Mix design of concrete for Grade M35, M30, M25, and M20 using IS Code

Concrete Mix Design

Input

What is mix design of concrete?

Mix design of concrete is the process of selecting suitable ingredients like cement, sand, and aggregate with proper proportions relative to each other. It helps to prepare the concrete in an economical way as well as improve the strength, workability, and durability of concrete.

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Principles of concrete mix design

The main principle of concrete mix design is to make the required concrete economical, durable, and have more strength to bear load safely.

Types of concrete mix design

1) Arbitrary method:- This method is accepted generally for small construction.

2) Minimum void method:- In this method void of aggregate is filled with fine aggregate and void of fine aggregate is filled with cement paste. The quantity of fine aggregate is 10% more than the voids in coarse aggregate and the quantity of cement is 15% more than voids of fine aggregate. The strength is achieved by reducing the void in concrete.

3) Maximum density method:- To get the maximum density of concrete based on gradation.

Where,

P = percentage weight of material finer (Cement and sand)

d =Maximum size for fine material

D = Maximum size for coarse material

4) Fineness modulus method:- For good concrete, the value of fineness modulus of coarse aggregate should be 6 to 8.5 and for fine aggregate, it should be 2 to 3.5.

Any increase in the value of FM of aggregate beyond the given range will result in a harsh mix while a decrease in the value of FM will give an uneconomical mix.

5) Water cement ratio method

Where,

P7 = Compressive strength of concrete at 7 days of curing

P28 = Compressive strength of concrete at 28 days of curing

x = water-cement ratio

6) American concrete institute method (ACIM):- According to ACIM mix design of concrete can be found with the help of try of construction in terms of slump and size of aggregate.

7) British Method

A curve showing wet density and w/c ratio

Weight of cement = weight of w/c ratio

FA + CA = wet density – Amount of cement – the amount of water

A curve showing the percentage of FA gives the FA

Then found coarse aggregate

8) Indian Standard method

9) Road note/Grading method

10) IRC method

11) Graphical method

Factors affecting mix design of concrete

The factor affecting the mix design of concrete are given below,

Types of cement selected for concrete

Water/Cement ratio used to prepare concrete

Shape and size of aggregate

The proportion of the ingredient used to prepare concrete

Environmental condition

Mix design of concrete is code

For India, two IS Code is necessary for Mix design of concrete. One of them is IS 456-2000 and another is IS 10262:2009

Advantages of concrete mix design

Concrete mix design enhances to make the economical structure.

It helps to prepare quality concrete i.e concrete having better strength.

It gives an idea to calculate the quantity of different materials like cement, sand, and aggregate.

It helps to manage the concrete materials in the required amount.

It helps to prevent concrete from defects like segregation and bleeding.

It prevents the concrete structure from unwanted creep and shrinkage.

It gives an idea of the process of placing, transporting, and finishing the concrete.

General Procedure to find Mix design of concrete

Required grade is found according to structure and strength

Mean target strength is fixed

Types of cement is selected

Water cement ratio is determined as per required strength

The strength of concrete proportion is found out which must br equal to target strength of concrete. if not then re-proportion is selected to achieve the target strength

Weight of ingredient per batch is found out.

Now, let us move toward the designing procedures and calculation. At first, we will see the mix design procedure for the M35 grade of concrete. After that, we can be able to calculate the Mix design proportion of M30 concrete, Mix design procedure for M25 grade of concrete, Mix design of concrete M20, etc easily.

Mix design procedure for M35 grade of concrete

For example, we have the given data which are required to design the mix of concrete.

Given data,

Grade of concrete = M35

Characteristic compressive strength required infield at 28 days = 35 Mpa = M/mm^{2}

Types of Exposure = Moderate

Design mix target slump = 150 mm to 175 mm

Maximum size of coarse aggregate = 20 mm

Fine aggregate = Zone – III

The specific gravity of cement = 2.93

The specific gravity of water = 1

Specific gravity of admixture = 1.21 (It is given in the packet or bags of admixture)

Specific gravity of coarse aggregate = 2.82 (This value should be calculated in laboratory)

Specific gravity of fine aggregate = 2.65

Water absorption of coarse aggregate = 0.86% – SSD (Saturated Surface Dry) Condition

Water absorption of fine aggregate = 1.25% – SSD (Saturated Surface Dry) Condition

SSD (Saturated Surface Dry) Condition (This is condition when surface of coarse and fine aggregate are dry but inner materails is wetted)

We should consider SSD conditon to get actual mix design of concrete.

Now, let us calculate the mix design of concrete according to the above data step by step.

1) Target Mean strength of concrete

To prepare a safe mix design, we have to take somewhat more value of compressive strength than the actual value of compressive strength at 28 days.

So, the formula to calculate the target mean compressive strength of concrete is given below.

F’_{ck}= Fck + 1.65 S

F’_{ck}= 35 + 1.65 * 5F’ck= 43.25 N/mm^{2}

Where,

F’_{ck}= Target mean compressive strength at 28 days in N/mm^{2}

F_{ck}= Characteristic compressive strength at 28 days in N/mm2, 35 N/mm^{2}

S = Standard Deviation, 5 N/mm^{2} (IS-456 2000)

Tolerance factor = 1.65

Value of Standard deviation is taken from IS Code 456:2000, Table-8 (Page no. 23)

2) Determination of water cement ratio

Water cement ratio depends on exposer (Given moderate)

As per Table 5, of IS 456-2000 (Page-20) given below.

Maximum water-cement ration = 0.5

Based on my experience as a civil engineer, adopt water-cement ratio = 0.5 – 0.05 = 0.45

The water cement ratio may be increased and decreased by the use of admixture.

3) Determination of water content

From table 2 of IS 10262:2009 (Page-3), We get that maximum water content for 20 mm coarse aggregate = 186 liters

(This value is for 25 to 50 mm slump range)

Now, Our target of the slump is 150-175 mm

(As per IS 10262-2009, clause 4.2, we can increase 3% for every additional 25 mm slump)

Estimated water content for 175 mm slump = 186 + (15/100 * 186) = 213.9 liters

As admixture is used, the water content can be reduced by 15% and above.

The actual water to be used = 213.9 – (15/100 * 213.9) = 181.82 Liters.

4) Calculation of Cement Content

As per table 5 of IS 456:2000 (Page-20), For moderate exposer minimum cement = 300 Kg/m^{3}

= 404 kg/m^{3} Which is greater than 300 kg/m^{3} , Hence OK

5) Calculation of volume of coarse and fine aggregate

From table 3 of IS 10262-2009, Volume of coarse aggregate corresponding to 20 mm size aggregate and fine aggregate Zone-III for water, cement ratio of 0.5 = 0.64

Now, Actual water cement ratia = 0.45

It is less by (0.5-0.45) = 0.05

As the water-cement ratio is reduced, it is desirable to increase the coarse aggregate proportion to reduce the fine aggregate.

The coarse aggregate is increased at the rate of 0.01 for every decrease in water ratio of 0.05

So, for a decrease of every 0.05 water-cement ratio coarse aggregate increased by 0.01

So, for a decrease of every 1 water-cement ratio coarse aggregate increased by 0.01/0.05

So, for a decrease of every 0.05 water-cement ratio coarse aggregate increased by 0.01/0.05 * 0.05 = 0.01

Concrete proportion of volume of coarse aggregate = 0.64 + 0.01 = 0.65

Since It is an angular aggregate of concrete is to be pumped, the coarse aggregate can be reduced by 10% (Given in IS 10262:2009 clause 4.4.1 Page-3)

By reducing 10% we get = 0.65 – (10/100*0.65) = 0.585 is the volume of coarse aggregate

Then, volume of fine aggregate is = 1-0.585 = 0.415

6) Design Mix Calculation (Ref-IS 10262-2009)

The mix design calculation per unit volume of concrete shall be as follows:-

a) Volume of concrete = 1 m^{3}

b) Volume of cement

= {Mass of cement/(Sp. gravity of cement*1000)}

= {404/(2.93*1000)}

= 0.138 m^{3}

c) Volume of water

= {Mass of water/(Sp. gravity of water*1000)}

= {181.82/(1*1000)}

= 0.182 m^{3}

d) Volume of admixture =

= {Mass of Admixture/(Sp. gravity *1000)}

Assuming dosage of 1.2% by weight of cementitious materials and assumimg specific gravity as 1.121.

= {(1.2/100) * 404} / (1.121 * 1000)

= 0.004 m^{3}

e) Volume of entrapped air

= 2% for 20 mm coarse aggregate

= 2/100 = 0.02 m^{3}

As per IS 10262-2009, the percentage of entrapped air is zero. Still, 2% is considered in practical experiences.

f) Volume of all aggregate ( Coarse + Fine)

= Volume of Concrete – ( Volume of cement + Volume of water + Volume of admixture + Volume of entrapped air)

= Volume of all aggregate * Volume of coarse aggregate * Sp. gravity * 1000

= 0.656 * 0.585 * 2.82 * 1000 = 1082 Kg

h) Mass of fine aggregate

= Volume of all aggregate * Volume of fine aggregate * Sp. gravity * 1000

= 0.656 * 0.415 * 2.65 * 1000 = 721.4 Kg

Hence, Final proportion of mix of concrete is given below.

Proportion for Mix design of concrete

Cement (Kg/m^{3} ) = 404

Cement (Kg/m^{3} ) = 181.82

Fine Aggregate (Kg/m^{3} ) = 721.4

Coarse Aggregate (Kg/m^{3} ) = 1082.2 (60% of 20 mm, 40% of 10 mm)

Admixture (Kg/m^{3} ) = 4.848

Water Cement Ratio (Kg/m^{3} ) = 0.45

In the same way, we can calculate the Mix design proportion of M30 concrete, Mix design procedure for M25 grade of concrete, Mix design of concrete M20.

If you have any problem in calculating any grade of concrete above, then you can contact us.

I hope this article on “Mix design of concrete” remains helpful for you.

"Structural Engineer" with over 5 years of experience in estimation, structural design, and surveying.I am passionate about using my skills to create safe and sustainable structures. I am also a keen writer, and I enjoy sharing my knowledge and experiences with others.

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