Paper 5 Repair & Rehabilitation of Rcc Structures Damaged by Corrosion

8/9/2019 Paper 5 Repair & Rehabilitation of Rcc Structures Damaged by Corrosion

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Introduction to Corrosion of RCC structures & Cost of Corrosion

Deterioration of RCC Structures

Corrosion Induced Damages and Condition Assessment of RCCStructures

Cases of Corrosion Induced damages to RCC Structures

Repair and Rehabilitation of Corrosion Induced Damages of RCCStructures

Corrosion Control of RCC Structures by Cathodic Protection

Conclusions

Repair & Rehabilitation of RCC StructuresDamaged by Corrosion- Outline


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Introduction to Corrosion Induced Damages To RCC

Structures and Cost of Corrosion


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Corrosion – Some Examples


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W Corrosion – Some Examples


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COSTS OF CORROSION

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COSTS OF CORROSION


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COSTS OF CORROSION


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“EPRI in its research report on the "Cost of Corrosion in theElectric Power Industry" estimated that the cost of corrosion inElectrical Industry of USA was of the order of US $ 34.5 Billion perannum in 2003. Based on the studies various corrosion problemsin the Fossil power plants were identified. Around US $ 11 billion

was due to boiler tube failures followed by US $ 6 billion due tocorrosion problems in turbines”

At present no such studies have been conducted for Indianpower sector. However, based on various corrosion relatedproblems being analyzed by NTPC - R&D and the literatureanalysis an attempt is being made to identify the high costcorrosion related problems where research & developmentefforts can be made to provide remedial measures and therebyreduce O&M costs & forced outages and improve performance.

COSTS OF CORROSION


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Corrosion Problem O&M Non- FuelRelated Corrosion

Cost US $

DepreciationCorrosion Cost

US $

Total CorrosionCost US $

All Corrosion Problems in FossilSteam Plants

3,43,50,00,000 1,14,20,00,000 4,57,70,00,000

Waterside/Steam side Corrosion

of Boiler Tubes

91,60,00,000 22,84,00,000 1,14,44,00,000

Turbine CF & SCC 45,80,00,000 14,27,50,000 60,07,50,000

Oxide Particle erosion of Turbines 27,48,00,000 8,56,50,000 36,04,50,000

Heat Exchanger Corrosion 27,48,00,000 8,56,50,000 36,04,50,000

Fireside Corrosion of Water walltubes

18,32,00,000 14,27,50,000 32,59,50,000

Generator clip to strand Corro 18,32,00,000 2,85,50,000 21,17,50,000

Copper deposition in turbines 9,16,00,000 5,71,00,000 14,87,00,000

Fireside Corrosion of SH & RH

tubes

9,16,00,000 5,71,00,000 14,87,00,000

COSTS OF CORROSION


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COSTS OF CORROSION

“According to the US Federal Highway Administration's National BridgeInventory, at least 59% of the nation's 586,000 bridges are reinforced

concrete structures. The durability of concrete is compromised by

corrosion of reinforcement in certain environments or exposure conditions.

This degradation has an impact on the operation of the structure and/or

results in the reduction of overall structural integrity. In addition, corrosion

can result in catastrophic failures, with accompanying loss of human lifeand significant impact on the local economy. With the limited availability of

maintenance and preservation funds, controlling corrosion has become a

top priority for many bridge owners”.

A recent cost-of-corrosion study determined that the annual cost of

corrosion to all bridges is $8.29 billion, and the indirect cost to theuser resulting from traffic delays and lost productivity can be morethan 10 times the direct cost of corrosion.


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Corrosion is a natural process and

is a result of the inherent tendency

of metals to revert to their more

stable compounds, usually oxides.

Most metals are found in nature inthe form of various chemical

compounds called ores. In the

refining process, energy is added

to the ore, to produce the metal. It

is this same energy that providesthe driving force causing the metal

to revert back to the more stable

compound.

CORROSION

General Corrosion

Pitting Corrosion

Under deposit Corrosion


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CORROSION IS A NATURAL PROCESS BY VIRTUE OF

WHICH THE METALS TEND TO ACHIEVE THE

LEAST ENERGY STATE – I.E. COMBINED STATE

M M2+ + 2e- ANODIC REACTION

N 2- + 2e N

CATHODIC REACTIONMIC

Dezincification

WHAT IS CORROSION


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W Corrosion Basics

• Corrosion requires:

– Oxygen & Water

– Rusting takes place in

presence of Air & Water

– No rusting will occur if either

water or air is removed


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Atmospheric Corrosion

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Relationship between corrosion rate and the moisture content of air shows the importance of

maintaining relative humidity below about 40%.


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Galvanic Series

• Ranking the reactivity of metals/alloys in seawater

Platinum

Gold

Graphite

Titanium

Silver

316 Stainless Steel (passive)Nickel (passive)

Copper

Nickel (active)

Tin

Lead

316 Stainless Steel (active)Iron/Steel

Aluminum Alloys

Cadmium

Zinc

Magnesium m o r e a n o d i c

( a c t i v e )

m o r e

c a t h o d i c

( i n e r t )


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W Galvanic Series


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PRACTICAL GALVANIC SERIES

Material Potential*

Pure Magnesium -1.75

Magnesium Alloy -1.60

Zinc -1.10

Aluminum Alloy -1.00

Cadmium -0.80Mild Steel (New) -0.70

Mild Steel (Old) -0.50

Cast Iron -0.50

Stainless Steel -0.50 to + 0.10

Copper, Brass, Bronze -0.20

Titanium -0.20

Gold +0.20

Carbon, Graphite, Coke +0.30

* Potentials With Respect to Saturated Cu-CuSO4 Electrode

Galvanic Series


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Deterioration of RCC Structures


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Concrete Interior (untreated)


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Reinforced Concrete


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Concrete Deterioration

Durable concrete is defined as concrete fit for the purpose for which itwas intended, under the conditions to which the concrete is expected,

and for the expected life during which the concrete is to remain in

service.

ACI 201.2R Guide to Durable Concrete – “Durability of hydraulic

cement concrete is determined by its ability to resist weathering action,chemical attack, abrasion, or any other process of deterioration”.

ACI 201 Deterioration Modes – Freezing & Thawing, Alkali-AggregateReaction (AAR), Chemical attack, Corrosion of embedded metal,

abrasion

Corrosion is one of the major modes of deterioration of concretestructures and is considered a big threat to the durability of the

structures especially for structures in contact with water/seawater.

R i & R h bilit ti f C i I d d


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W Repair & Rehabilitation of Corrosion InducedDamages to RCC Structures


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S.No Deterioration Mode Typical Appearance

1 Structural Failure:

Actual structural failure, or even structural

cracking is only rarely encountered but it

is important to differentiate between

cracking from structural and other

causes.

2 Crazing is a pattern of fine cracks that do

not penetrate much below the surfaceand are usually a cosmetic problem only.

They are barely visible, except when the

concrete is drying after the surface has

been wet.


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3 Plastic Shrinkage Cracking: When water evaporates

from the surface of freshly placed concrete faster than itis replaced by bleed water, the surface concrete shrinks.

Due to the restraint provided by the concrete below the

drying surface layer, tensile stresses develop in the weak,

stiffening plastic concrete, resulting in shallow cracks of

varying depth. These cracks are often fairly wide at the

surface.4 Drying Shrinkage: Because almost all concrete is mixed

with more water than is needed to hydrate the cement,

much of the remaining water evaporates, causing the

concrete to shrink. Restraint to shrinkage, provided by the

subgrade, reinforcement, or another part of the structure,

causes tensile stresses to develop in the hardened

concrete. Restraint to drying shrinkage is the most

common cause of concrete cracking. In many

applications, drying shrinkage cracking is inevitable.

Therefore, contraction (control) joints are placed in

concrete to predetermine the location of drying shrinkage

cracks.


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5 D-cracking is a form of freeze-thaw deterioration that has

been observed in some pavements after three or moreyears of service. Due to the natural accumulation of water

in the base and subbase of pavements, the aggregate

may eventually become saturated. Then with freezing and

thawing cycles, cracking of the concrete starts in the

saturated aggregate at the bottom of the slab and

progresses upward until it reaches the wearing surface.D-cracking usually starts near pavement joints.

6 Thermal cracks:

Temperature rise (especially significant in mass concrete)

results from the heat of hydration of cementitious

materials. As the interior concrete increases in

temperature and expands, the surface concrete may be

cooling and contracting. This causes tensile stresses that

may result in thermal cracks at the surface if the

temperature differential between the surface and center is

too great. The width and depth of cracks depends upon

the temperature differential, physical properties of the

concrete, and the reinforcing steel.


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7 Corrosion of Steel:

Steel reinforcement is normally chemically protected

from corrosion by the alkaline nature of the concrete.

If this alkalinity is lost through carbonation or if

chlorides are present which can break down this

immunity, then corrosion can occur. Obviously, when

cover is low, the onset of corrosion will be sooner.

8 Alkali Silica Reaction:

Alkali-aggregate reaction: Alkali-aggregate reactivity is

a type of concrete deterioration that occurs when the

active mineral constituents of some aggregates react

with the alkali hydroxides in the concrete. Alkali-

aggregate reactivity occurs in two forms—alkali-silica

reaction (ASR) and alkali-carbonate reaction

(ACR). Indications of the presence of alkali-aggregate

reactivity may be a network of cracks, closed or

spalling joints, or displacement of different portions of

a structure.

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9 Shrinkable Aggregates:

Some, mostly igneous, aggregates can contain inclusions of

weathered material in the form of clay minerals. These

minerals, in common with the clays encountered in the ground,

swell in the presence of moisture and shrink as they dry out.

They can cause excessive drying shrinkage of the concrete

and can cause a random crack pattern not unlike that

encountered with ASR

10 Chemical Attack:

Concrete buried in soils or groundwater containing high levels of sulfate

salts, particularly in the form of sodium, potassium or magnesium salts,

may be subjected to sulfate attack under damp conditions. An

expansive reaction occurs between the sulfates and the C3A phase toform calcium sulfoaluminate (ettringite) with consequent disruption to

the matrix. Past experience has shown that true sulfate attack is rare in

concrete, only occurring with very low cement content concretes, with

less than about 300 kg/m3 of cement. As a guide, levels of sulfate

above about 4% of cement (expressed as SO3) may indicate the

possibility of sulfate attack, provided sufficient moisture is present.

Sulfate attack requires prolonged exposure to damp conditions.

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11 Poor Quality Construction:

During construction lack of attention to proper

quality control can produce concrete which

may be inferior in both durability and strength

to that assumed by the designer. Particular

factors in this respect are compaction, curing

conditions, low cement content, incorrect

aggregate grading, incorrect water cement

ratio and inadequate cover to reinforcement.

12 Efflorescence:

In chemistry, efflorescence (which means "toflower out" in French) is the loss of water or a

solvent of crystallization from a hydrated orsolvated salt to the atmosphere on exposureto air.

Efflorescences can occur in natural and builtenvironments. On porous constructionmaterials it may present a cosmetic problemonly, but can sometimes indicate seriousstructural weakness.

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13 Patch Accelerated Corrosion –

Commonly referred to as "ring anode corrosion" or "halo

effect", patch accelerated corrosion is a phenomenon

specific to concrete restoration projects. When repairs

are completed on corrosion-damaged structures, abrupt

changes in the concrete surrounding the reinforcing steel

are created. Typical concrete repair procedures call forremoval of the concrete around the full circumference of

the reinforcing steel within the repair area, cleaning of

corrosion by-products from the steel, and refilling the

cavity with new chloride-free, high pH concrete. This

procedure leaves the reinforcing steel embedded in

adjacent environments with abruptly different corrosion

potentials. This difference in corrosion potential (voltage)

is the driving force for new corrosion sites to form in the

surrounding contaminated concrete. The evidence of

this activity is the presence of new concrete spalling

adjacent to previously completed patch repairs.

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Corrosion Induced Damages and Methods of

Condition Assessment of RCC Structures

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Corrosion Induced Damages To RCC Structures

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Corrosion Induced Damages To RCC Structures

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Visual Observations

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Concrete Quality at 165 m Level(Walkway)

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Concrete Quality at 165 m Level(Walkway)

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Anodic & Cathodic Reactions

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CORROSION OF STEEL IN CONCRETE


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Whatever the source of deterioration and the mechanism of its

development, corrosion of embedded reinforcement is recognizedas the major problem affecting the durability of concrete structures.

It has been found that 40% failure of structures is on account ofcorrosion of embedded steel in concrete. Therefore, corrosioncontrol of steel reinforcement is a subject of paramount importance.

Reinforcing steel in good quality concrete does not corrode even if

sufficient moisture and oxygen are available. This is due to the

spontaneous formation of a thin protective oxide film (passive film)

on the steel surface in the highly alkaline pore solution of the

concrete.

When sufficient chloride ions (from deicing salts or from sea water)

have penetrated to the reinforcement or when the pH of the pore

solution drops to low values due to carbonation, the protective film

is destroyed and the reinforcing steel is depassivated.

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CAUSES OF CORROSION

Following are the two most common contributing factors leading to reinforcement

corrosion:

(i) Localized breakdown of the passive film on the steel by chloride ions called

chloride attack.

(ii) General breakdown of passivity by neutralization of the concrete, predominantly

by reaction with atmospheric carbon dioxide called carbonation.

CARBONATION

Carbon dioxide, which is present in the air at around 0.3 per cent by volume,

dissolves in water to form a mildly acidic solution. This forms within the pores of the

concrete, here it reacts with the alkaline calcium hydroxide forming insolublecalcium carbonate. The pH value then drops from more than 12 to about 8.5.

In the case of carbonation, atmospheric carbon dioxide (CO2) reacts with pore

water alkali according to the generalized reaction,

Ca(OH)2 + CO

2 → CaCO

3 + H

2O

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CAUSES OF CORROSION

It consumes alkalinity and reduces pore water pH to the 8 –9 range, where steel isno longer passive.

The carbonation process moves as a front through the concrete, on reaching the

reinforcing steel, the passive layer decays when the pH value drops below 10.5. If

the carbonated front penetrates sufficiently deeply into the concrete to intersect

with the concrete reinforcement interface, protection is lost and, since both oxygenand moisture are available, the steel is likely to corrode.

CHLORIDE

The passivity provided by the alkaline conditions can also be destroyed by the

presence of chloride ions, even though a high level of alkalinity remains in the

concrete. The chloride ion can locally de-passivate the metal and promote activemetal dissolution. Chlorides react with the calcium aluminate and calcium

aluminoferrite in the concrete to form insoluble calcium chloroaluminates and

calcium chloroferrites in which the chloride is bound in non-active form. However,

the reaction is never complete and some active soluble chloride always remains in

equilibrium in the aqueous phase in the concrete. It is this chloride in solution that

is free to promote corrosion of the steel.

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MECHANISM OF CORROSION

The corrosion process that takes place in concrete is electrochemical in nature.Corrosion will result in the flow of electrons between anodic and cathodic sites on

the rebar. Concrete, when exposed to wet and dry cycles, has sufficient

conductivity to serve as an electrolyte.

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The corrosion of steel in concrete in the presence of oxygen but without chloridestakes place in several steps:

At the anode, iron is oxidized to the ferrous state and releases electrons

Fe Fe2+ + 2e-

These electrons migrate to the cathode where they combine with water and oxygento form hydroxyl ions

2e- + H2O + 1/2O2 2OH-

Fe2+ + 2OH- Fe(OH)2

In the presence of water and oxygen, the ferrous hydroxide is further oxidized to

form Fe2O3

4Fe(OH)2 + O2 + H2O 4Fe(OH)3

2Fe(OH)3 Fe2O3.2H2O

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At the anode, iron reacts with chloride ions to form an intermediate solubleironchloride complex

Fe + 2Cl - (Fe 2+ + 2Cl - ) + 2e -

When the iron –chloride complex diffuses away from the bar to an area with higher

pH and concentration of oxygen, it reacts with hydroxyl ions to form Fe(OH)2. This

complex reacts with water to form ferrous hydroxide.

(Fe 2+ + 2Cl - ) + 2H2O + 2e - Fe(OH)2 + 2H

+ + 2Cl-

The hydrogen ions then combine with electrons to form hydrogen gas

2H+

+ 2e-

H2

As in the case of corrosion of steel without chlorides, the ferrous hydroxide, in the

presence of water and oxygen, is further oxidized to form Fe2O3

4Fe(OH)2 + O2 + H2O 4Fe(OH)3

2Fe(OH)3 Fe2O3.2H2O

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C i I d d C ki f th C t


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Corrosion Induced Cracking of the Concrete

• Carbonation

• Chloride Contamination

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Corrosion Induced Deterioration of Concrete


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Corrosion Induced Deterioration of Concrete

caused by severe environment in Natural-Draft

hyperbolic Cooling Towers

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Corrosion Induced Deterioration of Concrete caused


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Corrosion Induced Deterioration of Concrete caused

by severe environment in Natural-Draft hyperbolic

Cooling Towers

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Corrosion Progress

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American Concrete Institute recommends the following limits for chloride in new

constructions (ACI 222R-01)

Category Chloride limits for New Constructions

(% by Mass of Cement)

Test Method

Acid Soluble Water Soluble

ASTM C 1152 ASTM C 1218 Soxhlet

Prestressed

Concrete

0.08 0.06 0.06

RCC in Wet

Conditions

0.10 0.08 0.08

RCC in dry

conditions

0.20 0.15 0.15

WRT Concrete = 0.03 – 0.04%

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ACI Building Code 318: Sulphate Attack on Concrete

Negligible attack: When the sulphate content is under 0.1 percent in soil,or under 150 ppm (mg/liter) in water , there shall be no restriction on thecement type and water/cement ratio.

Moderate attack: When the sulphate content is 0.1 to 0.2 percent in soil, or

150 to 1500 ppm in water , ASTM Type II portland cement or portlandpozzolana or portland slag cement shall be used, with less than an 0.5water/cement ratio for normal-weight concrete.

Severe attack: When the sulphate content is 0.2 to 2.00 percent in soil, or1500 to 10,000 ppm in water , ASTM Type V portland cement, with less than

an 0.45 water/cement ratio, shall be used.

Very severe attack: When the sulphate content is over 2 percent in soil, orover 10,000 ppm in water , ASTM Type V cement plus a pozzolanic admixtureshall be used, with less than a 0.45 water/cement ratio.

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Reinforced concrete structures that are partially or fully submerged in seawater

are especially prone to reinforcing steel corrosion due to a variety of reasons.

These include high chloride concentration levels from the seawater, wet/drycycling of the concrete, high moisture content and oxygen availability. Three

areas on concrete structures in marine environments can be distinguished

regarding corrosion:

The submerged zone (always below seawater); The splash and tidal zone (intermittently wet and dry); and The atmospheric zone (well above mean high tide and infrequently

wetted).

The characteristics of the corrosion differ from one zone to another. The

corrosion level on reinforced concrete structure located below water level islimited by low oxygen availability, and on the other hand lower chloride and

moisture content in the atmospheric zone limit the corrosion level above high

tide. Corrosion is most severe within the splash and tidal zones wherealternate wetting and drying result in high chloride and oxygen content.

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Corrosion Control Measures:

Epoxy -coated reinforc ing steel

Galvanized steel

Stainless steel

Cement and pozzolans

Water-cement i t ious mater ials rat io

Aggregate

Cur ing cond i t ions

Corros ion inhib i tors

Cathodic protect ion

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Corrosion Control Measures:

Cathodic protect ion (CP) is the on ly known means o f m it igat ing the

corros ion of re inforc ing steel , which is c aused b y the presence of the

chlor ide ion in ex is t ing s tructures.

Cathodic protection (CP) is a technique to control the corrosion of a metalsurface by making it work as a cathode of an electrochemical cell.

M → M+ + e- (metal) (soluble salt) (electron)

A common example is:

Fe → Fe++ + 2e-

2H+ + 2e- → H2 (hydrogen ions (gas) in solution)

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Condition Assessment of RCC Structures:

Parameter Test/MethodConcrete

Compressive Strength Rebound Hammer

Windsor Probe

Ultrasonic Pulse Velocity

Core

Capo

Pull out

Combination

Flexural Strength Break-off

Direct Tensile Strength Pull Off

Concrete quality,

Homogenity, Honeycombing,

Voids


Pulse Echo

Endoscopy

Gamma ray radiography

Damages – Fire/Blast Rebound Hammer


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Parameter Test/Method

Cracks – Pavements/Water Tanks Ultrasonic Pulse Velocity

Acoustic Crack detectorDye Penetration Test

X –Ray Radiography

Gamma Ray Radiography

Thermal Imaging

Crack Scope

SteelLocation, Cover, Size Rebar locator, Bar-sizer

Corrosion Half-Cell Potential

Resistivity

Carbonation

Chloride Content

Condition Endoscope/Boroscope

Integrity & Performance Tapping

Pulse echo

Acoustic Emission

Radar

PetrographyLoad Tests

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S.No. Potential (mV

Vs Cu/CuS04)

Corrosion

Condition

Electrical

Resistivity

(KiloOhm cm)

Corrosion

Condition

1 > - 200 Low > 20 Negligible

2 - 350 to - 200 Intermediate 10 to 20 Low

3 < - 350 High 5 to 10 High

4 < - 500 Severe < 5 Very High

W Survey Techniques include potential mapping


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CSE potential: volts Condition 0.20 Passive

0.20 to 0.35 Active or passive

0.35 Active

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Corrosion Rate measurements can be useful


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Cases of Corrosion Induced Damages To RCC Structures

W Case Studies on Corrosion Induced Damages toRCC Structures


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1. Chloride Induced Damages to Natural Draft Cooling Towers in Contact withSeawater:

Natural Draft Cooling Towers at a station operating on seawater with 35000 ppmchloride were found to be suffering from corrosion induced damages such as spalling

of concrete, rusted reinforced bars, cracks on the concrete, delaminated concrete,

etc.

Half-cell potential values – 165 to – 550 mv Vs. Cu/CuSO4 - severe corrosion of

reinforcement bars)Resistivity values - 0.4 – 26 kiloohms.cm - severe corrosion of rebarsChloride content - 0.03 to 0.8% by weight of cement- high chloridecontamination of concrete pH values lie between 8.0 to 12.5 - some chemical attack on concrete.Rebound hammer & core tests - some deterioration of concrete strength.

Repairs were carried out to some racker columns. Inspection after about 2 ½ yearsindicated that cracks/spalling at the point of repairs had resurfaced indicatingnormal patch repairs to chloride contaminated concrete are not successful andmore protective measures such as cathodic protection would be necessary for

ensuring durability of the structures.



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Condition Assessment of RCC Structures

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Results of Condition Assessment


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S.No Test Results Criteria Remarks

1 Visualobservations

Spalling of Concrete &Reinforcement bar corroded &

thickness reduced at many

locations (outer & inner

surfaces of NDCTs). Low

Concrete cover.

Corrosiondamages to RCC

structures. Less

damage at

Coated portions.

2 ReboundHammer Test CT (N) – 10-19 MPaCT 1 (S) – 14-42 MPaCT 2 (E) – 14-26 MPa

CT 2 (W) – 17-23 MPa

> 30 MPa Lower portionshave better

compressive

strength

3 Half CellPotential Test

(-) 165 to (-) 550 More negative

than (-) 350 MV –

90% Probability of

Corrosion

Most of the

readings are

more negative

than (-) 200 MV

4 ElectricalResistivity

CT 1 – 2.1 to 8.67 Kohm cm

CT 2 – 2.16 to 8.62 Kohm cm

>20 – no corrosion

< 5 – Severe Corr.

Mostly corrosive

5 Carbonation Only at outer layers Good concrete

6 Chloride

content

0.03 to 0.73% (Wt. of Cement) < 0.15% Major cause –

CP best option

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26.05.0908.05.11

01.07.12

W Raker Column of NDCT


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26.05.0908.05.11

01.07.12

W Condition of NDCTS


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Ring Beam of NDCT 1 in 2009Ring Beam of NDCT 1 in 2012



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Ring Beam of NDCT 1 in 2009Ring Beam of NDCT 1 in 2012



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Raker columns of NDCT 1 in 2009Raker Column of NDCT 1 in 2012

New Cracks observed



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Raker Column of NDCT 1 in 2012

New Cracks observedRaker Column of NDCT 1 in 2012

New Cracks with spalling of

Concrete observed



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Deteriorating condition of NDCT 1 in 2012



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Ring Beam of NDCT 2 in 2009 Ring Beam of NDCT 2 in 2012



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2. Chloride Induced Damages to RCC Structures such as Ash Handling, etc inContact with Seawater:

RCC structures such as Bottom Ash Hopper, ESP, Ash Slurry sump, etc at a stationoperating on seawater with 35000 ppm chloride were found to be suffering from

corrosion induced damages such as spalling of concrete, rusted reinforced bars,

cracks on the concrete, delaminated concrete, etc.

Half-cell potential values: – 382 to – 556 mv Vs. Cu/CuSO4 - severe corrosion of

reinforcement bars;Resistivity values: 1.8 – 9.2 kiloohms.cm - severe corrosion of rebars;Rebound hammer & core tests: some deterioration of concrete strength.

Patch repairs have been carried out to some of these damages and the repairs are

under observation.

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22 30 27

15 30 28

30 28 17

Rebound Hammer: (N/mm2)

-457 -444 -408

-455 -434 -411

-426 -436 -423

Half cell potential :(mV)

3.6

1.9 4.4 1.9

2.8

Electrical Resistivity :( kilo ohm cm)

Condition Assessment data

High Corrosion

Very High Corrosion

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Ash Slurry Sump


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46 52

48 48

47 46

Rebound Hammer: (N/mm2)

-382 -402 -417

-425 -395 -435

-410 -457 -415

Half cell potential :(mV)

3.1 2.8

2.1 2.4 2.3

1.8 2.1



High Corrosion

Very High Corrosion

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Bottom Ash Hopper


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Rebound Hammer: (N/mm2) Half cell potential :(mV)


40

45

35

45 26 35

38 25 48

-557 -441 -409

-522 -498 -432

-514 -458 -465

7.9 7.9

4.5

6.2

7.3

9.4

7.0


High to Very High Corrosion

High Corrosion



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ESP Buffer Hopper Structures



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Bottom Ash Structures



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3. Carbonation Induced Damages to Induced Draft Cooling Towers in Contact

with Fresh Water:

A station operating on fresh water as cooling water for more than 15 years reported

some damages like cracks, spalling, delamination, etc of concrete structures of

Induced Draft Cooling Towers.

Half-cell potential and carbonation tests indicated that the potential values are

between – 186 to – 293 mv Vs Cu/CuSO4 indicating that corrosion attack is low

to high. Carbonation tests indicated severe carbonation/chemical attack

(plant uses sulphuric acid for pH/alkalinity control in the cooling water system). It was

inferred that most of the damages were on account of carbonation/chemical attack.

Repairs have been suggested and are expected to be undertaken shortly.

Subsequently anti-corrosive coatings for complete structure which are water or watervapour touched, to be applied for further protection.



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4. Chloride Induced Damages to Induced Draft Cooling Towers in Contact with

Seawater:

A station in coastal region and using seawater as cooling water with about 40000ppm chloride reported severe damages to RCC structures of induced draft cooling

towers in less than 3 years’ time.

Visual observations indicated that generally efflorescence (salt deposition) was

prevalent on the structures and it appeared the concrete had high porosity, some

places rust spots could be observed, a few places reinforcements were exposed,some surface cracks were also seen. On one CT Salt along with coal dust was

deposited on the roof surface. Some expansion joints were found to be leaking. The

observations are depicted in following photographs.

The half-cell potential values: – 242 to – 489 mv Vs Cu/CuSO4 for older towersfor a new tower the value was +9 mv.

Chloride contamination: 0.1 to 0.9 % of the weight of concrete (acceptablevalue is < 0.03%) for the older towers.Negligible carbonationSevere corrosion induced damages are taking place on the older towerswhereas the new tower (yet to be put in operation) is not yet under durancefrom chloride. Cathodic Protection for older towers & PU coating for Tower



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5 Structural Damages to Dry Fly Ash Silo in Coastal Region:


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5. Structural Damages to Dry Fly Ash Silo in Coastal Region:

A coastal station reported cracking of RCC dry fly ash silo structure from where fly ash was

oozing out. The damaged portion was repaired by patch repair and inside surface was

inaccessible due to fly ash. Preliminary condition assessment by carbonation test, half-cellpotential measurement and chloride contamination test was carried out. The plant uses a blend

of imported and indigenous coal as fuel and the blend ratio is variable. The fly ash is alkaline in

nature with about 11.5% calcium oxide and around 12.8% iron oxide (such ashes are

hygroscopic in nature). The half-cell potential and chloride contamination tests do notindicate corrosion induced damages. Negligible carbonation was observed. Thus it wasinferred that the crack had developed due to tensile stresses on the concrete walls. The tensile

stresses could have developed due to any one or a combination of following reasons:

A large void (such as a horizontal arch or a vertical rathole) that forms within the body of thestored material and later collapses, resulting in a significant dynamic load on the silo walls.

Non-uniform pressures acting on a circular silo wall that are used by an off-center channel in

the material adjacent to the Wall.

Local peak pressure at a point where a funnel flow channel intersects a silo wall. Development of mass flow in a silo structurally designed for funnel flow.

Migration of moisture from wet to dry particles within the stored solids, which causes the dry

particles to expand and imposes strong radial loads on a silo.

Variation in operating practices in emptying the silos from design.

Variat ion in the qual i ty of f ly ash being s tored

Asymm etr ic pressures caused by ins erts (suc h as beams ) across the cyl ind er sect ion

of a si lo. (In p resent case some m odif icat ions had been carr ied o ut in the si lo, thism i h t h ave caused ten si le s tresses in th e s ilo .



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Repair & Rehabilitation of Corrosion Induced

Damages To RCC Structures



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Repair & Rehabilitation of Damaged RCC Structures:

Patch Repair:

By far the most common repair technique is the application of concrete patches to damaged

or deteriorated concrete. Furthermore, when other remediation techniques are being applied

in order to limit the extent of on-going corrosion mechanisms or to prevent their re-

occurrence. Patch repairs are also used to reinstate the spalled or delaminated areas of

concrete.

Electrochemical Process:

Conventional patch repair is, and will always remain the primary method of repair of

reinforced concrete structures suffering from corrosion damage to the

reinforcement. Electrochemical techniques provide a useful set of methods for preventing or

limiting further damage to structures affected by reinforcement corrosion.

Cathodic Protection (CP): In cathodic protection, the corroding anodic areas of steel aremade cathodic by the supply of electrons from an anode applied either to the concrete

surface or embedded. There are two ways of applying cathodic protection to structures:

Galvanic and Impressed Current CP



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Electrochemical Chloride Acceleration (ECE) – is also known as desalination or chloride

extraction (CE). The fundamental principle involved in ECE is similar to that of CP. The onlymajor differences are the period and level of current application. CP is essentially a

permanent installation involving an application of current in the region of 5-20 mA/m2 of steel

whilst ECE is a temporary treatment where a much higher current density in the range of 0.5-

2.0 A/m2 of steel is applied over a period of weeks. The chloride ions migrate to the concrete

surface where they are removed.

Electrochemical Re-alkalization is used for carbonated reinforced concrete structures andentails the re-establishment of alkalinity around the reinforcement and in the cover zone.

Alkali ions are electrically driven toward the steel which, with the production of hydroxyl ions

at the steel, repassivate the steel and reduce corrosion activity to a negligible level. The

electrolyte is highly alkaline and drawn into the carbonated cover concrete by electro-osmosis

where it acts as a buffer zone.

Corrosion Inhibitors: Concrete admixture inhibitors - used as a preventative measure. Surface applied and drilled-in inhibitors - used as a curative or preventative

measure.

These two generic categories can be further subdivided into anodic, cathodic and ambiodic

(mixed) inhibitors depending upon the formulation of the inhibitor.



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Surface Treatments:

Three generic types of Surface Treatment are available for the decoration and protection of

concrete surfaces, designed to control chemical ingress as well as moisture movement. They

are described as follows:

Pore-liners – these are hydrophobic impregnation treatments such as siliconeimpregnants, which line the pores of concrete. They repel water and therefore

prevent it from entering the concrete, but continue to allow water vapour to escape.

Pore blockers – these are materials that partially or completely block the inconcrete. They may accomplish this by either reacting with the concrete to produce

pore-blocking products or by physically blocking the pores.

Film-formers – these are coating systems based on either organic resins such asstyrene butadiene and acrylic copolymers or inorganic resins such as potassiumsilicate, which form a protective/decorative film on the surface of the concrete.

Coatings may be endowed with special properties, such as the ability to bridge moving cracks

whilst maintaining film integrity.

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Repair & Rehabilitation of Corrosion InducedDamages to RCC Structures


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SELECTING SUITABLE REPAIR AND REHABILITATION STRATEGIES FORDAMAGED RCC STRUCTURES:

To repair is defined as “to replace or refix parts, compensating for loss orexhaustion”.One definition of the word rehabilitate is “to restore to proper condition”.If we want to rehabilitate a structure we want to restore it, not necessarily toits original condition, because if we do, it may fail again because of intrinsicflaws.We want to establish its “proper” condition that is, resistant to corrosion. In other

words, to rehabilitate the structure we may need to improve it compared to its

original condition.

To repair is merely fixing the damage.

This implies that deterioration may continue.

Patch repairs are just what they say. They repair the damaged concrete. They willnot stop future deterioration and may accelerate it.

Cathodic protection and other electrochemical techniques can rehabilitate the

structure. They mitigate the corrosion process across the whole treated areas.

Coatings and barriers can also rehabilitate if applied well at the correct time.



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Conventional rehabilitation techniques, which consist of removing delaminated

areas of concrete, cleaning affected steel and patching with Portland cement

mortar, have proven to be ineffective for marine structures.

Repairs are often repeated every several years, which each successive repair

being increasingly greater in magnitude.

The presence of high levels of chloride ions remaining in the parent concrete will

allow the corrosion process to continue unabated.

The repair material also proves to be a problem since corrosion cells are

inadvertently created between steel embedded in the chloride-free repair materialand the steel embedded in the chloride contaminated concrete.

This result in corrosion damage along the periphery of the patch and eventually

complete failure will occur within the surrounding material and the repair itself.



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“Long-Term Performance of Corrosion Inhibitors Used in Repair ofReinforced Concrete Bridge Components” - Publication No. FHWA-RD-01-097,U.S. Department of Transportation, Federal Highway Administration, Research and

Development, USA –

“ ----- An analysis of the results of visual and delamination surveys, half-cell potential surveys,

corrosion rate measurements, and total chloride ion content determination conc luded thatneither of the corro sion in hibi to rs evaluated in this study, us ing the specif ied

repairs and exposed to the spec i fic env ironments , prov ided any co rros ion-

inhib i t ing benefit” .

“Long-Term Effectiveness of Cathodic Protection Systems on Highway

Structures” - Publication No. FHWA-RD-01-096

“After extensive research and test ing, the Federal Highw ay Adm inistrat ion,

USA issued the pol icy s tatement that the only rehabi l i tat ion techniqu e that

has p roven to stop corros ion in sal t contaminated b r idge decks, regardless

of the chlor ide con tent of the concrete is cathodic protect ion ”



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For undertaking repairs and rehabilitation of damaged RCC structures especially if they are incontact with water/seawater or situated in coastal regions, it is recommended to carry out

condition assessment of the structures so as to confirm the reasons of damages.

It may be advisable to carry out preliminary tests such as visual examinations;delamination; carbonation test; half-cell potential measurements; cover depthmeasurements; etc at random locations to check if the structures are suffering fromcorrosion or not. These tests can be carried out by the stations itself. Based on the resultsof the preliminary tests decision of detailed condition assessment can be taken.

Decision on appropriate repair & rehabilitation technique can be taken on thecondition of the structures and life expectancy of the structures/criticality of thestructures.

If the damages are corrosion induced than suitable corrosion protection measuressuch as cathodic protection need to be considered. The life of cathodically protectedstructures can be extended to 40+ years.

Documents like ACI 222R.01 or BS EN 1504 need to be considered while selectingrepair & rehabilitation techniques and materials for RCC structures.



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BS EN 1504: Products and Systems for the protection and repair of concretestructures – Definitions, requirements, quality control and evaluation of conformity



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BS EN 1504 – 9 Content



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BS EN 1504 – 9



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BS EN 1504 – 9



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BS EN 1504 – 9

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t0 = the time for the environment to penetrate into the concrete to a level where corrosion startst1 = the time for the corrosion rate to increase to significant levels

t2 = the time for cracking to occur, and a subsequent further increase in corrosion rate

t3 = the time for significant structural distress to be caused

t0 will depend on quality of concrete and corrosive environment present

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CONCRETE COVER


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Of the various standards the range of

values for minimum concrete cover are:

Marine Exposure 65-80 mm.

Below Grade Exposure 65-80 mm.

Above Grade Exposure 55-70 mm.

Indoor Exposure 40-50 mm.

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CONCRETE MODIFIERS


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The primary purpose of all concrete modifiers

is to decrease the chloride diffusion rate by

reducing the concrete permeability.

Therefore in areas subject to continuous

chloride exposure such as seawater and

saline groundwater, concrete modifiers willnot prevent corrosion, but only delay the

day at which it starts.

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INHIBITORS


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• “The primary benefit of calcium nitrate is toincrease the chloride threshold value for

corrosion initiation.”

• Grace Construction Products - NACE 1998,Paper 652

• Therefore in areas subject to continuous chloride

exposure such as seawater and salinegroundwater inhibitors will not prevent corrosion,

but only delay the day at which it starts.

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• “For 95% (of bridge decks) the epoxy coating will debondfrom the steel before the chloride arrives and thus provides no

additional service life.”

• Epoxy Coated Rebars are not a cost effective corrosion protection system for bridge decks in Virginia. Thus their use

should be discontinued

• Virginia Transportation Research Council - 1997

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CONCRETE COATINGS


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Coating concrete for corrosion prevention

provides a barrier against Chloride ingress.

Coating only slow the onsett of corrosion and in

harsh environments degrade long before the

end of the design life of most structures.

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Corrosion Control of to RCC Structures by

Cathodic Protection

W

THE CATHODIC PROTECTION CELLCathodic Protection ( Gain of Electrons / Ions )


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Cathode CathodeAnodeCurrent

Flow

Current

Flow

Electrolyte ( Ionic Flow )

Cathodic Protection ( Gain of Electrons / Ions )

Electron Flow Electron Flow e- e- e-

Current FlowCurrent Flow

OH-

OH-

OH-

OH-

OH-

H+ Cation Flow H+

OH Anion Flow 0H

e-

e-

e-

OH-

OH-

OH-

OH-

OH-

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Anode Mesh installation


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Positive Connection

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Shotcrete Overlay


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W

Anode Mesh


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Ribon Anodes


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HISTORY OF INTERNATIONAL APPROVALS


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124

• FHWA - 1982

• UK CONCRETE SOCIETY - 1989

• NACE RP0290-90 - 1990

• NACE RP0390-90 - 1990

• ACI 222 R- 01

• BS 7361 - 1991

• European Union Standard pr EN 12696-1 -

2000

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Official FHWA Policy Statement


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125

• Cathodic Protection is“…

.the onlyrehabilitation technique that has proven

to stop corrosion in salt-contaminated

bridge decks regardless of the chloride

content in the concrete.”

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QUOTATION FROM BS 7361


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126

• “….Cathodic Protection is a means,

possibly the only means, of indefinitely

extending the life of reinforced concretestructures which are suffering

reinforcing steel corrosion arising from

chloride intrusion….”

W Some RCC Structures Protected by CathodicProtection


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127

Some of the structures protected by cathodic protection systems

Structure System type Owner

JUPC Cooling Tower Mesh ribbon installed in slots JUPC, KSA

Sharq Cooling Tower Mesh ribbon installed in slots Sharq, KSA

Kayan Cooling Tower Mesh Ribbon Kayan, KSANCP Cooling Tower Mesh Ribbon NCP, KSA

Yansab Cooling Tower Mesh Ribbon Yansab, KSA

GPIC Intake Structure Mesh ribbon grouted in slots GPIC, Bahrain

Dubai Airport tunnel Mesh ribbon installed in slots Dubai, UAE

Ghazlan power plant Mesh ribbon installed in slots SCECO, KSAQarrayah Intake structure Mesh ribbon installed in slots SCECO, KSA

New Forced Draft CT Mesh Ribbon HPCL, Mumbai

(Under implementation at HPCL Mumbai Refinery)

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Conclusions

W Conclusions

1 RCC Structures are subject to deterioration through different


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129

1. RCC Structures are subject to deterioration through different

mechanisms.

2. RCC structures in contact with seawater or water are subject to

Corrosion induced damages such as chloride induced corrosion or

carbonation induced corrosion affecting the durability of the

structures.

3. The damaged structures are required to be Repaired and

Rehabilitated to restore their durability, however; structures affected

by corrosion need special treatment to care of corrosion besides

restoration of strength.

4. Before undertaking repairs & rehabilitation of damaged structures it

is necessary to carryout detailed condition assessment so that

suitable remedial measures are taken.

5. Preliminary tests such as half cell potential, carbonation tests, visual

inspections, etc can indicate if corrosion induced damages have

initiated.

6. Best remedial measure for chloride induced damages is application

of Cathodic Protection besides patch repairs

W


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