G.GUNA SRVEC

SEMBODAI RUKMANI VARATHARAJAN ENGINEERING COLLEGE

SEMBODAI 614 809DEPARTMENT OF CIVIL

ENGINEERINGCE 6002 – CONCRETE TECHNOLOGYUNIT – iii

Proportioning of concrete mix

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1. Concrete Mix Design

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Concrete Mix DesignConcrete mix design may be defines as the art of selecting suitable

ingredients of concrete and determining their relative proportions with the object of producing concrete of certain minimum strength & durability as economically as possible.Objectives :

The purpose of concrete mix design is to ensure the most optimum proportions of the constituent materials to fulfill the requirement of the structure being built. Mix design should ensure following objectives.To achieve the designed/ desired workability in the plastic stageTo achieve the desired minimum strength in the hardened stageTo achieve the desired durability in the given environment conditionsTo produce concrete as economically as possible.

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The following point must be considered while designing concrete mixesCostSpecificationWorkabilityStrength and Durability

CostThe cost of concrete is made up of

1. Material Cost2. Equipment Cost3. Labour Cost

The variation in the cost of materials arises from the fact that cement is several times costlier than aggregates. So it is natural in mix design to aim at as lean a mix as possible. Therefore, all possible steps should be taken to reduce the cement content of a concrete mixtures without sacrificing the desirable properties of concrete such as strength and durability.

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Specifications

The following point may be kept in mind while designing concrete mixes1. Minimum Compressive Strength required2. Minimum water/ cement ratio3. Maximum cement content to avoid shrinkage cracks4. Maximum aggregate / cement ratio5. Maximum density of concrete in case of gravity dams

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Basic ConsiderationsWorkability

The following points related to workability shall be kept in mind while designing concrete mixes.The consistency of concrete should no more than that necessary for placing, compacting and finishing.For concrete mixes required high consistency at the time of placing, the use of water-reducing and set-retarding admixtures should be used rather than the addition of more waterWherever possible, the cohesiveness and finishiability of concrete should be improved by increasing sand/ aggregate ratio than by increasing the proportion of the fine particles in the sand.

Strength and durabilityStrength and durability require lower w/c ratio. It is usually

achieved not by increasing the cement content, but by lowering the water at given cement content. Water demand can by lowered by throughout control of the aggregate grading and by using water reducing admixtures.

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Group Grade designation

Characteristics compressive strength of 150 mm cube at 28 days,

N/mm2Ordinary Concrete

M10M15M20

101520

Standard Concrete

M25M30M35M40M45M50M55

25303540455055

High Strength Concrete

M60M65M70M75M80

6065707580

Grade of Concrete

What is M 20 ?

M refers to Mix

20 refers to characteristic compressive strength of 150 mm cube at 28 days in N/mm2

The minimum Grade of Plain Concrete (PCC) shall be 15 N/mm2

The minimum grade of reinforced Concrete ( RCC) shall be 20 N/mm2

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Strength and Durability

Nominal Mix ConcreteThe wide use of concrete as construction materials has led to the use of mixes of fixed proportion,

which ensures adequate strength. These mixes are called nominal mixes.They offer simplicity and Under normal circumstances, has margin of strength above that specified. Nominal mix concrete may be used for concrete of grades M5, M 7.5, M10, M15 and M20.

Proportions of Ingredients in Nominal Mixes

Grade ProportionsC: FA: CA

M5 1: 5:10M 7.5 1:4:8M 10 1:3:6M 15 1:2:4M 20 1:1.5:3

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Strength and DurabilityDesign Mix Concrete

The concrete mix produced under quality control keeping in view the strength, durability, and workability is called the design Mix. Others factors like compaction equipment's available, curing method adopted, type of cement, quality of fine and coarse aggregate etc. have to be kept in mind before arriving at the mix proportion.The design mix or controlled mix is being used more and more in variety of important structures, because of better strength, reduced variability, leaner mixed with consequent economy, as well as greater assurance of the resultant quality.Factors Influencing Choice of Mix Design

According to IS 456:2000 and IS 1343:1980 the important influencing the design of concrete mix areGrade of ConcreteType of CementMaximum nominal Size of AggregateGrading of Combined aggregate Quality Control.Maximum Water/ Cement RatioWorkability Durability

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• The proportions of materials for nominal mix shall be in accordance

Grade of Concrete•The grade of concrete gives characteristic compressive strength of concrete. It is one of the important factor influencing the mix design•The grade M 20 denotes characteristic compressive strength fck of 20 N/mm2. Depending upon the degree of control available at site, the concrete mix is to be designed for a target mean compressive strength (fck) applying suitable standard deviation.

Type of Cement•The rate of development of strength of concrete is influenced by the type of cement.•The higher the strength of cement used in concrete, lesser will be the cement content. The use of 43 grade and 53 grade of cement, gives saving in cement consumption as much as 15 % and 25 % respectively, as compared to 33 grade of cement. For concrete of grade M25 it is advisable to use 43 and 53 grade of cement.

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Maximum Nominal Size of Aggregates•The maximum size of C.A is determined by sieve analysis. It is designated by the sieve size higher than larger size on which 15 % or more of the aggregate is retained. The maximum nominal size of C.A. should not be more than one-forth of minimum thickness of the member. •For heavily reinforced concrete members as in the case of ribs of main beams, the nominal maximum size of the aggregate should usually be restricted to sum less than the minimum clear distance between the main bars or 5 mm less the minimum cover to the reinforcement, whoever is smaller.•The workability of concrete increases with an increase in the maximum size of aggregate. But the smaller size of aggregates provide larger surface area for bonding with the mortar matrix which gives higher strength.

Maximum Water/ Cement RatioAbram’s water/Cement ratio states that for any given condition of test, the strength

of a workability concrete mix is dependent only on water/cement ratio. The lower the water/Cement ratio, the greater is the compressive strengthWorkability

Workability of fresh concrete determines the case with which a concrete mixture can be mixed, transported, placed, compacted and finished without harmful segregation and bleeding.

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Factors Influencing Choice of Mix Design

Durability•Durability require low water/Cement ratio. It is usually achieved not by increasing the cement content, but by lowering the water demand at a given cement content. •Water demand can be lowered by through control of the aggregate grading and by using water reducing admixtures

Method of Concrete Mix DesignSome of the commonly used mix design methods areI.S. MethodA.C.I methodRoad Note 4 method ( U.K. Method)IRC 44 methodArbitrary methodMaximum Density methodFineness modulus methodSurface area MethodNix design for high strength ConcreteMix design for pumpable ConcreteDOE (British) Mix design method

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Factors Influencing Choice of Mix Design

IS Method of Mix DesignThe Bureau of Indian Standards, recommended a set of procedure for design of concrete mix. The procedure is based on the research work carried out at national laboratories. Data for mix designThe following basic data are required to be specified for design a concrete mixCharacteristic Compressive strength only a few specified proportions of test results are expected to fall of concrete at 28 days (fck)Degree of workability desiredLimitation on water/Cement Ratio with the minimum cement to ensure adequate durabilityType and maximum size of aggregate to be used.Standard deviations of compressive strength of concrete.Target Strength for Mix DesignThe target average compressive strength (fck) of concrete at 28 days is given byFck= f ck + t.sWhere,Fck= target average compressive strength at 28 daysF ck= characteristics compressive strength at 28 dayss= Standard deviationt= a stastical value, depending upon the accepted proportion of low results and the number of tests.

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Factors Influencing Choice of Mix Design

The ACI Method of Mix Design

This method assumes that the optimum ratio of the bulk volume of coarse aggregates and on the grading of fineness aggregates regardless of shape of particles. This method also assumes that even after complete compaction is done, a definite percentage of air remains which is inversely proportional to the maximum size of aggregate.

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