PLASTERING AND DRY LINING

Building Construction III

Spring 2012

Dr.Hikmat Hammad

Lecture 2

Learning outcomes

After Studying this chapter, students should be

able to:

• Identify the purpose of plastering in a building

• I identify the materials used in plasters

• Identify different types of plasters

• Identify the application procedure

• Identify the common problems in plasters

• Identify the application of dry lining

General

Plastering can be defined as the process of covering rough surfaces with thin coat of plastic material to form a smooth durable and jointless surface.

• The coating of plastic material is termed as “plaster”.

• The term plastering is usually called the application to internal wall and ceiling finishes.

• The application to the outside surface

which are subject to severe atmospheric

effects is known as “Rendering” or

“Stucco”

• The main binding agent in “Rendering” is

portland cement

The purpose of Plastering

Plastering and Rendering are applied to uneven

surface to achieve the following objectives:

• To protect the surface against atmospheric

effect such as sun, rain, snow, etc.

• To provide smooth, hygienic surface

• To conceal defects in the background and

provide decorative effect.

• To provide thermal insulation, sound insulation

and fire protection

• The plastering work is wet and messy

process and is greatly effected by the

weather conditions.

• The application is difficult, time-consuming

and costly.

• Nowadays in modern construction the

application of plastering is reduced and

replaced by various types of dry lining on

interior surfaces and cladding on exterior

surfaces.

• Exterior plastering (rendering) requires the

consideration of hazards such as water

penetration, frost effect, corrosion of steel

accessories placed in the plaster, stresses

induced due to wide variations of

temperature and humidity, wind and dust.

• Therefore, the plaster mixes and

application techniques on outside surface

should be different than interior

application.

• Since it is subject to direct wetting due to

rain, rendering requires cementitious

binding material such as Portland cement,

and the Masonry Cement.

• The use of lime alone as a binder has

been practiced in the past but now it is

used as an admixture to the cement to

improve workability.

Materials used in Plastering

• Plastering is the application of plastic

mortars on the hard rough surfaces.

• The plastic mortars are consist of a

binding material, aggregate, water and

certain admixtures to improve the quality.

• The binding materials used in the mortars

are Portland cement, masonry cement and

lime.

• Portland Cement: due to its strong binding ability, normal Portland Cement is widely used in plastering work.

• It is available natural gray or white color.

• White cement can be used in finishing coat with colored aggregate to produce desired decorative effect.

• For areas subject to sulphate attack “Sulphate Resisting Cement” is recommended.

• In cold weather “high Early Strength Cement” should be used.

• It can gain higher strength in the first few days which minimizes the frost damage on the plaster.

• Masonry Cement: it is manufactured from

normal Portland cement by the addition of

certain admixtures such as lime and air

entraining agents to increase the

workability.

• Lime: the use of lime alone in the plaster mix is now rare. It is used as admixture

• Quicklime (CaO) is slaked by addition of water and the excess water is drawn out to obtain hydrated lime.

• Hydrated lime is added to the mortar during mixing time.

• Lime may have some impurities from natural source which effect the final properties.

• Magnesium, aluminum and Iron Oxides are common impurities.

• Magnesium oxide in the lime produce highly workable mixes but it takes longer time to hydrate.

• Aggregate: the aggregates for plastering are usually sand or lightweight aggregates such as perlite and vermiculite.

• sand used in the plaster mixes can be obtained from the rivers, from the sea or by crushing stones.

• Aggregate constitutes the major part of the mortar mixture by volume.

• Therefore, it is important that the aggregate should be clean, sound and well graded with particles ranging from fine up to a maximum size.

• For undercoats coarser sands are used

then the finishing coats.

• Grading of sand is very important to fill the

voids between larger particles and smaller

particles.

• This increases the workability and reduces

the shrinkage of the mix.

• B.S.1191 gives three different

grading of sand to be used in

undercoats and finishing coat of

plaster.

Type IIType IB.S.Sieve

1001005.0 mm

90-10090-1002.36 mm No. 7

70-10070-1001.18mm No.25

40-8040-80600 µm No.25

5-505-40300 µm No.52

0-100-10150 µm No.100

• Water: water for mixing and curing should

be clean and free from harmful materials

such as oils, acids, alkalies, salts or

organic substances that could effect the

final quality of the work.

• Any drinkable water may be used for

mortar preparation.

• Admixture: certain type of admixture such

as asbestos, animal, vegetable or glass

fibbers as well as certain chemicals can be

added to the plaster mortars.

• These are generally added to undercoat

plasters to improve bonding to the

background.

• All admixtures should be free from dirt and

chemicals harmful to the mortar.