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Construction of the Iowa Highway 60 Precast Prestressed Concrete Pavement Bridge Approach Slab Demonstration Project
Chapter 3. Iowa Highway 60 Demonstration Project
The intent of the Highway 60 demonstration project was to evaluate the viability of the PPCP concept for bridge approach slab applications. Although the ultimate goal is to utilize PPCP for rapid reconstruction of existing approach slabs in urban areas, with minimal lane closures, this initial project used PPCP for construction of the approach slabs on a new bridge that was closed to traffic during construction.
The Iowa demonstration project was located on a newly constructed section of northbound Highway 60 east of Sheldon, Iowa, as shown in figure 2. This project was incorporated into a larger project to realign Highway 60 around the city of Sheldon. The PPCP approach slabs were constructed at either end of the northbound bridge over Floyd River. The twin bridge for the southbound lanes was constructed with conventional cast-in-place approach slabs.
Figure 2. Illustration. Location of the Iowa precast pavement demonstration project.
Demonstration Project Goals
The overreaching goal of this demonstration project was to evaluate PPCP for bridge approach slab construction, with the intent that it will eventually be used for urban applications under stringent time constraints. This evaluation included all aspects of the design and construction processes, including the design details, fabrication, and installation operations. Because this project was constructed on a section of roadway closed to traffic, it provided an opportunity to carefully evaluate the construction process to identify refinements that can be made to both the design details and construction requirements for future projects.
Partnering and Project Coordination
As with any project utilizing a construction technique for the first time, coordination between all parties throughout the project is essential. For the Highway 60 project, IADOT designers and FHWA contractors worked closely together to develop a conceptual precast panel layout and initial design. These conceptual ideas were then used to solicit rough cost estimates from precasters. Once a precaster was identified, the project team worked closely with the firm to develop the precast panel details to ensure a viable solution. The designers continued to work closely with the precaster throughout the fabrication process and worked closely with the installation contractor throughout the construction of the project.
Figure 3 shows the Situation Plan for the Highway 60 bridge, including the layout of the precast panels. The specific details of the project site and the process by which the precast panel layout was selected are discussed below.
A tangent section of Highway 60 was selected for the location of the demonstration project. Although roadways with horizontal curves and superelevations may eventually be encountered, this project location allowed for evaluation of the overall process on a less complex pavement section. The roadway had a “rooftop” crown with a 2 percent cross slope on either side of the pavement centerline. This required a precast panel layout that could accommodate a crowned pavement section, common in Iowa.
Integral Bridge Abutment
The Highway 60 bridge was designed with an integral abutment. This type of bridge abutment moves horizontally with the expansion and contraction movement of the bridge itself. Normally, an expansion joint is provided between the abutment and approach slab to accommodate this movement. However, research by IADOT has found this expansion joint to be a source of water infiltration into the underlying embankment, which can lead to consolidation or erosion of the embankment material.(13)
A new approach slab detail being implemented by IADOT ties the approach slab to the abutment so that the approach slab moves with the abutment, shifting the expansion joint out to the end of the approach slab. This plan requires consideration of the length of the approach slab not only to optimize how far the expansion joint is moved away from the abutment, but also what length of pavement could feasibly be “pushed” and “pulled” by the abutment without causing excessive stresses in the bridge structure.
Precast Panel Layout Options
One of the underlying goals of this demonstration project was to develop a solution that is both practical from a fabrication and construction standpoint and adaptable to various approach slab configurations. Three options were considered for the precast panel layout to accommodate the approach slab characteristics listed above. These options were based primarily on precast panels that were used successfully on previous demonstration projects. The two preliminary alternate solutions are described below.
Skewed full-width panels with variable thickness
The first precast panel layout considered uses skewed full-width panels with variable thickness. Full-width panels have been used successfully in three previous demonstration projects in Texas, California, and Missouri,(2,3,6) and provide an efficient solution in terms of fabrication and installation as only half the number of panels are needed as compared to partial-width construction. Additionally, full-width panels with variable thickness were used successfully in California and Missouri to achieve the necessary change in cross slope in the pavement surface. Figure 4 shows a schematic of the layout of skewed full-width panels with variable thickness.
Full-width, variable-thickness panels presented several drawbacks for application on this project. First, the paving notch on the bridge abutment would have required modification since it was constructed with a crowned cross section and uniform 305-mm (12 in.) depth. A level (horizontal) paving notch would have been necessary for full-width panels with variable thickness. Paving notch modification for future rehabilitation projects with crowned paving notches will likely not be possible. Second, the complexity of fabricating full-width panels with both skewed edges and variable thickness would likely have significantly increased the cost of the panels. Finally, full-width panels would not have demonstrated single-lane (or lane-by-lane) construction. While single-lane construction was not required for the Highway 60 project, it will likely be required for future projects in urban areas to allow for traffic flow on the remaining lanes.
Skewed partial-width panels with uniform thickness
The second alternative precast panel layout considered uses skewed partial-width panels with a uniform thickness. Partial-width panels permit single-lane (lane-by-lane) construction, which will be important for future projects where construction staging will likely permit the closure of only one lane at a time. The use of panels with a uniform thickness also greatly simplifies the fabrication process over variable thickness panels. Figure 5 shows a schematic layout of this alternative.
The primary drawback to this alternative is the increased complexity of panel installation. With this layout, both the transverse and longitudinal post-tensioning tendons cross joints between adjacent panels at nonperpendicular angles, as shown in figure 5. During the stressing operation, this could cause horizontal slip in the joints, offsetting the panels from each other. To counter this slip force, additional “slip pins” would be required at all of the joints between panels, increasing the complexity of the panel fabrication and assembly processes. An additional drawback is the complexity of fabricating these panels. The acute angles at the corners of the panels would have required very strict dimensional tolerances on the panels to ensure that both transverse and longitudinal joints align properly.
The final selected precast panel layout consists of partial-width panels with a uniform thickness, as shown in figures 3 and 6. The total length of the approach slab layout is approximately 23 m (77 ft) at the pavement centerline. With this layout, the skew at the bridge abutment is “removed” with the first panel, transitioning to perpendicular joints and square panels for the remainder of the approach slab. This eliminates potential horizontal slip problems during post-tensioning. This solution also greatly simplifies the fabrication process as the majority of the precast panels are square, with only two trapezoidal panels required for each approach slab. The uniform panel thickness also simplifies fabrication and does not require modification of the paving notch.
Perhaps most importantly, this panel layout provides a great deal of flexibility in accommodating different bridge characteristics, such as variations in skew angle and approach slab width, length, and thickness. Partial-width panels will also permit single-lane construction for future applications.
Figure 7 shows the final precast panel assembly. The first two panels resting on the paving notch are pinned to the abutment with dowels drilled and grouted into the paving notch. This causes the approach slab to move with the bridge abutment during seasonal temperature cycles. At the far end of the approach slab, a standard IADOT “EF” expansion joint(14) is constructed between the approach slab and adjoining pavement.
The longitudinal joint at the centerline of the approach slab is a keyed grouted joint, as shown in figure 8, which is filled prior to completion of transverse post-tensioning. This type of joint provides tolerance for slight misalignment of the precast panels, and does not require a perfect fit between panels at the longitudinal joint.
The panel layout consists of essentially three types of panels. The “abutment panels,” shown in figure 9, transition the joint in the approach slab from skewed to perpendicular. These panels have a variable length that can be adjusted based on the bridge skew. The panels have keyways cast into the mating edge of the panel. The panels also have anchor access pockets and paving notch anchor pin sleeves cast into them. The pockets provide access to all of the longitudinal post-tensioning anchors and the transverse post-tensioning anchors in the skewed section of the pavement (figure 6). The anchor pin sleeves, 50 mm (2 in.) in diameter, receive dowels that anchor the approach slab to the abutment. The length of the abutment panels varies from 2.7 m (8 ft 10 in.) to 5.2 m (16 ft 11 in.) for the smaller of the two panels and 5.2 m (16 ft 11 in.) to 7.6 m (25 ft) for the larger of the two panels (see figure 6). The abutment panels are all 4.3 m (14 ft) wide and 305 mm (12 in.) thick.
“Base panels,” shown in figure 10, are the “standard” panels that make up the majority of the approach slab. Keyways are cast into the mating edges of the panels, and the transverse post-tensioning anchors are cast into the outside edges of the panels. Base panels for the Highway 60 project were 6.1 m (20 ft) long, 4.3 m (14 ft) wide, and 305 mm (12 in.) thick.
Finally, the “joint panels” located at the end of the approach slab provide the transition to the adjoining cast-in-place pavement. Sleeves for the dowels used in the EF joint at the end of the approach slab are cast into the ends of these panels, as shown in figure 11. The live end post-tensioning anchors for the longitudinal tendons are cast into the ends of these panels as well.