Grades of Concrete

GRADES OF CONCRETE

(Ref : IS 456: 2000 http://www.iitk.ac.in/ce/test/IS-codes/is.456.2000.pdf )

IS 456-2000 has designated the concrete mixes into a number of grades Say M10, M20 etc.,

In this designation the letter M refers to the mix and the number to the specified 28 day cube strength of 150mm size cube in N/mm²

Table 1: Grades of Concrete

Group	Grade Designation	Specified Characteristic Compressive Strength of 150mm cube at 28 days in (MPa) N/mm2
Ordinary Grades of Concrete	M10	10
	M15	15
	M20	20
Standard Concrete	M25	25
	M30	30
	M35	35
	M40	40
	M45	45
	M50	50
	M55	55
High Strength Concrete	M60	60
	M65	65
	M70	70
	M75	75
	M80	80

To concert Mpa to psi multiply by x145.038

1Mpa = 145.038 psi

Table 2: Nominal Mix Ratio

Grade Designation	Mix Ratio
M10	1:3:6
M15	1:2:4
M20	1:1.5:3
M25	1:1:2
M30 to M80	Design mix shall be carried out
Ratio of 1:3:6 refers to 1 Part of cement; 2 Parts of Sand and 3 Parts of aggregates
Calculation of materials for concrete can be obtained at https://civilengineeringdays.blogspot.com/2020/05/quantities-of-materials-for-concrete.html

Concrete of grades lower than those given in Table 1 may he used for plain concrete constructions.

Lean Concrete:

Lean concrete is one in which the amount of liquid in the strata is higher than the cement.

Other Grades

M5 – 1:5:10

M7.5 – 1:4:8

As per IS 456:2000, Grades less than M20 should be used for Reinforced Concrete

Properties of Concrete

1. Increase of Strength with Age

There is normally a gain of strength beyond 28 days. The quantum of increase depends upon the gradc and type of cement, curing and environmental conditions, etc.

The design should be based on 28 days charactcristic strength of concrete unless there is an evidence tto justify higher strengths for particular structure

For concrete of grade M 30 and above. the rate of increase of compressive strength with age shall be based on actual investigations.

2. Tensile Strength of Concrete

Tensile strength from the compressive strength, the following formula may be used:

Flexural strength, f_cr =0.7.(f_ck )^^0.5 N/mm²

where f is the characteristic cube compressive strength of concrete in N/mm²

3. Elastic Deformation

The modulus of elasticity of concrete can be assumed as follows:

E_c = 5000 (f_ck )^^0.5 N/mm²

E_c is the short term static modulus of elasticity in N/mm²

4. Shrinkage

The total shrinkage of concrete depends upon the constituents of concrete, size of the member and environmental conditions.

In the absence of test data, the approximate value of the total shrinkage strain for design may be taken as 0.0003

5. Creep of concrete

Creep of concrete depends, in addition to the factors listed in shrinkage, on the stress in the concrete. age at loading and the duration of leading

Age at loading	Creep coefficient
7 days	2.2
28 days	1.6
1 year	1.1

6. Thermal Expansion

The coefficient of thermal expansion depends on nature of cement, the aggregate., the cement content, the relative humidity and the size of sections.

Type of Aggreegate	Coefficient of Thermal Expansion for Concrete /oC
Quartzite	1.2 to 1.3 x 10-5
Sandstone	0.9 to 1.2 x 10-5
Granite	0.7 to 0.95 x 10-5
Basalt	0.8 to 0.95 x 10-5
Limestone	0.6 to 0.9 x 10-5