T3-2: Deterioration Process Flashcards
What are the two types of reinforcement corrosion, plus other types of concrete deterioration processes?
- Carbonation
- Chloride ion ingress
- Sulfate attack
- Alkali aggregate reaction
- High alumina cement conversion
- Freeze-thaw
- Fire damage
Describe the carbonation process (four steps)
- The high pH of concrete protects rebar from corrosion by forming a passive film
- Carbon dioxide diffuses into the concrete and reacts with the hydrated cement paste, producing calcium carbonate
- This removes OH- (hydroxide) ions from the pore solution and reduces the pH
- Corrosion begins when the carbonation front reaches the depth of the bar; pH protection is lost
What do these equations represent, and what is happening?
The reaction of calcium hydroxide with carbon dioxide
There is initial densification of the concrete, followed by breakdown of the C-S-H gel itself
[NAQ] graph showing the depletion of calcium hydroxide and pH reduction (by carbonation)
“Corrosion can begin when the carbonation front reaches the depth of the bar and pH protection is lost”
What does the rate of carbonation depend on (x3)?
- Relative humidity
- Permeability of concrete
- Cement content
For example, concrete that takes longer to cure (e.g. that uses fly ash) is a key contributor to carbonation
What does this equation represent?
Prediction for the rate of carbonation
Name three other factors affect the D_c (design chemical class, required concrete quality to resist chemical attack)?
- Generally poor correlation with physical properties (e.g. oxygen diffusion and water permeability)
- Higher rates of carbonation with blended cements (can be double)
- Best correlation with buffering capacity; concentration of Ca(OH)2 [calcium hydroxide]
[NAQ] table showing the estimated years for carbonation to reach reinforcement
NB. for external concrete sheltered from rain
In chloride-induced corrosion, how do chloride ions damage reinforcement?
Chloride ions don’t affect pH (unlike carbonation), but destroy the film on embedded reinforcement caused by high pH
Name different sources of chloride
- Penetrating sources from the environment (including de-icing salts, seawater, brackish water)
- Cast into the concrete (from admixtures, contaminated aggregates, brackish water etc)
When does chloride-induced corrosion begin?
What happens?
Corrosion can begin when the concrete surrounding the bar exceeds a chloride ion threshold
Chloride ions form complexes with cement hydration products
[NAQ] graph showing the total chloride ion content with depth
ie. lower depth = higher content
What factors influence chloride-induced corrosion risk (x6)?
- Concrete quality (porosity)
- Cement content
- Cement type
- Design detailing and construction
- Environmental loading factors (wet/dry cycles, saline spray)
- When the chloride was added (at mixing or in service)
In hardened concrete, what is the difference between fixed and free chlorides?
Fixed: chlorides are chemically or physically bound to cement minerals and hydration products as chloroaluminate complexes
Free: chlorides are present as ions in the pore water of the concrete
What equation is used to measure corrosion risk?
What structures are at risk from chlorides (x4)?
- Car parks
- Coastal/marine facilities
- Bridges/highways structures
- Industrial effluent containment
[NAQ] chloride corrosion initiation graph
NB. 2-3x faster corrosion rate at the propagation stage if chloride ions are present
What are three consequences of corrosion?
- Cracking
- Spalling
- Delamination
What does this image show?
Severe corrosion and spalling of a jetty
What does this image show?
A failed prestressing wire, showing salt crystallisation and corrosion
