It is also used in a number of exposure conditions, for example near the sea, which in some cases highlights one of the main weaknesses of reinforced concrete. This is the potential for the steel reinforcement or rebar to corrode, resulting in concrete façade staining, cracking and ultimately failure of the surrounding concrete. For over 50 years, hot dip galvanized zinc coatings have been used to effectively and economically to protect rebar from corrosion.
Concrete is a porous material made up of small pores and capillaries through which corrosive elements such as moisture, oxygen, chloride ions and carbon dioxide can diffuse through the concrete matrix, change the pH of the concrete and eventually corrode the rebar. For each concrete mix there is a threshold level for corrosive elements, causing steel to depassivate and corrosion to follow.
Concrete itself exhibits good compressive strength, but has a poor tensile strength at about generally one-tenth of the compressive strength. When rebar steel rusts, its corrosion products are two to ten times more voluminous than the original steel, generating stresses that exceed the tensile strength of the surrounding concrete, causing it to crack and spall. Once cracking has occurred, the structural capacity of the concrete element may be compromised and costly repairs are needed to extend its useful life.
The most cost-effective and efficient method to minimise the risk of rebar corrosion is to ensure that the concrete cover over the reinforcement is of an adequate thickness (>50mm), and that the concrete itself is dense and impermeable. Another important line of defence is protecting the rebar itself by hot dip galvanizing. Hot dip galvanized coatings form an impervious metallic zinc and zinc alloy barrier around the steel, isolating the steel surface from the surrounding concrete.
Galvanized rebar offers many advantages over conventional unprotected rebar, including:
These characteristics of galvanized rebar greatly reduce the risk of rebar corrosion, which causes rust staining, cracking and spalling of the concrete. Galvanized rebar extends the maintenance-free life of a concrete structure and greatly improves its overall lifecycle cost. For South Africa, this is especially critical at the coast, and consulting engineers should be designing coastal concrete structures with hot dip galvanized rebar to extend the life of the concrete.
Ends
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Notes to the Editor
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About the International Zinc Association
The IZA is the only global industry association dedicated exclusively to the interests of zinc and its users. Operating internationally and locally through its regional affiliates, the IZA helps sustain the long-term global demand for zinc and its markets by promoting such key end uses as corrosion protection for steel and zinc as being essential in human health and crop nutrition. IZA’s main programmes are Sustainability & Environment, Technology & Market Development and Communications.
In South Africa, the IZA plays a vital role in establishing the basis for the successful revitalisation of the zinc industry by increasing awareness of zinc and its applications and benefits in key sectors and markets, which will ultimately translate into the increased uptake of zinc.
International Zinc Association Contact
Simon Norton
IZA Africa Desk
Phone: (021) 788 9980
Cell: 082 831 2924
Email: zinc [at] iafrica [dot] com
Web: www.zinc.org
Media Contact
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Account Executive
NGAGE Public Relations
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Fax: 086 512 3352
Cell: 074 212 1422
Email: rachel [at] ngage [dot] co [dot] za
Web: www.ngage.co.za
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