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Utilizing unique construction practices, the city of Chicago has pioneered "green" urbanism. The city is committed to building its facilities in an environmentally sound manner, and is considered a leader in promoting green roofs as a sustainable alternative to the traditional roofing system.
On the ground, the Chicago Department of Transportation (CDOT) has implemented its Green Alley Program, along with a number of PCC and HMA green initiatives. Most recently, CDOT has focused its attention on the use of recycled asphalt pavement (RAP) to resurface its city streets.
With material costs and interest in recycling both at all-time highs, CDOT has specified the use of a low-volume HMA mix that includes up to 50-percent RAP and 15-percent recycled concrete. This Superpave bituminous concrete mix "is an N30 low ESAL, designated as an LC surface mix," says Cynthia Williams, quality assurance manager for CDOT.
"The mix is part of the overall environmental action plan the city has as far as green technologies and sustainable infrastructures," she says. "We have the support of the city management (including Mayor Richard M. Daley; Cheri Heramb, acting CDOT commissioner, and Tom Powers, 1st deputy commissioner, CDOT) to look into all green opportunities such as this."
Williams has been instrumental in overseeing development of the spec and getting asphalt producers to start using it. She has worked closely with Jay Behnke, P.E., president, and Fred Garrott, P.E., senior engineer, of S.T.A.T.E. Testing L.L.C., an independent engineering consulting firm, to make the high-recycled mix a reality.
"The city of Chicago's use of high-recycled HMA is a good example of how a local agency has stepped up and started to mine some of the mountains of RAP that have grown in the Chicago area," states Garrott. "Most of Illinois is rural enough that huge accumulations of RAP are not an issue, but it is a different problem in downtown Chicago because there is really a demand to get rid of those huge RAP piles that are accumulating."
During the initial year of its use, the CDOT's in-house construction crews used the mix to resurface mainly residential streets and not the city's major arterial streets. Chicago has 50 aldermanic wards, so CDOT's paving schedule is driven by specific needs in each ward.
CDOT has used the high-recycled HMA mix on numerous streets with much success. "We have been very pleased with the product and we have had very good results," says Williams. "The city has been pleased with it not only because it utilizes more RAP than any other low-volume mix, but also the fact that we are using recycled concrete in the mix. So, that is another way that we are using more existing products rather than using more virgin products."
Williams notes that approximately 7,000 tons of the mix were placed during the first year of the department's commodity contract. "We have a commodity contract for our in-house construction division to purchase asphalt," she explains. "The LC mix was specified in our commodity contract that went out in 2005, but 2006 was the first year the mix was used by CDOT."
The city currently has four contracts with area material producers — two with the joint venture of Ogden Avenue Materials/Reliable Asphalt Corp., and one each with K-Five Construction Corp. and Plote Construction Inc. Thus far, Ogden Avenue Materials/Reliable Asphalt Corp. JV has been the only vendor to produce the LC mix for CDOT. "I commend them for working with us to get the mix developed and produced," Williams says.
Anne Bigane Wilson, P.E., CPC, is president of Bigane Paving Co.,Chicago, a fourth-generation family firm. Wilson also owns Ogden Avenue Materials. "We have always had a very good working relationship with the city, not just as a producer but as a contractor," she says. "When I first heard about the mix design, I thought it was an interesting concept. In the city of Chicago, we have a tremendous amount of recycled asphalt that is generated every year, so we welcome working with a product that is going to utilize more of that recycled material."
Wilson, a past president of American Subcontractors Association, adds, "Recycling is always a bigger issue in any of the urban areas. Some people are afraid of the use of a higher-recycled mix. So I really applaud the city for not being afraid, and for trying something different and moving forward. The product is environmental friendly, and by reusing it we don't have to put it into a landfill. Also, anytime that you can use something that it going to use less oil — especially with the increasing price of oil — it is an advantage not only to us but obviously to all our customers as well."
Placing The Mix
During an early November visit to Chicago by Construction Digest, CDOT paving crews were placing the LC surface mix — produced by the Ogden/Reliable plant on South Pulaksi — on two city streets. Workers placed about 200 tons of asphalt on Fulton Street, between Clinton and Canal streets; then placed the mix on Bellevue Place, between State Street and Lake Shore Drive in the heart of Chicago's Gold Coast.
The thickness of the newly placed material ranged from 1-1/2 inches to 2 inches on average, varying with the subgrade conditions, according to Williams. "In the city, we have a hodgepodge of subbase conditions," she says. "For example, on the Fulton site the asphalt is sitting on a combination of brick and granite pavers and concrete patches/trenches, as well as existing base course."
Both street projects used a low ESAL mix containing 45-percent recycled asphalt and 10-percent crushed concrete, points out Behnke "The mix has natural sand in it and pea gravel," he says. "Even though it is a low ESAL mix, it is very high stability mix, and a lot of that has to do with the crushed concrete. The crushed concrete, being very angular and very absorbent, provides a natural balance to the pea gravel that is very round and has very low absorption. We have had very consistent QC/QA results with the mix."
LC Mix Features
Among its many attributes, CDOT and S.T.A.T.E Testing officials say the low ESAL mix is a very stable mix — one that provides an aesthetically pleasing surface with good rideability.
CDOT crews like resurfacing streets with the mix because it works well when paving around tight corners and other obstacles. "With the high stability of the mix, crews like working with it because they can place it quickly and traffic can get on it sooner without doing any damage to the surface," says Matt Galloy, senior field supervisor for ST.A.T.E. "A lot of times on a paving project, cars will travel on the freshly paved pavement and workers will have to go back and fix it. With this mix, they don't have togo back."
Vibratory rollers used on the Fulton and Bellevue projects hardly moved the high-recycled mix at all as they followed the paver. "Unlike a normal mix that compacts a quarter-inch for every inch, this mix only compacts down about an eighth of an inch, "Behnke says.
One of the key attributes of the LC mix is its ability to retain its temperature level. "We believe the mix will give workers a big advantage when laying asphalt in colder weather because of its ability to retain heat," says Behnke. "Traditionally, because city crews typically move from one location to another, the temperature of the mixes is kept on the hotter side — all the way up to 330 degrees. But with this mix, we found that if you made it at the regular temperature it was almost too hot to roll. Therefore, we have been gradually dialing down the temperature because it retains heat so well."
He continues, "By just looking at the way the new mix rolls out, it is probably one of the most stable mixes that you can lay. It is very comparable to a N90 mix F, which is more of a high-friction mix with manufactured sand in it that is used on interstates."
Looking Ahead
In a typical year, CDOT paving crews will place approximately 125,000 tons to 150,000 tons of asphalt a year, with four paving crews laying mix on a daily basis. "We put down a substantial amount of mix each year," says Williams. "The hope is that this LC mix becomes the basic bread-and-butter mix for thein-house crews for the residential streets, and we anticipate the tonnage of the mix will grow."
CDOT and S.T.A.T.E. are working to refine the LC mix. "Currently this mix is set up with a standard performance-graded PG 58-22 asphalt cement," says Behnke. "Because of the high-recycled content, one of the things we are looking at is using a softer bottom end liquid, say a 52-28, and possibly incorporating Ground Tire Rubber (GTR) with it. From an environmental standpoint, we have seen a lot of benefits using GTR."
