What is Concrete Mix Design?
The process of selecting suitable ingredients of concrete and determining their relative amounts as economically as possible is termed as the Concrete Mix Design. The proportioning of the ingredients of concrete is governed by the required performance of concrete in the two states. i.e the
- The Plastic State
- The Hardened State
If the plastic concrete is not workable, it cannot be properly placed and compacted. The property of workability is hence very important. The compressive strength of the hardened concrete which is generally considered to be an index of its properties mainly depends upon many factors. They are:
- Quality and Quantity of Cement
- Water and Aggregates
- Batching and Mixing
- Placing and Compaction
Objectives of Concrete Mix Design
The main objectives of concrete mix design are:
- To ensure that the most optimum proportions of the constituent materials are obtained in order to fulfill the requirement of the structure built.
- To achieve the desired and designed workability in the plastic state of concrete
- To attain the minimum strength at its hardened stage
- To achieve the desired durability in the given and considered environment conditions
- To produce concrete as economical as possible
Cost Consideration in Concrete Mix Design
The cost of concrete is made up of the cost of materials, plant, and labor. The variations in the cost of materials arise from the fact that the cement is several times costly than the aggregates. Thus the main aim is to produce a lean mix as possible. From the technical point of view, the rich mixes may lead to high shrinkage and crack in the structural concrete. The rich mix also results in the evolution of high heat of hydration in the concrete mass, that will also result in the cracking.
Types of Mixes in Concrete Mix Design
The two main types of mixes in Concrete Mix design are:
- Nominal Mix
- Design Mix
The Nominal Mix gains a fixed cement-aggregate ratio that will ensure adequate strength. These offer simplicity. These also provide a margin of strength above that is specified under normal circumstances. However, due to the variability of the mix ingredients the nominal concrete for a given workability varies widely in strength. These mixes are hence used for less important works. The nominal mix will gain a higher cement content which makes them uneconomical.
According to IS 456-2000, the nominal mix concrete may only be used for concrete with grade M20 or lower i.e. M5, M7.5, M10, M15, and M20. The proportion of the materials shall be in accordance with the following table as in IS: 456-2000.
The proportions of the materials for nominal mix design is given in the table-1.
|Table:1: Nominal Mix Proportion of Concrete Grade From M5 To M 20|
In this method, the mix proportion is calculated by using standard design procedures. The mix design depends upon various factors like the strength, durability, workability and many other factors. This approach results in the production of concrete with the appropriate properties most economically.
Other factors like compaction equipment available, the curing method adopted, the quality of fine and coarse aggregates, the type of cement etc have to be kept in mind before arriving into the mix proportion.
The design mix is implemented for more variety of important structures, because of the better strength attained. This also shows reduced variability, leaner mix with consequent economy and assurance for the resultant quality.