Concrete is a construction material made by mixing cement, sand, aggregate, and some admixtures with water. Portland cement is used as a cementing material in concrete which has great binding property. According to the requirements, different types and grades of concrete are made up and used. Almost for every construction work, concrete is used.
Concrete is the most widely used man-made material on the planet.
A) How to select materials for concrete?
It is important to select the appropriate material for making concrete. For concrete, we use cement, fine aggregate or sand, coarse aggregate, admixture, and water respectively. Since the quality of concrete depends upon these materials we use to make it. So we need to take major precautions while choosing materials.
Normally, Ordinary Portland Cement is used unless otherwise specified. The weight of O.P.C. is 1440 kg/m³. The minimum compressive strength of OPC is 175kg/cm² after 7 days and minimum tensile strength after 7 days should be 25kg/cm².
The initial setting time of cement should not be less than 30 min. and the final setting time should not be less than 10 hrs.
Also Read: How to store cement in a proper way?
b) Fine aggregate/ Sand
Generally, fine aggregate/sand obtained from the river is used. It shall consist of siliceous material having hard, durable particles free from dust, clay, etc. According to IS Code 383 – 1952 and the relevant portion of IS 515-1959 sand is selected for construction.
c) Coarse aggregate
Coarse aggregate made from hard crushed stone. They should be dense, durable, and free from the dust. Cubical shaped aggregates are suggested and used as far as possible. The flaky and elongated pieces of coarse aggregate are avoided. The absorption of water by coarse aggregates should not be more than 5 % by weight after 24 hrs. immersion in water.
As per the IS Code 383 – 1952 and IS 515-1959 selection of coarse aggregate for construction work is done.
The coarse aggregate used according to the thickness of the concrete:
- Mass cement concrete of 500mm and above is graded as 38mm and below.
- Normal cement concrete of 100 mm and above thickness is graded 20mm and below.
- Cement concrete topping thickness 40mm and above is graded as 12mm and below.
For making concrete and curing purpose potable water is used.. The water used in concrete should not be salty, alkaline, acidic or blackish. Only the water sources recommended by Engineers should be used.
Admixtures are used in concrete for promotion work-ability, improving strength, entraining air, etc with the proper approval of the engineer.
B) How to make concrete?
For the quality of concrete, mixing of materials in a proper and appropriate way is very important. So, while mixing cement with other materials for concrete you need to mix it in a proper way.
For the mixing of concrete, the proportion of cement, coarse aggregate, and sand should be taken by weight. Usually, by the volume of materials, the quantity of materials is taken which is not good for quality purposes.
You should follow the following grade method to mix concrete. Without proper measurement, you should not take any quantity of materials.
Concrete either mix by using a machine or by hand.
A) Mix concrete by machine
For important structures, mixing is done by machine. The materials are placed in a mixer arum and mixed at least for 1 minute before adding water. The mixing time should not be less than 2 min. The mixing time can be extended as per the Engineers direction. The mixture is completely cleaned before the next mixing is performed.
B) Mix concrete By hand.
- For less important structures, hand mixing can be done.
- Mixing shall be carried out on a smooth watertight platform of sufficient size to allow efficient turning of a mix.
- The platform shall be so arranged that mixing mortar shall not flow out and no foreign matter shall get mixed with mortar.
- The concrete shall be conveyed to the place required before the initial setting of concrete.
In very hot or cold weather to reduce the rate of loss of water by evaporation and loss of heat, concrete shall be transported in deep containers as directed by Engineer.
C) How to lay concrete in a proper way?
You need to know the standard procedure for laying concrete. Even the laying of concrete plays a vital role in the strength of concrete. Before laying the concrete, the surface where concrete is to be placed should be cleaned properly.
- The concrete shall be laid in layers not exceeding 15cm and vibrated thoroughly by
means of mechanical vibrators.
- Prior to placing concrete, formwork and reinforcement shall be approved.
- The placing of concrete shall be done before the initial setting time starts i.e. 30 min for OPC.
- Cement concrete work shall not be conducted in case of rains and separation of cement and sand is seen because of washing, the concrete shall be entirely removed.
- During cold weather casting of concrete shall not be done when the temperature falls below 4.5°c and during hot weather above 38°c.
- Slabs, beams, etc. shall be poured in one operation.
- When the placing of concrete is suspended or resumed on the next day, the necessary roughening of the surface shall be done before concrete is poured.
- In the column, joints shall be formed about 75mm below the lowest soffit of the beams framing into it.
- In beam, concrete shall be cast through without a joint unless unavoidable.
- The joint if have to be placed, shall be at the mid-span and shall be vertical. The same is followed for a suspended floor, slab.
- When resuming the concreting work another day, the skin of hardened cement is entirely
removed and surfaces are roughened by hacking, sandblasting, water jetting, air jetting, etc.
- The surface is then wetted for two hours and 1:1 cement-sand slurry is applied over it.
- The new concrete, advisably cold, shall be applied over this slurry before it sets.
D) Finishing of concrete work
- Freshly laid concrete shall be protected from rain, sun, wind, etc. 1 to 2 hours after laying when concrete begins to harden, moist gunny bags, sands, etc are used to protect the concrete against quick drying.
- After 24 hours, the surface is flooded with water of about 25mm depth for curing. It should be done for a minimum period of 14 days.
Also Read: Concrete Curing Methods
In this way, we can do perfect and best cement concrete work in the field.
- Cement Work: Tips for working with concrete
Are Cement and Concrete is the same thing?
No, Concrete is a mixture of 60–65% aggregates (sand, gravel, and aggregates), 12–20% water, and only 10–15% cement. Cement acts as binding materials when mixed with water, binds aggregates, and becomes hard rock-like mass called concrete.
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