SoE - Building Pathology

Question 1

Name and describe some deleterious materials.

Answer 1

• Asbestos – Fiberous material used for a variety of reasons from roof sheets to insulation and fire stopping. It is carcinogenic and needs to be removed if there is a risk of release of fibres.
• Lead – Risk of lead poisoning when used for pipes or paint. Contaminates drinking water and risk of inhalation of paint dust or potentially ingested when children chew on painted products.
• Machine Made Mineral Fibres – Potential Carcinogenic. The evidence leads to show there is a high risk and should be treated as such but has not yet been proved.
• Wood Wool Slabs – When used as permanent shuttering for concrete, it may lead to grout loss and inadequate coverage of steels which leads to inadequate fire protection. If used for roof decking, it is not considered deleterious.
• Calcium Silicate Bricks – Shrinks after construction with further movement from wetting. Thermal movement more likely than with clay bricks. Construction details must allow for movement and if designed and used correctly, performs well.
• High Alumina Cement – Gives high early strength and used in maritime buildings. Porosity is increased and resistance to chemical attack is reduced. Where water and chemicals are present in the mix, it can lead to the concrete becoming friable and lead to loss of strength.

Question 2

Are calcium silicate bricks deleterious?

Answer 2

The concern comes from the potential for movement that have given calcium silicate bricks the label of deleterious otherwise the bricks actually increase in strength when exposed to atmospheric carbon dioxide although does shrink it too. In highly polluted areas, sulphur dioxide in damp conditions causes the calcium silicate to decompose to form a gypsum crust.

Question 3

How would you recognise High Alumina Cement and in what condition would you expect to find it?

Answer 3

Used in buildings between 1954 and 1974. Precast pre-stressed concrete beams particularly in maritime buildings. Leaves concrete friable and sometimes browns it. Usually found in roof beams.

Question 4

Wood wool slabs are deleterious materials. Explain why and where their use would be acceptable.

Answer 4

When used as permanent shuttering for concrete, it may lead to grout loss, honeycombing or voids which can reduce fire resistance, corrosion protection and loss of strength. It is adequate for use in flat roof decking.

Question 5

What are the types of damp?

Answer 5

• Penetrating
• Rising
• Condensation ( + interstitial condensation)

Question 6

What is a protimeter / moisture meter?

Answer 6

Measures the electrical resistance between two electrodes. Generally used for moisture in wood.

Question 7

What is a speedy carbide meter?

Answer 7

Measures moisture in masonry through adding masonry dust into a container with calcium carbide which releases a gas in proportion to the moisture present.

Question 8

What is cold bridging?

Answer 8

A cold bridge is created when poor thermal insulators come into contact allowing heat to flow through the path. E.g. at the junction of a concrete slab and external walls.

Question 9

How would you identify condensation? What are the remedial measures?

Answer 9

Generally occurs in top corners of rooms where warm air rises and collects and due to the change in temperature, the warm air releases vapour onto the colder surface. It is an issue in poorly vented and cold buildings where there is high moisture volume. It can be remedied by venting the property better and maintaining regular internal temperature.

Question 10

How long would you leave brick/plaster to dry out before commencing work?

Answer 10

Generally should be left for 3-4 months but this is not practical.

Question 11

Discolouration at 450x900mm centres in grid formation? What is the remedy?

Answer 11

Lateral damp ingress from defected cavity ties. Due to mortar dropping onto the cavity tie which allows moisture to penetrate through the brick. Remedy is to remove the wall tie and replace it.

Question 12

What are the different ways moisture could enter the building? How would you differentiate between the types?

Answer 12

• Penetrating – Enters via a porous bridge transferring moisture through capillary action or there might be a hole in the building fabric.
• Rising – Moisture rising up from the ground through capillary action normally up to 1m high with tide marks. Usually from bridging of DPC or lack/failure of DPC.
• Condensation – Warm moist air comes into contact with cold surfaces which releases the moisture. Generally at high level.

Question 13

You are inspecting a building and identify several cracks in the external wall. Take me through your thought process.

Answer 13

Firstly, what is the building made from and what is the construction method? Are there local factors such as trees or high water tables? Have any alterations been carried out? Are there any nearby drains where the cracking is. What is the age of the building?

Question 14

What other sorts of cracks would you expect to see on a brick building and what might have caused them?

Answer 14

• Subsidence – shrinkage of clay
• Heave – saturated clay
• Settlement – Movement from increased load.
• Differential settlement – when parts of a building are constructed off different foundations or the ground has different soils, they move at different speeds.
• Lintel failure – Usually leads to triangular cracking to the brickwork above the window to drop.
• Internal alterations – drying of timbers or installation of steel beams without consideration of impact.

Question 15

What size cracks are of concern?

Answer 15

• BRE Digest 251
• Up to 2mm – very slight/aesthetic
• 2-5mm – slight / aesthetic
• 5-15mm – moderate/ serviceable
• 15-20mm – severe/ serviceable
• Over 25mm – very severe / stability issues

Question 16

Cracks between the bay window and main wall. What are the causes and remedies?

Answer 16

General rule of thumb is that it is caused by differential settlement due to different foundation types. It could also be because windows were swapped and the new window is not strengthened and causes the bay to drop.

Question 17

What are the types of timber defects?

Answer 17

• Insect attack
• Dry rot
• Wet rot
• Structural defects

Question 18

Describe the different types of rot and how you would recognise them?

Answer 18

Dry Rot – Wood shrinks and splits into cuboidal cracks. Wood is light in weight and crumbles under fingers. Usually mycelium on the surface which is grey when wet and yellow/purple when dry. Fruiting body usually brown/red in colour. Indoors only.

Wet Rot – Wood shrinks and splits. Wood darkens. Mycelium grows on surface which can be white, brown, green or amber in colour. Fruiting bodies can be different colours and occurs internally or externally.

Question 19

What are the remedial measures to dry/wet rot?

Answer 19

• Locate and remove the source of moisture and dry out the timber
• Remove rotten wood plus 450mm
• Apply fungicidal fluid and strengthen joists if required

Question 20

What is the moisture content needed for Dry and Wet Rot?

Answer 20

• 50-60% for Wet Rot. Will not survive below 44%.
• 25% for dry rot

Question 21

Ends of the joists in a timber floor built in an external wall have been seriously affected by dry rot. How will you recognise this type of rot and what action would you take to resolve the problem?

Answer 21

• Features include shrunken wood with cuboidal shapes. Cotton wool type mycelium on the surface. Timber crumbles under finger. Fruiting bodies are red/brown in colour and pancake shaped.
• Identify the rot and where it is at its worst. Remove the cause and strengthen the joists as required or fully replace. If the dry rot has passed through the masonry, this will need sterilisation too.

Question 22

Explain the life cycle of rot.

Answer 22

• Spores – with dry rot, it is a fine orange brown dust. The spores activate when in contact with timber.
• Hyphae – Timber and moisture are present, the spores will produce fine white strands which allows the rot to grow by feeding on timber.
• Mycelium – Hyphae mass is known as mycelium. It grows on various materials and over vast distances which allows it to progressively destroy the structural timber.
• Fruiting Body – Mushroom like form pumps spores into the air which is transferred by air currents and germinates it to create a new attack. Repeat the process

Question 23

Name some common insects known to attack timber. What remedial methods are available? How long do these remedial methods last?

Answer 23

• Category A (insecticidal treatment) – common furniture beetle, death watch, house longhorn beetle.
• Category B (treatment of rotten area only) – wood boring weebills.
• Category C (No treatment required) Bark borers
• Remedial methods – study flight holes to identify beetles and determine if it is still active or dormant. Apply boron-based treatment to affected areas

Question 24

How do you identify the presence of woodworm?

Answer 24

Holes in wooden item with frass around the holes. Typically 1-1.5mm holes.

Question 25

How do woodworms affect timber in buildings?

Answer 25

Adult beetles lay eggs on the timber. The grubs feed on the timber, hatch into beetles which breed, lay eggs and the process repeats.

Question 26

What is a woodworm infestation likely to indicate?

Answer 26

Most woodworm requires high moisture. May indicate an issue with the structure and damp.

Question 27

How is a woodworm infestation treated?

Answer 27

Chemical insecticides. Resolve damp issue as damp wood may be re-infected

Question 28

What is the lifecycle of a common furniture beetle?

Answer 28

• Furniture beetles and longhorn – 2-5 years
• Death-watch beetle – up to 10 years

Question 29

Deflected timber floor in early 19th Century building. What methods are available to deal with this if the client requires a level floor?

Answer 29

Confirm why the floor is deflecting. Notching to joists, not adequate for spans? Inadequate support? Can the issue be remedied without destroying the floor? Supplement the joists, overboard or remove it and replace.

Question 30

A newly cast reinforced concrete slab is exhibiting crazing and cracking on its surface. What are the causes?

Answer 30

Poor or inadequate curing – environmental conditions being conducive to evaporation and lack of protection. If the mixture is too wet or excessive floating which causes aggregate to sink, the cement paste on top will start to crack.

Question 31

Name the three types of asbestos and examples of where they are found.

Answer 31

• Chrysolite – most common asbestos in buildings. Corrugated asbestos cement sheets.
• Amosite – found in fire retardant materials
• Crocidolite – pipe insulation, plastics, cement products.

Question 32

What are nickel sulphide inclusions?

Answer 32

Occurs in glass production. Microscopic imperfections in the glass. When the glass is tempered, the nickel sulphide inclusions do not return to the normal low-temp state for several years and expands in the process which can cause cracks with no obvious cause.

Question 33

What else may have caused glazing to crack if it wasn’t nickel sulphide inclusions?

Answer 33

Structural movement above/below causing the frame to move or distort.

Question 34

How does cavity wall tile failure present itself? How would you fix it?

Answer 34

Horizontal cracking every 5 or so brick courses. Corrosion of old steel ties causes the cracks.
Remedy – use a borescope to determine the defect. Locate the wall ties and isolate and remove. Install replacement stainless steel mechanically fixed ties.

Question 35

What is nail fatigue?

Answer 35

Common with slates or tiles. Nails have rusted and expanded which cracks the tile allowing them to slip. Generally, if one nail has failed, several are likely to fail too.

Question 36

What would be the associated problems and defects with a “crinkly tin” shed approximately 15 years old? What defects might you find with profiled steel cladding?

Answer 36

• Plastisol degradation
• Cut edge corrosion
• Defective fastenings

Question 37

Why does cut edge corrosion occur? What can happen if it is not treated? How can it be remediated?

Answer 37

When cut edge of coated metal are exposed to elements, delamination may occur in the form of peel back of factory coating. The unprotected metal then rusts and corrodes. Can be remedied by applying a silicone paint. If the corrosion is bad, may be better to replace the panel.

Question 38

What coatings are usually found on sheet metal? How can it break down?

Answer 38

Plastisol is a PVC coating. Usually has a leather effect. Or use PVF2 which is a thinner material with a smooth finish. These are both effected by temperature, UV exposure which can lead to caulking and delamination.

Question 39

What is Japanese knotweed?

Answer 39

Herbaceous perennial plant.

Question 40

What does Japanese knotweed look like?

Answer 40

Hallow stems with raised nodes. Similar appearance to bamboo. Can reach 3-4m per year. Broad leaves with small white flowers

Question 41

Why is Japanese Knotweed and issue?

Answer 41

Invasive species which forms thick colonies that crowd and kill over species. Extend 7m horizontally and 3m deep making excavations difficult. Plant is resilient to normal weed killers and cutting. Needs herbicide application to kill it over a period of months.

Question 42

What legal restrictions are there in dealing with knotweed?

Answer 42

Offence to plant or grow it. Illegal to cause spread. Classed as controlled waste and requires a licence to dispose at licensed sites.

Question 43

How should it be dealt with?

Answer 43

Injected with herbicide or excavated with the roots and disposed.

Question 44

What typical defects are found in 60’s/70’s commercial buildings?

Answer 44

• Asbestos
• Concrete defects
• Cold roof
• Wood wool slabs

Question 45

What kind of defects did you find in industrial buildings?

Answer 45

• Cut edge corrosion
• Plastisol degradation
• Asbestos use in insulation

Question 46

What is concrete carbonation?

Answer 46

Carbonation occurs due to the ingress of atmospheric carbon dioxide reacting with water within the concrete, which forms carbonic acid that neutralises concrete alkalinity
This breaks down the passive layer around the reinforcement causing it to corrode due to the presence of water and oxygen. This happens at a slow rate (progressive)
Carbonation is more of a concern where there is poor or shallow cover over the steel reinforcement, or the concrete is poorly compacted
Carbonation is more rapid in porous concrete with low cement content and concrete with high water to cement ratio

Question 47

How would you identify concrete carbonation?

Answer 47

• Occurs to reinforced concrete buildings and structures – it is inevitable
• Fine cracks appear in the concrete causing the reinforcement to corrode
• Hairline cracks appear along the reinforcement line
• The reinforcement expands due to the rusting
• Concrete spalls and cracks exposing the reinforcement

Question 48

What are the remedial measures for concrete carbonation?

Answer 48

• Replacement – this involves removing defective concrete and cutting out the reinforcement to damaged members. New reinforcement is fixed but welding to existing and new concrete is cast. This is the simplest method and completed by specialist contractors.
• Gunting – a propriety process in which fine concrete is sprayed onto a concrete surface under repair of high velocity.
• Coating – coatings, sealants and membranes can be effective in reducing the carbonation process. however, if chloride is present at rebar depth then the coating will not prevent carbonation.
• Electrochemical re-alkalisation – the cathodic reaction around rebars produce hydroxyl ions. The steel cathodes also attack alkali metal ions towards its surface, so high alkalinity is restored around rebar. A temporary anode (steel mesh) is attached to concrete, the electrolyte penetrates the concrete and raises the alkalinity at the cover. An anti-carbonation coating is applied.
• Cathodic protection – this is a permanent and very expensive technique. an anode system (coating or mesh) are sprayed or fixed to the whole surface of concrete. A current is applied which passes to the reinforcement, which is then made cathode preventing corrosion of the reinforcement.

Question 49

What is HAC High alumina cement?

Answer 49

High Alumina Cement was used in the production of pre-cast concrete sections. This has very high strength at an early stage, however, loses strength as it ages and is susceptible to chemical attack in damp conditions.

Introduced in 1925, cement producer Lafarge commenced the UK manufacture of High Alumina Cement to provide concrete that would resist chemical attack, particularly for marine applications. Used in the UK from 1950-1976 in the manufacture of pre-stressed concrete beams

Question 50

What are the problems with HAC?

Answer 50

HAC concrete undergoes a mineralogical change known as conversion. During this process the concrete increases in porosity which in turn results in a loss of strength and reduction in resistance to chemical attack. The higher the temperature during the casting of the concrete the more quickly conversion takes place.

Question 51

Remedial measures for HAC?

Answer 51

• An investigation should be undertaken consisting:
• Sample testing
• Collect details of construction materials used – the local authorities hold records of HAC buildings
• If the strength has been reduced – reduce the live loads or strengthen
• In the case of reinforcement corrosion – localised repairs can slow further deterioration

Question 52

What is sulphate attack?

Answer 52

• Sulphate attack can cause serious damage to brickwork and concrete by creating expansive force that are, or can be, sufficient to disrupt mortar and create significant expansion, bowing and cracking of concrete floor slabs together with related collateral damage.
• In some subsoil there can be a problem with sulphate attack. It occurs when the fill material (hardcore) beneath the slab contains sulphates and these migrate into the concrete. The sulphates react with the concrete causing it to expand.
• Calcium, magnesium and sodium sulphates occur naturally in some clays and other sub soils
• The sulphates dissolve in the groundwater and permeate the concrete.
• This leads to an aggressive chemical reaction between the sulphates in solution and one of the chemicals in the cement.
• The resulting compound expands rapidly as it forms and this can crack the foundation concrete.
• If sulphate attack is a possibility it is wise to use sulphate resisting cement.

Question 53

Identification of sulphate attack?

Answer 53

• Located in brickwork, blockwork, render and concrete.
• Heave in concrete slabs and displacement of adjacent masonry
• In dwellings with timber floors, the sleeper walls may become displaced causing deflection in floors
• Cracking
• Spalling
• Loss of strength
• Increase surface area for further attack
• Friable
• Edges and corners susceptible
• Upwards bowing coupled with map pattern cracking and possibly displacement of brickwork at slab level are common indicators. Visual indicators of affected concrete may be the existence of flaky or crumbly concrete

Question 54

What is Alkali silica reaction?

Answer 54

Occurs where the chemical within the cement react with silica (in aggregates), forms a gel substance which absorbs water and expands creating cracks where water can enter. Further cracking may occur as a result of water freezing within the cracks.
* Cracking
* Small pop outs
* Map cracking
* Tensile strength reduced
* Gel absorbs water causing further cracking
* Cracks run parallel to reinforcement bars

Question 55

What are the ASR remedial measures?

Answer 55

• Remove one of the following and ASR will not take place:
• Critical silica in aggregate
• Sufficient moisture
• High alkalinity
• Use low alkali cement and avoid use of chemically reactive aggregates
• Consider complete replacement of an ASR affected section to maintain integrity and safety

Question 56

What is Chloride attack ? (salts)

Answer 56

• Chlorides may either ingress (from penetration of de-icing salts) or cast insitu as sea dredged aggregates or calcium chloride additive.
• Chloride ions attack the reinforcement can cause:
• In certain circumstances ‘pitting’ corrosion can occur – when the steel is effectively eaten away without the formation of expansive rust
• Corrosion of steel reinforcement is localised compared to carbonation and much more rapid.
• Spalling of concrete
• Significant loss of cross-sectional area
• Pit in bar surface – rebars could be completely eaten away
• Visual sign of cracking and spalling following expansive rusting

Question 57

Remedial measured for chloride attack?

Answer 57

• Cathodic protection
• Corrosion inhibitors
• Epoxy resin repairs

Question 58

What is Wet Rot?

Answer 58

• These can cause severe decay in timber. The timber cracks in cubes during the decaying process, brown rots cause decay and as a result, the timber loses strength quickly.
• The wood becomes darker in colour and dry i.e. it can crumble between the fingers
• White covering over the affected area – initial stages of a fruiting body
• Fruiting body grows which is brown
• Starts to spread across the timber – affects the structural integrity of the timber

Question 59

What is dry rot?

Answer 59

• The timber develops a fibrous consistency, similar to lint.
• The cracking that appears is often along the grain.
• The wood becomes lighter in colour i.e. looks bleached.
• Generally white rot prefers high moisture within timber than brown rots

Question 60

How do you identify subsidence?

Answer 60

Identified by diagonal cracking which is usually tapered (of uneven width) in the buildings structure/façade and verified by leaning or pulling of the building. Subsidence usually shows up as cracks that are typically wider at the top than the bottom. They commonly appear around windows, doors and other weaker areas. Doors and windows may stick when opening them, walls and floors may show signs of distortion.

Question 61

Remedial works for subsidence?

Answer 61

• Refer to BRE guidance to understand how to assess damage.
• Remedying the source of the problem can be effective, however it can sometimes make things worse i.e. removing a tree causes moisture to return = heave
• Underpinning is an option but it is expensive and disruptive
• Use pneumatic jacks to lift the building and reverse the movement

Question 62

o you have some examples of deleterious materials?

Answer 62

• Asbestos
• Calcium chlorides
• Calcium silicate bricks
• CFC’s
• Formaldehyde
• Hair plaster
• HAC (High Alumina Cement)
• Hollow clay pot floors
• Lead
• Nickel sulphides

Question 63

What are the defects linked to woodwool slabs?

Answer 63

• Concrete does not adequately get compacted.
• Risk of poor cover to the steel and hence loss of durability, possible reduction in strength, and loss of fire resistance.
• Voiding and honey combing due to vibrations. Poor bond or failed bond between the wood wool and the hardened concrete.
• Buildings constructed from 1950 to the mid-1970s are most likely to be affected by this form of construction.
• If permanent shuttering is discovered, intrusive investigations may be needed to determine the adequacy of concrete cover to the steel reinforcement. If voiding is present, the affected area can be exposed and repaired using hand-fixed concrete repair methods or, if over a larger area, sprayed concrete.

Question 64

What are the typical defects associated with industrial buildings?

Answer 64

• Cut edge corrosion.
• Delaminating plastisol finishes.
• Missing caps to fixings.
• Leaking gutter joints.
• UV damaged roof lights.
• Cracking – subsidence, heave, settlement etc.
• Impact damaged cladding.
• Asbestos.
• Carbonation.
• Cracked floor slabs.

Question 65

What defects are associated with GRP rooflights?

Answer 65

• Loss of transparency, embrittlement etc caused by ultraviolet light breaking down polymers in the rooflight plastic.
• Incorrect installation: 2 lines of sealant should be installed to each end of a rooflight (usually recommended by manufacturer’s).
• Embrittlement of foam fillers by UV degradation.

Question 66

What are the stages of plastisol coating deterioration?

Answer 66

1. Loss of colour
2. Caulking
3. Delamination

Question 67

What are the main types of roof covering that could be specified for a flat roof?

Answer 67

• Reinforced bitumen membranes: Constructed by bonding 2 or more layers of reinforced bitumen with hot bitumen (can also get
  single layer RBM systems too).
• Polymetric single-ply: PVC. Laid and can be fully fastened, fully adhered, or loose laid and ballasted.
• Mastic asphalt: Graded limestone aggregate bound together with an asphaltic cement (bitumen). Heated to 210 degrees and
  applied with a wooden float.
• Liquid roof systems: Monolithic, fully bonded, liquid-based coating with a rubberised finish.
• Profiled sheets.
• Lead, zinc, copper and steel sheet.

Question 68

What can cause ponding on a flat roof?

Answer 68

• Tapered insulation or roof deck not laid to correct falls (timber firrings used to set roof deck falls).
• Deflection of timber roof deck - through reduction in structural integrity of structural timbers - rot / insect infestation.
• Blocked rainwater goods.
• Subsidence / heave.

Question 69

What are the issues associated with interstitial condensation? How is it remedied?

Answer 69

Issues:

• Can cause dampening of insulation materials which can lead to reduction in performance and eventual physical deterioration.
• If forming against timbers, can promote rot and insect attack.

Rectification:

• Changing of the building detail if there is an inherent defect.
• Replacement of insulation if penetrated such as in a cold roof build up - insulation penetrated by fixings, vapour then passes through vapour control barrier and then into the very cold roof void allowing condensation to form on structural timbers.

Question 70

What are common defects associated with flat roofs and what are the remedies?

Answer 70

• Crazing - Surface crazing caused by lack of protection from UV exposure. No treatment is required if in small localised patches but should be monitored.
• Ponding - If the gradient is not sufficient and there are dips in the roof finish, it can lead to pools of water. It can lead to water ingress if there is a gap in the roof finish.
• Thermal Movement
• Cracks and splits can occur when there is thermal movement between the substrate and membrane. Usually requires localised repair to the area around the crack.
• Blistering - Caused when water vapour beneath the roof finish increases. The source of moisture should be traced and the blister opened and allowed to dry before patch repairing.
• Flashing and Falls around openings- Defective lead from deterioration or poor workmanship around openings causing water ingress. Depending on the issue with the lead, there are different repairs.
• People - Punctures caused by people from impact damage.
• Cracks and tears along the line of joists - Caused by thermal movement or saturation of insulation or sagging decking. Repaired by cutting felt back and allowing it to dry before patching over.

Question 71

What is Regents Street Disease? What are the methods of rectification?

Answer 71

Found in early 20th century load bearing steel framed buildings clad externally with stone, brick and terracotta that is notched around the steel frame and then the void filled with low grade mortar. Corrosion of the steel frame due to oxygen and moisture causes corrosion and expansion and with no void space causes cracking of the mortar and overlying cladding.
Can usually diagnose from cracks delineating the frame.
2 main ways to rectify:
1.Exposing the steel, cleaning it and protecting it and then create a void around steel frame to allow movement.
2.Use cathodic protection: system relies on concealment of discrete anodes into the stone joints and electrical connection to the steel frame and the introduction of an electric current to reverse the corrosion process. Requires detailed design to ensure harmful currents arnt produced and maintenance.

Question 72

What typical defects would you expect to find in a 1960s high rise office building?

Answer 72

Asbestos.
Concrete carbonation of the concrete frame.
HAC.

Question 73

What is radon? Which areas would you find it? How do you deal with it?

Answer 73

Radon is a natural, colourless, odourless, radioactive gas formed by the radioactive decay of uranium that occurs naturally in rock and soil.
Exposure to particularly high levels of it may increase risk of developing lung cancer.
Some areas of the UK have higher concentrations due to geology and soils e.g Cornwall, Devon and Sommerset.
Radon can build up in some underfloor areas and enter a building.

You can take some of the following steps:
* A well installed damp proof membrane (radon barrier).
* Air vents (with fans if needed).
* Radon sumps.
* There are others.

Question 74

What are the options for damproofing a basement?

Answer 74

Type A: Basement tanking used internally or externally (applied during the construction process). Any defects to the membrane will have damaging consequences and those defects to external tanking systems will be impossible to rectify. Internal tanking systems can be penetrated by fixings and can be displaced by build up of hydrostatic pressure.

Type B: Structurally integrated protection whereby waterproofing is provided by walls themselves (reinforced / prestressed concrete). Need to carefully consider jointing details as they can leak.

Type C: Drained cavity system whereby water is allowed to pass through the external skin into the cavity and is then pumped away by an electric pump. Can be problematic when there is a high water table and failure of pumping system (usually due to accumulation of fine silts etc).

Question 75

What does passivity mean?

Answer 75

Passivity is generated by the buildup of a layer of metal oxide on the surface of a metal. In order to provide passivity, this oxide layer must be stable. It is formed by corrosion insoluble components in the immediate environment of the metal. The metal oxide acts as a barrier by separating the metal’s surface from its environment and prevents corrosion until the reactants are able to diffuse through the oxide film. This diffusion may take a significant time to occur.