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McLean, VA, United States

Li X.,ESCINC | Gibson N.,FHWA
Asphalt Paving Technology: Association of Asphalt Paving Technologists-Proceedings of the Technical Sessions | Year: 2011

Some state DOTs are moving away from the standard Superpave N design tables and volumetric criteria and some researchers continue to suggest lower N design values. The concept of a gyration Locking Point (LP) in the Superpave gyratory compactor was developed as an alternative to the Superpave N design that would avoid over-compaction and possible damage to the aggregate skeleton. This paper investigates the changes in aggregate packing that occur during Superpave mix design by utilizing mechanical performance testing to explore and define a 'locking' phenomenon rather than a simplistic pattern of heights in SGC. Four asphalt mixtures, which were designed by the Superpave mix design method, are studied in detail. Gyration heights, air void content, VMA and axial, radial and volumetric strains measured during the flow number performance test were used to identify the presence of a locking point. Test specimens were evaluated at nine gyration levels from 8 to 125 gyrations. Locking points for each mixture were obtained from the compaction curves and the second instance of two consecutive gyrations resulting in the same sample height was found to produce a locking point measurably lower than the N design. A three-stage description of distinct aggregate packing phenomena during gyratory compaction is proposed ranging from rapid packing to aggregate reorientation. Aggregate degradation was detected in varying degrees depending on the aggregate softness and type of mixture. Recommendations were made regarding the best practical definition of locking point using only gyration heights. Fine aggregates should be included along with coarse aggregates when evaluating susceptibility to degradation during compaction. Finally, utilizing performance characterization tests for a follow-up study is necessary to develop appropriate insight as to the consequence of reducing design gyration levels. Source

Crawford G.L.,FHWA | Gudimettla J.M.,Global Consulting Inc. | Tanesi J.,Global Consulting Inc.
Transportation Research Record | Year: 2010

Coefficient of thermal expansion (CTE) is one of the sensitive inputs in the new Mechanistic-Empirical Pavement Design Guide (MEPDG). The most widely used test method to measure CTE of concrete is outlined in AASHTO TP60-00, Coefficient of Thermal Expansion of Hydraulic Cement Concrete. Several state highway agency materials laboratories and university research centers have custom-built manually operated or automated CTE measuring devices based on recommendations from AASHTO TP60. Automated CTE devices are also commercially available. With many states in the process of implementing the MEPDG, it is important that the CTE measurements from the various devices provide accurate and comparable results. Results from a nationwide CTE interlaboratory study conducted by FHWA are presented. Eleven custom-built and seven commercially purchased CTE units were used in the study. The 18 CTE devices were divided into four groups, with each group testing one 410 stainless steel (SS) specimen and two concrete specimens with low and high CTE. Statistical analysis was performed to determine overall variability of the CTE measurements from the various devices. Additionally, variability was determined between custom-built and commercially available CTE units and between two CTE measuring methodologies (AASHTO TP60 and Texas test method). Accuracy of the CTE measurements from the interlaboratory study was determined from the CTE value of a 410 SS specimen measured according to ASTM E228-06, Standard Test Method for Linear Thermal Expansion of Solid Materials with a Push-Rod Dilatometer. Source

Bergeron K.A.,FHWA
Public Roads | Year: 2011

Kathleen A. Bergeron reflects on the challenges facing leaders in the transportation community and the highway world authorities. Basic tenets of highway programs that have been around since the beginning of motorized transportation are no longer reliable. The new normal means higher interest rates, slower economic growth, an increasing number of retirees, less consumption, more savings, a more diverse population, more uncertainty in our personal and national futures, and more uncertainty about the future in general. Highway organizations face the challenge of insufficient resources to operate. Although much has been said about the possibility of a revised taxing structure, the dangers of outdated infrastructure, and the benefits of new technologies, the word does not seem to be getting to the people who matter: the motoring public. Pete Rahn was brought into the Missouri Department of Transportation (MoDOT) in 2004 to institute change. Previously, he had headed New Mexico's dynamic State highway program. Source

Public Roads | Year: 2011

Lane reduction can increase safety for pedestrians, bicyclists, and motorists while improving the quality of life in downtowns across the country. In some areas, crossing opportunities for pedestrians are located only at signalized intersections that are spaced at uncomfortable walking distances. In some cases, the number of lanes may be unnecessary for the actual volume of motor vehicle travel. The road diet approach involves narrowing travel lanes or shoulders or eliminating some of them to provide more space for pedestrians and bicyclists. In theory, road diets have potential drawbacks, but in fact, case studies in a number of States suggest that problems usually do not occur. Instead, this approach offers a number of benefits in terms of traffic operations, safety, and livability when applied in the appropriate situations. The reduction in vehicle interactions resulting from a road diet potentially can decrease the number and severity of crashes. Source

Valdez M.,FHWA | Cheu R.L.,University of Texas at El Paso | Duran C.,Walter P. Moore
Transportation Research Record | Year: 2011

This research examined the operations of a four-legged, two-lane modern roundabout with different combinations of approach volumes. Because of the insufficient number of roundabouts to collect field data, experiments were performed with the VISSIM microscopic traffic simulation model. The model's parameters were calibrated with real data collected at a four-legged, two-lane modern roundabout. Different combinations of approach volumes, which ranged from 400 to 1,600 vehicles per hour, were simulated to create relatively high or low traffic demand in one or two approaches. The performance of each roundabout approach was measured by average control delay and level of service. The results of the experiments highlight the potential operational issues of four-legged, two-lane modern roundabouts. The results are presented in three charts that may serve as a quick reference guide for engineers to determine whether a roundabout is a feasible type of intersection control for a given set of design volumes before in-depth engineering analysis is performed. Source

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