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

13 - Building Pathology - Concrete Flashcards

1
Q

What three mechanisms cause concrete to deteriorate?

A
  1. Chemical mechanisms - carbonation, chloride contamination, alkali-aggregate reaction and sulphate attack.
  2. Physical mechanisms
    - Environmental - freeze-thaw cycles, weathering and salt crystallization
    -Physical - structural loading, abrasion, impact damage, shrinkage, and expansion.
  3. Biological mechanisms - invasive plants and micro-organisms on the concrete surface

The most serious forms of deterioration are those that ultimately lead to the corrosion of the steel reinforcement. The reinforcement not only supplies crucial tensile strength that diminishes with corrosion, but as it corrodes also forms highly expansive products, which can cause the surrounding concrete to crack and, eventually, break off.

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2
Q

What are common defects in concrete buildings?

A
  1. High alumina cement
  2. Spalling
  3. Carbonation
  4. Suphate attack
  5. Alkali silica reaction
  6. Chloride attack
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3
Q

What is concrete spalling and how does it occur?

A
  1. Where the surface of the concrete begins to flake and peel away. This can be due to chemical reactions, natural weathering, or natural aging. It indicates a severe weakness in some parts or the entire structure.
  2. Spalling occurs when water penetrates the pores of the concrete and expands upon freezing, causing cracks that grow vertically and horizontally along their length. These cracks continue to grow, become unstable, and crumble away from each other due to thermal expansion forces.
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4
Q

How do you repair spalling in concrete?

A
  1. Install a new overlay on your concrete.
  2. Use a color-matching compound to patch the spalled area.
  3. Remove and replace the whole slab.
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5
Q

What are cracks in concrete and how do they occur?

A
  1. Is a whole or partial split of concrete into two or more portions caused by fracturing or breaking.
  2. Structural cracks caused by movement in the structure.
  3. Shrinkage cracks caused by water evaporating from the concrete mixture
  4. Concrete tends to shrink as it dries, allowing moisture to accumulate inside and causing the material’s surface to crack. It can also be caused by frost heave, improper mixing, humidity changes, poor quality concrete, or high water content.
  5. The cracks can be small or large depending on the amount of moisture present at the time.
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6
Q

How do you repair cracks in concrete?

A
  1. If the cracks are small, fill them with masonry cement.
  2. If the cracks are extreme, remove and replace the concrete.
  3. If you don’t like the appearance of the repairs, go for concrete resurfacing.
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7
Q

What is concrete efflorescence and how does it occur?

A
  1. Concrete efflorescence is the whitish powdery deposition that coats unfinished concrete surfaces. It’s salt that has been deposited on the surface of the concrete.
  2. Efflorescence occurs when vapor, carrying soluble salts, comes into contact with concrete. When the water evaporates, it leaves behind a white, powdery deposit. This defect occurs for several reasons, including poorly sealed or cured concrete, low temperatures, condensation, cracks or voids in concrete, use of salty water during concrete construction and deicing salt on concrete surfaces, and excessive humidity.
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8
Q

How do you repair concrete efflorescence?

A
  1. Apply pressurized water to dissolve efflorescence.
  2. Brush using a stiff bristle broom or brush.
  3. Use a mild acid rinse.
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9
Q

What is carbonation and how is it caused?

A
  1. Carbonation is the process by which carbon dioxide slowly penetrates concrete and dissolves in water present within its pores, forming a mildly carbolic acidic solution
  2. This acidic solution reacts with the alkaline calcium hydroxide (one of the components of concrete) to form calcium carbonate
  3. This results in a pH drop, reducing the alkalinity of the concrete (from more than 12.5 to approximately 8.5)
  4. This carbonation process progressively moves through the concrete over time
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10
Q

What problems are associated with carbonation?

A
  • Corrosion
  • Micro cracks and shrinkage
  1. The passive layer around reinforcing steel will deteriorate when the pH falls below 10.5
  2. Therefore, once carbonation reaches any steel, the concrete is insufficiently alkaline to protect the steel’s passive layer and thus becomes ‘active’ (aka depassivation)
  3. Moisture and oxygen ingressing through the porous concrete can now react with the steel, which may begin to rust and corrode
  4. If this happens, the steel will expand, which can cause cracking and spalling of the concrete cover, thus compromising its structural integrity
  5. This also makes it easier for aggressive agents to ingress towards the steel, thus further increasing the rate of corrosion
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11
Q

What factors affect the rate of carbonation?

A
  1. Quality and density of the concrete - good quality, well compacted concrete will carbonate at a much slower rate
  2. Exposure of the building to water and carbon dioxide (permanently wet conditions hinder penetration so carbonation will be low)
  3. Relative humidity of the atmosphere - carbonation is encouraged where RH is between 25-75% (optimum at 50-75%; anything over 75% usually hinders the rate of carbonation as the excess moisture slows the rate of carbon dioxide entry)
  4. Temperature - warmer temperatures increase the rate of carbonation (subject to RH)
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12
Q

Is the rate of carbonation greater internally or externally? Why?

A

The rate of carbonation is usually greater internally due to the higher relative humidity and temperature

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13
Q

Is carbonation more likely to cause corrosion in internal or external concrete? Why?

A

External concrete, due to the increased presence of moisture and oxygen that can penetrate the carbonated concrete

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14
Q

Under what circumstances is the risk of corrosion through carbonation particularly high?

A

If poor compaction and strength (perhaps caused by a too high water/cement ratio) is coupled with reinforcement with little cover

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15
Q

How would you identify carbonation?

A

Visual Appearance:

  • Longitudinal cracking along the line of any steel reinforcement (hairline cracking can occur as early as a few months after construction)
  • Brown stains as a result of the rusting steel
  • Over time, the expansion of rusting steel will result in further cracking and spalling of the surface concrete

Chemical Testing:

  • Used to determine depth of carbonation
  • Phenolphthalein solution is sprayed onto a fresh sample of the concrete
  • Non-carbonated areas will turn pink/purple (alkaline), whereas carbonated areas will remain colourless (neutral pH value due to reduced alkalinity)
  • The depth of penetration can then be measured
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16
Q

How can carbonation be addressed?
(Repair/treatment)

A

Firstly consider the likely rate of ongoing deterioration and the required life of the structure to assess the cost effectiveness of different protection and repair strategies

Options:

  1. Patch Repair
  2. Re-alkalisation by Diffusion
  3. Electrochemical Re-alkalisation
  4. Increase Resistivity
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17
Q

What is the process of patch repairing carbonated concrete?

A
  1. Clean surface
  2. Remove loose concrete
  3. Remove corrosion (e.g. grit blasting)
  4. Prime the reinforcement with alkali-based solution
  5. Reinstate concrete cover using patch repair mortar, sprayed concrete or conventional concrete (for large areas only)
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18
Q

What is the process of remediating carbonated concrete through re-alkalisation by diffusion?

A
  1. For concrete that has only suffered minor carbonation
  2. A thickness of fresh alkaline concrete is applied to the surface of the concrete
  3. Migration of alkalis from the fresh to the original concrete will allow for gradual re-alkalisation
  4. Not advisable to rely on this method alone if the average depth of carbonation exceeds 10mm, as moisture from the fresh concrete can ingress and increase the rate of steel corrosion
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19
Q

What is the process of remediating carbonated concrete through electrochemical re-alkalisation?

A
  1. A temporary anode mesh is fitted close to the surface of the concrete and is electrically connected to the steel reinforcement (cathode) and a power supply
  2. An electrolyte (usually a sprayed cellulose fibre) is applied around the anode mesh
  3. The steel cathode then attracts alkali metal ions towards it, so high alkalinity is restored around the steel
  4. Process takes approximately 3-10 days
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20
Q

How can you increase the resistivity of carbonated concrete?

A
  1. Surface coatings - designed to restrict the penetration of carbon dioxide (must still allow the concrete to dry out)
  2. Hydrophobic impregnants - designed to repel water
  3. Sheltering the concrete component - e.g. ventilated external cladding
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21
Q

What is chloride attack?

A

Chloride attack is the process by which chloride ions are introduced into concrete, which reduces its alkalinity.

Corrosion takes place as the chloride ions meet with the steel and the surrounding passive material to produce a chemical process which forms hydrochloric acid. The hydrochloric acid eats away at the steel reinforcement and thus leads to concrete cracking, spalling, and eventually failure.

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22
Q

What are the three main sources of chlorides?

A
  1. De-icing salts
  2. Marine environment
  3. Intrusive chlorides
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23
Q

What are two types of intrusive chlorides?

A
  1. Calcium chloride admixture (accelerator)
  2. Contaminated constituents eg marine aggregates
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24
Q

How can chloride ions be introduced into concrete?

A
  1. Introduced as an accelerator during the mixing process (calcium chloride)
  2. Introduced naturally (e.g. from the use of unwashed marine aggregates)
  3. Introduced as a result of external contamination (e.g. de-icing salt, exposure to salt spray etc.)
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25
Q

When was it common to introduce chloride ions into concrete as an accelerator?

A
  • Prevalent in the 1950s and 1960s - good for concreting in cold weather as the concrete or mortar would harden quickly, thus developing early resistance to freezing and thawing
  • Common until around 1978
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26
Q

What method of introducing chloride ions into concrete can cause the most damage?

A

External contamination as the concentration can be more erratic and the ions are not chemically bound

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27
Q

What problems are associated with chloride attack?

A
  1. Loss of alkalinity of the concrete removes its protective capability to stop any encased steel from oxidising (depassivation)
  2. Similar to carbonation, moisture and oxygen can then lead to expansion of the steel and cracking and spalling of the concrete cover, thus compromising its structural integrity
  3. Furthermore, as the chloride ions make contact with the steel and the surrounding passive material, hydrochloric acid is formed
  4. The hydrochloric acid will then eat away at the steel reinforcement (aka ‘pitting’) and could cause loss of section and serious structural failure
  5. Where high levels of chloride are present, corrosion of steel can occur even if the concrete is highly alkaline
  6. Where carbonation is also present, chloride attack can increase the rate of oxidisation of the steel reinforcement.
28
Q

What is meant by the term ‘pitting’?

A

Localised corrosion that leads to the creation of small holes in the metal

29
Q

What factors affect the rate of chloride attack?

A
  1. Physical characteristics of the concrete - i.e. calcium chloride used as an additive or introduced naturally
  2. Quality and density - denser concrete will be less porous and therefore decrease the rate at which chloride ions can reach the steel
  3. Physical condition - e.g. cracks and damage can speed up the transportation of moisture and ions to the steel (freeze thaw cycles can then exacerbate the process further)
  4. Location - sea walls, marine structures (sea water is a major source of chloride ions), areas where de-icing salts have been used and remain in-situ.
30
Q

How would you identify chloride attack?

A

Visual Appearance:

  • Can induce large cracking or bulging within the concrete of a more localised nature than carbonation
  • Black coloured rusting and pitting of the steel where aggressive hydrochloric acid has attacked
  • May be more difficult to see as pitting can occur where there is no cracking/spalling of the concrete

Chemical Testing:

  • Indicator solution is applied and if the liquid turns brown, significant chlorides are present
  • If it turns yellow/white, chlorides may be present and further investigation is required

Laboratory Testing.

31
Q

Name some of the different methods of obtaining samples to laboratory test for chloride attack.

A
  1. Lump sample - section is knocked off (ideally about 50g from a depth of at least 25mm) for testing, although corner samples may distort results as they may have been subjected to chloride ingress from two sides
  2. Dust drilling - dust is extracted using a rotary percussion drill, although cannot take incremental readings / profile
  3. Profile grinding - specialist grinder used to obtain concrete powder at selected, exact depth increments.
32
Q

Why is it important to take samples at different depths whilst testing for chloride attack?

A

To establish the concentration at different levels to determine whether the chloride content is a result of:

  • Airborne contamination (concentration highest towards the surface and gradually diminishing into the depth of the concrete)
  • Other sources (concentration at a more even distribution).
33
Q

How can chloride attack be remediated?

A
  1. Patch Repair
  2. Desalination (Chloride Extraction)
  3. Cathodic Protection
  4. Corrosion Inhibitors.
34
Q

What problems are associated with patch repairing chloride attack?

A
  1. Difficult due to the tendency for new corrosion cells to form at the boundary of the repair (aka incipient anode effect) - this can be minimised by removing where possible all concrete with significant chloride contamination
  2. Not sufficient for high levels of chlorides and long term protection.
35
Q

What can be introduced to help minimise the problems associated with patch repairing chloride attack?

A

The introduction of proprietary sacrificial zinc anodes embedded within the patch repair attached to the reinforcement can help reduce incipient anode effect.

36
Q

What is the process of remediating chloride attack through chloride extraction?

A
  1. Short-term process where negatively charged chloride ions can be electrochemically repelled from the steel
  2. A temporary anode mesh is fitted close to the surface of the concrete and is electrically connected to the steel reinforcement (cathode) and a power supply
  3. An electrolyte (usually a sprayed cellulose fibre) is applied around the anode mesh
  4. Once a current is applied for a period of time (may be up to 40 days), the chloride ions are transported from the concrete to its surface, where they are then carried out by water or removed with the temporary electrolyte.
37
Q

What problems are associated with remediating chloride attack through chloride extraction?

A
  1. Chloride that has penetrated deeper than 20mm can be hard to remove
  2. Total extraction is impossible, so risk of reoccurrence is likely
  3. Difficult to remove chloride ions bound in the mix at the time of construction (easier when chlorides had been introduced from external sources)
  4. Worries that it may generate Alkali Silica Reaction (ASR) - currently being researched
  5. Cannot be applied to prestressed concrete because of risk of hydrogen embrittlement (phenomenon where high-strength steel becomes brittle and fractures following exposure to hydrogen).
38
Q

What is the process of remediating chloride attack through cathodic protection?

A
  1. Similar to desalination, however current densities are generally lower and the system is designed for continuous use, not a short period of time
  2. The anodes are usually connected to a data-logging system so that current densities and corrosion rates can be monitored and corrected where necessary
  3. Tried and tested long-term solution for heavily chloride-contaminated structures (e.g. car parks).
39
Q

What is the process of remediating chloride attack through the use of corrosion inhibitors?

A
  1. Penetrates the concrete and creates a very thin protective layer around the steel
  2. Cost-effective alternative to conventional repairs.
40
Q

What problems are associated with remediating chloride attack through the use of corrosion inhibitors?

A
  1. Molecules are fairly large so can be slow to penetrate, particularly when the concrete mix is dense
  2. Applications in damp conditions may also reduce speed and effectiveness of treatment
  3. Considered to be more appropriate when used as part of a treatment system, not on its own.
41
Q

What is sulphate attack and how is it caused?

A
  1. Sulphate attack is a chemical reaction where water soluble sulphate salts are transported into cement mortar or concrete
  2. They react with the tricalcium aluminate (one of the components of Portland Cement) to form ettringite
  3. Ettringite is characterised by the formation of acicular crystals, which generate high expansive forces in the mortar or concrete
  4. For sulphate attack to occur, there must be sufficient sulphate and sufficient long-term water.
42
Q

Name some common sources of sulphates in construction.

A
  1. Soils containing high sulphate levels
  2. Contaminated hardcore (that containing high levels of black ash, burnt colliery shale, blast furnace slag and similar materials)
  3. Bricks
  4. Air pollution
  5. Exhaust gases of slow-burning fuel appliances
43
Q

What problems are associated with sulphate attack?

A
  1. Cracking, expansion and bulging due to loss of bond between cement paste and aggregates
  2. Sometimes the face of bricks spall, most commonly around their edges
  3. Often accompanied by frost attack due to water saturation
44
Q

What building elements are typically affected by sulphate attack?

A
  1. Chimney stacks
  2. Mortar joints
  3. Concrete floor slabs
  4. Internally where cement and gypsum are in contact (e.g. adding gypsum plaster to a cement/sand mix to accelerate its set) and remain wet for long periods
  5. Cement-based undercoat plasters if they contain ash (a sulphurous material) and if they remain wet for long periods
45
Q

Why are chimney stacks particularly at risk from sulphate attack?

A
  1. Very exposed to rain
  2. Additional sulphates are provided by exhaust gases from fires
  3. Additional moisture is provided by exhaust gases condensing inside the cold upper parts of the chimney
46
Q

How would you identify sulphate attack in chimney stacks?

A

Leaning due to different wetting and drying cycles between elevations (wetter side suffers most expansion)

47
Q

How would you identify sulphate attack in concrete floor slabs?

A
  1. Because it is restrained, upwards bowing towards centre coupled with map pattern cracking
  2. Displacement of brickwork at slab level
48
Q

How would you identify sulphate attack in brickwork?

A
  1. Expansion of brickwork along brick joints both horizontally (distinguishable from wall tie failure as it may occur in every joint) and vertically, particularly in rendered brick
  2. Bowing upwards, particularly if restrained
49
Q

What is High Alumina Cement (HAC) ?

A
  1. Composed of calcium aluminates which is found in certain types of clay
  2. Rapid strength development made HAC popular from 1950 to 1970.
  3. Used in maritime application and structural concrete.
  4. Also known as Calcium Aluminate Cement (CAC) - is available in New Zealand.
50
Q

What are the issues with High Alumina cement (HAC)?

A

Undergoes mineralogical change called conversion - increases its’ porosity, reduces it’s concrete strength and susceptibility to chemical attack.
2. Conversion is identified by concrete becoming friable and changes to a chocolate brown colour.

51
Q

How would you investigate HAC and confirm it?

A

If the presence of HAC is suspected, confirmation requires chemical or laboratory testing of samples. If the presence of HAC is confirmed, professional advice on its condition may be required.

  1. Identification - assessing the areas affected.
  2. Strength assessment - confirm the structural strength of the affected elements e.g precast concrete beam.
  3. Durability assessment - confirms the long term durability of the concrete and risk of chemical attach to associated reinforcement - involves petrographic analysis.
52
Q

Why was it banned in the UK?

A

HAC concrete was effectively banned for use in new structural concrete in the UK following a few well publicised collapses in the 1970s. Time and experience have shown that the primary causes of these collapses were poor construction details or chemical attack, rather than problems with the concrete itself.

53
Q

Is High Alumina Cement (HAC) / Calcium Aluminate Cement (CAC) used in New Zealand?

A

High aluminate cement, also known as calcium aluminate cement (CAC), is available in New Zealand from a number of suppliers. It is used in a variety of applications, including concrete repair, refractory mortars, and sewer rehabilitation.

e.g. Calcium Aluminate Cement (refractory cement) - High strength & refractoriness, good resistance to corrosion & wear, high thermal shock stability, and low thermal conductivity.

54
Q

There are two types of alkali aggregate reactions (AAR). What are they?

A
  1. Alkali silica Reaction (ASR)
  2. Alkali Carbonate Reaction (ACR)
55
Q

What is Alkali Carbonate Reaction (ACR)?

A

Alkali carbonate
reaction (ACR), occurs with some types of limestone aggregate. In New Zealand, limestone aggregate is not widely used, consequently no cases of ACR have been observed to date here. (as of 2021)

56
Q

What is Alkali silica reaction (ASR)?

A

Alkali-silica reaction (ASR) is a chemical reaction that occurs in concrete when alkaline substances in the concrete react with silica in certain aggregates. The reaction creates a gel that expands when it absorbs water, which can cause the concrete to crack.

Known as concrete cancer.

57
Q

What is the process for the Alkali Silica Reaction? How does it happen?

A
  1. Alkali hydroxide + reactive silica = alkali-silica gel
  2. Alkali-silica gel + moisture = expansion
  • The alkalis in concrete come from cement, admixtures, aggregates, or the environment.
  • The reaction occurs when the alkaline pore fluid in the concrete reacts with silica in aggregates.
  • The reaction creates a gel that absorbs water and expands, putting pressure on the concrete.
  • This hygroscopic gel swells and increases in volume when absorbing water: it exerts an expansive pressure inside the siliceous aggregate, causing spalling and loss of strength of the concrete, finally leading to its failure.
58
Q

What are the three conditions ASR needs?

A
  1. Moisture
  2. Reactive aggregates
  3. Alkali (high pH)
59
Q

How can ASR be detected?

A
  1. The first signs are often irregular surface cracks with gel material.
  2. It can take years or decades to appear.
    3.It can be detected using a test kit that stains the gel to indicate the presence of ASR.
60
Q

How can ASR be prevented?

A
  1. Limit concrete alkali content
  2. Use right amount of supplementary cementitious materials (SCM)
  3. Use non-reactive aggregates
  4. Use ASR control admixture
  5. Reduce concrete permeability
61
Q

How can ASR be tested?

A
  1. Visual : map cracking
    - spray surface with uranyl-acetate (appears bright yellow under UV)
  2. Petrographic analysis: examine concrete under microscope
  3. Chemical test ASTM C289
    4 .Mortar bar expansion test
  4. Concrete expansion tests
  5. Field performance of aggregates
62
Q

What are the NZ documents for ASR?

A

TR3 Alkali Silica Reaction: Minimising the Rise of Damage to Concrete
Author: ConcreteNZ
2021 edition

63
Q

What damage has been cause to structures in NZ by ASR?

A
  1. Damage in New Zealand structures due to ASR is usually
    minor compared to that observed overseas.
  2. Evidence indicates that minor ASR can occur even where low-alkali cement and pozzolan have been used and concrete alkali contents are less than 2.0 kg/m3.
  3. The extent of ASR observed indicates that given the highly reactive nature of some New Zealand volcanic rocks, the reaction has the potential to cause significant damage in concrete structures unless adequate controls
    are maintained.
64
Q

What damage has been seen?

A

Damage is normally confined to concrete where one or more sides are exposed to moisture. Visible damage
usually takes the form of areas of pattern cracking or isolated cracks not obviously due to structural causes
or normal concrete behaviour. Only in the more advanced cases is extensive pattern cracking observed and
apart from the darkening of crack margins, it is rare to observe alkali-silica gel on the surface of the concrete.

65
Q

What are the differences between ASR in New Zealand and Overseas?

A
  1. Freeze-thaw damage: Apart from some elevated and inland areas, freeze-thaw attack on concrete is rare
    in New Zealand and those few cases observed are usually minor. The severe damage due to the expansive
    freezing of water that has percolated into the cracks of concrete undergoing ASR has not been observed
    in this country. Similarly, de-icing salts are rarely used, so ASR is not exacerbated by this external source of
    alkali.
  2. .Salt-spray: A large portion of New Zealand is subject to deposition of salt spray (Balance & Duncan, 1985)
    carried in the prevailing winds. However, most areas receive sufficient rain throughout the year to wash salt
    off exposed surfaces. Thus augmentation of alkalis from deposited salt-spray is restricted to sheltered parts
    of structures and does not contribute significantly to ASR (see (c) below, and also section 4.2.3).
    (c) Humidity and condensation: Atmospheric relative humidity is commonly between 70-80% in coastal areas
    and approaches 60% inland. These conditions, combined with a high incidence of wind, effectively prevent
    serious condensation on the surfaces of structures that could otherwise increase the risk of ASR.
  3. Low-alkali cement: Since the 1950’s, low alkali cements have been used in all major public construction.
    Although there has been no mandatory limit on cement alkalis, almost all cements manufactured in
    New Zealand since 1970 (St John, 1988, April) have had alkali contents less than 0.6% Na2
    O equivalent.
    From 1974, NZS 3122 has allowed an alkali content of less than 0.60% Na2
    O equivalent to be specified if
    the cement is to be used with potentially reactive aggregate. Since 2015 the alkali limit of 0.60% has been
    the default for cement types GP and GB. Low-alkali cement has thus been the norm for use with reactive
    aggregates, unlike overseas where low-alkali cement is often not readily available. This is the principal reason
    for the low incidence of ASR damage in New Zealand structures. The use of low-alkali cement in New Zealand
    has minimised the damage due to ASR, and provides a ready means of minimising the problem in the future.
    However, higher-alkali cements (produced locally or imported) are likely to become available in New Zealand
    in the future because their production is cheaper and/or less energy intensive. If such cements are used, the
    recommendations in this document must be followed.
66
Q

What is the treatment if ASR in existing concrete?

A
  1. Seal it with silane
  2. Use lithium salt solutions
    - spray on surface
    -electrochemical impregnation
67
Q
A

APC Building Surveying (New Zealand) (16 decks)

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