Carbonation shrinkage occurs when the concrete is exposed to air containing carbon dioxide. When the hardened concrete is exposed to this type of air, the weight increases, and the concrete undergoes irreversible carbonation shrinkage. The carbonation shrinkage may be as great as the shrinkage due to air drying at seventy degrees Fahrenheit, fifty- percent relative humidity, and a saturated condition. The carbonation process proceeds slowly and usually produces small shrinkage at relative humidities below twenty five percent, or near saturation. The maximum effect of carbonation takes place when the concrete is in a dry environment with a relative humidity of about fifty percent. Carbonation in freshly placed concrete usually occurs when the concrete is placed in cold weather conditions that use unvented gas heaters. When the carbon monoxide reacts with the fresh concrete near the surface, it produces a soft, chalky surface, usually 0.00254 to 0.00762 meters thick.