Civil Engineering Reference
In-Depth Information
Since the 1930s, mortars based on
ordinary Portland cement (OPC) have
become the norm. These
would be batched on site from
cement, sand and usually some lime
to give added plasticity. Since the
1950s, a formulation plasticised by
small bubbles of air beaten into the
mix has been a popular alternative to
lime plasticised mixes. To make these
mixes more workable, an organic 'air-
entraining agent' is added to the mix.
All site-batched mortars can suffer
from batching problems which may
affect their performance and
durability. Occasionally too much
cement is added which can create
strong high bond, high shrinkage
mortar which leads to cracking of
masonry built with shrinkable units.
More commonly, too little cement is
added or too much water or air is
incorporated giving weak mortars
with poor durability.
Masonry cements are now covered
by BS 5224
(85)
and BS EN 413-2
(86)
.
Dry premixes of Portland cement,
hydraulically inert fillers and a
plasticising agent (air-entraining
admixture) are supplied for mixing
with sand and water on site. They
obviously eliminate some mixing and
batching problems and thus provide a
more precise control on quality than
wholly site-batched mortars.
Retarded ready-to-use mortars
perhaps offer the least room for error.
The complete mixing process is
carried out in the factory and mortar
can be used as received. Since an
OPC binder is used, a chemical
retarder has to be added to delay the
setting process. This is usually varied
to control the set to a period of two to
four days. This system obviously
gives the highest level of quality
control, but problems are still
possible if further additions are made
on site.
In view of difficulties which can be
experienced on site in sorting out the
various designations of mortars,
there is perhaps something to be said
for the use of a 'general use' mortar,
which is able to resist all but the most
severe exposure, and at the same time
be able to accommodate minor
movements. Such a mix would
consist of 1:1:5
1
⁄
2
ordinary Portland
cement:hydrated lime:Type S or G
sand plus an air-entraining agent.
Pointing technique, in addition to
altering the character of a brick or
block wall, can also play havoc with
its durability. Recessed joints, for
example, could lead to reduced
weathertightness, and should be
avoided in all but the most sheltered
locations.
Abutments
The most vulnerable situation for
solid masonry is the parapet. Not
only is it exposed on both faces, and
therefore can be wetted from both
faces as opposed to the wall on which
it is carried, it does not have the
benefit of heat gain from the building
to help it to lose excess moisture and
to keep it from freezing. Walls
beneath parapets arguably should
therefore have been protected with an
effective DPC at roof level, as well as
under the coping, otherwise they can
become very damp (Figure 2.10). In
certain cases, deterioration of the
masonry in parapets can be such as to
justify complete replacement; indeed
this has had to be done on flat roofed
industrial buildings constructed of
London Stocks at BRE after a life of
around fifty years. When replacement
is undertaken it is worthwhile putting
in an extra DPC.
Openings and joints
During rehabilitation a variety of
lintels may be found, including steel,
wrought iron, concrete, stone, timber
and arched brick. Lintels may be
found which are underdesigned; in
some cases they may be absent
altogether, reliance being placed on a
wall plate or the door or window
frame to support the wall above. In
these situations considerable
problems may be encountered during
window replacement.
Main performance
requirements and defects
Strength and stability
Most older brickwork was not
'designed' but built in accordance
with traditional rules of thumb.
Structural analysis of many existing
walls would indicate that they are
greatly over (or under) stressed.
Figure 2.10
Parapets are the most vulnerable parts of external walls
