Civil Engineering Reference
In-Depth Information
Strength and stability
Lime plasters will be comparatively
soft until carbonated, though there is
not usually a problem with gypsum.
The lightweight plasters
incorporating perlite or vermiculite
will offer less impact resistance than
the hardwall plasters, and should
therefore be specified with due
regard to risk.
Thermal properties
Where additional thermal insulation
is desirable, the substrate may be
plastered with a mix containing
perlite or vermiculite; however this
will give only marginal
improvement.
Case study
Plastering failure in a seventeenth
century building
A seventeenth century stable block of solid
brick wall construction had suffered from
detachment and collapse of new renovating
plaster around a staircase area. It was
decided that the cause was insufficient bond
strength to backing brickwork linked to a
high localised drying rate. Successful
replastering would involve much better
preparation of the background, control of
the drying rate of the plaster, and the use of
a sand:cement undercoat rather than a
gypsum undercoat to better isolate
contamination.
Fire
A plastered wall surface, whether
painted on not, will meet the Class 0
surface spread of flame criterion.
Dimensional stability, deflections
etc
Loss of adhesion of renders and
plasters is commonly due to
differential movement of the
background.
For coefficients of linear thermal,
and moisture expansion of plastering
and other background materials, see
Chapter 1.2.
Durability
Rendered or plastered surfaces that
have detached from the substrate are
one of the more common types of
failure. Especially in older properties
undergoing rehabilitation, it may be
better to strip the whole lot than to
struggle with patching (Figure 10.9).
Most brick or block surfaces can
be covered with little risk of failure,
but some backgrounds can prove
difficult.
As a background for cement:sand
render, in situ reinforced concrete
usually presents a dense smooth
surface with low absorbency. Render
should not be applied to this type of
background without providing an
adequate mechanical key to hold the
rendering both during and after its
application. It is important the bond
between the render and concrete is
strong enough to resist the stresses
set up as the rendering shrinks and to
avoid separation.
Smooth dense concrete surfaces
can be provided with an effective
textured key by:
●
Case study
Plastering failure in a further
education building
Cracking in plaster (and tiled) wall finishes
was the result of drying shrinkage
movement of the lightweight concrete block
background which had been transmitted
into wall finishes. Fine cracks should be
cleaned, filled with a coloured grout and the
repaired areas re-sealed. All cracked and
loose plasterwork at blockwork to column
and beam junctions should be removed.
Metal lath or proprietary movement joints,
or simply a straight cut in the plasterwork,
should be provided at changes in
background materials and the walls
replastered to match existing.
Consideration should be given to the
insertion of soft joints in tiled kitchen walls.
Dampness and condensation
Where walls are persistently damp,
and more particularly where the
surface has been contaminated with
soluble salts rising from below and
which may have become
hygroscopic, the original plaster may
need to be hacked off and the
substrate re-rendered in a
cement:sand mix, 1:5 for example.
One of the so-called renovating
plasters may be appropriate.
Case study
Plastering failure on aerated concrete
walls
The 'perlited' mortar undercoat had cracked
and lost adhesion to the aerated concrete
blockwork background. It would appear that
the blocks were dry when plastered and
were not wetted sufficiently to control the
suction. The cement content in the mortar
undercoat was low, resulting in a weak
friable material. The movements associated
with the walls and plaster were sufficient to
cause the cracking and loss of adhesion. It
was suggested that the plaster be removed
and the walls replastered using a bonding
undercoat.
abrasive blasting, tooling with a
needle gun, or bush hammering
applying a spatterdash, stipple
coat, or adhesive bonding agents
●
Another important factor in achieving
an adequate bond between a render
and its background is the suction of
the background. This can be checked
by splashing water on the surface of
the concrete. If the water is readily
absorbed the wall should be wetted.
Conversely it is important that the
wall is not too wet. Some suction is
necessary for the development of a
good bond. Using a spatterdash or
stipple coat overcomes many of the
problems associated with suction.
Figure 10.9
A one and a half brick external wall,
completely stripped. The double timber
lintels still seem to be in good condition,
but will need to be boxed before plastering
