Civil Engineering Reference
In-Depth Information
Terracotta
The term terracotta tends to be used
for the material rather than the unit,
and the dividing line between what is
a piece of terracotta and what is a
specially moulded and fired brick can
sometimes be a matter of opinion.
Terracotta is unglazed, though
partially vitrified; but when the clay
body is glazed it is usually termed
faience. Glazes may be either clear or
pigmented.
Terracotta, or faience, was
primarily used as a decorative
material bonded to the external face
of masonry, and is met with for the
most part in heritage buildings. The
material was first imported, mainly
from Italy, and used in England in
Tudor times (eg at Hampton Court
Palace); English manufacture did not
begin on a significant scale until the
first quarter of the eighteenth
century. The body is, of course, clay,
with the addition of various
quantities of other minerals such as
sand and ground flints, depending on
the desired colour and the need to
control firing shrinkage
(84)
. It is the
outer surface, or fire skin, which
gives the durable qualities.
Terracotta was much favoured
during the second half of the
nineteenth century for use on public
buildings where its decorative and
durable properties could be
exploited to the full. Frequently it
was made to imitate limestone. Units
fired to high temperatures under
highly controlled conditions tend to
be relatively uniform in appearance.
If they are laid with narrow mortar
joints the overall effect does not
show weathering in appearance as
does brickwork, and, for the most
part, the material can retain a rather
clinical and pristine appearance.
atmosphere and converting to
calcium carbonate. However, they
would always retain something of
their initial resiliency when
compared with mortars made from
Portland cement and this is why the
larger old buildings, which nowadays
would be provided with movement
joints, survive quite happily without
them.
Between the wars black-ash
mortars became popular in the
industrial areas of the UK. These
conform to no standard, but are
thought to be compounded of ashes
from local coal burning industries,
possibly mixed with some lime and
sand. These mixtures had very little
hydraulic set and only limited long
term set, but have survived, some
with repointing, to the present day.
Their main drawback is that they
often contained acids which attack
steel ties in cavity walls.
Fine (soft) sands, preferred for
mortar preparation in the UK because
they impart good plasticity and
workability, are largely supplied from
quarries unwashed. In this state they
contain some silt and clay particles
with the only requirement being a
limit of 8% passing a 75 µm sieve for
the general purpose (G) grade and a
5% limit for the structural (S) grade.
Sands with excessive amounts of clay
fines increase the amount of water
required in the mix which can lead to
lower strengths and higher shrinkage.
It is a common misconception that
mortars always need to be as strong
and as rigid as the bricks they join.
Provided they are sufficiently strong
to resist weathering, even relatively
weak mixes will normally be strong
enough for two storey work. It is only
when large loads have to be carried,
as for example in frameless
loadbearing masonry multi-storey
flats, that the question of matching
mortar strengths to units arises. On
the other hand, if there is insufficient
lime or cement to bind the sand,
durability problems will usually
ensue (Figure 2.9).
Mortars
The traditional mixes for mortars,
before Portland cement was invented,
were based on lime putty and sand.
Such mixes, depending on the
proportions, provided very workable
and user-friendly properties with a
long pot life, and developed their
strengths slowly over time as the
lime component hardened by
absorbing carbon dioxide from the
Figure 2.9
This old half brick extension contrasts with the stretcher bond elsewhere. The mortar joints
around the downpipe have eroded and repointing has been inadequate
