Dealing with a full-deck

June 20, 2005

A “fast-track” method of deck replacement has recently been completed in southern Missouri. The concept of full-deck replacement while maintaining traffic (in both directions on a two-lane bridge) during the daytime was proven successful on the Rte. 64 Bridge over Pomme de Terre Lake. (A few temporary lane closures were required to complete some minor construction tasks.)

A “fast-track” method of deck replacement has recently been completed in southern Missouri. The concept of full-deck replacement while maintaining traffic (in both directions on a two-lane bridge) during the daytime was proven successful on the Rte. 64 Bridge over Pomme de Terre Lake. (A few temporary lane closures were required to complete some minor construction tasks.)

This 1,684-ft-long continuous composite steel-rolled beam bridge was redecked in only six months with the bridge being shut down during each night of construction from 7 p.m. to 7 a.m. The key to this innovative method is match-cast panels that are post-tensioned together after installation. The full-width and full-depth panels are supported by four girders and provide a roadway width of 24 ft 10 in.

The bridge over Lake Pomme de Terre was originally designed and built by the Corps of Engineers in 1962. The bridge roadway is 22 ft wide and carries two 11-ft lanes. The superstructure consists of four composite continuous steel-plate girders. There are 17 90-ft spans and end spans of 76 ft 10 in.

The superstructure comprises five units (a typical unit is 360 ft long) that are joined by a pin and hanger connection located 18 ft from the pier. The grade of the bridge is perfectly level (0% grade). The substructure consists of two column bents with spread footings. Because of the large pier heights, the superstructure girders are post-tensioned to the pier cap beams.

Although the plate girders and substructure were in good condition, the 6-in. composite deck was rated deficient by the owner, the Missouri Department of Transportation (MoDOT), because of advanced deterioration brought on by deicing salts. Because the bridge connects the towns of Pittsburgh and Nemo, and the length of the nearest detour route was 28 miles, MoDOT deemed it essential that the bridge remain in service during construction. Additionally, the local economy of these towns is very reliant on the tourism industry, and most of their income is generated during the summer months.

Assembly by moonlight

To replace the deck, the initial choice was to use the typical method of cast-in-place stage construction. During the analysis of the existing deck for stage construction, it was determined that the interior deck cantilever overhang did not have sufficient strength to allow emergency vehicles or school buses. Another major concern was that the lane width during construction would only be 9 ft 6 in. on the bridge during Stage I traffic.

At this point in the project MoDOT District Maintenance Engineer Dave O’Connor proposed using some type of deck replacement that could be done in overnight closures. HNTB then studied various types of deck replacement schemes and recommended using full-width full-depth precast panels that are post-tensioned together longitudinally. The safety barrier curbs also would be cast with the segments in the casting yard. Additionally, a 1.5-in. overlay would provide a smooth riding surface and an extra layer of protection at the joints from chloride penetration.

The significant advantage of using the full-width full-depth panels is that they could be cast offsite ahead of time and then be brought to the jobsite for placement during the nightly closures. Match casting and post-tensioning of the panels was provided to eliminate closure pour joints in the deck. On other bridges using precast panels, closure joints had been found to be vulnerable to chloride penetration and thus deterioration. Figure 1 shows the typical cross-section with the locations of the post-tensioning bars.

To provide the superstructure with sufficient capacity, the deck was required to be composite (consistent with the original design). To accomplish this, 9-in. x 12-in. blockouts were provided in the panels to allow for shear connector installation on the girders after the panels were placed. See Figure 2 for details of a typical panel. Also note the partial-depth blockouts provided for the post-tensioning bar couplers.

The full panel size was 10 ft long and 27 ft 6 in. wide, which was dictated by shipping considerations. The panels that were placed directly over the pier locations required extra consideration because the post-tensioning bars connecting the girders to the pier extended into the deck. The solution to this problem was to provide a larger blockout (approximately 2 ft 4 in. x 3 ft 3 in.) in those locations. Because of these large blockouts, the shipping and handling stresses became very important in the design of these panels.

To fill the haunches and blockouts for the shear connectors and post-tensioning couplers, a flowable grout was required that would reach a compressive strength of 2,500 psi before traffic could be allowed on the bridge. After discussing this requirement with several concrete suppliers, several products were found that could provide this strength in as little as two hours.

Plan slow, work fast

The project was let on March 19, 2004, and Columbia Curb & Gutter (CCG) was awarded the contract based on low bid. Notice to proceed was issued on May 17, 2004, and CCG was allowed to have the first nightly bridge closure on June 21, 2004. The last panel was placed on Aug. 31, 2004.

For the overnight replacement concept to be successful, the contractor needed to have a reasonable amount of time to complete the work. Before the project was bid, HNTB and MoDOT determined a bridge closure window of 7 p.m. to 7 a.m. (with a work week of Sunday night through Thursday morning) and this was specified in the contract documents.

The HNTB design team had determined that it would be feasible (and demanding) to remove a significant amount of deck and replace it during the closure window. This inventive concept required many operations to occur in one night, including:

  • Remove the existing deck, including shear connectors;
  • Clean and prime the top flange of the steel girders;
  • Place and align the full-depth match-cast panels;
  • Connect post-tensioning bars;
  • Post-tension the panels together;
  • Weld new shear connectors; and
  • Fill the shear connector blockouts with grout.

The original concept was to perform all of the operations listed above in the 12-hour construction window, but CCG decided to split the operations into a two-step process that overlapped on consecutive nights. For a typical location, the deck removal and preparation of the top flange were performed on one night and then the panels were installed at this location the following night.

In order to accomplish this, the contractor fabricated a set of temporary bridge panels (consisting of steel grating and W sections) that could be reused each night. The contractor also proposed using a temporary barrier system utilizing part of the existing bridge barrier rail so that the permanent safety barrier curb could be slip-formed at the end of the project.

For the first few nightly closures, the contractor only placed two or three of the 10-ft-long panels. After the initial learning curve was over, however, the contractor was able to place five to seven panels on a given night. Since the connections of the existing vertical post-tensioning at the piers were required to be inspected, these locations impeded the contractor’s progress slightly. Additionally, when the contractor reached an expansion joint location, the panels were dimensioned to be a few feet from each side of the joint. At these locations, the contractor would “skip” over the joint and place the panels to start the next unit. After construction progressed into the next span during one of the nightly closures the new expansion joint was placed and a small area of deck poured. This pour also was required to reach a compressive strength of 2,500 psi before being opened to traffic.

After all of the deck panels were placed, a typical stage construction sequence with only one lane of the bridge open to traffic was set up to allow for slip-forming of the safety barrier curb followed by placement of the 1.5-in.-thick overlay on each half of the deck. By contract, the stage construction portion of the project could not start until after Labor Day (the traditional end of the tourism season). The total stage construction time with a median divider on this project was only seven weeks.

Stacking up the dollars

During the initial study, cost implications of using the full-depth panels were considered. The initial thought was that this concept would cost slightly more than conventional stage construction, but would be offset by reduced traffic-control costs. The low bid for this job was only about 9% higher than the original estimate, which included stage construction. (This project also included repainting the superstructure and retrofitting the pin and hanger connections.)

By looking at some other projects that were primarily deck replacements (see Table 1), the costs can be compared for the Pomme de Terre bridge. (Except for the Franklin County bridge over the Missouri River, all figures have been converted to 2005 dollars.)

Although the costs are highest for the Pomme de Terre bridge, the deck removal costs reflect the complexity of each project. This project has the highest unit cost, largely due to the fact that 90% of the work had to occur over water. Another important item that is not considered above is the benefit of reduced traffic-control costs.

The innovative method of using full-width full-depth precast panels proved to be cost-competitive with stage construction and beneficial for minimizing the disruption to the traveling public and local economies. The benefits to the local community included minimizing the disturbance on tourism, maintaining emergency vehicle and school bus access during the day and reducing the duration of construction from two years of daytime work to 2 1?2 months of overnight work. This “fast-track” concept could be further extended to replacing bridge decks in areas with high traffic-control costs, bridges with skewed supports or on larger bridge rehabilitation projects where construction time is a significant factor.

About The Author: Blakemore is a project engineer at HNTB Corp. Desai is an associate vice president at HNTB. Foster is a structural liaison engineer at the Missouri Department of Transportation.

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