Civil Engineering Reference
In-Depth Information
In a brickwork wall, it is possible
for rainwater to mobilise these salts
and transport them via capillaries
towards the external surface of the
wall (Figure 2.34). When evaporation
takes place from these external
surfaces, the salt concentration
increases to oversaturation and the
salts crystallise either at the surface
(efflorescence) or below the surface
(crypto-efflorescence), depending on
the water supply. Efflorescence
occurs when there is a plentiful water
supply, but if the water supply is
limited, the liquid front retreats
inwards, resulting in crypto-
efflorescence. Both processes can
take place simultaneously or
independently. Whereas
efflorescence is merely unsightly,
crypto-efflorescence causes
delamination, spalling and cracking
of the external brickwork surface as
a result of the expansive
crystallization of soluble salts within
the brick pore structure.
Crypto-efflorescence is not
unknown in the UK but our wet
climate usually ensures that the salts
come right to the surface where
they form the less harmful
efflorescence. Efflorescence is
essentially a temporary condition.
Once the salts have been brought to
the surface and removed, it is
unlikely to recur.
Crypto-efflorescence can occur if
old bricks are re-used as they may be
underfired 'place' bricks formerly
only used in the protected internal
walls of a building. If a brick is
underfired, any unwanted sulfates
present in the original clay will not
have been driven off but will remain
within the fired brick. If that brick
also has a very impervious vitreous
coating which will prevent the salts
reaching the surface it will be even
more susceptible to attack.
There is no chemical treatment
for efflorescence which can be
recommended. Dry brushing, which
is labour intensive, can remove
some of the material from masonry
with a sound surface, but most
deposits gradually reduce as wind and
driving rain take their effect over a
period of years.
Case study
Performance of concrete facing bricks
A group of factories were visited to investigate
defects and to recommend appropriate
remedial measures. The site had a driving rain
index in excess of 7 - a 'severe' exposure
grading.
The factories were single storey buildings,
steel framed with gabled pitched roofs; the
upper parts of the walls were faced externally
with PVC coated steel sheeting, lower parts
with concrete facing brickwork outer leaves
to cavity walls. The inner leaves were of
sandlime common bricks and the cavities
contained 50 mm of thermal insulation.
faces in certain parts of the walls. A sample
of some of the loose material was taken
from one of the bricks. On chemical analysis
this showed a Portland cement content of
2.32 % by weight (ie a cement:aggregate
ratio of 1:42 by weight, which was
equivalent to about 1:36 by volume). It was
concluded that the bricks which had eroded
were of defective manufacture.
Remedial work
The cracking that occurred to the main area
of the building, although readily visible, was
not conspicuous. While it might have been
considered unsightly it was unlikely to have
any real effect on any of the functions of the
walls. Any attempt to repair the brickwork by
raking and pointing cracks and, perhaps
replacing cracked bricks by good ones
would probably not be satisfactory.
Incorporating a movement joint in the
affected section might have been more
successful, but, despite this, a crack could
open between old and new walling rather
than on the line of the new movement joint.
The inevitable blemish resulting from
repair might well be more noticeable than
the defect. It was recommended that fine
cracks not visible at a distance of more than
a few metres should be left alone.
Condition of the walling
Cracking, characteristic of drying shrinkage
of the bricks and mortar, was observed in the
concrete brickwork of the main areas of
walling. This cracking was mainly confined to
the mortar joints and had affected the full
height of external brickwork but did not pass
below DPCs near ground level. The cracks
were no greater in incidence and severity than
would be expected from normal shrinkage of
the materials concerned. Their presence
could be due to the omission of movement
joints and restraint imposed by wrapping the
brickwork closely around corner stanchions.
The condition of some of the bricks was
poor, however, with flaking and erosion of brick
Figure 2.34
Severe efflorescence is present on this stretcher bond brickwork
