03 Cracking

Question 1

Why do buildings crack?

Answer 1

1. Drying shrinkage (sand-lime bricks, too strong rendering mixes)
2. Thermal movement (lack of vertical movement joints)
3. Frost action (freeze-thaw cycle)
4. Ground movement (settlement, subsidence, heave)
5. Wall tie failure
6. Chemical reactions (carbonation, chloride attack, sulphate attack, ASR)
7. Lack of lateral restraint (‘book-end’ effect)
8. Overloading (roof spread, increase of internal imposed loads)
9. Vibration

Question 2

Why is cracking a problem?

Answer 2

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)

Question 3

Name some of the causes of stepped cracking in brickwork.

Answer 3

???

Question 4

You see a vertical crack internally to a property’s wall - what may have caused it?

Answer 4

???

Question 5

What would cracking at DPC level indicate?

Answer 5

???

Question 6

How do you monitor cracking and what are the procedures?

Answer 6

1. 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
2. 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
3. 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

Question 7

Categorise the severity of cracking.

Answer 7

• 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

Question 8

How can cracked, worn or spalling bricks be repaired?

Answer 8

???

Question 9

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?

Answer 9

???

Question 10

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?

Answer 10

???

Question 11

What is subsidence?

Answer 11

Downward movement of a building foundation caused by loss of support beneath

Question 12

What are the common causes of subsidence?

Answer 12

Usually associated with volumetric changes in the subsoil, possibly due to:

1. Influence of trees on shrinkable (cohesive) soils
2. Washing away of non-cohesive soils (e.g. leaking drains, burst water mains or underground streams)
3. Change in ground-water levels (e.g. abstraction or land drainage)
4. Mining
5. Nearby excavations

Question 13

How can trees cause subsidence?

Answer 13

Influence of trees on shrinkable (cohesive) soils:

1. 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
2. Worse throughout periods of hot, dry weather (soils can become desiccated even without the presence of trees)
3. As a tree grows it will extract increasingly more water from the ground, which can still cause desiccation even without hot weather
4. Introducing new trees causes more water to be removed from the ground, thus heightening the problem

Question 14

How can subsidence be rectified?

Answer 14

1. Repair/alleviate the cause (e.g. repair/replace leaking drains, reduce height of trees)
2. Consult with an arboriculturalist when removing/gradually reducing the height of trees is concerned
3. Monitor the cracks and if no further movement occurs, repoint/patch/replace as necessary
4. Where movement is excessive and on-going, underpinning may be required
5. Where movement is slight and thought to be seasonal (usually where trees are concerned), it will be necessary to monitor the movement

Question 15

Who usually pays for damage caused by subsidence?

Answer 15

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

Question 16

What is heave?

Answer 16

Upward movement of a building foundation caused by the expansion or swelling of the subsoil

Question 17

What are the common causes of heave?

Answer 17

1. The removal of trees on shrinkable (cohesive) soils
2. The freezing of ground water in frost-susceptible soils

Question 18

How can trees cause heave?

Answer 18

1. The ground slowly regains moisture that was once taken up by the tree roots
2. Can last up to 10 years so care is needed when building on sites soon after trees have been removed

Question 19

How can freezing ground water cause heave?

Answer 19

1. After a period of high rainfall, water fills the voids between the particles and in freezing weather expands as it turns to ice
2. Additional damage can be caused when the ice thaws and the ground settles
3. Worse if ground has a high water table

Question 20

What is settlement?

Answer 20

1. Natural compaction of soil due to the load imposed by the building
2. Happens in all buildings and occurs soon after construction
3. 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)

Question 21

Explain the process of diagnosing ground movement cracking.

Answer 21

1. Inspect the cracks - location, size, direction, age
2. Investigate the site - ground profile, soil type, tree proximity, drains testing, mining area
3. Determine the cause of the cracks from steps 1 and 2
4. Begin remedial action - alleviate cause, provide underpinning if necessary
5. Monitor cracks - minimum 18 months
6. Assess action for trees (if necessary) with an arboriculturalist

Question 22

What features of a crack may indicate that it has been caused by ground movement?

Answer 22

1. Extends above and below the DPC
2. Affects both internal and external surfaces
3. Diagonal in direction (stepped along brickwork)
4. Tapered

Question 23

How does the width of a crack help determine which type of ground movement has caused it?

Answer 23

1. Heave - cracks are wider at the top and narrower at the bottom (both the cracks themselves and their location on the building)
2. Subsidence - cracks are narrower at the top and wider at the bottom (both the cracks themselves and their location on the building)
3. Local subsidence - cracks are wider at the top and narrower at the bottom

Question 24

Which tree types extract the most water from the ground?

Answer 24

Not all trees present the same risk, however the species that extract the most water are poplar, willow, oak, elm, horse chestnut and sycamore

Question 25

As a general rule, how far away should trees be from buildings?

Answer 25

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

Question 26

What RICS guidance is available in relation to ground movement and what information does it contain?

Answer 26

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

Question 27

What is underpinning?

Answer 27

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

Question 28

Explain the sequence of work involved in underpinning a strip foundation.

Answer 28

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

Question 29

Detail some alternative methods of underpinning.

Answer 29

• 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

Question 30

What guidance is available in relation to underpinning?

Answer 30

BRE Digest 352 (Underpinning) contains further information

Question 31

What is cavity wall tie failure and how is it caused?

Answer 31

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

Question 32

What is black ash mortar and how can it be problematic when used in cavity wall construction?

Answer 32

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

Question 33

What problems are associated with cavity wall tie failure?

Answer 33

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

Question 34

How would you identify cavity wall tie failure?

Answer 34

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

Question 35

What steps would you recommend to remediate cavity wall tie failure?

Answer 35

1. Remove old ties
2. Drill in new ties through the centre of the brickwork
3. Repoint/make good brickwork

Question 36

What different methods of replacing cavity wall ties are available?

Answer 36

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

Question 37

What guidance is available for replacing cavity wall ties?

Answer 37

BRE 329 (Inserting Wall Ties in Existing Construction) contains further remedial guidance

Question 38

How were the relevant British Standards changed to address cavity wall tie failure?

Answer 38

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