EXPLAINING THE CARBONATION OF CONCRETE IN SIMPLE WORDS

Carbonation of concrete is the chemical process that occurs when carbon dioxide (CO2) in the atmosphere reacts with the calcium hydroxide in the concrete resulting in a reduction in its alkalinity. It is a relatively slow process that is noticed over time on the surface of the concrete and gradually enters the material affecting its microstructure. The lime carbonate resulting from this reaction reduces the alkalinity of the concrete.

The chemical reaction that takes place is as follows:

Calcium Hydroxide + Carbon Dioxide → Calcium Carbonate + Water

Ca (OH)2 + CO2 → CaCO3 + H2O

The carbonation process is affected by factors such as the quality of the concrete, its porosity, environmental and climatic conditions, the concentration of carbon dioxide in the atmosphere, the presence of moisture in the material and the atmospheres, the presence of cracks and fissures on the surface that allow the penetration of carbon dioxide into the concrete. However, carbonation is not directly harmful to the concrete itself as it only reduces its alkalinity. In general, concrete as a material is highly alkaline and this alkalinity acts as a protective layer for the embedded steel of its reinforcement. By reducing the alkalinity, the protection of the reinforcing steel is compromised, and by extension prevents its corrosion and destruction.

If carbonation penetrates deep into the concrete, several negative effects can be caused to the buildings and their constructions. The most important effect is the corrosion of the reinforcement. As carbonation reduces the alkalinity that protects the reinforcement, the steel begins to corrode and deteriorate as it is now more exposed to moisture and water, environmental conditions, and oxygen of the atmosphere which causes its destruction, rusting it and finally, the construction acquires a reduced structural integrity. Also, the corrosion of the reinforcing steel weakens its connection with the surrounding concrete and, by extension, a reduction in its mechanical strength and structural stability. Cracking and peeling, chipping, or detachment of concrete fragments from the surface of the structure, leading to the final non-functional over time, if the necessary preventive measures are not taken.

REINFORCED CONCRETE IN SIMPLE WORDS

Steel reinforcement bars or rebars are used to improve the tensile strength of the concrete, since concrete is very weak in tension, but is strong in compression. Steel is only used as rebar because the elongation of steel due to high temperatures (thermal expansion coefficient) nearly equals that of concrete. The rebars are protected from corrosion by a thin surface layer of hydrated iron oxide formed due to the high alkalinity of the concrete (pH=13) which in turn is due to Ca (OH)2.

This layer can be locally perforated (pitting corrosion) by chloride ions when their concentration is greater than about 0.5 wt%. of cement, or to dissolve (depassivation of the steel), due to a decrease in the alkalinity of the concrete around the reinforcement at pH values around 9. Chlorides can come from the atmosphere (especially in coastal areas), from inside the concrete if collected aggregates or mixing seawater have been used, and from any anti-icing salts.

The reduction of concrete pH below 9 is caused by the reaction of Ca(OH)2 of the cement paste with atmospheric CO2, which gradually diffuses into the interior of the concrete through the pores. This reaction forms calcium carbonate and is called carbonation.

Carbonation greatly accelerates the action of chlorides:

The Ca(OH)2 of the cement paste reacts with the chlorides and binds them, thus limiting the amount that diffuses to the reinforcement below the approximately 0.5% needed to destroy the protective oxide. However, with the reaction of Ca(OH)2 during carbonation, the bound chlorides are released, thus attacking the steel.

The loss of the protective oxide of the reinforcement (hydrated iron oxide) can lead to oxidation, which is an electrochemical reaction with the pore water-electrolyte. Where the protective oxide is destroyed an anode is created, from where the iron cations dissolve in the pore water leaving behind free electrons. These electrons move through the armature to the cathode, which can be formed at any point on the rod. At the cathode, which can be the entire surface of the bar, the free electrons combine with the concrete pore water and with the oxygen dissolved in it (of the gas phase of the pores), giving hydroxyl, (OH)-. Hydroxyl anions move through the pore water to the anode, where they react with dissolved iron cations, thus forming iron oxides (rust) and recomposing the water that had been electrolyzed at the cathode. It is emphasized here that the presence of water is necessary for the reaction, even though the overall reaction is only between oxygen and iron.

Corrosion of the steel reduces the cross-section of the reinforcing bars (thus their effectiveness) but also their ductility. Furthermore, because the products of steel corrosion occupy a considerably larger volume (by 2-6 times) than that which produced them, the rust induces internal stresses in the surrounding concrete causing cracking along radial planes passing through the axis of the bar. Indeed, when the coating of the bars with concrete is of small thickness and/or the transverse reinforcement in the corroded bar is little (or non-existent), this cracking reaches the

From the above, it is concluded that for the reinforcement to oxidize, the pores of the concrete must be partially and not filled with water, so that a continuous supply of air is possible for the diffusion of oxygen inside the concrete mass. In fact, this explains why concrete structural elements that are permanently immersed in water or constantly wet by it do not have a corrosion problem. Also, if the relative humidity of the environment is low (e.g. below 50%) while the micropores are permanently filled with water, the capillary pores do not have a continuous layer of water in their walls to play the role of electrolyte. So there are frequent cases in areas of Greece with a dry climate where the concrete surrounding the reinforcement has been completely carbonized without it having been corroded.

The presence of water in the pores is also necessary during the preparatory stage of corrosion, for the transport of any chlorides from the outside to the rods, but also the reaction of CO2, with the dissolved Ca(OH)2, during carbonation. Chloride transport is faster when the pores are almost filled with water, i.e. for an ambient relative humidity close to 100% or when the structural element is partially submerged in water so the pores of the rest are almost always full (due to capillary rise). In contrast, carbonation requires partially filled pores to allow CO2 to diffuse inward, with the result that the rate of carbonation is maximal for relative humidities around 50%.

Because concrete expels water (by evaporation) more difficultly than it absorbs it when the relative humidity fluctuates and/or when the concrete is periodically wetted, the average amount of water in its pores is greater than that corresponding to the average relative humidity of the environment. The extra amount of water slows carbonation, which is why concrete in contact with the external environment carbonizes to a lesser depth than its counterpart in contact with internal spaces. The inside-outside difference is in the opposite direction for processes favored by a high degree of pore water saturation, such as chloride intrusion and corrosion progression after rebar ablation. Thus, when carbonation (absence of chlorides) is responsible for corrosion, the risk of corrosion is maximum for an ambient relative humidity of around 80%, while it decreases to 1/2 when the relative humidity is 60% or 95%, and to 1/5 when it is 50% or almost 100%. In the presence of chlorides, however, the risk of corrosion is maximum for a relative humidity of about 90%, while it is reduced to 1/2 when the relative humidity is 60% or 95%, and to 1/3 when it is 50% or almost 100%. So, the dry climate (like that of Greece) does not favor the corrosion of reinforcements, what usually causes it is the wetting-drying alternations. and in 1/3 when it is 50% or almost 100%. So the dry climate (like that of Greece) does not favor the corrosion of reinforcements, what usually causes it is the wetting-drying alternations. and in 1/3 when it is 50% or almost 100%. So the dry climate (like that of Greece) does not favor the corrosion of reinforcements, what usually causes it is the wetting-drying alternations.

About Carbonation:

Chemical Actions:

Ca (OH)2+ CO₂ à CaCO3 + H₂O

–           A small amount of water is required

Maximum percentage of carbonation, in an atmosphere with a relative humidity of 50-70%, in concrete saturated with water, the diffusion of CO2 into its pores is prevented.

–           It is limited as long as the diffusion of CO2, and humidity is prevented

Physical Activities:

–           Penetration of moisture, CO2, through porosity

–           It is limited by improving porosity

Consequences:

–           Lowering pH, increasing risk of reinforcement corrosion

The cost of maintenance and repair of buildings and structures that have suffered prolonged carbonation requires expensive repairs and maintenance. To avoid further damage, it is necessary to treat carbonation in its early stages, and even better to take the necessary preventive measures before it is noticed.

Mitigation and protection of structures and buildings from concrete carbonation are vitally important to ensure the longevity and durability of the concrete and to prevent corrosion of the reinforcing steel within the concrete. Some of the actions that contribute to this protection of structures and buildings are the selection of a high-quality concrete mix with a low water-cement ratio, during construction, but also the placement of sufficient material to cover the reinforcement. Using corrosion inhibitors as admixtures in the concrete mix and their repair slows down the corrosion process by reducing the rate at which the steel reacts with the carbonized concrete.

Concrete sealing, both on horizontal and vertical surfaces, protects the concrete from water and its exposure to atmospheric carbon dioxide. Choosing a suitable and quality waterproofing system, in addition to enhancing the protection of concrete against moisture penetration, excessive moisture can accelerate the carbonation process.

Carbonation depth testing to assess carburizing depth but also regular maintenance and immediate repair on structures and surfaces that show damage and any signs of carburizing, prevent further carbonation and corrosion contributing dramatically to repair costs. The implementation of prevention and repair measures can significantly enhance the protection of structures and buildings against the carbonation of concrete and extend their service life.