Construction Industry News
Concrete is the most widely used building material in the world. Although it is extremely durable, its main weakness is corrosion. This occurs most easily in older structures, but it can also attack new concrete in hot, humid coastal climates or in areas that rely heavily on de-icing salts. As such, rows and rows of green epoxy coated rebar are often seen on construction sites for added protection. However, it is important for engineers and contractors to understand that this may not be the end of their corrosion concerns, and that they should be aware of alternative methods to overcome the drawbacks of epoxy coated rebar.
If the epoxy coating on the rebar remains intact, it will provide exceptional protection by creating a thick barrier against corrosive materials. Unfortunately, damage to the coating is quite common. When epoxy coated rebar leaves the coating line and is packaged for shipment to the job site, it begins its most vulnerable stage. “When coated rebar is transported, bent, welded and installed, there is a high probability that the coating will be physically damaged. The damaged areas will become small anodes coupled with the large coated areas that will become cathodes, leading to pitting failure. The question then is how to find a backup technology that prevents corrosion at these points. Migrating corrosion inhibitors (MCIs) are an interesting, versatile and economical option.
When applied as an additive or surface treatment, the chemistry of ICMs makes its way through the pores of the concrete and forms a protective molecular layer on the surfaces of the rebar due to ionic attraction. Therefore, it can completely protect uncoated rebar or fill in molecularly damaged areas in epoxy-coated rebar where the metal is exposed. MCIs are classified as “hybrid” inhibitors because this molecular layer affects both the anodic and cathodic parts of the corrosion cell. MCIs can double or triple the time to corrosion onset and reduce the rates of Corrosion is significant once started. MCI additives such as MCI-2005 (USDA Certified Bio-Product) or MCI-2005 NS are easy to use when added to precast concrete at the time of construction.
Specific engineers can use service life prediction models to estimate and compare how much MCI is expected to extend service life in different environments compared to the expected performance of epoxy-coated rebar. The differences can be significant. On one Gulf Coast project, MCI-2005 life predictions exceeded the expected life of epoxy-coated rebar, saving the project hundreds of thousands of dollars. Although MCI can be used as a complete replacement for epoxy-coated rebar, those who want additional “insurance” and have the budget to do so can specify MCI in conjunction with epoxy-coated rebar to provide supplemental protection to supports in areas where the coating may be damaged.
Sometimes a contractor has already seen damage to the rebar coating, but it is too late to specify an MCI additive for an ongoing project. Fortunately, builders still have the option of adding MCI Surface Corrosion Inhibitor (SACI) once the structure is complete. MCI-2020 has the highest concentration of MCI of any SACI on the market and can be sprayed or rolled onto existing structures. It moves first through the concrete matrix by capillary absorption and then by vapor diffusion, gradually moving several inches into the concrete to reach and protect embedded rebar. Epoxy-coated rebar may seem like an attractive option until you take a closer look at the price and potential concerns about coating damage. Since MCI offers extended service life at a much smaller fraction of the total construction cost, engineers should carefully consider selecting MCI as a complement or complete replacement for epoxy-coated steel.
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