New pavement approaches

At the tactical level, the city is stretching its pavement management budget by changing the mill-and-fill specification for its 2012 arterial resurfacing program.

ChicagoDOT fields up to three asphalt paving crews and as many as 30 concrete crews at peak demand times, with most of the asphalt work directed at residential streets and alleys, and much of the concrete work focused on sidewalks and ADA (American Disabilities Act) ramp improvements.

Chicago’s traditional approach called for milling 2-1⁄2-inches deep, then filling with a special 1-inch binder course, and a 1-1⁄2 -inch surface course. Department engineers refer to the binder course as a “IL 4.75 sand mix,” a very dense blend of manufactured fine aggregate and a high ratio of performance-grade, polymer-modified asphalt. “It reduces the pavement’s permeability and it reduces reflective cracking,” explains Williams. “But it’s very expensive.”

To get more lane miles of resurfacing from its 2012 budget, the city is going to a conventional single-lift mill-and-fill design.

If that seems like a step backward, keep in mind that some of the mix designs the city is employing are anything but traditional. In particular, Chicago is one of Illinois’ most aggressive road departments in embracing Illinois DOT’s mix designs for high-recycle content in its asphalt pavements. The state now has specifications allowing up to 40 percent of the liquid asphalt binder in a mix to be “replaced” with the residual asphalt binder found in recycled materials. To achieve the highest amount of recycled material allowable a mix design must incorporate both recycled asphalt shingles (RAS) and fractionated reclaimed asphalt pavement (FRAP).The recycled material can include up to 7-½ percent RAS as a portion of the liquid asphalt, and to achieve the highest RAP content the mix designs require utilizing fractionated RAP.

“We have a huge supply of RAP and RAS available in this area,” says Williams, “and we see this spec as an opportunity to be more cost-efficient and greener at the same time.” The FRAP specification includes a PG 58-28 liquid asphalt to provide a longer lasting mix and enhance the workability of the mix.

“Ultimately,” Williams explains, “the new IDOT specification encourages multiple sources of recycled content and it increases the potential amount of recycled content, as well as the quality and consistency of the mixtures. It gives the contractors and producers options. And by allowing multiple sources of ingredient materials, the end product is not held hostage to variations in any one component.”

Looking for concrete solutions

While the department is emphasizing pavement maintenance and preservation in these difficult times, the city’s engineers are also exploring options for reconstruction and replacement. Some of Chicago’s low-traffic volume pavements are decades past their design lives, Williams notes, so reconstruction is sometimes an unavoidable option.

For better or worse the city has traditionally favored composite pavements — concrete with an asphalt overlay — but they are exploring a variety of options for dealing with the rigorous climate and traffic demands. These options include full-depth asphalt and in-place recycling, but perhaps the most interesting is the employment of roller-compacted concrete (RCC).

“We placed a trial RCC pavement on a residential street last fall,” says Williams. The opportunity arose when the city water department needed to replace a sewer beneath the street.

First impressions of the trial were promising, says Williams. The process produces what promises to be a strong pavement at a cost lower than conventional concrete.