Frequently asked questions - Hot dip galvanized reinforcement


Why are hot dip galvanized reinforcements used in concrete constructions?

The zinc coating on the reinforcement isolates the steel from the cement matrix and corrosion of the underlying steel will only commence once the coating has been completely corroded away. Because of the rate of corrosion of zinc in concrete is usually extremely slow, the loss of the coating in this way is a very long-term process and so corrosion of the steel is significantly delayed. Even if the coating has dissolved or been mechanically damaged such that the underlying steel is exposed, the remaining zinc on the adjacent surface becomes anodic and provides sacrificial cathodic protection to the bare steel. As such, the corrosion of the exposed steel is further delayed.


What are the differences between galvanizing and other coatings for reinforcing steel?

Unlike painting and epoxy coating on steel which are solely barrier-type coatings, galvanizing provides both barrier and sacrificial protection to the underlying steel. In a barrier coating, once the coating is damaged and the underlying steel is exposed, corrosion commences. This often leads to so-called under-film or filiform corrosion in which corrosion proceeds under the adjacent coating resulting in the further decohesion of the coating and continuation of corrosion. When usig galvanized steel, because of the more electronegative potential of the zinc in the surrounding area, it provides sacrificial cathodic protection to the exposed steel.


How long have galvanized steels been used in concrete?

The first reports on the use of zinc coated steel in concrete date to about 1908. Its first regular use as a reinforcing material was in the 1930s in the USA. In the post-WWII period the use of galvanized rebar became more common and by the 1960s and early 1970s a considerable tonnage of steel reinforcement was being galvanized especially for use in bridge and highway construction across the snow-belt states of the USA and Canada. Since this time, and especially over about the last 25-30 years, there has been a steady world-wide use of galvanized reinforcement in a wide variety of types of concrete construction and exposure conditions.


Are there different requirements in designing of reinforced concrete when using galvanized bars?

There are no special requirements for the design of galvanized reinforced concrete beyond that which apply to conventional reinforced concrete.


Can galvanized and black steel reinforcement be used together in concrete?

Because zinc is naturally protective to steel, galvanized reinforcement can be safely mixed with uncoated in concrete, especially if the connection point between the two materials is well embedded and sufficiently deep such that there is no corrosion risk for either material.


Does galvanizing affect the structure and properties of the reinforcing steel?

The microstructure and the mechanical properties of steels are primarily controlled by the temperatures to which they are heated during processing and the subsequent rate of cooling to ambient temperature. As a general rule, steels must be heated for a reasonable period of time above about 650°C for there to be any significant effect on either the microstructure or the mechanical properties of the steel concerned. In hot dip galvanizing, the maximum temperature reached in the zinc bath is about 450°C. This temperature is not sufficiently high to cause any noticeable heat treatment effect in structural steels and exhaustive testing of all types of reinforcing steel has consistently shown this to be the case.


Will concrete have an adequate adherence to the reinforcement if it is galvanized?

There is a vast body of evidence showing that concrete tightly adheres to galvanized reinforcement. In fact, this adhesion is better than is achieved with uncoated steel. The basis of this is the formation of the protective surface layer of calcium hydroxyzincate. This layer is not only tightly adhered to the zinc surface it also interacts with the adjacent cement matrix effectively creating a bridge between the bar and the matrix. There is also evidence to show that the zinc corrosion products released from the surface of the coating in these circumstances migrate (or diffuse) into a narrow interfacial zone between the bar and the concrete resulting in strengthening and densification of this zone.


What is the life extension achieved with galvanized reinforcement?

The delay in the onset of corrosion of galvanized steel compared to black steel is known as the extension of the service life. For galvanized reinforcement in concrete, this extension of life to the onset of corrosion has variously been reported to be 4-5 times longer than that for the corrosion of black steel in equivalent exposure conditions.


Are there special transport, handling or on site storage requirements?

There are no special handling or transport methods necessary when loading/unloading or job site handling of galvanized reinforcement. Regarding the on site storage, galvanized reinforcement (as any other hot dip galvanized product) can be stored directly on the ground without risk of significant damage to the coating. It is recommended that bundles of freshly galvanized products be stored on a slant to allow water drainage and air flow in order to reduce the aspect modifications that can occur.