Protection of Steel Reinforcement for Concrete

PROTECTION OF STEEL REINFORCEMENT FOR CONCRETE - A REVIEW

Vinod Kumar

R&D Centre for Iron & Steel, Steel Authority of India Limited RANCHI - 834 002 (BIHAR) INDIA

CONTENTS

1. Introduction 2. Reinforcement Corrosion 3. Mechanism of Reinforcement Corrosion 4. Preventing Reinforcement Corrosion

4.1 Corrosion Resistant Rebars 4.1.1 Micro Alloyed Rebars 4.1.2 Low Alloy Rebars 4.1.3 High Alloy Rebars 4.1.4 Dual Phase Rebars

4.2 Protective Coatings 4.2.1 Metallic Coatings

4.2.1.1 Sacrificial Coatings 4.2.1.2 Passive Film Forming Coatings

4.2.2 Organic Coatings 4.2.2.1 Epoxy Coating 4.2.2.2 Polyurethane Coating 4.2.2.3 Polymer Resin Coating

4.2.3 Cement-Slurry Coating 4.3 Cathodic Protection

4.3.1 Anode Cathodic Protection 4.3.2 Impressed Current Cathodic Protection

4.4 Electrochemical Removal of Chloride Ions 4.5 Concrete cover

4.5.1 Crack Width 4.5.2 Mix-Ratio 4.5.3 C170H- Ratio

317 Unauthenticated | 117.206.15.138Download Date | 11/20/12 9:22 AM

Vol. 16, No. 4, 1998 Protection of Steel Reinforcement for Concrete A Review

4.5.4 Additives 4.5.4.1 Inhibitors 4.5.4.2 Fly Ash

5. Concluding Remarks 6. References

remature failure of reinforced concrete structures occurs primarily due to early corrosion of steel reinforcement These reinforcements are prevented from corrosion by the formation of a passive oxide film in

the high pH environment of concrete. However, pH decreases in actual conditions due to the ingress of harmful ions like carbon dioxide, sulphur dioxide, chloride, etc., leading to localised corrosion. Of these, chloride ions are considered to be the most harmful, which may come from the deliberate additions of calcium chloride in the cement as accelerator or from the environment (marine/coastal regions, de-icing salts). The corrosion products thus formed, being more voluminous, result in expanded pressure in the surrounding concrete, leading to cracking of the concrete. The formation of such cracks accelerates further corrosion.

In view of the intensity of the problem, serious efforts have been made to prevent corrosion of reinforcement These have led to the development of varying types of prevention techniques/measures including coatings, electro-chemical techniques like cathodic and anodic protection, inhibitors, control of concrete mix and reinforcement with superior corrosion resistance. These preventive techniques have inherent advantages and disadvantages with respect to the type of structure and the surrounding environment. It is generally agreed that good concrete cover in terms of quality of ingredients, mix proportion, water-cement ratio, etc., has no substitute for the prevention of reinforcement corrosion. However, corrosion takes place due to the presence of fine cracks in the concrete assisting in the ingress of harmful ions. This necessitates an alternative preventive technique/measure which can be selected based on the type of structure to be protected, life expectancy, nature of environment, and cost of application.

The present review provides an understanding of the process of reinforcement corrosion, its mechanism, different protection techniques being adopted or under development and their comparative analysis.

ABSTRACT


Vinod Kumar Corrosion Reviews

KEYWORDS

Reinforcement, concrete, chloride ion, corrosion resistant rebars, stainless steel, galvanized steel, organic coatings, epoxy coating, polyurethane coating, polymer resin coating, cathodic protection, electrochemical removal, mix-ratio, Cl'/OH" ratio, inhibitors, fly-ash

1. INTRODUCTION

Premature failure of steel-reinforced concrete structures has been one of the major problems confronting civil engineers. The reinforcement used to provide strength to the concrete structure, in most cases, is found to be the main culprit. This is primarily due to early corrosion of steel reinforcement A number of methods and materials have been developed or are being developed to prevent corrosion of reinforcement steel. This paper reviews the work carried out so far on reinforcement corrosion, its mechanism and prevention.

2. REINFORCEMENT CORROSION

The reinforcement bars used to strengthen concrete structures are prevented from corrosion by the formation of a thin, adherent passive oxide film in the high pH environment of concrete /1,2/. The high pH (usually 12.5 to 13.5) is achieved by the formation of calcium hydroxide during hydration of cement. This high basicity level is good enough to maintain and protect the oxide film for a sufficiently long time. However, the presence of carbon dioxide and moisture in the environment gives rise to the formation of a weak carbonic acid which reacts with the calcium hydroxide in the concrete mix and forms calcium carbonate, thereby lowering the pH and in turn its passivating ability /3/. Passivity may be lost if pH falls due to carbonation to the range of 10.5 - 11.2. Calcium carbonate deposits in the pres of the paste and, since it occupies a greater volume than the hydroxide, the initial effect is partially to block the pores and give a denser paste. However, if the reaction goes to completion, pore water pH is reduced to 8.3 and certain products of hydration in the paste, notably basic silicates, basic aluminates and basic ferrites, become unstable and may begin to decompose. The steel no longer



remains passive and with the availability of oxygen and moisture it is open to the corrosion process IM. However, it has been observed that lowering of the pH of the concrete cover due to carbonation is not beyond 0.5 inch and, therefore, if the cover thickness is less, this may destroy the passive film 151.

In addition to this, a more serious problem arises with chloride ingress, particularly in coastal regions, since even in concrete with low permeability, chlorides in the marine environment or sea water can penetrate at a much faster rate well beyond the usual depths of concrete cover within a time shorter than the life time of the structure /4/. Pitting corrosion caused by the chlorides is most harmful. The chloride ions are able to destroy locally the passivated film even in an uncarbonated concrete cover. A very small anodic surface is then facing a larger cathodic surface still coated with the passivating layer. These unfavourable surface conditions cause highly accelerated and deep corrosion penetration into the steel. Chloride is used in an intermediate chemical reaction but in the end it is not consumed and acts as a permanent catalyst in the corrosion process. Furthermore, the hydrochloric acid simultaneously formed reduces the pH value at the corrosion pit. If the steel reinforcement is corroded by chloride penetration, then no superficial remedial treatment is available at present. The only possible solution is to replace the steel or rebuild it after demolishing the element /4/.

Chloride ions may come from the deliberate addition of calcium chloride to the cement as an accelerator, or from the environment (the presence of chloride ions in marine/coastal regions or from de-icing salts). The combined effect of carbonation and the presence of chloride can provide a very corro-sive environment for the embedded steel. The corrosion products thus formed are more voluminous than the parent material /2/; their formation and deposi-tion on the bars results in expansive pressure in the surrounding concrete leading to cracking of the concrete. Obviously, the formation of such cracks accelerates further deterioration 161.

Other factors may also combine to aggravate this situation. For example, a reinforced concrete structure at a liquefied gas/sulphur plant in the Arabian Gulf deteriorated shortly after completion of the construction 111. Investiga-tions revealed that the primary cause for this was corrosion of reinforcing steel due to the presence of chloride available from marine salt combined with an increased level of industrial pollutants. Other contributing factors included the use of sulphite-resisting Portland cement, the presence of hair-line cracks in the concrete, the elevated water/cement ratio, and insufficient



concrete cover. This was aided by simultaneous action of multiple environmental factors, such as the high salinity of Gulf water with a conse-quent high-rate fallout of marine salts, high relative humidities and air temperature, unfavourable wind speed and direction, intense solar radiation, and corrosive ground conditions.

3. MECHANISM OF REINFORCEMENT CORROSION /8/

As discussed earlier, the reinforcement remains unattacked under a highly alkaline environment (pH >13 due to the presence of sodium and potassium hydroxide Ι9Γ). This is due to the fact that, at this pH, steel passivates in the presence of oxygen by the formation of a surface layer of Fe203, which limits corrosion rate to that required to maintain passivity by the following anode reaction HOL

2Fe + 60H" -> Fe203 + 3H20 + 6e~ (1)

The corrosion of reinforcement generally takes place by the formation of galvanic cells in which the parent metal acts as the anode. The cathode may be mill scale, epoxy coating, or the passive oxide film The anodic reactions leading to the dissolution of iron are:

Fe —> Fe2+ + 2e~ (2)

The rate of corrosion is affected by oxygen transport through the concrete cover to the steel where cathodic reduction

can sustain the corrosion process /11,12/. The foregoing reaction products lead to the precipitation of hydroxides according to

lA02 + H 2 0 + 2e~ -h> 20H" (3)

Fe"" + 20H" Fe(OH) : 2+

(4)

2Fe(OH)2 + V2O2 + H20 2Fe(OH)3 (5)



The hydrated oxides in concrete can convert to various oxide species, such as magnetite (Fe304), hematite (Fe203), and geothite (FeOOOH), leading to a complex system of compounds depending on pH, oxygen availa-bility, and the composition of rebars /13/. FeOOH is more adherent to metal substrates than Fe203 and Fe304 /14/, and, therefore, could slow the corrosion rate at the interface between the oxides and the metal matrix.

In buried or fully saturated reinforced concrete, oxygen availability is restricted. Oxygen access to the steel may become so limited that the passive current cannot be sustained and general depassivation occurs /15/. The rate of corrosion is then limited by diffusion of oxygen or the kinetics of water reduction

2H20 + 2e~ ->· 20H~ + H2 (6)

This type of corrosion is characterized by low potentials, where Ecorr is more negative than -800 mV (SCE). It can occur, even in well oxygenated waters, if the concrete is fully saturated with sufficient depth of cover /12/. At low potentials the anode reaction may /15,16/ be the formation of Fe(OH)2

Fe + 20H" Fe(OH), + 2e" (7)

Under strongly alkaline conditions the formation of soluble FeO(OH)" is feasible /16/, but only at low concentrations.

Corrosion in the Presence of Chloride l l l l

The degree of protection provided by concrete may be reduced when a reinforced structure is exposed to de-icing salts /18/, acidic gases /19/, or immersed in sea water 1201. Ingress of aggressive species, e.g. chloride ions or carbon dioxide, can change both the concrete and its pore solution chemistry, leading to depassivation of the reinforcement /10/, and, therefore, accelerating the corrosion where metallic Fe at the anode is oxidized to ferrous ions (Fe24):

Fe Fe :+ + 2e" (8)

Initially, this reaction is balanced by cathodic reduction of dissolved oxygen to hydroxyl ions (OH)":



0 2 + 2H 2 0 + 4e" 4(0H)" (9)

The Fe2+ combined with CP drive to the anode by the corrosion current and form the H 2 0 soluble ferrous chloride (FeCl2):

Fe2+ + 2C1" -> FeCI2 (10)

Some FeCl2 migrates out of the corrosion pit and reacts with the cathodically formed sodium hydroxide (NaOH) to produce a ring of a white precipitate of ferrous hydroxide Fe(OH)2:

FeCl, + 2NaOH = Fe(OH)2 + 2NaCl (11)

The Fe(OH)2 is soon converted to hydrated ferric oxide (Fe203.H20), also known as ordinary red-brown rust, and to black magnetite (Fe3O.0, followed by the formation of green hydrated magnetite (Fe304.H20):

4Fe(OH)2 + 0 2 -> 2H 2 0 + 2Fe 20 3 .H 20 (12)

6Fe(OH)2 + 0 2 ^ 4H 2 0 + 2Fe203 .H20 (13)

Fe 30 4 .H 20 -> H 2 0 + Fe304 (14)

A tubercle is formed at the pit orifice, which consists mainly of Fe304 and Fe203.H20. This tubercle hinders the replenishment of dissolved 0 2 into the pit interior and prevents intermixing of the electrolyte trapped inside the pit with the bulk solution resulting in local acidification. This type of localized attack, when coupled with a large passive area of reinforcement, can cause structural damage. Apart from strength loss, expansive stresses due to corrosion products may lead to cracking and spalling of the concrete /21/.

Ionic Diffusion

The diflusion of chloride ion through the concrete cover can be estimated by using Fick's second law of difiiision,



(15)

where c is the concentration of chloride ion in concrete at a distance, x, from the surface at a time, t, and ce is the concentration at equilibrium (10%). Diffusion coefficient, D, of chloride ion through sound concrete has been reported to be 10"7 ~ 10-8 cnm2/sec 1221.

For boundary conditions:

c = 0 at t = 0 (0 < χ < c = ce at χ = 0 (0 < t < ») (16)

Based on the above equations, over 10 years would be required for chloride ions to reach the rebar surface by diffusion through a concrete cover of 28mm, but experiments /23/ showed it to be only 0.25 year. This was possible because micro-cracks in concrete cover might have facilitated the diffusion of chloride ions by an order of magnitude.

Ionic and molecular transport in a hardened concrete will depend upon the physical characteristics of the mix /8/. For example, the diffusion rates for oxygen and chloride are reduced when porosity is decreased by lowering the water/ordinary Portland cement (w/c) ratio /11,12,24-26/. In sea water, the pore solution will lose hydroxide ions and acquire chloride ions through ionic diffusion, and thus it is only a matter of time before sufficient chloride reaches the steel and pitting occurs. Cement composition influences both the rate of chloride diffusion and the ratio of bound-to-unbound chloride /24-27/. Measurements of ionic diffusion and oxygen transport have concentrated homogeneous pastes under "fully water saturated" conditions, whereas most concrete placed in real structures is unlikely to be homogeneous /15/. Areas of reinforcing bar corrosion are often associated with cracks and voids /15/, since such defects provide preferred diffusion pathways for both reactants and products. Inhomogeneity can also result from local variations in the mix during placement and subsequent cement hydration.

Interfacial Layers (Cement/Environment) /8/

Surface layers can affect transport processes in concrete and cement; Gjorv and Vennesland suggested that a dense cement-rich layer on the cast

c/ce = 1 - erf(x / 2VDt) (17)



surfaces of concrete increases its resistivity /26/ and limits oxygen diffusion /11/ to a greater extent than an equal thickness of mortar. Seawater exposure causes precipitation of CaC03 (Aragonite) and Mg(OH)2 (Brucite) layers in the surface pores of hardened cement, which limit subsequent Mg2"1" ingress and increase resistivity /28,29/ by modifying the pore size distribution There is evidence that these layers may also reduce oxygen transport /15/ and may be beneficial in repairing cracks.

Interfacial Layers (Steel/Cement) /8/

Page III identified a layer of Ca(OH)2 (Portlandite) precipitated on steel cast in hardened cement and proposed that this could act as a barrier to chloride ions, thus protecting passive steels. Defects in this layer could lead to pitting /1,30,31/. However, even at low C170H- ratios in the pore solution, electro-migration of these ions can evolve an environment that is locally acidified and concentrated in chloride, which produces local depassivation 1321. The Ca(OH)2 in the hardened concrete appears to have a dual role, providing both a physical barrier layer and a reserve of alkalinity to resist local acidification

4. PREVENTING REINFORCEMENT CORROSION

Researchers from different disciplines have tried to develop different preventive methods based on the experience related to their field For example, metallurgists or materials scientists developed corrosion resistant rebars and metallic coatings, chemical engineers or chemists developed organic coatings, whereas civil engineers developed better understanding of concrete control. These efforts led to the development of a number of preventive methods which are quite different from each other, though each one of them is claimed to be the most effective solution to reinforcement corrosion by the respective investigator. Therefore, thorough analysis is needed of various preventive methods, their advantages and disadvantages.

Kumar 133/ observed that any preventive method in isolation may not be sufficient and a combination of preventive methods may be needed to protect the reinforcement from corrosion. He 1331 explained this by comparing a RCC (Reinforcement-Cement-Concrete) structure with a king's fortress where the king employs a number of security measures to protect his



kingdom. A combination of preventive methods may be selected based on the severity of corrosion and the extent of protection needed.

Since corrosion is an electrochemical process, it involve a cathode, an anode and a suitable environment The corrosion takes place at the anode and simultaneous reduction of hydrogen or hydrogen evolution takes place at the cathode (in the case of an acid solution) or hydrolysis of water giving rise to hydroxide ions (in the case of a highly alkaline solution). The rates of both the anodic and the cathodic process are always equal. The environment (electrolytic solution) provides a way for electron as well as metal-ion transfer. The rate of corrosion can be restricted by the absence of any of these three, or by regulating them

The corrosion of reinforcement can be prevented by

(i) use of corrosion resistant steel rebars, (ii) use of a protective coating on the rebar,

(iii) use of an electrochemical technique, and (iv) control of concrete mix.

Each of these methods is discussed in detail in the following sections.

4.1. Corrosion Resistant Rebars

In recent years, efforts have been made to develop corrosion resistant rebars to prevent reinforcement corrosion (Table 1). These developments have been based on (i) alloy additions and/or (ii) microstructural control. Production of these rebars does not require any additional facility / capital investment and manpower as in the case of various coated rebar and cathodic protections and, therefore, the cost of protection is much lower. Nor does this call for any design changes.

Various types of corrosion resistant rebars currently being used are discussed in the following sections.

4.1.2. Low-Alloy Steel Rebars /37-48/

Alloying elements like Cu, Cr, Ni, W, P, etc. are added for this purpose. Chromium has been the common choice by most of the researchers; however, other alloys have also been added along with chromium. This is due to the fact that Cr makes the iron oxide more dense and adherent These steels still rust, but under highly alkaline conditions the rust formed becomes adherent and protective so that corrosion becomes less rapid than with ordinary steels.



Table 1 Corrosion Resistant Reinforcements

Rebar Quality Alloying Elements Addition Reference s

Microalloyed Microalloy * * 34-36 steel Low-alloy steel Cr 2% 37

Cr + Cu 0.75% min. 38-41 Cr + Cu + Ρ 1.62% max. 42-44 Cu + W * * 45-4.65 Ni 3.5% 46,47 Ni + W 1.0-5.5%Ni; 48

0.001-0.5%W High-alloy steel Cr 12% 49

Stainless steel Ferritic 50 Austenitic 51-53

Al + Cr + Cu/Ni + 20.0-37.3% Al 54 Ti/Nb/Mo/V/W/Cb/B 5.50 -10.0% Cr 55 (Non-magnetic steel) 0.1 -5.5% Cu/Ni

0.01-0.5% Microalloy

Unalloyed Dual phase — 56

** Not available.

SAIL has also developed both cold twisted deformed (CTD) as well as thermo-mechanically treated (TMT) rebars /38-41/ having superior corrosion resistance compared to conventional rebars. These rebars belong to high strength grades (yield strength in excess of 415 MPa) of IS: 1786-1985. Microalloying may be done up to 0.3% maximum to achieve high strength. TMT rebars are weldable due to their low carbon equivalent The low-alloy steel rebars are a cheaper option because of low alloying additions. Shimada and co-workers /46-48/ have found that nickel additions are most useful for improving salt corrosion resistance.

4.1.3. High-Alloy Steel Rebars /49-55/

These rebars belong to the stainless steel category where alloy additions are made in excess of 12%. Both ferritic /49,50/ as well as austenitic stainless



1521 steels have been used for reinforcement purposes. Cochrane 1521 ob-served that only austenitic stainless steel (Type 304) was wholly serviceable in the high chloride concentration (3.2% chloride ion equivalent to 5% by weight of cement) whereas ferritic stainless steels (Types 405 and 430) were affected by the same chloride concentrations. McDonald et al. 1531 have shown that the use of austenitic stainless steel for the reinforcement of con-crete bridges would add between 6% to 16% cost to the structure. It is reported /54/ that stainless steel rebars, besides showing superior corrosion resistance properties, show only negligible reduction in tensile strength and yield strength when exposed for two hours at 600°C.

Non-magnetic steels having superior corrosion resistance have also been developed for use in various concrete structures, such as magnetic floating high-speed railways, nuclear fusion facilities and marine structures and appliances where non-magnetic properties are required in addition to corrosion resistance /55/.

4.1.4. Dual Phase Steel Rebars 1561

This steel consists of martensite and ferrite produced by controlled processing. The superior corrosion resistance has been derived based on the small potential difference between ferrite and martensite as compared to a large potential difference between ferrite and martensite giving rise to galvanic corrosioa

All reinforcements should be free from mill-scale or corrosion product. It is reported that the corrosion rate of originally rusted steel rebar was two times higher than that of pickled steel /57/. Novak et al. 152,1 have also studied the different states of surface of rebar (machined, scaled, prerusted) on the corrosion rate and found that the prerusted reinforcement embedded in concrete showed the highest and technically unacceptable corrosion rate under all conditions, including concrete without chloride content. The scaled surface was comparable to a rust-free machined surface, has a corrosion rate acceptable for low chloride contents (0.4wt%/cement) and is not suitable for high chloride concentration /58/. Rust/mill-scale can be removed by weather-ing, by shot or sand blasting or by pickling in inhibited acid solutions. The last method is preferred, but should only be undertaken by an experienced staff. Care must be taken that the acid is properly inhibited and that the correct concentration of inhibitor is maintained throughout the pickling period.

Callaghan 754/ has also suggested that, if we are to use stainless steel or



the 12% Cr steels, it is essential that all scale or scale formed before and during bar deformation be removed, i.e. they be pickled and passivated or abrasive-blasted. He /54/ found that the presence of mill scale or hot rolled scale resulted in early signs of corrosion in 12%Cr steel and led to the initiation of pitting corrosion in type 316 and type 304 steels. However, Al-Tayyib et al. 1591 have shown that the initial rusting does not have an adverse effect on the corrosion resistance of rebars embedded in concrete.

4.2. Protective Coatings

The use of coatings for corrosion protection of structures has already been presented in detail by Giudice et al. 1601. Coated steel reinforcements have been widely used in many areas, like moderate-to-severe exposures, such as marine and coastal construction, industrial plants, water treatment and chemical processing facilities, power generation facilities, and bridges and highways. Coated reinforcements are also used in less severe applications in building and construction for cast-in-place and precast elements. Different types of coatings suitable and unsuitable for rebar are included in Table 2 and are discussed in detail in the following sections.

4.2.1. Metallic Coatings

The development of metallic coatings was based on their role as regards corrosion in combination with the parent metal (mild steel, in general). In view of this, two broad types of coatings have been developed. These are (i) passive film-forming coatings, e.g. Si-based coatings, Cr-based coatings, and (ii) sacrificial coatings, e.g. galvanizing. The first option is based on a passive Film formation and its adherence to the rebar. It may lead to localized attack, if broken during service, transportation, fabrication, etc. It should be noted that the parent metal acts as an anode compared to the cathodic passive film. However, in the second option, corrosion prevention is based on a sacrificial anode compared to the cathodic underlying parent metal, and, therefore, even if the coating is broken due to the above-mentioned reasons, the metal remains protected. This has led to the wide-ranging research on galvanized steels /61-92/ compared to others /80,84,93-100/. A scrutiny of the potential values against SCE (Saturated Calomel Electrode) suggests that only cadmium and zinc can provide galvanic protection to the steel under normal environmental conditions (Table 3). In seawater, copper, nickel, tin and lead were cathodic to steel. Lead, tin, cadmium and zinc were anodic to steel as



Table 2 Different Types of Coatings for Steel Rebar 7137/

System Disadvantages Coatings not suitable Red lead Deterioration in alkaline medium Coal tar enamel Brittle, subject to cold flow, sticky Asphalt Subject to cold flow Phenolic Deterioration in alkaline medium Urethane Hard, brittle, intolerant to poor surface preparation Neoprene Poor bond Vinyl No concrete to vinyl bond A l u m i n i u m No electrical insulation effect, rapid corrosion in presence

of chloride Coatings suitable Zinc/cadmium Sacrificial, rapid corrosion in the presence of chloride; no

electrical insulating effect Nickel/copper Not suitable for chloride exposure, no electrical insulating

effect, galvanic corrosion Epoxy Hard and brittle. In many formulations will not bend or

stretch Many formulations have poor bond. Only powder epoxy suitable.

Chlorinated May bond to both concrete and steel rubber

Table 3 Potential Values Against SCE 783/

Metal Potential, V Metal

Normal Sea Water Mortar Mortar containing 1% NaCl

Nickel -0.484 -0.210 +0.040 -0.140 Copper +0.110 -0.210 -0.200 -0.200

Steel -0.674 -0.720 -0.180 -0.450 Tin -0.370 -0.490 -0.980 -1.010

Cadmium -0.636 -0.760 -0.870 -0.830 Lead -0.360 -0.510 -0.650 -0.760 Zinc - 0 . 9 % -1.060 -0.450 -0.795

long as pH remained strongly alkaline, but the rates of self-corrosion were relatively higher 19,01. These potential values can serve as reference for selecting a coating metal for a particular environment



4.2.1.1. Sacrificial Coatings

Corrosion prevention is based on a sacrificial coating which acts as an anode compared to the cathodic underlying parent metal. Therefore, even if the coating is broken/removed during service, transportation or fabrication, the metal remains protected.

Galvanized reinforcement has been in use since the 1930s and is used in many types of concrete constructions /61,62/. Because the steel is hot-dipped into molten Zn at « 460°C, galvanizing produces a metallurgically alloyed coating integrally bound to the steel substrate. The alloy layers are strong, resist mechanical damage, and afford sacrificial protection to the steel 1631. Zn is passivated in concrete and can remain passivated to lower pH levels (perhaps as low as pH 9.5) than that which causes corrosion of black steel (pH <~11.5). Galvanizing thus offers substantial protection against the effects of carbonation in concrete. Galvanized reinforcement also can withstand exposure to chloride ion concentrations several times higher than that which causes corrosion of black steel in concrete /64/.

Various studies, however, have not shown total agreement on the corrosion resistance of zinc in concrete environment as in other aggressive media. Mange /65/ found that galvanized steel showed bad behaviour in structures in contact with chlorides where water retention and temperature were aggravators of the corrosion process on reinforcement. Experience /66-68/ showed that the use of galvanized steel involves an improvement in durability of the structures when they are in contact with chlorides. Unz /69/ studied both galvanized and non-galvanized steel plates, totally or partially immersed in Ca(OH)2 saturated solutions with additions of NaCl, and found that galvanic currents produced in non-uniform exposure conditions lead to local accumulations of chlorides with intense pitting corrosion, independent of the existence of passivating films. Griffen /70/ observed that galvanized steel has no advantages when exposed to marine conditions. Ishiwaka /71/, however, found that galvanized steel resists higher chloride concentrations than bare steel in saturated Ca(OH)2 solutions with additions of NaCl from 0.2 to 2.0%. Other authors also supported this finding Z72-74/, though their respective results differ as to the minimum chloride concentration needed to provoke pitting corrosion. This lack of clarity is largely based on the fact that hardened concrete is a complete electrolyte for the study of metallic corrosion processes. Marias and co-workers /75-78/ have carried out an extensive study on the behaviour of galvanized steel in alkaline solutions, simulating an aqueous phase present in the concrete pores, and found that the galvanized



steel becomes passivated in contact with electrolytes whose pH value is < 13.3 ±0 .1 and that above this value the galvanized coating is completely dissolved until it totally disappears within a period shorter than 33 days. Similarly, it was confirmed that the passivation product of the galvanized steel in these media was calcium hydroxyzincate, Ca(Zn(0H)3)2.2H20 /75-77/. In another study, Marias et al. /78/ have confirmed that the corrosion behaviour of galvanized steel in the presence of chlorides is controlled by a medium pH, which depends on the calcium or sodium salt, CaCl2 or NaCl, which provides the chloride ions. The latter hardly alters the medium pH, but for CaCl2, the Ca2+ ions have a certain buffering effect and the pH value remains inside a narrow range around 12 /78/.

Treadaway /79/ found that galvanized reinforcement offers several advantages over black steel. In particular, the onset of corrosion is deferred, the risk of cracking and rust staining is reduced, variability in the concrete mass (such as depth of cover to the reinforcement and gross porosity) can be tolerated better, and long-term protection is provided to the reinforcement prior to its being embedded in concrete. Yeomans /80/ alsop studied the performance of black (uncoated), galvanized and epoxy-coated reinforcing steels in chloride-contaminated concrete and found that galvanized steel resisted CI" levels in concrete «2.5 times the level that caused corrosion of black steel. Zinc provided sacrificial protection for a period of »4 to 5 times that for initiation of corrosion of black steel in equivalent conditions.

Subramanian /84/, however, observed that galvanizing is not a substitute for good concrete technology. As far as bond strength of galvanized bars is concerned, it is equivalent to that of uncoated bars.

The useful life of a zinc coating largely depends on the thickness of the coat, but owing to the sacrificial protective action of the zinc, a large amount of coating can be lost before the underlying metal is attacked. With well-galvanized steel it is usual to consider that as long as 10% of the original coat remains, serious rusting will be delayed. This is due not only to the sacrificial action of the zinc coat, but also to the fact that if the steel is properly galvanized, a series of protective alloy layers forms at the iron/zinc interface, which remain protective after the zinc coat itself has eroded away. The nature and thickness of the alloy layers are influenced by the composition of the underlying steel and by the galvanizing conditions. The presence of carbon and silicon in the steel encourages the formation of iron/zinc alloys and thus leads to the formation of heavier coats. Steels with a high silicon content are often used when very thick coats of galvanization are required.



4.2.1.2. Passive Film Formine Coatings

As discussed earlier, this is based on a passive film formation and its adherence to the rebar. It may lead to localized attack, if broken during service, transportation, fabrication, etc. It should be noted that the parent metal acts as an anode compared to the cathodic passive film. These include: Si, Ni, Ti, Mg, Cd, Ti-Ni and Si-Ti which can be applied by fluidized bed chemical vapour deposition (FBR-CVD), paint-and-heat, or FBR-plasma spray techniques. Of these, the paint-and-heat process appears most suitable for commercial application /93/. Sanjuro et al. /95/ showed that the Si-coated samples indicate a higher corrosion resistance in aggressive chloride environment in comparison to fusion-bonded epoxy coated or galvanized bars. In addition, less expensive Si-coated samples resisted scratching 1921. They /94/ also showed that the best coatings were obtained when Si and Ti were co-deposited at temperatures around 550°C. These coatings increased the corrosion resistance by more than an order of magnitude with respect to the uncoated rebars. Tomlinson et al. /100/ have shown that Ni and electro-less nickel were very stable in the passive range and the electrolytically induced pitting is not considered important in service. Titanium was very corrosion-resistant Tin was found to be highly resistant to pitting and the small amount of dissolution can possibly be reduced, to a large extent, by alloying. Magnesium was not found suitable since it corroded readily. Although Cd had excellent corrosion resistance, it has problems of toxicity /100/.

4.2.2. Organic Coatings

Organic coatings include epoxy coatings, polyvinyl chloride, poly-propylene, phenolic nitrite, polyurethane, etc. All these coatings act as a barrier to the aggressive ions, moisture and oxygen and remain cathodic with respect to the steel. Of these, epoxy coatings have been most popular.

4.2.2.1. Epoxy Coating

Fusion-bonded epoxy coated reinforcement (FBECR) was developed between the late 1960s and the early 1970s /101/ in respect to severe rebar corrosion problems in bridge decks in North America, and was first used in 1973 for the same purpose /80/. Its use is now varied and widespread, and epoxy coating is clearly the dominant coated reinforcement in general use /102-108/. Epoxy coatings provide excellent corrosion protection to steel, and the coating is not consumed in performing its corrosion protection function



/109-111/. The coating has veiy low permeability to chloride ion, is highly chemically resistant, and is essentially unaffected in many environments. An epoxy coating on steel acts as a veiy effective barrier to aggressive agents, particularly chloride ions, which will not easily diffuse through a continuous epoxy coating. Hence the steel surface is protected.

Muthukrishnan et al. /83/ have shown that only epoxy coatings (both liquid and powder) were not affected when tested for one year in 3M CaCl2

aqueous solution, 3M NaOH aqueous solution, saturated Ca(OH>2 and CaS04.2H20 with and without 0.5M CaCl2.

Hartley /101/ has presented the protection of rebar by FBECR. Corrosion of uncoated reinforcement is attributed to the local loss

of the passive surface film locally when chloride ions penetrate through the cover concrete; alternatively, the local concrete alkalinity is reduced by carbonation of concrete. The exposed steel then corrodes as an anode driven by the relatively large cathode of surrounding passive steel. Pitting results and is referred to as macro-cell corrosion. The presence of epoxy coating on FBECR acts as a barrier to chloride ions and prevents corrosion.

Allowance must be made for some normal damage to the coating during assembly and concrete casting The steel can corrode at any hole in the coating, but the extent of corrosion is controlled by the surrounding intact epoxy layer which stifles the cathodic reactions and prevents macro-cell corrosion. The steel surface at the hole has to sustain both the anode and the cathode, and, because the cathode reaction is much less efficient than the anodic reaction, the corrosion of the steel in the vicinity of the hole is 10 to 100 times less than it would be for uncoated reinforcement

Despite the excellent corrosion performance of epoxy-coated reinforce-ment in most situations, the effectiveness of epoxy coatings in veiy aggressive environments has recently been questioned /113-115/, primarily because the coating has naturally occurring holidays /116/ and may be damaged during transportation and fabrication Consequently, a great deal of emphasis is placed on the careful handling and storage of epoxy-coated reinforcement to minimize abrasion and mechanical damage /117/. Touch-up of damaged areas of the coating is usually required. Sagues /116/ has also observed, after inspection of over 30 bridges, that the corrosion-free service life in these structures will primarily be the result of concrete quality and thick cover, and not will not necessarily be due to the use of epoxy-coated rebar.

FBECR is specified to improve resistance of concrete structures to



corrosion; however, bond strength is reduced by the smooth surface of the epoxy coating. Various authors have recommended reductions in design bond strength based on comparisons of performance of epoxy-coated and non-coated bars under identical conditions. The reductions are approximately 15 -25% /117/.

Recently, attention in the epoxy-coated rebar industry has focused on service in hot/wet environments. New specifications mandate quality control programs that include cathodic-disbondment testing /118/. To meet this requirement, a number of systems are now available, including Chromate and non-chromate surface treatments and coatings for prefabricated rebar. The factors affecting cathodic-disbondment are application temperature, thickness of the coating, steel surface preparation and contamination, weight of treatment applied, testing time, temperature and pH /118/. 4.2.2.2 Polyurethane Coatinz

This coating method has been developed to overcome disadvantages accompanying thermosetting type polymers like epoxy. These are a build-up of thermal stress in the polymer during curing, poor film-forming capacity and a marked tendency to form pinholes and pores causing non-conductivity in the film, differential shrinkage built up in the polymer during curing and early weathering which is aided by UV radiation, oxygen, heat and moisture /119/. A comparison of the performance of typical systems as regards their concrete resistance in various environments is given in Table 4. Pathare /119/ observed that polyurethane coatings can be devoid of the above defects if the proper choice is made. Also, there is the widest diversity of compositions of polyurethane which can be designed to suit every requirement Polyurethanes are grouped into six types according to the method of curing, as per ASTM D-16 classification Of these, type V is the best choice as given in Table 5.

Polyurethane coatings have particularly high resistance to abrasion and wear. This is the reason why they are especially recommended for floors where the usual types of laid floor coverings are unable to meet the require-ments of particularly severe stresses. A typical example is the simultaneous exposure to chemicals and abrasion that occurs in store rooms, warehouses, wet-therapy rooms, kitchens, etc. It is, however, essential that the concrete beneath the coating is strong enough to take the loads without crumbling /II9/.

Polyurethane coatings are considered to be bacteriostatic since they do not provide nourishment to bacteria In moist and warm conditions,



Table 4 Comparative Performance of Topical Systems on Concrete Resistance

to Various Environments / 119/ System Acid Alkal i Salts Solve

nts W a t e r Oxid

at ion UV

Radi-at ion

A lira sion

Heat Biodegr ada t ion

Acry l i c s 8 8 8 5 8 8 9 8 7 7

A l k y d s 6 6 8 4 7 3 ,8 ft 8 3

B i t u m i n o u s 10 7 10 Ί 8 4 3 4 5

Chlor ina ted R u b b e r

10 9 9 7 8 6 8 5 6 8

E p o x y 9 10 10 9 10 6 6 9 7 8

Epoxy ester 5 5 7 3 7 4 6 6 7 4

Phenol ic 10 "> 10 9 10 6 6 6 7 7

S i l i cones 4 3 6 6 8 8 9 7 10 9

Po lyure thane (a l iphat ic)

9 9 10 9 10 9 10 9 8 10

microflora form very quickly and spread by multiplication which is mani-fested as follows: (i) appearance is impaired, (ii) hygiene is reduced since fungus-contaminated areas can be dangerous sources of infection, (iii) toxic effect of metabolic products of fungi (mycotoxins, alfatoxins) even in very small concentrations, (iv) destruction of the wall coatings or coverings, and (v) destruction and rotting of the plaster and subsequent destruction of concrete /119/. With properly formulated polyurethane coatings, most of these effects can be eliminated. Therefore, polyurethane coating is the ultimate choice for clean rooms in the pharmaceutical, electronic, hospital and foodstuff industries. 4.2.2.3 Polymeric Resin Coating

The use of an airtight protective coating of polymeric resin on the existing reinforced concrete surfaces would inhibit carbonation/120/. The suppression of rebar corrosion involves cutting off the source of moisture followed by application of a highly durable waterproof coating of silane monomer, methyl-silicone polymer or acrylic rubber separately or cojointly on the reinforced concrete surface. Of these, silane monomer coating provided the most effective characteristics of water barrier and corrosion suppressant /120Λ


Vi nod Kumar Corrosion Reviews

Table 5 Characteristics of Various Types of Polyurethane Coatings 7119/

Type Description Remarks I Air drying • Oil urethane system

• Oil or alkyds dry by air oxidation. • Hardness of the film and film properties are

slightly better than conventional enamel paints.

π Single component • Drying alkyds of their isocyanate function at the end of the linear polymer chain and this isocyanate portion of the polymer picks up atmospheric moisture and the film dries by formation of polyurea.

• Improvements can be made by substituting oil or alkyds with diols or polyesters.

• May undergo drying reaction with traces of moisture under improper handling or storage much before the intended application and hence may fail to function.

• Unpredictable behaviour under humid tropic weather.

m One or two com-ponents heat cure

• Not suitable for concrete.

rv Two-component catalyst cure

• Solution of simple and linear polyurethane in solvent

• Film is formed by evaporation of solvents on application and, hence, not advisable as a protective system on concrete.

V Two-component polyol/isocyanate cure

• Similar to type IV which need mixing just before application.

• Best and more convenient than type IV. • Possible to make a choice of individual com-

ponents as per properties of the coating desired (a distinct property of polyurethane).

• Polyurethane formation can occur directly on the surface to be coated after mixing the two components.

VI Solvent evaporation • Choice of composition is limited and poly-urethane linkages are already formed. Catalyst only brings about cross linking of the two polymers.



4.2.3. Inhibited Cement-Slurry Coating

Due to the failure of epoxy-coated reinforcing steel in marine-substructure service, interest has recently been revived in macro-cell corrosion studies /121-124/. The steel reinforcement embedded in concrete is surrounded by an alkaline medium; a coating based on cement is expected to be more compatible. A cement coating is a passivating type of coating and may have higher tolerance towards defects. Because of the surrounding concrete, which is again alkaline, the galvanic effect is likely to be pronounced. Various passivating treatments for reinforcement have been suggested, such as pickling in hydrochloric acid followed by treatment with phosphoric acid /125/ and treatment with a hydrolysable silicate or hydrated silica /126/. Simple preliminary coating of steel with a dense mortar is recommended to counteract acid fumes /127/. The application of coatings of cement containing paints with water proofing admixtures /128/, or of slurries of lime and cement with casein or bone glue /129/ is also recommended as an anti-corrosive measure. Bitumen paints are said to protect reinforcements from calcium chloride in concrete, but some paints of this type revent the formation of bond between the steel and the concrete /130/. Combinations of inhibiting agents in cement slurries are also proposed, notably sodium di-chromate /131/, sodium carbonate, sodium phosphate /132/, sodium benzoate /133,134/ and also barium Chromate /135/. But none of these methods appear to have made much headway, either because of their doubtful field per-formance or their adverse effect on bonding.

In a study, Rengaswamy et al. /136.137/ have shown that cement-sluny coated reinforcements were found to be three times more durable when com-pared to uncoated reinforcements under simulated macro-cell corrosion conditions. They observed that even under the most aggressive macro-cell corrosion conditions, the inhibited and sealed cement-sluny coating has performed very efficiently by showing only negligible superficial rusting. The application procedure for cement-sluny coating involved derusting, phosphating, and two coats of cement followed by sealing. This entire procedure takes about three days /137/.

4.3. Cathodic Protection

Cathodic protection is currently becoming one of the most useful methods in conosion protection because it controls the electrochemical conosion process itself. Wyatt /138/ has presented a review of the developments in



cathodic protection of reinforced concrete structures. The critical areas of this multi-discipline technology with respect to system performance and future development are the anode system, anode overlays, and assessment of cathodic protection performance /138/. Anodes include conductive overlay anodes, slotted anodes, conductive coating anodes, flame or arc sprayed zinc anodes, activated titanium mesh anodes. In another paper, Wyatt /139/ has already detailed the historical developments of various anode systems.

In this method, the structure to be protected is made into a cathode by supplying an electric current from an external d.c. source /140/. The cathodic protection method may be broadly classified into two categories: (i) sacri-ficial anode cathodic protection and (ii) impressed current method. Pithouse /141/ has studied both types of cathodic protection systems on concrete repair work and found that the current systems have been the most successful. He /141/ found that the distributed mesh systems are the latest development and have proved to be very effective.

One of the main criteria for effective cathodic protection is that the applied current should be uniformly distributed to the steel to be protected. A highly non-uniform current distribution may not only leave some of the reinforcement unprotected; it may also cause the formation of macrocells between protected and unprotected steel, as well as the danger of hydrogen evolution in the high current zones /142/.

4.3.1. Sacrificial Anode Cathodic Protection

This preventive method is based on the use of a sacrificial anode which is consumed during corrosion and the reinforcement remains unaffected. The standard electrode potential of the sacrificial anode must be lower than the standard electrode potential of the reinforcement so that during galvanic corrosion, the reinforcement acts as the cathode and remains unattacked. Zinc as the sacrificial anode is the unanimous choice for cathodic protection. This may be present in the form of a coating or separately, being electrically connected with the reinforcement. Zhang /143,144/ has developed a tech-nique based on the use of a zinc wire as sacrificial anode along the length of a rebar. He observed that the extent of galvanic protection of the steel and the galvanic corrosion rate of the zinc were functions of the concrete conductivity, overlay thickness, steel surface area, surface conditions, and other factors.

Zinc may be applied (i) by hot dipping of the rebar in the molten zinc followed by chromatizing treatment to produce galvanized rebars (discussed



earlier under metallic coating) or (ii) by flame or arc spraying, known as sprayed-zinc sacrificial anodes. It is generally used for rehabilitation work due to its ease of application and low cost. For field installations, the typical installed costs (including surface preparation, etc.) ranged from $6/ft2 to $12/ft2 ($65/m2 to $130/m2) compared to ~$40/ft2 ($370/m2) for conventional impressed-current cathodic protection systems on marine substructures, based upon experience by the Florida Department of Transportation /145/. Appli-cation of zinc in the field was preceded by removing the delaminated concrete and then cleaning the surface of the affected member by sand-blasting. Thus, the concrete and/or any exposed steel reinforcement surface was cleaned thoroughly and roughened, improving adhesion of the sprayed metal.

Periodical water contact (by sea water mist, splash, or weather exposure) was considered a necessary factor for long-term anode performance. Gao et al. 11461 have also found sprayed-zinc coating to be very well applicable to corrosion protection of reinforcement, because it could maintain cathodic polarization potential of reinforcement and potential decline field during 4 h of depolarization. An alloy of zinc like 85Zn-15Al has also been studied by Brousseau et al. /147/ for this purpose.

Recently, titanium-based anodes have also been studied which can also be applied to concrete by plasma, flame or arc spray techniques /148.149/. A process has also been developed for catalyzing the Ti coating, which results in long-term operation at low anodic potentials. This anode is expected to be useful especially where concrete overlays are undesirable and where long life and durability are important /148/.

4.3.2. Impressed Current Cathodic Protection

This system is more flexible but more complex than a galvanic anode system. The basic principle is the same for both systems, except that the impressed current system energizes the anode by means of an external electrical energy source. The DC current is passed into the electrolyte by means of an internal electrode like a lead-silver alloy or platinum. A number of studies have been directed towards this method /150-157/. McArthur /158/ has shown through modeling that the cathodic area is initially made very alkaline immediately after switch-on, and the anodic area becomes acidic in nature. This acidic area spreads out from the anodic electrode toward the cathodic area. It is found that this alkalinity is produced at the cathodically impressed rebar as the impressed current uses up the dissolved oxygen,



requires the hydroxyl ions to carry the ionic current, and produces hydrogen. For cathodic protection to work effectively, there must be a way for oxygen to difiuse to the cathodic area, so that it takes part in the cathodic reaction. The anodic area becomes acidic and the alkaline OH" ions are moved away from the rebar as a requirement for continuous current flow.

4.4. Electrochemical Removal of Chloride Ions

The conventional technique for repair of old structures consists of locating the corroding areas with the potential mapping technique /159.160/, determining the chloride content in corroding and passive zones and removing the chloride-contaminated concrete. Very often, sound concrete with high compressive strength has to be removed due to excessive chloride contamination. For these cases, where the concrete is chloride-contaminated but no corrosion or only slight corrosion of the rebars occurred, electro-chemical methods can be used to remove sufficient amounts of chlorides allowing the steel to repassivate. Trials are underway to determine if chloride removal by electrokinetics is an effective option /161/. An impressed cathodic d.c. current similar to cathodic protection of reinforced concrete structures is used to remove chloride ions from concrete. An anode and an electrolyte are applied to the structure surface and a current is passed between the anode and the reinforcing steel which acts as a cathode. Cathodic protec-tion is a permanent installation whereas electrochemical removal is only temporary /162/.

Miller /163/ has developed a process for rehabilitating internally reinforced concrete by electrical treatment. It involves applying to surface areas of the concrete a temporary sprayed-on layer of a self-adherent, electro-lytic material comprised of cellulosic pulp fiber mixed with liquid electrolyte. Voltage is applied between the reinforcement and the distributed electrode, preferably a wire grid to cause migration of chloride ions from the concrete to the electrolyte coating. When the chloride content of the concrete has been reduced to a desired level, the voltage is discontinued and the adherent electrolyte coating and distributed electrode are removed. Preferred electro-lytes include water or other solutions, such as calcium hydroxide. Desirably, the distributed electrode is formed of a ferrous material reactive with chlorine, to minimize the release of free chlorine gas into the ambient. Polder et al. /164/ have made a laboratory investigation using activated titanium meshes as anodes and saturated calcium hydroxide or 1M sodium carbonate



as electrolyte solution. They /164/ observed that the proportion of chloride ions removed from the core was a function of the total charge passed and the nature of the external electrolyte; the efficiency of chloride removal was much higher when saturated calcium hydroxide electrolyte was used.

4.5. Control of Concrete Cover

A good-quality, well-placed concrete cover provides the first and most efficient barrier to the aggressive ions and it is believed that it has no substi-tute in protecting reinforcement from corrosion. In view of this, it is appro-priate to understand its role towards reinforcement corrosion. The following sections have been devoted to various parameters related to concrete cover which may have an influence on reinforcement corrosion.

4.5.1. Crack Width

Concrete and cement mortar provide excellent corrosion protection to the reinforcement The most cost-effective and efficient means of avoiding corrosion of reinforcement is to provide an adequate cover of dense, impermeable concrete. The highly alkaline environment within concrete passivates the steel, and the steel will not corrode if this state is maintained /165,166/. However, when reinforced concrete is subjected to chloride ingress through exposure to de-icing salts and/or marine environments, chloride can induce corrosion of reinforcement. This can be traced back to a lack of concrete cover over the reinforcement and to the presence of regions of highly permeable concrete which allow rapid transport of chlorides, carbon dioxide and oxygen to the reinforcement Such corrosion finally leads to cracking along the reinforcing bars on account of the expansive nature of the oxidation products. Suda et al. /167/ have found that crystalline magnetite, geothite and lepidocrocite make up only 30% of the rust formed within concrete, as compared to approx. 45% for uncovered bars. They /167/ found through non-linear FEM analysis an equivalent volume expansion ratio of 2.9-3.2. Based on this it was observed that the loss of only several micrometers of the bar could lead to the formation of longitudinal cracks.

Beeby /168/ has reported that the quality of the concrete and the adequacy of cover over the reinforcing steel was more important than crack size in determining long-term corrosion performance. Mangat and Guruswamy /169/ showed that cracks under 0.2 mm (0.008 in) in width were subject to little CI" penetration in good quality concrete (water-to-cement ratio [w/c] ^ 0.4) with



proper concrete-cover. The American Concrete Institute (ACI) Building Code calls for low w/c and concrete cover of at least 38 mm (1.5 in). The ACI recommended that maximum crack sizes should be 0.15 mm (0.006 in) in marine environments and 0.18 mm (0.007 in) where de-icing salts are used /170.171/. The minimum requirements under discussion in Europe by Comitö Europ0en de Normalisation, Technical Committee 104, Working Group 3 (CEN/TC104/WG3) are 0.5 w/c, 25 mm (0.0984 in) of concrete cover, and cracks less than 0.4 mm (0.016 in) /172/.

4.5.2. MixRatio

The strength and density of the concrete primarily depend upon the mix ratio (Cement: Sand : Aggregate). If the concrete is of high strength and density, it can effectively protect the reinforcement against corrosion due to lower infiltration. It has been observed that 1:4:8 concrete is 100 times more permeable than 1:2:4 concrete /173/. It should be noted that the carbonation depth does not exceed 20mm in a dense concrete, while it is as high as 100mm in concrete with high permeability. The actual mix-ratio may vary with duration. This is due to conversion of free calcium hydroxide to calcium sulphate and hydrated calcium aluminates to more insoluble calcium sulpho-aluminates which increases the solid and volume to more than double and causes expansion and disruption of the concrete /173/. The pH value, which affects the passivation of reinforcement, also changes with the mix ratio as given in Table 6. The table shows that a high pH value is obtained for a rich

Table 6 pH Values of Various Mix Ratios 7173/

Mix Ratio PH

1 1:2 12.20 1 1.5:3 12.10 1 2:4 12.00 1 3:6 11.80 1 4:8 11.70

mix (greater proportion of cement in the concrete mix) and a low pH value for the lean mix This also indicates that there is a steep fall in pH and alkalinity with the decrease in cement content in the mix. It is important to note that the tolerable limit for corrosive ions like chloride and sulphate increases with an increase in the pH value /173/. In view of the above, ade-quate selection and control of the mix ratio is veiy important



The use of concrete with a minimum cement content with good compac-tion and w/c (water to cement ratio) < 0.4 is actively encouraged in structural applications /174/ as it improves strength and reduced permeability is obtained by minimizing paste porosity, and less drying shrinkage and cracking occur with these mixes. Newton et al. /8/ found that a reduction in w/c from 0.4 to 0.3 decreased mortar and cement paste porosity but did not improve the corrosion performance of embedded steel. This may be due to reduced compaction efficiency during placement and lower Ca(OH)2 content in the hardened cement paste /8/. Constantinou et al. /175/ found that the w/c ratio, the concrete cover and the curing time had a significant effect on the length of time taken to carbonate the specimen. However, the effect of these variables on the corrosion rate was negligible.

Gu et al. 11161 found that the corrosion reactions appeared preferentially to occur on the surface of the rebar embedded in a denser matrix for both the presence and absence of chloride ions. This phenomenon has been explained by the depletion of oxygen concentration at the steel/cement paste interface resulting in the formation of an "anodic area".

4.5.3. CT/OK Ratio

Chloride ions may enter the concrete, inadvertently, through the use of chloride contaminated water for mixing or aggregates /177/ or through calcium chloride used to accelerate hydration and hardening of the cement binder. These chloride ions react with the calcium aluminate and alumino-ferrite to form insoluble calcium chloroaluminates and calcium chloroferrites in which chloride is bound in non-active forms. The reaction, however, is never complete and some active soluble chloride remains in equilibrium in the pore fluid. This free chloride in the pore solution initiates corrosion in the reinforcement. Although codes of building practice specify a maximum chloride content, it is difficult to distinguish between free and bound chloride and their effects on steel corrosion /8/.

Chloride may also enter into concrete from the external environment to which it is exposed, such as sea water (tidal and splash zone), wind-carried salt spray, de-icing salts on roads and bridge decks or wetting and drying cycles under a chloride environment. This chloride does not combine with the hydration products of cement to the same extent as chloride in fresh concrete. The proportion of unbound (or free) chloride is, therefore, higher and the risk of corrosion is greater. Locke /178/ also found that a chloride ion which diffuses into hardened concrete will initiate corrosion less than that required



to initiate corrosion with chloride ion mixed in the concrete. It has been shown that pitting becomes more likely as the C170H" ratio in

the solution is increased /9.179/. The passivation of steel rebar may be dis-rupted in the presence of chlorides, if a certain Cl'/OH" ratio threshold is exceeded. However, a universal threshold level is not applicable to different materials and mixes (Table 7).

Table 7 Influence of C170H" Ratio on the Initiation of Corrosion

Conditions) c r / o f f Ratio

References Remarks

Calcium hydroxide solutions simulating real pore fluid in concrete

0.6 180-182

Steel fibers and rebar electrodes embedded in concrete in marine tidal and splash zone environment

>320 183

Steel electrodes "embedded in concrete of different composi-tions in terms of the Cl'/OH" concentration of their pore fluid

£4 184

Steel electrodes in mortar >30 185 Reinforcement electrodes em-bedded in concrete matrices of different cement and water/ cement ratios and exposed to marine environment

Not a dominant

factor

186 w/c is a dominant factor and not the cement content, acid-soluble chloride or CL/OH" ratio

Influence of pore fluid compo-sition on reinforcement corro-sion

187,188 Neither total chloride content nor CL/OH is a reliable indicator for corrosion risk.

Rebar electrodes embedded in concrete matrices containing different cement placements and exposed to marine en-vironment

Plain concrete Microsilica concrete

13 17-18

189 A universal threshold level is not applicable to different materials and mixes. CI" con-centration has a more significant influence on corrosion initiation than CL/OH"



4.5.4. Concrete Additives

4.5.4.1. Inhibitors

In addition to the above protective measures, corrosion inhibitors have been developed, most notably calcium nitrate (Ca(N02>2 /190-193/ which is effective in providing increased corrosion protection, especially when used in concrete of good quality /193/. NaNQ may also be added to prevent pitting on the reinforcement in the amount of about 4% of the concrete /194/. The common practice is to add them together with the mixing water of concrete; however, their use as a curative method, i.e. by penetrating the concrete from the outside of an old structure already contaminated, would be of great interest. Andrade et al. /195/ have studied both methods using Na2P03F as inhibitor. Results indicated that Na2P03F may act as an inhibitor of reinforcement corrosion through the hardened concrete. Dikkii et al. /196/ have studied the inhibiting effect of NaN02, Na3P04 and Na2Cr04 on a 3% solution of NaCl on reinforcement corrosion. They /196/ found that the greatest effect was provided by NaN02. The addition of 1% NaN02 increased the resistance to cracking by a factor of 14. At the same time, all inhibitors studied did not prevent the physico-chemical interaction of the solution with the metal of the crack-tip, and only slightly increased the resistance to crack propagatioa The mixture of NaN02 with a sodium phosphate derivative and an acyl derivative of tetraamine was found to be very effective for protection from corrosion and corrosion cracking.

Organic migrating corrosion inhibitors (MIC) have also been developed to protect reinforcement steel /197/. Miksic et al. /197/ found that MIC provide corrosion protection to rebar in chloride-contaminated concrete without any adverse effect on the structure.

Hansen /198/ has suggested drawing the chloride away from the metal and then applying a composition containing ammonium carbamate or ammonium carbonate to the concrete. An agent may also be added to the solution to prevent any calcium hydroxide contained in the concrete from precipitating the corrosion inhibitor /198/.

Collins et al. /199/ suggested chemical treatment of corroding steel reinforcement through ponding and/or placement of chemically treated mortar after removal of chloride-contaminated concrete. Calcium-nitrite based corrosion inhibitor was found to be most effective when applied as a ponding and when placed in backfilled mortar.



4.5.4.2. FlyAsh

Fly ash is a waste material and, therefore, needs disposal to avoid environmental pollution. In view of this, work has been carried out to study the addition of fly ash in partial substitution of Portland cement in concrete on concrete porosity and resistivity /200-202/ as well as on reinforcement corrosion /202,203/. Kayyali et al. /203/ have found that the use of fly ash was beneficial in immobilizing the chloride ions in non-supeiplasticized concrete. However, in concrete made with superplasticizers, the inclusion of fly ash was found to cause a greater release of chloride ions into the pore solution. The addition of fly ash reduces the corrosion susceptibility of steel in concrete /204-207/. Kouloumbi et al. /206/ found that use of a 30% fly ash mix provided the most effective protection.

5. CONCLUDING REMARKS

From the above discussion, it is evident that a good concrete cover in terms of quality of ingredients, mix proportion, water-cement ratio, etc., has no substitute for the prevention of reinforcement corrosion. Corrosion takes place only when a path exists for the ingress of chloride, sulphur dioxide, oxygen, etc., in the concrete cover. This situation is unavoidable in practice and the above path exists. This causes corrosion of reinforcement leading to premature failure. In view of this, a barrier must be present to prevent the ingress of harmful ions. This barrier may be in the form of a coating on the reinforcement or on the surface of the concrete cover. Epoxy coating is the most popular barrier coating; however, its mishandling during transportation, storage, fabrication, placement and construction may cause a serious corro-sion problems due to localized attack Apart from this, bond strength is also reduced due to the smooth surface of epoxy-coated rebars. Galvanized rebars are also popular. Zinc coating initially acts as a barrier and subsequently provided sacrificial cathodic protection. Other coating systems are either on a laboratory scale or have found limited application. The important criteria for selection of any coating are: its resistance to chemical attack, bond to steel, resistance to damage by rough handling, and feasibility of application

Steel reinforcement can also be prevented through cathodic protection. As mentioned above, a zinc-based sacrificial anode cathodic protection system is considered a better choice than the impressed-current cathodic protection method due to practical difficulties involved with the latter. These include



non-uniform distribution of current due to the complex non-homogeneous nature of concrete, electrical connections to all embedded rebars, large capital investment, and suitable provision in the structural design A large number of studies are also directed towards concrete additives like corrosion inhibitor and fly ash which have been found useful in preventing reinforcement corrosion.

In recent years, efforts have been made to develop reinforcement steel with better corrosion resistance. These corrosion resistant rebars do not require any special care during transportation, storage, fabrication, placement or even construction. Moreover, they do not require any skilled labour and capital investment, design changes, etc.

In summary, it can be concluded that the utmost care should be taken dining concrete-mix preparation and construction The choice of other pro-tection methods will depend upon the type of structure to be protected, life expectancy, nature of environment and cost of application

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15. N.J.M. Wilkins and P.F. Lawrence, in: Corrosion of Reinforcement in Concrete Construction, A.P. Crane (ed.), Chichester, Ellis Horwood, 1983; pp. 119.

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19. J. A. Gonzalez and C. Andrade, Br. Corros. J., 17,21 (1982). 20. O.E. Gjorv, Marine Sei. Comm., 1,198 (1970). 21. F.M. Lea and C.M. Watkins, "The Durability of Reinforced Concrete

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