Corrosion Prevention Using Epoxy-Coated Reinforcing Bars
Key Takeaways
- Epoxy-coated reinforcing bars demonstrated no corrosion-induced concrete distress after 21+ years in marine bridge environments.
- Even with coating damage or poor adhesion, many bars resisted chloride levels up to five times higher than uncoated steel thresholds.
- Modeling shows that without epoxy-coated reinforcement, these bridges would likely have experienced significant structural deterioration within decades.
In 2006, detailed field and laboratory evaluations were conducted by Wiss, Janney, Elstner Associates, Inc. for four concrete bridges in Georgia and North Carolina that contained epoxy-coated steel reinforcing bars. No concrete distress induced by corrosion of epoxy-coated bars was observed. It was further found that coating adhesion was a poor indicator of bar performance. Despite poor adhesion, a number of epoxy-coated bar segments were found to resist high chloride concentrations (up to five times the threshold normally assumed for uncoated bars). Modeling calculations suggested that the most critical areas of these bridges would have developed significant concrete damage if uncoated bars had been used. The use of epoxy-coated reinforcing steel provides extended service life, even though most bars examined from these bridges had greater coating damage and lower coating thickness than admissible by current standards.
INTRODUCTION
Epoxy-coated steel reinforcing bars have been used to improve the life in numerous marine bridges, many of which have been in service for more than 35 years.
To determine what level of corrosion prevention is currently being provided by the epoxy coating in aged marine bridges substructures, an investigation was performed on four coastal bridges that were constructed in the mid-1980s:
• MacKay River Bridge near Brunswick, Georgia; (1984)
• Ocean Isle Bridge in Ocean Isle, North Carolina; (1985)
• Holden Beach Bridge in Holden Beach, North Carolina; (1985)
• Atlantic Beach Bridge in Atlantic Beach, North Carolina. (1985)
METHOD
The investigation included a field survey, concrete and steel sampling, and laboratory analyses. An established statistical model considering distributions of concrete cover over the steel and chloride concentrations within the concrete was adapted to estimate the remaining service life of these structures.
The goals of this investigation were to obtain long-term corrosion performance data for epoxy-coated bars and to determine the service life of epoxy-coated bars in marine applications based on up-to-date data obtained from in-service structures.
OBSERVATIONS
The field investigation did not reveal any epoxy-coated bar corrosion-induced distress on any of these four bridges after 21 years of service. Three of the four bridges were found to have limited early-age thermal or shrinkage cracking. Core sampling at the cracked areas revealed no corrosion of the epoxy-coated reinforcing steel.
Approximately twelve cores were extracted from each bridge. The effective diffusion coefficient, D, and surface chloride content, Cs, were calculated based on the chloride concentrations measured at various depths from these cores. Based on the chloride profiles, the chloride concentration at the depth of the steel was estimated. Both the effective diffusion coefficient and the surface chloride content typically decrease with increasing elevation and can be grouped into two zones: tidal zone (between low and high tide) and splash zone (0 to 5 ft. above high tide).
The portions of reinforcing steel bar in the cores were extracted to allow evaluation of damage and corrosion, and for adhesion testing and coating thickness measurement. Coating damage varied, but the average amount of damage present was under 2% for each bridge. Many bar segments, especially those from the MacKay River Bridge, exhibited poor coating adhesion. Nevertheless, a number of epoxy-coated bar segments (14 unique bars) were found to have resisted high chloride concentrations (up to 0.156 wt%, or five times the 0.030 wt% threshold normally assumed for uncoated bars). This indicates that coating adhesion is not a good indicator of epoxy-coated bar corrosion performance.
Active corrosion was observed on three epoxy-coated bar segments (one from the MacKay River Bridge and two from the Ocean Isle Bridge) that had significant coating damage (2 to 5%). This coating damage appears to have been present since construction. These bars were all located in the tidal zone and exposed to chloride concentrations (0.183 to 0.251 wt%) well above the threshold for uncoated bar (0.030 wt%). Using corrosion modeling, it was calculated that corrosion of uncoated bars would have initiated around 1989. As there was no observed concrete distress in 2006, it was concluded that the propagation time for damaged epoxy-coated bar in the tidal zone is greater than 17 years.
Most bars examined from these bridges had greater coating damage and lower coating thickness than admissible by current standards governing the use of epoxy-coated reinforcing steel. The coating used in the MacKay River Bridge was an old generation of epoxy coating and this product demonstrated particularly poor performance in terms of adhesion and thickness. Modern epoxy-coated bars, which are required to have thicker coatings, should exhibit greater corrosion resistance than was observed in these four bridges.
Modeling calculations using values of D and C, suggested that the most critical areas of these bridges would have developed significant concrete damage if uncoated bars had been used. The concrete cover and diffusion were demonstrated to have a large effect on the projected service life and the service life extension provided by epoxy-coated reinforcement. This finding emphasizes the importance of sufficient concrete cover, highlights the additional benefits of epoxy coating, and may explain why relatively poor performance of epoxy-coated bars in tidal zones of marine sub-structures with low quality concrete and concrete cover has been observed in the past.
Characterization Of Coating
CONCLUSIONS
Based on this investigation, the following conclusions were drawn about corrosion prevention:
- No epoxy-coated steel bar corrosion-induced concrete distress in well consolidated concrete was observed.
- A considerable number of the epoxy-coated bars were found to have resisted up to 0.156 wt% chloride.
- Corrosion of epoxy-coated steel bars was observed on three epoxy-coated steel bars that had significant coating damage (2–5%). All bars were from the tidal zone and exposed to very high chloride concentration (0.183–0.251 wt% or 6–8½ times the threshold for black bar).
- Coating damage varied; however, the average value for the bar sections sampled was under 1%.
- The bar coating used in the MacKay River Bridge did not meet the ASTM A775-81 requirement for coating thickness and had poor adhesion performance. Modern epoxy coatings produced under strict quality control and good field practice would be expected to provide greater protection to reinforcing steel than was observed in this bridge.
- Coating adhesion is not a good indicator of epoxy-coated steel bars performance. Many coatings had lost adhesion completely.
- Bars with no coating damage were consistently in a corrosion-free condition despite the presence of high chloride concentrations that were well above the threshold for uncoated bars.
- Modeling calculations indicate that the most critical areas of these bridges would have developed more than 10% damage by 2006, if uncoated bars were used. Therefore, the use of epoxy-coated steel bars is likely responsible for the observed lack of deterioration of these structures.
Learn more about Epoxy and Corrosion Prevention here.
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Corrosion Prevention FAQs
How do epoxy-coated reinforcing bars support corrosion prevention?
Epoxy-coated reinforcing bars support corrosion prevention by creating a protective barrier between the steel and corrosive elements like water, oxygen, and chlorides. This coating limits direct exposure, reducing the likelihood of rust formation and extending the service life of concrete structures.
Are epoxy-coated bars effective for corrosion prevention in marine environments?
Yes, epoxy-coated bars are highly effective for corrosion prevention in marine environments. Field studies show they can resist chloride concentrations far beyond the threshold that typically causes corrosion in uncoated steel, even after decades of exposure to saltwater and tidal conditions.
Does coating damage reduce corrosion prevention performance?
Minor coating damage does not significantly reduce corrosion prevention performance. Research shows that epoxy-coated bars can still resist corrosion even with small areas of damage, though larger defects may increase the risk over time, especially in high-exposure zones like tidal regions.
How does epoxy coating compare to uncoated steel for corrosion prevention?
Epoxy-coated reinforcement provides significantly better prevention than uncoated steel. Uncoated bars can begin to corrode at relatively low chloride levels, while epoxy-coated bars have been shown to withstand much higher concentrations without corrosion-related damage.
Can epoxy-coated rebar eliminate corrosion completely?
No coating system can guarantee absolute corrosion immunity, but epoxy-coated rebar provides a proven barrier against the chlorides and moisture that drive rebar corrosion in concrete. Proper design, installation, and maintenance of concrete structures is needed maximize performance and ensure long-term structural integrity.
Why is corrosion prevention critical for bridge substructures?
Corrosion prevention is critical for bridge substructures because these elements are constantly exposed to moisture, chlorides, and environmental stressors. Without adequate protection, corrosion can lead to cracking, spalling, reduced load capacity, and costly repairs.