Durability of galvanized reinforcing steel
Galvanized reinforcing steel is in common usage and is an effective way to ensure the durability of a concrete structure at a much lower capital cost than using stainless steel reinforcing steel.
Galvanized reinforcing steel does not have the ongoing testing and maintenance costs associated with cathodic protection systems. The galvanizing process has no significant effect on the mechanical properties of reinforcing steel, and all available grades may be successfully galvanized. Galvanized coatings on reinforcing steel provide barrier protection, improved bond strength, a superior passivating layer and act as a sacrificial anode should the reinforcing steel beneath the coating be exposed. Unlike epoxy coatings, it has excellent abrasion resistance, is unaffected by UV light and has no special requirements for storage, transport, handling and fixing.
Galvanized reinforcing steel is passivated in wet concrete by the formation of an adherent film of calcium hydroxyzincate. In forming this film, the bond strength between the galvanized reinforcing steel and concrete is increased. Galvanized reinforcing steel is stable over a wide pH range and is completely unaffected by the carbonation of concrete.
Conservatively, galvanized reinforcing steel has a 2 to 2.5 times higher threshold to chloride attack when compared to uncoated reinforcing steel – this more than doubles the time to de-passivation of the reinforcing steel and corrosion initiation. Typically, galvanized reinforcing steel increases the service life of the structure by 4 to 5 times when compared to uncoated reinforcing steel. The time to corrosion initiation of galvanized reinforcing steel in concrete can be modelled using conventional industry chloride ion diffusion models based on Fick’s Second Law.
An interactive deterministic model based on Luping and Guiligers solution to Fick’s Second Law is available on the GAA website. The model predicts the time to corrosion initiation by calculating the chloride ion concentration profile through the concrete at a specified time, considering variables such as the thickness of cover, surface chloride level (environment), concrete age, chloride diffusion coefficients, type, and content of supplementary cementitious materials. Corrosion initiation of the reinforcement is predicted to occur when the chloride ion concentration at the bar surface reaches its critical chloride threshold and the bar becomes de-passivated. The higher critical chloride threshold of galvanized reinforcement can also be factored into probabilistic models.
The passive behaviour of galvanized reinforcing steel in concrete makes it suitable for use in aggressive environments and is ideally suited for external facades, precast panel joints and surface elements, indeed any application where carbonation or chloride ingress is of concern.
There are no special requirements for the design of concrete using galvanized reinforcing steel and no extra steel or overlay is required. In fact, the higher chloride threshold of galvanized steel allows the option for a thinner cover to be used compared to uncoated reinforcing steel while achieving the same durability.
Should galvanized reinforcing steel become de-passivated, zinc will corrode at a slower rate than iron, and the zinc coating provides a barrier to iron corrosion. And unlike iron, zinc corrosion products will migrate from the galvanized coating, and by reducing the porosity, will slow down the rate of chloride ingress. The relatively smaller volume of zinc corrosion products compared to iron, lessens the expansive pressure generated by the corrosion process, thereby reducing the size of any cracks which may form.
For further information see the Concrete section of this website.