In March, Concrete Products looked at peer-reviewed presentations — specific to ready mixed concrete and cast-in-place applications — from the 83rd annual meeting of the Transportation Research Board (TRB) in Washington, D.C. Here are informal summaries of presentations on precast/prestressed research. Some 10,000 delegates from the transportation engineering and research community attended the TRB meeting in January. More information on the meeting and organizers can be obtained from TRB, 2001 Wisconsin Avenue, NW, Green Building, Washington, D.C. 20007, or by visiting http://trb.org.

FIBER-REINFORCED POLYMER DECK DOES NOT EXHIBIT PANEL ACTION

A nonconcrete bridge deck made of prestressed FRP tubes — modeled after an existing FRP bridge deck in Delaware County, Ohio — did not show panel action in standing up to stresses. These results are reported by authors Zhenhua Wu, doctoral student, Amir Mirmiran, professor at North Carolina State University, and James Swanson, assistant professor at University of Cincinnati, in “Fatigue Behavior of Prestressed FRP Tubular Bridge Deck.”

FRP decks have emerged as a competitor to precast/prestressed or cast-in-place concrete designs as the transportation infrastructure community attempts to reduce the backlog of deteriorated bridge decks within existing financial constraints. “The main characteristics of FRP systems making them a viable alternative to traditional reinforced concrete decks include: better resistance to electro-chemical corrosion, higher strength-to-weight and stiffness-to-weight ratios, versatility of fabrication and potential for design optimization,” the authors write.

FRP interests have eyed the transportation market in the face of declining demand from the defense industry, the authors state. “Despite its successful introduction into the construction market, widespread acceptance of FRP decks by the civil engineering community has been slow, not only due to simple resistance to the change of common practice, but also by virtue of many legitimate technical and economical concerns,” they say.

This new prestressed FRP tubular deck system replaced a deck on an existing steel-truss and wood-deck bridge in Delaware County, Ohio, through the Federal Highway Administration's (FHWA) Innovative Bridge Research and Construction (IBRC) Program, which supports field research in FRP bridge decks (http://ibrc.fhwa.dot.gov/).

The Delaware County FRP deck is a modified version of a post-tensioned, concrete-filled FRP tubular system developed for a bridge near Columbus, Ohio. The bridge was disassembled, steel elements were regalvanized and reassembled with the FRP deck. The deck consists of a series of 4-in.-square pultruded FRP tubes laid side-by-side on existing stringers, perpendicular to the direction of traffic. “Due to low traffic, the county approved eliminating the concrete from all but the last few tubes at each end of the bridge,” the authors write, adding this would allow for a much lighter bridge deck with lower construction time and cost.

The tubes are post-tensioned up to 20 kips with 0.6-in.-diameter, seven-wire strands at mid-points between the stringers in the direction of traffic.

Tests were conducted at the University of North Carolina on an FRP bridge deck modeled after the Delaware County FRP deck. “Four double-span FRP deck specimens with two different tube sizes and three different span lengths were tested in flexural fatigue under AASHTO prescribed footprint of wheel loads for an HS20 truck,” the researchers write. “Panel action in the deck system was inadequate for the most part, as only the tubes directly under the load carried the majority of the load. Subsequent slippage between these tubes and their adjacent tubes can cause cracking in the asphalt overlay.”

Panel action, however, is generally improved at higher prestress levels, they say. “Prestressing also offers additional redundancy and reserve strength for the system.” While longer span decks fail in bending, shorter span decks generally suffer from local shear failure due to stress concentrations at the corner of the tubes under the applied load or at the support. The fatigue problem is less critical for longer span decks and smaller tube sizes.

TEXAS PRECAST METHOD SPEEDS BRIDGE ERECTION

A new precast bridge design for off-system bridges incorporating precast concrete deck panels on concrete or steel I-beams can significantly reduce bridge closure times, while maintaining quality and practicality of construction. So write R. Scott Phelan, Ph.D., P.E., and W. Pennington Vann, Ph.D., P.E., Department of Civil Engineering, Texas Tech University, Lubbock, in “Innovative Approach to Rapid Construction of an Off-System Bridge.” The authors were part of a research project sponsored by the Texas Department of Transportation (Texas DOT) to develop innovative design and construction methods for off-system bridges.

Possible solutions to these remote bridge replacements are limited by restrictions on the type of equipment available in a given area and able to be transported to a particular site, the overall project cost, and the desired long-term durability of the structure, they say.

“Texas DOT already has one of the fastest and least costly bridge superstructure construction strategies with the use of either steel or precast concrete girders and partial-depth precast panels,” the authors write. “Nevertheless, the cast-in-place (C-I-P) deck pour in the bridge construction process can consume one or more calendar months.”

Therefore, the authors focused on elimination of the C-I-P deck pour. Their proposed system is a modification of the current Texas DOT approach by the use of full-width, full-depth precast concrete panels set on specially matched girders and connected with multi-directional leveling and shear screws.

Although a grout pour is required, “top-only” construction is provided, further enhancing the speed as well as the safety of the method, they write. “The proposed system offers the possibility of completing superstructure installation in one day after the abutments and columns are ready,” Phelan and Vann say. “It also has the potential for easier replacement of the deck than with other systems. Longitudinal post-tensioning is optional.”

Only one 30-ton or smaller crane is required for the construction, they write. “For a 50-ft. span, by way of example,” they say, “the method is expected to require only one or two days of bridge closure. The system also offers the potential for much easier replacement in the future.”

Though overall cost is always an important design consideration, speed of construction and reduction of lane closure times were the primary focus of their research. For off-system bridges needing replacement, statistics indicate the majority have a single span over a stream, a narrow 24-ft.-wide roadway, and a small right-of-way. Using those parameters, the researchers developed their design solution.

NDT'S REVEAL TENDON DUCT CONDITION

The inner condition of tendon ducts in post-tensioned concrete structures may be ascertained by a combination of complementary, high-tech, nondestructive tests (NDTs). This topic is examined in depth by Christiane Maierhofer, Martin Krause and Frank Mielentz, Federal Institute for Materials Research and Testing (BAM), Berlin, in their paper “Complementary Application of Radar, Impact-Echo and Ultrasonics for Testing Concrete Structures and Metallic Tendon Ducts.”

“Nondestructive testing of concrete structures plays an increasing role in civil engineering, although up to now the full potential of these techniques has not been tapped,” the authors say.

For post-tensioned structures, the investigation of tendon ducts is one of the most essential testing problems. The location of tendon ducts, the determination of concrete cover, and especially, the detection and quantification of ungrouted areas inside the ducts are significant criteria, they report.

Radar, impact-echo and ultrasonics for the investigation of tendon ducts can be useful for these investigations, they say. But, there is a qualification: Although the results obtained on positioning and concrete cover determination are sufficient, the location of ungrouted areas is still a matter of research, they add.

“New approaches for this testing problem have to be considered,” they say. “[T]he combined use of complementary techniques offers a high potential to increase the reliability of the results.” These include the combined application of acoustic and electromagnetic impulse-echo methods, plus data fusion related to the investigation of tendon ducts.

These NDT methods were developed, tested and applied for the investigation of structural and material properties of concrete and masonry structures at the Federal Institute for Materials Research and Testing in Berlin. One of the main problems in assessing transportation infrastructure condition is the location and assessment of metallic tendon ducts, they write. “Since radar and ultrasonics give good results for the lateral position and concrete cover,” they say, “there is no NDT method besides radiography which provides reliable information on the inner condition of tendon ducts at this time.”

Impact-echo has been described as a useful tool for such inspection tasks, but there have been conflicting results about the reliability of these measurements, they contend. “The localization of artificial voids in tendon ducts by means of ultrasonics using synthetic aperture focusing techniques (SAFT) is feasible, but not yet fully consistent.”

For this research, radar was used for fast scanning of large surfaces to localize reinforcement bars and tendon ducts, detect voids and honeycombing, and to determine concrete cover and the geometry of structures that are only accessible from one side. For the performance of impact-echo measurements, a commercially available device was included in an automatic data acquisition system, enabling systematic scanning measurements.

“Typically, impact-echo is only used as a point method, making data analysis very difficult,” the authors write. “For ultrasonics, different scanning systems are used: an array of broadband transducers being moved over the surface, neighbored transmitting/receiving transducers, and a scanning laser vibrometer as receiver.”

Their data show that radar, impact-echo and ultrasonic-echo are well suited to determine the position of tendon ducts inside concrete slabs. From radar and ultrasonic data, the concrete cover can also be quantified. Since impact-echo tendon ducts are detected in most cases only by a shift of the back-side reflection, the determination of concrete cover is still difficult with this method.

Radar impulses are totally reflected at metallic tendon ducts, thus only ultrasonics and impact-echo have the potential to locate grouting faults. These have been investigated with ultrasonics using pressure waves and shear waves. Applying shear waves, it was demonstrated for the first time that the back side of a tendon duct containing strands is clearly imaged in the presence of good grouting.

In analyzing the intensity of the direct reflection of the ducts, location of artificial voids in tendon ducts by means of ultrasonic 3-D imaging is feasible, but not yet reliable. “Further research will be done in order to analyze the influence of the conditions of the boundary layer and the air pores,” the authors write.

The advantages of each method can be exploited by combination and data fusion. Since radar measurements can be performed very fast and without any defined coupling to the surface, this method is best suited for the location and concrete-cover determination of tendon ducts. Only in cases of very strong reinforcement, where most of the radar intensity is reflected, should radar be replaced by ultrasonics.

After tendon duct location, more detailed investigation on the inner condition can be performed with ultrasonics and/or with impact-echo. “The very simple data-fusion algorithm of the selection of maximum amplitudes already shows an increase in information, but further research is necessary to fully determine the most adequate fusion strategies,” the authors write. “The selection of conventional and new algorithms depends on the problem to be investigated and also on the stated goals.”

AUGERED PILES MAY SUBSTITUTE FOR PRECAST/PRESTRESSED PILES

New data indicate augered, cast-in-place (ACIP) piles may serve just as well as drilled-shaft or precast/prestressed concrete piles (PSC) in Florida DOT critical applications, at a lower cost, say Rudolph P. Frizzi, P.E., and Ramakumar V. Vedula, P.E., Langan Engineering and Environmental Services, Inc., Elmwood Park, N.J., in their paper “Augered Cast-In-Place And Driven Pre-stressed Concrete Pile Field Performance Comparison.”

A pedestrian bridge over SR A1A in Hollywood, Fla., required foundation design following Florida DOT guidelines, which demand drilled-shaft or prestressed concrete piles. But nearby, in the vicinity of the bridge foundation — at a hotel and parking garage — augered cast-in-place piles ranging from 14- to 24-in. diameter had been successfully installed and load tested to as much as 800 tons compression, the authors report.

“The ACIP pile data was compiled and presented to Florida DOT for their consideration in lieu of drilled-shaft or prestressed concrete pile deep foundations, which Florida DOT typically requires for ‘critical’ bridge structures,” they write.

The ACIP pile data indicate higher individual pile capacities could be attained, along with more uniform lengths, when compared to PSC piles. “This translated to potential cost and time savings should the ACIP pile alternative be used,” they assert.

Although, in this instance, ACIP piles ultimately were not utilized for the bridge foundations due to Florida DOT's guidelines, this project yielded data for use in evaluating the time and cost savings of an alternative ACIP pile foundation as compared to the driven PSC pile for a transportation-related project.

The authors conclude:

• A 14-in.-diameter augered cast-in-place pile was capable of sustaining a design load-carrying capacity of 100 tons, primarily in side shear within the upper limestone. Florida DOT mandated an 18-in.-square prestressed concrete pile be used to sustain a similar load-carrying capacity for the subject pedestrian bridge. To attain this required capacity, PSC pile lengths about half that of the ACIP pile were used in the bridge's west abutment. However, due to the driven piles' tendency to fracture the soft rock, piles about twice as long were required for the east abutment.

• The time to install ACIP piles is less than that required for PSC piles. Even after accounting for restrictions on installing adjacent ACIP piles on the same day, ACIP piles would appear to be installed in about two-thirds the time required for a similar number of PSC piles.

• At present, ACIP piles are about one-third the cost of PSC piles when equally compared on a cost-per-ton basis.

• Greater time and cost savings would likely be realized in this case, since field load tests demonstrated a feasible reduction of at least one-third in the ACIP pile embedment.

HOOKED-END STEEL FIBERS AND SECONDARY REINFORCEMENT

A tiny amount of hooked-end steel fibers may reduce the need for most secondary reinforcement in the anchorage zone of prestressed, post-tensioned concrete girders, according to Saif A. Haroon, Nur Yazdani, P.E., and Kamal Tawfiq, P.E., Florida State University College of Engineering, Tallahassee, in their paper “Feasibility of Steel Fiber Concrete in the End Zones of Post-Tensioned Bridge Girders According to AASHTO Special Anchorage Device Acceptance Test.” Their research can mean savings to precasters.

Precast post-tensioned girders are subjected to a high concentration of compressive stresses at the anchorage zone due to transfer of prestressing force at the girder end through bearing plates and anchors, they write. The corresponding transverse tensile stresses may cause longitudinal bursting cracks. The anchorage zone is, therefore, confined with secondary closed stirrups and/or spirals made of conventional steel rebars to prevent such cracks.

“The number of rebars used for this purpose causes congestion in the anchorage zone, posing difficulty in the placement of concrete, anchorages and post-tensioning ducts,” the authors note. “It is also labor-intensive to produce and place the secondary anchorage reinforcement.”

Their objective was to determine the feasibility of reducing or eliminating the secondary reinforcement, replacing it with steel fibers for post-tensioned anchor zones. The AASHTO Special Anchorage Device Acceptance Test was performed in this study. “Variations of spiral and skin reinforcement, with concrete strengths ranging from 3,500 to 5,000 psi, were utilized to investigate the performance of the two types and various amounts of steel fibers,” they write.

“The experimental results indicated that 1 percent hooked-end steel fiber could reduce a maximum of 79 percent of the secondary reinforcement for a minimum concrete strength of 4,710 psi,” they report. “Lower volumes of steel fibers may also result in reduction of secondary reinforcements.”