The
conductive concrete can be used as a structural material and bonds
well with normal concrete. The conventional mixing type is lightweight,
with only 70 per cent of normal concrete weight. Thermal stability
is comparable to normal concrete, production employs conventional
mixing and casting equipment, and application of the conductive
concrete is similar to that of conventional concrete. The conductive
concrete could be used along with specially configured electrodes
and an electric power supply to provide de-icing on roads, sidewalks,
bridges and runways. Placed as an overlay, conductive concrete
with very low resistivity can be used as a secondary anode in
existing cathodic protection systems, providing uniform current
distribution over its large surface area and reduced anodic current
density. At the same time, it provides excellent mechanical stability
due to its load-bearing capacity and its bond strength as an overlay.
And because conductive concrete attenuates electromagnetic and
radio waves, it can be used to shield computer equipment from
eavesdropping efforts and protect electrical installations and
electronic equipment from interference.
Stainless
steel differs significantly from carbon steel in its composition,
structure and properties. As its name suggests, stainless steel
is stainless and has superior corrosion resistance when compared
with the carbon steel. The most fundamental difference lies in
the composition. By definition, steel is considered to be stainless
when it contains at least 12% chromium. This is the minimum amount
of chromium required in an alloy/steel to maintain its "stainless"
surface appearance. The superior corrosion resistance of stainless
steel is due to the stability of an ultra thin surface oxide (mainly
chromium oxide) - the passive film. If the passive film is broken
or damaged due to chemical or mechanical actions, the chromium
element in the steel substrate can almost instantly repair the
damaged area by re-oxidization (re-passivation). It is this unique
self-healing process that makes stainless steel "stainless".
Chromium
is the essential element in all types of stainless steels and
alloys. Other alloying elements such as nickel and molybdenum
are also used to achieve certain required properties. The properties
of a stainless steel are determined by it structure which is in
turn determined by its alloying composition. There are three types
of stainless steels according to the structure: ferritic, austenitic
and martensitic. Austenitic grades 304 and 316 are the widely
used reinforcing material.
It
has long been well established that stainless steels have superior
corrosion resistance than carbon steels. Stainless steels and
alloys can maintain their natural passivity in seawater or 3.5%
NaCl solution. In comparison, the passive behaviour of carbon
steel rebars observed in concrete will be totally lost when carbon
steels are immersed in seawater or 3.5% NaCl solution, leading
to active dissolution and hence continuous loss of material. Research
has shown that when the pH of concrete pore water falls below
9, carbon steel rebar will depassivate. Stainless steels can even
maintain their passivity in acidic environment. Carbonation and/or
chloride ingress would not be able to destroy or depassivate stainless
rebars as the chromium oxides (the passive film on stainless steel
surface) is much more stable and hence has much greater resistance
to corrosion than the iron oxides (the passive film on carbon
steel surface). In recent years, more designers and engineers
have realised that solid stainless steels offer highest protection
against corrosion in concrete structures. With many other alternative
CP systems and coated products, the ultimate failure has often
been due to over-reliance on that lowest common denominator at
construction site level, namely the unskilled labour. This is
particularly so for coated/galvanized rebars -which, unlike solid
stainless steels, are not homogeneous. Careless handling and installation
(cutting, bending) can serously impair their corrosion resistance.
With solid stainless steel rebars, however, the integrity and
durability will not be affected at all.
Stainless
steels offer almost maintenance-free solution to the problem of
long-term durability of concrete structures and can be used within
existing design codes and practice in tow ways. The simplest but
more expansive option would be to use 100% stainless rebar to
replace existing carbon steel rebar. This would eliminate corrosion
and enable a design life in excess of 120 years. The total cost
can be competitive when life cycle costing is taken into account.
To reduce the cost, stainless steel rebar can be used intelligently
in areas of a structure such as bridge joints, splash zones, support
structures and column heads where carbon steel rebars are judged
to be at high risk of corrosion. If stainless rebars are used
in vunerable areas where cracking occurs and water enters, there
will be no significant corrosion and the structure will remain
sound. It has been estimated that the stainless steel content
on prestigious structures would be as little as 7~20%. The same
concept can be used in other structures such as general building
where peripheral or external walls could be constructed from stainless
steel rebar linking in to carbon steel rebar for the internal
structures where corrosion is not a problem.
