Plastering and pointing

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1. Plastering and Pointing

2. Plastering Process of covering rough surfaces of walls, columns, ceilings and other building components with thin coat of plastic mortars to form a smooth durable surface

3. Objects of plastering To protect external surfaces against penetration of rain water and other atmospheric agencies To give smooth surface in which dust and dirt cannot lodge To give decorative effect To protect surfaces against vermit To conceal inferior materials or defective workmanship

4. TYPES OF MORTAR FOR PLASTERING LIME MORTAR m ay be either fat lime or hydraulic lime Fat lime yields good putty after slaking; preferred more Hydraulic lime contains particles which slake very slowly; slow slaking may cause blisters Mix proportion(lime: sand)- 1:3 to 1:4 for fat lime - 1:2 for hydraulic lime Addition of “ gugal ” can improve the binding properties of lime mortar Mixing of chopped hemp can improve adhesive and tensile properties of lime mortar

5. CEMENT MORTAR Being non- absorbant , it is best for external plastering work Preferred in damp rooms and damp climates Stronger than lime mortar Does not possess sufficient plasticity Mix proportion (cement: sand)- 1:4 to 1:6 Mortar should be used before initial setting takes place Sand used should be clean, coarse and angular LIME-CEMENT MORTAR Contains properties of both lime mortar as well as cement mortar Addition of lime imparts plasticity resulting in smooth plastered surface Mix proportions (cement: lime: sand)- 1:1:6, 1:1:8 or 1:2:8

6. Preparation of background Joints should be raked to a depth of 10 mm in brick masonry and 15 mm in stone masonry for providing key to plaster Mortar droppings and dust should be removed with wire brush Unevenness is levelled before applying mortar Surface should be washed with clean water uniformly to produce optimum suction; excess soaking with water may cause sliding of mortar before it sets; less moisture may cause strong suction which withdraws moisture from mortar and makes it weak

7. METHODS OF PLASTERING LIME PLASTER- applied in 3 coats or in two coats Three-coat plaster Application of rendering/first coat : Mortar is forcibly applied with mason’s trowel and pressed well into joints and over the surface; Normal thickness-12mm (cover all inequalities of the surface); Surface is allowed to slightly harden and then scratched criss-cross ; left to set for 7 days (curing and drying) Application of floating/second coat: Rendering coat is cleaned off all dirt and loose material; Lightly wetted; Patches (15cm X 15cm or strips 10cm wide) applied at suitable spacing to act as gauges; Mortar is then thrown with mason’s trowel; spread and rubbed with wooden float; In case of lime- surkhi plaster, Surface is allowed to slightly harden and then scratched criss-cross ; left to set for 10 days (curing and drying)

8. Application of finishing coat : In case of lime-sand mortar, finishing coat is applied immediately after floating coat; It consists of lime cream and sand (4:1) applied with steel trowel and rubbed and finished smooth; It is left for 1 day and then curing is done for atleast 7 days; In case of lime- surkhi mortar, finishing coat is applied after 7 days of floating coat, after cleaning the surface; Finishing coat is rubbed hard and finished smooth Two-coat plaster Rendering coat of two-coat plaster is combination of rendering and floating coats of three-coat plaster; done under one continuous operation (except scratching of coats) Finishing coat is applied in a similar manner to that of three-coat plaster

9. CEMENT PLASTER AND CEMENT-LIME PLASTER Two-coat plaster Preparation of background If surface to be plastered is uneven, preliminary coat is applied to fill up the hollows Rendering coat is applied thickness equal to specified thickness of plaster less 2-3 mm In order to maintain uniform thickness of plaster, screeds are formed of plaster on wall surface by fixing dots of 15 cm X 15 cm. Two dots are formed in vertical line at a distance of about 2 m by means of plumb bob. A vertical strip of mortar known as screed is then formed. A number of such vertical screeds are formed at suitable spacing. Cement mortar is then applied on the surface between the successive screeds and surface is properly finished.

10. Rendering coat is kept wet for atleast 2 days, and then allowed to dry completely Rendering coat is again damped evenly before applying final coat; final coat is applied with wooden floats and finished with steel trowels. To eliminate joining marks, finishing coat should be applied starting from top towards bottom and completed in one operation. Three-coat plaster Intermediate coat known as floating coat is applied Thickness: rendering coat-9-10 mm (rough coat) floating coat- 6-9 mm (applied 4-7 days after applying first coat) finishing coat- 2-3 mm (applied about 6 hours after applying floating coat) Single-coat plaster used only for inferior quality work applied as two-coat plaster rendering coat is finished off immediately after it has sufficiently hardened

11. Special materials used in plastering Acoustic plaster contains gypsum mixtures applied as final coat undergoes chemical reaction resulting in production of gas bubbles and tiny opening in coat honey-combed openings absorb sound useful in interior walls of halls, auditoriums etc. applied in two coats each of 6mm Asbestos plaster made of cement, asbestos and finely crushed marble imparts marble-like finish

12. Barium plaster made from cement, sand and barium sulphate provided in X-ray rooms to protect persons working in it Granite silicon plaster used for superior type of construction quick setting possess highly elastic properties which eliminate cracks Gypsum plaster/ plaster of paris hardens within 3-4 minutes of adding water; retarders are added to increase setting time used in combination with lime used for ornamental work, repairing holes and cracks properties- fire-resisting, light in weight, sound-insulating, good adhesion to fibrous materials

13. Kenee’s cement plaster obtained by calcinating POP with alum very hard and sets in few days polishing characteristics used for angles, skirtings also useful for ornamental work and decorative plastering Martin’s cement plaster pearl ash(potassium carbonate) is calcined with POP has quick setting properties forms white hard surface on drying used for internal finishing work Parian cement plaster borax is calcined with POP used for interior works cheaper than Kennee’s cement

14. Scagliola plaster obtained by dissolving Kenee’s cement and coloring pigments in glue used for plastering panels, columns etc appears like marble Sirapite plaster obtained when POP is slaked in petroleum quick setting fire resisting produces white hard surface on drying

15. Defects in plaster Blistering of plastered surface formation of small patches of plaster swelling out beyond plaster surface caused due to late slaking of lime particles in plaster Cracking formation of cracks or fissures in plaster caused due to- imperfect preparation of background structural defects in building discontinuity of surface movements in background due to thermal expansion and drying movements in plaster surface itself faulty workmanship excessive shrinkage due to application of thick coat

16. Crazing Formation of series of hair cracks on plastered surface causes- same as that of cracking Efflorescence presence of whitish crystalline substance on the surface caused due to presence of salts in plaster-making substance and building materials affects adhesion of paint with wall surface Flaking formation of very loose mass of plastered surface caused due to poor bond between successive coats Peeling complete dislocation of some portion of plastered surface, resulting in formation of patch caused due to imperfect bond

17. Popping formation of conical hole in plastered surface due to presence of some particles which expand on setting Rust stains formed when plaster is applied on metal laths Uneven surface obtained purely due to poor workmanship

18. POINTING Finishing of mortar joints in masonry

19. Pointing consists of raking the joints to a depth of 10-20 mm and filling it with better quality mortar in desired shape.

20. What raking means?

21. MORTAR: pointing is done with following mortar mixes: Lime-mortar- 1:2 mix (lime: sand/ surkhi ) cement mortar- 1:3 mix (cement: sand) PREPARARTION OF SURFACE New work- joints raked down to depth of 20 mm while mortar is still soft; surface and joints cleaned and thoroughly wetted Old work- loose pointing and superfluous mortar on surface and joints removed; surface and joints cleaned and thoroughly wetted

22. METHOD OF POINTING Surface prepared, cleaned and wetted With the help of small trowel, mortar placed in joints in desired shape Mortar pressed to bring perfect contact between old interior mortar of joint and new mortar Care should be taken that mortar does not cover face edges in case of first-class brickworks Surface is kept wet for atleast a week or till it sets after application

23. TYPES OF POINTING Flash pointing formed by pressing mortar in raked joint and by finishing off flush with edge of masonry units edges are neatly trimmed with trowel and straight edge does not give good appearance durable as it does not provide any space for accumulation of dust, dirt etc. extensively used

24. Recessed pointing pointing is done by pressing mortar back from edges by 5mm or more face of pointing kept vertical gives very good appearance Rubbed, keyed or grooved pointing modification of flush pointing groove at its mid height formed with a pointing tool gives better appearance

25. beaded pointing formed by steel or ironed with a concave edge gives good appearance liabale to damage easily Struck pointing modification of flush pointing face of pointing is inclined, upper edge pressed inside the face by 10 mm drains water quickly

26. Tuck pointing formed by pressing mortar in racked joint, finishing flush with face groove or narrow channel (5 mm width, 3 mm depth) is cut in the centre while pressed mortar is green groove is tucked/ filled with white cement putty, projecting beyond face of joint by 3 mm if projection is done in mortar, it is called Bastard pointing or half- tuck pointing.

27. V- pointing forming V-groove in flush-finishing face Weathered pointing forming V-shaped projection