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NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and

technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Center for Computer Sciences and

Technology, and the Office for Information Programs. THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consistent system of physical measurement; coordinates

that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics —

Linac Radiation - — Nuclear Radiation '- — Applied Radiation - — Quantum

Electronics '• — Electromagnetics : — Time and Frequency 3 — Laboratory

Astrophysics : — Cryogenics 3

. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research lead- ing to improved methods of measurement, standards, and data on the properties of

well-characterized materials needed by industry, commerce, educational institutions, and

Government; provides advisory and research services to other Government agencies; and develops, produces, and distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Reactor

Radiation—Physical Chemistry. THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to pro- mote the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute also monitors NBS engineering standards activities and

provides liaison between NBS and national and international engineering standards

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divisions and offices: Engineering and Product Standards—Weights and Measures—Invention and

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1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted : mailing address Washington, D.C. 20234.

• Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

* Part of the Center for Building Technology.
- - . -

^t ^--s-d- ^snzi Nonmetallic Coatings for

-_ Concrete Reinforcing Bars

5 Coating Materials

James R. Clifton, Hugh F. Beeghly,

and

Robert G. Mathey

Structures, Materials, and Life Safety Division Center for Building Technology

Institute for Applied Technology

•-^tf'Jt National Bureau of Standards Washington, D.C. 20234

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NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of

transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

\ 33

'^exu of

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued April 1973
National Bureau of Standards Technical Note 768

Nat. Bur. Stand. (U.S.), Tech. Note 768, 40 pages (Apr. 1973) CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. 013.46:768). Price $0.65 domestic postpaid or $0.45 G.P.O. Bookstore
CONTENTS

Page

1

.

Introduction 1

2. Materials 2

2.1 Selection of Coating Materials for Evaluation 2 2.2 Curing Methods and Specimen Preparation 2 2.2.1 Two-Component Liquid Systems 2 2.2.2 One-Component Liquid Systems 6 2.2.3 Powder Systems . 6 2.2.4 Reinforcing Bars Coated by Applicators 7

3. Testing Procedures and Results 7 3.1 Immersion Testing , 7 3.1.1 Epoxy Disc Specimens 7 3.1.2 Coatings on Reinforcing Bars 10

3.2 Chloride Permeability 10

3.3 Impact and Abrasion Resistances of Epoxy Coatings on Steel Plates. 14 3.4 Inspection of Coated Reinforcing Bars 14

3.5 Physical Durabilities of Coated Reinforcing Bars 19 3.5.1 Bend Tests 19 3.5.2 Impact Tests 23 3.5.3 Pencil Hardness 23 3.6 Electrochemical Tests 23 3.6.1 Applied Voltage Studies 23 3.6.2 Electrical Potential and Resistance Measurements 24

4. Evaluation of Coatings 30

4.1 Evaluation Program 30 4.2 Chemical and Physical Durabilities of Coatings 30 4.3 Corrosion Protection. 31

5

.

Recommendations and Conclusions 32

6. References 34

iii

Nonmetallic Coatings for Concrete Reinforcing Bars. Coating Materials*

James R. Clifton

Hugh F. Beeghly

Robert G. Mathey

This work was undertaken to determine the feasibility of using organic coatings,

especially epoxies, to protect steel reinforcing bars embedded in concrete from the accelerated corrosion attributed to the depassivation of steel by chloride ions.

Coatings have been evaluated on the basis of their chemical and physical durabilities

as well as their protective qualities. In this study, attention also has been directed

to the following: application methods; surface preparation of the steel reinforcing

bar; and site of application.

Key words: Chlorides; concrete; corrosion; epoxy coatings; organic coating; steel reinforcing bars .

1 . INTRODUCTION

The premature deterioration of concrete bridge deckings, in 5-10 years, has become a major problem during the past decade [1-7]—. Often, this early deterioration has been attributed to accelerated corrosion of the steel reinforcing bars caused by chloride ions from deicing materials [8-9]. Use of the two more commonly applied deicing materials,

sodium chloride and calcium chloride, has increased substantially during the past decade. Normally, steel is passive towards corrosion when in an environment of high basicity (pH of about 13) inherent in portland cement concretes [l0]; chloride ions, however, are able to depassivate steel and thereby promote the active corrosion of steel [ll]. Corrosion of reinforcing bars results in spalling and cracking of concrete, necessitating extensive and expensive repairs.

Coating reinforcing steel bars with protective materials has been considered as a practical method to obviate the rapid corrosion of the bars. Much attention has been given

to the use of galvanized reinforcing bars [12-13]. Recent studies [14-15], however, indicate

that zinc does not provide long term protection to steel in the presence of chloride ions. Cadmium [ 15] and nickel [l6] have been reported to be satisfactory coatings for reinforcing

steel. Their cost, however, may be prohibitively high. The use of organic types of barrier

coatings for protecting reinforcing bars has been recommended L17-18 J . Tripler and co-workers evaluated a few nonmetallic coatings and suggested that an epoxy - coal tar type of coating

could have potential as protective coating for reinforcing steel [16]

.

The present study was undertaken to ascertain the feasibility of using organic coatings,

especially epoxy systems, to protect reinforcing steel bars. The evaluation of the physical

*/ Work performed under auspices of the Federal Highway Administration.

1/ Figures in brackets indicate the literature references at the end of this paper.
and chemical durabilities of coatings has been completed, along with evaluation of their potential protective qualities, and the results are presented in this report.

2. MATERIALS

2.1 Selection of Coating Materials for Evaluation

The selection of the coating materials to be evaluated as protective coatings on steel reinforcing bars was done on a generic basis and was restricted essentially to organic

formulations. Some of the most important criteria for selection were: inertness towards

the constitutents of cement paste and also chloride ions; creep characteristics; film integrity and protective qualities; as well as cost considerations.

The coatings selected for evaluation, listed in table 1 (also included, are a few unsolicited coatings submitted by the respective firms who handled them), are of commercial

origin. The coating materials have been assigned code numbers, for identification purposes, which will be used in this paper, exclusively. The code number sequence has no significance

other than indicating the chronological order in which the materials were received.

The polyurethanes and epoxies consist of two components and are classified as thermo- setting materials because their cure is accelerated by heat. Once cured, they normally

retain their shape up to their decomposition temperatures. The other coatings in table 1 are classified as thermoplastics as they soften and change shape when heated.

The emphasis has been on thermosetting materials and especially on epoxies because

these materials seem to best satisfy the chosen criteria. Altogether 36 epoxy coatings, both powder and liquid systems, have been evaluated to some extent.

Some materials, especially powders, were submitted only in the form of cured films on

steel reinforcing bars.

2.2 Curing Methods and Specimen Preparation

2.2.1 Two-Component Liquid Systems

The epoxy liquid systems consist of two components, an epoxy resin and a curing com- ponent. The curing of epoxies is attributed to chemical reactions between the resin and curing agents which lead to polymerization of the mixture.

In the present work, the ratio of curing component to epoxy resin and the mixing and curing times, were closely controlled and were the same as those specified by the manufacturers' accompanying instructions. The epoxy resins and curing components were mixed at room temperature, ca. 24°C, relative humidity of ca. 50 percent, using either an electric

stirrer (solventless systems) or a metal spatula (solvent containing systems). The two component urethanes and zinc-filled coating were similarly mixed.
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Test specimens of the coating materials were cast and in addition steel plates and

2/ steel reinforcing bars were coated with thin films. Specimen discs of 2 1/4^ inches diameter of thickness of ca. 3/8 inches (thicknesses of solvent containing systems were

reduced to 3/16 inches) were cast using aluminum weighing dishes as molds. The molds were

3/ stripped after the mixture had cured for seven days. Wet films of 3-7 mils— thicknesses were formed immediately after mixing, by applying the coatings with a Baker roller film

4/ applicator to the gel side of photographic paper or to sheets of Teflon—. Then the cured

films were stripped from the photographic paper by being immersed in water at room temper- ature for 16 hours. Cured films were easily stripped from Teflon sheets using a thin-bladed

spatula. Coating materials were applied to 4 x 4 x 0.050 inch cold-rolled steel plates and

to No. 6 steel reinforcing bars— using a paint brush. The steel plates had been degreased

previously using mineral spirits, and the reinforcing bars had been sand blasted to a white

surface [l9J

.

2.2.2 One-Component Liquid Systems

The two one-component liquid systems, a polyvinyl chloride and a phenolic nitrile, were hardened by the evaporation of solvents. Test specimens were formed as described above.

2.2.3 Powder System

No mixing of the epoxy powders was necessary since the two components are contained

within each powder particle. The powders were applied to steel and Teflon substrates pre- heated to ca. 200°C in an electric oven by immersing the substrates into a fluidized bed

[ 20] of the powders. Then the coatings were cured in the electric oven under the conditions

specified by the manufacturers. When allowed to cool to room temperature, the cured epoxy

films were easily removed from the Teflon substrate by using a thin-bladed spatula. Both

No. 6 steel reinforcing bars and 4 x 4 x 0.050 inch cold-rolled steel plates were coated with the powders.

Specimen discs were not fabricated from the epoxy powders because, when sufficient masses to make 3/16 inch thick discs were heated to their specified curing temperatures,

porous solids that had expanded over 100 percent were produced. Even four-fold reduction of the masses did not yield satisfactory specimens. Only one powder epoxy did not exhibit this expansion phenomenon. Possibly, some of the curing components are vaporized at the curing

temperatures causing the formation of porous structures in the thick castings. No similar

difficulties were encountered when films less than 20 mils were formed from the powder epoxies.

2/ One inch equals 0.0254 meter, exactly.

3>/ One mil equals 0.001 inch, exactly.

kj Certain instruments and materials are identified in this paper in order to adequately

specify the experimental conditions. In no case does such identification imply recom- mendations or endorsement by the National Bureau of Standards nor does it imply that the material or instruments are necessarily the best available for the purpose.

5/ No. 6 steel reinforcing bars have nominal diameters of 3/4 inch.
2.2. 4 Reinforcing Bars Coated by Applicators

The coating materials that were judged (on the basis of preliminary screening tests which included: resistance to chemicals; length of cure; film integrity; and evaluation of relative brittleness) to have the most promise as potential protective coatings for steel reinforcing bars were applied by the applicators or manufacturers handling the respective

coatings. No. 6 steel reinforcing bars, four feet in length having two different deformation

patterns, were supplied to each applicator. The surfaces of the bars were cleaned, usually

by sandblasting by the applicator; coatings applied; and the bars returned to the National

Bureau of Standards for evaluation.

3. TESTING PROCEDURES AND RESULTS

Descriptions of the test methods used to evaluate coating materials are given in this section along with the experimental results.

Not all of the 47 coating materials were subjected to the same degree of testing as some were quickly judged not to be acceptable for such reasons as: gel times for two component systems longer than eight hours, coatings Nos. 10 and 11 (table 1); poor film integrity and excessive entrapped air in the cured state, Nos. 12 and 13; the tendency of some thermoplastics to soften at 60°C, Nos. 14 and 47; rubber-like expansion qualities

(500 to 600 percent elongation) of two urethanes, Nos. 35 and 36 (obviously these two coatings would not pass the pull-out and creep requirements); and extreme brittleness of

some epoxy systems, Nos. 5 and 13.

3.1 Immersion Testing

3.1.1 Epoxy Disc Specimens

Disc shape castings of cured epoxy specimens were immersed in water, in an aqueous solution of 3M CaCl 9

, in an aqueous solution of 3M NaOH, and in a solution saturated with

Ca(OH) and CaS0.-2H and containing 0.5M CaCl . These test chemicals were selected because they are, with the exception of NaOH, probably the major chemicals most potentially deleterious to epoxy coatings present in concrete of bridge deckings. Ca(0H)„

is a reaction product of portland cement and water; it stabilizes the silicate gels which

are important constituents of durable concretes. CaSO, is often added as a set-regulator

to portland cement and also is frequently present in soil drainage water. CaCl, is one of the two most commonly used deicing materials. The solubility of Ca(0H)„ is low (0.2M at 25°C) and 3M NaOH was used in an accelerated-type of test to determine if hydroxide ions

are detrimental to the long term embedment of epoxy coatings in concrete. Water, in itself, can have a deleterious effect on coating materials. It was felt that these test solutions

are probably as aggressive or more so than those encountered in concrete. Therefore,

materials performing well in the immersion tests will probably not be degraded by long term embedment in concrete. The specimens were immersed in water for one or two minutes and wiped dry before measuring the original weights prior to the immersion studies. Original

weights of the discs varied from ca. 20 grams for solvent containing systems to ca. 50 grams

for the solventless epoxy systems. The temperatures of the test solutions were 24 ± 1°C.

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TABLE 3 IMMERSION TESTING OF COATINGS ON REINFORCING BARS-1/

Code Number 2/ 3M NaOH- 2/ Saturated Ca(0H)y

19 No Change A.

B.

3/ 4/ Rusted— -r. No Change22 No Change No Change

23 No Change No Change

24 No Change No Change

27 No Change No Change

28 No Change No Change

29 No Change No Change

30 No Change No Change

31 No Change No Change

32 No Change No Change

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Rusted 5/ 5/

41 No Change No Change

Uncoated

Rebar No Change No Change

— No. 6 reinforcing bar coated by firms handling the respective

coatings.

II—Immersion time of 45 days.

3/— A and B are specimens from companion bars. — Rusting took place during the first 15 days of immersion, after- wards no changes were visually observed. — B denotes bars that were only sandblasted prior to application

of the coating, while P indicates that their surfaces were also phosphatized prior to being coated.
The immersion data are presented in table 2. In some cases two separate castings were

made, indicated by two sets of data with different immersion times. The epoxies which in their uncured state contained solvents, generally lost weight and had greater weight changes

than the solventless epoxies. An exception is No. 7, a solvent containing epoxy, which had

the largest weight increase. The surfaces of both No. 7 and No. 9 were converted from smooth

to rough textures during the immersion period. No apparent deterioration was observed with

the other epoxy specimens.

3.1.2 Coatings on Reinforcing Bars

The chemical resistances of powdered epoxy systems were investigated by immersing

coated reinforcing bars, supplied by applicators, in aqueous solutions of 3M NaOH and of saturated Ca(OH)„. The coatings were visually inspected for evidences of softening, color

changes, disbonding, and changes in film integrity. The data are presented in table 3, which

includes three polyvinyl chloride coatings (Nos. 23, 24, and 30) as well as one one-component

liquid epoxy (No. 19) and 10 powder epoxy coatings.

The rusting of some coated reinforcing bars in saturated Ca(OH) solutions during the first two weeks of the immersion study is an interesting phenomenon, especially since the uncoated rebars were passive towards corrosion in a similar solution. Furthermore, the corrosion was only observed in the less alkaline solution, i.e. saturated Ca(0H)« (pH of 12.6) rather than 3M NaOH (pH of 14.5). The pH of saturated Ca(0H)

9

, however, is sufficient

to passivate steel. Therefore, the cause of the corrosion apparently lies in either the surface preparation or the composition of the coatings. Note that the surfaces of the rusting specimens of Nos. 38, 39, and 40 were phosphatized, while the surfaces of the non- rusting specimens were only sand blasted. It is not obvious at this date why the same rusting phenomenon was not observed when the rebars were immersed in 3M NaOH.

3.2 Chloride Permeability

The chloride permeability characteristics of thin films of cured epoxies were measured

using permeability cells of the type shown in figure 1. A cell consists of two glass com- partments separated by the epoxy film sandwiched between two glass plates, each having

centered one-inch diameter holes. One compartment contains 175 ml of 3M NaCl and the other

has 115 ml of distilled water. The activities of chloride ions passing through an epoxy membrane was measured using an Orion Specific Ion Meter Model 401, along with an Orion

Chloride Electrode Model 94-17, and an Orion Double Junction Reference Electrode Model 90-02. Activity readings have been converted into concentration values of moles per liter by using

a conversion diagram, constructed by plotting measured chloride ion activities versus known chloride ion concentrations.

Films selected for the permeability studies were carefully handled, and examined for any defects before installation in the cell.

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The permeability data (calculated at listed exposure times) are presented in table 4 in the form of both permeability units and concentrations. Many of the epoxy films appear to be essentially impervious to chloride ions. Only two films, No. 13 and 16, permitted chloride

ions to migrate through so that the corrosion threshold concentration of 0.02M chloride ion was approached [21] . Diagrams of the accumulative permeating chloride ion concentrations

versus time are reproduced in figure 2 for four epoxy films. These plots are representative

of the varying degrees of impermeabilities. The permeability rates were largest during the first six weeks of testing, afterwards the values were lower and more constant.

3.3 Impact and Abrasion Resistances of Epoxy Coatings on Steel Plates

Both direct and reverse impact resistances of cured epoxy coatings on 4 x 4 x 0.050 in. cold-rolled steel plates were determined in accordance with ASTM Designation G14-69T [22]. A Gardner Laboratory impact tester was used along with a four pound hammer.

The impact data are presented in table 5. The reverse impact is more severe than the direct impact and probably gives a better indication of the flexibility of a coating (reverse

impact values lower than 40 in. lb are indicative of brittle materials). However, it is felt

that the impact values for the powder materials (Nos. 25, 28, 29, 31, and 38-40) are too low

and are not reliable indicators of their properties. Possibly, the low values can be attributed to poor adherence to the particular steel substrate. The bend testing of coated reinforcing bars, discussed later, is considered to yield more reliable results.

The abrasion resistances of epoxy coatings on similar steel panels were determined in accordance with ASTM Designation D1044-56 [23] by using a Taber Abraser and Taber CS-10

wheels with lOOOg load per wheel. After each 200 cycles the wheels and specimens were gently cleaned with a soft bristle brush. The abrasion data are given in table 5 in units

of weight loss in mg per 1000 cycles. Two of the solvent containing materials, Nos. 3 and

16, had weight losses over 100 mg indicating poor abrasion resistances.

3.4 Inspection of Coated Reinforcing Bars

The film thicknesses, the number of holidays— (determined using a 67 1/2 volt holiday

detector) per unit bar length (4 feet), and the visual evaluations of coated reinforcing bars submitted by the respective firm handling the materials, are given in table 6. The film

thicknesses were measured with a Mikrotest Model 790000 Magnetic Gage.

The following tentative conclusions concerning the integrity of the coating films are implicit in the results given in table 6:

1. The effectiveness of the application methods in producing thin films free of defects decreases in the sequence; electrostatic spray gun? fluidized bed ? dipping> brush.

6/ Holidays denotes film defects such as pinholes that are locations of potential

corrosion.

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2. Powder coatings yield films of more uniform thickness than most liquid coatings.

3. Good application practices, including proper surface preparation, are imperative if

a coating material is to realize its fullest potential.

3.5 Physical Durabilities of Coated Reinforcing Bars

The physical durabilities of coated reinforcing bars were evaluated on the basis of bend tests, impact tests, and pencil hardness measurements (table 7). The tests were carried

out on either the same specimens, or companion specimens, listed in table 6.

3.5.1 Bend Tests

No. 6 bars coated by applicators were bent at a 120° angle with a radius of curvature

of ca. 3 in. using a Green Lee Tool Company Model 770 Bar Bender. Portions of the bars in contact with the bending machine were protected with rubber tubing of 1 1/2 in. o.d. and 3/4 i.d. to avoid mechanical damage to the coating so that any cracking in a coating occurring during the bend test could be attributed to stress failure of the coating.

Cracking and disbonding took place on the area of some bars that were under tension

during the bending (table 7). The four different polyvinyl chloride coated rebars (Nos . 23,

24, 26, and 30) gave excellent performances even though their film thicknesses ranged from

2 to 35 mils. A greater variation was observed for the epoxy coated rebars as some performed

well while a few were classified as failing. Generally, the epoxy coatings which did not perform well were either the most brittle epoxies or they were applied in film thicknesses

over 10 mils. The effect of the film thickness is well illustrated by comparing the coated reinforcing bars Nos. 22 and 31, which were both coated with the same materials, but applied

by different means. No. 22 has a film thickness of ca. 25 mils while the film thickness of No. 31 was ca. 8-9 mils. When bent, substantial cracking was observed in the film of No. 22, while No. 31 was completely free of cracking.

Another factor affecting the bending characteristics of coated reinforcing bars is the type of surface preparation of the substrate prior to application of the coatings. In two series of coated reinforcing bars, No. 28B and No. 32, epoxy coatings were applied to un- prepared surfaces which were still covered with mill scale. Almost total disbondment was observed when both series of bars were bent; while the epoxy adhered tenaciously to the mill

scale, the mill scale was disbonded from the steel substrate. A portion of the coated rebars

Nos. 38, 39, and 40 were both sand blasted and phosphatized prior to being coated while the remainder were just sand blasted. The sand blasted coated rebars gave no indications of coating failures when bent while the phosphatized bars were susceptible to varying degrees

of coating failures.

The temperature of the steel substrate, when being coated, can affect the flexibility

of the cured epoxy coating. For example, Nos. 41 and 42 rebars were coated with the same material. However, No. 41 rebars were heated to 191°C and immediately coated, while No. 42 rebars were at ambient temperature when coated. The epoxy coating was then cured at 177°C

on both sets of rebars. Excellent flexibility was exhibited by No. 41, whereas No. 42 cracked badly when bent.

19
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22
3.5.2 Impact Tests

The impact resistances of coatings were evaluated by dropping an 18 in. length of coated

No. 6 reinforcing bar on a slab of concrete so that impact occurred lengthwise as follows:

1. A single bar was dropped one meter from a horizontal position to the concrete.

2. The same bar was dropped from a height of two meters.

3. A companion specimen was taped loosely between two bare No. 7 bars of the same length and the assembly was dropped from a height of 2 meters to the concrete slab.

4. The bars were inspected after each drop for the following types of damages:

A. Shattering of the coating to expose bare metal.

B. Cutting of the coating to expose bare metal.

C. Cracking of the coating.

D. Disbonding of the coating from the steel substrate.

The coatings were rated on a relative basis (table 7). There is a fairly direct correlation

between the results of the bend tests and the impact tests.

3.5.3 Pencil Hardness

The pencil hardness values (table 7) were determined using a series of lead pencils

covering the hardness ranges from H to 8H with steps of one hardness increments. The hardness is designated as the softest lead that imparts a scratch in the coating. All of the epoxy coatings had ratings above 8H, while the polyvinyl chloride coatings were softer with

ratings of H, Nos. 24 and 30, and 8H, No. 23.

3.6 Electrochemical Tests

Electrochemical tests were undertaken to quantitatively rate the relative performance

of coatings exposed to solutions corrosive to steel.

3.6.1 Applied Voltage Studies

The effects of electrical and electrochemical stresses on the bond of barrier coatings

to steel were assessed by modifying the disbonding test, ASTM Designation G8-69T [24]. These stresses can be induced by cathodic protection devices, stray currents, or by cor- rosion processes. The cathode and anode were No. 6 reinforcing bars, 6 inches long, both

coated with the same material. The electrolyte was an aqueous solution of 7 percent NaCl

.

A potential of two volts was applied and the electrodes were visually observed periodically

for evolution of hydrogen gas at the cathode and for evidence of corrosion products of iron

at the anode. Before immersion, any bare ends or obvious mechanically damaged areas on the electrodes were covered with a film of silicone rubber, and no intentional holidays were

induced. The source of any corrosion, therefore, were holidays in the films. Therefore,

the applied voltage method serves as a sensitive holiday detector and can be used to ascertain if holidays are developing in a film because of degradation of the coating.

23
The results of the impressed voltage studies are listed in table 8. It is felt that coatings which permitted the evolution of hydrogen gas within 15 minutes are of doubtful value (when applied in the indicated thicknesses).

3.6.2 Electrical Potential and Resistance Measurements

Alternate means of assessing the protective qualities of barrier coatings are electrical

potential measurements on coated rebars immersed in corrosive solution and the simultaneous measurements of the resistance of the coating films. Such measurements were made on coated reinforcing bars partially immersed in 3 1/2 percent aqueous solutions of NaCl . The

electrode potentials of the coated reinforcing bars were measured using a Coleman Model 37A pH Meter with a standard calomel electrode as the reference electrode. Measurements of the resistance of the films on the coated reinforcing bars were made using a Yellow Springs Instrument Company Model 31 Conductivity Bridge along with a platinum electrode.

The electrical potential and resistance data are presented in table 8. Low resistance

readings, below 500 ohms, are indicative of films which either have many holidays or are permeable towards water and/or chloride ions. Coated reinforcing bars which were visually

observed to have several corrosion sites gave potential readings below -600 mV (the

electrical potential of uncoated reinforcing bars was -634 mV after 1000 hours). The thicker

films, above 15 mils, were free of holidays and had resistances higher than 24 x 10 ohm,

which are beyond the range of the measuring device. The same films did not permit the passage of any measurable current; therefore, the potential of these bars could not be measured.

Potential and resistance data for three sets of reinforcing bars each set coated with a different epoxy, are reproduced in figures 3, 4, and 5. The wide variance in the initial millivolts and ohmic readings of duplicate and triplicate specimens decreased rapidly the first 200 hours of testing so that after 1000 hours of testing good agreements were

obtained for companion specimens. A rapid decrease in the resistance of a coatings, probably,

can be attributed to the emergence of holidays; while an increase in resistance is, probably,

indicative of some type of healing mechanism.

The ratings of coatings, table 8, are based on their overall protective qualities. It

is felt that adequate protection is provided by coatings with ratings of 1 or 2. The coatings with ratings 3 may have performed badly because of poor application techniques, improper

curing or inadequate film thicknesses.

24
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Figure 3. Electrical potential and resistance measurements of reinforcing

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-

In the ordinate caption, M.V. vs. S.C.E. denotes millivolt

reading taken using a saturated calomel electrode as the reference.

27
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Figure 4. Electrical potential and resistance measurements of reinforcing

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.

In the ordinate caption, M.V. vs. S.C.E. denotes millivolt

reading taken using a saturated calomel electrode as the reference.

28
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Figure 5. Electrical potential and resistance measurements of reinforcing

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In the ordinate caption, M.V. vs. S.C.E. denotes millivolt

reading taken using a saturated calomel electrode as the reference.

29
4. EVALUATION OF COATINGS

4.1 Evaluation Program

The evaluation program, conducted to select the best coatings for reinforcing bars embedded in concrete, was roughly separated into three general test categories:

1. chemical and physical durabilities of cured coatings;

2. corrosion protection of steel reinforcing bars by coatings;

3. structural characteristics of coated reinforcing bars.

Categories 1 and 2 have been essentially completed and the overall results will be discussed

in this section. The structural characteristics denote creep and bond strengths of coated reinforcing bars embedded in concrete prisms. These tests are currently being performed and

the results will be given in a future report.

The major emphasis has been devoted to epoxies because of anticipated unacceptable

structural characteristics of reinforcing bars coated with thermoplastics. However, such coatings have been included in this study as they may serve well on dowel pins and other members of the steel frame work not subjected to high tensional forces.

4.2 Chemical and Physical Durabilities of Coatings

The resistance of coatings to chemicals was investigated by immersing both specimens of pure coatings (table 2), and coatings on steel substrates (table 3), in aqueous solutions

containing aggressive salts similar to those in portland cement concrete. The weight

changes listed in table 2 are in the range reported by others [25]. It is felt that with

the exception of two solvent containing epoxy systems, Nos. 7 and 9, the epoxy coatings

performed well and probably will not be degraded by long term embedment in concrete. The

long term durability of polyvinyl chlorides when embedded in concrete, however, is thought

to be of major concern for if considerable hydrolysis should take place sufficient amounts

of chloride ions to induce corrosion of the rebars will be liberated.

Phosphatizing the surface of metal substrates has been considered advantageous to inhibition of corrosion [26] . In the present study, however, the coated bars with phosphatized steel surfaces, Nos. 38, 39, and 40, prematurely rusted when immersed in saturated

Ca(OH)- (table 3). This corrosion terminated after about two weeks of exposure. No rusting

took place with reinforcing bars coated with the same epoxies applied to sand blasted surfaces.

Reinforcing bars are normally subjected to harsh physical treatment while being shipped

to the site of bridge construction and during the placement process. Furthermore, steel reinforcing bars are still being bent to form hooks, in accordance with the specifications of some state highway departments. Therefore, the abilities of coated bars to withstand such rough treatment with minimum coating damage is a necessary requirement.

The relative physical durabilities of coatings were ascertained by measuring the abrasion and impact resistance of coatings on steel plates (table 5), and by dropping and bending coated reinforcing bars (table 7). As discussed in section 3.3, the correlation

between the relative impact resistance of coated plates and the relative drop and bend

30
ratings of coated rebars was not good. Results of the bend and drop tests are considered to provide the more reliable indications of the relative performances of the coating films.

The performance of a coating on a reinforcing bar when subjected to bend stresses can be related to: the flexibility of a coating; proper cure of the coating; surface preparation

of the steel; and film thickness. Coatings with little flexibility will crack when under the tensional bend forces. Polyvinyl chlorides are inherently flexible materials and performed

well, even with film thicknesses up to 35 mils (No. 24). Although epoxies are intrinsically

more brittle than polyvinyl chlorides, the relative performance of epoxy films ranged from complete failure (e.g. No. 2) to excellent (e.g. No. 9). Interestingly, the flexibilities

do not appear to be directly related to the type of epoxy system, i.e. powder or liquid.

Flexibilities of epoxy coatings will often be decreased by improper cure caused by such factors as mixing incorrect ratios of resin to hardener or by curing powder epoxies at improper temperatures. The flexibilities of epoxy coatings decrease inversely with their film thickness. Based on the present study, it is recommended that the maximum allowable

film thickness should be determined for each epoxy coating and in no case should it exceed

10 mils.

The importance of proper surface preparation cannot be over emphasized, as noted for

the rebars coated with material No. 28. A set of bars was sand blasted prior to being

coated and gave good performance in the bend test; while with the B set of bars the epoxy was applied to mill scale surfaces and these coated bars performed badly in the bend test. Similar results were obtained for Nos. 38, 39, and 40, when epoxies were applied to freshly

sand blasted surfaces and to surfaces both sand blasted and phosphatized; the phosphatized

sets were subject to varying degrees of cracking while the sand blasted bars were free of cracks

.

The drop test does not merely supplement the bend test; some coatings have good bend characteristics but have a poor impact rating, for example, No. 18, and the reverse

situation was observed for No. 22. Both the drop and tend test, therefore, should be included in future coating evaluation programs.

Some of the epoxies are of doubtful value as protective coating for reinforcing bars because of bad chipping characteristics when coated rebars were struck together, Nos. 2, 17,

32, 33 and 34 (table 6), and poor abrasion resistances, Nos. 10, 18, and 43 (table 6).

4.3 Corrosion Protection

The relative effectiveness of barrier organic coatings in protecting reinforcing bars from accelerated corrosion attributed to chloride ions can be associated with the following:

physical and chemical durabilities of the coating (discussed in the previous section); intrinsic chloride ion permeability rates; film integrity; film thickness and corrosion

inhibitors added to coating formulations [ 26, 27] .

31
The present study confirms the results of others [25] that epoxies absorb measurable

amounts of water and, therefore, thin epoxy films, ca. 2-10 mils, are not entirely impervious

to moisture. However, chloride ion permeability rates may be much lower than those of pure

water. Little if any data on the permeability rates of chloride ions through epoxy films have been previously reported. The data listed in table 4 and partially reproduced in figure

2, indicate a large range of permeabilities exist for epoxy coatings. Films of materials

Nos. 1, 3, 17, 19, 31, 38, 39 were essentially impervious to chloride ions, during the listed

exposure periods. Films of materials Nos. 13 and 15 permitted the accumulation of chloride

ions in the originally distilled water compartment of the cell (figure 1) to approach, or reach, the chloride ion threshold concentration of 0.02M, i.e. the chloride concentration

reported felJ to induce corrosion of steel embedded in concretes. At the present, unequivocal

interpretations of the chloride permeability values are not possible, however, epoxy films with low values would probably provide the best protection for reinforcing bars.

The film integrity of coatings on reinforcing bars is an important consideration since holidays are potential sites of corrosion. In general, the coated rebars with few or no holidays (table 6) had acceptable corrosion ratings (rating of either 1 or 2). Holidays can

be produced by solvent evaporation, poor flow characteristics of coatings, and mechanical

damage. Note that in table 6, films of all of the solvent-containing systems had significant amounts of holidays, regardless of the application method. Liquid epoxies had the ten- dency to flow off the bar deformations leaving the high lying region exposed. The powdered

epoxies, in many cases yielded films with few holidays and uniform film coverage. The large

numbers of holidays in the powdered epoxy films Nos. 28, 29, 39, and 42, can be attributed

to either poor coating practices or to low film thicknesses. Holiday-free films can also

be obtained by thick film buildups, however, the maximum permissible film thickness must be consistent with good structural and flexibility characteristics.

Corrosion inhibitors can prevent the enlargement of small corrosion sites [27] . The apparent healing mechanisms previously noted for films Nos. 25 (figure 4) and 31 (figure 5) possibly, are associated with the presence of such corrosion inhibitors.

5. RECOMMENDATIONS AND CONCLUSIONS

Any recommendations concerning what type of coating to use, application methods, etc., must take into consideration the site of coating application; in the field or in an application plant. Bridge-construction-site application essentially limits the range of coating

systems to those of the liquid type and restricts their application to the less desirable methods such as air spray, brushing or dipping. It is our opinion that in-plant application,

soon after the reinforcing bars are fabricated, would yield the best quality of coated reinforcing bars.

The powder epoxies have, potentially, slightly better overall properties as barrier

coatings for reinforcing bars than the liquid epoxies; within the liquid epoxy system the solvent free generally perform better than the solvent containing systems. However, in order

for any coating to perform well, good application techniques are necessary.

32
Epoxy coatings should be applied to sand blasted steel surfaces as both phosphatized and mill scale surface are brittle and modest mechanical forces can cause disbondment.

The effectiveness of the application methods in producing thin films free of defects decrease in the sequence: electrostatic spray gun> fluidized bed> dipping -^ brushing.

33
6 . REFERENCES

1. Durability of Concrete Bridge Decks: A Cooperative Study, State Highway Commission of Kansas, Bureau of Public Roads and Portland Cement Association. Report 1 (1965).

2. Durability of Concrete Bridge Decks: A Cooperative Study, Michigan State Highway

Department, U. S. Department of Commerce, Bureau of Public Roads and Portland Cement

Association. Report 2 (1965).

3. Durability of Concrete Bridge Decks: A Cooperative Study, U. S. Department of Transportation, Bureau of Public Roads and Portland Cement Association. Report 3 (1967).

4. Durability of Concrete Bridge Decks: A Cooperative Study, Missouri State Highway

Commission, U. S. Department of Transportation, Bureau of Public Roads and Portland

Cement Association. Report 4 (1968).

5. Durability of Concrete Bridge Decks: A Cooperative Study; California, Illinois, Michigan,

Minnesota, New Jersey, Ohio, Texas, and Virginia Highway Departments, U. S. Department of Transportation, Bureau of Public Roads and Portland Cement Association. Report 5 (1969).

6. Durability of Concrete Bridge Decks: A Cooperative Study; California, Illinois, Kansas, Michigan, Minnesota, Missouri, New Jersey, Ohio, Texas, and Virginia Highway Departments,

U. S. Department of Transportation, Bureau of Public .Roads and Portland Cement

Association. Final Report (1970).

7. T. D. Larson, P. D. Cady, and J. C. Theisen, Durability of Bridge Deck Concrete, Report

No. 7, College of Engineering, The Pennsylvania State University, April (1969).

8. R. F. Stratfull, The Corrosion of Steel in a Reinforced Concrete Bridge, Corrosion, 13 , 173t (1957).

9. J. D. Mozer, A. C. Bianchini and C. E. Kelser, Corrosion of Reinforcing Bars in Concrete,

J. Am. Concrete Inst. Proceedings, bl, 909 (1965).

10. J. E. 0. Mayne, J. W. Menter and M. J. Pryor, The Mechanism and Inhibitions of the Corrosion of Iron by Sodium Chloride Solution, Part I, J. Chem. Soc, 3229 (1960).

11. F. N. Speller, Corrosion - Causes and Prevention, 3rd ed, McGraw-Hill Book Co., New

York, N. Y. , 1951.

12. K. S. Frazier, Value of Galvanized Reinforcing in Concrete Structures, Materials

Protection 4, 53 (1965).

13. I. Cornet and B. Bresler, Corrosion of Steel and Galvanized Steel in Concrete, Materials

Protection, 5_, (1966).

14. D. F. Griffin, Effectiveness of Zinc Coating on Reinforcing Steel in Concrete Exposed to

a Marine Environment, Technical Note N-1032, U. S. Naval Civil Engineering Laboratory,

Port Hueneme, (1969).

15. C. E. Bird and F. J. Strauss, Metallic Coatings for Reinforcing Steel, Materials

Protection, _6, 48, (1967).

16. A. B. Tripler, E. L. White, F. H. Haynie and W. K. Boyd, Methods for Reducing Corrosion

of Reinforcing Steel, NCHRP Report 23 (1966).

17. J. R. Castleberry, Corrosion Prevention for Concrete and Metal Reinforcing in the Construction Industry, Materials Protection, _7, 21 (1968).

18. R. C. Robinson, Design of Reinforced Concrete Structures for Corrosive Environments,

Materials Performance, ^,15 (1972).

34
19. Steel Structures Painting Council Surface Preparation Specification SSPC-SP5-63, amended 1971.

20. W. R. Pascoe, Plastic Coatings for Metals, Modern Plastics Encyclopedia Issue, McGraw-Hill, Inc., New York, N. Y. , 1968.

21. D. A. Hausmann, Steel Corrosion in Concrete, Materials Protection, 6, 19 (1967).

22. Tentative Method of Test for Impact Resistance of Pipeline Coatings (Falling Weight

Test), ASTM Designation G14-69T.

23. Standard Method of Test for Resistance of Transparent Plastics to Surface Abrasion, ASTM Designation D1044-56.

24. Tentative Method of Test for Cathodic Disbonding of Pipeline Coatings, ASTM

Designation G8-69T.

25. H. Lee and K. Neville, Handbook of Epoxy Resins, McGraw-Hill Book Co., New York, N. Y. 1967, pp. 6-45 to 6-52.

26. N. E. Hamner, Coatings for Corrosion Protection, Chapter 14, in NACE Basic Corrosion

Course, A.- des. Brasunas and N. E. Hamner, ed. , National Association of Corrosion

Engineers, Houston, Texas, 1970.

27. N. Hackerman and E. S. Snavely, Fundamentals of Inhibitors, Chapter 9, in NACE Basic

Corrosion Course, A. des. Brasunas and N. E. Hamner, ed., National Association of Corrosion Engineers, Houston, Texas, 1970.

r U. S. GOVERNMENT PRINTING OFFICE : 1973—511-325/228

35
FORM NBS-U4A 11-71) 2. Gov't Accession No.

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Nonmetallic Coatings for Concrete Reinforcing Bars. Coating Materials

5. Publication Date April 1973

6. Performing Organization Code

7. AUTHOR(S) James R. Clifton, Hugh F. Beeghly, Robert G. Mathey

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10. Project/Task/Work Unit No. 4616433

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16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) This work was undertaken in order to determine the feasibility of using organic

coatings, especially epoxies, to protect steel reinforcing bars embedded in concrete

from accelerated corrosion attributed to the depassivation of steel by chloride ions.

Coatings have been evaluated on the basis of their chemical and physical durabilities

as well as their protective qualities. In this study, attention has also been

directed to the following: application methods; surface preparation of the steel

reinforcing bar; and site of application.

17. key words (Alphabetical order, separated by semicolons) Chlorides; concrete; corrosion; epoxy

coatings; organic coating; steel reinforcing bars.

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