Opening to traffic in late December 1968, the original Marquette Interchange, or Central Interchange as it was known at the time, connected a network of highways that ran through the most populated region of Wisconsin. The interchange is an agglomeration of three major interstate highways in the Milwaukee area: I-94, I-43 and I-794. During the conception of the expressways, city traffic in Milwaukee had increased 100% between the years of 1945 and 1952. Residents voted in favor of a system, and an overall freeway plan for the Milwaukee area was released in 1955. It outlined the construction of 13 highways.
The interchange was designed to carry 150,000 vehicles daily. Actual volume increased to 210,000 and 300,000 in 1976 and 2000, respectively. The projected volume for 2025 is 375,000 vehicles daily. As the interchange aged, signs of its weakness became evident, and safety issues had begun to arise with an average of 700 crashes per year (between 1997 and 2000)—many of which were congestion-related. The decision was made, with much opposition from local activist groups, to reinvest in the infrastructure for another 75 years. With a dangling price tag of over $1 billion for the original design plans, the cost of the final design was reduced to just over $800 million through economizing on various components.
Laying it on thick
The new interchange has been designed with longevity in mind—from the high-performance concrete used in the bridge decks to the perpetual hot-mix asphalt (HMA) pavement used for mainline paving. Good engineers should always try to make infrastructure more durable, since it is an investment that affects the whole community. With a project of this size and importance, the engineers knew they needed to make smart decisions to get in and get out, and then stay out.
Two common modes of fatigue failure in HMA pavements are bottom-up cracking and rutting from heavy truck traffic. As the pavement is stressed due to repeated traffic loads, a crack may form at the bottom of the asphalt layer. This crack progressively grows larger as the pavement undergoes more loading. Eventually, this crack propagates through all of the asphalt layers, exposing the base and subgrade materials to moisture from the surface. In the rutting mode of failure, constant compressive stress applied to pavement causes the asphalt to literally “shove” the other asphalt until a rut forms; or on the other hand, the material (asphalt, base or subgrade) can slowly deform permanently due to this compression, thus creating a rut.
The design philosophy of a perpetual HMA pavement is to build sufficient pavement structure to retard or eliminate these modes of failure. For example, an HMA pavement of sufficient thickness and stiffness will reduce critical strains and retard or prevent cracks from forming at the bottom of the asphalt and consequently propagating through the thickness of the pavement. Likewise, a thicker pavement structure will reduce the compressive forces exerted on the weaker materials lower in the pavement structure, reducing the chances of rutting. Additionally, shoving of the asphalt can be eliminated through the use of a good HMA mix design. In a nutshell, the perpetual HMA pavement structure is designed to keep damaging stresses and strains very small, thus significantly extending the service life.
The final pavement structure selected for this project consisted of multiple base layers and a 13-in.-thick HMA layer. The native soils consist mainly of silty and clayey soils. An 18-in.-thick layer of select crushed material (with particle diameters near the 4- to 6-in. range) was then placed. This material was manufactured from recycled concrete from demolished materials on the site. On top of the select layer, a 6-in. layer of dense-graded crushed material (from the same recycled source) was placed, followed by a 4-in. open-graded layer of crushed virgin material.
The 13 in. of HMA was placed in four lifts, three of which were different mix designs. The 4-in. rich bottom layer was made with recycled asphalt pavement. A similar middle layer (without recycled material) comprised the next 7 in. (placed in two lifts) and was topped with a 2-in. stone-matrix asphalt (SMA) wearing surface.
How are you today, HMA?
The perpetual pavement instrumentation for the Marquette Interchange project was sponsored through the Wisconsin Highway Research Program by the Wisconsin Department of Transportation (WisDOT) and the Federal Highway Administration. The goals of this particular project were to install a wide variety of instruments into the perpetual HMA pavement in order to document the pavement’s structural response to the traffic and environmental loads. This will provide pavement engineers with valuable comparative data to test the validity of computer models developed to predict these output responses.
The pavement selected for instrumentation is located within the north leg contract of the Marquette Interchange reconstruction project, just north of the core of the interchange. The instrumentation is contained within a 50-ft section of the outer lane and adjacent shoulder within northbound I-43. The project was awarded to a team of engineers from the Transportation Research Center at Marquette University. The university lies adjacent to the tangles of the interchange and is the source of the roadway’s name.
A plan was formulated to set up a system that would measure and store dynamic pavement responses for every vehicle that passed over the selected test section.
Environmental, wheel-wander and weigh-in-motion (WIM) data for this location also would be collected and stored. A total of 25 strain sensors were used to measure strains in the bottom of the asphalt layer in both the longitudinal and transverse directions. In addition, four pressure cells also have been installed to measure vertical pressure in the subgrade and base layers. Capacitance probes were installed into the subgrade to measure fluctuations in the moisture content of the soils, and a large number of temperature probes have been placed throughout the entire pavement structure in order to generate temperature profiles. Other environmental sensors include pyranometers to measure solar radiation and an anemometer to measure wind speed. A closed-circuit camera also was installed to record a low-resolution snapshot image of passing vehicles.
The installation of these instruments required careful planning with the contractors. The instruments used for the project are very sensitive and required a large monetary investment—cutting through the wires during excavating or any other accidents were completely unacceptable and would compromise the project. No sensors were installed until all other underground work was complete. The instruments also needed to be placed precisely and with the required network of conduits to run the sensor wires. The site work contractor, Edgerton Contractors Inc. of Oak Creek, Wis., invested in a GPS locating system for doing their work and allowed the Marquette research team to utilize this resource for placing the instruments.
The strain sensor installation required a lot of cooperation with the paving company. Because of the layout and paving plans, the sensors could not be pre-placed and had to be installed at a rapid pace. The paving contractor, Payne & Dolan Inc. of Waukesha, Wis., brought in a material transfer vehicle to help the paving process go smoothly. Help during asphalt strain sensor installation was provided by Payne & Dolan and other WisDOT employees, significantly improving the outcome of the installations.
Other instruments installed into the HMA layers included temperature-gradient probes, a wheel-wander grid and a WIM system. The temperature-gradient probe measures pavement temperatures at 1-in. intervals throughout the thickness of the pavement. The wheel-wander sensor comprises three piezoelectric sensors arranged in a Z formation. With the as-built geometry of the Z and the signal pulses from the piezo sensors, the speed of the passing vehicle and the lateral location of the wheel can be measured. The WIM system was purchased from ECM Inc. and utilizes quartz piezoelectric sensors manufactured by Kistler Corp. The unique challenge for this project was finding a WIM system that would be accurate enough for the research needs, compatible with the data systems and, most important, compatible with the pavement material. Many Class 1 WIM systems in the U.S. are based on load cells placed in a concrete pad, which was unacceptable here due to cost and construction. The quartz piezo systems have been used extensively in other countries such as Germany and Japan with great success and are gaining popularity in the U.S. The WIM system installed for this project has been performing well since the start of the project.
All of the data is recorded with the use of a high-speed data acquisition system, and the information is transmitted wirelessly to one of the dormitories at Marquette University. Once inside the Marquette computing network, the data is stored on a database computer where it is available for researchers to analyze. With the amount of data being generated, analyzing the data requires the use of customized programs to run through the data and pull out the important information.
Already in the works is the production of data analysis tools. Data collection has been ongoing and was originally intended to last one year, the expected life of the strain sensors. However, data collection is completely automated and can continue as long as the sensors are alive.
A web page is available for anyone interested in viewing the status of the system. Live data coming into the database can be viewed from the web portal. Other information, such as the background of the project, details on the instruments and related project reports, is included as well. Giving other pavement engineers around the world access to the database for their own purposes will be of great significance.
Breeding confidence
A large concern on the Marquette Interchange project is the public’s perception of the work. The project was already the center of scrutiny with many political groups opposing the improvements, so repairs to any part of the project just prior to final completion might cause a flurry of unwanted criticism. Of the utmost importance, however, is that we, as employees to the taxpayers (and ultimately, to ourselves), must create facilities that provide the greatest benefit to the public. This research project, as well as others, aims to help the industry have more confidence and reliability in future work.
The ultimate goal of the instrumentation project is to calibrate the pavement design process used in Wisconsin so pavement engineers can more accurately design durable pavements. The pavement design process has been slowly evolving toward an analytical approach using the properties of the materials and traffic loads.
Until now, pavement design processes were largely based on experimental data from a handful of road tests conducted around the time that the Interstate Highway System was born. Complexities stemming from traffic characteristics, differing soil conditions, locally available materials and the performance of paving materials have always caused modern design processes to include this experimental knowledge. As technology and the ability to precisely measure and understand the behavior of pavements improves, we will continue to see pavements that are better suited to the needs of a particular project.
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