The article discusses concrete mix ratios, which are the proportions of various components of concrete such as cement, sand, aggregates, and water. The selection of these ratios depends on the type of construction and mix designs. Building codes provide nominal and standard concrete mix ratios based on previous experience and testing. These ratios are used for various construction works.
Types of Concrete Mix Ratio – Mix Designs
Nominal Concrete Mix Ratios
Concrete specifications in the past used to determine the required proportions of cement, fine and coarse aggregates. These prescribed mixes of fixed cement-aggregate ratio were known as nominal mixes, which ensured sufficient strength. Nominal mixes were preferred for their simplicity and, in most cases, had a strength margin above the specified level. However, because of the variability in mix ingredients, the strength of nominal concrete varied significantly for a given workability.
Nominal mix ratios for concrete were typically 1:2:4 for M15 and 1:1.5:3 for M20, among others. These ratios were established to ensure that the concrete mixture would have adequate strength for its intended use. Nevertheless, due to the inconsistent quality of ingredients, the actual strength of the concrete could vary considerably from the intended strength. As a result, nominal mixes were not always reliable in producing consistent results, which could pose a problem in construction projects.
Standard Mixes or Ratio
The nominal mixes of cement and aggregate are often fixed in a particular ratio by volume. However, such mixes can vary in strength and may result in either under or over-rich mixes. To address this issue, many specifications have included a minimum compressive strength requirement. These mixes are known as standard mixes and have been designated into several grades by IS 456-2000, namely M10, M15, M20, M25, M30, M35, and M40.
In this designation, the letter “M” represents the mix, and the number indicates the specified 28-day cube strength of the mix in N/mm2. The mixes of grades M10, M15, M20, and M25 roughly correspond to the mix proportions of (1:3:6), (1:2:4), (1:1.5:3), and (1:1:2), respectively.
Designed Mix Ratio of Concrete
The selection of mix proportions for concrete is a crucial aspect in determining its properties. In cases where specific materials possess unique characteristics, it is rational for the designer to specify the performance of the concrete, while leaving the determination of mix proportions to the producer. This approach ensures the production of concrete with the desired properties in a cost-effective manner. However, it should be noted that the designed mix may not guarantee the correct mix proportions for the desired performance.
For concrete with undemanding performance, nominal or standard mixes may be used. These mixes are prescribed in the codes by the quantities of dry ingredients per cubic meter and by slump. However, they are only suitable for small jobs and when the 28-day strength of concrete does not exceed 30 N/mm2. In such cases, no control testing is necessary, and reliance is placed on the masses of the ingredients.
The following table provides details of different types of concrete mix ratios and their corresponding strengths.
| Concrete Grade | Mix Ratio | Compressive Strength | |
| MPa (N/mm2) | psi | ||
| Normal Grade of Concrete | |||
| M5 | 1 : 5 : 10 | 5 MPa | 725 psi |
| M7.5 | 1 : 4 : 8 | 7.5 MPa | 1087 psi |
| M10 | 1 : 3 : 6 | 10 MPa | 1450 psi |
| M15 | 1 : 2 : 4 | 15 MPa | 2175 psi |
| M20 | 1 : 1.5 : 3 | 20 MPa | 2900 psi |
| 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 |