The concrete shall be in grades designated Group Grade designation **Characteristics compressive strength of 150 mm cube at 28 days, N/mm2.**

**Ordinary Concrete—**M10 M15 M20**Standard Concrete—**M25 M30 M35 M40 M45 M50 M55**High Strength Concrete—**M60 M65 M70 M75 M80**High performance concrete—**M 90, M95 M100, M105, M110, M115, M120.

Concrete mix can be classified as 1. Nominal mix & 2. Design mix

**1. Nominal mix:**

Nominal mix specifies the proportion of the cement , sand and aggregates **without making special effort to know their individual properties.** Nominal mix has Volumetric batching.

The proportions of materials for nominal mix shall be in accordance Grade Proportions C: FA: CA

**Grade | Mix ratio**

**M5 → 1:5:10 (Cement : sand: aggregate)**

**M7.5 →1:4:8**

**M10 → 1:3:6**

**M15 → 1:2:4**

**M20 → 1:1.5:3**

**M25 → 1:1:2**

**2. Design Mix Concrete:**

( From M30 onwards we need to design the concrete mix)

The **concrete mix produced under quality control keeping in view the strength, durability, and workability is called the design Mix.**

• Others factors like compaction equipment’s available, curing method adopted, type of cement, quality of fine and coarse aggregate etc. have to be kept in mind before arriving at the mix proportion.

• The design mix or controlled mix is being used more and more in variety of important structures, because of better strength, reduced variability, leaner mixed with consequent economy, as well as greater assurance of the resultant quality.

Eg:- M30, M35, M40, M45, M50 etc.

**And in counties like USA Grade of concrete denoted by, C15, C20, C25, C30 & C35 etc**

A concrete of Grade C25 means that the concrete will have a compression resistance of 25 N/mm2 per square millimeter in 28 days.

Proportion for grade C20.

**C20 is 1:2:4**(1 part cement, 2 parts fine aggregate, and 4 parts coarse aggregate.)

where as **C20 symbolizes a*** ***Cylindrical sample** in which cement, fine aggregates, coarse aggregates are mixed in such a ratio that its characteristic strength is 20 N/mm^2 after 28 days of curing

Generally **Strength of Cylinder sample**=** 0.8 x Strength of Cube.**

**example:- M20 is equivalent to C25**