INTRODUCTION TO STEEL:
Steel is an alloy having great properties such as formability and durability, steel are the most complex and widely used engineering material having thermal conductivity, excellent tensile strength and yield strength. As well as corrosion resistant. Due to its high tensile strength and lower construction cost, component employed in buildings, infrastructure, tools, ships, machines, automobiles, appliances, and weapons.
Steels is an alloys of iron and carbon and many other elements such asMn, Cr, Al, Si etc.
In Steels, carbon content is never greater than about 2.00%.while those containing over 2.00 wt% C are classified as cast iron.
INTRODUCTION TO REINFORCEMENT BAR:
Rebar is very important constituent of reinforced concrete structure and helps to strengthen and carry the concrete in compression. Concrete has weak tensile strength but excellent strength under compression. Rebar are stressed prior to subjecting the structure to loading. Patterned surface in order to get a better adhesion and bond with the concrete.
The wide range of rebar made up by carbon steel, typically consisting of hot-rolled round bars with deformation patterns. Steel and concrete have almost same thermal coefficient of expansion.
STEEL REBAR APPLICATIONS:
• Submerged in concrete in order to improve tensile strength.
• Specific patterned on the bar provide for better adherence to the concrete.
• Thermal co-efficient expansion properties compatible to concrete.
• Excellent ductility to with cyclic loading.
Deformation Patterns on the Rebar:
Bond strength between steel bar and concrete is essential for reliable performance of reinformced concrete structure. The conventional high yield strength deformed and thermomechanically treated bar mostly employed in the construction
How Bond Is Formed?
Three factor define the bond strength: chemical adhesion, friction resistance and mechanical interlocking of bar deformation’s ribs and concrete.
Rebar has ribs that bind it mechanically to the concrete, but it can still be pulled out of the concrete under high stresses. This first increases the friction locking the bar into place, while the second makes use of the high compressive strength of concrete.
The requirements for our project analysis are found in standard product specifications for steel bar reinforcing, such as ASTM A615 and ASTM A706.
DIFFERENT GRADES AND STANDARDS OF REINFORCEMENT BARS:
Rebar is available in different and variety of grades and specifications that vary in yield strength, chemical composition, ultimate tensile strength, and percentage of elongation.
The grade assigned is equal to the minimum yield strength of the bar in ksi (1000 psi) for example grade 60 rebar has a minimum yield strength of 60 ksi.
Rebar is most commonly manufactured in grades 40, 60, and 75 with higher strength readily available in grades 80, 100, 120 and 150, Grade 60 (420 MPa).
Some of commonly known standard that are most used in now-a-days are as follows.
Deformed and plain carbon-steel bars for concrete reinforcement, ASTM A615 has four grades (40, 60, 75 and 80) . ASTM A706/A706M:
Low-alloy steel deformed and plain bars for concrete reinforcement
ASTM A706 has only one grade which is 60.
Strength of 60 ksi (420 MPa), strength of 75 ksi (540 MPa), strength of 80 ksi (550 MPa), Strength of G-60 is of 60 ksi (420 MPa).
Corrosion defines as the chemical or electrochemical reaction between a material and its environment consequences deterioration of the material and its properties, for steel embedded n concrete, corrosion results in the formation of rust. Normally the concrete cover is able to provide a pH value higher than 12 avoiding the corrosion reaction.
Can Corrosion be avoided in reinforced concrete?
• Concrete is always dry, then there is no water to form rust.
• Concrete is always wet, then there is no oxygen.
• Cathodic protection is used to convert all the reinforcement in to cathode using a battery.
• A polymeric coating is applied to keep out aggressive agent.
TESTING OF REBARS:
Rockwell Hardness Test:
The Rockwell hardness test method, is widely used hardness test method. The test specimen is indented by an indenter and the hardness value is obtained at the gauge.
• Apply a minor load of 10kgf
• Then the dial is set to zero and then major load is applied
• Then apply major load 60 to 150 according to the scale used for 4 to 5 seconds
• Release the major load only
• Machine will show the Rockwell hardness number HR on the machine
• All these operations will be done by machine automatically.
Tensile ; Bend Test:
Universal testing machine which applies a tensile force in opposite direction to a specimen and material is stretched or pulling by tensile loading until it fractures and then measures that force and also the elongation. The following properties can be determined by tensile testing
• Modulus of toughness
• Modulus of resilience
The bend test is also known as flexure test measures the force required to bend a beam. The bend test is employed for assessing the workability of sheet and plate ductility test which is employed to evaluate the ability of metallic materials undergo plastic deformation under bending.
Bend test for ductility provide a simple way to evaluate the quality of materials by their ability to resist cracking or other surface irregularities during one continuous bend. No reversal of the bend force shall be employed when conducting these tests.
• The test specimen is placed on the supporting pins
• The loading force is applied in the middle by means of loading pin
A fatigue test , metal subjected to a repetitive or fluctuating stress will fail at a stress much lower than that required to cause failure on a single applicat.ion of load. Failure occurring under a condition of dynamic loading are called fatigue.
• Crack initiation
• Crack propagation
Classification based on Sequence of Stress Amplitude:
• Constant amplitude test
• Variable amplitude test
Fatigue tests are made with the object of determining the relationship between the stress range and the number of times it can be applied before causing failure, when the stress occurs a sufficient number of times, it causes failure by fatigue.
There are two type of fatigue failure.
• High cyclic fatigue
• Low cyclic fatigue
High cyclic fatigue:
It is type of fatigue failure in which specimen is subjected to low cyclic load results, specimen fails at high number of cycle.
Low cyclic fatigue:
It is type of fatigue failure in which specimen is subjected to high cyclic load results, specimen fails at low number of cycle.
• A cyclic load is applied to the specimen until it breaks in order to measure the fatigue resistance of the material, the fatigue life is indicated by the number of cycles to failure. N The fatigue testing is conducted by using a rotating beam fatigue tester the machine can record number of cycles to the failure with the counter. The number of cycles to failure indicates the lifespan of the specimen.
• The loading device is lowered to the same height as the drive shaft in orer to secure the specimen onto the machine.
• The lock nut is slide over the specimen. In the right end, the narrow shaft of the specimen is inserted into the bearing of the loading device.
• In the left end, the conical part is attached to the motor shaft and the lock nut is tightened thoroughly using wrenches.
• The counter is reset to zero and the loading control is adjusted.
• The power is switched on and the switch button is pressed to start the test.
• The number of load cycles (x10) on the counter is recorded once fracture occurs.
Test defines as the energy needed to initiate fracture and continue the fracture until the specimen is broken involves the application of sudden and dynamic load to fracture the specimen,
• Place the specimen onto the support with notch facing forwards the direction of striker of the striking direction.
• Using the setting gauge, center the notch to the reference level.
• Face the pointer to read 170J. latches the hammer
• Release the hammer. The pointer will indicate the amount of energy consumed by the specimen for its rupture.
INTRODUCTION TO EPOXY:
The first series of resins-reaction products was produced commercially on large scale between diglycidyl ethers and bisphenol-A. Epoxy is commonly known as polyepoxide they are a group of reactive prepolymers and polymers which contain epoxide groups. Epoxy resins cross-linked either with themselves through catalytic homopolymerisation, or with a wide range of co-reactants including polyfunctional amines, acids, phenols, alcohols. These hardening agent are often mentioned to as hardeners or curatives. Reaction of epoxy with themselves or hardeners forms a thermosetting polymer. Epoxy has wide scope of applications, including coatings of metal, metal mounting, also employed in electronics/electrical components/LEDs, high tension electrical insulators, paint brush manufacturing, fiber-reinforced plastic materials and structural adhesives.
The epoxy resins are most adaptable of the modern plastics. The fundamental properties can be altered in many ways: by blending of resins types, by correct choice of curing agents, and by the employ of modifiers and filler. 2 Easy Cure: Epoxy resins cure rapidly and easily under temperature range from 5?C to 150?C.
One of the biggest advantageous properties of the epoxy resins having low shrinkage during cure. Phenolic casting resins, which evolved water, reveal high shrinkage, Epoxy resins react with very little movement or rearrangement and with no volatile by-products being evolved.
High adhesive strength:
Due to the chemical constituent, mainly the presence of polar hydroxyl and ether groups, the epoxy resins are excellent adhesives. Adherences are not disturbed during cure. Adhesive strengths, without the need for either upon the time or high pressures, best obtainable in the contemporary plastic technology.
High mechanical properties:
The strength of properly developed epoxy resins usually better than that of the other types of the casting resins, result of their low shrinkage, which minimizes stresses that otherwise would be weaken mechanical structure.
High electrical insulation:
Epoxy resins have excellent electrical insulating properties.
Good chemical resistance:
The resistance against the chemical attack of the cured epoxy resin greatly depends upon on the curing agent used. Overall, most epoxy resins possess extremely high resistance to caustics and good to excellent resistance to acids.
The cured epoxy resins, maintain their shape under the prolonged stresses.
The epoxies cure without releasing water.
TYPES OF EPOSY RESIN:
There are following types of epoxy resin which are given below:
1. Bisphenol A epoxy resin
2. Bisphenol F epoxy resin
3. Aliphatic epoxy resin
4. Glycidylamine epoxy resin
Bisphenol A epoxy resin:
Epoxy is a synthetic resin that uses two chemical components (typically diglycidyl ethers and a combination of bisphenol A and epichlorohydrin) to harden or cure. Epoxy resins can be produced flexible or rigid..
Bisphenol F epoxy resin:
Bisphenol F is employed in the production of plastic and epoxy resin , BPF have lower melt viscosity and high mean constituent per gram than bisphenol A. Once cured gives high chemical inertness.
Novolac epoxy resin:
Phenols react with formaldehyde and glycidylation with epichlorohydrin produces novolacs. These types of epoxy are highly viscous and form a highly crosslinked polymer network showing and chemical resistance but low flexibility.
Aliphatic epoxy resin:
This type of epoxy formed by glycidylation of aliphatic alcohols or polyols. The resulting resins may be monofunctional,Difunctionaly and highly functionally.These resins having a low viscosity at room temperature.
Glycidylamine epoxy resin:
This epoxy resin are highly functionality epoxies which are formed when aromatic amine are treated with epichlorohydrin. The resins are low to medium viscosity at room temperature.
Uncured epoxy resin has their poor mechanical, heat and chemical resistance properties. Epoxy resin is then reacted with curatives to form 3-dimensonal cross-linked thermoset structure in order to get good properties, this process is commonly known as curing, it is an exothermic reaction.
Curing can be achieved by reacting an epoxy with homopolymerization or by forming a copolymer with hardeners.
The epoxy curing reaction can be accelerated by adding little amount of accelerators. Tertiary amines, carboxylic acids and alcohols (especially phenols) are effective accelerators.
TYPES OF HARDENERS:
FILLER AND POWDER:
Filler and powder are added in the epoxy resin and hardener in order to get much better corrosion resistance and curing. The curing is carried out under a temperature range between 175°-250°C for powder deposition. Filler contain at least 15 % by volume of barium sulfate or calcium carbonate and powder contain one-third by mica powder of total composition of epoxy resin.
Fusion Bonded Epoxy Coated Reinforcement Bar:
Fusion bonded epoxy coating, commonly known as fusion bond epoxy powder coating and commonly mentioned to as FBE coating, this type of coating is widely employed to protect and prevent concrete reinforcement bars from corrosion and oxidation. The most widely used types include acrylic, vinyl, epoxy, nylon, polyester, and urethane. The name fusion bond epoxy is because of resin cross linking and the application method.
Techniques for applying powders are divided into four basic categories.
1. fluidized bed process
2. electrostatic bed process
3. electrostatic spray process
4. plasma spray process
The electrostatic spray process is the most commonly employed method of applying powders.
Electrostatic spray process:
In this process, The epoxy powder is applied through electrostatic spray on hot rebar under specific temperature range. The powder come 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.
Materials Used in Fusion bonded Epoxy:
The main components of a powder coating are
2. Hardener or curing agent,
3. Fillers and extenders, and
4. Color pigments.
Chemistry Of fusion Bonded Epoxy:
• The resin and hardener combine together is known as the ‘Binder’. ‘Epoxy’ or ‘oxirane’ structure contains a three member cyclic ring one oxygen atom connected to two carbon atoms in the resin molecule.
• Most commonly employed FBE resins are derivative’s bisphenol A and epichlorohydrin. However, other types of resins are also widely used in FBE in order to achieve better properties.
• The second most important thing of FBE coatings is the curing agent. Curing agents react either with the epoxy ring, along the epoxy molecular chain. include dicyandiamide, aromatic amines, aliphatic diamines, etc.
In addition to these two major components, FBE coatings include fillers, pigments, extenders and various additives, to provide desired properties. These components control characteristics such as permeability, hardness, colour, thickness, gouge resistance etc
Process Flow Chart:
THE COATING PROCESS:
The application of this process involves in the following steps:
First cleaned Reinforcement bars from surface contaminations such as oil and grease etc. by solvent cleaning.
Secondly, blast clean the Reinforcement bars using abrasive grit or by shot blasting to near white metal finish. Mill scale and rust may be removes by shot blasting. It also roughens the surface to give it a textured anchor profile.
Heating the Reinforcement bars, under the temperature ranging from 200 C to 240 C using furnace.
Dry epoxy powder is then sprayed on the heated steel rebar through spray gun. As the powder leaves the spray gun, When the dry powder hits the hot steel, it melts and flows into the anchor profile. It also bonds with the steel. The heated steel cause chemical reaction to form complex cross linked polymers. Coating thickness in the range of 50 to 150 micrometers are usually obtained.
Curing and cooling:
The molten powder becomes a cure after few seconds after coating application. The resin part cause cross linking, which is known as ‘curing’ under the hot condition. Complete curing is attained with the help of residual heat on the steel. Full can be attained in less than one minute to a few minutes.
Inspection and testing:
Coating of FBE reinforcement bars are tested as per the requirements specified in standard. The adhesion of coated bars is also tested frequently by bending of the bar. Besides this, various other tests are conducted in laboratory such as chemical resistance, resistance in continuous boiling water, abrasion resistance and impact resistance etc.
Handling, transportation and working:
FBE coated reinforcement bars are to be handled with extraordinary care so that coating is not damaged meanwhile during storage. These reinforcement bars require cover contacts during transportation, stacking, handling till they are used in concrete.
This epoxy coating is deliberately employed to serve as barrier to protect the rebar. Concrete acts as passive but chlorine in the presence of water and oxygen, penetrate the concrete, Ph is reduced and the passive film has been destroyed and cathodic and anodic reaction take place.
At the anode, iron atom lose electron because of the broken passive layer, hence oxidation is carried out.
Fe › Fe2 + 2e-
The iron ions further reacts with the hydroxides to produce ferrous hydroxide.
Fe2 + 2OH- › Fe(OH)2
Ferrous hydroxides is then reacted with diffused oxygen to form rust
4Fe(OH)2 + O2 › 2Fe2O3(rust) + 4H2O
At the cathode, gain of electron take place through electron flow through the steel where they react with water and oxygen to form hydroxides ions.
4e- + 2H2O + O2 › 4OH-
Oxygen atom are reduced and form hydroxides ions that further persist the anodic reaction.
HOW FUSION BONDED COAED REBAR (FBECR) PROTECTS:
When Chlorine ions penetrate through the concrete, the unprotected steel is then corrodes. Epoxy coating on rebar acts as a barrier against chlorine ion which is easily penetrated. Steel can corrode at any hole but the extant of corrosion is suppressed by the epoxy layer which satisfies the cathodic reaction and prevents macrocell corrosion.
Performance of Fusion Based Epoxy Coating:
The epoxy coating performs well and minimizes the rate of degradation in high chlorine levels. Furthermore a comparison of research work has been carried out of performance of carbon steel, galvanized steel and stainless steel rebar under high chlorine environment but FBECR performs well than above discussed rebar but less well than austenitic stainless steel.
Table 1: Comparison of cost and performance of different reinforcing bar materials in concrete
Advantages of FBE coating:
FBE coating on reinforcement bars has the following advantages.
• Since the coating is conducted on the coating assembly lines, better quality control is achieved. The process permit uniform coating thickness.
• Excellent bonding and adhesive properties is attained on hot steel rebar.
• The coating does not get destroyed when the straight bar is bent, Because of flexibility.
• FBE coating has excellent corrosion resistance in salt containing environment.
Disadvantages of FBE coating:
FBE coating on reinforcement bars has the following disadvantages.
• Less adhesion between coated rebars and concrete.
• Extraordinary Handling are required.
• Corrosion can initiate at small damage area of FBE coating.
• Being a barrier type coating, it facilitates localized pitting corrosion through pinholes.
• Prolong sunlight cause degradation..
• FBE coated bar having poor alkali resistance.
Epoxy coated rebar or corrosion resistant rebar is employed instead of conventional reinforcing bar to provide strengthen the concrete and provide protection against corrosion. Epoxy coating is applied to factory In order to ensure corrosion resistance. Epoxy coated rebar is utilized in the following structures:
• Parking structures
• Marine structures
Factor Affecting Fusion Bonded Epoxy Coating:
1) The primary problem s caused by inorganic salt induced corrosion of steel in concrete primarily Chlorine.
2) Improper handling and transportation of steel can cause damage.
3) The initiation of macrocell due to differential aeration and chloride absorption.
4) Presence of oxygen to accelerate the corrosion process.
5) pH of concrete is very important.
6) Thickness of coating is yet another important parameter.
CORROSION TESTING TECHNQUES OF STEEL REBAR:
A lot of test are carried out in order to find the corrosion rate, some of which are as follows.
• Eddy current test
• Accelerated chlorine threshold level test
• Macrocell corrosion rate
• Concrete chlorine ion assessment test
• Modified G 109 test
• Linear Polarization Resistance test (LPR)
• Tefel Test
But in our metallurgical department, we have two type of corrosion testing facility which is conducted on a same testing machine which is LPR and Tefel test.
Linear Polarization Resistance:
The electrochemical techniques commonly known as linear polarization resistance, that allows corrosion rates to be measured. When metal electrode is immersed in an conducting electrolytes the metal corrode. Metal losses electron this part is called anode the adjacent sites where gain of electron take place commonly known as cathode
CHARACTERZATON OF STEEL REINFORCEMENT BAR:
Following characterization techniques and equipment are employed for rebar.
• Microscope (Optical microscope, Scanning electron microscope)
Spectroscopy is used to determined the chemical make up or composition of metal. It is study of interaction between electromagnetic radiation and matter depending upon the wavelength. Spectroscopy is often used in physical and analytical chemistry for the identification through the spectrum emitted from or absorbed by them. It is also used in astronomy.
A beam of electromagnetic radiation interact with a sample, the photons interact with the sample. They may be absorbed, reflected, refracted, etc. Absorbed radiation affects the electrons and chemical bonds. In some cases, the absorbed radiation leads to the emission of lower energy photons. Emitted and absorbed spectra can be used to gain information about the material. Because the interaction depends on the wavelength of radiation,
Classification of spectroscopy:
The type of spectroscopy depend upon the physical quantity measured i.e energy absorbed or produced, some of which are as follows.
• Absorption spectroscopy
• Emission spectroscopy
• Scattering spectroscopy
• X-ray spectroscopy
It uses the range of the electromagnetic spectra in which a substance absorbs. This includes atomic absorption spectroscopy and various molecular techniques such as infra-red spectroscopy in that region and nuclear magnetic resonance spectroscopy in radio range.
It uses the range of electromagnetic spectra in which a substance radiates (emits). The substance first must absorb energy. This energy can be from a variety of sources, which determines the name of the subsequent emission, like luminescence. Molecular luminescence techniques include spectroscopy.
It measures the amount of light that s substance scatters at certain wavelengths, incident angles, and polarization angles. The scattering process is much faster than the absorption/emission process. One of the most useful application of light scattering spectroscopy is Raman spectroscopy.
X-rays of sufficient frequencies interact with material and excite the atoms contained. Due to this excitation Auger Effect is produced and some excitation radiation are absorbed or evolved. X-ray absorption and emission spectroscopy is used in chemistry and material science to determined elemental composition.
Metallography is the study about the microstructure of all types of metallic alloys and observing and determining the chemical and atomic structure. Metallographic examination usually investigate grain size and shape, porosity, cracks, second phases, and fracture. Metallography is a power analysis tool, It’s also predict the mechanical properties of given material. Some steps of metallography are as follow.
• Optical microscope
Microscope is employed to enlarge the image. Optical microscope works on the principle of reflected light microscope. Metallurgical microscope is an important tool for metallurgist, fine structure details can be obtained with the help of this microscope. Specimen are opaque to light and light is reflected by sample. It is used to study the topographic or microstructural features on polish and etched surface.
There are two type of lens used in the optical microscope.
1) Objective lens
Objective lens is employed is used to resolve the microstructural details of the polish and etched surface.
It is employed to magnify the resolved details of the microstructure. It does not further resolve the microstructural details.
There are two type of defect in lenses.
1) Chromatic aberaation
2) Spherical aberration
In this aberration, lens will not focus different colors on exactly the same place commonly known as focus, when wavelength of color are focused at different position in the focal plane. It is cause by lens dispersion, with different colors of light travelling at different speeds while passing through a lens as a result blurred image is produce.
Remedy, a combination of converging lens with diverging lens, so that this aberration can be removed.
Spherical Aberration is an lens defect that occurs when all incoming light rays end up focusing at different points after passing through a spherical surface. This cause the blurred image.
Remedy, it can be minimized by bending the lens into its best form
Scanning Electron microscope:
Electron microscope is a instrument that use a beam of energetic electrons to examine objects.SEM that image the sample by scanning it with a high energy beam of electron in a raster scan pattern. The electron interact with the atom that make up the sample producing signals that contain information about the sample’s surface topography, composition and other properties.
Information obtained through SEM:
• Topography (surface features of an object )
• Morphology (shape and size of particle making up the object)
• Composition (chemical make up)
• Crystallographic information (arrangement of atom)