Plain cement concrete is the mixture of cement, fine aggregate(sand) and coarse aggregate without steel. PCC is an important component of a building which is laid on the soil surface to avoid direct contact of reinforcement of concrete with soil and water.
In this article, we study the procedure of laying PCC, dos and don’ts and advantages of laying plain cement concrete.
Material Used in Plain Cement Concrete
The general specifications of materials used in PCC are –
1. Coarse Aggregate
Coarse aggregate used in the PCC must be of hard broken stone of granite or similar stone, free from dust, dirt and other foreign matter. The stone ballast shall be 20 mm in size and smaller. All the coarse material should be retained in a 5mm square mesh and should be well graded so that the voids do not exceed 42%.
2. Fine Aggregate
Fine aggregate shall be of coarse sand consisting of hard, sharp and angular grains and shall pass through a screen of 5 mm square mesh. Sand shall be of standard specifications, clean and free from dust, dirt and organic matter. Sea sand shall not be used.
Portland Pozzolana cement (P.P.C) is normally used for plain cement concrete. It should conform to the specifications and shall have the required tensile and compressive stresses and fineness.
Water used shall be clean and reasonably free from injurious quantities of deleterious materials such as oils, acids, alkalis, salts and vegetable growth. Generally, potable water shall be used having a pH value not less than 6. The maximum permissible limits for solids shall be as per IS 456:2000.
Proportioning of Plain Cement Concrete
- The proportioning is done based on the requirement or given specification. Generally 1:2:4 or 1:3:6 mix is used.
- The measurement of material can be done by weight batching or volume batching.
- In volume batching, coarse aggregate and sand shall be measured by measuring box of 30cmx30cmx38cm of a suitable size equivalent to one bag cement of 1/30 m3or 0.035 m3.
- Sand shall be measured on the basis of its dry volume.
- While measuring the aggregate, sacking, ramming or hammering shall not be done.
Mixing of Plain Cement Concrete
Mixing of PCC can be done either manually or by the means of machines :
- PCC is allowed to be done by hand-mixing only for small-scale works.
- The base on which the concrete is mixed must be clean, watertight slab or a steel platform.
- Sand and cement are mixed thoroughly followed with the addition of coarse aggregate. Lastly, water is added and the mixture is mixed properly to gain an even colour and consistency.
- A measured quantity of dry coarse aggregate, fine aggregate and cement shall be placed in their respective hoppers.
- The dry materials shall be mixed in the mixing drum for at least four turns of the drum after which the correct quantity of water shall be added gradually while the drum is in motion.
- The total quantity of water for the mixing shall be introduced before 25% of the mixing time has elapsed and shall be regulated to achieve the specific water-cement ratio.
- The mixing shall be thorough to have a plastic mix of uniform colour.
Note: The quantity of water to be used for each mix of 50kg cement to give the required consistency shall be as follows:
- Not more than 34 lit – 1:3:6 mix
- Not more than 30 lit – 1:2:4 mix
- Not more than 27 lit – 1:1 ½:3 mix
- Not more than 25 lit – 1:1:2 mix.
Laying of Plain Cement Concrete
- The PCC is laid in the layers of not more than 150mm thick and
thoroughly vibrated by the means of mechanical vibrators till a dense concrete is obtained.
- Wherever needed, hand compaction shall be done with the help of wooden tamping rods so that concrete is thoroughly compacted and completely walked into the corners of the formwork.
- Compaction shall be completed before the initial setting starts that is within thirty minutes of addition of water to the dry mixture.
Curing of Plain Cement Concrete
- Freshly laid concrete shall be protected from rain by suitable covering.
- After the concrete has begun to harden- that is about one to two hours after it has been laid- it shall be protected with moist gunny bags, sand or any other materials against quick drying.
- After 24 hours of laying of concrete, the surface shall be cured by flooding with water of about 25mm depth or by covering with weight absorbent materials.
- Curing shall be done for a minimum period of 14 days.
Dos and Don’ts of PCC Works
- PCC shuttering should be of the exact size and thickness
- Water should be mixed with a bucket, in a measured quantity, as per w/c (water/ cement) ratio.
- Use the chute or additional labor to pour the concrete where the depth is more.
- Remove any loose material from the sides of the pit, so that no soil or other material will collapse in the pit during concreting.
- If the water table is high, then de-watering should be carried out at the same time during concreting.
- Do not mix the materials on bare land.
- Do not allow the PCC without formwork.
- Do not pour concrete without levelling and compacting.
- Do not pour concrete in the pit from a height of more than 1.5 m.
- Do not allow extra cement mortar on top of PCC. for smooth finishing.
Advantages of Laying Plain Cement Concrete
- Required cover to bottom reinforcement is ensured, as cover blocks rest on a firm PCC.
- The effective depth of RCC members is achieved as the formworks can be easily, uniformly and sturdily fixed, resulting in better dimension accuracy of foundation RCC member.
- Reinforcement steel bars placed on PCC, are never in touch with the ground soil, which may be chemically active and may lead to steel corrosion in the immediate future.
- Ease in the placement of steel cages and increased productivity.
- Concrete does not bond with ground soil (clay); PCC acts as a barrier to soil and bond well to overlayed structural grade concrete.
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