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Publication Number: FHWA-SA-96-022
Date: November 1996
Pavements take a beating from both traffic loads and the weather. But which factor-traffic or weather-causes most of the damage?
Engineers at the Mississippi Department of Transportation (MDOT) think the answer lies in experiment number 8 of the specific pavement studies (SPS) of the long-term pavement performance (LTPP) project. The SPS-8 experiment is an ongoing study designed to determine how environmental conditions affect pavement performance. By limiting load-related effects as a factor, SPS-8 sites attempt to isolate the effects of the environment. Test sections are located throughout the United States and Canada, allowing performance data to be collected in a wide range of environmental conditions.
The DOT was eager to participate in the SPS-8 experiment, but finding a site for a test section within the State took some wrangling, reports Al Crawley, research engineer with MDOT.
"MDOT has long been interested in the causes of pavement deterioration, particularly whether truck traffic or other factors played the most crucial role," says Crawley. "The SPS-8 experiment provided an excellent opportunity to gain some insights in this area."
The DOT initially set its sights on the Natchez Trace Parkway. But the Parkway's narrow shoulders didn't meet the specifications of the SPS-8 project. The State began looking elsewhere.
One of the DOT's district offices suggested that the asphalt concrete approach to a bridge on State Route 315, 80 km (50 miles) south of Memphis, might be a good candidate for an SPS-8 test section. With the bridge scheduled for replacement, the approach would have to be reconstructed-and it met the requirements for an SPS-8 site. But there was a hitch: requests for proposals for the bridge replacement project would soon be solicited, and all the bridge project plans and specifications were already approved.
"MDOT has a very ambitious highway construction and bridge replacement program underway," Crawley explains. "Construction is at an all-time high and it is very difficult to keep these programs on schedule. A general rule-of-thumb is that once project plans and specifications are complete and approved, there will be no changes in the project." MDOT's design division was thus reluctant to change the plans and specifications to make the project an SPS-8 test section.
Crawley turned to FHWA's regional LTPP office. "The design people wanted to hear from somebody outside the MDOT research division," said Crawley. Morris Reinhardt, LTPP regional engineer, persuaded the design division that any delay caused by changing the bridge project specifications would be more than offset by the value of the information to be gained from the SPS-8 project.
In the end, the project plans required only minor modifications to meet the requirements for an SPS-8 experiment, and a contract was let in February 1996-on schedule.
Crawley says that the SPS-8 and other LTPP experiments are important to the future of Mississippi's roads. "We want to make sure that the LTPP database and the design procedures that come out of the LTPP program represent conditions in Mississippi, because we plan to adopt those procedures when they come out," he says. "Mississippi and the country urgently need a thorough revamping of pavement design procedures to ensure that pavement design, construction, and maintenance are as cost-effective as possible."
Monte Symons of FHWA's Pavement Performance Division encourages more State highway agencies to follow MDOT's lead. "To me, the SPS experiments are critical. If you don't participate, the environmental conditions and highway practices in your State may not get included. If we don't have data or performance information, it's hard to factor it into the LTPP program and the products that result from it."
FHWA is still recruiting test sections for the SPS experiments. In the SPS-8 experiment, FHWA is looking for three more asphalt concrete test sections and nine more portland cement concrete test sections. "One way States can participate is to build a rigid pavement SPS-8 experiment as part of a bridge approach," says Symons (see sidebar).
To find out more about participating in the LTPP experiments, contact your local LTPP regional coordination office.
Construction is still under way, but MDOT is ready to post the sign identifying the bridge approach as being part of the SPS-8 experiments.
The SPS-8 experiment examines the effect of temperature and moisture on asphalt concrete pavements and portland cement concrete (PCC) pavements. To isolate the effects of moisture and temperature on pavement performance, SPS-8 test sites must be located on roads that carry at least 100 vehicles per day, but no more than 10,000 equivalent single-axle loads per year. Each test section must measure 150 meters (500 feet).
Finding sufficient numbers of PCC test sections for the SPS-8 experiment has been somewhat difficult. By barring pavements that carry heavy loads, the experiment excludes most PCC roadways. And most highway agencies are reluctant to build PCC pavements on roadways that carry only light loads. But the aprons linking a bridge to the adjoining roadway are usually constructed of PCC, making them good candidates for SPS-8 test sections.
FHWA's policy is that approach roadways to Highway Bridge Replacement and Rehabilitation Program (HBRRP) projects are limited to the length needed to render the bridge serviceable. However, in a July 1996 memorandum, FHWA stated "there may be cases where the approach roadway satisfies the physical conditions" for an SPS-8 test section. "In those cases, serious consideration should be given to identifying that portion of the HBRRP project as an SPS-8 site, thus enabling the States to construct the pavement with HBRRP funds."
For more information, contact the FHWA division office in your State.
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