Plastering

Plaster

Plastering is the application of a mortar coat to the surfaces of walls, pillars, ceilings, etc. to obtain a smooth finish. The mortar used for plastering may be lime mortar, cement mortar or lime-cement mortar. The lime mortar used will have a lime-to-sand ratio of 1:3 or 1:4. If hydraulic lime is used the mixing ratio (lime: sand) is 1:2. Cement mortar of 1:4 or 1:6 mix is usually used for plastering, with the rich mixture being used for exterior walls. To combine the cost-effectiveness of lime mortar and the good quality of cement mortar, many people use lime-cement mortar of 1:1:6 or 1:1:8 or 1:2:8 ratio (cement:lime: sand) use.

The purposes of plastering are:

1. To conceal defective workmanship

2. To give smooth surface to avoid dust catching

3. To give a good look to the structure

4. to guard the wall from rain water and other atmospheric agencies

5. To protect surfaces from vermitte.

Good plaster requires:

• It should easily follow the background.

• It should be hard and durable.

• It should be shielded from moisture.

• It should be cheap and economical.

• It should have good working ability.

• It should efficiently check for moisture penetration from the surface.

Plastering

Materials for plastering:

Lime mortar is typically applied in 3 coats whereas cement mortar is applied in two or three coats for stone and brick masonry. Cement mortar for concrete surfaces can be applied in two or three layers. Sometimes only one coat of cement mortar is applied to concrete building blocks. The first coat provides a means of achieving a flat surface. Final coat provides smooth surface. If three coats are used then the second coat is known as the floating coat. The average thickness of the primary coat is 10 to fifteen mm. The thickness of the middle coat is 6-8 mm. The final coat is simply 2 to three mm thick. If single coat is used then its thickness is kept between 6 to 12 mm. This type of coat is used on concrete surfaces that are not exposed to rain. Mortars used for plastering work are often classified into three categories:

• Lime Mortar: 

Contains equal amounts of lime and sand, both these ingredients are carefully ground in a mortar mill. Flat lime is suggested for plastering work.

• Cement mortar: 

Cement mortar consists of 1 a part of cement to four parts of unpolluted , coarse and angular river sand. The ingredients are mixed well in a dry state before watering. The mixing of ingredients is done on a waterproof platform.

• Waterproof mortar:

 This mortar is waterproof and is ready by mixing one part cement and two parts sand and powdered alum at the speed of 120 N per m3 of sand.

Plastering Method:

Plastering can be done on surfaces in either one, two and three coats. Plastering for two coats is as follows:

• Mortar joints are raked to a depth of 20 mm and the surface is cleaned and well watered. If it's found that the surface to be plastered is just too rough and uneven, a primary coat is applied to fill the hollow before the primary layer of plaster is applied to the surface.

• The first coat of plaster is now applied to the surface. The typical thickness of the first coat for brick masonry is 9 mm to 10 mm. To maintain a uniform thickness, screeds are made on the wall surface by fixing the dots.

• Cement mortar is continuously placed between the screeds and the surface is properly finished. The second coat is applied after six hours and the thickness of the second coat is 3 mm to 2 mm. The completed work is allowed to rest for 24 hours and then, the surface is kept well watered for the rest of the week.

Three coats are the same as two coats for plastering. The thickness of the first coat (rendering coat) is 9 to 10 mm, the second coat (rendering coat) 9 to 10 mm, and the third coat (setting coat) about 3 mm.

Techniques for plastering various surfaces:

• Interior plastering on brick and concrete surfaces:

Initially, the surface to be plastered will be cleaned. The level pegs on the walls shall be fixed with reference to off line for brick walls installed in the floor. (using center plumb bob and nylon thread). All brick walls will be watered before mortar is pasted on the walls. A first coat mortar filling (1:4 cement and sand) up to 15 mm shall be applied to surfaces where the required mortar thickness exceeds 25 mm. The walls and pillars shall be plastered with 1:4 cement and sand to obtain a semi-rough finished surface. Vertical joints of structural pillars/walls and brick walls shall be treated with 200 mm width chicken mesh with wire nails/concrete nails by centering the mesh at the vertical wall joint. All embedded service lines and provisions (conduit, box and etc.) will be completed on brick walls and checked with MEP drawings. The joints between the walls and beams shall be made up to a maximum of 20 mm and shall be sealed using a 30 minute fire rated flexible filler. (Material description to be submitted for approval of Engineer) Internal plastering on concrete columns, beams and walls surfaces, which are aligned with brick wall surfaces, to be plastered and other concrete surfaces with cement base easy plaster will be terminated. (Material details will be submitted for engineer's approval).

• Plastering of Exterior Wall:

Alignment and fixing level pegs on exterior wall surfaces shall be carried out using survey equipment / center plumb bob. After completing the leveling pegs on the walls the projection on the wall surfaces shall be cut and cleaned. A first coat mortar filling (1:4 cement and sand) up to 15 mm shall be applied to surfaces where the required mortar thickness exceeds 25 mm. Cement paste on concrete surfaces shall be applied to improve the bond of plaster to concrete surfaces. This drain will be filled with approved weather sealant. Exterior wall plaster will end up with a rough surface. The window sill will be formed while plastering the exterior of the window.

•Plastering Finishing with Cement Based Easy Plaster:

Slab soffits and beam edges and soffits which are to be painted on smooth surface shall be smoothed with easy plaster (Materials literature to be submitted separately) and the places where concrete surface is uneven shall be plastered with cement and sand mortar. will be leveled and leveled with. Before applying easy plaster to make the surface smooth.

  • Improvement of brick wall and structural concrete joints

  • A 200 mm wide chicken net will be installed at the joint.

  • Concrete surfaces shall be washed and cleaned.

  • The concrete surface to be plastered will be rough or spot cement slurry will be applied.

Various plastering techniques:

• Several plastering techniques are used to plaster ceilings and walls. It all depends on the requirements of the customer as well as the nature of the area in which plastering is required.

Let's take a look at some common plastering techniques:

Dry lining plastering

• During the last 3 to 4 decades dry plastering or dry lining plastering technology is being used instead. This method is preferred because of the ease of using plasterboard. After all, plasterboards are solid and are available in sheet form in a standard size of approximately 2.4 x 1.2 m. What's more, dry lining is a complete dry process that allows plasterers to quickly repair any mishaps. Paint can be easily applied to any surface ensuring a warm and welcoming finish.

• Most of the builders or plasterers prefer to use dry lining because of the speed of installation and less load on any type of structure. Because plasterboard walls are lighter, they offer better flexibility when it comes to planning interior or exterior spaces on a drawing board.

wet plaster

• Wet plastering is considered the most common of all plastering techniques used. This enables plasterers with Melbourne experience to achieve a clean and smooth finish by covering any surface requiring plastering with mortar, then smoothing it using a trowel. Dry wet plastered surfaces can be painted or papered according to different preferences. Like most plastering techniques, it requires efficient plastering and ample drying time. Wet plastering is prone to shrinkage, cracks, and needs to be re-plastered several times to cover any cracks.

Defects in plastering

1. Plaster Surface Blister:

 This is the formation of small patches of plaster swelling outside the plaster surface, resulting from late fall of lime particles in the plater.

2. Cracking:

 Cracking involves the formation of cracks or crevices in plaster work, resulting in the following reasons.

I. Incomplete preparation of background

structural defect in second building

ii. surface continuity

iii. Defective workmanship.

3. Efforescene:

 It is white crystalline substance which appears on the surface due to presence of salts in plaster making material as well as in building materials like brick, sand, cement etc. and even water. It gives a very bad look. This affects the adhesion of the paint with the surface of the wall. Efferesin can be removed to some extent by dry scrubbing and frequent washing of the surface.

Measures to reduce defects in plastering work:

• Brick work and plastering should be done with best workmanship.

• The bonding of the brickwork must be properly maintained.

• Rubbing brushes on damaged surfaces removes efflorescence. A solution of one part HCl or H2SO4 and five parts of clean water is prepared and applied to the affected area with the help of a brush. 

• Bricks of superior nature should be used only for brick work.

• Salt free water should be used for brick work and plastering work

• The surface to be plastered should be well watered so that it does not absorb water from the plaster.

• Moisture-proof courses should be provided at convenient locations in buildings.

• The overall construction should be such that moisture is contained.

• The freshly plastered surface should be protected from excess amounts of water such as rain and extreme heat such as the sun.