Civil Engineering Reference
In-Depth Information
Cast stone may be subject to
crazing. As
Principles of modern
building
(6)
remarked, crazing of
concrete products is a very variable
phenomenon, varying in scale from
wide cracks 100 mm apart to those
which can be seen only under a
microscope. The main cause of this
crazing is differential moisture
movement of the surface layer with
that immediately underneath. The
coarser mixes tend to craze less
than the finer, and trowelling to a
smooth surface tends to promote
crazing.
If the published recommendations
for external rendered finishes are
complied with there need be no
serious cracking of blockwork or its
finishes, including rendering (see
also Chapter 9.2). Shrinkage cracking
is the main risk. The following
aspects are particular important.
●
Case study
Sulfate attack in the walls of a church
The BRE Advisory Service was asked to
investigate the condition of the brickwork at a
church situated near the coast. The external
leaf of brickwork had bowed on the panels
that faced west. One panel was checked with
a plumb line and it was shown that the top of
wall just below the parapet had moved
backwards 34 mm from the vertical. At
between 2 m and 2.4 m the brickwork had
moved outwards 20 mm from the reference
point at DPC level. Examination of another
panel indicated a slight vertical bowing of the
wall (5 mm on a 1.5 m height).
Samples of the mortar were drilled from
the brickwork panel at one course above the
DPC, then at 1 m intervals to the top of the
wall. At each position the samples were
collected both from the outer 50 mm and the
remaining inner mortar. The outer drillings
were collected as a reasonably dry powder,
but further into the mortar it became very wet
and was collected as a paste.
The sulfate content for mortar samples
taken from the bowed panel was in excess of
that normally present in a mortar mix, but
other samples were not abnormal. Differential
thermal analysis showed that there were
significant amounts of ettringite present in the
samples taken from the bowed panel.
The bowing of the brickwork panels could
be explained by sulfate attack on mortar
joints. The bricks were an ordinary quality
facing brick with a sulfate content of 0.63%.
It was possible that the bricks could have
higher than recommended potassium
content which made the brickwork more
prone to sulfate attack. The brickwork was
afforded little protection from the rain. The
parapets did not provide sufficient overhang,
and the brickwork would remain damp for
long periods. Therefore, the three conditions
for sulfate attack were present.
The bricks were not the correct choice of
brick for the design and exposure of the
church. The spalling of the face of the bricks
could have been the result of frost damage,
and there were examples of the faces of
some bricks being forced off by ice. The
bricks would appear to have been rather
porous and readily absorbed moisture.
When replacing the brickwork, it would be
important that a brick be chosen that would
be low in sulfate content and had the
required durability for the known exposure
conditions and that the parapets be properly
detailed; that is they would be protected by a
coping with a good overhang, a damp proof
course under the coping, and a cavity tray.
Specification of mortar mixes: lean
mortars deform more readily than
rich ones and allow individual
blocks to move within the mortar
joint system without serious
disruption
Provision of vertical movement
joints: simple butt joints formed
during construction, or cut with a
power saw within a few days after
completion, at approximately 6 m
centres and filled with a resilient
sealant
●
Provision of reinforcement in areas
of high stress: galvanised or
stainless steel bed-joint
reinforcement located in mortar
joints at window and door
openings
●
Case study
Crypto-efflorescence
Crypto-efflorescence was investigated by BRE
in some sheltered brickwork walls in several
blocks of flats. The problem had arisen in
these areas because only a restricted amount
of rainwater fell on the bricks which carried
the dissolved magnesium sulfate salts
present in the bricks towards the surface,
drying relatively quickly through evaporation.
The salts crystallising below the surface led to
delamination and spalling of the outer skin of
brickwork 2 mm thick.
Two main types of brick were used: one
was an engineering brick and the other a
much lighter coloured wire cut, extruded
facing brick. It was the pale coloured facing
bricks which had deteriorated in places as a
result of the crypto-efflorescence. The
problem had been ongoing for a number of
years but it was unlikely that any reduction in
structural strength had occurred.
A sample of failed brick was examined in the
laboratory, using both optical and scanning
electron microscopy. Most of the brick was
composed of amorphous material containing
elastic quartz grains. No conclusive evidence
as to the origin of the salt could be determined.
However the brick did possess a number of
pores which appear to contain a magnesium-
bearing white precipitate. The outer 2 mm of
brick comprised a slightly more impermeable
higher glass content layer which is normally
found in extruded bricks of this type.
The salts responsible for the crypto-
efflorescence problem were found to be the
magnesium sulfate salts MgSO
4
.6H
2
O and
MgSO
4
.7H
2
O which originate from the raw
clay used for the brick manufacture. It was
unlikely that the bedding mortar was
involved in the process.
In a section of brick 100 mm thick, it
takes only a background mobile sulfate level
of 0.2% to build up and concentrate in a
1 mm thick crypto-efflorescence layer
containing 20% sulfate. There was therefore
plenty of potential for further problems to
arise and although this strongly depended
on the solubility of the sulfate compounds
present within the brick interior, it was highly
probable that further crypto-efflorescence
would occur in the vulnerable areas if the
problem was left untreated.
The possibilities for repair included:
Efflorescence and crypto-
efflorescence
Clay bricks may contain water-
soluble sulfate salts derived from the
constituent materials produced
during the firing process.
Additionally, but to a lesser extent,
sulfates can originate either in the
mortar, or from atmospheric pollution
or the ground. (Concrete bricks and
blocks, and calcium silicate bricks
are normally free from significant
amounts of sulfates).
rendering the brickwork, using a metal
lathing to support the render finish
●
removing and replacing the outer skin of
the affected brickwork
●
