CONCRETE MIX DESIGN – INDIAN STANDARD METHOD

IS-10262-2009-CONCRETE MIX DESIGN – INDIAN STANDARD METHOD

The following points should be remembered before proportioning a concrete mix a per IS-10262-2009.

• This method of concrete mix proportioning is applicable only for ordinary and standard concrete grades.
• The air content in concrete is considered as nil.
• The proportioning is carried out to achieve specified characteristic compressive strength at specified age, workability of fresh concrete and durability requirements.

Concrete Mix Design

This method of concrete mix design consist of following 11 steps

1. Design specification
2. Testing of materials
3. Calculating target strength for mix proportioning
4. Selecting water/cement ratio
5. Calculating water content
6. Calculating cement content
7. Finding out volume proportions for Coarse aggregate & fine aggregate
8. Mix calculations
9. Trial mixing and

10. Workability measurement (using slump cone method)

11. Repeating step 9 & 10 until all requirements is fulfilled.

Let us discuss all of the above steps in detail

STEP-1. DESIGN SPECIFICATIONS

This is the step where we gather all the required information for designing a concrete mix from the client. The data required for mix proportioning is as follows.

• Grade designation (whether M10, M15, M20 etc)
• Type of cement to be used
• Maximum nominal size of aggregates
• Minimum & maximum cement content
• Maximum water-cement ratio
• Workability
• Exposure conditions (As per IS-456-Table-4)
• Maximum temperature of concrete at the time of placing
• Method of transporting & placing
• Early age strength requirement (if any)
• Type of aggregate (angular, sub angular, rounded etc)
• Type of admixture to be used (if any)

STEP-2. TESTING OF MATERIALS

The table given below shows the list of most necessary tests to be done on cement, coarse aggregate, fine aggregate and admixture. After doing the test, store the test data for further calculation.

Concrete Ingredients	Tests to be done
Cement	Specific gravity	—	—	—
Coarse aggregate	Specific gravity	Water absorption	Free surface moisture	Sieve analysis
Fine aggregate	Specific gravity	Water absorption	Free surface moisture	Sieve analysis
Admixture (if any)	Specific gravity	—	—	—

STEP-3. TARGET STRENGTH CALCULATION

Calculate the target compressive strength of concrete using the formula given below.

fck’ = fck + 1.65s

Where,

fck’ = Target compressive strength at 28 days in N/mm2.

fck = Characteristic compressive strength at 28 days in N/mm2. (same as grade of concrete, see table below)

s = Standard deviation

The value of standard deviation, given in the table below, can be taken for initial calculation.

Sl.No	Grade of Concrete	Characteristic compressive strength (N/mm2)	Assumed standard deviation (N/mm2)
1.	M10	10	3.5
2.	M15	15
3.	M20	20	4.0
4.	M25	25
5.	M30	30	6.0
6.	M35	35
7.	M40	40
8.	M45	45
9.	M50	50
10.	M55	55

STEP-4. SELECTION OF WATER-CEMENT RATIO

For preliminary calculation, water cement ratio as given is IS-456-Table 5 (also given below) for different environmental exposure condition, may be used.

Note: Use Table-1 for finding out water-cement ratio of Plain Concrete and use Table-2 for finding out water-cement ratio of Reinforced Concrete.

Table -1
Sl.No.	Environmental Exposure Condition	Plain Concrete
		Minimum Cement Content (kg/m3)	Maximum Free Water-Cement Ratio	Minimum Grade of Concrete
1	Mild	220	0.60	—
2	Moderate	240	0.60	M15
3	Severe	250	0.50	M20
4	Very Severe	260	0.45	M20
5	Extreme	280	0.40	M25

Table -2
Sl.No.	Environmental Exposure Condition	Reinforced Concrete
		Minimum Cement Content (kg/m3)	Maximum Free Water-Cement Ratio	Minimum Grade of Concrete
1	Mild	300	0.55	M20
2	Moderate	300	0.50	M25
3	Severe	320	0.45	M30
4	Very Severe	340	0.45	M35
5	Extreme	360

Refer the table given below (As per IS-456) to choose right type of environment depending upon different exposure conditions to concrete.

Sl.No	Environment	Exposure condition
1	Mild	Concrete surfaces protected against weather or aggressive conditions, except those situated in coastal areas.
2	Moderate	Concrete surfaces sheltered from severe rain or freezing whilst wetConcrete exposed to condensation and rain Concrete continuously under water Concrete in contact or buried under non aggressive soil/ground water Concrete surfaces sheltered from saturated salt air in coastal area
3	Severe	Concrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing whilst wet or severe condensationConcrete completely immersed in sea water Concrete exposed to coastal environment
4	Very severe	Concrete surfaces exposed to sea water spray, corrosive fumes or severe freezing condition whilst wetConcrete in contact with or buried under aggressive sub-soil/ground water
5	Extreme	Surface members in tidal zoneMembers in direct contact with liquid/solid aggressive chemicals

 STEP-5. SELECTION OF WATER CONTENT

Selection of water content depends upon a number of factors such as

• Aggregate size, shape & texture
• Workability
• Water cement ratio
• Type of cement and its amount
• Type of admixture and environmental conditions.

Factors that can reduce water demand are as follows

• Using increased aggregate size
• Reducing water cement ratio
• Reducing the slump requirement
• Using rounded aggregate
• Using water reducing admixture

Factors that can increase water demand are as follows

• Increased temp. at site
• Increased cement content
• Increased slump
• Increased water cement ratio
• Increased aggregate angularity
• Decrease in proportion of the coarse aggregate to fine aggregate

The quantity of maximum mixing water per unit volume of concrete may be selected from the table given below.

Maximum water content per cubic meter of concrete for nominal maximum size of aggregate
Sl.No.	Nominal maximum size of aggregate	Maximum water content
1	10	208
2	20	186
3	40	165

The values given in the table shown above is applicable only for angular coarse aggregate and for a slump value in between 25 to 50mm.

Do the following adjustments if the material used differs from the specified condition.

Type of material/condition	Adjustment required
For sub angular aggregate	Reduce the selected value by 10kg
For gravel with crushed stone	Reduce the selected value by 20kg
For rounded gravel	Reduce the selected value by 25kg
For every addition of 25mm slump	Increase the selected value by 3%
If using plasticizer	Decrease the selected value by 5-10%
If using super plasticizer	Decrease the selected value by 20-30%

Note: Aggregates should be used in saturated surface dry condition. While computing the requirement of mixing water, allowance shall be made for the free surface moisture contributed by the fine and coarse aggregates. On the other hand, if the aggregate are completely dry, the amount of mixing water should be increased by an amount equal to moisture likely to be absorbed by the aggregate

STEP-6. CALCULATING CEMENTIOUS MATERIAL CONTENT

From the water cement ratio and the quantity of water per unit volume of cement, calculate the amount of cementious material. After calculating the quantity of cementious material, compare it with the values given in the table shown in Step-4. The greater of the two values is then adopted.

If any mineral admixture (such as fly ash) is to be used, then decide the percentage of mineral admixture to be used based on project requirement and quality of material.

STEP-7. FINDING OUT VOLUME PROPORTIONS FOR COARSE AGGREGATE & FINE AGGREGATE

Volume of coarse aggregate corresponding to unit volume of total aggregate for different zones of fine aggregate is given in the following table.

Sl.No.	Nominal Maximum Size of Aggregate (mm)	Volume of coarse aggregate per unit volume of total aggregate for different zones of fine aggregate
		Zone IV	Zone III	Zone II	Zone I
1	10	0.50	0.48	0.46	0.44
2	20	0.66	0.64	0.62	0.60
3	40	0.75	0.73	0.71	0.69

The values given in the table shown above is applicable only for a water-cement ratio of 0.5 and based on aggregates in saturated surface dry condition.

If water-cement ratio other than 0.5 is to be used then apply correction using the rule given below.

Rule: For every increase or decrease by 0.05 in water-cement ratio, the above values will bedecreased or increased by 0.01, respectively.

If the placement of concrete is done by a pump or where is required to be worked around congested reinforcing steel, it may be desirable to reduce the estimated coarse aggregate content determined as above, upto 10 percent.

After calculating volume of coarse aggregate, subtract it from 1, to find out the volume of fine aggregate.

STEP-8. MIX CALCULATIONS

The mix calculations per unit volume of concrete shall be done as follows.

a	Volume of concrete=	1m3
b	Volume of cement=	(Mass of cement/specific gravity of cement)*(1/1000)
c	Volume of water=	(Mass of water/specific gravity of water)*(1/1000)
d	Volume of admixture=	(Mass of admixture/specific gravity of admixture)*(1/1000)
e	Volume of total aggregate (C.A+F.A)=	[a-(b+c+d)]
f	Mass of coarse aggregate=	e*Volume of coarse aggregate*specific gravity of coarse aggregate*1000
g	Mass of fine aggregate=	e*Volume of fine aggregate*specific gravity of fine aggregate*1000

STEP-9. TRIAL MIX

Conduct a trial mix as per the amount of material calculated above.

STEP-10. MEASUREMENT OF WORKABILITY (BY SLUMP CONE METHOD)

The 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.

STEP-11. REPEATING TRIAL MIXES

If the measured workability of trial mix no.1 is different from 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 pre-selected value.

Trial-2 – increase water or admixture, keeping water-cement ratio constant

Trial-3 – Keep water content same as trial-2, but increase water-cement ratio by 10%.

Trial-4 – Keep water content same as trial-2, but decrease water-cement ratio by 10%

Trial mix no 2 to 4 normally provides sufficient information, including the relationship between compressive strength and water-cement ratio.