Civil Engineering Reference
In-Depth Information
prefabricated units which are hardened in an autoclave. Prefabricated units of
lightweight concrete are usually reinforced with steel. Aerated concrete is the only
commercial pure mineral block with good structural properties
and
a high ther-
mal insulation value. The material is very porous, and needs a surface treatment
which lets out/in vapour - hydraulic lime render, for example. If the water con-
tent becomes too high the material will easily be split by frost. The production of
this aerated concrete is dependent upon aluminium. The total contribution of alu-
minium in the external walls of a relatively large private house is 10-20 kg.
Aerated concrete normally has good moisture-regulating properties and does
not have any negative effects on the indoor climate, although the steel reinforce-
ment can increase the electromagnetic field in a building. Aluminium will have
completely reacted in the finished product, and in practice aerated concrete can
be considered inert and problem free as waste. Both prefabricated units and the
blocks can be re-used, depending upon how they were laid and the mortar used.
Strong mortars are used nowadays which make it difficult to dismantle the com-
ponents without damaging them. More appropriate mortars are weak lime
cement mortar and hydraulic lime mortar. Crushed aerated concrete can be used
as insulating granules for road building, and also as aggregate in lime sandstone,
different light mortars and light concretes.
Concrete with light aggregate
This is usually produced as blocks, slabs or floor beam units which are relative-
ly strong. There is a difference between products that have an organic and a min-
eral aggregate. Mineral insulating aggregate in concrete can be light expanded
clay, pumice, fossil meal and exfoliated vermiculite, perlite or slag. The first two
and expanded perlite have the lowest moisture absorption coefficient, and are
therefore best-suited to products used for insulation. The others have a very high
moisture absorption coefficient and are best used as insulation for high temper-
ature equipment.
Sawdust and chopped straw can be used as organic constituents in concrete.
Blocks are also produced using broken up, waste polystyrene, and it is possible
to produce lightweight concrete mixed with waste paper. With the exception of
woodwool slabs, discussed later in this chapter under 'Timber', concrete with
organic constituents generally has a low thermal insulation value compared to
rival products such as aerated concrete. In light expanded clay blocks it is becom-
ing more usual to cast in a thermal insulating membrane of expanded plastic,
usually polyurethane, discussed later in this chapter under 'Plastics'.
'Woodcrete', which contains the maximum proportion of sawdust possible,
achieves much higher thermal insulation values than normal concrete, and can
be compared with a light expanded clay block, for example. Woodcrete should
be a viable alternative because it provides a considerably warmer, softer surface
than pure concrete. It is also a good sound insulator, and will not rot because of
