1
Q
Why do buildings crack?
A
- 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
2
Q
Why is cracking a problem?
A
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)
3
Q
Name some of the causes of stepped cracking in brickwork.
A
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4
Q
You see a vertical crack internally to a property’s wall - what may have caused it?
A
???
5
Q
What would cracking at DPC level indicate?
A
???
6
Q
How do you monitor cracking and what are the procedures?
A
- 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
7
Q
Categorise the severity of cracking.
A
- 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
8
Q
How can cracked, worn or spalling bricks be repaired?
A
???
9
Q
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?
A
???
10
Q
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?
A
???
11
Q
What is subsidence?
A
Downward movement of a building foundation caused by loss of support beneath
12
Q
What are the common causes of subsidence?
A
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
13
Q
How can trees cause subsidence?
A
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
14
Q
How can subsidence be rectified?
A
- 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
15
Q
Who usually pays for damage caused by subsidence?
A
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
16
Q
What is heave?
A
Upward movement of a building foundation caused by the expansion or swelling of the subsoil
17
Q
What are the common causes of heave?
A
- The removal of trees on shrinkable (cohesive) soils
- The freezing of ground water in frost-susceptible soils
18
Q
How can trees cause heave?
A
- 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
19
Q
How can freezing ground water cause heave?
A
- 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
20
Q
What is settlement?
A
- 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)
21
Q
Explain the process of diagnosing ground movement cracking.
A
- 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
22
Q
What features of a crack may indicate that it has been caused by ground movement?
A
- Extends above and below the DPC
- Affects both internal and external surfaces
- Diagonal in direction (stepped along brickwork)
- Tapered
23
Q
How does the width of a crack help determine which type of ground movement has caused it?
A
- 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
24
Q
Which tree types extract the most water from the ground?
A
Not all trees present the same risk, however the species that extract the most water are poplar, willow, oak, elm, horse chestnut and sycamore
25
As a general rule, how far away should trees be from buildings?
1. 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
2. However, this does not necessarily mean they should be removed if closer, especially mature trees
26
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
27
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
28
Explain the sequence of work involved in underpinning a strip foundation.
1. The traditional method of underpinning involves the soil beneath the existing foundation being excavated and replaced in phases with foundation material, normally concrete
2. Bays of approximately 1500mm long are excavated in a strategic order so there are never adjacent bays being worked on at the same time
29
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
30
What guidance is available in relation to underpinning?
BRE Digest 352 (Underpinning) contains further information
31
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:
1. Rusting of the metal ties (due to water penetration)
2. Poor quality mortar reducing the bond
3. Not installing the correct number of ties
32
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
33
What problems are associated with cavity wall tie failure?
1. Cracking where rust has caused the ties to expand, which can in turn lead to damp penetration and further accelerate the rusting process
2. Bowing/bulging walls
3. Damage to the roof as the external leaf increases in height
4. Extensive expansion can cause rotation in the foundation as the loads from the roof can be transferred down the outer leaf
5. In severe cases, collapse of the outer leaf
34
How would you identify cavity wall tie failure?
1. 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
2. Intrusive inspection using a boroscope may also be used to physically inspect the cavity
3. Brickwork may also be removed to physically inspect cavity
35
What steps would you recommend to remediate cavity wall tie failure?
1. Remove old ties
2. Drill in new ties through the centre of the brickwork
3. Repoint/make good brickwork
36
What different methods of replacing cavity wall ties are available?
1. 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)
2. Mechanical - features a sleeve that expands when the tie is screwed up (only suitable when masonry is in a good condition)
3. Helical - long corkscrew with a wide thread is drilled trough both leaves
37
What guidance is available for replacing cavity wall ties?
BRE 329 (Inserting Wall Ties in Existing Construction) contains further remedial guidance
38
How were the relevant British Standards changed to address cavity wall tie failure?
1. In 1945, a British Standard was introduced that provided a minimum thickness for the galvanised layer
2. In 1968, the minimum thickness was reduced because it was thought the original thickness was excessive
3. In 1981, the minimum thickness was increased again
01 Building Pathology
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