PRACTICAL GUIDE TO CONCRETE MIX DESIGN

Different ingredients of concrete need to be mixed in appropriate proportions during the production of concrete. This can be done either by volume or by weight, the latter being more precise and scientific. It is essential that concrete mixes be “designed” for a particular set of given ingredients to produce specific properties of concrete in the most economical ways.

Rational proportioning of the ingredients of concrete, generally referred as “mix proportioning” or “mix designing” is a process by which one can arrive at the right combination of cement, aggregates, water and admixtures (if any) for producing concrete to satisfy given specifications. The purpose of mix proportioning is to obtain a product that will perform to certain predetermined requirements.

The objective of mix design is to ensure that the concrete:

1.            Complies with the compressive strength as laid down in the specifications

2.            Conforms to the specified durability requirements to resist the environment in which the structure will be serviceable during its design life

3.            has adequate workability

4.            is capable of being mixed, transported, laid down and compacted as efficiently as possible

5.            And last but not least, be as economical as possible.

To achieve an optimum mix proportion to fulfill the above parameters is a challenging task. The work of mix designing is a trial and error exercise, which need to be carried out by an experienced person in a laboratory.

The concrete mix needs to be designed to produce the grade of concrete having characteristic strength not less than the appropriate values as per IS 456:2000 Table 2 . The mix also needs to be designed for adequate workability so that it could be being mixed, transported, laid down and compacted as efficiently as possible. Depending upon the placing conditions, European Standard (DIN EN206) has recommended different range of workability and these are given in Table below. In addition, the concrete has to satisfy the durability requirements. Minimum cement content, maximum water cement ratio and minimum grade of concrete should be as specified by IS 456. It may be noted that the code has specified the minimum grade of concrete to be not less than M20 for reinforced concrete constructions.

Slump suitable for different placing conditions : 

 (Ref: DIN EN 206)

Guidelines for concrete mix proportioning

1.            Data for Mix Proportioning

The following data are required for mix proportioning of a particular grade of concrete:
a.            Grade designation
b.            Type of cement
c.             Maximum nominal size of aggregate
d.            Minimum cement content
e.            Maximum water cement ratio
f.             Workability
g.            Exposure conditions as per Tables 4 and 5 of IS456
h.            Maximum temperature of concrete at the time of placing
i.              Method of transporting and placing
j.             Early age strength requirements, if required
k.            Type of aggregates,
l.              Maximum cement content and
m.           Whether an admixture shall or shall not be used and the type of admixture and the condition of use.

2.            Target Strength for Mix proportioning

In order that not more than the specified proportion pf test results are likely to fall below the characteristics strength, the concrete mix has to be proportioned for somewhat higher target average compressive strength fck. The margin over characteristic strength is given by the following relation:

f’ck         =              fck + 1.65 * S

 where

 f’ck        = target mean compressive strength at 28 days.
fck          = characteristics compressive strength at 28 days, and S = Standard deviation

 Standard Deviation

The standard deviation for each grade of concrete shall be calculated separately.

Standard deviation based on test strength of sample

1.            Number of test results of samples: The total number of test strength of samples required to constitute an acceptable record for calculation of standard deviation shall be not less than 30. Attempts should be made to obtain the 30 samples (taken from site), as early as possible, when a mix is used for the first time.

2.            In case of significant changes in concrete: When significant changes are made in the production of concrete batches (for example changes in the materials used, mix proportioning, equipment or technical control), the standard deviation value shall be separately calculated for such batches of concrete.

3.            Standard deviation to be brought up to date: The calculation of the standard deviation shall be brought up to date after every change of mix proportioning.

Assumed Standard deviation

Where sufficient test results for a particular grade of concrete are not available, the value of standard deviation given in table 26 may be assumed for the proportioning of mix in the first instance. As soon as the results of samples are available, actual calculated standard deviation shall be used and the mix proportioned properly.

However, when adequate past records for a similar grade exist and justify to the designer a value of standard deviation different from that shown in table 26, it shall be permissible to use that value.

Assumed Standard Deviation
 (Ref: Table 1 of IS 10262:2007)

A. Selection of Mix Proportions

1.            Selection of water cement ratio

Since different cements, supplementary cementitious materials and aggregates of different maximum size, grading, surface texture, shape and other characteristics may produce concretes of different compressive strength for the same free water cement ratio, the relationship between strength and free water cement ratio should preferably be established for the materials actually to be used. In the absence of such data, the preliminary free water cement ratio (by mass) corresponding to the target strength at 28 days may be selected from the established relationship if available. Otherwise, the table 17 may be used as a starting point for selection of water cement ratio for respective environment exposure conditions.

Note: The supplementary cementitious materials that is , mineral admixtures shall also be considered in water cement ratio calculations in accordance with table 17.

The free water cement ratio selected should be checked against the limiting water cement ratio for the requirements of durability and the lower of the two values adopted

2.            Selection of Water content

The water content of concrete is influenced by a number of factors such as, aggregate size, aggregate shape, aggregate texture, workability, water cement ratio, cement and other supplementary cementitious material type and content, chemical admixture and environmental conditions. An increase in aggregates size, a reduction in water cement ratio and slump, and use of rounded aggregates and water reducing admixtures or fly ash will reduce the water demand. On the other hand increased temperature, cement content, slump, water cement ratio, aggregate angularity and a decrease in the proportion of the coarse aggregate to fine aggregate will increase water demand.

The quantity of maximum mixing water per unit volume of concrete may be determined from Table 27. The water content in table 27 is for angular coarse aggregate and for 25 mm to 50 mm slump range. The water estimate in table 27 can be reduced by approximately 10 kg for sub angular aggregates, 20kg for gravel with some crushed particles and 25 kg for rounded gravel to produce same workability. For the desired workability (other than 25 mm to 50 mm slump range) the required water content may be established by trial or an increase by about 3 percent for every additional 50 mm slump. This illustrates the need for trial batch testing of local materials as each aggregate source is different and can influence concrete properties differently. Water reducing admixtures or super plasticizing admixtures usually decrease water content by 5 to 10 percent and between 10 to 30 percent respectively at appropriate dosages.

Maximum water content per cubic metre of concrete for nominal maximum size of aggregate 

(Ref: Table 2 of IS 10262:2007)

** - Water content corresponding to saturated surface dry aggregates.

Note: These quantities of mixing water are for use in computing cementitious material contents for trial batches.

B. Calculation of Cementitious Material Content

The cement and supplementary cementitious material content per unit volume of concrete may be calculated from the free water cement ratio and the quantity of water per unit volume of concrete.

The cementitious material content so calculated shall be checked against the minimum content for the requirements of durability and greater of the two values adopted. The maximum cement content shall be in accordance with IS 456.

1.            Estimation of Coarse Aggregate Proportion

Aggregates of essentially the same nominal maximum size, type and grading will produce concrete of satisfactory workability when a given volume of coarse aggregate on rodded density basis, is used per unit volume of concrete. Approximate values for this aggregate volume are given in table 28. It can be seen that for equal workability, the volume of coarse aggregate in a unit volume of concrete is dependent only on its nominal maximum size and grading zone of fine aggregate. Differences in the amount of mortar required for workability with different aggregates, due to differences in particle shape and grading, are compensated for automatically by differences in rodded void content.

Volume of Coarse Aggregate per unit volume of concrete for Different Zones of Fine Aggregate 

(Ref: Table 3 of IS10262:2007)

** Volumes are based on aggregates in saturated surface dry condition

For more workable concrete mixes which is sometimes required when placement is by pump or when the concrete must be worked around congested reinforcing steel, it may be desirable to reduce the estimated coarse aggregate content determined using Table 28 up to 10 percent. However, caution shall be exercised to assure that the resulting slump, water cement ratio and strength properties of concrete are consistent with the recommendations of IS 456 and meet project specification requirement as applicable.

2.            Estimation of Fine Aggregate Proportion

With the completion of above procedure, all the ingredients have been estimated except the coarse and fine aggregate content. These quantities are determined by finding out the absolute volume of cementitious material, water and the chemical admixture; by dividing their mass by their respective specific gravity, multiplying by 1/1000 and subtracting the result of their summation from unit volume. The values so obtained are divided into coarse and Fine Aggregate fractions by volume in accordance with coarse aggregate proportion already determined. The coarse and fine aggregate contents are then determined by multiplying with their respective specific gravities and multiplying by 1000.

3.            Combination of Different Coarse Aggregate Fractions

The coarse aggregate used shall conform to IS 383. Coarse aggregates of different sizes shall be combined in suitable proportions so as to result in an overall grading conforming to Table 2 of IS 383 for particular nominal maximum size of aggregate.

Determination of mass per m3 yield and cement factor of freshly mixed concrete shall be carried out as per IS 1199.

C. Trial Mixes

The calculated mix proportions shall be checked by means of trial batches.

Workability of the trial mix no.1 shall be measured. The mix shall be carefully observed for freedom from segregation and bleeding and its finishing properties. If the measured workability of trial mix no.1 is different from the stipulated value, the water and /or admixture content shall be adjusted suitably. With this adjustment, the mix proportion shall be recalculated keeping the free water cement ratio at the pre selected value, which will comprise trial mix no.2. In addition two mor trial mixes no.3 and 4 shall be made with the water content same as trial mix no.2 and varying the free water cement ratio by +/- 10 percent of the pre selected value.

Mix no.2 to 4 normally provides sufficient information, including the relationship between compressive strength and water cement ratio, from which the mix proportions for field trials may be arrived at. The concrete for field trials shall be produced by methods of actual concrete production.

D. AN ILLUSTRATIVE EXAMPLE OF CONCRETE MIX PROPORTIONING

DESIGN MIX FOR M20

TEST DATA FOR RAW MATERIALS

1.            Cement used      OPC 53grade conforming IS12269

2.            Specific gravity of Cement            3.15

3.            Specific gravity of coarse aggregate          2.65

4.            Specific gravity of Fine aggregate               2.61

5.            Water absorption of coarse aggregate     0.5 percent

6.            Water absorption of Fine aggregate         1.0 percent

7.            Sieve analysis of Fine aggregate

8. Sieve analysis of Coarse aggregate

Design Mix calculation for M20 grade

Target Strength for Mix Proportioning F’ck = Fck + 1.65 * S

Where

F’ck = target average compressive strength at 28 days. Fck = Characteristics compressive strength at 28 days. S = Standard deviation

From IS 10262:2007, Table 1, standard deviation S = 4 N/mm2

Therefore, Target Strength = 20 + 1.65*4 = 26.66 N/mm2

 Selection of Water-Cement Ratio

From IS 456:2000, Table 5, Maximum water cement ratio = 0.55 Based on experience, adopt water cement ratio as 0.53

0.53 < 0.55, hence OK.

Selection of Water Content

From IS 10262:2007, Table 2, Maximum water content for 20 mm aggregate = 186 Kg (for 25 mm to 50mm slump range)

Estimated water content for 100 mm slump =186 + 3/100 * 186 =192 Kg

As super plasticizer is used, the water content can be reduced up to 30 percent.

Based on trials with super plasticizer water content reduction of 17 percent has been achieved. Hence the arrived water content = 192 * 0.83 =159.36 Kg

Calculation of Cement content

Water cement ratio = 0.53

Cement content =159/0.53 =300 Kg/m3

Check for exposure condition from IS 456, Table 5 Minimum cement content 300 Kg/m3

Hence, OK.

Proportion of volume of coarse aggregate and Fine aggregate content

From Table 3, Volume of coarse aggregate corresponding to 20 mm size aggregate and fine aggregate Zone II = 0.62

For Pumpable concrete these values should be reduced by 10 percent Therefore volume of coarse aggregate = 0.62*0.93 = 0.5766

Volume of fine aggregate content = 1 - 0.5766 = 0.4234

Mix Calculations

Volume of Concrete = 1 m3

Volume of cement           = (Weight. of cement/Sp.gravity of cement) * (1/1000)

                                       = (300/3.15) * (1/1000)

                                       = 0.0952 m3

Volume of Water              = (Weight. of water/Sp.gravity of water) * (1/1000)

                                        = (159/1) * (1/1000)

                                        = 0.159 m3

Volume of chemical = (Weight. of Admixture/Sp.gravity of Admixture) * (1/1000) Admixture      

                                         = (1.8/1.19) * (1/1000)

(@0.6% by mass of         = 0.0015m3 Cement)

Volume of all in = (1-(0.0952+0.159+0.0015)) Aggregate              = 0.7443 m3

Mass of Coarse  = 0.7443 * 0.5766 * Sp.gravity of C.A * 1000 Aggregate               

                                 = 0.7443 * 0.5766 * 2.65 * 1000

                                  = 1137.28 say 1130 Kg

Mass of Fine       = 0.7443 * 0.4234 * Sp.gravity of F.A * 1000 Aggregate

                                = 0.7443 * 0.4234 * 2.61 * 1000

                                = 822.50 say 810 Kg

MATERIAL REQUIRED FOR M20 GRADE - ONE CUM OF CONCRETE

01.         Cement    -            300 Kg

02.         Free Water - 159 Kg.

03.         River Sand - 570 Kg.

04.         Crushed Rock Fines   -     240 Kg

05.         Coarse Aggregate  - 20 mm - 660 Kg

                                                12 mm  - 470 Kg

06.         Admixture           - 1.80Kgs.

07.         Water for Absorption (CA & FA ) - 14.0 Kg

08.         Total Water - 173 Kg

Free Water Cement Ratio              - 0.53

Note:  Weight Batching correction for Surface Moisture in Aggregates Should be Carried out Regularly.