Carbonation of the concrete
The atmospheric carbonation degrades the reinforced concretes and is in particular responsible for the setting to naked of its steel reinforcements. The carbonation of the concrete involves problems of resistance in traction on the reinforced concrete structures because the steel bars, supposed to guarantee this resistance, do not adhere any more to the concrete when they are corroded.
Mechanism of the carbonation
At the time of the manufacture of the concrete, the quantity of water introduced for the hydration of its cement is always higher than the stoechiometric quantity necessary. So hydrated cement is always a porous environment of which the pores are initially filled with water which takes care in ions to respect chemical balance with the Hydrate S of cement (portlandite, HSC, AFt, AFm…). When the cementing material dries with the free air, it désature out of water and the pores fill partially of air. The CO2 naturally present in the atmosphere is then likely to diffuse through the gas phase of cement (the diffusion in the liquid phase is negligible. It is noted that cements completely saturated with water are carbonated only on their boundary layer because of an immediate filling of the pores by formation of Calcite).
The CO2 present in the gas phase of the pores dissolves in the interstitial solution to form ions carbonates which react mainly with the ions Calcium Ca2+ to form Calcite. The modification of chemical balance between the Hydrate S of the cementing matrix and the interstitial solution involves a dissolution of the Hydrate S. new chemical balance corresponds then to a solution much more acid than initially. pH passes from a value of 13 in the zone not carbonated to a value lower than 9 in the degraded zone. When the zone of carbonation reaches the steel reinforcements, the Corrosion of metal can start by producing species bulkier in particular than materials initially present. This explains the bursting of the concrete around the corroded reinforcements.
See too
- Concrete
- History of the composite concrete
- Material
- To make concrete
- Auguste Perret, precursory architect in the use of the reinforced concrete
- Concrete block
- Concrete reinforced
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