This post describes 7 different categories of most frequently seen defects in plaster (as explained below)
- non-structural cracks
- structural cracks
- lack of hardness
The causes and repair methods for each are discussed below.
1. non-structural cracks
Madness a network of fine cracks, usually in a hexagonal pattern, that measure between 5 and 75 mm across each hexagon. They are usually very fine and shallow and do not extend to the full depth of the plaster. they usually the result of an excess of a rich mixture (with a high cement content) or use sand with excessive amount of dust (more than 15% by month passing 0,075 mm sieve,
Crazing often occurs within hours of applying plaster to a wall and cracks are barely visible until dust or moisture makes them noticeable. Craze cracks are of little importance, do not open and close over time, and can be covered up using an appropriate quality paint. If necessary, glass fiber tissue can be applied during the painting operation.
map break is like madness except that it is usually deep (sometimes passing through plaster) and the hexagon of the pattern can measure up to 200 mm. These cracks generally occur when a plaster mix with a high cement content is used or the plaster is allowed to dry too quickly.
The reasons for the loss of moisture too quickly are:
- Evaporation if the wall is not protected from sunlight and wind.
- Suction in the walls if the bricks are absorbent and have not been soaked.
- Use of sand that is poorly graded and lacks finer material (less than 5% by mass passing through a 0.075 mm sieve).
- Not to use lime or masonry cement when sand is lacking in finer material.
When cracks are seen while the plaster is still plastic, they are often sealed, only to reappear after some time. These cracks can be filled with proprietary fillers and painted over. Glass fiber tissue can also be applied during painting.
Cracking which results in excessive water removal from plaster in the first hours after application is known as Plastic Shrinkage Cracking, Map cracking can be caused by shrinkage of plastic such as horizontal cracks that form at corners and between windows.
shrinkage cracks drying Moisture loss results after the plaster has hardened. Plaster is always shrinking and cracking, so it is desirable that a large number of fine, noticeable cracks develop at close spacing. Plasters with very high cement content and those made of poor quality sand with high water requirements develop few, widely spaced cracks. Plaster applied in very thick layers will also break in this way. These cracks are normally stable and can be filled with proprietary fillers and painted over.
2. structural cracks
Some cracks visible in the plaster may be due to the cracking of the wall. This may be due to differential movement of the foundation, expansion of moisture or shrinkage of the masonry units as they dry, or thermal movement of the roof. This type of crack often forms in straight vertical or horizontal lines, or in stepped diagonal lines, and can be quite unsightly. The width of the crack often varies with the season.
Since these cracks originate in the wall and not the plaster, it is ineffective to repair the plaster. A specialist should be called to establish the cause of the crack and recommend remedial measures.
Such measures may include structural changes that turn cracks into motion joints. Visible joints may be concealed by cover strips fixed to one side of the joint or sealed with elastomeric sealant.
Debonding of plaster This is often seen as a hollow sound when the surface is tapped. Plaster is inclined to curl and debond from the wall because the outer skin of plaster exposed to air will shrink at a different rate than plaster in contact with the wall. This is especially true for extremely thick plaster layers. Since debonding is usually the result of inadequate preparation of the substrate, it is important to ensure that the bond between the plaster and the wall is as good as possible. This can be done by:
- Thoroughly cleaning dusty or oily walls.
- Allowing the walls to reach the correct moisture content.
- Using cement slurry or spatterdash coat before plastering,
- Using bonding liquid and following the procedure recommended by the manufacturer.
Small areas of debonding (about the size of the plate) are not critical, but larger areas should be removed and replaced.
4. lack of hardness
There are no specifications covering the hardness or strength of plaster, and no reliable way to measure it. Evaluation is often done by scratching the surface with a hard sharp object such as a screwdriver or key, and is consequently quite subjective.
It is often better to have a slightly weaker plaster that is less likely to show significant cracking or debonding than a very strong one. However, very weak plaster will be unable to resist impacts, will reduce its resistance to water penetration and the picture nails will fall off. They also encourage the growth of moss on sheltered faces, especially if poor quality paint is used.
There are five common causes of soft plaster:
- insufficient cement
- Use of excessively dusty sand (more than 15% by mass passing through a 0.075 mm sieve)
- Use of mixtures with poor water retention properties
- Adding additional water sometime after the first mixing (a practice known as retempering)
- Rapid drying due to plastering in full sun or wind
The only one of these reasons that can be confirmed by testing hardened plaster in a laboratory is insufficient cement.
Unpainted plaster can be treated with methyl methacrylate products. These materials can increase the hardness to some extent.
Painted plaster can only be removed and replaced. The inconvenience of this option has to be weighed against living with a weak, unsatisfactory plaster. A coat of high quality exterior paint will normally reduce the risk of water penetration to acceptable levels if the plaster is strong enough to hold such paint. Areas that are particularly susceptible to impact, such as corners, can be rerouted with relatively little interference.
smiling The term given for the appearance of a plastered wall in mortar condition is Joints are clearly visible through plaster, This is due to the difference in suction between the masonry units and the mortar. Laying out mortar joints also leads to grout and thus the practice should be limited to soft clay brick work.
Although smiling can be awkward, it’s not likely to cause further cracking. The alternative is to live with it, or remove and replace the plaster. Applying an undercoat or splatterdash coat before plastering will help prevent grime.
These include swelling, softening, cracking of the crust and falling off of the plaster. This is usually due to the inclusion of proprietary gypsum-based products in the mix. Under moist conditions, the sulfate from the gypsum reacts with the Portland cement paste and forms compounds of increased volume that disrupt the plaster.
The only remedy for expansion-induced disruption due to gypsum in the mix is to remove and replace the plaster.
Pop outs are conical pieces that protrude from the surface of the plaster leaving holes that vary in size. They are caused by the presence of contaminant particles in the mixture which, by reacting with the moisture in the mixture, expand and cause cavities in the plaster. The contaminants are usually seeds, other organic matter or particles of dead burnt lime.
Once the cause of the pop out is removed, the hole can be filled with proprietary filler and painted over.