By Bob Humer, P.E.
In January 2006, Caltrans and the rest of California started to use the Superpave Performance Grade (PG) system for specifying neat asphalt binders. For years, California had used the Aged Residue (AR) grading system for their asphalt binders. These AR grades are expected to be phased out rapidly. For polymer modified asphalt binders, the Performance Based Asphalt (PBA) system will remain in place for at least another year.
During 2006, Industry and the California Department of Transportation (Caltrans) will sort out the proper PG grading for their polymer modified asphalt binders. The Gap Graded Binder grades and Modified Binder grades for rubber-modified asphalts are expected to remain in place. For more information on the asphalt grades being specified in California, visit www.asphaltinstitute.org and click on State Binder Spec Database.
Too Many Grades
The PG grade to be specified is determined by the climatic conditions at the project site. The wide range of climatic conditions in California would have required 20 PG grades, with five more for each grade bump.
Obviously this would not be practical for industry or the specifying agencies. Most hot mix plants have, at best, two asphalt binder storage tanks supplying multiple projects each day. Thus there was a need to limit the number of PG grades typically used in any one region.
A practical solution is for one “workhorse grade” asphalt for normal projects and one additional grade for heavy duty pavements. Such heavy duty, or special case, pavements can be heavily trafficked highways, container yards, busy intersections or airports. For those cases, the polymer-modified asphalt suitable for that area is recommended.
After many meetings, Caltrans and the asphalt industry agreed to keep things simple and designate four PG workhorse grades for four distinct climatic regions of the state.
These PG grades are:
- PG 64-10 for the Central Coast, Inland Valley and South Coast
PG 64-16 for the North Coast, Low Mountains and South Mountains
PG 64-28 for the High Desert and High Mountains This grade may require some type of modification.)
PG 70-10 for the Desert
For asphalt-rubber base stock, typically PG 64-16 is used, except in cold climate regions where a PG 58-22 base stock is required.
To avoid uncertainty in the supply chain, grade bumping for special loading conditions and/or designated routes has been limited to the recommended use of polymer-modified PBA grades (PBA 6, 6a, 6a*, and 7). One exception is the optional use of PG 70-10 and PBA 6a* in the Central Coast, Inland Valley and South Coast regions.
In order to limit the number of grades, some compromises have been made. For example, Death Valley would have required a PG 76-10 and Boca (I-80) would need a PG 58-34. The use of polymer-modified PBAs should suffice for these cases.
Why California Waited
With California being the last state to adopt the PG grading system, there is the valid question, “Why did this take so long?” As a crude oil producing state, California uses its own asphalt production, without the need to import. More than 50 percent of the California asphalts have difficulty complying with some of the PG specifications, because of their aging and temperature susceptibility characteristics.
Agencies and industry managed to build good performing roads, using these local asphalts in combination with the AR grading system and the Hveem mix design method.
In other words the system wasn’t broke. Therefore the decision was made to wait and see how well PG grading and Superpave would work in other states.
Since the advent of the Strategic Highway Research Program, Caltrans and the Pacific Coast Conference on Asphalt Specifications have been involved with the development and validation of the PG grading system. Caltrans adopted portions of the PG testing as early as 1995 for the PBA grades, using the Bending Beam Rheometer for low temperature testing and the Dynamic Shear Rheometer (DSR) for midtemperature testing.
The lack of validation of the PG fatigue parameter was a significant factor in the delay. However, now that it has become clear that the PG grading system is being accepted nationwide and that all future research will relate to PG binders, it was time for California to fall in line.
Developing the Database
Caltrans used FHWA’s LTTPBind computer program to determine the PG grades required. A database of the PG grades of the asphalts used in California was developed. This information was then used to select the four PG grades covering 90 percent of the climate conditions. The database comparing California’s existing AR and PBA graded asphalts to the PG grading system was developed over the last few years by cooperation among the laboratories of the FHWA, Caltrans, the local refiners, and the Asphalt Institute.
This significant laboratory effort has paid off by providing essential information for PG implementation. It showed that none of the AR grades exceeded a PG 64.
Thus the PG 70-10 is a new grade, higher in viscosity than what has previously been used.
Caltrans has adopted the AASHTO M320-04 specification for neat asphalts with the following exceptions:
- If the DSR limit is exceeded on the Pressure Aging Vessel (PAV) aged asphalt at the specified temperature, the sample shall meet the PAV-DSR requirement when tested at 3oC higher.
- A minimum ductility of 75 cm at 25oC on the RTFO sample is included.
- The PG 70-10 asphalt shall be PAV aged at 110oC only.
In the initial meetings between industry and Caltrans regarding the intended change, there were two very different positions. Caltrans was aiming for asphalt binder improvements by pushing the specification envelope beyond what was being supplied, while the industry was faced with the reality that, at the refinery, asphalt is in competition with fuels. If the specifications for asphalt become too complicated and costly, there will be a shift within the refinery to other, better margin hydrocarbon products.
Under the leadership of Terrie Bressette, of Caltrans’ Office of Flexible Pavement Materials, both parties worked together and agreed on a workable solution. With input from UC Berkeley and FHWA, the DSR-PAV temperature was eased as a reasonable compromise to avoid severely affecting as much as 55 percent of the asphalt supply in the state. This compromise is considered acceptable, especially in light of the lack of consistent correlation between the PG fatigue parameter and pavement fatigue performance.
The change to PG grades has forced some refiners to blend different crudes and/or adjust the refining process. Others had to modify the asphalt binder to stay within the specification. If the local refinery cannot supply a particular grade, it has to be obtained elsewhere. Hot mix plants have to be more careful when changing suppliers of asphalt binder. The next tanker truck does not typically arrive just when the storage tank at the plant runs empty. Some co-mingling will occur.
With PG specifications being tighter than the AR specifications, disagreements between labs regarding specification compliance can be expected. The Caltrans certificate of compliance (COC) program will help control this potential problem. Good quality control at the refineries and hot mix plants is essential. Details on the Caltrans COC program and the list of Caltrans Approved Suppliers for Modified and Non-Modified Binders can be seen at http://www.dot.ca.gov/hq/esc/Translab/ofpm/fpmcoc.htm.
For those projects that started before the change in 2006, Caltrans has left it to the contractor’s discretion to continue with the AR grade, or to switch to the appropriate PG grade. The AMRL certification of all laboratories testing PG asphalts will need to be completed by January 2007. Hveem mix design temperatures remain the same for the time being.
Also mix production and paving practices are not expected to change for the PG graded neat asphalts. However, as experienced in some other states, the switch to PG grades may result in the use of slightly stiffer binders, and so mixing and compaction temperatures may need to be raised slightly.
As a consequence of Caltrans implementing PG grading for neat asphalts, there was a need to educate the local agencies, contractors and Caltrans district personnel on grade selection and material testing. Through three venues, this has been achieved:
- The issue was discussed in local APA technical committee meetings and in the California Asphalt Magazine.”
- Caltrans contracted with UC Berkeley’s ITS Program to conduct 12 half-day informational seminars around the state.
- In September 2005, at Caltrans’ request, the Asphalt Institute conducted two successful hands-on Asphalt Binder Technology Workshops in California for local agency laboratory personnel to become familiar with the PG technology and testing procedures. For this effort, Mike Anderson, Mike Beavin and Shay Emmons, from the Asphalt Institute’s lab, worked two weeks in September at the Caltrans and L.A. County labs to prepare for and provide this hands-on laboratory training. A total of 42 people from the State, counties and cities were trained.
Fit or Adopt?
The next order of business for Caltrans and industry is to fit the polymer-modified asphalts which work for California into the PG grading system. The important question for Caltrans is, “Do we fit our PBA binders into PG grades, or do we adopt polymer- modified binders more similar to those used in Nevada?”
Bob Humer is the Asphalt Institute’s District Engineer in Los Angeles, California.