Plastering is a process of applying one or more coats of mortar to a concrete surface, brickwork, stone masonry or lathing. It must be durable so that it resists moisture penetration and is able to weather evenly. It should also be pleasing in appearance. These properties depend on the material used, the composition of the mixture, and the degree of mechanical bonding between the plaster and the backing surface and the workmanship.
The joint shall be raked to a depth of 15 mm for brickwork and 20 mm for stonework. For new work, where plastering is to be done later, raking of joints should be done while the work is in progress, when the mortar is still green. Dust or mortar powder (loose mortar) will be washed away. The entire surface shall be thoroughly cleaned and brushing and scraping shall remove the manne, if any. The surface is thoroughly rinsed with water, cleaned and vetted until the previous day and work begins, and will be kept very moist during the progress of plastering.
There are two types of bonds in cement mortar, one being mechanical in which the mortar squeezes into irregularities and interlocks when hardening and the other is due to the adhesive property of Portland cement when hardening. The degree of bond will therefore depend on the roughness of the treated surface and the quality of cement and sand used in the preparation of the mortar.
All monolithic concrete walls should be roughened with bush hammers or by hacking at close intervals with a chisel and hammer and then rinsed thoroughly with water to remove all dirt and loose particles. Monolithic concrete can be roughened with a heavy wire brush or a special handcraft tool if the forms are removed early. Forms for concrete, that is, receiving plaster, should not be given an excessive mold oil coating. Because it is likely to remain on the solid, interfering with the bond. Special care should be taken to remove the moldy oil coating before applying the plaster. If curing compound is used, it should also be completely removed before starting the plasterwork.
Brick and stone masonry:
Cement are excellent bases for the direct application of plaster. The surface should be hard, rough and clean. Joints should be racked. If the surface of the stone is very smooth it may be desirable to rough it with a pick or similar sharp tool.
Tools for plastering:
Following tools are used for plastering – gauging trowel, floats, floating rule, plumb bob, straight edge, bush, set square, spirit level, scratcher, plumb rules etc.
Materials conforming to the following requirements should be used.
Cement mixed with fine aggregates should produce a smooth, plastic, adhesive, strong and workable mortar. Cement plaster shall, unless otherwise specified, be of the following proportions and thicknesses. Mortar of specified mix shall be used.
|simple building||1:6||13 mm|
|important building||1:4||13 mm|
|Groove, Skirting, Dados, etc.||1:3||13 mm for drains, and 19 mm for skirting and shingles|
|septic tank, reservoir etc.||1:2||19 mm|
Presently 33 grades and higher grade cements such as 43 grades and 53 grades are in use. These are essentially recommended for use in concrete. It is also used in masonry and plastering work.
The sand shall be clean, sharp, suitably graded and free from all harmful and impure substances. Harmful materials exceeding a certain limit will adversely affect the hardness, strength, durability or appearance of plaster or cause corrosion of metal lathing or other metal in contact with plaster.
The most suitable particle size grading of sand plasterwork for interior and exterior walls and ceilings is given below:
|sieve size is||pass in)|
|4.75 mm||95 – 100|
|2.36 mm||95 – 100|
|1.18 mm||90 – 100|
|600 micron||80 – 100|
|300 micron||20 – 65|
|150 micron||0 – 50|
The water used in plasterwork must be of suitable quality for drinking. It should be free from chlorides and organic impurities.
Water Proofing Compound:
Generally, they are not specifically needed if the correct type and quality of other mortar materials are available. Where it is used, it should be spread evenly and mixed well in the mortar.
Plasticizers can be used in hot or hot weather conditions according to the requirements of the area.
It is always advisable to provide double scaffolding for plastering work. Easy to fix and remove at various heights without damaging masonry or plaster.
Mixing plaster ingredients:
It is better to mix the ingredients in a mixer. The dry mortar is mixed initially and then water is added to the dry mixture to obtain the required consistency. It is observed that excess mortar is often prepared and not used in time. Employees also take breaks for lunch and leave the wet mixed mortar to dry. Water is re-added resulting in less strength and greater shrinkage problems. The quantity of the solution made in one go should be such that it can be consumed within 30 minutes. Any mortar that falls to the ground in the process of application is thrown away and is not reused for any reason. If additional mortar is prepared it dries out either due to evaporation of water or due to water absorption by sand and/or water consumed by cement hydration. The adulteration of water should be carefully monitored and added in such quantity as to give the required functionality.
Application of plaster:
The walls shall be prepared as above and furnished with a mortar of cement and fine sand in specified proportions. At suitable intervals, 15 cm x 15 cm mortar sections for the full thickness of the base coat will be first placed over the entire surface of the wall to serve as a guide to ensure a flat, smooth layer of plaster.
Mortar must be dashed against the surface to be plastered firmly enough, and all joints and other surface depressions shall be thoroughly worked to ensure a permanent bond. The plaster surface will be rough and will not be beaten.
Ceiling plaster work will be completed before wall plastering begins. Plastering should be started from the top and worked down, filling all putlog holes before plastering as the scaffold is being moved down.
All vertices are generated, angles and junctions must be exactly vertical or horizontal, as the case may be, and must be carefully eliminated. Where necessary, rounding or chamfering corners, girths, junctions, etc. shall be done with the appropriate templates for the required shape.
At the end of the day the suspended plastering work will be left neat to line up both horizontally and vertically. Horizontal joints in plasterwork should not be on parapet tops and copings.
Coat for plastering:
The thickness of this coat should be approximately 10 mm to 12.5 mm and should be applied over the entire length of the wall or at natural breaking points such as doors and windows.
Base coat (in case of thick plaster):
The surface of the scratch coat should be evenly moistened before applying the base coat. This coat is approximately 10 mm thick depending on the overall thickness and then thickened with a wooden float to provide bond for the finishing coat. The second coat should be moistened for at least seven days and then allowed to dry.
Before applying this coat, the base cat is moistened evenly. Joints should be avoided and the finishing coat should be applied in one operation the thickness of which should not exceed 6 mm.
Exterior plaster is made in a richer cement mortar ratio than interior plaster. This is usually done in two layers. The first layer is 10 to 12.5 mm and the last layer is 6 mm thick. A waterproofing compound may be added if the plaster is exposed to severe wet conditions. The finish can be of the specified type.
Interior plaster is usually done in single layers of 12.5 mm.
The plaster should be finished to a true and plumb surface and the required level of smoothness. The work should be repeatedly tested as it is before a true straight edge with a long and plumb bob less than 2.5 m. All horizontal surfaces should be tested with a level and all jambs and corners should be tested with a plumb bob as work proceeds.
There are four different types of finishes that can be achieved with cement plaster.
When a smooth finish is desired, a minimum amount of work done should be applied to the wet surface and a wooden float should be used, not a steel trowel.
This finish is suitable for rural or coastal areas and severe exposure conditions. It is a finish, which is sprayed onto the surface as a wet mixture and left rough. The maximum size of sand, crushed stone or gravel varies from 12.5 mm to 6.3 mm.
Pebble Dash Finish:
It is the most durable of all finishes and is generally free from defects. This gives a rough texture and is achieved through small pebbles or crushed stone, graded from 12.5 mm to 6.3 mm and sprinkled over a fresh coat of mortar and left uncovered. This pebbles or stones are sometimes lightly pressed or tapped into the mortar.
Textured finishes are now becoming very popular and can be achieved in many different designs in a variety of ways. Special effects can be achieved by scraping the rendering surface with a straight edge hacksaw blade or with the edge of a steel trowel.
Curing will be started 24 hours after finishing plaster. The plaster shall be kept wet for 7 days during which period it shall be suitably protected from all damages at the expense of the contractor, as may be approved by the Engineer. The dates of the plaster should be clearly marked on different parts of the wall so that it can then be cured for the specified period.