While cracking of plaster is the most common problem encountered by contractors, there are also several other types of plastering defects that can cause headaches.
John Roxburgh, lecturer at The Concrete lnstitute’s School of Concrete Technology, deals with other types of plastering defects, what causes it, and what to do about it:
Debonding of plaster is often noticed as a hollow sound when the surface is tapped. Plaster tends to curl and debond from the wall because the outside skin of the plaster exposed to the air will shrink at a different rate than the plaster in contact with the wall. This is especially true of excessively thick plaster layers.
Small areas of debonding (about the size of a plate) are not significant, but larger areas should be removed and replaced.
Because debonding is generally the result of inad equate preparation of the substrate, it is important to make sure that the bond between plaster and wall is as good as possible.
This can be done by:
Cleaning dusty or oily wall surfaces thoroughly.
Allowing the walls to reach the correct moisture content.
Using a cement slurry or spatterdash coat before plastering.
Using bonding liquids and following the procedure recommended by the manufacturer.
Lack of hardness:
There are no specifications covering the hard ness or strength of plaster and no reliable way of measuring it.
Evaluation is often carried out by scratching the surface with a hard sharp object such as a screw driver or a key.
It is often better to have a slightly weaker plaster than one which is too strong. However, very weak plasters will be unable to resist imp acts, will have reduced resistance to water penetration and picture nails will tend to fall out.
They also tend to encourage moss growth on sheltered faces, particularly if poor quality paint is used.
There are five common causes of soft plaster:
The use of sand containing excessive quantities of dust (more than 15% by mass passing the 0,075mm sieve).
The use of a mix with poor water retention properties.
The addition of extra water too long after first mixing (a practice known as re-tempering).
Rapid drying due to plastering in full sun or wind:
Generally, causes of excessive early moisture loss are:
Evaporation, if the wall is not protected from sun and wind.
Suction into the walls, if the masonry units are absorbent and have not been dampened.
Use of a sand that is badly graded and lacks fine material (less than 5% by mass passing the 0,075mm sieve).
Not using building lime or a masonry cement when the sand lacks fine material.
Grinning is the term given to the appearance of a plastered wall when the positions of the mortar joints are clearly visible through the plaster. It is caused by the difference in suction between the masonry units and the mortar.
Raking out mortar joints also causes grinning and the practice should thus be limited to soft clay brickwork.
While grinning may be ugly, it is unlikely to lead to further cracking. The choice is to live with it or to remove and replace the plaster.
Application of an undercoat or a spatterdash coat before plastering will help to avoid grinning.
This includes swelling, softening, layer cracking and spoiling of the plaster. It is usually caused by the inclusion of proprietary gypsum-based products in the mix.
Under moist conditions, the sulphate from the gypsum reacts with the portland cement paste and forms compounds of increased volume which disrupt the plaster.
The only remedy for expansion-induced disruption due to gypsum in the mix is to remove and replace the plaster.
Popouts are conical fragments that break out of the surface of the plaster leaving holes that vary in size. These are 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.
Contaminants are usually seeds, other organic material, or particles of dead burnt lime.
Once the cause of the popout has been removed, the hole can be filled and painted over.
The Concrete Institute
Tel: (011) 315-0300