Civil Engineering Reference
In-Depth Information
undercoats. Mechanical keys will be
needed on backgrounds with very low
suction. These can best be provided
by, for example, raking out joints or
by applying cement spatterdash or
PVA bonding agents.
On the other hand, an important
factor in achieving a good bond
between the finishing coat and the
undercoat is that the undercoat has
developed sufficient suction. It is
important that both the background
and undercoat should have dried
sufficiently to allow this suction to
develop. The common practice of
applying the finishing coat within a
day or two of the application of the
undercoat, especially in damp or wet
weather conditions, is frequently a
contributory cause to hollowness and
bulging of the finishing coats.
Cracking due to movements of the
substrate may be repaired by cutting
out the cracks and filling to the
original specification. Shrinkage
cracks in finishing coats are probably
best disguised by surface decorations.
They should not recur.
Work on site
Storage and handling of materials
Particular attention should be paid to
storage conditions and shelf life.
Restrictions due to weather
conditions
Plastering should not take place on
frozen surfaces. In unheated
buildings in winter conditions, care
should be taken to protect the newly
plastered wall from freezing.
Inspection
The problems to look for are:
◊
plaster keys broken
Workmanship
High suction backgrounds should be
dampened with water before
proceeding with plastering
◊
detachment due to dubbing-out being
too thick
◊
hollowness and bulging due to lack of an
adequate scratch coat
◊
unsuitable choice of plaster for
conditions
◊
no scrim
◊
no metal or plastics corner beads
Case study
Skim coat detaching from the undercoat
The BRE Advisory Service was asked to
investigate the defective plaster to a number
of two storey terraced houses. The external
walls had been built with an outer leaf of
facing brickwork, 50 mm cavity filled with
mineral wool and a lightweight block inner
leaf. The separating walls between the
dwellings were constructed with two skins of
lightweight blockwork. The walls had been
plastered using a 1:6 cement:sand mix
incorporating a plasticiser undercoat and a
retarded hemi-hydrate skim coat.
It was after occupation of the houses that
the first evidence of the plaster finish shelling
from the undercoat was reported. Large areas
of the plaster had detached, typically on the
staircase walls. The skim coat was easily
removed to reveal the surface of the
undercoat. The amount of scratching to the
undercoat was significantly less than would be
recommended. Scratching to the surface
should be at approximately 20 mm centres to
a depth of 3-4 mm to provide a good
mechanical key for the finish coat. However,
in some areas where there was an absence of
scratching, the surface of the undercoat was
quite rough in texture and should have
provided a reasonable key for the plaster
finish coat.
The failure of adhesion in the plaster work
resulted from a combination of two factors -
a weak bond between the plaster and
background, or between successive coats of
plaster, and differential movements of the
plaster and background. The lightweight and
dense blocks used to construct the houses
inevitably shrank as they dried out especially
as they were wet when built in or had become
wet when laid. The cementitious undercoat
also shrank on drying. Little shrinkage
occurred in the gypsum finish coat on drying.
Consequently, unless the finish coat was well
bonded or adhered to the undercoat, the
differential movements could allow the finish
coat to break away from it causing
hollowness or bulging. If the finishing coat
had been well bonded to the undercoat, only
cracking might have developed.
The undercoat was analysed for its
cement:sand content. The results indicated
that the undercoat was of mix designation IV
(ie 1:8 cement:sand). This was not as the
specified designated mix Type III, which was
1:6 cement:sand. However BS 5492
(268)
recommended the use of 1:8 cement:sand
undercoats onto lightweight blockwork. A
visual examination of the undercoat seen
during the site inspection showed it to be
relatively strong and well bonded to the block
background, and there was no evidence to
suggest that the use of a Type IV mix had
contributed to the plaster failure.
It was advisable to delay repairs as long
as possible to allow the walls to complete
their drying shrinkage. The adhesion of the
remaining areas of finish should then be
tested with a knife, and any poorly bonded
areas stripped completely. The undercoat
should be carefully cleaned down, ensuring
the removal of dust particles. The suction of
the undercoat might well be high enough to
cause difficulties in applying and working
the new finish, and it would be advisable not
to attempt to control this suction by wetting
the wall. The use of a bonding agent for the
finishing coat should be considered,
applied to the surface of the undercoat in
strict accordance with the manufacturer's
instructions.
Case study
Cracking of lightweight plaster in an
industrial complex
Cracking of lightweight plaster and gloss
paint finish on lightweight concrete
blockwork walls was found mainly to be due
to moisture and thermal movement of the
blockwork. Some cracking and bulging of
plaster might have been due to deflection of
supporting reinforced concrete floor slabs.
Cutting back defective areas round cracks
and replastering could be carried out
immediately and repainting undertaken.
However, the original appearance might not
be achieved and consideration might have
to be given to applying a sheet material like
Anaglypta wallpaper. This could be
subsequently painted. Where other
defective areas existed, replastering would
be necessary though the existing bulging,
cracked wall between the affected room and
the corridor would have to be stabilised first.
