As a barrier, the galvanized coating on 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.

However, 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. The extent of coverage afforded by this reaction depends on many factors but primarily the conductivity of the surrounding environment, i.e. concrete in this case. Experimental data has shown that in sand-cement mortars with a w/c ratio of about 0.4, exposed steel to a distance of about 8 mm is protected by the presence of the zinc.

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. Acceptance of the use of galvanized reinforcement is also reflected in the number of national and international standards for the use of zinc coated (i.e. galvanized) reinforcement published in recent years, and the existence of many Codes and Specifications relating to galvanized reinforcement published by Federal and State bodies, especially in North America.

If corrosion of the black steel were to initiate at the connection, the zinc on the adjacent bar will simply act to cathodically protect the black steel. Clearly, the protection afforded by the dissolution of the zinc will cause the zinc to slowly dissolve and this is, of course, not the preferred outcome. To an extent this could be seen as wasting the benefit obtained by using galvanized steel in the first instance. So, to be safe, minimize the connections between galvanized steel and black steel as far as possible but if this is necessary then keep the point of connection deeply embedded in sound concrete where the risk of corrosion of the steel is minimal.

Zinc can be applied to steel in a number of ways including galvanizing, electroplating, thermal spraying and mechanical alloying. Each method produces a specific type of coating which will vary in its structure, thickness and performance, especially its anticipated life in different exposure conditions. This is because the life of a zinc coating is primarily determined by its thickness and composition.

Galvanizing is the most common method of applying zinc and should  be specified for the coating of reinforcing bar. The coating, produced by continuous or batch hot dipping,  is metallurgically bonded to the steel, and is quite tough and damage resistant. 

It is important to remember that the term galvanizing is often used to broadly mean the coating of steel with zinc. When used in isolation, it does not specifically identify the method of coating and so may be taken to allow coating by any of the available methods. This is the reason why it is important to be precise when specifying galvanizing in order that the requisite coating thickness and coating morphology will be obtained. Thus, for reinforcing steel, continuous or batch hot dip galvanizing should always be specified. 

There is some evidence that the earlier cold-twisted, high strength bars (around 400 MPa) which had been subsequently bent during fabrication may be embrittled by galvanizing. This problem was however effectively eliminated by the 1970s with the introduction of thermo-mechanically treated steels and micro-alloyed steels for high strength bars (minimum yield of 400 MPa). These conductive coated steel materials can be used. When splicing, a bar-lock coupler is recommended which can be either galvanized or stainless.

For welded splices, all welds should be touched up as recommended in appropriate Standards. It is also recommended to use appropriate protective masks and suitable ventilation when welding. Field cutting of reinforcement should be avoided and cut ends should be repaired using an appropriate touch-up procedure. In the concrete pour itself, no special handling or care is necessary.

The overall cost in using galvanized reinforcement in concrete construction depends largely on the extent to which it is used throughout the structure. For example, it is rarely necessary for the structural core or internal elements of a high rise building or the deeply embedded components of large abutments and foundations, to be galvanized. In these situations, it may only be necessary to use galvanized reinforcement in surface exposed elements or where foundations and the like may be affected by aggressive or fluctuating groundwater.

In building construction, it is generally found that the cost of galvanizing increases the overall cost of concrete as-placed by about 6-10% depending on the size and type of bar used, the galvanizing price and the quantity of steel per cubic meter of concrete. On average, the cost of the reinforcement would not be more than about 25% of the total cost of the concrete as placed. Considering that the cost of the structural frame and skin of a building normally represents only about 25-30% of total building costs, the additional cost of galvanizing reduces to between 1.5-3.0% of total building costs. This premium reduces to as little as 0.5-1.0% if galvanizing is restricted to surface panels only. However, when taken against the total project cost or final selling price, the added cost of galvanizing becomes very small indeed, often not more than 0.1-0.2%.

When the costs and consequences of corrosion damage to a reinforced concrete building are analyzed, this extra cost of galvanizing is often seen as a very small investment in long-term corrosion protection.

In essence, the use of galvanizing should not be at the expense of this basic quality and integrity of the concrete. In this way, the galvanizing can be considered to provide protection against those circumstances that may lead to premature corrosion of conventional reinforcement and deterioration of the concrete mass.

Standards for galvanized steel reinforcing bars are published internationally and many countries have their own specific specification.  

Reinforcing Steel Standards 

International:

ISO 14657, Zinc-coated steel for the reinforcement of concrete.

United States: 

ASTM A767, Zinc-coated (galvanized) steel bars for concrete reinforcement, 

ASTM A1094, Continuous Hot-Dip Galvanized Steel Bars for Concrete Reinforcement

United Kingdom: 

BS ISO 14657, Zinc coated steel for the reinforcement of concrete
France: 

NF A35-025, Hot-dip galvanized bars and coils for reinforced concrete
Italy:

UNI 10622, Zinc-coated (galvanized) steel bars and wire rods for concrete reinforcement
India: 

IS 12594, Hot-dip coatings on structural steel bars for concrete reinforcement specifications

In all national standards for the galvanizing of reinforcing steels, an average minimum thickness (or mass) of the coating is specified. This may vary depending on the thickness of the reinforcing steel, or by the class or type of zinc coating. It is important to specify coated reinforcing steel with the correct thickness of coating.  

Yes it can because when freshly galvanized reinforcing bar is embedded into wet concrete or cement paste, generally with a pH about 13.1, a tightly adhered layer of calcium hydroxyzincate salts forms on the bar surface which inhibits further attack on the coating due to its passivating effect. This reaction consumes about 10 µm of the original zinc coating at the surface of the coating and a more protective film is produced from pure zinc than from a zinc-iron alloy. This layer, know as a passivating film, isolates the zinc coating from the surrounding cement-rich matrix and once the concrete has hardened (which usually only takes a few hours) the reaction effectively ceases. The calcium hydroxyzincate layer is quite stable and remains intact on the bar surface as long as the passivating conditions at the bar surface are maintained.

When freshly galvanized steel comes in contact with wet cement, a reaction occurs at the zinc surface which passivates the coating by the precipitation of a protective layer of calcium hydroxyzincate (CHZ). A by-product of this reaction is the liberation of hydrogen and it has been suggested that the presence of the resulting gas pores in the concrete matrix may reduce the bond capacity of the reinforcement itself. To prevent the hydrogen evolution from occurring, chromates are used to passivate the zinc surface when the reinforcement is embedded in wet concrete. This has been seen as an important issue when using galvanized reinforcement to the extent that the requirement for chromate passivation is embedded in some reinforcing steel Standards.

It is to be noted that such treatment is only temporary as the chromate film, which is slightly soluble in water, can be washed off the surface. Though short-term exposure to rain water is not necessarily a problem, and wetting and drying cycles may in fact assist in the development of the protective oxide film, over time, exposure to such conditions will certainly deplete the chromate film. If the exposure conditions are quite aggressive, for example coastal, the chromate film may be lost in just a few weeks.

Another method of treatment is to add chromate salts or dilute chromic acid to the concrete mixing water. This practice has been extensively used in the pre-casting industry and addition rates in the range 70-100 ppm chromate are required. It is also to be noted that most cements contain small amounts of natural chromates which, if freely available as water soluble salts, may also contribute to the passivation of the zinc.

An important consideration here is that the hexavalent form of chromium (Cr⁶⁺) is a proven carcinogen while other forms of chromium such as Cr³⁺ also pose health risks. Chromates also present a significant environmental risk. These issues are severely limiting the use of chromates in industry generally and alternate passivation medium are being used.

An interesting consideration here is that the vast majority of quench passivated galvanized reinforcement is embedded in concrete quite likely long after the passive film has been lost, and this does not cause any reduction in the bond with the concrete. It is thus clear that chromate treatment is quite unnecessary and need not be specifically specified. If however chromate treatment is provided as a part of the galvanizing process, as is still common, this is simply an added but unnecessary benefit.

The result is that the galvanized bar has a high level of adhesion to the concrete which substantially increases the bond between the bar and the concrete. This situation is quite different to that found with black steel bars where there is in fact very little chemical adhesion between the bar and concrete. Similarly, with epoxy-coated bars, there is no adhesion, per se, of the concrete to the coating with the result that such coated bars show a reduced bond capacity to both black steel bars and also galvanized bars.

Black steel reinforcement embedded in concrete exhibits only limited adhesion to concrete and so its pullout strength is mainly determined by the geometry of the rib pattern. Galvanized reinforcement on the other hand is quite firmly adhered to concrete and, as a result, it usually displays a higher bond strength and reduced load-induced slip than equivalent black steel reinforcement.

Though these bond and slip improvements with galvanizing are realized in practice, this is not taken into account in the design of galvanized reinforced concrete and it is always assumed that the bond strength of galvanized reinforcement is no less than that of equivalent black steel reinforcement. That this may be somewhat higher than for black steel is purely taken as an added advantage. This approach simplifies the RC design process in that the same structural design considerations apply to galvanized reinforced concrete and conventional reinforced concrete. The same cannot be said however for epoxy coated reinforcement where the lack of adhesion and the low frictional effect necessitate greater embedment lengths of epoxy coated steel to achieve the same bond capacity as black steel.

These zinc salts (e.g. ZnO) retard the hydration of cement and so slightly delay the strength development of the concrete in this region. The result is that the increase in the bond strength of galvanized reinforcement lags slightly behind that of uncoated steel. This effect only lasts for the first week or so of curing and by 28 days it is usual for the concrete to have developed both its normal 28-day compressive and bond strength. Beyond this time as curing continues, the galvanized reinforced concrete will develop the typical higher bond capacity and reduced slip characteristics over than of black steel.

This is the reason why the bond strength of galvanized reinforcement may occasionally be reported as less than that of equivalent black steel in early age testing (i.e. less than 28 days). Beyond 28 days, the reduction is no longer evident and galvanized reinforcement exhibits the improved bond characteristics noted. Thus be cautious and always check the curing period when comparative bond strength test is being undertaken with black and galvanized steel.

In contrast, galvanized steel can resist the carbonation-induced reduction in pH since zinc has a very low rate of corrosion across a wide range of pH in the range pH 6-13. Certainly, zinc has a very low rate of corrosion below pH 12. This would indicate that galvanized reinforcement should perform well in carbonated concrete, and this has been confirmed by extensive research and field observation. In effect, it can be safely stated that galvanized steel does not corrode in carbonated concrete.

For galvanized steel in concrete, there is no universal agreement on what the chloride threshold may be. What is clear however is that a significantly higher chloride threshold is needed to initiate attack on the zinc coating. For example, in simulated cement solutions, it has been shown that zinc is attacked at chloride concentrations some 5-6 times higher than that required for black steel while in concrete specimens the chloride threshold is reported to be at least 2-2.5 times higher than that for black steel, and likely somewhat higher than this. Some isolated field results suggest a threshold up to 8-10 times higher. These high tolerance levels to chloride are a major contributor to the long-term durability of galvanized reinforcement in concrete exposed to aggressive chloride-containing environments.

It is also known that the resistance of the galvanized coating to chloride attack depends on the compactness of the passivating surface layer as well as the microstructure of the remaining coating. By the time chlorides reach the reinforcement during the life of the structure, the protective surface layer of calcium hydroxyzincate should have already formed. If this layer is compact and continuous and the remaining coating has a thick enough pure zinc layer to resist pitting attack, the galvanized coating will resist chloride attack quite well.

The issue of the extension of the life of galvanized coatings can be demonstrated by a simple calculation of the time to corrosion of black steel and galvanized steel in similar exposure conditions as follows. For black steel assume an upper threshold value of 0.4% Cl by mass of cement, while for galvanized steel assume a lower threshold of 1.0% Cl based on conservative experimental and field data. Also assume an equivalent exposure condition in a marine concrete with 0.35% chloride ion concentration at the concrete surface, a 30 mm cover to the reinforcement and a chloride diffusion coefficient D of 1.4 x 10-12 m2/s. By using Fick’s Law it is shown that for black steel corrosion of the reinforcement will initiate after 15 years, while for galvanized steel attack initiates after 44 years. This indicates a theoretical extension of life of 3 times for galvanized bar over black steel bar. In practice however, the extension is normally much longer.

As a result, it is dangerous to draw too heavily on the results of the accelerated testing of isolated specimens. What may be possible however is to make general comparisons between specimens of (say) different reinforcement in concrete of similar geometry in identical exposure conditions. If accelerated testing results can be correlated with data from natural and long-term exposure, this will be the most reliable comparison to make and of the greatest benefit.

Cold working may also render the steel susceptible to an effect known as strain ageing at the temperature of the molten zinc bath. This does cause a reduction in ductility of the bar and because of this stain ageing is sometimes mistakenly attributed to hydrogen embrittlement. An understanding of the composition and metallurgy of the steel will assist in avoiding such difficulties.

Though not particularly damaging, and with very little effect on the corrosion resistance of the coating, it does detract from the appearance of the galvanizing. There is no evidence to suggest that small quantities of white rust on the surface of galvanized reinforcement have any effect on the adhesion of concrete to the bar or the long-term corrosion resistance provided by the coating.

To overcome this potential problem in general galvanizing, it is standard practice in hot dip galvanizing facilities to cool the work by quenching it in water containing a low concentration of sodium dichromate (usually less than 0.5%). Chromate quench passivation as this is known provides initial protection to the zinc and gives it time to develop its own protective oxide film. It is to be noted however that such treatment is only temporary as the chromate passivation film, which is slightly soluble in water, can be washed off the surface. Short-term exposure to rain water is not necessarily a problem, and wetting and drying cycles may in fact assist in forming the protective oxide film, but over time exposure to such conditions will certainly deplete the chromate film.

Gray coatings detract from the appearance and uniformity of the galvanized coating and are often unacceptable on galvanized steelwork which is to be visually exposed. However, there is essentially no difference in the long-term corrosion resistance of gray coatings compared to bright coatings since the extent of corrosion protection is a function of the coating thickness not the coating structure.

As far as galvanized reinforcement is concerned, there is no effect on either the corrosion resistance or bond capacity if a gray coating be present. The only issue may be that the extra thickness of the coating and the absence of the pure zinc layer may cause some slight flaking during fabrication, but again this is not of concern. It should also be remembered that if galvanizing produces a gray coating this cannot be blamed on the galvanizer for it is the nature of the steel and not the galvanizing process which is the cause.

To avoid this situation, it is recommended that bar supports, spacers and reinforcement supports should all be hot dip galvanized and that 16.5 gauge or heavier galvanized tie wire should be used. Alternatively, solid plastic or non-conductive coated steel components may be used though care should be exercised to ensure that the non-conductive (i.e. plastic) coating is itself not damaged.

The female components are dipped prior to thread cutting and their threads are tapped slightly oversized, typically by about 0.4 mm. Though the female threaded components are uncoated they nevertheless remain protected by the adjacent zinc surfaces. Most proprietary reinforcing couplers have threads with a very slack fit enabling even the female components to be dipped without problems of fouling. To assist fixing and prevent galling, these threaded components should be coated with a lubricant prior to assembly.

Care should be exercised when welding galvanized steel to ensure that there is adequate ventilation and exhausting of fumes that may be generated. Because of the relatively low melting point of zinc, it is easily vaporized and converted to white zinc oxide fume. Though this fume is not toxic, it can cause discomfort if it is inhaled. In general, anything that can be welded before galvanizing can be welded after galvanizing. Advice on the welding of galvanized steels can be obtained from your local galvanizing association.

Overall, the repair can be easily undertaken by any of these methods and the results, though not as good as the original coating will provide adequate corrosion protection to the previously damaged region. Whenever a repair is undertaken it is important to appropriately clean the exposed metal surface and adjacent region. Advice on the repair of galvanized steels can be obtained from your local galvanizing association.

Continuous galvanized rebar has excellent formability and so can be produced as straight lengths and then fabricated with no issues. Batch hot dip galvanized rebar is less formable and so when produced in straight lengths and then fabricated often does require some repair of the coating at the bends. Both types of galvanized rebar will need touchup or repair at cut ends and welds. Though coating repair at bends on batch hot dip galvanized rebar can be easily and reliably completed, there are advantages to be gained if batch galvanizing can be undertaken after all fabrication has been completed.

For example, if a sizeable reinforcing cage for a column, beam, foundation or a precast panel can be fabricated from black steel and then galvanized, the coating will completely cover the bends, cuts and welds with no need for local repairs. In this case black steel tie wires can be safely used as will also be coated during the galvanizing process. The same principles apply to pre-fabricated stirrups and ties which are also regularly galvanized.

The question of whether pre- or post-galvanizing is best depends on the circumstances and the galvanizing process. If it is possible to pre-fabricate and then galvanize this is a good option to follow for batch galvanizing. Note however that the ability to do this may well be dictated by the capacity of the galvanizing bath, especially so if large complicated pre-fabricated sections are to be coated. This is a matter that needs to be discussed with the galvanizer well in advance with appropriate planning.

• light-weight precast cladding elements and architectural building features;
• surface exposed beams and columns and exposed slabs;
• prefabricated building units such as kitchen and bathroom modules and tilt-up construction;
• immersed or buried elements subject to ground water effects and tidal fluctuations;
• coastal and marine structures;
• transport infrastructure including bridge decks, roads and crash barriers;
• and high risk structures in aggressive environments.

Many examples exist around the world where galvanized reinforcement has been successfully used in a variety of types of reinforced concrete buildings, structures and general construction including:

• reinforced concrete bridge decks and pavements;
• cooling towers and chimneys;
• coal storage bunkers;
• tunnel linings and water storage tanks and facilities;
• docks, jetties and offshore platforms;
• marinas, floating pontoons and moorings,
• sea walls and coastal balustrades;
• paper and pulp mills, water and sewerage treatment works;
• processing facilities and chemical plants;
• power stations
• waste water and sewerage treatment facilities
• highway fittings and crash barriers; and also lamp posts and power poles.

Tilt-up panels are another good example and also ferro-cement construction in applications such as shelters, boats, pontoons and marine buoys where galvanized wire or mesh is often employed. The reasons for this are straightforward. Where the cover is intentionally reduced and/or thin elements may crack, the corrosion protection afforded by the zinc coating ensures that the reinforcement does not prematurely corrode.

A vitally important issue with such buildings, especially where large amounts of public funds have been expended, is that they maintain their pristine condition.

The key to this is that cracking and rust staining of exposed concrete must not be allowed to occur and so very high quality materials and construction methods are usually employed. To this end, galvanizing of reinforcement in precast cladding panels, facades and exposed structural elements has been widely used to ensure a long, trouble-free life.

The bright white sails of the Sydney Opera House, located as it is on the foreshores of Sydney Harbour, is a perfect case in point: it would be ‘catastrophic’ if those sails were to be stained by rust streaming from the precast cladding panels.

Galvanizers Association of Australia: www.gaa.com.au

American Galvanizers Association: www.galvanizeit.org

International Zinc Association: www.galvanized rebar.com

Hot Dip galvanizers Association of Southern Africa: www.hdgsa.org.za

European General Galvanizers Association: www.egga.com

Galvanizing Association of New Zealand: http:www.galvanizing.org.nz

Galvanizing Asia: www.galvanizingasia.com

Galvanizers Association UK: wwwhdg.org.uk

Asociación Latinoamericana de Zinc (LATIZA): www.latiza.com

India Lead Zinc Development Association: http: www.ilzda.com

• proper galvanizing procedures have no significant effect on the mechanical properties of the steel reinforcement;

• zinc coating furnishes local cathodic protection to the steel, as long as the coating has not been consumed;

• galvanized reinforcement provides protection to the steel during storage and construction prior to placing the concrete;

• corrosion of galvanized steel in concrete is less intense and less extensive for a substantial period of time than that of black steel;

• galvanized steel in concrete tolerates higher chloride concentration than black steel before corrosion starts;

• galvanized reinforcement delays the onset of cracking, and spalling of concrete is less likely to occur or is delayed;

• the concrete can be used in more aggressive environments, and so a standard design of concrete components can be retained for various exposure conditions by the use of galvanized steel in the most aggressive cases;

• lightweight and porous concretes can be used with the same cover as for normal concretes;

• poor workmanship resulting in variable concrete quality (poor compaction, high water/cement ratio), can easily be tolerated;

• accidentally reduced cover is less dangerous than with black steel;

• unexpected continuous contact between concrete and trapped water can be tolerated;

• repair of damaged structures can be delayed longer than with black steel;

• galvanized hardware is acceptable at the surface of the concrete, as it is for the joints between precast panels;

• the use of galvanized reinforcement ensures a clean appearance of the finished concrete with no trouble arising at cracks either from spalling or rust staining; and

• galvanized reinforcement is cleaner and easier to work with, and makes it possible to consider the use of thinner wires as welded fabrics.

The report goes on to say that “it is important to remember that even if these benefits are achieved, the use of galvanized reinforcement should not be considered as an alternative to the provisions of adequate cover of dense, impermeable concrete, unless special design criteria have to be met.

Galvanizing of reinforcement is a complementary measure of corrosion protection – a kind of insurance against the inability of the concrete to isolate and protect the steel”.