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Iowa Demonstration Project: Improvements to the 24th Street–I-29/80 Interchange in Council Bluffs
The focus of this project was to replace the existing 24th Street bridge as part of overall improvements to 24th Street and the diamond interchange. The existing four–span 215–ft by 53–ft pretensioned, prestressed concrete beam bridge was replaced with a two–span 350–ft by 105–ft steel welded girder bridge. The new bridge has an 82–ft roadway width, a 10–ft multiuse trail on the west side, and an 8–ft sidewalk on the east side. The 24th Street 2004 annual average daily traffic (AADT) across the bridge was 12,400 vehicles per day (vpd), and the estimated 2030 AADT is 27,700 vpd with 14 percent truck volume. For I–29/80, the 2004 AADT was 81,900 vpd and the estimated 2030 AADT is 124,400 vpd with 11 percent truck volume. Traffic was maintained on I–29/80 except during placement of the bridge girders and deck panels directly over the interstate, when traffic was routed onto the 24th Street ramps. Figure 1 shows the general project location.
Figure 1. General Poject Location
The new 24th Street bridge is wider than the old bridge to match the city's planned 24th Street improvements. The old bridge is shown in figure 2 and the new bridge, almost complete, is shown in figure 3.
Figure 2. Existing four–span bridge.
Figure 3. New 24th Street bridge near completion.
The superstructure of the new double–span bridge is comprised of 12 composite steel girders with a deck of 70 precast, post–tensioned panels and a 2–inch concrete overlay. Two construction phases were needed to build the superstructure and maintain traffic across the bridge. The bridge deck plan shown in figure 4 illustrates the phased construction.
Figure 4. Bridge deck plan
The bridge was constructed via the use of phased construction, maintaining at least one lane of traffic in each direction and left–turn lanes at all times on 24th Street. The interchange ramps, approach pavements, and westbound I–29/80 were reconstructed to the extent required to accommodate the proposed bridge location, roadway width, length, and grade. New interchange signals and lighting were incorporated to handle the new design geometry and traffic volumes.
The project widens westbound I–29/80 to the median side in preparation for the CBIS 12–lane reconstruction project scheduled for 2011. Traffic was shifted onto this widened section to allow the new single pier to be constructed. The new vertical profile on 24th Street was raised about 5 ft to gain the necessary vertical clearance over I–29/80. This grade raise required about 1,340 ft of 24th Street and portions of all four ramps in the diamond interchange to be reconstructed.
Innovative construction and contracting techniques used to bring this project to fruition are described in the following sections.
Cost–Plus–Time (A+B) Contract Bidding
Because of the size and scope of this project, it would have required at least two construction seasons to complete it using traditional methods. To reduce the project delivery time and open all lanes on the new bridge within one construction season (April through October), the Iowa DOT selected the A+B contract bidding process. This procedure selects the low bidder based on a monetary combination of the contract bid items (A) and the time (B) needed to complete the critical portion of the project. This method favors contractors that explore innovative construction methods to reduce cost and motivates contractors through incentives to minimize the delivery time.
During the planning stage, it was deemed impractical to impose additional constraints by further reducing construction time. As with any construction project, the additional cost for further reduction in construction time requires justification. Under these circumstances, the goal of construction acceleration by 50 percent was considered appropriate, given the need to maintain traffic during construction and the use of many innovations. From I–29/80, 24th Street provides vital access to several regional attractions and businesses in the area. These attractions include a casino, a convention and event center, and a large outdoor retailer. Both the city and the State made a commitment to provide access to these businesses during construction.
The decision to choose a maximum of one full construction season for this project was made after consulting with local contractors. All contractors at the constructability review meeting held to discuss accelerated construction methods for the project were in favor of a staged construction for one full construction season.
The accelerated project schedule was as follows:
Full–Depth Deck Panels and Self–Consolidating Concrete
The Iowa DOT uses partial–depth panels for low–volume bridges, but full–depth panels are still a new concept for high–volume corridors. The precast panels were transversely prestressed during casting and post–tensioned longitudinally after being placed on the bridge. These panels were cast offsite in a controlled environment, steam cured, and made with an innovative self– consolidating concrete to improve consolidation around the complicated arrangement of reinforcing and post–tension ducts. Figure 5 shows self–consolidating concrete flowing around the reinforcing steel in a panel form and three completed panels stacked after curing. Figure 6 shows panels in position at the job site.
Figure 5. Self–consolidating concrete being placed and three panels ready for delivery.
Figure 6. Panel being placed and several panels installed on the new bridge girders.
The deck panels span about half the width of the bridge and accommodated the two phases of construction traffic. Where the panels met near the bridge centerline, concrete was cast to join the two halves of the bridge deck (figure 7).
Figure 7. Longitudinal panel connection.
After the panels were placed, they were secured to the girders with shear stud connectors welded to the top flange of the girders. The shear stud pockets were then filled with grout and allowed to cure before the surface overlay was placed. This method gave the contractor flexibility to make field adjustments to the panels to ensure proper alignment.
Laboratory and field testing was undertaken by researchers at the Iowa State University Bridge Engineering Center1 to investigate constructability issues related to the panels and to evaluate the bridge during and after construction. Preconstruction laboratory testing on the full–depth panels is discussed in this section, and the field testing is described in the structural health monitoring section of this report.
Laboratory mockups of the stud pockets as specified in the bridge plans (figure 8) were built and investigated to determine the ability to test the shear studs once they are welded to the top flange of the girder and how to get adequate flow of grout into the haunch between the panel and top flange. The contractor was involved in the process and determined that the shear stud pocket as designed would work in the field.
Figure 8. Shear stud pocket detail.
Plans called for the transverse joints between the panels to be filled with grout before the surface overlay was placed. Researchers examined methods to splice the longitudinal post–tension ducts at the transverse joint to keep moisture or grout from infiltrating the ducts. The result of the investigation indicated that sealing the post–tension duct connections with waterproof duct tape or a combination of waterproof duct tape and butyl rubber would be adequate.
The influence of surface treatment on the transverse joint shear transfer between panels was also examined in the laboratory. Precast diamond plate texturing, chemical etching, and sandblasting were evaluated as possible surface treatments to promote bonding at the panel transverse shear key (figure 9). Sandblasting was found to deliver the highest shear bond of the three treatments.
Figure 9. Shear key detail.
HPS was used for the continuously welded plate composite girders (figure 10). Special low alloy 70 kilo–pound per square inch (ksi) steel gives these girders better corrosion resistance and increased fracture toughness over conventional steel. The expected benefit is longer service life with less maintenance during the life of the bridge. The Iowa DOT developmental specification DS–01065 sets standards for the materials as well as fabrication of the steel components that make up the girders.
Figure 10. High–performance bridge girders.
HPC is new to western Iowa due in large part to availability of materials. Iowa DOT developmental specification DS–01092 requires HPC to have 28–day compressive strength of 4,500 pounds per square inch (psi) and 5,000 psi for bridge deck and substructure, respectively. Permeability levels are specified to enhance the concrete’s resistance to chloride–related distresses. HPC was placed for all bridge components, including the prefabricated bridge panels, overlay, pier, and abutments (figure 11).
Figure 11 . Casting HPC at the bridge abutments
Fully Contained Flooded Backfill
A frequent problem in any bridge construction is development of differential settlement between the bridge and the adjacent pavement, which motorists commonly experience as a bump or dip just before the abutment joint. This project provided the opportunity for Iowa DOT engineers to mitigate this problem by using fully contained flooded backfill behind the abutments. This involves placing a granular wedge behind the abutment backwall, applying conventional compactive effort with a plate tamper (figure 12), and flooding the backfill with water (figure 13) to achieve consolidation. This method, designed to be superior to traditional compaction methods, minimizes settlement.
Figure 12 . Granular material behind the abutment.
Figure 13 . Flooding the self–contained backfill with water to achieve consolidation.
Intelligent Transportation System
The original ITS plan was to have five permanently mounted cameras and sensors positioned at the 24th Street interchange and on I–80/29. Because of funding difficulties, that system was not used. Instead, two portable cameras and two portable sensors were stationed on I–80 on either side of the interchange. Figure 14 shows the ITS equipment in service at a similar project on I–80 west of Council Bluffs.
The system had an automated feature to notify authorities in case of traffic–flow irregularities. If the sensors detected the traffic speed dropping below an expected rate, an e–mail was automatically sent via wireless cellular technology to officials, who could then determine the appropriate action. If sensors detected unusual congestion, a larger list of officials would be notified and the proper response assets activated to alleviate the cause of the congestion.
Figure 14 . ITS portable camera (left) and the ITS portable sensor (right).
Data were collected in both directions from each location on I–80. The sensors and cameras were bidirectional and were positioned about 1 mile east and west of the bridge, which allowed monitoring of traffic conditions from the vantage point of overlooking the interstate as traffic approached from both directions. A sample of the sensor data, shown in table 1, includes the traffic volume, lane occupancy, vehicle speed, and time of recording.
A late merge system was proposed that would have coordinated the cameras, sensors, and dynamic message boards to direct traffic merging maneuvers while lane closures on I–29/80 were in effect. This system provides the most benefit for moderate volume levels of mostly passenger vehicles. Consequently, the system was not deployed because of the few nighttime lane closures that did occur; the traffic was light and contained a relatively large percentage of trucks. Traffic conditions did not warrant the additional cost of using the system. Dynamic message boards were used, not as part of this proposed late merge system, but as a traditional nonautomated application to inform the traveling public of work zone conditions.
Structural Health Monitoring System
A structural health monitoring system was implemented through the coordination and expertise of the Iowa State University Bridge Engineering Center. Field data were collected during panel placement and after the bridge was completed, but the final results of this research effort have not been published. The innovative system involves corrosion monitoring of steel pre– and post–tension strands, monitoring of the panels during handling, and live load testing of the bridge.
Figure 15 shows corrosion sensor wiring for monitoring the long–term integrity of prestressed steel tensioning strands in the panels. Six pretensioned strands were instrumented during panel fabrication and six sacrificial post–tensioning strands were instrumented in the field.
Figure 15. Corrosion sensors installed in a panel before casting.
Two panels were instrumented with externally mounted strain gauges to document the performance of the panels from the point of shipping from the casting plant until the panels were placed on the bridge girders.
A series of strain gauges and deflection transducers were installed at critical locations on the steel girders of the completed bridge to test the performance of the bridge under semicontrolled live loads. These instruments collected time–history data from the bridge as it reacted to a fully loaded dump truck being driven across the bridge. The results will allow researchers to compare actual bridge performance to the expected design performance. The results from this Iowa State University research2 are expected to be completed by June 2010.
Public meetings were held during construction to gain input from the public and provide updates on the progress of the interchange and the concurrent CBIS project. Newsletters were distributed at key events throughout the development of the project. Meetings were held with area businesses impacted by the reconstruction to discuss the project. An advisory committee of local officials was used before and during construction as part of the CBIS project to keep local agencies abreast of the construction schedule and possible impacts on commerce.
A Web site was developed (www.iowadot.gov/cbinterstate.com) to provide background on the project as well as to notify the public about construction activities, road closures, detour routes, and schedules (figure 16). The Iowa DOT also covered the project in the Insight newsletter posted on its Web site to provide information to the public about construction progress and announcements for public meetings.
Figure 16 . Iowa DOT 24th Street bridge construction information Web site.
1Iowa State University Bridge Engineering Center, Laboratory Testing and Evaluation Report, 24th Street Bridge over I80/I29, Council Bluffs, Iowa, February 28, 2008.
2Evaluation of the 24th Street Bridge, Interstate 80/29, Council Bluffs, Iowa, (expected June 2010), Iowa State University Bridge Engineering Center.