The Road Most Traveled

Jan. 1, 2006

Lining the U.S. like blood vessels, the Interstate Highway System keeps the economy circulating day after day. Even though congestion has clogged some arteries, the vast network of highways has performed exceptionally well leading up to its 50th anniversary this year.

The following is an in-depth report on the shape of the Interstate Highway System in the four major regions of the country.

Lining the U.S. like blood vessels, the Interstate Highway System keeps the economy circulating day after day. Even though congestion has clogged some arteries, the vast network of highways has performed exceptionally well leading up to its 50th anniversary this year.

The following is an in-depth report on the shape of the Interstate Highway System in the four major regions of the country.

Providing an education

The interstate system of the Midwest has served as the center of research for the past half a century. Both the AASHO Road Test in 1956 and the Long Term Pavement Performance (LTPP) project in the late 1980s churned out mountains of research data.

The AASHO Road Test was probably the most famous of all. Constructed on the future right-of-way of I-80 southwest of Chicago, the test track contained 468 flexible and 350 rigid pavement sections. There were six pavement loops, five of which were under traffic.

The LTPP is spread out in 200 sections across the Midwest. “A couple of those have performed very well,” Gene Skok, an executive administrator of the International Society of Asphalt Pavements who worked on the project, told Roads & Bridges. “Down in Ohio they have concrete and full-depth asphalt projects.”

As a whole, the interstate system in the Midwest has produced the numbers of a pavement scholar. Recent data shows that 89% of the interstate highways in the region have present serviceability index values above 2.8, which is the score used to determine when a pavement needs rehabilitation. Twenty-five percent are higher than 3.9. There is over 13,000 miles of I-pavement in the Midwest, and in the beginning 90% of it was portland cement concrete.

“Overall I think it has held up fine,” Kurt Smith, program director for Applied Pavement Technology Inc., told Roads & Bridges. “The design standards have changed a lot over that 50-year period.”

In the early days of the interstate, designs primarily called for reinforced, jointed concrete pavements, concrete slabs as long as 100 ft long and 8- to 9-in.-thick pavements. Today you will see more jointed plain concrete pavement produced in 15- to 20-ft-long slabs with a pavement thickness of 10 to 12 in. Also, many pavements back in the 1950s were made to handle 4-6 million equivalent single-axle loads (ESALs), a number that was exceeded in the mid-1960s.

As for rebar, epoxy-coated continues to be the material of choice unless you are talking about dowel bars. DOTs are starting to experiment with solid stainless steel, stainless-steel clad and even fiber-reinforced composite materials. The problem with epoxy-coated rebar for the dowel bars is how it is handled out in the field. If the product is nicked or scratched it could lead to future corrosion. Before epoxy coatings, road crews would place black carbon steel dowel bars that might have been painted or greased.

A sometimes-wicked freeze-thaw cycle is the challenge of all DOTs in the middle of the country. One of the culprits, according to Smith, is decracking susceptible aggregate such as coarse limestone.

“A lot of the DOTs have gotten a fairly good handle on that and they know which aggregates are the bad ones and how to avoid those,” he said. “We need to make sure we have a strong, durable mix design and there has been a return-back-to-the-basics type of approach like making sure you are using a lower water-cement ratio and adding fly ash for increased durability.”

Drainage also can accelerate the aging process on the interstate road network of the Midwest. According to Skok, proper drainage requires constant maintenance. Every district has to make sure ditch and cross drains are operational.

“This year a lot of our region had a pretty warm January, and it turns out that is worse than having it completely frozen,” he said. “I know Minnesota and Wisconsin have had more deterioration because there has been water instead of ice in the system.”

As for the next 50 years, Smith sees longer pavement design lives like what is being placed on I-35 in Minnesota.

“They have started to adopt a pavement design life of 60 years,” he said. “Their intention was they were going to build the bridges to a 75-year design life so they wanted to build the pavement to last almost as long. I think they went to a thicker slab and paid a lot of attention to the durability of the mix design with a lower water-cement ratio and the use of fly ash. They had very strict requirements in the type of coarse aggregate that was allowed.”

The pavement in the Northeast could pass as the Midwest’s twin. Facing similar freeze-thaw cycles, many DOTs have taken similar steps to improve pavement design.

The Northeast, however, is home to some of the biggest road construction projects in the world. The King Kong of such jobs was the Central Artery/Tunnel Project, the “Big Dig.” Opened in 1959, the original Central Artery carried over 200,000 vehicles a day and cost motorists $500 million annually due to congestion. The solution was to replace the six-lane elevated highway with an 8- to 10-lane underground expressway. When it was all said and done, crews placed 3.8 million cu yd of concrete and excavated more than 16 million cu yd of soil for the 7.8 miles of highway.

The Pennsylvania Turnpike is another massive interstate system that has been fixed in the spotlight over the last five decades. Much of the system has or will be reconstructed by the year 2016. Receiving the most attention will be the Philadelphia region, where lane-widening work will cost almost $300 million. A 27-mile reconstruction of the turnpike near Harrisburg is estimated at $280 million.


Road trip through the South

The interstate pavements built in the southern states of the U.S. have held up quite well, considering. They were built to last—for 20 years at least. But now it is 50 years since the first interstates were built. Even considering that the majority of the system was not finished until 1970 or so, it is no surprise that the pavements are showing signs of wear.

Much higher design standards were used for the interstates, but nothing lasts forever. Arkansas was the first state to complete its planned interstate highway miles, constructed between the early 1950s and the mid-1970s. According to the March/April 2002 issue of Public Roads, “More than 30 years of wear and tear resulted in one of the roughest interstate systems in the country.”

Arkansas has recently repaired 60% of its 542-mile interstate system at a cost of about $950 million. They called it their Interstate Rehabilitation Program.

The Arkansas Highway and Transportation Department (AHTD) chose rubblization as one method for rehabilitating its 9- to 10-in.-thick reinforced, jointed concrete pavement. The highways had been patched and overlaid with portland cement concrete pavement for years, but the surface continued to deteriorate under increased traffic and heavier loads. The rubblized underlying portland cement concrete provides a base for a new hot-mix asphalt overlay. AHTD found the rubblization-and-overlay process fast and cost-effective.

One lesson learned by the AHTD was the increased importance of good subsurface drainage.

Michael Murphy, P.E., Ph.D., director of the Texas Department of Transportation’s Pavement and Materials Systems branch, agreed. Murphy told Roads & Bridges the importance of good drainage and a sub-base that will not erode were lessons that came out of the AASHO Road Test.

The Louisiana Department of Transportation and Development applied this lesson on I-10 in 1997-2002 when it completed 10 rubblization projects on jointed concrete pavement between the Texas border and Baton Rouge. The agency built a drainage system under the pavement before placing the overlay.

In Arkansas, the AHTD installed edge drains before the rubblization to drain any water that came up through the soil.

One aspect of pavements that is much different today than it was 50 years ago is the equipment and methods for collecting data.

“Florida has developed an automated data-collection vehicle that can operate at interstate highway speeds to collect ride, rut and visual distress data,” said Murphy. “That takes visual raters off the roadway and obtains the data that is necessary to populate the pavement management system and provide the information needed to manage the interstate. Another piece of equipment that’s been developed and used in several states . . . for doing forensic investigations and other types of investigations has been ground-penetrating radar, which provides us with layer thickness and subsurface condition information.”

Oklahoma recently evaluated their pavements by using a combination of a falling-weight deflectometer and ground-penetrating radar. Murphy commented, “Deflection data provides them with structural condition. GPR gives them layer thickness and subsurface condition.”

When Texas needed to know where to do repairs to I-20, an 8-in. continuously reinforced concrete pavement (CRCP), they used a rolling dynamic deflectometer, which was developed at the University of Texas. The device provides a continuous deflection profile as it rides down the roadway. It identified several places that needed repair. The overlay was removed, the CRCP was repaired and the new overlay was placed.

“We’ve found in Texas that thin overlays of CRCP pavement have performed very well,” said Murphy. “You can get easily 10 to 15 years of additional life before you have to do any further work on that pavement.”

In one section Murphy referred to, the pavement was opened to traffic in 1965. An overlay was placed in 1997. It is currently carrying 46,000 vehicles per day with 21% trucks. There are many sections similar to that in Texas.

Mississippi is another state that needs to do some road repairs. According to a report released in March 2005 by TRIP, Mississippi has the highest traffic fatality and bridge deficiency rates in the U.S. TRIP gave the state a C grade for pavement quality.

Eighteen percent of Oklahoma’s interstate pavements are in poor or mediocre condition, according to a May 2006 TRIP report. On the other hand, 70% of the state’s interstates are in good condition. Seventeen percent of the state’s interstate bridges are structurally deficient, and an additional 12% are rated functionally obsolete.

The Oklahoma DOT (ODOT) estimates that between 2006 and 2016, 219 lane-miles of the state’s interstate system will require major rehabilitation or reconstruction, and 123 interstate bridges will require significant repair or reconstruction. Also during that time, 48 interstate route miles will need added lanes to increase capacity.

The stumbling block is that Oklahoma will probably fall $625 million short of the funding necessary to rebuild the state’s aging interstates, according to an ODOT prediction. The TRIP report offered no hints about solutions to Oklahoma’s funding gap.


The Western lifestyle

In the western portion of the U.S., particularly California, Oregon and Washington, pavement and roadways have been well constructed since the beginning of the interstate system. Pavement structures have been able to withstand, in some cases, nearly 50 years of heavy traffic flow and variable weather conditions, while still preserving the movement of goods and people.

Two types of pavement—hot-mix asphalt (HMA) and portland cement concrete—have generally been used on the Western roadways and interstates. “It’s fair to say that both have performed very well,” Joe Mahoney, P.E., University of Washington, told Roads & Bridges. “They’re different, but they’ve both done great.”

While these types of pavements have been used for several decades, their thicknesses have changed over time in order to better accommodate increasingly heavy traffic loads.

According to Mahoney, the 1950s through the early 1970s saw concrete slabs with thicknesses of 8 to 9 in., while HMA was much less. Over the past half-century, concrete has increased by nearly 3 in. in thickness and now measures about 12 to 13 in. thick. HMA also has increased in thickness, and today it is almost equal to that of concrete.

By increasing the thickness of pavement, roadways have been able to withstand much more traffic for longer periods of time. According to Mahoney, traffic forecast predictions in the 1960s—particularly for trucks and buses—were much lower than what actually happened.

“Today, it’s typical on an interstate highway to see anywhere from 5,000 to 30,000 trucks and buses per day on a freeway. That’s a lot of traffic, and that really wasn’t terribly well anticipated several decades ago.”

With traffic flows that keep growing, it is becoming more and more difficult to rehabilitate urban roadways, especially in the Los Angeles area where there is a single freeway that runs about 400,000 vehicles a day. “If you go out there and interrupt that traffic flow, you are causing major disruptions,” said Mahoney. “More and more, the pavement community is looking to longer-lived pavements that will hopefully serve the public well going into the future.”

Despite heavy urban traffic, several major interstate rehabilitation and reconstruction projects have been successfully performed in recent years. In the Los Angeles area, one of the biggest and most heavily traveled freeways—I-17—is currently being reconstructed. The California Department of Transportation (Caltrans) also has been doing major work on I-5 and I-15. In Washington, a major multiyear reconstruction project has begun on I-5 through the western portion of the state, particularly the Seattle area. According to Mahoney, given space confinements and heavy traffic flows, it will be challenging to keep the roadways open to reasonable levels of traffic and do the reconstruction as well, which is why this particular project will take years to complete.


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