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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)

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