Michael Stanford, Colorado DOT
Where do you take the Longitudinal Joint (density) cores? Our current specification requires them to be taken from the center of the visible joint— the local contractors continually fight us on this stating that it should be taken over the top of the "wedge" created by the safety edge. Has there been any recent research on this topic?
Jerry Geib & John Garrity, Minnesota DOT
We have Superpave5 pilot projects.
Rob Crandol, Virginia DOT
VDOT and the Virginia Asphalt Association have cooperatively developed and delivered our first BMD certificate training, which will be required of any technicians working on BMD starting with the 2023 season. This is a separate, stand-alone course focused solely on BMD. In the meantime, course materials are being developed and incorporated in our existing asphalt plant and mix design materials certification school courses (through the Virginia Education Center for Asphalt Technology, VECAT).
The following responses have been received to questions shared in the previous issue.
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. -Greg Sholar, Florida DOT
Brian Hill, Illinois DOT
IDOT has experience with blisters. In general, extended rain events before placement of low permeability (4.75mm or 9.5mm fine-graded) mixtures led to higher blister potential. IDOT requires the existing surface to be dry for at least 24 hours in the case of 4.75 mm mixtures for constructability. However, it can take much longer for moisture within the pavement structure to evaporate.
Tony D. Collins, North Carolina DOT
We refer to road worms as blisters. Our findings are similar, trapped moisture and low bond strength. We have never done research on the issue to the same extent you have and find your research very interesting. We have been told by at least one expert that our mixes were "too tight" and this was causing moisture to be trapped coming up from the bottom.
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? -Oak Metcalfe, Montana DOT
Zane Hartzog, Alabama DOT
ALDOT Uses Ndesign = 60 for all traffic levels. We arrived at that number based on the concept of locking point, detailed in section 3.2.1 of NCAT Report 19-08. Page 5 of FHWA Technical Brief HIF-11-031 should also be helpful.
Michael Stanford, Colorado DOT
Colorado DOT does not adjust gyration levels for aggregate type. The gyrations are typically determined by traffic. 75 gyration mixes are our most common, along with a fair number of 100 gyration mixes. We also use an occasional 50 gyration mix for our thin lifts (3/8" NMAS).
Greg Sholar, Florida DOT
At the onset of Superpave, FDOT used five traffic levels in accordance with AASHTO R35 (we followed Note 11 and divided the 3 to 30 million ESALs category into two categories split at the 10 million ESAL level). We then downgraded the 0.3 to 3 million ESALs from Ndesign = 75 gyrations to Ndesign = 65 gyrations in accordance with an NCAT report that we thought AASHTO would adopt but never did. FDOT also reduced Ndesign for 30 million and greater ESALs to 100 to get more binder in the mixtures. Very recently, FDOT further decided to use only three Ndesign levels as follows: less than 3 million ESALs = Ndesign of 65; 3 to 10 million ESALs = Ndesign of 75; greater than or equal to 10 million ESALs = Ndesign of 100.
Brian Hill, Illinois DOT
IDOT back-calculated state-specific Ndesign levels for the entire state system. Mixture samples were collected and gyratory cylinders were compacted using Marshall designs that were known to perform well in specific traffic applications to identify the number of gyrations needed to reach 4.0% air voids. Traffic data was collected in conjunction with the mixture samples to tie the number of gyrations to 4.0% air voids to ESAL levels.
Richard Bradbury, Maine DOT
Maine DOT began specifying 65 gyrations for almost all projects several years ago. This decision was made primarily to streamline the design process for both DOT and industry by reducing the overall number of designs. We began deviating from the specified gyration levels in 2003 after noticing that some early Superpave projects were exhibiting distresses related to dry mixes. At that time, we expanded the use of 50 gyration mixes for many applications that should have been 75 gyrations according to the ESAL level. Over time, we observed that in most cases, producers were not using different aggregate blends; the only difference between 50 and 75 gyration mixes was a small change in design binder content. Consolidation to 65 gyrations seemed like a logical and low risk approach. Aggregate consensus properties are still specified based on ESALs, and we prevent rutting by specifying a stiffer binder grade.
Tony D. Collins, North Carolina DOT
We have never considered different gyration levels based on aggregate quality, as we have relatively good aggregates across the state. Our deviation was based on findings of NCHRP 9-9, which showed mixes designed using the levels originally published did not achieve in-place voids of 4% after construction compaction and traffic densification. We were trying to add more effective binder to our mixes, which is why we added VMA limits of +0.5% for most mixes. Our mix specs are across the board and not project-by-project.
Stacey Diefenderfer, VTRC—VDOT
Virginia initially planned to adopt 75 gyrations for Ndesign for the highest traffic level and 65 gyrations for lower traffic levels at the beginning of Superpave implementation. However, the 75 gyration level was dropped to 65 gyrations prior to full adoption in 2000 due to experiences with coarse mixes and lower asphalt contents during trials constructed in 1999. Concerns with lower asphalt contents have continued, and several studies were performed at VTRC to look at increasing binder content (see VTRC 03-R15 and VTRC 11-R5 by Maupin and VTRC 15-R8 by Boriak et al. at VTRC.virginiadot.org). In 2014-2015, VDOT initiated an effort to evaluate the reduction in design gyrations, along with other adjustments to gradation and volumetric requirements as a means of improving mixture performance. Several field trials were constructed in 2015 that included 50-gyration mixtures for evaluation and the in-service performance of those mixtures is still being monitored. During production and construction, evaluation indicated that overall, there were only slight changes in asphalt content; however, the 50-gyration mixtures showed improved density and permeability (VTRC 21-R11 by McGhee & Smith). Currently, all dense-graded mixtures are designed at 50 gyrations regardless of traffic level.
At what temperature should the Hamburg wheel tracking test be conducted? MN started at 50°C but we have moved to 46°C. -John Garrity, Minnesota DOT
Michael Stanford, Colorado DOT
Colorado DOT test temperatures are determined for the SHRP high temp PG as follows: PG 58: 45°C; PG 64: 50°C; PG 70: 55°C; PG 76: 55°C.
Greg Sholar, Florida DOT
FDOT conducts the test at 50°C.
Brian Hill, Illinois DOT
IDOT uses the Texas test temperature of 50°C and varies the number of passes based on the PG high temperature grade. IDOT adjusted the minimum number of passes for PG 58 and PG 64 high temperature grades during initial implementation.