Concrete Mix Design As Per Indian Standard Code

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Concrete Mix Design

Introduction

The process of selecting suitable ingredients of concrete and determining their relative amounts with the objective of producing a concrete of the required, strength, durability, and workability as economically as possible, is termed the concrete mix design. The proportioning of ingredient of concrete is governed by the required performance of concrete in 2 states, namely the plastic and the hardened states. If the plastic concrete is not workable, it cannot be properly placed and compacted. The property of workability, therefore, becomes of vital importance.

The compressive strength of hardened concrete which is generally considered to be an index of its other properties, depends upon many factors, e.g. quality and quantity of cement, water and aggregates; batching and mixing; placing, compaction and curing. The cost of concrete is made up of the cost of materials, plant and labour. The variations in the cost of materials arise from the fact that the cement is several times costly than the aggregate, thus the aim is to produce as lean a mix as possible. From technical point of view the rich mixes may lead to high shrinkage and cracking in the structural concrete, and to evolution of high heat of hydration in mass concrete which may cause cracking.

The actual cost of concrete is related to the cost of materials required for producing a minimum mean strength called characteristic strength that is specified by the designer of the structure. This depends on the quality control measures, but there is no doubt that the quality control adds to the cost of concrete. The extent of quality control is often an economic compromise, and depends on the size and type of job. The cost of labour depends on the workability of mix, e.g., a concrete mix of inadequate workability may result in a high cost of labour to obtain a degree of compaction with available equipment.

Requirements of concrete mix design

The requirements which form the basis of selection and proportioning of mix ingredients are :

a ) The minimum compressive strength required from structural consideration

b) The adequate workability necessary for full compaction with the compacting equipment available.

c) Maximum water-cement ratio and/or maximum cement content to give adequate durability for the particular site conditions

d) Maximum cement content to avoid shrinkage cracking due to temperature cycle in mass concrete.

Types of Mixes

1. Nominal Mixes

In the past the specifications for concrete prescribed the proportions of cement, fine and coarse aggregates. These mixes of fixed cement-aggregate ratio which ensures adequate strength are termed nominal mixes. These offer simplicity and under normal circumstances, have a margin of strength above that specified. However, due to the variability of mix ingredients the nominal concrete for a given workability varies widely in strength.

2. Standard mixes

The nominal mixes of fixed cement-aggregate ratio (by volume) vary widely in strength and may result in under- or over-rich mixes. For this reason, the minimum compressive strength has been included in many specifications. These mixes are termed standard mixes.

IS 456-2000 has designated the concrete mixes into a number of grades as M10, M15, M20, M25, M30, M35 and M40. In this designation the letter M refers to the mix and the number to the specified 28 day cube strength of mix in N/mm2. The mixes of grades M10, M15, M20 and M25 correspond approximately to the mix proportions (1:3:6), (1:2:4), (1:1.5:3) and (1:1:2) respectively.

3. Designed Mixes

In these mixes the performance of the concrete is specified by the designer but the mix proportions are determined by the producer of concrete, except that the minimum cement content can be laid down. This is most rational approach to the selection of mix proportions with specific materials in mind possessing more or less unique characteristics. The approach results in the production of concrete with the appropriate properties most economically. However, the designed mix does not serve as a guide since this does not guarantee the correct mix proportions for the prescribed performance.

For the concrete with undemanding performance nominal or standard mixes (prescribed in the codes by quantities of dry ingredients per cubic meter and by slump) may be used only for very small jobs, when the 28-day strength of concrete does not exceed 30 N/mm2. No control testing is necessary reliance being placed on the masses of the ingredients.

Factors affecting the choice of mix proportions

The various factors affecting the mix design are:

1. Compressive strength

It is one of the most important properties of concrete and influences many other describable properties of the hardened concrete. The mean compressive strength required at a specific age, usually 28 days, determines the nominal water-cement ratio of the mix. The other factor affecting the strength of concrete at a given age and cured at a prescribed temperature is the degree of compaction. According to Abraham’s law the strength of fully compacted concrete is inversely proportional to the water-cement ratio.

2. Workability

The degree of workability required depends on three factors. These are the size of the section to be concreted, the amount of reinforcement, and the method of compaction to be used. For the narrow and complicated section with numerous corners or inaccessible parts, the concrete must have a high workability so that full compaction can be achieved with a reasonable amount of effort. This also applies to the embedded steel sections. The desired workability depends on the compacting equipment available at the site.

3. Durability

The durability of concrete is its resistance to the aggressive environmental conditions. High strength concrete is generally more durable than low strength concrete. In the situations when the high strength is not necessary but the conditions of exposure are such that high durability is vital, the durability requirement will determine the water-cement ratio to be used.

4. Maximum nominal size of aggregate

In general, larger the maximum size of aggregate, smaller is the cement requirement for a particular water-cement ratio, because the workability of concrete increases with increase in maximum size of the aggregate. However, the compressive strength tends to increase with the decrease in size of aggregate.

IS 456:2000 and IS 1343:1980 recommend that the nominal size of the aggregate should be as large as possible.

5. Grading and type of aggregate

The grading of aggregate influences the mix proportions for a specified workability and water-cement ratio. Coarser the grading leaner will be mix which can be used. Very lean mix is not desirable since it does not contain enough finer material to make the concrete cohesive.

The type of aggregate influences strongly the aggregate-cement ratio for the desired workability and stipulated water cement ratio. An important feature of a satisfactory aggregate is the uniformity of the grading which can be achieved by mixing different size fractions.

6. Quality Control

The degree of control can be estimated statistically by the variations in test results. The variation in strength results from the variations in the properties of the mix ingredients and lack of control of accuracy in batching, mixing, placing, curing and testing. The lower the difference between the mean and minimum strengths of the mix lower will be the cement-content required. The factor controlling this difference is termed as quality control.

Mix Proportion designations

The common method of expressing the proportions of ingredients of a concrete mix is in the terms of parts or ratios of cement, fine and coarse aggregates. For e.g., a concrete mix of proportions 1:2:4 means that cement, fine and coarse aggregate are in the ratio 1:2:4 or the mix contains one part of cement, two parts of fine aggregate and four parts of coarse aggregate. The proportions are either by volume or by mass. The water-cement ratio is usually expressed in mass

Factors to be considered for mix design

ð The grade designation giving the characteristic strength requirement of concrete.

ð The type of cement influences the rate of development of compressive strength of concrete.

ð Maximum nominal size of aggregates to be used in concrete may be as large as possible within the limits prescribed by IS 456:2000.

ð The cement content is to be limited from shrinkage, cracking and creep.

ð The workability of concrete for satisfactory placing and compaction is related to the size and shape of section, quantity and spacing of reinforcement and technique used for transportation, placing and compaction.

Procedure

1. Determine the mean target strength ft from the specified characteristic compressive strength at 28-day fck and the level of quality control.

ft = fck + 1.65 S

where S is the standard deviation obtained from the Table of approximate contents given after the design mix.

2. Obtain the water cement ratio for the desired mean target using the emperical relationship between compressive strength and water cement ratio so chosen is checked against the limiting water cement ratio. The water cement ratio so chosen is checked against the limiting water cement ratio for the requirements of durability given in table and adopts the lower of the two values.

3. Estimate the amount of entrapped air for maximum nominal size of the aggregate from the table.

4. Select the water content, for the required workability and maximum size of aggregates (for aggregates in saturated surface dry condition) from table.

5. Determine the percentage of fine aggregate in total aggregate by absolute volume from table for the concrete using crushed coarse aggregate.

6. Adjust the values of water content and percentage of sand as provided in the table for any difference in workability, water cement ratio, grading of fine aggregate and for rounded aggregate the values are given in table.

7. Calculate the cement content form the water-cement ratio and the final water content as arrived after adjustment. Check the cement against the minimum cement content from the requirements of the durability, and greater of the two values is adopted.

8. From the quantities of water and cement per unit volume of concrete and the percentage of sand already determined in steps 6 and 7 above, calculate the content of coarse and fine aggregates per unit volume of concrete from the following relations:

formula-for-concrete-mix-design

where V = absolute volume of concrete

= gross volume (1m3) minus the volume of entrapped air

Sc = specific gravity of cement

W = Mass of water per cubic metre of concrete, kg

C = mass of cement per cubic metre of concrete, kg

p = ratio of fine aggregate to total aggregate by absolute volume

fa, Ca = total masses of fine and coarse aggregates, per cubic metre of concrete, respectively, kg, and

Sfa, Sca = specific gravities of saturated surface dry fine and coarse aggregates, respectively

9. Determine the concrete mix proportions for the first trial mix.

10. Prepare the concrete using the calculated proportions and cast three cubes of 150 mm size and test them wet after 28-days moist curing and check for the strength.

11. Prepare trial mixes with suitable adjustments till the final mix proportions are arrived at.

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Comments
  • bharat patel July 17, 2009 at 4:32 pm

    What is accetance criteria for 28 days compressive strength of M15,M20 and M25 grade mix design concrete ?

  • abdul hannan July 24, 2009 at 5:52 am

    I want to know about economical concret mix design?

  • ashokkumar.N July 24, 2009 at 6:18 am

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    i give a sugission to this site shows vedios and explane with images is very helpful to us

  • Mahatam Yadav July 30, 2009 at 10:38 am

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  • subhash August 1, 2009 at 2:37 pm

    definitely a powering link.i got a lot 2 know from the information provided here.
    will be better if u provide some information about the geotch. field and survey works.

  • Harikrishnan August 2, 2009 at 9:46 am

    Dear all,

    Please send the DOE methods of concrete mix design(British method)

  • parixit lathia August 5, 2009 at 5:34 am

    please send me highest grade of concrete which used in any building of the world

  • Santosh Shukla August 6, 2009 at 7:44 am

    How to calculate cement,sand,M1&2,fly ash,water,plasticer any type ofmix

    Pl.tell me short procedure.Any types of mix concrete.

    Thanks

  • jeevan August 6, 2009 at 7:57 am

    SIR,Please send me feasibility properties of M20 with(20mm aggregate)and M40 with (10mm aggregate) mix of concrete in the small
    resedential building and also cost analysis.And also gave some references regarding that one.

  • david August 9, 2009 at 8:04 am

    sir, can u show me how to design the 1 2 3 mix concrete

  • Anil kumar August 9, 2009 at 6:31 pm

    Sir Pls show me the calculation of mix concrete of ratio 1:3;6 And tell me how many bags of cement reuired in one cubic meter.

  • Navneet bhalerao August 12, 2009 at 5:26 am

    Sir plz tell me how to design high performance concrete of grade M70 and above…

  • s.venugopal August 13, 2009 at 6:12 am

    whycubes are tested for 7,14,28& 56 days,compressive strenght rather than for other days if any resonr

  • utkarsh mewada August 14, 2009 at 6:01 am

    Sir,
    Pls. show me the calcuation of mix concrete of 1:2:4 and tell me how meny bag
    of send and gread and cement of 1 cubic meter.

    Thank’s

    UTKARSH MEWADA

  • rajinder.kumar August 16, 2009 at 7:27 am

    sir pls tell me the calculations of m 20 grade mix design

  • Hari Paudel August 22, 2009 at 5:04 pm

    In the contest of country like Nepal which is the best method of concreting?

  • DINIL August 23, 2009 at 4:29 am

    sir, plz tell me
    1, Qty of jelly sand for 1 bag cement for m20 m25 grade concrete.
    2, how to calculate Qty of cement,sand,jelly for both m20, m25

  • yonas belihu August 24, 2009 at 10:46 am

    Sir,
    Please show me the calculation to produce M30 concrete for the following mix ingridients
    Coarse Aggregate and fine Aggregate
    Size 10mm -unit weight 1263kg/m3
    20mm -unit weight 1396kg/m3
    40mm -unit weight 1452kg/m3 and
    sand -unit weigt 1484kg/m3
    the M30 concrete is made with the above constituent materials and max
    imum cement content of 420kg/m3.

  • sundareswara ananth August 25, 2009 at 2:35 pm

    sir pls tell me M20(1:1.5:3) how to make calculate fine aggregate ,coarse aggregate ,cement and water ratio (per M^3)

  • Mohanakrishnan August 26, 2009 at 12:04 pm

    Sir I am a civil engineering student i need of Concrete mix design using quarry dust as fine aggregate(Super plasticizer admixtures) for M50 grade concrete.It will help for my project work.

  • Post a comment

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