By: Cynthia M. Williams and Fred Garrott, P.E.
North Michigan Avenue, the center of Chicago’s shopping district and affectionately referred to as the “Magnificent Mile,” got even more magnificent last year with a new coat of “green” pavement laid down in November by the Chicago Department of Transportation (CDOT).
The environmentally green asphalt-pavement surface includes three recycled construction materials—asphalt shingles (RAS), asphalt pavement (RAP) and ground-tire rubber (GTR). This unique combination of materials took CDOT to a new level in pavement recycling.
Rather than opt for a conventional dense-graded asphalt overlay, the CDOT team elected to keep pushing the recycling envelope on this high-profile project. That decision shaped the scope and resulted in a showcase project for Chicago.
The fast-tracked project was complete in time for the Magnificent Mile Lights Festival parade held in November, which has become an annual precursor to the busy holiday shopping season. Given a short schedule, a limited budget and an exclusive address, this high-profile project was deemed a success by all stakeholders.
A Magnificent way
The recycling statistics for this medium-sized 12-block project are impressive. Using conservative estimates, the Magnificent Mile project incorporated:
- Recycled shingles torn from the roofs of 130 houses;
- Recycled tire rubber from 2,200 discarded passenger car tires; and
- 24 truckloads of recycled asphalt pavement.
This stretch of North Michigan Avenue had not had a facelift since 2004, when the curb lanes were resurfaced. When the underlying base was investigated, the conditions were extremely inconsistent. Typically, CDOT would have performed resurfacing utilizing traditional binder and surface courses based on high-stability dense-graded Superpave mixes. The inconsistencies with the underlying sub-base conditions and the limited time and budget for the work caused the city to rethink its plan. CDOT worked with materials consultant S.T.A.T.E. Testing to determine the best course of action.
A stone-matrix asphalt (SMA) mix design constructed in a single 2-in. lift was chosen for this application because of its strength and durability. The single lift operation also reduced working days and limited traffic disruption.
From a green perspective, the high asphalt cement (binder) content of an SMA offers the opportunity to include a high amount of recycled binder. The mix design was developed by S.T.A.T.E. Testing, which worked with the general contractor, Arrow Road Construction, to develop a mix that consisted of ingredients already in use in the Chicago area. The combination was, however, unique to the Michigan Avenue project. The mix consisted of the following:
74% Quartzite aggregate, which provided strength, low absorption and high friction;
15% fractionated RAP (FRAP), which provided the majority of the fine aggregate in the mix. Arrow Road Construction processed its FRAP to a minus 3?8-in. size. The FRAP contributed about 1% of the 6% total binder content required in the mix;
7.5% RAS. Southwind RAS Co. provided the processed reclaimed post-consumer (tear-off) shingles, which were sized smaller than a 3?8-in. sieve. The 26% asphalt content in the RAS provided almost 2% of the total binder required in the mix;
3.3% GTR modified asphalt binder. Seneca Petroleum, a local producer, provided the terminal-blended GTR-modified binder. GTR-modified binder is an economical replacement for the polymer-modified PG binders that are more commonly used in Illinois. Because of its stiffness, GTR binder prevents draindown in mixes where fibers would otherwise be required. The base liquid for the Michigan Avenue SMA was PG 58-28, and when blended with 12% GTR, the final binder grade is considered equivalent to a polymer-modified PG 70-28.
A return to SMA
SMA has become the mix of choice for high-volume expressways in the Chicago area. A structure of strong coarse aggregate and stiff asphalt binder makes a mix that is resistant to rutting and very durable. Its relatively high cost, however, has limited use on non-truck arterial routes in Chicago. With 11 different bus routes running through the project limits, the city had a perfect opportunity to test the durability of this particular SMA.
“Whenever I’m asked to make a recommendation on a particular mix design for CDOT, I like to go back to the history of what has worked in the past,” said Jay Behnke of S.T.A.T.E. Testing, the creator of the concept and final design used on the project. “I went back to the successful North Avenue Bridge SMA overlay and South Michigan Avenue SMA bus pads. Both projects used SMA as the basic design but neither had structures and street returns to contend with during placement. This mix had to be more forgiving with all of the associated hand work. I think the stickiness of the GTR and the stiffness of the RAS worked extremely well together, limiting segregation.”
In order to ensure that the mix produced met the design, Arrow Road produced several test runs of the material at its Mount Prospect, Ill., plant. “I think the team effort by all parties and the city’s willingness to listen to our concerns and work with us made for a successful end result,” said John Healy, president of Arrow Road Construction.
More of the binder
The combination of recycled FRAP and RAS provided 45%, by weight, of the 6% asphalt binder in this mix. By traditional standards, this is a very high percentage.
Until very recently, most asphalt-paving specifications limited the contribution of binder from recycled material to less than 25% of the total asphalt binder. There are reasons for this restriction. The most basic is that the amount of recycled material that could be physically incorporated into the mix was limited by the thermal transfer efficiency of the aggregate dryer and mixer and the capacity of the RAP feeder. Also, there is consensus “conventional wisdom” that too much recycled binder “pre-ages” the mix, making it susceptible to premature cracking and reducing its useful life.
The concept of limited binder replacement is under question. The recent rise in asphalt cement costs and the availability of new recycled ingredients provide justification to take another look at the binder restrictions.
A holiday rush
This Michigan Avenue project provided another challenge to turn green theory into greener streets. There are multiple story lines in a project like this, including the urban working conditions, green objectives and tight schedules.
Night paving is commonly required in high-traffic locations. But urban streets provide more night-paving challenges than do expressways. Urban streets include intersections, turn lanes, signal loops, pavement markings, curb ramps, utility structures and pedestrians—lots and lots of pedestrians at all hours of the day and night.
In the case of Michigan Avenue, there also are commercial and retail businesses, residences and hotels to consider. With the impending holiday season, there also was a push by the local businesses and the Greater North Michigan Avenue Association to get holiday plantings, decorations and displays in place. Much of that work occurs at night.
Night or day, attention to scheduling and coordination is essential to all successful construction projects. Pablo Martinez from V3 Cos., the resident engineer, and Bulent Agar, CDOT’s construction manager for the project, attest to the complications associated with scheduling such a fast-tracked, unique and high-profile project.
“The scheduling and coordination of this project between the various city agencies, traveling public and the local businesses was a monumental task. It definitely helped that the local businesses were receptive to the project and the local alderman’s office really helped us to get the word out and to coordinate with all parties impacted by the work,” said Agar.
In addition to the challenges of working in a highly congested area, the contractor also had to react to a few unforeseen obstacles and permit restrictions during the construction.
“Arrow Road was spot-on with the schedule, put forth a great effort under difficult conditions and was responsive to the city and community’s needs,” said Agar.
This was an accelerated project from the beginning. Not on the original design schedule for 2011, a decision to move forward was not reached until late August. Construction permits were issued and structure adjustments began on Sept. 28; milling was completed on Oct. 19; paving was finished on Oct. 22, with loop detectors and striping substantially completed on Nov. 14.
To accommodate the businesses, hotels and residential buildings, as well as the highly sensitive traffic on this corridor, work-hour/noise restrictions were placed. Milling, paving, pavement-marking installation and detector-loop installation operations (more than 500 ft of signal detector loops were installed at 30 locations) were limited to nighttime hours between 8 p.m. and 6 a.m. Utility structure adjustment, as well as curb-and-gutter and sidewalk installation, was limited to daytime operations between 9 a.m. and 3 p.m. No work was allowed between Friday afternoon at 3 p.m. and Monday morning at 9 a.m.
The major portion of the work included milling 2 in. of deteriorated dense-graded asphalt overlay and placing a single 2-in. lift (3,500 tons) of SMA inlay for a 12-block length of Michigan Avenue. At the north end of the project, where Michigan Avenue becomes “inner” Lake Shore Drive, a more traditional Superpave surface mix was used.
The project included a “trackless” prime coat. The city’s recent trials showed improved bonding of the new asphalt surface vs. the traditional emulsion prime.
A total of 103 utility structures were adjusted during daytime hours, prior to milling operations, thereby minimizing the disruptive work required between milling and paving operations. The pavement around the structures was removed, the structures were adjusted and the surrounding pavement was replaced with rapid-setting calcium aluminate cement concrete mix provided by Chicago-based Henry Frerk & Sons in mobile concrete mixers. The concrete achieved strengths of 3,200 psi in four hours, allowing the restricted lanes to be fully reopened to traffic at the end of the work day.
Crosswalks on the new pavement were installed according to the city’s new “continental” standard, utilizing a 24-in.-wide white line at 4-ft centers, 10 ft wide. This new standard increases the visibility of the crosswalks, which is critical due to the large volume of pedestrian traffic.
Going deeper into green
Since the mid-2000s, CDOT has aggressively pursued green-paving technology. In multiple combinations, the department has incorporated waste shingles, scrap tires and mountains of old asphalt pavement into their paving programs. They also have been an Illinois leader in pursuing pervious paving surfaces and warm-mix asphalt (WMA).
CDOT is cooperating with the Illinois Department of Transportation and the Illinois Tollway to experiment with and evaluate several green technologies. The evaluations look at individual recycle options and combinations. Other projects being evaluated include different FRAP components, pervious pavements, WMA and softer grades of PG binder.
Traditional pavement research is time-consuming, drawn out by lab work, peer review, publishing, etc. The almost simultaneous appearance of FRAP, RAS and WMA have pressured highway agencies to use shorter research time frames. One way to evaluate higher binder replacement is to just build the pavement and measure the effects.
The use of recycled ingredients has the potential to lower costs and improve quality. Lower costs are a natural part of competitive bidding, as contractors choose to use recycled material in their mixes. The Michigan Avenue project was successful in getting this high-recycle mix on the street. The new street surface is expected to withstand years of Chicago’s four-season weather while looking good for the thousands of people who work, shop and live on the Magnificent Mile. R&B
About The Author: Williams is a quality assurance manager with the Chicago DOT. Garrott is with S.T.A.T.E. Testing LLC.
