Epoxy Coating of Steel Reinforcement
Epoxy coated rebar has been proven to be effective in protecting steel from corrosion, especially in steel exposed to seawater. This method has been used for painting reinforced steel in bridges and offshore structures since 1970. However, there are some shortcomings associated with this method. Precautions must be taken during the manufacturing and operation of painting the steel, such as avoiding friction between the bars to prevent erosion of the coating layer. Additionally, measuring the corrosion rate of epoxy coated rebar is challenging, making it difficult to predict corrosion performance or measure corrosion rate accurately.
In the United States and Canada, over 100,000 buildings use coated bars, which amounts to 2 million tons of epoxy-coated bars. ASTM A 775M/77M-93 sets allowable limits for epoxy coated rebar, including coating thickness (130-300 ?m), bending around a standard mandrel without cracks in the coating, number of pinhole defects (not more than six per meter), and damage area on the bar (not exceeding 2%). These deficiencies in the coating can occur during operation, transportation, and storage, and precautions must be taken to avoid cracks in the paint during these phases.
Painting steel reinforcement bars can reduce the bond between the concrete and steel, requiring an increase in the development length of steel bars to overcome this reduction. According to the American Concrete Institute (ACI) code, the increase in development length is typically 20-50%. The code also stipulates that, in the case of painting, the development length of steel bars must be increased by 50% when the concrete cover is less than three times the steel bar diameter or the distance between the steel bars is less than six times the bar diameter. In other circumstances, the development length should be increased by 20%. Painting smooth bars made of mild, low tensile steel at full bond strength is prohibited due to friction, as it would result in loss of bond strength. Therefore, it is important to avoid coating smooth bars with epoxy.
Care must also be taken to avoid increasing the thickness of the paint coating to more than 300 ?m, as excessive thickness can result in cracks and separation between steel bars and concrete. Some researchers have found that using a thickness of 350 ?m for the main steel reinforcement in concrete slabs resulted in too many cracks and separation between steel bars and concrete. Therefore, adherence to coating thickness limits is crucial in ensuring the effectiveness of epoxy coated rebar in protecting steel from corrosion.
The corrosion rates of steel bars, both coated with epoxy and uncoated, were compared when exposed to tap water and water containing sodium chloride and sodium sulfate. The results showed that the corrosion rate was slower for coated steel bars compared to uncoated ones, indicating that epoxy coating is effective in reducing corrosion. This method is widely used by workers and contractors in North America and the Middle East due to its cost-effectiveness. However, it is important to note that high-quality concrete and proper concrete cover must be maintained even when using epoxy-coated steel bars. Some steel manufacturers provide steel bars with pre-applied coating, which is a good alternative to on-site coating as it ensures consistent coating thickness, which may be difficult to measure accurately with on-site tools.