Traditionally, removing ice from pavement can be accomplished by a combination of several methods, such as plowing, natural melting, traffic movement and chemical treatment. Because the bond between ice and pavement is strong, removal by plowing alone may not be effective. Chemical treatment helps break the bond by melting into the ice and spreading under the ice layer.
Most highway winter maintenance depends on using chemicals and fine granular particles as a primary means for deicing and anti-icing. The use of road salts and chemicals for deicing is an effective method for ice removal but causes damage to concrete and corrosion of reinforcing steel in concrete bridge decks. This problem is a major concern to transportation and public works officials due to rapid degradation of existing concrete pavements and bridge decks. The search for improved deicing methods has been a research focus for a long time. The use of insulation materials and electric or thermal heating has been attempted, however, those techniques were either not cost-effective or could not meet the bridge deck strength requirements.
Conductive concrete is a relatively new material technology developed to achieve high electrical conductivity and high mechanical strength. Conductive concrete has been used for anti-static flooring, electromagnetic shielding and cathodic protection of steel reinforcement in concrete structures. However, applications of conductive concrete have been limited because the material did not meet strength requirements and was expensive to utilize in other applications.
In research sponsored by the Nebraska Department of Roads, we developed a conductive concrete mix specifically for bridge deck deicing. In this application, a conductive concrete overlay is cast on top of a bridge deck for deicing and anti-icing. The mechanical and physical properties of the conductive concrete mix were evaluated in accordance with the ASTM(8) and AASHTO(9) specifications.
Evaluation of the conductive concrete for bridge deck deicing was conducted in four stages; mixture proportioning and laboratory testing, durability evaluation, deicing and anti-icing in natural environment and implementation and demonstration project.
Mixture proportioning and laboratory testing
Conventional concrete is not electrically conductive. Approaches to improving the electrical conductivity of a concrete mix include: (1) use of conductive aggregates such as iron ore, slag, etc.; and (2) increasing the conductivity of the cement paste by adding conductive materials such as steel shaving, coke breeze, steel or carbon fibers, etc. In the mixes we developed, steel shaving and carbon and graphite products were used in addition to the steel fibers.
From 1998 to 2001 steel shaving and fibers were used as conductive materials. Over 150 trial mixes were prepared to optimize the volumetric ratios of the steel shaving and fibers in the mix proportioning. The evaluation criteria were mechanical properties (compressive and flexural strength), slab heating performance, power source (DC vs. AC), size effect, electric resistivity and electrode configuration. The optimized mix was evaluated in accordance with the ASTM(8) and AASHTO(9) specifications. The compressive strength, flexural strength, modulus of elasticity and rapid freeze and thaw resistance of the conductive concrete mix after 28 days have met the AASHTO(9) requirements for bridge deck overlay. Two concrete slabs, 7 ft x 7 ft and 4 ft x 12 ft (6 in. thick), were constructed with a 3.5-in. conductive concrete overlay for conducting deicing experiments in the natural environment.
In the spring of 2001, we developed a conductive concrete mix utilizing graphite and carbon products to replace steel shaving. Ten trial mixes with seven carbon and graphite products were included in the preliminary experimental evaluation. The evaluation criteria used for each trial batch were workability and finishability, compressive strength, heating rate and electric resistivity.
All mixes contained 1.5% of steel fibers per volume of conductive concrete, in addition to the carbon and graphite products used for conductive materials. The added carbon and graphite products amounted to 25% per volume of the trial mixes. The conductive concrete mix using 25% combined carbon and graphite products and 1.5% steel fibers per volume was tested extensively to evaluate its mechanical and physical properties. Material testing was conducted in accordance with the ASTM(8) and AASHTO(9) specifications.
A testing patch was constructed on Dec. 3, 1999, in one I-480 westbound lane over the Missouri River (near the Nebraska-Iowa border) for durability evaluation. The optimized mix design with steel shaving was used in this patch. The main objective of this test was to evaluate the performance of the conductive concrete mix under traffic loads.
Testing in natural environment
Several deicing and anti-icing experiments were conducted during the winter of 1998 and 2002. In the anti-icing experiments, the overlays were preheated two to 10 hours (depending on the initial temperature of the overlay) before and heated during the storms. In addition, deicing experiments, in which the overlays were heated only during the storms, were conducted to evaluate the heating rate of the conductive concrete.
In each experiment, the applied voltage, current going through each overlay, temperature distribution within each overlay, along with the air temperature, humidity and wind speed and direction, were recorded.
Average power of about 55 W/ft2 with a heating rate of 1°F/min. was generated by the conductive concrete to prevent snow accumulation and ice formation.
Conductive is productive
In the spring of 2003, the Nebraska Department of Roads opened to traffic a demonstration project at Roca, located about 15 miles south of Lincoln, Neb., where the conductive concrete has been successfully implemented. The Roca Spur Bridge has a 117-ft-long, 28-ft-wide conductive concrete inlay. A railroad crossing is located immediately following the end of the bridge, making it a prime candidate for a deicing application.
The Roca Spur Bridge is a three-span slab-type bridge and has a 150-ft-long, 36-ft-wide concrete deck. The slab thickness is 12 in. A 4-in.-thick inlay of conductive concrete was taken into account during the design phase. PVC conduits and junction boxes were embedded into the slab during construction. The conduits had no effect on the structural integrity of the bridge. Conventional concrete with 4,500-psi compressive strength was used to cast the slab.
The conductive concrete inlay used was 117 ft in length and 28 ft in width. The inlay consists of 52 individual 4-ft x 14-ft conductive concrete slabs. The slabs were divided into two groups separated by a 6-in. gap along the center line of the bridge to allow for the electrical connections.
Temperature sensors were installed at the center of each slab at about 0.5 in. below the surface to measure the slab temperature. The sensors were installed before casting the inlay. Separate PVC conduits were used to house the thermocouple wires.
A three-phase, 600 A and 220 V AC power was available from a power line nearby. A microprocessor-based controller system was installed in a control room to monitor and control the deicing operation of the 52 slabs. The system includes four main elements: a temperature-sensing unit, a power-switching unit, a current-monitoring unit and an operator-interface unit. The temperature-sensing unit takes and records the thermocouple readings of the slabs every 15 minutes. The slab’s power will be turned on by the controller if the temperature of the slab is below 40°F and turned off if the temperature is above 55°F. The power-switching unit will control power relays to perform the desired on/off function. To ensure safety, a current-monitoring unit will limit the current going through a slab to a user-specified amount. The operator-interface unit will allow a user to connect to the controller with a PC or laptop via a phone modem.
The conductive concrete heating system was fully operating in the spring of 2003, and at that time most of the winter storms were missed. However, the system was tested successfully under freezing temperature. Two powering schemes, alternating and simultaneous powering, were evaluated in the winter of 2003.
In the alternating powering scheme, the 52 slabs were divided into 26 groups with each group containing two consecutive slabs. To avoid power surge, the odd-numbered groups were started up in turn at three-minute intervals and energized at 208 V for 30 minutes before the even-numbered groups were started. During this powering scheme, the heating performance of the slabs was not as expected and the bridge deck was partially covered with snow. In the simultaneous powering scheme, all slabs were powered when the ambient temperature dropped below 30°F, and switched to alternating powering when the ambient temperature was above 30°F.
The maximum current recorded varied between 9 and 18 amperes, with an average of 12.5 amperes. The slab temperature distribution was very uniform across the deck during deicing operations, generally in the 25 to 50°F range. The average slab temperature was consistently about 18°F higher than the ambient temperature. The peak-power density delivered to the slabs varied between 33 and 52 W/ft2 with an average of 42 W/ft2. The total energy consumed by the conductive concrete slabs during the storms is summarized in Table 1. The energy consumed by the slabs varied from 47 to 70 kwh, with an average of 58 kwh per slab. The average energy consumption under simultaneous powering was about 3,200 kwh, which would cost about $260 for each major storm based on the rate of $0.08/kwh.
Price to be determined
The cost per unit surface area of the conductive concrete inlay is $59/ft2. This cost includes: (1) placing, finishing, curing and saw cutting conductive concrete; (2) procuring conductive concrete materials; (3) building and installing control cabinet with sensors and power relays; and (4) integrating and programming the deicing operation controller.
It is expected that the construction costs of conductive concrete overlay/inlay will drop significantly when the technology becomes widely accepted. In addition, other factors such as life-cycle costs, including system maintenance costs and deck repair costs and vehicle depreciation caused by deicing chemicals, should be used as the basis for cost-effectiveness comparisons of different deicing systems.