Civil Engineering Reference
In-Depth Information
Durability is clearly dependent on the quality of both workmanship and raw
materials, as well as the proportions of the mix and the environment of the build-
ing. In recent years it has become evident that certain types of air pollution
decompose concrete. Carbon dioxide and sulphur dioxide, both of which occur
in high concentrations around industrial areas and towns, are particularly dam-
aging.
It has been proved that carbon dioxide can carbonize up to 40 mm into con-
crete. The concrete loses its alkaline properties as a result and can be subject to
corrosive attack. The next phase of breakdown usually occurs quite quickly, and
involves the slow loss of the concrete. In the USA, one bridge per day is demol-
ished as a result of such processes.
Much of today's concrete contains organic additives, and these types of con-
crete break down even more quickly. Mortars with artificial resins have been seen
to decay within two to four years (Grunau, 1980).
The majority of Portland pozzolana concrete mixes have a much greater resis-
tance to pollution than pure Portland concrete. There is no long-term experience
of how lime sandstone and sulphur concrete last. The same can be said for lime
concrete, which is seldom used in northern countries.
Concrete can be protected through constructional detailing. There are certain
rules of thumb: avoid details that are continually exposed to rainwater. For
example, in horizontal concrete surfaces exposed to soot and other pollution, the
pollution is washed over the surface, intensifying decay of the concrete.
Recycling
The value of in-situ concrete in terms of recycling is low. It can, however, be
crushed and ground to aggregate. The majority of it has to be sorted and used
as fill. In theory, steel can be recycled from reinforcement, though this is a
complex process using machines for crushing the concrete, electromagnets for
separating, etc. Until 1950 smooth circular steel bars were used which were
much easier to remove from concrete. Fibre reinforcement has no recycling
potential.
Concrete units have considerably better recycling possibilities. By using
mechanical fixings or mortar joints that make it possible to dismantle the units,
the whole element can be re-used (see Figure 13.3).
The mortar used for constructions with concrete blocks is often Portland
cement. This construction is very difficult to disassemble without destroying the
blocks. Alternative are the different lime mortars, mainly based on hydraulic
lime. In some cases, weaker mortar may require compensation in terms of rein-
forcement. Larger concrete units are usually fixed together by welding or bolting,
which makes them easier to dismantle.
