APA 101 cvr - Department of Civil Engineering - Civil ... Pavement...Perpetual Pavements are con-fined to the surface, and that deep structural problems are eliminated or minimized

Perpetual PavementsPerpetual PavementsA Synthesis

NATIONAL ASPHALT PAVEMENT ASSOCIATIONASPHALT INSTITUTESTATE ASPHALT PAVEMENT ASSOCIATIONS

ASPHALT PAVEMENT ALLIANCE

APA 101

This document was prepared for the Asphalt Pavement Alliance (APA), a coalitionof the National Asphalt Pavement Association (NAPA), Asphalt Institute (AI), andState Asphalt Pavement Associations (SAPA). The content of this report reflects theviews of the author(s), who are responsible for the facts and accuracy of the datapresented herein. The contents do not reflect the decision-making process of NAPA,AI, or SAPA with regard to the advice or opinions on the merits of certain practices.This document does not constitute a standard, specification, or regulation.

The author(s) and the APA do not endorse specific products or manufacturers. Tradeor manufacturers' names appear herein because they are essential to this document.

Published 2002 Asphalt Pavement AllianceOrder Number APA 101

1/02

NATIONAL ASPHALTPAVEMENT ASSOCIATION


5100 Forbes Boulevard, Lanham, MD 20706-4407 Toll-free: 888-468-6499 Fax: 301-731-4621 [email protected] www.AsphaltAlliance.com

ASPHALT INSTITUTE

Introduction ................................................................................................ 5

Mechanistic-based Design ......................................................................... 6

Foundation ................................................................................................. 8

HMA Considerations ................................................................................ 11HMA Base Layer .......................................................................... 11Intermediate Layer ....................................................................... 12Wearing Surface ........................................................................... 13

Performance Goals .................................................................................. 14

Current Perpetual Pavement Efforts ........................................................ 15California ...................................................................................... 15Illinois ........................................................................................... 16Michigan ....................................................................................... 17Wisconsin ..................................................................................... 18Texas ............................................................................................ 19Kentucky ....................................................................................... 19Ohio and Virginia .......................................................................... 20United Kingdom............................................................................ 20

Summary .................................................................................................. 21

References ...............................................................................................23

5100 Forbes BoulevardLanham, MD [email protected]


APA 101

PERPETUAL PAVEMENTSA Synthesis

NATIONAL ASPHALT PAVEMENT ASSOCIATION

ASPHALT INSTITUTESTATE ASPHALT PAVEMENT ASSOCIATIONS

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Abbreviations used in this document:

AASHTO American Association of State Highway and Transportation Officials

AASHO American Association of State Highway Officials

AI Asphalt Institute

CBR California Bearing Ratio

DOT Department of Transportation

APEC Asphalt Paving Environmental Council

EAPA European Asphalt Pavement Association

ESAL Equivalent Single Axle Load

FWD Falling Weight Deflectometer

FHWA Federal Highway Administration

HMA Hot Mix Asphalt

IDOT Illinois Department of Transportation

KTC Kentucky Transportation Cabinet

LCPC Laboratoire Central de Ponts et Chasses

MAPA Michigan Asphalt Paving Association Inc.

Mn/ROAD Minnesota Road Research Project

NAPA National Asphalt Pavement Association

NCAT National Center for Asphalt Technology

NCHRP National Cooperative Highway Research Program

ODOT Ohio Department of Transportation

OGFC Open Graded Friction Course

PAIKY The Plantmix Asphalt Industry of Kentucky

PG Performance Grade

SHRP Strategic Highway Research Program

SMA Stone Matrix Asphalt

SST Superpave Shear Test

TxDOT Texas Department of Transportation

TxHMAPA Texas Hot Mix Asphalt Pavement Association

TRB Transportation Research Board

TRL Transport Research Laboratory

TRRL Transport and Road Research Laboratory

WSDOT Washington State Department of Transportation

WisDOT Wisconsin Department of Transportation

WAPA Wisconsin Asphalt Pavement Association

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Acknowledgements

This synthesis was prepared by Dr. David Newcomb of the National Asphalt Pave-ment Association.

A number of individuals contributed information contained herein. Dr. Mark Buncherof the Asphalt Institute and Mr. Jim Huddleston of the Asphalt Pavement Associationof Oregon both serve on the Asphalt Pavement Alliance resource team for PerpetualPavements. Dr. Buncher provided valuable insight during the preparation of this workand Mr. Huddleston is to be credited with many of the innovative ideas concerningPerpetual Pavements.

A workshop on Perpetual Pavements was held in October 2000 in Cincinnati.The following attendees discussed many of the topics presented herein:

Mr. Tom Blair – Cadillac Asphalt Paving Co.Mr. Dean Blake – The Plantmix Asphalt Industry of Kentucky Inc.Dr. Mark Buncher – Asphalt InstituteDr. Ray Brown – National Center for Asphalt TechnologyMr. Ron Collins – Pavement Technologies, Inc.Mr. Bill Fair – Flexible Pavements of OhioMr. Frank Fee – Citgo Asphalt Refining Co.Mr. Gary Fitts – Asphalt InstituteDr. Kevin Hall – University of ArkansasMr. Kent Hansen – National Asphalt Pavement AssociationMr. Gerry Huber – Heritage ResearchMr. Jim Huddleston – Asphalt Pavement Association of OregonDr. Joe Mahoney – University of WashingtonMr. Jack Mathews – Alabama Asphalt Pavement AssociationDr. Rich May – Koch Materials Co.Dr. David Newcomb – National Asphalt Pavement AssociationMr. Michael Nunn – Transport Research LaboratoryMr. Richard Schreck – Virginia Asphalt Association Inc.Mr. Jim Scherocman – Consulting EngineerDr. Marshall Thompson – University of IllinoisDr. Marvin Traylor – Illinois Asphalt Pavement AssociationMr. Cliff Ursich – Flexible Pavements of OhioMr. Harold Von Quintus – Fugro-BRE Inc.Mr. Brian Wood – The Plantmix Asphalt Industry of Kentucky Inc.

Credit is due Dr. Joe Mahoney for contributing valuable text concerning the treat-ment of frost heave and thaw weakening in soils, and to Dr. Marshall Thompson forproviding information concerning the practice of treating subgrade soils in Illinois.

The following reviewers gave a great deal of thought and comment on the presen-tation of the information in this synthesis: Messrs. Byron Lord, John Bukowski andJohn D'Angelo of the Federal Highway Administration; Dr. Marvin Traylor of the IllinoisAsphalt Pavement Association; Mr. John Becsey of the Michigan Asphalt PavingAssociation Inc.; and Dr. Joe Mahoney of the University of Washington. Their inputinto this document was invaluable.

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Notes

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A Perpetual Pavement is definedas an asphalt pavement designedand built to last longer than 50 yearswithout requiring major structuralrehabilitation or reconstruction,and needing only periodic surfacerenewal in response to distressesconfined to the top of the pavement.

The concept of PerpetualPavements, or long-lasting asphaltpavements, is not new. Full-depthand deep-strength asphalt pave-ment structures have been con-structed since the 1960s, andthose that were well-designedand well-built have been verysuccessful in providing longservice lives under heavy traffic.

Full-depth pavements are con-structed directly on unmodified ormodified subgrade soils, and deep-strength sections are placed ongranular base courses. One of thechief advantages of these pave-ments is that the overall section ofthe pavement is thinner than thoseemploying a thin asphalt layer overthick granular layers. As a result,the potential for traditional fatiguecracking may be eliminated, andpavement distress may be confinedto the upper layer of the structure.Both are advantages to PerpetualPavements. Thus, when surfacedistress reaches a critical level,an economical solution is to simplyremove the very top layer andreplace it to the same depth. Thepavement material that is removedcan then be recycled.

Recent efforts in materials selec-tion, mixture design, performancetesting, and pavement design offera methodology to obtain evenlonger-lasting performance from

Introduction

y y p

asphalt pavement structures(greater than 50 years) whileperiodically replacing the pave-ment surface. The structure,designed for durability, combinesa rut-resistant and wear-resistanttop layer with a rut-resistantintermediate layer and a fatigue-resistant base layer as shown inFigure 1. By applying the properstructural design and selectingmaterials appropriate to theirplacement within the structure,the designer can make the con-scious decision to obtain along-lasting pavement.

This approach can be takenon any pavement structure whereit is desirable to minimize rehabili-tation and reconstruction costsas well as minimize closures totraffic. While these considerationsare especially important for high-traffic-volume roadways and major

airports where user-delay costsmay be prohibitive, they couldcertainly be applied to lower-volume roads and general aviationairfields where the possibility offuture funding cuts may requiredeferred rehabilitation.

A life cycle cost analysis,including user-delay costs, shouldbe employed to evaluate differentpavement strategies. In thisprocess, consideration should begiven to the realistic expectationof the availability of future funding.If future funding for rehabilitationis uncertain, then the constructionof a thicker pavement initially maypreclude the need for costlyrehabilitation or reconstruction inthe future.

This synthesis is intended topresent discussions pertaining topavement design, materials andmix design, and construction

Figure 1

Perpetual Pavement Design Concept

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relevant to Perpetual Pavements.Since past empirical practiceshave not recognized the long-lifenature of thick Hot Mix Asphalt(HMA) pavements, pavementdesign is discussed in terms ofmechanistic-based design, oneof the key considerations beingthe pavement foundation. Beyondthe need for long-term stability,the pavement foundation playsa critical role in providing a

working platform for the place-ment and compaction of theasphalt layers. HMA materials areaddressed in terms of propertiesneeded in the various layers ofthe pavement. The proper con-struction for long-lasting pave-ment performance is discussedbecause without it the structuraldesign and materials selectionprocesses are incomplete. Theperformance of demonstrated

long-life HMA pavements isdiscussed in order to validatethe concept that distresses inPerpetual Pavements are con-fined to the surface, and thatdeep structural problems areeliminated or minimized. Finally,the experiences of agenciesworking toward the goal ofproducing long-life HMA pave-ment design proceduresare presented.

Mechanistic-based DesignThe basic premise of obtaining along pavement life is that anadequately thick HMA pavementplaced on a stable foundation willpreclude distresses that originateat the bottom of the pavement andthat eventually require expensivereconstruction to correct properly.Structurally, the pavement musthave the proper combination ofthickness and stiffness to resistdeformation in the foundationmaterial or the underlying sub-grade. Likewise, the HMA layersmust be thick enough and havethe right properties to resistfatigue cracking originating at thebottom of the structure.

Currently, most pavementdesign procedures do not con-sider each pavement layer and itscontribution in resisting fatigue,rutting, and temperature crackingin the structure. Since each pave-ment layer has its own uniquepart to play in performance, animproved structural designmethod is needed to analyzeeach pavement layer. Past empiri-cal methods such as the Califor-nia Bearing Ratio (CBR) or the

American Association of StateHighway and TransportationOfficials (AASHTO) structuralcoefficient procedures cannotconsider the contributions bydifferent HMA layers in the pave-ment, but the mechanistic-empirical approach can.

Mechanistic techniques forasphalt pavement design havebeen known since the 1960s,although wider development andimplementation started in the1980s and 1990s. States such asIllinois, Kentucky, Minnesota, andWashington are currently adoptingmechanistic design procedures,and a research project under theNational Cooperative HighwayResearch Program (NCHRP) isproceeding on the developmentof a new mechanistically basedpavement design guide, whichmay be adopted by AASHTO.

Mechanistic design is much thesame as the engineering ap-proaches used for structures suchas bridges, buildings, and dams.Essentially, the principles ofmechanics are used to determinea pavement's reaction to climate

and loading. Knowing the criticalpoints in the pavement structure,one can design against certaintypes of failure or distress bychoosing the appropriate materialsand layer thicknesses. Monismith(1992) thoroughly outlined themechanistic design approach inhis Transportation Research Board(TRB) Distinguished Lecturerpaper. His process is shown inFigure 2, wherein the materialproperties, traffic, climate, andperformance are interactivelycombined in determining therequired structural section.

The Washington State Depart-ment of Transportation (WSDOT)has been using a mechanistic-empirical design procedure fordesigning HMA overlays since thelate 1980s.The Washingtonapproach uses a fatigue transferfunction based on Monismith’slaboratory relationship betweentensile strain, asphalt mixturemodulus, and number of cycles tofailure. A shift factor of between4 and 10 is used to adjust thelaboratory fatigue relationship tothe field. Their structural ruttingtransfer function was proposed by

2

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Santucci (1977). In comparing theWSDOT overlay design methodwith the approach recommendedby the AASHTO 1993 pavementdesign guide, Pierce and Mahoney(1996) found the overlay thick-nesses determined by the AASHTOmethod to be overly conservative.

The Illinois DOT (IDOT) uses amechanistic approach to pavementdesign developed at the Universityof Illinois at Urbana-Champaign(Gomez and Thompson, 1984;Thompson and Cation, 1986;Thompson, 1987). This procedureis based upon the results of finiteelement analysis using the com-puter program ILLI-PAVE. A strain-based fatigue equation is usedwhich accounts for HMA propor-tioning, tensile strength, and fieldperformance. This procedure hasbeen implemented by IDOT.

The state of Minnesota hasbeen developing a mechanisticdesign procedure based on

information collected from theMinnesota Road Research Project(Mn/ROAD) (Timm et al., 1999).The layered elastic computer prog-ram WESLEA (Waterways Experi-ment Station Layered ElasticAnalysis) was used in computingpavement responses to load.Suggestions for seasonal changesin material properties and forperformance criteria were obtainedfrom Mn/ROAD data analysis. Thisprocedure is currently beingevaluated and modified for use bythe Minnesota DOT.

The British used a mechanisticdesign procedure developed byPowell, et al. (1984) to calculatepavement responses at criticallocations in the structure. This earlyprocedure was based upon theassumption of incremental damageoccurring in the pavement structuredue to repeated loads from com-mercial vehicles. Thus, regardlessof pavement thickness, it was

Figure 2

Mechanistic Design Flowchart (Monismith, 1992)

assumed that cracking or structuralrutting would eventually occur. Infact, Nunn and his colleagues(1997) found that in thick asphaltpavements, there is an upper limitto thickness beyond which bottom-up fatigue cracking and structuralrutting do not occur in well-con-structed pavements. The resultwas the establishment of a designchart in which the asphalt pave-ment thickness at 80 millionstandard axle loads (the same asan equivalent single axle load[ESAL]) does not change. Thisapproach to pavement design is anew paradigm: increasing trafficlevels do not automaticallynecessitate thicker flexible pave-ment structures. This is becausethere is a bending strain level atthe bottom of the HMA belowwhich fatigue damage will notoccur, and any additional HMAthickness to reduce strain will besuperfluous. This strain level isknown as the fatigue limit.

Mechanistic pavement designis being more readily adopted byvarious agencies as an improvedmethod for analyzing pavementstructures and assessing theimpact of changes in traffic andmaterials. The scheduled comple-tion of the new Guide for the Designof Pavement Structures in 2002should move the implementationof mechanistic design even faster.

Ultimately, whatever mechanis-tic approach may be adopted, itmust recognize the characteristicsinherent in the Perpetual Pave-ment, including the validity of thefatigue limit in bound layers forbottom-up load-related crackingand the preclusion of structuralrutting. The mechanistic designprocess for Perpetual Pavementwould conceptually be more of adesign for maximum strain than adesign for incremental damage.

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FoundationThe pavement foundation is criticalto the construction and perfor-mance of a Perpetual Pavement.During construction, the founda-tion provides a working platformthat supports the dump trucks andlaydown equipment placing theHMA layers. It also provides resis-tance to deflection under the rollersin order that the upper layers ofthe pavement may be firmly com-pacted. Throughout the perfor-mance period, the foundation iscritical in supporting the trafficloads and reducing the variabilityin support from season to seasondue to freeze-thaw and moisturechanges. Proper design andconstruction of the foundation arekeys to preventing volume changedue to wet-dry cycles in expansiveclays and freeze-thaw cycles infrost-susceptible soils.

Several northern states incorpo-rate frost design into their pave-ment structures in areas wherethe soils and conditions may leadto thaw weakening or non-uniformfrost heave. Generally, these statesrequire that the total pavementstructure thickness equal or exceed50 percent of the expected designfrost depth. This requirement isgenerally taken to be a minimum.Results from the American Asso-ciation of State Highway Officials(AASHO) Road Test and researchin other countries (such as Japan)suggest that a depth of up to 70percent may be required. Suchcriteria generally require that thepavement structure be constructedof non-frost-susceptible materials.

A pavement foundation may becomprised of compacted subgrade,chemically stabilized subgrade orgranular material, or unstabilized

granular material such as crushedrock or gravel. Regardless of thekind of material employed, thefoundation should meet someminimum requirement for stiffnessthroughout construction as well asduring the life of the pavement.Depending upon site conditions andpavement design, this may requirethe chemical or mechanical stabili-zation of soils or base coursematerials. Terrel and Epps (1979)provide excellent guidance on theselection of the stabilization proce-dures for unbound materials.Furthermore, the site and climatemay dictate that drainage featuresbe included in the pavement design,and guidance on subsurface drain-age may be found in the FederalHighway Administration (FHWA)manual (Moulton, 1980). The IllinoisDOT (IDOT) has put a great deal ofemphasis on subgrade soils asdetailed in their Subgrade Stability

Figure 3

Soil Strength for Support of Construction Equipment (IDOT, 1982)

Manual (IDOT, 1982). From aconstructability standpoint,Illinois requires a subgrade tohave a minimum CaliforniaBearing Ratio (CBR) of about 6to avoid excessive deformationduring the construction of subse-quent granular layers; they basethis requirement on the graphshown in Figure 3. This graphpresents the relationshipsbetween soil strength, sinkage,and the tire pressure under a40-kN load. Figure 4 shows theconditions under which IDOTrequires remedial procedures.Remedial action is required ifthe soil CBR is less than 6, it isoptional between a CBR of 6 and8, and it is considered unneces-sary above 8.

The remedial proceduresprovide a working platformadequate to facilitate pavingoperations, prevent overstressing

1 inch = 25 mm1 psi = 6.9 kPa

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1 inch = 25 mm

the subgrade, and minimize thedevelopment of surface ruttingfrom construction traffic.

The most frequently usedprocedure is to lime-modify (IDOT,2002) the fine-grained subgradesoils that predominate in Illinois.Undercutting and backfilling withgranular material (geo-fabricsare sometimes used) is also acommonly used procedure.The required thickness abovethe subgrade is typically 300 mm.For subgrade strengths less thana CBR of 4, the thickness isincreased as per Figure 4.

If the immediate CBR of thelime-modified soil is less than 10,a granular surface layer may benecessary. The combined thick-ness of the granular layer and thelime-modified soil is a minimum of200 mm and should follow theguidelines in Figure 4. The granu-lar material must be adequate interms of stiffness and strength toaccommodate the constructiontraffic. Highly plastic fines shouldnot be used in base materials.

Table 1 presents layer thick-ness requirements for lime treat-ment (IDOT, 2002) of low-strengthsoils according to various strengthand stiffness criteria, including themodulus of subgrade reaction (k),CBR, or Cone Index from thedynamic cone penetration test.This specification is more strin-gent than IDOT's lime-modifiedsoils specification. The increasedstrength of lime-stabilized soilmixtures permits a reduction inthe lime-treated layer thickness.

Seasonal modulus adjustmentfactors are used in Washingtonand Minnesota for subgrade andoverlying granular materials tocharacterize their behaviorsduring the design life. Seasonalmodulus adjustment factors forunbound materials differ between

Figure 4

Illinois Granular Thickness Requirement for Foundation (IDOT, 1982)

Subgrade Strength1 Minimum Lime-Soil Layer Thickness, inches 2

k, psi/in* CBR Cone Index 100 psi 3 200 psi 3

50 2 80 12 9125 4 160 12 9150 6 240 9 8200 8 320 9 8

1 In-situ subgrade strength 1 inch = 25 mm2 Strength before opening to traffic 1 psi = 6.9 kPa3 Unconfined compressive strength* Modulus of subgrade reaction

Table 1

Illinois Requirements for Depth of Lime Modification (IDOT, 1982)

Table 2

Seasonal Adjustment Factors for Unbound Materials Used in Washington State (Pierce and Mahoney, 1996)

Location Material SeasonSpring Summer Fall Winter

Eastern WA Base 0.65 1.00 0.90 1.10Subgrade 0.90 1.00 0.90 1.10

Western WA Base 0.85 1.00 0.90 0.75Subgrade 0.85 1.00 0.90 0.85

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eastern and western WashingtonState as shown in Table 2 (Pierceand Mahoney, 1996). The seasonsin Washington are assumed to beof equal length, and the baseseason is the summer with amultiplication factor of 1.00. Aslightly different approach is takenin Minnesota where the seasonsare considered to be of unequallengths as shown in Table 3,and the base season is the fall.Because the progression of thaw-ing results in different behavior inthe upper and lower regions of thepavement, the spring period isdivided into early and late spring.Ovik, et al. (1999) determinedthese seasonal factors from datacollected at the Minnesota RoadResearch Project (Mn/ROAD).The weakest condition for granularbase materials is in the earlyspring, and for the subgrade it isin the late spring. The very highmultiplication factors for the winterreflect a frozen condition in thebase and subgrade material. In thedesign of Perpetual Pavements, itis important to know how seasonalchanges in the moduli of unboundmaterials may affect the responseof the pavement. In other words,it may be necessary to considerthe worst condition in order topreclude undue damage duringa given season.

Nunn et al. (1997) encouragethe use of in-situ testing for pave-ment foundation materials. Theyformulated an end-result specifica-tion founded on nuclear densitytests and surface stiffness asmeasured by a portable dynamicplate bearing test. The foundationdesign practice in the UnitedKingdom is shown in Table 4(Nunn). The CBR of the subgradedictates the thickness of theoverlying granular layers calledthe capping and subbase layers.

For a subgrade CBR of less than15, a minimum 150-mm thicknessof subbase is required. Cappingmaterial may be considered similarin quality to a lower quality basecourse material in the UnitedStates, and the subbase may beconsidered a high quality basematerial. Transport ResearchLaboratory (TRL) set end-resultrequirements for the pavementfoundation, both during and afterits construction. Under a fallingweight deflectometer (FWD) loadof 40 kN, a stiffness of 40 MPawas required on top of the sub-grade and 65 MPa was requiredat the top of the subbase.

The German Ministry of Trans-portation (1989) requires a mini-mum subgrade surface modulus of45 MPa when tested using a staticplate-bearing test with a 300-mmdiameter plate. At the top of thesubbase layer, they require 120MPa for light traffic conditions and180 MPa for heavy traffic.

The French (Laboratoire Centralde Ponts et Chasses [LCPC],

Month Late Nov, March April, June, Sept, Oct,Dec, Jan, May July, early NovFeb August

HMA (120/150 pen asphalt) 2.5 2.1 1.3 0.37 1.0

Granular Base 28 0.65 0.80 1.0 1.0

Subgrade 22 2.4 0.75 0.75 1.0

1992) use an end-result specifica-tion for the constructed roadfoundation. For support of con-struction traffic, either of the twofollowing criteria must be met: adeflection of less than 2 mm undera 13-ton axle load, or a plate-bearing test modulus of more than50 MPa. For service conditions, therequired subbase stiffness is tiedto the strength of the subgrade.

The design and construction ofa strong, stable, and consistentfoundation is requisite to a Per-petual Pavement. The initial con-cern is support of constructiontraffic and a firm layer for provid-ing a reaction to compactionefforts. Long-term support of trafficloads and minimization of volumechange are crucial to performance.Thus, guidelines are needed forassessment of stiffness at thetime of construction, requiredstiffness for long-term performanceas input to mechanistic design,and provisions to minimize volumechange due to expansive behavioror frost heave.

Table 3

Seasonal Adjustment Factors for Mn/ROAD (After Ovik, et al.)

Table 4

Transport Research Laboratory Foundation Requirements (Nunn, 1997)

Subgrade CBR ≥ 2 2 – 5 > 5

Subase Thickness, mm 150 150 225

Capping Thickness, mm 600 350 —

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HMA ConsiderationsSince the Perpetual Pavement istailored to resist specific distressesin each layer, the materials selec-tion, mix design, and performancetesting need to be specializedfor each material layer. Themixtures' characteristics need tobe optimized to resist rutting orfatigue cracking, depending uponwhich layer is being considered.Durability is a primary concernfor all layers.

HMA Base LayerThe asphalt base layer must

resist the tendency to fatiguecracking from bending underrepeated traffic loads. One mix-ture characteristic that can helpguard against fatigue cracking is ahigher designed asphalt content(Figure 5a). A summary of fatigueresearch studies by Epps andMonismith (1972) established thatthis behavior is consistent in manyasphalt mixtures. Additional as-phalt, up to a point, provides theflexibility needed to inhibit theformation and growth of fatiguecracks. Combined with an appro-priate total asphalt thickness, thisensures against fatigue crackingfrom the bottom layer (Figure 5b).The concept of a “rich” or high-asphalt-content base is beingemployed in California (Monismithand Long, 1999a) and Illinois(IDOT, 2001).

Another approach to ensuringthe fatigue life would be to design athickness for a stiff structure suchthat the tensile strain at the bottomof the asphalt layers would beminimized to the extent that cumu-lative damage would not occur.This would allow for a single mix

design to be used in the base andintermediate layers, precludingthe need to switch mix types inthe lower pavement structure.This strategy is used in the TRLmethod proposed by Nunn andhis colleagues (1997) as well asthe French (EAPA, 1999).

The asphalt content in thebase should be defined as thatwhich produces low air voids inplace. This ensures a highervolume of binder in the voids inmineral aggregate (VMA), whichis critical to durability and flexibil-ity. This concept has been sub-stantiated by Linden et al. (1989)in a study that related higher-than-optimum air void content toreduction in fatigue life. Fine-graded asphalt mixtures havealso been noted to demonstrateimproved fatigue life (Epps and

Monismith, 1972). The asphaltgrade should have the high-temperature characteristics asdictated by the depth of the layerin the pavement. The low-tem-perature characteristics shouldbe the same as those of the inter-mediate layer. If this layer is to beopened to traffic during construc-tion, provisions should be madefor rut-testing the material toensure performance during con-struction, at a minimum. Consid-eration should be given to fatiguetesting this material using the four-point bending method describedby Tayebali et al. (1994a and1994b). This test has been stan-dardized by AASHTO (2001) in itsprovisional standard TP 8-94. Itshould be noted that fatigue test-ing requires substantial equipmentand training investment.

Figure 5

Fatigue Resistant Asphalt Base

Improve Fatigue Resistencewith High Asphalt Content Mixes

Minimize Tensile Strainwith Pavement Thickness

Figure 5a Figure 5b

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1996b). The same effect could beachieved with smaller aggregatesizes so long as stone-on-stonecontact is maintained. One test forevaluating whether this type ofinterlock exists is the Baileymethod (Vavrik et al., 2001).Segregation in coarse aggregatemixtures is an area of concern butproper handling of the materialduring manufacture, transport, andlaydown can prevent the problem(AASHTO, 1997).

The Performance Graded (PG)binder system is used to classifythe asphalt according to highand low service temperatures(Asphalt Institute, 1996a). Thehigh-temperature grade of theasphalt should be the same asthe surface to resist rutting. How-ever, the low-temperature require-ment could probably be relaxedone grade, since the temperaturegradient in the pavement is rela-tively steep and the low tempera-ture in this layer would not be assevere as in the surface layer

(Figure 6). For instance, if a PG70-28 is specified for the surfacelayer, a PG 70-22 could be usedin the intermediate layer.

The mix design should be astandard Superpave approach(Asphalt Institute, 1996b), and thedesign asphalt content should bethe optimum. Performancetesting should include rut testingand moisture susceptibility, at aminimum. Although a test forfundamental permanent deforma-tion properties is currently beingdeveloped in a National Coopera-tive Highway Research Programproject, it is recommended that arut-testing device be used in theinterim to evaluate mixtures inorder to protect against earlyrutting. A recent report onperformance testing is availablefrom the National Center forAsphalt Technology (Brown et al.,2001). They suggest that theconditions of rut testing need tobe selected considering the high-temperature grade of the PG

It is important to design pave-ments so that the bending strainat the bottom of the pavement isless than the fatigue limit of thematerial. The fatigue limit is thestrain below which the material willnot fail in fatigue. Japanese re-searchers (Nishizawa, et al., 1997)have suggested a fatigue limit forasphalt mixtures.

Because this layer is the mostlikely to be in prolonged contactwith water, moisture susceptibilityneeds to be considered. A higherasphalt content should enhancethe mixture's resistance to mois-ture problems, but it is advisableto conduct a moisture suscepti-bility test such as AASHTO T 283(AASHTO, 2000) during the mixdesign.

Intermediate LayerThe intermediate, or binder,

layer must combine the qualitiesof stability and durability. Stabilityin this layer can be obtained byachieving stone-on-stone contactin the coarse aggregate and usinga binder with an appropriate high-temperature grading. This isespecially crucial in the top 150mm of the pavement where highstresses induced by wheel loadscan cause shear failure.

The internal friction provided bythe aggregate can be obtained byusing crushed stone or gravel andensuring an aggregate skeleton.One option would be to use a largenominal maximum size aggregate,and guidance for the design oflarge-stone mixtures can be foundin Kandhal (1990) and Mahbouband Williams (1990). For mixtureswith a nominal maximum aggre-gate size up to 37.5 mm, theSuperpave mix design approachmay be used (Asphalt Institute,

Figure 6

Impact of Temperature Gradient on Asphalt Grade

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binder or criteria for the particulardevice. Another option for perfor-mance testing is the Superpaveshear test (SST) developed duringthe Strategic Highway ResearchProgram (SHRP) (Sousa et al.,1994), and standardized inAASHTO test method TP 7-01(AASHTO, 2001). Currently, mostmoisture susceptibility testing isperformed in accordance withAASHTO test method T 283-89(AASHTO, 2000).

In adjusting layer moduli forseasonal variations, the Washing-ton State DOT (Pierce andMahoney, 1996) and the Minne-sota DOT (Ovik et al., 1999) usemodulus-temperature relation-ships for asphalt concrete andseasonal multiplication factorsbased on estimated pavementtemperatures. For structuraldesign purposes, the HMAmodulus corresponding tothe mean monthly pavementtemperature is used.

Wearing SurfaceThe wearing surface require-

ments would depend on trafficconditions, environment, localexperience, and economics.Performance requirements includeresistance to rutting and surfacecracking, good friction, mitigationof splash and spray, and minimiza-tion of tire-pavement noise. Theseconsiderations could lead to theselection of Stone Matrix Asphalt(SMA), an appropriate Superpavedense-graded mixture, or OpenGraded Friction Course (OGFC).Guidance on mix type selectioncan be found in the HMA Pave-ment Mix Type Selection Guide bythe National Asphalt PavementAssociation (NAPA, 2000).

In some cases, the need forrutting resistance, durability,impermeability, and wear resis-tance would dictate the use ofSMA. This might be especiallytrue in urban areas with high trucktraffic volumes. Properly designedand constructed, an SMA willprovide a stone skeleton for theprimary load-carrying capacityand the matrix (combination ofbinder and filler) gives the mixadditional stiffness. Methods forperforming SMA mix design aregiven in NCHRP Report No. 425(Brown and Cooley, 1999).

The matrix in an SMA can beobtained by using polymer-modi-fied asphalt, with fibers, or inconjunction with specific mineralfillers. Brown and Cooley (1999)concluded that the use of fibers isbeneficial to preclude drain-downin SMA mixtures. They also pointout the need to carefully controlthe aggregate gradation, espe-cially on the 4.75-mm and 0.75-mm sieves.

In instances where the overalltraffic is not as high, or in caseswhere the truck traffic is lower, theuse of a well-designed, dense-graded Superpave mixture mightbe more appropriate. As withthe SMA, it will be necessary todesign against rutting, permeabil-ity, weathering, and wear. TheAsphalt Institute (1996b) providesguidance on the volumetric pro-portioning of Superpave mixtures.It is recommended that a perfor-mance test of dense-gradedmixtures, whether SMA or Super-pave, be done during mixturedesign. At a minimum, this shouldconsist of rut testing (Brown et al.,2001), but more fundamentaltests such as the SHRP SST(Sousa et al., 1994) could beemployed to estimate the perfor-mance of the material.

The PG grade used in the topdense-graded mixture should bebumped to at least one high-temperature grade greater thannormally used in an area, con-sistent with Brown and Cooley's(1999) recommendations. Toresist thermal cracking, the low-temperature grade should be thatnormally used for 95 percent or99 percent reliability in the area,depending upon availability andcost. With the possible use ofpolymer-modified asphalts, it willbe critical to avoid overheatingthe binder in the constructionprocess. New industry guidelineshave been developed to ensurethe proper handling and applica-tion of polymer-modified asphaltbinders (APEC, 2000).

OGFCs are designed to havevoids that allow water to drainfrom the roadway surface. Theseare often used in western andsouthern regions of the UnitedStates to improve wet-weatherfriction. Normally the mixturesare designed to have about15 percent air voids, but it hasbeen reported that void levelsapproaching 18 percent to22 percent provide better long-term performance (Huber, 2000).Fibers are sometimes used tohelp resist draindown of theasphalt during construction.Huber (2000) also reports thatthe use of a polymer-modifiedasphalt will help in providinglong-term performance. A mixdesign method for OGFC hasbeen developed by Kandhal andMallick (1999) using the Super-pave Gyratory Compactor.Guidance regarding the construc-tion and maintenance of OGFCsurfaces is found in Kandhal(2001).

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Performance GoalsTo maintain a Perpetual Pavementand ensure that it performs to itspotential, it is necessary to monitorthe pavement condition periodi-cally in order to keep all forms ofdistress in the top of the pave-ment. Thus, distresses such astop-down fatigue cracking, thermalcracking, rutting, and surface wearmust be confined to no deeperthan the original thickness of thewearing course. Once the dis-tresses have reached a pre-determined level, the resurfacingwould be programmed, and anevaluation of the pavementstructure would be undertaken.There are a number of studiesthat support the idea that thick,well-constructed asphalt pave-ments have distresses confinedto their surfaces.

A Dutch study (Schmorak andVan Dommelen, 1995) of 176pavement sections showed thatsurface cracking occurred inasphalt structures thicker than

160 mm, with cracks extendingabout 100 mm down into theasphalt layer. They concludedthat conventional fatigue failurewas very improbable and thatsurface cracking would be themain form of distress in thickasphalt pavements.

A 1997 report from the UnitedKingdom (Lesch and Nunn, 1997)showed that pavement deteriora-tion in thick asphalt structures wasmuch more likely to occur in thewearing course than deep in thepavement. This paper also dem-onstrated that the structural layersbecome stronger with time,instead of weakening as iscommonly assumed.

In a case study representativeof good-performing pavements, arecent review of thick (between160 and 475 mm) asphalt pave-ments on I-90 through the stateof Washington revealed that noneof these sections had ever beenrebuilt for structural reasons

(Baker and Mahoney, 2000).The pavement ages ranged from23 to 35 years, and thick asphaltpavements on this route comprise40 percent of the length (about225 out of 580 km). West of theCascade Mountains, near Seattle,the average age at resurfacingwas 18.5 years. On the easternside of the state, the average ageat first resurfacing was 12.4 yearsand the average time until secondresurfacing was 12.2 years.

The New Jersey DOT recentlyinvestigated distresses thatdeveloped on a 26-year-oldpavement surface on I-287 (Fee,2001). The structure was a 250-mm-thick asphalt pavement thathad received a minimum ofmaintenance. The surface showedfatigue cracking, longitudinalcracking in the wheelpaths, andruts deeper than 25 mm. A de-tailed examination of the pave-ment structure showed that noneof the distresses extended morethan 75 mm into the depth of theasphalt. As a result, the decisionwas made to mill off the top 75mm and replace it with a total of100 mm of HMA. This work wasdone in 1994, and a pavementsurvey done in 2001 showed nosigns of cracking or rutting.

In the event that certain charac-teristics may have changed, suchas a weakening of the underlyingsoil through increased moisturecontent, a slight additional thick-ness may be planned for theresurfacing to ensure the per-petual nature of the structure.The performance goal, however,is to minimize the amount ofadditional thickness required infuture overlays.

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Current Perpetual Pavement EffortsA number of cooperative efforts todevelop and implement PerpetualPavement are currently under wayin various states. California, Illinois,Michigan, Texas, Wisconsin,Kentucky, Ohio, Virginia, and theUnited Kingdom are among thosein the process of devising designsand specifications for the construc-tion of Perpetual Pavement.

CaliforniaAt this writing, California is

constructing a long-life asphaltpavement on Interstate 710 inLos Angeles County. Known asthe Long Beach Freeway, thisroad has a projected design lanetraffic of 100 million to 200 millionequivalent single axle loads(ESALs) for a 40-year period.The existing pavement is 200 mmof concrete, over 100 mm ofcement-treated material, over100 mm of aggregate base, over200 mm of subbase material. Theplans call for most of the concretepavement to be cracked andseated and overlaid with 200 mmof HMA (Monismith and Long,1999b), while the concrete pave-ment and cement-treated materialunder the bridges will be removedand replaced with 300 mm of HMA.A 25-mm Open Graded FrictionCourse (OGFC) will be placedover the entire length of the project(Monismith and Long, 1999a).

As shown in Figure 7, the full-depth asphalt section is to be atotal of 300 mm thick and willhave a fatigue-resistant 75-mmbottom layer in which the asphaltcontent will be raised by 0.5percent over optimum to 5.2percent. This increased binder

content can raise the fatigue life ofHMA by up to an order of magni-tude (Harvey, et al., 1997). Theintermediate 150 mm will beconstructed with the same aggre-gate gradation and binder as thebottom layer, but the asphaltcontent will be 4.7 percent. Theuse of a stiff asphalt (AR-8000)in the intermediate layer will helpguard against rutting. The upper75 mm of the pavement structurewill be constructed using a poly-mer-modified binder PBA-6A, andthis will be below a 25-mm OGFC.In tests using the California Accel-erated Pavement Test HeavyVehicle Simulator, this material wasfound to have less than half therutting of other asphalt mixtures.

The HMA overlay for the crackedand seated concrete is to be a totalof 200 mm thick, and will not havethe fatigue-resistant bottom layer.The cracked and seated concreteshould provide a stiff foundationfor the asphalt and prevent theexcessive bending associated

with bottom-up fatigue cracking.An asphalt-saturated fabric willbe placed over a 25-mm levelingcourse on top of the concrete toguard against reflective cracking.Other than this, the materials tobe used in the overlay are thesame as those planned for thefull-depth pavement. As with thefull-depth section, a 25-mmOGFC will be placed on top.

Laboratory testing of theasphalt mixtures included Hveemstabilometer at 60 oC, repeatedload simple shear (constantheight) at 50 oC and 60 oC,flexural fatigue tests at 10 oC,20 oC, and 30 oC. Reliability wasincorporated into the mixturedesign by requiring that theperformance of the material inlaboratory testing surpass theexpected demand of the materialin the field. This was done bymultiplying the expected numberof ESALs by a reliability factor,shift factor, and temperatureconversion factor.

Figure 7

California I-710 Sections

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The structural adequacy of thefull-depth pavement sections waschecked by limiting the bendingstrain in the HMA to less than70 and the vertical strain at thetop of the subgrade to less than200 , under an 80-kN singleaxle. The shear strain near theHMA surface was investigated toensure that rutting did not occurdue to the HMA. The shear strainto produce 5 percent permanentdeformation was over 6 timesthat computed for hot-weatherconditions in the field.

Construction of this pavementbegan in the summer of 2001 andis scheduled to be completed bythe summer of 2002.

IllinoisIn Illinois, the term Extended

Life HMA Pavement is used todenote Perpetual Pavement.The effort in this case was todevelop a methodology coveringstructural design, materialsselection, and construction.A panel of IDOT engineers,researchers, contractors, asphaltsuppliers, and national specialistsgathered to provide input to thedevelopment of the procedure.A draft document providingdetails of the Extended Life HMAPavement was prepared inDecember 2000, and a final ver-sion should be complete by 2002.

IDOT has decided to use itsown mechanistic-empirical ap-proach to pavement thicknessdesign. The typical pavementdesign in Illinois results in aHMA layer bending strain of lessthan 60 , because the fatiguecriterion in the Illinois procedureis conservative. As a result, nochanges were made to the IDOTmethod of pavement design.

The approach to designing themix for the bottom fatigue-resistantlayer is slightly different fromCalifornia's. In Illinois, the asphaltcontent to achieve an air void levelof 2.5 percent at the design num-ber of gyrations was the bench-mark set to obtain a binder-richmixture. The binder and aggregategradation would be the same ascurrently used in Illinois in theirapproach to Superpave. It isplanned that this bottom HMAlayer would be constructed inone 100-mm lift.

The intermediate asphalt layerwould be constructed using thesame binder and mixture specifi-cations currently used for IDOT'sdense-graded mixtures. The levelof gyrations for design would beset by the requirements for trafficduring a 20-year period.

In order to achieve a 20-yearlife in the renewable surface, theIllinois group decided to use StoneMatrix Asphalt (SMA) for the toplayer. The appropriate thickness ofSMA was determined by analyzingshear stresses near the surfaceand reviewing performance relativeto rutting and cracking. Most ofthe problems associated withdistresses were found to lie in thetop 100 mm of the pavement.Thus, for an extended-life HMApavement, the thickness of theSMA surface was associated withthe expected traffic. For low trafficlevels, the SMA thickness is 50mm, and for medium traffic, it is100 mm. One hundred-fifty mm ofSMA is used for high and veryhigh traffic levels. Although exactdefinitions for these traffic levelshave yet to be determined, it isexpected that the high traffic levelwill start at about 25 million ESAL.

In all pavements, the top 150mm of Hot Mix, regardless of mix

type, will contain a polymer-modified binder. This is considerednecessary to avoid thermal andload-induced cracking in thesurface. The PG binder grade tobe used in the structure is thatrequired in full-depth asphaltpavements in northern Illinois.Hydrated lime is required in allmixtures used throughout thepavement structure to preventmoisture damage.

The pavement foundationrequirements are currently beingreviewed, and two approachesare being considered. In oneapproach, the standard IDOTmethod of lime modifying the soilfor a depth of 300 mm underneatha full-depth pavement is proposed.In the other, a 300-mm granularsubbase would be used underthe pavement. Additionally, theissue of whether to require longitu-dinal underdrains at the side ofthe road is being discussed withrespect to structural benefits.

Controls on construction weredevised to help ensure the durabil-ity of the pavement. These include:

• Specifying a lift thickness of 3to 6 times the nominal maxi-mum aggregate size to facilitatecompaction.

• Requiring positive dust controlon the HMA plant.

• Requiring a polymer-modifiedtack coat between eachHMA lift.

• Requiring remixing of materialsduring laydown of all lifts.

• Revising density testing require-ments to ensure uniform densityacross the mat.

• Revising density requirementsfor the intermediate layer toobtain 93 percent of theoreticalmaximum density.

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20-Year Traffic Level, ESAL X 10 6 3 10 20 30

Total HMA Thickness, mm 290 345 370 405

SMA Thickness, mm — — 65 65

Superpave Thickness, mm 50 50 — —

Binder Course, mm 115 90 140 11 140 125 150

Base Course, mm 125 150 155 180 165 180 190

Aggregate Base, mm — — 330 430

Aggregate Subbase, mm 380 250 — —

Non-Frost Susceptible Soils, mm 345 315 220 200

Rehabilitation 1 Year 20 15 15 15

Mill-Overlay, mm 50–50 50–100 65–115 65–115

Rehabilitation 2 Year 32 30 30 30

Mill-Overlay, mm 50-50 50-50 50-50 50-75

• Longitudinal joint requirements,including:

– Use of screed extensions andgood paving practices.

– Density tests at two feet fromthe joint.

– Use of a polymer-modified tackcoat on vertical joint faces.

MichiganUnder a contract with the

Michigan Asphalt Paving Associa-tion Inc. (MAPA), Fugro-BRE Inc.developed a catalog of structuralsections for use as PerpetualPavement (Von Quintus, 2001aand 2001b). Von Quintus choseto use a mechanistic approachemploying the ELSYM5 computerprogram to calculate stresses andstrains in the pavement structure.This approach applied the conceptof cumulative damage to deter-mine the appropriate section fora design period of up to 40 years.Von Quintus used this methodol-ogy, in the absence of otherapproaches, as a way of deter-mining a reasonable rangeof pavement thicknesses forPerpetual Pavements.

The catalog of pavementdesigns is presented in Table 5where the structural sections arelisted according to the traffic levelsexpected in the first 20 years ofservice. The pavement designsand rehabilitation strategies listedat the bottom are intended for a40-year period.

The pavement foundation in theMichigan procedure consists ofone meter of non-frost-susceptiblematerial under crushed aggregatesubbase at the 3 million and10 million ESAL levels (20-year).The higher traffic levels of 20 and30 million ESAL (20-year) havea crushed stone base course.

Table 5 Design Catalog of Michigan Perpetual Pavement Sections (Von Quintus, 2001a)

Table 5 gives a suggested guideto the types of HMA mixtures tobe placed within the pavementstructure. The total HMA thicknessranges from 290 to 405 mm forthe four traffic levels presented.Von Quintus recommends thatthe asphalt mixture for the HMAbase layer be designed to have3 percent air voids to mitigatebottom-up fatigue cracking. Thesurface course mixture is a dense-graded Superpave in the case ofthe 3 million and 10 million ESALlevels, and an SMA in the case ofthe 20 million and 30 million ESALlevels (20-year).

For rehabilitation strategies,Von Quintus suggests a straightmill and fill for the lowest level oftraffic and mill and strengthening

for the higher levels of traffic.The need for strengthening agiven pavement should be investi-gated at the time of rehabilitation.The strategies presented in Table5 are for planning purposes only.

Von Quintus (2001b) wentthrough a similar process todevelop a Perpetual Pavementdesign for rubblized concretepavements. It can be seen inTable 6 that the total HMA thick-ness for a 40-year analysis periodranges from 215 to 370 mmdepending upon the 20-year trafficprojection. Again, use of a richbase mixture is recommendedalong with specifying an SMAsurface course at the two highesttraffic levels.

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WisconsinThe state of Wisconsin is

planning the construction of twotest sites for Perpetual Pavement.1

Five test sections were built onState Trunk Highway 50 in 2000,and two test sections will beconstructed at a truck weighstation near Kenosha in 2002. Thisis a cooperative effort between theWisconsin DOT (WisDOT) andthe Wisconsin Asphalt PavementAssociation (WAPA).

Table 6 Design Catalog of Michigan Perpetual Pavement Sections Over Rubblized Concrete (Von Quintus, 2001b)

Design 20-Year Traffic Level, 3 10 20 30Period, ESAL X 10 6

Years




20 Binder Course, mm 100 50 75 75

Base Course, mm — 115 130 150

Rehab. Year 20, Mill/Replace, mm 65/130 65/130 65/130 65/130






Base Course, mm 75 130 165 190







Base Course, mm 100 140 165 205



The design lane ESAL for theLake Geneva road is about 2 mil-lion for a 20-year period. ThreePerpetual Pavement sections willbe placed, along with two conven-tional pavement sections. The sec-tions differ primarily in the bindergrades and density requirements asshown in Table 7. There will be twocontrol sections, reflecting normalWisconsin construction procedures.All pavements in the Lake Genevasite will rest on 100 mm of an open-graded base course over 200 mmof a crushed stone base.

The truck station sections willbe subjected to about 75 millionESALs over a design life of 20years. The structure below theasphalt layers will consist of 100mm of an open-graded basecourse over 430 mm of crushedaggregate base course. The trafficconditions will be somewhatworse here than on a mainlinehighway because the truck trafficwill be channelized and moving atslow speed, increasing the poten-tial for rutting.

1 Personal communication with GeraldWaelti, Wisconsin Asphalt PavementAssociation, July 20, 2001.

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Test Site Test Section Layer Thickness, Asphalt In-Place Voidmm Grade Content, %

Perpetual Surface 50 58-28 6Pavement Binder 90 64-22 6(University Base 90 64-22 4 of Illinois) Total 230

Perpetual Surface 50 64-28 6Pavement Binder 90 58-28 6(WisDOT) Base 90 58-28 4

Total 230

Perpetual Surface 50 58-28 6Pavement Binder 90 70-22 6(WAPA) Base 90 70-22 4

Total 230

Standard Surface 50 58-28Pavement Binder 90 58-28

(9-in. Control) Base 90 58-28Total 230

Standard Surface 40 58-28Pavement Binder 70 58-28

(7-in. Control) Base 70 58-28Total 180

Surface 50 76-281 Binder 130 70-22

Base 100 58-28Total 230

Surface 50 70-282 Binder 130 70-22

Base 100 64-22Total 230

LakeGeneva

To BeDeterminedTruck

Station

TexasConstruction of a Perpetual

Pavement on I-35 near Waco,Texas, began in August 2001.2

This project was a joint effortbetween the Texas Department ofTransportation (TxDOT), the TexasHot Mix Asphalt Pavement Associa-tion (TxHMAPA), and the AsphaltInstitute. The existing pavement isbeing removed and replaced dueto moisture damage in the lowerasphalt layers.

Table 7 Wisconsin Perpetual Pavement Test Sections

One of the important factorsconsidered in the selection ofmaterials and the pavementdesign is the presence of highlyexpansive clays extending to4.6 m over a limestone bedrock.It is essential to limit changes inmoisture content for these materi-als as that will minimize seasonalmovements in the pavement. Itwas decided that an HMA mixturewith low permeability would berequired to address this problem.

So, an HMA subbase with2 percent design air voids wasselected. It was also thoughtthat this binder-rich layer wouldalso help to preclude crackingduring the early stages of thepavement's life.

The traffic level over a20-year period is expected tobe 48 million ESAL, and thesubgrade is classified as claywith a modulus of 82.7 MPa.The total pavement structure,not including an OGFC sur-face, is about 480 mm. Thelayers include 50 mm of SMAbelow the OGFC, over 80 mmof a19-mm Superpave mixture,over 250 mm of a 25-mmSuperpave mixture, with a100-mm impermeable HMA atthe bottom to help maintain aconstant moisture content inthe underlying clay. Perfor-mance testing for the mixtureswill include rut testing in theHamburg and Asphalt Pave-ment Analyzer devices, Super-pave Shear Tester, andComplex Modulus. In-situ test-ing of the pavement will includedynamic cone penetrometer,falling weight deflectometer,ground penetrating radar,portable seismic analyzer,and profile measurements.

KentuckyKentucky has a history of

using thick asphalt pavementsfor new construction andrehabilitation that result in longservice lives.3 The KentuckyTransportation Cabinet (KTC)currently uses a mechanistic-empirical design method thatusually results in relativelythick asphalt pavements.

2 Personal communication with Gary Fitts, Asphalt Institute, August 29, 2001.

3 Personal communication with BrianWood, The Plantmix Asphalt Industryof Kentucky, December 14, 2001.

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They often include about 100 mmof dense-graded aggregate (DGA)resting on clayey soils (sometimeschemically modified) beneath theHMA. In rehabilitation projects,broken and seated concretepavements are often overlaid witha thick layer of asphalt.

Mechanistic analyses haveshown that Kentucky's pavementsections have resulted in strainsbelow the 70 levels specified inCalifornia. Recently, KTC has letseveral projects where contractorscould bid alternate pavementtypes. As a result, KTC decidedto base the designs for theseprojects on 40-year traffic levels.

In 2000, KTC advertised analternate bid project on Interstate64 in Louisville which included therehabilitation of 5.3 km (two lanesin each direction) of Interstatepavement. The roadway carriesabout 100,000 vehicles per daywith about 10 percent truck traffic.A mechanistic-empirical designprocess concluded that a 280-mmasphalt overlay would result inless than 70 . At the same time,KTC found that a 280-mm asphaltsection matched their designassuming 1400 MPa for thebroken concrete pavement over-lay. The successful bidder chosethe 280-mm asphalt pavement.The asphalt section consisted ofdense-graded Superpave mix-tures with polymer-modifiedasphalt (PG 76-22) in the upperlayers.

A similar project on Interstate65 was recently let which allowedalternate bids on the rehabilitationand widening of an existingconcrete pavement near BowlingGreen. The successful bidderselected a design that includeda 280-mm HMA overlay of thebroken and seated concrete and

a 380-mm HMA section for thewidened portion of the road. Therehabilitation strategy assumed byKTC in their life cycle cost analy-sis (which includes a 90-mmoverlay in year 20) was amendedto include only shallow (40 mm)mill and overlay throughout the40-year analysis period. Thisrehabilitation strategy is consis-tent with the Perpetual Pavementconcept in that no additionalstructure will be needed duringthe design period. The mixturesspecified in the proposal are alsodense-graded Superpave mix-tures utilizing PG 76-22 in theupper layers. The KTC electednot to use the fatigue layer con-cept in either the I-64 or the I-65projects. As in California, thepresence of the stiff underlyingconcrete layer would most likelypreclude excessive bending in theHMA layer.

Ohio and VirginiaThese states have recently

begun devising their approachesto the design, specification, andconstruction of Perpetual Pave-ments.

The Ohio Department ofTransportation (ODOT) andFlexible Pavements of Ohio haveformed a Perpetual PavementCommittee to explore the designand construction of long-lifeasphalt pavements. The commit-tee is comprised of ODOT person-nel, Flexible Pavements staff,academia, and consultants.Subcommittees involved in thiseffort are addressing concerns inthe areas of design, specifica-tions, testing, and project selec-tion. The objective is to identify aproject to test the feasibility ofconstructing a Perpetual Pave-ment in the next year.

The Virginia DOT is developingits concepts on Perpetual Pave-ment based upon input providedby industry, represented by theVirginia Asphalt Association andthe Asphalt Institute, and by theVirginia Transportation ResearchCouncil. Issues being discussedinclude pavement design, materi-als selection, and life cycle costanalysis.

United KingdomPast practice in the United

Kingdom was to design a flexiblepavement structure with a life of40 years, and with a plannedstructural overlay at 20 years.Recent evidence shows it wouldbe more cost-effective, especiallyfrom a user-delay standpoint, todesign and build the structureadequate for the 40 years initially(Nunn et al., 1997). Thus, onlyperiodic milling and surfacerestoration is needed.

The structural section for thePerpetual Pavement in the UnitedKingdom includes the use ofgranular base and subbase layersbelow a thick asphalt pavement.The thickness of the asphalt issuch that traditional bottom-upfatigue cracking and structuralrutting are avoided. Nunn and hisassociates have found thatpavements having a total asphaltthickness of less than 180 mm areprone to structural rutting, whilethe rutting in thicker pavements isconfined to the top of the struc-ture. Rutting occurs mainly in thetop 100 mm of thick asphalt roadsin the United Kingdom. The TRLapproach allows for an adjustmentin asphalt thickness according tothe type of mix and stiffness of thebinder. The standard densebitumen macadam base uses a100-penetration asphalt binder.

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Using increasingly stiff bindersallows for the design of thinnersections according to the Britishapproach. However, the Britishresearchers placed an upper limiton asphalt thickness based uponobserved distresses. Studies ofthe performance of British roadsshow that additional pavementthickness, beyond that required for80 million ESAL, would notprovide additional benefit asshown in Figure 8. Nunn and hisassociates state that the fourthpower law, traditionally used fordescribing the relationship be-tween pavement damage and axleloads, is not appropriate for thickasphalt pavements.

Summarycalculate strains in pavementstructures so that, for a givencombination of materials, theoptimum pavement structure thatwill not require reconstruction maybe defined. Design criteria need tobe developed for different mixturecharacteristics. Work on this hasbegun in Illinois, California, andthe United Kingdom.

The pavement foundation willplay a key role in defining thequality of construction and thelong-term performance of Per-petual Pavements. Guidance oncriteria to use for pavementfoundations during design isavailable from the TransportResearch Laboratory (TRL) in theUnited Kingdom, the LaboratoireCentral de Ponts et Chasses(LCPC) in France, and the IllinoisDOT. Seasonal adjustments forconsidering the behavior of thepavement during its performance

Figure 8

TRRL Design Curve (Nunn et al., 1997)

The concept of the PerpetualPavement has been establishedby the performance of well-constructed, thick asphalt pave-ments. Although designed to becomparable in performance withmore conventional flexible pave-ments, deep-strength and full-depth asphalt sections have beenshown to confine distresses to theupper pavement layers. Thisallows for periodic removal of thesurface layer and replacementwith an HMA overlay, minimizingrehabilitation costs and userinconvenience. The pavementmaterial removed from the surfaceis recycled, conserving naturalresources.

A new approach to design isneeded to recognize that there isa point beyond which additionalthicknesses of HMA offer verylittle return on investment. Mecha-nistic methods offer a way to

period have been developed by anumber of agencies including theWashington State DOT, theMinnesota DOT, and TRL. Acombination of in-situ stiffnessand density measurements shouldbe made during construction toensure a good foundation for thepavement.

The Perpetual Pavement offersengineers the ability to design forspecific modes of distress in theHMA materials. Resistance tobottom-up fatigue cracking isprovided by the lowest asphaltlayer having a higher bindercontent or by the total thickness ofpavement reducing the tensilestrains in this layer to an insignifi-cant level. The intermediate layerprovides rutting resistancethrough stone-on-stone contactand the durability is imparted bythe proper selection of materials.The uppermost structural layer

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resists rutting, weathering, ther-mal cracking, and wear. SMAs ordense-graded Superpave mix-tures provide these qualities.

A number of agencies havestarted investigating and imple-menting Perpetual Pavementconcepts. California is in theprocess of building such a pave-ment on the Long Beach Freeway(I-710). Illinois is developing amethodology for extended-lifepavements and has set a numberof criteria pertaining to construc-tion. The Michigan Asphalt PavingAssociation Inc. has proposed adesign catalog for long-life pave-

ments, both in new design and inthe rehabilitation of concretepavements. The Wisconsin DOT isworking with its industry partnersto build test sections in that state.The Texas DOT is in the processof building a Perpetual Pavementon I-35 near Waco, utilizing thebest available technology in thematerials selection and mixdesign process. The KentuckyTransportation Cabinet is findingthat their existing design proce-dure results in long-lasting HMApavements. The states of Ohioand Virginia are at the beginningof a process in developing their

approaches to Perpetual Pave-ments. The Transport ResearchLaboratory in the United Kingdomhas put forward a well-docu-mented approach to the design oflong-lasting pavements.

The next step should be thedevelopment of national guide-lines and procedures to givepavement engineers the tools forsuccessfully designing andconstructing Perpetual Pave-ments. These guidelines shouldaddress the rehabilitation offlexible and rigid pavements inaddition to new or reconstructedpavements.

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American Association of State Highway and Transporta-tion Officials (1997) Segregation: Causes and Curesfor Hot Mix Asphalt, Washington, DC.

American Association of State Highway and Transporta-tion Officials (2000) Standard Specifications for Trans-portation Materials and Methods of Sampling and Test-ing, Part II – Tests, 20th Ed., Washington, DC.

American Association of State Highway and Transporta-tion Officials (2001) AASHTO Provisional Standards,Interim Edition, Washington, DC.

Asphalt Institute (1996a) Performance Graded Asphalt,SP-1, Lexington, Kentucky.

Asphalt Institute (1996b) Superpave Mix Design, SP-2,Lexington, Kentucky.

Asphalt Institute (1981) Thickness Design – Asphalt Pave-ments for Highways and Streets, MS-1, Lexington,Kentucky.

Asphalt Paving Environmental Council (APEC) (2000) BestManagement Practices to Minimize Emissions DuringHMA Construction, Report No. EC-101, NationalAsphalt Pavement Association, Lanham, Maryland.

Baker, M.J. and J.P. Mahoney (2000) Identification andAssessment of Washington State Pavements withSuperior and Inferior Performance, Report No. WA-RD437.1, Washington State Department of Transporta-tion, Olympia.

Brown, E.R. (1993) Experience with Stone Mastic Asphaltin the United States, Report No. 93-4, National Centerfor Asphalt Technology, Auburn University, Alabama.

Brown, E.R. and L.A. Cooley, Jr. (1999) Designing StoneMatrix Asphalt Mixtures for Rut-Resistant Pavements,Report No. 425, National Cooperative Highway Re-search Program, Transportation Research Board,Washington, DC.

Brown, E. Ray, Prithvi S. Kandhal, and Jingna Zhang(November, 2001) Performance Testing for Hot MixAsphalt, Report No. 2001-05, National Center forAsphalt Technology, Auburn University, Alabama.

Epps, J.A. and C.L. Monismith (1972) “Fatigue of AsphaltConcrete Mixtures – Summary of Existing Information,”STP 508, American Society for Testing and Materials,Conshohocken, PA.

European Asphalt Pavement Association (EAPA) (1999)“High Modulus Asphalt,” Breukelen, The Netherlands.

Federal Ministry of Transport (1989) Guidelines for the Stan-dardization of the Upper Structure of Traffic BearingSurfaces, RS+O 86, Bonn, Germany.

Fee, F., (July-August, 2001), “Extended-Life Asphalt Pave-ment,” TR News, No. 215, Transportation ResearchBoard, Washington, DC.

References

Gomez, M. and M.R. Thompson (1984) Mechanistic De-sign Concepts for Full Depth Asphalt Concrete Pave-ments, Transportation Engineering Series No. 41, CivilEngineering Studies, Department of Civil Engineering,University of Illinois at Urbana-Champaign.

Harvey, John T., John A. Deacon, Akhtar A. Taybali, Rita B.Leahy and Carl L. Monismith (1997) A Reliability-BasedMix Design and Analysis System for Mitigating FatigueDistress, Proceedings, Eighth International Conferenceon Asphalt Pavements, University of Washington,Seattle.

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Mission Statement

The Asphalt Pavement Alliance is a coalition of the

Asphalt Institute, the National Asphalt Pavement Associa-tion, and the State Asphalt Pavement Associations.

The Asphalt Pavement Alliance's mission is to further

the use and quality of Hot Mix Asphalt pavements.

The Alliance will accomplish this through research,technology transfer, engineering, education, and innovation.


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APA 101 cvr - Department of Civil Engineering - Civil ... Pavement...Perpetual Pavements are con-fined to the surface, and that deep structural problems are eliminated or minimized