T2-4: Chloride + CO2 Penetration Flashcards
(36 cards)
What is the most common cause of deterioration in concrete?
Corrosion of steel reinforcement
- formation of rust produces expansion
- surrounding concrete cracks and spalls
Describe the state of concrete at high pH (x2)
- ‘Sound’ concrete
- The high pH of the pore solution stabilises a virtually continuous oxide film on the steel [passive film]
- This prevents rusting
Describe the state of concrete at a lower pH?
The oxide film is not continuous and corrosion occurs
What causes a lowering of the pH of the pore solution?
Carbonation
What can chloride ions cause, even at a high pH?
Can cause a local breakdown of the passive film, leading to pitting corrosion
e.g. chloride ions from de-icing salts on road bridges
.
Describe the penetration/transport of chlorides and CO2 intro concrete
What does the cement paste consist of?
The transport of the corrosion reactants (water, oxygen, ions) and corrosion products occurs through the cement paste
Cement paste consists of solids (unreacted cement & hydration products) and a liquid (pore solution)
What are the three controlling parameters of reinforcement corrosion?
- The quantity and composition of the pore solution
- The porosity and pore structure (interconnectivity)
- The quantity of Ca(OH)2 [calcium hydroxide] and the nature of C-S-H in the hardened paste
What is the pore solution of a hardened cement paste (or concrete)?
What is the pH
It is “essentially one of alkali hydroxides”
- after a day, the only ions present in concentrations above a few mmol/L are K+, Na+, OH-
pH 12-13
Why is the pore size distribution and interconnectivity of the pores important (x2)?
- It determines the rate of penetration of chloride ions and CO2
- Determines the availability of oxygen and water at the steel surface
Otherwise, the passive film isn’t maintained
Why is the quantity of calcium hydroxide important, and how much is needed?
The higher the content of Ca(OH)2, the longer it takes for carbonation to penetrate the cover
- it provides a “pH buffer”, keeping the solution at a constantly high pH
What are the two time periods when active corrosion initiates?
- Initiation period, t_0
- Propagation period, t_1
Describe the propagation period for active corrosion
- what does the steel react with and what is formed?
What does it result in (x3)?
The steel reacts with oxygen and water to form iron oxides and hydroxides, resulting in:
- reduction in rebar diameter (section loss)
- cracking/spalling of concrete cover
- decrease of steel/concrete bond
What are three ways in which chloride ions may be introduced into concrete?
- Intentional
- Unavoidable
- De-icing salts
Describe the ‘intentional’ penetration of chlorides and CO2 into concrete
For example, what was used a lots in the 1960s/70s
- Addition of calcium chloride accelerates the hydration of Portland cement
- The chloride ions have deleterious (bad) effects on the passive film mean
Name three ways in which the incorporation of chlorides is ‘unavoidable’
Potable water is not available locally
- sea water or high-chloride water used in mix
Chloride-free aggregates are not available
- e.g. porous aggregates exposed to sea water used
Sea-water attack
- pH usually around 8
What (and why) is the risk of corrosion/ penetration of chlorides and CO2:
a) below the tidal zone
b) in and above the tidal zone
How is water drawn into the concrete?
a) risk of corrosion decreases with depth, due to lack of oxygen
b) corrosion can occur as oxygen is readily available:
- wetting/drying cycles occur
- water is drawn in by capillary action or enters as spray
Where and why are de-icing salts applied?
- Often applied to bridge decks and road slabs
- In cold climates prior to freezing conditions
What is the type of chloride penetration dependent on?
Moisture content
Where does the most rapid form of penetration occur?
What is the name given to this type?
In almost dry concrete
- capillary suction of salt water
- up to a few mm of penetration can occur in a few hours
Where does slow chloride penetration occur?
What is the driving force behind chloride penetration?
In wet or highly moist concrete
- slower diffusion of the CI- ions through the pore solution
Concentration gradient is the driving force
Finish the sentence: chloride ions are ___ in the cement paste
What is the impact of this?
Chloride ions are bound in the cement paste
Amount that can be bound influences the rate at which a critical concentration of CI- reaches the steel:
- only the CI- ions dissolved in the pore solution need to be available to attack the steel
What causes CI- ions to become chemically bound (x2)?
If the chlorides penetrate at a low degree of hydration, or are present in the initial mix; some can become chemically bound
They come chemically bound by reacting with the C3A to form Friedel’s salt, which is an AFm phase
- the amount of C3A in the cement affects the amount of CI- remaining in the pore solution
For the chemical composition and crystal structure of AFm phases, what different anions can serve as ‘X’?
What factors (x7) influence the degree of chloride binding?
- C3A content of the Portland cement
- OH- : CI- (ratio)
- Cement content
- W/C ratio
- pH of pore solution
- Presence of sulfate ions
- The nature of the C-S-H (including specific surface area)
