Mary M Robbins

Evaluation of Dynamic Modulus Predictive Equations for Southeastern United States Asphalt Mixtures

Mechanistic–empirical pavement design has recently come to the forefront of design, with many states looking to the Mechanistic–Empirical Pavement Design Guide (MEPDG) as their future primary design method. Central to any flexible pavement mechanistic–empirical design framework is the characterization of hot-mix asphalt (HMA) through the use of dynamic modulus E*. Because expensive and specialized equipment is required to measure E* in the laboratory, predictive E* equations that use binder and mixture properties to estimate E* at various frequencies and temperatures must be evaluated. There are currently three global E* predictive equations, two of which are used at Levels 2 and 3 in the MEPDG. These three models (Witczak 1–37A, Witczak 1–40D, and Hirsch) were evaluated with the use of 18 HMA plant-produced, lab-compacted mixtures (representative of general-use mixtures used in the southeastern United States) that were placed at the 2006 National Center for Asphalt Testing test track. E* predictions were made at three temperatures and three frequencies for direct comparison with measured values. The Witczak models had the greatest deviation from measured values, and the Witczak 1–40D model overestimated E* by approximately 61%. The Hirsch model most accurately predicted the moduli for the 2006 test track mixtures. Calibration of the Hirsch model for these mixtures indicated that the Poisson ratio selected for the asphalt binder had little effect on its prediction capabilities. The little improvement resulting from calibration proves that this step is unnecessary.

Analysis of Perpetual Pavement Experiment Sections in China

The rapidly growing highway infrastructure in China has led to the need for long-lasting, high-performing pavement structures. Historically, under frequent extremely heavy loads, expressways in Shandong Province that use pozzolana-treated base have needed to be fully rehabilitated after only 5 to 8 years. In 2005, a full-scale experiment was initiated to develop a better understanding of how these pavements behave under load and to explore perpetual pavement concepts. Five sections were constructed on a new expressway near Binzhou, Shandong Province, China. Three sections were designed as full-depth asphalt pavements that use perpetual pavement concepts, while the other two sections followed more-conventional Chinese designs. The original designs developed in 2004 used the best available information but did not represent in situ properties, climate, or traffic. Now that the sections have been used for 5 years, valuable data sets have become available to conduct an analysis of the as-built sections. This paper documents the relevant input parameters and analysis conducted by using PerRoad, a probabilistic mechanistic–empirical pavement analysis program. Surveyed thicknesses, material properties derived from backcalculation, and load spectra from an on-site weigh-in-motion system were entered into PerRoad, from which probabilistic distributions of pavement response were generated. Both full-bond and full-slip interfacial conditions were modeled. Tensile strain data showed the unlikeliness that any of the sections will experience bottom-up fatigue cracking. Compressive stress in the pozzolana-treated base could be contributing to distress, and stress distributions are proposed to control cracking in the pozzolana-treated material. Interface conditions also appear to be critical.

Further Evaluation of Limiting Strain Criteria for Perpetual Asphalt Pavement Design

Perpetual asphalt pavements have been designed with single threshold values for the horizontal strains at the bottom of the asphalt concrete layer and for vertical strains on top of the subgrade to prevent the occurrence of bottom-up fatigue cracking and subgrade rutting. Several thresholds have been utilized for design based on laboratory and field test results. Limiting strain distributions were recently proposed for perpetual pavement design instead of single threshold values for controlling the horizontal and vertical strains. These design criteria were developed with data collected from test sections at the National Center for Asphalt Technology pavement test track. The objective of this study was to conduct additional analyses of eight perpetual pavement sections located in different climatic regions, to support the proposed limiting strain criteria. These sections were simulated in the PerRoad perpetual pavement design software to determine the horizontal strains at the bottom of the asphalt concrete layer and the vertical strains on top of the subgrade. The strains were then analyzed to evaluate the proposed limiting strain criteria. Based on the simulations, the limiting horizontal strain distribution above the 60th percentile can effectively differentiate the calculated strain distributions of the eight perpetual pavement sections from those of the test sections that failed due to bottom-up fatigue cracking on the test track. In addition, the calculated vertical strains on top of the subgrade at the 50th percentile were lower than the proposed 200-microstrain limit. These results provide additional support for utilizing the proposed strain criteria in perpetual asphalt pavement design.

Adaptation and validation of stochastic limiting strain distribution and fatigue ratio concepts for perpetual pavement design

Traditional perpetual pavement thickness design is based, in part, on controlling strain levels at the bottom of the asphalt concrete layer below an endurance limit to prevent bottom-up fatigue cracking (FC). A field-based limiting strain threshold was developed from cumulative distributions of field-measured tensile strains in the 2003 and 2006 research cycles at the National Center for Asphalt Technology Pavement Test Track to understand the limiting strain necessary to control FC. Additionally, the fatigue ratio, the ratio of the nth percentile strain to the fatigue endurance limit, was developed. Both the tensile strain distributions and fatigue ratios showed a clear difference between sections that experienced bottom-up FC and those that did not. However, it is necessary to adapt these thresholds to strains predicted by perpetual pavement design tools. PerRoad, a stochastic perpetual pavement design programme, was used to predict strains for the same 2006 sections. Previously developed strain distrib...Traditional perpetual pavement thickness design is based, in part, on controlling strain levels at the bottom of the asphalt concrete layer below an endurance limit to prevent bottom-up fatigue cracking (FC). A field-based limiting strain threshold was developed from cumulative distributions of field-measured tensile strains in the 2003 and 2006 research cycles at the National Center for Asphalt Technology Pavement Test Track to understand the limiting strain necessary to control FC. Additionally, the fatigue ratio, the ratio of the nth percentile strain to the fatigue endurance limit, was developed. Both the tensile strain distributions and fatigue ratios showed a clear difference between sections that experienced bottom-up FC and those that did not. However, it is necessary to adapt these thresholds to strains predicted by perpetual pavement design tools. PerRoad, a stochastic perpetual pavement design programme, was used to predict strains for the same 2006 sections. Previously developed strain distributions and fatigue ratios were adjusted to reflect observed differences in predicted and measured strains. Cumulative distributions and fatigue ratios based on predicted strains for the 2009 research cycle validated the updated limiting strain distribution and maximum fatigue ratios for designing perpetual pavements to resist bottom-up FC.