Civil Engineering Reference
In-Depth Information
replacing the affected tiles, though
matching grouting may be a problem.
Careful selection, or even tinting of
the mix, may be necessary.
tile and substrate with adhesive is a
prime requirement to prevent loss
of adhesion
●
tiling should not proceed on new
render for at least 14 days, and on
new plaster for at least 28 days
●
thin bed adhesives should not be
used when the substrate surface
varies by more than 3 mm under a 2
m straight edge (Figure 10.5).
Problems will also occur if the
deviation is more than 6 mm for
thick bed adhesives
Work on site
Workmanship
The following points should be
noted:
●
evenly coating the joint between
Case study
Several reasons for loss of adhesion of tiles
The BRE Advisory Service was asked to
establish the reasons for wall tiles failing at an
institutional building. The background to the
tiling was a proprietary lightweight blockwork
with a sanded undercoat and retarded hemi-
hydrate finish. The tiles were fixed to the
plaster surface with a proprietary tile
adhesive. From the information available it
would appear at least four weeks elapsed
between plastering and tiling in the majority of
areas, except for one area where the interval
was less than four weeks.
The tiling adhesive failure had occurred at
three positions:
●
tile. This failure was due to the lack of
transfer of the adhesive to the back of
the tile. If the tiles were not tamped or
pressed firmly into position, leaving the
adhesive on the wall too long or allowing
it to skin and stiffen, then poor transfer
of adhesive to the tiles would occur
(Figure 10.4).
tiles without spacer lugs should not
be butt jointed
●
grouting should proceed between
24 and 72 hours after fixing
●
BS 5385 gives more information on
adhesives and backgrounds
(267)
.
Movement joints were not provided for in
the tiling. In any large areas of tiling,
movement joints should be incorporated in
the bedding and tiling at internal vertical
corners and at 3-4.5 m centres horizontally
and vertically. Although the areas of tiling in
the building were not particularly large,
except for the kitchen, it was possible that
movement joints at the internal corners
might have relieved some of the stresses
set up in the tiling.
The areas of hollow and missing tiling
needed to be replaced. In areas where the
plaster had separated the tiles should be
refixed directly to the undercoat surface. It
would be important that the surface be
thoroughly cleaned to remove all loose
plaster and dust. The tiles should then be
fixed with an adhesive suitable for the
conditions. It was recommended that a tile
adhesive manufacturer be consulted and
their advice sought as to the type of
adhesive to use, and the preparation of the
surface to which the tiles were to be fixed.
Inspection
The problems to look for are:
◊
the adhesive and plaster interface. In some
areas the adhesive was well adhered to the
tile but in other areas failure occurred at the
plaster surface and adhesive interface.
Visual examination of the plaster surface
and the adhesive surface in contact with the
plaster showed that the plaster surface
was quite dusty or powdery. The dusty
surface would result in a weak bond
between the plaster surface and adhesive.
The plaster should have been treated with a
binding coat of a suitable primer
cracking
◊
no movement joints or movement joint
intervals exceeding 4.5 m
◊
adhesive unsuitable for the conditions
◊
poor substrate quality
◊
adhesive incorrectly applied
◊
tiles applied too late after applying
adhesive
●
the skim coat plaster and undercoat
interface. The tiles had failed because the
bond between the skim coat of plaster and
the cement:sand undercoat was insufficient
to restrain the movements in the tiling. The
undercoat, when visually examined, had
good strength and its surface was not weak
or friable. However, it was apparent that the
amount of scratching to the surface of the
undercoat was minimal. It is important that
the surface of the undercoat is extensively
scratched to provide an adequate
mechanical key for the skim coat. The lack
of scratching had contributed to the weak
bond between the undercoat and plaster
finish. A microscopic examination of the
surface of the undercoat revealed a weak
layer of plaster. It was possible that failure
had occurred within the plaster skim rather
than at the undercoat/plaster interface. It
was not possible to confidently diagnose
the reasons for the weak, soft layer of
plaster without further more extensive
investigation
Not more than
3 mm for
thin bed
adhesives
2 m
Not more than
6 mm for
thick bed
adhesives
Figure 10.4
Lack of transfer of adhesive to the back
of the tile
●
the adhesive and tile interface. There was
only one small area of tiling where the
failure occurred between the adhesive and
Figure 10.5
Permissible deviations in the substrate for
successful fixing of ceramic wall tiles
