Minutes from the FHWA SUPERPAVE Mixture Expert Task Group Meeting
The meeting was convened at 1:00 pm on Tuesday, March 10, 1998 by Chairmen John Bukowski and Tom Harman of the FHWA.
Those members attending were:
Mike Anderson, Asphalt Institute
Bob Briggs, Washington DOT
John Bukowski, FHWA, HNG-44
Don Christensen, Penn State University
Dale Decker, NAPA
Doug Hanson, NCAT
Thomas Harman, FHWA, HNR-20
John Harvey, Univ. of California
Gerry Huber, Heritage Research
David Jahn, American Aggregates
Dave Andrewski, Indiana DOT
Ken Brown, Troxler
Ray Brown, NCAT
John D'Angelo, FHWA ,HTA-20
Frank Dalton, Pine Instrument Co.
Greg Harder, Suit-Kote Corp.
Dave Gearing, Interlaken
Lee Gallivan, FHWA, Indiana Division
Tom Kennedy, Univ. of Texas
Gale Page, Florida DOT
Those visitors attending were:
Larry Michael, Maryland DOT
Jim Musselman, Florida DOT
Allen Myers, KY Trans Cabinet
Julie Nodes, Arizona DOT
Brian Prowell, VA Trans Research Council
Mike Rafalowski, FHWA, HNG-20
Ronald Sines, New York DOT
Maghsoud Tahmoressi, Texas DOT
Haleem Tahir, AASHTO
Chuck Paugh, Contractor DP-90 Project
Katherine Petros, FHWA, HNG-40
Pedro Romero, FHWA contractor, HNR-20
Jim Scherocman, Consultant
Greg Schiess, FHWA, Florida Division
Mikael Thau, Superfos Construction
Chris Williams FHWA,HNR-30
Thomas White, Purdue University
Matt Witczak, Univ. of Maryland
Jack Youtcheff, SALUT
The meeting began with self introductions of the Expert Task Group (ETG) members and visitors. After introductions, John Bukowski and Tom Harman reviewed the objectives and structure of the Superpave Mixture ETG and the meeting agenda.
Subcommittee Reports
1. Flat & Elongated Aggregate Requirement
Jim Musselman briefed the ETG on the activities of the Mix ETG Subcommittee on Flat & Elongated Aggregate Requirements. This subcommittee, headed by Jim Musselman, had been tasked to develop ETG recommendations regarding the 3:1 ratio. Items they are to consider include: what fraction should the test be run on plus 4.36 mm (No. 4) or plus 9.5 mm (3/8") and what maximum criterion is appropriate. The subcommittee includes Jim Musselman, Becky McDaniel, Erv Dukatz, Eileen Connolly, and Jim Gehler (now retired).
A background on the flat and elongated specification was provided. There are some points of confusion in the current AASHTO MP-2. It is not clear, on what sieve size to perform flat-elongated and MP-2 does not include a size reference (+4 or +3/8 as in ASTM D4791). Musselman reviewed a number of State current practices. It was again emphasized that the test should measure and compare the shortest to longest aggregate dimensions.
The AASHTO Lead States have recommended changing the ratio from 5:1 to 3:1 because 5:1 does not seem to adequately differentiate between materials. There was considerable discussion among the ETG that there should be an engineering basis for the change and that research currently underway should be considered. Both the International Center for Aggregate Research (ICAR) and the University of Illinois are conducting research in this area. The University of Illinois study will be fabricating mixes with different ratios and seeing the effects on volumetrics. In addition, the AASHTO Subcommittee on Materials has formed a task group, led by Rick Smutzer, to look into this.
A draft guidance statement for agencies considering a 3:1 flat and elongated aggregate consensus criteria was presented to the ETG. The ETG recommended that the guidance statement be incorporated into a document that includes a background of the basis of the guidance. The Flat and Elongated Task Team will develop this document and will circulate it to the ETG by the end of March.
John Harvey presented the status of the Superpave Shear Test (SST)/Performance Tester Subcommittee. The other members of this subcommittee are Mike Anderson, Dale Decker, Doug Hanson, Ed Harrigan, Bob McGennis, Ron Sines, Julie Nodes, and Chris Williams. The subcommittee focus will be on the tests currently available on the SST and gathering information to try to relate SST test results to rutting performance.
The scope of the subcommittee's planned activities has been circulated among the subcommittee. Task 1 is to define the tests available on the SST and to provide guidelines on appropriate testing conditions. Task 2 describes the information that should be included with SST test results to permit data interpretation. Task 3 will develop a first approximation of performance criteria to be used with SST test results. The Task 3 activities will be conducted from March 1998 until March 1999.
There was some discussion that the Task reports need to be further reviewed before they are disseminated beyond the ETG. The Task reports will be reviewed by the SST/Performance test subcommittee and will then go to the whole ETG for review. It was agreed that these will be treated as working documents to only be distributed to the Mix ETG.
Julie Nodes explained that the purpose of this subcommittee is to review the Purdue FAA Study report. The subcommittee has not yet received the report. Members include David Jahn, Jon Epps, and Tim Aschenbrenner. Julie Nodes then turned over the presentation to Tom White who presented a status on the Purdue study.
The Purdue study involves the evaluation of one asphalt binder, PG 64-28, one coarse aggregate, and nine combinations of fine aggregates. Performance testing with the Purdue Loaded Wheel Tracking device (Purwheel) has been performed. The development of a Phase II proposal to extend this work is currently underway. Tom White will provide the FAA subcommittee with copies of the completed Purdue FAA report.
The National Pooled Fund Study #176 was also discussed. This study, which has the support of twenty-six States, is investigating FAA, VMA, and gradation criteria. Tasks include modeling of the Purwheel and looking at the sensitivity of the Purwheel and accelerated pavement tests.
UPDATE STATUS / DEVELOPMENT PRESENTATIONS
Tom Harman briefed the ETG on the status of gyratory compactors that have been tested under AASHTO PP-35. The Pine Jr.(portable version), Interlaken, and Test Quip devices have all met the criteria and can be considered comparable to the original Pine and Troxler devices. This means there are now five acceptable gyratory compactors. It was also noted that the gyratory being marketed by Gilson is the Test Quip device.
2. AASHTO Update
Haleem Tahir presented an overview of AASHTO's SHRP implementation functions. Organizationally, AASHTO relies on a framework of Task Forces and Committees that all have the objective of implementing SHRP technology. January 2000 is a milestone for SHRP implementation, even though some activities such as LTPP and the Superpave performance prediction models development will continue beyond that date.
In the area of Superpave implementation, by the end of this year, forty-seven States will have tried Superpave mixes. The uniform implementation of the Superpave system is a big push by AASHTO's Task Force on SHRP Implementation. Other areas of emphasis include promoting fast action on necessary specification changes and increasing the involvement of local governments.
AASHTO's Subcommittee on Materials (SOM) will publish a new complete edition of all Provisional Standards by June 1998. This edition will include sixty-nine standards, including PP-35 on evaluating gyratory compactors. Typically provisional standards exist for two to four years, for SHRP these have been extended to eight years. By 2001, the standards' provisional nature must be dropped.
At the conclusion of the NCHRP 9-7 (Field Procedures and Equipment to Implement SHRP Asphalt Specification) project, the panel asked that the report be put into a QC/QA standard. This draft standard is now complete at forty pages and dated 2/23/98.It has been sent to each of the State materials engineers for comment and Rick Harvey and Gary Robson are evaluating it for their respective Technical Sections of AASHTO's SOM. It was sent to the Mix ETG on May 12, 1998 with a request for comments to John Bukowski by June 1, 1998.
Larry Michael informed the ETG that a significant amount of testing has been completed on the WesTrack materials. The WesTrack forensic team was planning to meet that evening to discuss their report. Other members of the team are Ray Brown, Erv Dukatz, Gerry Huber, Jim Scherocman, Ron Sines, and technical support was provided by John D'Angelo and Chris Williams as FHWA representatives.
John Bukowski updated the ETG on activities underway by FHWA's Superpave TDT. A model project survey is being conducted of the Lead States by NCAT. The plan is to visit eight States by May 1998 and to gather data on approximately four projects in each State. The type of data currently under consideration for collection includes mix design, production, and test procedure data, as well as performance data such as rutting and cracking assessments.
The TDT is also developing protocols for a Superpave Swift Action Team (SWAT). The purpose of this team would be to quickly address Superpave issues/problems and to aid in the uniform adoption of Superpave.
John D'Angelo reported on the activities of the Superpave Binder ETG. A new specification for the direct tension test (DTT) has been developed and will be going to the AASHTO SOM within the next few weeks. The specification calls for new equipment, molds, and fluid. All of the original SHRP binders have been retested using the procedure. Ruggedness testing of the DTT procedure will be conducted in the spring of 1998. The bending beam rheometer and direct tension tests will be combined to address low temperature fatigue cracking.
The low temperature algorithm to convert air temperature to pavement temperature now includes latitude and is the LTPP algorithm that is based on Canadian data.
The NCHRP 9-10 study on the effects of modifiers is finding that the time-temperature superposition performed in the PG specification is incorrect for modified materials. The substitution of temperature for frequency does not work for modifiers and could result in errors.
The Binder ETG is considering adding an intermediate temperature to the PG specification. The resulting binder grade would appear as: PG HH-II-LL. The purpose of this addition would be to address fatigue.
Larry Michael informed the ETG that the AASHTO Lead States were planning to meet on March 12, 1998 following the Mix ETG meeting. The purpose of this meeting is to develop guidance on a number of Superpave related issues that are continually raised by the State highway agencies.
Jack Youtcheff presented the research he is conducting to develop a protocol for evaluating the moisture susceptibility of asphalt binders that is rapid, inexpensive, and reproducible. The experimental procedure measures the tensile and bonding strength of asphalt binder applied to a glass plate as function of time exposed to water. Asphalt binder, containing 1.0 wt % of glass beads, is applied to a porous disk which is then pressed onto a glass plate. The pressure necessary to debond the conditioned specimen at 25 oC is measured with a pneumatic adhesion tester. The study has demonstrated that the viscosity building structure provided by asphaltenes seems to be disrupted by the presence of water, and the resistance to moisture damage of the binder seems to depend on the properties of the maltenes. Tom Harman requested that any States with cores from stripped sections send one core per section to him or Jack for evaluation.
Julie Nodes provided the ETG with some background information on the SPS-9 project on US-93 that has experienced severe cracking. The SPS-9 was constructed in 1993 on new alignment along with some Arizona DOT test sections. One aggregate source was used and various structural sections were constructed. The SPS-9 contained two Superpave mixes, 19 and 25 mm, both of which graded below the restricted zone. The mix designs were done by the Asphalt Institute, but with limited AASHTO T-283 testing. ADOT added cement as an anti-strip to the sections that did not have Superpave mixes, but used no treatment on the Superpave sections. The cores taken from the cracked sections showed a significant stripping problem. This experience had made ADOT cautious and they are closely examining their Superpave sections planned for this year.
Kathy Petros presented an update on the status of Version 2.0 of the Superpave Software for volumetric mix design. Version 2.0 is Windows 95 based and in addition to the mix design module, it includes binder verification, QC, and QA modules. It has undergone beta testing by representatives from the State highway agencies, industry, the Superpave Centers, and FHWA. The Superpave Models contractor is incorporating the beta comments to the software and delivery to FHWA is expected in April. (This has now changed to May.) FHWA intends to turn the finished software over to AASHTO for distribution. AASHTO will assume ownership of the software and will be responsible for it user support and maintenance. The Models contractor will be providing limited user support until AASHTO is set to do so. Larry Michael added that Maryland plans to require the use of the software Statewide, and that using the software is a good way to standardize the Superpave mix design system.
10. NCHRP Superpave Related Projects
John Bukowski provided an update on the status of various NCHRP projects related to Superpave.
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 QC/QA standard has been developed and has been sent to the State materials engineers for comment.
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 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: Scheduled to have the report to the panel by the end of May/ beginning of June 1998.
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: Have evaluated the binders and are planning to begin the mixture testing.
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: Purdue with the Asphalt Institute PI: Becky McDaniel
Status: Panel will be meeting in May.
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 : University of Nevada, Reno PI: Jon Epps
Status: Award in September 1997. Results due by September 1998.
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.
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 study is divided into five tasks:
This study is taking a three-pronged approach to modifying the N-design table. Tasks 1 and 4 are two ways of looking at Ndesign, but the tasks related to a performance-related property (Tasks 2 &3) are the study's main focus. 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 current 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 15/20 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 4 will evaluate the change in volumetric properties with a change in design compaction. This task will evaluate/utilize laboratory work performed under research contract NCHRP 9-9.
The work in Task 2(A) is to evaluate several engineering properties of mixtures constructed in the WesTrack experiment to identify a property that is related to rutting on the road. Materials from five WesTrack sections were used to evaluate these properties. Two of the sections (2 & 18) were chosen because of their good rutting performance (average 7mm). The other three sections (3, 17, and 20) averaged 14 mm of rutting. All of these sections had fine gradations. Properties that have been evaluated include viscoelastic (G*, ), repeated load shear (slope vs. repetitions or strain at 5000 repetitions), and shear creep (strain at 1000 seconds). The repeated shear data shows the poorer performing sections deforming more rapidly than the better sections. The shear stiffness results from the frequency sweep tests had the highest R2 values, particularly at higher frequencies, but the point was raised that because the gradations are basically the same, the test is really looking at the binder or mastic stiffness.
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. Finally, Task 5 will be to make modifications, if necessary, to the table of design compaction levels based on all of the findings. It is planned that all tasks could be completed by the end of 1998. Copies of the Ndesign experimental plan were distributed to the Mixture ETG in August 1997.
12. Written Comments to the ETG from Randy West
Dale Decker initiated an ETG discussion on written comments submitted to the ETG by Randy West. The first issue raised was to clarify Table 9, Density Requirements, of AASHTO MP2-95. The ETG recommends that the required %Gmm values be written to the nearest 0.1%, i.e. 89.0, and that the < be replaced with .
The second point in Randy West's comments concerned clarifying the wording on the heating of compaction molds in AASHTO TP4. This has already been updated in the latest AASHTO specification.
Randy's final suggestion was that the ETG provide guidance on "Design ESALs" and "Average Design High Air Temperature." The ETG had a lengthy discussion on whether the SHRP design ESALs are based on a fifteen or twenty-year design life. The ETG recommends that the following note be added to AASHTO MP-2: Design ESALs used in the tables are based on a twenty-year design life.
Matt Witczak presented the ETG with some background on the tasks in the Superpave Models Phase II contract and then talked specifically about the task now underway to identify a fundamental performance test to be used with the Superpave volumetric mix design.
As a means of initially rating potential test devices for their utility to users in highway community, the Models Team sent a questionnaire to these prospective users asking them rate how important various reliability and operational factors are to them. The results of the approximately 100 responses returned to the Team as of March 1, 1998 were presented. Of all the reliability and operational factors, the one that was of greatest importance to users is the reliability of a test to identify inferior mixes. When users were asked to balance the test reliability against operational consideration such as equipment cost, testing time, etc., users felt that 62% of the emphasis should be on reliability and 38% should be on operational considerations. When asked what distress type the performance test should be related to, the overwhelming response was that rutting must be addressed. Fatigue cracking was considered to be of moderate importance, while a test for thermal cracking was not seen as needed to support the Superpave mix design procedure.
The Models Team will perform an assessment of each potential test device and then combine that assessment with weighting factors as determined by the survey. The top devices as determined from this analysis will be identified for a year long laboratory investigation. A report identifying the devices to be lab evaluated as well as the laboratory test plan will be submitted to FHWA by the end of April 1998. After the laboratory investigation, the Models Team will provide FHWA with a recommended performance test device and testing protocols in May 1999.
Mike Anderson reported on the status of ruggedness testing for the SST and the IDT procedures. 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.
One of the factors under evaluation is the tolerances on gyratory compacted samples. The tests are currently run at 7% ± 1% air voids, but a determination needs to be made on how high or low one could go from that value without adversely affecting the test results.
Specimen preparation is now underway. Laboratory samples of neoprene have been tested. Bob McGennis has the data for the SST and he may have extra samples available for those with an SST device. Don Christianson is analyzing/preparing the data for the IDT.
Dale Decker briefed the ETG on the distribution of NAPA's Superpave Construction Guidelines. NAPA has already sold 4000 copies and they are printing 4000 more. FHWA will be sending two copies to each of their Division Offices and asking their Divisions to send one of those copies to their State counterpart.
Dale Decker also presented the results of NAPA's Superpave Contractor Survey. The survey was conducted by interviewing one contractor per State and there were twenty States represented. The goal of the survey was to get an overview of Superpave successes and problems from a contractor's perspective. Overall, the survey revealed positive construction results with the biggest change resulting from the Superpave system being an increased attention to detail.
A few issues for further consideration were raised. Half of those surveyed said that they struggled to meet all of the volumetric criteria. Issues related to this include low/inconsistent voids, the need for more cubical material and more manufactured sand. The point was also raised that some States publish "generic" aggregate specific gravity values and tell the contractors to use those values. If the contractor is responsible for meeting volumetrics, these specific gravity values need to be determined regularly for specific aggregates.
On the topic of compaction, half reported that a greater compaction effort was needed with Superpave mixes. Two thirds of those surveyed noted tenderness in the mix, with temperature zones being critical. NAPA, working with FHWA, will put together a workshop in the next few months to discuss the technical reasons for the tenderness zone in compaction.
As a result of Dale Decker's presentation, a suggestion was made that the Superpave Lead States should review the NAPA survey to see where they could issue guidance.
Ray Brown briefed the ETG on the work ongoing under NCHRP 9-8. This study addresses to use of stone matrix asphalt (SMA) under the Superpave system. A lot of the things being done for SMA are also applicable to coarse graded Superpave mixes. One of the issues being investigated is the use and accuracy of nuclear density gauges. Other issues under review are the effect of flat and elongated properties on breakdown and volumetrics, and the effect that the binder/filler mastic has on mixing and compaction temperature.
An analysis of the ratio of the bulk specific gravity of gyratory compacted specimens to Marshall specimens indicates that 80 gyrations gives a value of 1.0. Maryland uses 100 gyrations and has had good performance. A value from 80-100 will probably be recommended.
The final report for NCHRP 9-8 is to be published this summer.
Ray Brown discussed his activities on the NCHRP 9-9 project. The intention of this study is not to change the Ndesign, but rather to look at consolidating the Ndesign tables. The values may change for low volume roads, though. Another possible change may be to perform all designs on specimens measured at Ndesign (not back calculated from Nmaximum ). They are also developing criteria for gap graded and large stone mixes, and evaluating whether base mixes need to be related to traffic.
The draft report is due to NCHRP by the end of May and the final report is due this summer.
Pedro Romero provided the ETG with the results of a study whose objective was to validate selected mechanical tests used to predict rutting in asphalt pavements using results from the FHWA's ALF. FHWA's ALF had twelve different pavement sections that were built to study rutting and fatigue. The sections had the same mix gradations (19mm and 37.5mm), but five different binder grades were used. Two of those grades were achieved through the use of modifiers (styrelf and novophalt).
The mix tests that were evaluated were the French pavement rut tester, the Georgia loaded-wheel tester, the Hamburg wheel-tracking device, and the Superpave shear tester. Most tests provided the right trend for the 19-mm mixtures, but not for both the 19 and 37.5 mm mixes. Possible reasons for the differences are that the specimen sizes may not be representative and the test configuration might not allow for proper boundary conditions.
Pedro also presented the results of the SST testing of the WesTrack cores conducted at Turner Fairbanks in response to a request from the WesTrack forensic team. Of the tests conducted (simple shear at constant height, frequency sweep, repeated shear at constant height), the repeated shear at constant height showed the closest relationship to the actual performance at WesTrack.
Lee Gallivan informed the ETG of his activities in developing a college level course in Superpave. The course curriculum is being developed by using the NCAT two week professor asphalt course as a foundation, and will provide students with a basic understanding of aggregates and asphalts. The course will be made available to the universities in Indiana, and will eventually go to all of FHWA's Division offices.
In addition to developing the course for universities to teach, training has been provided to university professors at the North Central Superpave Center and complete sets of the Superpave binder equipment were jointly purchased by the Indiana DOT, industry, and academia and provided to two of Indiana's universities.
David Jahn presented a perspective on the impacts of changing the flat and elongated requirements from a maximum of 10% at 5:1 to a maximum of 20% at 3:1. According to the data presented for the eastern United States, the number of stockpiles failing to meet the criteria would increase dramatically as a result of a change to 3:1.
21. Next Mix ETG Meeting
The next meeting of the mix ETG is planned for September 22 & 23, 1998 in Baltimore, Maryland. The meeting will begin at 1:00 pm on September 22 and end at 5:00 pm on September 23.