Civil Engineering Reference
In-Depth Information
attack as it contains low quantities of
soluble salts.
Sulfate attack is mostly found in
exposed brickwork (constructed
using N category bricks) of parapet
walls and in rendered brickwork
where the render has become cracked
and can no longer prevent water
penetration. It is also particularly
prevalent in exposed brickwork
adjacent to flues (irrespective of
brick type)
(92)
. However, it can also
occur in conditions of high exposure
to driving rain.
Case study
Sulfate attack in parapets
Problems of spalling and cracking were
investigated in the exposed, rendered
parapet walls on top of a prestigious
building in the south east of England some
five years after completion. The walls had
been constructed using N category bricks
and problems of drying shrinkage within
the render coats had caused very small
cracks to open up, allowing water to
penetrate to the brickwork and to become
trapped. Drying shrinkage of the render
had occurred because the second render
coat had either been richer in cement or
thicker than the first coat. It is probable
that some of the water for the reaction
was provided by the bricks being
inadequately protected in winter and the
render being applied over this wet
substrate thus sealing in the moisture.
Such wet conditions provide ideal
conditions for sulfate attack. A white
reaction product formed at the back of the
render coats and within the adjoining
bedding mortar between the bricks,
causing expansion to occur.
A complete rebuilding of the parapet
walls was necessary at a very high cost to
the building owners. The rebuilding was
undertaken using L category bricks, laid in
air entrained mortar, which leads to
improved durability of brickwork.
Work on site
Storage and handling of materials
Portland cements are the most
vulnerable item. They need to be
stored off the ground in covered
accommodation.
Restrictions due to weather
conditions
Bricklaying or repointing should not
proceed under the following
conditions:
●
air temperature below 2 °C or
when expected to fall below 2 °C
Efflorescence
Solid brickwork can suffer from
migration of salts to the surface, with
unfortunate effects on the appearance
of the brickwork. Unless there has
been a change in conditions affecting
old solid brick walls, it is likely that
any salts present will have been
mobilised and brought to the surface
long ago. Efflorescence is more likely
to be encountered in relatively new
brickwork in cavity construction;
a full discussion of this phenomenon
is to be found in Chapter 2.2.
when aggregates are frostbound
●
in driving rain
●
Workmanship
Even the old building skills such as
bricklaying need careful
specification, and high levels of skill
and supervision. Trying to solve an
unexpected problem under the
pressures of contract completion
dates and the unpleasant conditions
experienced on every site is highly
inefficient.
To minimise any risk of structural
movement or collapse, care is needed
in the selection and positioning of
temporary support for works to
openings in external walls when
replacing lintels or creating or
enlarging openings (Figure 2.18).
Before starting work, the size of load
Maintenance
Repointing should be the only
maintenance required on a durable
brick wall, and the frequency with
which this should be carried out
depends on the mortar, the finish of
the joint, and the degree of exposure
of the wall, though at least a 30 or
40 year life ought to be expected.
Hand raking the old mortar wherever
possible is preferred to the use of
mechanical equipment. BRE
investigators on site have seen the
appalling damage caused by
disc cutters.
Cleaning of terracotta must be
done with great care, in particular the
outer surface should not be
mechanically abraded or acid etched
to expose porous substrates, or
durability will be compromised.
Sulfate attack
From 1931 onwards, the Building
Research Station issued warnings
about the susceptibility of the mortar
in some brick walls to sulfate attack,
particularly when the wall was
rendered. A common factor to all was
detailing that did not ensure that the
brickwork stayed dry. And still
failures occur!
BS 3921 defines two categories of
clay bricks, normal (N) and low salts
(L) types, on the basis of their content
of water-soluble salts.
Salts in clay bricks will be largely, but
not exclusively, sulfates and are
usually of sodium, potassium,
calcium or magnesium. A brick that is
classified as 'normal salts content'
may release sufficient sulfates to
cause deterioration of Portland
cement based mortar or renderings in
rain saturated walls and so is
restricted in its application. The
second (L) type is not so susceptible
to problems associated with sulfate
Figure 2.18
Risks are occasionally taken when
replacing lintels in solid masonry external
walls. Here the old relieving arch has been
cut away with just a soldier supporting the
springing on one side
