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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)
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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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1

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

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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4

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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8

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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9

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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11

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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12

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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13

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.


26/26

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.

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