Durability of ConcreteDetail discrition of durability of concrete

8/10/2019 Durability of ConcreteDetail discrition of durability of concrete

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DURABILITY OFCONCRETE STRUCTURES

presented by Gopal

VIII sem

S. J. College of engineering


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INTRODUCTION

FACTORS GOVERNING DURABILITY

BEHAVIOUR OF CONCRETE

CAUSES FOR DETORIATION

IS CODE

DESIGN FOR DURABILITY OF CONCRETESTRUCTURES

RECOMMENDATIONCONCLUSION

REFRENCES

CONTENTS


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INTRODUCTION

During the recent past , the problem of earlydeterioration of concrete structure has posed a seriousproblem all over the world. In India also, this problem isbeing witnessed in the past few years, especially incoastal and industrial area as well as in other aggressiveenvironments.

WHAT IS DURABLE CONCRETE ?Durable concrete can be defined as one that is

designed, constructed and maintained to perform satisfactorilyin the expected environment for the specified designed life.


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Generally, concrete suffers from more than one causesof deterioration, which is generally seen in the form ofcracking, spalling, loss of strength, etc. It is nowaccepted that the main factors influencing the durabilityof concrete is its impermeability to the ingress ofoxygen, water, carbon dioxide, chlorides, sulphates, etc.

A detailed investigation of deteriorated structuresis essential before planning its remedial measures. Theinvestigations involve initial inspection, condition surveyfor cracks and other defects, sampling, measurement ofconcrete cover and assessing the material strength. Theintensity of damage can be assessed on the datacollected through Various investigations including NonDestructive techniques.


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FACTORS GOVERNING DURABILITY

Mix design Structural design

Reinforcement detailing

Concrete cover Curing

supervision

Quality of material used


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6Mix design


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7Structural design & detailing


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Concrete cover


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9Curing


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10Supervision


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BEHAVIOUR OF CONCRETE

The behaviour of concrete depends onseveral processes.

1. Physical

2. Chemical

3. Biological1.Physical process

Physical processes lead to gradual deterioration ofconcrete, and govern its long-term behaviour.

Cracking : Concrete cracks whenever tensile strainsexceed its tensile strain capacity. Cracks may occur ingreen concrete due to plastic shrinkage, settlement offorms and support movements


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Abrasion : The movements of person and traffic onconcrete surfaces cause abrasive wear. Industrial floorand bridge deck slabs are subjected to abrasive wear.

Frost & de-icing salts : The transition of water fromliquid state to solid state due to icing involves anincrease in volume by about 9%. In the porousconcrete, the freezing of water induces splitting forces.

Several cycles of freezing and thawing of water mayresult in spalling of concrete.The frost resistance ofconcrete depends upon several parameters.

2.CHEMICAL PROCESS

Acid attack: Acid attack involves conversion of calciumcompound to calcium salts. The rate of deteriorationdepends not only on the strength of the reactants butalso upon the solubility of the resultant salts and theirtransport.


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Sulphate Attack: Sulphate attack on aluminatecompounds, calcium and hydroxyl of hardened Portlandcement forming ettringite and gypsum. In the presence ofsufficient water, these reactions of delayed ettringiteformation cause expansion of concrete leading to irregularcracking. The cracking of concrete provides further accessto penetrating substances and to progressive deterioration.

Alkali attack : Alkalis react with silica containingaggregates and not with cement. The pore solution inconcrete is lime-saturated and contains potassium andsodium ions. Free alkalis present in cement dissolve in themixing water and forming a caustic solution, which attackthe reactive silica in the aggregate. The alkali silica gel soformed swells in the presence of moisture, and exertsosmotic pressure on the concrete internally.


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3. BIOLOGICAL PROCESS a)Environmental factors

The service life of the concrete structures depends onthe environmental factors as well. The nature, intensityand timing of environmental influences affect thebehaviour of materials. The permeability of concrete,concrete cover, structural form, type and location of

reinforcement, and nature of cement and aggregatesdetermine the response of concrete to environmentalinfluences.

Exposure conditions The general exposure conditions are as given below : Mild Conditions

Severe conditions

Very severe Conditions

Extreme Conditions


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B. Temperature and humidity

An increase in temperature increases the rate of reaction

The rate of corrosion is maximum, when relativehumidity is 90-95%.

Carbonation of concrete takes place rapidly, when therelative humidity is around 50-60%.

Water

Water is essential for most of the processes leading toconcrete deterioration.

Constant wetting and drying is more detrimental toconcrete than submerged conditions.

The concentration of aggressive substances in the porestructures increases as a result of cyclic wetting anddrying leading to corrosion.


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Aggressive elements :

Aggressive elements in nature include water and air. Theusual substance present in water and their actions

detrimental to concrete are listed below. Oxygen dissolved in water.

Carbon dioxide

Chlorides.

Acids in water.

Alkalis in water.

Sulphates.

Aggressive fumes from industrial processes.


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Marine conditions: Marine conditions are more severe thanthose occurring on land. Seawater contains MgCl 2 , MgS0 4 ,

CaSO 4 , KCl, K 2 SO 4 . The mean concentration of these salts isabout 35 gm/L. Apart from these salts, sea water alsocontains dissolved oxygen and carbon dioxide to add tocorrosive process.

The marine, environment may be classified in four zones

according to exposure conditions : Marine Atmosphere Zone

Splash Zone

Tidal Zone

Submerged Zone


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CAUSES OF DETERIORATION Concrete normally provides excellent corrosion

protection to embedded reinforcement. The high alkalinity ofconcrete, i.e. above pH 12.5, results in the formation ofprotective oxide film on steel bars. However, unless concrete iswell compacted and dense, it is susceptible to carbonation, andlooses its capacity to protect reinforcement. Some of thecauses for deterioration of concrete structures are,

Design and construction defects

Poor quality materials

Inadequate supervision

Environment

Corrosion of reinforcement

Inadequate understanding of materials


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IS CODE


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THE TWO IMPORTANT CRITERIA FOR THEDURABILITY OF CONCRETE STRUCTURES ARE

CONTROL OF DEFLECTION

CONTROL OF CRACKING


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CONTROL OF DEFLECTION

FOR FLEXURAL MEMBERS THE DEFLECTIONIS CALCULATED BY CONSIDERING THE SHORTTERM DEFLECTION, DEFLECTION DUE TOSHRINKAGE AND DEFLECTION DUE TO CREEP.

VERTICAL DEFLECTION LIMITS

CANTILEVER 7

SIMPLY SUPPORTED 20

CONTINUOUS 26

SPAN UPTO 10M


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CONTROL OF CRACKINGFLEXURAL MEMBERS

COMPRESSION MEMBERS

SPACING REQUIREMENTS

Minimum spacing

Maximum spacing


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1. Shape & size of members

Care should be taken to minimize any cracks that maycollect or transmit water.

2. Exposure condition

Exposure Nominal concretecover not less than(mm)

Mild 20

Moderate 30

Severe 45

Very severe 50

Extreme 75


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3.Freezing & thawing

Nominal maximum sizeaggregate(mm)

Entrained air percentage

20 5 1

40 4 1

Where freezing & thawing actions exist, enhanced durabilitycan be obtained by the use of suitable air entrainingadmixtures

4.Exposure to sulphate attack

Table 4 of IS 456(2000) shows recommendations for thetype of cement, maximum free water/cement ratio andminimum cement content, which are required at differentsulphate concentration in near-neutral ground water havingpH of 6 to 9.


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5.Concrete mix proportionThe free water-cement ration is an important factor in

governing the durability of concrete and should always be thelowest value. Appropriate values for minimum cement content

and the maximum free water-cement ratio are given below,

Exposure Plain concrete Min. cement Max. free Min. grade

content W/C ratio of concrete

Kg/m 3

Reinforced concreteMin. cement Max. free Min. grade

content W/C ratio of concrete

Kg/m 3

Mild 220 0.60 - 300 0.55 M20

Moderate 240 0.60 M15 300 0.50 M25

Severe 250 0.50 M20 320 0.45 M30

Very sever 260 0.45 M20 340 0.45 M35

Extreme 280 0.40 M25 360 0.40 M40


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6. Chlorides in concrete

There is an increased risk of corrosion of embedded metal

whenever there is chloride in concrete. The total amount of chloridecontent in the concrete at the time of pacing shall be given below,

Types or use of concrete Max. total acid soluble Clcontent (Kg/m3 of concrete)

Concrete containing metal andsteam cured at elevated temp. and

pre-stressed concrete.

0.4

Reinforced concrete or plain

concrete containing embeddedmetal

0.6

Concrete not containing embeddedmetal or any material requiring

protection from chloride.

3.0


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DESIGN FOR DURABILITY OF CONCRETESTRUCTURES

The main concept in the design is to minimize deflectionand cracking.

The procedure for control of deflection is to control spanto effective depth ratio. It assumes that the deflection of beam

and slab will depend on the following factors.1.The span/effective depth ratio

2.Type of supports as to whether simply supported , fixed orcontinuous

3.Percentage of tension steel or the stress level in the steel levelat service loads if more than the necessary steel is provided atthe section.

4.Percentage of compression steel provided.


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Design for limit state of deflection

Excessive deflection of beams and slab is not only aneyesore in itself but it can also cause cracking of portion.Asgiven in IS 456(2000) the commonly accepted limits ofallowable deflection are,

1 A final deflection of span/250 for the deflection of

horizontal bending members like slabs and beam due to allload so as to be noticed by the eye.

2 A deflection of span/350 or 20mm which is less for thesemembers after the construction of the partitions and finishesetc,to prevent damages to finishes and partitions.


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CRACKING

A crack is a complete or incomplete separation ofconcrete into two or more parts produced by breaking orfracturing. The crack in concrete is one, which cannot becompletely prevented but can only be controlled andminimized.

There are two types of crack1.Structural cracks

2.Non-structural cracks


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METHOD OF CRACK CONTROL

Method of crack control

To control the crack width the importantfactors to be considered are the following

1.Maximum and minimum spacing ofreinforcements2.Maximum and minimum area of steel in themember

3.Curtailment of reinforcement bars4.Anchorage of reinforcement bars

5.Cover to reinforcement.


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Exercising adequate care at every stage of planning,analysis, design and construction for the expected exposureconditions.

The performance of structures should be monitoredregularly from the stage of commencing.

RECOMMENDATIONS Good quality concrete mix with the lowest water cement ratiocompatible with practical placement and finishing techniques

should be used. Concrete should be properly placed, consolidated and cured.

Over stressing of structures should be avoided.

Application of flexible surface coatings to avoid concretesurfaces, which can effectively control the ingress of chlorides,sulphates, carbon dioxide, oxygen and moisture, can beconsidered as an effective corrosion control measure.


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CONCLUSION

Durability of concrete structures should be considered as asignificant aspect of structural design. A designer should beaware of the constructional aspects of structures, as well as, inorder to foresee durability problems due to any peculiarities ofstructural loads, layout as well as environment.


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REFERENCES

Concrete technology, Shetty.M.S

Limit state design of reinforced concrete,Varghese.P.C

IS 456 (2000)


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Durability of ConcreteDetail discrition of durability of concrete