Minutes of the SUPERPAVE Mixture Expert Task Group September 22-23, 1997
The next meeting of the ETG will be held in Orlando, Florida on March 10 & 11, 1998. At the request of a number of members the meeting is planned to start at 1:00 pm on March 10. Further hotel and site information will be provided.
The meeting was convened at 1:00 pm on Monday, September 22, 1997 by Chairman John Bukowski and Thomas Harman of the FHWA.
Those members attending were:
| Mike Anderson, Asphalt Institute | John Harvey, Univ. of California |
| Tim Aschenbrener, Colorado DOT | Gerry Huber, Heritage Research |
| John Bukowski, FHWA (HTA-20) | David Jahn, American Aggregates |
| Eileen Connolly, New Jersey DOT | Rebecca McDaniel, Purdue Univ |
| Dale Decker, NAPA | Bob McGennis, Univ of Texas |
| Erv Dukatz, Vulcan Materials | Larry Michael, Maryland DOT |
| Jon Epps, Univ. of Nevada | Jim Musselman, Florida DOT |
| Frank Fee, Koch Materials | Allen Myers, KY Trans Cabinet |
| Jim Gehler, Illinois DOT | Julie Nodes, Arizona DOT |
| Thomas Harman, FHWA (HTA-20) | Ronald Sines, New York DOT |
| Edward Harrigan, NCHRP | Maghsoud Tahmoressi, Texas DOT |
| Rick Harvey, Wyoming DOT | Haleem Tahir, AASHTO |
Those visitors attending were:
| Ken Brown, Troxler | Mike Rafalowski, FHWA, HNG-20 |
| Ray Brown, NCAT | Bob Rask, Asphalt Institute |
| John D'Angelo, FHWA (HTA-20) | Jim Scherocman, Consultant |
| Steve Jones, Asphalt Institute | Randy West, APAC Inc. |
| Pierre Langlois, LCPC, France | Shmuel Weissman, Univ. of California |
| Francis Moutier, LCPC, France | Chris William, FHWA HNR-30 |
| Chuck Paugh, Contractor DP-90 Project | Thomas White, Purdue University |
| Katherine Petros, FHWA, HNG-40 |
The meeting began with self introductions of the Expert Task Group (ETG) members and visitors. After introductions, John Bukowski and Thomas Harman reviewed the objectives and structure of the Superpave Mixture ETG and the meeting agenda..
1. WesTrack Evaluation Team
Larry Michael briefed the ETG on the preliminary findings of the WesTrack Evaluation Team. The team consists of Michael, Ray Brown, Erv Dukatz, Gerry Huber, Jim Scherocman, Ron Sines, technical support provided by John D'Angelo and Chris Williams as FHWA representatives.
WesTrack was opened to traffic in April, 1996 and applied nearly 3 million ESALs in the first year. The purpose of WesTrack was to quantify the variability of construction on performance and develop performance related specifications. Mixes were purposely produced at design level and at various levels of variability in order to observe failure conditions.
This summer, 10 sections were replaced due to severe rutting. Eight of these were replaced with a coarse graded mixture that also started to rut within a week of construction. The evaluation team looked into the reasons for this rutting. A summary of their preliminary findings was distributed to the group. A high design binder content (5.7%) and too low an asphalt cement stiffness (PG 64-22) were identified as having a major impact on the premature rutting. Another aspect that was discussed was the selection of design ESALs of 3 to 10 million for the mixture. Gerry Huber commented that if the initial WesTrack loading were continued for 15 years, a design ESAL closer to 75 million ESALs would have been needed. Larry Michael commented that Maryland has built several projects with similar aggregate gradation to the coarse graded mix used at WesTrack, and performance has been excellent, however he noted that the binders used in Maryland were much stiffer than that selected for WesTrack.
A final report, will be produced after additional mixture testing is performed, early in 1998. The ETG will be provided with copies of the report for comments. Based upon the report, the ETG will make recommendations to AASHTO concerning any modifications to Superpave that are needed in materials and construction criteria.
2. Lead States Superpave Meeting
Rebecca McDaniel reported on the recent Lead State meeting held in St. Louis. Some of the past activities of the Lead States are (1) provided a pool of expertise, (2) offer implementation plan guidance, (3) identified training needs, (4) developed national technical guidance, (5) conducted a survey of initial State implementation, (6) provided representatives to the Superpave Regional centers steering committees, (7) provided technical resolutions to the AASHTO subcommittee on materials.
She discussed a June 2, 1997 document on "Superpave Implementation - Lead State Guidance." This contains the guidance statements on N-design, aggregate properties, RAP, field construction, etc. issued as a result of the March 25/26 1997 Lead State Superpave meeting held in Baltimore, Maryland.
Some states are making arbitrary changes. A resolution is being written by AASHTO that will stress the importance of DOTs avoiding changes to the essential elements of the Superpave system. This is needed in order to evaluate and implement Superpave uniformly and to allow for comparisons. AASHTO specifications contain the current Superpave recommendations as well as the official source of any changes/modifications.
Superpave Lead States planed activities for 1998 are (1) to urge uniformity of Superpave usage. They will identify what are the essential/critical elements of the Superpave design (this will be coordinated with the ETG), (2) summarize and advocate QC/QA guidelines for use with Superpave. NCHRP 9-7 contains good information, but format needs to be re-worked to incorporate into State specifications - distribute generic specification, (3) survey States that have used Superpave and promote benefits to all States. Lead State activities conclude on September, 2000.
3. NCHRP Superpave Related Projects
Edward Harrigan briefed the group on NCHRP Projects related to Superpave and their status.
9-7: Field Procedures and Equipment to Implement Superpave
Objective: To develop comprehensive QC/QA procedure and equipment to insure that asphalt pavements meet Superpave specifications, and establish a training program framework to instruct technicians.
Contract Status: Completed, a statistical QC/QA plan was developed on the basis of 14 field projects between 1994 - 1996. A final report will be distributed in 1998. The plan is based on the SGC, measurement of volumetric properties, backed by Gmm, asphalt content, and gradation with an option for measurement of complex shear modulus with the SST. A "field shear tester" was used in the study, but inadequate data to correlate to existing SST. Continued use of the field shear tester will be influenced by findings under the current FHWA contract with the University of Maryland. A training package and software were developed as part of the study. The Lead States plan on developing a guide QC/QA plan in AASHTO format based on the study.
9-9: Refinement of Superpave Gyratory Compaction Procedure
Objective: Recommend revisions to the Superpave gyratory compaction procedures as set forth in AASHTO TP4. Areas of investigation will include compaction of open graded friction courses and SMA with the gyratory compactor, investigation of lab compaction temperature procedures, and investigation of separate compaction "N" values for base and surface mixes, and sensitivity of current values of N-Design.
Research Agency: Auburn University (NCAT) PI: Ray Brown
Contract Dates: 6/1/96 to 7/31/98.
Contract Status: First interim report has been completed and work begun on revised plan.
9-10: Superpave Protocols for Modified Binder Systems
Objective: To recommend any necessary modifications to the Superpave binder tests to ensure their applicability to modified asphalt binders, and identify any problems with the Superpave mix tests when applied to modified paving mixtures.
Research Agency: Asphalt Institute PI: H. Bahia
Contract Dates: 4/15/96 to 10/14/98.
Contract Status: First interim report submitted. A laboratory asphalt stability test (LAST) for storage and thermal stability is being evaluated, as is a particulate additive test.
9-12: Incorporation of Reclaimed Asphalt Pavement in the Superpave System
Objective: Develop guidelines for incorporation of RAP in the Superpave System on the basis of an objective measure of binder and mix performance, and prepare a manual for use by lab and field technicians. Will build on recommendations from the mix and binder ETGs.
Research Agency: Pending
. Status: Award in September 1997.
9-13: Evaluation of Moisture Sensitivity
Objective: Confirm the applicability of the AASHTO T-283 method for moisture sensitivity within the Superpave system. Consider the effects of factors such as 150 mm diameter specimens, short and long term aging, and gyratory compaction and suggest appropriate procedural modifications.
Research Agency : Pending
Status: Award in September 1997.
9-14: Investigation of the Restricted Zone in the Superpave Aggregate Gradation Specification
Objective: Determine through the performance properties of HMA if the restricted zone requirement is redundant with FAA and volumetric mix criteria and if appropriate, identify the traffic levels at which it is redundant.
Research Agency : Pending
Status: Award early 1998.
9-15: Quality Characteristics and Test Methods for Use in Performance-Related
Specifications of HMA Pavements
Objective: (1) Identify construction related quality characteristics for HMA pavement that effect long-term pavement performance, (2) identify quality characteristics of as-produced HMA that reflect compositional, volumetric and fundamental engineering properties in terms of long-term pavement performance, (3) select and prepare for use in PRS simple, practical and rapid field tests that measure the quality characteristics in the field.
Research Agency : Pending
Status: Award early 1998.
4-19: Aggregate Tests Related to Asphalt Concrete Pavements
Objective : To recommend a set of aggregate tests that relate to the performance of hot-mix asphalt concrete used in pavement construction.
Research Agency : Auburn University (NCAT) PI: K. Kandhal
Contract Dates : 2/1/94 to 1/31/97.
Contract Status: Completed, final report is being reviewed.
4. Survey of Field Experiences with Superpave Mixtures
Mike Anderson reported on an Asphalt Institute survey conducted by its field engineers in 18 states on 86 of the 93 Superpave projects constructed in 1996. Field practices and construction issues were the central theme of this survey. As a result of the field engineer's visits of State DOTs , a report was produced on construction experiences/solutions using Superpave. A copy of this report was sent to members of the ETG and Lead States for comment, and the report was distributed to all DOTs . About 1/3 of the initial projects experience some type of construction problem, usually related to compaction. Increased compaction efforts and lift thickness cured some of the problems, however a number of mixes experienced a tenderness during intermediate compaction temperatures. It has been proposed that this problem with coarse graded mixes may be the result of a combination of excessive moisture in the mix and binder stiffness.
As a complimentary part of this effort, the Lead States produced in July, 1997 an implementation survey of completed 1996 and planned 1997 Superpave projects.
These projects will form the basis for tracking projects in order to be able to provide a continuing data base of all future Superpave projects.
5. Aggregate Flat & Elongated Criteria
In response to a Lead State recommendation to include the Flat & Elongated 3:1 ratio as part of the Superpave Consensus Property Standards, Tom Harman presented data for discussion. The data collected as part of FHWA's DP 90, included stockpile results from 13 different states with a total of 27 individual stockpiles.
The current 5:1 ratio does not appear to distinguish between aggregates, and thus not of much use of anticipated performance differences. Some States report that all their materials pass the 5:1 at 10% maximum criteria.
Jahn presented a summary of his view of the impact of going to a 3:1 criteria.. He showed results on the basis of different geologic sources and crushing operations. However, his results were based on limited testing, in some cases of single stockpiles from each aggregate source. Regardless, a modified criteria of 3:1 will require some changes in aggregate production. Also, the impact is unclear because of what maximum criteria would be used with 3:1.
Flat and elongated shapes can effect performance. Flat and elongated particles will breakdown during the construction process changing gradation of the mix and the overall mix properties. Flat slivered aggregate have a tendency to lay flat in the pavement causing slip planes and reducing aggregate interlock.
Some DOTs are suggesting to set the maximum criteria for 3:1 at 20% as measured on the plus 4.75 mm sieve. This is similar to the SMA aggregate criteria that is currently in use. This criteria is more restrictive than the current Superpave and actual percentages as related to performance have yet to be determined.
6. Changes in Aggregates Using Ignition Oven
To address two questions raised by the ETG, Chuck Paugh presented data for discussion. First data was shown demonstrating the effect of the Ignition Oven on aggregate properties. Specifically, apparent and bulk specific gravity, and fine aggregate angularity. In many instances, the "differences" are less than the test precision. The purpose of the presentation was to explore the use of the Ignition Oven in assessing RAP in Superpave mix design. The ETG agreed this is an important issue.
Data was also shown looking at the difference between running the SGC to Ndes versus Nmax. The difference between the two methods was mix dependent as expected. However, mixes with steeper compaction slopes demonstrated the lowest difference. This issue will continue to be investigated and is a being investigated under the NCHRP 9-9 project.
7. Using SST to Evaluate Mixtures
Steve Jones made a presentation on the use of the SST to evaluate mix properties. In this evaluation the same aggregate structure and asphalt content were used, but a control binder of PG 64-22 was used and compared to various version of PG 76-22 (elastomer, plastomer and oxidized modifications). The hypothesis of the testing was that for a given mix all the 76-22 grades would have the same properties. SST tests were performed. For high temperature (52C & 58C) repeated shear constant height and stress ratio, and for intermediate temperature (25C & 37C) frequency sweep constant height and simple shear constant height were the tests utilized.
Permanent shear strain (3% air voids) differentiated between the control and PG 76-22 grades, the control being the less stiffer of the two and thus having the greater shear strain. Complex shear modulus (7% air voids) as would be expected was generally lower for the control, but repeatability between the tested pairs was quite variable. Coefficient of variability was 15% even when more specimens were utilized. While actual values needed to be further evaluated, results could possibly be used to rank mixes.
One observation was that as test temperatures increase, the performance indicators became less distinguishable between different binders, this can be observed by the test data being grouped closer at 58C than at 52C.
One suggestion was that since mixes are sensitive to air voids, we may need to be normalize all tests to 3-4%.
Mike Anderson also made a presentation on using the SST to evaluate mix properties. One aspect on the evaluation was to test a mix with fiber A and B and without any fiber, another with low binder content and low dust and high binder content and high dust. All were tested at 7% air voids and performed at 28C and 43C.
The data shows that mix with fiber A generally had a higher modulus and lower shear strain. In another part of the study mixes with high asphalt content and high dust (HH) had a greater shear strain and lower modulus. Again, while actual test values need refinement, results could possibly be used to rank mixes relative strength.
Next step is to evaluate most appropriate test parameters of temperature, air voids, etc. Procedures are being evaluated by Superpave Regional Centers for repeatability. More experience is needed with lab tests and actual field performance.
The FHWA is conducting work utilizing the SST as a mix performance indicator. These efforts are underway with the University of Maryland, University of California at Berkeley, and the Asphalt Institute as well as the FHWA's Turner Fairbanks Highway Research Center.
Bob McGennis reported on the status of ruggedness testing for the SST and the IDT procedures. Ruggedness is an evaluation of the procedure--does normal operator variation have significant impact on test results. This work is evaluating TP7 and TP9, and is being supported by the FHWA at various laboratories including the Superpave Centers, Turner Fairbanks Highway Research Center, University of California at Berkeley, and the Asphalt Institute.
Urethane specimens were fabricated for initial shakedown testing in the SST. The results of SSCH and FS testing gave similar results at all the labs. Urethane specimens appear to be the standard material for users of the SST to compare/initialize tests.
From the SST data shown very similar results were shown for all tests results from the Superpave Centers at Texas and Alabama using the Interlaken SST and the Asphalt Institute using the Cox SST, however differences were observed by the fourth test lab- the University of California at Berkeley. This lab is using the first prototype SST and it is believed experienced testing problems with defective LVDTs.
It is important for SST development that this work continues. Until this effort, a multiple laboratory round robin has never been conducted with the SST.
Kathy Petros presented an overview of the contract effort with the University of Maryland to develop a performance tester. This Phase II of the contract is to start on 10/1/97. One immediate task is to identify and perform limited evaluation on a simplified performance tester. Within 4 months, a number of likely candidates will be identified. For the next 14 months, the candidates will be evaluated in some limited lab analysis and comparisons made to various field projects (i.e. WesTrack, MnRoad, ALF, and SPS-9).
A supplementary effort to identify an SST test as a possible candidate performance tester is also being conducted during 1997 at University of California at Berkeley. Shmuel Weissman made a presentation on the various mechanical models associated with the various laboratory tests. The objective of this modeling efforts is to establish guidelines as to minimum specimen dimensions, specimen orientation, and effects of test imperfections on results. The RVE was identified, the representative volume element. This is the smallest material volume large enough so that the characteristics of the material are the same regardless of its position. While this needs to be further determined for asphalt mixes, typical values are expected to be 75 to 100 mm. For shear test at constant height, the diameter/height needs to be greater than 3. For restricted triaxial tests, the diameter needs to be greater than the RVE and the height needs to be greater than 2 times the diameter + RVE.
This work will be reported along with evaluation of SST tests by University of California at Berkeley by the end of 1997. Copies of Berkeley report will be made available to the ETG and the University of Maryland for review and comment.
Tom White reported on work being conducted at Purdue University, this work included validation of Superpave aggregate specifications using accelerated testing. In particular the FAA, VMA and gradation criteria will be reviewed.
The "Validation of Superpave Asphalt Mixture Specifications" project is a national pooled fund study # 176 with 26 participating states. The project has a budget of nearly $600,000 and is investigating VMA requirements, fine aggregate angularity, and gradation/restricted zone. Work planned for 1997 includes the examination of a limited number of samples to establish relationships and check the equipment sensitivity to property changes. In 1998, a full factorial design is planned. Related testing includes: characterization of materials in accordance with Superpave, triaxial testing on mixes, Purwheel testing, intermediate/complete Superpave mixture analysis, APT testing, and statistical analysis.
The "Fine Aggregate Angularity Study" is an Indiana DOT/ Joint Highway Research Program project (Tom White, PI). The study will use the Purwheel plus vane shear, Florida bearing ratio, Superpave Shear Test (SST), and petrographic analysis. The study is 12 months in duration. The goal of the study is to determine whether the fine aggregate angularity requirements are appropriate. The study is in response to industry concerns about the applicability of the test to distinguish between fine aggregates. This is of particular concern in Indiana since more than 40 Superpave projects are scheduled for 1997. The study is designed to answer specific industry questions about the test procedure. Final report of the FAA portion of the Purdue work is due in December, 1997.
Gerry Huber reported on activities in Indiana to construct Superpave mixes. PG binders have been required on all DOT projects since 1/1/97. Forty-five projects, representing over two million tons have been let utilizing Superpave criteria. These mixes represented major changes to Indiana surface mixes, utilizing less natural sand, higher binder contents, and more dust. For intermediate and base mixes less of a change.
Construction has been very satisfactory, but a few projects have experienced tenderness and density problems. Some flushing of variable severity experienced on 8 projects, mostly minor. However, I-74 project had major flushing problem. The results of a meeting on this project determined that the binder content was too high, incorrect specific gravity value used in calculations, possible mix moisture problems, improper mixture aging procedure used on plant produced mix, too low N-design value (3-10 million ESALs) used, and inappropriate binder grade.
As a result Indiana has increased binder grade to PG 64-22 on projects to eliminate flushing. Density specification and permeability of in-place pavements is being investigated. Moisture evaluation of mixes is also being conducted.
DOT believes implementation has been successful, with limited problems. DOT pleased that Superpave is addressing previous mixture shortcomings and has advanced asphalt pavement performance in the State. State planning on full Superpave implementation in 1998.
Eileen Connolly reported on activities related to the AASHTO Provisional Standards. The recommended revisions to MP 2 and PP 28 have been balloted . The ETG comments on TP4 and PP2 were provided to AASHTO, and these specifications were approved for re-issuance.
Tom Harman will send the following additional comments on AASHTO TP4-93, revised 1996-97 will be sent to Haleem Tahir.
Section 4
Section 4.1 "The compactor shall tilt specimen molds at an angle of 22 ± 0.35 mrad (1.25 ± 0.02°) and . . ." Change 22 to 21.8. 22 mrad is 1.26°.
Section 4.3 Ram Heads and Mold Bottoms. . . Change to Ram Heads / Mold Tops and Mold Bottoms throughout section. The Pine SGC has both a mold top and bottom.
Section 7
Remove Note 3; Table 1 is also to be removed.
Remove sections 7.3 and 7.4 and replace with "Lubricate required surfaces as
recommended by the manufacturer." Because each machine is unique, these sections do not always apply.
Section 8
Section 8.1 Remove the words "Level 2/3." This terminology is not longer being used.
Replace Note 4 with the following:
Note 4 - It may be necessary to produce a trial specimen to achieve this height requirement. Generally, the mass of mixture required to achieve this height is approximately 1970 to 1990 times the maximum specific gravity (Gmm or Rice). For a Rice value of 2.500, the required mass will be around 4925 grams (2.500 x 1970) to 4975 grams (2.500 x 1990).
Section 9
Section 9.5 Change 22.0 to 21.8
Section 12
Section 12.6 Change 0.01 mm to 0.1 mm.
Remove Table 1, Also note Table 1's Design ESAL's are not properly designated. If this
table is used in another specification please correct.
Provisional standards have a duration of two years, and then can be balloted for another two years. When provisionals are accepted, they are deleted from the provisional listing (TP33 is such a specification that has now become a regular AASHTO specification. AMRL is currently doing a round robin to determine procedures precision). There is typically some delay before accepted provisionals appear as a regular specification.
It was recommended by the mix ETG that PP2, the procedure for short term aging of asphalt mixtures, remain at four hours, with a note indicating that two hours could be an allowed exception under certain circumstances. This recommendation was sent out in letter ballot to the subcommittee on materials.
The 1997 edition of the Provisional Standards were issued earlier this year. To save on costs, they did not print all of the provisional standards. Instead they included only new provisional standards and any provisionals that have experienced major changes. The 18th edition of the AASHTO Specification book has a target date of 1997.
13. Protocol for Evaluating New Superpave Gyratory Compactors
Bob McGennis reported on lab evaluation of several new Superpave Gyratory Compactors using the protocol developed by the ETG.
Features of the SGC evaluation protocol include a "peer review process", no "approval" by FHWA. Elements of the protocol include comparison of new compactors to the performance of a pooled fund compactor, suggests laboratories to be used for the evaluation, and uses data from Demonstration Project 90 to establish guidelines for tolerances.
Some new compactors under development include models being manufactured by IPC, Pine, Interlaken, Rainhardt, Test QIP, Instron, Bork Longyear, and Troxler.
McGennis recommended some various additions to the protocol. These included (1) daily verification of each SGC calibration, (2) continually verify oven/mixture temperature, (3) randomization of sample testing, (4) set compaction temperature the same as aging temperature, (5) bulk specific gravity should all be performed by same person, (6) change section 1.2.10 criteria to 0.010 instead of just 0.01. These changes will be given to FHWA (Bukowski) for possible submission to AASHTO.
Gerry Huber presented an update on the N-design refinement study. The purpose of the study is to refine the relationship between densification in a gyratory compactor and under traffic.
The experiment to determine a design number of gyrations was conducted during the Strategic Highway Research Program (SHRP). The underlying hypothesis was based on the assumption that a number of gyrations in the Superpave gyratory compactor (SGC) would produce a density equal to the density in the wheel-path after trafficking. The experiment objective was to define a relationship between the number of gyrations in the Superpave gyratory compactor and the number of equivalent single axle loads (ESALs) within a given climate. The original N design experiment is documented in SHRP Report A-408 and elsewhere.
After SHRP was complete, concerns were expressed regarding the approach used to determine the Ndesign values. One concern was that the number of sections studied was small. Another was that the prototype SGC might not have met current FHWA specifications. With respect to the tabulation of Ndesign values, some believed that the table was too complicated for use by practicing engineers. Others believed that the climate zones were not adequately delineated because most locations in the U.S. had a design temperature in the first category. Still others questioned the absolute validity of the design numbers themselves, especially those in the extreme traffic categories.
Based on these concerns, the FHWA authorized researchers at the Asphalt Institute to extend the original experiment. Eight new sites were evaluated and the results were highly unusual. In some cases, the indicated design gyrations was extremely low, 15 to 20 gyrations. Other sites indicated a very large number of ESALs was needed to densify the mixture to four percent air voids. This experiment suggested that the design compactive effort probably should not be tied to density at the end of life.
In light of the aforementioned concerns and the most recent experiments, a very important consideration that must be recognized is that the current values of Ndesign appear to be successful. It is likely that the current values of Ndesign are sensible, or at least close to the "correct" values.
The work plan is proposed with the assumption that the design gyrations in current use are close to correct. An advisory group (Ray Brown, Sam Carpenter, Ray Bonoquest, and Scott Shuler) met earlier this year with Rich May, Gerry Huber, Bob McGennis and Tom Kenedy to help develop this plan. It uses a parallel research approach. That is, the tasks will support, but not necessarily depend on each other.
The effort in this work-plan is divided into five tasks:
1. Densification of Mixtures with Time.
2 (A) Identification of Performance-Related Property.
2 (B) Verification of Performance Related Property with SPS Sections.
3. Sensitivity of Performance Related Property to Change in Design Gyrations.
4. Sensitivity of Volumetric Properties to Change in Design Gyrations
5. Modification of Design Gyration Table.
Task 1 will evaluate both Superpave projects and mix designs currently used by states. By 1996, there were more than 100 Superpave projects built in the U.S., some as old as four years. The existing Ndesign table was used to design the existing projects. A sample of these projects will be evaluated. Air voids at different times in the pavement life will be obtained and projected to 15 years. The projected air voids will be compared to air voids of the plant mix in the compactor used for design, for example Marshall hammer for Marshall designs and Superpave gyratory for Superpave designs.
Task 2(A) will evaluate several engineering properties of mixtures constructed in the WesTrack experiment to identify a property that is related to rutting on the road. Task 2(B) will verify that the performance-related engineering property identified in the WesTrack experiment relates to rutting performance of Superpave mixtures constructed at other sites including SPS sites.
Task 3 is a laboratory study to evaluate the sensitivity of the performance-related property to changes in design compactive effort.
Task 4 will evaluate the change in volumetric properties with a change in design compaction. This task will evaluate laboratory work performed under research contract NCHRP 9-9.
Finally, Task 5 will be to make modifications, if necessary, to the table of design compaction levels based on all of the findings.
Conceptual Ndesign Values Resulting from Task 5
| PG Climate |
| Traffic Rate | 46 to 52 | 52 to 58 | 58 to 64 | 64 to 70 |
| Base* | Surface | Base | Surface | Base | Surface | Base | Surface | |
| Low | x | x | x | x | x | x | x | x |
| Medium | x | x | x | x | x | x | x | x |
| High | x | x | x | x | x | x | x | x |
| Each cell represents an Ndesign value for the conditions shown. |
| NCHRP 9-9 developing design compactive effort for lower layers. |
It is planned that all tasks could be completed by the end of 1998. A complete copy of the N-Design Plan will be distributed for comment and review by the ETG.
Pierre Langlois and Francis Moutier made a presentation on the development of gyratory compaction in France. French have used since 1972 and have built versions of their gyratory compactor for use in other countries. French device keeps mould heated during compaction. Uses a different angle than that in Superpave, but other factors are the similar. French relate % of voids to number of gyrations. N-design is a function of pavement thickness not traffic and corresponds to mix workability during construction. Number of gyrations function of thickness, roller compaction passes . A roller was used in 1980 correlation efforts to set the relation with the gyratory. This is their check of laboratory compaction to the field.
Basic difference in approach with the gyratory compactor is that the French use it not for mix design but rather to set field compaction targets.
16. Next Mix ETG Meeting
The next meeting of the mix ETG is planned for March 10 and 11, 1998 in Orlando, Florida. The meeting will begin at 1:00 pm on March 10 and end at 5:00 pm on March 11. Details of the hotel location of the meeting will be mailed separate from these minutes.
Members were requested to submit agenda items in writing with supportive information/data for the next meeting. Agenda items need to be submitted to John Bukowski no later than February 15, 1998. Only topics received in this manner can be assured of time on the next meetings agenda.
Meeting was adjourned at 5:00 pm.