Concrete Mix Design
Concrete mix design is the method of optimally proportioning various constituents of concrete, such as cement, aggregates, water, and admixtures, in order to manufacture a concrete at a lower cost that 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 slump for easy placing and consolidating.
2. Meets the strength requirements.
3. It 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 mean compressive strength is 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: A standard deviation can be calculated for each grade of concrete using at least 10 test samples, when a mix is used for 1st time.
In cases where 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 cementious material content per volume can be calculated from the free w/c ratio of step 2. The total cementious 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 : Concrete Mix Design
Following table gives the volume of coarse aggregate for different zones of fine aggregate for a w/c ratio of 0.5, which requires to be suitably adjusted for other w/c ratios.
Step 6. Identify the combination of different sizes of coarse aggregate fractions.:
Coarse aggregates from stone crushes 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 cementious 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 sum of the volumes 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 frictional volumes (already obtained in step 5) with the respective specific gravity and then multiplying by 1000.
Step 8. Perform trial mixes.: Concrete Mix Design
The calculated mix proportions should always be checked by means of trail batches. The concrete for trial mixes shall be produced by means of actual materials and production methods. Concrete Mix Design