Asphalt Forum

Randy Mountcastle, Alabama Department of Transportation

Are any States using ARB as a substitute for SBS?

Do any states use a double-layer open-graded friction course (OGFC)?

Don Watson, NCAT
AASHTO T 168/ASTM D 979 procedures for sampling HMA from a truck transport require taking samples at random from the truck in approximately three equal increments and combining to form one representative sample. Some of the large truck beds in use today make this virtually impossible without the technician having to climb over into the truck. How are states addressing this issue of getting random, representative samples from large truck beds without compromising the safety of the technician?

Most agencies specify that the HMA mixture temperature be within a certain tolerance of that specified in the mix design or Job Mix Formula for the project; otherwise, the mix may be rejected. Has anyone conducted research to show that mix produced at temperatures lower than the allowed tolerances is detrimental to HMA quality and long-term performance? How critical is the specified mix temperature if a contractor uses a materials transfer device to remix the material at the roadway to a uniform temperature and is able to meet the specified target density and smoothness requirements?

For agencies that have implemented PWL specifications for HMA acceptance, what were the greatest challenges to implementation? How did you overcome those challenges?

What changes are agencies anticipating or planning to make in order to meet the new FHWA regulations on noise? Do quieter pavements fit into those plans?

Dale Rand, Texas Department of Transportation
We are developing wording to address sample custody in all of our future HMA specification. The FHWA is particularly concerned about how we handle roadway cores to ensure that the opportunity for fraud is minimized or eliminated. We are not aware of any fraudulent activity; however, as a result of fraud in some other parts of the country this has become a high priority to the FHWA. We rely on the contractor to obtain the cores from the roadway and in some cases deliver them to our lab at the HMA production facility, which can be located an hour or two from the job site. We currently do not have a system in place that would ensure the cores could not be switched or altered in some way. We are exploring several ideas on how to address this situation in an effort to come up with something the FHWA will buy off on. Our situation, like many others, is that we typically only have one inspector available on the project and the inspector cannot take off for several hours to deliver cores to our testing lab. In the past, we instructed the contractor (actually the truck drivers) to transport these cores for us, but we have been informed by the FHWA (who is reviewed by the Office of Inspector General [OIG]) that we can no longer continue this practice. This all came to light when the OIG started reviewing stimulus projects in Texas. We would like to know if any other states are facing a similar situation and if so, what is your proposed solution?

Asphalt Forum Responses
The following responses have been received to questions shared in the Spring 2010 Asphalt Forum.

1. What criteria do agencies use to assess aggregate polishing characteristics for use in surface mixtures and/or chip seals? Other than the British Pendulum Test, what tests are used or recommended? (Don Watson, NCAT)

Texas Department of Transportation, Dale Rand

Texas has dropped the polish value test using the British Pendulum device. We now classify our aggregates based primarily on their acid insolubility results and secondarily on their magnesium sulfate soundness loss results.

Missouri Department of Transportation, Joe Schroer

Acid insoluble residue is used to measure non-polishing aggregates. Field measurements are made with a skid trailer.

Tennessee Department of Transportation, Mark Woods

Tennessee classifies its limestone sources for surface aggregate by a ranking system that is based on silica dioxide content, acid insoluble residue, BPN, and calcium carbonate content. Dependent on preliminary test results, source suppliers may be required to place two-year field test sections to be evaluated by ribbed-tire friction testing.

Alabama Department of Transportation, Randy Mountcastle

Alabama uses the BPN.

Florida Department of Transportation, Greg Sholar

For friction course aggregate properties, we run the LA Abrasion test (Florida Test Method) on coarse and fine aggregate. In addition, we run Acid Insolubility using the Florida Test Method. To measure field friction performance, we use either the Dynamic Friction Tester (ASTM E-1911) or locked wheel trailer (ASTM E-274) using a ribbed test tire (ASTM E-501). For monitoring surface texture loss we use either the Circular Track Meter (ASTM E-2157) or friction truck mounted texture laser (ASTM E-1845), measuring mean profile depth (MPD).

Ohio Department of Transportation, David Powers

Ohio DOT, through a series of research studies with University of Akron, is developing a method to polish and test 6-inch gyratory specimens for friction determination. The method would involve the polishing of specimens with a specially designed polishing machine and obtaining a friction number with the British Pendulum procedure. The process could be used for mix design approval and for quality assurance and/or control. The goal was to have a straightforward, less labor-intensive process for testing compared to other approaches.

Iowa Department of Transportation, Scott Schram

The Iowa DOT has revised frictional aggregate requirements for interstate surface mixes. Iowa classifies frictional characteristics of aggregate sources based on grain size and other quality measures (Types 1 through 5). Type 2 is the highest classification available in and around Iowa (quartzite and steel slag). For mixes containing more than 40 percent limestone, at least 25 percent of the combined fine aggregate and 30 percent of the combined coarse aggregate shall be Type 2. For mixes containing less than 40 percent limestone, at least 25 percent of the combined coarse aggregate shall be Type 2 (quartzite or steel slag). In addition, the fineness modulus of the proportion of Type 2 material in the total aggregate shall be at least 1.0. At least 80 percent of the coarse aggregate shall be Type 4. Limestone sources are defined as having less than 15 percent magnesium oxide (MgO) content.

2. Which states use VMA as an asphalt mixture acceptance criterion during daily production? If VMA is used for acceptance, how is G_sb determined? Is it based on test data at the time of production or historical data? (Don Watson, NCAT)

Texas Department of Transportation, Dale Rand

Texas uses VMA as a daily production test. We determine the VMA based on the effective specific gravity value that is back-calculated from the Rice Gravity (G_mm). We know this method is incorrect since it includes absorbed asphalt in the VMA. We accounted for this by increasing our VMA requirement. You could say we are extremely wrong but extremely accurate. We chose this path as the lesser of two evils since the "extremely correct" option is also extremely inaccurate based on our experiences. We have approximately 1500 certified HMA technicians. Our annual proficiency testing program shows that the Rice Gravity (G_mm) is the most precise test we have. When we did similar proficiency testing to determine G_sb values using the conventional methods, the results were highly variable to say the least. After about three years of trying to reduce the variability for G_sb (especially on fine aggregate) we opted to go the other direction. Since we determine asphalt content and a G_mm value for every sublot, we can easily determine a VMA value on each sublot (or at least our version of VMA).

Missouri Department of Transportation, Joe Schroer

Missouri uses VMA for acceptance during construction, which is determined with the G_sb from the job mix formula. We currently have pilot projects that determine the G_sb from the combined cold feed using the CoreLok device for the VMA calculation.

Tennessee Department of Transportation, Mark Woods

TDOT does not use VMA for acceptance.

Alabama Department of Transportation, Randy Mountcastle

In Alabama, G_sb is determined during design and only changes if the mix significantly changes.

Florida Department of Transportation, Greg Sholar

VMA is not an acceptance criterion during production. However, during mix design verification, both the contractor and the FDOT use historical G_sb data from the aggregate mines for VMA calculations. Typically, it is the average of the last 30 G_sb tests performed at the mine.

3. Has anyone stored warm mix overnight in a silo? If so, at what temperature? Did it contain RAP? (Chris Jones, Wiregrass Construction Company)

Texas Department of Transportation, Dale Rand

Texas does not allow storing HMA for more than 12 hours; however, at least one HMA producer has stored WMA with RAP overnight at roughly 270F, and they reported no problems with the mix the next day.

Florida Department of Transportation, Greg Sholar

FDOT is not aware that this has occurred in Florida.

NCAT invites your responses to these questions, which may be submitted to Karen Hunley at karen.hunley@auburn.edu. Questions and responses are published in each issue of Asphalt Technology News with editing for consistency and space limitations.