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Ultimate Concrete Protection.

Summary: THE REINFORCED CONCRETE CAN BE PROTECTED AND THE LIFESPAN PROLONGED IF APPROPRIATE COATING SYSTEMS ARE APPLIED TO THE CONCRETE SURFACE IMMEDIATELY AFTER CONSTRUCTION OR APPROPRIATE REPAIR WORK IS CARRIED OUT WHEN THE DETERIORATION HAS COMMENCED

For the majority of applications, reinforced concrete can be used with confidence to build complex structures which will have a long, low-maintenance life. However, even with the latest design codes, the life of the structure will ultimately depend upon the care taken during the design and construction. Because the required high standards are not always achieved, a proportion of reinforced concrete structures is affected by durability problems which lead to the need of early repair. Particularly in the Middle East the durability problems of reinforced concrete have been encountered with substantial adverse economic consequences, i.e. too early repair or even renewal of vital parts of the specific construction. The reasons for this situation are many. The fact that insufficient care is taken during the design and construction combined with harsh environment have led to premature failures in the past which could have been averted.

Experience shows that the durability of reinforced concrete in the Middle East varies from a few years to over a hundred years. However, many are the examples of buildings showing signs of serious deterioration after only 10-20 years since construction.

Eventually, when a code of practice has to be agreed upon it is of great importance that protection of reinforced concrete by surface coating systems is considered and included. In the recent years, it has been more and more realized that reinforced concrete can be protected and the lifespan prolonged if appropriate coating systems are applied to the concrete surface immediately after construction or appropriate repair work is carried out when the deterioration has commenced.

ANTI-CARBONATION COATINGS

For many years, coatings have been applied on concrete structures for aesthetic reasons. Depending on design and environmental requirements for the concrete waterproofing treatments have been widely used and also more durable paints have been produced. However, acknowledgment of the need to protect normally durable structural reinforced concrete by coatings has been really slow to develop. In fairness it is today recognized by several construction companies that in many instances the lifetime of reinforced concrete can be prolonged if anti -carbonation coatings are applied. These coatings or rather coating systems are specially designed to control carbon dioxide diffusion, oxygen diffusion, water permeability, and water vapour transmission. At the same time, these coatings can in many instances be made with high chloride diffusion resistance.

WHAT IS THE PROBLEM?

The main cause of failure in reinforced concrete is the oxidative corrosion of the steel reinforcement. The oxidation products (rust) can be as much as eight times greater in volume than the original steel, thus exerting enormous expansive forces on the protective concrete cover. This leads to spalling, cracking and rust staining. Theoretically this should not happen because concrete is highly alkaline (high pH) and should protect the steel. Under normal conditions, the alkaline pore water reacts with the steel surface to form a stable and protective (passivating) layer of an iron oxide on the surface. This prevents further reaction. There are two environmental effects which destabilize the passivated layer and allow corrosion to proceed. These effects are "carbonation" and "chloride intrusion".

CARBONATION

The water in the pores of the concrete, which contains calcium hydroxide, is highly alkaline (pH approx. 13.5). When an acidic gas such as atmospheric carbon dioxide permeates into the concrete it will react with the calcium hydroxide to form calcium carbonate. This neutralization is normally known as carbonation. Once the carbonated zone reaches the reinforcement the alkaline protection is removed and corrosion can proceed.

It is generally well known that once the pH falls below 9 the passivating layer breaks down and then there is little or no hindrance to corrosion.

The concrete quality will very much influence the carbonation rate. High water/cement ratio, high porosity concrete carbonates very rapidly. Low water/cement ratio, low permeability concrete carbonates up to 10 times more slowly. Even with good quality concrete, poor site practice can lead to low cover depths, poor compaction, cracks or blowholes. All this can lead to carbonation problems.

CHLORIDE INTRUSION

Chloride can be introduced into the concrete mix as admixtures, or by using contaminated water or aggregates. Chloride can also penetrate into concrete when it is exposed to deicing salts, industrial effluents, or marine spray, fog or mist. Chloride contents in concrete mixes are normally strictly controlled, and the main source of chloride contamination is ingress from the surrounding environment.

The chloride ions reaching the steel participate in electrochemical reactions that break down the passivating layer on the steel surface. Thus even in highly alkaline (high pH) environments typical pitting corrosion will occur once sufficient chloride is present. The ions also increase the electrical conductivity of the water in the pores (electrolyte) allowing the corrosion process to occur even more rapidly.

WHAT ARE THE RISK FACTORS?

Corrosion of reinforcement in con- crete is depending on a number of factors which are mainly the level of chloride ion, concentration of alkalinity and the availability of oxygen and moisture. It is therefore, not possible to define universal threshold limits at which corrosion of reinforcement due to these fac- tors will commence. The risk factor is of course dependent on the envi- ronment, i.e. temperature, humid- ity and of course concentration of chloride salts. Also the porosity of the concrete will influence strongly upon the rate of carbonation and chloride intrusion. Therefore due to the harsh environment in the Arabian Gulf and other places with similar conditions the risk factor will be very high due to the ex- tremely high carbonation rates and the high concentration of airborne chloride salts.

PROTECTION AGAINST REBARCORROSION

From the foregoing, it is evident that reinforced concrete structures can be protected against rebarcorrosion by:

- reducing water intrusion

- reducing carbon dioxide intrusion

reducing the intrusion of aggressive chlorides and other salts

CONTEX - Hempel's Anti-Carbonation coatings are specially developed to suppress the carbonation rate. The CONTEX treatment prevents to a very high extent intrusion of atmospheric carbon dioxide and moisture into the reinforced concrete, thereby creating the ANTI-CARBONATION effect: the rebars are protected from corrosion caused by carbonation, and the service life of the reinforced concrete structure is prolonged.

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Publication:Oman Economic Review (OER)
Date:Jun 15, 2014
Words:1066
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