|Development of New Standard Test Methods for Characterizing Low Temperature Properties of Asphalt Binders
by Raj Dongré, Ph.D., and John DAngelo, P.E.
This article describes the development of three new ASTM standards used to determine low temperature properties of asphalt binders. These standards in the chronological order of their development are D 6648, Test Method for Determining the Flexural Creep Stiffness of Asphalt Binder Using the Bending Beam Rheometer; D 6723, Test Method for Determining the Fracture Properties of Asphalt Binder in Direct Tension; and D 6816, Practice for Determining Low-Temperature Performance Grade of Asphalt Binders. They were developed under the auspices of Subcommittee D04.44 on Rheological Test Methods, part of Committee D04 on Road and Paving Materials. The new standards define the test protocol and/or calculation practice to measure rheological and failure characteristics (fundamental engineering properties) of asphalt binders. Described below is the need for new standards followed by a brief history of their development. Next, the scope and purpose of the standards are discussed followed by a description of potential users of these standards. The impact of these standards on the quality and safety of the highway pavements is also reported.
Why Are the New Standards Needed?
A majority of the roads (pavements) in the United States and around the world are constructed using a composite mixture of asphalt binder and aggregates blended to meet preset design requirements. In road building terminology, this composite of asphalt binder and mineral aggregates is called hot-mix asphalt or, simply, asphalt mixture.
The ultimate goal is to build better roads. A well-performing road is one that provides a safe and comfortable ride over its design life. Thermal (or low temperature) cracking of the road surface leads to poor performance due to the loss of structural integrity and, ultimately, premature deterioration of the entire pavement section. Low-temperature cracking, while found in most of the states, is more ubiquitous in the northern latitudes, such as the colder climates of North America and Canada. Resistance to thermal cracking is primarily provided by the asphalt binder. Cracked pavements require a lot of maintenance and are therefore expensive. Figure 1 shows an example of thermal cracking in asphalt pavements.
To build well-performing (with minimal thermal cracking) pavements performance based specifications derived from fundamental engineering properties of the asphalt binder are needed. Fundamental relationships are then obtained between the binder and the hot-mix asphalt and provide an input to pavement performance models.
Performance based purchase specifications enable engineers to specify high quality asphalt binders to build safe and long-lasting roads. Effective specifications require that standard test methods are available to measure the required physical properties of asphalt binders. The development of sound ASTM standards, such as those described in this article, is the key to measuring repeatable and reproducible fundamental engineering properties at low service temperatures.
Historical Development of the New Standards
In 1987, the U.S. Congress allocated US$150 million for an ambitious highway research program called the Strategic Highway Research Program to be completed in five years. A total of $50 million was allotted within SHRP to study asphalt binders. Upon completion of SHRP research in 1993, a set of performance-related specifications were developed for asphalt binders as well as mixtures. These were collectively called the Superpave pavement design system.
These specifications, which are purchase specifications, were adopted by the American Association of State Highway and Transportation Officials and are designated as AASHTO M320 (formerly designated as MP1) and AASHTO MP1a. The corresponding ASTM standard for AASHTO M320 is ASTM D 6373, Specification for Performance Graded Asphalt Binder. The ASTM standard corresponding to AASHTO MP1a is not complete at this time. AASHTO M320 was developed after completion of SHRP. AASHTO MP1a was developed later by the Low Temperature Task Group under the direction of the binder Expert Task Group sponsored by FHWA.
The principle difference between AASHTO M320 and MP1a is that MP1a incorporates a mechanistically based criterion to define low temperature performance of asphalt binders, whereas M320 uses empirical parameters related to performance. The properties obtained from ASTM D 6648 and D 6723 are used as input to ASTM D 6816 to obtain a critical cracking temperature (Tcr) as specified in AASHTO MP1a, whereas the properties obtained from ASTM D 6648 are used in the Superpave pavement design system under the purchase specification ASTM D 6373 (AASHTO M320).
The Superpave specification is based on test methods that were adapted and/or specifically designed to measure fundamental material properties as opposed to the empirical test methods employed by the earlier viscosity and penetration based specification. Two test methods and a calculation practice were developed for use in the Superpave specification to address low-temperature cracking performance of pavements:
A flexural creep test called the bending beam rheometer (BBR) to measure creep properties at low temperatures. The corresponding ASTM standard is D 6648 (Figure 2).
A uniaxial tension test at constant elongation rate called the direct tension test (DT) to measure failure properties at low temperatures (Tg<T<Tg+25°C). The constant elongation rate simulates the loading (in uniaxial tension) during thermal cooling of the pavement. The corresponding ASTM standard is D 6723 (Figure 3).
The critical cracking temperature (Tcr) specified in AASHTO MP1a is calculated using an algorithm that was developed into an ASTM standard calculation practice designated as D 6816 (Figure 4). This practice requires data input from both D 6648 and D 6723 to determine Tcr.
Scope and Purpose of the New Standards
The above BBR and DT test methods have been previously used in one form or another to characterize various materials including asphalt binder. A substantial redesign was needed in each case for use as a specification device. These devices were redesigned to produce repeatable and user-friendly test methods that can be performed at a moderate price.
The BBR test device was originally developed at Penn State University prior to the SHRP program in 1986 - 1987. (1) During the SHRP program, from 1988 to 1992, numerous enhancements were made to the BBR test device. For example, an air bearing had to be used because metal bearings used in the previous design had introduced errors. A user-friendly version of the software was written and the test protocol was further refined for specification type use. An important phenomenon called physical hardening of asphalt binders during prolonged exposure to low temperatures was discovered during the refinement efforts.
The BBR standard test method is specific in its design as a test to obtain creep properties at a 60-second loading time. Low temperature rheological characteristics for research use cannot be obtained using this test method beyond 240-second loading time where the test is terminated. Two parameters are obtained from D 6648 (BBR) for use in the specification. The stiffness, S, at 60-second loading time was found to relate to thermal cracking in the field. The slope of log-log plot of stiffness versus time, called the m-value, is used to control the shape of the stiffness master curve of the asphalt binder at low temperature. From laboratory tests performed on asphalt binder mixtures, m-value has been shown to be related to thermal fracture as well as thermal fatigue cracking. According to the specification, an asphalt binder will perform adequately at a given low service temperature if its S value is less than or equal to 300 MPa and the m-value is greater than or equal to 0.3.
Of the low-temperature test methods, the DT was the most challenging to design. This is because of the well-documented problems with testing brittle materials in tension and the unique gripping and loading problems presented by asphalt binder. Special fiberglass inserts were designed to grip asphalt binder for uniaxial testing to failure in the DT. The conventional compression type grips produce premature failure at the grips below the brittle-ductile temperature and excessive deformations above it.
The DT test method as initially proposed at the end of SHRP was found to be inadequate as a standard test method due to persistent problems with various aspects of the test system. The Federal Highway Administration in cooperation with Instron Corporation of Boston, Mass., further refined the DT test method as it exists in its current form. (2)
The DT test method is not limited to specification use. Low temperature rheological and failure characteristics of asphalt binders may be obtained using D 6723 (DT test method) at other test conditions such as different strain rates, stress control tests and low temperature fatigue. The mean failure strength value determined at a standard strain rate of 3 percent per minute is used along with BBR data to determine critical cracking temperature as described next.
The calculation practice, D 6816, was developed more recently. (3) It was becoming obvious that a mechanistically based criterion was needed to better characterize the low-temperature performance of modified asphalt binders. The low-temperature thermal cracking in flexible pavements is a result of the combination of three distress mechanisms: 1) single event thermal cracking, 2) thermal fatigue, and 3) load-associated thermal cracking. The AASHTO M320 or ASTM D 6373 specification does not discriminate among the various low-temperature distress mechanisms. Instead, the stiffness and the slope from the BBR creep data at a single loading time of 60 seconds are used as surrogate rheological parameters to control pavement thermal cracking at low temperatures. The low-temperature performance, however, is a function of a combination of rheological characteristics as well as fracture properties of the binder. Hence, a comprehensive model of low-temperature pavement performance must include rheological and fracture properties of the asphalt binder.
D 6816 is the result of the development of a comprehensive mechanistic model that enables better prediction of the performance of asphalt binders at low temperatures. In this practice, only single event thermal cracking is considered. Load induced and thermal cycling fatigue cracking will be included in the future versions of this practice.
This calculation practice uses as input the stiffness values from D 6648 (BBR) at six loading times and mean failure strength from D 6723 (DT). These properties are determined at two test temperatures as specified in the practice. The stiffness data from the BBR test is used to determine the thermal stress development in a typical pavement when the temperature changes at a cooling rate of 1°C per hour. The critical cracking temperature (Tcr) is the temperature where the thermal stress is equal to the strength of the asphalt binder. The calculation practice only considers single-event thermal cracking phenomenon as explained in the previous section.
Who Will Use These New Standards?
The new standards may be used by anyone interested in characterizing the low temperature behavior of asphalt binders. In the United States, the new standards are used in federal, state, city and local government laboratories involved in construction and maintenance of asphalt pavements. These standards are necessary for the implementation of ASTM D 6373 or AASHTO M320 and AASHTO MP1a specifications. In a recent survey conducted by Koch Pavement Solutions of Wichita, Kan., it was found that of the 51 transportation agencies (50 state departments of transportation and Washington, D.C.) surveyed, 39 use the AASHTO M320 specification which requires the use of ASTM D 6648. Ten DOTs either use or are planning to use the AASHTO MP1a specification which requires data from ASTM D 6723 and D 6816.
Impact on Safety and Quality of Roadways
As test methods and a calculation practice, the impact of these standards on safety and quality of highways is indirect in that they enable better characterization of material properties. In the Koch survey referred to above, it was found that 40 of the 51 transportation agencies have determined that the quality of asphalt binders has improved as a result of using the new test methods specified in the AASHTO M320 and MP1a specifications. Many DOTs have noticed a lower to almost negligible incidence of thermal cracking since the implementation and use of the new test methods. As more DOTs implement and adopt AASHTO MP1a, the quality of asphalt binders, especially polymer modified binders, will improve further, ultimately improving the safety and quality of asphalt highways. //
(1) R. Dongre and J. DAngelo, Effect of New Direct Tension Test Protocol on the Superpave Low Temperature Specification for Bitumen Binders, The Fifth International Conference on Bearing Capacity of Roads and Airfields, BCRA, Vol. 2, pp. 1035-1048, NTNU, Trondheim, Norway, 1998.
(2) D. A. Anderson, et. al., Binder Characterization and Evaluation SHRP A-369, Vol. 4, Binder Test Methods, Strategic Highway Research Program, National Research Council, 1994, Washington D.C.
(3) Bouldin, M.G., Dongre, R., Rowe, G.M., Sharrock, M.J., and Anderson, D.A., Predicting Thermal Cracking of Pavements From Binder Properties: Theoretical Basis and Field Validation, Journal of the Association of Asphalt Paving Technologists, Volume 69, pp. 455-496, 2000.
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