Civil Engineering Reference
In-Depth Information
Metal detectors are useful to locate
tie positions.
Alternatively an optical probe can
be inserted through strategically
drilled holes. However, it should be
noted that corrosion occurs mainly on
the part of the tie that is bedded in the
outer leaf, particularly that part
within the bed joint and close to the
cavity face. If inspection by optical
probe does not reveal corrosion of the
part of the ties spanning the cavity,
this should not be taken as firm
evidence without some sampling and
direct examination of parts bedded in
the outer leaf. Other, generally more
expensive, inspection techniques are
described in BRE Digest 329
(111)
.
In all cases of wall tie corrosion,
consideration should be given to the
possibility that the accompanying
expansion has distorted the wall to an
unacceptable extent or transferred to
the outer leaf loads
(eg roof loads) intended to be
carried by the inner leaf. Surveyors
should also be alert to the possibility
that cavity ties are used where there is
no cavity. For example, in some
cross-wall housing, separating
walls are projected beyond the face of
the building and their ends cloaked by
a half-brick skin carried up to the full
height; stability of the brick skin, in
these situations, can depend wholly
on the integrity of
the ties.
the reinstatement of cavity walls by
insertion of new wall ties without
recourse to demolition.
The range of techniques currently
available are described in BRE Digest
329 which deals with:
●
investigative test techniques
●
tests of inner leaf connections,
torque testing expanders and
examination using optical probes
and metal detectors
●
final visual acceptance checks on
the quality of repointing and
making good, and random
examination to ensure that the
overall quality is satisfactory
the classification of the main
problem (eg thick metal ties or
wire ties)
●
choosing appropriate remedial
actions to deal with any existing
structural deterioration and to
prevent further deterioration (eg
remove old ties or structurally
isolate ties)
●
choosing remedial tying systems
appropriate to the structure, fire
requirements, wall materials etc
(eg public buildings require longer
periods of fire resistance than
domestic dwellings)
quality assurance documentation
to verify the design basis of the
installation
●
Using modern remedial tie
technology, a complete cure of the
problem can be effected. The cost is
not excessive - from a few hundred
pounds for a small centre terrace
dwelling to perhaps £3,000 (at 1998
prices) for a large detached dwelling.
Maintenance
See the same section in Chapter 2.1.
The above reference is supported by
BRE Digest 401 which covers:
●
surveying the building to establish
the wall type and the density and
layout of existing ties
●
specifying tie sampling methods
and rates for some typical
situations
Work on site
Storage and handling of materials
On arrival on site, all materials,
especially lightweight blocks, should
be stacked and protected from rain
and snow so that they are kept as dry
as practicable both before and during
laying.
This recommendation is clearly
important wherever blocks will dry
out more or less completely (eg in
inner leaves of external walls and in
internal partitions) but research
suggests that it may be less relevant
to cracking in the wetter external leaf
situations. Doubts have arisen
because instances of minor cracking
have frequently been observed in
external leaves of lightweight
blockwork in the early life of the
buildings, occasionally even before
the rendering is applied. Typically the
cracks are fine, sometimes little more
than micro-cracks though possibly as
wide as 1-2 mm, and run vertically
through blocks and perpends,
extending through the full thickness
of the blocks including any surface
finish. On the coarser finishes they
can only be detected by close
inspection.
classifying old ties by visual
methods and by quantitative
measurement of remaining zinc
coatings; estimating remaining life
and recommending remedial work
in the short or long term
●
the density and layout of the
remedial ties; whether to specify
by prescription in accordance with
the Building Regulations and
British Standards Codes of
practice or by calculation
●
Location of ties
The most common technique is the
use of a metal detector. These are
specialised devices optimised to find
metal within a range of about
100 mm - 'treasure locators' are not
suitable. Also stainless steel
demands specialised devices.
Infrared thermography can be used
for detecting wall ties, and the
method is a smaller scale version of
that used on whole buildings to detect
heat losses. It is the action of ties as
thermal bridges that is used in this
technique. The test is totally non-
destructive and theoretically can be
used on any form of cavity wall
construction that uses metal wall ties.
the choice and validation of a
suitable remedial tie system to
ensure that the system can give an
effective fixing in both inner and
outer leaves of masonry
●
sampling techniques and rates for
the pre-contract testing of the
chosen system
●
quality control checks during
installation by operatives,
supervisors and independent
professionals to ensure that the
work is being carried out to
specification. They include visual
checks, checks on the removal of
debris and old ties, random proof
●
Replacement
Techniques have been developed for
