Building Pathology

Question 1

Why do buildings crack?

Answer 1

• Thermal movement (expansion and contraction of the fabric caused by hot and cold) - lack of expansion joints.
• Frost action (freeze-thaw cycle of moisture within the material).
• Ground movement.
• Wall tie failure.
• Chemical reactions (carbonation, sulphate attack, chloride attack).
• Overloading.
• Vibration.

Question 2

What are the issues of cracking?

Answer 2

• Can cause structural issues/become unsafe/unstable.

* Can lead to water penetration which can cause damp issues and further exacerbate the cracking.

Question 3

How to measure/monitor a crack?

Answer 3

• Use of tell-tales

* fixing screws either side and using a calliper to measure distance.

Question 4

Categories of cracking?

Answer 4

BRE Digest 251 table 1 gives 6 categories of cracking from 0-5, 0 being hairline and 7 being 25mm+

Question 5

What is subsidence?

Answer 5

Subsidence is the downward movement of a building caused by loss of support underneath.

Causes:
• Volumetric changes within the subsoil
• Influence of trees on cohesive soils (take up water and cause cohesive soil to become desiccated)
• Washing away of non-cohesive soils (leaking drain, burst water main)
• Nearby excavations
• Changes in ground water levels

Question 6

How do you remedy subsidence?

Answer 6

• Alleviate the cause (e.g. repair leaking drain, reduce tree height
• Monitor cracking - if no further movement repoint & patch repair
• If movement still occurs - underpin foundation

Question 7

What is Heave?

Answer 7

Heave is the upward movement of the building foundation, caused by expansion/swelling of the subsoil

Causes:
• Removal of trees on cohesive soils
• Freezing of ground water in frost susceptible soils
• Excess water in ground - Expands

Question 8

What is Settlement?

Answer 8

Natural compaction of soil due to load of building.
Only problematic is differential. (happens in different parts of the structure at different times - could be due to variation in ground conditions or different foundation depths.

Question 9

What is underpinning?

Answer 9

The process of strengthening the buildings foundation by transferring the load to a new level at lower depth.

Traditional method - Concrete below excavation 150mm long
Jack Pile - pre cast concrete pile hydraulically beneath original foundation.
Needles & Pile - Reinforced concrete means connected to small diameter bored piles.

Question 10

What is cavity wall tie failure?

Answer 10

A cavity wall tie failure is when the cavity wall tie holding the two walls together fails in keeping the two walls connected.

Causes:
• Rusting of metal ties
• Poor mortar - reduced bond
• Not enough ties installed

Question 11

How would you identify cavity wall tie failure?

Answer 11

1. Horizontal cracking every 450mm (6 course)
2. Use of borescope to inspect the cavity
3. Remove brickwork and inspect

Question 12

How would you remedy cavity wall tie failure?

Answer 12

1. Remove old wall ties
2. Drill in new ties (usually a high tensile strength bar)
3. Repoint and make good brickwork

BRE 329 - inserting wall ties into existing construction.

Question 13

How does water enter buildings?

Answer 13

1. Condensation (Condensation occurs when moist air cools below its dew point, and water condenses. This can occur as surface condensation, but also within the fabric of a building itself, as interstitial condensation).
2. Penetrating dampness
3. Rising damp (Rising damp is caused by incorrect placing of, faults to, or the absence of a damp-proof course, and is generally only apparent up to 1 m above ground level because of the limits of capillary action to draw moisture up through porous elements of the building structure).

Question 14

How do you record damp?

Answer 14

• Protimeter (measure of conductance between two points).
• Carbide meter/testing (Test a weight sample of drilled masonry - mixed with calcium carbide powder to produce gas & indicates moisture level).
• Oven drying (Gravimetric test) sample is weighed then dried then weighed - moss of water lost as a percentage.

Question 15

What is dry rot?

Answer 15

• Fungus known as Serpula Lacrymans
• Rot starts out as airborne spores which come into contact with damp timber that has a moisture content over 20%
• Most common in poor ventilated/damp air conditions
• The fungus feeds on the cellulose in the timber and reduces strength.

Question 16

Why is dry rot serious?

Answer 16

Dry rot can spread across other materials such as masonry to reach other timbers.

Question 17

How would you identify dry rot?

Answer 17

• The timber becomes dry and crumbly.
• The timber may become a dull brown colour
• Cuboidal cracking is evident
• Mycelium look like cotton wool
• Fruiting body is often pancaked shaped brown with white outer edge.

Question 18

What is the process of rot?

Answer 18

Airborne spores remain inactive until come into contact with timber moisture enough to make them active (typically 20%)
Spores develop into Hyphae (often white fine stringy strands)
Mycelium (collection/mass of hyphae) - can spread across masonry and behind wall plaster in search of timber.
The fruiting body is the final stage of the Dry Rot fungus’s life cycle. The fruiting body takes on a mushroom like appearance and it produces Dry Rot spores that it emits from the surface to be carried along by air currents out into the air.

Question 19

How would you remedy dry rot?

Answer 19

1. Remove cause of water source/ventilate space.
2. Remove all affected timber +300-450mm past infected area
3. Treat remaining timber with preservative (ACQ alkaline copper quaternary most popular)
4. All affected timber replaced with new treated timber.

(Walls can be treated with masonry fluid/irrigated).

Question 20

What is wet rot?

Answer 20

• Wet rot occurs when there are large amounts of moisture in timber, which then causes the timber itself to naturally decay.
• Thrives in timber with moisture content between 40-60%.
• fungus laches onto dap timber as food source.

Question 21

How would you identify wet rot?

Answer 21

The common signs of wet rot include:

• Darkened timber – darker than surrounding timber
• Soft and spongy timber
• Spitting or longitudinal cracking along the grain
• Timber that is damp to touch
• Localised fungus growth
• A damp, musty smell

Question 22

How would you remedy wet rot?

Answer 22

1. Eliminate the source of water (this may be a leaking pipe or roof covering).
2. Promote rapid drying (heating and ventilation).
3. Remove rotten wood (unless historical significance and needs to be retained).
4. Apply deep penetrative preservatives.
5. Replace any timber as necessary.

Question 23

Why is wet rot less severe than dry rot?

Answer 23

Wet rot will stay localised whereas dry rot can spread across other materials to get to other timbers.

Question 24

What is concrete carbonation?

Answer 24

The reaction of carbon dioxide in the environment with the calcium hydroxide in the cement paste. This reaction produces calcium carbonate and lowers the pH to around 9.

At this value the protective oxide layer surrounding the reinforcing steel breaks down and corrosion becomes possible.

Question 25

How do you test for concrete carbonation?

Answer 25

Phenolphthalein solution which reacts with carbonated cement by leaving a pink indicator - to see the depth.

Question 26

How to remedy concrete carbonation?

Answer 26

• Break out and replace.
• Spraying a dry mix of cement and sand to the surface of the concrete can create a larger amount of concrete cover to the reinforcement bar.
• Anti-carbonation coatings and sealants can be applied to the surface of the concrete.
Helps to prevent the ingress of C02 and other pollutants.
• Electrochemical Re-Alkalisation - A direct current can be applied between the steel bars and an external auxiliary electrode placed on the surface of the concrete. Aims to restore the loss of alkalinity and, re-establish a passive environment around the reinforcement

Question 27

What is chloride attack?

Answer 27

Calcium chloride was used as an accelerating admixture in concrete up until it was banned in 1976.
Chloride salts can also be found in sea-dredged (marine) aggregates.
Calcium chloride break down the protective film created by the alkaline state of the concrete & increase the risk of corrosion.
Corrosion of the steel occurs as a result of micro galvanic cells.
Rapid ‘pitting’ corrosion at the anode (bar eaten away).

Question 28

How to remedy chloride attack?

Answer 28

Difficult to repair as new corrosion cells form on boundary of repair incipient anode effect.
Assess extent of corrosion to rebar and depth of rebar.
Either shoring or demolition as it could be expensive to repair.

Question 29

What is HAC?

Answer 29

High alumina cement uses different cementing compounds to Portland cement.
Cures very quickly & after 24 hrs has same strength as Portland cement after 28 days.
Very popular in 1960s/70s.
Banned din 1970’s following collapses (one a swimming pool.

Question 30

What are the issues of HAC?

Answer 30

Under certain conditions of temperature and humidity, the cement hydrates converted over time to a different mineral form, which had a lower strength.
HAC concrete which is used for structural members that would experience wet or humid conditions at a temperature in excess of around 270C would experience conversion.

Question 31

How to identify HAC?

Answer 31

It is mainly found in buildings constructed between 1950-1970s.
External examination may show signs of failure including, excessive deflection, bowing, and lateral cracking.
Internal examination will show effects of conversion as the cement changes colour from grey to chocolate.
Extent of conversion can be established by cutting small samples and testing.

Question 32

What is sulphate attack?

Answer 32

Soluble sulphates which are present in ground water, soil, clay bricks etc. or hardcore beneath floor slabs react with Calcium Aluminate of cement in the presence of moisture.

This reactions forms calcium Sulphoaluminate which occupies a bigger volume and causes disintegration and weakening of concrete.

Can be both from within and external from the environment.

Question 33

How to remedy sulphate attack?

Answer 33

The most efficient method of resisting the sulphate attack is to use cement with the low C3A (calcium aluminate) content.

Question 34

What are the common wood boring inspects in the UK?

Answer 34

Common furniture beetle (woodworm)

Death watch beetle

Question 35

What is the life cycle of wood boring insects?

Answer 35

EGGS - laid by female insect in cracks, splits or holes created by previous insect attacks.
LARVA - most damage at this stage as larvae burrow into wood to obtain food.
PUPA/ADULT - the insect will bore its way out of the timber forming exit holes

Question 36

Describe woodworm/Common furniture beetle?

Answer 36

The woodworm is common for most insect damage in the UK. characterised by its cream coloured cigar shaped frass. Life cycle is 2-5 years with 1-2mm exit holes.

Most sever n damp timber. (timber with less than 12% moisture considered immune.

Question 37

What method of repair is available to woodworm?

Answer 37

Organic solvents or micro emulsion paste can be used to treat the timber.

Question 38

Describe the death watch beetle.

Answer 38

Common in older buildings in southern England, particularly Surrey, which contain oak, elm or other EU hardwoods. Characterised by oval discs of frass. Life cycle between 5-6 years with 2-3mm exit holes appearing in April-June.

Common in damp timber where fungi present.

Question 39

What method of repair is available to death watch beetle?

Answer 39

Smoke treatments or gas fumigation (only useful for moveable objects as have to enclose timber).

Reduce dampness and replace as necessary.

Pressure injection chemicals such as boron to extinguish the infestation.

Question 40

Describe the wood boring Weevil and how to remedy?

Answer 40

These attack timbers suffering from damp and fungal attack and destroys the integrity of the timber, leaving 1mm ragger exit holes.

Remedy - remove source of damp, dry out and replace affected timbers. Also add preservatives.

Question 41

Describe the longhorn beetle.

Answer 41

The longhorn beetle thrives in tropical conditions and few areas in UK. Life cycle is 5-7 years with 3-9mm exit holes appearing between June - August. Usually found in roof timbers.

Question 42

How to remedy the Longhorn beetle?

Answer 42

Apply organic solvents or micro emulsion paste. Inspect and remove powdered material to determine extent of infestation. Remove and burn affected timber and report to the BRE.