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Construction of the California Precast Concrete Pavement Demonstration Project
Chapter 3. California Demonstration Project
The intent of the California precast pavement demonstration project was to further evaluate and refine the precast pavement concept developed through the FHWA feasibility study(1) and to familiarize Caltrans and its contractors with this new technology. The goal of this demonstration project was to construct the test section under realistic time constraints on the main lanes of a major freeway.
The location selected by Caltrans for the demonstration project was a portion of a 5.1-km (3.2 mi) section of I-10, which was being widened from 8 lanes to 10 lanes to accommodate new high occupancy vehicle lanes. The location of the actual precast pavement test section is on eastbound I-10 just west of the Meeker Avenue overpass, approximately 3.2 km (2 mi) west of the San Gabriel River Freeway (I-605) in El Monte. The map in figure 7 shows the test section location.
Due to the small-scale nature of this project, Caltrans issued a change order to incorporate precast pavement into the existing I-10 widening project. The existing plans at the test section location called for widening the existing pavement by 11.2 m (37 ft), including 8.2 m (27 ft) of main lanes and 3.0 m (10 ft) of shoulder, as shown in figure 8. The existing pavement design called for 250 mm (10 in.) of jointed plain concrete pavement (JPCP) over 150 mm (6 in.) of lean concrete base (LCB) over 220 mm (8.5 in.) of Class 3 aggregate base over the retaining wall embankment fill. For the change order, the 250 mm (10 in.) of JPCP was replaced with precast prestressed concrete pavement. It should be noted from figure 8 that the precast pavement was installed between fixed structures: the existing pavement and a new retaining/sound wall. This position required strict tolerances on the precast panels to ensure they would fit properly.
The layout for the precast pavement demonstration project was dictated by the existing plans for the widening project. The goal of this demonstration project was simply to replace the original pavement design with precast concrete pavement—not to develop a new pavement structure and geometry for the precast pavement.
A section with a “simple” geometry was selected for the precast pavement demonstration project. While pavements with complex geometry (i.e., superelevations) will eventually be encountered, the goal of this initial project in California was to work out the details of precast pavement fabrication and construction on a less complex project first. The selected test site had no change in vertical curvature and very minimal horizontal curvature. To ensure that the precast panels would fit flush against the existing pavement, this slight horizontal curvature was removed by sawcutting a straight edge onto the existing pavement over the length of the precast pavement test section.
While the longitudinal geometry of the test section was simple, the transverse geometry was more complex, with a change in cross slope from 1.5 percent in the traffic lanes to 5 percent in the shoulder, as shown in figure 8. It was therefore necessary to incorporate this change in cross slope into the precast panels, as discussed below.
As shown in figure 8, the original plans called for widening the existing pavement by 11.2 m (37 ft), with 8.2 m (27 ft) of main lanes (traffic lanes) and 3 m (10 ft) of shoulder. Two alternatives were presented to Caltrans:
Due to the additional expense of the second option, requiring more panels to be fabricated and the addition of transverse post-tensioning, Caltrans selected the first option. To cast panels with a “flat bottom,” however, the cross slope of the LCB beneath the precast panels was changed to a uniform cross slope of 1.75 percent, rather than a range of 1.5 percent to 5 percent. While this resulted in an odd cross section for the precast panels, with 250-mm (10 in.) thickness at either end and 330-mm (13.1 in.) thickness at the edge of the traffic lanes, it greatly simplified the fabrication of the panels and installation over the LCB. Figure 9 shows the final cross section for the precast panels, which incorporate the cross slope change in the surface. Figure 10 shows one of the precast panels after fabrication.
The length of the demonstration project was contingent upon the funding available for construction. Based upon estimates from the fabricator and installation contractor, the total project length was limited to approximately 76 m (250 ft). The width of each precast panel was limited by transportation considerations. Although panel widths of 3 m (10 ft) or 3.7 m (12 ft) could have been fabricated, loads wider than 2.4 m (8 ft) required special permits for transportation. Therefore, panel width was limited to 2.4 m (8 ft). The total project length was 75.6 m (248 ft), or 31 panels. Although a single, post-tensioned slab could have been used for the full project length, it was decided to construct two slabs, each 37.8 m (124 ft) long. Each slab consisted of 12 base panels, 2 central stressing panels, and one-half of a joint panel at each end. The remaining half of each joint panel was tied into the existing pavement at either end of the test section.
Project coordination was a crucial aspect of this demonstration project. Because it was experimental in nature, Caltrans, the fabricator, and the installation contractor had no experience with this type of project. Prior experience by the design team with the Texas pilot project was very beneficial for the successful completion of the project. Flexibility on the part of Caltrans in meshing precast concrete and concrete pavement specifications was also very beneficial.
Several meetings were held during the development stage with the designers, Caltrans, precast fabricator, post-tensioning supplier, and installation contractor. Caltrans and the designers were involved throughout the fabrication and installation process. A pre-construction meeting involving all parties was held just before on-site installation began.