Civil Engineering Reference
In-Depth Information
can affect the level of radioactivity quite strongly. It can also be affected by prox-
imity to nuclear plants with (known or unknown) spillages.
Reinforcement
Steel is the most common material used to reinforce concrete. It is mainly recycled
from scrap metal, but it is normal to add 10 per cent new steel to improve the
strength. Steel reinforcement occurs in the form of bars or fibres that are 15 mm
long. Fibres are usually mixed in in proportions up to 2 per cent of the volume of
the concrete; the use of reinforcement bars takes up half as much volume as the
fibres. The advantage with the use of fibres is that they are better at taking up the
strains within the concrete and give a stronger concrete, which can reduce the
thickness of a slab by 30 per cent. The distance between the expansion joints can
also be increased considerably, therefore reducing the use of plastic joint mastics.
Other fibres have been introduced more recently in the form of glass and carbon.
Asbestos fibres were once used, but have been phased out because of their health
damaging properties. Any products or components that may contain asbestos
have to be identified and carefully removed from a site during demolition.
In smaller projects it is also possible to use fibres from plant material in a propor-
tion of 2 per cent volume. No research has been carried out to find out which types
are the most advantageous, but we can assume that long, strong fibres are well suit-
ed. They should be chemically neutral which is not always possible, but they should
at least be cleaned of all active substances before being used (see 'Woodwool cement
boards - production and use'). The most practical is hemp fibre (
Cannabis sativa
)
which is very strong. Timber fibres are also used, and in the former Soviet states
fibres from certain reed plants were tried, partly in industry and in schools up to
three stories. There have been experiments with bamboo reinforcement in both the
former Soviet Union and France in recent years with good results, even for larger
buildings.
Sinarunddinarianitida
is a tolerant species of bamboo which can be culti-
vated in Northern Europe.
Thamnecolomus murielae
is also a possibility.
Additives
It is quite normal to put a whole range of additives into cement and concrete
mixes (see Table 6.5). Additives are often organic and more or less volatile in
ready concrete, and many of them can cause problems in the indoor climate.
Evaporation of irritating substances from residues of oily fluids used on moulds
and temporary lathing during the casting process, is a large problem in many
concrete buildings.
Handling and demolishing concrete can cause a problem with dust from
colouring pigments which contain heavy metals, including chrome, lead and
cobalt. It is possible that the waste process allows seepage into the environment
of added tensides, aromatic hydrocarbons, amines, borates, etc. Melamine-based
plasticizers can develop poisonous gases during a fire.
