CONCRETE MIX DESIGN AS PER IS CODE

CONCRETE MIX DESIGN

Concrete Mix design is 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.

 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

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 simplicities 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.

DESIGN MIX REPORT FORMAT

(This is only for reference)

M-25 CONCRETE MIX DESIGN
As per IS 10262-2009 & MORT&H
A-1	Stipulations for Proportioning
1.	Grade Designation		M25
2	Type of Cement		OPC 53 grade confirming to IS-12269-1987
3	Maximum Nominal Aggregate Size		20 mm
4	Minimum Cement Content (MORT&H 1700-3 A)		310 kg/m3
5	Maximum Water Cement Ratio (MORT&H 1700-3 A)		0.45
6	Workability (MORT&H 1700-4)		50-75 mm (Slump)
7	Exposure Condition		Normal
8	Degree of Supervision		Good
9	Type of Aggregate		Crushed Angular Aggregate
10	Maximum Cement Content (MORT&H Cl. 1703.2)		540 kg/m3
11	Chemical Admixture Type		Superplasticiser Confirming to IS-9103
A-2	Test Data for Materials
1	Cement Used	xxx (Brand) OPC 53 grade
2	Sp. Gravity of Cement	3.15
3	Sp. Gravity of Water	1.00
4	Chemical Admixture	BASF Chemicals Company
5	Sp. Gravity of 20 mm Aggregate	2.884
6	Sp. Gravity of 10 mm Aggregate	2.878
7	Sp. Gravity of Sand	2.605
8	Water Absorption of 20 mm Aggregate	0.97%
9	Water Absorption of 10 mm Aggregate	0.83%
10	Water Absorption of Sand	1.23%
11	Free (Surface) Moisture of 20 mm Aggregate	nil
12	Free (Surface) Moisture of 10 mm Aggregate	nil
13	Free (Surface) Moisture of Sand	nil
14	Sieve Analysis of Individual Coarse Aggregates	Separate Analysis Done
15	Sieve Analysis of Combined Coarse Aggregates	Separate Analysis Done
15	Sp. Gravity of Combined Coarse Aggregates	2.882
16	Sieve Analysis of Fine Aggregates	Separate Analysis Done
A-3	Target Strength for Mix Proportioning
1	Target Mean Strength (MORT&H 1700-5)	36N/mm2
2	Characteristic Strength @ 28 days	25N/mm2
A-4	Selection of Water Cement Ratio
1	Maximum Water Cement Ratio (MORT&H 1700-3 A)	0.45
2	Adopted Water Cement Ratio	0.43
A-5	Selection of Water Content
1	Maximum Water content (10262-table-2)	186 Lit.
2	Estimated Water content for 50-75 mm Slump	138 Lit.
3	Superplasticiser used	0.5 % by wt. of cement
A-6	Calculation of Cement Content
1	Water Cement Ratio	0.43
2	Cement Content (138/0.43)	320 kg/m3
		Which is greater then 310 kg/m3
A-7	Proportion of Volume of Coarse Aggregate & Fine Aggregate Content
1	Vol. of C.A. as per table 3 of IS 10262	62.00%
2	Adopted Vol. of Coarse Aggregate	62.00%
	Adopted Vol. of Fine Aggregate ( 1-0.62)	38.00%
A-8	Mix Calculations
1	Volume of Concrete in m3	1.00
2	Volume of Cement in m3	0.10
	(Mass of Cement) / (Sp. Gravity of Cement)x1000
3	Volume of Water in m3	0.138
	(Mass of Water) / (Sp. Gravity of Water)x1000
4	Volume of Admixture @ 0.5% in m3	0.00134
	(Mass of Admixture)/(Sp. Gravity of Admixture)x1000
5	Volume of All in Aggregate in m3	0.759
	Sr. no. 1 – (Sr. no. 2+3+4)
6	Volume of Coarse Aggregate in m3	0.471
	Sr. no. 5 x 0.62
7	Volume of Fine Aggregate in m3	0.288
	Sr. no. 5 x 0.38
A-9	Mix Proportions for One Cum of Concrete (SSD Condition)
1	Mass of Cement in kg/m3	320
2	Mass of Water in kg/m3	138
3	Mass of Fine Aggregate in kg/m3	751
4	Mass of Coarse Aggregate in kg/m3	1356
	Mass of 20 mm in kg/m3	977
	Mass of 10 mm in kg/m3	380
5	Mass of Admixture in kg/m3	1.60
6	Water Cement Ratio	0.43

 

M-30 CONCRETE MIX DESIGN
As per IS 10262-2009 & MORT&H
A-1	Stipulations for Proportioning
1	Grade Designation	M30
2	Type of Cement	OPC 53 grade confirming to IS-12269-1987
3	Maximum Nominal Aggregate Size	20 mm
4	Minimum Cement Content (MORT&H 1700-3 A)	310 kg/m3
5	Maximum Water Cement Ratio (MORT&H 1700-3 A)	0.45
6	Workability (MORT&H 1700-4)	50-75 mm (Slump)
7	Exposure Condition	Normal
8	Degree of Supervision	Good
9	Type of Aggregate	Crushed Angular Aggregate
10	Maximum Cement Content (MORT&H Cl. 1703.2)	540 kg/m3
11	Chemical Admixture Type	Superplasticiser Confirming to IS-9103
A-2	Test Data for Materials
1	Cement Used	xxx (Brand name) OPC 53 grade
2	Sp. Gravity of Cement	3.15
3	Sp. Gravity of Water	1.00
4	Chemical Admixture	BASF Chemicals Company
5	Sp. Gravity of 20 mm Aggregate	2.884
6	Sp. Gravity of 10 mm Aggregate	2.878
7	Sp. Gravity of Sand	2.605
8	Water Absorption of 20 mm Aggregate	0.97%
9	Water Absorption of 10 mm Aggregate	0.83%
10	Water Absorption of Sand	1.23%
11	Free (Surface) Moisture of 20 mm Aggregate	nil
12	Free (Surface) Moisture of 10 mm Aggregate	nil
13	Free (Surface) Moisture of Sand	nil
14	Sieve Analysis of Individual Coarse Aggregates	Separate Analysis Done
15	Sieve Analysis of Combined Coarse Aggregates	Separate Analysis Done
15	Sp. Gravity of Combined Coarse Aggregates	2.882
16	Sieve Analysis of Fine Aggregates	Separate Analysis Done
A-3	Target Strength for Mix Proportioning
1	Target Mean Strength (MORT&H 1700-5)	42N/mm2
2	Characteristic Strength @ 28 days	30N/mm2
A-4	Selection of Water Cement Ratio
1	Maximum Water Cement Ratio (MORT&H 1700-3 A)	0.45
2	Adopted Water Cement Ratio	0.42
A-5	Selection of Water Content
1	Maximum Water content (10262-table-2)	186 Lit.
2	Estimated Water content for 50-75 mm Slump	160 Lit.
3	Superplasticiser used	0.5 % by wt. of cement
A-6	Calculation of Cement Content
1	Water Cement Ratio	0.42
2	Cement Content (160/0.42)	380 kg/m3
		Which is greater then 310 kg/m3
A-7	Proportion of Volume of Coarse Aggregate & Fine Aggregate Content
1	Vol. of C.A. as per table 3 of IS 10262	62.00%
2	Adopted Vol. of Coarse Aggregate	62.00%
	Adopted Vol. of Fine Aggregate ( 1-0.62)	38.00%
A-8	Mix Calculations
1	Volume of Concrete in m3	1.00
2	Volume of Cement in m3	0.12
	(Mass of Cement) / (Sp. Gravity of Cement)x1000
3	Volume of Water in m3	0.160
	(Mass of Water) / (Sp. Gravity of Water)x1000
4	Volume of Admixture @ 0.5% in m3	0.00160
	(Mass of Admixture)/(Sp. Gravity of Admixture)x1000
5	Volume of All in Aggregate in m3	0.718
	Sr. no. 1 – (Sr. no. 2+3+4)
6	Volume of Coarse Aggregate in m3	0.445
	Sr. no. 5 x 0.62
7	Volume of Fine Aggregate in m3	0.273
	Sr. no. 5 x 0.38
A-9	Mix Proportions for One Cum of Concrete (SSD Condition)
1	Mass of Cement in kg/m3	380
2	Mass of Water in kg/m3	160
3	Mass of Fine Aggregate in kg/m3	711
4	Mass of Coarse Aggregate in kg/m3	1283
	Mass of 20 mm in kg/m3	924
	Mass of 10 mm in kg/m3	359
5	Mass of Admixture in kg/m3	1.90
6	Water Cement Ratio	0.42