  |
   |
 |
 |
 |
 |
 |
 |
 |
 |
 |
May/June 2001
Using the Dynamic Modulus Test to Assess the Mix Strength of HMA
by Thomas HarmanHighway agencies have long been searching for a mechanistically based laboratory test to reliably characterize the strength and load-resistance of hot-mix asphalt (HMA) mixes. Researchers think they have the answer - the dynamic modulus (E*) test.
While E* has been known to some researchers since the 1960s, the use of E* by departments of transportation has not been widespread. However, two current major efforts in pavement research - revising the AASHTO Guide to the Design of Pavement Structures and modeling for SuperpaveTM - rely on the use of E*.
The current draft of the upcoming 2002 revision of the AASHTO Guide to the Design of Pavement Structures (AASHTO 2002 Design Guide) uses the dynamic modulus test (E*) to characterize mixes used on interstate highways and most other high-volume highways that require superior load resistance.
During the Strategic Highway Research Program (SHRP) and the development of the Superpave system, a similar approach for mixture testing was explored using shear testing. E* is one of the tests currently under consideration for addition to the Superpave mix design system as a simple performance test.
Before E* is adopted by the American Association of State Highway and Transportation Officials (AASHTO) for the AASHTO 2002 Design Guide, at least 15 states plan to take E* for a test run to make sure that it is practical for use by the states and that it produces reliable and usable results.
"We want to make sure the states are comfortable designing mixes using this test method," said Charles E. Dougan of the Connecticut Transportation Institute (CTI) at the University of Connecticut. CTI is working with the states to test the use of the existing ASTM test protocols for measuring E*. Toward that end, the CTI project staff is designing a test suite to address issues of ruggedness, precision, and bias. The goal is the eventual adoption of the E* protocol as an approved AASHTO test procedure.
NCHRP-Funded Research Developing More Mechanistic Approach for AASHTO 2002 Design Guide
The AASHTO Joint Task Force on Pavements (JTFP) is responsible for the development and implementation of pavement design technologies. This charge has been pursued by the JTFP since the AASHTO Road Test.
AASHTO's Guide for the Design of Pavement Structures, initially published in the 1970s and updated several times (most recently in 1993, with a 1998 supplement), is the primary document used to design new and rehabilitated highway pavements. JTFP kicked off an effort to develop the 2002 edition of the guide at a 1996 workshop in which pavement experts developed a framework for improving it. The workshop participants concluded that throughout the highway community, the major emphasis in pavement design is now on rehabilitation, for which empirical design approaches are often inadequate. Because mechanistic-empirical approaches more realistically characterize in-service pavements and improve the reliability of designs, the next generation of design approaches, which will be documented in the 2002 edition of the design guide, will be based on mechanistic principles.
However, because of gaps that exist in the knowledge base, mechanistic design methods need to be supported by empirical relationships, and many of the issues relating to the mechanistic-empirical approach need to be better defined before practical and realistic design procedures can be developed and put into use. Researchers have been at work since 1997 to fill in these knowledge gaps. The current research effort by ERES Consulting Inc. under NCHRP Project 1-37A, which is scheduled for completion in December 2001, will produce a second draft of the guide. Final deliverables will include a user-oriented computational software system and documentation based on the 2002 Guide for the Design of Pavement Structures and a program to train users and promote use of the new guide.
For more information on NCHRP Project 1-37A, contact Amir Hanna, National Cooperative Highway Research Program, (202) 334-1892.
Dynamic modulus testing will enable the highway community to accurately characterize the strength and load resistance of their asphalt mixes. The ultimate goal is better asphalt pavement. What is Dynamic Modulus?
The dynamic modulus is a linear viscoelastic test for asphalt materials that was originally developed at Ohio State University. E* was adopted by the Asphalt Institute as the "Modulus Test of Choice" in the late 1960s.Relating mixture modulus to temperature and time rate of loading has been an integral part of several mechanistic-empirical design procedures used throughout the world. The dynamic modulus is the basic protocol for characterizing asphalt concrete (AC) mixtures used in:
- The Asphalt Institute Airfield Design Procedure (MS-11).
- The Asphalt Institute Highway Design Procedure (MS-1)
- Several technical manuals developed for use with designs for U.S. Air Force, Army, and Navy military airfield and highway installations.
Advantages of the Dynamic Modulus Test Method
One of the most significant advantages for using E* is that researchers have accumulated over the last 30 years a wealth of historic laboratory data for the test's output variables. This continuously expanding database has served as the basis for the development of a series of predictive models that have been published in the technical literature by Dr. Matthew W. Witczak of Arizona State University and his colleagues. The most recent versions of the predictive models are based on a database of 2,750 test data points from more than 200 different AC mixtures, including a wide range of modified asphalts.The proposed E* approach for the 2002 AASHTO Design Guide will be completely compatible with the performance-graded (PG) binder specifications and test parameters being developed in the Superpave program. "The Superpave models project has led to the development of models that will allow the user to use current Superpave performance-graded binder properties in the dynamic modulus approach being developed for the AASHTO 2002 Design Guide," Witczak said.
How the Test Is Conducted
CTI's work is funded as an AASHTO pooled fund study, using money contributed by the participating states. CTI will provide the states with experience using the dynamic modulus test apparatus and protocols, starting at a meeting planned for August 2001 at Arizona State University. At the meeting, the study participants will be introduced to the equipment and tests. Researchers will show to the participants the results of E* tests on selected HMA mixes commonly used by various state transportation departments and will explain how the test results relate to various mix variables.Then the states will be able to see for themselves how well the test procedure enables them to accurately characterize the strength and load resistance of their mixes. If necessary, the test protocols and design procedures will be adjusted based on the states' experiences.
The dynamic modulus test was proposed by ERES Consultants Inc. in National Cooperative Highway Research Program (NCHRP) Study 1-37A, "Development of the 2002 Guide for the Design of New and Rehabilitated Pavement Structures: Phase II". A sinusoidal (haversine) axial compressive stress is applied to a specimen of asphalt concrete at a given temperature and loading frequency. The applied stress and the resulting recoverable axial strain response of the specimen is measured and used to calculate the dynamic modulus and phase angle.
One hundred millimeter- (four inch-) diameter cylinder specimens are tested at various temperatures and loading frequencies for a range of dynamic loads. Strain gauges affixed at various positions on the test specimens measure strain, and the data are automatically fed into a computer to calculate test results.
Equipment used to perform E* includes:
- Testing Machine. A servo-control testing machine produces a controlled sinusoidal (haversine) compressive loading. The testing machine can apply loads over a range of frequencies from 0.01 to 30 hertz and stress levels up to 2800 kilopascals (400 pounds per square inch).
- Environmental Chamber. The environmental chamber holds the temperature of the specimen constant at any set temperature range from -10 to 60 C (14 to 140 F). The chamber must be large enough to accommodate the test specimen and a dummy specimen with thermocouple mounted at the center for temperature verification.
- Measurement System. The system is fully computer-controlled. It measures and records the time history of the applied load and the axial deformations. The load is measured with an electronic load cell in contact with one of the specimen caps. Axial deformations are measured with linear variable differential transformers. Hardened steel disks transfer the load from the testing machine to the specimen.
- Superpave Gyratory Compactor (SGC). An SGC is used to prepare the laboratory specimens.
- Saw and Core Drill. A masonry saw and a water-cooled coring machine with a diamond bit are used for cutting test specimens
Regional Approach to Equipment Acquisition
Cost of the equipment (test frame, environmental chamber, measuring and recording systems) to run E* is in the $60,000 range. In addition, a Superpave gyratory compactor, coring rig, and other preparatory equipment are required."We are going to suggest that ultimately the states share equipment, perhaps through a regional contract," Dougan said. "To reduce the level of testing effort for each state, we suggest running a series of tests on the top mixes used in each region."
The initial testing will be conducted on equipment purchased for the pooled fund study, scheduled for delivery in June 2001. CTI will debug the equipment and run initial tests prior to the August workshop at Arizona State University.
Definitions While the term "dynamic modulus" is often used to denote any type of modulus that his been determined under "non-static" loading conditions, the strict technical definitions follow:
Dynamic Modulus ( E*): The dynamic modulus of a material is a viscoelastic test response developed under sinusoidal loading conditions. It is the absolute value of dividing the peak-to-peak stress by the peak-to-peak strain for a material subjected to a sinusoidal loading.
Linear Viscoelastic: Within the context of the dynamic modulus test, this refers to behavior in which the dynamic modulus is independent of stress or strain.
Phase Angle -<§> : The phase angle is one of the two output variables of the dynamic modulus test - the other being | E*|. The phase angle is a direct indicator of the elastic-viscous properties of the mix or binder material. The value of <§>= 0 is indicative that the material is behaving as a pure elastic material. A value of <§> = 90 indicates a pure viscous (Newtonian) material.
Thomas Harman is the Asphalt Pavement Team leader for FHWA's Office of Infrastructure Research and Development (R&D) at the Turner-Fairbank Highway Research Center (TFHRC) in McLean, Va. He both manages and conducts asphalt pavement research. His team is responsible for activities in asphalt pavement design, materials, chemistry, accelerated loading, and imaging. Harman joined FHWA in 1990, and he worked for the Office of Technology Applications, focusing on the implementation of the Strategic Highway Research Program (SHRP) Superpave system. In 1997, he moved to TFHRC. He services and participates in a host of expert task groups and committees. He has a bachelor's degree in civil engineering from the University of Maryland and a master's degree in civil engineering from the University of Illinois.
For more information on the E* pooled fund study, contact Charles E. Dougan at Connecticut Tranpsortation Institute, 179 Middle Turnkpike, U-202, Storrs, CT 06269-5202; telephone (860) 486-5535; fax (860) 486-2399; or e-mail cdougan@engr.uconn.edu.
Other Articles in this Issue:
5-1-1: Traffic Help May Soon Be Three Digits Away
Using the Dynamic Modulus Test to Assess the Mix Strength of HMA
The ITS Public Safety Program: Creating a Public Safety Coalition
Handling the Worst Crash Ever in Virginia
Moving Ahead - The American Public Speaks on Roadways and Transportation in Communities
Travelers Seek Byway Experiences
National Work-Zone Awareness Week Commemorated Across the Nation
Work-Zone Traffic Control: Survey of Contracting Techniques
