In 2002, the Ohio Department of Transportation (ODOT) created a task force to design a demonstration project using perpetual pavements. Subcommittees were established to address the various project elements. The process included developing the design procedures and specifications suitable for the selected site.
One of the subcommittees developed a layer thickness design based on mechanistic-empirical (ME) principles; the second developed material specifications for each layer; the third established testing design protocols, including laboratory testing, accelerated load testing and testing on the pilot project pavement; and the last subcommittee investigated potential locations for the test road. The final site selected included the westbound lanes of the 8-mile-long bypass around Wooster, Ohio, on U.S. Rte. 30 in Wayne County, designated as the WAY30 project.
All about ME
The traditional approach in designing asphalt pavement follows the 1993 AASHTO design guide, perhaps with some modifications to account for local experience. The perpetual pavement design methodology takes advantage of the development and adoption of ME design principles in asphalt concrete pavement design. ME design is based on sound principles of engineering mechanics. The starting point is an analysis or computational procedure (e.g., elastic layer system or finite element analysis), supplemented with accumulated experience. The inputs to the analysis procedure include material properties of each layer of the pavement structure (including effects of climatic factors), expected load configuration (magnitude of load and axle configuration) and number of loads. Comparing the pavement response to load from the analysis in the form of deflections, pressures and strains with the strength of the materials, the design life of the pavement can be determined, either in terms of years of service or number of loads.
The ME design process involves both analytical calculation of pressures, strains and deflections, as well as the development of permanent deformations and use of specialized software, which is currently being developed and refined. Parallel to the software refinement efforts are the construction and evaluation of instrumented test roads to verify the ME design of perpetual pavements.
To achieve the indefinite structural design life, the pavement design criteria included the critical load assumed to be the legal load plus 20%, while the strain at the bottom of the pavement would be limited to 70 microstrain. The resulting design of the perpetual pavement constructed on WAY30 consisted of four layers of asphalt on a dense-graded aggregate base. Verification of predicted response and performance required careful laboratory testing of specimens and field monitoring by condition surveys and data collection from extensive seasonal and pavement response instrumentation installed during construction.
Orchestrating instruments and data
The research effort on WAY30 is led by the Ohio Research Institute for Transportation and the Environment (ORITE) at Ohio University, with additional contributions from faculty at the Ohio State University. The main objective of the research is to verify that the ME design of the perpetual pavement accurately predicts the actual field response and performance. To accomplish this, instrumentation has been installed at specific test points to monitor environmental and load response parameters according to the typical layout. Environmental monitoring includes measurement of temperature (with thermistors) and moisture (with TDR probes) of the base and subgrade layers, and temperature within the pavement layer. In addition, an automatic weather station has been installed to gather climate data, including air temperature, precipitation (rain and snow), wind speed and direction, relative humidity and incoming solar radiation.
Load-response data being collected include displacements, pressures and strains (longitudinal and transverse) with linear variable displacement transducers (LVDTs), pressure cells and strain gauges. ODOT also has installed a weigh-in-motion system that counts traffic and cumulative axle loads.
Specific road tests planned include controlled vehicle and falling weight deflectometer tests conducted at regular time intervals after construction following SHRP protocols and analyzed by ORITE.
An additional research project being conducted by ORITE in conjunction with the University of Akron consists of the laboratory analysis of all materials used in the construction of the road. This includes samples mixed according to design specifications, samples collected during construction of the road and samples collected from the road in service on an annual basis. Tests being performed on the asphaltic materials include:
- Resilient modulus;
- Uniaxial dynamic modulus at 41, 77, 104°F and 1, 4, 16 Hz;
- Bulk modulus at 41, 77, 104°F;
- Static creep test;
- Confined and unconfined compression test with at least two strain rates at two temperatures;
- Indirect tensile strength test and determination of absorbed value;
- Flexural beam fatigue tests at four strain levels at 68°F;
- Moisture susceptibility test (AASHTO T-283);
- Thermal stress restrained specimen test;
- Repeated load permanent deformation test; and
- Georgia loaded wheel test.
The aggregate base was tested for resilient modulus and permeability, while the subgrade soil also was tested for resilient modulus.
The U.S. 30 bypass was opened to traffic in November 2005 and has already yielded significant results. The first controlled vehicle tests were conducted in December 2005 using a 28.2-kip single-axle truck and a 40-kip tandem-axle truck each traveling at 5 mph. Example plots of data collected are shown in Figure 1 (linear displacements) and Figure 2 (pressures).
The pavement layers of WAY30 are designed to support traffic loads and experience less than 60 microstrains. The strain due to a heavily loaded and slow-moving truck is less than half of that, so the pavement is performing well above the specified values. Figure 1 shows the deflections measured by the LVDTs measured while a 40-kip tandem-axle truck drove over the sensors at 5 mph. Figure 2 shows the pressure readings from the 40-kip truck. The deflection and pressure in the pavement is relatively low because of the stiffness of the pavement.
The next controlled vehicle test is scheduled for the summer of 2006, when the asphalt is warmer; thus the measured responses (strains, deflections and pressures) are expected to be somewhat higher. These tests will be conducted at a higher speed closer to actual road speeds, which will reduce the responses.