What is Design Mix? & Type of Mixes.
Concrete Mix Design
The concrete design simply means choosing well suited materials for good quality of concrete and determining their relative quantities. The main purpose of designing concrete mix is to produce workable and durable concrete with required strength.
Requirement of Concrete Design:
Requirements which are necessary for the selection of materials for concrete:
- Minimum compressive strength.
- Adequate workability required for full compaction.
- Maximum water-cement ratio/ maximum cement content.
Types of Mixes:
- Nominal Mixes:
Earlier, the specifications for concrete comprised of fixed proportions of cement, fine aggregates and coarse aggregates. These fixed proportions of cement-aggregate are known as Nominal Mixes. Due to varying quantities of ingredients, the workability of concrete becomes variable too.
- Designed Mixes:
This is the most rational method to select mix proportions for concrete. It takes into consideration the performance of the quantity according to which the mix is designed. This method does not guarantee accurate mix proportions as for the performance and so it is recommended to use nominal or standard mixes for undemanding concrete performance whose strength do not exceed 30 N/mm2 in 28 days.
Following proportions have been standardized:
- M10 – 1:3:6
- M15 – 1:2:4
- M20 – 1:1.5:3
- M25 – 1:1:2
Concrete Mix Design Procedure:
The basic steps to design concrete mix are as follows:
- Firstly, the mean target strength is calculated from the given characteristic strength
- For the calculated mean target strength, the water-cement ratio is calculated and checked for durability.
- The water content is determined for expected workability.
- Now, the quantity of cement can be found out from the W/C ratio in ii step and water-content in the 3 step.
- According to the cement content, the quantities of sand and coarse aggregates are also taken out.
- Trial mix proportions are estimated.
- These trial mixes are then tested for compressive strength and required adjustments are made to reach a final mix
Methods of Concrete Mix Design:
- Trial and Error Method,
- Road Note No. 4 Method,
- ACI Mix Design Method,
- The British Mix Design Method,
- The Mix Design Method according to IS recommended guidelines.