Carbonation is a surface affect on concrete that results from chemical conversion/reaction at the surface. The many calcium compounds in cement provide copious amounts of calcium that can react with CO2 in the air to create calcium carbonate. In addition other carbonates are created at the air interface.
Carbonation weakens the surface of concrete and is characterized by a pH in the range of about 8.5 to 9.5. This is lower than typical concrete pH of 11+ (after curing...even higher when fresh).
Check for carbonation by taking a freshly broken piece of concrete (do not use a sawcut piece or drilled piece), coat with phenolphthalien and watch for areas around the carbonation to turn purple which the actual carbonation layer will remain colorless. It usually shows as a distinct layer about 1/8 to 1/4-inch thick at the surface.
One other thing to consider with carbonation, part of the protection given to embedded steel reinforcement is derived from the high pH of the concrete. When calcium carbonate forms, and the pH lowers as mentioned by Ron, thus the protection given to the reinforcement is dramatically reduced.
Therefore when checking the depth of carbonation you are also looking to see how much protection is still being given to any reinforcement wihtin the concrete. It is possible to calculate the rate that carbonation is forming and relate this to life-span of the structure. The rate at which carbonation forms is related to the quality of the concrete, not just strength but also porosity, cement type etc...
It is a very smle tool in assessing structural concrete.
Surface carbonization is a common problem when pouring concrete floor slabs during the winter in buildings using temporary heaters. The heaters give off CO and CO2 and the wet concrete has a high affinity for it. Using a hand held CO detector is one way of monitoring (monitoring is also done to satisfy OSHA requirements when using temporary heaters) the presence of the gases.
I've seen a number of fixes used to prevent or correct the surface carbonization of a floor slab. One was to apply a high solids sealer right after finishing of the slab to prevent the CO and CO2 from getting to the concrete surface.
Another was to scrub the carbonized surface of the hardened surface to remove the soft, loose powdered layer, then apply a liquid hardener like Lapidolith to the floor surface.
