Conductive Concrete for Bridge Deck Deicing
By Christopher Y. Tuan, Ph.D., P.E., Associate Professor
of Civil Engineering, University of Nebraska
Heated deck of Roca Spur Bridge in Nebraska is the
world's first implementation using conductive concrete for deicing.
Roca Spur Bridge is a 46-m (150-ft) long and 11-m (36-ft) wide,
three-span highway bridge over the Salt Creek at Roca, located on
Nebraska Highway 77 South about 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 electrical deicing application.
The Roca Bridge project began in December 2001, and construction
was completed in November 2002. The bridge deck has a 36-m (117-ft)
by 8.5-m (28-ft) by 100-mm (4-in.) conductive concrete inlay, which
is instrumented with thermocouples to provide data for deicing monitoring
during winter storms.
Conductive concrete contains a certain amount of electrically
conductive components in the regular concrete matrix to attain stable
and relatively high electrical conductivity. The mix design used
in this project contained steel fibers and carbon products for conductive
materials. Steel fibers of variable lengths amounted to 1.5% and
the carbon products of different particle sizes amounted to 25%
per volume of the conductive concrete. Crushed limestone of 13-mm
(0.5-in.) maximum size and Nebraska 47B fine aggregate were also
used in the mix. Due to its electrical resistance and impedance,
a thin conductive concrete overlay can generate enough heat to prevent
ice formation on a bridge deck when connected to a power source.
It is expected that this project will demonstrate that conductive
concrete technology has national and international importance. Statistics
indicate that 10% to 15% of all roadway accidents are directly related
to weather conditions. This percentage alone represents thousands
of human injuries and deaths and millions of dollars in property
damage annually. Ice accumulation on paved surfaces is not merely
a concern for motorists; ice on pedestrian walkways accounts for
numerous slip and fall injuries. The payoff potential for this project
is tremendous: it could eliminate icy bridge roadways and save lives.
This revolutionary deicing technology is applicable to accident-prone
areas such as bridge overpasses, exit ramps, airport runways, street
intersections, sidewalks, and driveways.
A 100-mm (4-in.) thick inlay of conductive concrete was cast on
top of a 256-mm (10.5-in.) thick regular reinforced concrete deck.
The inlay consists of 52 individual 1.2-m x 4.1-m (4-ft x 14-ft)
conductive concrete slabs. In each slab, two angle irons were embedded
for electrodes (Figure 1). Coupling nuts were welded to one end
of the angle irons for making an electrical connection. A thermocouple
was installed at the center of each slab at about 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 PVC conduits
and are accessible from junction boxes along the centerline of the
|Figure 1: Two 89 x 89 x 6-mm (31/2 x 31/2
x 1/4-in.) angle irons spaced about 1 m (3.5 ft) apart were
embedded in each slab for electrodes in a back-to-back fashion.
The conductive concrete inlay was cast after the regular bridge deck had been cured for 30 days. The westbound lane was placed first. After hardening, the conductive concrete inlay was saw cut to a 100-mm (4-in.) depth along the perimeters of the individual slabs, and the gaps were filled with polyurethane sealant. There was a 150-mm (6-in.) gap along the centerline of the bridge to allow power cord connections with the coupling nuts of the angle irons. The gap was then filled with a non shrink, high-strength grout.
Integration of Power Supply, Sensors, and
A three-phase, 600 A and 220 V AC power source is available
from a power line nearby. In a control room a microprocessor monitors
and controls the deicing operation of the 52 slabs. The system includes
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 takes and records the thermocouple
readings of the slabs every 15 minutes. A slab's power will be turned
on by the controller if the temperature of the slab is below 4.5°C
(40°F) and turned off if the temperature is above 12.8°C
(55°F). The power-switching unit controls power relays to perform
the desired on/off function. To ensure safety, a current-monitoring
unit limits the current going through a slab to a user-specified
amount. The operator-interface unit allows a user to connect to
the controller with a PC or laptop via a phone modem. The operator
interface displays all temperature and electrical current readings
of every slab in real time. A user also has the option of using
a PC or laptop to download controller-stored data into a spreadsheet.
The deicing controller system was completed in March 2003 and was
tested successfully under snow storms in January and February 2004
(Figure 2). The system was activated in 2003 for an early April
storm with less than 6 mm (1/4 in.) of sleet. The slush on the bridge
deck was melted during the storm period. Temperature distribution
was uniform across the bridge. The controller system kept the slab
temperature about 9°C (16°F) above the ambient temperature.
|Figure 2: Deicing system in operation
in February 2004. (IMG14728)
The 52 slabs were energized in an alternating fashion to avoid a power surge. Groups of two slabs were started up in turn at 3-minute intervals and energized at 208 V for 30 minutes. This alternating form of energizing the slabs was followed throughout the storm. The maximum current recorded varied between 7 and 10 amps, with an average of 8. Peak power density delivered to the slabs varied between 360 and 560 W/m2 (33 to 52 W/ft2) with an average of 452 W/m2 (42 W/ft2). Energy consumed by the conductive slabs during the three-day period varied from 47 to 70 kW-hr, with an average of 58 kW-hr per slab. Total energy consumption was about 3,000 kW-hr.
The conductive concrete bridge deck will continue to be studied
for the next several winters to evaluate the effect of electrical
deicing and compare it to alternatives. This promising new technology
should prove to be a valuable tool in the fight against icy conditions
More information on the conductive concrete bridge deck project
can be found at: www.conductive-concrete.unomaha.edu
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