8 Factors Influencing the Choice of Mix Proportions
According to IS 456-2000 and IS 1343-1980, the designs of the concrete mix should be based on the following factors:
1. Grade Designation
The grade designation gives characteristic compressive strength requirements of concrete. It is the major factor influencing the mix design.
The concrete mix has to be designed for a target mean compressive strength which is somewhat higher than the characteristic compressive strength.
2. Types and Grades of Cement
The rate of development of compressive strength of concrete depends upon the type & grade of cement used. The choice of the type of cement depends upon the requirements of performance at hand.
Where very high compressive strength is required, Portland cement of grades 43 and 53 will be found suitable.
3. Maximum Nominal Size of Aggregates
The workability and compressive strength of concrete greatly depend upon the maximum size of aggregates. The workability increases with an increase in the maximum size of the aggregate.
On the other hand use of the large maximum size of aggregate, requires a smaller quantity of cement for a particular water-cement ratio. However, the smaller size aggregates provide a large surface area for bonding with the mortar matrix which increases the compressive strength.
4. Grading of Combined Aggregate
The grading of combined aggregate i.e., the relative proportion of the fine & coarse aggregates in a concrete mix is one of the important factors affecting the strength and workability of concrete. For dense concrete, it is essential that the coarse and fine aggregates be well grade.
5. Water-Cement Ratio
At a given age and under normal temperature, the compressive strength of concrete depends primarily on the water-cement ratio. The lower the water-cement ratio, the greater the compressive strength & vice-versa.
Insufficient workability of concrete may be liable for incomplete compaction of concrete which ultimately affects the strength, durability & surface finish of the concrete.
The durability of concrete is its ability to resist deterioration due to weathering action, chemical attack, abrasion etc. The requirements of durability are achieved by restricting the minimum cement content & the maximum water-cement ratio.
From the consideration of permeability, the water-cement ratio is usually restricted to 0.45 to 0.55. For a given water-cement ratio, the cement content should correspond to the required workability considering the placing conditions and the concentration of reinforcement.
In addition, the cement content is chosen to ensure sufficient alkalinity to provide a passive environment against the corrosion of steel.
8. Quality Control
The strength of concrete may vary from batch to batch over a period of time. The source of variation in the strength of concrete may be considered due to the variation in the quality of materials, mix proportion, mixing equipment, supervision & workmanship. The factor controlling this variation is quality control. The degree of control is ultimately evaluated by the variation in test results.