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PAVEMENT TEXTURE ANDFRICTIONPresentation to:SWIFTSeptember 2007Jim W. Hall, Jr.

Basics of Pavement FrictionFriction - force that resists the relative motion between a tireand pavement surfaceWeight, FWRotationDirection ofmotionFμ FWFriction Force, FExpanding the Realm of Possibility

Pavement Friction versus Tire Slip(Henry, 2000) Friction increases rapidlywCoefficientwith increasing slipwof FrictionPeak value between 10 and20 percent slip (critical slip)Friction decreases tocoefficient of sliding frictionat 100 percent slipDifference between the peakand sliding coefficients mayequal 50 percent of slidingvaluew0This disparity is muchw(free rolling)greater on wet pavementsAnti-locking brake system(ABS) uses this principlewPeak frictionwIntermittentwslidingwFullwsliding frictionwCritical slipwIncreased Brakingw100w(fully-locked)wTire Slip, %Expanding the Realm of Possibility

Levels of Surface TextureRunway RoughnessReference Lengthλ 20 in.Short Stretchof RunwayMega-textureλ 2 to 20 in.; A 0.005 to 2 in.TireMacro-textureλ 0.02 in.; A 0.04 to 20 milsPavement – TireContact AreaSingle AggregateParticleMicro-textureλ Wave LengthA AmplitudeExpanding the Realm of Possibility

Components of FrictionwTire RubberwFwVwAdhesionFunction of micro-texturewHysteresisFunction of macro-texturewTotal Braking Force, F, equals:F FA FHExpanding the Realm of Possibility

Components of Friction Hysteresis is:z force generated within the deflected, visco-elastic tire treadz a function of speedz mainly related to macro-textureAdhesion is related to micro-textureFor wet pavementsz adhesion drops off with increased speedz hysteresis increases with speedz above 56 mi/hr (90 km/hr) macro-texture accounts for over 90percent of frictionWinter friction on snow and ice is function of contaminantExpanding the Realm of Possibility

Micro and Macro Textures Micro-texture provides pavement friction at low speedsMacro-texture:z reduces potential for hydroplaningzprovides friction by hysteresis at high speedsExpanding the Realm of Possibility

Surface Texturing (PCC pavements) GrindingWidth of diamond blades(2.5 mm - 3.3 mm)Land area - varies depending onaggregate hardnessÎ2.0 mm typical for hard aggregateÎ2.8 mm in typical for soft aggregate GroovingSaw blade thickness2.5 mm19 mm3.2 mm min6.4 mm maxExpanding the Realm of Possibility

Surface Texturing (AC pavements) Grooved AC Porous FrictionSMAExpanding the Realm of Possibility

PIARC Texture ClassificationPavement Surface Characteristic ftWavelength 1μm 10μm 100μm 1mm10mm 100mm 1m10m100mPIARCMicrotextureMacrotexture MegatuxtureRoughnessCategoryRide Quality (Smoothness)Rolling ResistanceVehicle WearIn-Vehicle NoiseTire-Pavement NoiseSplash and SprayWet Weather FrictionKey:Dry Weather FrictionGoodBadTire WearExpanding the Realm of Possibility

Measurement of Textureand Friction (highways) Micro-texturezzzzASTM E 274 Locked WheelASTM E 524 (smooth tire)ASTM E 501 (ribbed tire)Smooth tire recommended for frictionmeasurementzSpot measurements are made on highwayssince tire wear would be a problem forcontinuous runszASTM E 1911 (Dynamic Friction Tester)ASTM E 303 (British Pendulum Tester)zExpanding the Realm of Possibility

Measurement of Texture and Friction(highways) Macro-Texturez ASTM E 2157 CircularTexture MeterzASTM E 965 SandPatchzASTM E 2389 OutflowMeterExpanding the Realm of Possibility

Measurement of Texture and Friction Macro-Texturez High-speed laser based devicesHigh-Speed InertialProfilersFHWA ROSANvExpanding the Realm of Possibility

Measurement of Textureand Friction (Runways) Microtexture (friction)zASTM E 274 Fixed slip (GripTester, MuMeter)zASTM E 1859 Variable slip (French IMAG;Norwegian Norsemeter RUNAR)zRunways tested with continuousmeasurements since tire wear is not aproblem for one or two milesMacro-texturezASTM E 2157 (Circular Texture Meter)zASTM E 965 (Sand Patch)Expanding the Realm of Possibility

Department of National DefenceFriction Testing Friction measurements with Grip TesterContinuous measurements for length of runwayMeasurements at 65 km/hr (40 mph) with waterdepth of 0.25 mmExpanding the Realm of Possibility

Transport Canada Friction Testing Continuous Friction MeasuringEquipment (CFME)zz Surface Friction Tester (SFT)Benchmark for standard runwaycoefficient of frictionSpot Measuring Devices(Decelerometers)zzzdeceleration rate of vehicle brakedto four wheel lock upfriction values available to pilotssurface conditions of snow, ice orcompacted snow made availableto pilots as CRFI (CanadianRunway Friction Index)Expanding the Realm of Possibility

International Friction Index (IFI) Developed by PIARC Based on ASTM E 1960F ( S ) F (60) e60 S()SPwSP 89.7*MPD 14.2F(S)F(60)SPSMPD adjusted value of friction for a slip speed of S measured friction value at slip speed of 60 km/hr speed number, km/hr measurement speed, km/hr mean profile depth (macro-texture), mm Sp from CT Meter or high-speed profilers (MPD) or relationships between MPDand MTD from Sand Patch or Outflow Meter F(S) is friction value from tests at slip speed SExpanding the Realm of Possibility

Guide for Pavement Friction (for highways)wNCHRP 1-43 NCHRP Project 1-43zThe “Guide for Pavement Friction” developed for adoptionby AASHTOzOutlines a friction management and design program forhighway pavement surfacesGuide provideszinformation on aggregates and mixture types for long-lasting,high-quality friction surfaceszinformation on friction-testing methods, equipment, andindiceszmethods for establishing investigatory and interventionfriction levelsExpanding the Realm of Possibility

Guide for Pavement Friction (for highways) Pavement Friction BasicswNCHRP 1-43zz Pavement Friction Managementzz Importance of Pavement FrictionPavement Friction PrinciplesDeveloping Pavement Friction Management PoliciesEstablishing the Pavement Friction Management ProgramPavement Friction DesignzzDeveloping Friction Design PoliciesProject-level Design GuidelinesExpanding the Realm of Possibility

Friction Management (for highways)wNCHRP 1-43Systematic approach to measuring/monitoring friction andcrash rates, identifying surfaces in need of remediation, andplanning/budgeting for treatments and reconstruction Key ComponentszzzzzzNetwork definitionNetwork-level data collectionNetwork-level data analysisAdequate monitoring of friction and crashesDetailed site investigationSelection and prioritization of short and long termrestoration treatmentsExpanding the Realm of Possibility

Friction Management (for highways) Two threshold levels of friction:wNCHRP 1-43zzInvestigatory – calls for detailed site investigation todetermine need for remedial action; actions are:¾Erect warning signs¾More frequent testing¾Further analysis of friction and crash data¾Short-term restoration treatmentIntervention – Remedial action required; actions are:¾Immediate restoration treatment¾Erect warning signs¾Program treatment in maintenance or construction work planExpanding the Realm of Possibility

Defining LevelsCrash RatesInvestigatory LevelIntervention LevelCrash RatesFriction NumberwNCHRP 1-43FrictionPavement Age, years Plot friction deterioration curve (for a specific friction demand category) Plot corresponding crash rates curve Set intervention level at significant increase in crash ratesExpanding the Realm of Possibility

Network-Level Friction DesignwNCHRP 1-43 Aggregate testingzRecommended tests that help characterize aggregatefrictional (micro-texture) propertieszBasic test criteria for discerning friction quality ofaggregatesAsphalt mix types and concrete texturing methodsz Typical macro-texture depthsFriction design categorieszMatching aggregate sources and mix type/texturingtechniquesExpanding the Realm of Possibility

Aggregate Properties wNCHRP 1-43 Coarse aggregate controls the friction properties of asphalt mixesFine aggregate controls the frictional properties of concrete mixesMineralogical and Petrographic Propertiesz Physical and Geometrical propertiesz Aggregate composition/structure and mineral hardnessAngularity, shape, and textureMechanical PropertieszAbrasion/wear resistancezPolish characteristicszDurability PropertieszSoundnessExpanding the Realm of Possibility

wNCHRP 1-43Aggregate Mineralogy Hard, strongly bonded, interlocking mineral crystals (coarsegrains) embedded in a matrix of softer minerals Differences in grain size and hardness constantly renewabrasive surface because of differential wear rates Mohs scratch hardness test for determining mineral hardnessz Mohs hardness 6 - resists wearz Mohs hardness of 3 to 6 - resists polishing but wearsquicklyExpanding the Realm of Possibility

Project Level Friction Design Project level design guidelineswNCHRP 1-43zzzStep 1 – Determine design friction levelStep 2 – Select aggregates (micro-texture)Step 3 – Establish surface mix types and/or texturingtechniques (macro-texture)¾ UseIFI equation to identify combination(s) ofaggregate and mix type/texturing technique that satisfydesign friction levelzzStep 4 – Develop construction specificationsStep 5 – Formulate design strategiesExpanding the Realm of Possibility

Transport Canada - Runway Friction StandardsCorrective ActionRunway Average COF less thanShall Be Planned (Investigatory)0.60Shall Be Taken (Intervention)0.50Corrective ActionRunway 100 Metre Section Avg.COF less thanShall Be Planned (Investigatory)0.40Shall Be Taken (Intervention)0.30Expanding the Realm of Possibility

Department of National Defence Runway Friction StandardsGripTesterFriction ValueAction 0.41 to 0.60Remedial work scheduled(Investigatory) 0.40Remedial work required(Intervention)Expanding the Realm of Possibility

Recommended Textures for Friction PCC Surfaces1. Transverse grooved (with quality aggregate)2. Transverse or skewed tining3. Diamond grinding4. Longitudinal tining AC Surfaces1. Aggregate type dictates texture/friction2. Open-textured friction course (PFC)3. Gap-graded mixes (SMA, NovaChip)4. Grooving for MacrotextureExpanding the Realm of Possibility

For more information:Jim W. Hall, Jr.Applied Research Associates, Inc.112 Monument PlaceVicksburg, MS [email protected] the Realm of Possibility

The “Guide for Pavement Friction” developed for adoption by AASHTO. z. Outlines a friction management and design program for highway pavement surfaces Guide provides. z. information on aggregates and mixture types for long-lasting, high-quality friction surfaces. z. information on fri