John L. Clarke, in Advanced Concrete Technology, 2003
26.2.1 Galvanized steel reinforcement
Galvanized steel reinforcing bars have been successfully used in several countries over the past 50 years (Australia, Bermuda, Netherlands, Italy, the UK, and the USA) and consumption is increasing. The main advantages of galvanized steel are:
it delays the initiation of corrosion and cracking
it has very good performance in carbonated concrete
it tolerates higher chloride migration levels than uncoated steel
it provides protection to the steel during storage
it has longer life in cracked carbonated concrete than uncoated bar.
Hot-dipped galvanized steel is produced by dipping clean and fluxed steel into a bath of molten zinc. The layer formed on the surface of the steel usually consists of a thin outer coating of pure zinc on a series of layers of zinc/iron alloys with increasing iron content.
The performance of galvanized steel in concrete as reported in the literature (Andrade et al., 1995) is contradictory. Although it has been used successfully in practice, laboratory studies suggest that its performance would not be cost-effective. The factors behind this divergence of views, currently the object of discussion, are:
the pH of the cement paste
the bond between the reinforcing bars and the concrete
chromate passivation of the galvanized steel
the structure and thickness of the zinc coating
the resistance of the zinc coating to corrosion induced by chloride ions
Zinc is passive in most cement pastes as the pH of uncarbonated cement pastes is 12–13.5. A passive layer would be formed when pH < 13.3, the upper limit for passivation, due to the formation of a layer of calcium hydroxyzincate, inhibiting further corrosion. The passivating process results in an homogenous zinc depletion of about 10 μm. A more protective film is produced from pure zinc than from an iron–zinc alloy. It is recommended that an external pure zinc layer of at least 10 μm and a total galvanized layer of at least 80–85 μm are needed to provide suitable protection when embedded in concrete.
In concrete made from a cement with exceptionally high soluble alkali, film formation could be inhibited during the setting period and corrosion of the zinc in the hardened concrete will depend on the environment (humidity, chloride penetration).
During the formation of the passive layer, hydrogen is evolved. Although the evolution of hydrogen raises the spectre of embrittlement, the reinforcing bars normally used in construction are not susceptible to hydrogen embrittlement. Similarly, the hydrogen evolved during the pickling process (pickling is part of the preparation of the surface prior to the application of the zinc, using a weak acid) before galvanizing does not cause a problem. Galvanizing is not generally recommended for steels with a tensile strength above 700–800 N/mm2, i.e. not for prestressing steels here the risk of hydrogen embrittlement is more severe than for unstressed reinforcement.
Several reports (Andrade et al., 1995) compare the reduction in bond strength of galvanized and uncoated steels, both plain and deformed. Reduction in bond is attributed to the formation of hydrogen bubbles at the interface between the bar and the concrete. It has been suggested (Andrade et al., 1995) that this can be overcome by adding chromate to the concrete mix or giving the bars a chromate passivation treatment. On the other hand, the zincates produced – which are less expansive and more soluble than iron corrosion products in the cement environment – could diffuse into the pores of the concrete and make the concrete more dense locally, increasing the bond strength above what would be expected for uncoated bar.
In practical terms, most construction is carried out with deformed bar and it is probable that the evolution of hydrogen will not affect the bond strength of galvanized deformed steel reinforcement. However, the use of a passivation agent is still debated. The most effective is a chromate but its use as a concrete admixture raises a number of serious environmental and health questions on-site and would certainly be rejected by cement manufacturers and contractors. It would be more appropriate to use chromated bars as, in the first instance, it would restrict the amount of chromate used and ensure it was where it was needed. It would furthermore provide additional corrosion protection before use and ensure that poor storage would not lead to white rust on the reinforcing bar.
Zinc coatings remain passive in carbonated concrete and the rate of corrosion is much lower than for uncoated steel. This makes galvanized steel reinforcement ideal for use in concrete which is at risk from carbonation.
As regards corrosion resistance in chloride-contaminated concrete, the distinction has to be made between cast-in chloride and that which penetrates from the outside. Cast-in chloride may attack the zinc coating before and during the formation of the passive calcium hydroxyzincate whereas chlorides penetrating from the outside will find the passive layer already formed and so may be less dangerous.
Though zinc can be depassivated and attacked in the presence of chloride ions, the tolerance of galvanized steel to chloride is higher than that of uncoated steel. Galvanizing protects the steel against chloride ingress because it is more tolerant to chloride, requiring a higher concentration for depassivation and it corrodes more slowly in chloride-contaminated conditions.