What Does Carbonation Mean?
Carbonation is the chemical reaction between carbon dioxide present in the air, and the hydration compounds of the cement in concrete structures. The rate of carbonation depends on the physical characteristics such as the design, on-site preparation, production and protection, as well as external factors, such as the location and degree of exposure to contaminants and other environmental factors. Carbonation may lead to the corrosion of the reinforcement steel and deterioration of concrete structures.
Corrosionpedia Explains Carbonation
The carbonation process starts immediately when concrete is exposed to air. Carbon dioxide (CO2) penetrates the concrete through the pores where it reacts with the calcium hydroxide and the moisture in the pores to form calcium carbonate. The carbon dioxide combines with the pore water to form a dilute carbolic acid which acts to reduce the concrete’s alkalinity.
Carbonation reduces the concrete’s natural alkalinity from pH13 to about pH8. Whereas a high pH provides a passivation layer around the steel, at pH below 9.5, the passivation layer breaks down and exposes the reinforcement steel to the corrosive effects of water and air.
When steel rusts, it expands in volume and exerts force on the surrounding concrete, causing the concrete to crack and spall at a rate that increases exponentially if the corrosion is not prevented.
Factors affecting concrete carbonation include:
- Concentration of CO2 gas — Usually high in cities due to pollution
- Humidity — Ideally 50%-70%, when lower, there is less water and when higher, water inhibits carbon dioxide diffusion
- Temperature — higher carbonation in hot environments
- Pore system of the concrete — Determined by water-to-cement ratio, degree of hydration and type of binder
- Porosity and permeability
Consequences of carbonation include:
- Deterioration of concrete structures through corrosion of the reinforcement steel
- Increased compressive strength of carbonated concrete
- Slight increase in the splitting strength of the concrete
- Increased electrical resistivity
- Reduced permeability and porosity
Carbonation is recognized by the presence of discolored areas on the surface of the concrete. If invisible, it can be visualized by using phenolphthalein, an indicator solution that is pink in unaffected concrete and clear in carbonated areas.
Carbonation can be prevented by:
- Following good building practices
- Understanding the environment
- Using low water-cement ratio
- Barrier coatings
- Mineral paints
Protection should be implemented in situations where carbonation penetration is deeper and nearly reaching the steel reinforcement. Once carbonation reaches the reinforcement, it cannot be prevented. However, it is still possible to protect steel from corrosion by waterproofing to prevent the ingress of water.