Concrete mix design is the calculation of proportion of contents (Like cement, aggregates, water and sometimes admixture also) to achieve desired degree of workability of fresh concrete and desired strength, durability, surface finish of hardened concrete. There are four variable factors for given materials to be considered in connection with specifying a concrete mix are:
- Water-Cement ratio – The mix must be workable so that it can be placed and finished easily.
- Cement content / cement-aggregate ratio – cement is the costliest ingredient in the mix, so use less.
- Gradation of the aggregates
Generally, all four of these inter-related variables cannot be chosen or manipulated arbitrarily. Usually two or three factors are specified, and the others are adjusted to give minimum workability and economy.
The mix shall be designed to produce the grade of concrete having the required workability and a characteristic strength not less than appropriate values given in table.
Methods of Concrete Mix:
Concrete Mix are primarily two Method –
Nominal Mix is generally used for lower grade (M10 – M20) of concrete, PCC work or for small scale constructions. This is defined as “fixed proportion based on volumetric ratio are called nominal mix”. In this type of mix, the mix ratios and concrete constituent proportions are prefixed and specified.
- M20 (1:1.5:3); the quantity of cement, sand and aggregate is batched in volume as per the fixed ratio 1:1.5:3.
- M15 – 1:2:4
- M10 – 1:3:6
Design mix concrete, now widely used in the country, is recommended for designing mixes for general types of construction, using the ingredients of concrete normally available. This is defined as “Mix design is a process of determining the proportion of materials to be mixed to producing concrete of certain minimum strength and durability as economically as possible”. It is based on the quality of actual sample of materials to be used are designed.
Note – Design mix proportions must be by weight while nominal mix proportion could be by volume also but by weight is preferable.
Data Required for Concrete Mix Design (as Per IS: 10262–1982)
A. Concrete Mix Design Stipulation
- Grade designation: ?
- Type of cement: ?
- Maximum nominal size of aggregate: In General 20mm
- Minimum cement content: From Table 5 of IS 456, minimum cement content for given exposure condition
- Maximum water-cement ratio: From Table 5 of IS 456:2000
- Degree of Workability: As per Placing Conditions
- Exposure condition: (for reinforced concrete)
- Method of concrete placing: ?
- Degree of supervision: ?
- Type of aggregate: ?
- Maximum cement content: ?
- Chemical admixture type
B. Test data of material (to be determined in the laboratory)
- Specific gravity of cement
- Compressive strength of cement at 7 days
- 1. Specific gravity of coarse aggregates:
2. Specific gravity of fine aggregates:
- Water absorption:
1. Coarse aggregate:
2. Fine aggregate:
- Free (surface) moisture:
1. Coarse aggregate:
2. Fine aggregate: Sieve analysis
C. Target mean strength for mix design:
In order that not more than the specified proportion of test results are likely to fall below the characteristic strength, the concrete mix has to be proportioned for higher target mean compressive strength. The target mean compressive (fck) strength at 28 days is given by –
Target mean strength f’ck= fck + kσ
fck = characteristic strength below which certain percentage of test results are expected to fall.
k = A constant depending on expected proportion of low results (risk factor) likely to fall below fck.
σ = Standard deviation.
According to IS: 456–2000 and IS: 1343–’80 Characteristic Strength of concrete is the value of strength of concrete below which not more than 5% of test results are expected to fail. in which case –
k = 1.65 and the equation for target strength reduces to –
Target strength = fck +1.65 σ
D. SELECTION OF MIX PROPORTIONS
1. Selection of Water-Cement Ratio –
Select the water-cement ratio based on experience for respective environment exposure conditions, using materials. Selected water cement ratio shall be less than from provisions provided in “Table 5 of IS 456” maximum water-cement ratio.
2. Selection of Water Content –
The water content of concrete is dependent on number of factors, such as aggregate shape and size, workability, water-cement ratio, cement content, chemical admixture and environmental conditions. The quantity of maximum water content per unit volume of concrete select from Table 2 given in IS 10262: 2009.
3. Calculation of Cementious Material Content –
The cement content per unit volume of concrete may be calculated from the free water-cement ratio and the quantity of water per unit volume of concrete. Calculated content checked against the minimum content requirements accordance with IS 456.
4. Estimation of Coarse Aggregate Proportion –
It can be seen that for equal workability, the volume of coarse aggregate in a unit volume of concrete is dependent only on its nominal maximum size and grading zone of fine aggregate.
5. Combination of Coarse and Fine Aggregate Fractions
Aggregate content can be determined from the following equations –
- V = absolute volume of fresh concrete, which is equal to gross volume (m3) minus the volume of entrapped air,
- W = Mass of water (kg) per m3 of concrete
- C = Mass of cement (kg) per m3 of concrete
- Sc = Specific gravity of cement
- P = Ratio of FA to total aggregate by absolute volume
- fa, Ca = Total masses of FA and CA (kg) per m3 of concrete respectively and
- Sfa, Sca = Specific gravities of saturated, surface dry fine aggregate and coarse aggregate respectively
6. Estimation of Fine Aggregate Proportion
E. TRIAL MIXES
Mix proportions for trial number – Cement Water, Fine aggregate, Coarse aggregate, Chemical admixture, and Water-cement ratio.
NOTE – Aggregates should be used in saturated surface dry condition. If otherwise. when 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 aggregates are dry, the amount of mixing water should be increased by an amount equal to the moisture likely to be absorbed by the aggregates. Necessary adjustments are also required to be made in mass of aggregates. The surface water and percent water absorption of aggregates shall be determined according to IS 2386.
Why concrete mix design is used?
Concrete mix design is an economical method of concrete mix for prepare the proportion of concrete ingredients for better strength and durability based on construction site.
Advantages of Concrete Mix Designs –
- Good quality concrete – This means the concrete possess the desired strength with the appropriate value of workability as prescribed in IS code.
- Economical – Nominal mix concrete may suggest more cement content, which is costlier than other materials and concrete mix designs gives the accurate quantity of cement consumption as is consistent with the attainment of desired properties. Thus, design mix is an economical solution for large projects. Design mix help to save up to 15 – 20% of cement content where M20 grade of concrete use. For higher grade concrete always choose design mix, that reduce the construction cost of project.
- Gives Desired Strength – The designed mix concrete produce with all tested materials, have desired properties based on project or construction requirements. Design test is performed day by day as material changes and design the required properties of concrete such as durability, strength, setting times, workability etc.
- Other benefits such as early de-shuttering can be done, pumpability, flexural strength