0005 India Regulation on Coating

For Official use only

GOVERNMENT OF INDIA MINISTRY OF RAILWAYS (Railway Board)

TECHNICAL LITERATURE ON CORROSION/CARBONISATION PROTECTION IN CONCRETE STRUCTURES

MARCH– 2008

BS-88

ISSUED BY RESEARCH DESIGNS AND STANDARDS ORGANISATION LUCKNOW - 226011

i

ii

CONTENTS

0

INTRODUCTION

1

1.0

CARBONATION OF CONCRETE

1

2.0 4

CHLORIDE ATTACK

3.0

CORROSION CONTROL

3.1 9

METLLURGICAL METHOD

3.2

CORROSION INHIBITORS

3.3 12

COATINGS TO REINFORCEMENT

3.4

METALLIC COATINGS

3.5 17

RE-ALKALIZATION

3.6

CHLORIDE REMOVAL

18

3.7

CATHODIC PROTECTION

18

3.8 21

COATINGS TO CONCRETE

4.0

REFERENCES

7

9

15

29

iii

0. Introductions Corrosion of steel in concrete is a complex phenomenon. There are different factors affecting the process of corrosion in concrete. The increase in volume of reinforcement after corrosion is one of the adverse effects on the structure apart from reduction in cross section area of reinforcement. Corrosion has been found one of the important reason causing weakness to concrete structures. Lot of research has been done on this subject in India and abroad to prevent the process of corrosion in concrete structures. Before discussing the methods to avoid or reduce corrosion, it will be more appropriate to understand some of the fundamentals of process of corrosion of reinforcement in concrete. The ingress of water and oxygen is must to start process of corrosion. The excess of Ca (OH) 2 as a result of hydration process of cement in concrete provides a passive environment around reinforcement to prevent corrosion. The pH value of normal concrete with OPC remains in the range of 13 to 17. Any chemical reaction with in concrete to reduce this pH value poses a potential threat for start of corrosion of reinforcement apart from other damages to concrete microstructures. Following are few commonly known processes causing deterioration to concrete.

1.

Carbonation of concrete

The microstructure of concrete is such that it has capillary pores to the extent of 28%. The extent of pores depends upon quality of concrete and the presence of water at the time of mixing of concrete. Making more dense concrete with less w/c ration reduces the amount of pores. These pores are created due to evaporation of excess free water during strengthening of concrete mass. These pores are inter connected and goes inside the concrete mass from surface of concrete structures. Carbonation of concrete is a process by which carbon dioxide from the air penetrates into concrete through pores and reacts with calcium hydroxide to form calcium carbonates. It has seen that the conversion of Ca(OH)2 into CaCO3 by the action of CO2 results in a small shrinkage. We shall see another aspect of carbonation, as CO2 by itself is not reactive. In the presence of moisture, CO2 changes into dilute carbonic acid, which attacks the concrete and also reduces alkalinity of concrete (i.e. ph value reduces). Air contains CO2. The concentration of CO2 in rural air may be about 0.03 percent by volume. In large cities the content may go up to 0.3 percent or exceptionally it may go up to even 1.0 per cent. In the tunnel, if not well ventilated the intensity may be much higher. The pH value of pore water in the hardened concrete is generally between 12.5 to 13.5 depending upon the alkali content of cement. The high alkalinity forms a thin passivating layer around steel reinforcement and protect it from action of oxygen and water. As long as steel is placed in a highly alkaline condition, it is not going to corrode. Such condition is known as passivation. 1

In actual practice CO2 present in atmosphere in smaller or greater concentration, permeates into concrete and carbonates the concrete and reduces the alkalinity of concrete. The pH value of pore water in the hardened cement paste, which was around 13, will be reduced to around 9.0. When all the Ca (OH)2 has become carbonated, the pH value will reduce up to about 8.3. In such a low pH value, the protective layer gets destroyed and the steel is exposed to corrosion. The carbonation of concrete is one of the main reasons for corrosion of reinforcement. Of course oxygen and moisture are the other components required for corrosion of embedded steel. Rate of Carbonation: The rate of carbonation depends on the following factors.  The level of pore water i.e. relative humidity.  Grade of concrete  Permeability of concrete  Whether the concrete is protected or not  Depth of cover  Time

Fig 1 Depth of Carbonation with respect to strength (grade) of concrete It is interesting to know that if pore is filled with water the diffusion of CO2 is very slow. But whatever CO2 is diffused into the concrete, is readily becomes dilute carbonic acid reducing the alkalinity of concrete. On the other hand if the pores are rather dry, that is at low relative humidity the CO2 remains in gaseous form and does not react with hydrated cement. The moisture penetration from external source is necessary to carbonate the concrete.

2

Fig 2 Depth of Carbonation for protected and un-protected concrete Depth of carbonation with age and grade of concrete

Table 1 Age-years 2 5 10 50

Depth of Carbonation (mm) M20 M40 5.0 0.5 8.0 1.0 12.0 2.0 25.0 4.0

The highest rate of carbonation occurs at a relative humidity of between 50 and 70 per cent. The rate of carbonation depth will be slower in case of stronger concrete for the obvious reason that stronger concrete is much denser with lower W/C ratio. It again indicates that the permeability of the concrete, particularly that of skin concrete is much less at lower W/C and as such the diffusion of CO2 does not take place faster, as in the case of more permeable concrete with higher W/C ratio. Fig 1 and table 1 shows th4e depth of carbonation in various grades of concrete. It is now well recognized that concrete needs protection for longer durability. Protective coating is required to be given for long span bridge girders, flyovers, industrial structures and chimneys. The fig.2 shows carbonation depth of protected and unprotected concrete. Depth of cover plays an important role in protecting the steel from carbonation. The table 2 shows relationships between W/C, depth of cover and time in years for carbonation depth to reach the reinforcement. Table 2 Approximate relations between W/C, depth of cover and time in years for carbonation depth to reach the reinforcement. Depth of cover (mm) WC Ratio 0.45 0.50

15 100+ 56

20 100+ 99 3

25 100+ 100+

30 100+ 100+

0.55 0.60 0.65 0.70

27 49 76 16 29 45 13 23 36 11 19 30 Time in years for carbonation

100 65 52 43

CO2

SO2

H2O

Fig 3 Concrete is under continuous attack by aggressive envoi mental agencies. Good concrete and sufficient cover is the answer for durability Measurement of depth of carbonation: A common and simple method for establishing the extent of carbonation is to treat the freshly broken surface of concrete with a solution of phenolphthalein in diluted alcohol. If the Ca(OH) 2 is unaffected to CO2 the colour turns out to be pink. If the concrete is carbonated it will remain uncoloured. It should be noted that the pink colour indicates that enough Ca(OH) 2 is present but it may have been carbonated to a lesser extent. The colour pink will show even up to a pH value of about 9.5. 2. Chloride Attack: Chloride attack is one of the most important aspects for consideration when we deal with the durability of concrete. Chloride attack is particularly important because it primarily causes corrosion of reinforcement. Statistics have indicated that over 40 per cent of failure of structures is due to corrosion of reinforcement. We have already discussed that due to high alkalinity of concrete a protective oxide film is present on the surface of steel reinforcement. The protective passivity layer can be lost due to carbonation. This protective layer also can be lost due to the presence of chloride in the presence of water and oxygen. In reality the action of chloride in inducing corrosion of reinforcement is more serious than any other reasons. One may understand that Sulphates attack the concrete whereas the chloride attacks steel reinforcements. Chloride enters the concrete from cement, water, and aggregate and sometimes from admixtures. The present day admixtures are generally containing negligible quantity of chloride or what they call chloride free. Chloride can enter the concrete by diffusion from environment. The Bureau of Indian Standard earlier specified the maximum chloride content in cement as 0.05 percent. But it is now increased the allowable chloride content in cement to 0.1 per cent. I S 456 of 2000 limits the chloride content as (Cl) in the concrete at the time of placing is shown in table 3.

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Fig 4 Pink Colour indicates that Ca (OH) 2 is unaffected by carbonization. the uncoloured portion indicates that concrete is carbonated. Table 3 Limits of Chloride Content of Concrete (IS 456 of 2000) Sl.No.

1.

2. 3.

Type of Use of Concrete

Maximum Total acid soluble chloride content. Expressed as kg/m3 of concrete Concrete containing metal and steam cured 0.4 at elevated temperature and prestressed concrete Reinforced concrete or plain concrete 0.6 containing embedded metal Concrete not containing embedded metal or 3.0 any material requiring protection from chloride

The amount of chloride required for initiating corrosion is partly dependent on the pH value of the pore water in concrete. At a pH value less than 11.5 corrosion may occur without the presence of chloride. At pH value greater than 11.5 a good amount of chloride is required. Limiting values of chloride contents, above which corrosion may be imminent, for various values of pH are indicated in table 4. The total chloride in concrete is present partly as insoluble chlorialuminates and partly in soluble form. It is the soluble chloride, which is responsible for corrosion of reinforcement. Table 4 Limiting Chloride Content Corresponding to pH of concrete PH 13.5 13.0

Chloride content g/litre 6.7400 2.1300 5

ppm 6740 2130

12.5 12.0 11.5 11.0 10.0 9.02

0.6720 0.2130 0.0670 0.0213 0.0021 0.0002

672 213 67 21 2 0.2

Chloride Permeability Based on Charge Passed (As per ASTM C 1202) Chloride Permeability High Moderate Low Very Low Negligible

Charges passed (Coulombs) ≤ 4000 2000 to 4000 1000 to 2000 100 to 1000 ≤ 100

Type of Concrete High water-cement ratio ≤ 0.6 Moderate W/C ratio (0.4 to 0.5) Water-cement ratio ≤ 0.4 Latex modified concrete Polymer Impregnated concrete

Corrosion of Steel (Chloride induced) Corrosion of steel in concrete is an electrochemical process. When there is a difference in electrical potential along the steel reinforcement in concrete, an electrochemical cell is set up. In the steel, one part becomes anode and other part becomes cathode connected by electrolyte in the form of pore water in the hardened cement paste. The positively charged ferrous ions Fe++ at the anode pass into solution while the negatively charged free electrons e- pass through the steel into cathode where they are absorbed by the constituents of the electrolyte and combine with water and oxygen to form hydroxyl ions (OH)-. These travel through the electrolyte and combine with the ferrous ions to form ferric hydroxide, which is converted by further oxidation to rust.The reactions are described below Anodic reactions Fe→ Fe++ +2 e++ Fe 2(OH) - →Fe (OH) 2 (Ferrous hydroxide) 4 Fe (OH) 2 + 2H2 O+O2→ 4 Fe (OH) 3 (Ferric oxide) Cathodic reaction 4 e- + O2+ H2 O→ 4 (OH) –

6

Fig 5 Simplified model representing corrosion mechanism

Fig 6 Shows that, depending on the oxidation state, metallic iron can increase more than six times in volume.

Fig 7 Diagrammatic representation of damage induced by corrosion cracking, spalling and delamination It can be noted that no corrosion takes place if the concrete is dry or probably below relative humidity of 60 percent because enough water is not there to promote corrosion. It can also be noted that corrosion does not take place if concrete is fully immersed in water because diffusion of oxygen does not take place into the concrete. Probably the optimum relative humidity for corrosion is 70 to 80 percent. The products of corrosion occupy a volume as many as six times the original volume of steel depending upon the oxidation state. Fig. 6 shows the increase in volume of steel depending upon the oxidation state. The increased volume of rust exerts thrust on cover concrete resulting in cracks, spalling or delamination of concrete. Refer Fig 7. With this kind of situation concrete loses its integrity. The cross section of reinforcement progressively reduces and the structure is sure to collapse. 3.0 Corrosion Control: From the literature survey and case studies it has been reported that 40% of failure of structures is on account of corrosion of embedded steel reinforcement in concrete. Therefore corrosion control of steel reinforcement is a subject of paramount importance. First and foremost for corrosion control is the good quality of concrete through good construction practices. It is a very vast subject touches the fundamentals of 7

choosing constituents material and good rules to be followed during various stages or production of concrete, in particular the use of lowest possible water/cement ratio having regard to workability. In view of the general availability of superplasticizers, it should be used to cut down the W/C ratio to make dense concrete.

Fig 8 Proper mix design, use of right quality and quantity of cement for different exposure conditions is to be adopted. Recently it has been realized that lower W/C ratio which has been always associated with lower permeability is not enough to make impermeable concrete contributing to high durability. Use of supplementary cementitious materials such as fly ash, ground granulated blast furnace slag (ggbs), silica fume etc. are required to be used as admixtures or in the form of blended cement in addition to lowest possible W/C ratio to make concrete dense. These materials improve more than one properties of concrete, which will eventually reduce corrosion of reinforcement. Tests on mortar containing ggbs have shown that water permeability is reduced by a factor up to 100. It is also reported that 60 per cent ggbs educed the diffusion of chloride ions into the concrete by as much as 10 times. Silica fume contributes to the all-round improvements in the quality of concrete, which are responsible for reducing corrosion of steel reinforcement. The improvement in the microstructure of hydrated cement paste is ultimately responsible for protecting the steel reinforcement from corrosion.

Fig 9 Crack formed due to bursting pressure on account of rusting of reinforcements Example of delamination of concrete cover

8

In short it can be said that if we make good concrete with low permeability and improved microstructure, it will be durable by itself and also it can take care of the reinforcement contained in it o a great extent. It is always not possible to make such ideal concrete, particularly, in view of the complex environmental and exposure conditions. Further the inherent long term drying shrinkage and micro cracks in concrete, the problems become more serious. This demands certain other measures to control the corrosion of steel reinforcement. They are listed and briefly explained.  Metallurgical methods  Corrosion inhibitors  Coatings to reinforcement  Re-alkalization  Chloride removal  Cathodic protection  Coatings to concrete 3.1 Metallurgical Methods: Steel can made more corrosion resistant by altering its structure through metallurgical processes. Different methods such as rapid quenching of the hog bars by series of water jets, or by keeping the hot steel bars for a short time in a water bath, and by such other process the mechanical properties and corrosion resistance property of steel can be improved. There are many situations where stainless steel reinforcements are used for long term durability of concrete structures. 3.2 Corrosion inhibitors: Corrosion inhibitors, which come in powder, gel and liquid form, retard the rate of the corrosion reaction. They are widely used in many industries to effectively reduce the corrosion rate of steel and other metals. Commercial products for the control of corrosion of steel reinforcement in atmospherically exposed concrete were first produced in the 1970's. They increase the time to the onset of corrosion and then act to reduce the rate of corrosion. They can be introduced into the concrete mix at the time of construction/ repair or (in a suitable formulation) applied to the surface of an existing concrete structure. There are three main types of inhibitors: 3.2.1 Anodic inhibitors, which retard the corrosion reaction at the anode: Corrosion can be prevented or delayed by chemical method by using certain corrosion inhibiting chemicals such as nitrites, phosphates, benzoates etc. At low dosage there is concern that they will suppress generalized corrosion but may fail to eliminate all anodic sites. Of the available materials, the most widely used admixture is based on calcium nitrite but have proved to be deleterious at high chloride concentrations. It is added to the concrete during mixing of concrete. The typical dosage is of the order of 10-30 litres per m3 of concrete depending on chloride levels in concrete.

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Fig 10 Corrosion inhibiting effects of calcium nitrite As mentioned earlier, in the high pH of concrete, the steel is protected by a passivating layer of ferric oxide on the surface of steel. However, the passivating layer also contains some ferrous oxide, which can initiate corrosion when the chloride ions reach the steel. The nitrite ions present in the corrosion-inhibiting admixture will oxidize the ferrous oxide to ferric oxide, thus stabilizing the passivating layer even in the presence of chlorides. The concentration of nitrite must by sufficient to cope up with the continuing ingress of chloride ions. Calcium nitrite corrosion inhibitor comes in a liquid from containing about 30 per cent calcium nitrite solids by weight. The more corrosion inhibitor is added, the longer the onset of corrosion will be delayed. Since most structures in a chloride environment reach a level of about 7 kg of chloride iron per m3 during their service life, use of less than 18 litres/ m3 of calcium nitrite solution is not recommended. Fig. 10 shows that without an inhibitor the reinforcing steel starts to corrode when the chloride content at the rebar reaches a threshold level of 0.7 kg/ m 3. Although the corrosion process starts when the threshold level is reacted, it may take several years for staining, cracking and spalling to become apparent and several more years before deterioration occurs. Adding calcium nitrite increases this corrosion threshold. When you add 20-litres/ m3, corrosion will not begin until over 7.7-kg/ m3 of chloride is present in the concrete at the rebar. 3.2.2 Cathodic inhibitors Which retard the reaction at the cathode and seek to prevent oxygen reaching the reinforcing steel. At low dosages, they are effective at reducing corrosion rates but are generally less efficient than the anodic type. 3.2.3 Bipolar inhibitors They retard the corrosion process both at the anode and the cathode. These combine the benefits of both anodic and cathodic inhibitors at relatively low dosages. In this category organic Migratory bipolar corrosion inhibitors are the most widely used. Migratory Bipolar Corrosion inhibitors are organic inhibitors. They protect the steel at both the anodic and the cathodic sites. The Bipolar corrosion inhibitor chemistry involves migration of its molecules by electron density distribution to both the anodic and 10

cathodic sites of the steel. By virtue of its high vapour pressure, very high affinity and virtue of diffusion these inhibitors migrate towards the steel in concrete and get deposited in a monomolecular layer. This is true even in dense concrete. This barrier coating then raises the chloride threshold concentration for corrosion. Further more the inhibitor within the concrete matrix reduces the rate of chloride ion migration towards steel. It also dislodges previously absorbed chloride ions and water molecules on the steel surface. The basic advantage of the product lies in the ease of use. Studies have proved that addition of these types of corrosion inhibitors has no deleterious effect on the properties of concrete. Concrete penetrating corrosion inhibiting admixture upon addition into the concrete matrix plays a major role in inhibiting the corrosion process. The European Committee for standardization (CEN) pr. ENV 1504-9 recommends application of Concrete penetrating corrosion inhibitors as a proven corrosion control strategy. SHRP-S-666 (Strategic Highway Research Programme) has recommended these types of corrosion inhibitors for concrete structures subjected to chloride-induced corrosion. General Building Corporation of Japan has evaluated this product in concrete extensively and has reported rebar life extension by six times. Two codes available internationally for testing these types of inhibitors are ASTM G 109 & JIS A 6205. They are available, both as surface applied inhibitors and as admixed inhibitors. Surface applied inhibitors are used by spraying on the complete surface of the structure being repaired so that protection to the unexposed reinforcement is taken care of. Admixed inhibitors are used in the fresh mortar/concrete being placed for strengthening the structure. Specification for surface applied Corrosion inhibitor

Base Colour Specific Gravity Viscosity at 250C pH Dosage Toxicity Evaluation

Bipolar Water based Organic inhibitor. Concrete penetrating type. Colorless Hazy liquid 1.01 – 1.02 at 25o C 11-12 sec by Ford B4 Cup. Minimum 9.5 To be sprayed at the rate of 4m2 / ltr Non-toxic , Eco-friendly. Should pass JIS - 6205 standard

Specification for Admixed corrosion inhibitor

Base Colour Specific Gravity Viscosity at 250C pH

Bipolar Organic inhibitor. Concrete penetrating type. Brownish 1.05 – 1.08 at 25o C 11-12 sec by Ford B4 Cup. Minimum 9.0 11

Dosage Toxicity Effect on concrete properties

3 kgs per cubic meter of concrete. Non-toxic, Eco-friendly. No adverse effect on physical properties in fresh & hardened concrete in the absence of any other admixtures. However it is Essential to carry out trial mix with desired admixtures along with Migratory Corrosion Inhibitor.

Compatibility for Thermal Cycles Evaluation

Higher Compatible for higher thermal cycles No deleterious effects even at high temperature. Effective even at higher temperatures. Should pass ASTM-G-109 standard & JIS - 6205 Should pass tropical climate test (thermal cycles)

Acceptance Criteria: Migratory Bipolar Corrosion Inhibitor should be tested as per ASTM-G-109/ JIS - 6205 from reputed laboratories like CSIR & I.I.T. Tropical Compatibility: Material shall have evaluated test reports indicating significant reduction in corrosion rate after minimum of 90 thermal cycles at 60oC followed by 8 weeks of accelerated corrosion. (Linear polarization method) The inhibitor shall be non-toxic & safe to plant and human life. The principle of most inhibitors is to develop a very thin chemical layer on the surface of the reinforcement. There is a very wide range of corrosion protection performance from different inhibitor formulations, even with generic classifications. Independent evaluation and certification of performance is desirable. However, such evaluations need to be representative of field concretes and conditions. As the true effect of an inhibitor can only be evaluated from corrosion rate measurements before and after application and by reference to a control area, such systematic evaluations are lengthy processes and are in their early stages. 3.3 Coatings to Reinforcement: 3.3.1 Cement Polymer Composite coating system (CECRI Karaikudi): Central Electro-Chemical Research Institute, Karaikudi, developed the system after years of research on cement-based coatings. Rebars embedded in concrete are surrounded by an alkaline medium and as such cement based coating is more compatible. Basically two coats are applied - Primer coat and Sealer coat. The primer and sealer products: have thereto-plastic acrylic resin as basic raw material. The sealer product is formulated with resin mixed with cement as a pigment. The principle of the system is that the base metal contains 'p' electrons, which get released in corrosive environment leading to formation of Iron Oxide (Fe203). To prevent oxidation, a surface coating capable of nullifying the released electrons is provided. The sealer coat is compatible with primer and alkaline environment. 12

System at a Glance: No. Parameter

Requirement

1.

Sand blasting to the near white metal

2.

Pre-treatment (Surface reparation) Primer coat

3.

Sealer coat

4.

Air curing

5.

Continuity of coating

To be given within 4 hours of sand blasting Within 30minutes of primer coat, this should be given. Thickness 150 microns ± 25 microns Six hours before use in the work.

7.

No defects such as cracking, bulging, peeling, no rust mark. Inspect visually. Adhesion of coating – Coated bars are bent @ 120 around a Test mandrel. No peeling or cracking should be observe don outer radius. Stacking Stack bars on buffer material.

8.

Cutting, bending, welding

6.

Codal Reference

Coated bars can be cut and bent. Cut ends and weld portion should be treated with same formulation.

3.3.2 Fusion Bonded Epoxy Coating: The System: Fusion bonded epoxy is basically 100% solid finely ground fused powder particles, which when heated; melt to form a continuous adherent film. Fusion Bonded Epoxy Coating (FBEC) system for rebars is a fully automatized online process and large quantities of rebars can be obtained by reproducible quality. In early seventies, this system was originally developed in USA and around 1988, it started in India. It involves coating of epoxy under factory conditions and such plants are established in the country.

13

Fig 11 Pros and Cons of the System: Though this system is being used in India for more than a decade and in USA for more than 3 decades, there are opinions expressed in favour and against this system.. Briefly, some points are highlighted here for the benefit of decision makers. Points in favour of the system:  Research carried out by The National Bureau of Standards (NBS) for Federal Highway Administration, USA concluded that epoxy coated reinforcement by fusion bonding process in excess of 102 microns in thickness would be capable of protecting steel from corrosion. Similarly, research done in other countries (Canada, Japan, UK) has shown that FBEC bars performed better compared to uncoated bars.  In India, Central Road Research Institute conducted the test on coated bars in the laboratory and in field for 3 years and evaluated that the performance of FBECR bars is satisfactory. It is possible to repair cut ends and damages to the fusion bonded epoxy coated bard by touch-up methods. Within few hours, it can be used. Points against the system: 

There is no passivating primer film provided in case of FBEC rebars.



This coating introduces a medium of weakness in the path of an intimate bond between rebar and alkaline concrete.



Investigation carried out on 40 bridges in Florida Key in USA has revealed that disbandment can occur easily in the FBEC rebars which lacked passivation layer of Fe 2 O 3 and is a precursor to corrosion. 

Higher co-efficient of Thermal Expansion of fusion bonded epoxies impose large thermal stresses in epoxy coating leading to its early failure.



Cost of FBEC rebars is 30 to 50% higher than uncoated bars.

Lot of precautions and care is required in transportation, handling and placement of these coated bars failing which there will be damages, cuts and abrasions which may lead to corrosion. 3.3.3 IP Net Rebar Coating System (CBRI Roorkee) CENTRAL BUILDING RESEARCH INSTITUTE [CBRI/CSIR] ROORKEE LICENCED SYSTEM It is Interpenetrating Polymer Network Coating System for Corrosion Protection of Reinforcing Steel.System Composition. The coating system consists of a primer coat and a topcoat of the same system. The Primer and Topcoat (both coats) have Epoxy phenolic base with mixing proportion of 1: 1 of resin and hardener by volume. METHOD OF APPLICATION OF COATING SYSTEM: The application of the IPNet-RB coating system comprises the following sequence. Properties: System shall conforms to 14

   

ASTM - D - 3963 - 86 A-1.4.5.1. (Adhesion test with reinforcing steel) IS - 2770 (Part-1) (Pull-out test of coated bar with concrete) ASTM - B - 117 - 64 Part 21 (Salt spray test) ASTM-D- 2370-73 (Elongation). The durability of the coated rebars is related to the coating quality. The more damages to the coating is more prone to corrosion. Good job site practices can minimize the coating damages. For this reason, standards from ASTM, NACE (USA) have laid down specific procedures for stacking and transporting coating bars. Some important guidelines are referred here. Specifications for patch repairing of coating damages a) Coating repairs is required when peeling off and other damages exist. Prior to repairs, the damaged area shall be cleaned by removing loose or deleterious material. In case where rust is present it shall also be mechanically removed to repair. b) After this, primer coat shall be brush applied. After curing (4 hours) top coat shall be applied and it is desirable to ensure a thickness of 150 microns. The object of coating to steel bar is to provide a durable barrier to aggressive materials, such as chlorides. The coatings should be robust to withstand fabrication of reinforcement cage, and pouring of concrete and compaction by vibrating needle. 3.3.4 Simple cement slurry coating It is a cheap method for temporary protection against rusting of reinforcement in storage. Central Electro Chemical Research Institute (CECRI) Karaikudi have suggested a method for prevention of corrosion in steel reinforcement in concrete. The steps involved in this process are: De-rusting: The reinforcements are cleaned with a de-rusting solution. This is followed without delay by cleaning the rods with wet waste cloth and cleaning powder. The rods are then rinsed in running water and air-dried. Phosphating: Phosphate jelly is applied to the bars with fine brush. The jelly is left for 45-60 minutes and then removed by wet cloth. An inhibitor solution is then brushed over the phosphated surface. Cement Coating: Slurry is made by mixing the inhibitor solution with Portland cement and applied on the bar. A sealing solution is brushed after the rods are air cured. The sealing solution has an insite curing effect. The second coat of slurry is then applied and the bars are air-dried. Sealing: Two coats of sealing solution are applied to the bars in order to seal the micropores of the cement coat and to make it impermeable to corrosive salts. The above is a patent method evolved by CECRI and license is given to certain agencies. Somehow or other this method has not become very popular. Some experienced consultants and engineers doubt the efficacy of this method. 3.4 Metallic coatings (Galvanized reinforcement): Although coatings can be provided by number of metals such as nickel, copper, lead, tin etc., coating provided by using zinc is more suitable and economical. This process is known as "Hot Dip Galvanizing" and involves steps such as picking, 15

rinsing, flux solution dipping, drying and coating. Galvanizing of reinforcement consists of dipping the steel bars in molten zinc. This results in a coating of zinc bonded to the surface of steel. The zinc surface reacts with calcium hydroxide in the concrete to form a passive layer and prevents corrosion.

Fig 12 Galvanization process adopted for corrosion resistance Minimum mass of zinc coating shall be provided as given in the following table: Mass of zinc coating – minimum Reference grams/m2 of surface

S. No.

Environment

1.

Aggressive surrounding 915 (125 microns) such as marine areas, chemical plants 16

ICJ Paper January 2004 by Mr. Pugazhendy

2.

Normal

610 (85 microns)

- do -

Zinc coating covers up any scratches/holidays that may occur in the coating due to electrochemical property of zinc. Galvanized bars can be bent without cracking or peeling up of coating due to ductility of zinc. Bond characteristics and weld-ability of zinc coated bars remain unaffected. Testing of Galvanized Bars: Following tests shall be carried out to determine suitability of galvanized bars and before using them in concrete: S.No.

Test

1.

Hammer Test (Hammer Wt.215 gms)

2.

Knife Test (Sharp Edge)

3.

4. 5. 6.

Criteria for Reference Acceptance Should not peel off IS – 2629

No scratches and no IS – 2629 peel off Thickness Test Zinc coating should IS – 3203 conform to specified thickness Preece Test (copper sulphate test for coating No copper deposits IS – 2633 uniformity) Hydrogen evolution test for purity of zinc Coating mass to be as ASTM – A – coating wt. specified 123 Stripping test 15% HCL solution Coating as specified IS - 6745 IS - 4759

3.5 Re-alkalization: This electro-chemical technique provides a means of restoring the alkalinity to carbonated but otherwise sound concrete. It involves the passage of a direct current between the reinforcement (the cathode) and an anode applied temporarily to the surface of the concrete. This process generates hydroxyl ions at the steel surface, which locally regenerates the alkalinity of the concrete raising its pH upto about 12. This helps restore the passivating surface oxide layer to the reinforcement. Under the applied voltage, alkali ions are drawn from the anode into the concrete. The use of sodium or potassium carbonate electrolyte is claimed to make the treatment more resistant to further carbonation. Several forms of anode may be employed. These are commonly either some form of mesh (titanium or steel) or electrolytic tanks (for vertical surfaces)/baths (for deck slab applications). Sprayed cellulose impregnated with the electrolyte is used with the mesh anode system. The introduction of sodium ions, when using sodium carbonate as an electrolyte, may exacerbate any potential the concrete has for ASR. In these cases, plain water has been used as an electrolyte. It is understood that a lithium electrolyte has been proposed and tested but is still a subject of research. The process typically takes 17

Remarks

between three and five days but sometimes may take several weeks. Successful treatment can be established by means of an acid/alkaline indicating solution. However, it should be noted that phenolphthalein changes colour at a pH of about 9.5 (unless a modified solution of phenolphthalein is used). This is not a passivating condition and an indicator (Universal indicator) with a colour change closer to pH 12 may be required to demonstrate that a passivating condition has been achieved. As with cathodic protection and desalination, consideration must be given to hydrogen evolution at the reinforcement. The re-alkalization process applies some 20 - 50 VDC between the anode and the steel that must be expected to achieve steel potentials at which hydrogen evolution could take place. It seems unlikely that realkalization would need to be applied to pre-stressed concrete structures. Re-alkalization requires electrical continuity of the steel in the areas to be treated, a reasonable level and uniformity in the conductivity of the concrete, no short circuits between the cathode and the anode and no electrically insulating layers in he cover zone/bar surrounds. The process requires fewer concrete repairs than the patch repair alternative. It is also able to treat the whole surface of the zone in question. There has been strong growth in the use of re-alkalization in recent years (since the late 1980's) presumably because of its greater convenience and cost advantage over patch repairs. 3.6 Chloride removal: Negatively charged chloride ions (Cl-) can be repelled from reinforcement and move towards an external anode by making the steel cathodic and passing a direct current through the concrete. This process is known by various names such as electrochemical chloride extraction, desalination and chloride removal. It is similar in operation to cathodic protection by utilizes a temporary anode and a much higher electrical power density. The cathode reaction generates hydroxyl ions that locally enhance the alkalinity of the concrete in the vicinity of the reinforcing bars and encourages their re-passivation. Treatment periods are in the order of 3 to 6 weeks. Electrolytes employed include water and saturated calcium hydroxide. The anode types employed are essentially the same as those used for Realkalisation protection, namely either mesh systems or liquid electrolyte systems contained within tanks. Sprayed cellulose impregnated with the electrolyte is used with the mesh anode system. These use either titanium or a steel mesh (which is consumed during the treatment). As with other electro-chemical systems, it is necessary to have electrical continuity across the zone to be treated, no electrical short circuits between anode and cathode together with a reasonable level and uniformity in the conductivity of the concrete. The approach minimizes the amount of concrete repair work necessary. It is claimed hat the technique can be used to treat the whole of the concrete surface and, on the basis of life cycle costs, that it should be applicable to a wide range of structures. Care needs to be exercised in relation to potential problems (as with the other electro-chemical methods, namely hydrogen evolution in the member concerned). 3.7 CATHODIC PROTECTION: The chloride extraction and re-alkalization repair techniques are temporary 18

processes. Cathodic protection is a similar technique but permanent. It is now well established and is increasingly becoming accepted as a practical long-term solution for the rehabilitation of reinforced concrete structures suffering from chloride induced corrosion. The basis of cathodic protection is to eliminate corrosion by reducing the potential of the steel to a more electronegative state, thereby converting the whole of the steel reinforcement into a large cathode. This is achieved by passing a small direct current between an external anode material and the steel reinforcement material. The anode material is connected to the positive pole of a rectifier and the negative to the steel reinforcement. The production of electrons (cause of corrosion), which are consumed by the oxygen and water in reduction reactions, does not occur at the steel reinforcement. Instead the system forces electrons into the steel to be consumed in these reactions and thus protect its integrity. The production of hydroxyl ions at the steel surface (cathodic reactions) causes the concrete to revert back to an alkaline state thus stopping the corrosion process. Cathodic protection of reinforcing steel can be achieved by using sacrificial anodes or an external direct impressed current source. However, sacrificial anodes may not be suitable in the atmospheric zones due to the high electrical resistivity of the concrete. Types of cathodic protection systems: 3.7.1 Titanium mesh anode/cementitious overlay system: This has been the system most widely used but other systems appear to have been overtaking it recently with regards to usage. It is applicable above ground/water level and provides even current distribution, which minimizes the risk of over protection. If necessary multiple layers can be used to protect large surface areas of steel. However, the cementitious overlay can be susceptible to de-lamination, if not applied correctly. It also imposes extra weight on the structure and can be susceptible to impact damage. 3.7.2 Slotted/grid anode system: A dense titanium mesh ribbon or strip is installed in slots cut into the concrete (generally 25mm x 25mm) and the slots are then backfilled with a cementitious mortar compatible with the parent concrete). It has a low risk of de-lamination and life can be enhanced by using a larger anode strip. Minimal concrete cover can affect the uniformity of current distribution. 3.7.3 Internal anode systems: The internal anodes are embedded in 12mm diameter holes drilled into the concrete at depths of upto 300 to 400mm. depending upon the length of anodes required and the structural component being protected. A graphite based backfill material or a conductive gel is injected into the holes and the 3mm titanium rods are then inserted into the anode backfill. The life of the system is basically controlled by the consumption of the graphite backfill, which is estimated by the manufacturer as 20 years, although recent use of the conductive gel suggests a life of at least 30 years. However experience is now showing that anodes of this type in service seem to have difficulties in meeting the 19

lifetime expectations of the manufacturers. The system is especially cost effective on large elements such as beams, piers and columns but is not suitable for thin sections. Careful design is required to minimize cable requirements and to ensure optimum spacing and critical positioning of anodes in the vicinity of the steel. 3.7.4 Electro-conductive tape grease/over-wrap system: A conductive tape/grease system which provides a conductive path to the concrete is wrapped around a column or pile followed by titanium mesh. A further layer of conductive tape over-wraps the mesh anode to secure and provide a contact surface for the outer face of the mesh anode. Mechanical protection is provided by either polyethylene or fiberglass impact resistance jackets. This system is difficult to install on large sections, is susceptible to impact loadings and appearance becomes a problem. 3.7.5 Electro-conductive tape Qrease/Ranel system: This system utilizes similar technology as the over-wrap system. However, it is pre-fabricated fiberglass impact resistant panel, which can be bolted into position. It is suitable for soffits, columns and beams and it is easy to install. The thickness of panels and their aesthetic appearance are its negative attributes. The expected life of the conductive gel also needs consideration. 3.7.6 Water/soil anodes: In this case remote anodes consist of proven materials such as high silicon cast it on, lead, silver or 3 - 6mm diameter titanium rod embedded in coke breeze (conductive backfill) and secure din geo-textile bags in a trench. In shallow waters, the anodes are dug into the mud whilst those installed in seawater are normally located flush with piers and housed in a slotted PVC' pipe for protection from boat damage. These anodes are powered by an independently controlled output from the Transformer/ Rectifier and protect large areas of reinforcing steel in immersed concrete structures. These systems are mainly used in conjunction with other systems to address the problem of current dumping. 3.7.7 Sacrificial zinc anode systems: Sacrificial zinc anodes can be clamped onto concrete columns, immersed below the waterline or dug into the mud in order to provide protection to the buried/submerged and tidal zones. The degree of polarization will vary as a result of several factors including the amount of current output from the anodes, the rate of polarization obtained in the submerged/buried section, tidal variations and tidal resistivity. Over-protection of the steel reinforcement is not a concern as zinc anodes' current output is self-regulating with low driving voltage of 0.9 to 1.1 volts. 3.7.8 Sprayed zinc CP systems: This is a very simple system with a very low initial cost outlay. It requires 20

the application of 99.9% pure zinc by arc-spray method at a total thickness of about 400 microns. It is a sacrificial system, which has a life expectancy of 12 to 15 years that can be extended by whip blasting and re-spraying additional zinc material. It can also be installed as an impressed current system and a protective coating may be applied over it to further extend the useful life of the system. Sacrificial zinc anodes are also use din combination with these systems, which protect the submerged/ tidal zones and minimize current dumping. 3.7.9 Impressed current systems: These require electrical connections to distribute the impressed current across the anode, a DC power supply and an associated control system together with embedded monitoring probes providing data by which adjustments can be made to the voltage and currents applied. An installation will normally be divided into a number of zones, each with its own power supply. The design of the zones needs to take account of a number of factors such as the  Variation in moisture and chloride contents (and hence the conductivity, of the concrete) across the structure.  Continuity of reinforcement in different areas,  Presence of joints in the structure,  Requirements for different anode types  Variation in reinforcement provision. Commissioning is a very important stage in achieving an effective and durable CP system. It provides the opportunity to perform a variety of tests and trials establishing the initial behaviour of the CP system, make adjustments to current and voltage supplies and to verify control criteria. Once it has been established that an impressed current CP system is providing protection to all reinforcement, it is essential that the operation of the system be monitored and that it be properly maintained. Changes occur in the concrete over the first few months of operation (increase in resistivity due to removal of chloride ions and drying out of concrete). The objective of the monitoring is to ensure that all reinforcement remains effectively protected. For concrete structures with old types of pre-stressing steel, the risk of hydrogen-embitterment shall be analyzed when considered the implementation of impressed current CP systems. 3.8 Coatings to Concrete: In the past it was believed that concrete by itself is a durable material, which needs no protection or maintenance. This belief is no more hold good particularly on account of environmental pollution, industrial fumes and contamination of ground water. In addition to the coating of reinforcement by appropriate material, a surface coating to the concrete member is given to increase the durability further. The coatings serve the dual purpose of protection and decoration. Fig2 shows the reduction in depth of carbonation of the protected concrete. Giving protective coatings to major concrete structures such as bridges, flyovers, industrial buildings and chimneys have become a common specification in India as in

21

other countries. Four km long bridge on national highway at Cochin was recently coated. Almost all the flyovers at Mumbai are being coated for additional durability. There are number of approved coating systems available in the country however selection of coating depends on the severity of environment and the component of the bridge. 3.8.1 Epoxy Painting System by CECRI System: Central Electro-chemical Research Institute at Karaikudi has developed a coating system for controlling corrosion. This system is applied on the concrete surface and helps in controlling carbonation and weathering effects. The system comprises of 4 coats such as primer coat, middle coat and top coat all these are epoxy based. The fourth coat is finish coat of polyurethane and is recommended wherever the surface or part of the structure is exposed to ultra violet radiation. Before application, the substrata is thoroughly cleaned to remove the dust, hardened cement slurry, oily residues etc., by scrubbing with coarse wire brushes, grinding or sweep blasting methods depending on site requirements. Residual amounts of de-moulding agents, curing agents should be completely removed. Any cracks, crevices or surface blemishes should be treated with sealant prior to primer application. The subsequent coats shall be applied by brush or spray with an interval of 24 hours between the two coats. Application: Concrete surface to be painted shall be allowed maturation time of minimum 28 days before applying primer coating. Primer coating shall be applied to the cleaned surface after surface preparation within the pot life. After air-curing, intermediate and top coatings should be applied with time lag as per manufacturer's specifications. The coating shall be applied by brush or air-less spray gun. The paint application should aim to achieve minimum dry film thickness as specified by CECRI. Some typical thickness used are as under:    

Primer 100 microns Middle coat 100 microns Top coat 120 microns Finishing coat 40 microns

The DFT (Dry film thickness) shall be measured on 100x100mm steel plate attached with epoxy on the concrete surface at the rate one per l0m2. 3.8.2 Epoxy Phenolic IP Net (Inter-Penetrating Protective Coating system (CBRI Roorkee):

Polymer

Network)

MATERIALS 1. The coating materials shall meet the standards specified by various codes and formulation set forth by the patenter. 22

2. A written certification shall be furnished to the Department that properly identifies the number of each batch of coating material used in the work, material, quantity represented, date of manufacture, name and address of manufacturer and a statement that the coating material used must meet the requirements specified by CBRI-Roorkee. 3. The coating material shall be stored in the manner as per recommendations of the manufacturer until ready for use. The coating material shall be used within the manufacturer’s written recommended shelf life. 4. When a representative sample of the material is to be sent to outside laboratory, then the sample shall be packaged in an airtight container and identified by batch number. The sample will be got tested at Tenderer/ Contractor’s cost. SPECIFICATION OF COATING MATERIAL S. No.

Description

Primer coat

Middle coat

1.

Base

Interpenetrating Polymer (Epoxy phenolic)

Interpenetrating Polymer (Epoxy phenolic)

2.

Pot life

1 Hour for 2 lt.mix

1 Hour for 2 lt.mix

Top coat Interpenetrating Polymer (Aliphatic Polyurethane) 1 Hour for 2 lt.mix

3.

Curing

Air curing

Air curing

Air curing

4.

Colour

5.

Shelf Life

Clear Yellow/Grey Yellow/Grey One year in tightly One year in tightly One year in tightly sealed container One sealed container sealed container

6. 7. 8.

Dry film thickness 55-65 per coat (Microns) Coverage 5-6 sq.mt/lt. Recommended One No. of coats

9.

Recoatibility

10.

Mix proportion

90-100 per coat

40-50 per coat

4-5 sq.mt/lt

6-7 sq.mt/lt

One

One

4 hours to 7 days. Subsequent coat shall Ensure the surface is be applied after 6 dust and deposit free N. A. hours to 7 days prior to application. Base:1 PBV*/ Base:1 PBV*/ Base:1 PBV*/ Curing Agent:1 PBV* Curing Agent: 1PBV* Curing Agent: 1PBV* *PBV-parts volume

by *PBV-parts volume

by *PBV-parts volume

Material should conform to following properties: a. Tensile strength: Minimum tensile strength of the coating must be 15 N/mm 2 and it should be determined as per ASTM D-2370-73 b. Elongation: Minimum elongation of the coating must be 15% and it should be determined as per ASTM D-2370-73 23

by

c. Specific permeability: The maximum value must be 0.15 mg/cm2/mm/24hr and it should be determined as per ASTM D-1653-74 d. Adhesion with concrete: The minimum adhesion with concrete by pullout method must be 2.5N/mm2 and it should be determined as per BS-3900-E-10-79

SURFACE PREPARATIONS In order to have better bonding, the concrete surface should be clean, dry and mechanically sound. The surface of the concrete structure to be coated shall be cleaned of all traces of mould oil, laitance, salt deposits by mechanized means. The surface should be thoroughly scrubbed using power tools/sweep blasting. Finally, the surface should be washed with clean water jet to remove any salt deposits. The surface should be dried. All the protrusions should be removed and cracks, joints should be sealed with sealant. APPLICATION OF COATING 1. Mix the base and curing agent in prescribed proportion by volume thoroughly and allow it to remain in a container for ten minutes. 2. A primer coating of IPN polymer shall be applied to the cleaned surface after surface preparation within the pot life. 3. After air curing, intermediate and top coating should be applied with time lag as per manufacturer’s specification. 4. The coating shall be applied by airless spray or other approved means. COATING THICKNESS During the application of IPNet systems clean, abraded steel plates of approximately 10cm x 8cm shall be adhered to the concrete surface by means of putty/ adhesive in such a way that these can be detached. IPNet system can be applied over the plates in the course of application over the concrete surface. Dry film thickness (DFT) can be measured using magnetic electrometer. DFT measurement should be done at every 500-to 600-sqm areas or as per the direction of Engineer-in-charge. For acceptance purpose, at least 60% of all recorded total thickness measurement of the coating after curing shall be 200 ± 10 microns (minimum). Thickness measurement below 200 ± 10 microns shall be considered cause for rejection. The upper thickness limit does not apply to repaired areas of damaged coating. PERMISSIBLE COATING DAMAGE AND REPAIR OF DAMAGED COATING 1. All coating damage shall be repaired with patching material by the contractor at his own cost. 2. Repaired areas shall have a minimum coating thickness of 200 ± 10 microns 3. Repair of damaged coating shall be done in accordance with the patching material manufacturer’s written recommendations within the accepted rates.

24

3.8.3 Wet surface compatible Substructure coating system (CECRI Karaikudi): MATERIALS 1. The coating materials shall meet the standards specified by various codes and formulation set forth by the patenter. 2. A written certification shall be furnished to the Department that properly identifies the number of each batch of coating material used in the work, material, quantity represented, date of manufacture, name and address of manufacturer and a statement that the coating material used must meet the requirements specified by CSIR/CECRI Karaikudi. 3. The coating material shall be stored in the manner as per recommendations of the manufacturer until ready for use. The coating material shall be used within the manufacturer’s written recommended shelf life. 4. When a representative sample of the material is to be sent to outside laboratory, then the sample shall be packaged in an airtight container and identified by batch number. The sample will be got tested at Tenderer/ Contractor’s cost. SPECIFICATION OF COATING MATERIAL The coating system shall be moisture compatible for applying in wet/dry conditions as well as foundations and subsoil structures. The coating system shall conform to the following: 

Base

Quick curing two component moisture compatible Resin System



Drying time (touch dry)

2 hours



D.F.T. in two coats

300 – 350 microns



Chemical resistance

Excellent against chlorides, salts, sulphate, alkalies



Salt spray test

Should pass as per ASTM-B-117 1000 hrs minimum



Adhesion

3.0 KN minimum as per ASTM-D-4541



Resistance (Impedance)

108 ohms



Surface preparation

As per manufacturers specification or as per relevant IS codes

SURFACE PREPARATIONS In order to have better bonding, the concrete surface should be clean, dry and mechanically sound. The surface of the concrete structure to be coated shall be cleaned of all traces of mould oil, laitance, salt deposits by mechanized means. The surface should be thoroughly scrubbed using power tools/sweep blasting. Finally, the surface should be washed with clean water jet to remove any salt deposits. All the 25

protrusions should be removed and cracks, joints should be sealed with sealant as per Central Electrochemical Research Institute, Karaikudi (CECRI). APPLICATION OF COATING 1. Mix the base and curing agent in prescribed proportion by volume thoroughly and allows it to remain in a container for ten minutes. 2. First and Top Coat should be applied with time lag as per manufacturer’s specification. 3. Repair of damaged coating shall be done in accordance with the patching material manufacturer’s written recommendations within the accepted rates. COATING THICKNESS 1. Measurement of coating thickness shall be made using thickness measuring gauge and Elcometer. The minimum total thickness of coating (1st& Top coat) must be 300350 microns 2. For acceptance purpose, at least 60% of all recorded total thickness measurement of the coating after curing shall be 300-350 microns (minimum). Thickness measurement below 300 microns shall be considered cause for rejection. The upper thickness limit does not apply to repaired areas of damaged coating. PERMISSIBLE COATING DAMAGE AND REPAIR OF DAMAGED COATING 1. All coating damage shall be repaired with patching material by the contractor at his own cost. 2. Repaired areas shall have a minimum coating thickness of 300-350 microns 3. Repair of damaged coating shall be done in accordance with the patching material manufacturer’s written recommendations within the accepted rates. 3.8.4 Solvent/Water based Acrylic Elastomeric Coating 3.8.5 MATERIALS a. The coating materials shall meet the standards specified by various codes and formulation set forth by the patenter. b. A written certification shall be furnished to the Department that properly identifies the number of each batch of coating material used in the work, material, quantity represented, date of manufacture, name and address of manufacturer c. The coating material shall be stored in the manner as per recommendations of the manufacturer until ready for use. The coating material shall be used within the manufacturer’s written recommended shelf life. d. When a representative sample of the material is to be sent to outside laboratory, then the sample shall be packaged in an airtight container and identified by batch number. The sample will be got tested at Tenderer/ Contractor’s cost.

26

SPECIFICATION OF COATING MATERIAL S. Description Primer coat Middle coat No. Aqueous based Aqueous 1. Base Acrylic Acrylic

Top coat based Aqueous Acrylic

based

2.

Curing

Air curing

Air curing

Air curing

3.

Colour

Clear

Pigmented

Pigmented

4.

Shelf Life

One year in tightly One year in tightly One year in tightly sealed container sealed container sealed container

5.

Coverage

7-8 sq.mt/lt.

4-5 sq.mt/lt

4-5 sq.mt/lt

6.

Recommended No. of coats

One

One

One

Recoatibility

24 hours to 7 days. Subsequent coat shall Ensure the surface is be applied after 24 dust and deposit free N. A. hours to 7 days prior to application.

7.

Dry Film Total DFT in Three Coats 225±20 microns Thickness Material should conform to following properties: 8.

1.

Specific Gravity

IS-345

1.32 to 1.40

2.

Solid Content

IS-345

70% ± 3

3.

UV Resistance

ASTM-G-53

Should Pass

4.

Adhesion

ASTM-D-4541-02

Should Pass

5.

Bend Test (Conical Mandrell)

ISO-6860

Should Pass

6.

Difusion Dioxide

Tested like ASTM-E-96

Should Pass

7.

Resistance to water immersion

ISO-2812-2

Should Pass

Against

Carbon

Electrochemical Polarisation Tafel Extrapolation Should Pass Test SURFACE PREPARATIONS In order to have better bonding, the concrete surface should be clean, dry and mechanically sound. The surface of the concrete structure to be coated shall be cleaned of all traces of mould oil, laitance, salt deposits by mechanized means. The surface should be thoroughly scrubbed using power tools/sweep blasting. Finally, the surface should be washed with clean water jet to remove any salt deposits. The surface should be dried. All 8.

27

the protrusions should be removed and cracks, joints should be sealed with sealant as per manufacturers recommendation. APPLICATION OF COATING 1. Thoroughly mix the entire contents of the packaged tin prior to use. 2. A primer coating of the coating shall be applied to the cleaned surface after surface preparation. 3. After air curing, intermediate and top coating should be applied with time lag as per manufacturer’s specification. 4. The coating shall be applied by airless spray or other approved means. COATING THICKNESS During the application of the coating, clean abraded steel plates of approximately 10cm x 8cm shall be adhered to the concrete surface by means of putty/ adhesive in such a way that these can be detached. The coating can be applied over the plates in the course of application over the concrete surface. Dry film thickness (DFT) can be measured using magnetic electrometer. DFT measurement should be done at every 500-to 600-sqm areas or as per the direction of Engineer-in-charge. For acceptance purpose, at least 60% of all recorded total thickness measurement of the coating after curing shall be 225 ± 20 microns (minimum). Thickness measurement below 225 ± 20 microns shall be considered cause for rejection. The upper thickness limit does not apply to repaired areas of damaged coating. COATING CONTINUITY The coating shall be visually inspected after curing for continuity of the coating and shall be free from holes, voids, contamination, cracks and damaged areas discernible to the unaided eye. PERMISSIBLE COATING DAMAGE AND REPAIR OF DAMAGED COATING 1. All coating damage shall be repaired with patching material by the contractor at his own cost. 2. Repaired areas shall have a minimum coating thickness of 225 ± 20 microns 3. Repair of damaged coating shall be done in accordance with the patching material manufacturers written recommendations within the accepted rates. Bridge piers and girders are of considerable dimensions. Freshly made concrete members contain plenty of water in the pore structures. It takes long time to dry. Such freshly made concrete structures should not be coated with epoxy or other materials, which will seal off and prevent the internal moisture from going out in consonance with atmospheric conditions. The moisture trapped inside the concrete can do untold harm to the durability of concrete in addition to damaging the protective coating itself. For better durability, the concrete should be able to “breathe” i.e. water vapour should be able to migrate from inside to outside and from outside to inside. But water as it is, should not be able to enter from outside to inside. The protective coating given to the concrete should be of the above characteristics. Therefore, it is pointed out that the epoxy coating, which does not allow the concrete to breathe, should not be used for coating concrete members. Instead, the protective coating should be based on acrylics which retains the breathing property of concrete, while protecting the concrete from other harmful environmental agencies, in particular entry of water and carbonation. 28

In addition, epoxy based coating material is not resistant to ultra violet rays when exposed to sunlight and also it is not flexible. Whereas the coating material based on acrylic polymer is resistant to ultra violet rays of sun and is flexible. Coating is not only required for bridges, flyovers and industrial structures, it is also required for very thin members like fins, façade, sun breakers and other delicate concrete structures where specified amount of cover can not be given. Therefore, acrylic based protective cum decorative coatings can be given for additional durability of such concrete members.

Biggest world map was drawn on cooling tower in Germany using Emce Colour flex for protection of concrete subjected to aggressive acidic environment

Acrylic based protective cum decorative coating given to J-J Flyover at Bombay. particularly being in coastal region.

a

REFERENCE :-



Specifications for Road & Bridge Works issued by MORT & H and published by Indian Road Congress.



IRC – Highway Research Board – Special Report on “State of Art : Corrosion & Corrosion Protection of Pre-stressed Concrete Bridges in Marine Environment.”

29



Steel Corrosion in Concrete by Aron Bentur, Sidney Diamond & Neal S.Berke published by E&FN Spon – London.



Corrosion of Reinforcement in Concrete Construction by C.L. Pagci P.B. Bamforth, J.W. Figg (U.K.).



Corrosion Reinforced Steel in Concrete by Tonini-Gaides - ASTM Special Publication 713.



Concrete bridge Practice (Construction, Maintenance & Rehabilitation by Dr. V.K. Raina.



FIP Recommendations – Corrosion Protection of Unbonded Tendons.



FIP Guide to Good Practice – Maintenance of Pre-stressed Concrete Structures.



IS 12594 : 1988 HOT – DIP Zinc Coating on Structural Steel Bars for Concrete Reinforcement.



CRRI Paper on “Critical Evaluation of Fushion Bonded Epoxy Coated Reinforcements and other Protective Coating on Reinforcement” by M.V. Bhaskar Rao.



FIP Guide to Good Practice – Practical Construction.



Technical Report by Mr. P. Xercavins. PX consultants, France ( Formerly Technical Director of STUP, France).



Corrosion Handbook, a Joint Publication of The Electrochemical Society of India and Associate (Data) Publishers Pvt. Ltd.



Report on Investigation of Sharavathi Bridge, by STUP Consultants Limited/Hyder Consulting Limited, U.K. and STATS, U.K.

30