Nominal Mix
Definition: The concrete is extensively applied as construction materials and several mixes of fixed ratio are now being used with concrete to retain adequate strength. These mixes are known as nominal mixes.
Under normal situation, it contains margin of strength over that specified.
Nominal mix concrete is applied for concrete of grades M5, M7.5, M10, M15 and M20.
Ratios of ingredients in nominal mixes
| Concrete Grade | Mix Ratio | Compressive Strength |
|---|---|---|
| M7.5 | 1 : 4 : 8 | 7.5 MPa |
| M10 | 1 : 3 : 6 | 10 MPa |
| M15 | 1 : 2 : 4 | 15 MPa |
| M20 | 1 : 1.5 : 3 | 20 MPa |
With 1:1.5:3 ratio, it signifies that these should be measured as 1 kg of cement, 1.5 kg of fine aggregate and 3 kg of coarse aggregate.
But, usually they are selected on the volume basis and multiplied with 1.55 constant (in Nominal Mix Concrete Calculation) to obtain the bulk volume of the material.
Design Mix
Definition: The concrete mix created under quality control on the basis of the strength, durability and workability in the laboratory tests is known as the design mix.
Prior to arrive at the mix ratio, several other factors should be taken into consideration which range from accessibility of equipment for compaction, curing process selected, type of cement, quality of fine and course aggregate etc.
The design mix or controlled mix is extensively utilized for different types of vital structures due to superior strength; leaner mixed with consequent economy and improved quality.
In design mix, the concrete is formed in weight basis especially for greater volume where the load is vital. It is required to examine each and every property of the ingredients mentioned below.
Weight of each ingredient
• Brands
• Mix Proportions
• Type of exposure
• Properties of Cement: Initial & Final Setting Time, Specific gravity, Cement Grades
• Properties of Fine & Coarse Aggregates – Particle Size, Silt Content, Unit Weight, Fineness Modulus, Crushing Value
| Standard Grade of Concrete | |||
| M25 | 1 : 1 : 2 | 25 MPa | 3625 psi |
| M30 | Design Mix | 30 MPa | 4350 psi |
| M35 | Design Mix | 35 MPa | 5075 psi |
| M40 | Design Mix | 40 MPa | 5800 psi |
| M45 | Design Mix | 45 MPa | 6525 psi |
High Strength Concrete Grades
| M50 | Design Mix | 50 MPa | 7250 psi |
| M55 | Design Mix | 55 MPa | 7975 psi |
| M60 | Design Mix | 60 MPa | 8700 psi |
| M65 | Design Mix | 65 MPa | 9425 psi |
| M70 | Design Mix | 70 MPa | 10150 psi |
On the basis of the above things, the mix design will be set up as per requirements and then examined with trial mix. As soon as the trial mix is examined and passed on 7th & 28th Days for compressive strength, then it will be considered for construction purposes.
Civil-Ideas 