*DURABILITY OF CONCRETE STRUCTURES

*INTRODUCTION FACTORS GOVERNING DURABILITYBEHAVIOUR OF CONCRETE CAUSES FOR DETORIATIONIS CODEDESIGN FOR DURABILITY OF CONCRETE STRUCTURESRECOMMENDATIONCONCLUSIONREFRENCESCONTENTS

*INTRODUCTIONDuring the recent past , the problem of early deterioration of concrete structure has posed a serious problem all over the world. In India also, this problem is being witnessed in the past few years, especially in coastal and industrial area as well as in other aggressive environments.WHAT IS DURABLE CONCRETE ?Durable concrete can be defined as one that is designed, constructed and maintained to perform satisfactorily in the expected environment for the specified designed life.

*Generally, concrete suffers from more than one causes of deterioration, which is generally seen in the form of cracking, spalling, loss of strength, etc. It is now accepted that the main factors influencing the durability of concrete is its impermeability to the ingress of oxygen, water, carbon dioxide, chlorides, sulphates, etc.

A detailed investigation of deteriorated structures is essential before planning its remedial measures. The investigations involve initial inspection, condition survey for cracks and other defects, sampling, measurement of concrete cover and assessing the material strength. The intensity of damage can be assessed on the data collected through Various investigations including Non Destructive techniques.

*BEHAVIOUR OF CONCRETE The behaviour of concrete depends on several processes. Physical Chemical Biological 1.Physical process Physical processes lead to gradual deterioration of concrete, and govern its long-term behaviour. Cracking : Concrete cracks whenever tensile strains exceed its tensile strain capacity. Cracks may occur in green concrete due to plastic shrinkage, settlement of forms and support movements

*Abrasion : The movements of person and traffic on concrete surfaces cause abrasive wear. Industrial floor and bridge deck slabs are subjected to abrasive wear. Frost & de-icing salts : The transition of water from liquid state to solid state due to icing involves an increase in volume by about 9%. In the porous concrete, the freezing of water induces splitting forces. Several cycles of freezing and thawing of water may result in spalling of concrete.The frost resistance of concrete depends upon several parameters. 2.CHEMICAL PROCESS Acid attack: Acid attack involves conversion of calcium compound to calcium salts. The rate of deterioration depends not only on the strength of the reactants but also upon the solubility of the resultant salts and their transport.

*Sulphate Attack: Sulphate attack on aluminate compounds, calcium and hydroxyl of hardened Portland cement forming ettringite and gypsum. In the presence of sufficient water, these reactions of delayed ettringite formation cause expansion of concrete leading to irregular cracking. The cracking of concrete provides further access to penetrating substances and to progressive deterioration. Alkali attack : Alkalis react with silica containing aggregates and not with cement. The pore solution in concrete is lime-saturated and contains potassium and sodium ions. Free alkalis present in cement dissolve in the mixing water and forming a caustic solution, which attack the reactive silica in the aggregate. The alkali silica gel so formed swells in the presence of moisture, and exerts osmotic pressure on the concrete internally.

*3. BIOLOGICAL PROCESS a)Environmental factors The service life of the concrete structures depends on the environmental factors as well. The nature, intensity and timing of environmental influences affect the behaviour of materials. The permeability of concrete, concrete cover, structural form, type and location of reinforcement, and nature of cement and aggregates determine the response of concrete to environmental influences. Exposure conditions The general exposure conditions are as given below : Mild Conditions Severe conditions Very severe Conditions Extreme Conditions

*B. Temperature and humidity An increase in temperature increases the rate of reactionThe rate of corrosion is maximum, when relative humidity is 90-95%. Carbonation of concrete takes place rapidly, when the relative humidity is around 50-60%.

Water Water is essential for most of the processes leading to concrete deterioration. Constant wetting and drying is more detrimental to concrete than submerged conditions. The concentration of aggressive substances in the pore structures increases as a result of cyclic wetting and drying leading to corrosion.

*Aggressive elements : Aggressive elements in nature include water and air. The usual substance present in water and their actions detrimental to concrete are listed below. Oxygen dissolved in water. Carbon dioxideChlorides. Acids in water. Alkalis in water. Sulphates.Aggressive fumes from industrial processes.

*Marine conditions: Marine conditions are more severe than those occurring on land. Seawater contains MgCl2, MgS04, CaSO4, KCl, K2SO4. The mean concentration of these salts is about 35 gm/L. Apart from these salts, sea water also contains dissolved oxygen and carbon dioxide to add to corrosive process. The marine, environment may be classified in four zones according to exposure conditions :Marine Atmosphere ZoneSplash ZoneTidal ZoneSubmerged Zone

*CAUSES OF DETERIORATION Concrete normally provides excellent corrosion protection to embedded reinforcement. The high alkalinity of concrete, i.e. above pH 12.5, results in the formation of protective oxide film on steel bars. However, unless concrete is well compacted and dense, it is susceptible to carbonation, and looses its capacity to protect reinforcement. Some of the causes for deterioration of concrete structures are,Design and construction defects Poor quality materials Inadequate supervision Environment Corrosion of reinforcement Inadequate understanding of materials

*IS CODE

*THE TWO IMPORTANT CRITERIA FOR THE DURABILITY OF CONCRETE STRUCTURES ARE CONTROL OF DEFLECTION CONTROL OF CRACKING

*CONTROL OF DEFLECTIONFOR FLEXURAL MEMBERS THE DEFLECTION IS CALCULATED BY CONSIDERING THE SHORT TERM DEFLECTION, DEFLECTION DUE TO SHRINKAGE AND DEFLECTION DUE TO CREEP.VERTICAL DEFLECTION LIMITSCANTILEVER 7SIMPLY SUPPORTED 20CONTINUOUS 26SPAN UPTO 10M

*CONTROL OF CRACKINGFLEXURAL MEMBERSCOMPRESSION MEMBERSSPACING REQUIREMENTSMinimum spacingMaximum spacing

*Shape & size of membersCare should be taken to minimize any cracks that maycollect or transmit water.2. Exposure condition

ExposureNominal concrete cover not less than (mm)Mild20Moderate30Severe45Very severe50Extreme75

*3.Freezing & thawingWhere freezing & thawing actions exist, enhanced durability can be obtained by the use of suitable air entraining admixtures4.Exposure to sulphate attack Table 4 of IS 456(2000) shows recommendations for the type of cement, maximum free water/cement ratio and minimum cement content, which are required at different sulphate concentration in near-neutral ground water having pH of 6 to 9.

Nominal maximum size aggregate(mm)Entrained air percentage20514041

*5.Concrete mix proportionThe free water-cement ration is an important factor in governing the durability of concrete and should always be the lowest value. Appropriate values for minimum cement content and the maximum free water-cement ratio are given below,

ExposurePlain concrete Min. cement Max. free Min. grade content W/C ratio of concrete Kg/m3Reinforced concreteMin. cement Max. free Min. grade content W/C ratio of concreteKg/m3Mild220 0.60 -300 0.55 M20Moderate240 0.60 M15300 0.50 M25 Severe250 0.50 M20 320 0.45 M30 Very sever260 0.45 M20 340 0.45 M35Extreme280 0.40 M25 360 0.40 M40

*6. Chlorides in concreteThere is an increased risk of corrosion of embedded metal whenever there is chloride in concrete. The total amount of chloride content in the concrete at the time of pacing shall be given below,

Types or use of concreteMax. total acid soluble Cl content (Kg/m3 of concrete)Concrete containing metal and steam cured at elevated temp. and pre-stressed concrete.0.4Reinforced concrete or plain concrete containing embedded metal0.6Concrete not containing embedded metal or any material requiring protection from chloride.3.0

*DESIGN FOR DURABILITY OF CONCRETE STRUCTURES The main concept in the design is to minimize deflection and cracking.The procedure for control of deflection is to control span to effective depth ratio. It assumes that the deflection of beam and slab will depend on the following factors.1.The span/effective depth ratio2.Type of supports as to whether simply supported , fixed or continuous3.Percentage of tension steel or the stress level in the steel level at service loads if more than the necessary steel is provided at the section.4.Percentage of compression steel provided.

*Design for limit state of deflection Excessive deflection of beams and slab is not only an eyesore in itself but it can also cause cracking of portion.As given in IS 456(2000) the commonly accepted limits of allowable deflection are, 1 A final deflection of span/250 for the deflection of horizontal bending members like slabs and beam due to all load so as to be noticed by the eye.2 A deflection of span/350 or 20mm which is less for these members after the construction of the partitions and finishes etc,to prevent damages to finishes and partitions.

*CRACKING A crack is a complete or incomplete separation of concrete into two or more parts produced by breaking or fracturing. The crack in concrete is one, which cannot be completely prevented but can only be controlled and minimized.There are two types of crack1.Structural cracks2.Non-structural cracks

*METHOD OF CRACK CONTROLMethod of crack control To control the crack width the important factors to be considered are the following1.Maximum and minimum spacing of reinforcements 2.Maximum and minimum area of steel in the member 3.Curtailment of reinforcement bars 4.Anchorage of reinforcement bars 5.Cover to reinforcement.

*Exercising adequate care at every stage of planning, analysis, design and construction for the expected exposure conditions. The performance of structures should be monitored regularly from the stage of commencing.

RECOMMENDATIONSGood quality concrete mix with the lowest water cement ratio compatible 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 concrete surfaces, which can effectively control the ingress of chlorides, sulphates, carbon dioxide, oxygen and moisture, can be considered as an effective corrosion control measure.

*CONCLUSIONDurability of concrete structures should be considered as a significant aspect of structural design. A designer should be aware of the constructional aspects of structures, as well as, in order to foresee durability problems due to any peculiarities of structural loads, layout as well as environment.

*REFERENCESConcrete technology, Shetty.M.S Limit state design of reinforced concrete, Varghese.P.CIS 456 (2000)

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