Comparing The Performance Of Epoxy-Coated And Galvanized Rebar
Key Takeaways
- Epoxy-coated reinforcing steel consistently outperforms galvanized rebar in corrosion resistance and long-term durability.
- Bridge data shows epoxy-coated steel is significantly less likely to require repair compared to galvanized alternatives.
- Laboratory and field studies confirm epoxy-coated steel provides higher chloride thresholds and reduced cracking in concrete.
Epoxy-coated reinforcing steel was first used in 1973 on the Schuylkill Bridge near Philadelphia, PA, as a method to reduce corrosion damage to bridge structures. It remains the principal method for protection of concrete structures in North America against corrosion damage and is commonly specified in the Middle East and Asia. This document illustrates why epoxy-coated reinforcing steel has been chosen in preference to galvanized reinforcing steel.
INTRODUCTION
During the past 40 years, substantial research has been conducted comparing the performance of epoxy-coated and galvanized reinforcing steels. This document outlines several research studies demonstrating why epoxy-coated reinforcing steel has become the material of choice in protecting concrete structures against corrosion.
Based upon the 2011 National Bridge Inventory, there are more than 74,097 bridge decks using epoxy-coated reinforcing steel covering an area of 885 million sq ft, while only 1,072 decks covering an area of 9.9 million sq ft use galvanized steel. Thus, epoxy-coated reinforcing steel has been used in over 67 times more bridge decks covering over 90 times more area than galvanized reinforcing steel. Between 2010 and 2011 the National Bridge Inventory reported an increase of 3,231 bridge decks containing epoxy-coated reinforcing steel and only 19 using galvanized reinforcing steel.
Epoxy-coated reinforcing steel has been used in over 67 times more bridges covering over 90 times more area than galvanized reinforcing steel.
MATERIALS
Epoxy-coated Reinforcing Steel:
Epoxy-coated reinforcing steel bars are typically specified to meet either ASTM A775 Standard Specification for Epoxy-Coated Steel Reinforcing Bars or A934 Standard Specification for Epoxy-Coated Prefabricated Steel Reinforcing Bars. Coatings may be applied to ASTM A615, A706 or A996 reinforcing steel with yield strengths from 40 to 80 ksi. Epoxy-coated welded wire reinforcing is also available, meeting ASTM A884 Standard Specification for Epoxy-Coated Steel Wire and Welded Wire Reinforcement; however, it is less commonly used than reinforcing bar.
CORROSION OF STEEL IN CONCRETE
When steel is placed into concrete it develops a passive oxide film due to the high pH of the concrete. This passive film prevents further corrosion. Bars extracted from very old concrete may exhibit no evidence of corrosion.
The protective film on reinforcing bars may be disrupted by carbonation of the cement paste, which reduces the pH surrounding the bar, or through the ingress of chloride ions into the concrete, from either deicing salts or sea water. The rate of carbonation and penetration of chloride ions is governed by the permeability of the concrete, which may be reduced using concrete with lower water-cement ratios or additions of materials such as fly ash, silica fume or slag cement. The presence of cracks may also enable either carbonation or chloride ingress to be accelerated. Carbonation is generally not considered a major issue in North America due to the use of low water-cement ratio concretes.
The amount of chloride ion to initiate corrosion of uncoated steel in concrete is generally considered to be 1.2 to 2.0 lb/yd³ by weight of concrete. Once this level is reached, the passive film on the steel is disrupted and corrosion initiates. As the volume of corrosion products that result from the corrosion are greater than the initial metal, cracking and damage to the concrete occurs, leading to expensive concrete repairs.
Various methods to reduce concrete damage have been used, including: reducing the concrete permeability by using lower water-cement ratios and pozzolans, surface sealers and membranes; using corrosion inhibitors in the concrete mixture; and changing the type of reinforcing steel.
Epoxy-coated reinforcing steel is generally provided from dedicated plants that manufacture epoxy-coated reinforcing steel using requirements of the Concrete Reinforcing Steel Institute (CRSI) Certification Program for Epoxy-coated Manufacturing Bar Plants. This program outlines requirements for quality control, training, and compliance with industry standards.
Many State Departments of Transportation require bars to be manufactured under this CRSI certification program.
Galvanized Reinforcing Steel:
The majority of hot dip galvanized reinforcing steel is generally processed alongside other products. While there a few galvanizers that specialize in the coating of reinforcing steel they lack any independent certification programs for galvanized reinforcing steels.
Galvanized bars are created by dipping reinforcing steel into a bath of molten zinc at about 840°F. This results in layers of iron, zinc-iron alloys and pure zinc. The silicon content of the steel influences the formation of these layers and may result in thick layers of zinc-iron alloys, which are brittle and susceptible to flaking during bending. The performance of the bars may be strongly affected by the thickness of the outermost pure zinc layer. As reinforcing bar chemistry varies due to the type of scrap steel used in its manufacture, the performance of galvanized bars may be expected to vary considerably.
PERFORMANCE OF GALVANIZED COATINGS IN CONCRETE
Extensive research has evaluated how galvanized rebar performs in concrete environments, particularly under exposure to chlorides and varying pH conditions. While galvanization provides an initial protective layer, findings across multiple studies show inconsistent long-term performance.
These findings suggest that while galvanized rebar can offer short-term protection, its effectiveness is highly condition-dependent and less predictable over time.
Performance Comparison Studies
When compared directly to other reinforcement types, the limitations of galvanized rebar become more pronounced, especially in aggressive environments.
Research comparing uncoated, galvanized, and epoxy-coated reinforcement shows:
- In chloride exposure, zinc coatings may corrode faster than conventional steel, although they can temporarily delay corrosion of the underlying bar.
- Long-term exposure studies demonstrate that uncoated steel experiences severe corrosion across all chloride levels, while galvanized rebar shows delayed cracking but still develops significant corrosion in higher chloride conditions.
- Epoxy-coated reinforcing steel consistently shows no corrosion or cracking under comparable lower chloride exposures.
- In the high-pH environment of fresh concrete, the zinc coating reacts with the alkaline pore solution — consuming zinc and evolving hydrogen gas at the bar surface. This reaction not only degrades the coating itself but can disrupt the bond interface where the concrete is meant to adhere to the bar.
- By contrast, epoxy coatings — — are chemically inert in the concrete environment. The coating does not react with the cement paste, so there is no interfacial chemical disruption and the concrete-to-bar bond remains undisturbed and particularly textured epoxy meeting ASTM A1124 the bond strength is improved to be equal to or greater than uncoated black bar.
Additional electrochemical testing reinforces these findings:
- Epoxy-coated bars exhibit dramatically lower corrosion activity, with macrocell voltage measured at approximately 9 times lower than galvanized reinforcement.
- Corrosion activity in epoxy-coated steel has been recorded at only a fraction of that observed in uncoated systems.
These results highlight a clear performance gap, with epoxy-coated reinforcement delivering more reliable and sustained protection.
Chloride Threshold Performance
The chloride threshold required to initiate corrosion is a critical measure of durability in reinforced concrete.
Comparative testing shows:
- Black steel begins to corrode at approximately 1.63 lb/yd³ of chloride ions
- Galvanized rebar increases this threshold modestly to 2.57 lb/yd³
- Epoxy-coated reinforcing steel significantly outperforms both, with a threshold of 7.28 lb/yd³
This substantial increase demonstrates that epoxy-coated steel can withstand far greater chloride exposure before corrosion begins, making it a stronger solution for demanding environments.
Evaluation of Long-Term Bridge Performance
Field data from the National Bridge Inventory provides real-world validation of these performance differences. Many agencies use a condition rating of 5 or lower to indicate when bridge repairs are likely required.
Analysis of bridge decks constructed between 1973 and 1983 shows:
- Approximately 32 percent of decks with uncoated reinforcement fell below this threshold
- 22.5 percent of decks with galvanized rebar required repair
- Only 8.9 percent of decks with epoxy-coated reinforcement reached this condition
This means bridge decks reinforced with epoxy-coated steel are approximately 2.5 times less likely to require repair than those using galvanized rebar.
These long-term results reinforce laboratory findings and confirm that epoxy-coated reinforcing steel provides more durable and reliable protection against corrosion-related damage.
Conclusion
When comparing epoxy-coated reinforcing steel and galvanized rebar, the data is clear. Epoxy-coated steel provides superior corrosion resistance and reduced maintenance requirements.
For projects where durability, performance, and lifecycle cost matter, epoxy-coated reinforcing steel continues to be the preferred choice.
REFERENCES
Clear, K. (1981). Time-to-Corrosion of Reinforcing Steel in Concrete Slabs.
Darwin, D. et al. (2009). Critical Chloride Corrosion Threshold.
Haran, B. et al. (2000). Galvanized Carbon Steel Studies.
Macias, A. and Andrade, C. (1987). Corrosion of Galvanized Steel.
McDonald, D. B. et al. (1998). Corrosion Evaluation of Reinforcing Bars.
O’Reilly, M. et al. (2011). Evaluation of Corrosion Protection Systems.
Pianca, F. and Schell, H. (2005). Ontario Bridge Studies.
Rasheeduzzafar et al. (1992). Performance of Corrosion-Resisting Steels.
Saraswathy, V. and Song, H. (2005). Performance of Galvanized and Stainless Steel.
Shimida, H. and Nishi, S. (1983). Seawater Corrosion Attack.
Treadaway, K. and Davies, H. (1989). Performance of Epoxy-Coated Steel.
Frequently Asked Questions About Galvanized Rebar
Does galvanized rebar prevent corrosion completely?
No, galvanized rebar does not fully prevent corrosion. It can delay corrosion through sacrificial protection, but studies show corrosion can still occur, especially in high-chloride environments.
How does galvanized rebar compare to epoxy-coated rebar?
Galvanized rebar offers some corrosion resistance, but epoxy-coated rebar provides a more consistent and durable barrier. Research shows epoxy-coated steel performs better in both laboratory and field conditions.
Is galvanized rebar more cost-effective?
While galvanized rebar may have a lower upfront cost in some cases, epoxy-coated reinforcing steel often results in lower lifecycle costs due to reduced maintenance and longer service life.
How long does galvanized rebar last in concrete?
The lifespan of galvanized rebar depends on environmental exposure, concrete quality, and chloride levels. In aggressive environments, its service life may be shorter than epoxy-coated alternatives.