Emerging Corrosion Control Technologies for Repair
and Rehabilitation of
Concrete Structures*
Dr. Jianhai Qiu
School of Materials Engineering
Nanyang Technological University
Nanyang Avenue, Singapore 639797
5.Conductive
Concrete
5.1 The
Nature of Conductive Concrete
Conventional
concrete is excellent in durability and mechanical properties but is a poor electrical conductor, especially under
dry conditions. Durable concrete that is excellent in both mechanical and electrical conductivity properties may
have important applications in the electrical, electronic, military and construction industry (e.g. for CP system,
de-icing road from snow).
Conductive
concrete may be defined as a cementitious composite which contains a certain amount of electronically conductive components
to attain stable and relatively high electrical conductivity. The principle behind it is the use of cement to bind
together electrically conductive materials such as carbon fiber, graphite and 'coke breeze' - a cheap by-product
of steel production - to make a continuous network of conducting pathway. The design formulation is based on the
'electrical percolation' principle by which the composite conductivity increases dramatically by several orders
of magnitude when the content of the conductive phase reaches a critical 'threshold' value. Further increases in
the conductive phase content boost composite conductivity only slightly. The design specifies an amount just over
the threshold content, assuring high conductivity and mechanical strength as well as good mixing conditions.
Concrete
bridge decks are prone to ice accumulation. The use of road salts and chemicals for deicing is cost effective but
causes damage to concrete and corrosion of reinforcing steel in concrete bridge decks. This problem is a major
concern to transportation officials and public works due to rapid degradation of existing concrete pavements and
bridge decks. The use of insulation materials for ice control and electric or thermal heating for deicing have
been attempted and met limited success. Based on the results of a transient heat transfer analysis, a thin conductive
concrete overlay on a bridge deck has the potential to become a cost effective deicing method. When connected to
a power source, heat is generated due to the electrical resistance in the cement admixture with metallic particles
and steel fibers. Small-scale slab heating experiments have shown that an average power of about 520 W/m2
(48 W/ft2) was generated
by the conductive concrete to raise the slab temperature from -1.1oC (30oF) to 15.6oC (60oF) in 30 minutes.
This power level is consistent with the successful deicing applications using electrical heating cited in the literature.
5.2 Applications
of Conductive Concrete
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.
The
Benefits
Conductive
concrete has excellent mechanical and electrical conductivity properties.
-
It is
much lighter in weight than conventional concrete.
-
It can
be produced easily, without special equipment.
-
It will
reduce the need of salts and save millions in dollars in snow removal costs.
-
It warms
by power taken off-line, it uses an AC current. It is also safe for a person crossing a charged concrete pathway.
-
It can
also be used for protecting structures against static electricity and lightning, and preventing steel structures
and reinforcing layer of steel in concrete structures from corroding.
-
It absorbs
over 90% of the electromagnetic energy and it is cheaper and more convenient than the existing ways of blocking
out electromagnetic energy.
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