IMPORTANT PROPERTIES OF CONCRETE

The following are the important properties of concrete are to be noted by designer-

1. Through it consists of different materials like cement, sand and jelly the intimate mixture is so good that for all practical purposes it may be assumed as homogeneous.
2. For concrete characteristics strength is defined as compressive strength of 150 mm cube at 28 days in N/mm2, below which not more than 5 percent cubes gives the result. Based on the characteristics strength concrete is graded as given below:
   Grades of Concrete
   

   Group	Grade Designation	Characteristics Strength (N/mm2)
   Ordinary Concrete	M10 M15 M20	10 15 20
   Standard Concrete	M25 M30 M35 M40 M45 M50 M55	25 30 35 40 45 50 55
   High Strength Concrete	M60 M65 M70 M75 M80	60 65 70 75 80

   Now a days ultra high strength of grade M500 are also produced in the laboratories amd M250 concrete has been used for the construction of some bridges.                      Minimum Grades of concrete for different exposure with normal weight aggregates of 20 mm nominal maximum size.

   S. No.	Exposure	Minimum Grade of Concrete
   1	Mild	M20
   2	Moderate	M25
   3	Sever	M30
   4	Very Sever	M35
   5	Èxtreme	M40

3. Stress Strain Relationship: Stress strain curve depend on strength of concrete as well as on the rate of loading. The sort term stress strain curve is to be obtained for a constant rate of straining of 0.01 percent per minute or for a constant rate of stress increases of 14 N/mm2 per minute.
4. Tensile Strength: A designer may use the following expression for the flexceral tensile strength of concrete: 
                                                    Fcr = 0.7√fck N/mm2                                                Where fck= Characteristics compressive strength of concrete.
5. Modulus of Elasticity: The short term modulus of elasticity for concrete may be taken as:                                                                                                                                                                                 Ec = 5000√fck 
6. Poisson's Ratio: It may be taken as 0.1 for high strength concrete and 0.2 for weak concrete. Usually it is taken as 0.15 for strength and 0.2 for serviceability calculations.
7. Shrinkage: Total amount of shrinkage in concrete depends on the various factors including the  amount of water present at the time of casting. In the absence of data the approximate value of the total shrinkage strain may be taken as 0.0003.
8. Creep: It depends on various factors including the age of loading, duration of loading and stress level. The creep coefficient which is defined as the ratio of ultimate creep strain to elastic strain at the age of loading may be taken as shown below:

   S. No.	Age of Loading	Creep Coefficient
   1	7 Days	2.2
   2	28 Days	1.6
   3	1 Year	1.1

 (IS: 456-2000)