Concrete

        The mixture of cement, sand, water and aggregate is known as Concrete. Aggregate must be coarse aggregate (which is also called as Khadi) and fine aggregate (i.e. sand).

Difference between coarse aggregate and fine aggregate:

Coarse aggregate

The aggregates having size greater than 4.75mm is known as coarse aggregate 

Or 

The aggregates which does not passing through 4.75mm IS sieve is known as coarse aggregate.

Fine aggregate 

The aggregates having size less than 4.75mm and greater than 75microns is known as fine aggregate.

Various grades of concrete

    Depending upon cement, sand, aggregate and water content the concrete can make in different grades with different properties and strength. Following are the different grades of concrete with their ratio.
1. M10 (1:3:6)
 i.e. 1 part of cement 3 part of sand and 6 part of coarse aggregate.
2. M15 (1:2:4)
3. M20 (1:1.5:3)
4. M25 (1:1:2)

Here M stands for mix and number for compressive strength of cube which is tested after 28 days and it's unit is N/mm2. The cube specimen size will be 15cm x 15cm x 15cm using for testing of compressive strength of concrete.
The compressive strength of concrete is tested either in CTM (i.e compression testing machine) or UTM (i.e Universal testing machine).

Properties of concrete 

    The freshly mixed concrete or when the concrete in plastic state having the following properties 
1. Workability
It is defined as the ease with which concrete can be placed, finished and consolidated without loss of its homogeneity (or minimum loss of homogeneity).The workability depends on it water cement ratio.

Factors affecting workability of concrete:

1. Water cement ratio 

2. Mix proportion

3. Size of aggregate

4. Shape of aggregates

5. Surface texture of aggregates

6. Grading of aggregates

7. Use of admixtures

2. Segregation
It can be define as the separation of the constituent of materials of concrete
Or
Segregation means non uniform distribution of constituent of materials of concrete.
Types of segregation
1. Coarse aggregate settles down when concrete poured
2. Matrix paste separate from the paste
3. Water separates from the ingredients of concrete

3. Bleeding
In case of bleeding water comes to the top of concrete. It is particular form of segregation.
When water comes the top surface of concrete the certain quantity cement also comes with water which is bad for concrete to some extent.
Bleeding can be reduced by:
1. Proper proportioning of materials
2. Unifor and complete mixing
3. Using air entraining agents
4. Using cement with low alkali content.

When the concrete in hardened state should have following properties.
1. Compressive Strength
2. Durability
3. Impermeability
4. Shrinkage
5. Creep
6. Thermal expansion.