Designing adequate corrosion protection is essential at the beginning of a project.
Adding value to construction
Dr John Muirhead, global functional technical manager of Jotun Powder Coatings UAE, provides a generalised account of the corrosion protection of steel reinforcement bars (rebars), the various corrosion protection systems available and the value of epoxy-coated rebars.
01 March 2003
The use of epoxy-coated rebars 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 construction using concrete with rebars.
In the last 25 years, 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 US alone, it has been reported that 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 need rehabilitation.
Before the use of epoxy-coated rebar, specifiers 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 US had significant corrosion problems. The use of epoxy-coated reinforcement and improved concrete practices have dramatically improved these statistics, and enhanced the durability of reinforced concrete structures over the last 25 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 chlorides (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 bar will start to lose its reinforcement properties.
Corrosion creates rust (a mixture of the various iron oxides and hydroxides) which is characterised by a higher volume than the original metallic iron. This expansion creates internal stresses on the surrounding concrete producing staining, spalling and cracking.
The chloride ion is the main culprit and when chlorides penetrate concrete from external sources, such as de-icing salts and seawater, carbon steel rebar corrodes; 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 roles in corrosion and structural longevity. The weather is hot and humid with a high chloride level. These items, combined with high water tables (hot subkha), irrigation and saline waters surrounding the Gulf region create the perfect environment for severe corrosion.
The world's seas have an average salinity of between 33 and 41 (parts per thousand). The salinity of Red Sea and Arabian Gulf waters is 40-41, 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 US or Europe.
It is therefore logical that structures in the Gulf should be designed to combat corrosion and prolong their life.
Corrosion protection
There are several techniques that can be employed to reduce corrosion and/or its rate. Some are more expensive, and others require greater technical skill 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.
A brief summary of some corrosion protection methods is given below in Table 1
 |
The balance of cost and avoiding flaw is the essence of selecting the appropriate anticorrosion system.
For 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 problems 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 costs of maintenance, repair and others which appear after the structures have been commissioned. The result is a structure with minimal corrosion 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; and be flexible and durable enough for handling.
Fusion-bonded epoxy coating satisfies all of these requirements. It is a thermoset 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 catalysed 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 environmentally-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: the coating reduces the size and number of potential cathode sites, which will limit the rate of any corrosion reaction that could occur. For macrocell 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
An excerpt from a document entitled, "Corrosion Protection: Concrete Bridges", Federal Highway Administration, U.S. Department of Transportation:
"To better manage construction costs 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 per cent to the cost of uncoated reinforcement. As use and production grew, the cost to specifiers 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 rebar 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 costs 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 US 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 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 differences between the decks that were studied. Previous experience on decks not using epoxy-coated steel has produced widely varying percentages of delamination reaching as high as 60 to 80 per cent although five to 20 per cent 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 US are fusion-bonded epoxy-coated rebars (FBECR), which have been used in approximately 20,000 reinforced concrete bridge decks. This rebar has performed very well in alleviating the problem of corrosion-induced deterioration of concrete bridge decks. It is estimated that its use in the last 25 years has saved US taxpayers billions of dollars.
To bring home the importance of providing a corrosion protection system with rebar, Table 2 (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 rebar in construction.
 |
• Although actual costs 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 the Federal Highway Administration's study "Corrosion Resistant Reinforcement for Concrete Components." This study evaluated epoxy coatings under a series of severe, "worst-case" test conditions. In actual practice, with the use of quality material, and proper handling and construction practices, service life extensions in excess of these values may be possible;
• The repair/rehabilitation cost estimate includes costs for patching, engineering, traffic control and user delays.
Specifying
When specifying a corrosion protection system for rebar, 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.
References:
(1) Epoxy-Coated Rebar Delivers Cost Effective Value, Concrete Reinforcing Steel Institute.
