The Illinois Tollway is well under way with its multibillion Congestion-Relief Program, which will rebuild and restore nearly the entire 274-mi. (440 km) system, add lanes to many miles of existing road, provide congestion relief by converting mainline toll plazas to non-stop open road tolling, and extending I-355 12.5 mi. south from I-55 to I-80 to accommodate the needs of growing communities in the area.
Since the program’s launch in 2005, the Tollway has faced the same challenges posed to other transportation agencies: managing increasing costs of construction and materials within current program budgets. One avenue the Tollway is researching is increasing RAP usage in its HMA and SMA mixtures, without sacrificing the quality and long-term durability of its roadways.
The Illinois Tollway will rebuild and widen 14 mi. of the Jane Addams Memorial Tollway (I-90) near Rockford, Ill., during 2008 and 2009. The project’s design is nearly complete, and advance work is in progress to prepare for mainline work. The advance work involves utility work, strengthening shoulder pavements to carry mainline traffic, and constructing median crossovers and emergency pull-offs. It is within this advance work that the Tollway has begun its “higher RAP” research.
of Rubblized Concrete
The existing pavement on I-90 consists of an asphalt overlay of concrete pavement. The new pavement will consist of a rubblized concrete base, which will support 12 in. of HMA pavement. The roadway also will be widened, with the added lane consisting of a 15-in. HMA pavement supported by a 12-in. aggregate base.
The Tollway’s pavement design consultant, Applied Research Associates, developed the pavement structure, beginning with a 4-in. “rich bottom” base, then using progressively-stronger dense-graded mixtures in the middle of the structure, and capping it off with two lifts of stone matrix asphalt (SMA).
The Tollway’s research involves incorporating the process of fractionating RAP (FRAP), to increase the use of RAP in the lower HMA courses, to introduce the use of sand-sized RAP with high quantities of residual asphalt in the two layers of SMA, and to provide improved control of the mix properties and performance for all of the high-RAP HMA mixtures produced.
The Tollway has recently adopted the Illinois Department of Transportation’s (IDOT) Superpave mix design process.
IDOT’s maximum allowable RAP for dense-graded mixtures ranges from 10 to 25 percent depending on the application, with up to 50 percent allowed for the HMA subbase mixture. The Tollway is researching higher allowable RAP levels for dense-graded HMA: up to 40 percent in binder courses, and up to 25 percent in shoulder surface courses.
One caveat is included in the use of the higher RAP percentages: the RAP must be fractionated into at least two sizes, to ensure that the control of the RAP materials will be at such a level to achieve consistency in the produced mixture.
One new step for Illinois is the use of RAP in SMA. A key concern when incorporating RAP into SMA is the properties of the RAP aggregates. To assure the performance of SMA, both the coarse and fine aggregates need to be crushed, angular materials. A benefit for the Tollway in the search for RAP containing these qualities is that previous specifications required mainline Tollway HMA to use 100 percent crushed aggregates. The Tollway has a readily available source for high-quality crushed aggregate RAP in its existing mainline pavements.
The Tollway’s current “RAP in SMA” research allows the use of up to 15 percent fine aggregate RAP — material passing the No. 4 (4.75 mm) sieve. With the availability of mainline RAP containing 100 percent crushed aggregates, the Tollway contractors have the materials available to include the fine aggregate RAP in its SMA mixtures.
The prime contractor for the I-90 advance work is Rockford Blacktop Construction Company of Loves Park, Ill. It has included a subcontract with Rock Road Companies of Janesville, Wis., to complete a portion of the HMA construction. Rockford Blacktop and Rock Road have embraced the Tollway’s higher RAP research activities, and are producing test mixtures to include in the analysis.
Table 1 summarizes the mixtures that are being proposed for analysis. At the time of publication, two of the SMA mixtures and the N70 19.0-mm binder have been successfully produced and placed.
The production and placement process of each test mixture occurs over two production days. Each day involves placing about 500 tons of HMA.
Mix test results from the first day of production (voids, ignition burns for asphalt content and gradation, and in-place density) are evaluated to determine if the produced mixture is matching the mix design. Small mix adjustments can be made going into the second day’s production. It is during the second day’s production that mix samples are obtained for conducting additional analysis.
Several other aspects of the Tollway’s higher-RAP research are evident in Table 1:
• Three SMA mixtures are being evaluated, each with a different type of coarse aggregate.
• The SMA mixtures will be a continuation of the research the Tollway has conducted regarding ground tire rubber (GTR) modified asphalt.
• The N50 19.0-mm and N50 Subbase mixtures are being used to compare the properties of PG 58-22 and PG 58-28 asphalt binders in higher-RAP mixtures.
The last item listed above involves determining the degree of “softness” required when incorporating higher percentages of RAP into HMA. The industry agrees that a softer PG grade is required when using higher percentages of RAP. The analysis of the mixtures that contain the same mix proportions and vary the PG grade will help the Tollway determine which softer grade will be the most acceptable for its HMA.
Through the Tollway’s advance work contract on I-90, the higher-RAP HMA mix designs have already been created, the high-speed processors to fractionate RAP have been set up, and several of the mixes have been produced successfully by Rockford Blacktop and Rock Road Companies to verify that the specified volumetric properties of each mix can be obtained.
These higher-RAP dense-graded and SMA test production mixes were developed through the Tollway’s materials consultant, S.T.A.T.E. Testing, and will be analyzed for fatigue and durability at the University of Illinois, and for moisture susceptibility by IDOT. The test results may be part of an upcoming University of Illinois research project sponsored by IDOT to study the impact of higher RAP contents on pavement structural performance. That study would be performed using high-RAP laboratory mixes with specific material resources taken from the Tollway’s I-90 project to allow for future comparison to high RAP production mixes to be placed on the I-90 project.
Ross Bentsen of Bloom Consultants LLC is providing materials engineering services to the Illinois Tollway. Steve Gillen is the materials manager of the Illinois Tollway.
(This article was reprinted from HMAT magazine, January/February 2008, Volume 13, No. 1.)