AASHTO Designation: M Release: Group 3 (August ). American Association of State Highway and Transportation Officials North Capitol Street. Find the most up-to-date version of AASHTO M at Engineering AASHTO M Standard Specification for Performance-Graded Asphalt Binder. standard by American Association of State and Highway Transportation.
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Refining Superpave asphalt binder characterization
By Dwight Walker, P. Superpave Superior Performing Asphalt Pavements was a product of that research effort. Although Superpave was a aashtoo improvement over earlier approaches, one of the concerns was that it was based primarily on unmodified asphalts.
Many industry and agency experts have discussed and debated the applicability of the specification for modified binders since it was introduced. In particular, the permanent deformation or rutting criteria, the fatigue cracking and the low-temperature cracking aspects of the specification have been reviewed. This article summarizes some of the latest developments in refining the PG asphalt binder system. The use of polymer-modified asphalts to reduce pavement rutting has grown steadily since the late s.
To ensure that the modified binders contain polymer, a number of tests have been introduced as additions to the PG specification. It is designed to be performance related rather than empirical and to be blind to the type of modifier used. Both neat unmodified and modified binders were tested in the development of the MSCR test.
The MSCR test starts at the lowest stress level and increases to the higher level at the end of 10 cycles, with no time delay between cycles. The average non-recoverable strain for the creep and recovery cycles is divided by the applied stress for those cycles to yield the non-recoverable creep compliance, Jnr.
Asphalt Binder Specification Tests
Numerous evaluations of the MSCR test have shown that it is able to measure the difference in rutting potential of various modified and unmodified asphalt binders. These evaluations were performed on several types of mixes using a number of evaluation means, including loaded wheel test devices, accelerated load testers and actual m302 sites.
For neat binders, a Jnr of 4. Under the existing approach to binder selection, grade mm320 is used to select stiffer or stronger binders to reduce the potential for rutting of these high stress pavements. For these cases, binders with a higher temperature grade would be selected.
ASTM Standards for Asphalt Binders
With the grade-bumping approach, the specification criteria remain the same but the test temperature at which the criteria are applied is increased. The asphalt binder is tested at temperatures the pavement is not likely to actually experience. This assumption is not accurate because aashro binders do not have the same temperature susceptibility. A new high-temperature binder specification would require the Jnr value, determined at a ashto stress of 3.
For heavy traffic the Jnr is reduced to 2. The terminology would also be changed to move away from increasing the high temperature designation. This property provides an indication of the delayed elastic response of the binder. A high response value indicates aashfo significant elastic component. In addition to developing a better relationship between high temperature binder properties and rutting, considerable research has been done to identify an improved binder fatigue test.
This parameter is measured after a relatively small number of load cycles and the non-linear behavior or damage is not apparent. Work done under the NCHRP research project used the time sweep test to apply sufficient cycles to induce damage, but the amount of time required to perform the test was a problem.
So, more recent efforts have been geared to developing accelerated fatigue procedures. One of the new procedures being considered to characterize fatigue cracking performance of asphalt binders is the Linear Amplitude Sweep LAS test. Aasgto is a relatively fast test, but it has some serious mechanics and calculations backing it up.
The LAS test is performed on aged asphalt binder at an appropriate intermediate temperature using a frequency sweep test to generate rheological properties and a stepped shear strain amplitude test to generate accumulated damage in the asphalt binder sample.
The LAS test can be performed on the newer DSRs with a total testing time of about 12 minutes the relatively fast and easy part. Then the data is exported to a spreadsheet and the viscoelastic continuum damage VECD approach is used to calculate the fatigue resistance of the binder the serious mechanics and calculation part. In cold conditions, as the temperature drops, an asphalt binder contracts more than the aggregate in an asphalt pavement.
This shrinkage causes thermal stresses and when these stresses exceed the tensile strength of the mixture, a low temperature crack forms.
To prevent m3220 cracking, the S value was limited to a maximum of MPa. The rate at which the binder stiffness changes with time at low temperatures is regulated through the m-value. The DTT pulls an asphalt specimen in tension at a very slow rate to simulate shrinkage as a pavement cools.
Like the high and intermediate temperature properties, cold temperature characterization is being refined to address limitations that have been identified in the existing procedures. Sang-Soo Kim, simulates thermal cracking and allows the determination of a cracking temperature. The introduction of the Superpave asphalt binder system was a major step forward in characterizing the behavior of asphalts.
With the refinements underway, the PG system is continuing to be improved. The Asphalt Institute AI believes the MSCR test and specification are improvements to the current PG system that will allow for better characterization of the high-temperature performance-related properties of asphalt binders.
Improved high-temperature performance predictions The use of polymer-modified asphalts to reduce pavement rutting has grown steadily since the late s. Intermediate temperature In addition to asahto a better relationship between high temperature binder properties and rutting, considerable research has been done to identify an improved binder fatigue test. Cold temperature In cold conditions, as the temperature drops, an asphalt binder contracts more than the aggregate in an asphalt pavement.
Conclusion The introduction of the Superpave asphalt binder system was a major step forward in characterizing the behavior of asphalts.