As soon as concrete is formed the carbon dioxide in the air reacts with the calcium hydroxide, forming calcium carbonate, a process known as carbonation. Carbonation has beneficial and deleterious effects on concrete, in particular on reinforced concrete, which has embedded steel bars, plates or grids .
The carbonation process creates calcium carbonate beginning at the surface of the concrete that is open to air and working downward. Several factors affect the penetration rate of carbonation: the density, humidity and porousness of the concrete. The water-to-cement ratio of the concrete changes the carbonation rate: more water means faster carbonation. The same is true for porousness. If concrete is cracked, carbonation will penetrate deeper.
Carbonation improves the compressive and tensile strength of concrete. Carbonation also reduces the porosity of dense and compact concrete, protecting it against water and chloride ion infiltration. Chloride ions can corrode the steel placed in reinforced concrete. These advantages are important to keep in mind because carbonation is an inevitable process for concrete to undergo.
The main danger from carbonation of concrete is the effect on embedded steel. Carbonation lowers the alkalinity of concrete. High alkalinity, however, protects steel from corrosion. At a pH lower than 10, corrosion may begin to occur. Reinforced concrete that has been thoroughly penetrated by carbonation will likely dip below this threshold, exposing the rebar in it to damaging rust.
All concrete that you can see has undergone some degree of carbonation, but it is not possible to tell just by looking at the surface how deep the process has gone. Engineers can drill a small hole in concrete and treat the lower exposed area with phenolphthalein, which turns pink or purple at high alkalinities. Wherever the concrete changes color, carbonation has not yet occurred.