Carbonation of concrete is associated
with the corrosion of steel reinforcement and with shrinkage. However, it also
increases both the compressive and tensile strength of concrete, so not all of
its effects on concrete are bad.
Carbonation is the result of the
dissolution of CO2 in
the concrete pore fluid and this reacts with calcium from calcium hydroxide and
calcium silicate hydrate to form calcite (CaCO3). Aragonite may form
in hot conditions.
Within a few hours, or a day or two
at most, the surface of fresh concrete will have reacted with CO2 from the air. Gradually, the process
penetrates deeper into the concrete at a rate proportional to the square root
of time. After a year or so it may typically have reached a depth of perhaps 1
mm for dense concrete of low permeability made with a low water/cement ratio,
or up to 5 mm or more for more porous and permeable concrete made using a high
water/cement ratio.
Fundamental
Principle for Carbonation Testing:
Carbonation of concrete occurs when
the carbon dioxide, in the atmosphere in the presence of moisture, reacts with
hydrated cement minerals to produce carbonates, e.g. calcium carbonate. The
carbonation process is also called depassivation. Carbonation penetrates below
the exposed surface of concrete extremely slowly.
Testing
for Carbonation:
The
affected depth from the concrete surface can be readily shown by the use of
phenolphthalein indicator solution. This is available from chemical suppliers.
Phenolphthalein is a white or pale yellow crystalline material. For use as an
indicator it is dissolved in a suitable solvent such as isopropyl alcohol
(isopropanol) in a 1% solution.
The phenolphthalein indicator
solution is applied to a fresh fracture surface of concrete. If the indicator
turns purple, the pH is above 8.6. Where the solution remains colorless, the pH
of the concrete is below 8.6, suggesting carbonation. A fully-carbonated paste
has a pH of about 8.4.
The phenolphthalein indicator
solution is applied to a fresh fracture surface of concrete. If the indicator
turns purple, the pH is above 8.6. Where the solution remains colorless, the pH
of the concrete is below 8.6, suggesting carbonation. A fully-carbonated paste
has a pH of about 8.4.
In practice, a pH of 8.6 may only give a faintly discernible slightly pink color. A strong, immediate, color change to purple suggests a pH that is rather higher, perhaps pH 9 or 10.
In practice, a pH of 8.6 may only give a faintly discernible slightly pink color. A strong, immediate, color change to purple suggests a pH that is rather higher, perhaps pH 9 or 10.
Normal
concrete pore solution is saturated with calcium hydroxide and also contains
sodium and potassium hydroxide; the pH is typically 13-14. Concrete with a pore
solution of pH 10-12 is less alkaline than sound concrete but would still
produce a strong color change with phenolphthalein indicator. It therefore
follows that the indicator test is likely to underestimate the depth to which
carbonation has occurred.
Range
and Limitation of Carbonation Testing:
The phenolphthalein test is a simple and cheap method of determining the depth of carbonation in concrete and provides information on the risk of reinforcement corrosion taking place. The only limitation is the minor amount of damage done to the concrete surface by drilling or coring
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