Revisiting a topic that sparked keen interest two years ago, new research presented at the 84th annual meeting of the Transportation Research Board (TRB) targeted self-consolidating concrete (SCC) — a material growing in popularity due to its faster placement and cost-reducing potential. In 2003, a presentation by researchers at the Virginia Transportation Research Council (VTRC) examined the application of self-consolidating concrete for production of bridge components. VTRC's 2005 study brings that research up to date.

Over 9,000 delegates traveled to the nation's capital in January to attend the meeting. TRB is a unit of the National Research Council, a private, nonprofit institution serving as the principal operating agency of the National Academy of Sciences and the National Academy of Engineering. More information on the studies presented can be obtained from TRB at 2001 Wisconsin Ave., N.W., Green Building, Washington, D.C. 20007, or at the web site: http://trb.org. Of particular note from this year's program are the following topics:

SCC IN BULB-T GIRDERS

In a paper two years ago, Celik Ozyildirim, Ph.D., P.E., principal research scientist, and D. S. Lane, senior research scientist, Virginia Transportation Research Council, demonstrated that SCC can enhance strength and durability of precast bridge elements. A lower viscosity and consequent greater flow rate distinguish SCC from conventional mixes. Lower pumping pressure, in turn, reduces wear and tear on pumps and the need for cranes to deliver concrete in buckets at the job site.

SCC is formulated with limits on aggregate nominal maximum size (NMS), amount, and grading in order to achieve a high flow rate and avoid obstruction by closely spaced reinforcing. Consolidating under its own mass, SCC has the ability to eliminate inadequate consolidation in thin sections or in areas of congested reinforcement, where a large volume of entrapped air voids may compromise the strength and durability of precast components.

When flow rate is high, however, the potential for segregation and loss of entrained air voids increases. The remedy lies in a mix design incorporating a high fine-to-coarse-aggregate ratio, a low water-cementitious material ratio (w/cm), good aggregate grading, and a high-range water-reducing admixture (HRWRA). Viscosity modifying admixtures (VMA) are also used to reduce segregation and enhance the stability of the air-void system.

Ozyildirim and Lane examined in 2003 a variety of mix designs to identify those with good flow characteristics, requiring no additional consolidation, yet providing adequate compressive strength, low permeability, shrinkage control, and resistance to freeze/thaw cycles. These mix designs were demonstrated at two concrete operations, while a field installation involving a 25-unit, precast arch bridge in a residential neighborhood was undertaken. The study indicated that, with mix-proportion adjustments, SCC proved beneficial to users and facility owners.

Now, in his 2005 paper “Virginia Department of Transportation's Early Experience with Self-Consolidating Concrete,” Ozyildirim reports VDOT is evaluating SCC in bulb-T beams. Incentives for investigating this application are clear: “Strands and shear reinforcement create difficulty in consolidating concrete in prestressed beams,” he explains. “In addition, the ability for faster construction and the attainment of a smooth finish, together with its self-consolidating nature, make SCC appealing to precasters.”

Accordingly, two 45-in.-deep and 60-ft.-long bulb-T beams were fabricated and tested for transfer length. “Tests for development length, flexural strength, and shear strength are continuing. If successful, actual bridge beams will be cast with SCC,” Ozyildirim affirms.

SCC flow properties depend on the characteristics of the element cast, including flow distance, level of reinforcement, shape, potential for segregation, and appearance, Ozyildirim notes. “Desired slump flow was from 22 to 26 in.,” he says. “One of the tested batches had a lower slump flow; however, it had a good filling height in the U-box. A rise of 12 in. or more is a very good indication that concrete is self-consolidating. The concretes had the specified 28-day compressive strength of 8,000 psi and had a much lower permeability than the specified 1,500 coulombs. The drying shrinkage values were about 400 microstrains at 28 days.”

The concrete was placed using discharge trucks with augers, he reports. The concrete was dropped at one end of the 60-ft. beam and allowed to flow to the other end. Although the slump flow or the U-box did not exhibit segregation, slight bleeding in the beams was attributed to the long travel distance and resistance from reinforcement as the concrete was flowing. As each new load traveled over the previous layer, the bleed water was pushed to the end and seeped out the end plate. Shorter flow distances of 10 to 15 ft. will be tried in the future, Ozyildirim asserts.

Precasters are driving demand, he implies. In Virginia, precasters wanting the benefits of SCC took the lead in developing and requesting permission to use it, the author says. “SCC is an engineered, value-added product and requires special attention and proper use of admixtures,” he emphasizes. “SCC is also beneficial for cast-in-place applications. VDOT is looking for opportunities to incorporate SCC in transportation facilities in both precast and cast-in-place applications.”

BRIDGE SYSTEMS UPGRADE

New overseas technologies can change and improve how precast or prefabricated bridges are implemented in the United States, contend Henry Russell, Henry G, Russell, Inc.; Mary Lou Ralls, Ralls Newman, LLC (formerly with the Texas DOT); and, Benjamin M. Tang, FHWA Office of Bridge Technology, in their paper “Prefabricated Bridge Elements and Systems in Japan and Europe.” An April 2004 scanning tour of Japan, the Netherlands, Belgium, Germany and France, sponsored by the Federal Highway Administration (FHWA) and American Association of State Highway & Transportation Officials (AASHTO), was conducted to observe bridge construction methods being used to minimize traffic disruption, improve work zone safety, minimize environmental impact, improve constructibility, increase quality, and lower life-cycle costs.

“Based on information obtained from the tour, 10 technologies were identified for further consideration and possible implementation in U.S. practices,” say Russell, Ralls and Tang. “These included two technologies that allow bridges to be built off-site and then moved to their final location in a short time, three superstructure systems and four deck systems that facilitate faster and safer construction, and one substructure system.”

The two technologies for moving bridges, they note, were self-propelled modular transporters and other moving systems involving skidding or sliding, incremental launching, floating, rotating, and lifting bridges into place. The superstructure systems included a precast concrete deck system known as the Poutre Dalle system, the use of partial depth concrete decks prefabricated on steel or concrete beams, and U-shaped precast concrete segments with transverse ribs.

The deck systems comprised full-depth precast panels, special cast-in-place closure joint details, hybrid steel-concrete deck systems, and a multiple-level corrosion-protection system. The substructure system consisted of stay-in-place precast panels serving as both formwork and structural elements for solid and hollow bridge piers.

More information on these technologies in the form of a May 2004 preliminary summary of the scanning tour report may be downloaded in pdf format at www.fhwa.dot.gov/bridge/prefab/pbesscan.pdf, or in html at www.fhwa.dot.gov/bridge/prefab/pbesscan.htm.

SEISMIC-DESIGN LESSONS FROM JAPAN, CALIFORNIA

Lessons learned from the 2004 international scanning tour of precast seismic bridges (described above) stand shoulder-to-shoulder with designs implemented in California, according to Eric E. Matsumoto, Department of Civil Engineering, California State University, Sacramento, and Jim Ma, Caltrans Office of Structure Design. Their paper, “Precast Concrete Bridge Systems and Details for Seismic Regions,” provides an overview of precast bridge superstructure and substructure systems for seismic regions. Its results were based on projects identified during the 2004 FHWA/AASHTO International Scan on Prefabricated Bridges and two projects recently completed in California.

Among the primary systems and components detailed are partially precast segmental piers and bent caps, full-depth pretensioned concrete decks, U-shaped precast superstructure segments with transverse ribs, and continuity features for deck joints and superstructure-to-substructure connections. “All systems have potential or have already demonstrated viability for application in the U.S.,” Matsumoto and Ma assert.

The authors note that Japan was included in the study tour due to its extensive research in seismic design. Similarly, Caltrans in recent years has developed innovative precast connection systems and features for projects, such as the San Mateo-Hayward Bridge Widening and the Sacramento River replacement, sited in highly seismic regions.

Matsumoto and Ma conclude:

• The use of precast bridge systems for seismic regions in Japan is highly advanced; implementation of such systems in California is increasing. To date, applications in Japan and California have been highly successful.

• These systems have provided benefits over traditional cast-in-place construction, including reduced traffic disruption, environmental impact, and life cycle costs; and, improved work zone safety, constructibility and quality of finished product.

• Both superstructure and substructure systems in Japan show high potential for eventual U.S. implementation, but may require further study, research, and discussions with Japanese engineers. Systems used in California have demonstrated their suitability for high-seismic regions.

• The full-depth pretensioned deck system described in the tour should be compared with existing U.S. technology and current research. Further validation and enhancement of connection details should precede implementation. The U-shaped precast segment with horizontal rib warrants continued study and consideration for precast segmental bridge applications, especially where site constraints limit on-site production. As continuity details from Japan and Europe are examined through research and evaluated, differences in construction quality and tolerances between these countries and the U.S. should be considered.

• When continuity is properly designed, the spliced girder system provides a highly advantageous solution for seismic regions.

• Regarding substructure, partially precast systems for bent caps and piers offer wide-ranging benefits for construction in seismic regions. Design fundamentals and connection details, however, need to be refined. Research funding is needed to develop a design methodology, connection details, and specifications for a range of precast bent cap-to-column connections.

In 2001, Matsumoto participated with other researchers to publish a 351-page report on precast bent cap systems. The complete paper, “Development of a Precast Bent Cap System,” can be downloaded from the web site of the Center for Transportation Research-University of Texas at Austin: http://www.utexas.edu/research/ctr/pdf_reports/1748_2.pdf.

PRECAST DECK REHAB SAVINGS

Precast panels save money and minimize traffic closure times when contending with a vintage steel span, according to “Solutions For Rapid Rehabilitation Of A Truss Bridge.” The report was presented at the 2005 TRB meeting by Munindra Talukdar, P.E., S.E., senior structural engineer, WSDOT Bridge & Structures Office in Olympia. The historic Lewis and Clark Bridge, designed by Joseph B. Strauss (of Golden Gate Bridge fame) and built in 1930, spans the Columbia River between Longview, Wash., and Rainier, Ore. The bridge comprises a 2,720-ft. main through-truss section and a 927-ft. deck truss section on the Oregon side; and, a 168-ft. deck truss and 1,507-ft., 12-span rolled-beam section on the Washington side. WSDOT Bridge and Structures prepared a deck replacement design in October 2002.

Traffic loads dictated that the work could only take place from 9:30 p.m. to 5:30 a.m. Night closures were limited to 120 days and single-lane closures were limited to 200 days. As a result, precast panels were put into play.

“The WSDOT Bridge Office designed a method to replace the existing concrete deck on the main through-truss and deck trusses, and for widening the existing deck on the rolled beam spans, using precast concrete deck panels,” Talukdar says. “A total of 103 36-ft.-wide precast panels in lengths ranging from 25 to 45 ft. was placed on the trusses.” For the rolled-beam spans, 46 4-ft.-wide precast panels in variable lengths of 58 to 70 ft. were placed. The total cost of the project was $27 million.

Per-cubic-yard mix design for the panels was portland cement 600 lb., fly ash 80 lb., fine aggregate 1,158 lb., coarse aggregate 1,114 lb., total water 270 lb., air entrainment (Daravair) and 3.2 oz., water reducer (WRDA 64) 34 oz. The water/cement ratio was 0.40 and slump 4±1 in. The weight of new deck panels was only about 5 percent lower than the deck section removed. Talukdar explains, “The replacement lightweight precast deck panels had a pre-installed 1-in.-thick latex modified concrete overlay to provide long-term durability.”

“The precast concrete deck panel system showed that rapid replacement of the deck in truss bridges and widening of the deck in the rolled beam spans is possible, without closing down the bridge for more than eight hours at night,” the author affirms. “Using this concept would be appropriate for rehabilitation of other truss bridges subjected to similar traffic and time constraints.

RESEARCHER: COST-BENEFIT ANALYSIS FAVORS SCC

In his 2005 update on self-consolidating concrete (pages 56-57), Virginia Transportation Research Council's Celik Ozyildirim tells the Transportation Research Board:

• SCC can be produced with locally available material; however, even going to distant places for material may be cost-effective, considering the overall cost and consistency of the product.

• High-range water reducing agents (HRWRA) based on polycarboxylates provide large water reductions with minimal effect on setting.

• To determine if concrete is self-consolidating, specimens made with and without consolidation may be tested for strength or permeability.

• The segregation can be detected by observing the halo of the slump spread or vertical-cut section of hardened concrete in a tube. However, lengthy travel of concrete through reinforcement may induce segregation not detected in the tests. Flow distance of SCC may be limited to ensure no bleeding or segregation is induced.

• SCC may have low freeze/thaw resistance because of a poor air-void system. Increasing the total air content or selecting the right combination of admixtures can improve the air-void system and make concretes resistant to frost damage.

• To reduce shrinkage, larger size coarse aggregate, more coarse aggregate, and a low water content may be used.

• During delivery of SCC in ready-mixed concrete trucks, holding back some of the water or the HRWRA enables easy delivery of full loads. Otherwise, small loads must be delivered to prevent spillage during transit.