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APC Building Surveying (New Zealand) > 13 - Building Pathology - Masonry > Flashcards

13 - Building Pathology - Masonry Flashcards

1
Q

What are the causes of cracking in buildings?

A
  1. Drying shrinkage (sand-lime bricks, too strong rendering mixes)
  2. Thermal movement (lack of vertical movement joints)
  3. Water penetration and 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 (neighbouring works, earthquakes)
  10. Underpinning
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2
Q

Why is cracking a problem?

A
  1. Cause the building to be structurally unsafe
  2. Can lead to water penetration, leading to damp problems, which can exacerbate the cracking in some cases (frost action, carbonation, wall tie failure).
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3
Q

How do you monitor cracking and what are the procedures?

A
  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
  4. Specialised cameras - provides a precise and efficient means of tracking structural changes over time. These cameras can detect even the smallest of cracks, offering a high level of accuracy that is essential for effective monitoring.
  5. Crack monitoring discs: Provide a reference point for accurate measurements of concrete crack movements
  6. Crack width gauges: Can measure cracks up to 7 mm and work well in corners
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4
Q

Why should you monitor cracking?

A
  1. Regular monitoring helps ensure the safety and integrity of buildings, bridges, and other critical infrastructure.
  2. Even minor cracks can be indicative of significant structural issues that could compromise safety. Early detection through crack monitoring allows for timely maintenance and repairs, thereby preventing minor issues from escalating into major, costly problems.
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5
Q

Categorise the severity of cracking.

A

Category 1 and 2
- Less than 5mm
- Aesthetic problems only
- Unlikely to indicate a significant structural issue

Category 3 and 4
- Between 5mm and 25mm
- Cause serviceability issues (e.g. sticking doors, penetrating damp)

Category 5
- More than 25mm
- Require structural intervention

  • BRE Digest 251, Table 1 gives 6 categories of cracks based on size
  • Anything under 5mm (category 2) are not regarded as severe
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6
Q

What are the different types of cracking?

A
  1. Hairline cracks: Very thin cracks that are usually less than 1 mm wide. They are usually not a major concern, but should be monitored.
  2. Vertical cracks: Can be caused by normal settlement, but wider cracks at the top or bottom may indicate a serious foundation shift.
  3. Horizontal cracks: Can indicate severe structural stress, such as from soil pressure or improper construction.
  4. Diagonal cracks: Can be caused by differential settling of the foundation.
  5. Stair-step cracks: Can indicate significant foundation movement and are often serious.
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7
Q

What are the crack shapes and possible causes?

A
  1. Horizontal - repeated at regular intervals
    - Corrosion of wall ties or other metals within the wall
  2. Horizontal - Single horizontal joint - High level
    - Roof spread; crack two or three causes below the eaves
    - Wall tie corrosion
    - Deterioration of the purlins within the roof or absence of diagonal bracing
    - High wind vibration can cause roof movement; no holding down straps
  3. Horizontal - Single horizontal joint - Above windows
    - Concrete lintels
    - Deterioration of brick slips set onto concrete lintels
    - Timber bressumer failures behind a brick arch
    - Collapse of a soldier arch

pg 181 Malcolm Ellis book.

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8
Q

What is subsidence?

A

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

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9
Q

What are the common causes of subsidence?

A

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
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10
Q

How can trees cause subsidence?

A

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
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11
Q

How can subsidence be rectified?

A
  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
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12
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

(relevant in NZ?)

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13
Q

What is heave?

A

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

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14
Q

What are the common causes of heave?

A
  1. The removal of trees on shrinkable (cohesive) soils
  2. The freezing of ground water in frost-susceptible soils
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15
Q

How can trees cause heave?

A
  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
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16
Q

How can freezing ground water cause heave?

A
  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
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17
Q

What are the issues that trees can have on buildings?

A
  1. Foundation damage - from roots
  2. Subsidence - roots extract moisture from the soil
  3. Blocked drains and pipes - roots
  4. Roof damage - falling branches
  5. Pest infestations - harbor pests that move into building.
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18
Q

What are the problems with the removal of trees?

A
  1. Heave - ground can swell
  2. Subsidence
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19
Q

What are the problems of planting a tree near a building?

A
  1. Ground movement - Downward movement - Tree roots can exacerbate soil shrinkage and ground movement, which can cause foundation settlement.
  2. Tree roots can grow into foundations
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20
Q

What is the difference between settlement and subsidence?

A

Settlement is applied to the failure of the components of the building.

Subsidence is used where the building is damaged by a failure in the ground.

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21
Q

What is settlement?

A
  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)
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22
Q

Explain the process of diagnosing ground movement cracking.

A
  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
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23
Q

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

A
  1. Extends above and below the DPC
  2. Affects both internal and external surfaces
  3. Diagonal in direction (stepped along brickwork)
  4. Tapered
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24
Q

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

A
  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
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25
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

26
Q

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

A
  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
27
Q

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

A

Subsidence in Relation to Insurance Claims GN, 1st Edition, 2011:

  • 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
28
Q

What is underpinning?

A

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

29
Q

Why would underpinning be necessary?

A
  1. The original foundation is simply not strong or stable enough.
  2. The usage of the structure has changed.
  3. The properties of the soil supporting the foundation may have changed (possibly through subsidence) or were mischaracterized during design.
  4. The construction of nearby structures necessitates the excavation of soil supporting existing foundations.
  5. To increase the depth or load capacity of existing foundations to support the addition of another storey to the building (above or below grade).
  6. It is more economical, due to land price or otherwise, to work on the present structure’s foundation than to build a new one.
  7. Earthquake, flood, drought or other natural causes have caused the structure to move, thereby requiring stabilisation of foundation soils and/or footings.
30
Q

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

A
  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
  3. Underpinning is accomplished by extending the foundation in depth or in breadth so it either rests on a more supportive soil or distributes its load across a greater area.
31
Q

Detail some alternative methods of underpinning.

A
  1. 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
  2. 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
  3. Expanding foam/resin injection:
    - Relatively modern technique where foam is injected into foundations, filling any voids and solidifying
    - No excavation required
  4. Injecting polyutrethane resin beneath a foundation (e.g. Teretek) - NZ
    - The resin expands, gently lifting the structure
    - Non-invasive
  5. Computer controlled, multi-point grout injection system (e.g. JOG) - NZ
    - It works by injecting cementitious grout with a tailored set time beneath the building’s foundation.
32
Q

What guidance is available in relation to underpinning?

A

BRE Digest 352 (Underpinning) contains further information

33
Q

What is the difference between TC1, TC2 and TC3 Land?

NZ

A

TC stands for Technical Category.

  1. TC1 land:
    - Unlikely to experience liquefaction-related damage in future earthquakes
    - Is suitable for rebuilding or repairing homes after earthquakes.
    - Generally considered safe for standard concrete slab or timber floor foundations
  2. TC2 land:
    - Categorized as having medium liquefaction vulnerability.
    - Requires a waffle slab which is more expensive than a standard foundation
  3. TC3 land:
    - Categorized as having high liquefaction vulnerability.
    - Requires deep geotechnical investigating and a specially designed foundation
34
Q

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

A

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
35
Q

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

A

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

36
Q

What problems are associated with cavity wall tie failure?

A
  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
37
Q

How would you identify cavity wall tie failure?

A
  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
38
Q

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

A
  1. Remove old ties
  2. Drill in new ties through the centre of the brickwork
  3. Repoint/make good brickwork
39
Q

What different methods of replacing cavity wall ties are available?

A
  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
40
Q

What guidance is available for replacing cavity wall ties?

A

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

41
Q

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

A
  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
42
Q

What are bed joints?

A

A bed joint refers to the horizontal layer of mortar on which masonry, such as bricks, are laid, acting as the surface that supports the weight of the next layer.
Bed joints are crucial in distributing loads evenly across a structure and to help bond the individual units together, helping create a strong and stable masonry wall.

43
Q

What are bed joint cracks and how can you repair them?

A
  1. Cracks in bed joints can indicate structural stress, inadequate mortar, or soil movement.
  2. Cracks in bed joints can be repaired using helical bars, which are inserted into the mortar joint to reconnect the masonry on either side of the crack.
44
Q

What is efflorescence in masonry?

A

Salts in brickwork can appear as a white, crusty deposit called efflorescence. Efflorescence can be caused by water moving through brickwork and carrying salts to the surface.

45
Q

What are calcium silicate bricks?

A

Calcium silicate (sand lime and flint lime) bricks are manufactured by mixing lime, sand and/or crushed silicaceous or flint stone together, with enough water to allow the mixture to be moulded under high pressure. The bricks are then steam autoclaved so that the lime reacts with the silica to form hydrated calcium silicates. Pigments can be added during the mixing stage. In their natural state, calcium silicate bricks are white to a creamy off-white colour, but the addition of ochres (buff or cream colours), iron oxides (pink, red, brown or black) or chrome oxide (green) can enable a very wide variety of colours to be produced.

46
Q

What are the issues with calcium silicate bricks?

A

Calcium silicate bricks can crack due to shrinkage, thermal movement, and inappropriate design.

  1. Shrinkage
    - Shrink after construction, which can cause cracking. This shrinkage is usually even throughout the brickwork.
    - Calcium silicate bricks should not be used in solid work with clay facings because clay expands, while calcium silicate shrinks.
  2. Thermal movement
    - Can experience thermal movement, which can be about 1.5 times that of clay brickwork. If the design doesn’t account for this movement, it can lead to cracking.
  3. Inappropriate design
    - If the wall ties aren’t flexible enough to allow for differential movements, it can lead to cracking.
    If there’s not enough discontinuity around cavity closers, it can lead to cracking.
  4. Other factors
    Calcium silicate bricks can be more vulnerable to light damage than clay bricks.
    Cracks in calcium silicate bricks can split brick units, which can be more difficult to repair than cracks in clay bricks.
47
Q

What is cold bridging?

A
  1. Cold bridging occurs in localised spots where the nature of the construction allows heat to escape through the structure at a higher rate than normal
  2. This creates situations where there may be quite low internal surface temperatures which can encourage patches of local condensation, sometimes interstially which is inside the surface.
48
Q

Where would you expect to find cold bridging?

A
  1. Concrete lintels crossing cavity walls
  2. Uninsulated box-section lintels
  3. Jambs and sills with returned blockwork inner leaf
  4. Floor slabs lacking insulated upstands
49
Q

What are the causes of cold bridging?

A
  1. Gaps in insulation
  2. Penetrating elements
  3. Interruptions in building fabric (e.g. studs, wall ties)
  4. Junctions (e.g. roof and wall, wall and floor)
  5. Warm structure meeting a colder one
50
Q

What are the effects of cold bridging?

A
  1. Allows heat to pass through more easily
  2. Lead to energy loss, reduced comfort, and moisture-related issues.
  3. Summer - temperatures can rise quickly.
51
Q

What are the solutions to cold bridging/ thermal bridging?

A
  1. Insulation
  2. Thermal breaks (materials with low thermal conductivity)
  3. Air sealing
  4. Proper ventilation
  5. Advanced framing techniques (e.g. staggered studs)
  6. Architectural design changes
52
Q

What is spalling in brickwork?

A

‘Spalling’ refers to the breakdown of bricks such as cracking, peeling, crumbling or chipping, which eventually lead to the surface of bricks breaking away from the main body

53
Q

How do you repair spalling brickwork?

A
  1. Identify the source of moisture: Before repairing, determine the cause of the spalling, which is usually excess moisture from leaks or poor drainage, to prevent future damage.
  2. Remove damaged brick: Carefully chisel away the spalled brick pieces, ensuring to remove all loose material.
  3. Clean the area: Thoroughly clean the exposed area with a brush and water to remove debris.
  4. Prepare mortar: Mix a batch of mortar that matches the existing brickwork.
  5. Replace brick: Insert a new brick that matches the size and color of the old one into the cavity and secure it with the prepared mortar.
  6. Cure the mortar: Allow the new mortar to properly cure according to manufacturer instructions.
54
Q

How do salts get into brickwork?

A
  1. Water from rain, snow, sprinkler systems, cracks, and gaps in the ground can dissolve salts in building materials.
  2. Water used in cement and mortar mixtures can dissolve salts in building materials.
55
Q

What is the result of salts in brickwork?

A
  1. Efflorescence: When water evaporates from the surface of brickwork, the salts are left behind as a crystalline deposit. This is known as efflorescence.
  2. Salt crystallization: The crystallization of salts in brickwork can cause pressure crystallization and expansive corrosion products.
  3. Pitting and powdering: Soluble salt crystallization can cause pitting and powdering of surfaces.
56
Q

How can efflorescence be removed?

A
  1. Use abrasive techniques like dry brushing, light water blasting, or light sandblasting
  2. Use products available from stone dealers
  3. Use muriatic acid followed by flushing with clean water
57
Q

How can efflorescence be prevented?

A
  1. Use low alkali cement in mortar and grout
  2. Store finished bricks wrapped and off the ground
  3. Ensure proper curing practices
58
Q

What are stone walls?

A

Stone walls are made of natural stone, cut stone, or reconstituted stone. They can be built with or without mortar.

Typically limestone, granite, sandstone, ashlar.

The quality of building stone varies greatly, both in its endurance to weathering, resistance to water penetration and in its ability to be worked into regular shapes before construction.

59
Q

What are the issues with stone walls?

A
  1. Stone deteriorates through the action of moisture and frost.
  2. Pointing - Stone naturally absorbs water and then dries – a process generally referred to as ‘breathing’. The use of cement mortar can prevent this breathing process, leading to damage.
  3. Stone decay - from moisture/leaks, chemicals and gas combustion, incorrect bedding, or natural weathering
  4. Stone cleaning - can cause damage
  5. Plant growth - Moss and plant growth can build up, especially on horizontal courses of stone.
  6. Rusting cramps - Cramps are used to hold stones together. If they are not rustproof, these metal cramps, when exposed to wetting over long periods, will expand as they rust, breaking the stone
  7. Salt damage next to treated roads.
60
Q

How do you repair a stone wall?

A
  1. Pointing - Repoint with a lime mortar that matches the existing.
  2. Stone decay - professional survey, remove old repairs, may be possible to get stone redressed
  3. Indent repairs - Used where stone has weathered by more than 30 – 44mm, indenting is where a defective area of a stone is cut out and new matching stone inserted using stainless steel dowels.
  4. Matching stone - Analyse samples of existing stone and suggest suitable matches from currently active quarries.
  5. Plastic repairs - Where a mortar material is used to repair damaged stonework. Rather than use cement, best practice today would be to use natural hydraulic lime mortar for the repair as this allows stone to breathe better.
  6. Dressing back stone - Some less badly worn and flaking stone can be treated with a bristle brush or (unlisted buildings only) carefully dressed back with a hand chisel.
  7. Cracked lintels - Quite common, and although they can be repaired, complete replacement is a better long-term solution.
  8. Stapling cracks - Some hairline cracks in stone can be repaired by inserting a stainless steel staple or cramp into holes on either side of the crack and recessing the staple.
  9. Treating rusted cramps or staples - Ideally remove and replace with stainless steel cramps. If not possible, treat the rusted cramp before repairing.
  10. Protecting string courses and cornices - Protect these horizontal courses of stone with a lead flashing. Ideally, the flashing should be shaped to fall away from the building, so rain splashes bounce away from the building.

APC Building Surveying (New Zealand) (16 decks)

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