Asphalt Forum

Greg Sholar, Florida DOT
Florida has a problem with a pavement distress known as "road worms" or "blisters." It has been documented since the early 1970's. There has been much speculation, but most believe it is caused by trapped moisture that vaporizes in hot weather and increases in volume between pavement layers, causing a blister to form on the surface. Debate exists of whether the moisture comes from the surface and penetrates into the pavement or comes from the lower granular layers and rises. A recent FDOT funded research project with ARA examined this issue and involved substantial field investigation of five roadways from various regions of Florida. Each roadway had road worm and control sections. The consensus for these five projects is that the moisture was coming from the top, and pavements with low in-place density and low interlayer bond strength had a propensity for exhibiting road worms.

Example of road worms, photo courtesy of Greg Sholar.

John Garrity, Minnesota DOT
At what temperature should the Hamburg wheel test be conducted?

Oak Metcalfe, Montana DOT
How do other states determine when or if they can deviate from the AASHTO R 35 specified gyration level? In Montana, traffic over 3 million ESALs isn't all that common, so we decided several years ago to just use 75 gyration mixes. Obviously, we have numerous roads with less than 0.3 million ESALs, but we chose 75 for consistency. Now that states are using 30, 50, 60, etc. gyrations, I wonder: how do you get there? As an example, we have an out-of-state limestone quarry that wants to get into our market, but they claim they can't develop a 75 gyration design and want us to allow a 60 gyration design. This area of our state doesn't have good aggregate so a nearby limestone source would significantly reduce costs. This mix has performed in the supplier's local area. How do we adjust our contract language to take advantage? Do other states adjust gyration level by aggregate type? Is it strictly by traffic or have you conducted other research that resulted in a systemic decision to use a non-standard gyration level? Is it simply a project by project decision?

Asphalt Forum Responses

The following responses have been received to questions shared in the previous issue.

What experiences have other states had with balanced mix design? What were the challenges of BMD and performance testing, and how have you dealt with them? If using a bonus/penalty system for pay with volumetrics, how did you use the same system for performance testing? -Zane Hartzog, Alabama DOT

Michael Stanford, Colorado DOT
CDOT is currently working with the Colorado Asphalt Pavement Association to develop a balanced mix design specification utilizing HWT and Ideal CT as our preferred performance tests.

Greg Sholar, Florida DOT
Florida has held off implementing BMD for the main reason that no proven cracking test exists yet. In addition, during production, normal variations in mixture gradation and AC values can have overly significant effects on BMD test values. These two issues need to be worked out. 

Brian Hill, Illinois DOT
IDOT currently specifies the Illinois Flexibility Index Test (I-FIT) for cracking and the Hamburg wheel tracking test for rutting. The minimum criteria for each test are required to be met in both mix design and production phases. Detailed information on the implementation effort is shown in the IDOT case study report by Hajj, Aschenbrener, and Nener-Plante shown on NCAT’s BMD resources webpage (aub.ie/bmd). IDOT does not use an incentive/disincentive program with performance tests.

Nathan Awwad, Indiana DOT
We are still experimenting with BMD. We’ve just completed an interlaboratory study between DOT labs and producer labs. It is difficult to obtain similar results even with split samples. It seems that the hindrance of BMD and performance tests is the difficulty in producing specimens and interpreting the data. At this time, it appears performance testing would be more labor intensive than standard volumetric testing.

Oak Metcalfe, Montana DOT
Montana has had a Hamburg rut specification for many years, both in design and production, but it has been a "go/no go" specification in the field. This causes problems because there are limited devices in our state, the test takes a full day to run, and we're so spread out geographically. It's the contractor's risk to proceed or wait for the results. We specify that we will turn results around ASAP but give ourselves seven days, so that's not the best situation. Also, up until recently, we were somewhat draconian with the specification, paying full price for mix with less than 13mm and requiring R&R as soon as the rut was deeper than 13mm. The R&R specification compounds the delay in results and has caused some problems, especially when a contractor chooses to proceed, the job is short, or paving is completed before we have a result. We have now instituted a sliding scale so mix can be left in place with a penalty, but there are still some kinks to work out. As far as the bonus/penalty for volumetrics, our construction managers have the leeway to take a sample for HWTT at any time, so if a lot has a failing HWTT but is still left in place, any bonus is rescinded. This is, of course, only one half of a true BMD system. We're working on incorporating IDEAL-CT, which we hope will be smoother given the relative ease and speed of running the test.

Kevin Suitor, Oklahoma DOT
OK DOT is placing BMD projects across the state. We are working to verify specification limits and refine procedures for short-term oven aging and time limits for test completion. We have not implemented pay factors at this time.

What type of mix design programs are being used by contractors in your state? How does your state track or record binder source changes on a job mix formula (JMF)? Do you require a revision to the JMF or notification from the contractor when a source changes? -Tony Collins, North Carolina DOT

Zane Hartzog, Alabama DOT
Mixes are designed by various ALDOT procedures indexed here: https://www.dot.state.al.us/publications/Materials/TestingManual/ProcedureTopics.html. ALDOT does not record binder source changes on JMFs. From ALDOT Standard Specifications for Highway Construction Section 410.02 (d) 2. paragraph 2 :"...A change in liquid asphalt binder source and anti-stripping agent will be allowed without a new job mix formula provided the design criteria is met by a one-point check of the mixture. The one-point check shall include the air void content, VMA, stability, flow, and TSR (tensile strength ratio) and may be determined during the production of the mix..."

Michael Stanford, Colorado DOT
CDOT requires the contractors to submit their mix design. Our regional / district materials engineers then set the volumetric target, and approve the JMF. A new JMF is required when any material source changes (aggregate, binder, lime, etc.).

Greg Sholar, Florida DOT
If the first question is referring to computer programs, I do not know what specific programs contractors are using. In the past, many used FDOT's Excel mix design workbook. However, since FDOT went to a new construction and mix design database system called MAC five years ago, this system also is comprehensive and could be used as a sole source program by some contractors. For binder source changes, FDOT does not require a revision to the design. Any binder source used must be on FDOT's Approved Products List and must meet the binder grade required by the contract. This information is verified on asphalt binder delivery tickets obtained at the asphalt plant by FDOT inspectors.

Brian Hill, Illinois DOT
IDOT staff created an Excel-based program that includes a section on HMA mix design. This program is updated as needed to address specification changes. The asphalt binder source is listed on the mix design cover sheet and the HMA daily plant reports used for reporting stockpile percentages, air voids, VMA, etc. Any source changes would be shown in the HMA daily plant reports. The contractor is not required to revise the job mix formula assuming that the same asphalt binder performance grade is used.

Nathan Awwad, Indiana DOT
We rely on AASHTO R 35. In the past, we had a standard spreadsheet that was required for submittal. We recently upgraded this to an online cloud-based program called DMF Entry that mimics the old sheet. It’s much more efficient, automated and much easier to data mine. Per our specification, "A PG binder grade or source change will not require a new mix design. If the upper temperature classification of the PG binder is lower than the original PG grade, a new TSR value is required." Bottom line, we are approving the aggregate design.

John Garrity, Minnesota DOT
Minnesota does not track binder sources other than the bill of lading. Binder source changes are not tracked on the JMF and we do not require a revision to the JMF.

Oak Metcalfe, Montana DOT
As far as tracking mix designs, Montana uses the AASHTOWare project suite of programs for materials. It has a function to store mixes. Once a mix is approved, the data is stored in the system and assigned a number. When there are changes, the contractor must submit a form indicating what has changed. The department reserves the right to re-verify a mix or not, but documents the "new" mix with a new number to keep the JMFs separate in the system. This only works for "start up" as we allow changes during production as long as performance parameters are met (volumetrics, density, Hamburg, etc.) that are not tracked. We require what we refer to as "Target Set" before 2000 tons. The contractor must submit their "targets" for VMA, VFA, VTM, and D/A, within the specified range. Once those are submitted, the contractor is allowed to make adjustments without a new JMF. If the contractor requests new targets, a new JMF must be submitted and verified.

Charlie Pan, Nevada DOT
Contractors propose their JMF based on the mix design performed/approved by the department. Source changes will require a new mix design.

Kevin Suitor, Oklahoma DOT
OK DOT requires a formal request from the contractor and a new design number is generated. Along with the request, the contractor submits results from T-283 using the new source and a new set of Hamburg tests for OK DOT testing.

Neal Fannin, Pennsylvania DOT
Producers generally use Excel programs developed by our district materials units on an ad hoc basis for mix design. Design information is entered into our centralized construction and materials management system for review and approval. If there is a change in binder source, a boil test is required, followed as soon as possible by AASHTO T 283 for moisture susceptibility.

Cliff Selkinghaus, South Carolina DOT
SCDOT wrote a mix design program in Excel for contractors to use and submit paperwork electronically. This is updated every several years as we change gradations and likely increase performance testing for BMD. Contractors may elect to change sources of neat binders (64-22) without having to re-verify mix designs; however, if a PMA (76-22) is different, then we require re-verification of volumetric properties and performance tests at optimum binder content if applicable.

What is the minimum and maximum lift thickness allowed in the specifications in terms of nominal maximum aggregate size (NMAS)? Virginia currently has a minimum 2.5 times and a maximum 4 times of NMAS. Are you satisfied with outcomes if you implement 5 or even 6 times? -Sungho Kim, Virginia DOT

Zane Hartzog, Alabama DOT
ALDOT does not specify maximum lift thickness in terms of NMAS. Maximum lift thickness is specified in the ALDOT Standard Specifications for Highway Construction section 410.03 (f) 1. paragraph 6: "Unless otherwise provided in the following sections of these specifications, or shown on the plans, the average rate placed and compacted in one layer shall not exceed 350 pounds per square yard {200 kg/m2} for base or binder layers, and 225 pounds per square yard {120kg/m2} for surface layers. Where the amount to be placed exceeds these limits, it shall be placed and compacted in two or more approximately equal layers or as shown on the plans."

Michael Stanford, Colorado DOT
As a general rule of thumb CDOT also has a minimum of two-and-a-half times and a maximum four times of NMAS requirement.

Greg Sholar, Florida DOT
Type SP-9.5: 1.0 to 1.5 inches
Type SP-12.5: 1.5 to 3.0 inches
Type SP-19.0: 2.0 to 4.0 inches
We are satisfied with these values, but it should be pointed out that we have increased the maximum thickness slightly over the years. 

Brian Hill, Illinois DOT
IDOT uses a minimum compacted lift thickness of three times the NMAS. In general, IDOT uses a maximum compacted lift thickness of six times the NMAS unless a 4.75mm NMAS mixture is used. A 4.75mm NMAS mixture has a maximum compacted lift thickness of 6.5 times the NMAS. IDOT is satisfied with the density of mixtures placed at the maximum compacted lift thicknesses.

Nathan Awwad, Indiana DOT
In the past, we required two to four times the NMAS. Our pavement designs target three times. We feel this is too conservative, and have updated to at least allow up to five times. We have tried to encourage internally that 2" of 9.5mm is much better than 2" of 12.5mm. We have allowed intermediate and base up to six times NMAS in special situations with no ill effects.

John Garrity, Minnesota DOT
Minnesota has recommendations regarding aggregate size and minimum lift thickness but no specification requirement.

Charlie Pan, Nevada DOT
Maximum 3" lift thickness for up to 3/4" NMAS.

Neal Fannin, Pennsylvania DOT
9.5 mm, 1 to 1.5 inches
12.5 mm, 2 to 3 inches
19 mm, 2.5 to 4.5 inches
25 mm, 3 to 6 inches
37.5 mm, 4.5 to 8 inches

Cliff Selkinghaus, South Carolina DOT
We commonly use three to four times NMAS. We are experimenting with some rapid construction rebuild sections in select areas to reduce chances of delamination between lifts and to help with limiting length of road closures. SCDOT sponsored Section S-9 on the NCAT Test Track, which uses a ½ NMAS in a single lift up to 8”. We are learning that this technique should only be used when the mix is placed in a milled section where there is confinement and works best when placed in cold winter months to reduce mat temperature more rapidly. Paving thick lifts in warm to hot weather has caused less than desirable smoothness values, which often require milling or diamond grinding.