Road salts and deicing chemicals are usually used for deicing and ice removal. However, infrastructure deterioration and groundwater contamination are the main drawbacks for using deicing chemicals. Other methods, such as heating the bridge deck using embedded cables, heated fluids, infrared heat lamps or insulation on the underside of the deck, have been attempted and have met with limited success.
Over the past 10 years, an innovative material called “conductive concrete” has been developed and evaluated for bridge-deck deicing. Conductive concrete is a cementitious mixture containing electrically conductive components to enable conduction of electricity. Because of 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.
The original conductive concrete mix was developed at the University of Nebraska and later modified at Western Michigan University (WMU). The mix was implemented in a bridge-deck deicing application. Conductive concrete is currently being investigated at WMU for other applications such as electromagnetic shielding and cathodic protection of reinforcement.
The first generation of conductive concrete was developed in 1998 utilizing steel fibers and steel shavings. The main objective was to develop a conductive concrete mix to achieve high electrical conductivity and high mechanical strength. Christopher Tuan, a professor of civil engineering at the University of Nebraska at Lincoln, and I evaluated the mechanical and electrical properties of the mix in accordance with the ASTM and AASHTO specifications. The compressive strength, flexural strength, freeze-thaw resistance, permeability, shrinkage, thermal conductivity and electric resistivity of the mixture have been determined. Laboratory testing and results showed that a thin conductive concrete overlay on a bridge deck may become a very cost-effective deicing method.
In 2001, Tuan and I developed a conductive concrete mix utilizing carbon powder and steel fiber. The carbon powder was used to replace the steel shavings that were used during the early development of the conductive concrete mix. The mixture has a compressive strength of 4,500 psi and provides an average thermal power density of 55 W/sq ft with a heating rate of 0.25°F/min in a winter environment. This mix was used in the implementation project at Roca, Neb.
As a follow-up effort, the research team at WMU in 2004 modified the conductive concrete mix with carbon powder and steel fibers. The main objectives of the research are to enhance the stability of the mix during the deicing operation; reduce the power requirements for the deicing operation; and utilize the conductive concrete mixture in new applications, such as electromagnetic shielding and cathodic protection of reinforcement.
Laboratory results of the modified mix showed that a reduction of the power requirements and stable heating performance can be achieved. The modified mix was selected for a bridge-deck deicing application in Calhoun County, Mich. The evaluation of other applications is in progress.
A demonstration project in Roca, Neb., was constructed in 2002. The conductive concrete mix with carbon powder and steel fibers was used to cast a 4-in. inlay for the deicing application.
Roca Spur Bridge is a 150-ft-long and 36-ft-wide, three-span slab-type bridge over the Salt Creek. The bridge deck has a 117- x 28-ft by 4-in. conductive concrete inlay. The inlay consists of 52 panels, each 4 x 14 ft, that are instrumented with thermocouples for deicing monitoring during winter storms. A microprocessor-based controller system was installed to monitor the slab temperature and current and to maintain the slab temperature below 55°F and above 35°F.
The conductive concrete deicing system has shown excellent deicing performance during the past five years.
The average energy consumption is 2,821 kW/hr, and the average unit cost is $0.07 per sq ft during the five years of operation. The air temperature during many snowstorms was in the range of 20°F to 10.9°F. Deicing salts are usually not effective under this temperature range. The performance of the Roca Bridge has demonstrated the effectiveness of using conductive concrete for deicing, while the deicing chemicals would become ineffective under similar conditions.
The modified conductive concrete mix developed at WMU was planned to be used in a demonstration project in Calhoun County, Mich. The 4-in.-thick conductive concrete inlay was to be utilized for bridge-deck deicing. The bridge is 222.4 ft long and 42.5 ft wide. It consists of three-span box girder precast units, which will be post-tensioned in the transverse direction. The conductive concrete inlay is divided into 96 panels, each 12 ft x 4 ft. PVC conduits and junction boxes would have been used for the electrical connections. The conduits have no effect on the structural integrity of the bridge. Compressive strength of the conductive concrete is in the range of 4,500 to 5,500 psi. However, the conductive concrete overlay was later canceled (during the construction phase) from this project because of the loss of the additional construction funds.
Current practice in slowing or preventing the corrosion process of reinforcement involves the use of cathodic protection methods. The first method uses sacrificial anodes (metals) to provide a protective current to the cathode (protected metal). The second method involves using an anode, with an impressed (protective) current provided by an external direct current power source to protect the cathode. The second method is the commonly used one with concrete structures.
The main objective of the current research effort at WMU is to utilize the conductive concrete bridge-deck deicing overlay as an anode of a cathodic protection system at the same time. Several specimens are being tested to optimize the slab thickness and to evaluate the efficiency of the proposed configuration. The initial results (after a year of testing) showed that the system is meeting all the ASTM requirements for cathodic protection systems.
In the field of electromagnetic shielding, the current practice in protecting electronic equipment is to install a welded steel liner or a layer of carbon fiber sheet on walls. The proposed system is to use the conductive concrete as a barrier between the source of electromagnetic waves and the equipment to be shielded. Several slabs will be prepared to evaluate the efficiency of the conductive concrete material for electromagnetic shielding.
The conductive concrete material has the potential to be a cost-effective method for bridge-deck deicing. In addition, conductive concrete can be utilized in many other infrastructure applications such as cathodic protection of reinforcing steel, electromagnetic shielding and bridge-deck health monitoring.