PLASTERING
OBJECTS OF PLASTERING:
Plastering is done to achieve the following objects
1.To protect the external surfaces against penetration ofrain water and other atmospheric agencies
2.To give smooth surface in which dust and dirt cannotlodge
3.To give decorative effect
4.To protect surfaces against vermin.
5.To conceal inferior materials or defective workmanship
REQUIREMENTS OF GOOD PLASTER:
It should adhere to the background and should remainadhered during all variations of the climatic changes.
It should be cheap and economical
It should be hard and durable.
It should be possible to apply it during all weatherconditions
It should be effectively check the entry or penetration ofmoisture from the surface
It should possess good workability
TYPES OF PLASTER1. SAND FACED PLASTER
The first coat of sand faced cement plaster is done withcement mortar in 1:4 with coarse sand after curing theplaster for seven days the second coat of cement plaster1:3 ratio will be done on the first coat.
Sponge is used in the second coat and it is applied whenthe second coat is wet, it is thus worked that the densityof sand grain appear on surface is equal and uniform.
The surface is kept well watered at least for 15 days.
External Sand Faced Plaster
2. ROUGH CAST PLASTER
This plaster is a mixture of sand and gravel in specifiedproportion dashed over a freshly plastered surface.
The plaster base consists of two coats, under layer 12 mmthick and top layer 10 mm thick having the cement mortarratio 1:3.
In order to make the base more plastic, about 10% ofhydrated lime by volume of cement shall be added whileprepare mortar.
Rough Cast Plaster
3. DE PETER PLASTER:
This is just another form of rough cast finish.
The rendering coat of 12 mm thick is prepared as in caseof pebbled dash finish.
While this coat is wet, pieces of gravel are pressed withhand on the surface.
Thus it is possible to have beautiful patterns andornamental designs on the surface by selecting materialsof different colors.
4. PEBBLE DASH PLASTER
It is similar to rough cast finish except clean pebble ofsize from 6 mm to 12.5 mm are dashed against the surfaceso that there are laid in position by mortar appliedalready.
Pebble dash plaster
5. SMOOTH CAST PLASTER OR PLAIN FACEPLASTER
This finish is just similar to sand faced finish except finegrained sand is used instead of coarse sand.
No sponging is done to expose the sand grain .
It is normally done on internal walls.
Smooth cast plaster
DEFECTS IN PLASTER
The following defects may arise in plaster work.
Blistering of plastered surface:This is the formation ofsmall patches of plasterswelling out beyond theplastered surface, arisingout of late slaking of limeparticles in the plaster.
CRACKING:
Cracking consists of formation of cracks or fissures in theplaster work resulting from the following reasons.
Imperfect preparation of background
Structural defects in building
Discontinuity of surface
Movements in the background due to its thermalexpansion or rapid drying
Movements in the plaster surface itself, either due toexpansion or shrinkage.
Excessive shrinkage due to application of thick coat
Faulty workmanship.
Cracking of plaster surface
EFFLORESCENCE
It is the whitish crystalline substance which appears onthe surface due to presence of salts in plaster makingmaterials as well as building materials like bricks, sand,cement etc and even water.
This gives a very bad appearance. It affects the adhesionof paint with wall surface.
Efflorescence can be removed to some extent by drybushing and washing the surface repeatedly.
Flaking : It is the formationof very loose mass of plasteredsurface, due to poor bondbetween successive coats.
Peeling: It is the completedislocation of some portionof plastered surface, resultingin the formation of a patch.This also results from imperfectbond.
Popping: It is the formationof conical hole in the plasteredsurface due to presence ofsome particles which expandon setting
Rust Stains: These aresometimes formed whenplaster is applied on metallaths
Uneven surface: This isobtained purely due to poorworkman ship.
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