Content of this Article:

Concrete, Requirements of good concrete mix, Grades of concrete, The characteristic compressive strength of concrete

R.C.C. consists of concrete and reinforcing material. The strength of an R.C.C. section depends upon the kind of concrete and reinforcement used.

  Concrete

The properties of concrete depends upon the proportions and type of its ingredients. A properly designed concrete mix is very durable.

Requirements of good concrete mix

(i)        The concrete should be mixed thoroughly to from a homogeneous mix.

(ii)       Concrete should be compacted properly to prevent it from being porous.

(iii)     Sufficient curing of concrete is required for developing full strength.

(iv)      The water cement ratio should be appropriate, considering the strength and workability criteria.

(v)       The concrete mix should be designed properly and should have all the ingredients in right proportions.

(vi)      The water used for mixing should be free from all harmful organic substances.

(vii)    The aggregate should be hard, durable and properly graded. For most R.C.C. works, 20mm size of aggregate is suitable.

(viii)   The cement used for R.C.C. work should be of good quality and measured by weight only, and not by volume.

Concrete is graded or designated on the basis of its compressive strength. As per IS456:2000, concrete is graded into fifteen types as given in the Table 1.1 (Table 2, IS456:2000)

TABLE 1.1. Grades of Concrete

Group Designation Characteristic Compressive Strength, fck (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

Concrete grades are expressed by letter M followed by a number. The letter ‘M’ refers to the mix and the number represents the characteristic compressive strength of concrete in N/mm2.

The specified characteristic strength is determined for 150mm size cube at 28 days.

The characteristic compressive strength of concrete

The characteristic compressive strength of concrete is defined as that strength below which not more than 5 percent of the test results are expected to fall. The various grades of concrete as per their use are listed below :

  1. For R.C.C. work – not lower than M20
  2. For post tensioning works – M35 and above
  3. For pretensioned prestressed concrete – M40 and above

Concrete of grades lower than M20 may be used for plain concrete works, lean concrete, simple foundations, masonry walls and other simple constructions works.