|FHWA > HfL > Projects > Iowa Demonstration Project: Accelerated Bridge Construction on US 6 over Keg Creek > Project Details|
Iowa Demonstration Project: Accelerated Bridge Construction on US 6 over Keg Creek
The need to standardize ABC technology for use nationwide brought together the Iowa DOT and the SHRP 2 project R04 project team to collaborate on this HfL demonstration project. Efforts also involved industry leaders and researchers from the Iowa State University to help develop national ABC standards.
The Iowa DOT chose the Keg Creek site because the three-span bridge configuration is typical throughout Iowa and other States, making the lessons learned from this project valuable to as many designers as possible. Incorporating a combination of precast elements and innovative materials into the bridge design and reducing the 6-month closure to 16 days showcased the viability of the ABC concept.
This project represents the first time in Iowa that steel girder/concrete deck modules were jointed on site with UHPC. Durability of UHPC made it possible to join the deck panels and open the bridge to traffic without an overlay otherwise required to protect the joints made from standard materials. Eliminating the overlay saved time during the accelerated construction schedule and helped keep the closure to a minimum. Even though the bridge deck was fully functional after opening to traffic, the bridge may be overlaid with asphalt sometime in the future to protect the joints from water and deicing salts. An overlay would also hide the bridge deck’s unusual appearance as a result of the many closure pours and lifting pocket pours.
The bid price for only the bridge portion of the project, excluding roadwork traffic control and drainage improvements, was $2.3 million, which is essentially double the estimated cost of a similar three-span conventional bridge. The Iowa DOT received a $400,000 HfL grant and a $250,000 grant from the SHRP 2 program to offset some of the additional costs incurred.
The project was located about 10 miles east of Council Bluffs, as indicated in Figure 1. Local geology was typical of western Iowa. Fine-grained soils surrounded the project with sands and gravel along Keg Creek and sedimentary bedrock at about 75 ft below the creek. Keg Creek flowed continuously at seasonal levels during the time of construction.
The average annual daily traffic (AADT) was 3,890 vehicles per day with 9 percent trucks in 2009 and is estimated to increase to 5,380 vehicles per day in 2029.
Figure 1. Map. Project location (source: Google Maps).
The existing 28-ft-wide by 180-ft-long three-span continuous concrete hunched girder bridge (FHWA # 043230) was constructed in 1953 and was classified as structurally deficient with sufficiency rating of 33. The replacement bridge has the same three-span configuration but is 47 ft wide by 210 ft long, consisting of a 70-ft interior span and two 67-ft, 3-inch end spans. The new and old bridge alignments were set at a zero skew. Figure 2 shows the deteriorated existing bridge, and Figure 3 shows the newly reconstructed bridge.
HNTB Corporation furnished the bridge design and the Iowa DOT provided construction engineering inspection. Godberson-Smith Construction was awarded a $2.3 million contract to reconstruct the bridge ($2.7 million total project) with the following requirements:
Originally, a 14-day closure was part of the contract however due to a survey error in the abutment piling and the addition of post-tension hardware retrofitting the closure lasted 16 days. Iowa DOT added 3 days to the 14 day closure to account for the additional post-tensioning retrofit work. Therefore the contractor was awarded one day of incentive pay ($22,000).
Although it was not fully detailed in the design plans, the contractor was allowed to propose a precast concrete modular alternative. The steel modular option was chosen based on early discussions with local contractors and fabricators.
Figure 2. Photos. Existing bridge.
Figure 3. Photos. Newly reconstructed bridge.
The major bridge elements above ground were precast on site using conventional construction equipment. The pier columns, pier caps, abutment walls, approach slabs, and modular superstructure sections were precast in a farm field converted to a temporary staging area adjacent to the bridge. Figure 4 shows the wood supports used in the staging area to support the steel/concrete modular deck panels during fabrication. These wood piles and beams were used in the staging area to create mock pier caps and abutments so the modular sections could be prefabricated in the planned bridge arrangement. This allowed the contractor to understand how the sections fit together at ground level in the staging area before setting them into place.
Figure 5 is a view during the simultaneous casting of the modular sections. Note the precast pier columns in the upper part of the image and the precast abutment elements just right of the pour.
Figure 4. Photo. Temporary supports used for the bridge modules during fabrication.
Figure 5. Photo. View during the modular section pour.
Figure 6 shows the modular section plan and cross section taken at an abutment location. Six rows of modular sections spanned the length of the bridge with 3 sections per row for a total of 18 sections.
Figure 6. Diagram. Modular section plan and cross section.
Exterior modules were cast with an integrated barrier wall, as shown in Figure 7, but were otherwise similar to the interior modules. Both types consisted of two parallel W30x99 steel beams topped with an 8.5-inch reinforced concrete deck. 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. Slight sag can be noticed in the bottom flange of the uncambered beams in Figure 3.
HPC was used to form all precast elements, including the modules. The joining edges of the concrete deck were concave with protruding hairpin reinforcing bars that overlapped when the modules were joined. Joint openings were typically 6 inches wide.
Figure 7. Diagram. Cross section of a typical exterior module.
Crews moved modules from the staging area to the bridge with a flat-bed semi-truck and set them into place using two cranes. Figure 8 shows an exterior module being loaded on the delivery truck. Figure 9 is an aerial view of the two cranes lifting an interior module into place. Most of the staging area was bare earth except for a riprap access road across the creek.
At this point in the accelerated construction schedule, the effect of a substantial rain could have turned the exposed ground to mud and negatively impacted the contractor’s ability to move the modules and the heavy precast substructure elements safely. Fortunately, dry weather prevailed during the ABC period.
Figure 8. Photo. Lifting an exterior module into place.
Figure 9. Photo. Aerial view showing placement of bridge modules.
The first stage of construction involved excavating the area under and around the bridge to accommodate equipment traffic and then installing 6-ft-diameter drilled shafts outside the existing bridge footprint. Figure 10 shows a shaft being drilled and workers installing reinforcing steel in one of the shafts. Once the drilled shafts were complete, the ABC portion (second stage) of the project began, at which time traffic was detoured and the existing bridge demolished.
Figure 10. Photos. Drilled shafts next to existing bridge (left) and workers positioning rebar.
Substructure work, such as assembling the precast columns, setting the precast pier cap beams, and setting the abutment elements, occurred in the second construction stage. Grouted splice couplers were used to make the connection between the drilled shaft and columns and pier caps (refer to Figure 11 and Figure 12). H-piles were driven to bedrock to support the precast abutment stems and wingwalls.
Figure 11. Diagram. Cap to column detail.
Figure 12. Diagram. Detail of the drilled shaft to column connection.
During this critical stage, a surveying error was discovered during the west abutment pile installation resulting in extra effort to keep the schedule on track while correcting the mistake. Once adjustments were made in response to the survey error (resulting in an abutment alignment offset of 4 inches) the pile pockets in the abutments and abutment closures were filled with high early strength SCC. The third construction stage involved assembling the modular superstructure, placing backfill and placing precast approach slabs.
Fully-Contained Flooded Granular Backfill
The backfill process began after the superstructure modules were set. The end modules incorporated a suspended abutment backwall designed with a step to support the approach slabs. Figure 13 shows the suspended backwall detail. Figure 14 shows the suspended backwall of an interior module in place over the abutment.
Figure 13. Diagram. Suspended backwall detail.
Figure 14. Photo. Suspended backwall of an interior module (source: HNTB Corp.).
Workers placed fully-contained flooded granular backfill behind the abutments. They used standard materials, and the backfilling procedure was specified in the plans directing the contractor to line the excavation and backwall with geotextile filter fabric, place a 4-inch subdrain at the toe of the rear of the excavation backslope, and then partially fill the excavation with porous material fill. The remaining work involved placing granular material in layers, as shown in Figure 15, surface flooding, as shown in Figure 16, and finally compacting the material with a vibrating compactor.
The contractor had positive experience placing the backfill on this project. Other bridge sites in Iowa where this backfill technique has been applied are currently being reviewed for evidence of any under-pavement void development or approach pavement distress.
Figure 15. Photo. Workers spread granular backfill behind an abutment.
Figure 16. Photo. Water is applied to consolidate the fully-contained flooded backfill.
Before the approach slabs were set in place, standard modified subbase material was placed on top of the backfill and leveled to receive the approach slabs. Precast sleeper slabs were used to support the approach panels. Figure 17 details the approach panel and sleeper slab plan.
Figure 17. Diagram. Approach slab cross section.
UHPC is characterized by its ability form strong bonds with compressive strengths up to 32,000 lb/in² and flexural strengths up to 7,000 lb/in². The material is not only strong, but ductile, in the sense that it can deform and support flexural and tensile loads even after initial cracking. These qualities, coupled with inherently high abrasion resistance similar to natural rock and superb impermeability, made UHPC the chosen material to join the modular superstructures.
Key to the material's ductility is the inclusion of steel fibers intended to arrest initial micro cracking while providing the capacity for the joint to flex under loading. The aggregates used in the mix were of uniform size to produce a compact arrangement of particles while providing space between the particles for the fibers. The fibers used in this project were brass-coated to aid in their manufacturing.
Both the longitudinal and pier joints were filled with UHPC. This demonstration project was the first time for using UHPC to form the critical moment-resisting joints positioned over the piers. UHPC was chosen partly because of its ability to achieve a high strength bond between the modules at such an important connection. A bonding agent was applied to the transverse panel edge in lieu of sand/water blasting. The contractor used a pressure washer to water blast all the precast joint surfaces as well. Figure 18 shows an example of the pier joint reinforcement.
Figure 18. Photo. Pier joint reinforcement.
The UHPC constituents were measured by hand and blended in an on-site mixer. Motorized buggies, like the one shown in Figure 19, were used to transport and pour the mix into the joints. The contractor made a custom wooden funnel to expedite placing the UHPC into the narrow joint.
Once mixed, the UHPC was fluid (similar to grout) and easily flowed into the joints and around crowded reinforcing steel. The fluid property of the mix challenged the contractor to contain the material at the intended depth in the joints because the mix would flow lengthwise in the joint, resulting in the UHPC dropping slightly below the top of the joint. In this situation, the surface of the UHPC would begin to harden before the next batch could be mixed and poured. The contractor's solution was to secure wood strips along both sides of a joint (seen in the lower left of Figure 19) and pour the UHPC about ¾ inches high, allowing for some slump and insuring the joints were not under filled. Additionally, temporary acrylic bulkheads were placed ahead of the pour to help contain the UHPC.
Figure 19. Photo. Workers place UHPC in a longitudinal joint.
As part of the design, the entire bridge deck was diamond ground to a maximum depth of ½ inch (making the effective deck thickness 8 inches), and any UHPC left too high in the joints was removed by the grinding.
Prior to construction, the contractor participated in mockup construction of the UHPC joints as the SHRP2 R04 project team conducted laboratory testing on full-scale transverse test joints at the Iowa State University (ISU) to verify the strength properties as well as handling characteristics of the fresh mix and the serviceability of the bridge joints. Strain gauges embedded in the test specimen and fixed to the surface measured strains during 1 million simulated service load cycles and during ultimate failure loading.
Although UHPC has the proven ability to penetrate the surface of cured concrete and create a strong bond, ISU’s test results6 submitted to HNTB suggested a lack of adequate bond performance at the interface between the UHPC joint material and the HPC deck, which could limit the joints’ ability to resist moment forces. As a precaution, and to prevent water and deicing chemicals from penetrating the interface between the UHPC and the bridge modules, the transverse joints over the piers were retrofitted with 1 inch diameter high-strength steel post-tensioning bars anchored to the upper web of the deck beams to limit strain levels at the joint.
Figure 20 illustrates the post-tension retrofit design, and Figure 21 show workers installing the hardware on an exterior module. Because the post-tensioning was added to prevent cracking of the HPC deck (a serviceability issue), the retrofit was not tested under cyclic loads for fatigue. The retrofitted connection was, however, tested to AASHTO Service Level II loading which is intended to represent a fatigue limit state.
Figure 20. Diagram. Post-tension retrofit design.
Figure 21. Photo. Post-tension retrofit installation.
6Laboratory Testing of Ultra High Performance Concrete Deck Joints for use in Accelerated Bridge Construction, 2011, Iowa State University.