How to reduce surface preparation cost by Ervin Test ?

Testing in industrial life is routine and mundane thing but some tests you remember esp when its impact results in out proportion saving to bottom-line. Surface preparation with abrasive blasting is carried out in many activities like coating. Use of right abrasive results in right quality & best economy. I would like to share one such pleasant experience about Ervin test.

It is very small equipments consists of a wheel with two blades, a target and a re-circulating device-just like a miniature version of shot blasting machine.

Ervin test machine enables us evaluation of quality of metal abrasives esp durability [life] and transmitted energy [impact energy] of abrasives under controlled conditions, which are two key measures of the value and quality of metal abrasive. Durability of abrasives are determined by number of passes against the target that are required to reduce the abrasive to unusable size ie 100% replacement by test quantity.

We had calibrated the system using S-550 calibration standard shots and then tested abrasives available at our end, results were as follows:

AbrasivesDurability
[No.s of passes to replace 100%]
calibration standard- SS 5502946 [2900+-50 as per standard]
A-grit GP 252893
A-shots SS 3303043
B-grit GP 251091
B-shots SS3301483

Supplier A & B were both good name in industry B-shots gives half life compared to that of A-shots and B-grit gives one third life compared to that of A- grits. Results at that time were shocking, but resulted in very hefty saving in surface preparation cost.

This facility may be utilized for evaluation of shot and grits and for new vendor development.

It can also be used with a frequency inverter to establish optimum shot velocity of abrasive mix.

Thus Ervin test with simulation of real shot blasting process, helps to demonstrate media properties such as hardness, size, density and durability and how their interaction, determines the efficiency of energy transfer and the economy of abrasive cleaning process.

How to tell the difference between paint and fusion bonded epoxy coating

Hundreds of paint formulations are being sold in market each claiming the best benefits. Each paint has its own chemistry and set of properties so it is essential to match it with requirement of application to get the maximum advantage esp when value addition difference is too wide and the opportunity cost difference is too high eg. In functional coatings like rebars.

When pigments are fine tuned, it is difficult to differentiate between regularly painted rebar and fusion bonded epoxy coated rebar as visually both look alike. However with careful observations and some tests to validate the same it can be easily identified & differentiated between normally painted rebar and fusion bonded epoxy coated rebars.

ParameterPaint Fbec
Image
Raw materialContains VOC+solvents-pollution
Liquid
Lot of wastage due to overspray
Environment friendly
Powder
100% used
ProcessSand blasting
-health hazard
-poor Ra-poor adhesion
Primer coat+top coat
Brush applied, air cured, poor adhesion
Site application-unreliable quality
Abrasive blasting
Single uniform coat
Fusion bonded-strong adhesion
Plant applied-controlled quality
Properties VisualDull matt-like finish
Bulging
Air pockets
Sagging, dripping
Solid smooth finish
Almost uniform thickness
DamageEasily damaged
Corrosion spreads underneath film
Chips & scratches easily
Hard & tough film
Prevents spread of corrosion around damaged area
Scratch resistant
Sharp edgesDifficult to cover, less thicknessMore thick cover
Contour ribsFilled & coveredLess thickness
DurabilityFew years80-100 years
Tests
Cathodic Disbondment>4 mm<3mm
Applied Voltage ResistenceH2 Evolution & Rusting within a weeklasts more than 30 days
GlossLowHigh
Dielectric resistancepoorstrong
Film ThicknessLow and non-uniformUniform
Hardness< 16 Knoop>16Knoop
Salt spray-scratched panelvery less>1000Hrs
Boil Adhesion TestFailPass

Above table summarizes the salient differences between paint and FBEC which will be helpful to site engineers in achieving desired design life of RCC structures by protecting their rebars from undue corrosion.

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.
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.

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. .