Case Study 2: Thin Whitetopping in Illinois: Highway 4, Piatt County

Introduction and Design

In 2000, the Illinois Department of Transportation (IDOT) rehabilitated a 4.94-mi (8-km) section of Highway 4 in Piatt County using a 5-inch (127 mm) overlay of an existing asphalt pavement. The existing asphalt, after milling, was 4 inches (102 mm) thick and rested on a cement-treated base. The overlay was constructed using two different panel sizes: 5.5 ft by 5.5 ft (1.7 m by 1.7 m) and 11 by 11 ft (3.4 by 3.4 m) with skewed transverse joints. The traffic volume as of 2013 was 2,150 vehicles per day, which included 7.2 percent heavy commercial truck traffic (King and Roesler 2013).

Section Performance

Winkelman (2005) reported that the section experienced very little cracking in its first 4 years of service: 0.2 percent of the 5.5-ft (1.7 m) panels were cracked, whereas 1.0 percent of the 11-ft (3.4 m) panels had cracked. It was noted that many of these reported cracked panels were in fact “sympathy” cracks: that is, cracks induced by neighboring distressed panels or neighboring distresses in the shoulder. For this project, the construction of a driveway in 2003 induced cracking in neighboring panels.

King and Roesler (2013) later summarized a condition survey by IDOT, which found that after 8 years of service the section experienced severe faulting. The observed cracking was found to be longitudinal cracking and corner breaks. During the final survey in 2012, IDOT observed that 1.4 percent of 5.5-ft (1.7 m) panels were cracked, whereas 17.8 percent of the 11-ft (3.4 m) panels had cracked. However, the extent of cracking in the 11-ft (3.4 m) panels may have been as much a local issue as a global one: King and Roesler (2013) note that over half of the reported cracking for the 11-ft (3.4 m) panels is restricted to a local portion of the total project, and this portion of the roadway was slated for rehabilitation as of 2013.

Conclusions

The extent of this case study provides a good basis for comparing the effects of panel size. Both the 4-year and 8-year reports note that the smaller panel size was associated with significantly less panel cracking. However, the correlation between cracking and panel size may be difficult to establish, due to the use of skewed joints. Furthermore, the fact that the majority of cracking was found to be a local phenomenon in this case study points to the importance of pre-overlay repairs and preparation.