Plastering is the process or item
in the construction, where (in general) mixture of cement + sand + water is
applied on wall, ceilings, soffit, etc. to make the surface level, or to bring
it to required flatness or to decorate it or to make it ready to receive the
subsequent applications like painting, tile / stone installation. This will
also, hide the structural process of making it or hides error. Ex, construction
joints, defects like cracks, plumpness level difference, etc. As per the usage
areas, modern technology provides us many types of admixture for use with
plastering to enhance the strength of the plaster and extend the strength and
durability of plastering.
BULGING: Swelling, softening,
layer cracking and spalling of the plaster 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. Pop outs are conical
fragments that break out of the surface of the plaster leaving holes which vary
in size. They 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. The only remedy - remove and replace the plaster.
CRACKS: Cracks in plastering is considered
as Non-structural cracks, found in the form of network of fine cracks, usually
in a hexagonal pattern, which measure between 5 and 75 mm across each
hexagon. They are usually very fine and shallow and do not extend through the
whole depth of the plaster. They are usually the result of over trowelling
a rich mix of mortar. These cracks do not open and close with time, and
can be covered by applying a scratch coat of pain / putty during the painting
operation. Map cracking is similar to crazing but normally occur when a
plaster mix with high cement content is used or the plaster is allowed to dry
too quickly. Excessive evaporation, absorbent surface, no curing of applied
plaster also shows up cracks. Fundamentally, all because of shrinkage of the
cement. Cracks may develop at the centre portion or at the junction / corners.
STRUCTURAL CRACKS: Some cracks
visible in the plaster may result from cracking of the wall. This can be caused
by differential movement of the foundations, moisture expansion or drying
shrinkage of masonry units, or thermal movement of the roof. This type of crack
often forms in straight vertical or horizontal lines, or in stepped diagonal
lines, and may be quite unsightly. The crack width will often vary with the
seasons. Visible joints can be hidden by cover strips fixed on one side of the
joint or sealed with elastomeric sealants. Additional – localized expansion
joints shall be introduced in between.
GRINNING: Crack line clearly visible
of line though the plaster at dis-similar materials and at construction joints.
This is because of suction between the masonry units and the mortar or because
of not filling the mortar fully. Can be hidden by scratch coat of paint / putty
before painting.
DEBONDING: Debonding of plaster is
often noticed as a hollow sound when the surface is tapped. Plaster is inclined
to curl and de-bond 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. This is especially true of excessively thick plaster
layers. Removing and reinstalling fresh mortar with proper surface preparation is
important to make sure that the bond between plaster and wall is as good as
possible, use bonding slurry if possible.
HARDNESS: Scratching the
surface with a hard sharp object such as a screwdriver or a key, and is
consequently quite subjective to find out the hardness of the plaster. Weak
plasters will be unable to resist impacts, will have reduced resistance to
water penetration. This is because of Insufficient cement, contaminated sand,
poor water retention in mortar, more water cement ratio, rapid drying due to
plastering in full sun or wind, etc. The solution is to go with paint having
elongation property or remove and re-do it depending on how much plaster is not
hard.
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