Session 3-6Session 3-6

HMA OverlaysHMA Overlays

Learning ObjectivesLearning Objectives

Describe the characteristics of typical Describe the characteristics of typical Hot-Mix Asphalt (HMA) overlaysHot-Mix Asphalt (HMA) overlays

Identify best applications Identify best applications

Describe preoverlay repair need and Describe preoverlay repair need and feasibilityfeasibility

Describe thickness design approachDescribe thickness design approach

Describe key construction issuesDescribe key construction issues

IntroductionIntroduction Most popular methodMost popular method

Relatively fast and cost-effective means Relatively fast and cost-effective means for:for: Correcting deficienciesCorrecting deficiencies Restoring user satisfactionRestoring user satisfaction Adding structural capacityAdding structural capacity

Poor performance is NOT uncommonPoor performance is NOT uncommon

DefinitionsDefinitions Functional performance - Ability to Functional performance - Ability to

provide a safe, smooth riding surfaceprovide a safe, smooth riding surface

Structural performance - Ability to carry Structural performance - Ability to carry traffic without distresstraffic without distress

Empirical - Design based on past Empirical - Design based on past experience or observationexperience or observation

Mechanistic - Design based upon Mechanistic - Design based upon engineering mechanicsengineering mechanics

Improve functional and/or structural Improve functional and/or structural characteristicscharacteristics

Wide range of applicationsWide range of applications Road surface categoriesRoad surface categories Climate and support conditionsClimate and support conditions

Purpose and ApplicationsPurpose and Applications

Characteristics of Typical Characteristics of Typical HMA OverlayHMA Overlay

Dense graded HMADense graded HMA

Flexible or rigid surfaceFlexible or rigid surface

25 to 200 mm (1 to 8 in) thickness25 to 200 mm (1 to 8 in) thickness

Mill and FillMill and Fill

Best ApplicationsBest Applications

What is the best What is the best application for HMA application for HMA

overlays?overlays?

Limitations and Limitations and EffectivenessEffectiveness

Why do we have premature failures?Why do we have premature failures?

Improper selectionImproper selection

Wrong typeWrong type

Inadequate designInadequate design

Insufficient preoverlay repairInsufficient preoverlay repair

Lack of consideration of reflection Lack of consideration of reflection crackingcracking

Limitations and Limitations and EffectivenessEffectiveness

What limits the effectiveness of HMA What limits the effectiveness of HMA overlays?overlays?

Distress exhibited in HMADistress exhibited in HMA

Intended design life of the overlayIntended design life of the overlay

Availability of quality materialsAvailability of quality materials

Limitations and Limitations and EffectivenessEffectiveness

How can we improve our overlays?How can we improve our overlays?

Preoverlay treatmentsPreoverlay treatments

Better materials and practicesBetter materials and practices

Sound engineering judgmentSound engineering judgment

Thin Overlay Thick Overlay

Surface Defects Structural Defects

Overlay Selection to Overlay Selection to Correct DeficienciesCorrect Deficiencies

What Are Considerations What Are Considerations in Overlay Selection?in Overlay Selection?

Construction feasibilityConstruction feasibility Traffic controlTraffic control ConstructibilityConstructibility Vertical clearancesVertical clearances UtilitiesUtilities

Performance periodPerformance period

FundingFunding

Preoverlay Treatment and Preoverlay Treatment and RepairRepair

Dependent upon:Dependent upon: Type of overlayType of overlay Structural adequacy of existing Structural adequacy of existing

pavementpavement Existing types of distressExisting types of distress Future trafficFuture traffic Physical constraintsPhysical constraints CostCost

To Repair or Not to To Repair or Not to Repair?Repair?

Types of Preoverlay Types of Preoverlay TreatmentsTreatments

Localized repair (patching)Localized repair (patching)

Surface levelingSurface leveling

Controlling reflection crackingControlling reflection cracking

Drainage improvementsDrainage improvements

% Area repaired

Repaircost

Overlaycost

Localized Repair Localized Repair

% of Area Repaired

Total Cost

Minimum Total CostOptimum

Localized Repair Localized Repair

Surface Leveling Surface Leveling PurposePurpose

Rut fillingRut filling Restore cross slopeRestore cross slope Improve longitudinal profileImprove longitudinal profile

MethodMethod Cold millingCold milling Leveling course Leveling course

Controlling Reflection Controlling Reflection Cracking Cracking

What do you use to control reflective What do you use to control reflective cracking?cracking?

Geotextiles or fabrics Geotextiles or fabrics

Stress relieving or stress absorbing Stress relieving or stress absorbing membrane interlayersmembrane interlayers

““Band aid” type crack sealantsBand aid” type crack sealants

Reflective Crack Reflective Crack Treatments Treatments

Geotextile: varied results on its Geotextile: varied results on its effectivenesseffectiveness

SAMI: varied results alsoSAMI: varied results also

Aggregate Interlayers: effective when Aggregate Interlayers: effective when designed correctlydesigned correctly

Route and seal: does not prevent but Route and seal: does not prevent but effective at limiting deteriorationeffective at limiting deterioration

Drainage Corrections Drainage Corrections Drainage surveyDrainage survey

Identify moisture / drainage related Identify moisture / drainage related distressesdistresses

Develop solutions that address moisture Develop solutions that address moisture problemsproblems

Two Aspects of Two Aspects of Overlay DesignOverlay Design

Asphalt mixtureAsphalt mixture Fatigue crackingFatigue cracking Permanent Permanent

deformationdeformation Thermal Thermal

crackingcracking Moisture Moisture

susceptibilitysusceptibility

Overlay ThicknessOverlay Thickness Engineering Engineering

judgmentjudgment Deflection Deflection

approachapproach Structural Structural

deficiencydeficiency Mechanistic Mechanistic

approachapproach

Mix DesignMix DesignSuperpaveSuperpave

Goal – integrate mixture/structural designGoal – integrate mixture/structural design Performance Graded (PG) asphalt Performance Graded (PG) asphalt

cementscements Mix designMix design

MaterialsMaterials Aggregate structureAggregate structure Binder contentBinder content Moisture susceptibilityMoisture susceptibility

50 100 150 2000 250

0.0

0.4

0.8

1.2

1.6

2.0

Overlay Thickness, THov (mm)

Def

lect

ion

,

(

mm

)

Limiting

Deflection

Original SurfaceDeflection

HMA Overlay

HMA Layer

Granular Layer

Natural Soil

Simulated 80-kN Axle Load

THov

150 mm

75 mm

Structural DesignStructural DesignDeflection ApproachDeflection Approach

TrafficTraffic

PSIPSI

2.52.5

1.51.5

OverlayOverlay

SCSC

SCSC

SCSC

SCSC

effeff

ooLL

ff

Structural Design Structural Design Structural Deficiency Structural Deficiency

SCSCOL OL = SC= SCf f – SC– SCeffeff

Problem: Determining SCProblem: Determining SCeffeff

Distress surveyDistress survey Remaining lifeRemaining life Deflection testingDeflection testing

Structural DesignStructural DesignAASHTO ApproachAASHTO Approach

Natural Soil

Subbase

Base

OriginalHMA SurfaceLayer

HMA OverlayTH

TH

TH

TH

E

E

E

E

1

2

3

4

1

2

3

4

Thickness Young’s Modulus

ov

HMA

SimulatedDesign Axle Load

σV

Structural Design Structural Design Mechanistic-EmpiricalMechanistic-Empirical

Critical Stress or Strain

AllowableTraffic

RuttingFatigue

Structural Design Structural Design Mechanistic-Empirical Mechanistic-Empirical

HMA Overlay Thickness, THov (mm)

Allo

wab

le A

xle

Load

App

licat

ions

(8

0 -

kN E

SA

Ls,

mill

ions

)Simulated 80 - kN Axle Load

THov

THhma

THbg

ov HMA overlay

BaseNatural soil

Design HMAOverlay Thickness

25 50 75 125100 150 175

1

2

3

4

Design 80-kN ESALS, W

Original HMA layer (fatigued)

80

Structural DesignStructural DesignMechanistic-EmpiricalMechanistic-Empirical

1

2

3

Distance Along Roadway

Des

ign

Ove

rlay

Th

ickn

ess

Structural Requirement Structural Requirement Varies Along RoadwayVaries Along Roadway

Key Construction IssuesKey Construction Issues

Lift ThicknessLift Thickness Old – 2X maximum aggregate sizeOld – 2X maximum aggregate size

New – 3X nominal maximum aggregate New – 3X nominal maximum aggregate sizesize

Lift thickness versus mix designLift thickness versus mix design

CompactionCompaction

ConsequencesConsequences Rutting Rutting RavelingRaveling OxidationOxidation StrippingStripping

KeysKeys Proper Proper

equipmentequipment Roller Roller

immediately immediately behind paverbehind paver

Newer mixes more difficult to achieve densityNewer mixes more difficult to achieve density

Ride QualityRide Quality Public’s No. 1 issuePublic’s No. 1 issue Ride specificationRide specification

Method of measurementMethod of measurement Incentive/disincentiveIncentive/disincentive

Increased pavement Increased pavement performanceperformance

Key – continuous, steady Key – continuous, steady operationoperation

ReviewReview What are the characteristics of an HMA What are the characteristics of an HMA

overlay?overlay?

Where are HMA overlays applicable? Where are HMA overlays applicable?

What types of pre-overlay repair should What types of pre-overlay repair should be considered?be considered?

Name three structural design Name three structural design approaches?approaches?

What are some of the key construction What are some of the key construction issues?issues?

Key ReferencesKey References Daleiden, J. F., A. Simpson, and J. B. Rahut. Daleiden, J. F., A. Simpson, and J. B. Rahut.

1998. 1998. Rehabilitation Performance Trends: Rehabilitation Performance Trends: Early Observation from Long-Term Pavement Early Observation from Long-Term Pavement Performance (LTPP) Specific Pavement Performance (LTPP) Specific Pavement Studies (SPS). Studies (SPS). FHWA-RD-97-099. Federal FHWA-RD-97-099. Federal Highway Administration, Washington, DC.Highway Administration, Washington, DC.

Brown, E. R. 1997. Brown, E. R. 1997. Superpave Construction Superpave Construction Guidelines. Guidelines. Special Report 180. National Special Report 180. National Asphalt Pavement Association, Lanham, MD.Asphalt Pavement Association, Lanham, MD.

Key References Key References (continued)(continued)

Huber, G. A. 1999. Huber, G. A. 1999. Methods to Achieve Rut-Methods to Achieve Rut-Resistant Durable Pavements.Resistant Durable Pavements. Synthesis of Synthesis of Highway Practice 274. Transportation Highway Practice 274. Transportation Research Board, Washington, DC.Research Board, Washington, DC.

Collura, J., T. El-Korchi, K. Black, M. Chase, Collura, J., T. El-Korchi, K. Black, M. Chase, and L. Jin. 1997. and L. Jin. 1997. Guidelines for Ride Quality Guidelines for Ride Quality Acceptance of Pavements Final Report. Acceptance of Pavements Final Report. New New England Transportation Consortium, University England Transportation Consortium, University of Connecticut, Storrs, CT.of Connecticut, Storrs, CT.

Key References Key References (continued)(continued)

American Association of State Highway and American Association of State Highway and Transportation Officials (AASHTO). 1993a. Transportation Officials (AASHTO). 1993a. AASHTO Guide for Design of Pavement AASHTO Guide for Design of Pavement StructuresStructures. American Association of State . American Association of State Highway and Transportation Officials, Highway and Transportation Officials, Washington, DC.Washington, DC.