Civil Engineering Reference
In-Depth Information
Sands, aggregates
(238)
and pigments
●
Sands should be well-graded; that
is to say, having a good mix of
coarse and fine particles, and
ideally should comply with
Clause 5 of BS 1199
(239)
●
Although sands not complying
with the above can give good
results, great care should be taken
when contemplating the use of fine
sands
●
Coarse aggregates for dashed coats
should comply with
BS 882
(64)
or BS 63-1
(240)
, or,
perhaps more appropriately,
BS 63-2
(241)
Strength and stability
Impact resistance of the insulating
renders is the main concern, and this
should be checked before
specification. Those renders
incorporating polymers minimise the
risks of cracking.
Pigments should comply with
BS 1014
(242)
●
Water
●
Dimensional stability, deflections
etc
Loss of adhesion of renders due to
differential movement of the
background is frequent.
For coefficients of linear thermal
and moisture expansion of rendering
and other background materials see
Chapter 1.2.
Map-pattern cracking is
characteristic of shrinkage in the
render itself. Cracks are often fine
enough to be both obscured and
sealed by the application of a
masonry paint coating, but clearly-
defined cracks - often the result of
applying a strong finish coat over a
weak backing coat - may well
penetrate the full thickness of the
render, with the risk of rain
penetration and, if on clay brickwork,
of sulfate attack.
Predominantly linear vertical
cracks in a render are likely to
indicate a change of substrate
material at those locations. Since
differential size changes in the
substrate are likely to continue, these
cracks should be raked out and filled
with a suitable sealant.
Cracks due to sulfate attack and to
corrosion of wall ties are readily
distinguishable from those due to
moisture-induced size changes. If
movement joints are provided at
centres appropriate to the movements
expected of both render and substrate,
cracking should be avoided (Figure
9.15).
Water of potable quality is suitable
Finer sands should be used for final
coats with tooled finishes
Reinforcement
●
Natural fibres used in specialist
applications should be dry, clean
and grease and oil free
●
Coarser sands should be used for
final coats with scraped finishes
●
Mineral and polymer fibres
should be specified only in
accordance with manufacturers'
recommendations
●
Render stop
Admixtures
●
Plasticisers should comply with BS
4887-1
(243)
Plasticisers should not be used
with renderings made with
masonry cement
●
Waterproofing agents should be
used in accordance with
manufacturers' instructions
●
If render below
DPC, DPC must
project
Bond strength, resistance to rain
penetration and durability are
improved by polymer dispersions
such as SBR (styrene-butadiene
rubber) and acrylics, which should
be used in accordance with
manufacturers' instructions
●
Not less than 150 mm
DPC
Figure 9.14
Criteria for rendering at the base of a wall
Weathertightness, dampness and
condensation
Although renderings are often used to
enable weather resistance of a basic
wall material to be increased, the
more exposed the wall to be rendered,
the more restricted becomes the
choice of specifications available.
These are the factors.
Figure 9.15
Movement joints have been provided on this newly rendered façade, and this should avoid
cracking
