Asphalt Forum

NCAT invites your comments and questions, which may be submitted to Karen Hunley at karen.hunley@auburn.edu.

Greg Sholar, Florida DOT: What lift thickness is optimal for SP-4.75 mixtures? What thickness would be considered too thin for good paving practices? Is there a maximum thickness where rutting becomes an issue? What binder grade is being used in this mix type?

Don Watson, NCAT: What practices have agencies successfully used to prevent reflective cracking?

Don Watson NCAT: There seems to be growing interest in recycled asphalt shingles (RAS), but are agencies actually seeing an increase in shingle recycling? Are there different handling procedures or mix design guidelines for tear-offs than for manufacturing rejects, since tear-offs are much stiffer?

Don Watson, NCAT: There is renewed interest in the use of ground tire rubber (GTR). What mesh-size particles have worked best and in what proportions?

Don Watson, NCAT: What are some of your agency’s priority needs in the area of asphalt research?

Don Watson, NCAT: What challenges do you foresee regarding workforce enlistment and development?

Denis Boisvert, New Hampshire DOT: New Hampshire’s experience with WMA to date has been favorable. Our specification is of a permissive nature, so most contractors are opting for foaming technologies since they cost less. They are mainly interested in the compaction aid over the fuel savings. 

We specified three WMA projects within our maintenance district overlay program, requiring temperatures that were lower than the foaming technologies are capable of to force the contractors to try the new technologies. All projects were completed with a chemical additive, that being the easiest to implement without committing capital investment.  Both DOT and contractors were pleased and excited with the results.

Dale Rand, Texas DOT: We recently completed some in-house research on a porous friction course (PFC) mix with shingles to see if we could eliminate the use of additional fibers. When using up to 5 percent RAS, we were not able to get anything to pass the drain-down test unless additional fibers were added. We concluded that the amount of fibers could be reduced but not eliminated when using RAS in PFC mixes.

Asphalt Forum Responses

The following responses have been received to questions shared in the Fall 2011 Asphalt Forum.

  1. What is the maximum amount of time that warm-mix asphalt (foam method) has been stored in a silo? At what temperature was it originally placed in the silo? (Chris Jones, Wiregrass Construction)
    1. Mark Woods, Tennessee DOT
      TDOT  Specification 407.13, “Mixing,” applies to all mixtures and permits storage for 48 and 96 hours depending on the mixture type.
    2. Dale Rand, Texas DOT
      Our specifications allow storing the mix in a silo for a maximum of 12 hours.
  2. What types of asphalt mixes/asphalt binders/pavement designs are other states using at interstate weigh station areas that are subject to heavy, slow-moving truck traffic? (Pat Upshaw, Florida DOT)
    1. Joe Schroer, Missouri DOT
      Missouri has had no problems with PG 76-22 and Superpave mix designs at 125 gyrations or stone-matrix asphalt (SMA).
  3. The Nevada DOT is experimenting with warm-mix asphalt (WMA). Statewide, we require 1.5 percent hydrated lime to be applied to the aggregates, which are wetted to approximately SSD moisture and stockpiled for a minimum of 48 hours before use. Do any other states have experience with incomplete drying of aggregates in the drum and related moisture-sensitivity issues? (Darin Tedford, Nevada DOT)
    1. Denis Boisvert, New Hampshire DOT
       New Hampshire does not apply hydrated lime.  We have not experienced any moisture-sensitivity issues with WMA to date, after an experimental season in 2010 (14,000 tons or 2 percent) and a full season in 2011 (257,000 tons or 40 percent).
  4. Has any state seen any satisfactory bid numbers, in terms of cost per square yard, for any type of in-place recycling? Has anyone considered alternate contracts such as mill, surface and binder versus hot-in-place (HIP) and surface? Thoughts? (Mark Woods, Tennessee DOT)
    1. Joe Schroer, Missouri DOT
       Missouri does not have many in-place recycling projects in place, but we believe we are getting fair prices. For example, a 4-inch cold in-place (CIR) project will have roughly the same cost as 2 inches of mill-and-fill HMA. With HIP recycling (HIR), the pricing gets trickier because the savings of the HIR can be made up by the value of the RAP to the HMA contractor. We had a full-depth reclamation (FDR) project with 12 inches of FDR at $55,000 per mile and a 3-inch overlay at $83,000 per mile. We bid projects with a multi-lift overlay versus in-place recycling and an overlay with about half going each way.
  5. Are any states having to import aggregate to meet hot-mix asphalt surface friction requirements? What tests are performed to evaluate friction properties of new aggregate sources? (Mike Heitzman, NCAT)
    1. David Powers, Ohio DOT
      Ohio imports limited aggregate in some areas for friction purposes. We currently base aggregate acceptance on actual field performance. However, in the future we will be looking to implement a new mixture lab polish method that has been developed through a series of research studies at the University of Akron. The method is highly correlated to field projects. In this method, 6-inch gyratory mix specimens are polished and then measured with the British Pendulum.
    2. Dale Rand, Texas DOT
       In some areas of the state we do have to import aggregate. The primary test we use to classify aggregates is acid insolubility. Aggregates that are highly soluble in acid tend to polish quickly and tend to produce the lowest skid numbers.
    3. Darin Tedford, Nevada DOT
      No, our open-graded mixes supply friction course.
    4. Denis Boisvert, New Hampshire DOT
      No need in the Granite State.
  6. What states are making plans to implement Asphalt Mixture Performance Tester (AMPT) requirements for HMA mix design? (Mike Heitzman, NCAT)
    1. Joe Schroer, Missouri DOT
      At this time, Missouri does not plan to implement it as part of the mixture design. We are using it to compare performance between similar mixtures for new additives and materials and will use it to check mechanistic-empirical design guide (MEPDG) calibration as our mixtures evolve. At some point in the future, it may figure into a performance specification as we try to move that direction.
    2. Darin Tedford, Nevada DOT
      We are not planning to implement the AMPT requirements at this time, but maybe after further research.
    3. Mark Woods, Tennessee DOT
      Tennessee is still awaiting arrival of our pooled-fund AMPT. Once the machine is here and we have time to work with it, we will determine its place in our program.
    4. Denis Boisvert, New Hampshire DOT
      New Hampshire will make use of the AMPT in the mix design approval process. The specifics and timing are not formulated yet, since we need experience with the device.
  7. What states are using ground-penetrating radar (GPR) or some other form of non-destructive testing for forensic investigation? (Mike Heitzman, NCAT)
    1. Pat Upshaw, Florida DOT
      We currently use air and ground-coupled GPR systems for pavement thickness evaluation and forensics (sinkhole, etc.) evaluations.
    2. Darin Tedford, Nevada DOT
      We have only used GPR in research/demonstrations, and we seldom use the falling-weight deflectometer (FWD).
    3. Denis Boisvert, New Hampshire DOT
      We have only used GPR for concrete applications, particularly on bridge decks.
    4. Dale Rand, Texas DOT
      We use GPR for forensic investigation. The equipment does a good job of detecting moisture and delamination in the pavement structure. We have also used GPR to quantify end-of-load segregation.
  8. Is anyone aware of research regarding whether it is best to remove the aggregate with cut faces from cores before testing Gmm, Pb or gradation? (Don Watson, NCAT)
    1. Joe Schroer, Missouri DOT
      We did an informal in-house look at this in Missouri 15 years ago and concluded we were altering the gradation and Pb more by removing the cut faces than by using as is. One of our contractors suggested mixing (we use a wire whip) the softened mixture briefly before performing the Rice test to coat the cut faces. We think this has improved the reliability of the Gmm.

    (Note: The following questions were asked in the Spring 2011 newsletter; however, Joe Schroer’s responses were mistakenly left out of the Fall 2011 newsletter.)

  9.  It would be interesting to hear how other states approve HMA mix designs prior to production. More specifically, what materials are submitted/tested (if any), and what is the department ultimately trying to evaluate? (Mark Woods, Tennessee DOT)
    1. Joe Schroer, Missouri DOT
      Missouri accepts contractors’ designs by paper verification if they participate in the AASHTO Proficiency Sample Program and achieve a score of 3 or higher. Otherwise, aggregates and binders are sent to the central laboratory for verification of the contractor’s design within the limits of the specification. Compaction on the roadway, resistance to segregation, etc., are considerations left up to the contractor.
  10. Has any agency used bonded wearing courses (i.e., Novachip process) and seen a noticeable improvement in performance? (Greg Sholar, Florida DOT)
    1. Joe Schroer, Missouri DOT
      Missouri uses bonded wear courses, and we believe they perform well both as a seal and when used as a single lift on concrete pavements.
  11. Has any agency experienced problems with end-of-load segregation and, if so, how is this being rectified? (Greg Sholar, Florida DOT)
    1. Joe Schroer, Missouri DOT
      Missouri adopted a version of the method developed by Kansas, which uses the nuclear gauge for determining segregation. It is used as a referee method when inspectors and the contractor disagree on the extent of segregation. The specification states no segregation is allowed.
  12. Have any agencies implemented the multiple stress creep recovery (MSCR) test for asphalt binders? Has the test eliminated any binders that would otherwise by acceptable?  (Don Watson, NCAT)
    1. Joe Schroer, Missouri DOT
      Missouri accepts binder in accordance with AASHTO MP 19 to take advantage of the MSCR test.  No binders were eliminated, but most binder is now being supplied by this specification at a lower cost.