Multi-storey car parks

Essentially, carbon dioxide in the atmosphere reacts with cement hydrates to form calcium carbonate. The process lowers the alkalinity of the concrete, which then leads to the depassivation of the steel reinforcement. Corrosion does not necessarily follow, but depassified steel is certainly at risk of corrosion given exposure to moisture and oxygen.

Carbonation spreads from the outside of the concrete inwards, so good quality, well-compacted concrete carbonates at a much slower rate than poor quality materials. If poor compaction and strength (perhaps caused by too high a water:cement ratio) is coupled with misplaced reinforcement with little cover, the corrosion risk is high.

Corrosion caused by carbonation will be accelerated in damp conditions, while in dry areas, the rate of carbonation will be higher. Carbonation also liberates a certain number of chloride ions that may have been incorporated in the original mix and would otherwise be chemically bound in. So, there is a risk that electrical conductivity in the concrete could increase, with the consequent increase in corrosion currents and therefore corrosion.

In certain cases carbon dioxide released from vehicle exhausts can lead to greater carbonation rates in concrete than would be experienced in well-ventilated conditions. Such problems can occur in crowded, poorly-ventilated city centre car parks where vehicles are left running for longer periods.