*Concrete Defects

Question 1

What is carbonation and how is it caused?

Answer 1

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

Question 2

What problems are associated with carbonation?

Answer 2

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

Question 3

What factors affect the rate of carbonation?

Answer 3

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)

Question 4

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

Answer 4

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

Question 5

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

Answer 5

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

Question 6

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

Answer 6

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

Question 7

How would you identify carbonation?

Answer 7

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

Question 8

How can carbonated concrete be addressed?

Answer 8

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

Question 9

What is the process of patch repairing carbonated concrete?

Answer 9

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)

Question 10

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

Answer 10

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

Question 11

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

Answer 11

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

Question 12

How can you increase the resistivity of carbonated concrete?

Answer 12

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

Question 13

What is chloride attack?

Answer 13

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

Question 14

How can chloride ions be introduced into concrete?

Answer 14

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.)

Question 15

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

Answer 15

• 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

Question 16

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

Answer 16

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

Question 17

What problems are associated with chloride attack?

Answer 17

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

Question 18

What is meant by the term ‘pitting’?

Answer 18

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

Question 19

What factors affect the rate of chloride attack?

Answer 19

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

Question 20

How would you identify chloride attack?

Answer 20

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

Question 21

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

Answer 21

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

Question 22

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

Answer 22

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)

Question 23

How can chloride attack be remediated?

Answer 23

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

Question 24

What problems are associated with patch repairing chloride attack?

Answer 24

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

Question 25

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

Answer 25

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

Question 26

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

Answer 26

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

Question 27

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

Answer 27

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)

Question 28

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

Answer 28

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)

Question 29

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

Answer 29

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

Question 30

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

Answer 30

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

Question 31

What is sulphate attack and how is it caused?

Answer 31

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

Question 32

Name some common sources of sulphates in construction.

Answer 32

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

Question 33

What problems are associated with sulphate attack?

Answer 33

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

Question 34

What building elements are typically affected by sulphate attack?

Answer 34

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

Question 35

Why are chimney stacks particularly at risk from sulphate attack?

Answer 35

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

Question 36

How would you identify sulphate attack in chimney stacks?

Answer 36

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

Question 37

How would you identify sulphate attack in concrete floor slabs?

Answer 37

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

Question 38

How would you identify sulphate attack in brickwork?

Answer 38

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

Question 39

What level of sulphate is usually considered harmful to cement/concrete?

Answer 39

Laboratory testing as per BS 1881-124 - anything over 5% sulphate content of cement (assuming cement is 15% of mass of concrete) can be harmful

Question 40

What steps would you recommend to remediate sulphate attack?

Answer 40

1. Keep the concrete dry by installing a DPM to serve as a barrier to moisture to prevent salt migration
2. Reconstruct affected elements (floors, walls, foundations) with sulphate-resisting cement
3. Sulphate-resistant bricks may also be used

Question 41

How can rendering brickwork often exacerbate problems caused by sulphate attack?

Answer 41

1. Dense cement renders often crack and let water in, but restrict it from drying out
2. Soluble sulphates in the bricks themselves can then dissolve out to react with the cement mortar, causing it to expand

Question 42

What is ASR and how is it caused?

Answer 42

ASR is a reaction produced when highly alkaline pore water in concrete mixes with silica molecules in certain rocks and minerals

For ASR to occur, 3 interrelated factors must be present:

1. High alkalinity (either from cement or other external sources)
2. Sufficient moisture
3. Critical silica in the aggregate

Question 43

What problems are associated with ASR?

Answer 43

1. The chemical reaction produces a gel, which absorbs water, expands and can cause the concrete to crack or disrupt
2. Cracks then allow more moisture to enter to fuel the reaction, thus producing greater amounts of gel
3. Durability of the concrete can thus be compromised
4. In extreme cases, the tensile strength of the concrete component can be reduced

Question 44

How would you identify ASR?

Answer 44

Visual appearance:

1. Pattern of map cracking occurs - applies to unreinforced concrete
2. In other occurrences, small ‘pop-outs’ (approx. 30-50mm) form - like concrete with acne
3. Spalling will often reveal a reservoir of sticky gel behind it or gel will be exuding from cracks
4. When carbonated, the gel appears as a whitish opaque coating - like bad efflorescence

Laboratory testing can confirm diagnosis

Question 45

What steps would you recommend to remediate ASR?

Answer 45

1. Existence of ASR is not necessarily fatal to a structure (depends on severity and elements affected)
2. A risk-based analyse is recommended before deciding upon replacement strategies
3. Without completing replacing the affected concrete, there are no full remedies, only mitigation measures

Question 46

What measures can be undertaken to mitigate the affects of ASR?

Answer 46

1. Application of penetrating breathable sealers - cannot be used when structure is permanently wet and must be reapplied every 5 years at most
2. Crack filling with flexible caulking - only benefit is it slows water ingress
3. Apply physical constraint to confine/strengthen structure - may be difficult to achieve and does not stop the process of ASR occurring
4. Over-cladding - may trap moisture and cause difficulty in future inspection

Question 47

What is ACR and how is it caused?

Answer 47

1. Very rare as it only occurs with certain impure forms of dolomitic limestone
2. The aggregate reacts with the dissolved potassium and sodium alkalis within the pore fluid
3. This then alters the crystal structure of the aggregate, causing it to expand

Question 48

What is meant by the term concrete cancer?

Answer 48

• Another name given to Alkali Aggregate Reactions (of which ASR is the most common in the UK)
• This term could be misleading and is therefore best avoided

Question 49

What different types of concrete tests are there?

Answer 49

1. Schmidt Hammer Test (aka rebound hammer test) - used to determine the compressive strength of concrete
2. Chemical testing (e.g. phenolphthalein for Carbonation)
3. Laboratory testing (e.g. profile grinding for Chloride Attack)

Question 50

What guidance is available in relation to concrete repair methods?

Answer 50

• BRE 444-3 (Corrosion of Steel in Concrete) - remedial measures and guidance for appropriate repair methods
• BS EN 1504 - 10 parts covering different concrete repairs
• The Concrete Society - Technical Report 69 ‘Repair of concrete structures with reference to BS EN 1504’ (2009) - explains the concepts provided for in the BS

Question 51

The reinforcement bars on a concrete building are exposed and corroding. What are the causes and remedial works strategies?

Answer 51

• Likely to be either carbonation, chloride attack or both
• Remedial measures depend on the extent of the damage
• Combination of the following may be appropriate:

1. Patch repair
2. Re-alkalisation
3. Desalination
4. Cathodic protection
5. Apply corrosion inhibitor