PE 459 LECTURE 2- natural gas basic concepts and properties
Defects in plastering
1.
2. 1. Plaster De-bonding
• Bond failure between plasterwork and background
• This is often the result of background movement or deflection.
• However, it can also be due to a weak bond caused by poor bonding
of the background
• Inadequate control of suction,
• Use of the wrong grade of plaster or the presence of residual
moisture in the structure
• Excessive thick plaster
DEFECTS IN PLASTERING
Prevention / Remedies
• Cleaning dusty or oily wall surfaces thoroughly.
• Allowing the walls to reach the correct moisture content.
• Using a cement slurry
• The background should be prepared and a coat of chemical Bond-it applied prior to the
application of a suitable grade of plaster
3. 2. Bond failure between finish plaster and sand/cement undercoat
• It normally results from applying plaster over a ‘green’ undercoat (i.e. insufficient time allowed for
the sand/cement undercoat to complete its drying shrinkage) and/or
• Inadequate scratching of the undercoat surface to provide a mechanical key.
• It can also be caused by local exposure to excessive heat.
• The affected plasterwork should be stripped off. Re-plastering can take place once the background
is thoroughly dry after first applying Thistle Bond-it in accordance
DEFECTS IN PLASTERING
4. 3.Cracking
• There are many types of cracks appears in plastering – some are non-structural and some
are structural.
• Structural cracks are mainly formed due to the movement of the structure. Such as, if a crack formed
in the masonry wall, then a crack will also be formed on the plastered surface.
• This is usually due to movement in the background/surrounding structure or to drying shrinkage in
sand/cement undercoats.
• When the cracking is due to movement, it takes the form of localised cracks taking a definite line.
• On walls, they commonly appear around lintels and window sills due to settlement and/or thermal
movement.
• When cracking results from drying shrinkage, it takes the form of hair-line cracks on the surface of
the finish plaster.
causes for forming non-structural cracks are-
• If the high cement content mix is used for plastering.
• If the plaster dries too quickly.
DEFECTS IN PLASTERING
5. 4.Crazing/dry-out of finish plaster
• In hot weather conditions, or in situations where the finish plaster has been applied exceptionally
thin, rapid drying out of the plaster can occur before it has properly set.
• This can result in a soft and powdery surface.
• The plasterwork should be stripped off and any dust removed from the undercoat surface prior to re-
plastering.
DEFECTS IN PLASTERING
6. 5. Dampness within the plaster
• This can result from rain penetration, moisture ingress, defective DPC or poor drying conditions
preventing the plaster from drying out.
• The source of the problem must be identified and corrected and the plasterwork allowed to dry out.
If the plasterwork fails to develop adequate strength, it will need to be removed, followed by
suitable background preparation and re-plastering.
DEFECTS IN PLASTERING
7. 6. Lack of Hardness
• We normally use a key or a coin or nail to check the hardness of a plaster surface.
• With a key, we just make a scratch on a plastered surface. If the scratch is left a deep sign on the
surface then we think it as lack of hardness. Otherwise, it is a hard plaster.
• Using insufficient cement in the mortar.
• Having dust in the sand (more than 15% by mass passing through the 0.075mm sieve).
• Adding extra water to the mortar while it starts to dry.This is called re-tempering.
• Maintain the right proportion of sand and cement in the mortar.
• Use washed sand
• Don’t add extra water after preparing the mortar
• Don’t allow the plastered surface to dry rapidly
• Use fresh drinking water in the mortar mix
DEFECTS IN PLASTERING
8. 7. Grinning
• It is a condition of plastered surfaces where mortar joints of masonry blocks are visible through
plaster.
• Grinning is mainly caused by the difference in suction between the mortar and the masonry
blocks or bricks
• The only solution to this defect is to apply an undercoat before plastering.
DEFECTS IN PLASTERING
