Thomas E. Freeman

MEASUREMENT OF VERTICAL COMPRESSIVE STRESS PULSE IN FLEXIBLE PAVEMENTS: REPRESENTATION FOR DYNAMIC LOADING TESTS

Testing at Virginia Smart Road allowed determination of the vertical compressive stress pulse induced by a moving truck and by falling weight deflectometer (FWD) loading at different locations beneath the pavement surface. Testing was performed on 12 different flexible pavement sections. Stress and temperature were measured using pressure cells and thermocouples, respectively, that had been installed during construction of the road. Target testing speeds were 8 km/h, 24 km/h, 40 km/h, and 72 km/h. The considered depths below the pavement surface were 40 mm, 190 mm, 267 mm, 419 mm, and 597 mm. A haversine or normalized bellshape equation was found to be a good representation of the measured normalized vertical compressive stress pulse for a moving vehicle. Haversine duration times varied from 0.02 s for a vehicle speed of 70 km/h at a depth of 40 mm to 1 s for a vehicle speed of 10 km/h at a depth of 597 mm. For the FWD loading, a haversine with a duration of 0.03 s was found to approximate the induced stress pulse at any depth below the pavement surface. Currently, laboratory dynamic testing on hot-mix asphalt (HMA) specimens is performed using a haversine wave at loading duration of 0.1 s. Because HMA is a viscoelastic material, the loading time affects its properties and, therefore, it is recommended that the loading time of HMA dynamic tests be reduced to 0.03 s to better match loading times obtained from moving trucks at average speed and from FWD testing.

DATA COLLECTION AND MANAGEMENT OF THE INSTRUMENTED SMART ROAD FLEXIBLE PAVEMENT SECTIONS

The flexible pavement research facility at the Virginia Smart Road consists of 12 different designs. All sections are closely monitored through a complex array of sensors located beneath the roadway embedded during construction. The environmental sensors include thermocouples for temperature measurements, time domain reflectometry probes to measure moisture content in the base layers, and resistivity probes to measure frost penetration. The dynamic sensors include pressure cells and strain gauges to measure stresses and strains, respectively, induced at different layers from truck loading. Environmental data are collected daily every 15 min for temperature, every hour for moisture, and every 6 h for frost penetration. Truck testing is performed every week with different loading configurations. The loading variables include three load levels, three wheel inflation pressures, and four different speeds. Data are managed by saving environmental data from different instruments separately using date and section number. Truck loading data are saved by test type (based on loading configuration, inflation pressure, and speed), date of test, and section number. A database is being generated for all 12 sections to study the effect of all tested variables on the different flexible pavement designs. The performance of the used instruments and collected data are presented, and the techniques used to manage the overwhelming data are discussed. In addition, based on instrumentation responses, a preliminary discussion of the load distribution in a tested pavement system, the effect of speed on pavement stress and strain responses, and the effectiveness of drainage layer are discussed.

Pavement response to dual tires and new wide-base tires at same tire pressure

Although concern was raised about the introduction of radial tires due to their higher inflation pressure compared with that of bias tires, radial tires have been proven to reduce the strain at the bottom of the hot-mix asphalt (HMA) layer. However, conventional wide-base single tires have been shown to be more damaging to pavement than dual tires. The damage mainly depends on the tire tread width and inflation pressure. It has been suggested that wide-base tires may produce damage equivalent to that of dual tires if the maximum load per tire is limited to 11.6 kg/mm of tire tread width. Recent advances in tire design and material have led to the design of a new wide-base tire that is wider and flatter in the crown area to provide a uniform contact stress distribution. It operates at an inflation pressure of 690 kPa for 151-kN tandem axle load. An experimental program studied the effects of the newly developed wide-base tire on a flexible pavement section at the Virginia Smart Road under different loading and environmental conditions. Testing results have shown that the newly developed wide-base tires induce approximately the same horizontal tensile strains under the HMA layer as do equivalent dual tires. Hence, the fatigue damage expected from these newly developed wide-base tires is the same as that produced by dual tires. However, the vertical compressive stresses induced by the wide-base tire are greater on the upper HMA layers of the pavement. The difference in stresses diminishes with depth and becomes negligible at the bottom of the subbase layer.

Contracting for pavement distress data collection

Many agencies responsible for managing pavements have adopted pavement management systems (PMS) to help manage their pavement networks more cost-effectively. One of the most costly parts of operating a PMS is collecting condition information, especially pavement distress information. Many agencies have started to contract for pavement distress data collection. Some of the agencies have experienced problems with the data collected by contract. A study for agencies in Washington and Oregon to define the accuracy of data needed by the agencies with an evaluation of certain participating vendors using semiautomated data collection methods is described. Issues about quality control and quality assurance faced by agencies considering contracting for automated data collection also are raised. These issues need additional study to develop appropriate guidelines. The initial set provided is based on discussions with some of the agencies currently contracting for pavement distress data collection.

PERFORMANCE OF PAVEMENTS SUBJECT TO HIGHER TRUCK WEIGHT LIMITS IN VIRGINIA

A study was mandated by Virginia’s General Assembly to determine if pavements in the southwest region of the state carrying vehicles operating under higher allowable weight limit provisions have greater maintenance and rehabilitation requirements than pavements bound by lower weight limits elsewhere. Detailed field surveys were conducted at 18 in-service pavement sites representing the range of roadway and traffic conditions found on Southwest Virginia’s primary and secondary highways. Traffic classification and weight surveys, an investigation of subsurface conditions, and comprehensive structural evaluations were conducted at all sites. The results were used to estimate the cost of damage attributed only to the net increase in allowable weight limits. Consistent with results of more comprehensive research conducted by and for other highway agencies, the authors found that pavement damage increased drastically with relatively small increases in truck weight for all vehicle classes affected by Virginia House Bill 2209. The cost of structural damage to mainline pavements attributable to the net weight increase in the seven affected counties alone was estimated at

QUANTITATIVE FIELD EVALUATION AND EFFECTIVENESS OF FINE MIX UNDER HOT-MIX ASPHALT BASE IN FLEXIBLE PAVEMENTS

28 million over 12 years. Costs for necessary improvements to roadway geometry, reduced service lives of bridges, increased motorist delays through work zones, and the safety implications of heavier vehicles operating in mountainous terrain were not included in the estimate. The report highlights the experience of one state highway agency with increasing political pressures to raise legal load limits.

Deterioration Prediction Modeling of Virginia's Interstate Highway System

Testing was conducted with the main objective of predicting the effect of incorporating a fine hot-mix asphalt (HMA) layer under an HMA base on the long-term fatigue performance of flexible pavements. Testing at the Virginia Smart Road allowed the determination of the vertical compressive stress and horizontal transverse strain induced by a steering-axle tire of 25.8 kN (5,800 lb) under the HMA layer of two pavement designs, one of which included a fine surface mix below a base mix. Stresses and strains were measured for four different speeds [8, 24, 40, and 72 km/h (5, 15, 25, and 45 mph)], for three tire inflation pressures [552, 655, and 724 kPa (80, 95, and 105 psi)], and at different temperatures. Stresses were measured with pressure cells, while strains were measured with H-type strain gauges embedded in the HMA layers during construction. As expected, temperature was found to significantly affect the vertical compressive stresses and horizontal transverse strains measured under the HMA layer. Speed, on the other hand, did not affect the magnitude of the vertical compressive stress measured in any of the layers but did affect the loading time. However, speed was found to significantly affect the horizontal transverse strain measured under the HMA layer. The compressive stress and horizontal transverse strain measured at the bottom of the HMA layer at depths greater than 150 mm (6 in.) were found to be independent of tire inflation pressure ranges from 552 to 724 kPa (80 to 105 psi). It appears that incorporation of a fine HMA at the bottom of a HMA base layer would increase the fatigue lives of flexible pavements.