Plastering

UNIT – III

Chapter – 6 (E)

PLASTERING

Plastering: Plastering is the process of covering the rough inner and outer faces of walls, beams, columns and ceilings with a protective layer. This layer is of cement mortar or lime mortar.

Plaster: The mortar used for plastering is called Plaster.

1. PURPOSES OF PLASTERING

1. Smooth Appearance: Plastering provides even, smooth, regular and clean surfaces to walls, columns, beams, etc., to improve appearance.

2. Durability: Plastering improves the durability of the exposed surfaces of walls, etc.

3. Concealing Defects: Plastering conceals the un-evenness and other defects in workmanship. It conceals the use of inferior and porous materials used in masonry walls, concrete beams, etc. Also, it conceals the defects in the joints of the masonry work.

4. Effects of Atmospheric Agencies: Plastering prevents damping of the walls, etc., due to atmospheric agencies like rain, sun, wind, etc.

5. Base: Plastered surface forms a satisfactory base for white washing, colour washing, distempering or painting.

2. REQUIREMENTS or QUALITIES OF A GOOD PLASTER

1. Workability: A good plaster should possess good workability property.

2. Adherence: A good plaster should adhere to the surface firmly.

3. Appearance: It should provide the surface a decorative appearance.

4. Durability: A good plaster should be durable.

5. Atmospheric Agencies: A good plaster should resist the effects of atmospheric agencies.

6. Smooth and Even Surface: It should provide a smooth, even and washable surface.

7. Shrinkage and Cracking: A good plaster should not shrink while drying. Shrinkage may cause cracking of the surface.

8. Insulation: A good plaster should have good insulation property against sound and heat.

3. TYPES OF MORTARS USED FOR PLASTERING

1. Cement Mortar

Cement Mortar is a mixture of Portland cement and coarse sand. The usual mix for cement mortar for plastering varies from 1 : 3 for the surfaces in contact with water (bathroom, etc.) to 1:4 to 1 : 6 for other surfaces.

'ment and sand are thoroughly mixed in a dry state. The mixture is added with required cement of water to make a plastic mass or paste of desired consistency and workability.

Precaution: The wet mortar should be used within 30 minutes, because the cement starts setting after about 30 minutes.

2. Lime Mortar

Lime Mortar used for plastering is prepared as follows: Equal volumes of lime and fine sand are thoroughly mixed. The mixture is ground in a mortar mill by adding required quantity of water to form a paste of required consistency and workability.

3. Water-Proof Mortare

It is prepared by mixing 1 part of cement with 2 parts of sand and pulverized alum. Soap water is added to this dry mixture to make it water-proof and to obtain required consistency and workability. This mortar is used for water-retaining structures.

Mortars Used for Different Plastering Works

For interior and exterior plastering, we may use the following proportions:

Cement Mortar (Cement-Sand Plaster) 1:4

Lime Mortar (Lime Plaster) 1:1

Lime Surkhi Plaster 1:2

4. DIFFERENT STAGES IN PLASTERING

Number of Plaster Coats

Plastering may be done in one or two or three coats. One coat of plaster is used in Cheap and Unimportant Buildings. Two coats may be applied for Ordinary Buildings. Three coats are applied for Superior Buildings. Partially-set mortar should not be used for plastering.

Plastering is always started from the top of the surface to be plastered and worked downwards. The following procedure is adopted for plastering:

1. Mortar Joints: Mortar Joints of wall to be plastered are racked out to a depth of 20 mm.

2. Background Surface: Background surface is made even and uniform by removing the projections. It is cleaned by brushing and is roughened. Surface is then wetted.

3. Preparation of Plaster: The plaster is prepared in suitable proportion depending on the requirement. The ceiling is plastered first and then the walls.

4. Preparation of Screeds: See Fig. 1.

To obtain uniform thickness of plastering, a Dot of 15 cm x 15 cm square and 10 mm thick is marked using a small quantity of mortar on the wall surface. Another dot is fixed 2 m vertically below.

These two dots are checked by plumb bob. These are joined with a vertical strip of mortar, called Screed.

Screeds are placed at a horizontal distance of 2 m. Plaster is applied between them to obtain uniform thickness.

5. Plastering of First Coat: The mortar placed as above between successive screeds is called First Coat. The surface is levelled using trowel and finished. Curing is done for 7 days.

6. Plastering of Second Coat: For Two Coat Plastering, the surface of first coat should not be finished smooth, but left rough. Second coat is applied after 2 days. Curing is done for 7 days.

7. Plastering of Third Coat: Depending of the appearance, the Third Coat is as follows:

Rough Coat Finish: The finishing mortar of 1 : 3 mix for rough coat finish is prepared with coarse grained sand. It is dashed against the surface to be plastered. The surface is roughly finished by light movements of a wooden float.

Smooth Coat Finish: Smooth Coat Finish is just similar to rough coat finish. The only difference is fine grained sand is used instead of coarse grained sand.

8. Plastering of Decorating Piece: It should be finished before the finishing coat is applied.

9. Rounding of Corners: Rounding of corners and edges should be carried out along with the finishing coat in order to prevent any joint mark appearing after some time.

5. PLASTERING DEFECTS AND PRECAUTIONS