Repairing damage caused by corrosion is a multi-billion dollar problem. Observations of numerous structures show that corrosion of reinforcing steel is either a prime factor, or at least an important factor, contributing to staining, cracking and/or spalling of concrete structures. The effects of corrosion often require costly repairs and continued maintenance during the life of the structure.
Galvanized reinforcing steel is effectively and economically used in concrete where unprotected reinforcement will not have adequate durability. The susceptibility of concrete structures to the intrusion of chlorides is the primary incentive for using galvanized steel reinforcement. Galvanized reinforcing steel is especially useful when the reinforcement will be exposed to the weather before construction begins. Galvanizing provides visible assurance that the steel has not rusted and requires no on-site repair, unlike most other coatings.
Reinforcing steel can be galvanized to retard corrosion, providing barrier and sacrificial protection. As the corrosion products of zinc are much less voluminous than those of steel, the cracking, delamination, and spalling cycle is greatly reduced when using galvanized rebar. Laboratory data support, and field test results confirm, that reinforced concrete structures exposed to aggressive environments have a substantially longer service life when galvanized rebar is used as opposed to bare steel rebar.
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Corrosion Resistance of Hot-Dip Galvanized Reinforcing Bar in Concrete
Under normal conditions, concrete is alkaline (pH of about 12.5) due to the presence of calcium hydroxide. In such an environment, a passivating iron-oxide film forms on the steel, causing almost complete corrosion inhibition. As the pH of the concrete surrounding the reinforcement is reduced by the intrusion of salts, leaching, or carbonation, the system becomes active and corrosion proceeds.
The presence of chloride ions can affect the inhibitive properties of the concrete in two ways. The presence of chloride ions creates changes in the iron oxide, resulting in pitting corrosion. Carbonation can lower the pH and increase the corrosion rate. Any additional lowering of the pH will accelerate the bare steel corrosion rate.
The iron corrosion products that form on steel have much greater volume than the metal that is consumed in the corrosion reaction. This increase in volume around the bare steel rebar exerts great disruptive tensile stress on the surrounding concrete. When resultant tensile stress is greater than the concrete tensile strength, the concrete cracks (as shown in the figure below), leading to more changes by allowing rain and chlorides direct access to the steel bar. Corrosion cracks are usually parallel to the reinforcement and are quite distinct from transverse cracks associated with tension in the reinforcement caused by loading. As the corrosion proceeds, the longitudinal cracks widen and, together with structural transverse cracks, cause spalling of the concrete.
The reason for the extensive use of hot-dip galvanized steel is the twofold nature of the coating. As a barrier coating, galvanizing provides a tough, metallurgically bonded zinc coating that completely covers the steel surface, sealing it from the corrosive action of the environment. Additionally, zinc’s sacrificial (cathodic) action protects the steel even where damage or minor discontinuity occurs in the coating.
Zinc is characterized by its amphoteric nature and its ability to passivate due to the formation of protective reaction product films. Reaction of zinc with fresh cement leads to passivity by formation of a diffusion barrier layer of zinc corrosion products. Comparison of the two lines in the figure below emphasizes the importance of the passivating layer for corrosion protection against chlorides. The corrosion potential is much lower for zinc that has been passivated, prior to exposure to chlorides, than zinc that has not been passivated.
Both steel and zinc are normally passive in the highly alkaline environment of concrete. However, penetration of chloride ions to the metal surface can break down this passivity and initiate rusting of steel or sacrificial corrosion of the zinc. The susceptibility of concrete structures to the intrusion of chlorides is the primary incentive for using galvanized steel reinforcement.
Galvanized reinforcing steel can withstand exposure to chloride ion concentrations several times higher (at least 4 to 5 times) than the chloride level that causes corrosion in black steel reinforcement. While black steel in concrete typically depassivates below a pH of 11.5, galvanized reinforcement can remain passivated at a lower pH, thereby offering substantial protection against the effects of concrete carbonation.
These two factors combined – chloride tolerance and carbonation resistance – are widely accepted as the basis for superior performance of galvanized reinforcement compared to black steel reinforcement. The total life of a galvanized coating in concrete is made up of the time taken for the zinc to depassivate (which is longer than that for black steel, because of its higher tolerance to chloride ions and carbonation resistance), plus the time taken for the dissolution of the alloy layers in the zinc coating. Only after the coating has fully dissolved in a region of the bar will localized corrosion of the steel begin.
Galvanizing protects the steel during in-plant and on-site storage, as well as after embedment in the concrete. In areas where the reinforcement may be exposed due to thin or porous concrete, cracking, or damage to the concrete, the galvanized coating provides extended protection. Since zinc corrosion products occupy a smaller volume than iron corrosion products, the corrosion that may occur to the galvanized coating causes little or no disruption to the surrounding concrete. Tests also confirm that zinc corrosion products are powdery, non-adherent and capable of migrating from the surface of the galvanized reinforcement into the concrete matrix, reducing the likelihood of zinc corrosion-induced spalling of the concrete (Yeomans).
Good bonding between reinforcing steel and concrete is essential for reliable performance of reinforced concrete structures. When protective coatings on steel are used, it is essential to ensure that these coatings do not reduce bond strength. Studies of the bonding of galvanized and black steel bars to Portland Cement concrete have been investigated. The results of these studies indicate:
- Development of the bond between steel and concrete depends on age and environment.
- The fully developed bond strength of galvanized and black deformed bars is the same. In the chart below, the bond strength of galvanized bars is greater than for similar black bars.
Bond Strength Test
The bond of the hot-dip galvanized reinforcing bar to the concrete can be tested according to ASTM A 944. The bond strength relies heavily on the deformation of the bar and not as much on the actual bond between the zinc and the concrete. For plain bars with no deformation, the bond between the zinc and the concrete becomes very important. Pullout strength of hot-dip galvanized reinforcing steel has been tested many times, and the values of bond strength are equivalent to, or better than, black steel bond strength, as illustrated in the chart above.
Zinc Reaction in Concrete
During curing, the galvanized surface of steel reinforcement reacts with the alkaline cement paste to form stable, insoluble zinc salts accompanied by hydrogen evolution. This has raised the concern regarding the possibility of steel embrittlement due to hydrogen absorption. Laboratory studies indicate that this liberated hydrogen does not permeate the galvanized coating to the underlying steel and the reaction ceases as soon as the concrete hardens.
Most types of cement and many aggregates contain small quantities of chromates. These chromates passivate the zinc surface, minimizing the evolution of hydrogen during the reaction between zinc and the concrete. If the cement and aggregate contain less chromate than will yield at least 100 ppm in the final concrete mix, the galvanized bars can be dipped in a chromate solution or chromates can be added to the water when the concrete is mixed.
Removal of Forms
Because cements with naturally low occurring levels of chromates may react with zinc, it is important to ensure that forms and supports are not removed before the concrete has developed the required strength to support itself. Normal form removal practices may be utilized if the cement contains at least 100 ppm of chromates in the final concrete mix or if the hot-dip galvanized bars are chromate-passivated according to ASTM A 767.
Ductility and strength of reinforcing steel are important to prevent brittle failure of reinforced concrete. Studies of the effect of galvanizing on the mechanical properties of steel reinforcing bars have demonstrated that the tensile, yield and ultimate strength, ultimate elongation, and bend requirements of steel reinforcement are substantially unaffected by hot-dip galvanizing, provided proper attention is given to steel selection, fabrication practices, and galvanizing procedures.
The effect of the galvanizing process on the ductility of steel bar anchors and inserts after being subjected to different fabrication procedures also has been investigated. The results demonstrate conclusively that, with correct choice of steel and galvanizing procedures, there is no reduction in steel’s ductility.