The Science Behind How Galvanized Rebar Reacts in Concrete

Galvanized rebar is used in many DOT projects because of its robust corrosion protection in reinforced concrete; it is common in construction ventures across the country. To fully understand and appreciate the extent of this protection, you have to carefully examine the science behind how galvanized rebar reacts in concrete.

At South Atlantic, you can get the galvanized rebar you need without the hassle. We make it easy to order the length and weight you need, and because we have an extensive inventory, it’s ready to ship to your job site quickly and efficiently. Want to know more? Contact us to get a quote.

In this article, we’ll go further in depth to explore the principles behind how galvanized rebar reacts in concrete, examining the benefits it can bring to your project.

How Does Galvanized Rebar React in Concrete?

Galvanized rebar is steel rebar that is coated with a protective layer of zinc through the process of hot-dip galvanizing. This zinc forms a protective coating, and the zinc “sacrifices” itself in order to protect the material underneath. The result is corrosion resistance that can last well over 100 years.  

Galvanized Rebar in Concrete: Breaking the Myths

There are a lot of common misconceptions about how galvanized rebar reacts in concrete. One of the main myths centers around zinc’s reaction in concrete, and that the hydrogen gas generated by the interaction of zinc and high pH liquid concrete can weaken the bond of the concrete to the rebar.

A Closer Look at the Chemical Reactions

When hot-dip galvanized (HDG) rebar is placed into fresh concrete, a chemical reaction occurs when the zinc coating touches the wet cement paste (which has a high pH level). 

As a result, a reaction occurs that precipitates calcium hydroxy-zincate crystals onto the surface of the bar. These crystals adhere strongly to the bar’s surface and expand into voids within the surrounding concrete, including micro bubbles caused by the hydrogen gas that is a byproduct of this reaction, increasing the bond strength between the steel and the hardened concrete matrix.

This crystalline structure also acts as a passivation layer which has a significantly higher corrosion threshold than the passive oxide film that forms on uncoated ‘black’ rebar, and will isolate the zinc alloy beneath it from corrosive agents making their way through the concrete.

The Performance Advantages of How Galvanized Rebar Reacts in Concrete

There are several performance advantages to how galvanized rebar reacts in concrete. These include:

• The passivation protection provided by the calcium hydroxy zincate shell significantly delays corrosion initiation, even in areas of cracked concrete
• The zinc alloy between the passivation shell and the steel bar provides a secondary layer of slow reacting protection that will not create expansive byproducts that will crack and spall the surrounding concrete. 
• Providing “sacrificial” protection. If corrosive agents make it through the zinc alloy layer and reach the steel bar, the surrounding zinc will take damage in order to keep the material underneath from rusting. There can never be under-film corrosion of steel with galvanized bar

What Issues Can These Reactions Cause?

Reduced Bond Strength During Early Stages

Years of testing in various laboratories around the world have consistently shown that galvanized rebar after 28 days has a higher bond strength than black rebar. As discussed above, the crystalline Calcium hydroxy zincate grows into any voids along the bar surface so gas microbubbles created during the initial pour do not create a problem.

The Zinc / Liquid Concrete Reaction Diminishes the Overall Coating Thickness Layer 

As part of these early reactions, up to ½ mil of zinc may be consumed in formation of the crystalline shell. Fortunately this is a very small percentage when compared to the overall rebar coating thickness, and the product it becomes is even more resistant than the original zinc.

Chromate Treatment

All ASTM standards covering galvanized rebar in the United States require that bars be treated with a chromate solution after galvanizing to help control the amount of gas generated by the initial reaction of zinc and liquid concrete. This additional step is only added under US codes and is not used in other countries, and the US specifications allow architects and engineers to waive this requirement.

South Atlantic Reinforcing Has You Covered with the Strength and Flexibility You Need in Galvanized Rebar

Our rebar is not only strong, but it won’t flake or crack when bent and is in stock ready to ship. With reliable delivery dates and stellar customer service, you won’t have to worry about whether or not your material will get to you on time. We also qualify under “Buy America.”

Want to see how we’ve got you covered? Contact us for a quote today.