Pretty in Zinc

Sept. 19, 2002

Corrosion of rebar has proven to be one of the leading causes of premature failure of concrete structures. Moisture wicks its way into just about anything, and concrete is no different. Galvanized rebar is an economical and effective way to eliminate this problem.  

Corrosion of rebar has proven to be one of the leading causes of premature failure of concrete structures. Moisture wicks its way into just about anything, and concrete is no different. Galvanized rebar is an economical and effective way to eliminate this problem.  

How failure occurs is quite simple. As moisture and the environment attack the rebar, iron oxides form, taking up more space than the rebar did initially. This creates stress cracks in the concrete, which in turn accelerates the corrosion, creating more stress. Eventually spalling and failure will occur.

Three layers of protection

The galvanizing process is equally simple and easy to understand. It's primarily a cleaning process, because zinc will not form a bond with a surface with even the slightest amount of impurities evident.

Once pickled clean, the rebar is slowly lowered into a liquid zinc bath. It remains in the zinc until it reaches kettle temperature, which is 850°F. It's then cooled in a water and chromate bath to seal the fresh surface. This process leaves 3-4 mills of zinc and zinc-iron alloy layers on every surface of the steel. 

Galvanizing uses three different methods to protect steel from corroding. First, the zinc acts as a barrier, totally encapsulating the product in zinc, which keeps moisture and the ravages of the environment from the steel. If the environment can't reach your steel, it can't rust.

It takes over 2,500 psi to remove galvanizing, while most paints come off at 250 psi or less. This tough barrier is developed in the kettle. Molten zinc is 850°F, and with steel melting at 2,200°. There is a lot happening at the surface of the steel when it's in the kettle. Three distinct zinc-iron alloy layers are formed, each of which is harder than the base piece of steel. This tremendous barrier is "grown" on every surface of a galvanized piece of steel, inside and out.

The second way in which galvanizing protects steel is by providing cathodic protection to the steel. When a steel surface is exposed, zinc corrodes sacrificially so that the steel will not rust. This is an electro-chemical process, some describe as self-healing, in which scratches up to 1?4 in. wide are protected from rusting. This is a very common way of preserving steel. Many types of cathodic protection are offered to rebar users but none provides the longevity, reliability and ease of use that galvanizing does.

The third way zinc protects steel is by protecting itself. As the zinc layer oxidizes, its byproducts form a carbonate that inhibits further oxidation. 

It can go anywhere

Galvanized rebar also has other beneficial properties for use in concrete structures. The bond that develops between galvanized rebar and concrete is similar to and often stronger than that of black and epoxy-coated rebar. This superior bond results in shorter development lengths, less cost and stronger structures. The chromate used in quench tanks further passivates the zinc surface when it goes into the fresh cement, further promoting bond strength. Storage, installation and general handling of galvanized rebar is less of a problem because galvanized rebar needs no special precautions, the surface of the bar is harder than the steel under it. 

One of the greatest misconceptions surrounding galvanized rebar is that it cannot be fabricated after galvanizing. Rebar can be bent after galvanizing with a radius of no less than 6 to 10 times bar diameter, depending on bar size. Any coating discontinuities can be repaired using zinc-rich paint, according to ASTM standard A767. 

Another advantage galvanizing has over the widely popular epoxy-coated rebar is coating durability, before and after it's in place. No "field guide to handling rebar" is available or necessary.

Additionally, rebar that is refabricated onsite can be repaired by simple methods, eliminating work stoppages due to field changes. Field cutting and welding does little to degrade the long-term protection afforded by galvanizing. 

Zinc has been used successfully for decades to protect rebar from corrosion. Galvanizing has been used in many different climates, from the subtropics to the far north with equal success. Recently an elevated roadway project was built near Pulaski, Pa., on Rte. 56 using galvanized rebar exclusively.

The reasons for this were many. Storage space was limited and, knowing that materials were going to be stacked up, the specifier chose galvanizing because it could stand up to the abuse of being moved around often. Because of the superior bond strength of zinc to concrete, development lengths could be reduced, which also improved storage problems. All of these benefits offset the cost of the galvanizing. High-performance concrete, which has been known to crack, was used in this application. Galvanizing was again chosen to provide long-term protection from road salts and the environment that might find an easy path to the rebar. Another reason was longevity. With a goal of 40 years to first maintenance, many parts of this structure received a galvanized coating, not just the rebar.  The cost increase of a fully galvanized project vs. an epoxy rebar and painted bridge is minimal compared to the cost of maintaining or replacing a corroding structure.

Crash tested

Highway guardrail is one of the most commonly galvanized products there is and if there was a better and cheaper way of protecting steel from corrosion, it would definitely be used on guardrail. Few coatings or materials can match the durability of galvanizing in this hostile environment.

Galvanizing can provide the same extension of useful life for rebar that it does for guardrail at a minimal cost premium over epoxy bars. This will save the owner or DOT money.

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