Concrete - definition of concrete

Concrete 

After water, concrete is the most used thing in the world. The mixer of cement ,water and coarse aggregate is called Concrete.

Properties of concrete:- Concrete is divided into two stages:-

1. Plastic stage
2. Hardened stage

• Plastic stage :-The time duration when concrete begins to take their original shape after the removal of stress or strain. At the time of plastic stage concrete should have:-

• Good workability
• Freedom from segregation
• freedom from bleeding 

2.Hardened stage:-The time duration when concrete fails to take their original shape after the removal of stress or strain.

Properties of concrete in hardened stage:-

• Strength
• Durability
• Impermeability
• Dimensional change

1. Strength :-

       (1) Compressive strength:-

      1.For road & Bridge -20 to 40 N/mm2.
       2. 20 N/mm2 for building
       3. 60 N/mm2 for nuclear construction

      

      (2) Tensile strength :- The concrete has low tensile strength is range from 8 to 12% of its compressive strength.(split test) 

        (3) Bond strength :- The resistance of concrete to the slipping of reinforcing bar embedded in concrete is called bond strength.It is taken as 10% of the compressive strength. 

        (4) Dimension change:- 

                                   1. Shrinkage

                                   2. Creep

                                   3. Thermal Expansion

                                   4. Elasticity

•  Elasticity :- The modulus of elasticity is the measure of resistance of concrete to deformation and it's depend on strength, moisture content and type of aggregate and age of concrete.
•  Creep :- When a concrete structure is subjected to load, it undergoes some changes which are disappear after the removal of load. but this load is sustained for a long time, the deformation continuous even when the load is kept constant.
•  Thermal expansion:- The thermal expansion is defined as change in length per unit length degree change in temperature.It is highest with quartz and lowest with limestone aggregate.

Admixture

Admixture are the material added in basic constitute before or during mixing,Admixture are added in concrete in order to modify, the property of concrete with desire performance and strength.

Some types of admixture :-

• Plasticizers:-It help in obtaining higher workability without using excess of water.The organic  and inorganic substance when added to concrete change its properties, so that for a given workability, water content reduce termed as plasticizer.
• Superplasticizer:- It also help in obtaining higher workability without using excess of water At a given workability water content reduced by an extent up to 30%.
• Accelerator:- Speedly development of early strength.calcium chloride was the most common use material as an accelerator in earlier days.It help in making the cement paste hardener in less than 5 minute
• Retarders :-Retarders are the admixture that is used to increase the initial setting time.It reduce the rate of hydration of cement which keep concrete plastic and workable for long time.
• Fly Ash:-Making high strength and high performance concrete.It control environmental pollution.It has some cementitious property that's why we use in PPC cement.

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