Iowa Demonstration Project: Accelerated Bridge Construction on US 6 over Keg Creek
Project Overview and Lessons Learned
The project consisted of replacing a bridge located on US 6 over Keg Creek in Pottawattamie County, Iowa, about 10 miles east of Council Bluffs. The new bridge was designed to increase the structural capacity of the bridge, improve roadway conditions, and enhance user safety by providing a wider bridge and approaching roadway.
The focus of this demonstration project is the innovation of combining several cutting edge ABC materials and methods together in a single bridge design and construction project that can help guide similar projects in the future. Featured in the project are prefabricated superstructure and substructure systems, ultra-high-performance concrete (UHPC), self-consolidating concrete (SCC), and fully contained flooded backfill.
The technologies incorporated into this bridge project have been used successfully in other constructed projects drawn from around the US, albeit on a limited basis, such as the HfL demonstration project in Washington, DC, featuring a prefabricated substructure and steel/concrete modular superstructure system1. The fact that several diverse structural systems have been assembled and incorporated into a single project reinforces the concept that innovation does not necessarily mean creating something completely new, but rather facilitating incremental improvements in a number of specific bridge details to fully leverage previously successful work.
Under traditional construction methods and considering the rural locale and relatively low amount of traffic, the Iowa DOT estimated the bridge would need to have been closed for a 6-month period to accommodate conventional cast-in-place construction2. Central to the ABC approach adopted on this project was condensing the bridge closure to only 16 days, which was enough time to facilitate both removal of the old bridge and construction of the new bridge. The positive benefits of such work zone management techniques have been demonstrated in other HfL projects such as Minnesota’s TH 36 project.3
Construction prior to the full closure included drilling pier shafts outside the footprint of the existing bridge and casting footings. Meanwhile, the farmland around the bridge was used as a casting yard to precast the steel/concrete modular superstructure elements, piers, pier caps, abutments, wingwalls, and approach slabs. Both longitudinal and pier joints were filled with UHPC, and finally the whole deck was diamond ground prior to reopening to traffic.
The Iowa DOT partnered with the Transportation Research Board (TRB) Strategic Highway Research Program 2 (SHRP 2) project R04 (Innovative Designs for Rapid Renewal ) research team to further advance and implement the use of standardized approaches to ABC. The success of this project will validate the SHRP 2 effort and will pave the way for the introduction of standardized ABC design details and construction methods.
Videos of this project as well as discussion of the SHRP2 program can be found on the TRB website4. Three videos are available:
- "ABC for Everyday Bridges"-details how typical bridges can be quickly and cost effectively replaced using ABC techniques.
- "One Design–10,000 bridges"–discusses new ABC tools for designing and constructing bridges.
- "Time-Lapse Video of Keg Creek Bridge Replacement"–shows the bridge construction during the accelerated closure period.
A concrete drainage flume separate from the bridge, was also part of the overall contract but was outside of the HfL scope and is not addressed in this report. The standard reinforced concrete flume was designed to carry storm water from an adjacent ditch to Keg Creek.
In Iowa's previous HfL project, the DOT successfully applied the following ABC techniques to accelerate the reconstruction of a busy interchange in Council Bluffs5:
- Cost-plus-time bidding to reduce the time required to deliver the project.
- Full-depth, precast bridge deck panels made with self-consolidating, high-performance concrete (HPC) to ensure quality, increase speed of construction, and improve safety.
- HPC used throughout the bridge and high performance steel (HPS) welded plate girders to increase quality of the completed bridge.
- Incorporation of a structural health monitoring system to evaluate and document the performance of the in-service materials after project completion.
- Fully contained flooded granular backfill installed behind the abutments to mitigate settlement that inevitably occurs with conventionally compacted backfill.
- Intelligent transportation system (ITS) technology used to optimize traffic control during construction.
HfL Performance Goals
Safety, construction congestion, quality, and user satisfaction data were collected before, during, and after construction to demonstrate that innovations can be an integral part of a project while simultaneously meeting the HfL performance goals in these areas.
- Work zone safety during construction—As expected, no incidents occurred during the entire construction period including the full closure period, which meets the HfL goal of achieving a work zone crash rate equal to or less than the preconstruction rate.
- Worker safety during construction—No workers were injured on the project, so the contractor achieved a score of 0.0 on the OSHA Form 300, meeting the HfL goal of less than 4.0.
- Facility safety after construction—The additional bridge width and updated side barriers and beam guards are improvements over the existing bridge. The net effect that these safety improvements will have on the HfL goal of 20 percent reduction in fatalities and injuries in 3-year crash rates after construction is yet to be determined.
- Construction Congestion
- Faster construction—Compressing the time it took to replace the bridge from an estimated 6 months to only 16 days under the ABC approach drastically reduced the impact to motorists and went beyond the HfL goal of a 50 percent reduction in the time traffic is impacted compared to traditional construction methods.
- Trip time— Considering the cumulative trip time over the 16-day detour compared to 6 months of detour estimated for traditional construction, motorists experienced a reduction in trip time, meeting the HfL goal of no more than a 10 percent increase in trip time compared to the average preconstruction conditions.
- Queue length during construction—The project met the HfL goal of less than a 0.5-mile queue length in a rural area, as there were no traffic backups along the detour route.
- Smoothness —Smoothness increased across the bridges. IRI decreased from 221 in/mi before construction to 179 in/mi after construction. Motorists will notice a smoother ride, although the HfL goal for IRI of 48 in/mi—typically expected to be attainable on long, open stretches of pavement—was not met on this project.
- Noise—The sound intensity (SI) data showed a noticeable 3.2 dB(A) increase in noise from a preconstruction value of 98 dB(A) to 101.2 dB(A) after construction which does not meet the HfL requirement of 96.0 dB(A) or less. The new texture of the bridge surface—while aiding traction and increasing safety—is prone to increasing noise.
- User satisfaction—Most motorists surveyed were satisfied with the finished highway and the way the project was carried out, which met the performance goal of 4 or more points on a 7-point Likert scale.
The costs and benefits of this innovative project approach were compared with those of a project of similar size and scope delivered using a more traditional approach. A comprehensive economic analysis that accounted for construction, road user, and safety costs revealed that Iowa DOT’s innovative approach realized a cost savings of $0.44 million, or 29 percent, less than conventional construction practices. A significant amount of the cost savings stemmed from avoiding the delay costs to the road users through the use of ABC techniques.
Through this project, Caltrans gained valuable insights into the innovative technologies and materials used. The agency learned what contributed to the project's success and what issues need improvement or more careful consideration in future project deliveries. The following are some of the lessons learned:
General Items Regarding ABC
- The condensed bridge closure duration was adequate for demolition and construction of a rural bridge of this size. There was enough time to completely remove the existing bridge and set the precast elements in place, and fill the deck joints with UHPC. The contractor worked long shifts from dawn to a few hours after dusk each day during the closure but did not need to work around the clock.
- A major rain event could have made the bridge closure window problematic. Flooding of Keg Creek was the major concern, as the land around the bridge was mostly bare earth, easily turned to mud. A muddy job site would have made moving the heavy bridge modules and approach panels especially difficult.
- Regarding risk, the construction tasks (i.e., moving the bridge elements, placing UHPC, installing the approach panels) were less of a risk compared to the possibility of heavy rain occurring once the bridge closure began.
- Isometric drawings detailing how the superstructure connects to the abutment would have been helpful, as the joint proved to be complex and difficult to visualize in the field. Less steel reinforcing in the joint would have made erection easier. Overall, fit-up was not an issue but difficulties arose due to a survey error in the abutments.
Steel/Concrete Bridge Modules and Substructure Elements
- The contractor used more of a custom construction approach to building the modules in contrast to the earlier Iowa HfL project on the 24th Street Bridge project, which had many similar deck panels fabricated at a precast plant in a repetitive technique. Custom construction ensured each module fit with the others as one continuous superstructure.
- On-site module construction allowed the contractor to build in a comfortable level of tolerances, such as a slightly thicker concrete deck to account for some loss of thickness after diamond grinding.
- A precast plant likely would have difficulty handling the steel beams associated with the deck modules. A typical precast plant would have forms and equipment readily available to make standard concrete panels and beams but not steel/concrete composite bridge modules.
- The sheer weight of the deck modules would have likely presented an obstacle for precast plant fabrication.
- The weight (in excess of 70 tons) and size of the individual deck modules would have made overland transport difficult and would have involved obtaining load permits.
- Multiple trucks would have been needed to ship and temporarily store the modules on site on the day(s) of placement.
- The bridge module construction approach was well suited for a spacious site with ample access to both ends of the bridge.
- Making the modules at a precast plant may be economical if multiple bridges were built under one or a group of contracts and if the bridges were similar.
- On-site construction made it possible for the contractor to build all the modules in the final configuration so the proper fit of the modules could be guaranteed.
- Cambering the beams would likely make connecting the steel beams easier. It was the designer’s decision not to camber the beams since it did not affect the structural integrity and there were no vertical clearance issues.
- Use embedded lift points and eliminate the pockets. The deck pockets were patched after module installation but the patches could become a durability problem in time.
- Most of the risk on this project was from predicting the weather during the short bridge closure and the possibility of damaging a module during handling.
- There also was risk associated with the new and unfamiliar type of construction.
- The contractor chose to cast the modules on site, as opposed to at their own yard, due in part to the difficulties and risk associated with transporting the heavy bridge elements.
- The contractor likely would have cast the sleeper slabs over the backfill instead of trying to bring the backfill to the perfect elevation for the precast sleeper slabs. On future projects, the DOT could leave this construction detail to the contractor to decide how best to set the approach slabs.
- ABC projects like this one should have two independent construction surveys as part of the contract specifications because mistakes can be costly if discovered during the ABC bridge closure period.
- The cost of furnishing a thick stone haul road as part of the contract specifications should be considered on future projects. This should be done regardless of ABC project or not, and permits for the temporary haul road should be obtained ahead of time (which is typical of most projects to have as part of the contract plans). Unique to this project was that the stone for the haul road was reused as riprap for a drainage flume next to the bridge.
- To limit the risk (and associated cost) of weather impacting construction progress during the short closure period, the contract could be structured to limit disincentives to a maximum of 10 percent of the contract value or another manageable/predictable amount.
- The contract could allow for the closure period to be extended for extreme weather events without penalty, which would encourage competitive bidding.
- The incentives in the actual contract helped to offset some of the risk in the contractor's bid decision making process. The contractor was awarded $22,000 for one day of incentive.
- UHPC is sensitive to temperature and wind. The ambient air temperature should be between about 32 and 73 degrees Fahrenheit for optimum placement and cure.
- Once mixed, UHPC is liquid and challenging to handle.
- Forms need to be water tight. Leakage at the abutment was a critical problem. Next time, the pour should be strategically bulkheaded, sealed, and tested with water before pouring UHPC.
- Casting the deck joints 3/4 inches high and then grinding to the designed height ensured the joints were never underfilled. The contractor secured 3/4-inch wood boards along the joint to act as forms so the joint could be overfilled initially at one end of the joint in an effort to keep the joint from being underfilled as the UHPC flowed ahead of the pour.
- Grinding removed the exposed surface of the UHPC, which likely will increase durability because this removes the skin formed during curing and any of the material that would otherwise have been subject to surface shrinkage cracks.
- Bottom forming the deck joints was necessary. The contractor had no problem doing this.
- UHPC has the proven ability to penetrate the surface of cured concrete and create a strong bond. Research into texturing the joints by sandblasting or other chemical and mechanical means may further improve bonding of the UHPC to the deck concrete.
- Iowa DOT will monitor the finished joints in the bridge deck for leakage but is considering a thin asphalt overlay to seal the joints from rain and snow melt.
- Plan for bulkheads inside the joint when using UHPC. Even though the bulkheads could be considered a "means and method" it would be prudent to show the bulkheads in the contract plans to focus the contractor’s attention on the need to check the flow of the UHPC.
- For this ABC project, UHPC had constructability challenges but was a suitable solution to close the deck joints because UHPC is 1) very strong, rigid, and durable, 2) develops high strength very quickly, and 3) affords short embedment development strength for reinforcing steel.
- Considering future projects, "buy American" waivers may be needed to ensure prompt acquisition of the metal fibers for the UHPC. This is important, as some or all of the fibers or other UHPC constituents may come from outside the US. Acquiring the UHPC constituents could otherwise impact the construction schedule.
The Iowa DOT gained valuable insights into the use of several innovative ABC techniques and materials, such as prefabricated superstructure and substructure systems, HPC and UHPC, SCC, and fully contained flooded backfill. These innovations were key to successfully achieving the HfL performance goals of increasing safety, reducing congestion, and increasing quality.
1Reconstruction of Eastern Avenue Bridge over Kenilworth Avenue on Washington, DC, August 2011. Federal Highway Administration. http://www.fhwa.dot.gov/hfl/summary/projects_summary.cfm
2Price and Sivakumar, "Two-week notice," June 2011. Roads & Bridges.
3Reconstruction of Trunk Highway 36 in North St. Paul, June 2010. Federal Highway Administration http://www.fhwa.dot.gov/hfl/summary/projects_summary.cfm
5Improvements to the 24th Street Bridge–I29/80 Interchange in Council Bluffs, November 2009. Federal Highway Administration. http://www.fhwa.dot.gov/hfl/summary/projects_summary.cfm