Concrete:

• Composite mixture of binder, aggregate, water and sometimes admixture.
• Commonly used building material.

Merit:

• Ability to cast in any shape
• Economic
• Durable
• Fire and water resistant
• High strength

Demerit:

• Low tensile strength
• Low toughness
• Low curing time

Uses:

• Resistance to weather and fire resistant
• Strength increases with time
• Economical and energy efficient
• Durable

1.2 Constituent of concrete:

1. Coarse and Fine aggregate:

• Chemically inactive material i.e. inert material.
• Obtain from disintegration of strong rock. Example : gravel, pebble, sand etc.
• Contributes 70 to 80 % of volume of hardened concrete.

Properties of good aggregate:

• Must be clean, hard, strong, dense, durable, properly shaped, well graded.
• Inertness and soundness.
• High strength and toughness.
• High bond strength.
• Angular shape aggregate are good due to good interlocking.

2. Cement:

• Chemically active ingredient of concrete which shows binding properties after reaction with water.
• Manufactured by dry and wet process.
• Components used for making cement are:

i. Limestone, clay, chalk-lime (60%)

ii. Silica (20%)

iii. Aluminium (10%)

Iv. Others: Iron oxide, carbon dioxide etc.

Bougie compound:

Bougie compound are formed during hydration of cement.

1. Tricalcium silicate: Responsible for initial setting and early strength.

2. Dicalcium silicate: Provide good ultimate strength.

3. Tricalcium aluminate: First compound to react with water.

4. Tetra calcium aluminate ferrite: Poor cementing value and less active.

Types of cement:

• Ordinary Portland cement
• Portland pozzolana cement
• Rapid hardening cement
• White cement
• Colored cement
• Acid resistant cement
• Low heat cement
• Quick setting cement

Test of cement:

• Fineness
• Setting time
• Soundness
• Specific gravity
• Tensile strength
• Compressive strength

3. Admixture:

• A material other than basic ingredient like cement, aggregate and water which are used to alter the properties of concrete for specific purpose such as workability, w/c, setting time and strength can be changed.
• Added in powder or liquid form.

Purpose:

1. To modify fresh properties:

• Increasing the workability without changing water cement ratio.

(This process is also known as plasticizer.)

• Retard or accerate the intial setting time.
•  Modify rate of bleeding.

2. To modify harden properties:

• Reduce the heat of evolution.
• Accelerate the rate of strength developed at early stage.
• Increase durability.

Types of admixture:

1. Chemical admixture:

• Material is form of powder or fluid that are added to concrete to give certain characteristics.
• Less than 5% added at the time of mixing.
• Accelerator: Increase rate of hydration.
• Retarder: Slow down rate of hydration.
• Entrapped air:

               Air bubble are formed inside concrete unintentionally due to less compaction.

• Entrained air:

Air bubble is added intentionally to concrete to change the properties of concrete.

2. Mineral admixture:

• Very fine grain inorganic material which are added to concrete to improve properties of concrete.
• Needs large amount of volume compare to chemical admixture.

Use:

• Increase water tightness.
• Increase early strength.
• Increase workability.
• Decrease heat evolution.

References:

• Dayaratnam, P. Brick and reinforced brick structure.
• Neville, A.M. Properties of Concrete. England: Pearson Education Limited.
• Hendry, A.W., Sinha, B.P. & Davies, S.R. Design of Masonry Structure. London: E & FN Spon.

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