Pavement Design Basics

Saving Lives, Time and ResourcesPavement Design BasicsDarlene Goehl, P.E.Texas A&M Transportation Institute2018 Transportation Short CourseOctober 16, 2018

Overview History of Pavement Design3-6 Rigid Pavement Design7-10 Flexible Pavement Design11-14 Pavement and Material Evaluation15-32 Pavement Design Report33-34

History TxDOT established 100 years ago Pavement design largely concerned with protecting thesubgrade until the end of WWII Initiation of highway road tests following WWII AASHO develops relationship between repetitive axleloads and pavement damage/serviceability AASHO Empirical design is developed based on Road Test;formalized in early 1970’s.

History TxDOT initializes research into developing own empiricalflexible design system early 1970s into 1980s. FPS-19 introduced early 1990s using material moduli andelastic layer theory AASHTO ME design released 2006 TxME development continues . . .

Pavement Pavement Structure. Combination of surface course and basecourse placed on a subgrade to support the traffic load anddistribute it to the roadbed The primary structural difference between a rigid and flexiblepavement is the manner in which each type of pavementdistributes traffic loads over the subgrade.

Rigid vs. Flexible Pavement A rigid pavement has a very high stiffness and distributesloads over a relatively wide area of subgrade –– a major portion of the structural capacity is contributed by theslab itself. Flexible pavement’s load carrying capacity is dependent onthe layers.– load-distributing characteristics of the layered system is criticalLoadRigid PavementSubgradeLoadFlexible PavementBaseSubgradeLoad distribution for a typical flexible pavement and a typical rigid pavement.

Overview of Rigid Pavement Thickness Design TxCRCP-ME Design Program for CRCP Developedunder department research project 0-5832,“Develop Mechanistic/Empirical Design for CRCP.– Uses Punchouts as main distress AASHTO Rigid Pavement Design Procedure forCPCD– The 1993 AASHTO Guide for Design of PavementStructures

Overview of Rigid Pavement Subbase Design Base layer combinations for concrete slabsupport (CRCP or CPCD):– 4 in. of hot-mix asphalt (HMA) or asphalt treated base(ATB), or– a minimum 1 in. hot-mix asphalt bond breaker over 6in. of a cement treated baseLoadLoadRigid PavementRigid Pavement4” HMA or ATBStabilized Subgrade1” HMA “Bond Breaker”6” CTBStabilized Subgrade(if needed)SubgradeThese are non-erodible subbases.TxDOT chose this strategy instead ofrequiring subbase drainage designs.Subgrade

Rigid Pavement Design Inputs -TXCRCP-MEParameterDesign Life (years)Punchouts per MileThickness of ConcreteNumber of LanesDesign TrafficSoil Classification ofSubgradeRigid Pavement Design Inputs - New PavementRangeUsual Input30107" to 15"GW, GP, SW, SP, GM, SM, GC,SC, ML, OL, MH, CL, CH, OHAsphalt treated base (ATB),Base Layer Requirementshot mix (HMA)or cement treated base(CTB)Modulus of Base Layer100-700(ksi)Estimated depthin One DirectionOne-Way total 18-kip ESALsDetermine from subgradetestingCommentsMay lower depending on Roadway.TxCRCP-ME program will evaluate depthentered to determine if it is adequate,however this may be a trial and errorprocess to determine the minimumthickness required.Refer to Typical sectionRefer toUnified Soil Classification SystemSelect most cost effective base layerHMA or ATB use 400CTB use 500

Rigid Pavement Design Inputs – 1993 AASTO FormulaParameterDesign Life (years)Rigid Pavement Design Inputs - New Pavement, CPCDRangeUsual Input30Thickness of Concrete7" to 15"Estimated depth28-day Conc. Modulus of Rupture, psi (MOR)620 - 680620Reliability, %90% - 95%90% or 95%Overall Standard DeviationLoad Transfer Coefficient (J)28-day Concrete Elastic Modulus, psi2.5 – 2.9Serviceability IndicesDesign TrafficEffective Modulus of Subgrade Reaction, psi (k)Drainage Coefficient0.392.95,000,000Initial: 4.5Terminal: 2.5One-Way 18-kip ESALs300 - 800Annual Rainfall (in.)58-5048-4038-3028-2018-8300Drainage 11-1.16CommentsAASHTO equation can be used with trial and errorprocess to determine depth or depending on equationcalculator program may be able to calculate thicknessneeded.Use 90% when 5 M ESALsUse 95% when 5 M ESALsJCP with tied concrete shoulderDifference: 2.0Can apply lane distribution factor(LDF) whentotal lanes are 4 lanes.For 6 lanes, LDF 0.7, 8 lanes, LDF 0.6Assuming 4” of ACP or 6” of CTB 1” ACPSelect Drainage Coefficient based on annual rainfall.

Overview of Texas Flexible PavementDesignProcessFWDDataDesignInfoMODULUSFPS 21HMAh1Baseh2SubgradeTexas TriaxialCheck(Overloads on thin)MechanisticCheck(Rutting and Cracking)11

Flexible Pavement Loading and ResponseδTensionCompressionDeflection under LoadpStrainStressDeformation (ε)Load/Area (σ)

Service Histories of Several Trial DesignsTrial BServiceability Index, SITrial A2Trial ATrial A1Min. acceptable SIRequired analysis period or design lifePerformance periodTATdTime13

Failure & Serviceability Structural– Material fails to point can no longer function to distribute loading.– May require complete rehabilitation Functional– Loss of Ride– Loss of Friction– May only require repair of riding surface Serviceability– Can be correlated to pavement roughness Initial should be a good smooth ride Final will be rough ride

Available Pavement Evaluation Tools FWD structural strength, includingsubgradeGPR thickness variability; identify majorproblem areas; sampling locationsDCP in-site strengths of lower layersTPAD structural strength of concretepavement, weak subgrades, poor loadtransferLIDAR ditch depth; horizontal andlongitudinal slopes; drainage evaluations1

GPR dataBottom HMABottom base16

Rolling Deflectometer in Action1

TPAD DATADeflectionsSensors 1 and 2

Falling Weight Deflectometer - FWD19

Stress Distribution and Deflections Under FWD LoadingSurfaceBaseSubgradeCL20

Raw Deflection Data IndicesW7W3W1W2Indices:BCISCIW1 Overall Pavement StiffnessW1-W2 (SCI) Top 8”W2-W3 (BCI) 8” to 16”W7 Subgrade 48”Normalized to 9 kip Drop Load21

Interpreting Relative Deflection Bowl ShapeA Good Base/Good SubgradeB Bad Base/Good SubgradeC Good Base/Bad Subgrade22

Subgrade Evaluation http://websoilsurvey.nrcs.usda.gov/app/– USDA soil information webpage Sample and Test Subgrade6-18 in Brown clay PI 32 M 25.8 %0-6 in Sandy clay PI 6 M 10.2%18-30 in Tan clay PI 52 M 32.3%

Site Coring validating defects - additional testing24

Subgrade and Existing Pavement Layers FPS Priority should be to use the project-specific backcalculated subgrade modulus. Defaults by county are available in the FPS design program. Typical Design Modulus range is 8-20 ksi. Typical Poisson’s Ratio range is 0.35-0.45 Wetter or more highly plastic materials warrant higher Poisson ratios. New location construction deflection testing on an adjacent highway, or intersecting highways can provide data for backcalculation. Alternatively, elastic modulus correlations to field or laboratory derived CBR or the program defaultmay be used.

Stabilized SubgradeMaterial Type2014 SpecDesign ModulusPoisson’sRatioLime orCement Treated SubgradeItem 260, 27530 - 45 ksi0.30Emulsified AsphaltTreatment (Subgrade)Item 314, various specialspecs15 - 25 ksi0.35

BaseMaterial Type2014 SpecDesign ModulusPoisson’sRatioAsphalt Treatment (base)Emulsified AsphaltTreatment (Base)Item 292Item 314, various OTUspecial specs250 - 400 ksi0.3550 - 250 ksi0.350.35Flexible BaseItem 247If historic data not available,modulus shall be no greaterthan 3-4 times the subgrademodulus or use FPS default,whichever is lower. Typicalrange 40-70 ksi.Lime Treated BaseItem 260, 26360 - 75 ksi0.30 - 0.35Cement Treated BaseItem 275, 27680 - 150 ksi0.25 - 0.30Fly Ash or Lime-Fly AshTreated BaseItem 26560 - 75 ksi0.30

Guidance FDR projectsPoison’sRatioCohesiometerValue for MT check0.35na 3 times the subgrade modulus0.35na100 ksi0.380065 ksi0.365035 ksi0.3530070 ksi0.35na 3 times the subgrade modulus,but not Districts default modulusvalue (40-70 ksi)0.35naMaterial DescriptionModulus ValueExisting Material(not reworked, including Subgrade)Existing Pavement –Scarified, Reshaped and Compactedmostly granular baseStabilize Blend(75% or more base)of Existingblend subgrade & basePavement &(50% to 75% base)Subgrademostly subgrade( 50% base)New Flexible Base over Stabilized Subbase(mostly granular base)500 ksi or Back-calculated fromFWD data,1st 6” lift of new flexible base(when multiple lifts are required)

Sampling Projects for FDR or CIR HMA Sections 2 insKeep HMA and Base Samples Separate29

TxDOT new Small Sample Mix Design procedure Traditional sample sizes– 110 pounds per design Small samples– 15 pounds per design Oftentimes 4 to 8 different mixtures underconsideration Complete lab design in 5 working days“Traditional” vs “Small”Sample Sizes

HMAMaterial Type2014 SpecDesign ModulusPoisson’sRatioSeal Coat(nonstructural layer)Item 316200 - 250 ksi0.35Limestone Rock Asphalt PavementItem 330200 - 350 ksi0.35Hot-Mix Cold-Laid ACPItem 334300 - 400 ksi0.35Combined HMA thickness:Dense-Graded Hot-Mix AsphaltItem 340, 341 4 in. use 500 ksi0.35 4.0 in. use 650 ksiPermeable Friction CourseItem 342300 ksi0.35Combined HMA thickness:Superpave MixturesItem 344 4.0 in. use 650 ksi4 in. T 6 in. use 750 ksi0.35 6.0 in. use 850 ksiStone-Matrix AsphaltItem 346Same as Item 3440.35

Hot MixFine PFCOPEN GRADED(24% )AV THIN MIXES TOM-CGAPTOM-FDENSE30% Cost savings over traditional mixes - lifts of 1 inch or lessMust pass Rutting (HWTT) and Cracking (OT) performance testsMin 6% PG 76-22 SAC A Rock, 100% passing 3/8” NO RAP or RAS

Pavement Design Report – Supporting Documentation Background Material – As-Built PS&E – Sitemanager Construction Records– Maintenance History– PMIS data– 4 year Plan scope of work– Traffic Data– Previous Pavement Design ReportsDrainage analysis Design exceptions and approvalsDocument AssumptionsExisting Material testing ReportsCore ReportLife-cycle cost analysisPG binder selection criteriaSurface Aggregate Selection Form 2088Traffic DataResults of NDT to characterize theexisting structural condition– Falling Weight Deflectometer(FWD) MODULUS, including thebackcalculation summary– TPAD– Ground Penetrating Radar (GPR)Soils Report– Material Description: Tex 141-E,142-E– PI: Tex 104-6-E– Sulfate Content: Tex-145-E, Tex146-E– Organic Content, Tex-148-E– PVR: Tex-124-E Material Designs– Treatment: Tex 120-E, 121-E, 127E, 122-EDesign Program Inputs and Results– FPS 21 summary, modified Texas Triaxial check, mechanistic checks, stress analysis, etc.,– Alternate pavement design AASHTO (DARWin 3.1) designsummary for CPCD rigidpavements. TxCRCP-ME Design summaryfor CRCP rigid pavements.

Report Content – Summary Page Clearly define the pavement layers– Description, thickness, and materials with estimate rates(if applicable) Note: An underseal, Prime Coat, etc. will not be modelled in the design software. It needs to be shown on the summary andproposed typical section, don’t assume the designer knows that it is needed.– Controlling specification item for each layer Proposed Typical Section– Changes within the limits of the project (more than one typical section) Frontage Roads vs. MainlanesCross overs and intersectionsWidening detailsStage Construction and detours.Undercut areas or Soil mitigation areas (include details for replacement or treated materials)Include special details such as widening, hot mix tapers, stage construction details Summary Cost Analysis– Comparison of alternates considered– Life cycle cost analysis34

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Pavement Design Basics Darlene Goehl, P.E. Texas A&M Transportation Institute. 2018 Transportation Short Course. October 16, 2018. Overview History of Pavement Design 3-6 Rigid Pavement Design 7-10 Flexible Pavement Design 11-14 Pavement and Mate