Concrete Mix Design

Concrete mix design is the method of proportioning various constituents of concrete such as cement, aggregates, water, and admixtures optimally in order to manufacture a concrete at lower cost and will have specified properties of workability and homogeneity in the fresh state and strength and durability in the hardened state.

Purpose of Concrete Mixes

The main purpose of concrete mix proportioning is to make a concrete that has the following features:
1.Satisfies workability requirements in terms of a slump for easy placing and consolidating.
2.Meets the strength requirements.
3.Can be mixed, transported, placed, and compacted efficiently.
4.Economical to manufacture.
5.Fulfills durability requirements to resist the environment in which the structure is expected to serve

Mix Proportioning Procedure

Step 1. Calculate the target mean compressive strength for mix proportioning:

The 28-day target means compressive strength as per clause 3.2 of IS 10262.
                                              ft = fck + 1.65 s 
     where ft  = target mean compressive strength at 28 days
              fck = Characteristic compressive strength 
            s = standard deviation.
Note: Standard deviation can be calculated for each grade of concrete using at least 10 test strength of samples when a mix is used for 1st time.
In case sufficient test results are not available, the values of standard deviation may be assumed 
1. s=3.5 for M10 & M15.
2. s=4.0 for M20 & M25
3. s=5 for M30 & other. 

Step 2. Select the w/c ratio.: 

It is preferable to establish the relationship between the compressive strength and free water-cement ratio for a given set of materials.  If such relationship is not available, maximum w/c ratio for various environmental exposure conditions as given in Table 5 of IS 456. Any w/c ratio assumed based on the previous experience for a particular grade should be checked against the maximum value permitted.

Step 3. Select the water content.

The quality of water considered per cubic meter of concrete decide the workability of the mix.  The use of water reducing chemical admixtures in mix help to achieve increased workability at lower water content.

Step 4. Calculate the content of cementitious material.:

The cement and supplementary cementitious material content per volume can be calculated from the free w/c ratio of step 2.  The total cementitious material calculated should be checked against the minimum content for the requirements of durability and the greater of two values adopted.

Step 5. Estimate the proportion of coarse aggregate.: 

The following table gives the volume of coarse aggregate for different zones of fine aggregate for a w/c ratio of 0.5, which requires being suitably adjusted for other w/c ratios.

Volume of coarse aggregate per unit volume of total aggregate
 for different zones of fine aggregate

Step 6. Identify the combination of different sizes of coarse aggregate fractions.: 

Coarse aggregates from stone crushers are normally available in two sizes (20 mm and 12.5 mm). Coarse aggregates of different sizes can be suitably combined to meet the gradation requirements.

Step 7. Estimate the proportion of fine aggregate.: 

The quantity of cementitious material, water, and the chemical admixture is found by dividing their mass by their respective specific gravity and multiplying by 1/1000. The volume of all aggregates is obtained by subtracting the summation of the volume of these materials from the unit volume. From this, the total volume of aggregates, the weight of coarse and fine aggregate, is obtained by multiplying by their friction of volumes (already obtained in step 5) with the respective specific gravity and then multiplying by 1000. 

Step 8. Perform trial mixes.: 

The calculated mix proportions should always be checked by means of trails batches.  The concrete for trial mixes shall be produced by means of actual materials and production methods.