Christopher Y. Tuan

CONDUCTIVE CONCRETE OVERLAY FOR BRIDGE DECK DEICING

The use of road salts and chemicals for deicing concrete bridge decks is an effective method for ice removal but causes damage to concrete and corrosion of reinforcing steel. The use of insulation materials for ice control and electric or thermal heating for deicing have been attempted and have met limited success. Conductive concrete may be defined as a cementitious composite that contains a certain amount of electronically conductive components to attain stable and relatively high electrical conductivity. When connected to a power source, heat is generated because of the electrical resistance in the cement admixture with metallic particles and steel fibers. Based on the results of a transient heat transfer analysis, a thin conductive concrete overlay on a bridge deck has the potential to become a cost-effective deicing method. Small-scale slab heating experiments have shown that an average power of approximately 520 W/sq m (48 W/sq ft) was generated by the conductive concrete to raise the slab temperature from -1.1 deg C (30 deg F) to 15.6 deg C (60 deg F) in 30 min. This power level is consistent with the successful deicing applications using electical heating cited in the literature. The work described in this paper is part of an ongoing research project being conducted for the Nebraska Department of Roads. Two large slabs are under construction for a bridge deck deicing experiment in the natural environment to monitor power consumption and deicing performance. The construction costs and experimental data will be used to evaluate the cost effectiveness of using a conductive concrete overlay for bridge deck deicing or anti-icing.

CONDUCTIVE CONCRETE OVERLAY FOR BRIDGE DECK DEICING: MIXTURE PROPORTIONING, OPTIMIZATION, AND PROPERTIES

Bridge pavement surfaces are prone to ice accumulation, making wintry travel hazardous. Current practice is to use road salt and deicing chemicals that cause damage to concrete and corrosion of reinforcing steel. A thin conductive concrete overlay can generate enough heat to prevent ice formation on a bridge deck. Conductive concrete is a cementitious admixture containing electrically conductive components to attain stable and high electrical conductivity. Because of the electrical resistance and impedance in conductive concrete, heat is generated when connected to a power source and can be used for deicing or anti-icing. Conductive concrete is a relatively new material technology developed to achieve high electrical conductivity and high mechanical strength. A conductive concrete mixture has been developed at the University of Nebraska-Lincoln specifically for bridge deck deicing. Steel fibers and shaving were added to the concrete as conductive materials. Over 100 trial batches of conductive concrete were prepared and their properties evaluated. In this study, optimization of the conductive concrete mixture proportioning for bridge deck overlay is discussed in detail. The mechanical and physical properties of the optimized mixture are presented. The mechanical and physical properties of the conductive concrete mixture after 28 days have met the American Society for Testing and Materials and the American Association of State and Highway Transportation Officials specifications for bridge deck overlay. Slab heating experiments using AC and DC power have shown that the conductive concrete overlay has the potential to become the most cost-effective bridge deck deicing method.

Simulation of Soil Behavior under Blast Loading

A viscoplastic cap model was previously developed to address the high strain rate effect on soil behaviors. Although the model is an improvement over the inviscid cap model, it does not update soil density and bulk modulus as the shock wave propagates through the soil. Further, soil should be modeled as a three-phase porous media to accommodate various degrees of water saturation. This is especially true for the soil mass surrounding the source of energy release because each of the three phases responds differently to shock loading. A revised cap model comprising a Gruneisen equation of state for each of the three phases has been developed. These equations of state for solid, water, and air have been integrated with the viscoplastic cap model to simulate behaviors of soil with different degrees of water saturation. Numerical results from this revised soil cap model compared closely with experimental data from explosive tests in both dry and saturated soil.

Flexural Behavior of Concrete-Filled Circular Steel Tubes under High-Strain Rate Impact Loading

Nine simply supported circular steel concrete-filled tubes (CFTs), two circular steel posttensioned concrete-filled tubes (PTCFTs), and one circular steel fiber–reinforced concrete-filled tube (FRCFT) have been tested in an instrumented drop-weight impact facility. The weight and the height of the drop-weight were varied to cause failure in some test specimens. The failure modes and local damages in those specimens have been investigated extensively. Failure in the steel tubes was commonly tensile facture or rupture along the circumference. Concrete core in the impact area commonly crushed under compression and cracked under tension. The use of prestressing strands and steel fibers significantly restrained the concrete tension cracks in the PTCFT and FRCFT specimens, respectively. The experimental results are analyzed in the context of principles of energy and momentum conservation.

Thin Conductive Concrete Overlay for Bridge Deck Deicing and Anti-Icing

Concrete bridge decks are prone to ice accumulation. Bridge decks freeze before the roads approaching them freeze, making wintry highway travel treacherous. Road salts and deicing chemicals are effective for ice removal but cause damage to concrete and corrosion of reinforcing steel in concrete bridge decks. The resulting rapid degradation of existing concrete pavements and bridge decks is a major concern to transportation and public-works officials. The use of insulation materials for ice control and electric or thermal heating for deicing have been attempted, with unsatisfactory results. Conductive concrete is a cementitious admixture containing electrically conductive components to attain high and stable electrical conductivity. 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. In 1998, Yehia and Tuan, at the University of Nebraska–Lincoln, developed a conductive concrete mix specifically for bridge deck deicing. In this application, a conductive concrete overlay is cast on the top of a bridge deck for deicing or anti-icing. The mechanical and physical properties of the conductive concrete mix after 28 days have met ASTM and AASHTO specifications. Two concrete slabs were constructed with a 9-cm (3.5-in.) conductive concrete overlay for conducting deicing experiments in the natural environment. Deicing and anti-icing experiments were conducted in five 1998 snowstorms. Average power of about 590 W/m2 (55 W/ft2) was generated by the conductive concrete overlays to prevent snow and ice accumulation.

Evaluation and Repair Procedures for Precast/Prestressed Concrete Girders with Longitudinal Cracking in the Web

This report establishes a users manual for the acceptance, repair, or rejection of precast/prestressed concrete girders with longitudinal web cracking. The report also proposes revisions to the AASHTO LRFD Bridge Design Specifications and provides recommendations to develop improved crack control reinforcement details for use in new girders. The material in this report will be of immediate interest to bridge engineers.

Ponding on Circular Membranes

Nonlinear circular membrane responses under partial and full ponding loads have been solved by the fourth-order Runge-Kutta numerical integration and by finite element simulation with good accuracy. Under partial ponding loads, a discontinuity exists in the curvature of the deformed membrane at the fluid boundary while an inflection point forms inside the ponding. An iterative finite element algorithm has also been developed for solving membrane ponding problems where inelastic material response becomes significant. The finite element simulation procedure was proven to be accurate when validated against test data from membrane forming experiments.

Conductive concrete as an electromagnetic shield

Conductive concrete mixture was originally developed for surface de-icing purposes but can be designed to perform as an electromagnetic shield. Testing procedures have been developed to measure the attenuation provided by conductive concrete without the cost and labor of building a large structure. This paper provides a description of the design, testing methods, and results obtained from the development of conductive concrete as an electromagnetic shielding material.

Evaluation of Ice-Melting Capacities of Deicing Chemicals

Common deicing chemicals include sodium chloride, magnesium chloride, calcium chloride, calcium magnesium acetate, potassium acetate, potassium formate, and corn or beet-based deicer solution. Liquid deicers are commonly used for pre-wetting road salt, sand or other solid deicers, or mixed with salt brine as liquid deicer. Although manufacturers provide performance data under specific conditions, a standardized test is very much needed. Samples of sodium chloride, magnesium chloride, calcium chloride, potassium acetate, and beet juice-based chemical deicers were selected for performance evaluation. The SHRP Ice-Melting Capacity Test has been used in many research projects, but the results do not always correlate well with field data. A simple and economical test has been developed to evaluate the ice-melting capacities of deicing chemicals using a martini shaker, which shows some potential to become a standardized test for ice-melting capacity evaluation. Field data was collected by the Nebraska Dept. of Roads using automatic vehicle location (AVL) and the maintenance decision support system (MDSS) installed on some plow trucks. The AVL takes roadway pictures from the cab and records vehicle location. The MDSS collects weather data from area weather stations. Although initial shaker test results correlate well with known deicer performance and limited field data provided by MDSS, further development work is necessary before the shaker test can be considered for official use.

Ionically Conductive Mortar for Electrical Heating

An innovative conductive composite, ionically conductive mortar, is developed in this study. The directional migration of ions under external voltage makes the mortar conductive. The electrical resistance of the mortar causes the mortar to generate heat, which is used for deicing. To ensure conductivity, the number of free ions and the moisture content in the mortar must stay relatively high. The specimens were soaked in electrolyte solutions for 96 hours to saturation and coated with epoxy resin. Subsequent electrical heating tests showed that the specimens could achieve a heating rate of 19.7°C (35.5°F) in 120 minutes under 30 V AC. This heating performance would improve with increasing applied voltage.