01 June 2005

Corrosion of reinforcing bars is a significant concern for the integrity of reinforced concrete structures, especially in the harsh climate of the Mideast. RICHARD COWL of the UAE-based Jotun Powder Coatings reviews the problem and explains how fusion-bonded epoxy powder coating can help in tackling it.

The NACE (National Association of Corrosion Engineers) has put the cost of corrosion in the US alone at a staggering $276 billion per year (1). In a study issued by the American Federal Highway Administration, the cost of repairing or replacing bridge structures deteriorated by corrosion is estimated at more than $20 billion and it is said to be increasing at a rate of $500 million per year (2).
In the backdrop of the studies, not only has cost become a concern but also the rate at which structures experience corrosion problems. The American National Institute of Standards and Technology noted that bridges employing uncoated bars were experiencing deterioration from corrosion within five to 10 years from construction – and similar figures for other structures are also being heard.

Corrosion
The alkaline environment of concrete provides a natural degree of protection to the steel reinforcing bar against corrosion. This is because the concrete reacts with the steel to form a film that passivates and protects the steel. However, the intrusion of chlorides and other ions can undermine these protective qualities and can cause steel to corrode if oxygen and moisture are also present. The high humidity, and excessive heat and salinity of the environment in the Arabian Gulf provide excellent conditions for reinforcement bar corrosion. Table 1 shows the relative salinity of the oceans highlighting that found in the Arabian Gulf.
A secondary cause of damage for the protective passivation state is known as carbonation whereby carbon dioxide from the air reacts with the concrete to reduce concrete’s pH. The lowering of pH results in accelerated corrosion rates for unprotected steel.
Cracks in concrete, galvanic effects due to contact with dissimilar metals, concrete resistivity and permeability, temperature and depth of cover are all factors that will influence the rate of corrosion of the reinforcement.
When steel corrodes, it forms rust that occupies a volume much greater than the steel itself. This volumetric change exerts large expansive pressures on the surrounding concrete. Because the concrete is low in tensile strength, these stresses can cause cracking and spalling which, in turn, permits faster ingress of water, oxygen and ions, accelerating corrosion even further.

Anti-corrosion systems
Several methods are employed today to try and overcome reinforcement bar corrosion and concrete degradation. A few of these systems are:
• Modification of concrete make-up: Improvements in concrete formulations have minimised cracking and reduced the permeability to water. Examples of changes in make-up are the modification of the water to cement ratio, addition of pozzolan or blended cement and the addition of admixes based on organic or inorganic corrosion inhibitors;
• Changing the chemical environment: Minimising the presence of corrosion-causing materials such as chlorides with improved material sourcing and modification of the environmental pH to more alkali;
• Electrochemical techniques: Electrochemical protection via impressed currents and sacrificial anodes;
• Barrier films: Waterproofing, fusion-bonded epoxy powder coatings and other types of coatings, both of organic and metallic nature; and
• Design: Increased concrete cover, specialist maintenance procedures, and usage of non-corroding materials.
There are many methods available today to try and reduce the impact of steel reinforcing bar corrosion and concrete degradation. However, these systems will have both advantages and disadvantages such as maintenance issues, material quality assurance, longevity and cost.

FBECR
Fusion-bonded epoxy-coated reinforcing bars (FBECR) were first used in the US in 1973 in West Conshohocken, Pennsylvania and since then thousands of structures – parking lots, building foundations, bridges, causeways, refineries and so on –  have been constructed employing this system. Many structures built in the 70s and 80s using FBECR have now been evaluated for corrosion protection performance. Field research has shown that structures using epoxy-coated rebar are performing very well. FBECR is significantly increasing the structures service life and in turn reducing life cycle costs.
However, what is powder coating?
Powder coating is the technique of applying dry paint to an object where the powdered paint has to melt and coalesce to form a continuous film.
During the curing process (in the oven), a chemical cross-linking reaction is triggered at the curing temperature and it is this chemical reaction, which gives the powder coating its most desirable properties.
The chemistry is affected primarily by the choice of binder (for example, epoxy, polyester, acrylic, polyurethane) and the resinous component that binds all the other components together in the continuous film. Other components include pigment, reinforcement minerals, additives which provide colour, strength and resistance and modification of gloss, flow and adhesion.
In the case of corrosion protection of concrete reinforcement bars the resin of choice to give good anticorrosion and chemical resistance properties is epoxy.
Epoxy offers an excellent bond to a correctly treated steel substrate, which will resist adhesion loss in the conditions prevailing in a concrete structure. It also offers excellent oxygen and moisture barrier properties as well as the mechanical resistance required in the construction of reinforced structures.
The usual application process of the fusion-bonded epoxy powder coating to the steel reinforcing bar is:
Cleaning the steel bar by grit/shot blasting to SA 2.5 or better and (in some plants) chemical pre-treatment.
Heating the bars to a temperature of around 240 deg C using a heating technique that does not impair cleanliness of the bar – such as induction coil heating.
Application of the coating powder to the hot bar by a spray technique. (The electrically charged particles are attracted to the earthed bar)
Curing of the coating by the residual heat of the bar, followed by water quenching.
Prefabricated reinforcement cages can be coated using the fluidised bed method. The cages are first fabricated by welding bars at all intersections before blasting, heating and dipping in a tank containing the fluidising epoxy powder. Again, the residual heat of the bar provides the curing.

Track record & experience
Performance evaluation has taken place on the FBECR structures on a continual basis. The results have been extremely positive but with a few noted exceptions such as Florida Keys.
Failure analysis of structures showing stress indicated the need on following the correct coating application and handling procedures. Common causes for failure have been as shown in Table 2:
For Florida Keys a combination of many of the above stated items provided the cause for the failure. Of interest to note is that even those bars with incorrectly applied epoxy coating still performed better than uncoated black bar.
To remedy incorrect procedures during coating, several revisions of the specifications have been issued which, when followed correctly, will provide a system that will deliver a significant reduction in rebar corrosion.
The specifications covering the application of FBECR are ASTM A775, ISO 14654/14656 and NACE RP 0395
In today’s world we hear of bad news quicker than good news and the story of FBECR is no different. Although thousands of structures have been constructed employing FBECR and showing excellent performance it is the minority of structures with stress that capture the headlines. Some examples of performance are given below.
In West Virginia the Department of Transportation performed an evaluation of eight side-by-side bridges – four bridges were constructed with FBECR and four without. After 20 years of service, corrosion induced concrete damage is widespread on the decks built with uncoated bar and is virtually non-existent on the decks with FBECR in the top mat of reinforcement.(3)

Cost & service life
Although the use of FBECR slightly increases the structure’s cost (approximately 1 per cent in a heavily reinforced structure), the benefits of extended service life outweighs any cost factor. Substantial information is readily available on the cost benefits of employing FBECR in structures.
The Iowa Department of Transportation states in a 2000 publication that the predicted service life for Iowa bridge decks considering corrosion of FBECR was over 50 years. This illustrates that FBECR can significantly extend the service life when compared with bridges constructed with black reinforcement bars.
CRSI (Concrete Reinforcing Steel Institute) concludes that there is a threefold increase in the service life of bridges constructed with epoxy coated reinforcing bars over those constructed with uncoated steel (5).
“At present, epoxy-coated rebar is the most common protection system used by 48 US highway agencies. To date, FBECR has been used as the preferred protection system in about 20,000 bridge decks, which represent roughly 95 per cent of the new deck construction since the early 1980s. And, the use of FBECR has extended to other structures, such as continuously reinforced concrete pavements, parking garages, nuclear power plants, coal plants, aquariums, buildings exposed to the marine environments, and wastewater treatment tanks. At present, there are approximately 100,000 structures containing ECR. Due to the success and the confidence gained by using FBECR over the last 20 years, there are about 35 coating plants.”

Conclusion
Several potential solutions exist to minimise the impact of corrosion of concrete structures that are steel reinforced. Fusion bonded epoxy coating is one of the major systems currently in use with an extensive and positive track record.
Continuous improvement by coating manufacturers, steel bar coating applicators and on-site handling procedures together with tougher control and specifications will ensure that FBECR continues to be a mainstay solution in the future.
Final confirmation of the benefits of FBECR is derived from the minimal cost implication to a project but the significant life service extension provided.

References
1: www.corrosioncost.com
2: www.crsi.org
3: http://www.tfhrc.gov/pubrds/fall96/p96au6.htm
4: Performance of epoxy coated rebars in bridge decks, 1996, US Dept of Transportation
5: CRSI – bridge decks constructed with epoxy coated reinforcing bars – Research series No 10
6: http://www.tfhrc.gov/structur/corros/results.htm

• This paper was presented at the Annual Concrete Technology and Corrosion Protection Conference 2004 held during the Gulf Construction Conference Week in Dubai last November.

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