The subject of carbonation is confusing. Carbonation can occur while the concrete is plastic or after it has hardened, but the effects are very different.
Carbonation in the plastic state usually develops during cold-weather construction. Heaters produce a great deal of carbon dioxide, which enters the mix water readily and becomes carbonic acid. This reacts with the calcium hydroxide in the mix water, which is necessary for the development of strength in the cement paste, to form insoluble calcium carbonate. If this reaction is carried far enough, no calcium hydroxide is left in solution to react with the silica and alumina to form the hydrates that give concrete its strength.
To prevent this, you must vent combustion products to the outside.
The contractor has only two possible choices for rectifying this problem—both expensive. The first is to remove the soft layer and replace it with a polymeric mortar. The second is to remove and replace the entire slab. Which one is more economical must be determined on a case-by-case basis.