affected areas.
5. Treat infected timbers and install new pre-treated timber.
- Not as severe structurally as dry rot but can eventually cause failure.
How do trees negatively affect buildings?
- Can pull more moisture of soil, particularly during dry seasons causing subsidence and then once removed can lead to excess moisture and heave of soils.
- Roots dont usually directly affect buildings, they damaged water and drainage which then cause issues.
- Oak and Poplar have strong root activity.
- Safe building distances vary but good rule of thumb is to limit proximity of tree equivalent of that of its full maturity height.
How do you interpret cracks?
Subsidence cracking: Usually diagonal, runs through masonry and blockwork and usually wider at top than bottom.
Heave cracking: Usually wider at base and as above.
Appoint a structural engineer
What are the options for damproofing a basement?
Type A: Basement tanking used internally or externally (applied during the construction process). Any defects to the membrane will have damaging consequences and those defects to external tanking systems will be impossible to rectify. Internal tanking systems can be penetrated by fixings and can be displaced by build up of hydrostatic pressure.
Type B: Structurally integrated protection whereby waterproofing is provided by walls themselves (reinforced / prestressed concrete). Need to carefully consider jointing details as they can leak.
Type C: Drained cavity system whereby water is allowed to pass through the external skin into the cavity and is then pumped away by an electric pump. Can be problematic when there is a high water table and failure of pumping system (usually due to accumulation of fine silts etc).
What is heave and what causes heave?
The upward movement of the ground beneath a building as a result of the soil expanding.
Causes: Tree removal (increase in soil moisture), frost heave, burst sub terrain water pipes.
What is subsidence and what causes subsidence?
The ground beneath a building sinks, pulling the property’s foundations down with it.
Causes: Prolonged dry spells particularly with clay rich soils, trees drawing moisture and collapsed mines.
What is settlement and what causes settlement?
Downward movement due to compression of soil due to weight of building within 10 years from construction.
Causes: occurs after construction.
What is Regents Street Disease?
Found in early 20th century load bearing steel framed buildings clad externally with stone, brick and terracotta that is notched around the steel frame and then the void filled with low grade mortar. Corrosion of the steel frame due to oxygen and moisture causes corrosion and expansion and with no void space causes cracking of the mortar and overlying cladding.
Can usually diagnose from cracks delineating the frame.
How would you remediate Regents Street Disease?
2 main ways to rectify:
- Exposing the steel, cleaning it and protecting it and then create a void around steel frame to allow movement.
- Use cathodic protection: system relies on concealment of discrete anodes into the stone joints and electrical connection to the steel frame and the introduction of an electric current to reverse the corrosion process. Requires detailed design to ensure harmful currents are not produced and maintenance.
What concrete defects are you aware of?
- Concrete Carbonation
- HAC
- Sulphate Attack
- Regent’s Street Disease
- Chloride Attack
- Alkali Silica Reaction
If you identify movement in a building, what further investigations might you wish to undertake?
- Environmental matters, i.e. trees in close proximity
- Neighbouring dwellings
- Engineers structural survey
- CCTV Drainage survey
What the positives and negatives of cold and warm flat roofs?
Cold roofs:
- As the roof void is cold there can be issued with interstitial condensation as warm air from inside the building enters the unheated roof space allowing condensation to form on the timber roof joists and deck - this make them susceptible to rot and therefore structural failure eventually. Require vented 50mm gap between bottom of roof deck and top of insulation to try and prevent.
More complicated to construct - insulation between the joists rather than just on top of roof. - Greater chance of thermal bridging - ceiling joists are not insulated.
- Benefit: as insulation is between joists there is no change to roof height - this could affect roof height relative to the parapet walls - may need to extent them etc
Warm Roofs
- Easier to install.
- Can be retrofitted.
- Less chance of thermal bridging.
- Drawback - external roof height will be raised.
What information could you consult with regards to dealing with Japanese Knotweed?
Property Care Association Code of Practice.
