Carbonation Attack

How does carbonation attack takes place in concrete?

Concrete carbonation is a process that can cause the corrosion of steel reinforcement and shrinkage; however, it can also increase both compressive and tensile strength. This process occurs when CO2 in the concrete pore fluid reacts with calcium from calcium hydroxide and calcium silicate hydrate to form calcite (CaCO3), or in hot conditions, aragonite. Within hours or a day or two, the surface of fresh concrete reacts with CO2 from the air. The carbonation process then gradually penetrates deeper into the concrete, with the rate proportional to the square root of time. For low permeability dense concrete made with a low water/cement ratio, the carbonation may typically reach a depth of around 1 mm after a year or more, whereas for more porous and permeable concrete made using a high water/cement ratio, the depth can be up to 5 mm or more.

Visually identifying carbonation in concrete

To identify carbonation, one may observe a discolored area on the concrete surface, which can vary in color from light gray (difficult to identify) to bright orange (easier to spot). Thymolphthalein can be used to visualize carbonation.

Under an optical microscope, carbonation is identified by the presence of calcite crystals and the absence of calcium hydroxide, ettringite, and un-hydrated cement grains. The porosity in the carbonated zone remains unchanged or is lower.

Testing for Carbonation in Concrete

To test for carbonation in concrete, phenolphthalein indicator solution is commonly used. This solution, made by dissolving phenolphthalein in a suitable solvent like isopropyl alcohol, can be obtained from chemical suppliers. To use this solution, apply it to a fresh fracture surface of the concrete. If the solution turns purple, the pH is above 8.6. If the solution remains colorless, the pH of the concrete is below 8.6, indicating carbonation. A fully-carbonated paste has a pH of about 8.4.

However, a pH of 8.6 may only give a faint pink color, while an immediate color change to purple suggests a higher pH, possibly 9 or 10. Normal concrete pore solution typically has a pH of 13-14 due to its saturation with calcium hydroxide, sodium, and potassium hydroxide. Concrete with a pore solution of pH 10-12 is less alkaline than sound concrete but still produces a strong color change with the indicator. As a result, the indicator test may underestimate the depth of carbonation.

Microscopy, either optical microscopy using thin-sections or scanning electron microscopy using polished sections, can confirm carbonation effects at greater depths than the indicator suggests. Nonetheless, the indicator test is useful for a quick and easy initial assessment.

When applying the indicator, the near-surface regions that have not changed color suggest carbonation to a depth of at least 4 mm from the top surface and 6 mm from the lower surface. In the center of the slab, where the indicator has turned purple, the pH of the concrete pore fluid remains high (above 8.6, probably closer to 10). It is unclear whether the cement paste in this region is entirely uncarbonated, despite the strong purple color, so microscopic examination is necessary for a complete assessment. Any untested concrete retains its original color.

Finally, the carbonation depth is approximately proportional to the square root of time. For example, if the carbonation depth is 1mm in one-year-old concrete, it will be about 3mm after 9 years, 5mm after 25 years, and 10mm after 100 years.

Bi-carbonation of concrete

In some instances, concrete may undergo a process known as bi-carbonation. This process can occur in concrete with a very high water-to-cement ratio, resulting in the formation of hydrogen carbonate ions at a pH lower than 10. Unlike normal carbonation, bi-carbonation causes an increase in porosity, resulting in soft and friable concrete. To identify bi-carbonation, one may observe the presence of large, "pop-corn" like calcite crystals and a highly porous paste.