Editing Reinforced concrete (section)

=== Carbonation ===
[[File:Concrete wall cracking as steel reinforcing corrodes and swells 9058.jpg|left|thumb|Concrete wall cracking as steel reinforcing corrodes and swells. Rust has a lower density than metal, so it expands as it forms, cracking the decorative cladding off the wall as well as damaging the structural concrete. The breakage of material from a surface is called ''spalling''.]]
[[File:Concrete wall cracking as its steel reinforcing cracks and swells 9061v.jpg|left|thumb|Detailed view of spalling probably caused by a too thin layer of concrete between the steel and the surface, accompanied by corrosion from external exposure.]]

{{main|Carbonation}}
Carbonation, or neutralisation, is a chemical reaction between [[carbon dioxide]] in the air and [[calcium hydroxide]] and hydrated [[calcium silicate]] in the concrete.

When a concrete structure is designed, it is usual to specify the [[concrete cover]] for the rebar (the depth of the rebar within the object). The minimum concrete cover is normally regulated by design or [[building code]]s. If the reinforcement is too close to the surface, early failure due to corrosion may occur. The concrete cover depth can be measured with a [[cover meter]]. However, carbonated concrete incurs a durability problem only when there is also sufficient moisture and oxygen to cause electropotential corrosion of the reinforcing steel.

One method of testing a structure for carbonation is to [[drill]] a fresh hole in the surface and then treat the cut surface with [[phenolphthalein]] indicator solution. This solution turns [[pink]] when in contact with alkaline concrete, making it possible to see the depth of carbonation. Using an existing hole does not suffice because the exposed surface will already be carbonated.