Civil Engineering Reference
In-Depth Information
was extracted, 83% of which became a source of aluminium.
Aluminium is extracted from bauxite by the Hall-Héroult elec-
trolysis process. Four tonnes of bauxite makes two tonnes of
alumina which, in turn, produces one tonne of aluminium
Aluminium can be alloyed with traces of other elements such
as magnesium, manganese, chromium and silicon. A strict def-
inition of an aluminium alloy is one that contains at least 99%
by mass of aluminium and stays within the following limits:
Exfoliation
■
■
Stress cracking.
In fire, aluminium alloys will melt at around 550°C to 650°C
and start to lose strength at temperatures in excess of 100°C.
Aluminium alloys can be repeatedly recycled. Each oper-
ation only takes about 5% of the energy used for the original
manufacture of new alloys. Good quality scrap is recycled for
the production of extruded and rolled products. Depending on
the type of scrap material, recycling rates in the UK can be as
high as 98%. It is reported that the recycling rate for alumin-
ium cans in Japan and Brazil is over 90%.
Aluminium and aluminium alloys are classified using an
alpha-numeric system. To give but one example BS EN573-1
gives the European designations as follows.
EN AW-5154A: EN shows it is a European designation listed
in a European Code. EN is followed by a blank space. A rep-
resents aluminium and W represents a wrought product. After
the W the hyphen is followed by the international designation
consisting of four digits representing the chemical compos-
ition and, if required, a letter identifying a national variation:
this designation is attributed by the Aluminium Association via
an international registration procedure.
The following are some typical ranges of property values:
A total content of iron and silicon not greater than 1%.
■
A content of any other element not greater than 0.10% except for
■
■
copper which may have a content up to 0.20% provided that nei-
ther the chromium nor the manganese content exceeds 0.05%.
Aluminium alloys can be classified as:
Heat treatable (can be strengthened by thermal treatment).
■
Non-heat
treatable
(cannot
be
strengthened
by
thermal
■
treatment).
Castable (by sand, die and/or centrifugal casting methods).
■
Aluminium alloys are corrosion-resistant in many environ-
ments due to the inert film of aluminium oxide which forms
on its surface. Certain types are weldable using both MIG
(metal inert gas) and TIG (tungsten inert gas) systems. For
structures at modest stress level and under conditions of tight
quality assurance/control procedure it is also possible to use
adhesives to joint structural members. Alloys are defined by
a complex alpha-numeric coding system (see below) which
defines properties such as strength, temper, weldability and
others. Decorative effects can be achieved by anodising and
other techniques. Examples of use include:
Tensile strength 55 N/mm
2
to 580 N/mm
2
Proof stress (0.2%) 60 N/mm
■
■
2
to 520 N/mm
2
Modulus of elasticity 69 000 N/mm
2
to 80 000 N/mm
2
Coefficient of linear expansion 16 × 10
■
■
-6
to 24 × 10
-6
per °C.
14.6 Concrete
Concrete is not a new material: it is claimed that the first example
was its use in the floor of a hut in Yugoslavia in 5600 bc. Concrete
is strong in compression but weak in tension. Until the late nine-
teenth century it was, for the most part, unreinforced and per-
formed well in arched structures such as short span bridges. At
the end of the century under the influence of Coignet, Hennebique
and others methods of reinforcing the material were introduced
and
ferroconcrete
was born.
In building structures,
in situ
and/or precast concrete may be
used. In precast structures care must be taken with the detail-
ing of joints and connections to achieve the required stability
of the whole structure. In water retaining structures, such as
reservoirs, watertightness may be achieved by best practice
design, detailing and construction.
Before, and immediately after, the Second World War a
cubic yard of concrete might contain Portland cement, all-in
ballast, or a mixture of natural coarse and fine aggregates (usu-
ally crushed rock) and sufficient water for adequate hydration
of the cement; reinforcement would probably be plain round
mild steel bars. The concrete would most probably have been
volume batched. Compaction might have been achieved by
hand tamping or early types of surface or poker vibrators.
Lightweight prefabricated buildings
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Space frames
■
Motorway gantries
■
Roofing members
■
Structural members in aircraft
■
Offshore heli-decks
■
Pipework and ducting
■
Scaffolding
■
Curtain walling and other types of cladding
■
Lighting columns
■
Transport vehicles (where weight reduction can lead to reduced
■
fuel demand).
Aluminium may be subjected to the following types of
corrosion:
Galvanic (bi-metallic corrosion)
■
Pitting
■
Intergranular
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