Fusion Bonded Epoxy Coated Reinforcement Bar
Fusion Bonded Epoxy Coated Reinforcement Bar
The integrity of concrete structures is largely dependent upon the durability of the reinforcement steel bars. Corrosive elements penetrate the permeable concrete subjecting the reinforcement bar (rebar) to corrosion. The resulting corrosion products which form on steel occupy a much greater volume than the iron that they replace. The volume occupied by corrosion products is as much as 20 times to 30 times the original volume of steel. The volume increase causes tremendous internal pressure within the concrete. As corrosion continues, the pressure eventually exceeds the tensile strength of the concrete. This internal pressure in turn, causes the concrete to crack and spall, allowing greater access of corrodents to the steel, further accelerating the deterioration of the concrete structure.
The pressure because of the volume expansion induces cracks in the surrounding concrete, which aggravates further corrosion of steel and results in total loss of interaction between steel and concrete. A solution to minimize such corrosion problems is to apply protective coating on reinforcing steel. Fig 1 shows damage to the concrete structure because of the corrosion of the reinforcement bar.
Fig 1 Damage to concrete structure due to corrosion of rebar
Corrosion of reinforcement steel bar in concrete is one of the major deterioration mechanisms for reinforced concrete systems which affects the overall safety and serviceability of the concrete structures. Several reinforced concrete infrastructure constructions are designed for a corrosion-free service lives of more than 100 years. To achieve such long service lives, ‘fusion bonded epoxy coated steel reinforcement bars’ are widely used strategy in anticipation of delaying the initiation of corrosion among other available strategies such as use of supplementary cementitious materials, and corrosion inhibitors.
The corrosion problems are caused primarily by chloride-induced corrosion of steel in concrete. Chloride penetrates the concrete from sources such as road deicing salts or sea exposure. It can also be built in through the use of salt-contaminated aggregate, seawater in the concrete or chloride-based admixtures. Upon achieving a sufficiently large concentration, the chloride causes the depassivation and subsequent rapid corrosion of the steel.
The most important requirements of the coating system for steel reinforcement bars are (i) strong adhesion to the surface, (ii) long-term chemical and mechanical resistance of coating materials at all temperatures during all seasons under working environment, (iii) high mechanical impact strength, and (iv) penetration resistance to all aggressive chemicals. The reinforcement bar coating system is to be equipped with a wide range of performance properties which include (i) low permeability to water vapour and gas pressures, (ii) mechanical protection against handling and transportation damage, (iii) superior resistance to disbonding, (iv) outstanding dielectric properties, (v) aging resistance against heat and resistance against oxidation damage, and (vi) strong resistance to ambient conditions such as corrosive soils, salt water, micro-organisms, and penetration of plant roots. One of the popular coating systems is the fusion bonded epoxy (FBE) coating.
Fusion bonded epoxy coating is widely used to protect concrete reinforcement bars, steel pipes, and piping connections etc. used in construction. Fusion bonded epoxy coatings are in the form of dry powder at normal atmospheric temperature. The powder when applied electrostatically on to the surface cleaned, hot steel bars, that it fuses, melts, flows and cures to an adherent continuous chemically cross-linked protective film. Fusion bonded epoxy coating is used to protect reinforcement steel in concrete in the most adverse environments that which prone to corrosion.
Fusion-bonded epoxy coating principally protects against corrosion by serving as an electrochemical and a physical barrier that isolates the steel from the oxygen, moisture, and chloride ions which cause corrosion. Epoxy coating has high electrical resistance, which blocks the flow of electrons that make up the electrochemical process of corrosion. In addition to serving as a circuit breaker, the coating protects in way which is less obvious i.e., coating reduces the size and number of potential cathodic sites, which limits the rate of corrosion reaction that can occur. As a matter of fact, for macrocell corrosion to take place, a large area of steel surface is needed to serve as the cathode where oxygen reduction can occur. The coating almost eliminates such cathodic reaction.
The micro-climate at the steel-coating interface influences the corrosion of fusion bonded epoxy coated reinforcement bars. The presence of hydrogen and oxygen in the interface between the coating and steel surface is essential to enable cathodic reaction. If only water and hydrogen are in contact with the steel without additional ions, an extended diffuse double layer can form, which prevents electro-chemical reactions. However, any kind of existing ions at the steel-coating interface affects the development of the electric double layer. If aggressive ions such as chloride ions reach the interface, then anodic reactions are also possible which results in the corrosion initiation of steel. The chloride threshold can be defined as the minimum chloride concentration needed at the steel surface to initiate corrosion.
In case of fusion bonded epoxy coated steel reinforcement bars, chlorides have to travel through the cover concrete or mortar and then through the epoxy coating to reach the steel surface beneath. In reinforced concrete structures with good quality epoxy coating (say, chloride diffusion coefficient around 10 square meter per second to 20 square meter per second), the time taken for chlorides to diffuse through the coating can be considerable and is to be considered while estimating the corrosion-free service life.
The mechanisms of prevention of corrosion of steel using fusion bonded epoxy coatings are (i) physical barrier or shielding from the surrounding environment (oxygen, moisture, and other deleterious elements), (ii) limiting the formation of anodes and cathodes on steel surface, and (iii) high electrical and ionic resistance making it difficult to form corrosion cells or circuits. Fig 2 shows corrosion of epoxy coated steel in concrete.
Fig 2 Corrosion of epoxy coated steel in concrete
Fusion bonded epoxy coated reinforcement bars have been in use in concrete structures for over thirty years. Evaluations of hundreds of structures in several different environments have shown that epoxy-coated reinforcement bars are performing to reduce the corrosion which damages the concrete structures. Several studies have shown the way to provide even improved corrosion performance of the fusion bonded epoxy coated reinforcement bars. These studies have resulted in significantly improved standards and application processes which provides even better performance in the future. Fusion bonded epoxy coated reinforcement bars are a sound choice if the reinforcement steel bars in the concrete are to be exposed to corrosive agents such as salt. Life cycle cost analyses produced by the Concrete Reinforcing Steel Institute have shown that the fusion bonded epoxy coated reinforcement bars are a cost-effective solution even if structure life is extended only by 1 year to 2 years.
The evaluations of several concrete structures has shown continued successful performance of fusion bonded coated reinforcement bars to protect structures from corrosion induced deterioration. New information generated by these evaluations and the several research studies are changing and improving the coating and construction industry. Significant improved attention to detail has resulted in far superior coated reinforcement compared to only a few years ago.
Epoxy-coated reinforcement steel bars is defined by ASTM A775/A775M Standard Specification for Epoxy-Coated Steel Reinforcing Bars as ‘reinforcing bars with protective epoxy coating applied by the electrostatic spray method’. Fusion bonded epoxy-coated reinforcement bars have the same weight per meter as normal ‘black’ carbon reinforcement steel bars.
Fusion bonded epoxy coating, also known as fusion bond epoxy powder coating is an epoxy-based powder coating which is widely used to protect concrete reinforcement bars from corrosion. Fusion bonding refers to the process used to apply the coating of the reinforcement steel bars. The steel bar is cleaned and the surface is heated to 200 deg C to 230 deg C and then passed through an electrostatic spray containing fine epoxy powder. The powder is attracted to the bars based upon electrostatic forces. When the epoxy encounters the heated bars, it melts and fuses forming a thermosetting polymer. The resultant coating is significantly more uniform in thickness than can be achieved by other methods, such as paint coating or dipping.
The epoxy coating reduces the bond between the reinforcement and concrete. Bond in reinforced concrete is described by three components viz., chemical adhesion, friction, and mechanical interaction between the bar ribs and the surrounding concrete. The resultant of these forces acts inclined to the bar and can be resolved into two components namely, radial force and bond force as shown in Fig 3.
Fig 3 Transfer of forces at the interface
The fusion bonded epoxy coating process for reinforcement bars was developed in United States in 1960s and its use was strongly recommended in coastal areas. Since Its introduction, fusion bonded epoxy coating formulations had gone through vast improvements and developments. Today, different types of fusion bonded epoxy coatings, which are tailor made to meet several requirements are available. Modern application techniques for applying powders fall into four basic categories. These are (i) fluidized bed process, (ii) electrostatic bed process, (iii) electrostatic spray process, and (iv) plasma spray process.
The electrostatic spray process is the most commonly used method of applying powders. In this process, the electrically conductive and grounded object is sprayed with charged, non-conducting powder particles. The charged particles are attracted to the substrate and cling to it. The epoxy powder is applied by electrostatic spray on hot steel on preset temperature level. The powder, when in contact with hot bar, melts flows, gels, cures, cools, and produces a well adhered continuous corrosion resistant protective coating. The thermosetting of the epoxy is an irreversible process and provides a good protection to reinforcement bars against corrosion. It prevents attack of chloride ion on the metallic surface and the occurrence of electro chemical reaction initiating corrosion of steel.
Fusion bonded epoxy coatings are thermoset polymer coatings. The most widely used types include acrylic, vinyl, epoxy, nylon, polyester, and urethane. Most popular for reinforcement bars is epoxy coating. The name fusion bond epoxy is because of resin cross linking and the application method, which is different from a conventional paint. Fusion bonded epoxy coatings are 100 % solid coatings applied as dry powders and formed into a film by heating.
In epoxy coated bars, the frictional resistance seems to be low and hence the bond force decreases. Studies on coated and uncoated plates show that the average coefficient of friction with uncoated and coated bars is 0.52 and 0.46 respectively. Bond strength of epoxy coated reinforcement bar splices confined with nominal lateral reinforcement is because of the decrease of the bond strength of coated bars. Based on the experimental results, it has been reported that the ratio of the coefficient of friction of coated and uncoated plates is 0.875.
The coating on reinforcement bars also lowers the relative rib area and reduces the transfer of force from concrete to the reinforcement bar because of decrease in mechanical interaction. The coating on the reinforcement bar is a weak layer between the reinforcement and the surrounding concrete. The shear deformation of the coating improves the slip between the concrete and the reinforcement. Hence, epoxy coating on the reinforcement influences the bond strength and hence the serviceability of reinforced concrete structures. These negative effects of coating can be minimized by (i) providing more splice length, (ii) large cover to the reinforcement, and (iii) lateral confinement over the splice region.
Increasing the cover to the reinforcement can be effective before cracking of the concrete. Most of the slip of the reinforcement bar occurs after cracking of the surrounding concrete. After cracking, the lateral confinement provides resistance to slipping of reinforcement bar. Hence, the behaviour of the coated reinforcement bars is improved by providing lap length as needed for the uncoated reinforcement bars with increased confinement. Also, overlapping of reinforcement bars with inadequate confinement causes brittle failure. Hence the epoxy coated lap splices are to be provided with adequate confining reinforcement to ensure ductile failure. The lateral confinement can be accomplished by transverse steel reinforcement or by fiber reinforcement or fiber reinforced primer (FRP) wrapping.
Chemistry of fusion bonded epoxy coating
The main components of a powder coating are (i) resin, (ii) hardener or curing agent, (iii) fillers and extenders, and (iv) colour pigments. The resin and hardener together are known as the ‘binder’. The resin part is an ‘epoxy’ type resin. ‘Epoxy’ or ‘oxirane’ structure contains a three members cyclic ring (one oxygen atom connected to two carbon atoms) in the resin molecule. Most commonly used fusion bonded epoxy resins are derivatives of bisphenol A and epichlorohydrin. However, other types of resins (e.g., bisphenol F type) are also commonly used in fusion bonded epoxy formulations to achieve different properties, combinations, or additions. Resins are also available in various molecular lengths, to provide unique properties to the final coating.
The second most important part of fusion bonded epoxy coatings is the curing agent or hardener. Curing agents react either with the epoxy ring or with the hydroxyl groups, along the epoxy molecular chain. Several types of curing agents, used in the fusion bonded epoxy manufacture, include dicyandiamide, aromatic amines, and aliphatic diamines, etc. The selected curing agent determines the nature of the final fusion bonded epoxy coating such as its cross-linking density, chemical resistance, brittleness, and flexibility etc. The ratio of epoxy resins and curing agents in a formulation is determined by their relative equivalent weights.
In addition to these two major components, fusion bonded epoxy coatings include fillers, pigments, extenders and different additives, to provide desired properties. These components control characteristics such as permeability, hardness, colour, thickness, and gouge resistance etc.
There are several factors which affect the characteristics of fusion bonded epoxy coating. The desired properties of epoxy resin can be achieved by alteration of chemical composition or by controlling the curing time of epoxy coating. For example, mechanical properties such as modulus of elasticity and mechanical strength of fusion bonded epoxy coating can be controlled by curing time and suitable dosage of nano-materials. Also, the desired corrosion resistance can be achieved with thin coating, say a minimum of around 200 micrometers. The coating thickness of greater than 420 micrometers can result in a reduction of the bond between coated steel and concrete. It has also been reported that the coating thickness of less than 100 micrometers can result in several manufacturing defects such as pinholes, discontinuous coating, lower impact resistance, and lower flexibility. The pinholes in the coatings can be reduced or eliminated by using additives and fillers. For example, the use of nano-particles such as Fe2O3 (ferric oxide), clay, and carbon black can improve the packing of epoxy coating.
Use of around 1 % of clay or Fe2O3 can improve the corrosion resistance of steel by providing denser microstructure of the coating and reducing the water penetration through the coating. Whereas, controlled curing and the use of carbon black nano-particles can improve the mechanical properties. Also, the tensile strength of coating and bonding between steel and coating can be improved by using barium sulphate. Similarly, corrosion resistance of coated steel can be improved by increasing the cross-linking and adhesion forces.
It has been reported that the dielectric and interface properties of the coated steel reinforcement bars can be improved by the use of barium titanate. It has also been reported that the addition of around 8.5 % of polyamide to epoxy can increase the density and resistance of epoxy coating. Also, it has been reported that the use of around 10 moles per litre of CaCl2 (calcium chloride) can increase the electrical resistance of the coating.
The use of additives can also improve the resistance to ultra-violate (UV) degradation. For example, the use of around 2 % of carbon black nano-particles can improve the ultra-violate degradation resistance by two times. The use of photo-stabilizers such as TiO2 (titanium oxide) and ZnO (zinc oxide) are widely used additives to improve the resistance to ultra-violate degradation. Also, uniform distribution of such photo-stabilizers in the coating material is important for achieving effective and uniform ultra-violate degradation resistance. Typically, such chemical compositions are also used for the manufacture of the fusion bonded epoxy coating.
Standards such as ASTM A775 and other standards specify the different coating characteristics of the fusion bonded epoxy coated reinforcement bars. These characteristics are coating thickness, coating continuity, coating flexibility, limitation on damage to the coating, salt spray resistance, resistance to chloride permeability, abrasion resistance, impact resistance, and resistance to cathodic disbondment. The comparison of specifications of standards indicates that a few of these specifications need to be more stringent. For example, ASTM A775 specifies that the maximum quantity of repaired damage is not to exceed 1 % of the total surface area of coated steel reinforcement bars in each 0.3 meters length of the reinforcement bar.
However, photographic evidence is available, showing the widespread use of fusion bonded epoxy coated steel reinforcement bars with significant damage to the coating. These include macro-scale damage such as scratches and cracks and micro-scale damage such as pinholes and micro-cracks. Also, the quantification of such damage on coating is very challenging. ASTM A775 recommends using the holiday detector for assessing the coating continuity which beeps when there is an electrical short circuit between the sensor and the underlying steel. The assessment of fusion bonded epoxy coated steel reinforcement bars using the holiday detector is time-consuming because of the small-damaged surface area and large quantity of coated steel reinforcement bars. Fusion bonded epoxy coating can shrink and crack when exposed to sunlight or ultra-violate (UV) rays. At early exposure times, the cracks are not deep enough to get the electrical short circuit between the sensor and underlying steel. Hence, the assessment of coating continuity using holiday detector at the manufacturing unit alone (i.e., before the possible sunlight exposure) is not adequate. In addition, ASTM A775 recommends to repair all the visible damage at construction sites. However, there can be defects, which are not visible to the naked eyes such as pinholes, UV-induced cracking, which need to be considered in the specifications. Also, the effect of repair of scratch damage at sites by repair epoxy on their electrical resistance is unknown. In short, the existing specifications need revision.
Several studies have been carried out on the performance of the fusion bonded epoxy coated steel in different environments and service conditions. Most of the laboratory test results confirmed that the fusion bonded epoxy coated material, applied under controlled conditions, passed successfully all qualification and service simulated tests. However, in a few cases, field samples showed poor adhesion to the extent of delamination and disbonding of the fusion bonded epoxy reinforcement bar. Frequently, the term delamination is being used as the definition for a ‘coating failure,’ rather than corrosion, or concrete distress.
Corrosion control of the fusion bonded epoxy coating is a function of the coating’s ability to provide a barrier against water, oxygen, chloride, and other aggressive elements which prevents permeation through the coating film to attack the metal substrate. There are critical properties needed for corrosion protection fusion bonded epoxy coatings which include adhesion and wetting ability to the reinforcement bar. Reduction in adhesive strength increases the delamination process rate.
A study into the delamination of fusion bonded epoxy coatings in a simulated pore solution environment suggested the delamination mechanism which includes (i) delay time before initiation of observable delamination processes can be a function of water penetration through the coating to the interfacial or interphasial coating / substrate region, (ii) delamination of fusion bonded epoxy coatings from steel substrates is predominantly caused by hydroxyl ions, (iii) rate of fusion bonded epoxy delamination is controlled by transport processes from a pore in the coating and along the delaminated coating / substrate interface to the disbondment front, (iv) the locality of failure of coating adhesion is in the interfacial or interphasial coating / region, (v) the rate of fusion bonded epoxy delamination in near-passive conditions is controlled by hydroxyl ion migration from the bulk external solution to the coating / substrate disbondment front, and (vi) the rate of fusion bonded epoxy delamination in the condition of under film corrosion is controlled by hydrated cation movement to the cathode site.
Design of fusion bonded epoxy coating powder for steel reinforcement bar coating
New technologies are under continual development to optimize the properties of the fusion bonded epoxy coating to improve coating utility. The stoichiometric ratio is to be controlled by the equilibrium between the curing group and the epoxy group. For example, increasing the level of curing agent can reduce the cross-link density and increase flexibility, while decreasing chemical resistance. Impact resistance or hardness is a function of the cross-link density. Higher densities can be achieved using low molecular weight curing agents which show tightly cross-linked structures. Adding non-reactive diluents can interfere with this structure, providing the end product with more flexibility but less toughness.
Mechanical adhesion is the gripping force which results from the roughness of the substrate, (i.e., peaks and valleys). Changing from a round to angular surface profile and increasing the depth of the valleys can improve this type of adhesion. Polar adhesion is the hydrogen bonding which occurs between the substrate and epoxy coating. Chemical bonds are formed through electron sharing by groups on the substrate and epoxy resin. These bonds are by far the strongest and contribute most to adhesion. Groups such as nitrogen and oxygen can bond with iron and silica.
For coated steel, an occasionally expressed concern is about macrocell pit corrosion. Macrocell driven pit corrosion implies a large cathode area driving a small anode such as in a holding tank with coated sides, but an uncoated bottom. The very nature of damage to the epoxy coating on the reinforcement bar makes that scenario unlikely. Even in the case of uncoated bottom mats and coated top mats, there are few, if any, reports of pit corrosion resulting in considerable loss of cross section. This is likely due to the relatively limited interconnectivity between bars because of the insulating characteristics of the coating.
In general, uncoated bars provide better bond strength than coated bars. There are three components to bond strength namely (i) adhesion, (ii) friction, and (iii) mechanical bearing of the concrete on the steel deformations. Both adhesion and friction relate to roughness of the steel (or coating). Since fusion bonded epoxy coated reinforcement bar is smooth and concrete does not adhere well to its surface, bond strength develops primarily through mechanical bearing. Different studies have given different results, but most give values of 65 % to 90 % as the relative level of bond strength for epoxy-coated bar compared to black bar.
There are several other factors which considerably affect the bond strength of reinforcement bar whether coated or uncoated. These are cover, casting position, concrete slump, and degree of consolidation. There is a nearly linear increase in bond strength with increasing concrete cover. Casting position affects bond strength since increasing the quantity of concrete below the bar increases settlement and bleeding which lowers bond strength. Ultimate bond strength decreases with increasing slump. Lack of vibration reduces bond strength. In summary, while there are several other significant variables, coating on reinforcement bar does reduce relative bond strength, which means that increased development length is needed for splice and anchorage lengths and there is no requirement for increased cross-sectional area of the steel. In practical terms, this has very little influence on the overall cost of a construction project. For example, using a typical bridge design for a bridge having a length of 920 metre and a width of 15 metre, the added splice length is around 370 metre of additional reinforcement bar for which an additional cost is to be incurred. This results in around 0.005 % increase in total cost of the project.
Because of concern raised about the effect of loss of adhesion of epoxy coating to steel on bond strength, a study comparing pullout strength among uncoated bars, coated bars, and debonded coated bars has been done. The findings have shown that there are measurable differences, but they are not large enough to constitute a structural safety problem. The conclusion of the study is that ‘a 20 % to 30 % degree of disbondment between the epoxy coating and its steel substrate for bars used as the main flexural reinforcement of a one-way slab does not compromise the slab’s flexural capacity’.
The process of coating fusion bonded epoxy on reinforcement bars
Fusion bonded epoxy is very fast curing, thermosetting protective powder coating. It is based on specially selected epoxy resins and hardeners. The epoxy is formulated in order to meet the specifications related to protection of steel bars as an anti-corrosion coating. The application of fusion-bonded epoxy to reinforcement steel bars is straightforward and uncomplicated and consists of the steps which are (i) surface preparation, (ii) induction pre-heating to proper temperature, (iii) application of fusion bonded epoxy powder, (iv) cooling and coating of coating, (v) inspection and testing, and (vi) handling, storage, and transportation. However, the details are important and are to be understood and implemented to assure a quality coating which extends the working life of a structure in a corrosive environment. The same steps apply whether the steel is fabricated before or after coating. However, the equipment configuration and the powder coating gel and application characteristics need to be designed to meet the coating process. Fig 4 shows process of coating fusion bonded epoxy on reinforcement bars.
Fig 4 Process of coating fusion bonded epoxy on reinforcement bars
Fusion bonded epoxy coating is in the form of dry powder at normal atmospheric temperature. The powder when applied electrostatically on to the surface cleaned, hot steel reinforcement bars, that it fuses, melts, flows and cures to an adherent continuous chemically cross-linked protective film.
Surface preparation – Surface preparation is necessary for ensuring the ability of the coating to bond to the reinforcement bar surface. This bonding is important to eliminate the environmental fluid migration between the substrate and the reinforcement bar coating.
Reinforcement bars are first cleaned from surface contaminations such as oil and grease etc. by solvent cleaning or by burn off. After degreasing, the surface is cleaned using steel grit to achieve surface profiles of 40 micrometres to 110 micrometres. During blast- cleaning, the bar surface temperature is to be more than 3 deg C above the dew point. The bar surface temperature is always to be more than 5 deg C. The relative humidity is not to be higher than 85 %. Abrasives are to be stored and used dry.
Reinforcement bars are blast-cleaned to a near white metal finish using abrasive grit in a shot blaster. The shots clean the surface of the bar. The grits provide an anchor to the bar surface, which cleans the steel of contaminants, mill scale, and rust. It also roughens the surface to give it a textured anchor profile. During this process, salt and other mineral contamination are removed.
Induction pre-heating – The blasted bars are heated to required temperature specified by the epoxy powder manufacturer (ranging from 200 deg C to 240 deg C) by passing them through an electric induction heater. However, in some installations, gas fired heating is also used.
The bars moved at a scheduled speed. Uniform heating is of paramount importance for good coating properties. Induction heating is the only method for this coating process. It is essential to serve a clean oxide-free surface to the heater in order to achieve a constant application temperature.
Fusion bonded epoxy powder application – The heated steel is passed through a powder-spray booth where the dry epoxy powder is sprayed from a number of spray nozzles. As the powder leaves the spray gun, an electrical charge is imparted to the particles. These electrically charged particles are attracted to the grounded-steel surface providing even coating coverage. It also bonds with the steel. The heat also initiates a chemical reaction that causes the powder molecules to form complex cross-linked polymers which give the material its beneficial properties. Coating thickness in the range of 50 micrometers to 150 micrometers are normally obtained, even though lower or higher thickness ranges can be specified, depending on the service conditions.
When the dry powder hits the hot steel, it melts and flows into the anchor profile (i.e., the microscopic peaks and the valleys on the surface) and conforms to the ribs and deformations of the bar. The melted dry powder quickly gels as a film on the reinforcement bars and on its deformations while the residual heat cures the coating.
The heat also initiates a chemical reaction which causes the powder molecules to form complex cross-linked polymers which give the material its beneficial properties. The melted epoxy resin reacts with the curing agent present in the fusion bond epoxy and bonds to the substrate, providing a highly cross-linked polymer with a sophisticated network of covalent and coordinate bonds. These high energy bonds provide excellent adhesion between the coating and substrate.
The amongst three adhesion forces between the fusion bond epoxy and substrate, the two of them i.e., chemical and polar-polar adhesion are directly related to the number of bondable sites available on the substrate. Hence, to provide the maximum bondable site to fusion bond epoxy, the high peak heights are achieved by abrasive cleaning. It also depends upon the viscosity of the fusion bond epoxy system used and the application temperature.
Fusion bond epoxy has to achieve at least 97 % cross-linking (cure) in order to have the optimum properties. The curing process is a heat-related phenomenon. It depends on the initial steel temperature and the duration which the bar retains the heat. Even if the initial steel temperature is low, one can reduce the bar travel speed in the coating line to allow enough heat energy for the curing process to be completed. All the over-sprayed powder is recollected and automatically recycled.
Cooling and curing – The molten powder becomes a solid coating, when the ‘gel time’ is over, which normally occurs within few seconds after coating application. The resin part of coating undergoes cross linking, which is known as ‘curing’ under the hot condition. Complete curing is achieved either by the residual heat on the steel, or by the help of additional heating sources. During curing time, the coating gets hardens to a solid. Depending on the fusion bonded epoxy coating system, full cure can be achieved in less than one minute to a few minutes in case of long cure fusion bonded epoxies.
The coated bars after curing are passed through a cooling tunnel. In this tunnel, water is sprayed on to the bars to cool them. Water quench quickly reduces the bar temperature to facilitate testing and handling.
Inspection and testing – The cured coated bars are kicked off on to the final inspection rack for testing and inspection. The testing on coated bars is carried out as per the specification requirements. Fusion bonded epoxy coated reinforcement bars are tested at the coating plant as per the requirements specified in the relevant standard. The acceptance tests normally conducted are thickness measurements, flexibility test, and holiday test. On line and off line holiday checks and thickness checks are carried out. The adhesion of coated bars is also tested frequently by bending of the bar. Besides this, different other tests for every batch are done in laboratory such as chemical resistance, short spray, resistance in continuous boiling water, abrasion resistance and impact resistance etc. Once the bars are inspected, they are bundled / strapped for dispatch to the job site for fabrication.
Handling, transportation and working – Fusion bonded epoxy reinforcement bars are to be handled with extreme care so that coating is not damaged during storage, transportation, and handling / concreting. The fabrication and field handling of epoxy-coated reinforcement steel bars is covered by ASTM 208 D3963 and in the Appendixes of ASTM A775/A775M or A934/A934M.
Fusion bonded epoxy coated reinforcement bars need padded contacts during transportation, stacking, handling till they are used in concrete. Synthetic or padded slings are to be used and at no time fusion bonded epoxy-coated reinforcement bars be lifted using bare chains or cables. The cut ends, welded spots and handling damages are required to be repaired with special liquid epoxy compatible with the coating.
When lifting individual bars or bundles of epoxy-coated reinforcement bars, spreader bars or strong backs with multiple pick-up points are to be used to minimize sags. Bundles of fusion bonded epoxy coated reinforcement bars are to be stored off the ground on suitable materials, such as timber cribbing. Fusion bonded epoxy coated reinforcement bars are to be stored separately from uncoated carbon steel reinforcement bars to prevent abrasion of coating. During the storage and shipping, all contact points (e.g., trailers, storage racks) are to be wood or plastic-lined.
Fusion bonded epoxy coated reinforcement bars are to be covered using opaque polyethylene sheeting or other suitable opaque material if they are to be stored outdoors for more than a month. These reinforcement bars are to be protected against coating damage through appropriate lifting, handling, placing, and concrete placement operations. During placement, these reinforcement bars are to be lifted and set in place. Fusion bonded epoxy coated reinforcement bars are not to be dragged into place and other materials are not to be dragged across placed epoxy coated reinforcement bars. Movement of personnel and materials across the fusion bonded epoxy coated reinforcement bars are to be minimized.
Prior to concrete placement, fusion bonded epoxy coated reinforcement bars are to be inspected and damaged coating repaired with a two-part epoxy material meeting ASTM 235 A775/A775M or A934/A934M requirements. Plastic-headed vibrators are to be used to consolidate the concrete. When placing fusion bonded epoxy coated reinforcement bars, all wire bar supports, spacers, and tying wire are to be coated with dielectric material, for example, an epoxy-coated or plastic-coated material compatible with concrete.
Fusion-bonded epoxy coatings can undergo surface discoloration and chalking from exposure. If extended exposures occur, then it is strongly recommended that the reinforcement bars are carefully inspected and any site of damage or localized corrosion is repaired following ASTM D3963 using a two-part epoxy, recommended for use on epoxy-coated steel reinforcement bars.
Field evaluations of fusion bonded epoxy-coated reinforcement steel has shown that if corrosion does occur, it can cause bond loss of the coating to the steel, if an actual cathodic / anodic cell develops. Focused corrosion in the area of damage which can compromise the structural performance does not normally occur.
Fusion bonded epoxy coated reinforcement bars can be used in structures with other reinforcement steel bars. However, when using epoxy-coated reinforcement steel bars in decks, it is recommended that all the deck steel is coated as this reduces the overall rate of corrosion if the coating is damaged. In piers, the use of fusion bonded epoxy-coated reinforcement steel bars is to be continued into an area above the splash zone to minimize corrosion risks.
The development length is the length of bar embedded in concrete needed to achieve yield of the steel. This value is increased for fusion bonded epoxy-coated reinforcement bars as the coating can reduce the bond to the concrete by 15 %. Fusion bonded epoxy coatings are stable in high pH materials and do not degrade in concrete.
All exposed steel in the fusion bonded reinforcement bars is to be coated using a two-part epoxy. Normally, end coating is conducted as part of standard fabrication practice. The process for repairing damaged coating on fusion bonded epoxy coated reinforcement steel bars involves cleaning any corrosion off the bars at the damage site using a wire brush followed by application of a two-part epoxy repair material, typically using a paint brush.
Fusion bonded epoxy coated reinforcement steel bars can be spliced using either lap splices or mechanical couplers. Use of the particular method depends on several factors and this becomes likely an economic decision. For smaller sizes of the reinforcement bar, the ‘extra’ length of fusion bonded epoxy coated reinforcement steel bar to facilitate the splice requirements is likely to be cheaper than the selected coupler. For the larger sizes of the reinforcement bars, the coupler becomes more economical than the ‘extra’ length of bar used to make the splice. A mechanical coupler can, however, be a better alternative given job specific constructability conditions, congestion issues, and / or spacing requirements.
Several mechanical couplers are commercially available in standard size threaded couplers. Some of these are coated with epoxy coating, while others are uncoated and protected using a water-proof sleeve at the jobsite. As with any coupler, test data are to be utilized to determine suitability of available products. When couplers are used, they are to be inspected for any coating damage prior to the placement of concrete. If damage is observed, the steel is to be lightly cleaned to remove any surface corrosion and coated with an approved two-part epoxy coating, formulated for use with fusion bonded epoxy coated reinforcement steel bar.
Fusion bonded epoxy coated reinforcement steel bars without damage perform better than the bars with undamaged coatings. However, field and laboratory data has shown that even bars with damage perform considerably better than uncoated bars.
Fabrication of fusion bonded epoxy-coated reinforcement bars uses the same process as for carbon reinforcement steel bars except that the bending pins are covered with a polymer outer wrap. However, the contact surfaces of equipment used to fabricate or handle fusion bonded steel reinforcement bars are to be protected using plastic or other material to protect the bars against damage.
Fusion bonded epoxy coated steel reinforcement bars have the several sustainable attributes such as (i) manufactured using reinforcement bars which are made using almost 100 % recycled steel, (ii) can be readily recycled after use, (iii) manufactured using low quantities of energy compared with other systems, (iv) the coating process produces no VOCs (volatile organic compound) during manufacture or use, (v) structures which use fusion bonded epoxy coated steel reinforcement steel bars are more durable than those which do not use these bars, and (vi) fusion bonded epoxy-coated reinforcement bars are easily available.
Fusion bonded epoxy coating is widely used coating in the reinforcement steel bars. It is the preferred over other protective systems, because of its several advantages. It acts as a barrier to corrosive chemicals and moisture. It is applied on preheated steel as a dry powder, which melts and cures to a uniform coating thickness. The corrosion control property of the coating is dependent on its ability to be an excellent barrier against water, oxygen, chloride, and other aggressive elements through concrete to the metal surface. However, for a fusion bonded epoxy coating to be an effective long-term corrosion protection system, it is necessary that it stays bonded to the substrate during the entire life of the structure.
Advantages and disadvantages of fusion bonded epoxy coated reinforcement bars
Fusion bonded epoxy coating on reinforcement bars has several advantages which include (i) since the coating is done on the coating lines, better quality control is achieved, (ii) fusion bonding process gives uniform coating thickness, (iii) there is good bonding of coating with the steel as fusion bonded epoxy has very good adhesive properties, (iv) because of flexibility, the coating does not get damaged when the straight bar is bent during fabrication on a special mandrill, (v) fusion bonded epoxy coating acts as insulator for electro chemical cells and offer barrier protection to steel which prevents chloride ions through it, (vi) there are well established criteria for acceptance of fusion bonded epoxy coating in different standards, (vii) fusion bonded epoxy coated reinforcement bars provide the most effective corrosion protection to the reinforcement bars, (viii) fusion bonded epoxy coated reinforcement bars provide extended service life to the concrete structures, (ix) fusion bonded epoxy coated reinforcement bars have cost effective life-cycle and need low initial investment
The disadvantages of fusion bonded coating on reinforcement bars include (i) there is reduction in the bond strength between coated rebars and concrete, (ii) as the technology is plant based, there is need for double handling and transportation of reinforcement bars, (iii) handling of coated bars is to be done with utmost care to avoid damage to the coating, (iv) performance of the fusion bonded epoxy coated rebars is heavily dependent upon least defect in the coating, (v) patching in the defective area is not always effective, (vi) even a small damage in the coating can initiate corrosion in severe environment, since the coating has no cathodic protection and because of this, corrosion cells are set up in the damaged area of the reinforcement bars which leads to first de-lamination of the epoxy coating and then rusting, (vii) being a barrier type coating, it facilitates localized pitting corrosion through pinholes, (vii) fusion bonded epoxy coated reinforcement bars undergoes degradation on long term exposure to sunlight, and (viii) fusion bonded epoxy coated reinforcement bars normally shows poor alkali resistance.
Specific concern for the fusion bonded epoxy coating is the pitting corrosion. In a severe corrosion environment, where coating damage penetrates to the steel substrate, the corrosion takes place. There is no exception based on size or location of the damage. This is different from pit corrosion, which is an extremely localized attack occurring when steel passivity is destroyed only locally, forming a small anodic area. For uncoated steel, the larger surrounding cathodic areas drive the anodic reaction resulting in a pit. In a sense, the pit cathodically protects the surrounding metal. In the case of coated reinforcement bar, however, the coating restricts the availability of surrounding cathodic areas and restricts the corrosion activity, alleviating its severity.