SWUTC Research Project Description
A Comprehensive Characterization of Asphalt Mixtures in Compression
University: Texas A&M University
Principal Investigator:
Yuqing Zhang
Texas Transportation Institute
(979) 458-0893
Project Monitor:
Anastasia Muliana
Department of Mechanical Engineering
Texas A&M University
3123 TAMU
College Station, TX 77843-3123
Funding Source: USDOT and State of Texas General Revenue Funds
Total Project Cost: $59,880
Project Number: 600451-00006
Date Started: 4/1/12
Estimated Completion Date: 3/31/13
Project Summary
Project Abstract:
Asphalt pavements exhibit various distresses such as rutting, cracking and moisture damage if asphalt mixtures are not well designed and constructed based on material fundamental mechanical principles. Many research efforts had been made on the investigation of the permanent deformation and cracking, which were normally investigated individually in existing studies. However, they are companions and can be affected by each other. Thus it is urgently necessary to investigate the rutting and cracking simultaneously by comprehensively analyzing the mechanistic properties of asphalt mixtures under a compressive load. In addition, the existing mechanistic model could not predict the material performance due to their deficiencies in addressing some crucial characteristics of the asphalt mixtures, such as the anisotropy including inherent anisotropy and stress-induced anisotropy, the hydrostatic-dependent yield surface satisfying convex requirements and considering the differences between compression and extension in a full range of friction angle, and the viscofracture properties of the asphalt mixtures that occurs in the tertiary stage in compression.
This proposed research will develop an advanced constitutive model to comprehensively characterize the anisotropic, viscoplastic, and viscofracture properties of the asphalt mixtures in compression. A systematic testing protocol and analyzing formulations will be proposed to rapidly and accurately determine the parameters of the constitutive model and relate these parameters to measurable and understandable engineering material properties. The permanent deformation and cracking will be investigated simultaneously with using the proposed mechanistic model, which include the accelerating effect among them, the effect of binder, air voids and aging on the evolution of damages. In addition, practical distress prediction models for rutting and cracking will be proposed based on the results of the mechanistic analysis.
Project Objectives:
The objective of this study is to provide pavement engineers and researchers with fundamental mechanistic models and efficient, reliable, and user-friendly testing methods to comprehensively characterize the engineered properties of the asphalt mixtures in compression and to promote the understandings and the predictions of the pavement distresses in the field. More specifically, the following research objectives will be achieved:
1) Model the constitutive behaviors of the asphalt mixtures using a comprehensive equation which accounts for the viscoplasticity, viscofracture and the anisotropy including inherent anisotropy and stress-induced anisotropy.
2) Develop a systematic testing protocol and analyzing formulations to rapidly and accurately determine the parameters of the constitutive model and to relate the parameters of the constitutive model to the measurable and understandable engineering material properties.
3) Investigate the permanent deformation and cracking simultaneously using the proposed
mechanistic model, which includes the mutually promotive effects of the two damages and the effects of binder type, air void content and aging on the evolution of damages. In addition, practical distress prediction models will be proposed based on the results of the mechanistic analysis.
Task Descriptions:
Task 1: Literature Review
The objective of this task is to conduct an extensive literature review to gather information on available mechanistic models and related parameter acquisition methods for characterizing the permanent deformation and cracking of asphalt mixtures. The research team will perform literature searches on the following aspects:
1) Constitutive models and characterization methods for the modeling of the viscoplasticity,
viscofracture and anisotropy of asphalt mixtures;
2) Testing protocols and procedures used to determine the model parameters and material
properties;
3) Distress prediction models for the rutting and cracking of asphalt pavements;
4) Effects of material volumetric characteristics and aging conditions on the rutting and fracture performance of the asphalt mixtures.
Task 2: Development of a Comprehensive Characterizing Model
The objective of this task is to develop a comprehensive constitutive model to characterize the anisotropic, viscoplastic, and viscofracture properties of the asphalt mixtures. Based on the preliminary study, the widely used Perzyna’s viscoplastic model will be employed as a basic modeling framework which will be modified to consider the effects of the anisotropy and the viscofracture on the evolution of the damages including permanent deformation and cracking. The following developments and modifications will be taken into accounts during the modeling:
1) Anisotropic tensors will be integrated into the constitutive model to account for both the
inherent anisotropy due to the horizontally preferential oriented aggregates and the stress
induced anisotropy due to the growth of cracks in compression;
2) A new yield surface function will be proposed to address the hydrostatic stress dependent yield surface as well as the differences of yielding between compression and extension. The new yield surface will satisfy the convex requirement and simultaneously allow the internal friction angle of the asphalt mixtures ranges from 0 to 90 degrees;
3) A non-associated plastic potential function and a strain hardening law will be incorporated in the anisotropic viscoplastic-viscofracture model;
4) Evolution models will be developed to characterize the growth rate of the viscoplasticity and viscofracture of the asphalt mixtures under repeated loading.
Task 3: Model Parameters Acquisition and Testing Protocol Design
The objectives of this task are to provide a systematic testing protocol and analyzing formulations to rapidly and accurately determine the parameters of the constitutive model using commonly accessible testing equipment. In addition, to make the model understandable to most civil engineers, some of the model coefficients will be derived to be related to the commonly-used and understandable engineering material properties such as modulus, cohesion, and internal friction angle. Basically, the model parameter can be divided into three categories. Each category and corresponding testing protocols are addressed as follows:
1) Strength-related coefficients in yield surface and plastic potential functions. Testing protocol may include uniaxial compressive strength tests and triaxial compressive strength tests at multiple confining pressures conducted on different asphalt mixtures. Since the triaxial tests require special apparatus, e.g. triaxial cell, which may not be accessible for some civil engineers, the indirect tensile strength tests will be examined to determine if the strengthrelated model coefficients can be obtained by only conducting the uniaxial compressive strength tests and the indirect tensile strength tests. To determine the yield surface on an octahedral plane, a simple shear test with confining pressure will be developed to determine the yielding properties due to the differences between compression and extension.
2) Viscoplasticity-related parameters in Perzyna’s model and the strain hardening law.
Testing protocol may include a destructive repeated load test, in which the total strain is
recorded and then decomposed into elastic, viscous, plastic, viscoplastic and viscofracture
strains using the strain decomposition technique. The plastic and viscoplastic strain will be
utilized to determine the coefficients in the Perzyna’s viscoplastic strain model and the strain
hardening function.
3) Anisotropy-related parameters in the anisotropic tensor formulations. Testing protocol may include a nondestructive test to determine the inherent anisotropic tensor. The stress induced anisotropy will be represented by an anisotropic damage density that is a lost area ratio on a specific cross section. The anisotropic damage density will be determined based on the separated viscofracture strains in both axial and radial directions in the destructive repeated loading tests.
Task 4: Laboratory Experiments and Model Validations
The objectives of this task are to perform laboratory experiments according to the testing protocol designed in Task 3 on a variety of asphalt mixtures with different volumetric and aging properties and to propose a performance test to validate the proposed characterizing model for both permanent deformation and cracking. Specifically, the following work will be conducted:
1) Prepare materials including aggregates, binder and mineral fillers, and fabricate asphalt
mixtures specimens in the lab;
2) Perform the laboratory tests according to the testing protocol designed in Task 3 on different asphalt mixtures that vary by asphalt binder, air void content and aging condition;
3) Conduct different performance tests, predict the testing results using the proposed models and validate the models based on the comparisons between modeling and testing; and
4) Based on the proposed mechanistic model and the performance testing results, propose quick distress determining methods to evaluate the material resistance to permanent deformation and cracking.
Task 5: Technology Transfer and Final Report
The objective of this task is to document and report the performed research in a written format which includes the proposed model, test protocol, theoretical analysis, calculation methods and experimental results and findings. The main results will be summarized in a couple of technical articles for presentation at the next annual Transportation Research Board meeting and for potential publication in several learned professional journals.
Implementation of Research Outcomes:
Obviously, the asphalt mixtures that are used in pavement construction are under compression when a rolling tire passes over them. Under such a high compressive load, irrecoverable deformation occurs and contributes to rutting which can trap water and lead to wet weather accidents. The behaviors of asphalt mixtures have not been characterized completely due to the complexity of the material. This project overcame this shortcoming of the existing models and provides a comprehensive mechanistic model and characterizing testing methods for asphalt mixtures in compression. The model impacts the theories, knowledge and research methods of pavement engineering by accounting for following characteristics of asphalt mixtures that have not been considered in previous models:
1) anisotropy of the asphalt mixtures which means the material has different properties when measured in different directions. Without considering anisotropy, the rutting depth would be underestimated;
2) the asphalt mixtures crack under compressive load, which is one of the discoveries of the project. The compressive cracks accelerate the development of the rutting and was taken into account in the proposed model of this project.
The aforementioned two properties were integrated into a comprehensive viscoplastic theory which accounts for anisotropy, the compressive cracking, the fact that all asphalt mixtures have a friction angle greater than 22 degrees, the hardening beyond the yield point, and the associated flow rule. All of the properties are fundamentals of the asphalt mixtures, which must be considered during the performance prediction of the asphalt pavements. The model developed in this project provides an innovative and straight-forward method to account for these material properties and comprehensively characterize the asphalt mixture in compression.
Products developed by this research:
Presentation: Constitutive Modeling of Anisotropic Viscoplasticity of Asphalt Concrete, Y. Zhang and R. Lytton, presented to the 12th U.S. National Congress for Computational Mechanics, Raleigh, North Carolina, July 2013.
Presentation: Mechanistic Modeling of Fracture in Asphalt Mixtures under Compressive Loading, Y. Zhang, R. Luo and R. Lytton, presented to the Transportation Research Board 92nd Annual Meeting, Washington, C.D., January 2013.
Presentation: Modeling Asphalt Concrete in Compression, Y. Zhang, R. Luo and R. Lytton, presented to the 49th Petersen Asphalt Research Conference, Laramie, Wyoming, July 2013.
Technical Paper: Characterization of Viscoplastic Yielding of Asphalt Concrete, Y. Zhang, R. Luo and R. Lytton, Construction and Building Materials, 47, 671-679.
Presentation: Development and Validation of a Generalized Viscoplastic Yield Surface Model for Asphalt Concrete, Y. Zhang, R. Luo and R. Lytton, submitted for presentation at the 93rd Transportation Research Board Meeting, Washignton, D.C., January 2014.
Journal Article Submitted for Review: Anisotropic Modeling of Compressive Crack Growth in Tertiary Flow of Asphalt Mixtures, Y. Zhang, R. Luo and R. Lytton, Journal of Engineering Mechanics, American Society of Civil Engineers (ASCE)
Journal Article Submitted for Review: Development and Validation of a Generalized Viscoplastic Yield Surface Model for Asphalt Concrete, Y. Zhang, M. Bernhardt, G. Biscontin, R. Luo and R. Lytton, Journal of Engineering Mechanics, American Society of Civil Engineers (ASCE)
Impacts/Benefits of Implementation:
In addition to directly impacting and benefiting the state of good repair of the highway transportation system, the theory developed in this project is general and applies to many other materials used in the industries. Some good examples include:
1) The compressive cracking theory can be employed to study the time-dependent cracking of rocks under high compressive earth pressures.
2) The viscoplastic deformation theory including flue rule, yield surface and plastic potential can be used to predict the permanent deformation of many geo-materials such as soils, aggregates, sands, cement, and concrete.
3) The energy-based fracture method can be used to predict the time-dependent cracking of polymer-based materials used in the industries as diverse as the aerospace, medicine, adhesives, and infrastructures.
Web Links:
Final Report