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This report encompasses the properties, applications and the advantages of conductive
concrete along with a case study on the subject.
1. PROPERTIES OF CONDUCTIVE CONCRETEConductive concrete is a cement-based composite that contains a certain amount of
electronically conductive components to attain stable and relatively high conductivity. In
essence, the aggregates normally used in concrete can be largely replaced by a variety of carbon-
based materials to achieve electrical conductivity in conductive concrete. This is achieved while
retaining the desired engineering properties, as indicated in Table 1. The conductivity is usually
several orders of magnitude higher than that of normal concrete. Normal concrete is effectively
an insulator in the dry state, has unstable and significantly greater resistivity characteristics than
conductive concrete, even when wet.
Table 1. Conductive Concrete Properties (source:http://www.engineeringcivil.com/electrically-
conductive-concrete-properties-and-potential.html)
Electrical Resistivity (omega cm) 1-40
Compressive Strength (MPa) 30
Flexural Strength (MPa) 515
Density (kg/m3) 14501850
Conductive concrete can be produced using conventional mixing techniques. The mixing
process can be controlled, permitting design of mix formulations that are reliable, and achieve
electrical resistivity values within the overall target design range.
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1.1ELECTRICAL RESISTIVITY
Conventional concrete is not electrically conductive. The electric resistivity of normal
weight concrete ranges between 6.5411 kWm. A hydrating concrete consists of pore solution
and solids, including aggregates, hydrates, and unhydrated cement. The electric resistivity of the
pore solution in the cement paste is approximately 0.25 to 0.35 kWm. Most common aggregates
(for example, limestone) used in concrete, with electric resistivity ranging between 3 x 102 and
1.5 x 103kW.m are considered nonconductive.
The approximate values of the impedance and the electric resistivity of conductive
concrete are given by the following equations
()
and
()
where Z is the impedance; V is the applied AC voltage, I is the AC current, is the average
electric resistivity of the specimen, L is the spacing between the electrodes and A is the cross-
sectional area of the test slab parallel to the electrodes
In the tests conducted by Tuan and Yehia S.(2004), two trial mixtures, EC-All and Slag +
25% EL showed high electrical conductivity and heating rates. Experimental data from the
heating tests of these two mixtures are presented in Figure 1 and Figure 2, respectively. The EC
designations are used to distinguish carbon products of different particle sizes. Earth Link (EL) is
the trade name of graphite cement, which contains approximately 70% Portland cement and 30%
graphite powder
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Figure 1 Electric resistivity versus temperatureEC-All mixture(source:
http://www.cement.org/tech/cct_con_design_conductive.asp )
Figure 2 Electric resistivity versus temperatureSlag +25% EL mixture(source:http://www.cement.org/tech/cct_con_design_conductive.asp)
http://www.cement.org/tech/cct_con_design_conductive.asphttp://www.cement.org/tech/cct_con_design_conductive.asphttp://www.cement.org/tech/cct_con_design_conductive.asphttp://www.cement.org/tech/cct_con_design_conductive.asphttp://www.cement.org/tech/cct_con_design_conductive.asphttp://www.cement.org/tech/cct_con_design_conductive.asp8/13/2019 Thus Haraccccc
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The electric resistivity of these materials is a function of temperature. As temperature
increases, the materials become more electrically conductive. The higher electrical conductivity
is probably due to the good gradation of carbon particles in the EC-All and the added slag in the
Slag + 25% EL mixtures.
2. THE TECHNOLOGYThe principle behind conductive concrete 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 increase 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.
Conductive concrete particles and fibers are added to conventional aggregate and cement
paste compositions to achieve the conductive concrete, which can be fabricated by two methods.
The first one is by conventional mixing, which has relatively higher resistivity and high
compressive strength. The second one is by slurry infiltration. This method can increase
compressive and flexural strengths, and lower resistivity can be obtained.
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 obtained by
conventional mixing 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
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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
.
3.1IMPROVING THE CONDUCTIVE PROPERTY OF CONCRETE
The conductive property is the first element of conductive concrete. The material resistivity
is an index which reflects the conductive property of the concrete. In order to obtain the
resistivity as low as possible, the scientists experimented many different kinds of and different
specifications of conductive materials, adopted different proportions and preparation processes,
finally found a relatively reasonable optimization scheme after balancing the cost factors.
THE CONDUCTIVE CONCRETE WITH GRAPHITE AS CONDUCTIVE
MATERIACONDUCTIVE CONCRETE
SEMINAR REPORT
Submitted in partial ful fi llment of the
Requirements for the award of the
Degree of Bachelor of Technology in Civil Engineering
Of the University of Kerala
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Submitted By
VYSAKH MURALI V
Guided By
DR. JAYASREE P K
DEPARTMENT OF CIVIL ENGINEERING
COLLEGE OF ENGINEERING TRIVANDRUM
TRIVANDRUM 16
2012-2013
DEPARTMENT OF CIVIL ENGINEERING
COLLEGE OF ENGINEERING TRIVANDRUM
TRIVANDRUM 16
CERTIFICATE
This is to certify that this Seminar Report on Conductive Concrete is a bonafide record of the
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seminar report submitted by VYSAKH MURALI V,
5 APPLICATIONS 10
6 ADVANTAGES 11
7 CASE STUDY- ROCA SPUR BRIDGE 11
7.1 Construction Sequence 11
7.2 Intergration of Power Supply, Sensors and Control Units 13
7.3 Safety concerns 13
7.4 Deicing operations 14
7.5 Construction costs 17
8 CONCLUSIONS 19
9 REFERENCES student of Civil Engineering, College of Engineering Trivandrum,
in the partial fulfillment of the requirements for the award for B-Tech Degree in Civil
Engineering of the University of Kerala during the academic year 2012.
Guided by U G Professor,
DR Jayasree P.K. Prof Joisy M. B.
Assistant Professor UG Professor
Department of Civil Engineering. Department of Civil Engineering.
College of Engineering College of Engineering
Trivandrum Trivandrum
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ABSTRACT
The recent advancements in construction technology have given rise to innovative
techniques such as conductive concrete. Conductive concrete particles and fibers are added to
conventional aggregate and cement paste compositions to achieve the conductive concrete,
which can be fabricated by two methods. The first one is by conventional mixing, which has
relatively higher resistivity and high compressive strength. The second one is by slurry
infiltration. This method can increase compressive and flexural strengths, and lower resistivity
can be obtained. The invention of electrically conductive concrete promises many opportunities
to utilize this material in potential applications such as deicing of roads, roads that can charge
electric cars as they drive, improved grounding for power plants, security shielding for sensitive
data handling and storage facilities, and massive electrical storage batteries that be built intobuildings, roads or parking lots.
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CONTENTS
1. INTROUDCTION 12. PROPERTIES 2
2.1 Electrical Resistivty 3
3. THE TECHNOLOGY 5
3.1 Improving the Conductive Property of Concrete 6
3.1.1 The conductive concrete with graphite as conductive material 6
3.1.2 Composite conductive concrete with carbon fiber 7
4 ELECTRIC CONDCUTION MECHANISM IN CONDUCTIVE 9
CONCRETE
20
LIST OF FIGURES
3 Electric resistivity versus temperatureEC-All mixture 4
2 Electric resistivity versus temperatureSlag +25% EL mixture. 4
3 Comparison of Different Graphite 7
4 Comparison of Graphite and Composite 8
5 Conductive concrete panel layout (dimensions in mm). 12
6 Power cord and angle iron connection. 12
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5 Roca Bridge deck deicingFebruary 5, 2004. 15
6 Ambient versus average slab temperature. 16
7 Average current/temperature relationship 17
LIST OF TABLES
2 Conductive Concrete Properties 2
3 Deicing performance of Roca Spur Bridge 14
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3. INTRODUCTIONConductive concrete may be defined as a cementitious composite that contains a certain
number of electrically conductive components in regular concrete matrix to attain stable and
relatively high electrical grounding and electro-magnetic pulse (EMP) shielding. Concrete
has been used for many years as a composite material that has excellent mechanical
properties and durability for construction. However, concrete is a poor electrical conductor,
especially under dry conditions. The electric resistivity of normal weight concrete ranges
between 6.54 11 km 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 de-icing road from snow). In 1998, Yehia and Tuan from
the University of Nebraska developed a conductive concrete mixture design specifically for
bridge deck deicing. The mixture design contained 1.5% of steel fibers and 20% steel
shavings per volume of concrete. A 1.2 x 3.6m and 150mm-thick conventional concrete slab
was constructed to simulate a concrete bridge deck. A 90mm-thick conductive concrete
overlay with two steel strips embedded with electrodes was cast on top of the concrete slab
for conducting a deicing experiment in a natural environment. The mixture had an average
compressive strength of 31 MPa (4500 psi) and provided an average thermal power density
of 590 W/m2(55 W/ft2) with a heating rate of approximately 0.14 C/min (0.25 F/min) in a
winter environment. The average energy cost was approximately $0.8/m2 ($0.074/ft2) per
snowstorm.
Steel shavings are waste materials produced by steel fabricators in the form of small
particles of random shapes. The drawbacks noticed when using steel shavings during
development of the conductive concrete are as follows, a lack of consistency in size andcomposition from various sources of steel shavings, the steel shavings were usually
contaminated with oil and required cleaning and the steel shavings required a specialized
mixing procedure to ensure uniform dispersal in the concrete. As a follow-up effort to
eliminate the drawbacks, carbon and graphite products were used to replace steel shavings in
several trial conductive concrete mixture designs.
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2
3.1.1 L
Graphite is a kind of good conductive material. It is widely used in the areas related to
electric conduction and also a popular material used in the conductive concrete due to its good
conductivity, stable property, wide material source and no pollution to environment. By
comparing tests in different proportions by taking respectively the soil shape common graphite
samples with carbon content more than 80% (B) and the flaky high grade graphite samples with
carbon content more than 95% (A) and fineness of 500 mesh. See Figure 3 Comparison of
Different Graphite for the test result.
Figure 8. Comparison of Different Graphite (source: Yehia et al., 2000)
We can see from the figure, the conductive property of the high grade flaky graphite with
high carbon content and high fineness has the conductive property higher than that of the soil
shape common graphite with low carbon content, and the conductive property of the concrete
will improve with increase of the graphite proportion. However, the graphite inherent lubricity
greatly decreases the strength of the concrete with high graphite proportion and the cost of the
concrete greatly increases with increase of the graphite proportion. Obviously, it is not an ideal
way to use the high price and high grade graphite and raise the graphite proportion in the
concrete to improve the conductive property, the comprehensive property and economic result(it
is not an ideal way to improve the conductive property, the comprehensive property and
economic result by using the high price, high grade graphite and raising the graphite proportion
in the concrete).
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3.1.2 Composite Conductive Concrete with Carbon Fiber
When the composite conductive concrete with a new conductive fiber material was used
as the dominant material and other materials as the auxiliary ones for improving the conductive
property of the conductive concrete since the conductive concrete mixed with pure graphite
cannot get the satisfactory conductive property, strength and economic result. As it easily forms
the conductive channel between the materials, the conductive fiber material can evidently
improve the conductive property of the conductive concrete, as shown in Figure 4.
Figure 9. Comparison of Graphite and Composite(Yehia, S. A. Tuan, C. Y. Ferdon, D. andChen, B. 2000)
Comparison of Graphite and Composite Material in the figure shows, the graphite used
for comparison is the high grade flaky graphite with 95% carbon content and in the proportion of
15%. According to the used material and its proportion, the conductive concrete is the very good
graphite conductive concrete. But compared with the composite conductive concrete, the
conductive property of the graphite conductive concrete is evidently not in the same order of
magnitude. Moreover, the resistivity of the graphite conductive concrete gradually will rise with
the curing time extending, but the resistivity of the composite conductive concrete changes
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slightly and its conductive property is very stable. This is a big advantage of the composite
conductive concrete, that i
Feb 4-6,2004 198 -7.2 19 2797 1.00 Simultaneous
The power was turned on for 6 to 8 h before the snowstorms to preheat the slabs. The 52
slabs were divided into 26 groups with each group containing two consecutive slabs. Thus,
Group 1 contains Slabs 1 and 2, Group 2 contains Slabs 3 and 4, and so on. During the December
8 storm, the odd-numbered groups were energized for 30 min and off for 30 min when the even-
numbered groups were powered. This alternating form of energizing the slabs could not keep up
with the low temperature, high wind, and a snow rate of about 25 mm/h (1in./h). As a result, the
deck was partially covered with snow. The scheme was revised to energize all the slabs when the
ambient temperature dropped below 1 C (30 F) and switched to alternating powering when
the ambient temperature was above 1 C (30 F). The revised scheme seems to have worked
well in the later storms. Figure 7 shows that the deck was free of snow cover during the February
5 storm.
Figure 10 Roca Bridge deck deicingFebruary 5, 2004(source: Krause, A.P, 2012)
The slab temperature distribution was very uniform across the deck during deicing operations,
generally in the 4 to 10 C (25 to 50 F) range. As shown in Figure 8, the average slabtemperature was consistently approximately 10 C (18 F) higher than the ambient temperature.
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Figure 11 Ambient versus average slab temperature.(source: Krause, A.P, 2012)
The maximum current recorded varied between 7 and 10 amps. Figure 9 shows that the
electrical conductivity of the conductive concrete increased with higher average slab
temperatures. The peak power density delivered to the slabs varied between 360 and 560 W/m 2
(33 to 52 W/ft2) with an average of 452 W/m2. The total energy consumed by the conductive
concrete slabs during the storms is summarized in Table 4. The energy consumed by the slabs
varied from 47 to 70 kW-h, with an average of 58 kWh per slab. The average energy
consumption under simultaneous powering was approximately 3200 kW-h, which would cost
approximately $260 for each major storm based on the rate of $0.08/kW-h. The Roca Spur
Bridge project has demonstrated that using conductive concrete for deicing has the potential to
become the most cost-effective roadway deicing method in the future.
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Figure 12 Average current-temperature relationship (source:http://rebar.ecn.purdue.edu/ect/links/technologies/civil/conductive.aspx)
3.1. CONSTRUCTION COSTS
The construction costs of the conductive concrete inlay are itemized as follows:
Placing, finishing, curing, and saw cutting conductive concrete: $50,020 Procuring conductive concrete materials: $80,620
Building and installing control cabinet with sensors and power relays: $43,685
Integrating and programming the deicing operation controller: $18,850
Therefore, the total construction cost of the Roca Spur Bridge deicing system was
$193,175. The cost per unit surface area of the conductive concrete inlay is $635/m2 ($59/ft2).
The heated deck of Roca Spur Bridge is the first implementation in the world using conductive
concrete for deicing. The initial construction cost was high compared with the $377/m2 ($35/ft2)
cost of a propane-fired boiler heating system recently installed in the Buffalo River Bridge in
Amherst, Va., in 1996.14 Life-cycle costs including system maintenance costs, deck repair costs,
and vehicle depreciation caused by deicing chemicals, however, should be used as the basis for
cost-effectiveness comparisons of different deicing systems. In addition, the construction costs of
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conductive concrete overlay/inlay are expected to drop significantly when the technology
becomes widely accepted.
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4. CONCLUSIONProperties of the conductive concrete are far superior to that of conventional concrete. It has
superior conductivity and lower density than the conventional concrete. Although the initial cost
of construction is significantly higher, in the long run it is more economical when we factor in
the low maintenance of the conductive concrete. Furthermore new cost effective techniques are
being experimented on. Detailed studies have exposed the drawbacks of using the steel shavings
in the mixture. As a follow-up effort, carbon products were used to replace the steel shavings in
the conductive concrete mixture design. The use of graphite has successfully increased the
workability, conductivity without compromising the strength. A conductive concrete deck using
the EC-All mixture has been implemented for deicing on a highway bridge at Roca, located
approximately 24 km south of Lincoln, Nebraska. Temperature and current sensors installed inthe Roca Spur Bridge enabled the scientist to monitor the heating performance of the bridge
during winter storms. The deicing system has worked well in four major snowstorms in the
winter of 2003 and delivered an average power density of 452 W/m 2 to melt snow and ice. The
conductive concrete deck deicing system at Roca Spur Bridge will continue to be monitored for
the next several winters to evaluate its cost-effectiveness against other deicing technologies.
As of now constructive concrete remains a new and promising technology that is slowly
making its way in to the commercial construction sector and is poised to replace conventionalconcrete in many sectors.
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REFERENCES
1. Christopher, Y. T. and Yehia, S. (2004). Evaluation of Electrically Conductive
Concrete Containing Carbon Products for DeicingACI material journal,101-32, PP
287-293
2. Yehia, S. A. Tuan, C. Y. Ferdon, D. and Chen, B.(2000) Conductive Concrete
Overlay for Bridge Deck Deicing: Mix Design, Optimization, and Properties, ACI
Materials Journal, V. 97, No. 2, pp. 172-181.
3. Krause, A.P (2012). "Conductive Concrete for Electromagnetic Shielding Methods
for Development and Evaluation Degree of Master of Science Thesis, University of
Nebraska, USA
4.
http://www.engineeringcivil.com/electrically-conductive-concrete-properties-and-potential.html accessed on 31/6/2013
5. http://rebar.ecn.purdue.edu/ect/links/technologies/civil/conductive.aspx accessed on
31/6/2013
http://www.cement.org/tech/cct_con_design_conductive.asp accessed on 31/6/2013s, the
concrete already has a good and stable conductive property in its initial setting stage. Although
the graphite conductive concrete has a relatively good conductive property in its initial setting
stage, the conductive property will gradually decrease and finally reach a stable value as time
goes on. When the conductive concrete is used as the resistance reducing agent, the composite
conductive concrete can be used for the effect test soon after pouring. The test values measured
in this stage are basically consistent with those measured after setting of concrete. This can not
only improve the work efficiency and shorten the work cycle, but also increase the economic
benefit.
Although the conductive fiber material can greatly improve the conductive property of
the concrete, the original preparation process makes it very difficult to apply such fiber material
in the construction site. In order to bring it into utmost play, the fiber material must be evenly
distributed in the concrete, which needs to be accomplished by adding dispersant. The dispersant
can be dissolved only in the high temperature water. It is not very difficult to solve the problem
in the lab because it is a small-batch test. But in the construction site, to dissolve the dispersant in
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the high temperature water will be a Gordian knot. If this problem is not solved, the wide
application of the fiber material in the engineering practice will be certainly limited.
5. ELECTRIC CONDUCTION MECHANISM IN CONDUCTIVECONCRETE
Conduction of electricity through concrete may take place in two ways: electronic and
electrolytic. Electronic conduction occurs through the motion of free electrons in the conductive
media, whereas electrolytic conduction takes place by the motion of ions in the pore solution.
Whittington, McCarter, and Forde investigated conduction of electricity through conventional
concrete using cement paste and concrete specimens. The electric resistivity was found to
increase with time for both specimens because conduction in these specimens depended on the
ions motion in the pore solution. In addition, the electric resistivity of the concrete specimens
was higher than that of the cement paste specimens due to the restricted ion movement from
nonconductive aggregates used in the concrete specimens. Farrar in 1978 used marconite, a
carbon by-product from oil refining, to replace sand in a conductive concrete mixture. The
electric resistivity of the conductive concrete using marconite ranges between 0.5 to 15 Wcm.
The use of marconite was limited to small-scale applications such as electromagnetic shielding
and antistatic flooring because it was expensive. Conduction of electricity in this case was
through the movement of electrons, and the particles must be in continuous contact within the
concrete. This phenomenon is called electrical percolation in concrete.
Because the conductive components added only amounted to 25% by volume of the total
materials, there are probably not enough conductive fibers and particles to form a fully
interconnected electronic circuit within the concrete. Instead, these dispersed conductive
materials would act as capacitors when a voltage is applied across the material. Electrical current
will flow through the material if the applied voltage is high enough to cause dielectric
breakdown of the material. There is a critical threshold of voltage, above which large current will
go through the material like a short circuit. If the applied voltage is kept below this breakdown
voltage, a controllable amount of current proportional to the voltage will go through the material.
This behavior is similar to that of a surge protector used in computers,
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6. APPLICATIONS Electrical heating: Electrical heating using conductive concrete has excellent potential for
domestic and outdoor environments, especially for de-icing of parking garages,
sidewalks, driveways, highway bridges, and airport runways. This method of heating
would eliminate or dramatically reduce the need for using salt, thus providing an
effective and environmentally friendly alternative. Conductive concrete itself is the
heating element, and thus is able to generate heat more uniformly throughout the heated
structure.
Electrical grounding. Grounding is required for virtually every electrical installation. The
main purpose of electrical grounding is to protect the equipment and occupants in the
event of an electrical systems failure, or in special situations such as the presence of
lightning or static electricity. The protection is achieved through a proper electrical
connection between the systems usually by embedding an electrode underground.
The establishment of an effective, economical and durable electrical grounding system
has always presented problems for the electrical engineer, but now many of them can be
solved through use of conductive concrete. Conductive concrete grounding uses include
creation of equipotential floors in such disparate applications as dairy barns, where small
voltage differences can reduce production, through to electronics fabrication and
handling areas, where the potential for costly damage to high-value semi-conductors and
associated equipment caused by static charges can be high.
With its excellent structural engineering properties, conductive concrete is also a good
candidate for grounding in a variety of utility uses. These include communications, and
electrical transmission towers, as well as electrical transformer locations.
7.
ADVANTAGES
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.
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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 and a 120 Volt plug.
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.
8. CASE STUDY - ROCA SPUR BRIDGERoca Spur Bridge is a 46 m-long (150 ft) and 11 m-wide (36 ft), three-span highway bridge
over the Salt Creek at Roca, located on Nebraska Highway 77 South approximately 24 km (15
miles) south of Lincoln. A railroad crossing is located immediately following the end of the
bridge, making it a prime candidate for deicing application. The Roca Bridge project was let in
December 2001 and construction was completed in November 2002. The bridge deck has a 36 x
8.5 m (117 x 28 ft) by 102 mm (4 in.) conductive concrete inlay, which is instrumented with
thermocouples for deicing monitoring during winter storms.
8.1.CONSTRUCTION SEQUENCE
A 102 mm- thick (4 in.) inlay of conductive concrete using the EC-All mixture was cast on
top of a 256 mm-thick (10.5 in.) regular reinforced concrete deck. As shown in Fig. 5, the inlay
consists of 52 individual 1.2 x 4.1 m (4 x 14 ft) conductive concrete slabs. In each slab, two 89
x 89 x 6 mm (3.5 x 3.5 x1/4 in.) angle irons spaced 1067 mm (3.5 ft) apart were embedded forelectrodes. Thread sleeves were welded to one end of the angle irons for making electrical
connection. A Type TX thermocouple was installed at the center of each slab at approximately
13 mm (0.5 in.) below the surface to measure the slab temperature. The power cords and
thermocouple wiring for each slab were secured in two polyvinyl chloride (PVC) conduits and
are accessible from junction boxes along the centerline of the bridge deck.
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Figure 13 Conductive concrete panel layout (dimensions in mm)(source: Krause, A.P, 2012)
The conductive concrete inlay was cast after the regular bridge deck had been cured for
30 days. After hardening, the conductive concrete inlay was saw cut to a 102 mm (4 in.) depth
along the perimeters of the individual slabs and the gaps were filled with polyurethane sealant.
There was a 152 mm (6 in.) gap along the centerline of the bridge to allow power cord
connections with the thread sleeves on the angle irons, as shown in Figure 6. The gap was then
filled with a non-shrink, high-strength grout.
Figure 14- Power cord and angle iron connection (source: Krause, A.P, 2012)
8.2.INTEGRATION OF POWER SUPPLY, SENSORS, AND CONTROL UNITS
A three-phase, 600 A and 208 V AC power supply was provided by a power line nearby. A
microprocessor-based controller system was installed in a control room to monitor and control
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14
the deicing operation of the 52 slabs. The system included four main elements: 1) a temperature-
sensing unit; 2) a power-switching unit; 3) a current-monitoring unit; and 4) an operator-
interface unit. The temperature-sensing unit took and recorded the thermocouple readings of the
slabs every 15 min. The slabs power was turned onby the controller if the temperature of the
slab was below 1.7 C (35 F) and turned off if the temperature was above 12.8 C (55 F). The
power-switching unit controlled power relays to perform the desired on/off function. To ensure
safety, a current-monitoring unit limited the current going through a slab to a user-specified
amount. The operator-interface unit allowed a user to connect to the controller with a PC or
laptop by a phone modem. The operator interface displayed all the temperature and electrical
current readings of every slab in real time. A user also had the option of using a PC or laptop to
download the controller-stored data into a spreadsheet.
8.3.SAFETY CONCERNS
The use of high voltage and high current causes a safety concern, even though the
conductive concrete behaves as a semi-conductor. A model commonly used to describe the
behavior of a diode2 as a resistor in parallel with a variable resistor and a capacitor may be used
to describe the electrical conduction behavior of the conductive concrete. The isolated
conductive particles within the concrete act as capacitors when a voltage is applied across the
material. The current flows through the material due to dielectric breakdown. The summation of
the potential drops of all the viable current paths between the two electrodes is equal to the
applied voltage. Likewise, the total current going through all the viable paths is equal to the
current corresponding to the applied voltage. This behavior has been confirmed by field
measurements. Several measurements were taken at different locations on the inlay surface under
208 V during heating experiments, and step potential readings were in the range of 10 to 20 V.
The current readings were in the range of 15 to 30 mA. These voltage and current levels pose no
hazard to the human body. On another occasion, the authors touched the surface of the 1.2 x 3.6
m (4 x 12 ft) conductive concrete slab containing steel fibers and shavings during deicing
experiment without feeling any electric shock while the slab was energized with 410 V of AC
power and had approximately 10 amps of current going through it.
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A potential safety hazard exists, however, if some steel fibers were in direct contact with
electrodes and exposed on the surface. An effective measure to eliminate potential stray current
on the surface is to apply 1.6 to 3.2 mm (1/16 to 1/8 in.) coating of a low-modulus and low-
viscosity epoxy on the conductive concrete surface. Fine aggregate will then be spread on before
the epoxy sets to form a skid-resistant surface. Although the power will be turned on only when
snow/ice storms are anticipated, it may be prudent to monitor the step potential and stray current
to ascertain that there is no electric shock hazard to the public.
8.4.DEICING OPERATION
The deicing controller system was completed in March 2003. Although major snow storms
of 2002 were missed, the system was tested successfully under freezing temperatures. The 52
conductive concrete slabs were activated for deicing during four major snow storms in the winter
of 2003. The climatic data of these storms were obtained from the National Climatic Data
Center,13 a weather station in Lincoln, Nebr., and are summarized in Table 2.
Table 2- Deicing performance of Roca Spur Bridge(source: Christopher, Y. T. and Yehia, S.2004)
Storm date Snow depth
(mm)
Air
temperat
ure 0c
Wind
speed
Km/h
Energy kWh Unit
cost,$/m2
Power scheme
Dec 8-9,2003 165 -6.3 36 2023 .54 Alternating
Jan 25-26,2004 257 -11.1 23 2885 .75 Simultaneous
Feb 1-2,2004 145 -10.0 18 2700 .71 Simultaneous
6.