State-of-the-art lab paves way for highway future

Most U.S. highways are now carrying more and heavier – trucks than they were designed to bear. In 1992, combination trucks drove 240 percent more miles across U.S. roads than in 1960, and the total tonnage carried by trucks in 1993 had increased by nearly 300 percent above 1950 levels.

One of today’s fully weighted 18-wheel semi-trailer trucks has an impact comparable to that of 10,000 cars. And, more of the nation’s domestic tonnage is moving via the highways: 45 percent in 1993 versus only 26 percent in 1950. This dramatic increase in heavy traffic results in faster road breakdown at greater cost to taxpayers.

Moreover, the design standards still used to build and repair highways date from the 1950s and ’60s. Due to changes in pavement materials, truck weights, tire pressures, suspension systems and axle configurations, these standards need updating.

Now, road researchers are getting an unprecedented look at how truck traffic and weather affect highway pavements, thanks to millions of bytes of live data generated since summer 1994 at the Minnesota Road Research Project (Mn/ROAD). Built at a cost of $25 million by the Minnesota Department of Transportation (Mn/DOT), Mn/ROAD’s data will update the decades-old information that road designers still use.

Using 4,572 electronic sensors embedded in pavement layers of bituminous, concrete and underlying aggregate and soil, the Mn/ROAD project can measure road stress, movement, temperature, moisture and a host of other variables. The sensors generate 30 megabytes of data daily on pavement pressure, movement and strain due to truck loads and environmental conditions such as temperature, moisture and frost depth.

The sensors are strategically embedded at the six-mile “live” traffic site, located 40 miles northwest of Minneapolis/St. Paul in Otsego, Minn., within the layers of 40 500-foot test sections – or cells – of varying pavement designs.

The cells are distributed on two roadways. Twenty-three are located on a 3.5-mile segment of traffic diverted from westbound Interstate 94 in Otsego, which provides data about the effects of actual interstate commercial traffic. This section switches traffic at regular intervals to the existing 1-94 for sensor maintenance and collection of distress survey data.

The remaining 17 cells are located on a 2.5-mile closed loop that simulates a low-volume city or county road. Variations in loading are applied by a Mn/DOT semi-tractor/trailer combination with different tire pressures and axle weights to examine their effects on pavement performance.

Both segments run parallel to I-94 at a site chosen for its suitability for studying both traffic and environmental conditions. For example, the site’s high water table allows researchers to examine the effects of the freeze/thaw cycle.

Readings from the static sensors, which measure moisture and temperature conditions, are taken every 15 minutes and are augmented by two on-site weather stations. Dynamic sensor measurements, which measure the effect of truck traffic, are triggered by heavy vehicles and take readings 2,000 times per second, measuring weight, strain, deflection, acceleration and pressure.

In addition to the sensors, the construction of Mn/ROAD required 245 miles of sensor wires, 26 miles of fiber optic cable, 26 roadside cabinets to house electronic equipment, 38 data acquisition computers, five protocol computers and an on-site RISC 6000. The thousands of sensors collect the data, which is transmitted to the Mn/DOT research lab in Maplewood, Minn., and stored in an Oracle data-base system. State-of-the-art communications equipment then sends the data to researchers at the University of Minnesota and eventually around the world via the Internet.

Data from the sensors will help Mn/ROAD’s researchers meet a number of research objectives, such as evaluating the effect of heavy vehicles on pavement, evaluating seasonal changes in paving materials and improving design and performance of low-volume roadways.

Data collection is projected to continue for at least 20 years.

Seventy-five initial research projects are now underway using this data. Within three years, project managers will have collected sufficient data to begin revising historically established measurements such as pavement coefficients and relative strengths of base and subgrade materials. In five to seven years, the pavement designs for city and county low-volume roads and higher-volume state highways may be substantially changed.

“What we learn in Minnesota will have broad implications for road planning in other states and countries,” says Mn/ROAD Director Richard Sullivan.

The prospect of longer lasting roads has attracted the partnership of the Finnish Government’s National Road Administration (FinnRA), whose goal is to test a low-cost paving method for gravel roads using a cold asphalt pavement technology developed two years ago for environmental purposes.

From its inception, Mn/ROAD has been envisioned as a joint project involving researchers, governments and private industry, both nationally and internationally.

Designed and developed by Mn/DOT in partnership with the University of Minnesota,, Mn/ROAD has partnering arrangements with the U.S. Army Corps of Engineers, the Illinois State Department of Transportation and the Universities of Minnesota, Illinois and North Carolina.

Some partnerships are sources of funding, while in others, Mn/DOT will furnish data from Mn/ROAD and receive the benefits of the research completed by those organizations.

For example, as experiments in some of the test cells are concluded, partners will be able to arrange with Mn/ROAD to conduct new projects in those cells.

Sensor manufacturers, for instance, can test their products in real-world conditions just as Mn/ROAD is testing its own specially modified resistivity probe for more extensive implementation.

“Our goal is to build a better foundation for the future of highways and roads,” says Commisioner Denn. “In that regard, many other countries and states are looking to Minnesota as pioneers in this research.”