One of many technologies reported at the 83rd annual meeting of the Transportation Research Board (TRB) in Washington, D.C., Jan. 11-15, conductive concrete was described as a potential weapon in the war against snow and ice, leading to safer highways and reduced chloride damage. Research supporting this claim was presented to a record number of delegates — over 9,000 — attending the TRB meeting. More than 500 technical sessions, 40 workshops, and 350 TRB committee meetings, covering all aspects of transportation, were included on the meeting agenda.

Organized as the Highway Research Board in 1920, TRB is a division of the National Academies, including the National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council. More information can be obtained by contacting the Transportation Research Board, 2001 Wisconsin Avenue, N.W., Green Building, Washington, D.C. 20007, or visiting TRB's web site at http://trb.org.

CONDUCTIVE CONCRETE FIGHTS ICE, SNOW

Conductive concrete may help fight snow and ice while preserving bridge deck reinforcing steel, contend Sherif Yehia, Western Michigan University, and Christopher Tuan, University of Nebraska-Lincoln, in their paper, “Conductive Concrete Overlay for Bridge Deck Deicing at Roca, Nebraska.” The authors explain, “Conductive concrete is a cementitious admixture containing electrically conductive components to attain stable and high 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 research sponsored by the Nebraska Department of Roads, Yehia and Tuan developed a conductive concrete mix specifically for bridge deck deicing. In this mix, steel shavings with particle size ranging between 0.15 and 4.75 mm (0.007 to 0.19 in.) and steel fibers of four different aspect ratios between 18 to 53 were added to the concrete as conductive materials.

“Over 150 trial mixes were prepared to optimize the volumetric ratios of the steel shaving and fibers in the mix proportioning,” they report. The optimized mix was evaluated and found to be in compliance with ASTM and AASHTO specifications.

Due to problems inherent in the use of steel shavings exclusively, Yehia and Tuan developed a conductive concrete mix in spring 2001 utilizing graphite and carbon products to partially replace steel shavings. Ten trial mixes with seven carbon and graphite products were included in the preliminary experiments. All mixes contained 1.5 percent of steel fibers per volume of conductive concrete, in addition to the carbon and graphite products, which amounted to 25 percent per volume of the trial mixes.

“Findings of the Phase I research showed that the conductive concrete overlay had the potential to become the most cost-effective bridge deck deicing method,” Yehia and Tuan attest. Prompted by this evidence, Nebraska Department of Roads in 2001 approved a demonstration project at Roca.

The Roca Spur Bridge is a three-span, slab-type structure whose 150-ft.-long and 36-ft.-wide concrete deck features a conductive inlay. Conventional concrete of 4,500-psi compressive strength was used to cast 12-in.-thick slabs. The inlay is 117 ft. long, 28 ft. wide and 1 in. deep, consisting of 52 individual 4 × 14 ft. conductive concrete slabs.

Temperature sensors and a microprocessor-based controller system were installed to monitor and control the deicing operation of the inlay. Upon completion of construction, the bridge was opened to traffic in spring 2003. Data from the first deicing event showed that an average of 46 W/ft² was generated by the conductive concrete, raising the slab temperature about 16° F above the ambient temperature. In the meantime, Nebraska Department of Roads approved a five-year plan for monitoring the conductive concrete overlay.

“The heated bridge deck of Roca Spur Bridge is the first implementation in the world using conductive concrete for highway bridge deicing,” the authors contend. “The new mix design containing carbon powder and particles is found superior to using steel shavings, in that the electrical conductivity and the heating rate are improved without the drawbacks. The construction costs and deicing performance of the heated bridge deck would demonstrate its cost-effectiveness as opposed to other existing deicing technologies.”

CEMENTITIOUS COMPOSITE EXHIBITS METAL-LIKE DUCTILITY

A new, highly crack-resistant, ultra-ductile cementitious composite may offer an alternative to conventional concrete, which is often brittle and tends to crack when used in transportation infrastructure — resulting in a lack of durability and frequent need for repair. In “Crack-resistant Concrete Material for Transportation Construction,” Victor C. Li and Michael Lepech, Advanced Civil Engineering Materials Research Laboratory, University of Michigan-Ann Arbor, propose that a cementitious composite may offer new options to structural designers and providers of concrete. This new material, dubbed Engineered Cementitious Composites (ECC), has a tensile strain capacity over 300 times that of normal concrete, the authors report.

“ECC is a new class of cementitious materials and meets nearly every major characteristic sought by highway engineers for a highly durable concrete repair material,” say Li and Lepech. “This type of ultra-ductile, high-performance, fiber-reinforced cementitious composite (HPFRCC) exhibits ductility similar to metals, along with inherently tight crack widths for excellent durability and corrosion protection. Additionally, this material shows excellent performance in durability testing.”

The characteristic that most distinguishes ECC from other concrete repair materials is an ultimate strain capacity between 3 and 5 percent, depending on the specific ECC mixture, the authors contend. This high strain capacity is realized through the formation of many closely spaced microcracks. Carrying increasing load after formation, these cracks enable the material to exhibit strain hardening similar to many ductile metals, as seen in a typical uniaxial tensile stress-strain curve.

“This is uniquely different from typical concretes or fiber-reinforced concretes (FRC), which form a single crack when loaded,” note Li and Lepech. “In the case of normal concrete, the crack opens wide with a rapid drop in load capacity. In the case of FRCs, the crack opens with a gradual drop in load, exhibiting a tension-softening behavior. While the mechanism behind concrete and FRC deformation is similar to ECC in that it cracks, all deformation is localized at a single section (i.e., the crack face) and the concept of gauge length, and consequently strain, ceases to exist.”

While the composition of ECC — a mixture of cement, sand, water, fibers, and a small amount of commercial admixtures — is similar to many other FRCs, the distinctive ECC characteristic of strain hardening through microcracking is achieved through micromechanical tailoring of the components, along with controlled interfacial properties between constituents, the authors explain. “Fracture properties of the cementitious matrix are controlled through mix proportions,” they emphasize.

Coarse aggregates are not used due to their adverse effect on performance. According to the authors, “These large aggregates are found to dominate the micromechanical properties of the composite, leading to poor fiber dispersion and lower overall performance.”

While most HPFRCCs rely on a high fiber volume to achieve high performance, ECC uses low amounts (typically 2 percent by volume) of short, discontinuous fiber, Li and Lepech report. This low fiber volume, along with the common components, allows for conventional mixing in a gravity mixer, while many HPFRCCs with fiber fractions exceeding 5 percent cannot conform to conventional mixing practices.

ECC has undergone durability and performance tests, such as restrained shrinkage tests, fatigue and bonding tests, freeze-thaw exposure, wearing/abrasion tests, and accelerated environmental tests. Also investigated were the long-term strain capacity and early-age strength development of ECC.

“The use of ECC materials in both completed and planned demonstration projects with the Michigan Department of Transportation, including bridge deck patching and a link slab project, reveal that ECC is a plausible and preferable repair/retrofit material for various transportation applications,” Li and Lepech contend. “The introduction of high performance materials, such as ECC, is expected to set a new standard for transportation repair materials resulting in fewer repairs, less maintenance, and lower lifetime costs.” More information on ECC technology and applications can be found at http://ace-mrl.engin.umich.edu.

PROPER AIR-ENTRAINMENT OFFERS BEST FREEZE-THAW RESISTANCE

Concrete subject to saturation and exposure to cycles of freezing and thawing must be properly air entrained for long-lasting service; otherwise, cracking will occur, with resultant deterioration of the concrete, asserts Celik Ozyildirim, Ph.D., P.E., principal research scientist, Virginia Transportation Research Council, in “Air-Void Characteristics of Concretes in Different Applications.”

The author elaborates, “Air voids must be small in size, closely spaced, and uniformly distributed to ensure adequate resistance to freezing and thawing and satisfactory strength. An increase in air volume results in a reduction in strength.”

Air content is affected by material properties, including water and admixture content, as well as construction practices, including pumping and consolidation. Further, special consideration must be given to the air entrainment and air-void system in concretes having a stiff consistency, that are placed by pumping, or contain a large amount of high-range water-reducing admixture. “Under these conditions, an improper air-void system may result and may have an adverse effect on the concrete's resistance to freezing and thawing,” Ozyildirim says.

The author concludes that air-void size and distribution are more important than total air content for satisfactory resistance to freezing and thawing. In view of the effect that stiffness of the mixture, pumping, and use of an HRWRA can have on the air-void system, he says, “Testing the air-void system of the in-place concrete in accordance with ASTM C 457 or testing the resistance to freezing and thawing in accordance with ASTM C 666 would indicate if a proper air-void system was achieved and if the concrete had adequate protection.”

Ozyildirim offers several observations:

• In stiff concretes, entraining air is more difficult. Furthermore, vibration of stiff concretes requires a greater consolidation effort, which may dissipate the bubbles. Thus, in addition to the difficulty of entraining air in low-slump mixes, the marginal air-void system may be compromised through vibration. Over-sanded mixtures are prone to segregation and loss of air. By contrast, the adverse effect of vibration is minimized with uniform aggregate grading and a proper ratio of coarse and fine aggregates in the concrete mix.

• Pumping vertically in a downward direction reduces the air content of freshly mixed concrete. When loss of air occurs in large bubbles, resistance to freezing and thawing remains adequate; if air escapes in bubbles of varying size, resistance may be compromised. A steady flow of concrete during pumping and the elimination of a large, free drop of concrete in the pump line generally result in satisfactory resistance despite a total air content lower than specified. When in doubt, testing for freeze-thaw resistance or the air-void system would indicate the level of protection.

• The addition of an HRWRA can result in coarser bubbles that may adversely affect resistance to freezing and thawing. Two possible solutions are to increase the total air content or to select a new formulation of an HRWRA with defoamers, enabling a satisfactory air-void system.

Accordingly, the Virginia researcher recommends:

• Check the air-void system or the resistance to freezing and thawing of the specific mixtures in stiff concretes after consolidation. Concretes with a low air content may still have enough small bubbles for adequate freeze-thaw protection.

• Since pumping affects the air-void system of freshly mixed concrete, sample the concrete under the same conditions as the concrete being placed on the deck. When an adequate relationship is established between the pumped and the delivered concrete, test the delivered concrete for convenience.

• When large amounts of HRWRA are used, as in self-consolidating concretes (SCC), determine the air-void system or test for resistance to freezing and thawing to predict if adequate protection is possible. If the spacing factor is above the 0.2-mm limit generally accepted for satisfactory resistance, test for resistance to freezing and thawing to determine if protection is adequate.

DENSE-GRADED MIX WITH FLY ASH YIELDS HPC FOR TENNESSEE DECKS

A dense-graded, high performance concrete (HPC) that exceeds Tennessee DOT standard specs — using fly ash, but no silica fume — has the potential to serve as the department's standard bridge-deck mix, observe University of Tennessee-Knoxville's Rohi Salem, Ph.D., research associate, and Edwin Burdette, Ph.D., P.E., professor of civil engineering.

In “Evaluation of a Well-Graded High-Performance Concrete Mixture for Tennessee Bridge Decks,” the authors assess a lab investigation of the development of a well-graded HPC mixture for Tennessee cast-in-place bridge decks using local aggregates. Their main objective was to develop a dense-graded aggregate mixture providing good workability, decreased permeability, reduced cracking potential, adequate strength, and economical savings.

Four mixtures were investigated, while Tennessee DOT conventional Class D concrete served as the control mix. The other mixtures were developed by incorporating various aggregate sizes and varying the quantity of fly ash, slag and silica fume.

A number of characteristics relevant to bridge-deck performance were evaluated, including compressive strength, drying shrinkage, freeze-thaw durability, and chloride ion permeability. All modified control mixes exhibited higher compressive strength than the corresponding Class D control mix.

Adjusting the typical gap-graded mix used by Tennessee DOT to a dense-graded mix with 25 percent fly ash replacement of cement resulted in a high-performance concrete mix (mix 2F1) with very low permeability, adequate compressive strength, high frost resistance, and low drying shrinkage — all without the addition of silica fume. The authors recommend, therefore, that this dense-graded high performance concrete mix exceeding DOT specifications be a potential replacement for Tennessee's Class D concrete mix.

SOUTHERN STATES FAVOR S-I-P METAL DECK FORMS

Stay-in-place metal forms (SIPMFs) for bridge deck construction are used commonly throughout the U.S., but users in southern states appear better satisfied with their performance. Nabil F. Grace, professor and chairman, James L. Hanson, associate professor, and Walid Farahat, research assistant, Department of Civil Engineering, Lawrence Technological University, Southfield, Mich.; and, Roger D. Till, engineer of structural research, Michigan DOT, examine trends in “Survey of State DOTs on Performance of Concrete Bridge Decks Constructed Using Stay in Place Metal Forms.” The researchers developed a comprehensive survey to evaluate the practice of using stay-in-place metal formwork for concrete bridge deck construction in the U.S.

“Stay-in-place metal formwork provides cost savings and increased construction safety,” the authors note. However, they add, after over 30 years of use, response to performance of SIP forms ranges from highly satisfied to unsatisfied.

Following distribution of the survey to all U.S. state DOTs, a total of 39 responses were received. The survey results presented on a geographic basis indicate that 67 percent of the DOTs responding to the survey allow the use of SIPMFs.

From survey results, the researchers observe:

• A total of 26 DOTs allow, and 13 do not allow, SIPMFs in concrete bridge deck construction. Most of the 26 DOTs that use SIPMFs are satisfied with the performance of this bridge deck system. The majority of DOTs that do not use SIPMFs are concerned with the inability to visually examine and access the bottom of the deck slabs.

• Among DOTs that use SIPMFs, five (all in the East) have more than 1,000 SIPMF bridges each, 15 DOTs have less than 100 bridges each, and the remaining six DOTs have between 100 and 1,000 SIPMF bridges each. Nearly half of the DOTs that permit the use of SIPMFs (11) have been using SIPMFs for more than 30 years. The earliest applications represent only a portion of the total number of bridges in these states.

  Filling the corrugations of SIPMF with Styrofoam to reduce the dead weight of bridge decks is not a common practice among the majority of the DOTs that allow the use of SIPMF in bridge decks.

• The use of epoxy-coated steel bars in bridges with SIPMF is a common practice in most states. The majority of the DOTs did not observe a difference in performance between SIPMFdecks constructed with bare steel reinforcement and those constructed with epoxy-coated steel reinforcement.

• The majority of the DOTs use conventional inspection approaches, such as visual inspection and hammer sounding, for periodic examination of their SIPMF bridge decks. The typical period between each inspection ranges from one to three years.

• Most of the DOTs do not believe that the SIPMF increase the long-term durability of bridge decks. A majority of the DOTs reported that the use of SIPMFs is not linked to any deck deterioration. Corrosion of SIPMFs was observed by 12 DOTs, while 14 did not observe corrosion. Most of the reported zones of SIPMF corrosion occurred at water-leakage and joint locations. Corrosion of deck reinforcement on bridges constructed with SIPMFs was reported by six DOTs, whereas 18 DOTs did not observe corrosion in deck reinforcement. Reported corrosion occurred in the top reinforcement and at the span ends.

• Overall acceptance of SIPMF construction methods and performance evaluations for bridge decks with SIPMFs are generally more favorable among Southern states as compared to those of the North.

• Comparing results of the survey to a similar survey administered in 1974, an increase in the overall use of SIPMFs was observed. However, some DOTs remain hesitant to adopt widespread use of SIPMF for concrete bridge deck construction.

ULTRATHIN WHITETOPPING PAPER LANDS TRB AWARD

A Transportation Research Board technical paper, “Performance of Ultrathin Whitetopping Intersections on U.S. 169,” won a national research award for its author in January. Julie Vandenbossche, P.E., assistant professor in the Department of Civil and Environmental Engineering at the University of Pittsburgh, is the winner of TRB's Fred Burggraf Award. Recognizing excellence in transportation research by individuals 35 years of age or under, the award named in honor of Fred Burggraf, TRB's executive director from 1951 to 1964, was established in 1966 to stimulate and encourage young researchers to contribute to the advancement of knowledge in the field of transportation.

Vandenbossche's paper evaluates the use of ultrathin whitetopping (UTW) in the rehabilitation of asphalt pavements. Studies were conducted by Minnesota DOT personnel on three test sections of varying dimensions on U.S. 169 and repeated on the hot-mix asphalt (HMA) pavements of I-94. The results reveal cracking failures on U.S. 169, which were not apparent on I-94.

Strain and deflection measurements indicate that the HMA provides support, although this support can be eroded with temperature increase or raveling. Accordingly, the study endorses UTW as a viable option when the HMA maintains uniform thickness and no sign of stripping and raveling is evident.

Vandenbossche received her award, including a cash prize, Jan. 14 at TRB's Chairman's Luncheon.