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North Dakota Demonstration Project: Whitetopping on U.S. 2 West of Rugby
The purpose of this project was to improve the ride, improve the safety, and reduce the maintenance costs for 8.24 mi (13.26 km) of westbound U.S. 2 west of Rugby (figure 1). The original HMA road was a four-lane divided highway with limited access and no bridges within the project limits. Construction began in spring 2008, and the paving was completed and open to traffic in November 2008. The bid construction cost was $7,670,203 of which the Highways for LIFE grant was $1 million or about 13 percent of the project cost.
Figure 1 . Project location. (Source: Google Maps)
Ride and distress ratings of this interregional highway were poor and the maintenance costs were dramatically increasing with time. Distresses such as rutting, alligator cracking, and severe transverse and longitudinal cracking were present over the length of the project. Figure 2 shows the original distressed pavement surface and a closeup of the surface texture.
Figure 2 . Project location. (Source: Google Maps)
Most of the existing pavement in this portion of U.S. 2 was 6 in of HMA over stabilized gravel base or plain aggregate base. Shoulders on the existing roadway were steeply sloped at 4:1. Figure 3 details the existing pavement sections. Several median crossovers are located within the project limits for crossroads and access to farms. The existing crossovers did not have turn lanes and offered only minimum queue storage area. The new design called for turn lanes and wider median crossovers, which will improve future traffic safety. The shoulders were reshaped from 4:1 to 6:1 to decrease the chance of vehicle rollover in the event of a roadway departure incident. Safety was improved not only by reshaping the embankment slopes and installing turn lanes, but also by eliminating hazardous rutting in the wheel paths with a new non-rutting concrete surface.
Figure 3 . Existing pavement sections.
New construction consisted of milling about 1 in of existing asphalt surface from both westbound lanes and overlaying the milled surface with 7 in of PCC pavement. This whitetopping paving method was the main innovation on this project. Figure 4 shows the rough milled surface of U.S. 2 before the whitetopping was placed. The asphalt material left over from the milling operations was recycled as base material for the new shoulders.
Figure 4 . Rough-milled surface of the original HMA pavement.
A key construction detail is using the pavement milling machine to create a rough surface on the existing HMA to facilitate bonding between the HMA and PCC layers. This bond is crucial because it allows the layers to flex together under traffic loading. The bonded pavement layers optimize the material properties by placing the upper layer of concrete in compression and the lower layer of asphalt in tension. The optimized pavement structure reduces the need for a thicker PCC, which would be required if the asphalt was removed.
The bond between the asphalt and concrete is relied on to hold the concrete panels together. As a result, it eliminates the need for dowel bars to provide load transfer across transverse joints and tie bars to keep the longitudinal joints closed. The lack of steel lowers the construction cost compared with conventional concrete pavements. The absence of steel also simplifies the construction process and will make future panel repairs easier and less expensive. Recycling the pavement at the end of its service life will also be simplified because the need to separate the steel from the concrete is eliminated.
Transverse joints were made with a single sawcut at 7-foot (ft) intervals, and longitudinal joints were sawcut at 7 ft for the driving lane and 6.5 ft for the passing lane. Joint sealant was not used because little movement is expected from the thermal expansion of the small-size panels. Figures 5 and 6 show details of the new pavement section and the joint detail. Paving was done late in the construction season, so large temperature swings raised concerns about early-age cracking. The contractor paid close attention to sawcut timing and made relief cuts during the first few days of paving to keep uncontrolled cracking in check.
Figure 5 . New pavement section.
Figure 6 . Joint detail.
Whitetopping exists on one short test section in North Dakota, but a full whitetopping project had never before been constructed in the State. This project served as a full-scale trial to introduce the whitetopping innovation to practitioners and builders in North Dakota. Figures 7 and 8 show the finished pavement. Figure 8 provides a close look at the longitudinally tined surface texture and the sawcut joints.
Figure 7 . Completed whitetopped pavement.
Figure 8 . Closeup of the completed whitetopping showing the longitudinally tined surface texture and sawcut joints.
Whitetopping, rather than a conventional asphalt overlay, was chosen for this project for several practical reasons, which are outlined in the American Concrete Pavement Association (ACPA) Engineering Bulletin Whitetopping—State of the Practice (EB210P,1998) and presented in table 1.
NDDOT developed new specifications for this whitetopping project, using whitetopping specifications from Colorado, Illinois, Iowa, and Michigan as a basis.
A large portion of the project success stems from the use of full lane closure to route westbound and eastbound traffic head to head on the eastbound lanes, similar to interstate median crossover work zones. By closing both lanes at once, the contractor was able to complete paving in one pass rather than the two used in staged construction, reducing paving time. Moreover, separating live traffic from the paving operations greatly enhanced safety for workers and the traveling public. The safety and construction congestion benefits of the full lane closure are quantified in the next section of this report.