1) Classification according to binding material
According to binding material used concrete are classified into two types.
- Cement concrete
- lime concrete
i) Cement concrete
- The concrete consisting of cement, sand and coarse aggregates mixed in a suitable proportions in addition to water is called cement concrete. In this type of concrete cement is used as a binding material, sand as fine aggregates and gravel, crushed stones as coarse aggregates.
In cement concrete useful proportions of its ingredients are 1 part cement : 1-8 part sand : 2-16 parts coarse aggregates.
- Uses – cement concrete is commonly used in buildings and other important engineering works where strength and durability is of prime importance.
ii) lime concrete
- The concrete consisting of lime, fine aggregates, and coarse aggregates mixed in a suitable proportions with water is called lime concrete.
- In this type of concrete hydraulic lime is generally used as a binding material, sand and cinder are used as fine aggregates and broken bricks, gravel can be used as coarse aggregates.
Placing of lime concrete
- Placing of concrete shall be completed within three hours of adding water in case of concrete is prepared with hydraulic lime.
- Concrete should be well cured for a period of at least 10 days.
- Uses – Lime concrete is generally used for the sake of economy in foundation works, under floors, over roof and where cement is not cheaply and easily available in required quantity.
2) Classification according to design of concrete
- Plain cement concrete (PCC).
- Reinforced cement concrete (RCC).
i) Plain cement concrete (PCC)
- The cement concrete in which no reinforcement is provided is called plain cement concrete or mass cement concrete. PCC is also called as Cement Concrete (CC) or Blinding Concrete.
- Uses – Plain cement concrete is commonly used in for foundation work, concrete roads flooring of buildings etc.
Why is plan cement concrete not used in major construction?
- Concrete has enormous compressive strength – the ability to withstand heavy weights or forces on it.
- But it has almost no tensile strength – the ability to withstand pressing or stretching.
- Concrete can resist the compression but will break under the tension. Concrete cracks in roads and slabs are largely due to tension
- To give concrete tensile strength — to support beams in a bridge or building, reinforcing steel is added. Steel has great tensile strength; it bends without breaking.
- The combination produces the strength needed for bridge girders, roadways, building walls and other construction. To solve this problem, RCC & Pre-stressed concrete is used.
ii) Reinforced cement concrete (RCC)
- The cement concrete in which reinforcement is embedded for taking tensile stress is called reinforced cement concrete.
- In this type of concrete the steel reinforcement is to be used generally in the form of round bars,6mm to 32mm dia. This concrete is equally strong in taking tensile, compressive and shear stresses.
Usual proportions of ingredients in a reinforced concrete are 1part of cement : 1-2 parts of sand : 2-4 parts of crushed stones or gravel.
- RCC is commonly used for construction slabs, beams, columns, foundation precast concrete etc.
NOTE: Stress is a quantity of deforming force applied on an object per unit area.
3) How is concrete prepared?
The operation involved in the production and execution of the items of concrete can be broadly divided in following stages:
- Batching of materials
- Mixing of concrete
- Transportation of mixed concrete
- Placing of concrete
- Compaction of Concrete
- Curing of concrete
i) Batching of materials
- In order to ensure uniformity in the quality of concrete it is necessary that all materials that go into its production are measured accurately.
- Batching of material can be done manually or by batching plant.
ii) Mixing of concrete
- Thorough mixing is essential for the production of uniform, high-quality concrete of correct proportions and consistency.
- Mixing is done manually on a steel plate, 2m x 2m in size or on a clean hard water tight surface. The method is resorted to when the quantity of concrete needed for the work is small.
- Sand and Cement in appropriate proportions are mixed first in dry state. The coarse aggregate is then added and the whole mixture is mixed thoroughly with the help shovels
- The predetermined amount of water is then sprinkled over the mix , till the color of concrete obtained is homogeneous and workable mix is obtained.
- Concrete should normally be mixed in a mechanical mixer. The main part of the mixer is a drum in which the ingredients are mixed thoroughly by mechanically rotating the drum. The drum is made of steel plates, with a number of blades put in inclined position in the drum.
- The mixers are either operated electrically or else are driven by oil engines attached to them.
- Coarse aggregate should be fed first, then sand and lastly cement.
- In revolving state, when the three get thoroughly mixed, water should be added.
- Mixing should be continued until there is a uniform distribution of materials.
Transportation of mixed concrete
- Not all concrete is mixed on the actual construction site and could require some significant travel.
- Concrete should be handled from the place of mixing to the place of final deposition as rapidly as practical , by methods which will prevent the segregation or loss of any of the ingredients.
- If the segregation does occur during transport, the concrete should be remixed before being placed.
- The main objective in transporting concrete is to ensure that the water-cement ratio, slump or consistency, air content, and homogeneity are not modified from their intended states
Placing of concrete
- After mixing of concrete it should be placed within 30min of adding of water.
- It should be quickly transported to the place .
- Before placing concrete, it should be ensured that forms are rigid, in their correct position, well cleaned and oiled.
- Oiling of these forms will prevent the concrete from sticking to it, and will then be easier to remove the forms when they are no longer required.
- In placing, the surface is coated with neat cement grout after which concrete is laid in thin uniform layers. Each layer being thoroughly consolidated, before the next one is laid.
- When work has to be resumed on a surface which has hardened, such surface should be roughened, then be swept clean, thoroughly wetted and covered with a 13 mm layer of mortar, composed of cement & sand in the same ratio as in case of concrete.
- Concrete should not be dropped from a height as it would cause segregation of aggregates.
- In case concrete has more of water or it has been laid in thick layers then on compaction water and fine particles of cement comes at the top forming a layer of weak substance known as laitance
Compaction of Concrete
- The removal of entrapped air during production of concrete and the uniform dense arrangement of the constituents of concrete are effected during the compacting of corners.
- The density, strength and durability of concrete depend upon this operation.
- Concrete should be thoroughly compacted during the operation of placing , worked around reinforcements , embedded fixtures and into corners of form work.
- Concrete is compacted by vibrators, during which vibration of low amplitude is caused in concrete making it fluid in nature , filling all the air gaps to make concrete more stable and strong.
- Vibrators are of 3 general types:
x) Internal vibrators
y) External vibrators
z) Surface vibrators
- Internal or immersion vibrators consists of a vibrating element enclosed in a casing which is immersed in fresh concrete & transmit vibrations
- External or form vibrators are fastened to the form work by a clamping device & transmit vibrations to concrete through form.
- Table or surface vibrators ,mostly used in precast concrete members. They vibrate the entire mass of concrete uniformly through working platform.
- Over vibration or vibration of very wet mixes is harmful & should be avoided.
- Alternatively, concrete may be compacted manually by rodding , hammering etc.
Improperly consolidated Concrete
Curing of concrete
- The process of keeping concrete wet to enable it to attain full strength is known as curing.
- The objective of curing is to prevent loss of moisture from concrete due to evaporation or because of any other reasons.
- Curing should be done for a period of three weeks but not less then 10 days.
- Efficient curing increases impermeability , durability and also increases resistance of concrete , reduces shrinkage
- To do curing, any one of the following method can be used.
i) The surface is coated with a layer of bitumen or similar other waterproofing compound which gets into the pores of concrete and prevent loss of water from concrete.
ii) Concrete surface is covered with waterproof paper or with a layer of wet sand. It could also be covered with gunny bags.
- A common misconception is that concrete dries as it sets, but the opposite is true – damp concrete sets better than dry concrete.
8) Applications of Concrete
- Concrete is suitable for the construction of –
ii) Roof Slabs
iii) Columns and Beam
- RCC is used in the construction of small, medium and large scale bridges. It results in more aesthetically good and economical structures as compared to steel bridges.
- Reinforced Concrete is also used in the construction of Earth Retaining Structures as well as retaining walls for embankments.
- It is suitable for water retaining structures like ground and overhead tanks and sports stadiums and conference halls.
- R.C.C piles both pre-cast and cast-in situ have been used for the foundations of structures like bridges and building.
- Multistory reinforced concrete buildings are adopted for both residential and office complexes.
- It is used in the pavements for highways and airports runway.
- Concrete has flexibility and is used to design free forms.