Construction of a Precast Prestressed Concrete Pavement Demonstration Project on Interstate 57 Near Sikeston, Missouri
Chapter 3. Missouri Demonstration Project
The intent of the Missouri PPCP demonstration project was to familiarize the Missouri Department of Transportation (MoDOT) and local contractors with this new technology, while also further evaluating and refining, in another unique application, the precast pavement concept developed through the original FHWA feasibility study.(1)
The Missouri demonstration project was located on northbound I-57 approximately 16 km (10 mi) north of the I-55 interchange in Sikeston, as shown in Figure 6. This project was incorporated into a larger rehabilitation project for I-57, which included diamond grinding of the existing pavement, asphalt overlays and full-depth pavement replacement.
Figure 6. Illustration. Location of the Missouri PPCP demonstration project.
The existing pavement on I-57 was constructed in 1959 and consisted of a jointed reinforced concrete pavement, 200 mm (8 in.) thick with 18.7-m (61.5 ft) joint spacing over 100 mm (4 in.) of granular base. The existing pavement was 7.3 m (24 ft) wide with a 50-mm (2 in.) crown and asphalt-treated shoulders. Longitudinal edge drains had been added to the pavement at a later date.
Demonstration Project Goals
The primary goal of this demonstration project was to provide MoDOT an opportunity to evaluate PPCP technology as a solution for rapid pavement construction and rehabilitation. As such, this initial project was constructed on a rural section of interstate pavement that could be closed to traffic throughout the duration of construction, ensuring that any problems encountered would not significantly impact traffic on this critical section of I-57. Although PPCP is intended for use in urban areas where construction impacts on traffic must be minimized, this project permitted MoDOT an opportunity to evaluate and refine the construction process in a less critical environment first.
This project also permitted MoDOT to evaluate PPCP "side by side" with a conventional jointed concrete pavement constructed just south of the precast pavement section at the same time. Furthermore, this section of I-57 carries a significant amount of truck traffic, which will provide a good test of long-term pavement performance.
A secondary goal of this demonstration project was to help familiarize local contractors with PPCP technology. Precast pavement is a unique product for precast producers to produce and for highway contractors to install. If precast pavement technology is ever to be embraced by the precast and highway construction industries, they must have the opportunity to become familiar with the technology first, before it is required for time-critical urban applications.
As with any project utilizing new technology that is experimental in nature, coordination with all parties involved throughout the project is essential. To get precast producers involved early in the process, several meetings were held with MoDOT and regional precast producers during the project development stage. These meetings were used to not only give the producers a better understanding of precast pavement technology, but to also solicit their input in decision-making for design and construction details. As a result, many of the unknowns were addressed before the bidding process began, likely resulting in lower bids for the project.
In addition to the meetings during the project development stage, a pre-construction meeting was held with both the precast producer and installation contractor. This meeting was used to address any concerns the producer or installation contractor had as well as to emphasize the importance of the project to the contractors.
The layout for the demonstration project was dictated by the existing pavement on I-57. The length of the precast pavement section was governed by the unit bid cost for the project and the available funding for construction.
For this initial demonstration project, a tangent section of I-57 with essentially no change in vertical curvature was selected. While complex geometries with superelevations are anticipated for future projects, a simple geometry permitted MoDOT to focus on overall construction details and procedures for this first project.
The pavement cross section was one of the more complex aspects of the project, with a "crowned" pavement cross section required to match the existing pavement. The original pavement section consisted of a concrete pavement, 7.3 m (24 ft) wide with asphalt shoulders. Based on current design standards, however, MoDOT elected to utilize integral shoulders for the precast pavement section. The resulting precast pavement section was 11.6 m (38 ft) wide, including two traffic lanes, each 3.7 m (12 ft) wide, an inside shoulder 1.2 m (4 ft) wide, and an outside shoulder 3 m (10 ft) wide.
Three alternatives were considered for achieving the 11.6-m (38 ft) pavement width with crowned cross slope:
- Partial-width panels with a uniform thickness: Two sections of panels (one section 4.9 m [16 ft] wide and the other 6.7 m [22 ft] wide) tied together with transverse post-tensioning with the longitudinal joint located at the pavement crown. The underlying base would be trimmed to the crowned cross section.
- Full-width panels with uniform thickness: Single panels 11.6 m (38 ft) wide with the crown cast into the panel. The underlying base would be trimmed to the crowned cross section.
- Full-width panels with variable thickness: Single panels 11.6 m (38 ft) wide with a flat bottom and variable thickness to achieve the crowned cross section. The underlying base would be trimmed to a uniform horizontal grade.
The first alternative was ruled out due to the additional expense of fabricating and installing twice the number of panels and additional transverse post-tensioning. However, if lane-by-lane construction had been required for the project, this option would have been the most viable alternative. Alternative 2 was ruled out due to the impracticality of fabricating a "bent" precast panel and grading the base material to such a tolerance that it would properly support the precast panels.
Alternative 3 was selected as the most practical solution for achieving both integral concrete shoulders and the necessary pavement crown. From a fabrication standpoint, casting panels with a flat bottom and sloping (crowned) surface is easily achievable. From a construction standpoint, grading the base to a uniform, horizontal profile is more viable than grading to a cross slope, and installing a single precast panel to achieve the full pavement width is more efficient than using two or more panels.
To match the existing pavement's 200-mm (8 in.) thickness at either end of the precast pavement section, the thickness of the precast panels was specified as a minimum of 200 mm (8 in.) for the traffic lanes. To achieve the 2 percent crown in the pavement surface, the thickness was varied from 276 mm (10 7/8 in.) at the peak of the crown down to 178 mm (7 in.) at the edge of the inside shoulder and 143 mm (5 5/8 in.) at the edge of the outside shoulder, as shown in Figure 7.
Figure 7. Illustration. Cross section of the precast panels, incorporating the change in cross slope into the surface of the panel. (Note: 1 in. = 25.4 mm)
Figure 8 shows a profile view of the precast panels at the fabrication plant, showing the variable thickness panel. Because of the variable thickness of the panels and the need for the keyways to be parallel to the flat bottom of the panels, it was not possible to extend the keyways across the full width of the panels. As a minimum requirement, the keyways were maintained across both traffic lanes and extended 0.3 m (1 ft) into each shoulder to ensure load transfer from the keyways within the traffic lanes.
Figure 8. Photo. Precast panels at the fabrication plant after removal from the forms showing the variable thickness cross section.
During the design process (described in Chapter 4), the amount of horizontal slab movement and the resulting expansion joint widths during daily and seasonal temperature cycles were estimated. It was determined that a 76.2-m (250 ft) "standard" post-tensioned slab length (between expansion joints) would meet the necessary criteria for maximum expansion joint width for the prevailing site conditions. Considering the available funding for the demonstration project, it was determined that 305 m (1,010 ft), four, of these standard post-tensioned slabs could be constructed.