Building Construction 14

Dr Nabil El-Sawalhi

Associate professor

Engineering Projects Management

Plaster

• Plaster is a wet mixed material applied to internal walls as a finish to fill in any irregularities in the wall surface and to provide a smooth continuous surface suitable for direct decoration.

• The plaster finish also needs to have a good resistance to impact damage.

CEMENT AND CEMENT-LIME PLASTER FINISH

• Preparatory Work

– Preparation of Background for Application of Plaster

• a) For the durability of the plaster or rendering, it is vital to obtain a satisfactory bond between the background and the first plaster coat and also to ensure that the bond is maintained sub-sequently.

• b) It is also important to clean the background; remove dust, laitance, etc.

• The background should be made rough and then moistened, joints shall be raked.

• Suction of the back- ground should be suitably adjusted.

• The back- ground shall be even.

• The strength and elasticity of plaster shall be compatible with the background.

• Precautions against discontinuity of the background should be taken to prevent cracking; the best treatment appears to be to separate the two backgrounds by a neat cut through the plaster at the junction.

• c) Brickwork or Hollow Block Background –

• The masonry shall be allowed to dry out for sufficient period so that the initial drying shrinkage is fairly complete and suction adjustment is possible during plastering.

• Joints shall be raked out and projecting bricks trimmed off.

• Old brick work shall be thoroughly brushed down and low spots dubbed out by a mix similar to the first coat to be applied.

• d) Concrete Background –

• The surface shall be clean and rough. The surface shall be evenly wetted (not saturated) to provide correct suction.

• All projections shall be removed.

• Sequence of Operations

• For external plaster, the plastering operations may be started from the top floor and carried downwards.

• For internal plaster, the plastering operations may be started wherever the building frame and cladding work are ready and the temporary supports of the ceiling resting on wall or the floor have been removed.

• Constant supply of plaster shall be ensured.

• The first undercoat is then applied to ceilings and walls.

• It is an advantage to plaster the ceiling first to remove the scaffolding before the plastering work on the wall.

• After a suitable interval, not more than 5 days, the second coat may be applied.

• After a further suitable interval, the finishing coat may be applied first to the ceilings and then the walls.

• Plastering of cornices, decorative features, etc, shall normally be completed before the finishing coat is applied.

• Holes in walls left by scaffolding shall be made good and plastered true, even and smooth in conformity with the rest of the wall.

• Where corners and edges have to be rounded off, this should be done along with the finishing coat, so that no sign of any joint shows out later.

Reinforcement for angles

• A range of galvanised steel beads and stops is made for use with plaster and plasterboard as reinforcement to angles and stops at the junction of wall and ceiling plaster and plaster to other materials. An angle bead is pressed from a steel strip to form reinforcement to angles. The bead has expanded metal wings, as shown in Figure 10.7. The wings of the bead are bedded in plaster dabs on each side of the angle. The bead is then squared and plaster

• A metal stop with an expanded metal wing is pressed from steel strip and used as a stop to make a neat finish at the junction of plaster with other materials at angles, skirtings and around doors and windows,

Thickness of Plaster

• Finishing coats (and single coat work, when employed) shall be such minimum thickness that provides sufficient body material to harden satisfactorily under-site conditions in any particular case.

• The total thickness of two coat work, excluding keys and dubbing out shall be about 20 mm thick and not more than 15 mm thick in case of in-situ concrete soffits.

• The thickness of three coat work shall be about 25 mm. The thickness of an individual coat shall be generally as recommended in Table 10.1.

Number of Coats

• a) The ideal number of coats, where practical, is two, namely, the undercoat followed by finishing coat.

• This is possible on reasonably plane backgrounds of brick, concrete and similar materials.

• However, for very rough surfaces, such as, rough stone masonry, three coat plastering may be necessary.

• Metal lathing requires three coat plaster finish. • Renovation work on wooden laths should also be

carried out in three coats.

Application of Undercoats

• First Coat/Rendering Coat –

• The rendering coat shall be at least 10 mm thick and carried to the full length of the wall.

• Before it hardens it shall be roughened to provide key to the next coat.

• The coat should be trowelled hard and tight.

• On smooth concrete walls, the surface shall be roughened and then the first coat shall be dashed on it.

• Second Coat/Floating Coat • The surface of the first coat shall be dampened

evenly, the second coat shall be 3 mm to 8 mm thick.

• Finishing Coat • The second coat shall be dampened. • Coloured cement work - The pigment may be

intimately ground with cement clinker, or pigments are added to the white cement or ordinary grey cement to get the required shade.

• In case of coloured cement plastering, waterproofing compounds should be added on the undercoats to minimize the risk of efflorescence.

• Curing

• a) Each coat shall be cured for a maximum period of 7 days and-shall be kept damp till the next coat is applied.

• b) After completion of the finishing coat, the plaster shall be kept wet at least for 7 days

EXTERNAL RENDERED FINISHES

• ‘Rendering’ denotes generally external plaster and allied finishes, plain or textured, applied for the purpose of protection and decoration.

• Rendered finish can withstand different exposure conditions, such as, severe or moderate exposure to rain; dusty, clear or corrosive atmosphere, or subject to large variations in temperature or other climatic cycles.

• Rendering increases durability and ensures better performance.

• Types of Rendered Finishes – Trowelled or Floated Plain Finishes

• Steel trowelled finishes are not recommended for ex- ternal renderings. Plain floated finishes require a high standard of workmanship to minimize the risk of cracking, crazing and irregular discolouration.

– Scraped or Textured Finishes, Hand Applied • Scraped or textured finishes are generally less liable to

crack or craze than plain finishes and are easier to bring to a uniform appearance; this is specially important when coloured renderings are used.

• Although rough texture offers more lodgement for dirt, it tends to an evenness of discolouration; rain water is distributed reducing the risk of penetration through the rendering.

– Hand-Thrown Finishes, Rough Cast and Dry-Dash, etc

• These finishes have all the advantages of scraped or textured finish. Under severe conditions of exposure these are more satisfactory from weather proofing, durability, reistance to cracking and crazing

– Machine Applied Finishes

• There are various types, mainly proprietary materials and processes operated by specialists. The types are as follows:

– Finishes in which the material is thrown at random –

• These have an open porous structure, behave similarly to hard-applied scraped finishes and are equal to the latter and to hand- thrown finishes in water proofness, durability and resistance to cracking and crazing.

– Finishes which are applied by gun spray

• These give low density porous finishes. There are other finishes solely applied by means of cement gun under pressure which produce somewhat similar appearance to roughcast but less attractive. They are more dense and therefore give very effective protection.

• Nature of Background in Relation to Choice of Rendering

– Broadly the background may be classified under the following types so far as application of rendering is concerned:

– a) Dense, strong and smooth materials;

– b) Moderately strong and porous materials;

– c) Moderately weak and porous materials;

– d)No-lines concrete; and

– e) Lathing or other similar backing materials.

Recommended Mix Proportions

• Mixes suitable for different types of renderings are given in Table 10.4.

• Where alternate mixes are given selection shall be made on the following considerations:

– The mix for each successive coat shall never be of a type richer in cement than the mix used for the coat to which it is applied; and

– Richer type of mix shall be preferred in winter conditions.

Preparatory Work • Independent scaffolding may be used wherever possible to avoid put

log holes.

• The background shall be brushed with a stiff bristle or wire brush.

• Laitance if any on fresh concrete shall be removed.

• The background shall be roughened and moistened.

• Joints in masonry shall be raked to a depth of 1 cm.

• Adequate time shall be allowed between erection and application of rendering to make the surface suitable for suction adjustment.

• The backing shall be wetted, if it is dry, for this purpose; it shall not be soaked but dampened evenly.

• Ensure evenness of background before applying rendering.

• Differential movements of mixed backgrounds shall be provided for.

Application of Rendering Coats

• A)General • -When using high alumina cement, the work shall

not be done in hot weather. Between coats, give 24 h interval in hot and dry weather and more in cold weather.

• B) First Undercoat • - This may be applied either by laying or throwing

from the trowel. It shall be as uniform as possible up to a thickness of 8 to 12 mm. It shall be rough and then combed and scratched to provide a key to the next coat.

• C)Subsequent undercoat, if necessary, may Abe laid similarly to (b).

• D)Plain Finishing Coats - The finishing coat shall be not less than 3 mm or more than 8 mm thick; the coat shall be laid with a trowel and finished with a wood, felt, cork or other suitably faced float.

• E)Scraped or Textured Coats - Various scraped or textured finishes can be obtained by hand or machine application. The thickness of the final coat is governed by the texture required. For scraped finishes the final coat is of a thickness of 6 to 12 mm of which about 3 mm is removed in the scraping process. For textural finish the surface skin of the mortar shall be removed to expose the aggregate.

• F)Roughcast Finish -The final coat, the mix of which shall be quite wet and plastic, is thrown on by means of a trowel or scoop.

• G) Dry-Dash Finish – 1) For this finish the undercoat shall be up to a

thickness of 8 mm and slightly pressed. – 2)The aggregate used for dashing shall be well washed

and drained. – 3) To ensure satisfactory bond between the dashing

and mortar, the aggregate may be slightly tapped into the mortar with a wood float or to a trowel.

• H)Machine-Applied Finishes - All damaged adjacent surfaces shall be masked. The finish shall be applied at such a rate as to ensure the desired texture.

• Curing

– The moistening shall commence as soon as the plaster has hardened; the surface shall be kept wet for at least 7 days by using a fine fog spray. Soaking of wall shall be avoided.