MSCR tune up
When any new system makes its way into an industry, there is always a break-in period. During this time, the system is evaluated for ruggedness, reasonable variability limits are determined and potential users can take time to become familiar with the new parameters.
The Multiple Stress Creep Recovery test (T 350) and M 332, the associated PG-related system, has been undergoing this process for the last several years. Up until the last two to three years, MSCR has been primarily a report-only parameter. But now, with many agencies either partially or completely implementing MSCR, Asphalt Institute wanted to reinforce some concepts related to the system and test. We have worked closely with industry and agency during the research, evaluation and early implementation of the M 332 system and have provided expertise to educate our industry in its proper use. There is, however, some confusion remaining regarding test parameters and the system as a whole. We hope that the following tune-up clears up some of the confusion.
MSCR is performed on RTFO conditioned binder and can either be run immediately after traditional M 320 RTFO DSR testing on the same specimen or loaded and tested only as MSCR. If both tests are performed on the same specimen, it is important to run MSCR last because the strains induced during the test damage the specimen. Further testing will produce unreliable data.
The test is performed first at a low stress and then at a high stress. Each stress is applied for ten cycles. In each cycle a one second loading period is followed by a nine second rest period. During the loading and rest periods, the material is sheared (strained) and then is allowed to recover. In other words a stress is applied to the loaded specimen causing the top plate to move from the starting position, straining the binder, and then is removed allowing the binder to pull the top plate back toward the starting position. Because asphalt binders are viscoelastic, the recovery will never be complete. Depending on the base binder properties and modification there can be very large differences in recovery between different grades. Results from each of the ten cycles are then averaged.
The percent recovery can then be calculated by comparing the amount of total recovered strain to the unrecovered strain. You could stop here and use this data as a quick substitute for the elastic recovery test or you could continue by calculating the non-recoverable creep compliance (Jnr
) which is the principal parameter in the M 332 system. To do this, simply divide the non-recoverable strain at the end of each cycle by the applied stress that caused the strain. The result is used as an indicator of rutting resistance under various traffic loads and environmental temperatures. The smaller the Jnr
value the less rutting is likely to occur at higher traffic loads.
The MSCR test and the associated M 332 specification were designed to use environmental temperatures only, not grade bumped temperatures. Grade bumping was introduced as a way to get a stiffer binder at the expected high pavement temperatures. This practice, however, requires G*/sind to be determined at temperatures higher than the environmental temperatures at the job site. There was a different mindset behind the M 332 system: Why test at temperatures higher than will be encountered at the job site and use a stiffness parameter that may not always correlate to rutting? Why not test at actual environmental temperatures and use a recovery based parameter directly related to traffic load?
To determine an appropriate MSCR grade you need two pieces of information: expected traffic load and environmental temperature at the job site. Based on traffic load a designation is assigned. There are four traffic designations- Standard, Heavy, Very heavy and Extreme which correlate to the following traffic conditions.
Standard (S) – Normal speed/normal volume
Heavy (H) – Slower speed and/or heavier volume
Very heavy (V) – Slow and/or very high volume
Extreme (E) – Very slow and very high volume
An example M 332 grade is 64V-22. This grade would be used at a job site where the expected high pavement temperature will seldom exceed 64o
C, the traffic load is expected to be slow and/or high volume and the expected low pavement temperature will seldom drop lower than negative 22o
C. This grade has either very similar or identical properties to that of a standard PG grade of 76-22 but instead of increasing the test temperature to 76 for very heavy traffic and using a stiffness based parameter, the test temperature is not increased. Instead, the required Jnr
value is changed depending on traffic load. An M 320 PG grade of 76-22 could be used for either a normal traffic load in a very hot climate or in a cooler climate with very heavy traffic. If the product were to be used in the very hot climate, the 76 does indicate environmental temperature but if the same product were to be used in the cooler climate, the 76 would be a grade bumped temperature. 76-22 does not make that distinction. An M 332 grade tells the user at a glance the proper application of the product. Important note:
If testing for MSCR on our example PG76-22 that has been selected to accommodate heavy traffic and not for environmental reasons, the appropriate MSCR temperature would not be 76o
C. The lower environmental temperature should be used.
Another parameter in the M 332 system is the ‘Jnr
difference’. The intent of this piece of information was to give some assurance that, if higher stresses occur in the field than are simulated in the MSCR test, the binder will still perform adequately with no dramatic change in recovery properties. There has been some concern over variability in this parameter which probably stems from the very small Jnr
values typical in H, V and E grades. Solutions to the issue are still being discussed at the time of this article.
Common tests and parameter changes
Original DSR testing is still performed when using the M 332 system, but because only environmental temperatures are used, not grade bumping, the O DSR can be run at the same temperature for four different MSCR grades. Because of this, only an S grade would have any risk of failing this parameter. The H, V and E grades will yield values far higher than the minimum requirement of 1 kPa.
PAV DSR testing is also still performed in M 332. Again, because all MSCR grade temperatures are environmentally based, PAV DSR G*.
sinδ values will be considerably higher for H, V and E grades than are found when using the M 320 system. For this reason, the maximum 5000 kPa parameter is changed to 6000 kPa for those grades. It does not change for S grades.
There are no differences between M 320 and M 332 in rotational viscosity, flash point and BBR testing.
Look for more information about M-332 and the MSCR test here.
Take the PRESSURE out of aging
Just as the ‘inside the eggshell egg scrambler’ revolutionized the world's eating habits and drastically reduced catastrophic egg beating related injuries, a "new" device being embraced by our technicians here at Asphalt Institute is changing what we thought possible. Its far-reaching benefits in aging are just now being revealed.
It’s a drill – with a lithium ion battery – and you can attach stuff to it.
We are talking pressure aging as in the Pressure Aging Vessel or PAV. Take what was attached to your ratchet and stick it on the drill and now you can seal and unseal the PAV with speed and ease.
We used to do this:
Now we do this:
So take the "pressure" out of aging, eliminate all those unnecessary forearm exercises and give yourself a much deserved break.
- Mike Beavin, Asphalt Institute Technical Training Coordinator
Arizona Department of Transportation
Transportation Construction Technician III
Wei has worked in the asphalt industry for 8.5 years. In his current role he performs PG, PG plus, emulsion and crumb rubber asphalt testing.
"The NBTC training helped me to improve my knowledge on how to eliminate mistakes and errors on test procedures, and minimize variability in each step," said Wei.