Plastering is the process of covering rough surface with a plastic material to obtain an even smooth, regular, clear and durable surface. Plastering conceals defective workmanship and cover up unsound and cheap quality material. External plastering also termed as rendering is done with the object of improving the resistance of the surface to rain water penetration and other atmospheric influences.
The plastic material or plaster is made by working together a mixture of building material which may be cement, lime or clay, fine aggregate and water.
Preparation of surface for plastering
The durability of plaster depends to a great extend upon its adhesion with the back ground. The preparation of surface is of prime importance. All the projection which extend by more than 13mm from the general surface of the masonary wall face are knocked off to obtain a uniform surface and to reduce consumption of plaster.
Rust and loose mortar is brushed out of the raked joint. The surface is rust free from oily, greasy spots and efflorescense. If any is removed by brushing and scrapping. In case of old walls the smoothness of the surface is destroyed by hacking it with some tools. The surface of the patches of plaster serves the gauges for maintaining even thickness of the wall being applied. Mortar is then applied on the wall between the screed with trowel.
Pointing may be done by using lime or cement mortar. The mortar for lime pointing is made by taking equal parts of fat lime and fine sand and then grinding the mix thoroughly in a mortar mill. The mortar for cement pointing is made by mixing cement and sand is proportion of 1:2 or 1:3. The mortar is pressed into the raked joints with a pointing trowel and rendered flash surk or raised according to the type of pointing required. The desired type of pointing is then made with the proper tool and the surface of the masonary cleaned of all superfluous mortar. The pointing should be kept wet for about a week. The pointing lines should be regular truly horizontal and vertical. In case of random rubble masonry where the joints are starting the line of the joints from different directions should preferably meet at or near the junctions.
The aim of evolving different methods of proportionating concrete as so get strongest and densest mix with least amount of cement. The different methods of proportionating concrete are
1. By arbitrary standards
2. By minimum void method
3. By trial or maximum density method
4. By fineness modulus method
5. By water cement ratio low
The machine used for mixing concrete is termed as concrete mixer. Two types of concrete mixer are in common use.
1. Continuous mixer
2. Batch mixer
The machine required careful supervision so as to obtained the concrete mix of desired consistency.
In batch type of concrete mixer the desired proportions of materials are fed into the hopper of the drum in which the materials get mixed by the series of blades or baffles inside the mixer. Batch mixers are further devided into two main types
1. Tilting drum type
2. Closed drum type
While using the mixture coarse aggregates should be fed first, sand and then cement should be put afterwards. In the revolving state the componants get mixed while water is poured with help of a can. The concrete should be mixed for at least 1 ½ to 2 min.
In plastering usually 3 numbers of coats are applied. First coat also known as rough coat is applied on the surface in a layer of 6 to 10 mm in thickness. Second coat is then applied over the first coat after the lattez has set and rough thickness of mortar in second coat vanes between 6 to 10 mm. Third coat or finished coat is applied after 5 days the second coat. This coat does not exceed 3 mm in thickness and usually consist of mortar made with cream of white or flat lime. With lime white sand taken in the ratio of 1:2. But on the side time period between two coats was of 4 days. For first coat bricks or precast cement blocks then selves provide gripping and bean and columns, where punched for getting better surface.
The first layer plaster was scratch with the help of trowel these scratches provide necessary gripping medium.
Diagram
Diagram
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Necessary handling equipment
1. Hand gloves
2. Bucket
3. Spread
4. Pan
5. Trowel
6. Float
7. Plumb bob
Precaution to be taken
1. The surface should be clean.
2. It should wet before plastering.
3. Required amount of water should be provided.
4. The cement, sand ratio should be maintained.
5. After plastering water well spread on plaster every day.
Defects in plaster work
- Old surface not being properly prepared.
- Movement in the plaster itself on account of the expansion or shrinkage.
- Excessive shrinkage of plaster due to the application of mortar in thick coat.
- The adhesion of the plaster to the back ground may not be perfect.
- Excessive thermal changes in plaster or backing.
- In-adequate curing of the plaster.
- The bond between the successive coat of plaster may not be perfect.
- The section of backing material may not be uniform. At the places where the backing material absorbs excessive amount of water the plaster loses its strength as well as the mechanical bond between the two surfaces.
