DURABILITY OF CONCRETE STRUCTURES RURAL POLYTECHNIC TARIHAL presented by Arjun Narasannavar V sem RURAL POLYTECHNIC TARIHAL

CONTENTS INTRODUCTION FACTORS GOVERNING DURABILITY BEHAVIOUR OF CONCRETE CAUSES FOR DETORIATION IS CODE DESIGN FOR DURABILITY OF CONCRETE STRUCTURES RECOMMENDATION CONCLUSION REFRENCES

WHAT IS DURABLE CONCRETE ? INTRODUCTION During 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.

FACTORS GOVERNING DURABILITY Mix design Structural design Reinforcement detailing Concrete cover Curing supervision Quality of material used

Mix design

Curing

Supervision

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 Abrasion Frost & de-icing salts

2.CHEMICAL PROCESS Acid attack Sulphate Attack Alkali attack

3. BIOLOGICAL PROCESS Environmental factors Temperature & Humidity Marine atmosphere zone Splash zone Tidal zone Submerged 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. 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 CRACKING FLEXURAL MEMBERS COMPRESSION MEMBERS SPACING REQUIREMENTS Minimum spacing Maximum spacing

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 ratio 2.Type of supports as to whether simply supported , fixed or continuous 3.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 crack 1.Structural cracks 2.Non-structural cracks

METHOD OF CRACK CONTROL To control the crack width the important factors to be considered are the following 1.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.

RECOMMENDATIONS Good 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. 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.

CONCLUSION Durability 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.

REFERENCES “Concrete technology”, Shetty.M.S “Limit state design of reinforced concrete”, Varghese.P.C IS 456 (2000)

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