42907 Chap.6 Carbonation induced corrosion of reinforcement
Carbonation induced corrosion of reinforcement
Week 4 reaches the same failure as the chloride chapters by the opposite route: instead of adding an aggressive ion, carbonation removes the alkalinity that keeps the passive film stable.
The chapter follows the reaction through the pH ladder from 12.5 down to about 9, sets out the root of time front law with its coefficient in millimetres per root year, and records the two measured effects that dominate it, a twelvefold swing with curing and a rise with supplementary binder content.
It closes on the humidity scissor: the front advances fastest near 65 per cent humidity while corrosion is fastest near 90 to 95 per cent.
What this chapter covers
- 01
Carbonation lowers the pH with the ingress of carbon dioxide, depassivating the bars in a damp environment
- 02
Diffusion, infiltration, reaction: the three step mechanism
- 03
The reaction with portlandite and with calcium silicate hydrate, and the five things it does
- 04
The pH ladder: 12.5 to 13 sound, above 10.5 protected, below 10.5 depassivating, about 9 at the bar
- 05
Phenolphthalein colours, and why the indicator front is not the depassivation front
- 06
Intrinsic factors: reactive hydrate content and the diffusivity of the pore structure
- 07
External factors: carbon dioxide concentration, temperature and humidity
- 08
The optimum humidity for carbonation, about 65 per cent, and why an optimum exists at all
- 09
The root of time law, and why the coefficient is in millimetres per root year
- 10
Curing moving the coefficient from 6.0 to 0.5 on one mix, and supplementary binders raising it
- 11
Accelerated testing, and accelerated coefficients running about ten times the natural ones
- 12
Service life as initiation plus propagation, with the corrosion depth for cracking at 100 micrometres
- 13
Corrosion rate against relative humidity, for carbonated and for chloride contaminated concrete
- 14
Assembling a design coefficient: base value, binder correction, climate correction
From a field survey to a remaining initiation period
- +1Back out the coefficient from the survey: the depth divided by the square root of the age, that is 20 divided by 5, which is 4.0 millimetres per root year.
- +1Invert the front law at the cover: the initiation period is the cover divided by the coefficient, squared. That is 35 over 4.0, which is 8.75, squared, which is 76.6 years, truncating conservatively to 76.
- +1Compare with the design life: 76 years is beyond the 50 year target, so the front does not reach the shallowest bar inside the design life and no propagation period has to be counted.
- +1Check which cover was used. Had the mean cover of 45 mm been used the answer would have been 126 years, a comfortable margin that does not exist for the worst bar on the member. Corrosion starts at the shallowest bar, not at the average one.
Key terms
- Carbonation
- The reaction of atmospheric carbon dioxide, dissolved in the pore solution, with the alkaline hydration products of the cement. It consumes hydroxide ions, lowers the pH and precipitates calcite into the pore space.
- Carbonation front
- The boundary between carbonated and uncarbonated concrete, advancing inward as the square root of time. Its depth at any age is the carbonation coefficient multiplied by the square root of the exposure period in years.
- Carbonation coefficient
- The single parameter of the front law, in millimetres per root year. A value of 2.5 means a front at 2.5 mm after one year and 25 mm after a hundred, not 2.5 mm per year.
- Phenolphthalein indicator
- A solution sprayed on a freshly broken surface to reveal the carbonation depth. It is pink on uncarbonated concrete around pH 12.5 and colourless on carbonated concrete around pH 9.
- Portlandite
- Calcium hydroxide left over from cement hydration, which supplies the alkalinity reserve the carbonation front has to consume. A high content resists carbonation, which is why pozzolanic binders make it worse.
- Corrosion depth for cracking
- The loss of bar radius that opens a crack of 0.2 to 0.3 millimetres in the cover, taken in this subject as 100 micrometres. It sets the end of the propagation period that a design may count.
- Propagation period
- The corrosion depth for cracking divided by the corrosion rate. Carbonation design may include it within the service life, but only up to cracking, because after the cover cracks the rate is no longer the one used.
- Accelerated carbonation test
- A laboratory test holding specimens at a raised carbon dioxide concentration, between 2 and 50 per cent, at 60 to 70 per cent humidity. Its coefficients run about ten times the natural ones, so the conversion back matters as much as the test.
Carbonation induced corrosion of reinforcement FAQ
Why does carbonation need a particular humidity rather than simply as little water as possible?
Because it needs two incompatible things. Carbon dioxide diffuses through air filled pores, so a saturated network chokes it, but the reaction itself happens in the pore solution, so a bone dry network gives it nowhere to occur. The subject places the optimum at about 65 per cent relative humidity and states it twice.
That is the front; the corrosion that follows peaks somewhere around 90 to 95 per cent, which is why a member that cycles between the two is the dangerous case.
Do supplementary binders help or hurt against carbonation?
They hurt, which is the opposite of their effect on chloride and one of the most common errors in this subject. Carbonation depth increases with fly ash content, and slag has the same effect above about 35 per cent replacement. The mechanism is the pozzolanic reaction itself: it consumes the portlandite the front would otherwise have had to react through.
Good curing substantially reduces the penalty, which is why extended curing rather than a different binder is the usual response.
How much does curing really matter here?
More than anything else in the chapter. On one identical mix the measured coefficient runs 6.0 at one day of curing, 2.0 at seven days and 0.5 at twenty eight, a twelvefold swing with no change to the materials at all. Since the initiation period goes as the inverse square of the coefficient, that is a factor of well over a hundred in time to depassivation, bought with site time rather than with money.
Why is the phenolphthalein depth described as conservative in one direction and optimistic in the other?
Because the colour change and the corrosion threshold sit at different pH values. The indicator turns colourless around pH 9, while the passive film begins to break down below pH 10.5, which is reached earlier and therefore deeper. The sprayed depth is a safe reading of where the concrete is fully carbonated and an unsafe reading of where the steel is still protected. Treat it as a lower bound on the affected zone.
Can a carbonation design count the propagation period the way a chloride design in a severe class cannot?
Yes, and that difference is deliberate. The subject allows a carbonation design to include both the initiation period and the propagation period up to cover cracking within the service life, while the severe chloride classes are required to reach the design life on initiation alone.
The reason is the rate: in carbonated concrete below about 70 per cent humidity the tabulated corrosion rate is effectively zero, whereas chloride contaminated concrete is still corroding at tens of micrometres a year at the same humidity.
Assessment move
Study this chapter alongside the chloride chapters rather than after them, because almost every fact here is best learned as a contrast: air rather than water, general rather than pitting attack, blended binders harmful rather than helpful, propagation countable rather than excluded.
Memorise the front law and its inversion, and practise both directions until moving between a measured depth and a predicted initiation period is automatic. Be careful with the coefficient's unit, since millimetres per root year behaves nothing like millimetres per year.
When you use the corrosion rate table, say out loud which column you are in, and where the subject's own sources disagree, as they do at high humidity, write both readings down and name the one you used.
Working through Carbonation induced corrosion of reinforcement in 42907? Sia is AskSia’s AI Engineering tutor — ask any 42907 Carbonation induced corrosion of reinforcement question and get a clear, step-by-step explanation grounded in how 42907 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
