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.
What are the three types of loading on buildings?
- Dead load: load imposed by the structure itself.
- Live load: load imposed by occupants and objects in a structure.
- Environmental load: load imposed by rainfall, snow and wind.
How would you test for damp?
What are the pros and cons of each of these methods?
Carbide meter:
Sample of mortar, plaster or brick taken and weighed.
Sample then placed in one chamber, a weighed amount of calcium carbide powder is placed in a separate chamber.
Container then shaken to allow mixing of two materials. The amount of acetylene gas produced indicates the amount of moisture present.
Pros: accurate.
Disadvantages: intrusive, more time consuming that protimeter.
Moisture meter:
Measures level of conductivity, water has high conductivity so increases the reading when present.
Pros: Quick, non-intrusive (relatively).
Disadvantages: Calibrated for timber, can give false high readings from highly conductive materials such as salts, foil backed plasterboard and black ash mortar.
Deep wall probe:
Essentially a long protimeter than can measure conductivity deeper within the wall.
What typical defects would you expect to find in a 1960s high rise office building?
- Asbestos
- Concrete carbonation of concrete frame
- High Alumina Cement
Give me some examples of defects you would find in residential buildings
Georgian
- Water ingress through parapet walls and valley gutter details.
- Missing or poorly altered load-bearing timber partitions.
- Failure of roof trusses
- Poorly applied or painted stucco limiting water evaporation from walls
Rotten or stuck sash windows
Victorian
- Defective slates and nail sickness.
- Deflection of roof timbers caused by installation of heavier concrete tiles (cheap).
- Blocked air vents to suspended timber floors from removal or raising of external pavement level.
- Lack of lateral support between terrace houses (known as the book end effect); joists run parallel with part walls.
Modular Housing - Post Second World War
- Use of asbestos sheets to roofs.
- Corrosion of steel frames used.
- Alkali Silica Reaction affecting concrete frames (gel
Modern Housing
- Cracking and movement of walls due to timber frame being wet and then shrinking once dried out.
- Heave / subsidence.
Give me some examples of defects you would find in industrial buildings
- Impact damaged cladding
- Cut edge corrosion
- UV degraded roof lights
- Asbestos roof sheets
Give me some examples of defects you would find in office buildings
**Office Buildings (60-80s)
- Hollow clay pot floors - can exhibit loss or strength due to honeycombing of the structural concrete ribs of the floor system which reduces structural integrity and fire stopping capabilities. Incorrect removal of topping screed if structural to gain floor height can lead to structural failure.
- Wood wool cement board - WWCB is formed of softwood shavings which are coated and bounded by Portland cement. WWCB was commonly used as permanent shuttering in flat slab or ribbed reinforced concrete floors. There is risk of poor concrete cover to the steel structural members therefore reduces durability, structural integrity and fire resistance. If discovered the shuttering be removed to determine coverage and parch repairs undertaken by hand if necessary.
Can you confirm another scenario where you may have penetrating damp through a wall?
Solid walls: leaking mortar joints (need to be raked out to a depth equivalent to their height and then repointed), masonry water repellent to less exposed elevations, defective jointing details around windows etc.
Cavity walls: Rarer but could have bridging of cavity by wall cavity ties sloping inwards (will see regular spots of ingress), bridging of the cavity wall by it being blocked above the DPC from mortar droppings and other debris, missing cavity trays.
What are the main ways you can eradicate Japanese Knotweed?
Dig and dump (excavation of the plant and roots)
- JK infested soil excavated (up to 3m vertically and 7m horizontally from the area of growth), removed off site and disposed in an appropriately licenced waste management facility as it is classed as controlled waste under the Environmental Protection Act 1990.
- Benefits: quick, no restrictions on site for development, works can continue on site immediately.
Drawbacks: Expensive (£70 per tonne), reduced valuable landfill capacity (should be last resort), increases risk of spread (disturbing the plant through excavation)
On-site burial
Trench is excavated on the site in suitable location and JK then excavated and buried in the trench which will need to have at least 5m of overburden. Unless JK can be buried with 5m of overburden then a specialist root barrier membrane must be used to partly or full encapsulate the JK. Area is then monitored for at least two successive growth seasons and treated with herbicides if needed.
Benefits: More cost effective than dig and dump, works can continue immediately, quick to remove.
Drawbacks: Need suitable set aside area where the JK will not be disturbed, can’t be used if water table is high (allows distribution of JK into waterways), if root barrier used it needs to be installed correctly as any weaknesses / tears can be exploited by JK, JK can be accidently spread during excavation.
Herbicidal treatment
Herbicides applied to JK usually combined application of spraying and stem injection.
Benefits: Cost effective, can be carried out in situ (less risk of spread), quickly reduces spread and ability to impact structures, can be used on restricted sites that don’t have a set aside area.
Drawbacks: JK remains in situ do could be disturbed and spread (particularly if works being carried out), not appropriate for sites where there is planned development (excavation), take several years to achieve complete eradication with two years of monitoring to ensure growth has ceased.
Other options:
Biological control - testing being undertaken - still not used.
Screening / sifting - JK excavated along with soil which is then sifted to separate rhizomes which are disposed of appropriately, the soil is then screened and stored on site in a suitable location, treated with herbicide and monitored.
What other types of invasive species are you aware of?
- Giant Hogweed
- ## Horse Tail
What causes spalling masonry?
- Incorrect application of cement pointing: In solid wall buildings, if lime based mortar is replaced with cement mortar the mortar joints cannot release the interstitial moisture within the wall, it therefore is released through the brickwork which leads to spalling due to excess moisture release and makes the bricks more susceptible to freeze thaw as they have a higher moisture content.
- Freeze thaw: Water absorbed through the porous surface of the bricks and then expands upon freezing.
- Crystallisation of salts (cryptoflorescence): this occurs just below the surface of the bricks leading to spalling. Occurs from large build up of salts and usually occurs where older, weaker bricks are re-used inappropriately in areas of excess dampness (at ground level which is susceptible to splashback - should use engineering bricks).
What is nail fatigue/sickness?
As a roof ages the fixings which are often iron nails start to fail resulting in tile or slate slippage especially in windy weather conditions.
Replace existing iron nails with copper nails, these do not corrode.
Explain why sulphates in hardcore can result in sulphate attack.
As they are not protected with a DPM when they comes into contact with a water source they expand and can transfer this up into the slab above
What are common defects associated with flat roofs and what are the remedies?
** Crazing**
Surface crazing caused by lack of protection from UV exposure. No treatment is required if in small localised patches but should be monitored.
** Ponding**
If the gradient is not sufficient and there are dips in the roof finish, it can lead to pools of water. It can lead to water ingress if there is a gap in the roof finish.
Thermal Movement
Cracks and splits can occur when there is thermal movement between the substrate and membrane. Usually requires localised repair to the area around the crack.
Blistering
Caused when water vapour beneath the roof finish increases. The source of moisture should be traced and the blister opened and allowed to dry before patch repairing.
Flashing and Falls
around Openings
Defective lead from deterioration or poor workmanship around openings causing water ingress. Depending on the issue with the lead, there are different repairs.
Punctures
caused by people from impact damage.
Cracks and tears along the line of joists
Caused by thermal movement or saturation of insulation or sagging decking. Repaired by cutting felt back and allowing it to dry before patching over.
What is a deleterious material?
Any material that is harmful to health, safety, environment, not suitable for their purpose or pose a risk for where it is used.
Name and describe some deleterious materials.
Asbestos
Fiberous material used for a variety of reasons from roof sheets to insulation and fire stopping. It is carcinogenic and needs to be removed if there is a risk of release of fibres.
Lead
Risk of lead poisoning when used for pipes or paint. Contaminates drinking water and risk of inhalation of paint dust or potentially ingested when children chew on painted products.
Machine Made Mineral Fibres
Potential Carcinogenic. The evidence leads to show there is a high risk and should be treated as such but has not yet been proved.
Wood Wool Slabs
When used as permanent shuttering for concrete, it may lead to grout loss and inadequate coverage of steels which leads to inadequate fire protection. If used for roof decking, it is not considered deleterious.
*Calcium Silicate Bricks**
Shrinks after construction with further movement from wetting. Thermal movement more likely than with clay bricks. Construction details must allow for movement and if designed and used correctly, performs well.
High Alumina Cement
Gives high early strength and used in maritime buildings. Porosity is increased and resistance to chemical attack is reduced. Where water and chemicals are present in the mix, it can lead to the concrete becoming friable and lead to loss of strength.
Wood wool slabs are deleterious materials. Explain why and where their use would be acceptable.
When used as permanent shuttering for concrete, it may lead to grout loss, honeycombing or voids which can reduce fire resistance, corrosion protection and loss of strength. It is adequate for use in flat roof decking.
When was DPC made compulsory?
1875
