Concrete is one of the most famous materials in construction industry, and is comprised of a mixture of several materials such as cement, fine and coarse aggregates, and water. In fact, the aggregates bond together by cement and water within the mixture. Concrete specifications rely on the type, quality, and quantity of these ingredients in the mixture.

Among all characteristics, concrete strength is mostly determined by its water-to-cement ratio, and for example, having two identical fresh batches of concretes with the same mixture, adding more water to it lowers its final strength.

Grade of Concrete

Concrete mixes are classified by their grade, which pertains the minimum strength they will have after 28 days of construction with standard quality control and measurements and the strength is the amount of pressure needed to crush a 15 cm x 15 cm x 15 cm cube, which is measured in MPa. As an example, the grade of a concrete with strength of 10 MPa is denoted by M10. Each of the standard grades correspond to a specific mix proportion either.

Regular Concrete Grades

According to IS 456:2000, concrete is graded into 15 types which is shown in the below table:

Concrete Grade	Mix Ratio	Compressive Strength (MPa)	Compressive Strength (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

Regularly used grades of concrete are M15, M20, and M25. For construction projects with plain cement concrete, generally M15 grade is used and for reinforced concrete construction minimum grade of M20 is used.

Types of Mix Ratios

Concrete mix ratios indicate the proportions of cement, sand, aggregates and water components within concrete. The mixes are selected according to the construction type, project requirements, and mix designs. There are three types of mix rations, which are nominal mixes, standard mixes, and design mixes, that are described below.

• Nominal Mix

As the first standards, only the proportion of components, i.e. cement, fine and coarse aggregates, sand, and water, which ensured adequate strength were used to classify concrete grades. These mixes (Nominal Mixes) offer simplicity to users and under normal conditions provides concrete with strength above the specified minimum. However, since many parameters such as the variability of ingredients and their qualities play important role in the concrete strength, the nominal mixes vary widely in strength.

• Standard Mix

As it was mentioned above, the nominal mixes of fixed cement-aggregate ratio can result in widely variable strength. Therefore, in standard mixes, the minimum compressive strength has also been included in the specifications. As shown in the above table, the concrete mixes are categorized by IS 456-2000 into different grades such as M10 with mix proportions 1:3:6, M15 1:2:4, etc.

• Design Mix

In the design mix, the concrete strength is specified by the designer and the mix proportions of the concrete components are determined by the concrete manufacturer. In this way, the designer ensures that the mix properties and concrete strength are appropriate for the project and the manufacturer ensures the most economic production procedure is implemented. However, the designed mix cannot be used as a guide as the correct mix proportions for the required performance is not guaranteed.