Discuss about concrete and its nominal proportions?
Concrete is a composite construction material made by mixing cement, fine aggregate (sand), coarse aggregate, and water in suitable proportions. Sometimes admixtures are also added to improve specific properties.
1. Nominal Proportion of Concrete
Nominal mix means concrete in which the ingredients are mixed in fixed, prescribed proportions, generally based on volume, without detailed laboratory mix design.
Common nominal mix proportions are:
Grade
Cement : Sand : Coarse Aggregate
M5
1 : 5 : 10
M7.5
1 : 4 : 8
M10
1 : 3 : 6
M15
1 : 2 : 4
M20
1 : 1.5 : 3
For example, M15 = 1 : 2 : 4 means:
- 1 part cement
- 2 parts sand
- 4 parts coarse aggregate
The required amount of water is added to obtain the desired workability.
2. Characteristics of Nominal Mix
- Proportions are predefined.
- Usually suitable for small or less critical construction.
- It does not account precisely for variations in aggregate properties, moisture, grading, etc.
- The actual strength obtained can vary.
- For important structural work, design mix is generally preferred.
3. Nominal Mix vs Design Mix
Nominal mix: Proportions are prescribed beforehand.
Design mix: The proportions of cement, water, fine aggregate and coarse aggregate are determined through calculations and/or laboratory trials to achieve the required strength, durability and workability.
Example:
If concrete is specified as M20, a nominal proportion historically associated with M20 is 1 : 1.5 : 3, but under modern practice, structural concrete is normally proportioned by design mix, rather than simply assuming this nominal ratio.
Thanks for Visiting:)
