This document discusses plaster and common defects found in plaster work. It describes several types of non-structural cracks like craze cracking and map cracking that occur due to high cement content or rapid drying. Structural cracks are caused by foundation movement, moisture expansion, or drying shrinkage. Debonding causes a hollow sound when tapped and curling from uneven shrinkage rates. Other defects include lack of hardness, grinning from mortar joints, expansion from improper additives, and popping from contaminants. The document also outlines plaster finishing types and their uses.

1. COURSE NO: ARCH 2101
COURSE TITLE: CONSTRUCTION II : FINISH MATERIALS
Lecture 2
Plaster

2. PLASTER

3. DEFECTS IN PLASTER WORK
A lot of different types of defects we have seen after completing the finishing works.
Some of them are given below:
A Text Book of Engineering Materials- by Dr. M. A. Aziz
1. Non-structural cracks
2. Structural cracks
3. Debonding
4. Lack of hardness
5. Grinning
6. Expansion
7. Popping

4. Non-structural cracks
• Craze cracking: Usually in a hexagonal
pattern. They are usually very fine and
shallow and do not extend through the
whole depth of the plaster.
• Map cracking: is similar to crazing
except that it is usually deeper. Normally
occur when a plaster mix with a high
cement content is used or the plaster is
allowed to dry too quickly.
A Text Book of Engineering Materials- by Dr. M. A. Aziz

5. Structural cracks
• Caused by
I. differential movement of the
foundations,
II. moisture expansion or
III. drying shrinkage of masonry units
• Forms in straight vertical or
horizontal lines, or in stepped diagonal
lines
• Ineffective plaster
A Text Book of Engineering Materials- by Dr. M. A. Aziz

6. Debonding
• As a hollow sound when
the surface is tapped.
• Inclined to curl and debond
from the wall because the
outside skin of the plaster that
is exposed to the air will shrink
at a different rate from the
plaster in contact with the wall.
A Text Book of Engineering Materials- by Dr. M. A. Aziz

7. Lack of hardness
• Evaluation is often carried out
by scratching the surface
with a hard sharp object such
as a screwdriver or a key
• Slightly weaker plaster have
less likely to show
significant cracking or
debonding than one which is
too strong
A Text Book of Engineering Materials- by Dr. M. A. Aziz

8. Grinning
• When the positions of the
mortar joints are clearly
visible through the plaster.
• Caused by the difference in
suction between the masonry
units and the mortar
• Raking out mortar joints also
causes grinning
A Text Book of Engineering Materials- by Dr. M. A. Aziz

9. Expansion
• Swelling, softening, layer
cracking and spalling of the
plaster.
• Caused by the inclusion of
proprietary gypsum-based
products in the mix
A Text Book of Engineering Materials- by Dr. M. A. Aziz

10. Popping
• Conical fragments that break
out of the surface of
the plaster leaving holes which
vary in size
 Caused by the presence of
contaminant particles in the
mix which, reacting with the
moisture in the mix, expand
and cause cavities in the
plaster.
A Text Book of Engineering Materials- by Dr. M. A. Aziz

11. SPECIAL TYPES OF FINISHING:
Smooth Cast
It is a finish which presents leveled and smooth surface.
A Text Book of Engineering Materials- by Dr. M. A. Aziz

12. Pebble Dash
• Pebble dash is a
coarse plaster surface used on
outside walls that consists
of lime and
sometimes cement mixed
with sand, small gravel, and
often pebbles or shells
A Text Book of Engineering Materials- by Dr. M. A. Aziz

13. Rough Cast
It is a finish in which the
mortar for the final coat
contains a proportion of fairly
big size coarse aggregates.
The mortar for the finish is
made by mixing cement fine
sand & coarse aggregates in
the ratio of 1: ½ : 3.
A Text Book of Engineering Materials- by Dr. M. A. Aziz

14. Textured Finish
In this finish, ornamental patterns or textured surfaces are produced by
working with various tools on the freshly applied final coat.
A Text Book of Engineering Materials- by Dr. M. A. Aziz

15. LIME PUNNING
• Thin coat of lime plaster and usually applied on ceilings and walls.
• Consist of fresh slakes lime and good sand in equal volumes.
• Thickness not more than 1/16 inch and the surface is made smooth.
A Text Book of Engineering Materials- by Dr. M. A. Aziz

16. WHITE WASH:
• This is applied to give better appearance to the surface as well for sanitary
reasons because lime is said to have antiseptic and disinfectant
properties.
• Some adhesive substance like gum or paste of boiled rice are used in the
white wash and to prevent glamaer as well to give pleasant effect, a small
quantity of copper sulphated is some times added to it.
NOTHING BUT WHITE WASH WITH SOME DESIRABLE COLORING
INGREDIENTS ADDED TO IT AND MAINLY USED FOR DECORATIVE
PURPOSE.
A Text Book of Engineering Materials- by Dr. M. A. Aziz

17. A Text Book of Engineering Materials- by Dr. M. A. Aziz

18. Pointing
• Pointing is the finishing off the mortar in the rough brick joints of walls to
give the surface a beautiful appearance.
• For cement pointing the plaster should consist of cement and clean sand in
the proportion 1:2.
• Walls are raked out to a depth not less than ½ inch and all dusts are
brushed off.
• Various types of pointing such as tuck pointing, bastard pointing, cut
pointing, rule pointing, v-pointing and the choice of a particular type
depends upon the owner.
• It gives attractive appearance of walls.
• Pointing projects joints from dampness, and makes the joints become
loose and defective.
A Text Book of Engineering Materials- by Dr. M. A. Aziz

19. Pointing
A Text Book of Engineering Materials- by Dr. M. A. Aziz

20. Pointing
A Text Book of Engineering Materials- by Dr. M. A. Aziz

21. ADVANTAGES OF PLASTER
• Easy in application
• Plaster is the most common form of interior wall finishing
• If properly mixed and applied, a plaster coating creates a stronger and more
durable.
DISADVANTAGES OF PLASTER
• When plaster cracks then difficult to repair.
• Repairing is very expensive.
• Difficult to maintain the wet condition.
A Text Book of Engineering Materials- by Dr. M. A. Aziz

22. RECOMENDED REFERENCES
A Text Book of Engineering Materials- by Dr. M. A. Aziz
https://www.afrisam.co.za/media/13707/Common_defects_in_plaster.pdf
http://civilblog.org/2014/11/03/what-are-the-defects-found-in-plaster-work/