Galvanized Rebar Concrete Investment

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HOTDIPGALVANIZEDREINFORCINGSTEEL

A CONCRETEINVESTMENT

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IntroductionReinforced concrete is one of the mostwidely used modern construction materials.It is inexpensive, readily available, has a rangeof attractive properties and characteristics

and is suitable for a vari-ety of building and con-struction applications. It

is also used in a numberof exposure conditions,which in some caseshighlights one of themain weaknesses of rein-forced concrete: thepotential for the steelreinforcement (rebar) tocorrode, resulting in

staining, cracking and failure of thesurrounding concrete.

For over 50 years hot dip galvanized (zinc)coatings have been used around the world toeffectively and economically protect rebarfrom corrosion.

The Rebar Corrosion ProblemConcrete is a porous material made up ofsmall pores and capillaries through which

corrosive elements such as water,chloride ions, oxygen, carbon diox-ide and other gases travel into theconcrete matrix, eventually reach-ing the rebar. For each concretemix at some threshold level of cor-rosive elements, steel depassivatesand corrosion starts. Concrete

itself exhibits good compressive strengthbut has poor tensile strength, generallyabout one-tenth of the compressivestrength. When iron rusts, its corrosion prod-ucts are 2-10 times more voluminous thanthe original steel which generate stressesthat exceed the tensile capacity of the sur-rounding concrete, causing it to crack andspall. Once cracking has occurred, the struc-

tural capacity of the element may be com-promised and costly repairs may be neededto extend its useful life.

Preventing Rebar CorrosionThe most cost effective and efficientmethod of minimizing the risk of rebar cor-rosion is to ensure that the concrete coverover the reinforcement is of adequate thick-ness and that the concrete itself is dense and

impermeable.Another important line of defense is pro-tecting the rebar itself by hot dip galvaniz-ing. Hot dip galvanized coatings form animpervious metallic zinc and zinc alloybarrier around the steel that isolates thesteel surface from the surrounding concrete.Galvanized rebar offers many advantagesover conventional unprotected rebar,including:

Zinc has a higher chloride concentrationthreshold for corrosion than bare steel. Thissignificantly delays the initiation of corro-sion from the ingress of chlorides to thegalvanized rebar surface.

The corrosion rate of zinc in con-crete is lower than that of steel,

HOT DIP GALVANIZED REINFORCING STEEL

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Sydney Opera House

Effect of corroding reinforcing steel


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and zinc corrosion products, once theyform, do not create the same deterio-rating internal stresses that occurwhen steel corrodes in concrete.

Zinc coatings sacrificially protectsteel, which means that at any break inthe coating the surrounding zinc will corrodepreferentially and electrochemically protectthe adjacent exposed steel from corroding.

As a result, a galvanized coating cannot beundercut by iron corrosion products as is thecase with other barrier coatings (e.g.,epoxy).

The better corrosion resistance of galva-nized rebar allows greater tolerance of

concrete variability andplacement.

The zinc coating pro-

vides corrosion protec-tion of the rebar prior toit being embedded inconcrete.

These characteristics ofgalvanized rebar greatly reduce the risk ofrebar corrosion that causes rust staining,cracking and spalling of the concrete.Galvanized rebar extends the maintenancefree life of a concrete structure and greatlyimproves its overall lifecycle cost.

Hot Dip Galvanized RebarThe hot dip galvanizing process applies acontinuous metallic zinc coating to steelrebar by immersing the bars in a bath ofmolten zinc at about 450C. A metallurgicalreaction takes place between the steel andthe zinc producing a coating made up of aseries of iron-zinc alloy layers which form atthe steel/zinc interface with an outer layerof pure zinc (theeta phase) adhering

to the outer surface as the rebar is removedfrom the zinc bath.

The unique structure of a hot dip galvanizedcoating offers many important advantagesover other coatings. The galvanized coatingis metallurgically bonded to the underlying

steel, producing a coating that has an orderof magnitude greater adhesion than, forexample, fusion bonded epoxy coatings. Inaddition, the iron-zinc alloy layers of thecoating are actually harder than the underly-ing steel which, combined with a soft outerlayer of pure zinc, produces an extremely

tough and abrasion resistant coating.Galvanized rebar can generally be treated inthe same manner as uncoated rebar and doesnot require special handling precautions toprotect the coating during handling, trans-port to, and placement at, the job site.Further, the bond strength of galvanizedrebar to concrete is no less than that ofuncoated bar, and in many cases is somewhatbetter. This allows the same reinforced con-crete design specifications (bar size, lap

lengths, etc.) to be used for galvanized rebaras unprotectedrebar.

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Straight lengths of

galvanized rebar

Galvanized rebar stored

at job site

Microstructure of a hot dip galvanized coating

Rebar emerging from the molten zinc bath

Eta(100% Zn)70 DPN HardnessZeta(94% Zn 6% Fe)179 DPN HardnessDelta(90% Zn 10% Fe)244 DPN HardnessGamma(75% Zn 25% Fe)250 DPN HardnessBase Steel159 DPN Hardness


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Applications of Galvanized RebarGalvanized rebar and other fittings (includ-ing bolts, ties, anchors, dowel bars and pip-ing) have been widely used in a variety ofreinforced concrete structures and elements.Particular circumstances where the galvaniz-ing of reinforcement is likely to be a costeffective engineering decision include:

transport infrastruc-ture, including bridge

decks, road surfacesand crash barriers;

light weight precast cladding elements andarchitectural building features;

surface exposed beams and columns, andexposed slabs;

prefabricated building including units such

as kitchen and bathroom modules and tilt-up construction;

immersed or buried elements subject togroundwater effects and tidal fluctuations;

coastal and marine structures; and

high risk structures in aggressiveenvironments.

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North American

bridge decks

Sewage pipesPrecast window panels


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Many examples exist around the world wheregalvanized rebar has been successfully usedin a variety of types of reinforced concretebuildings, structures and general construc-tion, including:

reinforced concrete bridge decks and

pavements;

cooling towers and chimneys;

coal storage bunkers;

tunnel linings and water storage tanks andfacilities;

docks, jetties and offshore platforms;

marinas, floatingpontoons andmoorings;

sea walls andcoastal balustrades;

paper mills, water and sewage treatmentworks;

processing facilities and chemical plants;

highway fittings and crash barriers; and

concrete lamp posts and power poles.

Some prominent examples, many of whichare well known buildings and major struc-tures from around the world, are describedon page 10.

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Floating marina

Sea wall

Steelworks - coke ovens storage

Sea water cooling channels

Rail tunnel

Safety barrier

Tunnel


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How Zinc Protects Rebar inConcreteThe corrosion protection afforded by gal-vanized rebar in concrete is due to a com-bination of beneficial effects. Of primaryimportance is the substantially higherchloride threshold (2-4 times) for zinccoatings to start corroding compared to

uncoated steel. In addition, zinc has a muchgreater pH passivation range than steel,making galvanized rebar resistant to the pHlowering effects of carbonation as the con-crete ages. Even when the zinc coating doesstart to corrode, its corrosion rate is consid-erably less than that of uncoated steel.

Why Galvanized Rebar MaintainsConcretes IntegrityZincs corrosion products are loose, powderyminerals that are less voluminous than ironcorrosion products and are able to migrateaway from the galvanized rebar surface intothe adjacent concrete matrix. As a result,corrosion of the zinc coating causes verylittle physical disruption to the surroundingconcrete.

There is also evidence to suggest that thediffusion of zincs corrosion products helpsfill pore spaces at the concrete/rebar inter-face, making this area less permeable andhelps to reduce the transport of aggressivespecies such as chlorides through this inter-face zone to the zinc coat-ing. The reactions betweenzinc and concrete, and theresulting corrosion prod-uct diffusion, also explainswhy galvanized rebar has

such good bond strengthwith concrete.

Initial Reaction of Zinc in FreshConcreteZinc reacts with wet concrete to formcalcium hydroxyzincate accompanied by theevolution of hydrogen. This corrosion prod-uct is insoluble and protective of the under-lying zinc (provided that the surroundingconcrete mixture is below a pH of about13.3). Research has shown that during this ini-

tial reaction period until coating passivationand concrete hardening occurs, some of thepure zinc layer of the coating is dissolved.However, this initial reaction ceases once theconcrete hardens and the hydroxyzincatecoating has formed. Studies of galvanizedrebar recovered from field structures indi-cate that the coating remains in this passivestate for extended periods of time, evenwhen exposed to high chloride levels in thesurrounding concrete.

For concretes of high pH, or where somebackground chlorides are expected, the zincsurface can be passivated, using a range ofproprietary post treatments, as a safeguard

against excessive hydrogenevolution that may, in seriouscases, reduce the pulloutstrength of the bar. For normalconcrete conditions, researchhas shown no statistical differ-

ence in bond strength betweengalvanized rebar that waspassivated and not passivated.


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SEM image of the interfacialzone between the galvanizedcoating (white phase, left)and the concrete matrix. Theplume of zinc-rich corrosionproducts (light grey phase,left) can be seen migratinginto the cement matrix. Largegrey particles are fine sand.(Magnification 100X)

Deep Tunnel Sewage System, Singapore

Reinforced wall


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Cost of CorrosionFor reinforced concrete structures exposedto aggressive environments, such as coastalmarine and industrial climates, or de-icingsalts, the problem of rebar corrosion is wide-spread and severe and requires a corrosion-abatement strategy. A 2001 study commis-sioned by the United States Federal Highway

Administration estimated the cost of corro-sion in the US to be approaching $300 billionper year. Highway bridges, a large consumerof reinforcing steel, alone contributed over$8 billion to this total, mostly resulting fromstructurally deficient concrete reinforceddecks and substructures caused by rebarcorrosion.

Economics of Galvanized

ReinforcementHot dip galvanizing is a small but importantinvestment. It is used extensively throughoutthe world annually to protect millions oftons of steel against corrosion. Hot dipgalvanizing is therefore a widely availableservice that is cost competitive with otherrebar protective coating systems. Whenmeasured against total building and con-struction costs, and the enormous potentialcosts associated with premature repair ofdamaged concrete, or failure of thestructure, the premium paid for galvanizedrebar is very small and easily justified.

Codes of Practice and StandardsThe specification for hot dip galvanizing ofsteel reinforcing bars is handled in differentways around the world. Some countries treatsteel reinforcing bars in the same way as anyother steel product, and the hot dip

galvanizing of rebar falls under a generalgalvanizing standard (Table 2). Several othercountries have developed product specificstandards for the hot dip galvanizing of steelreinforcing bars. These are also listed in Table 2.

SummaryOver a long period of time, hot dip galvaniz-

ing has proven to provide cost effective andreliable corrosion protection to steel in con-crete in a variety of exposure conditions. Hotdip galvanizing is one of a number of corro-sion protection measures used to improvethe overall durability of reinforced concrete.The convenience of manufacture and supplyof the product, the ease of handling, trans-portation and installation, its provendurability and the fact that no special designrequirements are needed, means that hot dip

galvanized steel has been accepted in manycountries worldwide for a wide range ofconcrete construction.

Service Life Model of Uncoated vs. Galvanized Rebar

Time

Stress

ConcreteCracks

Concrete Spalls

UncoatedRe

bar

Galva

nizedReb

ar

Cl-Pro

fileat

Reba

rsu

rfac

e

Stress level at whichconcrete cracks

Cl-concentration whengalvanized rebar starts to corrode

Cl-concentrationwhen uncoated rebarstarts to corrode

2 - 4X Cl -

Zinc coatings have a higher chloride (Cl-) corrosion threshold(2-4 times) than that of uncoated steel, significantly extendingthe time until corrosion initiation. Once corrosion of the zinccoating does occur, the properties of the corrosion products andtheir ability to migrate into the concrete matrix reduces stressgeneration in the surrounding concrete, further extending thelife of the reinforced concrete structure.

9Bridge deck

Corroding concrete piling


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HOTDIPGALVANIZEDREINFORCINGSTEEL

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Australia / New ZealandSydney Opera House: 35 mm thick panels for cladding of sails and seawall unitsHydro-Electricity Commission, Hobart: clad with 950 galvanized precast panelsTelecom Exhibition Exchange, Melbourne: clad with precast panelsIntercontinental Hotel, Sydney: 1549 precast windows and fascia unitsLibrary Tower, Sydney: galvanized bar in external columns and panelsHigh Court and National Gallery, Canberra: galvanized bar in critical areasNew Parliament House, Canberra: 1800 galvanized cladding panels

National Tennis Centre, Melbourne: precast stadium support beamsNew Zealand Parliament House, Wellington: clad with precast fascia panels

AsiaThe Lotus Temple, India: galvanized rebar in white precast fascia panelsOffshore piers at Ominichi, Japan: galvanized reinforcement throughoutDeep Tunnel Sewage System, Singapore: 10,000mt of galvanized rebar

United States and CanadaBank of Hawaii, Waikiki: thin decorative precast arches with galvanized barFinancial Plaza of the Pacific, Honolulu: precast cladding panelsCrocker Building, San Francisco: galvanized reinforced structural elementsLevi Strauss Building, California: precast panelsUniversity of Wisconsin: precast panels and in-situ concrete in numerous buildingsWrigley Field Sports Arena, Illinois: precast panels in seating decksFootball Hall of Fame Stadium, Canton, Ohio: galvanized reinforcing steelGeorgetown University Law Center, Washington, DC: precast panelsUS Coast Guard Barracks, Elizabeth City, NC: galvanized bar in 237 precast panelsBridge decks and road construction in New York, New Jersey, Florida, Iowa, Michigan,

Minnesota, Vermont, Pennsylvania, Connecticut, Massachusetts, South Carolina,Ontario and Quebec

Staten Island Community College, New York: brilliant white precast panelsIBM Data Processing Division, White Plains, New York: galvanized reinforcement in

precast facade panels

Arkansas Civic Centre: galvanized reinforcement in slim external columnsEuropeNational Theatre, London: over 1000t of galvanized bar in exposed parapet wallsEastbourne Congress Theatre, UK: cladding panels and window mullionsCollegiate Buildings, University College, London: galvanized bar and meshUniversity Sports Hall, Birmingham: 37 mm thick panels using galvanized barNew Hall, Cambridge University: galvanized mesh in roof segmentsBarclays Bank, City of London: galvanized precast window surroundsOffices, Westminster Bridge, London: galvanized reinforced white facing panelsDome of the Mosque, Rome, Italy: galvanized reinforcementANDOC North Sea Oil Rig: 2000t galvanized bar in roof of storage tank

Power Station, Spijk, Netherlands: fully galvanized reinforced cooling water ductsCoke quenching towers, Dunkirk, France: galvanized structural reinforcementToutry Viaduct, St Nazaire Bridge and Pont dOuche Viaduct, France: galvanized

reinforcing barsOffshore piers Riva di Traiano, Rome, Italy: galvanized reinforcement throughout

the structure Deep Tunnel Sewage System, Singapore

Court House, Townsville, Australia

ASER Tower, Adelaide,Australia

The Lotus Temple, India

Examples of Prominent Structures Utilizing Galvanized Reinforcing Steel

North American bridge decks


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Further Information

For further information and details about hot dip galvanized reinforcedsteel, please visit: www.galvanizedrebar.com or [email protected]

This publication is based on information provided in the book entitledGalvanized Steel Reinforcement in Concrete edited by Stephen R. Yeomans,University of New South Wales, Canberra, Australia, published byElsevier.

www.elsevier.com

ISBN: 008044511X

This document is for information purposes only. The publisher, International Zinc Association, has taken allreasonable care to ensure accuracy of the information contained within this brochure, but accepts no liabilityfor loss or damage to any party howsoever caused, arising from its use. This brochure may not be reproduced,in whole or in part, or in any form whatsoever, without the prior written consent of the publisher.

Table 2. Standards for Hot Dip Galvanizing of Reinforcing Steel.*

Designation Title of StandardHot Dip Galvanizing Standards

Australia/ AS/NZS 4680 Hot dip galvanizing (zinc) coatings onNew Zealand fabricated ferrous articles

Canada CAN/CSA G164 Hot dip galvanizing of irregularly shapedarticles

South Africa SABS ISO 1461 Hot dip galvanized coatings on fabricatediron and steel articles

Europe EN ISO 1461 Hot dip galvanized coatings on fabricatediron and steel articles

International Standards ISO 1461 Hot dip galvanized coatings on fabricatedOrganization iron and steel articles

Reinforcing Steel Hot Dip Galvanizing Standards**

United States ASTM A 767 Zinc-coated (galvanized) steel bars forconcrete reinforcement

France NF A35-025 Hot dip galvanized bars and coils forreinforced concrete

Italy UNI 10622 Zinc-coated (galvanized) steel bars and

wire rods for concrete reinforcementIndia IS 12594 Hot dip coatings on structural steel bars

for concrete reinforcement specifications

International Standards ISO 14657 Zinc-coated steel for the reinforcement ofOrganization concrete

* Further quidance and support in creating a hot dip galvanized rebar specification is available from regional galvanizing associations,as listed on: www.galvanizedrebar.com

**A new European standard Steel for Reinforcement - Galvanized Reinforcing Steel is currently under development. Also, a Germannational approval/quality specification exists Z-1.4-165 Allgemeine Bauaufsichtliche Zulassung - Feuerverzinkte Betonsthlein English General approval by the building authorities - Galvanized reinforcing steel


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International Zinc Association (IZA), 168 Avenue de Tervueren, B-1150 Brussels - BelgiumTel: 32.2.7760070 Fax: 32.2.7760089 Email: [email protected] Internet: www.zincworld.org

www.galvanizedrebar.com

A website dedicated to advancing the knowledge ofhot dip galvanized rebar.

Technical questions can be directed to:

[email protected]

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nuary2006

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Galvanized Rebar Concrete Investment

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