Concrete – Properties, Types and Slump - Materials & Construction

What is Concrete?

Concrete is a mixture of cement, sand, crushed rock and water which when placed in the skeleton of forms and allowed to cure, becomes hard like a stone. The subject which deals with the fundamental principles of concrete is known as Concrete Technology. The practical utility of Concrete Technology enables the civil engineers to know how to stock materials properly for concrete and perform different tests on concrete.

Properties of Concrete

1. Fresh Concrete Properties :

i) Workability : It is defined as the ease with which it can be mixed, transported and placed in position in a homogenous state. It depends upon quality of water, grading of aggregate and percentage of fine materials. A concrete should have good workability.

ii) Segregation : It is defined as the separation of cement paste from aggregate. It results in honey combing, decrease in density and loss in strength of hardened concrete. The concrete should be free from segregation.

iii) Bleeding : It is defined as the separation of water or water-cement mixture from freshly mixed concrete. It makes concrete porous and weak.

iv) Harsh : It is the resistance offered by concrete to its surface finishing. The surface of harsh concrete remains harsh and porous.

2. Hardened Concrete Properties :

i) Strength : The strength of hardened concrete should be high enough to resist heavy loads of the structures. It should ≥ 15.5 N/mm².

ii) Durability : The hardened concrete should be durable to resist the effects of rain, frost action etc. This property is mainly affected by water-cement ratio.

iii) Impermeability : The hardened concrete should have sufficient water tightness so that it can resist the entry of water inside the structure.

iv) Shrinkage : The hardened concrete should exhibit minimum shrinkage.

v) Creep : The continuous strain under the action of external loads is called creep. The hardened concrete should be subjected to minimum creep.

vi) Thermal Expansion : The hardened concrete should have minimum thermal expansion so as to provide good resistance to fire.

Types of Concrete :

1. Plain Cement Concrete (PCC) : The PCC consists of cement, sand and coarse aggregates mixed in suitable proportions in addition to water. The cement act as binding material, sand as fine aggregate and gravel, brick or crushed stone as coarse aggregates. PCC is strong in taking compressive stresses but has negligible tensile strength.

Usual Proportions – Cement ( 1 part) : Sand ( 1.5 to 8 parts) : Coarse aggregate (3 to 16 parts).

2. Lime Concrete : The lime concrete consists of lime, fine aggregate and coarse aggregate mixed in suitable proportions in addition to water. The lime is used as a binding material, sand as fine aggregate and broken bricks or stones as coarse aggregate.

Usual Proportions – Lime (1 part) : Sand ( 2 to 3 parts) : Coarse aggregate (3 to 4 parts).

3. Reinforced Cement Concrete (RCC) : The RCC is a type of concrete in which reinforcement is embedded. The RCC is equally strong for taking tensile, compressive and shear stresses.

4. Pre-stressed Cement Concrete : The pre-stressed cement concrete is a concrete in which high compressive stresses are artificially induced before its actual use. This type of concrete can take up high tensile and compressive stresses without development of cracks.

5. Light Weight Concrete : The Light weight Concrete is prepared by using coke-breeze, cinder or slag as aggregate in cement concrete. This type of concrete possesses high insulating property. It is used in making precast structural units for partition and wall lining purposes.

6. Cellular or Aerated Concrete : This type of concrete is prepared by mixing aluminium in cement concrete. It is light in weight and spongy in structure. It is used for roof slab and precast units in partitions.

7. Saw Dust Concrete : This concrete is prepared by mixing Portland cement with saw dust in specified proportions in the concrete. It is used as a heat and sound insulating material.

8. Vacuum Concrete : The vacuum concrete is a concrete from which entrained air and excess water are removed with a vacuum pump, after placing it in position. The removal of excess air helps in increasing the strength of concrete by 15% to 20%. It is used for all reinforced concrete work.

What is Slump in Concrete?

Slump is the measure of consistency of fresh concrete before it sets. It is performed to check the workability of fresh concrete. Slump is performed using slump cone test.

Slump for different concrete works are as follows :

Important Note :

1. Recommended Proportioning for different grades of concrete :

M 5 – 1 : 5 : 10

M 7.5 – 1 : 4 : 8

M 10 – 1 : 3 : 6

M 15 – 1 : 2 : 4

M 20 – 1 : 1.5 : 3

M 25 – 1 : 1 : 2

Remembering Technique = 1 : n : 2n

Cement : Sand : Coarse Aggregate

2. For lean concrete and simple foundations for masonry walls M 5 and M 7.5 grades of concrete may be used.

3. For RCC work ≥ M 20 grade of concrete must be used.

4. w/c ratio for different grades of concrete :

M 5 – 1.2

M 7.5 – 0.9

M 10 – 0.68

M 15 – 0.64

M 20 – 0.60

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