Galvanized Steel Reinforcement Bars
Galvanized Steel Reinforcement Bars
Galvanized steel reinforcement bars (also called galvanized steel rebars) are the normal reinforcement steel bars which are coated with a protective layer of zinc (Zn) metal. Zn coating is usually carried out by hot dip galvanizing process. The Zn coating serves as a barrier to the corrosive environment which the rebars are exposed to when embedded in concrete. In addition to the barrier protection, Zn also provides cathodic protection where Zn corrodes preferentially when in contact with unprotected steel. This means that in case of any gap in Zn coating the surface of bare steel is protected by the surrounding Zn.
The reaction between steel and molten Zn produces a coating on the steel made up of a series of iron -zinc alloy layers (gamma, delta and zeta) which grow from the steel-zinc interface with a layer of essentially pure Zn (eta) at the outer surface. What distinguishes galvanizing from other types of coatings is that the coating is metallurgically bonded to the steel. It actually becomes an integral part of the steel, as compared to the paints and epoxy coatings which are simply attached to the steel surface by physical bonding. The alloy layers in the coating are harder than the base steel resulting in a coating that is not only firmly adhered to the steel but is tough and hard and can resist abrasion and fairly heavy handling. It also allows the galvanized rebar to be handled, transported and fabricated in the same manner as ordinary steel. A typical galvanized coating structure is shown in Fig 1.
Fig 1 Galvanized coating structure
The first regular use of galvanized coating was done in USA during 1930s. Since then, and especially during the last 25 -30 years, it is being used in a variety of concrete construction in several countries. Acceptance of the use of galvanized reinforcement is also reflected in the significant number of international standards for the use of galvanized rebars (e.g. ISO 1461, AS/NZS 4680, ASTM A767, CAN/CSA G164, SS-EN ISO 1461, BS EN ISO1461, and ABS/ISO 1461) as well as technical publications, codes of practice and specifications relating to galvanized reinforcement which have been published in recent years.
There are no special requirements for the design of galvanized reinforced concrete beyond that which apply to conventional reinforced concrete. In particular, splice and lap lengths are the same as for normal steel rebar, as are bond and load transfer considerations.
Cracking of the galvanized coating
One of the major concerns with galvanized rebars is the cracking of the galvanized layer during bending and whether this causes a loss of adhesion to the steel. However, if appropriate bend radii are used the risk of cracking and any effect on the coating adhesion can be minimized.
Many bend tests have shown that the extent of cracking and the width of cracks in the Zn coating are influenced by the bend radii, the diameter of the rebars, the angle of bend, and the thickness of the coating. In general, the smaller the bend radius, the larger are the cracks. Also the thicker is the coating the greater is the intensity of cracking. Cracking in the coating invariably occurs at right angles to the length of the rebar and if the cracking causes local de-bonding between the coating and base steel, the durability of the coating may be compromised. Hence it is safer to galvanize rebars after bending if it is at all possible.
The corrosion susceptibility of bent galvanized rebars with cracked coatings has shown that corrosion does not preferentially occur at the cracks produced by bending. It appears that ZnO corrosion products blocked the cracks in the coating thereby preventing localized corrosion of the base steel. The Zn coating also has good inherent resistance to abrasion and impact. It has also been shown that transportation of galvanized rebars, and normal concreting operations, do not damage the coating.
Mechanical properties of galvanized steel rebars
Extensive testing has shown that galvanizing does not adversely affect the tensile properties of conventional reinforcement steels (around 250 MPa) providing such steels have not been excessively cold worked prior to galvanizing (e.g. bending and re-bending). There is some evidence that cold twisted, high strength rebars (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 (TMT) reinforcement steels and micro alloyed steels for high strength rebars (minimum yield of 400 MPa). More recently, higher strength reinforcement to 500 MPa yield has been introduced and it has again been seen that the mechanical properties of these rebars are not adversely affected by galvanizing.
On the other hand, Zn has low fatigue strength and consequently the fatigue behaviour of galvanized rebars is affected more than their static properties. While not a major concern, this may need to be taken into account for structures designed to withstand earthquake forces.
Bond and slip characteristics
The bond between concrete and the rebars is essential for developing the full capacity of the reinforcement. The single largest contributor to bond between the rebars and the concrete is whether the rebars are ribbed or plain. In case of plain rebars the bond strength is solely due to chemical adhesion and frictional between the rebar and the concrete. If the rebars are ribbed, the bearing of the concrete on the ribs and the shear strength of the concrete between the ribs become predominant.
In the case of galvanized rebars the principle issue is whether there is any significant difference in bond strength and slip compared to the normal black steel rebars. Experience with the use of galvanized rebars has shown the following.
- The time needed to develop full bond strength for galvanized rebars may, in some circumstances, be longer than that for black steel rebars though this effect is usually overcome prior to 28 days of curing.
- Galvanized plain rebars have superior bond strength to equivalent black steel rebars, though in some cases not as good as pitted and rusty steel rebars, thereby indicating the sensitivity of the bond strength to the roughness of the surface.
- There is no significant difference in the ultimate bond capacity of ribbed black steel or galvanized steel rebars
- In beam tests, at ultimate load there is no significant difference in the free end slip of galvanized and black steel rebars. At intermediate loads there is a noticeable reduction in slip for galvanized rebars compared to black steel rebars.
Usually the bond strength of galvanized rebars is not less than that of equivalent black steel rebars. In fact it may be higher than that of the black steel rebars. In practice however, though the bond strength is usually somewhat higher than that with black steel rebars, this is not taken into account in the design of galvanized reinforced concrete. It is simply assumed that the element will behave as if it is reinforced with black steel rebars.
Despite this, there are doubts about the effect that hydrogen evolution, as a result of the reaction between zinc and wet cement, may have on the bond strength, and the use of chromates to overcome this perceived issue.
The issue of hydrogen evolution
When galvanized steel rebars come in contact with wet cement, the formation of calcium hyrdoxyzincate at the surface of the rebars is accompanied by the evolution of hydrogen gas bubbles. Since this reaction ceases once the cement starts to harden after the first few hours, only quite small quantities of hydrogen are produced.
In mass concrete with significant quantities of coarse aggregate and entrained air, the hydrogen bubbles are well distributed in the concrete matrix and can rarely be separately identified. In light weight concretes with low volume fractions of coarse aggregate, such as may be used in pre-casting, the bubbles may accumulate and rise though the concrete cover. This can result in variations in the texture of the concrete at the surface of the precast panel which may be aesthetically undesirable.
While the evolution of hydrogen may occur, the issue of whether this will reduce the bond strength of galvanized rebars in concrete is often over-emphasized. As seen above, there is no reduction in bond strength for galvanized rebars compared to equivalent black steel rebars. A key aspect of this is that the hydrogen evolution from galvanized steel immersed in cement paste occurs on surfaces where iron and Zn are both present and not from pure Zn. This suggests that it is the zinc-iron alloy layers in the coating which initiates the formation of hydrogen and that this should not occur to any significant extent on the pure Zn outer surface of bright galvanized steel rebars.
The hydrogen evolution, however, can be effectively eliminated if the coating is passivated by another means. This can be achieved by treatment of the freshly galvanized steel rebars with a variety of chemicals, the most common of which are chromate salts.
The prevention of hydrogen generation on the surfaces of galvanized steel, by precluding the reaction between Zn coated reinforcement rebars and fresh cement, can be achieved by the application of a dilute chromate solution to freshly galvanized steel.
Chromate passivation as it is called can be achieved by quenching freshly galvanized rebars in water containing 0.2 % sodium dichromate or a 0.2 % chromic acid solution. The bath needs to be at a temperature of at least 32 deg C and the rebar needs to be immersed for at least 20 seconds. If the rebar has cooled after galvanizing, sulphuric acid (0.5 % -1.0 %) needs to be added to activate the rebar. An alternative method is to add chromates to the concrete mix water in the form of sodium or potassium dichromate at a rate of 70 ppm expressed as CrO3 by mass of cement.
In addition to extensive laboratory data, evidence from field applications has clearly shown that galvanizing extends the life of rebars in concrete and provides a safeguard against premature cracking and rust staining of the concrete. As shown earlier, the corrosion protection caused by galvanizing is due to a combination of beneficial effects. Of primary importance is the substantially higher chloride threshold for Zn coated steel rebars in concrete compared to black steel rebars. In addition, galvanized rebar is resistant to the effects of carbonation of the concrete mass.
The net effect of the presence of the Zn coating is that it not only delays the initiation of the corrosion process, but it continues to provide barrier protection during that period when the coating is reacting (i.e. dissolving) but remains intact. Even when the coating is breached, the Zn sacrificially protects the steel thereby further extending the life of the reinforcement.
Considerable research has been carried out in USA to investigate the use of galvanized rebars for concrete bridge and highway construction exposed to high levels of accumulated chlorides due to the application of deicing salts or in marine exposure. In bridge decks for example, where both top and bottom rebar mats were galvanized, very low corrosion current densities resulted compared to black steel rebar mats, and the extent of corrosion on the galvanized rebars was significantly less. It has also been shown that when galvanized rebars were used in the top mat only, though some corrosion of the Zn occurred there was very much less corrosion compared to black rebars in equivalent conditions.
Economics of galvanized rebars
When the costs and consequences of corrosion damage to a reinforced concrete building are analyzed, the extra cost of galvanizing is seen as a small investment in corrosion protection. While the initial cost of galvanizing may add up to 50 % to the cost of the rebars, the cost of using galvanized rebars as a percentage of total building cost is always significantly less than this.
General cost analysis for building construction reveals that the galvanizing of rebars increases the overall cost of reinforced concrete by around 6 % to 10 %. The actual value varies depending on many factors including the type of rebar and the galvanizing cost, the amount of steel used per cubic meter of concrete poured, and the unit cost of the concrete mass. Further considering that it is rarely necessary to galvanize all steel in the structure, and 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 % to 3.0 % of the total building cost.
Applications of galvanized rebars
Galvanized steel rebars can be used widely in a variety of reinforced concrete structures since the Zn coating provides a safeguard against early corrosion of the rebars. Particular cases where the use of galvanized rebars is likely to be a cost effective and sound engineering decision include the following.
- Light weight precast cladding elements and architectural building features
- Surface exposed beams and columns and exposed slabs
- Prefabricated building units 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
- High risk structures in aggressive environments
Many examples exist around the world where galvanized rebars have been successfully used in a variety of types of reinforced concrete buildings, structures and general construction.
Benefits of the use of galvanized rebars
The benefits of the use of galvanized rebars are as follows.
- Proper galvanizing procedures have no significant effect on the mechanical properties of the steel rebars
- Zn coating furnishes local cathodic protection to the steel, as long as the coating has not been consumed
- Galvanized rebars provide protection to the steel during storage and construction prior to placing them in the concrete
- Corrosion of galvanized steel rebars in concrete is less intense and less extensive for a substantial period of time than that of black steel rebars
- Galvanized steel rebars in concrete tolerates higher chloride concentration than the black steel rebars before the start of the corrosion
- Galvanized rebars delay the onset of cracking. Spalling of concrete is less likely to occur or is delayed
- Concrete can be used in more aggressive environments, and a standard design of concrete components can be retained for various exposure conditions by the use of galvanized steel rebars
- 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 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
- Use of galvanized rebars ensures a clean appearance of the finished concrete with no trouble arising at cracks either from spalling or rust staining
- Galvanized rebars are cleaner and easier to work with. They make it possible to consider the use of thinner wires as welded fabrics.
It is important to note that even if these benefits are achieved, the use of galvanized rebars 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 rebars is a complementary measure of corrosion protection. It is a kind of insurance against the inability of the concrete to isolate and protect the steel.
Leave a Comment