NCAT invites your comments and questions, which may be submitted to Christine Hall. Questions and responses are published with editing for consistency and space limitations.
Eric Biehl, Ohio DOT
Ohio DOT has a micro surfacing bond research project that's looking at determining bond strength as well as if and how much (diluted) tack is needed. Phase 2 field trials are to be placed in Spring 2019.
The following responses have been received to questions shared in the previous issue.
With the development of the regressed air void concept, balanced mix designs, and greater emphasis on higher field densities, the asphalt pavement community seems to be more comfortable with mixes designed to have less than the 4.0% air void target traditionally used for Superpave design. What is the minimum air void level for in-place compacted pavements at which the mix would not be acceptable for your organization? -Don Watson, NCAT
Michael Stanford, Colorado DOT
Below 92%, would not be acceptable. Our specification requires a density of 92 to 96% of the daily theoretical maximum specific gravity.
Greg Sholar, Florida DOT
We currently require removal of asphalt that has an in-place density below 89.5% Gmm measured of roadway cores using vacuum drying. Our target densities are 93.0% Gmm for vibratory compaction and lifts greater than 1.0" thick and 92.0% Gmm for roadways with mandatory static compaction and/or layers 1.0" thick. FDOT is monitoring national research but has not made a decision to lower target design air voids to a level below 4.0%.
Rick Bradbury, Maine DOT
Our lower specification limit is 92.5% of Gmm for all dense-graded mixes, accepted using PWL. AQL = 90 PWL; RQL = 50 PWL. Based on research into permeability, in-place density <92.0 % Gmm is the point where permeability is unacceptable for 9.5mm - this increases for coarser mixes.
Kevin Kennedy, Michigan DOT
Michigan regresses mix designs to 3% air voids. We have a PWL specification for pay. We define spec limits for lab compacted air voids as ± 0.9%, and sublot rejectable quality limits of -1.5%, +2.0%.
Eric Biehl, Ohio DOT
In our current 446/447 (field cores for density acceptance) specifications, if the average of 10 mat cores is between 97.0 and 97.9% density then the contractor receives a 6% deduction in pay. For surface layers, an in-place density of 98.0% or greater would be removed and replaced; for other courses, a 40% deduction would be applied if the layer is left in place. Section 446 includes three "joint" cores in the average, and Section 447 has two tables (one for mat average density and the other for joints that's a PWL approach). Average mat densities for all of Ohio have been mid 93% for the past four to five years, so we most likely don't see those high of densities. We have talked about increasing our density acceptance values. Our mixes are designed at 4.0% or 3.5% air voids depending on mix type.
Cliff Selkinghaus, South Carolina DOT
We do not have a maximum for in place density. An average of 94.0% of Gmm or more receives a 5% incentive.
Howard J. Anderson, Utah DOT
The air void target in Utah is 3.5% at Ndesign. With our PG plus specification that requires all our binders to be modified, and with the Hamburg rut test in place, we allow in place materials to be compacted up to 97.5% of the Gmm. This is measured from cores. We made this change along with our longitudinal density specification a couple of years ago.
In theory, job mix plus/minus tolerances should be allowed to exceed the upper or lower limit of the specification band. However, there are local agencies in Hawaii that do not allow this when checking production mixes What is the policy in other states? Can the job mix formula +/- the tolerance exceed the upper or low limit of the specification band? What is the effect on the mix if either limit is exceeded? -Jon Young, Hawaii Asphalt Paving Industry
Michael Stanford, Colorado DOT
The quality level of all elements is constantly monitored. If the quality level drops below the tolerance, production may be suspended until the source of the problem is identified and corrected.
Greg Sholar, Florida DOT
FDOT has target gradation values on the mix design and PWL tolerances based on those targets during production for the -8 and -200 sieves (different sieve sizes apply for OGFC mixes). FDOT also has a master range, which exceeds the PWL limits, for the -200 sieve. If that master range is exceeded then an investigation occurs, which may include just a data examination of the other material properties, a follow-up sample, coring of pavement to determine material properties, or a combination of the above.
Rick Bradbury, Maine DOT
Maine DOT's specification requires that gradation tolerances (+/- from the JMF target) cannot exceed the Superpave control point limits. For example, for a 12.5 mm mix, the control point on the 9.5mm sieve is 90% maximum. If the production tolerance for the 9.5 mm sieve is +/- 5%, a contractor could design a JMF with a target % passing the 9.5 mm sieve of 89%, but the upper specification limit during production would be 90% passing, not 94%. This is to prevent contractors from designing what is essentially a 9.5mm mix when a 12.5 mm mix is specified in order to meet VMA requirements.
Kevin Kennedy, Michigan DOT
Michigan uses a PWL specification where exceeding those limits affects pay factors. There are also QC action limits, QC suspension limits, etc. to prevent a contractor from continually running outside of those tolerances without taking actions to correct the problem.
Greg Johnson/John Garrity, Minnesota DOT
If the specification limits are exceeded then a monetary deduction is applied; if extreme, the material must be removed and replaced.
Eric Biehl, Ohio DOT
Our mix gradation bands in our specification book are considered "design" gradation bands with a few exceptions. The mix design is required to meet the gradation band before we will approve it. During production, there are tolerances based on the approved job mix formula for gradation for four sieves that can exceed the "design" gradation band. For the No. 4 sieve, the mixture’s macrotexture can be impacted if too large of a gradation change occurs (we have limestone and gravel mixes mainly). One of the exceptions is for 12.5 mm mixtures (used for heavy traffic surfaces). For this mix, the percent passing the No. 4 sieve cannot exceed 63% during production. The other item that limits P200 content during production is the fines to asphalt (dust to binder) ratio. A cheap way for contractor to get density/low air voids is to add dust.
Cliff Selkinghaus, South Carolina DOT
We are sticking with the USL and LSL for gradation only to be sure that we are getting the correct mix type. The binder content, air voids, and VMA may extend outside of the USL/LSL with the +/- tolerances applied. The main focus is to ensure that we get enough percent binder in our mixes, especially with higher recycled contents.
Our specifications require a thickness tolerance for all asphaltic concrete laid. If the thickness is not within the required tolerances, the layer is not accepted. Do any agencies have a method for rectification or acceptance of the works? -Wimal Silva, Sri Lanka
Michael Stanford, Colorado DOT
In Colorado, asphalt is paid by the ton. Our field engineers constantly check the yield (tons vs. area paved) to verify the thickness is correct. This is typically not a problem in our state.
Greg Sholar, Florida DOT
FDOT measures spread rate based on an average of five to ten truckloads of mix and then calculates daily averages. There are multiple steps along the way, but basically if the average spread rate differs by more than 5%, then monitoring occurs. If this happens for two days in a row, then shutdown occurs until the problem is identified. When an individual spread rate is beyond plus or minus 20% of the target spread rate, construction is halted and the unacceptable pavement is evaluated via coring to determine limits for removal and replacement, if necessary. Too thin of pavements is the biggest concern.
Kevin Kennedy, Michigan DOT
We pay for HMA by the ton so thickness is usually not an issue although we check yields in the field. On jobs where we pay by the square yard we do thickness checks and penalize accordingly.
Greg Johnson/John Garrity, Minnesota DOT
If the thickness exceeds planned thickness plus the tolerance, then materials placed in excess may be excluded from payment. If pavement thickness is below planned thickness, the engineer will decide on reduced payment under our standard unacceptable work specification.
Eric Biehl, Ohio DOT
We pay by the cubic yard so we typically get the thickness specified. If the layer is too thick (more CY than the plan quantity), then we just pay the plan quantity. In the rare situation where the layer is too thin, we would evaluate the pavement to make sure the pavement structure is adequate. If not, then the contractor is required to make corrections (another lift, mill/fill, or a combination of both).
Cliff Selkinghaus, South Carolina DOT
We specify a placement rate in pounds per square yard. The department's roadway inspector checks the placement rate every 200 tons placed and ensures that the HMA contractor makes adjustments throughout the day to average out as close as possible to the contract placement rate (stay within the plan quantities).
Howard J. Anderson, Utah DOT
In Utah, if the cores show they are deficient in thickness by 0.25 inches or more, the contractor has to place another layer. I have copied language directly from our standard HMA specification for your use:
Thickness requirements are based only on mat cores. The thickness requirement may be waived when matching up to existing pavement, curb and gutter for pavement in or next to intersections.
The department accepts a lot for thickness when (1) The average thickness is not more than 1/2 inch greater or 1/4 inch less than the total design thickness specified, (2) No individual sublot shows a deficient thickness of more than 3/8 inch.
Excess thickness – the engineer may allow excess thickness to remain in place or may order its removal. The Department pays for 50% of the mix for material in excess of the +1/2 inch tolerance when excess thickness is allowed to remain in place.
Deficient thickness – place additional material where lots or sublots are deficient in thickness. The department pays for material necessary to reach specified thickness. The department pays for 50% of the mix for additional material over specified thickness necessary to achieve minimum lift thickness. Minimum compacted lift is three times the nominal maximum aggregate size.
Thickness tolerances established above do not apply to leveling courses. Check final surfaces in stage construction. Check thickness regularly with a depth probe and take corrective action as necessary for thin overlay pavement.