Properties and Classification of Concrete in Building Materials3 min read

Introduction:


Concrete is a composite man-made material and is the most widely used material in the construction Industry.

It consists of a rationally chosen mixture of binding material such as lime or cement, well-graded fine and coarse aggregate, water, and admixtures sometimes (to produce concrete with special properties).

Freshly mixed concrete before set is known as wet or green concrete whereas after setting and hardening it is known as a set or hardened concrete.

Most of the ancient structures and historical buildings had been constructed with lime concrete. With the advent of cement, the use of lime concrete has been confined to making bases for concrete foundations and roof terracing.

The major factors responsible for the wide usage of cement concrete are modulability, early hardening, high early compressive strength, development of desired properties with admixtures to be used in adverse situations, suitability for guniting, pumpability and durability.

Properties of Concrete:


  • It should have high compressive strength as compared to its own tensile strength.
  • Free from corrosion and there is no appreciable effect of the atmospheric agent on it.
  • It should be more economical than steel for RCC work.
  • It should harden with age and the process of hardening continues for a long time.
  • It binds the steel material in RCC work so that the bond strength increases.
  • The concrete should have a tendency to shrinkage.
  • The basic requirement of good concrete is that it should have good strength in a hardened state and remain fresh during transportation, placing, and compaction.

Classification of Concrete:


1. Based on Cementing Material

Concretes are classified as lime concrete, gypsum concrete, and cement concrete.

2. Based on Perspective Specification

Cement concrete is specified by the proportions (by weight) of different ingredients. e.g.: 1 (cement): 1.5 (fine aggregate): 3(coarse aggregate)

It is presumed that by adhering to such perspective specifications satisfactory performance may
be achieved.

This type of concrete mix is known as a nominal mix.

Conventional nominal mix proportions have limited significance since the quantity of fine aggregates is fixed irrespective of the cement content, water-cement ratio, and maximum size of aggregates to be used.

Mixture proportion of cement concrete:

Grade of concrete M5 M10 M15 M20 M25
Mix
proportions
1:5:10 1:3:6 1:2:4 1:5:3 1:1:2
Perspective
characteristics
strength (N/mm²)
5 10 15 20 25

Here M refers to mix, IS:456 restricts the use of nominal mix up to M20 grade only.

3. Based on Performance-Oriented Specifications

When the concrete properties such as strength, water-cement ratio, compaction factor, slump, etc. are specified then concrete can be classified as designed mix concrete.

In a design mix concrete, the mix is designed to produce the grade of concrete having the required workability and a characteristics strength not less than the specified values.

4. Based on the Grade of Cement Concrete

On the basis of strength (N/mm²) of concrete cubes (150 mm sides) at 28 days, concrete is classified as:

Grade

Characteristics

M5 M7.5 M10 M20 M25 M30 M35 M40 M45 M50 M55
Strength (N/mm²) 5 7.5 10 20 25 30 35 40 45 50 55

It is further classified as:

(i) Low strength concrete (<20 N/mm²)
(ii)Medium strength concrete (20-55) N/mm²
(iii) High strength concrete (> 60 N/mm²)

5. Based on Bulk Density

Extra light weight < 500 kg/m³
Light weight 500-1800 kg/m³
Dense weight 1800-2500 kg/m³
Super heavy weight > 2500 kg/m³

6. Based on Place of Casting

When concrete is made and placed in position at the site it is known as in-situ concrete and when used
as a material for making prefabricated units in a factory is known as precast concrete.

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