Concrete mix ratio - Various grades of concrete - Concrete mix design

What is concrete?
Concrete is a stone like material obtained by permitting a carefully proportional mixture of cement, sand and gravel or other coarse aggregate and water to harden in forms of the shape and dimensions of the desired structure. Cement and water interact chemically to bind the aggregate particles into a solid mass. Concrete with a wide range of properties can be obtained by adjustment of the proportion of the constituent materials. The strength of this concrete mix is determined by the proportion on which these cement, sand, stones or aggregates are mixed. There are various grades of concrete available in the market based on these ratios. Some of them are: 

• M10
• M20 
• M30
• M35 etc. 

What really does M10 or M20 mean or represent ? 

“M” stands for “mix”. Mix represents concrete with designated proportions of cement, sand and aggregate. And the number following “M” represents compressive strength of that concrete mix in N/mm² after 28 days. For example, for M20 grade of concrete mix, its compressive strength after 28 days should be 20 N/mm².

Concrete mix ratio table

The standard chart table of Mix design of concrete. It is  showing various grades of concrete mix design along with their respective ratios of cement, sand and aggregates required.

Grades of Concrete	Ratios of Concrete mix design (Cement:Sand:Aggregate)
M5	1:5:10
M7.5	1:4:8
M10	1:3:6
M15	1:2:4
M20	1:1.5:3
M25	1:1:2
M30	1:0.75:1.5
M35	1:0.5:1
M40	1:0.25:0.5

As you can see in the table above, volume of sand is always kept half of that of aggregates in these standard mix designs. You can measure and maintain these ratios by using buckets or some other standard cubes which could be easily used throughout the project. It is necessary to maintain consistency in each and every concrete mix prepared during the entire project. It is one of the important job of site engineer/supervisor to inspect and enforce it.

Water content ratio in concrete mix

Besides water content also largely determine the strength & work-ability of concrete. Greater the amount of water, higher will be the work-ability of concrete (more fluid) however it reduces the strength of concrete. But if you keep water too low, work-ability of water will also reduce. Therefore, it will be difficult to place such concrete in the structure. Amount of water required may vary for same volume of concrete for various grades of concrete. Hence, a balance has to be found in the construction site during concrete mixing.

What is Grade of Concrete?

Grade of concrete is defined as the minimum strength the concrete must posses after 28 days of construction with proper quality control. Grade of concrete is denoted by prefixing M to the desired strength in MPa. For example, for a grade of concrete with 20 MPa strength, it will be denoted by M20, where M stands for Mix.

These grade of concrete is converted into various mix proportions. For example, for M20 concrete, mix proportion will be 1:1.5:3 for cement:sand:coarse aggregates.

 How to select suitable concrete grade for construction?

Grade of concrete construction is selected based on structural design requirements. There are two types of concrete mixes, nominal mix and design mix.

Nominal mix concrete are those which are generally used for small scale construction and small residential buildings where concrete consumption is not high. Nominal mix takes care of factor of safety against various quality control problems generally occurring during concrete construction.

Design mix concrete are those for which mix proportions are obtained from various lab tests. Use of design mix concrete requires good quality control during material selection, mixing, transportation and placement of concrete. This concrete offers mix proportions based on locally available material and offers economy in construction if large scale concrete construction is carried out.

Thus, large concrete construction projects uses design mix concrete.

So, suitable grade of concrete can be selected based on structural requirements. Nominal mixes for grades of concrete such as M15, M20, M25 are generally used for small scale construction.

Large structures have high strength requirements, thus they go for higher grades of concrete such as M30 and above. The mix proportions of these concretes are based on mix design.

Regular Grades of Concrete and their Uses

Regular grades of concrete are M15, M20, M25 etc. For plain cement concrete works, generally M15 is used. For reinforced concrete construction minimum M20 grade of concrete are used.

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