Building Pathology Flashcards
What is Regent’s Street Disease?
- The corrosion of the steel frames within masonry clad buildings.
- Its so called because a lot of the buildings on regents street are built around early to mid 1900s and have masonuary packed tightely around their steel frames.
- These leaves no room for expansion, if moisutre gets inside the facade the steelwork can corrode, it then expands massively and causes damage to the stonework such as cracking and spalling and in severe cases falling masonary.
- 1900s to 1950 builders packed masonry tight around the framework, leaving no room for the steel to expand.
- Rain can permeate the porous facade stone or brickwork corrode the steelwork or it can be moist air inside the cavity from inside the building.
- Corroded steel exapands in volume and can cause stonework to spall and leading to falling masonary which is dangerous
What remedial works might you suggest for Regent’s Street disease?
- Temporary resin stitch repairs to tie loose masonary
- Retained facade, renew steel structure
- Remove cladding, remove rust from steel and treat with a corrosion inhibitor and repair
- Cathodic protection.
- There are temporary solutions such as stitching loose masonry together with resin on fixed stainless-steel bars at the affected area. Other methods may help just delaying the inevitable.
The only way to resolve it permanently, as has been successfully done on many buildings in Regent Street, is to retain the façade but replace the entire steel frame with a new, purpose-built frame, constructed back from the masonry. The masonry is then fixed to the new “web” using stainless steel secondary fixings.
Repairing the façade steelwork is an extensive but effective method – provided it is carried out correctly.This work involves removing the masonry cladding and the surface rust by blasting the existing steel before treating it with a protective coating/paint.
Cathodic protection, a less invasive method, involves reducing the corrosion process by running an electrical current through the corroded steelwork.
Cathodic protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. A simple method of protection connects the metal to be protected to a more easily corroded “sacrificial metal” to act as the anode
DC current from the environment into the protected metal surface to reverse the direction of electric currents associated with the corrosion process. It does not make good previous corrosion but suppresses the continuation of the process.
Can you name some typical defects you might find in a victorian property?
- roof spread - poorly supported roofs
- nail fatigue
- Structural issues -soil movement shallow foundations make more suceptible
- Weathered brickwork from pollution acide rian, masonary bees, in appropriate repairs.
- penetrating damp throguh single skin walls failure of the damp proof course.
- chimney leaning sulphate attack (products of burning, hydroscopic salts), poor flashing, water ingress missing flanching, weathering. missing pots
- Defective rainwater goods cast iron
- Lead paint and lead water supply pipes
- Water ingress through parapet walls -
- Bay window differential movement
- Poor sub floor ventilation - dry rot
- Dated service installations
- later additions 1960s back boilers
- Asbestos likely to have been added later on
- Water ingress to basements
Can you name som concrete defects?
- Carbonation - phenol phalein test
- Chloride attack
- Concrete cancer Alkali-silica Reaction (moisture and high alkali content)
- High alumina cement
- Sulphate attack
- Mundic and Bungaroosh
- Fire damage
- Lack of concrete cover and honeycombing (poor concrete mix)
9.
- Can you tell me about the defects you may encounter in a flat roof?
- Intersitial Condensation
- Missing termination bar
- blisters (liquid applied coating)
- ponding
- solar degradation
- Age related (cracking and crazing) of asphalt
- Woodwall slab roof deck
- Rotten timber deck due to water ingress
- Lack of appropirate falls to drainage
- lack of insulation
Why might you specify a retention period of 12 months?
To allow enough time for any defects to show themselves, particularly those which might be due to seasonal variations such as flash flooding, snow etc.
What is the difference between dry and wet rot?
- Wet rot is confined to the area of damp timber whereas dry rot can spread and affect timbers some distance away from the source of damp.
- They are cause by different species of fungus
- Wet rot is caused by Coniophora puteana
- Dry rot is caused by serpula lacrymans
- They prefer different moisure levels
- Wet rot grows at a higher moisurte level 50% +
- Dry rot 20-30%
- Wet rot you tend to see the wood turn black whereas dry rot you see rusty dust like brownmarks where you can see a fruiting body which contains the spores. Mycelium are white turning grey with age.
- Dry rot grows in a dark humid environement inside the building such as in a roof void or under a timber foor whereas wet rot can be on external timbers.
- Dry rot you might see cuboidal cracking on the skirting boards but this tends to be hidden behind paintwork, timber is crumbley.
- Wet rot the wood turns soft nd spongey with the fibres showing
- Dry rot smells of mushrooms!
- Recognising wood rot and insect damage in buildings. BR 453
What are the different types of damp?
- condensation -
- warm moist air within a property condenses ona cold surface which is below the dew point forming water droplets on the surface.
- Black mould growth on cold surfaces such as outside wall when internal relative humidity is high and there is insufficient ventilation
- Interstitial condensation occurs within or between the layers of the building envelope, e.g. water vapour enters the roof void and condensation has occurred to the cold underside of flat roof decking
- Rising damp
- damp which rises from ground level and wicks up porous materials by capilliary action
- Caused by
- High water table
- failure or bridging of damp proof course
- wavy tide mark up to 1 metre high of salts and water staining on internal finishes
- Penetrating damp
- rainwater entering the building - defective rainwater goods, poorly fitting windows
- Services leaks
- Infrared thermography can be useful here, because this technique will identify surface temperatures that can be linked to symptoms of dampnes
- flooding
- Reference BRE
Understanding dampness BR 466
Can you tell me three different invasive plant species and how you would identify them and deal with them?
- Japanese knotweed
- green, purple-speckled, bamboo-like stems around 3m tall
- Grows on volcanos introduced to Uk mid 1800s to stablise railway cuttings
- heart- to shield-shaped leaves
alternate leafing pattern along stems
4. completely hollow stems that can be snapped easily. 5. Rhizome fragments are also easily excavated and spread when soil is disturbed 6. Can break up concrete tarmac grow through foundations destabilizing properties. 7. Dies back in winter, canes can be removed. 8. Contaiminated waste 2. Giant Hogweed 1. hollow, purple (or green speckled with purple) and covered in fine hairs. 2. Toothed Leaves. 3. large white umbrella like flowers, dies back in winter 4. sap is an irritant to skin causes sun sensitivity, can cause blindeness, teratogenic and carcinogenic 5. Causes destabilisation, bad for crops 6. Exacavate using equipment and PPE, herbicide remove to licensed refuse. 3. Himalayan Baslam 1. Pink flowers, red stems, grows on riverbanks, spreads by seed 2. Increases flooding risks by blocking water courses. 3. Herbicide, excavate remove, grazing 4. All are controlled waste Environmental Protection Act 1990 - licensend landfil site,
Can you tell me different patterns of structural cracking and what they might mean for a building?
- Horizontal cracking to brick work/stone
- wall tie failure (cavity walls)
- Regents street disease
- Stepped cracking wider at bottom than the top
- Heave
- Stepped cracking wider at the top than the bottom throuhg brick and mortar
- subsidence
- Stepped cracking either side above a lintel/brick arch
- lintel failure
- Hairline cracking at wall and ceiling joints
- Shrinkage
- Settlement - usually of a new build
- Large 5mm+ cracks or multiples of 3mm between a house and an extention or a bay window
- Differential movement.
- Vertical cracking on a steel framed structure or between two different materials
- thermal expansion
- Large scale cracking and sudden damage.
- sink holes
- mines.
What is a cold bridge and which approved document refers to them?
A thermal bridge (sometimes referred to as thermal bridging, a cold bridge or thermal bypass) describes a situation in a building where there is a direct connection between the inside and outside through one or more elements that are more thermally conductive than the rest of the building envelope.
As a result, there will be wasteful heat transfer across this element, its internal surface temperature will be different from other, better insulated internal surfaces and there may be condensation where warm, moist internal air comes into contact with the, potentially cold, surface. This condensation can result in mould growth.
Thermal bridges are common in older buildings, which may be poorly constructed, poorly insulated, with single skin construction and single glazing.
In modern buildings, thermal bridging can occur because of poor design, or poor workmanship. This is common where elements of the building penetrate through its insulated fabric, for example around glazing, or where the structure penetrates the building envelope, such as at balconies.
The Approved Documents to Part L of the building regulations (Conservation of fuel and power) state that ‘The building fabric should be constructed so that there are no reasonably avoidable thermal bridges in the insulation layers caused by gaps within the various elements, at the joints between elements and at the edges of elements such as those around window and door openings.’
- Can you explain what is meant by lateral restraint?
- Can you explain how there can be a failure of lateral restraint in a victorian building and what this might cause?
- Lateral restraint - means litereally sideways support. As a masnonary wall gets taller it needs to be tied in to supports otherwise it would bend and topple outwards.
- In a house the force of the roof pushing out at the top should be balanced by beams at floor and ceiling level which tie the walls into the structure.
- If those same beams go rotten and the wall loses its connection to the floor joists you have a lack of lateral restraint and the wall will start to pull away creating a bulge seen from outside, inside the nearby floors may have excessive movement or spring because of the same lack of support.
- There may also be horizontal cracks appearing and eventually the wall could collapse.
- A surveyor is more likely to come across this problem when a roof covering is changed from something lightweight, such as slate, to something much heavier. The common example is when heavy concrete tiles are used on a late Victorian terrace’s roof, which would originally been covered with slates: the increased load causes the top of the walls to be pushed out, and because masonry walls do not have much flexibility, horizontal cracks will appear just a few courses down from the eaves.
Apart from subsidence, what else might cause diagonal cracking around a window in a masonary building?
Lintel failure
Where a lintel does not have enough bearing; that is, it may not be extended far enough into the surrounding wall.
When this happens, one end of the lintel can often slip where the pressure of the wall above causes a diagonal crack through that wall.
Usually the lintel will stop in a secure position and the cracking will be minor, in category 1 or 2. More severe movement and cracking – category 3 or 4 – can result in sticking doors or windows and repairs will be required.
What is subsidence and how might you diagnose it?
Subsidence is when cohesive soils such as clay shrink usually because a tree is drawing up a lot of moisture. This shrinkage then creates voids in the soil which can lead to the downward movement of the foundation.
Visible symtons:
- Diagnoal cracking
- visible both inside and outside the property
- Cracks extending below the damp-proof course
- Sticking doors and windows due to warping.
- BRE has a guide to cracking 5 steps.
What is heave
- Heave is when there is suddenly more mositure in the soil and the ground under a building swells and moves upwards
- for example
- following removal or death of a tree.
- Change in ground water levels
- leaking drains
- Normally associated with clay soil.
- Cracking is often vertical
- Doors sticking
- Where ground heave is likely to be a problem, cellular raft foundations may be installed to reduce the upward force of heave from transmitting to the structure above (includes voids to accomodate upward movement).
How does water enter buildings?
- Condensation
- Penetrating dampness
- Rising dampness
- Leaks (e.g. from pipework)
- Trapped construction water (new builds)
Describe the main consequences caused by dampness within buildings.
- Health hazard
- Reduce strength of building materials
- Cause movement in building elements
- Lead to timber decay (dry and wet rot, insect attack)
- Cause chemical reactions in building components
- Reduce effectiveness of insulation
- Damage decorations
How can you record damp in buildings?
- Oven Drying (Gravimetric Testing)
- Conductance Meter (aka Protimeter)
- Carbide Testing (aka Speedy Meter)
Explain the process of oven drying to measure damp.
- Sample is weighed, dried in an oven and then weighed again
- Moisture content = (wet weight - dry weight x 100) / dry weight
Explain how a conductance meter can be used to measure dampness.
- When materials absorb water, they can conduct electricity
- Conductance meters have two metal probes (electrodes) which are firmly pressed into the material being tested
- Electrical resistance between the two probes can then be measured
Explain how carbide testing can be used to measure damp.
- Used for masonry products (e.g. bricks, blocks, mortars etc.)
- Material is drilled slowly to minimise heating (and thus drying) then weighed and placed in a container
- Specific amount of calcium carbide is added and container is sealed
- Container vigorously shaken so two materials mix
- Moisture in sample reacts with calcium carbide to produce acetylene gas, causing pressure inside the container, which gives a reading on the pressure gauge
Explain some of the limitations of oven drying to measure damp.
- Destructive
- Little practical use on site
Explain some of the limitations of using a conductance meter to measure damp.
- Calibrated for timber, so not accurate for other materials (only comparative readings)
- Readings may be higher if timber has been treated with water-based preservatives
- Electrical conductive surfaces (e.g. aluminium foil-backed wallpaper) may cause inaccurate readings
- Only surface readings can practically be taken (deep probes with insulated sides needed otherwise)
- Salts naturally present in walling materials conduct electricity and can be confused with damp problems
- Hygroscopic salts left by previous dampness may cause high reading, not necessarily ongoing damp problems
Explain some of the limitations of carbide testing to measure damp.
- Knowledge of material being tested is required as different materials will differ in the amount of moisture they can contain and still be regarded as ‘dry’
- Destructive - requires several readings for accuracy
What guidance is available in relation to dampness in buildings?
- BRE BR 466 - Understanding Dampness
- BS 5250:2011 - Code of practice for control of condensation in buildings
- BRE Digest 245 - Rising Dampness in Walls: Diagnosis and Treatment
- BS 6576:2005 - Code of practice for diagnosis of rising damp in walls of buildings and installation of chemical DPCs
What is condensation and how is it caused?
- Condensation - change of water vapour naturally present in air into liquid water
- The amount of water vapour the air can hold depends on its temperature (the warmer the air, the more water vapour it can hold)
- If moist air comes into contact with a cold surface, the air will be cooled and its ability to hold water will reduce
- Once the air falls to a temperature where it can no longer hold the amount of water vapour present (i.e. it becomes saturated), liquid water will form on the cold surface
What is meant by the terms ‘dew point’ and ‘relative humidity’?
- Dew point - the temperature at which the air becomes saturated and will condense
- Relative humidity - moisture content present within the air, referred to as a percentage of the amount of water vapour it can hold at that temperature
How does modern living standards affect the occurrence of condensation within buildings?
- Double-glazed windows can create a more ‘sealed’ building that lacks adequate ventilation
- Trickle vents in windows (where present) are often kept closed
- Balanced flue boilers (instead of open fires) reduce natural ventilation
- Central heating systems are often used intermittently, meaning cold surfaces may coincide with high humidity levels
What problems are associated with condensation?
- Mould growth, particularly where RH remains above 70% for long periods (usually more than 12 hours)
- Health risks to the elderly, young children, asthmatics and those with weakened immune systems
- Can encourage timber decay where timbers are sublect to prolonger moisture exposure
How would you identify condensation within a building?
- Wall has a ‘misty’ surface
- Stains or streaks of water runnin gdown a wall (particularly in bathrooms, kitchens and below windows)
- Damp patches with no definitive edges
- Dampness behind wall cupboards or inside wardrobes against external walls (areas where air circulation is restricted)
- Localised dampness at potential ‘cold bridges’
- Patches of mould growth
- Humidity (measured using a hygrometer), insulation and ventilation levels as well as heating and living patterns must also be taken into account
What steps would you recommend to eliminate condensation?
- Short term - mould-affected areas can be washed using a fungicidal or bleach solution - do not use water as this will just spread the mould
- Long term - combination of the following:
- Reduce moisture generation (lids on pans, dry clothes outside, vent tumble dryers externally, do not use paraffin or bottled gas heaters, put cold water in bath before hot)
- Increase ventilation to remove moisture-laden air (open trickle vents, open windows, mechanical ventilation)
- Increase air temperature by heating - warmer air can hold more water vapour without condensing
- Increase surface temperature by thermal insulation (external or internal)
What is interstitial condensation and how would you deal with it?
- Interstitial condensation is condensation that occurs within the structure of an element, as opposed to on its surface
- For example inside a flat roof on the roof deck.
- Check factors affecting this- blocked ventilation, lack of vapour barrier
- Check timbers for signs of rot and repalce/treat with preservative as necesary
- Additional insulationad
- unblock ventialtion
- vapour barrier membrane on WARM SIDE of the insulation
- If it occurs in a material such as timber, action will be needed to prevent risk of rot, usually by installing additional wall insulation
- If internal inuslation is added, a vapour barrier must be provided on the warm side of the insulation to act as a barrier to interstitial condensation
What are the health risks associated with the presence of mould in buildings?
- Moulds produce allergens, irritants and sometimes toxic substances, so inhaling or touching mould spores can cause allergic reactions (e.g. sneezing, runny nose, red eyes, skin rash etc.) and respiratory problems (e.g. asthma attacks)
- There is contradicting research that certain toxigenic moulds can cause rare health conditions such as bleeding in the lungs - research is ongoing
To avoid mould growth, what level should the relative humidity be kept under?
70%
What guidance is available in relation to condensation in buildings?
BS 5250:2011 - Code of practice for control of condensation in buildings
What is penetrating damp?
Water that ingresses through the structure of a building
What are the different mechanisms of water ingress relating to penetrating dampness?
- Gravity
- Capillary action
- Surface tension
- Kinetic energy (splashing)
- Wind force
- Differential air pressure (inside and out)
What are the common causes and routes of penetrating dampness?
- Slipped roof tiles
- Inadequate chimney/parapet flashing
- Copings without drips or not bedded on DPCs
- Leaking gutters (lack of correct support, damaged joints, lack of regular clearing)
- Overflowing hopper heads
- Leaking downpipes (broken joints, rusting cast iron downpipes to rear against wall)
- Continuously running cistern overflows (not discharging water clear of wall)
- Blocked gulleys (resulting in water splashing against wall)
- Cracked render or movement cracks in brickwork
- Defective pointing (recessed joints that could lead to frost action)
- Cavity ties (upside down so drip ineffective, mortar droppings resting on ties, uneven courses resulting in ties sloping toward inner leaf)
- Inadequately fixed cavity insulation boards causing bridging of the cavity from outer to inner leafy
- Poorly fitted windows and doors
- Sills without drips
- Poorly designed thresholds
- Missing vertical DPCs
- Driving rain on solid walls in particularly exposed situations (on cliff sides/west side of the Pennines)
- Vegetation growth to damp/shaded brickwork
- Repointing older buildings with cement mortar - stronger but less breathable than lime mortar, reducing the rate of evaporation. Also not as flexible so more likely to crack through drying shrinkage, thus allowing water ingress
- Applying strong external renders (for the same reasons as using strong mortars)
- Walls built in denser materials (e.g. granite or engineering bricks) - lack the ability to absorb water so most water runs down the face, which makes minor cracks more critical in allowing moisture ingress. Evaporation is also restricted
What problems are associated with penetrating damp?
- Lead to outbreaks of dry or wet rot under the right conditions
- Reduce the strength of building materials, such as chipboard and plasterboard
- Cause chemical reactions in building components (e.g. sulphate attack)
- Reduce the effectiveness of insulation
- Damage decorations
How would you identify penetrating damp within a building?
- Distinct damp patches with well-defined edges
- Often in localised areas
- Moisture readings show sharp change from wet to dry
- Patches of efflorescence (crystallisation of sulphates and carbonates present in building materials)
- Timber in area of damp has high moisture content
- External inspection may reveal obvious defects (e.g. cracked render/brickwork, damaged downpipes etc.)
- Deep wall probes indicate high readings in centre of wall
- Line of dampness on internal plasterwork corresponding with mortar joints where cement mortar/dense wall materials have been used
- Measure wall temperature, air temperature and RH to eliminate condensation
- Salt analysis shows zero level of nitrates and chlorides, eliminating rising damp
What steps would you recommend to eliminate penetrating damp?
- Identify the source of penetrating damp and remove or provide a barrier, for example:
- Replace defective rainwater goods
- Introduce DPCs beneath copings or vertical DPCs around openings
- Repair cracked render/brickwork
- Unblock gulleys/rainwater goods
- Replace poorly installed cavity ties
- Application of a hydrophobic masonry paint (repels water but still allows the wall to breathe) may also be beneficial for solid walls that experience excessive rain penetration
What is rising damp and how is it caused?
- Ground water rising by capillary action through pores of the wall or floor material
- Causes:
- Lack of DPC/DPM
- Inadequate lapping of DPC/DPM
- Bridging of an existing DPC/DPM (often by external rendered finishes or raised ground levels)
- DPC/DPM failure through natural deterioration or damage caused by building movement
- Splashing from rain or downpipes where DPC is less than the recommended 150mm above ground (Approved Document C)
- Increase in ground water levels (e.g. man-induced changes to the water table, leaking drains, blocked land drainage systems, leaking water mains and springs) - likely if rising damp occurs fairly suddenly
What height can rising damp reach and what factors can affect this?
- Rarely higher than 1.5m
- Depends on:
- Supply of water
- Pore structure of materials
- Rate of evaporation
- Heating within building
- Chemicals in ground and walls - efflorescence can block capillaries through which water evaporates, thus driving water further up the wall
What problems are associated with rising damp?
- Lead to outbreaks of dry or wet rot under the right conditions
- Reduce the strength of building materials, such as chipboard and plasterboard
- Reduce the effectiveness of insulation
- Damage decorations
How would you identify rising damp within a building?
- Visual inspection of possible causes (lack of / bridging of DPC/DPM etc.)
- Characteristic tide mark that does not extend beyond the lower part of the wall
- Damp contours can be pinpointed with a moisture meter
- Damp limited to usually 1m-1.5m above ground and readings above peak will drop quickly
- High percentage of moisture content in timber skirtings
- Salt analysis using a calcium carbide meter determines a high level of nitrates and chlorides, which are contained naturally in the subsoil
- Areas of dampness appear to get wetter in humid conditions due to hygroscopic salts (nitrates and chlorides) brought up from the ground, which attract water in from the atmosphere
- BRE Digest 245 (Rising Damp in Walls - Diagnosis and Treatment) contains detailed guidance on rising damp identification and remediation
What steps would you recommend to eliminate rising damp?
- Identify the source of rising damp and remove or provide a barrier, for example:
- Lower the ground level (where DPC is breached or ground level is not 150mm below DPC)
- Repair leaking drains/water mains
- Unblock land drainage systems
- Replace DPC or provide new where non-existent
- Replace plaster/finishes where hygroscopic salts may still be present
- Renew to height 300mm above level of rising damp
- Re-plaster with a cement-based plaster (not gypsum plaster as most of these cannot prevent the passage of hygroscopic salts and quickly breakdown in wet conditions), preferably containing a waterproofer or salt inhibitor
What different methods of installing/replacing DPCs are available?
Installation of any type of DPC must be by a reputable company and members of the British Wood Preserving and Damp-Proofing Association (BWPDA) offering an insurance backed guarantee for the works:
- Physical replacement - more expensive, disruptive and can only be laid on horizontal course (not suitable for rubble walls)
- Chemical injection - cheaper, lines the pores with a water-repellent solution (usually silicone/latex) however effectiveness depends on its successful penetration of the wall and lack of ‘viscous fingering’ (chemical spreads out and does not form a continuous barrier)
- Electro Osmotic DPC - a titanium wire (anode) is secured around the wall at DPC level and connected to a small power supply, which causes moisture molecules to repel down the wall back into the ground (not supported by the BRE)
What guidance is available for rising damp problems?
- BRE Digest 245 (Rising Damp in Walls - Diagnosis and Treatment) - rising damp identification and remediation
- BS 6576:2005 - Code of practice for diagnosis of rising damp in walls of buildings and installation of chemical DPCs
What methods can be used to minimise dampness within a basement?
- Dense Monolithic Concrete
- Cementitious Tanking
- Mastic Asphalt Tanking
- Bund Wall System
- Drained Cavity System
How is dense monolithic concrete used to waterproof a basement and what are its main disadvantages?
- Walls and floor are constructed from high quality dense monolithic concrete to form a watertight barrier
- May not always be water vapour proof so some form of lining (tanking) may be required
- Only applies to new-builds
What is the difference between cementitious tanking and mastic asphalt tanking?
Cementitious Tanking:
- Certain additives are added to a cement based medium and applied to the base slab and walls
- Not very good at withstanding substantial levels of hydrostatic pressure
Mastic Asphalt Tanking:
- Provides a continuous waterproof membrane applied to the base slab and walls
- Can be applied internally or externally depending on the circumstances on site (i.e. external may not be possible in existing buildings)
- Membrane needs additional protection by building an inner skin wall backfilled to keep the membrane adhered to the earth-retaining wall
What are the disadvantages of using tanking as a method of waterproofing a basement?
- Water is not drained, merely pushed to other areas around the structure, which could cause problems elsewhere
- Only external tanking will protect the structure from aggressive sulphates that may be present in the surrounding soil, which is not always possible
What is a bund wall and how can it be used as a method of waterproofing a basement?
- Construction of an inner non-load bearing wall to form a cavity joined to special triangular tiles laid to falls
- This enables moisture to collect in the cavity and drain away into a sump, where it can be pumped into the surface water drainage system
- Cavity should be ventilated
What is a drained cavity system and how can it be used as a method of waterproofing a basement?
- Plastic membrane in an egg-crate type formation applied to the wall and floor with properly bonded overlap joints in one continuous system
- Allows air and moisture to circulate and drains water into a sump, where it can be pumped into the surface water drainage system
What guidance is available for waterproofing basements?
BS 8102:2009 ‘Protection of Below Ground Structures against Water from the Ground’
How would you differentiate between rising damp and penetrating damp?
For rising damp:
- Positive salt analysis (containing nitrates and/or chlorides)
- Visible tide mark to lower part of wall
- Limit of dampness usually 1m-1.5m above ground
- Moisture readings quickly drop above tide mark
- Areas of dampness appear to get wetter in humid conditions (due to hygroscopic salts drawn up from the ground)
- External inspection may indicate missing or bridged DPC
How would you differentiate between penetrating damp and condensation?
For penetrating damp:
- Moisture content is usually localised/isolated
- Moistures readings identify an epicentre of the water entry
- High moisture content within fabric of element, not just on its surface
- Evidence of an external defect (e.g. wall cracking, defective downpipe etc.)
- Measure wall temperature, air temperature and RH to eliminate condensation
- Mould growth unusual
How would you differentiate between rising damp and condensation?
For condensation:
- Water is usually on the wall face when wiped with hand
- Negative salt analysis (no nitrates or chlorides)
- Moisture readings may occur across the full height of a wall (although higher readings at lower levels as warm air rises so less chance of condensation)
- Moisture content of skirting normal but may contain staining due to water run-off
- Deep wall probes indicate low readings in centre of wall
- Mould growth likely
- Surface temperature is below dew point temperature (established by measuring air temperature and RH)
What are hygroscopic salts and are they problematic?
- Salts that absorb moisture in from the air
- As they absorb water, they continually re-dissolve, which prevents any crystallisation
- Often associated with rising damp, as nitrates and chlorides (both of which are hygroscopic) naturally present in sub-soil are drawn up through the wall with water and are left behind on the surface once the water evaporates, causing surfaces to become wetter from moisture in the air as well
What is efflorescence and is it problematic?
- Temporary white powdery substance often seen on the face of new brickwork and in cases of rising damp
- Caused by sulphates and carbonates naturally present in building materials crystallising as water evaporates, due to their relatively insoluble nature
- These salts are not hygroscopic and merely indicate that moisture is evaporating from the structure
- They may only be problematic in cases of rising damp, as the crystals can sometimes block pores in brickwork, thus preventing evaporation and driving damp higher up the wall
- Can be brushed off if appearance is causing a concern
What is cryptoflorescence and is it problematic?
- Crystallisation of salts (often magnesium) below the surface of the brick
- Can cause spalling where old, relatively weak bricks are re-used inappropriately, particularly in areas of excessive dampness
- Can also occur through salts deposited by the run-off from limestone or from air pollution
- Damage can also occur where bricks are covered by a surface treatment (as salts may crystallise behind it)
- The effect on the bricks is similar to that caused by frost attack
- water penetration poorly sealed joints missing gaskets
How do you test to establish if service pipes are leaking?
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What is the adverse effect of well-insulated buildings?
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When carrying out a survey of a Victorian house, what potential pathology issues could lead to damp problems in the building?
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What problems are associated with vegetation growth to damp/shaded brickwork?
Can retain moisture and cause pentrating dampness
What measures should be adopted to repair a property affected by flooding?
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What would be required from a damp specialist before you recommended them?
Must be by a reputable company and members of the British Wood Preserving and Damp-Proofing Association (BWPDA) offering an insurance backed guarantee for the works
What are the main defects typically associated with timber?
- Wood-rotting fungi
- Wood-boring insects
What is the average moisture content for internal and external timber?
- Internal timber: 2-16% (depending on level of heating within building)
- External timber: +20% (depending on weather conditions)
What is dry rot and how is it caused?
Type of brown rot that thrives and spreads rapidly in damp buildings and caused by a certain type of fungus present in the air latching onto damp timber to use as a food source under specific conditions, namely:
- Moisture content is between 20-35%
- Temperature is between 0-26°C
- Space is not ventilated
What is the Latin name for dry rot and what is its meaning?
Serpula Lacrymans (literal meaning: ‘creeping tears’)
What is the life cycle of dry rot?
- Spores - microscopic fungus spores omnipresent in the air land on timber surfaces
- Germination - if the timber is damp, the spores germinate (grow)
- Hyphae - upon germination, the spores begin to grow fine white strands (not unlike cobwebs) known as hyphae. Hyphae reach out in search of moisture and it is through this process that it feeds on the timber and causes decay
- Mycelium - a mass of hyphae forms, known as mycelium, which continues to feed on the organic matter and is capable of spreading over large distances, including through masonry and plaster
- Fruiting body - a sporophore within the mycelium develops which thrives on the moisture being brought back to it from the hyphae. The fruiting body releases further spores that travel on air currents to other susceptible areas for the lifecycle to start over
What problems are associated with dry rot?
- Timber becomes dry and crumbly
- Reduces structural integrity of timber
- Can spread through an entire building given the right conditions
- ‘Softened’ timber becomes more easily attacked by wood-boring insects
How would you identify dry rot?
- Decayed wood has dull brown colour with deep cuboidal cracking along and across the grain, light in weight and can crumble between fingers
- The rot has left no skin of sound wood
- Hyphal strands are white/grey in colour and 2-8mm thick
- Silk-white sheets or cotton wool-like mycelium
- Rusty red coloured spores
- Reddish brown fruiting body with grey/white edges, usually pancake or bracket-like in shape
- Conditions (such as lack of ventilation, moisture content of timber etc.) should also be used in identification
What steps would you recommend to remediate dry rot?
Set out in BRE 299 (Dry Rot: Recognition and Control):
- Establish the size and significance of the attack, particularly if structural timbers are affected as measures may be needed to secure structural integrity
- Locate and eliminate sources of moisture
- Promote rapid drying of the structure through heating and ventilation
- Introduce support measures (e.g. ventilation pathways between sound timber and wet brickwork, barriers such as DPMs or joist hangers etc.)
- Remove all rotted wood and cut away timber 300-450mm beyond last evidence of rot
- Do not retain timber infected by dry rot without seeking expert advice
- Strip back affected plaster and contain fungus within wall by applying surface biocides or fungicidal paints/renders
- Apply localised superficial preservative treatment only to timbers that are likely to remain damp
- Replace any timbers necessary only with preservative pre-treated timbers
What is wet rot and how is it caused?
Type of white or brown rot that thrives in wetter conditions (mainly external joinery) and is caused by a certain type of fungus present in the air latching onto the damp timber to use as a food source
What problems are associated with wet rot?
- Reduces structural integrity of timber
- ‘Softened’ timber becomes more easily attacked by wood-boring insects
How would you identify wet rot?
- Where species is a white rot, wood becomes lighter (as if bleached) and cracked along the grain
- Where species is a brown rot, wood becomes darker with cuboidal cracking (but not as severe as that found in dry rot)
- A thin veneer of sound wood remains
- Often found where wood is repeatedly wetted (e.g. as a result of faulty plumbing or leaking gutters)
- Hyphae, mycelium and fruiting bodies differ between species, however typically:
- Hyphae (not always present) is thinner than dry rot and flexible when dry, usually creamy-white in colour
- Mycelium (usually not present in daylight areas) is generally creamy-brown in colour
- Fruiting body is not very common but where present is usually flat and plate-like with a greenish-brown centre and yellow margins
What steps would you recommend to remediate wet rot?
Set out in BRE 345 (Wet Rot: Recognition and Control):
- Establish the size and significance of the attack, particularly if structural timbers are affected as measures may be needed to secure structural integrity
- Locate and eliminate sources of moisture
- Promote rapid drying of the structure through heating and ventilation
- Introduce support measures (e.g. ventilation pathways between sound timber and wet brickwork, barriers such as DPMs or joist hangers etc.)
- Remove all rotted wood, however there may be occasions when it can be retained (e.g. large beams or where there are conservation considerations)
- Where retained, deeply penetrating preservative treatments should be applied to rotten areas in conjunction with other repairs (e.g. resin bonding systems)
- Apply localised superficial preservative treatment only to timbers that are likely to remain damp
- Replace any timbers necessary only with preservative pre-treated timbers
What is the difference between dry and wet rot?
- Location:
- Dry rot - rare outside
- Wet rot - stays localised to moisture source
- Type:
- Dry rot - a brown rot
- Wet rot - can either be a white or brown rot
- Hyphae:
- Dry rot - 2-8mm in and brittle when dry
- Wet rot - thinner and flexible when dry
- Mycelium:
- Dry rot - silky white cotton wool-like sheets
- Wet rot - brown branching strands
- Decaying wood:
- Dry rot - deep cuboidal cracking with no skin of sound wood
- Wet rot - smaller cuboidal cracking and a thin veneer of sound wood remains
- Fruiting body:
- Dry rot - reddish brown with grey/white edges
- Wet rot - not very common but usually greenish-brown centre with yellow margins
- Conditions for growth:
- Dry rot - 20-35% timber moisture content, 0-26°C
- Wet rot - 45-60% timber moisture content, -30 to +40°C
How would you identify whether rot was wet or dry?
- Location - if outside, likely to be wet rot
- Conditions - if space is unventilated, could be dry rot
- Appearance - deep cuboidal cracking, no skin of sound wood, rusty red spores, fruiting body and mycelium would indicate dry rot
- Smell - mushroom smell would indicate dry rot
- Moisture content - dry rot would be between 20-35%, wet rot would be 45-60%
What is the difference between brown rot and white rot?
- White rots - cause wood to become lighter in colour and fibrous in texture, without cross-cracking
- Brown rots - cause wood to become darker in colour and crack along and across the grain, forming cubes in the decaying process. When dry, very decayed wood will crumble to dust
Timber decay is often found to suspended timber floors in traditionally constructed buildings. What does the occupier commonly do to the property to lead to this problem?
- Often block the air bricks
- Inadequate ventilation will allow the RH of the air below the floor to rise (particularly if the ground is wet), as the air steadily absorbs water
- At the same time, the moisture content of the floor structure will slowly settle into equilibrium with the damp air, causing its moisture content to rise, thus putting it at risk of decay
Is there a potential for wet rot turning to dry rot when drying out?
???
How would you treat a rotting timber fence?
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Your client owns a Grade I listed building and has found evidence of extensive dry rot. How might this be dealt with to cause minimum disturbance to the building?
- Splice repairs - rotten timber is removed and reclaimed timber sections are joined to the existing timber (often incorporating internal reinforcement rods) where required
- Resin bonding systems - used for localised repairs (e.g. window frames) where the rotten timber is removed and a 2-part epoxy resin is mixed and applied to the timber, effectively ‘filling in’ the voids left by the rotten timber, hardening and sanded to a smooth finish
- Introducing supplementary structures - allows existing timbers to be left in place whilst new supplementary supports perform its function (e.g. doubling up timber, fixing discrete metal plates etc.)
What are wood-boring insects and why are they a problem?
- Wood-boring insects are insects that use wood as a food source, as well as a habitat, and can cause damage to timbers within buildings by eating away at it
- Can get into buildings through open windows, doors, fresh-air vents, gaps in eaves etc.
What is the general lifecycle of a wood-boring insect?
- Larva (worm): 1-5 years
- Pupa (larva to adult): 6 weeks
- Adult (beetle): 2-3 weeks
Name some of the common wood-boring insects.
- Common furniture beetle (aka ‘woodworm’)
- Deathwatch beetle
- House longhorn beetle
- Lyctus powderpost beetle
- Ptilinus beetle
What are the two most common wood-boring insects?
- Common furniture beetle (aka ‘woodworm’)
- Deathwatch beetle
What are the key indicators to identify different wood-boring insects?
BRE Digest 307 (Identifying Damage by Wood-Boring Insects) contains extensive guidance, including:
- Size of flight holes
- Bore dust (frass) colour
- Geographical location
- Type of wood attacked
Where is the Longhorn beetle most known to attack?
Only common in the south-east, particularly Surrey
Name some harmless insects to timber.
- Woodlice
- Silver fish
- Ants
- Earwigs
- Millipedes
What works should be undertaken to eliminate and deal with the effects of wood-boring insects?
- Depends on the type of insect and the extent of the attack
- Often down to a specialist company
- Usually involves treatment with an organic solvent, emulsion or paste (or sometimes smoke for the deathwatch beetle)
- Sometimes when wood is so wet and rotten, preservative treatment is pointless and timbers require replacing
- New timbers may need to be placed alongside affected timbers, or completely replaced depending on the extent of the damage
What changed within the building regulations to deal with a certain wood-boring insect?
House longhorn beetle - Approved Document A prescribes geographical areas where softwood timber for roof construction must be treated against infestation
When must the BRE be notified of attacks from wood-boring insects?
BRE must be notified of every house longhorn beetle attack
What are the main defects typically associated with timber?
- Wood-rotting fungi
- Wood-boring insects
What is the average moisture content for internal and external timber?
- Internal timber: 2-16% (depending on level of heating within building)
- External timber: +20% (depending on weather conditions)
What is dry rot and how is it caused?
Type of brown rot that thrives and spreads rapidly in damp buildings and caused by a certain type of fungus present in the air latching onto damp timber to use as a food source under specific conditions, namely:
- Moisture content is between 20-35%
- Temperature is between 0-26°C
- Space is not ventilated
What is the Latin name for dry rot and what is its meaning?
Serpula Lacrymans (literal meaning: ‘creeping tears’)
What is the life cycle of dry rot?
- Spores - microscopic fungus spores omnipresent in the air land on timber surfaces
- Germination - if the timber is damp, the spores germinate (grow)
- Hyphae - upon germination, the spores begin to grow fine white strands (not unlike cobwebs) known as hyphae. Hyphae reach out in search of moisture and it is through this process that it feeds on the timber and causes decay
- Mycelium - a mass of hyphae forms, known as mycelium, which continues to feed on the organic matter and is capable of spreading over large distances, including through masonry and plaster
- Fruiting body - a sporophore within the mycelium develops which thrives on the moisture being brought back to it from the hyphae. The fruiting body releases further spores that travel on air currents to other susceptible areas for the lifecycle to start over
What problems are associated with dry rot?
- Timber becomes dry and crumbly
- Reduces structural integrity of timber
- Can spread through an entire building given the right conditions
- ‘Softened’ timber becomes more easily attacked by wood-boring insects
How would you identify dry rot?
- Decayed wood has dull brown colour with deep cuboidal cracking along and across the grain, light in weight and can crumble between fingers
- The rot has left no skin of sound wood
- Hyphal strands are white/grey in colour and 2-8mm thick
- Silk-white sheets or cotton wool-like mycelium
- Rusty red coloured spores
- Reddish brown fruiting body with grey/white edges, usually pancake or bracket-like in shape
- Conditions (such as lack of ventilation, moisture content of timber etc.) should also be used in identification
What steps would you recommend to remediate dry rot?
Set out in BRE 299 (Dry Rot: Recognition and Control):
- Establish the size and significance of the attack, particularly if structural timbers are affected as measures may be needed to secure structural integrity
- Locate and eliminate sources of moisture
- Promote rapid drying of the structure through heating and ventilation
- Introduce support measures (e.g. ventilation pathways between sound timber and wet brickwork, barriers such as DPMs or joist hangers etc.)
- Remove all rotted wood and cut away timber 300-450mm beyond last evidence of rot
- Do not retain timber infected by dry rot without seeking expert advice
- Strip back affected plaster and contain fungus within wall by applying surface biocides or fungicidal paints/renders
- Apply localised superficial preservative treatment only to timbers that are likely to remain damp
- Replace any timbers necessary only with preservative pre-treated timbers
What is wet rot and how is it caused?
Type of white or brown rot that thrives in wetter conditions (mainly external joinery) and is caused by a certain type of fungus present in the air latching onto the damp timber to use as a food source
What problems are associated with wet rot?
- Reduces structural integrity of timber
- ‘Softened’ timber becomes more easily attacked by wood-boring insects
How would you identify wet rot?
- Where species is a white rot, wood becomes lighter (as if bleached) and cracked along the grain
- Where species is a brown rot, wood becomes darker with cuboidal cracking (but not as severe as that found in dry rot)
- A thin veneer of sound wood remains
- Often found where wood is repeatedly wetted (e.g. as a result of faulty plumbing or leaking gutters)
- Hyphae, mycelium and fruiting bodies differ between species, however typically:
- Hyphae (not always present) is thinner than dry rot and flexible when dry, usually creamy-white in colour
- Mycelium (usually not present in daylight areas) is generally creamy-brown in colour
- Fruiting body is not very common but where present is usually flat and plate-like with a greenish-brown centre and yellow margins
What steps would you recommend to remediate wet rot?
Set out in BRE 345 (Wet Rot: Recognition and Control):
- Establish the size and significance of the attack, particularly if structural timbers are affected as measures may be needed to secure structural integrity
- Locate and eliminate sources of moisture
- Promote rapid drying of the structure through heating and ventilation
- Introduce support measures (e.g. ventilation pathways between sound timber and wet brickwork, barriers such as DPMs or joist hangers etc.)
- Remove all rotted wood, however there may be occasions when it can be retained (e.g. large beams or where there are conservation considerations)
- Where retained, deeply penetrating preservative treatments should be applied to rotten areas in conjunction with other repairs (e.g. resin bonding systems)
- Apply localised superficial preservative treatment only to timbers that are likely to remain damp
- Replace any timbers necessary only with preservative pre-treated timbers
What is the difference between dry and wet rot?
- Location:
- Dry rot - rare outside
- Wet rot - stays localised to moisture source
- Type:
- Dry rot - a brown rot
- Wet rot - can either be a white or brown rot
- Hyphae:
- Dry rot - 2-8mm in and brittle when dry
- Wet rot - thinner and flexible when dry
- Mycelium:
- Dry rot - silky white cotton wool-like sheets
- Wet rot - brown branching strands
- Decaying wood:
- Dry rot - deep cuboidal cracking with no skin of sound wood
- Wet rot - smaller cuboidal cracking and a thin veneer of sound wood remains
- Fruiting body:
- Dry rot - reddish brown with grey/white edges
- Wet rot - not very common but usually greenish-brown centre with yellow margins
- Conditions for growth:
- Dry rot - 20-35% timber moisture content, 0-26°C
- Wet rot - 45-60% timber moisture content, -30 to +40°C
How would you identify whether rot was wet or dry?
- Location - if outside, likely to be wet rot
- Conditions - if space is unventilated, could be dry rot
- Appearance - deep cuboidal cracking, no skin of sound wood, rusty red spores, fruiting body and mycelium would indicate dry rot
- Smell - mushroom smell would indicate dry rot
- Moisture content - dry rot would be between 20-35%, wet rot would be 45-60%
What is the difference between brown rot and white rot?
- White rots - cause wood to become lighter in colour and fibrous in texture, without cross-cracking
- Brown rots - cause wood to become darker in colour and crack along and across the grain, forming cubes in the decaying process. When dry, very decayed wood will crumble to dust
Timber decay is often found to suspended timber floors in traditionally constructed buildings. What does the occupier commonly do to the property to lead to this problem?
- Often block the air bricks
- Inadequate ventilation will allow the RH of the air below the floor to rise (particularly if the ground is wet), as the air steadily absorbs water
- At the same time, the moisture content of the floor structure will slowly settle into equilibrium with the damp air, causing its moisture content to rise, thus putting it at risk of decay
Is there a potential for wet rot turning to dry rot when drying out?
???
How would you treat a rotting timber fence?
???
When must the BRE be notified of attacks from wood-boring insects?
BRE must be notified of every house longhorn beetle attack
What changed within the building regulations to deal with a certain wood-boring insect?
House longhorn beetle - Approved Document A prescribes geographical areas where softwood timber for roof construction must be treated against infestation
What works should be undertaken to eliminate and deal with the effects of wood-boring insects?
- Depends on the type of insect and the extent of the attack
- Often down to a specialist company
- Usually involves treatment with an organic solvent, emulsion or paste (or sometimes smoke for the deathwatch beetle)
- Sometimes when wood is so wet and rotten, preservative treatment is pointless and timbers require replacing
- New timbers may need to be placed alongside affected timbers, or completely replaced depending on the extent of the damage
Name some harmless insects to timber.
- Woodlice
- Silver fish
- Ants
- Earwigs
- Millipedes
Where is the Longhorn beetle most known to attack?
Only common in the south-east, particularly Surrey
What are the key indicators to identify different wood-boring insects?
BRE Digest 307 (Identifying Damage by Wood-Boring Insects) contains extensive guidance, including:
- Size of flight holes
- Bore dust (frass) colour
- Geographical location
- Type of wood attacked
What are the two most common wood-boring insects?
- Common furniture beetle (aka ‘woodworm’)
- Deathwatch beetle
Name some of the common wood-boring insects.
- Common furniture beetle (aka ‘woodworm’)
- Deathwatch beetle
- House longhorn beetle
- Lyctus powderpost beetle
- Ptilinus beetle
What is the general lifecycle of a wood-boring insect?
- Larva (worm): 1-5 years
- Pupa (larva to adult): 6 weeks
- Adult (beetle): 2-3 weeks
What are wood-boring insects and why are they a problem?
- Wood-boring insects are insects that use wood as a food source, as well as a habitat, and can cause damage to timbers within buildings by eating away at it
- Can get into buildings through open windows, doors, fresh-air vents, gaps in eaves etc.
Your client owns a Grade I listed building and has found evidence of extensive dry rot. How might this be dealt with to cause minimum disturbance to the building?
- Splice repairs - rotten timber is removed and reclaimed timber sections are joined to the existing timber (often incorporating internal reinforcement rods) where required
- Resin bonding systems - used for localised repairs (e.g. window frames) where the rotten timber is removed and a 2-part epoxy resin is mixed and applied to the timber, effectively ‘filling in’ the voids left by the rotten timber, hardening and sanded to a smooth finish
- Introducing supplementary structures - allows existing timbers to be left in place whilst new supplementary supports perform its function (e.g. doubling up timber, fixing discrete metal plates etc.)
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.
???
You see a vertical crack internally to a property’s wall - what may have caused it?
???
What would cracking at DPC level indicate?
???
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?
???
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?
???
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
What is heave?
Upward movement of a building foundation caused by the expansion or swelling of the subsoil
What are the common causes of heave?
- The removal of trees on shrinkable (cohesive) soils
- The freezing of ground water in frost-susceptible soils
How can trees cause heave?
- The ground slowly regains moisture that was once taken up by the tree roots
- Can last up to 10 years so care is needed when building on sites soon after trees have been removed
How can freezing ground water cause heave?
- After a period of high rainfall, water fills the voids between the particles and in freezing weather expands as it turns to ice
- Additional damage can be caused when the ice thaws and the ground settles
- Worse if ground has a high water table
What is settlement?
- Natural compaction of soil due to the load imposed by the building
- Happens in all buildings and occurs soon after construction
- Only problematic if it is differential (i.e. happens in different parts at different times, perhaps due to variations in ground conditions or using old/different foundation depths)
Explain the process of diagnosing ground movement cracking.
- Inspect the cracks - location, size, direction, age
- Investigate the site - ground profile, soil type, tree proximity, drains testing, mining area
- Determine the cause of the cracks from steps 1 and 2
- Begin remedial action - alleviate cause, provide underpinning if necessary
- Monitor cracks - minimum 18 months
- Assess action for trees (if necessary) with an arboriculturalist
What features of a crack may indicate that it has been caused by ground movement?
- Extends above and below the DPC
- Affects both internal and external surfaces
- Diagonal in direction (stepped along brickwork)
- Tapered
How does the width of a crack help determine which type of ground movement has caused it?
- Heave - cracks are wider at the top and narrower at the bottom (both the cracks themselves and their location on the building)
- Subsidence - cracks are narrower at the top and wider at the bottom (both the cracks themselves and their location on the building)
- Local subsidence - cracks are wider at the top and narrower at the bottom
Which tree types extract the most water from the ground?
Not all trees present the same risk, however the species that extract the most water are poplar, willow, oak, elm, horse chestnut and sycamore
As a general rule, how far away should trees be from buildings?
- General rule - tree should be at least its full mature height away from a building and one and a half times when part of a group/row
- However, this does not necessarily mean they should be removed if closer, especially mature trees
What RICS guidance is available in relation to ground movement and what information does it contain?
Subsidence in Relation to Insurance Claims GN:
- Guidance for professionals involved in subsidence claims, including:
- Causes of subsidence, heave and landslip
- Causes of other building fractures
- Policy cover details
- Handling claims
- Repair techniques
- Recoveries and contribution
- Relevant voluntary agreements
What is underpinning?
Process of strengthening the foundation of an existing structure by transferring the load carried by the foundation from its existing bearing level to a new level at a lower depth
Explain the sequence of work involved in underpinning a strip foundation.
- The traditional method of underpinning involves the soil beneath the existing foundation being excavated and replaced in phases with foundation material, normally concrete
- Bays of approximately 1500mm long are excavated in a strategic order so there are never adjacent bays being worked on at the same time
Detail some alternative methods of underpinning.
- Jack pile underpinning:
- Used where depth required is too deep for traditional underpinning
- Precast concrete pile is hydraulically jacked beneath existing foundation
- Existing foundation must be in good condition as it must rest on the pile caps once the pile has been inserted
- Needle and pile underpinning:
- Used where condition of existing foundation is unsuitable for traditional or jack piling
- Reinforced concrete beams (aka ‘needles’) are connected to small diameter bored piles which take the load
- Can be cantilevered where access beneath existing foundation is restricted
- Expanding foam:
- Relatively modern technique where foam is injected into foundations, filling any voids and solidifying
- No excavation required
What guidance is available in relation to underpinning?
BRE Digest 352 (Underpinning) contains further information
What is cavity wall tie failure and how is it caused?
Cavity wall tie failure is when the wall ties fail to tie the two leaves together, which can be caused by:
- Rusting of the metal ties (due to water penetration)
- Poor quality mortar reducing the bond
- Not installing the correct number of ties
What is black ash mortar and how can it be problematic when used in cavity wall construction?
The use of black ash mortar (a product of coal mining which was sometimes added to mortar to give it a black colour) in Victorian buildings can exacerbate the problem of wall tie failure, as high sulphur content in the black ash produces sulphuric acid when wet for long periods, which can corrode the wall ties
What problems are associated with cavity wall tie failure?
- Cracking where rust has caused the ties to expand, which can in turn lead to damp penetration and further accelerate the rusting process
- Bowing/bulging walls
- Damage to the roof as the external leaf increases in height
- Extensive expansion can cause rotation in the foundation as the loads from the roof can be transferred down the outer leaf
- In severe cases, collapse of the outer leaf
How would you identify cavity wall tie failure?
- Horizontal cracking usually every 6 courses / 450mm, however less bulky wall ties (e.g. butterfly wire ties) will not generally produce enough expansion to induce cracking, therefore failure may occur without any outwardly visible signs
- Intrusive inspection using a boroscope may also be used to physically inspect the cavity
- Brickwork may also be removed to physically inspect cavity
What steps would you recommend to remediate cavity wall tie failure?
- Remove old ties
- Drill in new ties through the centre of the brickwork
- Repoint/make good brickwork
What different methods of replacing cavity wall ties are available?
- Resin/grouted - pre-drilled holes are filled with resin before new ties are pushed in, then more resin injected through the tie to fill around the end in the inner leaf (not suitable for porous masonry)
- Mechanical - features a sleeve that expands when the tie is screwed up (only suitable when masonry is in a good condition)
- Helical - long corkscrew with a wide thread is drilled trough both leaves
What guidance is available for replacing cavity wall ties?
BRE 329 (Inserting Wall Ties in Existing Construction) contains further remedial guidance
How were the relevant British Standards changed to address cavity wall tie failure?
- In 1945, a British Standard was introduced that provided a minimum thickness for the galvanised layer
- In 1968, the minimum thickness was reduced because it was thought the original thickness was excessive
- In 1981, the minimum thickness was increased again
What is carbonation and how is it caused?
- Carbonation is the process by which carbon dioxide slowly penetrates concrete and dissolves in water present within its pores, forming a mildly carbolic acidic solution
- This acidic solution reacts with the alkaline calcium hydroxide (one of the components of concrete) to form calcium carbonate
- This results in a pH drop, reducing the alkalinity of the concrete (from more than 12.5 to approximately 8.5)
- This carbonation process progressively moves through the concrete over time
What problems are associated with carbonation?
- The passive layer around reinforcing steel will deteriorate when the pH falls below 10.5
- Therefore, once carbonation reaches any steel, the concrete is insufficiently alkaline to protect the steel’s passive layer and thus becomes ‘active’ (aka depassivation)
- Moisture and oxygen ingressing through the porous concrete can now react with the steel, which may begin to rust and corrode
- If this happens, the steel will expand, which can cause cracking and spalling of the concrete cover, thus compromising its structural integrity
- This also makes it easier for aggressive agents to ingress towards the steel, thus further increasing the rate of corrosion
What factors affect the rate of carbonation?
- Quality and density of the concrete - good quality, well compacted concrete will carbonate at a much slower rate
- Exposure of the building to water and carbon dioxide (permanently wet conditions hinder penetration so carbonation will be low)
- Relative humidity of the atmosphere - carbonation is encouraged where RH is between 25-75% (optimum at 50-75%; anything over 75% usually hinders the rate of carbonation as the excess moisture slows the rate of carbon dioxide entry)
- Temperature - warmer temperatures increase the rate of carbonation (subject to RH)
Is the rate of carbonation greater internally or externally? Why?
The rate of carbonation is usually greater internally due to the higher relative humidity and temperature
Is carbonation more likely to cause corrosion in internal or external concrete? Why?
External concrete, due to the increased presence of moisture and oxygen that can penetrate the carbonated concrete
Under what circumstances is the risk of corrosion through carbonation particularly high?
If poor compaction and strength (perhaps caused by a too high water/cement ratio) is coupled with reinforcement with little cover
How would you identify carbonation?
Visual Appearance:
- Longitudinal cracking along the line of any steel reinforcement (hairline cracking can occur as early as a few months after construction)
- Brown stains as a result of the rusting steel
- Over time, the expansion of rusting steel will result in further cracking and spalling of the surface concrete
Chemical Testing:
- Used to determine depth of carbonation
- Phenolphthalein solution is sprayed onto a fresh sample of the concrete
- Non-carbonated areas will turn pink/purple (alkaline), whereas carbonated areas will remain colourless (neutral pH value due to reduced alkalinity)
- The depth of penetration can then be measured
How can carbonated concrete be addressed?
Firstly consider the likely rate of ongoing deterioration and the required life of the structure to assess the cost effectiveness of different protection and repair strategies
Options:
- Patch Repair
- Re-alkalisation by Diffusion
- Electrochemical Re-alkalisation
- Increase Resistivity
What is the process of patch repairing carbonated concrete?
- Clean surface
- Remove loose concrete
- Remove corrosion (e.g. grit blasting)
- Prime the reinforcement with alkali-based solution
- Reinstate concrete cover using patch repair mortar, sprayed concrete or conventional concrete (for large areas only)
What is the process of remediating carbonated concrete through re-alkalisation by diffusion?
- For concrete that has only suffered minor carbonation
- A thickness of fresh alkaline concrete is applied to the surface of the concrete
- Migration of alkalis from the fresh to the original concrete will allow for gradual re-alkalisation
- Not advisable to rely on this method alone if the average depth of carbonation exceeds 10mm, as moisture from the fresh concrete can ingress and increase the rate of steel corrosion
What is the process of remediating carbonated concrete through electrochemical re-alkalisation?
- A temporary anode mesh is fitted close to the surface of the concrete and is electrically connected to the steel reinforcement (cathode) and a power supply
- An electrolyte (usually a sprayed cellulose fibre) is applied around the anode mesh
- The steel cathode then attracts alkali metal ions towards it, so high alkalinity is restored around the steel
- Process takes approximately 3-10 days
How can you increase the resistivity of carbonated concrete?
- Surface coatings - designed to restrict the penetration of carbon dioxide (must still allow the concrete to dry out)
- Hydrophobic impregnants - designed to repel water
- Sheltering the concrete component - e.g. ventilated external cladding
What is chloride attack?
Chloride attack is the process by which chloride ions are introduced into concrete, which reduces its alkalinity
How can chloride ions be introduced into concrete?
- Introduced as an accelerator during the mixing process (calcium chloride)
- Introduced naturally (e.g. from the use of unwashed marine aggregates)
- Introduced as a result of external contamination (e.g. de-icing salt, exposure to salt spray etc.)
When was it common to introduce chloride ions into concrete as an accelerator?
- Prevalent in the 1950s and 1960s - good for concreting in cold weather as the concrete or mortar would harden quickly, thus developing early resistance to freezing and thawing
- Common until around 1978
What method of introducing chloride ions into concrete can cause the most damage?
External contamination as the concentration can be more erratic and the ions are not chemically bound
What problems are associated with chloride attack?
- Loss of alkalinity of the concrete removes its protective capability to stop any encased steel from oxidising (depassivation)
- Similar to carbonation, moisture and oxygen can then lead to expansion of the steel and cracking and spalling of the concrete cover, thus compromising its structural integrity
- Furthermore, as the chloride ions make contact with the steel and the surrounding passive material, hydrochloric acid is formed
- The hydrochloric acid will then eat away at the steel reinforcement (aka ‘pitting’) and could cause loss of section and serious structural failure
- Where high levels of chloride are present, corrosion of steel can occur even if the concrete is highly alkaline
- Where carbonation is also present, chloride attack can increase the rate of oxidisation of the steel reinforcement
What is meant by the term ‘pitting’?
Localised corrosion that leads to the creation of small holes in the metal
What factors affect the rate of chloride attack?
- Physical characteristics of the concrete - i.e. calcium chloride used as an additive or introduced naturally
- Quality and density - denser concrete will be less porous and therefore decrease the rate at which chloride ions can reach the steel
- Physical condition - e.g. cracks and damage can speed up the transportation of moisture and ions to the steel (freeze thaw cycles can then exacerbate the process further)
- Location - sea walls, marine structures (sea water is a major source of chloride ions), areas where de-icing salts have been used and remain in-situ
How would you identify chloride attack?
Visual Appearance:
- Can induce large cracking or bulging within the concrete of a more localised nature than carbonation
- Black coloured rusting and pitting of the steel where aggressive hydrochloric acid has attacked
- May be more difficult to see as pitting can occur where there is no cracking/spalling of the concrete
Chemical Testing:
- Indicator solution is applied and if the liquid turns brown, significant chlorides are present
- If it turns yellow/white, chlorides may be present and further investigation is required
Laboratory Testing
Name some of the different methods of obtaining samples to laboratory test for chloride attack.
- Lump sample - section is knocked off (ideally about 50g from a depth of at least 25mm) for testing, although corner samples may distort results as they may have been subjected to chloride ingress from two sides
- Dust drilling - dust is extracted using a rotary percussion drill, although cannot take incremental readings / profile
- Profile grinding - specialist grinder used to obtain concrete powder at selected, exact depth increments
Why is it important to take samples at different depths whilst testing for chloride attack?
To establish the concentration at different levels to determine whether the chloride content is a result of:
- Airborne contamination (concentration highest towards the surface and gradually diminishing into the depth of the concrete)
- Other sources (concentration at a more even distribution)
How can chloride attack be remediated?
- Patch Repair
- Desalination (Chloride Extraction)
- Cathodic Protection
- Corrosion Inhibitors
What problems are associated with patch repairing chloride attack?
- Difficult due to the tendency for new corrosion cells to form at the boundary of the repair (aka incipient anode effect) - this can be minimised by removing where possible all concrete with significant chloride contamination
- Not sufficient for high levels of chlorides and long term protection
What can be introduced to help minimise the problems associated with patch repairing chloride attack?
The introduction of proprietary sacrificial zinc anodes embedded within the patch repair attached to the reinforcement can help reduce incipient anode effect
What is the process of remediating chloride attack through chloride extraction?
- Short-term process where negatively charged chloride ions can be electrochemically repelled from the steel
- A temporary anode mesh is fitted close to the surface of the concrete and is electrically connected to the steel reinforcement (cathode) and a power supply
- An electrolyte (usually a sprayed cellulose fibre) is applied around the anode mesh
- Once a current is applied for a period of time (may be up to 40 days), the chloride ions are transported from the concrete to its surface, where they are then carried out by water or removed with the temporary electrolyte
What problems are associated with remediating chloride attack through chloride extraction?
- Chloride that has penetrated deeper than 20mm can be hard to remove
- Total extraction is impossible, so risk of reoccurrence is likely
- Difficult to remove chloride ions bound in the mix at the time of construction (easier when chlorides had been introduced from external sources)
- Worries that it may generate Alkali Silica Reaction (ASR) - currently being researched
- Cannot be applied to prestressed concrete because of risk of hydrogen embrittlement (phenomenon where high-strength steel becomes brittle and fractures following exposure to hydrogen)
What is the process of remediating chloride attack through cathodic protection?
- Similar to desalination, however current densities are generally lower and the system is designed for continuous use, not a short period of time
- The anodes are usually connected to a data-logging system so that current densities and corrosion rates can be monitored and corrected where necessary
- Tried and tested long-term solution for heavily chloride-contaminated structures (e.g. car parks)
What is the process of remediating chloride attack through the use of corrosion inhibitors?
- Penetrates the concrete and creates a very thin protective layer around the steel
- Cost-effective alternative to conventional repairs
What problems are associated with remediating chloride attack through the use of corrosion inhibitors?
- Molecules are fairly large so can be slow to penetrate, particularly when the concrete mix is dense
- Applications in damp conditions may also reduce speed and effectiveness of treatment
- Considered to be more appropriate when used as part of a treatment system, not on its own
What is sulphate attack and how is it caused?
- Sulphate attack is a chemical reaction where water soluble sulphate salts are transported into cement mortar or concrete
- They react with the tricalcium aluminate (one of the components of Portland Cement) to form ettringite
- Ettringite is characterised by the formation of acicular crystals, which generate high expansive forces in the mortar or concrete
- For sulphate attack to occur, there must be sufficient sulphate and sufficient long-term water
Name some common sources of sulphates in construction.
- Soils containing high sulphate levels
- Contaminated hardcore (that containing high levels of black ash, burnt colliery shale, blast furnace slag and similar materials)
- Bricks
- Air pollution
- Exhaust gases of slow-burning fuel appliances
What problems are associated with sulphate attack?
- Cracking, expansion and bulging due to loss of bond between cement paste and aggregates
- Sometimes the face of bricks spall, most commonly around their edges
- Often accompanied by frost attack due to water saturation
What building elements are typically affected by sulphate attack?
- Chimney stacks
- Mortar joints
- Concrete floor slabs
- Internally where cement and gypsum are in contact (e.g. adding gypsum plaster to a cement/sand mix to accelerate its set) and remain wet for long periods
- Cement-based undercoat plasters if they contain ash (a sulphurous material) and if they remain wet for long periods
Why are chimney stacks particularly at risk from sulphate attack?
- Very exposed to rain
- Additional sulphates are provided by exhaust gases from fires
- Additional moisture is provided by exhaust gases condensing inside the cold upper parts of the chimney
How would you identify sulphate attack in chimney stacks?
Leaning due to different wetting and drying cycles between elevations (wetter side suffers most expansion)
How would you identify sulphate attack in concrete floor slabs?
- Because it is restrained, upwards bowing towards centre coupled with map pattern cracking
- Displacement of brickwork at slab level
How would you identify sulphate attack in brickwork?
- Expansion of brickwork along brick joints both horizontally (distinguishable from wall tie failure as it may occur in every joint) and vertically, particularly in rendered brick
- Bowing upwards, particularly if restrained
What level of sulphate is usually considered harmful to cement/concrete?
Laboratory testing as per BS 1881-124 - anything over 5% sulphate content of cement (assuming cement is 15% of mass of concrete) can be harmful
What steps would you recommend to remediate sulphate attack?
- Keep the concrete dry by installing a DPM to serve as a barrier to moisture to prevent salt migration
- Reconstruct affected elements (floors, walls, foundations) with sulphate-resisting cement
- Sulphate-resistant bricks may also be used
How can rendering brickwork often exacerbate problems caused by sulphate attack?
- Dense cement renders often crack and let water in, but restrict it from drying out
- Soluble sulphates in the bricks themselves can then dissolve out to react with the cement mortar, causing it to expand
What is ASR and how is it caused?
ASR is a reaction produced when highly alkaline pore water in concrete mixes with silica molecules in certain rocks and minerals
For ASR to occur, 3 interrelated factors must be present:
- High alkalinity (either from cement or other external sources)
- Sufficient moisture
- Critical silica in the aggregate
What problems are associated with ASR?
- The chemical reaction produces a gel, which absorbs water, expands and can cause the concrete to crack or disrupt
- Cracks then allow more moisture to enter to fuel the reaction, thus producing greater amounts of gel
- Durability of the concrete can thus be compromised
- In extreme cases, the tensile strength of the concrete component can be reduced
How would you identify ASR?
Visual appearance:
- Pattern of map cracking occurs - applies to unreinforced concrete
- In other occurrences, small ‘pop-outs’ (approx. 30-50mm) form - like concrete with acne
- Spalling will often reveal a reservoir of sticky gel behind it or gel will be exuding from cracks
- When carbonated, the gel appears as a whitish opaque coating - like bad efflorescence
Laboratory testing can confirm diagnosis
What steps would you recommend to remediate ASR?
- Existence of ASR is not necessarily fatal to a structure (depends on severity and elements affected)
- A risk-based analyse is recommended before deciding upon replacement strategies
- Without completing replacing the affected concrete, there are no full remedies, only mitigation measures
What measures can be undertaken to mitigate the affects of ASR?
- Application of penetrating breathable sealers - cannot be used when structure is permanently wet and must be reapplied every 5 years at most
- Crack filling with flexible caulking - only benefit is it slows water ingress
- Apply physical constraint to confine/strengthen structure - may be difficult to achieve and does not stop the process of ASR occurring
- Over-cladding - may trap moisture and cause difficulty in future inspection
What is ACR and how is it caused?
- Very rare as it only occurs with certain impure forms of dolomitic limestone
- The aggregate reacts with the dissolved potassium and sodium alkalis within the pore fluid
- This then alters the crystal structure of the aggregate, causing it to expand
What is meant by the term concrete cancer?
- Another name given to Alkali Aggregate Reactions (of which ASR is the most common in the UK)
- This term could be misleading and is therefore best avoided
What different types of concrete tests are there?
- Schmidt Hammer Test (aka rebound hammer test) - used to determine the compressive strength of concrete
- Chemical testing (e.g. phenolphthalein for Carbonation)
- Laboratory testing (e.g. profile grinding for Chloride Attack)
What guidance is available in relation to concrete repair methods?
- BRE 444-3 (Corrosion of Steel in Concrete) - remedial measures and guidance for appropriate repair methods
- BS EN 1504 - 10 parts covering different concrete repairs
- The Concrete Society - Technical Report 69 ‘Repair of concrete structures with reference to BS EN 1504’ (2009) - explains the concepts provided for in the BS
The reinforcement bars on a concrete building are exposed and corroding. What are the causes and remedial works strategies?
- Likely to be either carbonation, chloride attack or both
- Remedial measures depend on the extent of the damage
- Combination of the following may be appropriate:
- Patch repair
- Re-alkalisation
- Desalination
- Cathodic protection
- Apply corrosion inhibitor
What is carbonation and how is it caused?
- Carbonation is the process by which carbon dioxide slowly penetrates concrete and dissolves in water present within its pores, forming a mildly carbolic acidic solution
- This acidic solution reacts with the alkaline calcium hydroxide (one of the components of concrete) to form calcium carbonate
- This results in a pH drop, reducing the alkalinity of the concrete (from more than 12.5 to approximately 8.5)
- This carbonation process progressively moves through the concrete over time
What problems are associated with carbonation?
- The passive layer around reinforcing steel will deteriorate when the pH falls below 10.5
- Therefore, once carbonation reaches any steel, the concrete is insufficiently alkaline to protect the steel’s passive layer and thus becomes ‘active’ (aka depassivation)
- Moisture and oxygen ingressing through the porous concrete can now react with the steel, which may begin to rust and corrode
- If this happens, the steel will expand, which can cause cracking and spalling of the concrete cover, thus compromising its structural integrity
- This also makes it easier for aggressive agents to ingress towards the steel, thus further increasing the rate of corrosion
What factors affect the rate of carbonation?
- Quality and density of the concrete - good quality, well compacted concrete will carbonate at a much slower rate
- Exposure of the building to water and carbon dioxide (permanently wet conditions hinder penetration so carbonation will be low)
- Relative humidity of the atmosphere - carbonation is encouraged where RH is between 25-75% (optimum at 50-75%; anything over 75% usually hinders the rate of carbonation as the excess moisture slows the rate of carbon dioxide entry)
- Temperature - warmer temperatures increase the rate of carbonation (subject to RH)
Is the rate of carbonation greater internally or externally? Why?
The rate of carbonation is usually greater internally due to the higher relative humidity and temperature
Is carbonation more likely to cause corrosion in internal or external concrete? Why?
External concrete, due to the increased presence of moisture and oxygen that can penetrate the carbonated concrete
Under what circumstances is the risk of corrosion through carbonation particularly high?
If poor compaction and strength (perhaps caused by a too high water/cement ratio) is coupled with reinforcement with little cover
How would you identify carbonation?
Visual Appearance:
- Longitudinal cracking along the line of any steel reinforcement (hairline cracking can occur as early as a few months after construction)
- Brown stains as a result of the rusting steel
- Over time, the expansion of rusting steel will result in further cracking and spalling of the surface concrete
Chemical Testing:
- Used to determine depth of carbonation
- Phenolphthalein solution is sprayed onto a fresh sample of the concrete
- Non-carbonated areas will turn pink/purple (alkaline), whereas carbonated areas will remain colourless (neutral pH value due to reduced alkalinity)
- The depth of penetration can then be measured
How can carbonated concrete be addressed?
Firstly consider the likely rate of ongoing deterioration and the required life of the structure to assess the cost effectiveness of different protection and repair strategies
Options:
- Patch Repair
- Re-alkalisation by Diffusion
- Electrochemical Re-alkalisation
- Increase Resistivity
What is the process of patch repairing carbonated concrete?
- Clean surface
- Remove loose concrete
- Remove corrosion (e.g. grit blasting)
- Prime the reinforcement with alkali-based solution
- Reinstate concrete cover using patch repair mortar, sprayed concrete or conventional concrete (for large areas only)
What is the process of remediating carbonated concrete through re-alkalisation by diffusion?
- For concrete that has only suffered minor carbonation
- A thickness of fresh alkaline concrete is applied to the surface of the concrete
- Migration of alkalis from the fresh to the original concrete will allow for gradual re-alkalisation
- Not advisable to rely on this method alone if the average depth of carbonation exceeds 10mm, as moisture from the fresh concrete can ingress and increase the rate of steel corrosion
What is the process of remediating carbonated concrete through electrochemical re-alkalisation?
- A temporary anode mesh is fitted close to the surface of the concrete and is electrically connected to the steel reinforcement (cathode) and a power supply
- An electrolyte (usually a sprayed cellulose fibre) is applied around the anode mesh
- The steel cathode then attracts alkali metal ions towards it, so high alkalinity is restored around the steel
- Process takes approximately 3-10 days
How can you increase the resistivity of carbonated concrete?
- Surface coatings - designed to restrict the penetration of carbon dioxide (must still allow the concrete to dry out)
- Hydrophobic impregnants - designed to repel water
- Sheltering the concrete component - e.g. ventilated external cladding
What is chloride attack?
Chloride attack is the process by which chloride ions are introduced into concrete, which reduces its alkalinity
How can chloride ions be introduced into concrete?
- Introduced as an accelerator during the mixing process (calcium chloride)
- Introduced naturally (e.g. from the use of unwashed marine aggregates)
- Introduced as a result of external contamination (e.g. de-icing salt, exposure to salt spray etc.)