BY:ABHISEK PANDA12.0 PLASTERING

12.1 INTRODUCTION AND TERMINOLOGY:

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

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

i. Back ground: It’s the surface on which plastering work isstarted.

ii. Hacking: It’s the process of making the back ground rough foradequate bondage with the plaster and formation of key.

iii. Key: This is the indentations (scratch marks) on the background to which plaster forms a bond and do not get displacedor spalls off.

iv. Ground: It’s the wooden strip fixed to back ground on whichprimary finishing is secured.

v. Dado: It’s the lower part of plastered wall finished with neatcement slurry to make it better water resistive.

vi. Rendering coat/Under coat: This is the coat applied underfinishing coat.

vii. Floating coat: It’s the second coat applied just before thefinishing coat

viii. Finishing coat: It’s the final coat known as skimming coat orsetting coat.

ix. Blistering: It’s the development of local swelling caused dueto residual unslaked lime and is located on the finishedplastered surface.

x. Dot: It’s the small projection of size 15 15 cm2 plaster madeintentionally for fixing of screeds or similar materials.

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xi. Peeling: It’s the dislodgement of plastered work from the background of plaster.

xii. Cracking and Crazing: Cracking is the development of one ormore fissures on/in the plastered surface due to back groundmovement or vibration while crazing is development of haircracks 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 rainwater 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 offduring bad weather.

ii. It should be inert and should not chemically react withreinforcement 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 arecategorized as general plastering material and special plasteringmaterial. General plastering materials are the materials used for

BY:ABHISEK PANDAplastering everywhere and irrespective of any special requirement andthese forms the parent material even for special plastering materialsand 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 typesof 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 overhydraulic lime though hydraulic lime gives a stronger andharder surface than fat lime just because of the reason thathydraulic lime is very slow in slaking and even continuesmonths (6 to 8 months) and normally used in dry condition withground dry with sand for twice in an interval of 2 to 3 weeks.Also blisters may form if unslaked particles remained inslaked hydraulic lime. However, fat lime can be slaked wet andeasy to use. Lime mortar can be used with proportion lime :sand as 1:3 to 1:4 and 1:2 for hydraulic lime mortar andcement : lime : sand as 1:1:8 to 1:2:8 proportion. For purelylime mortar with no cement, a binding material named gugal canbe added @ 1.6 kg/m3 to the mortar. Also the tensile propertiesof lime is increased with the addition of chopped hemp @ 1kg/m3

of mortar which ultimately prevents the formation of tensilecracks.

ii. Cement: Usage of cement as plastering material is normallyadopted for external wall though in today’s scenario, cementplasters are used in all most all type of plastering. Theadvantages lie in quick finishing, high workability, noadditional binder requirement, and hardened surface as well asnon absorbent property of cement. Details of cement arealready discussed earlier. The commonly used cement forbuilding works is Portland Pozzolana Cement or PPC of grade 43or 53 as available in the market. Also Portland slag cements(PSC) is used for the purpose. The ratio of cement to sand

BY:ABHISEK PANDA

varies from 1:4 to 1:6 in cement plaster and for lime-cementplaster, its ratio varies between cement : lime : sand as1:1:6 to 1:2:8.

iii. Sand: Sand is classified under fine aggregate category wherethe 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 isnot used any type of construction work. For plastering ofroofs or heavy duty floors, where greater hardness is desired,instead of fine sand, broken granite chips of smaller size isused. But sand is a major material for all type of plastersand even on heavy duty floors, coarse sand can replace thesmaller broken granite chips.

iv. Water: Water is another constituent in plastering. Desiredamount of water is added to plastering for making the mortarworkable. Also after plastering is hardened, for obtainingharder surface and adequate performance of heat of hydrationreaction, water is used for curing. The water used should nothave a pH below 6.0 and should be free from debris and othermaterials like chloride, sulpher etc. should remain withinacceptable limits as per IS code.

12.3.1.2 Special Plastering Materials:

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

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

BY:ABHISEK PANDA

ii. Acoustic Plaster: This is plaster of gypsum mixture withplaster to obtain a sound proof plastering which is applied asfinishing coat in two layers of 6 mm each. The soundinsulation property is due to honey combing of the coat onundergoing chemical reaction. These are used therefore in theinterior walls of auditorium, picture halls and conferencehalls etc.

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

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

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

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

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

viii. Snowcrete and Colorcrete Cement: The first one is white cementwhile the second one is colored cement which is used forgiving a good looking appearance to the buildings. The namesare trade names only.

12.3.2 Tools:

BY:ABHISEK PANDAi. 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 metalfloat is also known as laying trowel. The wooden float is knownas skimming float and mainly used for finishing plaster work. Theblade size of steel float generally is 10 30 cm2 and the woodenfloat blade size varies from 10 30 to 11 33 cm2. Sometimes thedevil float having nail projection of 3 mm is used for makingzigzag on the plaster surface for further work upon it.

iii. Trowel: A trowel is used for throwing the mortar on to theplastering surface by the mason. It’s also used for gauging smallquantities of material and applying mortar to moldings, cornersetc. The end of the trowel blade may be pointed or may be of bullnose type.

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

BY:ABHISEK PANDA

12.4 NUMBER OF COATS OF PLASTER :

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

BY:ABHISEK PANDA

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 byIndian standard code. The first coat is known as rendering coat orunder-coat which strengthen and level an uneven surface. It seals thesurface wall and prevents rain penetration. The second coat is knownas floating coat. The 3rd or final coat provides the smooth surface andis known as finishing coat or setting coat. The average thickness ofrendering 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 asingle coat its thickness should not exceed 12 mm neither should beless than 6 mm.

12.5 TYPES OF PLASTERING:

Following types of plastering is generally adopted in practice. Mostof them are termed as per their finishing. Others are as per theirnumber 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 facedcement plaster is done with cement mortar in 1:4 (12 mm thick)with coarse sand after curing the plaster for seven days thesecond coat of cement plaster 1:1 (8mm thick) ratio will be doneon the first coat.

Sponge is used in the second coat and it is applied when thesecond coat is wet. The surface of the final coat is finished byrubbing clean and washed sand of uniform size by means of woodenfloat. It ensures the uniform density of sand grain that appearson surface.

BY:ABHISEK PANDA

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

12.5.2 Smooth cast plaster or plain face plaster:

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

In this case no sponging is done since sand grains are requiredto 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 andhence 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 specifiedproportion which is dashed over a freshly plastered surface.  

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

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

This finish is water proof, durable and resistant to racking andcrazing. 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 mmto 20mm are dashed against the surface so that there are laid inposition by mortar applied already. The mortar is in the proportion of1:3 as cement:sand.

12.5.5 Washed stone Grit Plaster:

This plaster is done in two layers, under layer of 12 mm cementplaster in 1: 4 ratio.

This is furrowing the under layer with scratching tools applyingcement slurry on the under layer at the rate 2 kg ofcement/m2 and on the top layer 15 mm cement plaster of 1 cement :

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0.5 coarse sand : 2 stone chipping 10 mm nominal size in panelswith groove all around as per approved pattern.

Top layer is scrubbed with brushes and washed with water toexpose 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 differentcolors.

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:

BY:ABHISEK PANDAThis is otherwise called as patching plaster. This plaster is a mix ofbinding 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 manufactureprocess.

12.5.10 Multi finish Plaster:

This is a top coat plaster which is suitable for a great finish on allthe 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 impactresistance 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 inworkshops 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 basedplaster and mostly used for resurfacing a wall after installation ofnew D.P.C. Similarly another type of plaster is board finish plastermostly used on surfaces of low-medium suction. Another type of plasteris external wall cladding plaster, which is done by Dholpur stone.Another type of plaster is Texture Finish which is basically used withstucco plastering to produce ornamental works.

12.6 METHODS OF PLASTERING:

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

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Preparation of Background:

All masonry joints should be raked 10 mm in case of brick masonry and15 mm in case of stone masonry for providing key to the plaster beforeonset of plastering operation. All mortar drops, dusts and freshlylaid concrete or laitance should be removed with the help of a stiffbrush. Rendering is applied after any existing unevenness is removed.For a three coat finishes the local projection should not be more than10 mm and local depression should not exceed 20 mm. For two coatplaster these limitations are reduced to 5 and 10 mm respectively. Thesurface should be kept damp for suction and it should be cleaned1st.The surface should not be kept soaked and neither it should be keptless wet so as to cause strong suction which withdraws the water frommortar and makes it weak and porous. During the application of plasteron old surface, all dirt, oil, paint etc should be removed along withcomplete removal of all loose and crumbling plaster to its fullthickness. The back ground of the plaster should be exposed clearlybefore new plaster is applied. The surface should be kept damp tomaintain optimum suction.

12.6.1 LIME PLASTER:

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

12.6.1.1 Three Coat Plaster:

i. Rendering coat: This is the very first coat applied tothe back ground and its thickness is normally of 12 mm.However, this thickness may increase as per surfaceunevenness. This plaster coat covers all theirregularities and is pressed against the wall forciblyby the help of trowel. This coat is allowed to harden forseven days ad cured well. The surface is already made

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scratched just after the work is over and the surface islittle bit set.

ii. Floating coat: The rendering coat is cleaned and thefloating is applied. Then the patches of 15cm × 15cm or10 cm wide strip is made for acting as gauge followed bythrowing of mortar with the help of mason’s trowel whichis later spread with the help of wooden float. In case oflime-sand plaster, the finishing coat is appliedimmediately while in case of lime-surkhi plaster, it’sallowed to harden little bit and cross marks are providedwith wooden float edge at a spacing of 4 cm which iscured for 10 days approximately to obtain a set surface.

iii. Finishing coat: As discussed above, the finishing coatfollows immediately after the floating coat while in caseof lime-surkhi plaster, it’s applied after getting a setand little hard surface. The lime cream to cement ratiois kept 4:1 in finishing coat which is applied with thesteel trowel and made smoothen. The surface is left forone 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 renderingand floating coat is applied at a time with out any gap in between theoperation. Also the thickness of the combined coat is kept 12 mm andthe 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 mmdepth, followed by cleaning operation of the surface and wateringit.

2) For very uneven surface, a preliminary coat is applied to even itand to fill the voids.

3) The 1st coat or rendering coat is applied which thickness is equalto the specified thickness of plaster less 2 to 3 mm. In order to

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maintain uniformity, screeds are formed on the surface by fixingdots of size 15 cm × 15 cm where vertical line spacing of 2m iskept in between them. They are plumbed by means of plumb bob. Anumber of screeds are formed at suitable spacing. The cementmortar is then applied on the successive screeds and theplastering is finished.

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

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

12.6.2.2 Three Coat Plaster:

BY:ABHISEK PANDAThree coat plaster is quite similar to that of three coat plaster oflime mortar. The procedure of applying the three coat plaster is sameas two coat plaster except one intermediate coat or floating coat isapplied. Its purpose is to bring the plaster surface in to more evenand more protection to the plaster surface. Hence it’s generally doneon the outer surfaces. The thickness of rendering, floating, andfinishing coat is 9-12, 6-9 and 2-3 mm respectively which may berevised as per the requirements. The floating coat is applied 4-7 daysafter 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 plasterexcept the finishing coat. The rendering coat after sufficientlyhardens become the 1st and final coat. The rendering coat thereforeshould 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 smallpatches of plaster which swells out beyond the plastered surface,arising out of late slaking of lime particles in the plaster. Itmay also arise due to presence of un slaked particles in slakedlime.

b. Cracking: It consists of formation of cracks and fissures on theplastering 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 cracking

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d. Flaking: Due to poor bond between successive coats very loosemass of plastered surface is formed which is called as flaking.

e. Peeling: It’s the complete dislocation of plastered surfaceresulting from formation of patches. Formation of imperfect bondis also another reason of this type of defects.

f. Pooping: It’s the formation of conical hole on the plasteredsurface due to the presence of some expandable materials.

g. Rust stains: These are sometimes formed when plaster is appliedon metal laths due to rusting action.

h. Efflorescence: This is another defect by which the whitishcrystalline material which appears on the plastered surface dueto presence of salts present in the plastering material orbuilding material. It gives the bad appearance and even affectsadhesion during painting. It can be removed to some extent bywashing 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 defectsexplanation. Hence no further explanation is sighted here.