concrete technology civil engineering

Introduction about concrete

The civil concrete products is obtained by mixing cement, aggregates and water in required propertions, with or without a suitable admixture. The subject which deals with the fundamental principles of concrete in civil  is known as concrete technology civil engineering . the practical utility of concrete technology for civil engineers is to enable them to know how to stock properly the raw materials required for concrete block and to perform different tests for Concrete. 

concrete

properties of concrete in civil engineering

The concrete admixture, in the plastic state (i.e. freshly mixed concrete) should have the the following properties of concrete in civil engineering:

(a) workability of concrete. The concrete should have good workability form . It is defined as the ease with which it can be mixed, transported and placed in position in a homogeneous state. It depends upon the quantity of water, grading of aggregate and percentage of fine materials in the mix concrete grade.

(b) segregation of concrete.  The concrete mix should be free from segregation effect. It is defined as the breakdown of cohesion (separation of coarse aggregates) in the mass of concrete. It results decrease in density. The ultimately loss of strength of hardened concrete.

segregation of concrete

(c). bleeding in concrete technology. The concrete admixture should have no bleeding. bleeding in concrete technology is defined as the separation of water or water-cement mixture from the freshly mixed concrete. It causes the concrete porous and weak.

bleeding in concrete

(d) harsh in concrete. harsh concrete meaning is the resistance offered by concrete to its surface finishing. The surface of harsh in concrete remains rough and porous. 

The concrete, in the hardened state, should have the following properties : 

1. Strength.  The hardened concrete should have high compressive strength so that it can resist the heavy loads of the structures. It should not be less than 15.5 N/mm².

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

3. Impermeability.  The hardened concrete should have sufficient impermeability or water tightness so that it can resist the entry of water inside the structure.

4. shrinkage of concrete.  The hardened concrete should exhibit minimum shrinkage. This property is directed by water-cement ratio.

shrinkage of concrete

5. creep concrete.  Hardened concrete should have minimal creep. It is a continuous strain, which the concrete undergoes due to the application of external load. 

Creep in Concrete

6. Thermal expansion. Hardened concrete should have minimal thermal expansion so as to provide good resistance to fire.

 classification of cement concrete 

The concrete is generally classified as follows :

1. plain cement concrete grades.  Plain cement concrete grade consists of cement, sand and coarse aggregate mixed in appropriate proportions in addition to water. Cement is used as a binding material, sand as fine aggregate and shingle, gravel, broken brick or crushed stone as coarse aggregates.

The usual proportions of ingredients in plain cement concrete ratio are : Portland cement (1 part), clean sand (1.5 to 8 parts) and coarse aggregate (3 to 16 parts).

2. reinforced cement concrete (R.C.C.) in civil engineering.  The reinforced cement concrete (also called ferro concrete) is a cement concrete in which reinforcement is embedded. The usual proportions of ingredients in reinforced cement concrete are : Portland cement (1 part), clean sand (1 to 2 parts) and coarse aggregates (2 to 4 parts).

3. prestressed cement concrete (P.C.C.).  The pre-stressed cement concrete is a cement 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.

function of different ingredients of cement

The functions of different ingredients of an ordinary portland cement, as mentioned above, are as follows : 

1. Lime.  

2. Silica.

3. Alumina.

4. Iron oxide.

5. Magnesium oxide.

6. Sulphur trioxide.

7. Alkalies.

cement clinker composition

The cement clinkers (which are formed when calcareous and agrillaceous raw materials are mixed and burned in rotary kilns) consist of the following major compounds :

cement clinker

clinker

1. tricalcium silicate in cement (40%). The presence of tricalcium silicate in cement allows it to hydrate more rapidly. This generates more heat of hydration. It develops high starting strength and possesses less resistance to sulphate attack.

2. dicalcium silicate in cement (32%).  The presence of dicalcium silicate in cement hydrates slowly. It generates less heat of hydration. It hardens more slowly and provides greater resistance to sulfate attack. It provides good ultimate strength to the cement.

3. tricalcium aluminate in cement (10.5%).  The presence of tricalcium aluminate (c3a) causes initial setting of cement. It reacts fast with water and generates large amount of heat hydration. It is the first compound which reacts with water and generates large amount of heat hydration. It is the first compound which reacts with water when mixed with cement.

4. tetracalcium aluminoferrite in cement (9%).  The presence of tetracalcium alumino ferrite in cement has poor cementing value. If reacts slowly with water and generates small amount of heat hydration.

Notes : (a) The high percentage of tricalcium silicate and low percentage of dicalcium silicate in cement results in rapid hardening. high early strength, high heat of generation and less resistance to chemical attack.

(b) The low percentage of tricalcium silicate and high percentage of dicalcium silicate in cement results in slow hardening, much more ultimate strenth, less heat of generation and greater resistance to chemical attack. 

setting and hardening reaction of cement

The chemical reaction between cement and water is called hydration of cement or simply cement hydration. The phenomenon by virtue of which the plastic cement changes into a solid mass is known as setting of cement. The phenomenon by virtue of which the cement paste sets and develops strength is known as hardening of cement. 

The rate of setting and hardening of cement, the rate of evolution of  heat and resistance to sulphate attack are affected by the proportions of different cement compounds. The sum of  percentage of  tricalcium silicate and dicalcium silicate for Portland cement varies from 70 to 80%. The tricalcium silicate hydrates more flat out or rapidly than dicalcium silicate and develops strength in cement for the first 7 days. The tricalcium aluminate and tetracalcium alumino ferrite compounds are responsible for the initial setting of cement.