fhwabridgecoating studyyields preliminary … · 2016-10-24 · 38 jpcl january 2011...

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ince the coatings industrychanged in the 1970s fromcoating systems with lead-based primers to systemswith zinc-based primers,the typical service life of azinc-rich coating has beenassessed to be about 30years before a major

touch-up is required. Typical concernswith zinc-rich systems include the costof removing mill scale before coatingapplication, the time and spacerequired for shop application, and thelogistics of moving heavy steel mem-bers from the shop to the field. A goodalternative to addressing these cost

Sissues is to extend the service life ofthe existing coating system on steelbefore any maintenance or replace-ment is required of the existing coatingsystem.1

To search for inexpensive anddurable coating systems, the FHWACoatings and Corrosion Laboratory(CCL) at Turner-Fairbank HighwayResearch Center initiated the FHWA100-year coating study. This in-housestudy was initiated in August 2009under the U. S. Congress’s mandatedprogram on high-performance steel.The objective of this study is to identi-fy and evaluate coating materials thatcan provide 100 years of virtually

FHWA BRIDGE COATINGSTUDY YIELDSPRELIMINARYTEST RESULTS

By Pradeep Kodumuri,SES Group & Associates; andSeung-Kyoung Lee,Federal Highway Administration,Turner-Fairbank HighwayResearch Center

Test Panels and Test ConditionsTypically, for its in-house coating stud-ies, FHWA uses conventional, rectan-gular-shaped, 4-inch x 6-inch test pan-els. For this study, in addition to usingconventional panels, FHWA adopteda new test panel design to closely sim-ulate the steel coated on highwaybridges. The new panels are 18 inchesx 18 inches, with a welding joint andtwo angle attachments. All 4-inch x 6-inch panels will be referred to as TypeI; 18-inch x 18-inch panels will bereferred to as Type II panels. Figure 1shows a typical Type II panel with a v-notch (welding joint), a T-shaped angleattachment and a wide-angle attach-

ment. The corresponding dimensions of each component arealso shown.All Type I test panels were coated according to each man-

ufacturer’s recommendations for dry film thickness (DFT).The test surface of a Type II panel consisted of three areas ofvarying DFT values:(a) Area 1—Target DFT(b) Area 2—DFT is 20% less than the target DFT(c) Area 3—DFT is 20% more than the target DFTTest results from these three DFT areas will help show

how DFT areas thinner than target DFT and DFT areas

maintenance--free service life for the steel bridge structuresat costs comparable to those of existing coatings. Coatingsystems were selected based on past experience and resultsfrom previous FHWA studies that had evaluated coatingsystems such as moisture-cured urethanes, waterborneacrylic and epoxy systems, two-coat zinc-rich rapid deploy-ment systems, overcoating systems, and one-coat systems.2-6

This article presents an overview of the work plan as well asthe first data sets collected through six 360-hour accelerat-ed test cycles and one 6-month outdoor exposure test cycle.The article also describes design innovations in test panelsand testing.

Experimental Procedure Selected Coating Systems Table 1 summarizes eight coating systemsstudied. Two 3-coat systems were used ascontrols; the remaining coating systemsincluded another 3-coat system, four 2-coatsystems, and a 1-coat system. The acronymsused for all the coating systems can also beseen in the far right column of Table 1. Thisarticle will use these acronyms to identifythe coating systems. All the coating systemswere applied to steel substrates prepared toSSPC-SP 5 (White Metal Blast).

39www.paintsquare.com J P C L J a n u a r y 2 0 1 1

SystemNumber

1

2

3

4

5

6

7

8

Category

Control: conventional 3-coat shop system

Control: 3-coat organic zincshop system

3-Coat fluoro-topcoat system

2-Coat fast dry coating

2-Coat polysiloxane

Metallizing (conventional) + topcoat

Organic zinc-rich epoxy (zinc flake) / linear epoxy

Calcium sulfonate alkyd

Generic Coating Name

Inorganic zinc / epoxy / aliphatic polyurethaneZinc-rich epoxy / epoxy / aliphatic polyurethane

Moisture-cured urethane-zinc /epoxy / fluorourethaneZinc-rich epoxy /

aliphatic polyurethaneInorganic zinc / polysiloxane

Thermal sprayed zinc / linear epoxy

Experimental primer / topcoat

High-ratio, single-coat CSA

Acronym

IOZ/E/PU

E/E/PU

MCU/E/F

E/PU

Zn/PS

TSZ/LE

ZnE/LE

HRCSA

Table 1: Selected Coating Systems

Fig. 1: Type II Test Panels

Editor’s Note: This article is based on apaper the authors will present at SSPC 2011featuring GreenCOAT, the conference ofSSPC: The Society for Protective Coatings.The conference will be held in Las Vegas,NV, January 31-February 3.

Dimension (Not to Scale)

thicker than target DFT compare to thetarget DFT area for the coating sys-tems tested. Figure 1 also shows thephysical locations of these test areason the surface of the Type II test panel. In all, 100 Type I and 27 Type II test

panels were prepared for acceleratedand outdoor testing, respectively.Tables 2 and 3 list the types and num-ber of test panels. Outdoor exposuretests were arranged in the backyard ofTurner-Fairbank Highway ResearchCenter (TFHRC) with and without saltspray. Another outdoor test was con-ducted at Golden Gate Bridge (GGB) inSan Francisco, California. Each coating system was sprayed on

12 Type I panels. Five panels were test-ed in accelerated conditions, and fivepanels were tested outdoors in naturalweathering and natural weatheringwith salt spray. None of the Type I pan-els were tested at the Golden GateBridge. The two remaining Type I pan-els were used exclusively for physicaltesting such as adhesion strength andFourier Transform InfraredSpectroscopy analysis. Four uncoatedsteel panels were also deployed on theoutdoor exposure racks, two on eachexposure rack with and without saltspray. For outdoor exposure testing on

three racks—two for natural weather-ing (with and without salt spray) atTFHRC and one at GGB—3 Type IIpanels were coated with each coatingsystem; 3 uncoated steel panels werealso employed for each, making a totalof 27 panels. An independent coating laboratory

prepared the test panels and deliveredthem to TFHRC. After their as-received condition was documented,half of the Type I test panels werescribed and the other half remainedunscribed. DFT areas of all of the TypeII test panels were scribed. Three out of the 5 (Type I) panels for

accelerated testing and 3 out of 5 (TypeI) for outdoor testing were scribed

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Group

UncoatedsteelControlOthersSubtotal

Number ofCoatingSystems

268

Accelerated Lab Testa

ALT

103040

Outdoor Tests

NW NWS

2b 2b5c 5c15c 15c22c 22c

Physical Testingd

41216

TotalNumber ofTest Panels

42472100

Table 2: Type I Test Panels

Group

UncoatedsteelControlOthersSubtotal

Number ofCoatingSystems

268

NW

1268

NWS

1268

GGB*

1268

Total

361827

Table 3: Type II Test Panels

Item

Each Cycle

TargetDuration(20 cycles)

FreezeExposure(Hours)

24

480

UV-CondensationExposure (Hours)

168

3,360

ProhesionExposure (Hours)

168

3,360

TotalExposure(Hours)

360

7,200

Table 4: Accelerated Lab Testing of Type I Panels

ALT – Accelerated Laboratory Testing; NW – Natural Weathering; NWS – Natural Weatheringwith Salt Spraya 3 scribed + 2 unscribed = 5 panels/coating systemb 2 panels for periodic salt spray outdoor exposure and 2 panels for natural outdoor expo-sure (no salt spray), respectivelyc 2 scribed and 1 unscribed panels/coating system for periodic salt spray outdoor exposureand 1 scribed and 1 unscribed panels/coating system for natural outdoor exposure = total 5panels/coating systemd 2 unscribed panels/coating system

*GGB – Golden Gate Bridge

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according to ASTM Method D 1654.Using a mechanical scriber, researchersmade a 2-inch long, diagonal scribe oneach panel. All test areas of Type II test panels

were scribed (Fig. 1). A mechanicalscribing tool used for Type I panels wasnot suited for the large size test panels;hence, the Type II test areas werescribed using a high-speed Dremel toolwith a rotary bit. C-clamps were used to support a metallic guide, along whichthe scribing was done using the Dremelbit. The exposure conditions used in the

100-year study are summarized inTables 2 and 3.

Accelerated TestingTable 4 summarizes the test conditionsfor accelerated testing and the totalnumber of cycles. Each accelerated labo-ratory test cycle was carried out for360 hours and a total of 20 cycles willbe carried out. Upon completion ofevery 360-hour cycle, the panels areexamined for their performance. Adetailed description of each 360-hourtest cycle is shown below. 1. Freeze: 24 hours Temperature: -23 C (-10 F)

2. UV/Condensation: 168 hours (7 days)Test cycle: 4 hours

UV/4-hour condensation cycle UV lamp: UVA-340 UV temperature: 60 C (140 F) Condensation temperature: 40 C (104 F)

3. Prohesion (Cyclic Salt-Fog, ASTMG85): 168 hours (7 days) Test cycle: 1-hour wet/1-hour dry Wet cycle: A Harrison Mixture of

0.35 wt% ammonium sulfate and 0.5 wt%sodium chloride was used. Fog wasintroduced at ambient temperature. Dry cycle: Air was preheated to 35

C (95 F) and then was purged to thetest chamber.

Natural Weathering with and without Salt SprayType I and Type II test panels weredeployed on wooden racks inclined at30 degrees facing south. Figures 2(a)and 2(b) show the test panels deployedinitially in the back yard at TurnerFairbank Highway Research Center inMclean, Virginia. A 15 wt% sodium chloride solution

was sprayed onto these test panelsevery 24 hours by anautomatic salt spraysystem. This systemwas built in-house. Itworks with a timingswitch turning on anelectro-mechanicalpump every 24 hoursto spray these testpanels for a short peri-od (15 seconds) withthe salt solution. Aftera week of salt spray,due to excessive saltdeposit buildup, the

salt solution was changed from 15wt% sodium chloride to the Harrisonmixture (0.35 wt% ammonium sulfateand 0.5 wt% sodium chloride). Testpanels are evaluated every 6 monthsfor coating performance.

Golden Gate BridgeThe outdoor exposure test conditionsat Golden Gate Bridge can be consid-ered harsh because of the severe fog,which also contains airborne chlorides.Figure 3 shows Type II panelsdeployed near the south abutment atthe Golden Gate Bridge. Test panels areevaluated every 6 months for coatingperformance.

Initial Coating Characterizationand Performance Monitoring Coating systems in this study werecharacterized through Fourier


Fig. 2: (a) Natural weathering with salt rack and (b) natural weathering rack

Fig. 3: Type II panels deployed at the Golden Gate Bridge

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Transform Infrared (FTIR)Spectroscopy and pull-off adhesion.7

The tests were performed on twoextra panels for each coating system.The adhesion tests were conductedwith a hydraulic pull-off adhesiontester, and the nominal adhesionstrength of a coating system was cal-culated by averaging six readingsfrom two panels.

DFT and the number of holidayswere measured on individual panelsusing SSPC-Paint ApplicationSpecification No. 2 and ASTMD51628,9 before the panels wereexposed to accelerated or outdoortesting. Digital photographs were alsotaken before testing to document theiras-received conditions. A low voltageholiday detector was used for the hol-

iday measurement. ElectrochemicalImpedance Spectroscopy (EIS) wasused to determine initial impedanceproperties of selected test panels andmonitor their subsequent changesupon exposure. The gloss and color ofeach panel were measured by meth-ods described in ASTM D523 andD2244, respectively.10,11

During the tests, test panels wereevaluated periodically by monitoringchanges in the number of holidaysand physical condition. Measurementswere made after every 360 hours oflaboratory testing and after every sixmonths for outdoor exposure testing.Digital photographs were taken todocument progressive changes of indi-vidual panels. Growth of rust creep-age at the scribe was monitoredaccording to ASTM D7087.12

Preliminary Test Results This section presents initial coatingcharacterization results and prelimi-nary test results from accelerated test-ing and outdoor exposures pertainingto color and gloss, DFT, pull-off adhe-sion, surface appearance, holidays, andcreepage. Some supplementary pho-tographs are included.

DFT Figure 4 shows the mean DFT, stan-dard deviation, and coefficient of vari-ance (CV) of each coating system. DFTvaried significantly, from 7.5 mils to17.8 mils, depending on the coatingsystem. None of the mean DFTs wereless than 7 mils, and three systems(E/PU, Zn/PS, and HRCSA) werebetween 7 and 10 mils. All three-coatsystems (IOZ/E/PU, E/E/PU, andMCU/E/F) had mean DFTs between10 mils and 15 mils. The remainingtwo-coat systems (TSZ/LE andZnE/LE) had DFTs exceeding 15 mils.Variation of DFT data, in terms of CVand standard deviation, was the high-est for the last group of coating sys-tems.


Fig. 4: Mean DFT values (Type I panels)

Fig. 5: Mean adhesion strength values (Type I panels)

Average

IOZ/E/PU E/E/PU MCU/E/F E/PU Zn/PS TSZ/LE ZnE/LE HRCSA

IOZ/E/PU E/E/PU MCU/E/F E/PU Zn/PS TSZ/LE ZnE/LE HRCSA

DFT (mils)

Coefficient of Variance (%)

20

18

16

14

12

10

8

6

4

2

0

Average Adhesion Strength (psi)

2500

2000

1500

1000

500

0

100

90

80

70

60

50

40

30

20

10

0

Coefficient of Variance (%)

100

90

80

70

60

50

40

30

20

10

0

CVStandard Deviation

Average CVStandard Deviation

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Pull-Off Adhesion Initial mean adhesion values and theirvariability are shown in Fig. 5.ZnE/LE exhibited the highest adhesionstrength of 3160 psi (CV 20%) whileHRCSA demonstrated the lowestadhesion strength of 259 psi (CV 44%).All remaining three-coat and two-coatsystems except TSZ/LE showed adhe-sion strengths from 1,000 to 1,500 psi(CV 15–25%). TSZ/LE had the secondhighest adhesion strength of 1,834 psi(CV 13%). The two control coating sys-tems had adhesion strengths of 1,119and 1,173 psi respectively (CV 30 and35%).

HolidaysFigure 6 shows the cumulative (prelim-inary) number of holidays detectedduring the accelerated laboratory test-ing. When excessive holidays weredetected, discrete defect spots couldnot be identified, and an arbitrarynumber of 100 was entered in the datasheet. Excessive numbers of holidayswere observed on the surface of theTSZ/LE coating system. Initial assess-ment of this coating system showed noholidays on the surface. However, oneof the panels demonstrated more than20 holidays after 1,080 hours of test-ing, and the number of holidaysincreased excessively after two accel-erated cycles that followed (after 1,440and 1,800 hours of testing). These sur-face defects were then followed byexcessive blistering and cracking of thesurface of the coating system.The rest of the three-coat, two-coat,

and one-coat systems demonstratedeither zero or minimal coating defectson the surface after 2,160 hours ofaccelerated testing. Test panels coated with the Zn/PS

coating system had initial defects tobegin with on the low DFT area ofType II panels. After outdoor exposureof six months, none of the test areas onthe Type II panels had developed anynew holidays. All Type II test panels

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mm) and E/E/PU (1.42 mm) haveshown more than 1.0 mm creepage at2,160 hours. HRCSA, IOZ/E/PU,E/PU and Zn/PS showed creepagegrowth of less than 1 mm. Based on thecreepage values at the end of 2,160hours of accelerated testing; the coatingsystems can be ranked in the followingorder of highest to lowest rust creepage: MCU/E/F > E/E/PU > E/PU > Zn/PS > HRCSA > IOZ/E/PU

It is interesting to note that the inorganiczinc three-coat control has the best per-

formance for rust creepage, followed bythe HRCSA. The latter coating systemhas performed well in earlier studies,5

and a similar trend is being observedhere up to 2,160 hours of acceleratedtesting. The surface of the TSZ/LE coating

system at 2,160 hours of acceleratedtesting indicated surface blistering andpeeling off of the coating system all overthe surface for both scribed andunscribed panels. After six months of outdoor exposure

at TFHRC in natural weathering withsalt spray, two of the Type I panels coat-ed with ZnE/LE coating system devel-oped severe rust creepage of 7.6 mm. Ona high DFT area of ZnE/LE on Type IIpanels, high rust creepage (11.8 mm)developed. The high DFT area of thethree-coat control IOZ/E/PU was theonly other coating system that showedmoderate creepage of 1.9 mm after sixmonths of exposure in natural weather-ing with salt spray.

Surface Deterioration Figures 8 through 10 show selected Type Itest panels representing coating systemswith the best (HRCSA), moderate (E/PU),and worst (TSZ/LE) performance duringthe accelerated laboratory testing at 0;1,080; and 2,160 hours. Similarly, Figs. 11


had coating defects in areas such asnuts, bolts, the underside of the T-attachment, and the wide angle attach-ment. These areas appeared to havedeveloped rusting in areas of impropercoating application. They will be care-fully monitored over time for coatingdegradation and rusting.

Rust Creepage Figure 7 shows rust creepage data foreight coating systems after 2,160 hoursof accelerated testing. MCU/E/F (2.0

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Fig. 6: Cumulative holiday development during accelerated testing

100

90

80

70

60

50

40

30

20

10

0

Number of Holidays

0 500 1000Time (hrs)

1500 2000 2500

Shown are coating systemswhich developed holidays

only

E/E/PU (panel 19)

E/PU (Panel 42)

Zn/PS (Panel 57)

TSZ/LE (Panels 66,67,68,69,70)

HRCSA (Panels 90,91,92,93,94)

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TSZ/LE coating system had the worstsurface deterioration, as indicated bythe development of holidays, surfaceblistering, and coat-ing peel-off. The visual obser-

vation of surfacechanges was fol-lowed up with digitalmicroscopy of thesurface of the testpanels. Digitalmicroscopy of theTSZ/LE coating sys-tem showed the

J P C L J a n u a r y 2 0 1 1 47www.paintsquare.com

and 12 are representative of Type II pan-els that showed the best (HRCSA) andworst (ZnE/LE) surface deteriorationafter six months of exposure in naturalweathering with salt spray. The three-coat inorganic zinc control

(IOZ/E/PU) had the best surface reten-tion properties in terms of holidays,rusting, and blistering. However, this isexpected of a control coating system.The next best performing coating sys-tem among the candidate coating sys-tems was identified as the HRCSA. TheE/PU had a moderate development ofholidays and rust creepage growth. The

phases of its progressive deterioration.Certain areas still had blisters intactwhile some areas had half-peeled offcoating or detachment of the coatingfrom the surface. White residual zincoxide was forming on the surface in theareas where coating had peeled off. Digital microscopy at 1,440 hrs of

accelerated testing yielded anotherimportant experimental observation.The surface of the ZnE/LE coating sys-tem demonstrated micro-cracks all overthe surface of the coating system.Cracks appeared to have spread acrossthe coating surface. The central pointsat which these micro-cracks originatedseemed to be a few microns in size, andthe cracks themselves were a few hun-dred microns in size. However, the sur-face of these test panels did not demon-strate any holidays, indicating that the

Fig. 7: Rust-creepage growth with time accelerated testing

Fig. 8: Progressive changes of HRCSA (1-coat system)

0 hours 1080 hours 2160 hours

Rust creepage (mm)

0 500 1000Time (hrs)

1500 2000 2500

Shown are coating systemswhich developed rust creepage only

IOZ/E/PU (panel 7)

E/E/PU (Panels 18,19,21)

MCU/E/F (Panels 30,31,33)

E/PU (Panels 42,43,45)

Zn/PS (Panels 54,57)

HRCSA (Panel 91)

2.5

2.0

1.5

1.0

0.5

0.0


appeared after accelerated testing. During outdoor exposure testing,except for ZnE/LE, none of the Type Ipanels and the Type II panel test areasdeveloped holidays or rust creepage atthe scribe. Figures 11 and 12 show theType II test panels for the HRCSA andZnE/LE coating systems.

Summary of Preliminary Findings In this study, a new test panel was


cracks did not develop through thecoating thickness and may have origi-nated only on the surface. The densityof the cracks seemed to increase as thetime of accelerated testing increased. Comparison of digital microscopy

images before and after exposure indi-cated that these crack-originating loca-tions were on the panel surface beforetesting. However, the cracks that prop-agated from these locations have

designed, and 27 of the new panelswere used. They include welding jointsand angle attachments. On the newdesign, three DFT areas of varyingthickness were selected to simulatefield conditions. Based on the initialcoating characterization—six accelerat-ed test cycles and one 6-month outdoorexposure cycle—some preliminaryfindings are summarized below. 1. Two three-coat systems were cho-

Fig. 9: Progressive changes of E/PU (2-coat system)



Fig. 10: Progressive changes of TSZ/LE (2-coat system)

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sen as the controls, and another three-coat system, four two-coat systems,and one one-coat system were selectedas candidate coating systems. 2. Adhesion strength of the three-coatsystems was comparable to that of thethree-coat systems, but four times higherthan the adhesion strength of the one-coat system. 3. The TSZ/LE coating system devel-oped the highest number of defectsduring accelerated testing up to 2,160hours, but no holidays developed onthe surface after outdoor testing. Thedefect formation was progressively followed by blisters and by the coatingpeeling from the surface. 4. MCU/E/F developed the highestamount of rust creepage (almost twice)in comparison to the rest of the coatingsystems in accelerated testing. As wasexpected, the inorganic three-coat control (IOZ/E/PU) had the lowestcreepage, followed by the HRCSA. 5. In outdoor exposure testing,ZnE/LE developed severe rust creepageon both Type I and Type II panels. 6. During accelerated testing, ZnE/LEdeveloped micro cracks on the surface. 7. All coating systems developed nonew holidays and surface deteriorationafter the first outdoor exposure cycleat the TFHRC and the Golden GateBridge.

[Note below received close to press time—Ed.]Note: Due to unexpected premature fail-ures of certain coating systems, theFHWA 100-year coating study has beenterminated in December 2010.

Fig. 11: Progressive changes of HRCSA Fig. 12: Progressive changes of ZnE/LE

0 months 6 months 0 months 6 months



Jan. 29–Feb. 1, 2006. 5. S-L. Chong and Y. Yao,

“Selecting Overcoats For Bridges,” Public Roads, Sept./Oct., 2007.

6. S-K. Lee, R. Kogler, and Y. Yao, “Outdoor Performance of One-Coat Systems Applicable to New Steel Bridges,” Proceedings of PACE 2010, Phoenix, AZ, Feb. 7–10, 2010.

7. ASTM D4541-02, “Standard Test Method for Pull-Off Strength for Coatings Using Portable Adhesion Testers.”

8. SSPC- PA 2, “Measurement of Dry Paint Thickness with Magnetic Gauges.”

9. ASTM D5162-01, “Standard Practice for Discontinuity (Holiday) Testing of Nonconductive Protective Coating on Metallic Substrates.”

10. ASTM D523-89 (1999), “Standard Test Method for Specular Gloss.”

11. ASTM D2244-07, “Standard Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates.”

12. ASTM D7087-05a, “Standard Test Method for an Imaging Technique to Measure Rust Creepage at Scribe on Coated Test Panels Subjected to Corrosive Environment.”

Pradeep Kodumuri, Ph.D.,is a senior chemist/researchscientist for the SES Groupand Associates, and a contract chemist for FHWA’sCoatings and CorrosionLaboratory at the Turner-

Fairbank Highway Research Center(TFHRC) in McClean, VA. He is active inSSPC and NACE.

Seung-Kyoung (SK) Lee,Ph.D., is the research corrosion engineer andmanager of the Coatingsand Corrosion Laboratory at the FHWA’s TFHRC. A member of SSPC and

NACE, he is a NACE-certified CathodicProtection Specialist and is NACE CIP-Certified-Level 2 with Bridge Specialty.

References1. E. Kline, “Steel Bridges: Corrosion

Protection for 100 Years,” JPCL, May 2008.

2. S-L. Chong and Y. Yao, “Laboratory and Test-site Testing of Moisture-cured Urethanes on Steel in Salt-rich Environment,” FHWA Publication, FHWA-RD-00-156, Dec. 2000.

3. S-L. Chong and Y. Yao, “Performance of Two-Coat Zinc-RichSystems on Steel Bridges,” JPCL, June 2006.

4. S-K. Lee, S-L. Chong and Y. Yao, “Performance Evaluation of Bridge Overcoating Materials Using Electrochemical Impedance Spectroscopy,” Proceedings of PACE 2006, Tampa, FL,

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• Provides an impermeable barrier, adding years to service life• 1 or 2 coat applications• No abrasive blasting required, can be applied over blasted metals• Surface tolerant over rust and existing coatings• Apply as low as 0ºF - extend painting season 3 months• Dries quickly to a firm film


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fhwabridgecoating studyyields preliminary … · 2016-10-24 · 38 jpcl january 2011...

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