Galvanizing for Corrosion Protection (AGA)
Upcoming SlideShare
Loading in...5
×
 

Like this? Share it with your network

Share

Galvanizing for Corrosion Protection (AGA)

on

  • 3,114 views

 

Statistics

Views

Total Views
3,114
Views on SlideShare
3,114
Embed Views
0

Actions

Likes
0
Downloads
175
Comments
0

0 Embeds 0

No embeds

Accessibility

Categories

Upload Details

Uploaded via as Adobe PDF

Usage Rights

© All Rights Reserved

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Processing…
Post Comment
Edit your comment

Galvanizing for Corrosion Protection (AGA) Document Transcript

  • 1. Galvanizing for Corrosion Protection: A Specifier’s Guide to Reinforcing SteelAm e r i c a n Ga l v a n i z e r s As s o c i a t i o n
  • 2. Table of ContentsCORROSION & PROTECTION OF STEEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 Corrosion of Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 How Zinc Protects Steel from Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 Barrier Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 Cathodic Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4THE HOT DIP GALVANIZING PROCESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Surface Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Fluxing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Galvanizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6PHYSICAL PROPERTIES OF GALVANIZED COATINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 The Metallurgical Bond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Impact and Abrasion Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Corner and Edge Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Complete Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9MECHANICAL PROPERTIES OF GALVANIZED STEEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 Strength and Ductility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 Fatigue Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 Mechanical Properties of Galvanized Steel in Concrete . . . . . . . . . . . . . . . . . . . . . . . . .10 Zinc Reaction in Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11DESIGN, SPECIFICATION, FABRICATION AND INSTALLATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 Design Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 Steel Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 Detailing of Reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 Dissimilar Metals in Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 Bending Bars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 Storage and Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 Local Repair of Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 Removal of Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13FIELD PERFORMANCE OF GALVANIZED REINFORCED STEEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 Horizontal Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 Vertical Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
  • 3. Corrosion & Protection of SteelINTRODUCTION mences. Galvanizing can provide visible assur- ance that the steel has not rusted, as well as an Corrosion and repair of corrosion damage are additional safety factor after installation.multi-billion dollar The intention of thisproblems. Observations guide is to provide infor-on numerous structures mation on the various fac-show that corrosion of tors involved so specifiersreinforcing steel is can draw their own conclu-either a prime factor, or sions as to when to specifyat least an important galvanized reinforcement.factor, contributing to The guide also providesthe staining, cracking guidelines on the specifica-and spalling of concrete tion and practices involvedstructures. These for galvanized reinforce-effects of corrosion ment.often require costly Corrosion of reinforced steel creates severely damaging spalling (above). Galvanizing rebar helps prevent corrosion (below).repairs and continuedmaintenance during thelife of the structure. C ORROSION OF Under normal ser-vice conditions in a S TEELnon-aggressive envi- Rust, the corrosionronment, Portland product of iron, is thecement concrete pro- result of an electrochemi-tects the reinforcing cal process. Rust occurssteel against excessive because of differences incorrosion if the con- electrical potentialcrete permeability is between small areas on thelow and the steel-con- steel surface involvingcrete interface is free of discontinuities such as anodes, cathodes and an electrolyte. These differ-voids, cracks, etc. However, when a structure is ences in potential on the steel surface are causedexposed to an aggressive environment, or if the by:design details or workmanship are inadequate, theconcrete protection may break down and corrosion · variations in composition/structureof the reinforcement may become excessive. · presence of impurities Galvanized reinforcing steel is effectively · uneven internal stressand economically used in concrete in those situa- · presence of a non-uniform environmenttions where black reinforcement will not have ade- These differences in the presence of an elec-quate durability. Galvanized steel reinforcement is trolyte, a medium for conducting ions, create cor-especially useful where the reinforcement must be rosion cells. Corrosion cells consist of micro-exposed to the weather before construction com- scopic anodes and cathodes. Because of differ- 2
  • 4. ences in potential within the cell, negatively hydroxide. In such an alkaline environment, a pas-charged electrons flow from anode to cathode and sivating iron oxide film forms on the steel, causingiron atoms in the anode area are converted to pos- almost complete inhibition of corrosion. As the pHitively charged iron ions. The positively charged of the concrete surrounding the reinforcement isiron ions (Fe++) of the anode attract and react with reduced by intrusion of salts, leaching or carbona-the negatively charged hydroxyl ions (OH-) in the tion, the passivity is reduced and corrosion mayFigure 1 electrolyte to form iron proceed. oxide, or rust. The presence of chloride ions can affect the Negatively charged elec- inhibitive properties of the concrete in two ways. trons (e-) react at the The presence of chloride ions creates lattice vacan- cathode surface with cies in the oxide film, thus providing defects in the positively charged film through which metal ions may migrate more hydrogen ions (H+) in rapidly and permit corrosion to proceed. This cre- the electrolyte to form ates pitting corrosion. Also, if the hydroxyl ion hydrogen gas. A simpli- concentration is reduced, for example by carbona-Fe++ + 2OH- FeO + H2O fied picture of what tion (reaction of atmospheric carbon dioxide with 2H+ + 2e- H2 gas occurs in this corrosion calcium hydroxide), the pH is lowered and the cor- cell is shown in Figure 1. rosion proceeds further. In the presence of oxygen, Impurities present in the electrolyte create an inhibition of corrosion occurs at a pH of 12.0. Buteven better medium for the corrosion process. For as the pH is reduced, the corrosion rate increases.example, these impurities can be the constituents With reduction of pH to 11.5, the corrosion ratein which the steel is immersed, or present in increases by as much as five times the corrosionatmospheric contaminants, including sulfur rate at a pH of 12.0.oxides, chlorides or other pollutants present in At an active anodic site, particularly in pits,damp atmosphere or dissolved in surface moisture. the formation of positively charged ferrous ionsCalcium hydroxide, present in hardened concrete, attracts negatively charged chloride ions, givingwill also act as an electrolyte in the presence of high concentrations of ferrous chloride. Ferrousmoisture. chloride partially hydrolyzes, yielding HCl and an Under normal conditions, concrete is acid reaction. These reactions reduce protec-alkaline with a pH of about 12.5, due tion at the steel-concrete interface.to the presence of calcium At a corroding surface, the pHFigure 2 may be 6.0 or less.1. Corrosion of steel is an electrochemical As mentioned before, thereaction. Minute differences in structure ofthe steel’s chemistry create a mosaic pat- anode and cathode areas ontern of anodes and cathodes containing a piece of steel are micro-stored electrochemical energy. 2. scopic. Greatly magnified,Moisture forms an electrolyte which com- 1 2pletes the electrical path between the the surface might appear asanodes and cathodes, spontaneously releasing the mosaic of anodes andthe stored electrochemical energy. A small cathodes pictured in Figure 2,electrical current begins to flow, carrying away all electrically connected by theparticles of the anode areas. The particles given upcombine with the environment to form rust. When salt or 3 underlying steel.acid is added to the moisture, the flow of electric current, and Moisture in the concrete providescorrosion, speeds up. 3. At this stage, the anodes are corroded and cathodes the electrolyte and completes the electrical pathprotected. However, the instability of the metal itself causes the anodes tochange to cathodes and the corrosion cycle begins again, resulting in uniform cor- between the anodes and cathodes on the metal sur-rosion of the entire surface. face. Due to potential differences, a small electric current begins to flow as the metal is consumed in the anodic area. The iron ions produced at the3
  • 5. anode combine with the environment to form the BARRIER PROTECTIONloose, flaky iron oxide known as rust. Zinc is characterized by its amphoteric nature As anode areas corrode, new material of dif- and its ability to passivate due to the formation offerent composition and structure is exposed. This protective, reaction product films. Reaction ofresults in a change of electrical potentials and also zinc with fresh cement paste leads to passivity bychanges the location of anodic and cathodic sites. formation of a diffusion barrier layer of calciumThe shifting of anodic and cathodic sites does not hydroxy-zincate. Comparison of the two curves inoccur all at once. In time, previously uncorroded Figure 4, emphasizes the importance of the passi-areas are attacked and a uniform surface corrosion vating layer for corrosion protection againstis produced. This process continues until the steel chlorides.is entirely consumed. Figure 4 The corrosion products which form on steelhave much greater volume than the metal from Effect of Chloride Concentration on the Criticalwhich they form. This increase in volume exerts Pitting Potential (after Duval).great disruptive tensile stress on the surrounding 0concrete. When the pressure is such that the ten- -200sile stress in the concrete cover is greater than itstensile strength, the concrete cracks (Figure 3), -400 mVleading to further corrosion. Corrosion cracks are (SCE)usually parallel to the reinforcement, and are thus -600quite distinct from transverse cracks associated -800with tension in the reinforcement caused by load- Zincing. As the corrosion proceeds, the longitudinal Zinc Passivated by -1000 Ca(OH)2 for 15 dayscracks widen and, together with structural trans-verse cracks, may cause spalling of the concrete. -1200 10-2 10-1 1Figure 3 Concentration of Cl-(N) CATHODIC PROTECTION Table 1 shows the galvanic series of metals and alloys arranged in decreasing order of electri- cal activity. Metals toward the top of the table, often referred to as less noble metals, have a Before Corrosion Build-up of Further Corrosion Eventual Spalling Corrosion Products Surface Cracks, Corroded Bar greater tendency to lose electrons than the more Stains Exposed noble metals. Thus metals higher in the series pro- vide cathodic or sacrificial protection to those met-HOW ZINC PROTECTS STEEL FROM als below them. Because zinc is anodic to steel, the galvanizedCORROSION coating will provide cathodic protection to The reason for the extensive use of hot dip exposed steel. When zinc and steel are connectedgalvanizing is the two-fold nature of the coating. in the presence of an electrolyte, the zinc is slowlyAs a barrier coating, it provides a tough, metallur- consumed, while the steel is protected. Zinc’s sac-gically bonded zinc coating which completely cov- rificial action offers protection where small areasers the steel surface and seals the steel from thecorrosive action of the environment. Additionally, of steel are exposed, such as cut edges, drill holes,the sacrificial action of zinc protects the steel even scratches, or as the result of severe surface abra-where damage or minor discontinuity occurs in the sion. Cathodic protection of the steel from corro-coating. sion continues until all the zinc in the immediate 4
  • 6. Table 1 galvanized reinforcement compared to equivalent CORRODED END black steel reinforcement. The total life of a gal- Anodic or less noble Arrangement of Metals in vanized coating in concrete is thus made up of the (Electronegative) Galvanic Series: time taken for the zinc to depassivate, which is Magnesium Any one of these metals and known to be longer than that for black steel, Zinc alloys will theoretically cor- because of both its higher tolerance to chloride Aluminum rode while offering protection Cadmium ions and carbonation resistance, plus the time to any other which is lower in Iron or Steel taken for the dissolution of the alloy layers in the Stainless Steels (active) the series, so long as both are electrically connected. coating. Only after the coating has fully dissolved Soft Solders Lead in a region of the bar will localized corrosion of Tin In actual practice, however, the steel commence (Figure 5). Nickel zinc is by far the most effec- Figure 5 Brass tive in this respect Bronzes Copper Acceptable Limit of Damage Nickel-Copper Alloys Zn+Fe Stainless Steels (passive) Silver Solder Fe Corrosion Silver Gold Platinum Zn PROTECTED END A B Time Cathodic or most noble C D E (Electropositive) Adapted from Yeomans & Kinstlerarea is consumed. Galvanizing protects the steel during in- Both steel and pretreated zinc are normally plant and on-site storage, as well as after embed-passive in the highly alkaline environment of con- ment in the concrete. In areas where the reinforce-crete. However, penetration of chloride ions to the ment may be exposed accidentally, due to thin ormetal surface can break down this passivity and porous concrete, cracking, or damage to the con-initiate rusting of steel or sacrificial corrosion of crete, the galvanized coating provides extendedthe zinc. The susceptibility of concrete structures protection. Since the corrosion products of zincto the intrusion of chlorides is the primary incen- occupy a smaller volume than the corrosion prod-tive for use of galvanized steel reinforcement. ucts of iron, the corrosion which may occur to the Galvanized reinforcing steel can withstand galvanized coating causes little or no disruption toexposure to chloride ion concentrations several the surrounding concrete. Recent tests also con-times higher (at least 4-5 times) than what causes firm that the zinc corrosion products are powdery,corrosion in black steel reinforcement. While nonadherent and capable of migrating from theblack steel in concrete typically depassivates surface of the galvanized reinforcement into thebelow a pH of 11.5, galvanized reinforcement can concrete matrix reducing the likelihood of zincremain passivated at a lower pH, thereby offering corrosion induced spalling of the concrete. Ansubstantial protection against the effects of car- additional advantage is that zinc’s corrosion prod-bonation of concrete. ucts are grayish white and do not produce unsight- These two factors combined, namely chloride ly reddish-brown staining.tolerance and carbonation resistance, are widelyaccepted as the basis for superior performance of5
  • 7. The Hot Dip Galvanizing Process The hot dip galvanizing process consists a flux. The method of applying the flux to the steelof three basic steps: surface preparation, flux- depends upon whether the “wet” or “dry” galva-ing and galvanizing. Each of these steps is nizing process is used. Dry galvanizing requiresimportant in obtaining a quality galvanized the steel to be dipped in an aqueous zinc ammoni-coating (Figure 6). um chloride solution and then thoroughly dried. This “preflux” prevents oxides from forming onSURFACE PREPARATION the material surface prior to galvanizing. The wet It is essential for the material surface to be galvanizing process uses a molten flux layer float-clean and uncontaminated in order to obtain a uni- ed on top of the molten zinc. The final cleaningform, adherent coating. Surface preparation is occurs as the material passes through the flux layerusually performed in sequence by caustic (alka- before entering the galvanizing bath.line) cleaning, water rinsing, acid pickling, andwater rinsing. The caustic cleaner removes organic contam- GALVANIZINGinants, including dirt, paint markings, grease, and The material to be coated is immersed in aoil. Next, scale and rust are removed by a pickling bath of molten zinc maintained at a temperature ofbath in hot sulfuric acid (150 degrees F) or about 850 degrees F. A typical bath chemistryhydrochloric acid at room temperature. Water used in hot dip galvanizing is 98.5 percent purerinsing usually follows both caustic cleaning and zinc. The time of immersion in the galvanizingacid pickling. bath varies, depending upon the dimensions and Surface preparation can also be accomplished chemistry of the materials being coated. Materialsusing abrasive cleaning as an alternate to, or in with thick sections will take longer to galvanizeconjunction with, chemical cleaning. Abrasive than those with thin sections.cleaning is a mechanical process where sand, Surface appearance and coating thickness aremetallic shot or grit is propelled against the mate-rial by air blasts or rapidly rotating wheels. controlled by the galvanizing conditions. These include: steel chemistry; variations in immersionFLUXING time and/or bath temperature; rate of withdrawal The final cleaning of the steel is performed by from the galvanizing bath; removal of excess zincFigure 6 6
  • 8. by wiping, shaking or centrifuging; and control of developed procedures for galvanizing reinforcingthe cooling rate by water quenching or air cooling. steel, (i.e. “Process Manual for Hot Dip The American Galvanizers Association has Galvanized Concrete Reinforcing Steel”) to assure the galvanized coating will meet not only the min-Table 2 imum coating weights for galvanized reinforce-Coating Class Weight of Zinc Coating ment specified in ASTM A 767 “Standard min, oz/ft2 of Surface Specification for Zinc-Coated (Galvanized) SteelClass I Bars for Concrete Reinforcement,” (Table 2) but Bar Designation Size No. 3 3.00 Bar Designation Size No. 4 & larger 3.50 also the other requirements of the standard.Class II Bar Designation Size 3 & larger 2.00 A galvanizer removes reinforcing steel from the bath of molten zinc. Excess zinc running off the bars is visible, but enough zinc has bond- ed to the steel to protect the steel from corrosion for decades.7
  • 9. Physical Properties of Galvanized CoatingsFigure 7 THE METALLURGICAL BOND Hot dip galvanizing is a factory applied coat- ETA ing which provides a combination of properties (100% Zn) ZETA unmatched by other coating systems because of its (94% ZN, 6% Fe) DELTA unique bond to the steel. (90%Zn, 10% Fe) The photomicrograph in Figure 7 shows a GAMMA (75%Zn, 25% Fe) section of a typical hot dip galvanized coating. Steel The galvanized coating consists of a progression of zinc-iron alloy layers metallurgically bonded to the base steel. The metallurgical bond formed by the galvanizing process ensures no underfilm corro- sion can occur.Figure 8 Organic coatings, on the other hand, merely add a film to the steel which can be penetrated. As illustrated in Figure 8, once the film is broken, cor- rosion begins as if no protection existed.This is what happens This is what happens This is what hap- IMPACT AND ABRASION RESISTANCEat a scratch on galva- at a scratch on paint- pens at a scratch on The ductile outer zinc layer provides goodnized steel. The zinc ed steel. The exposed steel coated with acoating sacrifices steel corrodes and less active metal, impact resistance to the bonded galvanized coat-itself slowly to pro- forms a pocket of such as copper.tect the base steel. rust, which lifts the The exposed steel ing. The photomicrograph in Figure 9 shows theThis sacrificial paint film from the corrodes fasteraction continues as metal surface to form than normal to pro- typical hardness values of a hot dip galvanizedlong as there is zinc a blister, which will tect the more noblein the immediate area continue to grow. metal. coating. The hardness of the zeta and delta layers is actually greater than the base steel and provides exceptional resistance to coating damage from abrasion.Figure 9 CORNER AND EDGE PROTECTION Eta Layer Corrosion often begins at corners or edges of 70 Hardness Vickers Zeta Layer products which have not been galvanized. Organic 179 Hardness Vickers coatings, regardless of application method, are Delta Layer thinnest at such places. 244 Hardness Vickers Base Steel However, the galvanized coating will be at 159 Hardness Vickers least as thick, possibly thicker, on corners and 8
  • 10. edges as on the general surface. This provides required, so a fully protected item is delivered toequal or extra protection in these critical areas (see the job site. This assures the customer will notFigure 10). receive a coating which is not properly bonded to the steel surface.A GALVANIZED COATING IS A COMPLETE Figure 10COATING Because galvanizing is accomplished throughtotal immersion, all surfaces of the article are fullycoated and protected, including areas inaccessibleand hard to reach with organic coatings.Additionally, the integrity of any galvanized coat-ing is ensured because zinc will not metallurgical-ly bond to unclean steel. Thus, any uncoated area is immediatelyapparent as the work is withdrawn from the moltenzinc. Adjustments are made on the spot, whenBecause of galvanizing’s unique, tough coating, there’s no tip-toeing around the work site. A galvanized surface is actually tharder than the base steel, so galvanized rebar is extremely resistant to damage from abrasion and other installation elements.9
  • 11. Mechanical Properties of Galvanized SteelSTRENGTH AND DUCTILITY 2. Galvanized steel exposed to calcium Strength and ductility of reinforcing steel are hydroxide solution and subjected to full stressimportant to ensure good performance of rein- reversal in a rotary bending tester performedforced concrete and to prevent brittle failure. significantly better than black steel.Studies of the effect of galvanizing on the mechan- 3. Deformed reinforcing steel, exposed to anical properties of steel reinforcing bars have aggressive environment prior to testing underdemonstrated that the tensile yield and ultimate cyclic tension loading, performed better whenstrength, ultimate elongation, and bend require- galvanized.ments of steel reinforcement were substantiallyunaffected by commercial hot-dip galvanizing, MECHANICAL PROPERTIES OF GALVANIZED STEELprovided that proper attention is given to steel IN CONCRETEselection, fabrication practice and galvanizing pro- Good bonding between reinforcing steel andcedures. concrete is essential for reliable performance of The effect of the galvanizing process on the reinforced concrete structures. When protectiveductility of steel bar anchors and inserts after being coatings on steel are used, it is essential to ensuresubjected to different fabrication Figure 11procedures has also been investi-gated. The results demonstrated Concrete-Reinforcing Steel Bondconclusively that, with correct Study A Study B Study Cchoice of steel and galvanizing 1000procedures, there was no reduc- Stress in Pounds Per Square Inchtion in the ductility of the steel. 800FATIGUE STRENGTH An extensive experimental 600program examining the fatigueresistance of galvanized steel rein-forcement showed that: 400 1. Concrete beams exposed 200 to cyclic loading in a corro- sive environment performed better when reinforced with 1 3 12 1 3 12 1 3 12 galvanized steel. Months of Curing Galvanized Source: University of California, Berkeley Black 10
  • 12. that these coatings do not reduce bond strength.Studies of the bonding of galvanized and blacksteel bars to Portland cement concrete have beeninvestigated. The results of these studies report thefollowing: 1. Development of the bond between steel and concrete depends on age and environ- ment. 2. In some cases, the time required for devel- oping full bond strength between steel and concrete may be greater for galvanized bars than for black, depending on the zincate At a construction site, galvanized fabricated rebar has been cement reaction. installed and is ready for concrete to be placed. 3. The fully developed bond strength of gal- vanized and black deformed bars is the same. ZINC REACTION IN CONCRETE For plain bars, the bond strength of gal- As stated previously, during curing the galva- vanized bars is greater than for similar black bars (Figure 11). nized 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 of the possibility of embrit- tlement of the steel due to hydrogen absorption. Laboratory studies indicate that this “liberated” hydrogen does not permeate the galvanized coat- ing to the underlying steel and the reaction ceases as soon as the concrete has hardened. Reaction of zincates with fresh Portland cement mortar may retard set and early strength development, but later, setting occurs completely with no detrimental effects on the concrete. In fact, a positive strength increase occurs. Most types of cement and many aggregates contain small quantities of chromates. These chro- mates passivate the zinc surface, which is then resistant to attack by fresh concrete. If the cement and aggregate contain less chromate than will yield at least 20 ppm in the final concrete mix, the Because of the strong bond strength between galvanized steel and concrete, galvanized rebar is used successfully in a vari- galvanized bars can be dipped in a chromate solu- ety of applications to provide reliable corrosion protection. tion or chromates can be added to the water when the concrete is mixed.11
  • 13. Design, Specification, Fabrication, and InstallationDESIGN CRITERIA to exhibit corrosive reactions as long as the two metals remain passivated. To insure this is the When galvanized steel is specified, the design case, the depth to the zinc/steel contact should notrequirements and installation procedures be less than the cover required to protect blackemployed should be no less stringent than for steel alone under the same conditions. Therefore,structures where non-galvanized reinforcement is when galvanized reinforcement is used in con-used. There are, in addition, some special require- crete, it should not be coupled directly to largements to be observed when galvanized steel is areas of black steel reinforcement, copper or otherused. The following recommendations are intend- dissimilar metal. Bar supports and accessoriesed as a guide to designers, engineers, contractors should be galvanized. Tie wire should be annealedand inspectors. They are intended as a supplement wire, 16 gauge or heavier, preferably galvanized.to other codes and standards dealing with design, If desired, polyethylene and other similar tapes canfabrication and construction of reinforced concrete be used to provide insulation between any dissim-structures, and deal only with those special consid- ilar metals.erations which arise due to the use of galvanizedsteel in place of black steel reinforcement. BENDING BARSSTEEL SELECTION Hooks or bends should be smooth and not sharp. Cold bending should be in accordance with The concrete reinforcing steel to be galva- the recommendations of CRSI. When bars are bentnized shall conform to one of the following ASTM cold prior to galvanizing, they need to be fabricat-specifications: A 615 (A 615M), A 616 (A 616M), ed to a bend diameter equal to or greater than thoseA 617 (A 617 M) or A 706 (A 706M). specified in Table 3. Material can be cold bent tighter than shown in Table 3, if it is stressDETAILING OF REINFORCEMENT relieved at a temperature from 900 to 1050 degrees Detailing of galvanizing reinforcing steel F for one hour per inch of bar diameter.should conform to the design specifications for Galvanizers find it difficult, and thereforenon-galvanized steel bars and to normal standard costly, to handle bars of small diameter bent intopractice consistent with the recommendations of complicated configurations. It is therefore recom-the Concrete Reinforcing Steel Institute (CRSI). mended that the bars be bent after galvanizing when possible. When galvanizing is performedDISSIMILAR METALS IN CONCRETE Table 3 Another consideration whenusing galvanized reinforcement in Minimum Finished Bend Diameters- Inch-Pound Unitsconcrete is the possibility of estab- Bar No. Grade 40 Grade 50 Grade 60 Grade 75lishing a bimetallic couple betweenzinc and bare steel (i.e. at a break in 3,4,5,6 6dA 6d 6d ...the zinc coating or direct contact 7,8 6d 8d 8d ...between galvanized steel and black 9,10 8d 8d 8d ...steel bars) or other dissimilar met- 11 8d 8d 8d 8dals. A bimetallic couple of this type 14,18 ... ... 10d 10din concrete should not be expected Ad= nominal diameter of the bar 12
  • 14. before bending, some cracking and flaking of the WELDINGgalvanized coating at the bend may occur and is Welding of galvanized reinforcement shouldnot a cause for rejection. The tendency for crack- conform to the requirements of the current editioning of the galvanized coating increases with bar of the American Welding Society (AWS) Standarddiameter and with severity and rate of bending. Practice AWS D19.0 “Welding Zinc-Coated Steel.” Welding of galvanized reinforcement poses no problems, provided adequate precautions are taken. These include a slower welding rate and proper ventilation. The ventilation which is normally required for welding operations is con- sidered adequate. Also, heat damaged areas need to be repaired. LOCAL REPAIR OF COATING Local removal of the galvanized coating in the area of welds, bends, or sheared ends will not significantly affect the protection offered by galva- nizing, provided the exposed surface area is small compared to the adjacent surface area of galva- nized steel. When the exposed area is excessive, and gaps are evident in the galvanized coating, the area can be repaired with a paint containing zincSTORAGE AND HANDLING dust conforming to ASTM A780 “Standard Practice for Repair of Damaged and Uncoated Galvanized bars may be stored outdoors with Areas of Hot-Dip Galvanized Coatings.”complete assurance. Their general ease of storagemakes it feasible to store standard lengths so thatthey are available on demand. Another important REMOVAL OF FORMScharacteristic of galvanized reinforcing steel is that Because cements with low natural occurringit can be handled and placed in the same manner as levels of chromates may react with zinc and retardblack steel reinforcement, because of the great hardening and initial set, it is important to ensureabrasion resistance of galvanized steel. 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 20 ppm of chromates in the final concrete mix or if the hot dip galvanized bars are chromate passivat- ed per the requirements of ASTM A 767, Section 5.3. Standard size reinforcing steel, both straight and fabricated can be galvanized in advance and easily stored until needed (top left). The abrasion resistant galvanized coating requires no spe- cial handling procedures (bottom left).13
  • 15. Field Performance of Galvanized Reinforcement VERTICAL CONSTRUCTION The Empire Center at The Egg, a performingarts center in Albany New York, was a massiveundertaking of architecture, combining aestheticsand function, and concrete and steel designed toservice the citizens of New York state fordecades. Despite it’s name and elegantly simpledesign, The Egg is a pillar of strength— literally.The Egg balances on a concrete and steel stemextending six stories into the ground. The “shell” of The Egg is shaped by a heavi- ly reinforce concrete “girdle” which helps keep the egg’s shape and directs the weight of the structure onto the supporting pedestal and stem. Adding even more durability to this decep- tively fragile structure are thousands of miles of galvanized rebar, weaving in and out of the shell and stem. The Egg, underwent construction in 1966, (left) and took 12 years to build. Today, The Egg remains a beautiful piece of rust-free architecture. The housing barracks Extensive use of galvanized reinforcement was speci- at the U.S. Coast fied for a hospital in Australia, including this surrounding wall Guard Academy were (above). Galvanizing will help keep corrosion from creating built with galvanized severe spalling problems in this structure, located in coastal reinforcing steel to city Katingal, which is home to a severely corrosive marine envi- protect the building ronment. from corrosion and spalling (left). 14
  • 16. HORIZONTAL CONSTRUCTION nized rebar. The year was 1948, and since then the bridge has per- formed beautifully in this highly corrosive atmosphere. Inspection 20 years after con- struction showed no evidence of deterioration of the concrete, and core samples found the galvanized rebar retained about 98 percent of its zinc coating. This lead officials to predict another 80 years of maintenance free service for the Longbird Bridge. Currently, 12 bridges in Bermuda In order to combat the corrosive marine envi- are fully galvanized, and the Ministry of Worksronment in Bermuda, the U. S. Army Corps of and Engineering continues to specify galvanizedEngineers built the Longbird Bridge, the first ever reinforcement because of its exceptional perfor-bridge deck exclusively constructed with galva- mance. For over 20 years galvanized rebar has provided the Boca Chica Bridge near Key West, Florida (below) with maintenance free corrosion protection. Galvanized rebar has helped avoid traffic-snarling repairs of this 2,573 foot-long and 42 foot-wide s l w bridge. Despite heavy traffic and humid, salt water conditions, core samples showed the galvanized rebar to have an average thickness of 4 mils and no signs of corrosion are detectible. The state of Pennsylvania’s DOT makes extensive useof galvanized rebar. The bridge deck of the Schuylkill RiverExpressway in Philadelphia (above), is protected by 400 tons ofgalvanized rebar. After nearly a decade of service, the rebar isin excellent condition, even in areas where the concrete coveringis thin.15
  • 17. Additional ResourcesACI Committee 222. “Corrosion of Metals in Concrete”; American Concrete Institute, 222R-85, 1985.Adnrade, C. et al. “Corrosion Behavior of Galvanized Steel in Concrete”; 2nd International Conferenceon Deterioration and Repair of Reinforced Concrete in the Arabian Gulf; Proceedings Vol. 1, pp. 395-410, 1987.Arup, H. “The Mechanisms of the Protection of Steel by Concrete”; Society of Chemical IndustryConference of Reinforcement in Concrete Construction; London, June 1983.Structures - A Scientific Assessment”; CSIRO Paper, Sydney, 1979.Bird, C.E. “Bond of Galvanized Steel Reinforcement in Concrete”; Nature, Vol. 94, No. 4380, 1962.Breseler B. and Cornet I. “Galvanized Steel Reinforcement in Concrete”; 7th Congress of theInternational Association of Bridge and Structural Engineers, Rio de Janeiro, 1964.Chandler, K.A. and Bayliss, D.A. “Corrosion Protection of Steel Structures”; Elsevier Applied SciencePublishers, pp. 338-339, 1985.Cornet, I. and Breseler, B. “Corrosion of Steel and Galvanized Steel in Concrete’; Materials Protection,Vol. 5, No. 4, pp.69-72, 1966.Concrete Institute of Australia. “The use of Galvanized Reinforcement in Concrete”; Current PracticeNote 17, September 1984. ISBN 0 909375 21 6.Duval, R. and Arliguie, G.; Memoirs Scientifiques Rev. Metallurg.; LXXI, No. 11, 1974.Galvanizers Association of Australia. “Hot Dip Galvanizing Manual”; 1985.Hime, W. and Erlin, B. “Some Chemical and Physical Aspects of Phenomena Associated with Chloride-Induced Corrosion”; Corrosion, Concrete and Chlorides; Steel Corrosion in Concrete: Causes andRestraints; ACI SP-102, 1987.Hosfoy, A.E. and Gukild, I. “Bond Studies of Hot Dipped Galvanized Reinforcement in Concrete”; ACIJournal, March, pp. 174-184, 1969.India Lead Zinc Information Centre. “Protection of Reinforcement in Concrete, An Update,Galvanizing and Other Methods”; New Delhi, 1995.International Lead Zinc Research Organization. “Galvanized Reinforcement for Concrete - II”; USA,1981. 16
  • 18. ADDITIONAL RESOURCES CONTINUEDKinstler, J.K. “Galvanized Reinforcing Steel - Research, Survey and Synthesis”; International BridgeConference Special Interest Program, Pittsburgh, PA, 1995.MacGregor, B.R. “Galvanized Solution to Rebar Corrosion”; Civil Engineering, UK, 1987.Page, C.L. and Treadway, K.W.J. “Aspects of the Electrochemistry of Steel in Concrete”; Nature,V297,May 1982, pp. 109-115.Portland Cement Association. “An Analysis of Selected Trace Metals in Cement and Kiln Dust”; PCApublication SP109, 1992.Roberts, A.W. “Bond Characteristics of Concrete Reinforcing Tendons Coated with Zinc”; ILZROProject ZE-222, 1977.Tonini, D.E. and Dean, S.W. “Chloride Corrosion of Steel in Concrete”; ASTM-STP 629, 1976.Warner, R.F., Rangan, B.V., and Hall, A.S. “Reinforced Concrete”; Longman Cheshire, 3rd edition, pp.163-169, 1989.Worthington, J.C., Bonner, D.G. and Nowell, D.V. “Influence of Cement Chemistry on Chloride Attackof Concrete”; Material Science and Technology; Vol., pp. 305-313, 1988.Yeomans, S.R. and Hadley, M.B. “Galvanized Reinforcement - Current Practice and DevelopingTrends”; Australian Corrosion Association Conference, Adelaide, 17pp., November, 1986.Yeomans, S.R. “Corrosion Behavior and Bond Strength of Galvanized Reinforcement and EpoxyCoated Reinforcement in Concrete”; ILZRO Project ZE-341, June, 1990.Yeomans, S.R. “Comparative Studies of Galvanized and Epoxy Coated Steel Reinforcement inConcrete”, Research Report N0. R103, University College, Australian Defense Force Academy, TheUniversity of New South Wales, 1991.Yeomans, S.R. “Considerations of the Characteristics and Use of Coated Steel Reinforcement inConcrete”; Building and Fire Research Laboratory, National Institute of Standards and Technology,U.S. Department of Commerce, 1993.17