Carbonation (concrete)

Reinforcement corrosion with concrete spalling as a result of carbonation and poor concrete cover

Damage below a bridge on the A 661 motorway

In construction, carbonation (sometimes also carbonation ) is a chemical reaction that takes place in every concrete in the presence of carbon dioxide and moisture.

This process does not damage the concrete directly. The formation of limestone during carbonation even increases the strength , which in principle is to be assessed positively. In the case of reinforced concrete , however, the loss of the alkaline environment ( depassivation ) caused by the process enables reinforcement corrosion , which results in serious damage to the component.

Chemical reaction in concrete

Carbonation is the chemical conversion of the alkaline components of the cement stone by CO ₂ into calcium carbonate.

Carbonation reaction of cement paste:

${\displaystyle \mathrm {Ca(OH)_{2}+CO_{2}\ \rightarrow \ CaCO_{3}+H_{2}O} }$

Calcium hydroxide from the concrete ( portlandite ) and carbon dioxide from the air react to form limestone and water

The following partial reactions take place:

Dissolve the crystalline portlandite

${\displaystyle \mathrm {Ca(OH)_{2}\ \rightarrow \ Ca^{2+}+2OH^{-}} }$

Dissolving CO ₂ in the alkaline pore water

${\displaystyle \mathrm {CO_{2}+H_{2}O\ \rightarrow \ H_{2}CO_{3}} }$

Neutralization of Ca (OH) ₂ by H ₂ CO ₃

${\displaystyle \mathrm {Ca(OH)_{2}+H_{2}CO_{3}\ \rightarrow \ CaCO_{3}+2\,H_{2}O} }$

The pH value of the cement stone drops from an average of 12.5 to below 10 and the pore structure of the cement stone changes.

The speed of carbonation from the concrete surface into the concrete depends on various factors:

• Moisture content - A maximum of the carbonation rate occurs at 50% to 70% concrete moisture. For this reason, dry concretes carbonate more slowly indoors or in weather-protected installation locations than concretes that are exposed to the elements.
• w / c value and the compressive strength of the concrete.
• Porosity of the concrete - Due to the larger surface, porous concretes carbonate faster than dense concretes.
• Age of the concrete - the rate of carbonation decreases with increasing age of the concrete according to the root-time law . Based on this relationship, statements can be made about the progress of carbonation.

Depending on these factors, carbonation can come to a standstill at a certain depth.

Concrete breakout with carbonation up to behind the top layer of reinforcement, verification with phenolphthalein solution

The progress of carbonation (carbonation depth) is visible on fresh concrete breakpoints or drill cores by spraying with 1 percent ethanolic phenolphthalein solution . At pH values ​​between 8.2 and 9.8, the color changes from colorless (neutral) to purple (alkaline).

Damage to reinforced concrete

Damage to a reinforced concrete ceiling

Carbonation is disadvantageous for the reinforcing steel (close to the surface) . If the pH of the concrete is above 10, a passivation layer forms on the surface of the reinforcing steel embedded in the concrete , which permanently protects the steel from reinforcement corrosion. If the pH value in the concrete drops , the oxide layer around the reinforcing steel is dissolved ( depassivation ). As a result, the steel surface is ready to corrode and begins when unfavorable parameters are present, e.g. B. sufficient moisture to corrode. Since this process is associated with an increase in volume (approx. Factor 2.5), tensile stresses arise in the concrete structure in the vicinity of the reinforcing steel. If they exceed the inherent strength of the concrete, they cause cracks in the concrete structure, and later the concrete cover will flake off. On the one hand, the erosion of the near-surface concrete zone causes the loss of the bond between reinforcement and concrete and, on the other hand, the ingress of corrosive media is further promoted. As a result, structural damage to the reinforced concrete structure usually occurs.

The service life of reinforced concrete is thus determined by two factors: On the one hand, it is the initial phase, i.e. the period within which the carbonation reaches the reinforcement layer. On the other hand, it is the destruction phase, here the reinforcement corrodes. Recognized models are available for calculating the introductory phase. Booklet 510 of the German Committee for Reinforced Concrete (DAfStb) provides a probabilistic model. The current state of knowledge (July 2006) hardly provides any recognized models for the destruction phase.

See also

• Damage to concrete structures
• Concrete repair

literature

• Christoph Gehlen: Probabilistic service life assessment of reinforced concrete structures . Ed .: DAfStb (=  DAfStb-Heft . Volume 510 ). Beuth, 2000, ISBN 3-410-65710-X .

Web links

Commons : carbonation  - collection of images, videos and audio files

• Concrete corrosion (English)