Galvanized-Steel Rebars

Galvanized-steel rebars can be used as a preventative measure to control corrosion in reinforced concrete structures exposed to carbonation or mild contamination with chlorides, such as chimneys, bridge substructures, tunnels and coastal buildings. 

Galvanized reinforcement offers significant advantages compared to carbon steel under equivalent circumstances. These include an increase of initiation time of corrosion a greater tolerance for low cover, e. g. in slender (architectural) elements, and corrosion protection is offered to the reinforcement prior to it being embedded in concrete. 

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Galvanized steel rebar in concrete reduces the corrosion rate to acceptable levels (i.e., <0.5 pm/year in most of the cases) as shown by the data in Table 4.81. 

F. Tittarelh, T. BeUezze, Investigation of the major reduction reaction occurring during the passivation of galvanized steel rebars, Corros. Sci. 52 (2010) 978—983. 

As far as practice at the construction site is concerned, it should be observed that compared with other types of corrosion-resistant rebars (such as epoxy-coated or galvanized steel), corrosion resistance is a bulk property of stainless steel. Therefore, the integrity of stainless steel is unaffected if its surface is cut or damaged... 

Often the use of stainless-steel reinforcement is hmited to the outer part of the structure (skin reinforcement) or to its most critical parts for economical reasons. Furthermore, when stainless-steel bars are used in the rehabihtation of corroding structures, they are usually connected to the original carbon-steel rebars. Concern has been expressed with regard to the risk of galvanic corrosion of carbon steel induced by coupHng with stainless-steel bars. Actually, the galvanic corrosion that can arise when stainless steel is used in partial substitution of carbon steel has to be compared with that which takes place in the absence of stainless steels [30]. 

The passive film of galvanized rebars is stable even in mildly acidic environment, so that the zinc coating remains passive even when the concrete is carbonated. The corrosion rate of galvanized steel in carbonated concrete is approximately 0.5-0.8 pm/y, therefore a typical 80 pm galvanized coating would be expected to last over 100 y. [44]. The corrosion rate of galvanized bars remains negligible in carbonated concrete even if a low content of chloride is present. 

R. Fratesi, G. Moriconi, L. Coppola, The influence of steel galvanization on rebar behaviour in concrete , in Corrosion of Reinforcement Construction, SCI, The Royal Society of Chemistry, Cambridge, 1996, 630-641. 

The most promising corrosion-resistant rebars are galvanized (zinc-coated) rebars, stainless steel-clad rebars, and solid stainless steel rebars. Titanium has also been considered as a rebar metal, but its cost is prohibitive although it is highly corrosion-resistant. 

The extent of galvanic corrosion has been measured between steel rebars and chromium containing steel rebars in concrete reinforcement [29]. The corrosion rates were estimated in both chlorinated and carbonated environments. 

Care must be taken not to allow contact between galvanized steel parts outside a concrete construction (e.g., underneath moist insulation materials) with ungalvanized (black) rebars in the concrete. The chance of corrosion of ungalvanized and galvanized steel parts is summarized as follows ... 

The fibre surface has a considerable influence on the composition of the transition zone. For highly corrosion resistant glass fibres with specially coated surface, like CemFILl, this zone is very porous. This is in contrast to the strong interface formed around steel and asbestos fibres. In various composites with different kinds of fibres and matrices, the transition zone is formed as a result of chemical affinity, quality of the fibre surface and the penetration of the cement paste into the bundles of fibres. Furthermore, the ITZ may be different above and below a fibre due to bleeding and water lenses below fibres. Higher porosity of the ITZ around steel galvanized reinforcements than around ordinary steel rebars was observed by Belaid et al. (2001). 

Bridge deck in 1-295 highway near Trenton, New Jersey 1985 Carbon steel rebars with t5fpe 304 cladding. Exposm to winter deicing salts. If ends of clad products are exposed, these represent a galvanic corrosion risk. 

Presence of different metals. Rebars of carbon steel in certain cases can be connected to rebars or facilities made of stainless steel or copper. This type of coupling, which in other electrolytes would provoke a considerable degree of corrosion in carbon steel by galvanic attack, does not cause problems in the case of concrete any different from those provoked by coupling with normal passive steel. In fact, the corrosion potential of passive carbon steel in concrete is not much different... 

Structures immersed in seawater. Macrocells may form between rebars reached by chlorides and passive rebars on which, for any reason, oxygen is available. Macrocell current is then controlled by the amount of oxygen that can be reduced on the passive rebars. The galvanic coupling lowers the potential on these rebars and produces alkalinity on their surface. Therefore the macrocell contributes to maintaining the steel passive. 

Rebars not entirely embedded in concrete. Macrocell corrosion can occur when there are macroscopic defects in the concrete (cracks with large width, honeycombs, delaminations, etc.) or when there are metallic parts connected to the rebars that are only partially embedded in the concrete. This case is important for structures immersed in seawater or in aggressive soil. Besides being subjected to direct attack, those parts in direct contact with water or soil may also undergo more severe attack caused by the galvanic coupling with steel embedded in concrete. 

P. Pedeferri, Stray Current Induced Corrosion in Reinforced Concrete Structures Resistance of Rebars in Carbon, galvanized and Stainless Steels (in Italian), La Metallurgia (10 ... 

In order to install cathodic protection, continuity can be established by welding in extra rebars. However, at Florida DOT one approach has been to expose bars in damaged areas, grit blast them clean and apply arc sprayed zinc directly onto the steel and then across the steel surface. This provides galvanizing directly on the steel and SACP to the steel embedded in the concrete. Multiple continuity connections are established by the sprayed zinc. 

It may be very risky to apply electrochemical treatments to galvanized reinforcing steel. Very severe pitting can result. NCT, the patent holders on the realkalization and desalination techniques, do not recommend their use on structures containing galvanized rebar (Miller, 1995). 

A characteristic feature for the chloride induced corrosion of steel in concrete (pitting) is the development of macrocells, that is the coexistence of passive and corroding areas on the same rebar forming a short circuited galvanic element with the corroding area acting as anode and the passive surface as cathode (Fig. 8-11). The cell voltage. 

Galvanized reinforcement, i.e. zinc coatings formed by dipping clean rebars in a bath of molten zinc, can protect steel in concrete from corrosion attack. However, the performance reported in the literature is contradictory (Bentur et al., 1997). Galvanized rebars remain passive in carbonated concrete and the corrosion rate is much lower than with black steel. In situations where chloride induced corrosion prevails, a delay in the initiation of corrosion can be expected, but at high chloride concentrations depassivation cannot be avoided completely. 

Creating a barrier between the rebar steel and the internal concrete environment, for example, epoxy coating and galvanization. 

The importance of concrete cracks in rebar corrosion has also been highlighted by Niirnberger. Both carbonation and chloride ion diffusion, two important processes associated with rebar corrosion, can proceed more rapidly into the concrete along the crack faces, compared with uncracked concrete. Niirnberger argued that corrosion in the vicinity of the crack tip could be accelerated further by crevice corrosion effects and galvanic cell formation. The steel in the crack will tend to be anodic relative to the cathodic (passive) zones in uncracked... 

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