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Dowel bars in concrete slab

Concrete has to face lot of hostile environmental forces. Concrete can expand or contract because of temperature, stresses due to various kind of load etc to maintain functional capacity in such way so that it does not get cracked, allow transfer of load without faulting at joints, in practice this is achieved by small steel rods called Dowel bars.

corrosion risk -dowel bars
                           Dowel bar

Dowel bars are place at transverse joints of concrete pavements to provide a connection between two slabs. Dowel bars are used in new construction like roads and rehabilitation projects such as slab replacement.

  1. Dowel bars are the device through which wheel loads are transferred from one slab of pavement to the next.
  2. Dowel Bars will significantly reduce the magnitude of the stresses and deflections at the slab joints.
  3. Dowel Bars prolong the useful life of concrete pavements.

 

Dowel bars  will extend service life of the project by improving load transfer and prevent faults at joints. Dowel bar of  30, 32, 36, 38 & 40 mm  Dia and 510 mm long are generally used.

 

Higher Corrosion Riskcorroded dowel bar in slab

Dowel bars are at higher risk of corrosion due to following reasons:

  1. Joints allow direct passage to aggressive agents like oxygen, moisture, chloride etc to the surface of dowel bar. It is further aggravated due to direct exposure to high and low temperature.
  2. Bond between dowel bar and concrete also allow easy diffusion to aggressive ions due to cyclic horizontal movements between the slabs.

 

Corrosion prevention

Dowel bars should be protected from corrosion although joints are sealed to keep water penetration to a minimum; water will seep over time and may corrode unprotected bars.

Typically dowel bars are protected from corrosion by application of epoxy coating or stainless steel clad bars. Epoxy coating’s smooth and almost friction-less surface provides easy horizontal movement in concrete so that slabs move independently which prevents slabs from excess stresses.

Fusion bonded epoxy coated bars for durable construction of Airports

airportAirport project falls into extremely important public infrastructure projects where durability and safety are of most important. Airport areas are prone to heavy localized corrosion such as pitting and crevice corrosion, the factors other than regular corrodants present everywhere excess corrosion due to exhaust emissions from the aircraft jet engines and high sulfur compounds present in such environment.  Concrete structure in such areas  suffer from severe rebar corrosion leading to issues regarding durability, public safety , loss of productive time and recurring  maintenance cost. Looking to size of public infrastructures in general the amount of money needed to correct this problem is staggering especially considering the current state of economy.

Corrosion                

the deterioration of a material or its properties due to a reaction of that material with its chemical environment – has been with us forever. People have recognized, accepted, coped with and, occasionally, battled corrosion for millennia. In the 19th century, we began taking steps to understand, prevent, and treat corrosion, and we have gradually expanded these efforts ever since. But recently, corrosion has become a major concern, partly because our demands for more complex and sophisticated systems and products have been satisfied by materials that are more susceptible to corrosion. The insidious and pervasive effects of corrosion have now reached the point where it is a major cost for our economy and quality of life – in fact recent studies estimate the direct cost of corrosion in the United States to be nearly $300 billion dollars per year.

The problem is caused primarily by inorganic-salt induced corrosion of steel in concrete. The salt, primarily chloride, penetrates the concrete from sources such as  sea exposure. It can also be built in through the use of salt-contaminated aggregate, seawater in the concrete, or chloride-based admixtures.

The chloride ion initiates and catalyzes the corrosion reaction. The iron corrosion products resulting from the reaction occupy a much greater volume than iron and cause tremendous pressure on the concrete. The pressure causes the concrete to crack and spall, allowing even greater access of corrodents to the steel and accelerated deterioration of the structure.

Mechanism of Reinforcing Steel Corrosion in Concrete     

rebar corrosionThe traditional view of the reinforced concrete structure is that the concrete is protective to the reinforcing steel bars through the combined effects of the chemical reactions between the steel and the cement hydration products and the environmental barrier provided by the concrete cover. If these conditions are maintained within the concrete mass, the steel bars do not corrode and the structure should have the expected trouble-free life span

Poor quality reinforced concrete structure contributes to a faster deterioration of the steel reinforcing bars. Low degree of compaction, excess water in the concrete mix, and the hydration process are considered the main factors to create voids within the concrete and make the concrete structure porous.

Porosity of concrete allows penetration and ingress of aggressive elements (e.g., chloride, oxygen, carbon dioxide, and other materials that vary from one location to another) to the embedded steel rebar and to initiate corrosion.

The primary factors controlling the initiation of the steel corrosion and its mechanism in concrete are summarized in the following points:

  • The rate of steel depassivation
  • The initiation of the macrocells due to the differential aeration and chloride absorption
  • The low resistivity attributed by the concrete pore water
  • The presence of oxygen to accelerate the corrosion process

The corrosion of steel in concrete is an electrochemical process, which results in the formation of a corrosion cell. The following corrosion mechanism is the most likely for steel rebar embedded in the concrete when significant variations exist in the surface characteristics of the steel. The steel surface initiates cathodes and anodes electrically connected through the body of the steel bar. The “half cell reaction” takes place, by inducing an electromotive force known as standard redox potential when the metal is connected to a hydrogen electrode – see Equation 1.

Equation 1

For iron: Fe –à Fe+2 + 2 e – (Anode)

The electrons liberated at the anode migrate to the cathode and react in various ways dependant upon the pH value and the availability of oxygen. See Equation 2, Equation 3, and Equation 4.

Equation 2

2e + 2H + ½ O2 ——à H2O

Equation 3

2e + H2O + ½ O2 —–à H2O

Equation 4

2e + 2H —————- à H2

The anodic and cathodic reactions are autocatalytic and result in the transformation of metallic iron (Fe) to rust. The rust formation is accompanied by a significant increase in the volume, suggested as large as seven times that of the original Fe volume. The volume increase causes concrete cracking and spalling.

Effect of Chloride Ions

When the steel is placed in a highly alkaline solution (pH >11.5), even in the presence of oxygen, corrosion will not be initiated. In fact, slightly rusted bars will be dissipated when placed in strong alkali. That is the reason why, during construction, slightly rusted steel bars do not create a concern.

The chloride ions ingress does not lower the pH in the concrete. However, it destroys the passive layer on the steel bars. The depassivated steel bars do not corrode in the presence of the chloride ions only. The corrosion occurs after the presence of the carbon dioxide lowers the pH below 11, thus contributing to corrosion initiation.

Sources of chloride are either in the concrete mix, mainly from the sand, aggregates, or the water used, or as chloride ingress from the environment, such as in the marine atmospheric environment.

Effect of Carbonation

Carbonation is the alkalinity loss in the concrete mass. The product of the reaction between carbon dioxide in the normal outside air and the alkaline products, mainly the calcium hydroxides, is calcium carbonate. In case of high water/concrete ratio, carbonation continues to the depth where the reinforcing steel bar is embedded.

When carbon dioxide penetrates through the concrete cover in the presence of water in the pores, it drives the pH to lower values which depassivates the steel

Other hydration products in the cement can go through the same reaction with carbon dioxide causing a significant quality loss of the cement and faster deterioration of the concrete mix.

Effect of other Elements

Sulfide can be found in the cement as a contaminant (more than 0.2%). The sulfide ion has been found more destructive to the steel rebar embedded in the concrete if it goes higher than the regulated percentage shown. Regardless of the sulfide ion source, it has been the cause of several cases of hydrogen embrittlement – particularly in pre-stressed rebar.

Mechanism of FBE coated steel corrosion in concrete

fbe Corrosion control of the FBE  coating  is a function of the coating’s ability to provide a barrier against water, oxygen, chloride, and other aggressive elements  that prevents permeation through the coating film to attack the metal substrate. There are critical properties required for corrosion protection FBE coatings that include adhesion and wetting ability to the rebar.

 Epoxy coatings significantly reduce the corrosion rates of reinforcing steel. Epoxy-coated reinforcing steel maintains low initial and life-cycle costs over a 75-year life-cycle and use of epoxy-coated

reinforcing steel was found to be substantially more cost-effective than either using uncoated reinforcing steel in concrete containing corrosion inhibitors or stainless-steel reinforcing.

Epoxy coatings are the workhorses of the protective coatings industry. They have excellent chemical and

corrosion resistance, high mechanical strength, good adhesion to a variety of substrates and a

combination of other properties that have made them a material of choice for providing cost effective, long term protection on industrial, marine and offshore structures.

Epoxy Bar Use

 2nd most common strategy to prevent reinforcement corrosion

–Following increased concrete cover

  • 850,000,000 ft2 of decks

–>70,000 bridges in the US alone

–>600,000 ton/yr or 10 – 15% of all rebar in NA

  • USA, Canada, Middle East, Japan, and India

Apart from above

 

Conclusion: 

Apart from properties listed above advantages to  airport projects are as follows:

  1. Enhanced durability & life span of concrete structure at low Life cycle cost.
  2. Reduction in recurring cost of maintenance.
  3. Enhance public safety an availability of productive assets.
  4. AS FBEC is perfect barrier film it provides one more advantages from EMI- interferences in operation.

Predicting Durability and building life expectancy

Concrete is one of the most used engineering materials on tonnage basis. Life cycle Cost of projects depends on durability of building materials and components hence durable concrete is very very important with regards to structure’s life span. Durability can be defined as material’s ability to resist environmental (internal & external) conditions while maintaining desired engineering properties.

Factors affecting durability of concrete structure are divided into two parts:

  1. Concrete degradation
  2. Rebar corrosion

Concrete durability depends on :

factors affecting durability of concrete

Contrary to common belief, concrete is a complex composite material, whose structure and properties can change over time.

It is generally recognized that the environmental degradation of the concrete infrastructure is a serious, large scale and costly problem in many parts of the world

Transport mechanism of environmental degradents in concrete

Some weaknesses or new factors of degradation added to this because of operational practices eg.

  1. Concrete mix esp at site
  2. w/c ratio and its distribution
  3. vibration & mixing uniformity
  4. curing conditions
  5. nature of bond between cement and aggregate etc

Concrete vulnerability variable-some well-known factors are listed in above two figures- are huge in numbers so  quantitative estimation and control of durability and retaining engineering properties at a desired level becomes very complex task, a model and a software to carry out accurate calculations was much needed  to simplify this.

Designing reinforced concrete structure wrt various concrete mix, exposure condition, ingredient & life cycle cost and expected service life is very complex due to huge numbers of variables. To tackle this Life-365 consortium was formed in USA by various concrete related organizations so that collective knowledge & expertise can be combined to develop life-365 service life prediction model which allows users to input local concrete-chloride profile to customize the model to their worldwide location and environments.

You can download Life-365 service life prediction here : Life-365

Life-365 is software designed to estimate the service life and life-cycle costs of alternative concrete mixture designs proportions and life-cycle costs of alternative concrete mixture designs proportions and corrosion protection systems.  It follows research-based methodology developed by the Life-365  Consortium I and II groups of companies, that gives estimates on the effects of design, chloride exposure, environmental temperature, high-performance concrete mixture proportions, surface barriers, and steel types on the service life and life-cycle cost of steel-reinforced concrete structures.

This simple and transparent model provides a fundamental tool for design consultants to use for estimating the service life and life cycle costs of alternate protection systems in their design of steel-reinforced concrete structures that will be exposed to chlorides.

 

view of epoxy coated rebar in foundations

why epoxy coated rebar are better for foundation?

The present experience in construction and service of reinforced concrete structures shows that there are numerous problems related to foundations  causing severe damage, and often, compromising the bearing capacity of structures and the durability. The reasons for this are its interaction with soil, incorrect assessment of moisture and water effects on:

􀁸  Foundations soil and

􀁸  Concrete of the foundations.

Foundations  are exposed to following aggressive environmental effects :

􀁸  Physical -The most drastic form of the physical impact leading to concrete degradation is frost action. Namely, water which is retained in pores and cracks freezes in low temperatures and exposes concrete to often very high pressures (up to 220 MPa).

􀁸  Biological -The biological effects comprise the impact of vegetation, which causes the existing cracks to widen as the root systems of trees expand Particularly detrimental in these terms are fig, willow and liquidambar (it is common in warm climates, grown locally as decorative tree whose leaves and fruit resemble those of a chestnut tree)

􀁸  Chemical effects.-

–  Aggregate expansion,

– Salt weathering,

– Carbonation,

– Leaching.

Since the foundations, after the construction has been completed, are usually hardly accessible, it is necessary to pay attention to prevention of the adverse impacts of potentially aggressive actions, than to repair the damage.

epoxy coated rebar in foundation

 

As Fusion bonded Epoxy coatings are inert to alkaline as well as acidic media and have very low diffusion rate for corrosive elements like chlorides, water, oxygen etc  epoxy coated rebar is the  best corrosion protection strategy for foundations.

FRP rebar-will it replace steel bars?

FRP rebars have generated lot of hope in construction industry largely as a result of high decibel marketing and aspirations generated due to its earlier applications in aircraft industry.

The use of high performance FRP in primary structural applications, however, has been slower to gain acceptance although there is much development activity.

Their benefits of corrosion resistance and light weight have proven attractive in many low stress applications. Composites present immense opportunities to play increasing role as an alternate material to replace timber, steel, aluminium and concrete in buildings.

What is FRP

Demand for steel rebar in India in 2016-17 was 30.39 million ton, it is projected to grow in 2020-21 to 41.82 million ton. Due to increased competition,  producers have focused on branding and value adding to their products.

FRP materials were developed primarily for aerospace and defense industries in the 1940s and are widely used in many industries today, including aeronautic, marine, automotive and electrical engineering. With the continuing cost reduction in high-performance FRP materials and the growing need for new materials to renovate civil infrastructures, FRP materials are now finding some acceptance among civil engineers

A Fiber Reinforced Polymer (FRP) composite is defined as a polymer (plastic) matrix, either thermo set or thermoplastic, that is reinforced (combined) with a fibre or other

A Fiber Reinforced Polymer (FRP) composite is defined as a polymer (plastic) matrix, either thermo set or thermoplastic, that is reinforced (combined) with a fibre or other reinforcing material with a sufficient aspect ratio (length to thickness) to provide a discernable reinforcing function in one or more directions. FRP composites are different from traditional construction materials such as steel or aluminium. FRP composites are anisotropic (properties apparent in the direction of the applied load) whereas steel or aluminium is isotropic (uniform properties in all directions, independent of applied load). Therefore, FRP composite properties are directional, meaning that the best mechanical properties are in the direction of the fiber placement.

FRP composites are composed of:

Epoxy-transfers load to reinforcing fibres and acts as glue

Reinforcenments-  provides mechanical properties aramid, carbon and glass. In construction mainly glass fibers are used due to cost

Filler-improve performance and reduce the cost of resit & fibres

Additives-expand the usefulness of polymers, enhance process ability

 

Why it looks so attractive

. corrosion proof

.  high tensile strength

.  light weight

.  electrical and magnetic properties

Some time facts are more fictitious than reality.  let us examine the claim of advantages of FRP bars in construction.

.  Corrosion proof

steel gets corroded and its value gets deteriorate, it’s a so deep rooted fact that when we consider alternative we conveniently degradation mechanism of the material say FRP.    FRP too deteriorates but it follows different mechanism.  So let’s consider environmental conditions of concrete constructions and its impact on durability on FRP:

              -moisture  : 

water molecules act as plasticizer disrupting van der waals bond in polymerchains. This will  lead to                                  changes in modulus, strength, strain to failure and toughness. Swelling stresses may cause                                                 permanent polymer  matrix cracking, hydrolysis and  a degree of fibre-matrix debonding Effects                                       will aggravates further with rise in temperature. FRP have 0.5 % water retention.

               -chlorides :

In broad terms, CFRP bars show very little degradation with time, exposure or temperature. AFRP                                   and GFRP elements may show reductions in strength and stiffness of up to 50%, pre-stress                                                 relaxation of up to 30% and moment reduction in reinforced beams of up to 20%.

               -alkali     :

Performance of FRP reinforcement and pre-stressed tendons in alkali varies with the materials                                        (fibres and resins) used and manufacturing processes. Literature suggests that bars or tendons                                          deteriorate much faster in alkaline solution than in concrete, probably due to the relative mobility                                    of  OH+ ions.  Specific observations for generic material types are given below:

 

􀁸 With GFRP extensive degradation can be caused by exposure to high

Temperature alkaline solutions. Bars embedded in concrete at various

temperatures and with good fibre–resin combinations show only limited

degradation, but this increases with temperature and stress level.

 

􀁸 Alkali affects AFRP bars and tendons less than GFRP, but a combination of

frp

Alkali solution and high tensile stress (in the order of 0.75 UTS) may damage AFRP bars significantly.

 

                 -temperatue :

Deterioration by thermal action may occur in composite materials when constituents have                                                  different coefficients of thermal expansion. With FRP-reinforced concrete members, transverse                                        thermal expansion is particularly important for a good bond.

Resins will soften due to excessive heat. The tensile, compressive, and shear properties of the                                             resin diminish when temperatures approach the Glass Transition Temperature, Tg.  Tg values are                                    approximately 250oF (120oC) for vinylestes resins which are typically used with GFRP rebars.  Tg                                     lowers as a  result of moisture absorption.

Thermal expansion in longitudinal and radial direction different generally 2 or more times than                                       concrete

            – UV radiation :

                                   Ulta Violet  rays may cause degradation during storage or if the FRP is used in external                                                   environment. Degradation mechanism is benign to the surrounding concrete unlike steel that                                           expands and causes failure of the member. So its silent killer.

 High tensile strength

strength depends on type and composition of fibre.

 Following Factors Affects  Material Characteristics:

 Type of fiber

 Fiber volume

 Type of resin

 Fiber orientation

 Dimensional effects

 Quality control procedures during manufacturing

 Rate of curing

 Void content

 Service temperature

Tensile property of some FRP is comparable to steel but GFRP- which is mainly used in construction , there is not much gain.

Steel                                                           GFRP

Tensile strength. in mpa                             483 to 1600                                               483 to 690

 

. Light weight

This benefit is of not much functional value except very long bridge(with low stress) other than transportation and installation cost.

Limitations:

 Other than different degradation mechanism FRP has following inherent limitations or disadvantages

  • Low ductility and No yield point-fickly plastic behavior- structure will fail suddenly, heavy safety risk
  • Susceptible to local unevenness
  • High cost
  • Low shear strength
  • Tensile strength of bent bar is significantly lower
  • Brittle – so risky in earthquake prone zone, area with high wind velocity and presence of machinery vibrations
  • Different design criteria so cannot replace steel directly  in use.
  • Anisotropic so bend, shear strength, dowel action will be affected
  • Creep rupture endurance time can decrease in high temperature, UV radiation, high alkalinity, wet & dry cycle, freezing & thaw cycle.
  • Resin will control much of the property as dia of bar increase, strength decreases.
  • Recycling is either not easy or not possible
  • Fatigue- temperature and moisture decrease the fatigue endurance
  • Anchorage- gripping the end of FRP bar is difficult and costly
  • Lower modulus of FRP bars, design for serviceability controls
  • Endurance in fire and elevated temperature is less than steel

As the material is in its infancy as far as rebar application is concerned, only guidelines for use are formed, extensive data on durability is not available, FRP lacks in value proposition compared to epoxy coated bars, not only that from review of limitations we can say that FRP will never replace even steel bars except in some low stress applications.

How epoxy coating protects?

What is epoxy ?

The basic building blocks of an epoxy resins are Epichlorhydrin and    Bisphenol -A continued polymerization reactions create higher molecular weight solid epoxy resins.

                                                                   CH                                                            CH3
           2CH2 – CH – CH2 +  HO – R – C – R – OH à CH2 – CH – CH– O – R – C – R – O – CH2 – CH – CH2
                    O                                            CH3                       O                                  CH                  O
         EPICHLORHYDRIN             BISPHENOL – A          –                EPOXY RESIN   ( R = Aromatic group)

Why epoxy ?

To effectively protect reinforcing steel against corrosion a coating must provide a continuous film that will:

  • Resist penetration by salt ions,
  • Resist the action of osmosis,
  • Adhere to and expand/contract with the steel substrate,
  • Resist breakdown from weathering and exposure,
  • Be flexible and durable enough for handling,
  • Strong resistance to oxygen & chloride &
  • Highly insulating with very low conductivity & high dielectric resistance.

Fusion-bonded epoxy coating satisfies all of these requirements.

It is a thermoset material, meaning that once it is cured, the coating will not tend to soften with higher temperatures. It achieves its beneficial properties as a result of a heat catalyzed chemical reaction.

Epoxy coating starts out as a dry powder. The powder is produced by combining organic epoxy resins with appropriate curing agents, fillers, pigments, flow control agents. When heated, the powder melts and its constituents react to form complex cross-linked polymers.

cross section of epoxy coated rebar in concrete
fbec

Epoxy coatings are environmentally friendly materials. Unlike many paints, the fusion-bonded epoxy coatings used for steel reinforcement do not contain appreciable solvents or other environmentally hazardous substances. Systems used to apply the coating are very efficient, resulting in little material loss to the atmosphere and little waste disposal

How Epoxy Coating Protects

Fusion-bonded epoxy coating principally protects against corrosion by serving as a barrier that isolates the steel from the oxygen, moisture, & chloride ions that are needed to cause corrosion.

Epoxy coating also has a 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 that is less obvious: coating reduces the size and number of potential cathodic sites, which limits the rate of  corrosion reaction that could occur.

In order for macrocell corrosion to take place, a large area of steel surface is needed to serve as the cathode where oxygen reduction can occur.

Fusion bonded epoxy coating process

The application of fusion-bonded epoxy to reinforcing steel is straightforward and uncomplicated: clean the steel, heat it to the proper temperature, apply the powdered-epoxy coating material, allow the coating to cure, and inspect. However, the details are important and must be understood and implemented to assure a quality coating that will extend the working life of a structure in a corrosive environment. .

Do Corrosion Resistant Steel bars resist corrosion?

A number of manufacturers have come up with a new type of higher strength re-bars which are claimed to be corrosion resistant generally known as CRS.

These bars have dual micro structures i.e. the surface layer is tempered martensite while the inner shell is ductile ferrite-pearlite to increase corrosion resistance of these bars certain elements like copper , chromium,  and phosphorus are also added.  Copper plugs pores in the rust. Phosphorous in the form of P2O5  acts as an inhibitor and chromium helps in spinel oxide layer.These  protective layers is formed  on the surface when they come in contact with atmospheric oxygen and  moisture. This layer is formed over a period of time. In concrete oxygen is  not available. Hence, the protective coating may not form when such bars  are used as reinforcement in concrete. Obviously, there appears to be no advantage in using such bars in place of mild steel. Even, when exposed to atmosphere in marine environment the chlorides present in the air break the protective layer thereby reducing corrosion resistance. Therefore, in marine atmosphere these bars do not give good resistance to corrosion as in normal atmosphere. Secondly, their chemical composition is such that it may lead to pitting corrosion which is highly undesirable. In mild steel bars conforming to IS:1786, although is less corrosion resistance, but corrosion occurs all over  the surface without pitting.

Effects of alloying element

Phosphorus :          Higher phosphorus content contributes to the increase in strength and Corrosion resistance                                            properties but brings brittleness due to the formation of low euctoid phosphicles in the grain                                          boundary. Also lowers the impact Value at sub zero temperature level (transition temperature)                                      and tenacity to cracking during welding

Copper  :                Being a pearlite stabiliser, it increases the strength and corrosion resistance Property [only when                                   wet and dry cycle present]

Chromium :            Present as an impurity from the scrap and influences carbon  equivalent; Weldability and                                                increases corrosion resistance property.  High chromium bars not suitable for welding.

Result of Laboratory test on FBEC and CRS bars:

appvolt

 

   Result  of Applied voltage test:

  1. FBEC : after 30 days no sign of corrosion.
applied voltage resistance test

 

  CRS :   after 24 Hrs most of the bar gets dissolved in similar condition testing.

CRS

 

  1. What various standard recommends about CRS

 

aci

 

Best site practices for epoxy coated reinforcing steel

Unloading

unloading coated bar

Use spreader bar or strong back with

multiple pick-up points to minimize sags.

 For lifting, use nylon or padded slings; not

bare chains or cables.

 Storage

storage of coated bar

Unload as close as possible to the point of

Concrete placement to minimize rehandling.

Store coated and uncoated steel separately.

 Store bundles on suitable materials, such as

timber cribbing.

 Space timber cribbing to minimize sagging.

 Use nylon strapping or other non-abrasive

material.

covered bar

 If outdoor storage is to exceed 30 days, cover

coated bars with suitable opaque material and

minimize condensation

Shearing & Bending

bending

 Contact points on shearing and bending

equipment to be a suitable protective material.

 Inspect bars after bending and repair any

Cracks with patching material.

Cutting 

Power shears or chop saw (avoid flame cutting).

cutting

Repair cut ends.

Welding

 1.  Remove coating from the areas to be welded.

2.   Cleaned it with brush, remove any dust particles
3.   Weld properly prepared area
-reinforcing steel should be welded according to the American Welding Society,

AWS D1.4/D1.4M Structural Welding Code – Reinforcing Steel.
-While in situ welding of epoxy coated bars adequate ventilation to dissipate fumes must be provided
-Tack welding is not permitted.
4.   Clean all carbon deposits & slag from weld and HAZ
5.   Apply patch up  as usual practice.
– the damaged areas should be repaired using patch materials meeting ASTM  A775/A775M.

Patch up Material 

Use 2-part patching material, approved

by the coating manufacturer.

 Follow manufacturer instructions.

Repair of damaged epoxy coating, cut ends,

cracks and abrasions

step 1

Step 1.

Remove rust and contaminants from the

damaged area to be patched with a wire

brush

step 2

step 2       

Mix the patching material according to the

manufacturer’s instructions. Use patching

material prior to end of pot life.

step 3

Step 3.          

Apply the patching material to the repaired

area. Follow the patch material manufacturer’s

instructions.

step 4

 Step 4.

Allow the repaired area sufficient curing time,

as specified by the manufacturer’s instructionsbefore placing concrete.

Bar Placement

bar placing

Lift and set the steel into place without dragging.

Use plastic or bar supports coated with non-conductive material.

Use pvc coated tie wire.

Use power shears or chop saw to cut steel and not flame cutting.

Concrete Operations

concrete opration

Minimize traffic over the steel.

Avoid traffic and concrete hoses on placed steel.

Use plastic headed vibrators to consolidate concrete.

Corrosion rate measurement by Half-cell potential

The half-cell potential measurement is an electrochemical technique commonly used by engineers to assess the severity of corrosion in reinforced concrete structures.

Corrosion of steel reinforcement is a major factor in the deterioration of highway and bridge infrastructure. A survey of the condition of a reinforced concrete structure is the first step towards its rehabilitation. A rapid, cost-effective and non-destructive condition survey offers key information on the evaluation of corrosion, and aids in the quality assurance of concrete repair and rehabilitation and in the prediction of remaining service life.

 

                       Schematic showing basics of the half-cell potential measurement technique

 

Traditional way to assess the severity of steel corrosion is to measure the corrosion potential, since it is qualitatively associated with the steel corrosion rate. One can measure the potential difference between a standard portable half-cell, normally a copper/ copper sulphate (Cu/CuSO4) standard reference electrode placed on the surface of the concrete with the steel reinforcement underneath. We have developed use of standard calomel reference electrode[ at almost fraction of cost]  available in our lab as  illustrated above. The reference electrode is connected to the negative end of the voltmeter and the steel reinforcement to the positive.

    Half cell potential reading      Corrosion activity
>-80 mv Low risk of corrosion{10 % risk}
-80 mv to -230 mv Intermediate corrosion risk
<-230 mv High corrosion  risk {90 % risk}
<-380 mv Severe corrosion

      

         Criteria for corrosion of steel in concrete for calomel reference electrode

 

A more negative reading of potential is generally considered to indicate a higher probability of corrosion. However, this general “rule” may not always apply because of the complexity of today’s concrete and repair technologies. eg Not directly applicable to epoxy coated & galvanized bars

It must be stressed that the half-cell potential measurement only reveals the corrosion probability at a given location and time. Long-term monitoring of the half-cell potential reading is more meaningful.

Concrete Coatings-limitations

Various external hostile environmental substances, such as, water, carbon dioxide, oxygen, chlorides, sulphides and biological organisms are transported from the atmosphere into the concrete and attack steel and concrete in different mechanisms causing premature deterioration of reinforced concrete challenging its durability resulting in premature failure of the structures.

  1. Concrete is a porous material having high gas, vapour and liquid permeability leading to deterioration of reinforced concrete structures. chemicals can penetrate the pores and attack the paste. The paste and aggregate can also be worn down by physical impact and abrasion. Water can penetrate concrete, freeze and expand inside it when the temperature drops, and ultimately weaken the concrete from within. In addition, if the concrete has reinforcing steel bar (rebar) to impart additional strength and other properties, the rebar can corrode if moisture, oxygen and chloride ions penetrate the concrete. Corrosion of rebar contributes to the deterioration of concrete.

2.  The protection of concrete should actually begin at the conceptual stage and meticulous strategies are adopted            for protecting the concrete from both internal and external environments. Various coating materials and                       application methods for concrete surface repair and strengthening have been developed. However, selection                 criterion for these materials has not been established yet at the current moment. Selecting procedures of                       concrete coating materials must focus on deteriorating mechanisms diagnosed carefully by the conditions of                 target structures. For instance, in case of salt damage, repair policy should consider corrosion environment and          deteriorating condition to determine symptomatic indications such as, 1) removal of permeated chloride ions, 2)           penetration block of chloride ions, moisture, and oxygen, 3)derusting of rebar, 4) corrosion-control method                (coating or potential control). However, it is still ambiguous to determine which is the best material and coating          system, because there is not enough, durability data to estimate.

concrete coatings
concrete coatings

 

3.  it should be noted that all resin materials are not totally resistant and impermeable to all aggressive agents                  and do not provide a total protection. Chemical/physical degradation of resins and debonding of coatings are              the major phenomena affecting the durability of surface protection. The mechanisms of destructive processes             in such heterogeneous materials as resin composites are complicated and not completely understood.                             Degradation of resins mainly involves swelling, dissolution and scission of molecular chain bonds. A wide                     variety of reactions is possible for resin degradation. The transport of gases and liquids aggressive to substrate             into or through the coating is the major problem of its delamination. There are many parameters that                             influence  the deterioration process of coatings, such as chemical agents, temperature, solar radiation,                           pressure, abrasion, cyclic temperature-moisture changes etc. All these parameters can occur simultaneously or             they can be complementary to one another.

  1. Diffusion rate of corrosive ingredients is higher than FBEC

5.  Concrete coatings suffers from poor durability of the coating and loss of corrosion protection in the areas                       where the coating is damaged

  1. Concrete requires continuous maintenance and therefore recurring cost.