Concrete TechnologyCivil Engineering Department

Concrete TechnologyThe word concrete originates from the Latin verb concretus, which means to grow together.

Objectives To explain the basic concepts of concreteTo explain briefly the properties of freshly mixed concreteTo explain briefly the properties of hardened concreteTo explain briefly mix design of concrete

What is Concrete?Concrete is one of the most commonly used building materials. In the United States almost twice as much concrete is used as all other building materials combined. It is a mixture of cement, water, fine aggregates, coarse aggregates and in some cases, admixtures. Cement and water form a paste that hardens and bonds the aggregates together. Concrete is a versatile material that can easily be mixed to meet a variety of special needs and formed to virtually any shape.Concrete is often looked upon as man made rock. Demand for concrete with higher strength and better quality, coupled with larger and faster mixer trucks, led to the emergence of the ready-mix concrete industry in the post-World War II period.

Cement PowderCement + Water Cement PasteCement Paste + Fine Aggregate (FA) MortarMortar + Coarse Aggregate (CA) ConcretePortland cement, water, sand, and coarse aggregate are proportioned and mixed to produce concrete suited to the particular job for which it is intended.When paste and aggregates are mixed together the particles of aggregates get coated with the paste.Because of the chemical reaction of the cement and water the paste hardens and binds the aggregates together to form a rocklike mass called Concrete Definition of CementCement is a binding material that is used to bind different aggregates (coarse and fine) in the presence of moisture (water) through a chemical process known as hydration. Cement contains limestone, clay , cement rock and iron ore blended and heated to 1200 to 1500 C. The resulting product "clinker" is then ground to the consistency of powder. Gypsum is added to control setting time.

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Fine AggregateNormally called sand, this component can be natural sand or crushed stone, and represents particles smaller than 4.75mm Generally accounts for 30%-35% of the mixture. Coarse AggregateMay be either gravel or crushed stone. Makes up 40%-45% of the mixture, comprised of particles greater than 4.75mm.Chemical AdmixturesMaterials added to alter the properties of concrete including:Air entrainment Set accelerators Set retardersWater reducers Air entraining admixtures add microscopic air bubbles to the concrete, enhancing its resistance to freeze/thaw cycles and makes the concrete easier to finish.

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Set accelerators speed the set-time of the mixture, enabling finishing operations to begin sooner, useful during cold weather pours.Set retarders have the opposite effect, slowing the set and enabling delivery to distant sites and finishing during hot weather.Water reducers are used to reduce the amount of water required to produce a given slump. They also provide a ball bearing effect, making the concrete easier to finish, and produce better cement hydration. By reducing the amount of water required, cement amounts can be reduced because concrete strength is directly related to the water/cement ratio. YieldIt is the volume of fresh concrete produced from known quantities of component materials. Generally expressed in cubic yards or cubic meters.

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Advantages of concreteConcrete has many environmental advantages, including durability, longevity, heat storage capability, and chemical inertness.Ability to be castEconomicalDurableFire resistantEnergy efficient.Low maintenance.On-site fabricationNeeds little or no finish or final treatments. Chemically inert concrete doesn't require paint to achieve a given colour; natural -mineral pigments and colouring agents can be added at the mixing to provide a rainbow of options.Can be reused or recycled. Concrete can be reused with bituminous asphalt as road base materials, can be recycled and reused by crushing into aggregates for new concrete or as fill material for road beds or site works.

Disadvantages of concreteLow tensile strengthLow ductilityVolume instabilityLow strength to weight ratio

Progress in Concrete TechnologyLightweight ConcreteHigh-Strength ConcreteHigh Workability or Flowing ConcreteShrinkage Compensating ConcreteFiber-Reinforced ConcreteConcrete Containing polymersHeavyweight ConcreteMass ConcreteRoller-Compacted Concrete

Quality of ConcreteAggregate Requirement Aggregates should not contain deleterious materials such as chloride, sulfate, etc. that will cause deterioration of concrete.The size, shape and particle size distribution (gradation), of the aggregates should be such that the least amount of paste is required to fill the spaces between aggregate particles.Paste RequirementQuality of paste greatly affects the quality of the concretePaste should be sufficient enough to completely coat each particle of aggregate and to fill spaces between aggregate particles completely.The quality of hardened concrete is determined by the amount of water used in relation to the amount of cement (W/C ratio of the paste).

Advantages of reducing the W/C ratio of the pasteIncrease compressive strength and flexural strength.Lower permeability, thus increased water-tightness and lower absorption.Increased resistance to weathering.Better bond between successive layers and between concrete and reinforcement. Less volume change from wetting and dryingReduced shrinkage cracking tendencies. Uses of Admixtures to Improve the Quality of ConcreteAdjusting setting time or hardening in abnormal conditions of concreting such as cold and hot.Reducing water demandIncreasing workability in case of a very low w/c ratioIntentionally entrained air in concrete

All the ingredients of concrete should be mixed properly so as to get a homogeneous mixture.The sequence of charging ingredients into the mixer plays an important part in the uniformity of the finished product.Size of the batch in relation to the size of the mixer. The elapsed time between batching and mixing.Design, configuration, and condition of the mixer drum and blades. Consolidation RequirementsConsolidation of the freshly placed concrete removes voids in concrete and improves the quality of concrete on account of increased density.For a highly workable concrete the consolidation should be done by hand rodding. For stiffer mixtures having low w/c ratio the consolidation should be done by vibration.

Mixing Requirements

Portland CementPortland cement was named for the Isle of Portland, a peninsula in the English Channel where it was first produced in the 1800's.Since that time, a number of developments and improvements have been made in the production process and cement properties. The production process for portland cement first involves grinding limestone or chalk and alumina and silica from shale or clay. The raw materials are proportioned, mixed, and then burned in large rotary kilns at approximately 1500C until partially fused into marble-sized masses known as clinker. After the clinker cools, gypsum is added, and both materials are ground into a fine powder which is portland cement. Three types of portland cement are used for construction purposes Type I Type II Type III

Type Icement is the general purpose and most common type. Unless an alternative is specified, Type I is usually used. Type IIIt releases less heat during hardening. It is more suitable for projects involving large masses of concrete , heavy retaining walls, or for suspension bridges. Type III It produces concrete that gains strength very rapidly. It is very finely ground and sets rapidly, making it useful for cold weather jobs.

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WaterGood water is essential for quality concrete. It should be good enough to drink--free of trash, organic matter and excessive chemicals and/or minerals. The strength and other properties of concrete are highly dependent on the amount of water and the water-cement ratio. AggregatesAggregates occupy 60 to 80 percent of the volume of concrete. Sand, gravel and crushed stone are the primary aggregates used. All aggregates must be essentially free of silt and/or organic matter. AdmixturesAdmixtures are ingredients other than portland cement, water, and aggregates.Admixtures are added to the concrete mixture immediately before or during mixing.

These are the most commonly used admixtures for agricultural concrete. produce microscopic air bubbles throughout the concrete. Entrained air bubbles: improve the durability of concrete exposed to moisture and freeze/thaw action. Improve resistance to scaling from deicers and corrosive agents such as manure or silage. Retarding admixturesare used to slow the rate of concrete hardening. They are useful for concrete that is placed during hot weather. Accelerating admixtures such as calcium chloride, are used to increase the rate of hardening--usually during cold weather.

Air Entraining agents

Water-to-Cement RatioThe ratio of water-to-cement, or w/c, is the single most important parameter with regards to concrete quality and used to determine the water used in the mix.Theoretically, about 0.22 to 0.25 is required for complete hydrationPractically, the useful limit is around 0.33Reducing the water for a given amount of cement will move the cement particles closer together, which in turn densifies the hydrated cement pasteThis increases strength and reduces permeabilityIt also makes the concrete more difficult to workIn combination, the w/c and degree of hydration control many of the properties of the hardened concrete .

WorkabilityWorkability is the most important property of freshly mixed concrete. It is desirable that freshly mixed concrete be relatively easy to transport, place, compact and finish without harmful segregation. A concrete mix satisfying these conditions is said to be workable.There is no single test method that can simultaneously measure all the properties involved in workability. It is determined to a large extent by measuring the consistency of the mix. Strongly associated with the slump test

Properties of fresh concrete

Slump Test is related with the ease with which concrete flows during placement Slump Cone Test

10 cm 20 cm30 cmThe slump cone is filled in 3 layers. Every layer is evenly rodded 25 times.Measure the slump by determining the vertical difference between the top of the mold and the displaced original center of the top surface of the specimen.

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Factors Affecting WorkabilityMethod and duration of transportationQuantity and characteristics of cementing materialsAggregate grading, shape and surface textureQuantity and characteristics of chemical admixturesAmount of water Amount of entrained airFluidity, stability, pumpability, compactability and finishabilityThese properties are affected by:Design of mixAdjustment to the mix constituentsEnvironmental Conditions:Ambient and concrete temperatureRelative humidity and the air speedDegree of agitation and friability of aggregratesElapsed time since mixing

ConsistencyConsistency is the fluidity or degree of wetness of concrete. It is generally dependent on the shear resistance of the mass. It is a major factor in indicating the workability of freshly mixed concrete. Test methods for measuring consistency are:Flow test measures the amount of flowKelly-Ball test measures the amount of penetrationSlump test (Most widely used test!)

SegregationSegregation refers to a separation of the components of fresh concrete, resulting in a non-uniform mix.The primary causes of segregation are differences in specific gravity and size of constituents of concrete. Some of the factors affecting segregation:Larger maximum particle size (25mm) and proportion of the larger particles.Improper mixing, improper placing and improper consolidation. High specific gravity of coarse aggregate. Decrease in the amount of fine particles. Particle shape and texture. Water/cement ratio.

BleedingBleeding is the tendency of water to rise to the surface of freshly placed concrete. Undesirable effects of bleeding are:With the movement of water towards the top, the top portion becomes weak and porous (high w/c). Thus the resistance of concrete to freezing-thawing decreases.Water rising to the surface carry fine particles of cement which weaken the top portion and form laitance. This portion is not resistant to abrasion.Water may accumulate under the coarse agg. and reinforcement. These large voids under the particles may lead to weak zones and reduce the bond between paste and agg. or paste and reinforcement. It is caused by the inability of solid constituents of the mix hold all of the mixing water as they settle down.A special case of segregation.It is decreased by:Increasing the fineness of cementIncreasing the rate of hydration (C3S, C3A and alkalies)Adding pozzolansReducing water content

Mixing of concreteThe aim of mixing is to blend all of the ingredients of the concrete to form a uniform mass and to coat the surface of aggregates with cement paste. Ready-Mix concrete: In this type ingredients are introduced into a mixer truck and mixed during transportation to the site. Wet Water added before transportationDry Water added at siteMixing at the siteHand mixedMixer mixedMixing time should be sufficient to produce a uniform concrete.The time of mixing depends on the type of mixer and also to some properties of fresh concrete. Undermixing non-homogeneity Overmixing danger of water loss, brekage of aggregate particles

Ready Mix Concrete

Mixing at site

Methods of transporting concreteWheelbarrowsHoistsTrucksChutesPumpsPipelines

TrucksTrucks range from 1.8 mini trucks to 7.5 metres capacityWhen using trucks we may need to specifySize of truck i.e 6 wheeler or 8 wheelerA cubic metre concrete weighs 2.5tA 7.5 metre truck will carry 18.75t of concreteThis will need to be taken account of when driving over structuresAlways be aware that after trucks discharge concrete they actually get higherChutesA convenient method to transport concreteItems that can be used to make a chuteRoof sheetingPlywood

Pumps and pipelinesConcrete pumps come in two types, line and boom pumps Concrete pumpsWhen ordering pumps considerationA pumps footprint (a 42m boom pump may need up to 12m in width to operate properly)Generally boom pumps will take up 2 lanes of road traffic and traffic management plans will need to be put in placeBoom pumps can reach approximately 15 floors after this concrete line pumps can only be used

Placing of ConcretePlaced vertically and near its final positionIf it needs to be moved it needs to be done with shovelsDropping of concrete should be restricted to 1.8m to avoid segregationSequence should be planned to avoid cold joints on large slabs CompactingCompaction is required to achieveMaximum StrengthWatertight ConcreteFill in Sharp CornersGood bond to reinforcementGood surface appearance

Inadequate CompactionThis has a severe effect on concrete strength

Consolidating concreteInadequate consolidation can result in:HoneycombExcessive amount of entrapped air voids (bug holes)Sand streaksPlacement lines (Cold joints)

Vibration of concreteThe process of compacting concrete consists essentially of the elimination of entrapped air. This can be achieved by:Tamping or rodding the concreteUse of vibrators Internal vibrator: The poker is immersed into concrete to compact it. The poker is easily removed from point to point.External vibrators: External vibrators clamp direct to the formwork requiring strong, rigid forms.

Radius of ActionInternal Vibrator

Internal VibratorsTo aid in the removal of trapped air the vibrator head should be rapidly plunged into the mix and slowly moved up and down. The actual completion of vibration is judged by the appearance of the concrete surface which must be neither rough nor contain excess cement paste.

Adapted from ACI 309Contd..

Diameterof head,(mm)Recommended frequency, (vib./min.)Approximate radius of action, (mm)Rate of placement, (m3/h)Application20-409000-15,00080-1500.8-4Plastic and flowing concrete in thin members. Also used for lab test specimens.30-608500-12,500130-2502.3-8Plastic concrete in thin walls, columns, beams, precast piles, thin slabs, and along construction joints. 50-908000-12,000180-3604.6-15Stiff plastic concrete (less than 80-mm slump) in general construction .

External VibratorsForm vibratorsVibrating tables (Lab)Surface vibratorsVibratory screedsPlate vibratorsVibratory roller screedsVibratory hand floats or trowelsExternal vibrators are rigidly clamped to the formwork so that both the form & concrete are subjected to vibration. A considerable amount of work is needed to vibrate forms. Forms must be strong and tied enough to prevent distortion and leakage of the grout.

Vibrating Table: used for small amounts of concrete (laboratory and some precast elements)Contd..

Systematic VibrationCORRECTVertical penetration a few inches into previous lift (which should not yet be rigid) of systematic regular intervals will give adequate consolidationINCORRECTHaphazard random penetration of the vibrator at all angles and spacings without sufficient depth will not assure intimate combination of the two layers

Curing of concreteProperties of concrete can improve with age as long as conditions are favorable for the continued hydration of cement. These improvements are rapid at early ages and continues slowly for an indefinite period of time. Concrete that has been specified, batched, mixed, placed, and finished "letter-perfect" can still be a failure if improperly or inadequately cured.Curing is the procedures used for promoting the hydration of cement and consists of a control of temperature and the moisture movement from and into the concrete.Curing is usually the last step in a concrete project and, unfortunately, is often neglected even by professionals. The primary objective of curing is to keep concrete saturated or as nearly saturated as possible. Hydration reactions can take place in only saturated water filled capillaries. A seven-day (or longer) curing time is recommended.

Curing MethodsMethods which supply additional water to the surface of concrete during early hardening stages.Using wet coversSprinkling Ponding or immersionKeep water on the concrete during the curing period.

2. Methods that prevent loss of moisture from concrete by sealing the surface.Water proof plasticsUse liquid membrane-forming compoundsForms left in placeSuch methods provide some cooling through evaporation, which is beneficial in hot weather.

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3. Methods that accelerate strength gain by supplying heat & moisture to the concrete.By using live steam (steam curing)Heating coils.Contd..

Curing RequirementsCuring of a placed and finished concrete is done for maintaining a satisfactory moisture and temperature in concrete for some defined period after placing and finishing, to allow the complete hydration of the cement. Advantages of curing More strength and abrasion resistanceMore watertigtnessLess volumetric changes; more volume stability.More resistance to freezing and thawing and deicer salts.More durability Effect of curing on Strength

Hot Weather ConcreteRapid hydration early setting rapid loss of workabilityExtra problems due toLow humidityWind, excessive evaporationDirect sunlight SolutionsWindbreaksCooled Concrete IngredientsWater ponding (cooling due to evaporation)Reflective coatings/coveringsCold Weather ConcreteKeep concrete temperature above 5 C to minimize danger of freezing SolutionsHeated enclosures, insulationRely on heat of hydration for larger sectionsHeated ingredients concrete hot when placedHigh early strength cement

Uniformity of concreteConcrete uniformity is checked by conducting tests on fresh and hardened concretes.Slump, unit weight, air content testsStrength testsDue to heteregeneous nature of concrete, there will always be some variations. These variations are grouped as:Within-Batch Variations : inadequate mixing, non-homogeneous natureBatch-to-Batch Variations : type of materials used, changes in gradation of aggregates, changes in moisture content of aggregates

The principal properties of hardened concrete which are of practical importance can be listed as: 1. Strength 2. Permeability & durability 3.Shrinkage & creep deformations 4.Response to temperature variations 5.Control of cracking

out of these compressive strength is the most important property of concrete. BecauseConcrete is used for compressive loadsCompressive strength is easily obtainedIt is a good measure of all the other properties.

Properties of Hardened concrete

Strength Development and Strength MeasurementAggregates glued together by cement paste to form concreteCement hydration is a chemical reaction which requires waterStrength gain reflects degree of hydrationStrength gain depends onType of cementTemperature history temperature and timeCuringAdmixturesFactors Affecting StrengthEffect of materials and mix proportionsProduction methods Testing parameters

Strength of concreteThe strength of a concrete specimen prepared, cured and tested under specified conditions at a given age depends on:w/c ratioDegree of compaction

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Compressive strengthCompressive Strength is determined by loading properly prepared and cured cubic, cylindrical or prismatic specimens under compression.Cubic: 15x15x15 cmCubic specimens are crushed after rotating them 90 to decrease the amount of friction caused by the rough finishing.Cylinder: h/D=2 with h=15To decrease the amount of friction, capping of the rough casting surface is performed.

Tensile StrengthTensile Strength can be obtained either by direct methods or indirect methods. Direct methods suffer from a number of difficulties related to holding the specimen properly in the testing machine without introducing stress concentration and to the application of load without eccentricity.Direct Tensile strength

Due to applied compression load a fairly uniform tensile stress is induced over nearly 2/3 of the diameter of the cylinder perpendicular to the direction of load application.Split Tensile Strength

The advantage of the splitting test over the direct tensile test is the same molds are used for compressive & tensile strength determination.The test is simple to perform and gives uniform results than other tension tests.P: applied compressive loadD: diameter of specimenl: length of specimenSplitting Tensile StrengthContd..

The flexural tensile strength at failure or the modulus of rupture is determined by loading a prismatic concrete beam specimen. The results obtained are useful because concrete is subjected to flexural loads more often than it is subjected to tensile loads.Flexural Strength

Factors Affecting the Strength of Concrete1. Factors depended on the test type:Size of specimenSize of specimen in relation with size of agg.Support condition af specimenMoisture condition of specimenType of loading adoptedRate of loadingType of test machine2. Factors independent of test type:Type of cementType of agg.Degree of compactionMix proportionsType of curingType of stress situation

Permeability is important because:The penetration of some aggresive solution may result in leaching out of Ca(OH)2 which adversely affects the durability of concrete.In R/C ingress of moisture of air into concrete causes corrosion of reinforcement and results in the volume expansion of steel bars, consequently causing cracks & spalling of concrete cover.The moisture penetration depends on permeability & if concrete becomes saturated it is more liable to frost-action.In some structural members permeability itself is of importance, such as, dams, water retaining tanks. The permeability of concrete is controlled by capillary pores. The permeability depends mostly on w/c, age, degree of hydration.In general the higher the strength of cement paste, the higher is the durability & the lower is the permeability.

Permeability of concrete

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Failure modes of concrete Normal Abnormal

If acceptable materials are used, the properties of concrete, such as durability, freeze/thaw resistance, wear resistance, and strength depend on the cement mixture.A mixture with a sufficiently low ratio of water to cement plus entrained air, if specified, is the most desirable. These properties--and thus the desired concrete quality--can only be fully achieved through proper placement and finishing, followed by prompt and effective curing.A durable concrete is the one which will withstand in a satisfactory degree, the effects of service conditions to which it will be subjected.Factors Affecting Durability:External EnvironmentalInternal Permeability, Characteristics of ingredients, Air-Void System.

Durability

Structure of un-damaged ConcreteMacrostructureAggregates (CA, FA)Hydrated cement paste (hcp)Entrapped air voidsMicrostructureHydrated cement paste (Hydration products: C-S-H, ettringite, monosulfate; porosity: gel, capillary pores entrained/ entrapped air voids)Transition zone (TZ)MacrostructureMicrostructure

Structure of damaged ConcreteMacrostructureVisible cracks in hcp and aggregates due to volume changes (to understand cause of cracks, microstructure should be examined)MicrostructureAlkali-silica reaction: Reaction product forms at TZ and expandsFrost action: Water freezes in capillary pores and expandsSulfate attack: reaction products form in hcp and expand

Leaching & EfflorescenceWhen water penetrates into concrete, it dissolves the non-hydraulic CH (and various salts, sulfates and carbonates of Na, K, Ca)Remember C-S-H and CH is produced upon hydration of C3S and C2SThese salts are taken outside of concrete by water and leave a salt deposit.

Sulphate AttackGround water in clayey soils containing alkali sulfates may affect concrete.These solutions attack CH to produce gypsum. Later, gypsum and calcium alumina sulfates together with water react to form ettringite. Formation of ettringite is hardened cement paste or concrete leads to volume expansion thus cracking.Moreover, Magnesium sulfate may lead to the decomposition of the C-S-H gel. Seawater contains some amount of Na and Mg Sulfates. However, these sulfates do not cause severe deleterious expansion/cracking because both gypsum and ettringite are soluble in solutions containing the Cl ion. However, problem with seawater is the frequent wetting/drying and corrosion of reinforcing steel in concrete.To reduce the sulfate attackUse low w/c ratio reduced permeability & porosityUse proper cement reduced C3A and C3S Use pozzolans they use up some of the CH to produce C-S-H

CorrosionElectrochemical reactions in the steel rebars of a reinforced concrete structure results in corrosion products which have larger volumes than original steel.Thus this volume expansion causes cracks in reinforced concrete. In fact, steel is protected by a thin film provided by concrete against corrosion. However, that shield is broken by CO2 of air or the Cl- ions.

Freezing and ThawingWater when freezes expands in volume. This will cause internal hydraulic pressure and cracks the concrete.Important for exterior concrete. If aggregates or concrete absorbs so much water that when the water freezes and expands the concrete cannot accommodate the build up of internal pressure, popouts may occur.

To prevent the concrete from this distress air-entraining admixtures are used to produce air-entrained concrete.

Cement Concrete Mix Design means, determination of the proportion of the concrete ingredients i.e. Cement, Water, Fine Aggregate, Coarse Aggregate which would produce concrete possessing specified properties such as workability, strength and durability with maximum overall economy.Methods of Concrete Mix DesignI.S. MethodBritish MethodA.C.I. Method These Methods are based on two basic assumptions Compressive Strength of Concrete is governed by its Water-Cement Ratio Workability of Concrete is governed by its Water Content

Concrete Mix Design

Data required for concrete mix design Grade of Concrete e.g.: RCC-M30-A20Slump required in mm e.g.: 25 75 mmDegree of Site Control e.g.: GoodType of Exposure e.g.: ModerateGrade of Cement e.g.: OPC 43 GradeWorkability (Clause 7.1, IS:456-2000)I.S. Method of concrete mix design

Placing ConditionsDegree of WorkabilitySlump(mm)Blinding Concrete; Shallow Sections; Pavements using paversVery LowSee 7.1.1Mass Concrete; Lightly reinforced sections in Slabs, Beams, Walls, Columns; Floors; Hand placed Pavements; Canal lining; Strip FootingsLow25-75Heavily reinforced sections in Slabs,Beams, Walls, Columns;Slip form work; Pumped Concrete.Medium50-100Trench fill; In-Situ Piling; Tremie ConcreteHigh 100-150

*Degree of Site Control (Table 8, IS:456-2000)Approximate Quantity of Materials required for concrete mix design Cement : 200 Kg.Fine Aggregate : 240 Kg.Coarse Aggregate : 180 Kg. (20 mm) 180 Kg. (10 mm)

GoodSite control having proper storage of cement; weigh batching of all materials; Controlled addition of water, regular checking of all materials, aggregate grading and moisture content; and periodical checking of workability and strength.FairSite control having deviation from the above.

Type of Exposure (Table 3, IS:456-2000)

Sl.No.EnvironmentExposure Conditionsi)MildConcrete surfaces protected against weather or aggressive conditions, except those situated in coastal area. ii)ModerateConcrete surfaces sheltered from severe rain or freezing whilst wet.Concrete exposed to condensation and rain.Concrete continuously under water.Concrete in contact or buried under non-aggressive soil/ground water.Concrete surfaces sheltered from saturated salt air in coastal area.iii)SevereConcrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing whilst wet or severe condensation.Concrete completely immersed in sea water.Concrete exposed to coastal environment.iv)Very SevereConcrete exposed to sea water spray, corrosive fumes or severe freezing conditions whilst wet.Concrete in contact with or buried under aggressive sub-soil/ground water.v)ExtremeSurface of members in tidal zone.Members in direct contact with liquid/solid aggressive chemicals.

Step I:- Determine the physical properties of concrete ingredients.

I. Cement (OPC 43 Grade)Steps Involved in concrete mix design

Sl.No.Particulars of TestSpecifications As per IS:8112-19761Standard consistency (% by weight) 30-322Setting Time in minutesa) Initialb) Final30 Minimum600 Maximum3Compressive Strength in N/sq.mm at the age of a) 3 days b) 7 days c) 28 days23 Minimum33 Minimum43 Minimum4Specific Gravity2.5-2.75Fineness in Sq.m/Kg225 Minimum

Fine Aggregate Sieve Analysiscontd. III.20.0mm Coarse aggregate Sieve Analysis

Sieve SizeSpecifications for ZoneII (passing) As per IS:383-197010.0 mm1004.75 mm90-1002.36 mm75-1001.18 mm55-90600 micron35-59300 micron8-30150 micron0-10

Sieve SizeSpecifications As per IS:383-1970GradedSingle Sized40.00mm10010020.00mm 95-10085-10010.00mm25-550-204.75mm0-100-5

IV.Mechanical properties V.10.0mm Coarse aggregateSieve Analysiscontd.

Sl.No.Particulars of TestSpecifications As per IS: 383-19701Crushing Value in %30 Maximum For wearing surfaces45 Maximum For other concrete2Impact Value in %30 Maximum For wearing surfaces45 Maximum For other concrete3Los Angeles Abrasion Value in %30 Maximum For wearing surfaces50 Maximum For other concrete

Sieve SizeSpecifications As per IS:383-1970GradedSingle Sized12.50mm 10010.00mm 85-1004.75mm0-202.36mm0-5

Step II:-Compute Target Mean Compressive Strength:Fck =fck + t * SWhere,Fck=Target Mean Compressive Strength at 28 days in N/Sq.mm

fck=Characteristic Compressive Strength at 28 days in N/Sq.mm

S=Standard Deviation in N/Sq.mm

t =A Statistic, depending on accepted proportion of low results. = 1.65 for 1 in 20 accepted proportion of low results

contd. Assumed Standard Deviation (Table 8, IS:456-2000)

Grade of ConcreteAssumed Standard Deviation (N/Sq.mm)Good Site ControlFair Site ControlM10, M153.54.5M20, M254.05.0M30, M35M,40,M45M505.06.0

Step III:- Select the Water-Cement ratio of trial mix from experience contd. Step IV:- Select the water content per cubic meter of concrete from table 2 of I.S: 10262-2009.

S.No.Concrete GradeMinimum expected W/C1M100.92M150.73M200.554M250.505M300.456M350.407M400.358M450.30

Maximum size of Aggregate (mm)Water Content per cubic meter of concrete (Kg)102082018640165

contd. Approximate water content (Kg) per cubic meter of concrete (Table 32, SP:23-1982)Volume of Coarse Aggregate per Unit Volume of Total Aggregate (Table 3, IS:10262-2009)

Slump (mm)Maximum Size of Aggregate (mm)10204030-5020518516080-100225200175150-180240210185

Maximum Size of Aggregate (mm) Volume of Coarse Aggregate per Unit Volume of Total AggregateZone IVZone IIIZone IIZone I100.500.480.460.44200.660.640.620.60400.750.730.710.69

contd. Step V:- Compute the quantity of cement as follows. Water Cement = ------------- W/C RatioStep VI:- Then we find the quantities of Fine & Coarse aggregate by absolute volume method.V = (W+C/Sc+(1/p) * (fa/Sfa)) * (1/1000) {for fine aggregate} andV = (W+C/Sc+(1/(1-p)) * (ca/Sca)) * (1/1000) {for coarse aggregate}

Where V = Absolute volume of fresh concrete = 1 m3 W = Mass of Water (Kg) per m3 of concrete C = Mass of Cement (Kg) per m3 of concrete p = Percentage of fine aggregate. fa = Mass of fine aggregate ca = Mass of coarse aggregate Sc = Specific gravity of cement. Sfa = Specific gravity of fine aggregate. Sca = Specific gravity of coarse aggregate.

Step VII:- Make slump trials to find out the actual weight of water to get required slump. Make corrections to the water content & %FA, if required.

Step VIII:- Compute 2 more trial mixes with W/C ratios as 0.40 & 0.50, taking %FA as 34% and 38% respectively.

Step IX:- Cast atleast 3 cubes for each trial mix.

Step X:- Test the cubes for compressive strength at 28 days.

Step XI:- Draw a graph between compressive strength Vs C/W Ratio.

Step XII:- From the graph, find the W/C ratio for the required target mean compressive strength.

Step XIII:- Calculate the mix proportions corresponding to the W/C ratio, obtained from the graph.

Step XIV:- Check the cement content & W/C ratio against the limiting values given in Table-5 of I.S: 456-2000 for given type of exposure & type of Concrete.

contd.

Table-5 (IS:456-2000) Minimum Cement content Maximum Water-Cement ratio and Minimum Grade of Concrete for different exposures with normal weight of aggregate of 20mm nominal maximum size.contd.

Sl.No.

ExposurePlain ConcreteReinforced ConcreteMinimum Cement Content kg/m3Maximum Free Water Cement RatioMinimum Grade of ConcreteMinimum Cement Content kg/m3Maximum Free Water Cement RatioMinimum Grade of Concretei)Mild2200.60-3000.55M20ii)Moderate2400.60M153000.50M25iii)Severe2500.50M203200.45M30iv)Very Severe2600.45M203400.45M35v)Extreme2800.40M253600.40M40

ConclusionConcrete is a highly versatile construction material, well suited for many agricultural applications. It is a mixture of Portland cement, water, aggregates, and in some cases, admixtures. Strength, durability, and many other factors depend on the relative amounts and properties of the individual components. A perfect mix can result in poor quality concrete if correct placement, finishing, and curing techniques under the proper conditions of moisture and temperature are not used. It is a fairly simple exercise to determine the correct selection of curing system needed for a particular application. However, it is a far easier selection to choose to cure a concrete structure or not. It is not possible for any individual to know even 10% of the total available knowledge about concrete. But it is possible to learn how to function as a forward looking, receptive, discriminating and contributing member of the concrete fraternity.

**Fig. 10-5. Schematic of a ready mix plant.*Fig. 11-9. Honeycomb and rock pockets are the results of inadequate consolidation. (50207)Bottom: Sand streaks*Table 11-1. Range of Characteristics, Performance, and Applications of Internal Vibrators.Generally, extremely dry or very stiff concrete does not respond well to internal vibrators.While vibrator is operating in concrete.Distance over which concrete is fully consolidated.Assumes the insertion spacing is 112 times the radius of action, and that vibrator operates two-thirds of time concrete is being placed. These ranges reflect not only the capability of the vibrator but also differences in workability of the mix, degree of deaeration desired, and other conditions experienced in construction.*129-18*