Consider the performance based asphalt binder specification
By Kevin Carlson
The concept of using creep and recovery testing to measure the high temperature properties of asphalt binder has been around since the late 1990s and was used in the NCHRP 9-10 research to determine rutting performance of modified asphalt binders.
Additional studies have found that the non-recoverable creep compliance (Jnr) calculated from multiple stress creep recovery (MSCR) testing used in AASHTO MP 19 to be a better predictor of rutting performance than G*/sinδ used in the current performance-graded (PG) asphalt binder specification (AASHTO M320), for both modified and unmodified asphalt binders. Research studies have also indicated that the percent recovery calculated from MSCR test data could also serve as a replacement for PG “plus” specifications that are used to measure the presence of polymer modifier in the binder.
AASHTO has published provisional specifications for both the MSCR test (AASHTO TP 70) and performance graded asphalt binder incorporating the MSCR test (AASHTO MP 19). Despite the positive findings of the research and the availability of published specifications, adoption of MSCR-based specifications has progressed at a slow rate. For those hesitant to make the switch, I offer the following as a list of 10 reasons why you should consider making the switch to AASHTO MP 19.
1. New testing equipment should not be required.
The MSCR test referenced in AASHTO MP 19 uses the same dynamic shear rheometer (DSR) required in AASHTO M 320. The same 25mm test plates are used to run both tests. Virtually no financial investment in equipment should be needed, as nearly all modern DSRs are capable of performing the test. Minor software updates or new test scripts may be needed to run the MSCR test, but the hardware should be fully capable of running the MSCR test.
2. The MSCR test can be completed quickly and easily.
The MSCR test procedure in AASHTO TP 70 follows the same sample conditioning and temperature conditioning prior to testing as the rolling thin film oven (RTFO) aged DSR test in AASHTO M 320. The MSCR test consists of one second of constant stress followed by a nine second zero-stress recovery period. The loading and recovery sequence is repeated 30 times. The first 10 cycles are at 0.1kPa stress level to condition the test specimen, followed by 10 more cycles at 0.1 kPa stress levels, and 10 more cycles at 3.2 kPa stress level. The creep and recovery cycles take only five minutes to complete.
3. The MSCR test can effectively work with both modified and non-modified binders.
The DSR tests used to measure rutting performance in AASHTO M320 worked well on non-modified asphalt binders, but failed to fully capture the response of the polymer network in a polymer modified asphalt binder. The MSCR test can better characterize modified binders. The polymer network can be damaged or rearranged at the higher stress levels used in the MSCR test, giving a better indication of binder performance. The higher stress levels also work well for non-modified binders that tend to not be as sensitive to the stress level in the test. MP 19 could be considered truly blind to the method of modification used to produce the binder, which was one of the original objectives of the M 320 specification.
4. MSCR is a better indicator of rutting resistance.
Research studies have found that Jnr from the MSCR test has better correlation with actual asphalt mix rutting performance than G*/sinδ. The Federal Highway Administration Accelerated Load Facility study, as well as a Mississippi Interstate 55 study are the most notable early research efforts to support this. Both modified and non-modified binders were included in those research studies. Using MP 19 should give you a better indication of the mix rutting performance than M 320.
5. No more “grade bumping” to account for traffic loading.
Using the MP 19 specification, binders are tested at realistic temperatures they are expected to see in the field. You follow the same high and low temperature selection process you would if using M 320, with the exception of the grade bumping. Instead of grade bumping you choose the appropriate “S,” “H,” “V,” or “E,” grade within that temperature grade.
“S” grade (Standard Grade) is suggested to be used in places where less than 10 million equivalent single axle loads (ESALs) and traffic speeds over 70 km/h are expected with a maximum Jnr of 4.0 kPa-1. “H” grade (High Grade) is suggested for use in places where between 10 million and 30 million ESALs or traffic speeds in the range of 20 km/h to 70 km/h are expected with a maximum Jnr of 2.0 kPa-1. “V” grade (Very High Grade) is suggested to be used in places where more than 30 million ESALs or traffic speeds less than 20 km/h are expected with a maximum Jnr of 1.0 kPa-1. “E” grade (Extreme High Grade) is suggested to be used in places where more than 30 million ESALs and traffic speeds less than 20 km/h are expected with a maximum Jnr of 0.5 kPa-1.
Under M 320, this is accomplished by “bumping” up the high temperature grade of the binder. The “bumped” binders are tested 6-18°C above the highest temperature the binder would likely ever be exposed to in the pavement. The high testing temperatures could result in the use of a highly modified binder that may not necessarily be needed at the actual temperatures in the pavement.
6. PG “plus” tests can be eliminated.
AASHTO M 320 did not do a good job of addressing polymer-modified binders. This has led to a proliferation of PG “plus” specifications, where specifying agencies are requiring tests in addition to those in M 320 to ensure the presence of polymer modifiers in the binder. The “plus” tests are mostly empirical and can aid the identification of the presence of polymer, but they do not necessarily do a good job of identifying the performance of a polymer-modified system and can take hours to complete. The MSCR test can help out with this.
One of the properties that can be calculated from the TP 70 test data is the percent recovered strain, also known as percent recovery. The percent recovery can be used to identify the presence of polymer modifier, and TP 70 contains a graph that can be referenced to identify if the binder is modified with an acceptable elastomeric polymer. With the MSCR test, you can test for high temperature rutting performance with Jnr and identify the presence of an acceptable amount of polymer in one test that can be completed in minutes versus hours. It is recommended that if the MSCR percent recovery is used to identify an acceptable amount of polymer modification, that other PG “plus” tests such as elastic recovery, force ductility and toughness and tenacity be dropped from specifications.
7. Stress sensitive binders can be identified.
A stress sensitivity calculation is included in TP 70 and included in the MP 19 specification. The criterion is that the difference in Jnr at the 3.2 kPa and 0.1 kPa stress levels must be less than or equal to 75 percent of Jnr at the 0.1 kPa stress level. This can identify binder that has a high potential to perform poorly in the pavement when exposed to high stresses. This can also be used to identify weak polymer structure in binders that either breaks down or undergoes substantial reorientation when under high stress, which could lead to poor pavement performance. This may also help identify binders made by unconventional methods that may perform poorly under high stress. M 320 does not have a means to measure or identify this property at the current time.
8. Improve the efficiency of your laboratory.
The MSCR test can also lead to increased efficiency of your laboratory. Since the high temperature testing will be done at fewer temperatures due to the elimination of grade dumping or bumping, you can spend less time waiting for the DSR to change temperatures and more time testing. Compared to most PG plus tests that can involve hours of preparation and testing time, the MSCR test can be completed in minutes, freeing up valuable laboratory time. In a time when most people are being asked to do more with less, the MSCR test has potential to get increased throughput out of your laboratory without hiring more people or buying more equipment.
9. A standard specification would benefit the asphalt industry.
One of the major inefficiencies in the asphalt industry right now is the proliferation of asphalt binder specifications. The specifications are not standard from state-to-state in most cases, making it difficult for suppliers to supply all the grades that are needed efficiently. MP 19 can address most, if not all, of the concerns of those specifying agencies had that led to the specification proliferation that we have right now. Jnr and percent recovery should be able to address most of those concerns.
10. There are people willing to help you with the transition.
If you have questions or concerns about switching to AASHTO MP 19 and the MSCR test that are preventing you from moving forward, help is available. Contact the asphalt binder suppliers in your region. They should be able to provide you guidance on what products they are able to offer using the MP 19 specification that are best suited for your region’s climate and traffic conditions. The Asphalt Institute is also an excellent resource for questions related to AASHTO MP 19. The Asphalt Institute has guidance documents available to assist with the transition from M 320 to MP 19. Check out www.asphaltinstitute.org for more information.
AASHTO MP 19 offers many advantages over AASHTO M 320 as your performance graded asphalt binder specification. MP 19 has the versatility to address both modified and unmodified asphalt binders and can replace most PG plus tests. The MSCR test used in MP 19 can be completed quickly and efficiently, without the need of new equipment in most cases, and produces results closely tied with actual field performance. I would encourage you to consider switching to the MP 19 asphalt binder specification, as it is a big improvement over M 320 as a performance based specification and should benefit the asphalt industry for years to come if it is widely used.
Kevin Carlson is the General Manager of Research and Development at Jebro Inc. in Sioux City, Iowa.