12.0 PLASTERING

12.1 INTRODUCTION AND TERMINOLOGY:

Plastering is a process of obtaining smooth surface on the rough surfaces of walls, roofs, columns and ceilings etc. for long lasting purposes and to meet its estimated design life period. Also plastering make the rough surfaces smooth thereby make it good looking and attractive. Also it helps in preventing damp proofing. The coat formed on the rough surface is called plaster when applied inside of the building and it’s called as rendering when applied outside of the building i.e. at outer wall exposed to direct environmental condition.

The following terms are used frequently in plastering. They are defined as follows:

i. Back ground: It’s the surface on which plastering work is started.ii. Hacking: It’s the process of making the back ground rough for adequate bondage with the

plaster and formation of key.iii. Key: This is the indentations (scratch marks) on the back ground to which plaster forms a

bond and do not get displaced or spalls off.iv. Ground: It’s the wooden strip fixed to back ground on which primary finishing is secured.v. Dado: It’s the lower part of plastered wall finished with neat cement slurry to make it better

water resistive.vi. Rendering coat/Under coat: This is the coat applied under finishing coat.vii. Floating coat: It’s the second coat applied just before the finishing coatviii. Finishing coat: It’s the final coat known as skimming coat or setting coat.ix. Blistering: It’s the development of local swelling caused due to residual unslaked lime and is

located on the finished plastered surface.x. Dot: It’s the small projection of size 15 15 cm2 plaster made intentionally for fixing of

screeds or similar materials.xi. Peeling: It’s the dislodgement of plastered work from the back ground of plaster.xii. Cracking and Crazing: Cracking is the development of one or more fissures on/in the

plastered surface due to back ground movement or vibration while crazing is development of hair cracks in a disorderly manner.

xiii. Gauging: It’s nothing but mixing of various plaster materials.xiv. Grinning: It’s the appearance of joint pattern in the back ground or similar areas on the

plastered surface.xv. Laitance: This is layer of fine cement particles formed due to excessive trowelling on the

plastered surface.

12.2 OBJECTIVES AND REQUIREMENTS OF A GOOD PLASTER:

12.2.1 Objective:

i. Protection of surface against environmental condition and rain water as well as sun light.ii. Protection against insects and fungus etc.iii. Provides aesthetically beautified construction.iv. Defective construction is patched and a little strength is gained.

12.2.2 Requirements of a Plaster:

i. It should stick to the back ground and should not spall off during bad weather.ii. It should be inert and should not chemically react with reinforcement or other decorative

finishing or floorings.iii. It should be economical and easily available.iv. It should possess water resistance nature and should be environmental friendly as far as possible.v. It should be durable, hard and possess good workability.

12.3 MATERIALS & TOOLS USED FOR PLASTERING:

12.3.1 Materials:

The materials used for plastering is commonly of two types. These are categorized as general plastering material and special plastering material. General plastering materials are the materials used for plastering everywhere and irrespective of any special requirement and these forms the parent material even for special plastering materials and commonly used in residential as well as commercial buildings. Special materials are used as per requirement of the environment, radiation, atmospheric exposure and type of decoration etc. Both types of materials are discussed below.

12.3.1.1 General Plastering Materials:

General plastering materials are as follows:

i. Lime: Lime used for plaster is of basically two types i.e. flat lime and hydraulic lime. Fat lime is preferred over hydraulic lime though hydraulic lime gives a stronger and harder surface than fat lime just because of the reason that hydraulic lime is very slow in slaking and even continues months (6 to 8 months) and normally used in dry condition with ground dry with sand for twice in an interval of 2 to 3 weeks. Also blisters may form if unslaked particles remained in slaked hydraulic lime. However, fat lime can be slaked wet and easy to use. Lime mortar can be used with proportion lime : sand as 1:3 to 1:4 and 1:2 for hydraulic lime mortar and cement : lime : sand as 1:1:8 to 1:2:8 proportion. For purely lime mortar with no cement, a binding material named gugal can be added @ 1.6 kg/m3 to the mortar. Also the tensile properties of lime is increased with the addition of chopped hemp @ 1kg/m 3 of mortar which ultimately prevents the formation of tensile cracks.

ii. Cement: Usage of cement as plastering material is normally adopted for external wall though in today’s scenario, cement plasters are used in all most all type of plastering. The advantages lie in quick finishing, high workability, no additional binder requirement, and hardened surface as well as non absorbent property of cement. Details of cement are already discussed earlier. The commonly used cement for building works is Portland Pozzolana Cement or PPC of grade 43 or 53 as available in the market. Also Portland slag cements (PSC) is used for the purpose. The ratio of cement to sand varies from 1:4 to 1:6 in cement plaster and for lime-cement plaster, its ratio varies between cement : lime : sand as 1:1:6 to 1:2:8.

iii. Sand: Sand is classified under fine aggregate category where the size below 4.75 mm is considered as fine aggregate. However, in general practice, sands are termed as coarse sand,

medium sand and fine sand and then silts. In plastering works, coarse sand and medium sand is used and not fine sand. Silt is not used any type of construction work. For plastering of roofs or heavy duty floors, where greater hardness is desired, instead of fine sand, broken granite chips of smaller size is used. But sand is a major material for all type of plasters and even on heavy duty floors, coarse sand can replace the smaller broken granite chips.

iv. Water: Water is another constituent in plastering. Desired amount of water is added to plastering for making the mortar workable. Also after plastering is hardened, for obtaining harder surface and adequate performance of heat of hydration reaction, water is used for curing. The water used should not have a pH below 6.0 and should be free from debris and other materials like chloride, sulpher etc. should remain within acceptable limits as per IS code.

12.3.1.2 Special Plastering Materials:

These are the materials used to meet specific requirements as discussed earlier. They are described as follows:

i. Plaster of Paris (Gypsum Plastering): Plaster of Paris is obtained from Gypsum powder on heating it at 1600C to 1700C and is kept dry always as it gets hardened when comes in contact with water within 3 to 4 minutes. Its general use is with lime combination though it may be combined with other combinations also. They are used for ornamental works, sound insulation, fire proofing, to reduce shrinkage, finishing of wood or timber or metal floorings etc due to adhesive property to them. A major drawback of its use is the water solubility tendency of gypsum which should be taken care of.

ii. Acoustic Plaster: This is plaster of gypsum mixture with plaster to obtain a sound proof plastering which is applied as finishing coat in two layers of 6 mm each. The sound insulation property is due to honey combing of the coat on undergoing chemical reaction. These are used therefore in the interior walls of auditorium, picture halls and conference halls etc.

iii. Barium Plaster: Barium sulphate is used for plastering in barium plaster with cement and sand. This is X-ray resistive and hence protects the pathologists and others working in the X-ray room frequently.

iv. Asbestos Plaster: It’s the plaster where combination of cement, asbestos and marble chips is used for plastering; resulting a marble finish which gives an aesthetically attractive looking.

v. Granite Silicon Plaster: These are costly plastering and superior in quality due to the elastic nature of the plastering. This is aesthetically good looking and eliminates any cracking too due to its elastic behavior. Hence in superior finishing works, it’s used.

vi. Scagliola Plaster: It’s the plaster obtained due to mixing of Kenee’s cement and coloring pigments. Its use is on columns and panels etc. for giving marble flooring appearance.

vii. Kenee’s Cement Plaster: In this type of plaster, Kenee’s cement is used which is obtained from the calcination of plaster of paris with alum. It has self polishing property and takes glas like polishing. It’s also very hard and sets in few days thus giving an ornamental look. Hence for precious or decorative plastering, it’s used.

viii. Snowcrete and Colorcrete Cement: The first one is white cement while the second one is colored cement which is used for giving a good looking appearance to the buildings. The names are trade names only.

12.3.2 Tools:

i. Floating Rule: It’s used for checking the plastering surface whether it’s leveled or not.ii. Float: It’s used for applying and spreading mortar on the surface. It’s made up of either by wood

or steel. Steel or metal float is also known as laying trowel. The wooden float is known as skimming float and mainly used for finishing plaster work. The blade size of steel float generally is 10 30 cm2 and the wooden float blade size varies from 10 30 to 11 33 cm2. Sometimes the devil float having nail projection of 3 mm is used for making zigzag on the plaster surface for further work upon it.

iii. Trowel: A trowel is used for throwing the mortar on to the plastering surface by the mason. It’s also used for gauging small quantities of material and applying mortar to moldings, corners etc. The end of the trowel blade may be pointed or may be of bull nose type.

iv. Other Miscellaneous Tools: In this type other tools like steel plate (used for discharging mortar from one place to the working place), set square (used for measurement works), plumb line (used by mason to know the leveling), edge brushes (used for cleaning the plaster surface) etc are used.

12.4 NUMBER OF COATS OF PLASTER :

The back ground over which plaster is to be made decide the number of coats like random rubble, brick masonry, stone masonry, coarse rubble masonry, cement block etc. Therefore plastering is applied in 1 or 2 or 3 coats. Generally lime plaster is applied in 3 coats and cement plaster in two coats.

TABLE 12.1 (PLASTER COATING NUMBERS)

BACKGROUND OF PLSATER NUMBER OF COATS

Stone work 3or 2

Concrete cast in situ 2or 1

Brick work or Hollow clay tile 3, 2 or 1

Building blocks 3, 2 or 1

Fiber building board 2 or 1

Wood wool slabs 3or 2

Wood or metal lath 3

Cork slabs 2 or 1

The above table showing number of coats of plaster is recommended by Indian standard code. The first coat is known as rendering coat or under-coat which strengthen and level an uneven surface. It seals the surface wall and prevents rain penetration. The second coat is known as floating coat. The 3 rd or final coat provides the smooth surface and is known as finishing coat or setting coat. The average thickness of rendering and floating coat may be 10-15 mm and 6-9 mm respectively. The final coat may be of 2-3 mm thick. If plastering is done in a single coat its thickness should not exceed 12 mm neither should be less than 6 mm.

12.5 TYPES OF PLASTERING:

Following types of plastering is generally adopted in practice. Most of them are termed as per their finishing. Others are as per their number of coats and resisting capacity. They are described as below.

12.5.1 Sand faced plaster:

This is a two coat plaster in which the first coat of sand faced cement plaster is done with cement mortar in 1:4 (12 mm thick) with coarse sand after curing the plaster for seven days the second coat of cement plaster 1:1 (8mm thick) ratio will be done on the first coat.

Sponge is used in the second coat and it is applied when the second coat is wet. The surface of the final coat is finished by rubbing clean and washed sand of uniform size by means of wooden float. It ensures the uniform density of sand grain that appears on surface.

Proper leveling is ensured and the surface is kept well watered for at least 15 days.

12.5.2 Smooth cast plaster or plain face plaster:

This finish is just similar to sand faced finish except fine grained sand is used instead of coarse grain sand and the ratio of cement to sand id kept 1:3.

In this case no sponging is done since sand grains are required to be exposed. It is normally done on internal walls. For external walls, steel float is not used; instead wooden float is used as steel float provides

excessive smooth surface and hence the surface may crack due to environmental conditions.

12.5.3 Rough Cast Plaster or Spatter dash plaster:

This plaster is a mixture of sand and gravel in specified proportion which is dashed over a freshly plastered surface.  

The plaster base consists of two coats, under layer 12 mm thick and top layer 10 mm thick  having the cement mortar ratio 1:3. In the final coat of this plaster, concrete is used in the ratio of cement : sand : coarse aggregate as 1:1.5:3 (M20). The coarse aggregate is of the range 3mm to 12mm.

The base is made more plastic, about 10% of hydrated lime by volume of cement shall be added while preparation of mortar.

This finish is water proof, durable and resistant to racking and crazing. Hence it may be used for external renderings.

12.5.4 Pebbled dash plaster:

It is similar to rough cast finish except clean pebble of size 10 mm to 20mm are dashed against the surface so that there are laid in position by mortar applied already. The mortar is in the proportion of 1:3 as cement:sand.

12.5.5 Washed stone Grit Plaster:

This plaster is done in two layers, under layer of 12 mm cement plaster in 1: 4 ratio. This is furrowing the under layer with scratching tools applying cement slurry on the under layer

at the rate 2 kg of cement/m2 and on the top layer 15 mm cement plaster of 1 cement : 0.5 coarse sand : 2 stone chipping 10 mm nominal size in panels with groove all around as per approved pattern.

Top layer is scrubbed with brushes and washed with water to expose the stone chippings.

12.5.6 Depeter Plaster:

This is just another form of rough cast finish. The rendering coat of 12 mm thick is prepared as usual in the case of pebbled dash finish. The coat is made wet; pieces of gravel are pressed with hand on the surface. Thus it is possible to have beautiful patterns and ornamental designs on the surface by selecting

materials of different colors.

12.5.7 Bonding Plaster:

This is the under coat or first coat plaster to be applied to a new (or to be patched) wall. When it is trowelled off, it is scratched with a nail to give a “key” for the top coat, or finish plaster to adhere to it. Bonding plaster has got high “stickabilty” and does not rely on an absorbent surface to bond to.

Bonding has got advantages that it can be applied to really dense concrete blocks or engineering bricks and even concrete itself. These surfaces are where you would use bonding plaster as a scratch coat. Bonding does not need the wall underneath to be scratched or have a mechanical “key” and bonding agents, such as latex SBR adhesive are usually applied to the wall before the bonding plaster itself.

12.5.8 Browning Plaster:

Browning plaster is classified under undercoat plastering for use on more absorbent surfaces. Browning plaster works much better on surfaces with a mechanical key and you will often see brick layers “raking out” the joints of block and brick walls which are to have a Browning plaster covering.

12.5.9 One coat Plaster:

This is otherwise called as patching plaster. This plaster is a mix of binding plaster and multi finish. It is designed to fill patches in one go and can be “laid on” to far greater thicknesses than normal undercoat plasters; anything up to 50mm depending upon the manufacture process.

12.5.10 Multi finish Plaster:

This is a top coat plaster which is suitable for a great finish on all the other surfaces. Multi finish is an ideal for finishing plaster when there are a variety of backing surfaces to be covered.

12.5.11 Hard wall Plaster:

Hard wall is similar to Browning except that it offers a higher impact resistance and quicker drying surface. It is the most often used undercoat plaster these days, and can be applied to most masonry surfaces. Due to its impact resisting factor, it’s mostly preferred in workshops and motor rooms or any such similar impact expected buildings.

12.5.12 Tough coat Plaster:

Even tougher than Hard wall type plaster as an undercoat plaster and has a greater coverage based on the nominal depth of 11mm. It’s also suitable for most masonry walls.

Some other type of plaster is Dri-Coat Plaster which is a cement based plaster and mostly used for resurfacing a wall after installation of new D.P.C. Similarly another type of plaster is board finish plaster mostly used on surfaces of low-medium suction. Another type of plaster is external wall cladding plaster, which is done by Dholpur stone. Another type of plaster is Texture Finish which is basically used with stucco plastering to produce ornamental works.

12.6 METHODS OF PLASTERING:

There are basically three types of plastering i.e. lime plaster and cement plaster or combination of both. Before plastering, however, the back ground should be prepared since the adhesion of plaster is highly affected by the back ground. Hence at first back ground is prepared.

Preparation of Background:

All masonry joints should be raked 10 mm in case of brick masonry and 15 mm in case of stone masonry for providing key to the plaster before onset of plastering operation. All mortar drops, dusts and freshly laid concrete or laitance should be removed with the help of a stiff brush. Rendering is applied after any existing unevenness is removed. For a three coat finishes the local projection should not be more than 10 mm and local depression should not exceed 20 mm. For two coat plaster these limitations are reduced to 5 and 10 mm respectively. The surface should be kept damp for suction and it should be cleaned 1 st.The surface should not be kept soaked and neither it should be kept less wet so as to cause strong suction which withdraws the water from mortar and makes it weak and porous. During the application of plaster on old surface, all dirt, oil, paint etc should be removed along with complete removal of all loose and crumbling plaster to its full thickness. The back ground of the plaster should be exposed clearly before new plaster is applied. The surface should be kept damp to maintain optimum suction.

12.6.1 LIME PLASTER:

After preparation of back ground, lime plaster is applied in three coats normally. However, two coats of plaster may be applied too. They are discussed as follows:

12.6.1.1 Three Coat Plaster:

i. Rendering coat: This is the very first coat applied to the back ground and its thickness is normally of 12 mm. However, this thickness may increase as per surface unevenness. This plaster coat covers all the irregularities and is pressed against the wall forcibly by the help of trowel. This coat is allowed to harden for seven days ad cured well. The surface is already made scratched just after the work is over and the surface is little bit set.

ii. Floating coat: The rendering coat is cleaned and the floating is applied. Then the patches of 15cm × 15cm or 10 cm wide strip is made for acting as gauge followed by throwing of mortar with the help of mason’s trowel which is later spread with the help of wooden float. In case of lime-sand plaster, the finishing coat is applied immediately while in case of lime-surkhi plaster, it’s allowed to harden little bit and cross marks are provided with wooden float edge at a spacing of 4 cm which is cured for 10 days approximately to obtain a set surface.

iii. Finishing coat: As discussed above, the finishing coat follows immediately after the floating coat while in case of lime-surkhi plaster, it’s applied after getting a set and little hard surface. The lime cream to cement ratio is kept 4:1 in finishing coat which is applied with the steel trowel and made smoothen. The surface is left for one day for drying and then cured for seven days.

12.6.1.2 Two Coat Plaster:

This is nothing but combination of all three coats but the rendering and floating coat is applied at a time with out any gap in between the operation. Also the thickness of the combined coat is kept 12 mm and the finishing coat is finished thereafter.

12.6.2 CEMENT AND CEMENT-LIME PLASTER:

12.6.2.1 Two Coat Plaster:

1) The back ground is prepared by raking the surface up to 20 mm depth, followed by cleaning operation of the surface and watering it.

2) For very uneven surface, a preliminary coat is applied to even it and to fill the voids.3) The 1st coat or rendering coat is applied which thickness is equal to the specified thickness of

plaster less 2 to 3 mm. In order to maintain uniformity, screeds are formed on the surface by fixing dots of size 15 cm × 15 cm where vertical line spacing of 2m is kept in between them. They are plumbed by means of plumb bob. A number of screeds are formed at suitable spacing. The cement mortar is then applied on the successive screeds and the plastering is finished.

4) Before rendering hardens, mechanical key for finishing coat is provided in form of scratching it in criss-cross manner. The rendering coat is applied forcing mortar into the joints and over the plaster surface. It’s allowed to wet at least for two days in case of cement plaster and in case of lime plaster 7 days curing is needed and then the surface is left to dry completely.

5) The final coat may be of 2-3 mm thickness. Before application of the final coat, the rendering coat is damped evenly. The final coat is applied with a float and finishing is done using trowel. The coat should be done from top to bottom and should be done in one single operation to eliminate the joint marks.

12.6.2.2 Three Coat Plaster:

Three coat plaster is quite similar to that of three coat plaster of lime mortar. The procedure of applying the three coat plaster is same as two coat plaster except one intermediate coat or floating coat is applied. Its purpose is to bring the plaster surface in to more even and more protection to the plaster surface.

Hence it’s generally done on the outer surfaces. The thickness of rendering, floating, and finishing coat is 9-12, 6-9 and 2-3 mm respectively which may be revised as per the requirements. The floating coat is applied 4-7 days after applying the 1st coat. The finishing coat may be applied after 6-7 hours of application of floating coat.

12.6.2.3 Single Coat Plaster:

This is applied for inferior works. It’s same as two coat plaster except the finishing coat. The rendering coat after sufficiently hardens become the 1st and final coat. The rendering coat therefore should be polished and made even with high quality of workmanship.

12.7 DEFECTS IN PLASTERING:

a. Blistering of plastered surface: This is the formation of small patches of plaster which swells out beyond the plastered surface, arising out of late slaking of lime particles in the plaster. It may also arise due to presence of un slaked particles in slaked lime.

b. Cracking: It consists of formation of cracks and fissures on the plastering surface due to following reasons like :

i. Defective preparation of back groundii. Structural irregularities in buildings

iii. Discontinuity of plastered surfaceiv. Movements in the back ground due to the thermal expansion or rapid drying or shakingv. Movement in the plaster surface itself either due to expansion or shrinkage or due to

creepvi. Excessive shrinkage caused by application of thick coat

vii. Faulty workmanshipc. Crazing: It’s the formation of a series of hair cracks on plastered surface due to the same reason

as for crackingd. Flaking: Due to poor bond between successive coats very loose mass of plastered surface is

formed which is called as flaking.e. Peeling: It’s the complete dislocation of plastered surface resulting from formation of patches.

Formation of imperfect bond is also another reason of this type of defects.f. Pooping: It’s the formation of conical hole on the plastered surface due to the presence of some

expandable materials.g. Rust stains: These are sometimes formed when plaster is applied on metal laths due to rusting

action.h. Efflorescence: This is another defect by which the whitish crystalline material which appears on

the plastered surface due to presence of salts present in the plastering material or building material. It gives the bad appearance and even affects adhesion during painting. It can be removed to some extent by washing the surface continuously and by dry brushing.

i. Uneven Surface: This is obtained purely due to poor workman ship.

The remedy to the above defects lies it self in the defects explanation. Hence no further explanation is sighted here.