Civil Engineering Reference
In-Depth Information
thicker glass giving improved values.
Sealed double windows will give up
to around 43-45 dB.
usually be done with heritage
buildings where careful cutting out
and replacement, provided matching
bricks are available, is preferred.
Brickwork may be deteriorating
due to frost action
†
or sulfate attack,
bricks below the DPC being more
vulnerable. Salt crystallization may
also be found. Mortar may be too
strong, resulting in cracking, or too
weak, resulting in washout by rain
and enhanced frost damage. Bricks
used for sills and below DPC level
may not be of M or O quality to
BS 3921
(91)
and therefore will be
more susceptible to frost damage than
the F quality.
Concrete elements are often found
within external walls, for example as
lintels, and cheeks to bays or
canopies; these may often be of thin
section and with inadequate thickness
of concrete over the steel
reinforcement. Precast lintels may
also be found installed upside down.
Rendering may have areas that are
unbonded or cracked (see also
Chapter 9.2). Timber lintels,
bonding timbers and internal joinery
may be decayed, and reconstructed
stone lintels weak - problems which
often come to light only when
replacing windows.
White spirit used as a solvent for
chemical injection DPCs can affect
bitumen DPCs. (The possibility of
this should be checked with the
supplier.)
Certain metals, such as lead and
aluminium can corrode when in
contact with alkaline mortars; for
example when they are used as
flashings or when they are
subjected to run-off from fresh
mortars or concrete. (It is widely
recommended that they are protected
by bituminous paints where they are
tucked into mortar joints but this is
rarely done in practice.) Run-off from
corrodable metals can stain masonry.
It is a design responsibility to see that
this does not occur, for remedial
action is not always feasible.
Durability
Traditional walling materials of fired
clay generally have a very long life
expectancy (Figure 2.16). Some
terracotta units dating from medieval
times have survived in a superb state
of preservation, though the glaze on
glazed faience units sometimes
crazes, giving rise to accumulation of
dirt. However, some of the pale
coloured bodies popular in late
Victorian times have not proved
durable, since they were underfired in
order to develop the required
shades
(90)
. Generally the higher the
firing temperature, the more durable
the material. Unfortunately,
inconspicuous repair is not feasible.
So far as actual bricks are
concerned, the durability of these has
varied according to the type of clay
and firing temperatures reached.
Taking London Stocks as an example,
their structure tended to be porous.
Although first grade stocks were very
durable, and second grade possibly
so, milds and commons were not very
durable. Many older, especially
Victorian, buildings used mild stocks
internally where they are perfectly
serviceable, but if such bricks are re-
used in external walling they can
suffer rapid and severe frost attack.
Gaults as a class were not very
durable, but some particular
specimens were. Some of the
Keuper marl bricks were not very
durable. Flettons, of course, are not
suitable for the highest exposures,
and the manufacturers offer advice on
their specification.
Where the normal life of faced
work has been exceeded or
maintenance has been neglected,
radical remedial action may be
required. While bricks or blocks in a
wall may be sound, the associated
mortar, DPCs or render may be
failing (Figure 2.17). It may be
appropriate to limit further
deterioration by providing extra
protection to a wall (eg rendering or
overcladding) rather than attempting
to carry out a full restoration of the
masonry. However, this cannot
Figure 2.16
Roman bricks re-used in the tower of
St Alban's Abbey
Figure 2.17
A solid wall in a poor state of repair. While
the bricks themselves may be in a
reasonable condition, the paving is too high
in relation to the slate DPC, the
replacement pointing has failed, and there
is evidence of movement
† A freeze thaw test has been devised by British Ceramic
Research Ltd which is expected to become a CEN and later
a British Standard.
