• Home
• Contact
• Sitemap

Rebar - Adding Value to Construction

The use of epoxy-coated re-bars should be seen as part of a greater package to extend the longevity of any structure, especially those subject to harsh, corrosive environments. There is a firm belief that fusion-bonded epoxy coating adds value to structure using concrete with rebars.

The cost of maintaining, rehabilitating and reconstructing corrosion-damaged reinforced concrete structures has rapidly escalated, necessitating more cost-effective corrosion protection systems. The deterioration of concrete infrastructure is the largest civil engineering challenge facing the developed world. In the U.S. alone, it has been reported that more than 600,000 road bridges are scheduled for repair at an estimated cost of $200 billion, four times their original construction cost. Thousands more bridges in Europe and Asia needs rehabilitation.

Before the use of epoxy-coated rebar, designers were becoming acutely aware of the short life expectancy of structures built without corrosion protection. In 1974, the National Bureau of Standards reported that Bridges are experiencing deterioration within five to 10 years of service. In 1979, The General Accounting Office reported that 160,000 bridges in the U.S. had significant corrosion problems. The use of epoxy-coated reinforcement and improved concrete practices have dramatically improved these tactics, and enhanced the durability of reinforced Concrete structures over the years.

The corrosion process is a result of the inherent tendency of metals to revert to their more stable compounds, usually oxides. Normally the alkalinity of the concrete provides a passive environment around the bars in which corrosion will not occur, though among other factors, the presence of chloride (in the original mix or entering the concrete) will break down the alkali passive state. A corrosion cell can now be generated with the additional presence of water and oxygen, corrosion begins and the bars will start to lose its reinforcement properties.

The chloride ion is the main culprit and when chlorides penetrate concrete from external sources, such as deicing salts and seawater, carbon steel rebars corrode; rust forms, occupying a volume about three to seven times that of the original steel, and the surrounding concrete cracks and spalls.

Climatic Conditions

It has been calculated that actively corroding steel will typically corrode at a rate of up to one mm per year in normal environments. In harsh climates, conducive to corrosion, however, this rate is increased.

In the Middle East, climate and environment play important role in corrosion and structural longevity. The weather is hot and humid with a high chloride level. These factors, combined with high water tables (hot subkha), irrigation and saline waters surrounding the Gulf region create the perfect environment for severe corrosion.

The worlds seas have an average salinity of between 33 and 41 (parts per thousand). The salinity of Red Sea and Arabian Gulf waters is 40 to 41, exceeding 50 in limited areas of the latter. The high salinity is the result of extremely high evaporation, insignificant rainfall and river inflow, and restricted exchange with the open ocean. Due to temperature and solar radiation, annual rates of corrosion in the Gulf are 10 to 20 times faster than in parts of the U.S. and Europe.

It is therefore logical that structures in the Gulf should be designed to combat corrosion and prolonged their life.

Corrosion Protection

There are several techniques that can be employed to reduce corrosion and its rate. Some are more expensive, and others require greater technical skills during planning, design, application and construction.

Corrosion control can be seen as active protection, such as cathodic protection which actively controls the environment to reduce corrosion, or passive protection, which involves creating a barrier to one or more of the chemical requirements for corrosion.

For optimum performance, a corrosion protection system will depend on several components in combination with each other.

Brief Summary of Corrosion Protection Methods:

The balance of cost and avoiding flaw is the essence of selecting the appropriate anticorrosion system.

For a better, clearer and more informed understanding of the suitability of the various corrosion protection systems, two areas need careful evaluation:

The real performance of corrosion protection systems:

This can be obtained from suppliers’ literature, market-sponsored reviews and articles found on the internet. The greatest difficulty is the analysis of results, as many publications derive their findings from structures that have suffered stress. These highlight the problem of corrosion protection systems rather than address the greater number of buildings that do not show stress or corrosion problems.

The cost of building in corrosion protection systems:

Today, it is necessary more than ever to reduce costs to remain competitive. A quotation including substantial corrosion protection components will be higher than one without corrosion protection. There is a tendency to look at the initial cost of a project and gloss over the cost of maintenance, repair and others which appear after the structures have been commissioned. The result is a structure with minimal protection which will start to show problems a few years after commissioning. The various people initially involved in choosing the construction package are likely to have moved on, and therefore liability becomes an issue.

Designing adequate corrosion protection at the beginning of a project is therefore a logical step.

To effectively protect reinforcing steel against corrosion, a coating must provide a continuous film that will:

• Resist penetration by salt ions
• Resist the action of osmosis, adhere to and expand/contract with the steel substrate
• Resist breakdown from weathering and exposure
• Be flexible and durable enough for handling.

Fusion-bonded epoxy coating satisfies all of these requirements. It is a thermostat material, meaning that once it is cured, the coating will not tend to soften with higher temperatures. It achieves its beneficial properties as a result of a heat catalyzed chemical reaction.

Epoxy coating starts out as a dry powder. The powder is produced by combining organic epoxy resins with appropriate curing agents, fillers, pigments and flow control agents. When heated, the powder melts and its constituents react to form complex cross-linked polymers.

Epoxy coatings are environmental friendly materials. Unlike many paints, the fusion-bonded epoxy coatings used for steel reinforcement do not contain appreciable solvents or other environmentally-hazardous substances. Systems used to apply the coating are very efficient, resulting in little material loss to the atmosphere and little waste disposal.

How Epoxy Coating Protects

Fusion-bonded epoxy coating principally protects against corrosion by serving as a barrier that isolates the steel from the oxygen, moisture, and chloride ions that are needed to cause corrosion. Epoxy coating also has a high electrical resistance, which blocks the flow of electrons that make up the electrochemical process of corrosion. In addition to serving as a circuit breaker, the coating protects in a way that is less obvious: it reduces the size and number of potential cathode sites, which will limit the rate of any corrosion reaction that could occur. For macro cell corrosion to take place, a large area of steel surface is needed to serve as the cathode where oxygen reduction can occur.

Getting the right system in place at the beginning is cost efficient.

Here is an excerpt from document entitled, Corrosion Protection: Concrete Bridges, Federal Highway Administration, U.S. Department of Transportation:

To better manage construction cost and to assure maximum return on investment, many owners and specifiers are employing life-cycle cost analysis to evaluate future expenditures and to justify corrosion protection strategies. Since its first use in the early 1970s, the cost of epoxy-coated reinforcement has dropped significantly.

Early on, epoxy coating added 80 to 120 cent per cost of uncoated reinforcement. As use and production grew, the cost to consumers decreased. Presently, the cost of epoxy coating typically adds about $0.10 to $0.20 per pound to the cost of steel reinforcement. For most structures, coating all rebars will usually only increase the total structural cost by one to three per cent. For a typical bridge deck, epoxy coating the reinforcement adds in the range of $0.70 to $1.40 per sq. ft. For parking decks (with less reinforcement) the added cost is typically in the range of $0.40 to $0.80 per sq. ft. Compare these to the high cost of maintenance, repair and reconstruction. Patching repair can run $20 to $35 per sq. ft. or more. Combined with the potential for user delay, loss of service and revenue plus increased accident rates, the cost of corrosion-induced damage is very high compared to the low cost of epoxy-coated reinforcement.

Independent Evaluation

Numerous research studies have confirmed that epoxy-coated reinforcement is significantly extending the life of concrete bridges, parking garages and other structures in corrosive environments.

The results obtained from a study on bridge decks in the U.S. were published in a 1994 article in the Concrete Reinforcing Steel Institute (CRSI) research series entitled West Virginia Department of Transportation Division of Highways Materials Inspection Report. The study compared bridge decks using epoxy-coated steel and those not using coated reinforcement.

The study indicated that while the use of epoxy-coated reinforcement does not necessarily reduce the number of transverse cracks found in a bridge-deck, the damage incurred to the deck by allowing water to penetrate through these cracks and accelerating the corrosion of the uncoated steel is greatly reduced, if not eliminated. No patching was observed on any of the decks. The delamination surveys provide the most striking difference between the studied decks. Previous experience on decks not using epoxy-coated steel has produced widely varying percentages of delamination, reaching as high as 60 to 80 percent although five to 20 percent is more common. Comparing this to the uniform absence of any measurable reinforcement associated delamination in the decks investigated in the study leads to only one conclusion: that the epoxy-coated reinforcement must be directly responsible for the lack of delamination in these decks.

The Final Solution

The preferred primary corrosion-protection systems in many parts of the U.S. are fusion-bonded epoxy-coated rebars (FBECR), which have been used in approximately 20,000 reinforced concrete bridge decks. These rebars have performed very well in alleviating the problem of corrosion-induced deterioration of concrete bridge decks. It is estimated that its use has saved U.S. taxpayers billions of dollars.

To bring home the importance of providing a corrosion protection system with rebar, the table below has been extracted from the CRSI website (www.crsi.org). It highlights how, with a long-term view, using epoxy coatings as a protection system is a cost-effective means of using rebars in construction. The repair/rehabilitation cost estimate includes costs for patching, engineering, traffic control and user delays.

Although actual cost may differ, this analysis and the relative life-cycle costs are representative of many typical bridge decks. The above costs are provided for illustrative purposes only; their validity should be verified for any given project. The actual cost of construction and of epoxy-coated reinforcement will vary depending on factors such as location, complexity, design, loading and timing.

The estimated service life extensions for the epoxy-coated alternatives are based on conservative estimates derived from Federal Highway Administration’s study Corrosion Resistant Reinforcement for Concrete Components. This study evaluated epoxy coating under a series of severe, worst-case test conditions. In actual practice, with use of quality material and proper handling and construction practices, service life extensions in excess of these values may be possible.

Specifying

When specifying a corrosion protection system for rebars, standard specifications for epoxy-coated reinforcing steel are available from the American Society for Testing and Materials (ASTM A775, ASTM A934 and ASTM D3963) and the American Association of State Highway and Transportation Officials (AASHTO M284). In addition, ASTM standard specification A884 is available for epoxy-coated welded wire fabric.

Reference:

• Epoxy-Coated Rebar Delivers Cost Effective Value, Concrete Reinforcing Steel Institute
• Gulf Construction Magazine, Vol. XXIV, No. 3, March 2003 Dr. John Muirhead, Global Functional Technical Manager Jotun Powder Coatings, UAE