It is not excessive to say that reinforced concrete (RC) is the most popular and successful constructive material since approximately 12 billion tonnes of RC are manufactured annually, which is more than any other man made material. In most cases when RC structures are adequately designed and carefully casted or fabricated, they perform well throughout their service life. However, RC can be seriously affected under the attacks of the environment, especially the steel. Corrosion of the steel reinforcement is considered to be the main cause of RC structure deterioration in marine conditions. Therefore, it can be called as a "major problem facing civil engineers today".
II/ Causes of corrosion in RC structures
The basic mechanism of corrosion in reinforced concrete is how does steel corrode in concrete?
Initially, the reinforcement can be fully protected when they are embedded in the concrete despite the fact that concrete is porous and contains moisture. Concrete is alkaline thanks to the high concentration of soluble calcium, sodium and potassium oxides; thus, can create a very condition of pH 12-13 in the solution with the presence of water. This alkaline condition of the concrete helps to create a ‘passive' thin film over the surface of the reinforcing steel. In normal conditions, the passive film is believed to fully protect the steel under the attacks of corrosion. The whole protective mechanism of RC described above is so-called "passivity" of steel in concrete.
There are two different mechanisms that can break down the passivity of RC, that are blamed to be main causes of corrosion of steel in concrete, namely carbonation and chloride attack.
The high quantity of calcium hydroxide dissolved in the pore water as the product of the cement hydration process helps maintain the pH at the safety thresholds of 12-13. This alkaline condition protects the steel reinforcement by constituting the thin film (2-20 nanometres thick) of iron oxide on the metal surface. Carbonation is defined as the alkalinity neutralization process of the concrete.
CO2 + H2O + Ca(OH)2 à CaCO3 (calcium carbonate) + 2H2O
In case all of the alkaline ingredients might be neutralized and the pH decreases to under a value of 9, the normal mechanism of passive protection is no longer operative. Thus, the reinforcement is corroded.
The rate of carbonation varies due to the effects of both environmental factors: humidity, temperature, CO2 concentration and concrete composition: alkalinity and permeability. The most ideal condition that promotes carbonation is from 60-75% of RH. In addition, the carbonation rate also increases when the content of CO2 in the air and temperature increases. On the other hand, the cement content is the important factor that reserves the alkali and resists carbonation. Besides, as carbonation is the function of thickness, the concrete cover is also a major factor of corrosion reduction.
Carbonation is a slow process, especially in the temperate climates and can be measured and prevented. However, it can be a serious issue of aging structures.
2. Chloride attack
Chlorides in the concrete can come from several ways. They can be cast into the structure by the use of deliberate admixtures (CaCl2, but forbidden now), or the chloride ions can appear in the mix (mixing water, aggregates) unknowingly or deliberately. However, the major cause of chloride-induced corrosion in most part of structures is the diffusion of chlorides from the environment due to:
· direct exposure with marine environment;
· uses of deicing salts and chemicals.
Similar to the carbonation, chloride attack process does not directly corrode steel reinforcement, except it breaks down the protective film of iron oxide and promotes the corrosion rate. Other words, chlorides play a role as catalysts to corrosion5. However, the mechanism of chloride diffusion into concrete is different with carbonation in that it attacks the passive layer without the requirement of pH reduction.
Figure 2: Pitting corrosion due to the high concentration of ion Cl- on steel bar in RC.
There are four different mechanisms of chloride transport into crack-free concrete:
· capillary suction;
· diffusion due to the high concentration of chloride ion on the surface;
· permeation under pressure;
· and migration due to electrical potential gradients.
Structures in reality usually work under the mutual attacks of these two mechanisms. It has been shown that chloroaluminates (AlCl-4), which are formed by the reaction of chlorides with C3A in the cement paste, can reduce the quantities of chloride; hence, decelerate the corrosion process. Unfortunately, as carbonation causing the reduction in pH, AlCl-4 will be broken down. Consequently, structures which are under attacks by both chlorides and carbonation are more sensitive to corrosion than those under only one kind of problem.
III/ Prevention of corrosion in RC structures
The quality of the concrete and sufficient concrete cover over the reinforcing bar are of the first defenses against corrosion of steel in concrete. The concrete quality must has a water-to-cement ratio (w/c) low enough to slow down the penetration of chloride salts and the development of carbonation. The w/c ratio should be less than 0.50 to slow the rate of carbonation and less than 0.40 to minimize chloride penetration. ACI 318 recommends to use a minimum concrete cover of 1.5 inches (38.1 mm) and at least 0.75 (19.05 mm) inch larger than the nominal maximum size of the coarse aggregate and increase the minimum cover to 2.5 inches (63.5 mm) for marine exposure (ACI 357).
The correct amount of steel will help keep cracks tight. ACI 224 helps the design engineer to minimize the formation of cracks that could be detrimental to embedded steel. In general, the maximum allowable crack widths are 0.007 inches (0.178 mm) in deicing salt environments and 0.006 inches (0.152 mm) in marine environments.
The concrete must be adequately consolidated and cured. Moist curing for a minimum of seven days to 70°F (21oC) is needed for concrete with a 0.40 w/c ratio, whereas six months is needed for a 0.60 w/c ratio. Numerous studies show that concrete porosity is reduced significantly with increased curing times and, correspondingly, corrosion resistance is improved. Thus, ensure that the concrete is adequately cured.
Other protection techniques include protective membranes, epoxy-coated reinforcing bars, galvanized coated bars, and concrete sealers (if reapplied every four to five years) and especially cathodic protection.
The mechanisms of cathodic protection and its application will be discussed further in the next paper.
Duy An Le