CONCRETE mix DESIGN



CONCRETE MIX DESIGN 
IS: 10262 

1. Introduction

The ingredients of a concrete mix are expressed in terms of parts or ratios of cement, fine and coarse aggregates. For example, a 1:2:4 concrete means that it contains 1 part cement, 2 parts fine aggregates and 4 parts coarse aggregates by volume or mass. The water cement ratio of the mix is usually expressed in mass. Concrete mix design involves determination of the ingredients of the mix to obtain a concrete of the required strength, durability, workability and to the possible extent, economy. The proportioning is governed by two factors viz., workability and strength. If the concrete in the plastic state is not workable it cannot be properly placed and compacted. The compressive strength of concrete in the hardened state is considered to be its most important property and also as an index of other properties.  The selection and proportioning of the mix ingredients are governed by the following:

(i) The minimum compressive strength required from structural considerations as per specification.

(ii) Sufficient workability to fully compact the concrete with the available equipment. (iii) Maximum water-cement ratio under the prevailing site conditions.

(iv) Minimum cement content to ensure durability of the structure.

(v) Maximum cement content to avoid shrinkage cracking due to evolution of high heat of hydration especially in mass concrete.

2. Types of mixes

The concrete mixes are generally classified as nominal mixes and design mixes. As per IS: 456-2000, the concrete mixes are designated into a number of grades viz., M10, M15, M20, M25, M30, M35 and M40. The letter „M? stands for mix and the number to the 28 days compressive strength in N/mm2. a) Nominal mixes: In nominal mixes, the specifications of concrete prescribe the proportion of cement, fine aggregates and coarse aggregates by volume. The minimum compressive strength required is also included in many specifications to nominal mixes. The proportions of materials for nominal mix concrete shall be in accordance with table 1 (Ref: Table 9 of IS: 456).

Table 1: Ingredients for nominal mix per 50 kg of cement 

Grade of concrete Total quantity of dry aggregates (CA+FA)by mass(kg Quantityof water (litres)
M5 800 60
M7.5 625 45
M10 480 34
M15 330 32
M20 200 30

b) Designed mixes:

In these mixes, certain performance requirement of concrete is specified by the structural engineer.  The concrete mix is then designed in the laboratory, after checking the properties of al the ingredients, site exposure conditions and level of supervision available at site to achieve the minimum prescribed strength and durability.  The designed mix is then put into site trials and sometimes slight adjustments may be required to the laboratory proportions to achieve a workable mix. Design mix is preferred to nominal mixes. In any case design mix is mandatory for concrete grade M25 and above. 3. Factors affecting the selection of mix ingredients The various factors affecting the mix design are:

a) Compressive strength:

It is the most important property of concrete and influences many other 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.

(b) Workability

The degree of workability required for the mix depends on three factors viz., the size of the section concreted, the amount of reinforcement, and the method of compaction 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.

(c)  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.

(d) Maximum nominal size of aggregate:

In general, larger the maximum size of aggregate, smaller is the cement requirement for a particular water-cement ratio. 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, IRC: 112-2011 and IS 1343:1980 recommend that the nominal size of the aggregate should be as large as possible.

(e) 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. 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. Uniformity in aggregate grading can be achieved by mixing different size fractions.

(f) 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. By exercising proper Quality Control, the difference between the mean and minimum strengths of the mix is maintained low. This will also help to keep the cement-content to the design minimum.

 4. Procedure for mix design

a) Determine the mean target strength (ft) from the specified characteristic compressive strength at 28-days (fck) and the level of quality control. ft = fck + kS, where S is the standard deviation based on the grade of the concrete obtained from table 2 below (Ref: Table 8 of IS: 456-2000).

Grade of concrete

Assumed standard deviation (MPa)

M10 & M15

3.5

M20 & M25

4.0

M30, M35, M40, M45 & M50

5.0

 The value of k depends on the probability of results falling below the specified characteristic strength. Table 3 gives the value of k for different probabilities. k = 1.65 (As per IS: 456-2000/IRC112-2011).

Sl. No.

Probability of value falling below fck

k

1

1 in 5

0.84

2

1 in 10

1.28

3

1 in 15

1.5

4

1 in 20

1.65

5

1 in 40

1.86

6

1 in 100

2.33

b) For initial design, free water-cement ratio corresponding to the 28 day target mean strength for the requirement of durability as per table 3 below (Ref: table 5 of IS: 456-2000) may be used for building works and table 4 (Ref: table 14.2 of IRC 112:2011) for bridge works.

c) The grades of concrete and minimum cement content are also selected from the tables 4 and 5. 

Table 4 – Minimum cement content, maximum water-cement ratio and minimum concrete grade  for different exposure conditions for building works. 

   PCC    RCC 
Exposure Min. cement content kg/m3 Max water-  ratio Grade of concrete Min. cement content kg/m3 Max water- cement ratio Grade ofconcrete
Mild 220 0.6 - 300 0.55 M20
Moderate 240 0.6 M15 300 0.5 M25
Severe 250 0.5 M20 320 0.45 M30
Very severe 260 0.45 M20 340 0.45 M35
Extreme 280 0.4 M25 360 0.4 M40

Table 5 – Minimum cement content, maximum water-cement ratio and minimum concrete grade for different exposure conditions for bridge works.

Exposure  PCC   RCC 
Min. Max   Min. Max  
cement content Kg/m3 water- cement ratio Grade ofconcrete cementcontent Kg/m3 water- cement ratio Grade of concrete
Moderate 340 0.5 M20 340 0.45 M25
Severe 360 0.5 M25 360 0.45 M30
Very severe 380 0.45 M35 380 0.4 M40
Extreme 400 0.4 M40 400 0.35 M45

d) The minimum cement content indicated in tables 4 and 5 shall be adjusted for 10 mm and 40 mm aggregates as given in table 6 (Ref: table 6 of IS: 456-2000 & Table 14.3 of IRC 112:2011).

Table 6 - Adjustments to minimum Cement content for nominal MSA 

Nominal maximum aggregate size (mm)

Adjustment to minimum cement content in tables 4 & 5 (kg)

10

+40

20

0

40

-30

(e) The quantity of maximum water per cubic metre of concrete may be determined from table 7 (Ref: table 6 of IS: 10262-2009)

Table 7 - Maximum water content and percentage of sand for nominal MSA 

Nominal maximum aggregate size (mm)

Maximum Water Content kg

% of sand of total aggregate by volume

 

For grades up to M35

 

10

208

40

20

186

35

40

165

30

 

For grades above M35

 

10

200

28

20

180

25

(f) For changes in conditions mentioned in table 7 above and based on the grading zone of fine aggregate. Hence the sieve analysis of the fine aggregate is carried as per IS: 2386 and the grading zone is determined.  Necessary adjustments in water content and percentage of sand the grading falls under zone I, III or IV shall be made as given in table 8 (Ref: IS: 10262).

(g) The water content in table 7 is for angular coarse aggregates and for 25 mm to 50 mm slump range. According to clause 4.2 of IS: 10262-2009, suitable corrections shall be made to maximum water content for other conditions as given in table 9.  

Table 8 – Adjustments for water content and percentage of sand 

Change in condition Correction to water Correction
content to % of sand
Sand in grading zone I 0 1.50%
Sand in grading zone III 0 -1.50%
Sand in grading zone IV 0 -3.00%
Increase/decrease of CF by 0.1 ±3.0% 0
For each 0.05 increase or    
decrease in free-water cement 0 ±1.0%
ratio    
For rounded aggregate -15 kg/m3 -7%

(h) The cement content per m3 of concrete is calculated from the free water-cement ratio and the corrected quantity of water per m3 of concrete. The cement content thus arrived is checked against the minimum value prescribed in table 4 or 5 and greater of the two values is selected for the design. The maximum cement content excluding fly ash and ground granulated blast furnace slag shall not be in excess of 450 kg/m3 as per IS:456-2000 and IRC:112-2011.

(i) The amount of entrapped air in the concrete, based on nominal maximum aggregate size, is estimated from table 10. 

i) The percentage volume of fine aggregate to total aggregate for different nominal maximum aggregate size is obtained from table 6.

The quantities of fine and coarse aggregates are computed from the equation below.

V = [W + C/Sc + 1/p x fa/Sfa] x 1/1000 for fine aggregate and

V = [W + C/Sc + 1/(1-p) x ca/Sca] x 1/1000 for coarse aggregate where,

V = Absolute volume of fresh concrete = gross volume – volume of entrapped air.

W = Mass of water per m3 of concrete kg or litre.

C = Mass of cement per m3 of concrete kg.

Sc = Specific gravity of cement normally assumed as 3.15.

p = Ratio of fine aggregate to total aggregate by absolute volume. 

fa, ca = Total quantity of fine and coarse aggregates in kg per m3 of concrete. Sfa, Sca = Saturated surface dry specific gravities of fine and coarse aggregates.

For example, if the design cement content is 378 kg, water content 170 kg and sand content is 30% of the absolute volume, for 1% entrapped air absolute volume of concrete,

V is given by 1-0.1= [170 + 378/3.15 + 1/0.3 x fa/2.6] x 1/1000 (Assuming fa = 2.6) Simplifying, fa = 546 kg/m3.

Similarly, 0.99 = [170 + 378/3.15 + 1/0.7 x ca/2.7] x 1/1000 (Assuming ca = 2.7)

Simplifying, ca = 1323 kg/ m3. Thus mass of the ingredients of 1 m3 concrete will be cement = 378 kg, water = 170 kg, fine aggregate (SSD) = 546 kg and coarse aggregate (SSD) = 1323 kg. Water cement ratio is 0.45 and the proportion of cement: fine aggregate: coarse aggregate will be 1:1.44:3.5.  

5. Report The mix design report shall include the following details:

i) The name of project, type of structure and/or component.

ii) Grade of concrete and slump requirement.

iii) Type of cement, grade and specific gravity.

iv) Maximum size of aggregate.

v) Source of aggregates and its physical properties such as specific gravity, strength, grading etc.

vi) Fineness Modulus, silt content, grading zone etc of fine aggregates.

vii) Source of water and test reports to confirm its suitability for using in the concrete.

viii) Design water cement ratio and cement content per m3 of concrete.

ix) Proportioning of fine and coarse aggregate.

x) Details of admixture if recommended and its dosage.

xi) Test reports of compressive strength of 150 mm test cubes after 3-days, 7-days and 28days curing.

xii) Quantities of ingredients for 1 m3 of concrete by weight as well as volume.

xiii) Any specific recommendations on the mix to be selected.

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