The Pavement Testing Facility is a permanent pavement testing laboratory located on the grounds of the Turner Fairbank Highway Research Center (TFHRC). The facility was originally established in 1986 with space available for the construction of eight full scale test pavement sections. It was expanded to 12 test pavement sections in 1989 and to 24 in 1993. Traffic loading for the test pavements is provided by a machine called the Accelerated Loading Facility (ALF). As the first of its kind in North America, the ALF has the capability of simulating 20 years of traffic loading in six months or less.

Research at the facility is directed at understanding the complex mechanisms governing the response and performance of pavement systems. The facility provides researchers with the capability to monitor the behavior of full scale pavement systems under controlled loading conditions. Data obtained from tests conducted at the facility are used to validate pavement design concepts and to provide guidance on the use of new materials in pavement construction.

The facility''s main feature is the ALF machine used to simulate traffic loading. The ALF is a 29 m long structural frame containing a moving wheel assembly. The wheel assembly models one half of a single axle and can apply loads ranging from 44.5 to 100.1 kN. It travels 18.5 km/h over a 9.8 m test pavement section. To simulate highway traffic, the ALF loads pavement in one direction, and the loads are laterally distributed to simulate the side-to-side wander of trucks. The ALF is computer controlled, permitting operation 24 hours per day, seven days a week.

Test sections represent typical high way pavements and are constructed using normal highway materials, equipment, and procedures. Various instruments and equipment are available to measure load associated responses such as stresses, strains, and deformations; pavement performance characteristics such as rutting, cracking, and rough ness; and environmental effects including pavement temperature and base and sub grade moisture conditions. A falling weight deflectometer is also available for nondestructive pavement testing. Data acquisition and analysis are performed using personal computer-based systems.

Pavement Testing Facility Projects

Research at the Pavement Testing Facility has focused on factors affecting the performance of conventional flexible pavement systems. These pavements consist of asphalt concrete surface and unbound aggregate base layers constructed on a prepared subgrade soil. Recently completed studies have evaluated the damaging effects of high tire pressure and wide based single tires in support of FHWA''s research on truck/pavement interaction. Current research at the facility is directed at obtaining pavement performance data to validate the performance-based asphalt binder and mixture specifications proposed during the Strategic Highway Research Program (SHRP).

Tire Pressure

Advances in tire technology resulted in the widespread replacement on trucks of bias ply tires with radial tires. Accompanying this replacement was an increase in the average tire inflation pressure from 550 kPa to 690 kPa. Pavement engineers were concerned that increased tire pressures were resulting in greater pavement damage. Tests conducted over a wide range of tire pressure on instrumented pavements at the Pavement Testing Facility showed that tire pressure was a second order effect for relatively thick asphalt layers (greater than 125 mm) produced from properly designed mixes. The effect of tire pressure on flexible pavement response and performance was found to be less significant than the effects of load and pavement temperature.

Wide-Based Single Tires
 
Another concern of pavement engineers that was initiated by advances in tire technology is the replacement of conventional dual tires with wide-based single tires. Although not currently popular in the United States, wide-based single tires have been used extensively on trucks in Europe. Since they offer potential economic advantages over dual tires, their use is expected to increase here in the future. Because wide-based single tires have a smaller footprint than conventional dual tires, pavement engineers are concerned that they may be more damaging to flexible pavements. Comparative tests between dual and wide-based single tires conducted at the Pavement Testing Facility validated this concern. Under the same loading, wide based single tires resulted in four times greater fatigue damage, and two times greater rutting than conventional dual tires.

SHRP Validation

The recently completed SHRP asphalt research program produced asphalt binder and mixture analysis specifications and tests that are fundamentally related to pavement performance. Demonstrating the capability of these specifications and tests is a key component of the SHRP research initiative. The Pavement Testing Facility is currently being used to obtain this valuable validation data. FHWA researchers worked with the SHRP staff and contractors to develop an experimental design for the validation study. The experiment that was designed emphasizes the effect of binder properties on pavement performance. In combination with extensive asphalt binder and mixture testing, this accelerated load testing provides the capability to validate the underlying concepts of the SHRP binder specification, the suitability of the proposed accelerated laboratory mixture tests, and the rationality of the SHRP pavement performance models. The SHRP validation study is expected to take two to three years to complete.