CONCRETE MIX DESIGN AS PER THE GUIDELINES OF IS 10262.

Vinod Kumar SinghCo-founder, www.Happho.com

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INTRODUCTION TO CONCRETE MIX DESIGN

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CONCRETEBasic Definition:Concrete is a composite material that essentially consists of water, a binding

medium embedded with Fine Aggregate (typically sand) and Coarse Aggregate (typically gravel) with or without chemical and mineral admixture and filler.

Constituents:

•mixture of aggregate and paste→paste 30 to 40%

portland cement 7% to 15% by Vol. water 14% to 21% by Vol.

→Aggregates 60% to 70% coarse aggregates Fine aggregates

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Typical Concrete Proportion by Volume:

CONCRETE

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CONCRETE MIX DESIGNThe selection of concrete proportion involves a balance between economy and

requirement of workability, consistency, density, strength, and durability, for the particular application.

• Workability: The property of the concrete that determines its capacity to be placed and consolidated properly and be finished without harmful segregation.

• Consistency: It is the relative mobility of the concrete mixture, and measured in terms of the slump; the greater the slump value the more mobile the mixture.

• Strength: The capacity of the concrete to resist compression at the age of 28 days.

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CONCRETE MIX DESIGN• Water-cement (w/c) : Defined as the ratio of weight of water to the weight of

cement. This ratio is used in mix design and considerably controls concrete strength.

• Durability: Concrete must be able to endure severe weather conditions such as freezing and thawing, wetting and drying, heating and cooling, chemicals, deicing agents, and the like. An increase of concrete durability will enhance concrete resistance to severe weather conditions.

• Density: For certain applications concrete may be used primarily for its weight characteristics. Examples are counterweights, weights for sinking pipelines under water, shielding from radiation, and insulation from sound.

• Generation of heat: If the temperature rise of the concrete mass is not held to a minimum and the heat is allowed to dissipate at a reasonable rate, or if the concrete is subjected to severe differential or thermal gradient, cracking is likely to occur."

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CONCRETE MIX DESIGNBACKGROUND DATA:

The following information for available materials will be useful in designing a CMDfor the intended application:

• Sieve analyses of fine and coarse aggregates.

• Specific gravities and absorption of aggregates.

• Mixing-water requirements of concrete developed from experience with available aggregates.

• Relationship between strength and water-cement ratio.

• Specific gravity of Portland cement and other cementitious materials, if used.

• Optimum combination of coarse aggregates to meet the maximum density grading for mass concrete.

Other data which can be useful are silt content (Fine Aggregate), Flakiness andElongation Index (Coarse Aggregate)

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BASIC COMPONENTS IN CONCRETE

CEMENT Coarse Aggregate NATURAL SAND

WATER 8

BASIC FUNCTION OF PORTLAND CEMENT• Dry powder of very fine particles

• Forms a paste when mixed with water

• Chemical reaction-Hydration

• Glues other materials

• Paste coats all the aggregates together

• Hardens and forms a solid mass

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• Needed for two purposes:• chemical reaction with cement• Workability

• Only 1/3 of the water is needed for chemical reaction

• Extra water remains in pores and holes

• Results in porosity

• Good for preventing plastic shrinkage cracking and workability

• Bad for permeability, strength, durability.

BASIC FUNCTION OF WATER

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FUNCTIONS AND IMPORTANCE OF AGGREGATE IN CONCRETE APPLICATIONS.

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CONCRETE MIX DESIGN AS PER IS 10262 GUIDELINES

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AGGREGATE• Aggregate is relatively inexpensive and does not enter into complex chemical

reactions with water; it has been customary, therefore, to treat it as an inert filler in concrete.

• However, due to increasing awareness of the role played by aggregates in determining many important properties of concrete, the traditional view of the aggregate as an inert filler is being seriously questioned.

• It is true that aggregate strength is usually not a factor in normal concrete strength because, with the exception of lightweight aggregates, the aggregate particle is several times stronger than the matrix and the interfacial transition zone in concrete. In other words, with most natural aggregates the strength of the aggregate is hardly utilized because the failure is determined by the other two phases.

• There are, however, aggregate characteristics other than strength, such as the size, shape, surface texture, grading (particle size distribution), and mineralogy which are known to affect concrete strength in varying degrees.

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BASIC FUNCTIONS OF AGGREGATE • Cheap fillers

• Hard material

• Provide for volume stability

• Reduce volume changes

• Provide abrasion resistance

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NOMENCLATURE & CLASSIFICATIONAggregates are generally classified according to particle size, bulk density, or source sieve).

• Coarse aggregate is used to describe particles larger than 4.75 mm, and the term fine aggregate is used for particles smaller than 4.75mm; typically, fine aggregates contain particles in the size range 75 µm to 4.75 mm, and coarse aggregates from 4.75 to about 50 mm, except for mass concrete which may contain particles up to 150 mm.

• Most natural mineral aggregates, such as sand and gravel, have a bulk density of 1520 to 1680 kg/m3and produce normal-weight concrete with approximately 2400 kg/m3 unit weight.

For special needs, aggregates with lighter or heavier density can be used to make correspondingly lightweight and heavyweight concretes. Generally, the aggregates with bulk densities less than 1120kg/m3 are called lightweight and those weighing more than 2080 kg/m3 are called heavyweight

For the most part, concrete aggregates are comprised of sand, gravel, and crushed rock derived from natural sources and, therefore, are referred to as natural mineral aggregates ( are further classified as granite, limestone, basalt etc. depending upon their parent rock source)

On the other hand, thermally processed materials such as expanded clay and shale, which are used for making lightweight concrete, are called synthetic aggregates. Aggregates made from industrial by products, for instance, blast-furnace slag and fly ash, also belong to this category. 15

AGGREGATE CHARACTERISTICS AND THEIR SIGNIFICANCE

Generally, aggregate properties affect not only the concrete mixture proportions but also the behavior of fresh and hardened concrete. Due to considerable overlap between the two, it is more appropriate to divide the study of aggregate properties into three categories that are based on microstructural and processing factors.

•Characteristics dependent on porosity: density, moisture absorption, strength, hardness, elastic modulus, and soundness

• Characteristics dependent on prior exposure and processing factors: particle size, shape, and texture

• Characteristics dependent on chemical and mineralogical composition: strength, hardness, elastic modulus, and deleterious substances present

•A knowledge of certain aggregate characteristics (i.e., density, grading, and moisture state) is required for proportioning concrete mixtures. Porosity or density, grading, shape, and surface texture determine the properties of plastic concrete mixtures.

•The mineralogical composition of aggregate affects its crushing strength, hardness, elastic modulus, and soundness which, in turn, influence various properties of hardened concrete containing the aggregate.

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AGGREGATE PROPERTIES• Absorption capacity is defined as the total amount of moisture required to

bring an aggregate from the oven-dry to the SSD condition.

• SSD condition :When all the permeable pores are full and there is no water film on the surface, the aggregate is said to be in the saturated-surface dry condition (SSD).

• Surface Moisture: The amount of water in excess of the water required for the SSD condition is referred to as the surface moisture.

• Specific Gravity :defined as the density of the material including the internal pores.

• Bulk Density :defined as the mass of the aggregate fragments that would fill a unit volume.

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AGGREGATE PROPERTIESSoundness:An aggregate is considered unsound when the volume changes in aggregate

induced by weather (e.g., alternate cycles of wetting and drying, or freezing and thawing) ,result in the deterioration of concrete.

IS limit:• Fine Aggregate = 10% (weight loss of five cycles with Na2SO4)• Fine Aggregate = 15% (weight loss of five cycles with MgSO4)• Coarse Aggregate = 12% (weight loss of five cycles with Na2SO4)• Coarse Aggregate = 18% (weight loss of five cycles with MgSO4)

Shape:• Flakiness Index :Thickness being 0.6 times their mean dimension, contributes

more surface area for a unit volume occupied.

• Elongation Index :Greatest dimension being 1.8 times their mean dimension, contributes more surface area for a unit volume occupied.

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AGGREGATE PROPERTIESShape:• Flakiness Index :Thickness being 0.6 times their mean dimension, contributes

more surface area for a unit volume occupied.

Flakiness Index Apparatus Flaky Aggregate

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AGGREGATE PROPERTIESShape:• Elongation Index :Greatest dimension being 1.8 times their mean dimension,

contributes more surface area for a unit volume occupied.

Elongation Index Apparatus Elongated Aggregate

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AGGREGATE PROPERTIESIdeal Aggregate:

Neither Elongated Nor Flaky

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AGGREGATE PROPERTIES• Mechanical Properties: Crushing strength, impact value abrasion resistance, and elastic modulus of aggregate

are interrelated properties, that are greatly influenced by porosity. Aggregates from natural sources that are commonly used for making normal-weight concrete, are generally dense and strong; therefore they are seldom a limiting factor to strength and elastic properties of concrete.

Indian Standard (IS) limit:• Crushing and Impact Value :

Wearing surface = 30% & Non-wearing surface = 45%.• Abrasion Resistance :

Wearing surface = 30% & Non-wearing surface = 50%.

• Fineness Modulus: Empirical factor called the fineness modulus is often used as an index of the fineness of aggregate.

The fineness modulus is computed from screen analysis data by adding the cumulative percentages of aggregate retained on each of a specified series of sieves, and dividing the sum by 100. The sieves used for determining the fineness modulus are: No. 100 (150 µm), No. 50 (300 µm), No. 30 (600 µm), No. 16 (1.18 mm), No. 8 (2.36 mm), No. 4 (4.75 mm), 10 mm,20mm,40mm etc.

• Slit Content :Material finer than 75-µm (No. 200) sieve are generally called slit. They affect the workability as water demand increases, strength is also influenced along with bonding. IS limit is 3% by weight. 22

AGGREGATE PROPERTIESSize and Grading :• Grading is the distribution of particles of a granular material among various size ranges, usually

expressed in terms of cumulative percentage larger or smaller than each of a series of sizes of sieve openings, or the percentage between certain range of sieve openings.

• Size: The maximum size of aggregate is conventionally designated by the sieve size on which 15 percent or more particles are retained. In general, the larger the maximum aggregate size, the smaller will be the surface area per unit volume which has to be covered by the cement paste of a given water-cement ratio.

Since the price of cement may be 10 to 15 times as much as the price of aggregate, any action that saves cement without reducing the strength and workability of concrete can result insignificant economic benefit

• There are several reasons for specifying grading limits and maximum aggregate size, the most important being their influence on workability and cost.

For example, very coarse sands produce harsh and unworkable concrete mixtures, and very fine sands increase the water requirement (therefore, the cement requirement for a given water-cement ratio) and are uneconomical.

• Aggregates that do not have a large deficiency or excess of any particular size produce the most workable and economical concrete mixtures.

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AGGREGATE PROPERTIES

IS Sieve Size (mm)

WeightRetained (gms)

Cum.WeightRetained

(gms)

%Retaine

d

%Passin

g

10 0 0 0 1004.75 120 120 4 962.36 450 570 19 811.18 390 960 32 68

0.600 870 1830 61 390.300 750 2580 86 140.150 360 2940 98 2Pan 60 3000 - -

Fineness Modulus = Col.04/100 = 300/100 = 3As per our experience Fine Aggregate with F.M of 2.7 to 3.0 are best suited concrete application

Fineness Modulus (F.M) solved example :

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IMPORTANCE OF AGGREGATE• Aggregate primarily acts as a inert filler, but has secondary influences on various concrete

properties.

• Awareness about the role played by aggregate in concrete can be instrumental in exploiting the use of the same in achieving concrete properties as per intended requirements, which would be of high performance and economical.

• It is inappropriate to treat the aggregate with any less respect than cement.

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DESIGN STIPULATIONS• Characteristic Compressive Strength (basic mix design criteria, required to ascertain Target mean strength)

• Maximum size of Aggregate(Governs water and cement content)

• Degree of workability( basic placement requirement, governs water content)

• Degree of quality control(Assumption of standard deviation, depending upon site quality control)

• Type of Exposure (To fix minimum cement content ,maximum water - cement ratio and minimum

grade of concrete)

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TEST DATA FOR MATERIALSINGREDIENTS TO BE PHYSICALLY CHARACTERIZED

• CEMENT

ꟷType & Grade ( w/c ratio for target mean strength)

ꟷSpecific Gravity (calculation of various ingredients)

• AGGREGATE

ꟷSpecific gravity (calculation of various ingredients)

ꟷWater absorption (Site adjustments)

ꟷFree surface moisture (Site adjustments)

ꟷSieve Analysis (proportioning of fine & coarse aggregate)

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STEP-01 :TARGET MEAN STRENGTH• In order that not more than the specified proportion of test results are likely to

fall below the characteristic strength.

f t.m.s = f ck + t X s

f t.m.s = target mean strength at 28 days.f ck = characteristic compressive strength at 28 days.

t = a statistic, depending upon the accepted proportion of low results = 1.65s = standard deviation (as per table 01)

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STEP-01 :TARGET MEAN STRENGTH• Assumed standard deviation as per IS 456:2000

Table – 01

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STEP-02 : SELECTION OF WATER CEMENT RATIO• Different cements and aggregates of different maximum size ,grading surface texture, shape and other characteristic may produce concretes of different compressive strength for the same water cement ratio

•The relationship between strength and water cement ratio should be preferably established for the materials actually to be used.

•In the absence of such data, the preliminary water- cement ratio corresponding to the target strength at 28 days may be selected from the relation shown in fig 01,alternatively by fig 02.

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STEP-02 : SELECTION OF WATER CEMENT RATIO

Fig 01 Fig 02

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STEP – 03 ESTIMATION OF AIR CONTENT•Approximate amount of entrapped air to be expected in normal ( non-air entrained) concrete given in Table 02.

Table 02

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STEP -04 : ESTIMATION OF WATER CONTENT AND FINE TO TOTAL AGGREGATE RATIO

•Approximate sand and water content per cubic meter of concrete for grades upto M35

Table 03

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STEP -04 : ESTIMATION OF WATER CONTENT AND FINE TO TOTAL AGGREGATE RATIO

•Approximate sand and water content per cubic meter of concrete for grades above M 35.

Table 04

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ADJUSTMENTS OF VALUES IN WATER CONTENT AND SAND PERCENTAGE FOR OTHER CONDITIONS

Table 05

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STEP – 05: CALCULATION OF CEMENT CONTENT•Water – cement ratio arrived at step-02•Water content arrived at step – 04

Cement Content = Water content / water-cement ratioArrived cement content and water - cement needs to be checked with Table-06

Table – 06 (as per IS 456:2000)

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ENVIRONMENTAL EXPOSURE CONDITIONS.Table – 07 (defined as per IS 456:2000):

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STEP -06 : CALCULATION OF AGGREGATE CONTENT

•The total aggregate content per unit volume of concrete may be calculated from the following equations

V = [ W + C/SC + 1/p * fa /Sfa] x 1/1000&

V = [ W + C/SC + 1/1-p * ca /Sca] x 1/1000WhereV = absolute volume of fresh concrete, which is equal to gross volume (m3) minus the volume of assumed entrapped air.W = mass of water (kg) per m3 of concrete [arrived in step -04]C = mass of cement (kg) per m3 of concrete [arrived in step -05]p = ratio of fine aggregate to total aggregate by absolute volume [arrived in step -06]fa, ca =total mass of fine aggregate and coarse aggregate (kg) per m3 of concrete respectively.Sfa, Sca = specific gravities of saturated surface dry fine aggregate and coarse aggregate respectively. [test data of materials]

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STEP -07 COMBINATION OF DIFFERENT AGGREGATE FRACTIONS

•Coarse aggregate of different sizes should be combined in suitable proportions as to result in an overall grading confirming to IS 383 – 1970 for the particular nominal maximum size of aggregate.

•Combined gradation criteria for 20mm graded maximum size of aggregate.IS Sieve Sizes (mm) Combined % passing (as per IS:383)

2010

4.752.36

95 -10025-550-10

-

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MIX PROPORTION

•Arrived initial trial mix proportion :

Water Cement Fine Aggregate Coarse AggregateStep -04 Step -05 Step -06 Step -06

For Example:

Water Cement Fine Aggregate Coarse Aggregate

191.6 416.5 597 1179

0.46 1 1.43 2.83

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GUIDELINES FOR SUBSEQUENT MIX TRIALS• Subsequent trial mixes may be undertaken if the results are not meeting desired fresh and hardened concrete properties.

Observation/Results Guidelines1.Undersanded2.Non Cohesive

3.Lacks workability

4.T.M.S not achieved

Increase sand content in multiples of 5%.Vary proportions of aggregates amongst themselves

Increase water content (maintaining w/c ratio i.e increasing cement content)Reduce w/c ratio in multiples of 5% (maintaining total water content i.e increasing cement content)

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GOOD CONCRETE = GOOD MIX DESIGN + GOOD QUALITY CONTROL

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