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.
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)|
|High Strength Concrete||M60
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 :
- For R.C.C. work – not lower than M20
- For post tensioning works – M35 and above
- 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.