Bare (or black) steel reinforcement bars rely entirely on protection provided by the surrounding concrete. However, concrete permits the passage of chlorides from sea salts or other corrosive substances to the rebar because of its natural permeability, and also through cracks and expansion joints. Even carbon dioxide from the air will eventually result in rebar corrosion.
Corrosion can be managed by reducing concrete permeability through optimal water/cement ratios; appropriate compaction and curing conditions; the use of concrete impregnation methods or membrane-type concrete coatings; and by providing a good depth of concrete cover over the rebar.
All of these measures can delay the corrosion of rebar, but not prevent it. The use of galvanized rebar has real benefits in improving the safety and reliability of reinforced concrete, even when the measures described above are used.
There are two basic types of coatings: barrier and sacrificial. Most coatings can be classified as barrier because they provide basic protection from air and water penetration to the steel they are covering. Sacrificial or zinc coatings offer barrier protection, but also provide a secondary line of defence if the barrier coating is damaged as the zinc sacrifices itself, or corrodes preferentially before the steel.
All coatings are not equal
All coating systems are designed to provide barrier protection to the substrate they cover. Barrier coatings are only effective as long as the coating remains intact. Any scratch, cut, or abrasion exposes the underlying steel to corrosive forces.
When steel protected only by a barrier coating is damaged or weathered, corrosion will initiate at the unprotected surface and quickly expand from that point outward. This is caused by the growth of iron oxides under the coating, which stresses the coating, causing failure and subsequent expansion of the unprotected area.
Galvanized zinc coatings form an impervious metallic zinc barrier around the steel to isolate the steel surface from the surrounding concrete. This barrier is the first line of defence in protecting the steel from corrosion
The excellent corrosion protection offered by zinc coatings or galvanizing derives from both the low natural corrosion rate of zinc coupled with its ability to extend protection to adjacent exposed steel areas, an effect known as cathodic protection. The coating also exhibits strong adhesion to the underlying steel surface due to its unique metallurgical bond that, together with the inherent toughness of a metallic coating, provides superior resistance to mechanical damage. The combination of these features results in a very durable coating, enabling concrete structures to be more tolerant of variability in concrete quality and reinforcement placement.
The use of galvanized reinforcement is uniquely advantageous:
It offers excellent resistance to chloride salt attack and is unaffected by concrete carbonation.
Zinc’s cathodic protection inhibits corrosion at any minor coating discontinuity and also prevents ‘undercutting’ of the coating, confining any corrosion risk to the local area of exposed steel.
Zinc corrosion results in little accompanying volume change. Unlike with steel corrosion, there is no adverse impact on the surrounding concrete. Research shows that any corrosion products simply diffuse into the adjacent concrete, helping to fill micro porosity that further inhibits corrosion.
A strong metallurgical bond between the steel rebar and the zinc coating is created during the galvanizing process. Galvanizing is metallurgically bonded to the steel up to 25 MPa, meaning when rebar is dropped, kicked, stepped on, or rubbed against existing concrete or other rebar pieces on the job-site, the protective coating will remain tightly adhered. This durable zinc coating will withstand the effects of UV light, temperature extremes, and exposure to rain or snow, thus protecting the rebar both as it waits to be used and while in use. From installation through the use phase, galvanized rebar is abrasion resistant and durable.
A Sustainable Material
Material specifiers and product engineers in key end-use markets such as building, construction, and transportation are increasingly interested in selecting materials that have the best environmental profile while meeting traditional cost, quality, and technical performance criteria.
Measuring the impact and resource requirements associated with zinc production against the impact and the benefits of using zinc during other stages in the product life cycle show zinc as a very sustainable material. The environmental footprint of galvanized coatings has also been documented.
Galvanizing can extend the life of steel and concrete structures to 100 years or more, enabling huge conservation of natural resources by reducing the waste inherent with premature end-of-life. Energy savings are also accrued through minimized maintenance and upkeep. The end-of-life recycling of zinc-coated steel also adds to this conservation because energy requirements for re-melting steel and recovering the zinc are less than those required for producing the original metals.
The zinc and galvanizing industries understand that environmental and sustainability programs are integral to their future and are committed to updating the already favourable life-cycle information.