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
- Mix calculations
- 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
- 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|
|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
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)|
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.
|Sl.No.||Environmental Exposure Condition||Plain Concrete|
|Minimum Cement Content (kg/m3)||Maximum Free Water-Cement Ratio||Minimum Grade of Concrete|
|Sl.No.||Environmental Exposure Condition||Reinforced Concrete|
|Minimum Cement Content (kg/m3)||Maximum Free Water-Cement Ratio||Minimum Grade of Concrete|
Refer the table given below (As per IS-456) to choose right type of environment depending upon different exposure conditions to concrete.
|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 rainConcrete continuously under waterConcrete in contact or buried under non aggressive soil/ground waterConcrete 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 waterConcrete 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
- 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|
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.||NominalMaximumSize ofAggregate(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|
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 be decreased 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.