Khanna A.S. (Ed.) High-Performance Organic Coatings: Selection, application and evaluation

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Organic coatings for concrete 297

other concrete ingredients as a preventative measure for new construction

or repair work. Migrating inhibitors are used in mature reinforced concrete

structures that are showing signs of reinforcement corrosion or need addi-

tional protection to inhibit future corrosion [6]. They are applied to the

surface as liquids or solid ‘plugs’, inserted in drilled holes, which migrate

through the concrete to the reinforcement.

14.10 Cathodic protection

the potential of the steel more negative, i.e. the potential of the metal is

lowered from active corrosion potentials to immunity or passivity potentials

[3]. In this way, all anodic or corroding areas of the metal are made cathodic,

thereby stopping the corrosion process. When the cathodic protection

system is energized, the O

reaching the steel surface is reduced to OH

−

ions

−

thus eliminating one of the main mechanisms of corrosion. Cathodic protec-

an electrolyte to the metal (which acts as a cathode), using an external

d.c. current source (impressed current source) or connecting the steel to a

corrosion rate becomes negligible is the ‘protection potential’ [3, 4]. The

quality of concrete and the corrosive species dictate the protective potential

required to stop corrosion of steel bars in concrete.

Cathodic protection of steel is considered effective when the steel poten-

tial is shifted into the range of −85 mV

SCE

to −105 mV

SCE

[3]. However, in

the case of steel in concrete, the situation is more complex. A −300 mV

SCE

shift in the negative direction from its original potential is a widely accepted

criterion for cathodic protection. Another criterion is a 100 mV shift of the

instant potential on switching off the current, which in many cases indicates

complete cathodic protection [3]. The potential instantly measured in the

‘off’ position is called the ‘depolarization potential’, though the measure-

ments may be adversely affected by high chloride contents [3].

Summing up, experience is the best guide in selecting the criteria for

cathodic protection as it is strongly affected by quality of concrete, water

saturation level, degree of contamination and waker to cement ratio.

14.11 Protective coatings

As the name indicates, protective coatings aim at protecting the substrate/

material when applied in a thin continuous layer to that substrate/material.

Protective coatings are a viable method for extending service life while

Cathodic protection is defined as the reduction or elimination by making

and the alkalinity generated restores the passive film on the metal. As a

Zn, Mg or Al anode (sacrificial anode system). The potential at which the

consequent of the current flow, Cl ions are removed from the steel surface,

tion is achieved by generating a direct current flow from an anode through

298 High-performance organic coatings

providing improved appearance and additional value to concrete struc-

tures. Coatings protect concrete façades by providing a barrier between the

concrete and the aggressive elements of its environment. These protective

coatings protect a concrete structure by acting as either water barriers,

carbon dioxide screens or chloride screens.

Concrete coatings may be either coatings directly applied to the concrete

surface or coatings applied before casting on the rebar surface itself. The

property of the coating material is decided according to the type of environ-

ment present. The criteria for coating selection are resistance from water

ingress, resistance from chloride ions, CO

diffusion resistance, UV light

resistance, elastic/crack bridging abilities, chemical resistance, abrasive

resistance, ease of application and aesthetic appearance [4].

14.12 Concrete coatings

A major difference exists between a steel bar and the concrete surface and

[4]. Some of the important essentials for a concrete coating are:

• Elastic enough to withstand the dynamic loading in the case of bridges,

etc.

• Impermeable to moisture and pollutants

• Aesthetics (important in the case of monuments and buildings)

• Suitable for use in the food chain (not toxic to animals or humans in

any way).

use:

• Preservation – maintain original state and appearance

• Protection – crack bridging; mold, fungi and algae resistance; chloride

and carbon dioxide permeability resistance; alkali resistance

• Decoration.

Coatings for concrete must have adequate innate adhesion characteristics

and not depend on the so-called ‘porosity’ of concrete to ‘gain adhesion by

penetration’ [7].

For concrete coatings a minimum of two coats, each coat having a dry

considered essential for an adequate period of longevity before the coating

starts to disintegrate [7].

Some organic resin-bound coatings ‘dry’ or are hardened by means of

a co-reactant, i.e. at least two ‘resins’ or components are mixed together

prior to application. This co-reaction, like the reaction of inorganic cement

hence the requirements of coating specifications for both are also different

Concrete coatings can be classified as follows, based upon their end

film thickness (dft) of 100 μm, i.e. a total dry film thickness of 200 μm, is

Organic coatings for concrete 299

with water, forms a ‘glue’ or coating with exothermic heat being produced

in the chemical crosslinking action. Two-pack epoxy, polyurethane (‘ure-

thane’ for short), polyester, 100% solids two-pack acrylic resin coatings

and high-performance cementitious coatings all fall into this category

[7].

absorbent lime-bound cement renders is the silicate type. This type is avail-

able in colors and sometimes as a transparent clear coating. Silicate paints

to result in excellent adhesion [7]. Although invented in 1895 in Germany,

it is doubtful whether this type has yet been tested for pull-off tests on

concrete substrates. Some types may even be quite suitable for concrete,

particularly for uncured porous existing concrete.

Rubber has very high adhesive qualities and typically demonstrates high

adhesion relative to other polymeric materials [7]. Suitable synthetic rubbers

can be co-polymerized with acrylic monomers to manufacture acrylic–

rubber co-polymer binding resins for concrete coatings.

Acrylic rubber co-polymer resins available to coatings manufacturers

may be used to make excellent clear and colored single-pack coatings for

concrete. These feature higher adhesion with similar or often superior

durability and wear resistance compared with single-pack coatings bound

with ‘straight’ (100%) acrylic or styrene–acrylic co-polymer resins.

All concrete coatings perform better when appropriately selected accord-

14.13 Galvanic coatings

Hot-dip galvanized rebar is simply bare steel that is coated with zinc. Hot-

dip galvanizing produces a tough and adherent coating on steel which

resists abrasion and fairly heavy handling, and which can be stored, handled

and transported in much the same way as black steel [8].

Although zinc can be applied to steel by a number of commercial

processes, each producing a characteristic range of thickness and coat-

involves the immersion of the steel bars in molten zinc at about 450°C

lurgically bonded coating of zinc and zinc–iron alloys on the base steel [9].

steel products greater than 5 mm thick is 84 μm, equivalent to a coating

mass of 600 g/m

per surface [9]. In routine processing, however, hot

dipping results in coatings that are generally at least 100–120 μm thick

[9].

Another very durable film-forming coating type suitable for hardened

chemically bond to lime-bound rendered/applied mortar finished surfaces

ing to the specific requirements.

ing structure, hot dipping should always be specified. Hot dipping

and holding for a sufficient period to allow the development of a metal-

According to AS/NZS 4680, the minimum specified coating thickness on

300 High-performance organic coatings

Zinc is an amphoteric metal that reacts with both strong acidic and strong

basic solutions, the attack being most severe below pH 6 and above pH 13.

At intermediate pH ranges, the rate of attack on zinc is very slow due to

the formation of protective surface layers. When embedded in concrete,

zinc is passivated for pH values between about 8 and 12.5, again due to the

insoluble below pH 12.5 [9]. While zinc reacts with wet cement, this reaction

effectively ceases once the concrete has hardened and the barrier layer of

calcium hydroxyzincate has formed.

The corrosion protection afforded by galvanized rebar in concrete is due

stantially higher chloride threshold (2–4 times) for zinc coatings to start

corroding compared to uncoated steel. In addition, zinc has a much greater

pH passivation range than steel, making galvanized rebar resistant to the

pH-lowering effects of carbonation as the concrete ages. Even when the

zinc coating does start to corrode, its corrosion rate is considerably less than

that of uncoated steel.

including zinc oxide and zinc hydroxide. A unique feature of these corro-

sion products is that they are friable (loose, rather than blocky) minerals

and they migrate well away from the bar and into the adjacent concrete

cause very little disruption to the surrounding matrix and this helps to

maintain the integrity of the concrete itself.

The rationale for the use of galvanized steel in concrete centers on

the concept that the coating provides a safeguard against early or unex-

pected corrosion of the reinforcement. The coating itself should not

be used as the primary or sole means of corrosion protection; rather it

should be used in conjunction with an adequate cover of a dense imper-

meable concrete suited to the type of structure and the exposure conditions

[9].

However, there is always a concern regarding the performance of galva-

nized coatings when they come into contact with a highly basic concrete

environment, i.e. an environment with pH > 13 [10]. At about pH = 13.3,

the zinc coating dissolves quickly in an active state with the evolution of

hydrogen until the coating disappears totally [10]. In such cases, a minimal

value, the rate of corrosion of zinc seemed to increase approximately with

increasing pH value and protection is ineffective in concrete containing

higher chloride contents.

Thus, in cements with low alkali contents, protection against corrosion

of galvanized coatings has been successful, but in cements with high alkali

contents the performance results are unsatisfactory [10]. Thus, performance

formation of a protective surface film of corrosion product that is relatively

to a combination of beneficial effects. Of primary importance is the sub-

A number of minerals have been identified in the corrosion products,

matrix where they fill voids and micro-cracks. As a result, these products

lower chloride content is sufficient to initiate corrosion. Above this pH

Organic coatings for concrete 301

of galvanized rebars in concrete has mostly led to contradictory results in

the literature.

14.14 Epoxy coatings

The organic material that has developed into that used most commonly as

a protective coating for steel reinforcement is epoxy resin coatings. Epoxy

resin is a thermosetting plastic that is resistant to solvents, chemicals and

water. Research has shown that defect-free epoxy coatings resist the diffu-

sion of chloride and oxygen, but are not impermeable to moisture [11].

They impart mechanical properties such as high ductility, low shrinkage

during curing, and heat resistance, which make them desirable for use as a

mately 1.2 and withstand temperatures of 125°C [11].

Epoxy coatings for reinforcing steel (ECR) are generally prepared and

applied by one of two methods: two-part liquid or powder.

14.14.1 Liquid epoxy coatings

Liquid epoxy is commonly prepared by mixing stoichiometrically balanced

proportions of a two-part system, epoxy resin and polyamine [11]. The

liquid epoxy is applied by brushing or spraying, or by immersing the bars

observed damage on reinforcing bars prior to concrete placement. The

liquid coatings commonly contain solvents that, upon evaporation, may

result in pore formation within the cured coating. However, shrinkage due

to solvent loss and curing is also a concern with the use of liquid epoxy

coatings [11].

IPNet RB is a two-pack epoxy/phenolic-based interpenetrating polymer

coating system (source: Krishna Conchem Products Pvt. Ltd, Mumbai,

India). It is a novel type of polymer formulation consisting of more than

two polymers crosslinked in a network form [12]. This interpenetrating

coating system has high crosslinking density and low MTVR (moisture

vapor transmission rates). Studies have shown that the performance of

IPNet was poor in terms of bending, corrosion resistance and bond strength

[13]. However, these properties were shown to improve on addition of

polyvinyl butyral (PVB) [13]. PVB is an important member of the polyvinyl

acetol family and its important coating properties are excellent adhesion to

cation has shown tremendous improvement in properties and has proved

IPNet to be compatible with FBE coatings [13].

reinforcing steel coating. Most epoxies have a specific gravity of approxi-

to be coated. The former method is most appropriate for field dressing of

to water and other chemicals. The modified IPNet coating has shown good

corrosion resistance under various surface pretreatments [13]. The modifi-

metals, film formation property, abrasion resistance, flexibility, resistance

302 High-performance organic coatings

14.14.2 Fusion bonded epoxy coatings

The most commonly used fabrication method for ECR involves fusion

bonding of epoxy. This is a process in which epoxy powder is applied by

electrostatic spray on hot steel at a preset temperature level. The electri-

cally charged powder particles are attracted to the preheated steel, and melt

coating (Fig. 14.2).

FBE coatings are ideally suited for protective coating on the complex

applied, the coating quality could be well maintained.

The epoxy coating cures through a combination of physical and chemical

curing processes. The physical process involves the melting and sintering

of powdered constituents at elevated temperature, to form a continuous

fuse, a reactive cure is also initiated, where a polymer network is formed

from the poly-addition reaction between components in the thermosetting

powder. Curing time, shrinkage and pore development within the coating

solvents.

As per ASTM A 775-81, the thickness of epoxy powder coating in order

Blast clean

Preheat

FBE application

Cure time

Quench

Electrical

inspection

Stockpile

Induction

heat

Final surface

treatment

Grind surface

defects

Surface

inspection

14.2 Electrostatic coating process.

to form a continuous film, and then the system is quenched to form a solid

surface configuration of a reinforcing steel bar. Electrostatic deposition

specifications. Rapid gel sets the coating quickly and, since they are plant-

film from the separate powder particles. As the powder particles melt and

are significantly reduced, since powder epoxy coatings do not require

combined with controlled melt and flow can attain uniform coating to any

to fulfill flexibility, bonding and corrosion protection requirements should

Organic coatings for concrete 303

be between 130 μm and 300 μm [10]. Integrity of the coating is essential for

cracks and damaged areas.

The use of epoxy coatings free of essential defects guarantees complete

rioration of reinforced concrete. However, studies have reported that epoxy

lose their stability at temperatures of about 200°C [10]. The bond between

epoxy and substrate may be affected by softening and melting of the coating.

It has also been reported that epoxy coatings result in reduction in bond

strength and increasing slip between reinforcing bars and concrete in com-

parison to uncoated rebars, with these effects increasing with increasing

thickness [10].

One of the biggest limitations of FBE coatings is their susceptibility to

mechanical damage during handling, transportation and storage and to the

damage occurring when coated bars are intentionally bent (after being

These are designed through the addition of microencapsulated materials,

resulting in a self-healing FBE coating. The mechanical damage which

weakens conventional coatings instead ruptures the microcapsules which

in turn ‘heal’ the FBE coating, preserving its barrier properties [14].

Microencapsulation is a technique that encapsulates materials, such as oils,

within a seamless, solid shell. A wide variety of shell wall materials are

available – the appropriate choice is determined by a combination of the

application, the material to be encapsulated, and the desired stimulus to

rupture the capsule (e.g., impact, pH, or solution chemistry). In general,

microcapsules are between 5 μm and 200 μm in size with wall thicknesses

of the order of 1 to 2 μm [14]. Fill materials may be either aqueous or non-

to a coating system without adversely affecting its properties.

Thus, upon damage, unlike in a conventional FBE, the microcapsules are

ruptured, releasing their protective chemistry (Fig. 14.3). As a result, the

formance of the coating in the damaged state as compared to conventional

FBE materials [14].

and tested in our laboratory, is the use of Dual Fusion Bond Epoxy (DFBE).

The DFBE consists of a two-coat system applied by a powder coating

method. The primer layer consists of a FBE (∼100 μm). This is followed by

effective corrosion protection. The film must therefore be free of pores,

protection in carbonated concrete and significant reduction in rate of dete-

coatings may have a finite tolerance limit for chlorides. Also, epoxy coatings

14.15 Modified FBE coatings

coated) to form a structure. Modified FBE coatings provide the solution.

aqueous in nature. Once encapsulated, liquid-based fill materials behave as

a solid, and as a result larger quantities of the fill material may be added

damaged FBE coating is healed, thereby significantly improving the per-

Another modification of FBE, which has been carried out by DuPont

304 High-performance organic coatings

another layer of FBE with a different composition (mixed with silica). The

primer layer has strong cathodic disbondment resistance while the outer

layer is hard and rough. The latter helps to protect the coating from damage

during handling and transportation and also in enhancing its bonding with

the concrete, which when tested in our laboratory was found to be 125%

compared to that of bare bar and 84% in the case of single-layer FBE.

Unfortunately, there is still no practical experience to date [15].

14.16 Cement polymer composite coatings (CPCC)

The CPCC technology has been developed by CECRI, Karaikudi, India. It

evolved and has been custom designed from consorted inputs originating

from polymer science and electrochemistry (corrosion is an electrochemical

reaction) to meet the structural engineering requirements of steel rein-

forcement rods. CPCC is a specially formulated indigenous technology only

for reinforcement steel and pre-stressing steel and cables. CPCC is a two-

coat polymer-based cement incorporating a coating system. The coating

thickness required is around 150–175 μm and this coating can be applied

by brushing, dipping or any other suitable method [16].

with the water repellency characteristics of quinine-based polymers, are

carefully tailored through the formation of a single phase in the polyblend

which offers the necessary mechanical and physical properties in the coating.

Newly sealed

layer

Self healing

Damage

14.3 Schematic illustrating the function of a single primer layer

containing corrosion-preventative microencapsulated additions.

Besides, this precursor has the possibility of accepting rust as filler in the

polyblend film, which facilitates the application of a surface coating derived

The flexibility of acrylate copolymer, its film-forming capability along

Organic coatings for concrete 305

from this polymer precursor or rusted steel surface, thus making surface

preparation non-critical. Further, the cement polymer composite based on

these polymers is expected to serve as a precursor for the formulation of a

topcoat which will take care of the alkaline environment that is anticipated

in the concrete structure. An additional feature of this system is that the

requirement.

The highlight of this particular system is its tendency to keep the rein-

forcement steel in its natural passive environment and to maintain its

natural pH. This is equivalent to the nature and environment of uncoated

steel in concrete. The reason is the presence of cement blended with the

steel in its natural passive environment, which is necessary to prevent cor-

rosion, and also it hydrates along with the curing of concrete and keeps the

bond strength intact [16].

Even in the case of localized corrosion initiation due to damage or

pinhole, the corrosion is not allowed to progress due to the presence of

primer, which acts as a buffer and stabilizes the pH and arrests corrosion

[16].

14.17 References

1. Girish A. Bhagwat, ‘Concrete deterioration: causes and its prevention’, Paint

India, August 1999.

2. G. K. Glass, N. R. Buenfeld, in Reinforced Concrete – The Principles of its

Deterioration and Repair, edited by S. MacDonald, Donhead Publishing,

Department of Civil Engineering, Imperial College, London, 1996.

3. Zaki Ahmad, ‘Concrete corrosion’, Chapter 12 in Principles of Corrosion

Engineering and Corrosion Control, Elsevier Publications, 2006, pp. 609–645.

4. A. S. Khanna, ‘Paint coatings for rebars, protective coatings and linings for

corrosion protection’, Course Study Material, NACE International India

Section, 2000, pp. 112–141.

5. ‘Admixtures’, www.fhwa.dot.gov/infrastructure/materialsgrp/admixture.html.

6. Richard Stehly, Jessica Jackson Meyer, Alla Furman, Marlin Hanson, ‘Active

coating systems that minimize corrosion of engineering systems’, ICORR,

2003.

coatings intended for concrete surfaces’, www.abilityproducts.com.

8. S. R. Yeomans, ‘Performance of black, galvanized, and epoxy-coated reinforc-

ing steel in chloride-contaminated concrete’, Corrosion, 50(1), 1994, pp. 72–81.

9. S. R. Yeomans, ‘An overview of the use of galvanized reinforcement in concrete

construction’, MaSTEC 2003, Hong Kong, 15–17 January 2003, www.hkpc.org/

hkiemat/mastec03_notes/46.pdf.

10. Ulf Nürnberger, ‘Supplementary corrosion protection of reinforcing structures’,

Otto-Graf Journal, Volume 11, 2000.

curing time of the coating can be manipulated to suit the specific process

polymer resin. Cement as an active filler serves dual purpose: it keeps the

7. Robert F. Barber, LSCAA, ‘Some important requirements for film-forming

306 High-performance organic coatings

11. Michael Carey Brown, ‘Corrosion protection service life of epoxy coated rein-

forcing steel in Virginia bridge decks’, PhD dissertation, University of Virginia,

3 May 2002.

rebars for reinforced concrete structures under various conditions of surface

treatments’, M. Tech Dissertation, Corrosion Science and Engineering, IIT

Mumbai, India, 1999.

13. A. S. Khanna, K. S. Sharma, A. K. Sinha, Emerging Trends in Corrosion

Control: Evaluation, Monitoring, Solutions, Volume I, Akademia Book

International, New Delhi, 2000, pp. 519.

14. D. G. Enos, J. A. Kehr, C. R. Guilbert, ‘Improving the damage tolerance, and

extending the service life of fusion-bonded epoxy coatings’, paper no. 01643,

Corrosion, 2001, downloadable from www.nace.org/nacestore/department.asp?

ID=286.

15. A. S. Khanna, Dual Fusion Bond Epoxy, Evaluation and Testing Report, IIT

Bombany (unpublished).

16. Technology Information, Forecasting and Assessment Council, ‘Cement

polymer composite coating system for corrosion protection of reinforcing and

prestressing steels’, www.tifac.org.in.

12. Riju Bhargav, A. S. Khanna, ‘Development of modified epoxy coatings for