Carlos E. Cary

Resilient Modulus for Unsaturated Unbound Materials

ABSTRACT The suitability of the current resilient modulus test protocol (NCHRP 1-28A) for its application to unsaturated soils was assessed. Modifications to the stress state conditions of the protocol are necessary due to the axis-translation needed during the test when measuring matrix suction. This study presents the modulus of unbound materials resulting from tests performed under unsaturated soil conditions. Two different materials were tested. The base material was tested under drained and undrained boundary conditions, while the subgrade was tested under drained boundary condition. The results allowed for the enhancement of the Universal Model for resilient modulus prediction by incorporating suction as a stress state. This model predicts the resilient response of unbound materials as a function of external stress state and matrix suction levels and therefore, it is independent of moisture variation.

Enhanced Model for Resilient Response of Soils Resulting from Seasonal Changes as Implemented in "Mechanistic–Empirical Pavement Design Guide"

The present study deals with the revision of the current model in the Mechanistic–Empirical Pavement Design Guide (MEPDG) used to predict the environmental factor for unfrozen unbound materials (FU), which is used to adjust the resilient response of soils resulting from seasonal changes. A large database with data from the existing literature and studies at Arizona State University was developed to evaluate the model. The results suggest that the environmental factor is underestimated for fine-grained materials with high plasticity under dry (arid) conditions. However, insufficient data were available to enhance the FU models for wetter conditions. Three fundamental factors that may have impacts on the FU values were evaluated in this study: stress state, compaction energy (soil density), and soil type. The stress state was found to have little to no impact on the predictions of FU. But density changes and soil type were found to be important. The potential for soil index properties to be predictive variables was assessed. Models dependent on enhanced moisture content accounting for the effect of soil type are proposed for nonplastic and plastic materials. The range of predicted FU values is in close agreement with the actual measured FU values found from laboratory studies. It is recommended that the new models be adopted in the revision of the MEPDG model for the drier conditions described in this report and that research be conducted to enhance the FU approach in the current MEPDG for wetter conditions brought on for a variety of reasons (e.g., groundwater table change, increased rainfall, and frost effects).

Data Collection to Support Implementation of the Mechanistic-Empirical Pavement Design Guide for County Roads

Evaluation and calibration of the Mechanistic–Empirical Pavement Design Guide (MEPDG) has been attempted by various agencies throughout the United States. Agencies interested in adopting the MEPDG procedure must prepare a practical implementation plan that fits local conditions. The first step in the implementation plan is collection of design input data and establishment of a database for inputs. A 3-year study was conducted at Arizona State University to establish a database to support MEPDG implementation for the Maricopa County, Arizona, Department of Transportation. The implementation program included testing of asphalt binders, hot-mix asphalt dynamic modulus, and unbound-materials resilient modulus; development of climatic weather stations and training material; and collection of traffic data. The collected information can be used to calibrate the MEPDG distress models to county conditions and verify such models. The input parameters can serve as a framework for similar highway agencies and help ensure the successful implementation of the MEPDG.

A New Generation of Resilient Modulus Characterization of Unbound Materials

Pavement damage is heavily influenced by seasonal variations in the environmental regime and by externally applied loads. The state of the art methodology currently evaluated by pavement engineers focuses upon coupling the effects on pavement performance of moisture content/matric suction properties of unbound materials with the external stresses. The literature review showed that several equations have been proposed to incorporate moisture variation effects on resilient modulus; however, the limited models available lack fundamental principles or have not been properly validated. A feasibility study on cohesionless granular base material is presented, whereas matric suction was controlled during drained and measured during undrained resilient modulus tests. It is apparent that certain modifications in the recommended stress state conditions of the NCHRP 1-28A protocol may be necessary when measuring matric suction due to the axis-translation needed during the test. Difficulties associated with the test include the current confinement fluid, which allows for air diffusion during the suction equilibration stage. Preliminary results show that the k1-k2-k3 parameters used in the approved AASHTO Mechanistic Empirical Pavement Design Guide (ME-PDG) may be functions of the suction applied to the specimen during the test protocol.

Effect of Pavement Structure on the Mechanical Response and Performance of Perpetual Pavements at the National Airport Pavement Test Facility

Construction Cycle 7 (CC7) conducted at the FAA National Airport Pavement Test Facility (NAPTF) was aimed to study the effect of hot mix asphalt (HMA) layer thickness and develop perpetual pavement design criterion for airfield flexible pavements. Four fully instrumented perpetual test pavements were designed, constructed, and tested, with all test items trafficked under heavy aircraft loads using a three duals in tandem configuration. Pavement condition was monitored using heavy weight deflectometer tests, distress surveys, and surface profiles. Comprehensive response data analysis revealed that increasing HMA layer thickness significantly reduced the tensile strain at the bottom of the HMA layer, permanent deformation in the unbound layer, and vertical stress at top of the subgrade. Thicker HMA test items exhibited better rutting performance, as evidenced in both multiple depth deflectometer and surface profile data. The interaction between HMA layer temperature and thickness was captured in the pressure cell responses. More tests are planned to determine the threshold HMA strain to prevent fatigue cracking.

Evaluation of Asphalt Concrete Layer Response Using Asphalt Strain Gauges and Fiber Optic Strain Gauges

Four flexible pavements were constructed on the north side for Construction Cycle 7 (CC7) at the Federal Aviation Administration’s (FAA) National Airport Pavement Test Facility (NAPTF) to develop flexible perpetual pavement design criterion and validate/refine/modify the fatigue model for asphalt concrete (AC). H-Bar asphalt strain gauges (ASG) were installed in four test sections to measure transversal and longitudinal strain at the bottom of the AC layer. Fiber optic strain gauges (FOSG) were also installed in one of the test sections (8-inch thick asphalt concrete layer). Full-scale tests were performed on these pavement test sections under heavy aircraft gear loads using the National Airport Pavement Test Vehicle (NAPTV). Three traffic speeds, four gear configurations and three wheel loads were used to evaluate the mechanical response of the AC layer. It was found that response of fiber optic strain gauges is comparable to the asphalt strain gauge. The effects of traffic speed, gear configuration, and wheel load on the asphalt concrete strain were investigated. Strain basins were established based on five wander positions.

Integrating National Database of Subgrade Soil–Water Characteristic Curves and Soil Index Properties with Mechanistic–Empirical Pavement Design Guide

The database developed under NCHRP Project 9-23A, Development of a National Catalog of Subgrade Soil–Water Characteristic Curves (SWCC) Default Inputs to Use in the MEPDG, included not only measured soil index properties needed in all hierarchical levels of the enhanced integrated climatic model but also SWCC parameters, which are key in the implementation of Level 1 analyses. A set of maps in portable document format displaying the location of every soil unit identified within the continental United States, Hawaii, Alaska, and Puerto Rico and a simple interface in MS Excel to aid in querying of data were also developed for the project. Under NCHRP Project 9-23B, Integrating the National Database of Subgrade Soil–Water Characteristic Curves and Soil Index Properties with the MEPDG, a second research effort was directed to integrate an enhanced version of the geographic information system–enabled database with the Mechanistic–Empirical Pavement Design Guide. Specifically, NCHRP Project 9-23B aimed at the implementation of an interactive tool that allows the design guide users (currently DarWin-ME users) to retrieve both appropriate soil unit maps and soil properties relevant to a particular user-specified location by entering either state milepost information or geographical coordinates. The final product was integrated into a public website that can be accessed by design guide users, agencies, industry, and academicians through a simple link. Details of the development of this useful search tool and its main features are presented.

Comparison of Design Thicknesses for Flexible Airfield Pavement Based on Agency Limiting Subgrade Strain Criteria

A newly developed mechanistically based design procedure for airfield hot-mix asphalt pavement is studied. The pavement performance is calculated in terms of critical strains on the basis of multilayer theory. Rutting failure criteria from the Asphalt Institute, Shell Oil, and the revised U.S. Army Engineer Waterways Experiment Station (USACE-WES) are used to calculate the thickness requirements necessary for a range of design input variables. The program has been implemented in ZAPRAM, an event-driven, user-friendly educational computer program that runs in the Excel 2007 environment, coupled with Visual Basic programming. Results of the design comparison indicate a significant difference between the three common airfield pavement design procedures used. Differences between the Asphalt Institute procedure and the revised USACE-WES procedure are relatively moderate for all design situations. The greatest deviation is found between the Shell Oil approach and the other two criteria. The design difference depends heavily on several major factors. An analysis of heavy aircraft (B-747) operating on a low-support subgrade foundation may yield pavement design differences of 2 to 3 ft of granular subbase material. A major effort to enhance the state of the art for airfield pavement design models is recommended.