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Impact of Asphalt Thickness on Pavement Quality (2019)

Chapter: Appendix D - Tabulated Industry Survey Responses

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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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Suggested Citation:"Appendix D - Tabulated Industry Survey Responses." National Academies of Sciences, Engineering, and Medicine. 2019. Impact of Asphalt Thickness on Pavement Quality. Washington, DC: The National Academies Press. doi: 10.17226/25498.
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103 A P P E N D I X D Tabulated Industry Survey Responses This section summarizes the responses to the survey as reported by the industry representatives. Numbers in brackets or on the Totals line in tables are the number of responses for each option. Not every respondent answered every question, so totals may not match from question to question. 1. In which state(s) do you routinely work? In which state do you most commonly work? Routinely Most Commonly (Number per State) AL, AZ, AR, CA, CO, CT, FL, IL, IN, KS, KY, MA, MN, MO, NE, NV, NH, NY, NC, OH, OK, OR, PA, RI, SC, VT, VA, WA, WY [Total 29 states] AL (2), CA (4), CO (1), FL (3), IL (3), IN (13), KS (1), KY (3), MA (6), MO (3), NE (1), NY (1), NC (1), OH (1), OK (1), PA (3), SC (3), VA (7), WA (1), WY (1) [59 responses] 2. Are you aware of any established policies or guidelines regarding appropriate asphalt lift thicknesses relative to the nominal maximum aggregate size (NMAS) of the mixture in any of the states in which you work? If yes, how are these policies or guidelines enacted? (Click all that apply.) Mostly Work In: Routinely Work In: Policies/ Guidelines? How Enacted? AL AL Yes Design Memo AL AL Yes Other (Guideline For Operation) CA CA Yes Specifications CA CA Yes Specifications CA CA Yes Specifications CA CA, MN No CO CO Yes Pavement Design Manual FL FL Yes Specifications FL FL Yes Specifications FL FL Yes Specifications IL IL Yes Specifications IL IL, IN Yes Specifications IL IL Yes Specifications IN IN Yes Pavement Design Manual IN IN Yes Specifications IN IN Yes Pavement Design Manual IN IN Yes Specifications IN IN, KY Yes Other (Separate Memo) IN IN Yes Specifications IN IN Yes Specifications IN IN Yes Specifications IN IN Yes Specifications IN IN Yes Pavement Design Manual IN IN Yes Specifications IN IN Yes Pavement Design Manual IN IN Yes Other* (continued on next page)

104 Impact of Asphalt Thickness on Pavement Quality Mostly Work In: Routinely Work In: Policies/ Guidelines? How Enacted? KY KY Yes Other (KY DOH “Warrants”) KY KY, OH Yes Specifications MA CT, MA, NH, VT Yes Specifications MA CT, MA Unsure MA MA Yes Specifications MA MA Yes Specifications MA MA, RI Unsure MA MA, NH Yes Specifications MO KS, MO Yes Specifications MO AR, MO, OK No MO KS, MO Yes Specifications NE NE Yes Pavement Design Manual NY NY Yes Pavement Design Manual NC NC Yes Specifications OH OH Yes Pavement Design Manual OK OK Yes Other (ODOT Suggested Guidelines) OR OR No PA PA Yes Specifications PA PA Yes Pavement Design Manual SC SC No SC NC, SC Yes Specifications SC SC Yes Specifications VA VA Yes Specifications VA VA Yes Specifications VA VA Yes Specifications VA VA Yes Pavement Design Manual VA VA Yes Specifications VA VA Yes Specifications VA VA Yes Specifications WA WA Yes Pavement Design Manual WY AZ, CA, KY, NV, WY Unsure Totals 52 Yes, 4 No, 3 Unsure *Not sure how they were enacted, just aware they exist and most abide by. KS KS Yes Specifications KY IN, KY Yes Specifications

Tabulated Industry Survey Responses 105 3. How do you typically measure pavement density for quality control? (Please click all that apply.) Mostly Work In: Routinely Work In: Nuclear Gauge Non-Nuclear Gauge Cores Other AL AL • • • AL AL • • • • CA CA • • • CA CA • • CA CA • • CA CA, MN • • CO CO • • FL FL • • • FL FL • • • FL FL • IL IL • • IL IL, IN • • • IL IL • • IN IN • IN IN • • IN IN • • IN IN • IN IN, KY • • IN IN • • IN IN • IN IN • • IN IN • IN IN • IN IN • • IN IN • IN IN KS KS • • KY IN, KY • KY KY • • • KY KY, OH • • MA CT, MA, NH, VT • • MA CT, MA • • MA MA • • • MA MA • MA MA, RI • • MA MA, NH • • MO KS, MO • MO AR, MO, OK • • • MO KS, MO • NE NE • • NY NY • • • NC NC • • • OH OH • • • (continued on next page)

106 Impact of Asphalt Thickness on Pavement Quality Mostly Work In: Routinely Work In: Nuclear Gauge Non-Nuclear Gauge Cores Other OK OK • • OR OR • PA PA • • PA PA • SC SC • • • SC NC, SC • • • SC SC • • VA VA • • VA VA • • VA VA • • VA VA • • VA VA • • VA VA • • • VA VA • • WA WA • • WY AZ, CA, KY, NV, WY • • Totals 36 26 54 3 No responses indicated “not applicable/ state measures.” Comments: o Contractor Quality Control. For pay, Contractor cuts cores and [agency] tests cores. o Contractors QC by nuclear gauges calibrated to cores. Acceptance using cores. o Nuclear gauge to determine roller pattern/control strip - occasionally for acceptance. Mostly cores for acceptance.

Tabulated Industry Survey Responses 107 4A. Have you observed more difficulty in obtaining required field densities since implementing the Superpave mix design system in the states where you work? Mostly Work In: Routinely Work In: Yes, initially but less so now Yes, continuing to be difficult Yes, with certain mixes/ applications No Unsure Have not implemented Superpave AL AL • AL AL • CA CA • CA CA • CA CA • CA CA, MN • CO CO • FL FL • FL FL • FL FL • IL IL • IL IL, IN • IL IL • IN IN • IN IN • IN IN • IN IN • IN IN, KY • IN IN • IN IN • IN IN • IN IN • IN IN • IN IN • IN IN • IN IN KS KS • KY IN, KY • KY KY • KY KY, OH • MA CT, MA, NH, VT • MA CT, MA • MA MA • MA MA • MA MA, RI • MA MA, NH • MO KS, MO • MO KS, MO • • MO AR, MO, OK • NE NE • NY NY • NC NC • OH OH • OK OK • OR OR • PA PA • PA PA PA • SC SC • SC NC, SC • SC SC • (continued on next page)

108 Impact of Asphalt Thickness on Pavement Quality Mostly Work In: Routinely Work In: Yes, initially but less so now Yes, continuing to be difficult Yes, with certain mixes/ applications No Unsure Have not implemented Superpave VA VA • VA VA • VA VA • VA VA • VA VA • VA VA • VA VA • WA WA • WY AZ, CA, KY, NV, WY • Totals 20 1 18 8 7 4 4B. If the situation has improved, what changed? Mostly Work In: Routinely Work In: Lift Thicknesses Finer Mixes Mix Design Parameters Smaller NMAS Used Other AL AL • • • AL AL • • • CA CA, MN • • CO CO • • IN IN • IN IN • IN IN • KY KY • • • MA CT, MA • MA MA • • • • MA MA, RI • MA MA, NH • • • NY NY • • • NC NC • OH OH • • • PA PA • • • SC SC, NC • • VA VA • • • VA VA • • VA VA • Totals 3 13 14 7 6 No one responded that density requirements have been revised. Comments: o Running rollers in tandem. o Warm mix asphalt, higher performance compaction equipment. o Roller operators learned the amount of compacting effort required. o I’ve learned to design mixes to better help with compaction by trying to increase film thickness and VMA, rather than just meet volumetric parameters. I’ve also adjusted many of my field placement and rolling procedures. o Mixes have evolved by adding more fines and liquid. o Got used to it ... adjusted mixes.

Tabulated Industry Survey Responses 109 4C. Have you observed any increased difficulty obtaining adequate density in mixtures with these components or in these applications in any of the states where you work? (Please click all that apply.) Mostly Work In: Routinely Work In: R A P RA S PM B G TR Ce rt ai n PG G ra de s A ng ul ar A gg re ga te s Ce rt ai n A gg re ga te s Th in L ift s A t L ow Te m ps O ve r Co nc re te O th er FL FL • • FL FL • • • FL FL • IL IL, IN • • • IN IN • • IN IN, KY • • • IN IN • • • • • • IN IN • • • IN IN • • • KY IN, KY • • • • • MO KS, MO • • • • MO AR, MO, OK • • PA PA • • SC NC, SC • • • • • VA VA • • • VA VA • WA WA • • • • • • • WY AZ, CA, KY, NV, WY • • • • • Totals 2 4 5 2 4 4 3 14 9 8 5 Comments on Binder Grades: o High polymer. o PG 76-22 does better with higher initial compaction temperatures. o PG 64-22 and 76-22. o Polymerized binders (PG 70-22 or higher). Comments on Certain Aggregate Types: o Some manufactured aggregates such as steel furnace slag or blast furnace slag. o ACBF (not in all cases, but generally with high percentages of coarse slag products). Other Comments: o Existing asphalt to be overlaid in poor condition. o High VMA designs can be coarse, but not to the point of SMA. Can cause segregation. o [DOT] barely meets three times the nominal aggregate size in their mixes. o Initial Superpave mixes took longer to achieve density (more compactive effort). o When compacting over milled pavement with layer separation.

110 Impact of Asphalt Thickness on Pavement Quality 5. Do any of the agencies you work for allow the use of warm mix asphalt technologies at conventional temperatures as a compaction aid? If you are allowed to use WMA as a compaction aid, how often do you use it in those states where it is allowed? Mostly Work In: Routinely Work In: WMA for Compaction? If allowed, how often do you use? Always Frequently Sometimes Never AL AL Yes • AL AL Permissive • CA CA Yes • CA CA Permissive • CA CA Yes • CA CA, MN Yes • CO CO Yes • FL FL Yes • FL FL Yes • FL FL Yes • IL IL Yes • IL IL, IN Yes • IL IL IN IN Yes • IN IN Yes • IN IN Yes • IN IN Yes • IN IN, KY Yes • IN IN Yes • IN IN Yes • IN IN Yes • IN IN Yes • IN IN Yes • IN IN Yes • IN IN Yes • IN IN KS KS Yes • KY IN, KY Yes • KY KY Yes • KY KY, OH MA CT, MA, NH, VT Yes • MA CT, MA Yes • MA MA Yes • MA MA Yes • MA MA, RI Yes • MA MA, NH Yes • MO KS, MO Yes • MO AR, MO, OK Yes • MO KS, MO Yes • NE NE Yes • NY NY Yes •

Tabulated Industry Survey Responses 111 Mostly Work In: Routinely Work In: WMA for Compaction? If allowed, how often do you use? Always Frequently Sometimes Never NC NC Yes • OH OH Permissive • OK OK Permissive • OR OR Yes • PA PA Yes • PA PA PA PA Yes • SC SC • SC NC, SC Yes • SC SC Yes • VA VA Yes • VA VA Permissive • VA VA Yes • VA VA Yes • VA VA Yes • VA VA Yes • VA VA Yes • VA VA No WA WA Permissive WY AZ, CA, KY, NV, WY Yes • Totals 49 Y, 1 N, 6 P 11 28 8 8

112 Impact of Asphalt Thickness on Pavement Quality 6. Have you observed any asphalt pavement performance problems that are perceived to be related in whole or in part to inadequate compaction during construction? (Please click all that apply.) Mostly Work In: Routinely Work In: In cr ea se d Ru tti ng In cr ea se d Cr ac ki ng In cr ea se d Pe rm ea bi lit y D ec re as ed D ur ab ili ty Sh or te ne d Se rv ic e Li fe In cr ea se d M ai nt en an ce O th er AL AL • AL AL • CA CA • • • CA CA • • • • CA CA • • • • • • • CA CA, MN • • • CO CO • • • FL FL • • • FL FL • • FL FL • IL IL • • IL IL, IN • • • • • IL IL IN IN • • • • IN IN IN IN • • • • IN IN • • • IN IN, KY • • • IN IN • • IN IN • • • IN IN • • • IN IN • • • IN IN • • • • IN IN • • • • • • IN IN • • • • • IN IN KS KS KY IN, KY • • • • KY KY • KY KY, OH MA CT, MA, NH, VT MA CT, MA MA MA • • • MA MA • MA MA, RI • • MA MA, NH • • • • • • MO KS, MO • • MO AR, MO, OK • •

Tabulated Industry Survey Responses 113 Mostly Work In: Routinely Work In: In cr ea se d Ru tti ng In cr ea se d Cr ac ki ng In cr ea se d Pe rm ea bi lit y D ec re as ed D ur ab ili ty Sh or te ne d Se rv ic e Li fe In cr ea se d M ai nt en an ce O th er MO KS, MO • • • • NE NE • • NY NY • • NC NC • OH OH • • • • OK OK • • • • OR OR • • • • • PA PA • PA PA PA PA • • • • • • SC SC SC SC • • • • SC NC, SC • VA VA • • VA VA • • • • VA VA • • VA VA • • • • • • VA VA • • • • VA VA • • VA VA • • • • • • WA WA • WY AZ, CA, KY, NV, WY • • • • • • Totals 15 23 25 31 36 24 9 Comments: o Nothing significant. o Most all performance issues are related to compaction. o Many times inadequate density is self-induced due to poor planning, equipment breakdown, unexpected weather. o We are experiencing decreased durability, shortened pavement service life, and the need for increased maintenance but not because of perceived issues related to inadequate compaction during construction. We have the same issues but the perceived root cause is not compaction related. o I haven’t noticed. I work mostly out of the laboratory now. o No. o Joints failing. o Surface raveling. o Occasional surface spalling in cold climate conditions.

114 Impact of Asphalt Thickness on Pavement Quality 7. Have you encountered situations during project planning or construction where you have requested exceptions or waivers to the lift thickness requirements? In what situations would you consider requesting changes to the specified lift thicknesses? (Please click all that ap ply.) On past projects where you have requested exceptions, are you aware of any construction or performance issues that developed that may have been related in whole or in part to the lift thickness? Mostly Work In: Routinely Work In: Re qu es te d W ai ve r? Reason to Request Waiver/Exemption? Pe rf or m an ce Pr ob le m s? A nti ci pa te d Co m pa cti on Is su es Sm oo th ne ss Re qu ir em en ts M O T Is su es La te S ea so n Pa vi ng O th er AL AL Yes • • No AL AL Yes • • No CA CA No CA CA No CA CA Yes • • Yes CA CA, MN Yes • • • Yes CO CO No FL FL No FL FL No FL FL No IL IL Yes • • No IL IL, IN No IL IL IN IN No IN IN No IN IN Yes • No IN IN Yes • • • No IN IN, KY No IN IN Yes • • No IN IN Yes • • • • Yes IN IN No IN IN No IN IN Yes • • Yes IN IN Yes • • No IN IN No IN IN KS KS No KY IN, KY Yes • • • • No KY KY No KY KY, OH MA CT, MA, NH, VT No MA CT, MA Yes • • No MA MA Yes • • • No MA MA No MA MA, RI Yes • • • No MA MA, NH Yes • Yes MO KS, MO No MO AR, MO, OK Yes • • Yes MO KS, MO Yes • • No NE NE No NY NY No NC NC No

Tabulated Industry Survey Responses 115 Mostly Work In: Routinely Work In: Re qu es te d W ai ve r? Reason to Request Waiver/Exemption? Pe rf or m an ce Pr ob le m s? A nti ci pa te d Co m pa cti on Is su es Sm oo th ne ss Re qu ir em en ts M O T Is su es La te S ea so n Pa vi ng O th er OH OH Yes • • • No OK OK Yes • • No OR OR No PA PA Yes • • Yes PA PA PA PA No SC SC Yes • • No SC NC, SC No SC SC No VA VA No VA VA Yes • • • Yes VA VA Yes • • No VA VA Yes • • • No VA VA VA VA Yes • • No VA VA Yes • • • No WA WA Yes • • • Yes WY AZ, CA, KY, NV, WY Yes • • • • • Yes Totals 26N, 28Y 29 21 6 11 5 19N, 10Y Comments on Other Reasons for Exemptions: o Anticipated segregation issues or unsuitable base conditions. o Dropoff concerns, leveling, concerns with raveling. o Material specified for job was incorrect for application rate but agency would not allow for material substitution to get correct density. o Construction issues when deep-excavating next to live traffic and bringing back to a safe transition in a day. o Surface texture and appearance issues. Comments or Resulting Performance Issues: o Thin overlays on distressed or poor base structure - raveling, cracking, etc. o Premature raveling due to a low lift thickness:NMAS ratio. o Variable depth QC/QA pavement that should have had wedge and level performed prior to a uniform thickness overlay. o Difficulty achieving density when the lift thickness is at the minimum for the NMAS and the gradation is coarse. o Lack of compaction led to over-rolling the mix, which led to cracking. o Thin overlays on low-volume routes with inferior subgrade, density difficult to achieve - rutted existing surface creates bridging issues with steel wheel rollers. o Thin lifts cause compaction issues, so over-rolling to achieve compaction results in poor smoothness. o Placing minimum thickness on irregular surfaces. This causes the new course to vary above and below the min. thickness. o Thin lifts over concrete pavements or bridge decks, especially in late season paving. o Density, texture, and appearance - performance is expected to suffer as well.

116 Impact of Asphalt Thickness on Pavement Quality 8. Have you had to change your mix designs, compaction, or other processes to cope with compaction difficulties? If yes, what changes have you chosen to make and how successful would you judge them to be? Mostly Work In: Routinely Work In: M ad e Ch an ge s? How successful were the changes? Fi ne r D es ig n Le ss A ng ul ar A gg re ga te s O th er C ha ng es W M A M or e Eff or t Ro lle r Pa tt er ns Ro lle r Ty pe s In te lli ge nt Co m pa cti on Th er m al Im ag in g O th er AL AL Yes L M M AL AL Yes M M M M M M CA CA No CA CA No CA CA Yes M M L L M N N CA CA, MN Yes L M M L L M M M CO CO Yes M M FL FL Yes M M M M M M N N FL FL Yes L L L L FL FL No IL IL Yes N M M M IL IL, IN Yes L M L N L L L IL IL IN IN Yes L M M L L L M IN IN Yes L M L IN IN Yes M M M IN IN Yes L M N L L L IN IN, KY Yes M L L M M IN IN Yes M M M M L L L M M IN IN Yes M M M M M M M IN IN Yes L L IN IN Yes M L L IN IN Yes L M M M M M L IN IN Yes N M M N L IN IN Yes L M IN IN KS KS No KY IN, KY Yes L N N M M M KY KY Yes L N M L M M M M M KY KY, OH MA CT, MA, NH, VT Yes M M M M M MA CT, MA Yes L M L L L M N N MA MA Yes L M L L L L L N M MA MA No MA MA, RI Yes M N L L L L L N N

Tabulated Industry Survey Responses 117 Mostly Work In: Routinely Work In: M ad e Ch an ge s? How successful were the changes? Fi ne r D es ig n Le ss A ng ul ar A gg re ga te s O th er C ha ng es W M A M or e Eff or t Ro lle r Pa tt er ns Ro lle r Ty pe s In te lli ge nt Co m pa cti on Th er m al Im ag in g O th er MA MA, NH Yes M N L M M M M N M L MO KS, MO Yes M M M L L L N N MO AR, MO, OK Yes M M M M M M MO KS, MO No NE NE Yes L M M M L L M NY NY Yes L L L L NC NC No OH OH Yes M M M L M M N OK OK Yes L M N L L L M M M OR OR Yes L L L L M M N N L PA PA Yes L L L PA PA PA PA Yes L M L M SC SC SC NC, SC Yes L L M M SC SC Yes L L L VA VA Yes M M M M M M VA VA Yes L M L L M M M N M VA VA No VA VA Yes M N L L M L M VA VA Yes L M M L M M VA VA Yes M M M VA VA Yes M M M L L L M WA WA Yes M N M M M L L L WY AZ, CA, KY, NV, WY Yes M M L L M L L Totals 8 N, 47 Y 21 L 18 M 0 N 1 L 17 M 5 N 11 L 17 M 2 N 14 L 20 M 5 N 20 L 19 M 0 N 22 L 18 M 0 N 11 L 21 M 1 N 1 L 7 M 10 N 0 L 9 M 6 N 5 L 0 M 0 N L = Largely Successful; M = Marginally Successful; N = Not Very Successful. Comments on Other Changes: o Increase VMA and PGAB content. o Adding asphalt binder. (2 responses) o Dropping down to lower NMS mix design to achieve 4×NMS.

118 Impact of Asphalt Thickness on Pavement Quality 9. Do you see a need for additional research into any of the following? (Please click all that apply.) Mostly Work In: Routinely Work In: Fi el d V al id ati on o f t/ N M A S Im pr ov in g M ix Co m pa cti bi lit y A ch ie vi ng M at D en si ty Te st s/ Te ch no lo gy Eff ec ts o f M ix Pr op er ti es O th er AL AL • AL AL CA CA CA CA • CA CA • CA CA, MN • • CO CO • • FL FL • • • FL FL FL FL • IL IL • • IL IL, IN • • • • • IL IL IN IN • • • IN IN • • • • • IN IN • IN IN • • • IN IN, KY • • IN IN • IN IN • IN IN • • • IN IN • • IN IN • • • • • IN IN • IN IN • • IN IN KS KS KY IN, KY • • KY KY • • KY KY, OH MA CT, MA, NH, VT • • • MA CT, MA • • • MA MA • • • • • MA MA • • MA MA, RI • • MA MA, NH • MO KS, MO • • • MO AR, MO, OK • • • • MO KS, MO • • • NE NE • • • • • NY NY • • NC NC • OH OH • • OK OK • • • OR OR •

Tabulated Industry Survey Responses 119 Mostly Work In: Routinely Work In: Fi el d Va lid ati on o f t/ N M AS Im pr ov in g M ix Co m pa cti bi lit y Ac hi ev in g M at De ns ity Te st s/ Te ch no lo gy Eff ec ts o f M ix Pr op er tie s O th er PA PA • PA PA PA PA • SC SC • • SC NC, SC • SC SC • • • VA VA • • • VA VA • • • • • VA VA • • VA VA VA VA • • VA VA • • • VA VA • VA VA • WA WA • WY AZ, CA, KY, NV, WY • Totals 30 22 13 23 18 12 Comments: o Better understanding of the effect that the liquid portion in recycled products will have on the end product. o Education in design to spec the right mix and not copy 20-year-old specs from somewhere. o Effects of the underlying surface condition on compaction and overall mix performance. o Agencies to apply the correct paving thicknesses of material to increase compaction and therefore pavement life. o We use 15-ton rollers to beat the mix into submission. Meets compaction, not good for the mix. o There seems to be a focus on density results (percent and uniformity) and not on the cause for lack of density or lack of uniformity. For instance, a DOT's decision to not mill or prelevel wheel ruts before paving will increase variability, and after the project the DOT will use the variability as a reason to “tighten” the specification. We need research on how common sense should be applied to postproject density numbers. If we have coarse, rut-resistant mixes placed on a surface of variable conditions, should the contractor be penalized for lack of density or uniformity in density numbers? We have specifications that are getting more and more complex with respect to data and documentation (e.g., intelligent compaction), and DOTs that do not appear to understand how to evaluate the cause of variability in density testing or how to determine if a contractor is following reasonable compaction methods. o Speed of construction - how thick is too thick? Section to be placed on NCAT Test Track in 2018 - ½ NMAS - 7 in. in one pass. o Correlation of lab permeability to field nuclear density to minimize destructive testing. o Most (70%) of our density issues stem from poor execution in the field, 25% from poor field conditions, and 5% from mix production. o Balanced mix design–air void regression mix designs to slightly increase binder content. o I think information on all of these topics is readily available - the problem is getting people to pay attention. There is still often a huge divide between engineers and spec writers and those who build the work.

120 Impact of Asphalt Thickness on Pavement Quality 10. If you have any additional thoughts or information on your organization’s experience with the effects of asphalt lift thickness on pavement performance, please describe briefly here or indicate if you are willing to be contacted for further information. o Thicker pavement sections tend to improve all aspects of pavement performance. o Historically, lifts less than 2 in. have, at times, been a challenge to obtain density. o What we typically see is public and private entities using minimal thicknesses. I haven’t run into too many instances that require minimizing lift thickness to achieve better pavement performance. o We have observed the thicker the mat the easier to achieve density. Mostly running our rollers in echelon with rollers with 84-in. drums has eliminated most of our density issues, regardless of mat thickness. o I believe the performance issues … are related more to low or inadequate asphalt binder content (as allowed by the specifications) and are not the result of inadequate compaction or lift thickness. o Many design engineers are unaware of the impact on lift thickness and nominal size of the aggregate; we see specs calling for 1/2-in. stone mix to be installed at 1 in. o My personal experience as it applies to lift thickness and its effects on pavement performance is that if the lift thickness is within 2.5 to 4 times the nominal maximum aggregate size, it significantly reduces your chance of issues. These issues can include but are not limited to segregation (physical and thermal), compaction, bond, and workability. All of these issues can lead to an overall failure in performance. This has not been too much of an issue in our state contracted work, but has been more of an issue in our private work (parking lots, town roads, etc.). Any time we have been under the 2.5× nominal max, we have noticed these issues arising more frequently. A lot of the time these specifications do not take into account that a paver is designed to scalp high spots and fill in low spots over the underlying surface in order to make it flat. When a high spot is scalped in the underlying surface, it reduces your lift thickness and if not accounted for can be detrimental to the overall quality of the final surface. o Lower the gyrations on Superpave. This would allow easier compaction that the federal highway has already approved. Lower the air voids to 2-5 instead of 3-5. States don’t like change, they like to take deducts…. o Field validation of increased permeability versus lift thickness would be a good avenue to go. In terms of mix designs, most, if not all, contractors know how to change their mixes to give them the best chance at compaction success. o The DOT typically specifies lift thickness for surface and intermediate courses that are near the minimum for compaction based on NMAS. This is done for economy. We support this practice since pavement type is largely selected competitively on the basis of initial cost. But, it does make achieving compaction and smoothness more challenging. o We achieve superior quality when the plans call for 4× or 5× NMS lift thickness. This is probably the MOST important factor in compaction and ride quality. This will ensure longer pavement life. o Other negative effects like aggregate crushing or dragging under the screed. o See last option in last question … flip side of min thickness to NMA size.... How thick is too thick? This is rehab interstate pavements where FDP or reconstruction may not be an option. o When used appropriately lift thickness and NMAS correlate well together. Mixes designed with the correct NMAS and placed in the correct lift thickness have sufficient structural aggregate skeleton to support the design traffic loads and be compacted well enough to be impermeable to prevent water infiltration, mix stripping, and subgrade degradation. o We have found that lift thicknesses may be increased with no significant issue obtaining density. The key is ensuring communication and execution of increasing compactive effort to match. The crew has to be very intentional about achieving density and not delay on a single part of the process. We've found this to be true for base, binder, and surface mixes. o Many agencies are moving toward thin lifts without considering the need to increase paver speed or decrease production, add rollers or decrease production, without considering the reduced time available to compact, and without considering the increased difficulty in evaluating density - more influence of underlying layers with nuclear gauges, and more difficulty in obtaining and measuring cores. I would like to see more details on some of the mix changes some agencies are employing for thin lift applications and their effectiveness.

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 537: Impact of Asphalt Thickness on Pavement Quality documents transportation agency policy for lift thickness and minimum compaction requirements on resultant asphalt pavement quality.

To achieve expected pavement performance, it is important that asphalt concrete (AC) have adequate density. A critical factor in achieving this density is the ratio of lift thickness to nominal maximum aggregate size (t/NMAS).

The information in the report is designed to help make agencies aware of a range of practices other agencies use to achieve a desired t/NMAS ratio, ensuring that density of AC is adequate to meet expected pavement performance.

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