IS-10262-2009 CONCRETE MIX DESIGN – INDIAN STANDARD METHOD AND MIX CONCRETE MANUALLY ON SITE
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
Design specification
Testing of materials
Calculating target strength for mix proportioning
Selecting water/cement ratio
Calculating water content
Calculating cement content
Finding out volume proportions for Coarse aggregate & fine aggregate
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.
Proper mixing of concrete ingredient is very much necessary as it affects the quality of concrete in its fresh state as well as in hardened state. A concrete is said to be well mixed if it fulfills the following requirements.
Concrete mix should be uniform in colour
Concrete should achieve proper consistency for which it is designed
Complete blending of all concrete ingredients
Cement paste should cover all the surface of the aggregate
Hand mixing is adopted for small works and where the quantity of concrete used is small. The following procedure is followed to mix concrete by hand.
PROCEDURE
STEP-1 [MIXING PLATFORM]
Clean and water tight platform should be used for mixing concrete. Construct a platform of bricks or lean concrete or iron sheets, of a size as per requirement of the volume of concrete to be mixed. Commonly a platform made of iron sheet having 3m2 in size with strips or kerbs are used.
STEP-2 [DRY MIXING OF CEMENT & SAND]
Spread out the measured quantity of sand uniformly on the mixing platform. Dump the cement on the sand and distribute it uniformly. Mix the sand and cement with the shovels, turning the mixture over and over again until it is uniform in colour.
STEP-3 [DRY MIXING OF C.A. WITH CEMENT-SAND MIX]
Spread out the sand-cement mixture once again uniformly on the mixing platform. On top of this, spread the measured quantity of C.A (coarse aggregate). Don’t dump at one place; otherwise the bigger particles will tend to roll out. Mix again at least three times by shoveling from the center to side, and then back to the center and again to the side.
STEP-4 [ADDITION OF WATER]
Make a depression in the middle of the mixed pile and pour slowly into half to three-quarters of the total quantity of water required. While the material is turned in towards the centre with shovels, add the remainder of the water slowly, turning the mixture over and again until the colour and consistency are uniform throughout the pile. This will indicate that all ingredients are thoroughly combined. The water should be poured slowly into the mix. This is best done by means of a gardener’s water can fitted with a rose-head. Throwing water from a bucket all at a time will result in its running away and taking cement with it.
Note
Time of mixing does not exceed 3 minutes
Mixing platform is cleaned at the end of the day’s work, so that it is ready for use the next day.
CHECK FOR CONSISTENCY
A rough estimation of consistency of concrete can be made by doing the following simple & quick field test.
Take a handful of concrete and squeeze it tightly in the hand. If the concrete is mixed properly, then after releasing the hand pressure, it will retain its shape. Also the surface of the sample will be moist without water dripping out of it.
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