Corrosion of embedded metal components, especially reinforcing bars, is a principal cause of deterioration and failure in concrete structures. Corrosion expands the diameter of the rebar, which puts pressure on the surrounding concrete and leads to cracks, delamination, and spalls. Rebar corrodes most readily when it’s exposed to water and dissolved chlorides (salt), so concrete structures in marine environments and those treated with deicing salts are particularly at risk. (See “Preparing Concrete for Durable Repairs,” Concrete Construction, April 2013, which describes the corrosion mechanism in more detail.)
Good design and construction practices — including appropriate mix design and careful placement, consolidation, and curing — can go a long way toward preventing or slowing rebar corrosion. In the most critical applications, however, it’s often wise to provide extra protection. For projects such as bridge decks, parking structures, marine structures, sewage treatment or chemical plants, and others, the use of alternative, more corrosion-resistant reinforcing materials is advisable. Several such products are available and worth considering.
The most common alternative to standard black steel rebar is epoxy-coated reinforcement (ECR). It is produced by subjecting standard steel reinforcement to a four-step process of blast-cleaning, heating to about 450 degrees F, spray-application of dry epoxy powder, and brief curing.
ECR is covered in ASTM A775 “Standard Specification for Epoxy-Coated Steel Reinforcing Bars,” and special requirements for jobsite handling are included in the Appendix of A775 and in D3963. The epoxy coating protects the steel from exposure to water and chlorides, and the handling requirements are designed to limit damage and provide procedures for repair of damage to the coating that could reduce its effectiveness. The handling requirements include provisions for properly loading and unloading, transporting, and storing bars to avoid sagging and abrasion; placing bars without dragging or walking on them; inspecting for and patching any cuts or breaches in the coating; and using a rubber or nonmetallic vibrator head when consolidating concrete to minimize the risk of damage.
The first ECR project was the Schuykill Bridge near Philadelphia in 1973, and by 2008, the material had been used on more than 60,000 bridges nationwide. David McDonald, head of the Epoxy Interest Group within the Concrete Reinforcing Steel Institute (CRSI), estimates that ECR represents 10% to 15% of the total rebar market, and that about 85% of it goes into bridges. It is widely accepted and frequently required by state DOTs for bridge decks and other transportation structures.
McDonald says that the increased initial cost of epoxy-coated bar over uncoated bar is generally 25% to 50%, depending on market fluctuations. The development length for coated bars is slightly longer as well, which contributes another modest cost factor. Together though, these factors typically add only 0.5% to 2% to the overall cost of a project. McDonald says the cost premium is more than justified by the improvement in performance and increased service life that ECR makes possible.
Recent reports from Michigan Department of Transportation using field data from 766 bridge decks with ECR indicates a 70-year design life for their bridge decks. Recent data from Florida shows an expected 100-year design life for piers in most of the 300 bridges with ECR.