Carbon dioxide in the air reacts with the calcium hydroxide in the hydrating cement paste to form calcium carbonate:
Ca(OH)
2+CO2=CaCO3+H2O
The reaction results in a significant reduction in the pH of the cement paste (from about 12.5 to about 9.5).
The surface of concrete exposed to the air carbonates very rapidly, forming a carbonated layer of micron thickness. In good quality concrete, the rate of penetration is very slow and depends on many factors, the principal ones being porosity, permeability and moisture content. The floor and walls of the majority of swimming pools are not exposed directly to the air once the back-filling has been completed. It is therefore reasonable to assume that the carbonation of the concrete shell is unlikely to endanger the reinforcement during the lifetime of the pool. 3.4 Chloride-induced corrosion of reinforcement
3.4.1 General considerations
Corrosion of rebars can occur in un-carbonated concrete due to the presence of chlorides. Chlorides are present either because they were added to the concrete mix, or were present in the aggregates, and/or the mixing water, or they had penetrated into the hardened concrete from an outside source.
When present, chlorides are in solution in the pore water. When a salt is dissolved in water it is immediately split up into electrically charged particles known as ions:
NaCl=Na++Cl-
It is the negatively charged chloride ions which destroy the passivity (the layer of ferric oxide) on the surface of the rebars.
In practice, the chloride ions present in the pore water exist in two forms, free chloride ions and combined chloride ions. The combined chloride ions are combined with the hydration products in the cement paste, mainly the tricalcium aluminate (C3A). It is generally agreed that it is the free chloride ions which damage the passivity of the steel, resulting in the corrosion of the rebars.
It can thus be seen that the higher the concentration of C3A in the hydrating cement paste, the higher the percentage of chloride ions which will be ‘locked up’ and not free to attack the steel. From the point of view of dealing with potential chloride attack on reinforcement, the use of a cement with a high C3A content is to be preferred. Ordinary Portland cement has a C3A content in the range 8–12%, compared with sulphate-resisting Portland cement which has a C3A content not exceeding 3.5% (see BS 4027).
The formation of rust by chloride attack can cause cracking and spalling of the concrete due to the considerable increase in volume of the steel when it is converted into rust (an increase of three to four times the original thickness of steel).
There are two main types of corrosion of rebars, general corrosion and local corrosion (pitting). The general corrosion is more likely to cause cracking and spalling of the concrete, but local corrosion can be more serious due to significant reduction in the diameter of the rebars at the ‘pits’. These pits may penetrate the rebars by more than 50% of the bar diameter; even a careful visual examination of the concrete may not detect localised/pitting corrosion.
Localised/pitting corrosion is more likely to be the result of chloride ions in the concrete in contact with the steel than carbonation of the concrete in contact with the rebars.
3.4.2 Chlorides in swimming pool water
Concern is sometimes expressed about the durability of reinforced concrete in continuous contact with chlorinated swimming pool water. A careful literature search has not revealed any authorititive research or detailed study of this problem. This suggests that significant corrosion of steel reinforcement in pool shells due to chlorides introduced into the pool water for disinfection of the water has so far not been detected and/or has not caused serious concern. However, a brief discussion on the subject is considered justified.
A great deal has been written about the durability of reinforced concrete marine structures, and consideration of the characteristics of sea water is worthwhile as this water contains a high percentage of dissolved salts, mainly chlorides. Comparatively few swimming pools contain sea water; the majority of pools in the UK contain water of drinking quality. For reasons of hygiene, the water in the pool is filtered and treated with various chemicals including a disinfectant. The
disinfectant in most general use in public swimming pools in the UK is chlorine which is generally produced by dosing the water with sodium or calcium hypochlorite.
Sea water contains a high percentage of dissolved salts. Figures given by Lea, in The Chemistry of Cement and Concrete, 3rd edition, are quoted below:
This Table can be converted to:
The dosage of chlorine into swimming pool water depends on a number of factors as the aim is to maintain the ‘free chlorine residual’ at about 0.5–1.0 ppm. If it is assumed for the purpose of this discussion that the actual dosage of chlorine is as high as 5 ppm, when the bathing load is heavy, then it can be seen that the concentration of chloride ions in swimming pool water is less than 0.01% of that in sea water. On this basis, the chance of chloride-induced corrosion of reinforcement occurring in a properly constructed swimming pool shell can be considered as insignificant, and can be reasonably disregarded until authentic research proves otherwise.