Overview of Asphalt Pavement Design

8/12/2019 Overview of Asphalt Pavement Design

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Overview of Asphalt ConcretePavement Design



“...it is the native soil which reallysupports the load.”

John Loudon McAdam

Overview of Asphalt Concrete Pavement Design



Soil Subgrade

PR ESSUR E


Design the pavement thick enough to ensure thestrength of the subgrade is not exceeded for the

loads to which it will be exposed.



S NITR A


When a pavement is too thin the strength of thesubgrade is exceeded and the pavementexperiences high strain causing it to fatigue andeventually fail.



S NITR A


strain = elongation



STRAIN


Design the pavement thickness to ensurebending results in STRAIN < 100 me(NCHRP 9-38)



Pavement Thickness Design Methods• AASHTO

(Empirical & Mechanistic Empirical – MEPDG)

• ODOT (AASHTO Empirical)

• Asphalt Institute (Mechanistic)

• FPO (AASHTO Empirical)




Design Factors – all design methods

Traffic Loading (heavy trucks)

Soil Subgrade StrengthPavement Materials Characteristics(strengths of materials comprising the pavement

build-up)

Environmental Conditions (Its effect onsoil and pavement material strength)




Design ConsiderationsThe method of design provided in the AASHTO Guideincludes consideration of the following items:

Pavement performance, Traffic, Roadbed soil, Materials of construction Environment, Drainage, Reliability, Life cycle costs, and Shoulder design





Pavement performance – the pavement’sstructural and functional performance.

• STRUCTURAL PERFORMANCE - The expectation ofthe pavement thickness to provide sufficientstructural strength to sustain the traffic loads overthe performance period;

•

FUNCTIONAL PERFORMANCE - The expectation ofthe level of “service” a pavement type will provideto the road user over its life. The dominantcomponent of serviceability is riding comfort or ridequality. Safety is also a consideration.




Pavement performance (continued) – AASHTOGuide… “The serviceability -performance concept isbased on five fundamental assumptions, summarized asfollows:1. Highways are for the comfort and convenience of

the travelling public (User)

2. Comfort or riding quality, is a matter of subjectiveresponse or the opinion of the User.3. Serviceability can be expressed by the mean of the

ratings given by all highway Users and is termed theserviceability rating.

4. There are physical characteristics of a pavementwhich can be measured objectively and which canbe related to subjective evaluations. This procedureproduces an objective serviceability index.

5. Performance can be represented by theserviceability history of a pavement.”





Pavement performance,Traffic – consists of the amount, type and weight ofvehicles that are expected to use the roadway. Onlytruck use of a roadway facility is considered since itis these types of vehicles that are sufficiently heavyto damage the pavement.





Pavement performance, Traffic,Roadbed soil – roadbed soil is the foundation onwhich the pavement will be constructed. Soilstrength must be known such that the pavementthickness is sufficient to spread the load induced by

heavy vehicles on the soil without the soil deforming(rutting).





Pavement performance, Traffic, Roadbed soil,Materials of construction – the types of materialsthat will be used in the pavement buildup (asphalt,concrete, crushed stone, rubblized base, etc.) their

respective thickness and strengths.





Pavement performance, Traffic, Roadbed soil, Materials of construction,Environment – addresses the impact ofenvironment on foundation/subgrade strength.Seasonal impacts of wet, dry, freeze, non-freezeenvironments will affect strength of soil and non- stabilized materials (e.g. crushed stone base).





Pavement performance, Traffic, Roadbed soil, Materials of construction Environment,Drainage – drainage (or lack thereof) impactsfoundation/subgrade strength, and as such, impactsthe pavement thickness. Saturated soil is weakerthan dry soil. Weak soils require greater thickness.ODOT always assumes that drainage will be providedin all pavement build-ups.





Pavement performance, Traffic, Roadbed soil, Materials of construction Environment, Drainage,Reliability – Provides consideration of uncertainties inboth traffic predictions and performance predictions.Reliability is used as a safety factor. A higher level ofreliability is used in the design computations when greaterassurance is needed that the pavement will not fail duringits life.





Pavement performance, Traffic, Roadbed soil, Materials of construction Environment, Drainage, Reliability,Life cycle costs – the cost of a pavement toconstruct initially and maintain over the “analysisperiod” - in Ohio, the analysis period is typically 35years.





Pavement performance, Traffic, Roadbed soil, Materials of construction Environment, Drainage, Reliability, Life cycle costs, andShoulder design – Related to rigid pavement design, AASHTO considers the impact of tied shoulders onpavement life.




Log10(W18) = Z R x S o + 9.36 x log 10(SN +1)

PSI4.2 - 1.5[ ]log10

0.40 +1094

(SN + 1) 5.19

+ 2.32 x log 10(MR ) - 8.07

- 0.20 +

Overview of Asphalt Concrete Pavement Design AASHTO Equation for the design of flexible pavements.



AASHTO Eqn. Thickness Design Inputs• ALWAYS USE THE AVERAGE

CONDITION/VALUE!• USE “RELIABILITY” FACTOR AS THE

MEANS BY WHICH A SAFETY FACTORIS INCLUDED.• USING CONSERVATIVE INPUTS AND

A HIGH RELIABILITY RESULTS IN

EXCESSIVELY THICK PAVEMENTS.




AASHTO Eqn. Thickness Design Inputs• Performance Period (years) –

o The period of time used indetermining pavement thicknessand as the basis for forecastingfuture traffic loads;

o In Ohio, typically assumed to be20 years in length.




AASHTO Eqn. Thickness Design Inputs• Performance Period (years)• Soil Strength –

o AASHTO uses resilient modulus (M r) asthe measure of soil strength,accounting for seasonal variation in soilstrength;

o ODOT utilizes Group Index and

correlates to California Bearing Ration(CBR). A multiplier is used to estimate “effective” resilient modulus, EM r

o (EMr = CBR X 1200)






AASHTO Eqn. Thickness Design Inputs• Performance Period (years)• Soil Strength• Traffic Loading Over the

Performance Periodo Measured in Equivalent 18,000 lb.

Single Axle Loads (ESAL or W 18);o ESAL is a means by which the

pavement damage caused by different

axle configurations and truck weightscan be normalized;

o For determining the pavementthickness use the accumulated ESALs

over the Performance Period.






• Traffic Loading (continued)o Consider directional distribution of heavy

trucks (i.e. is the truck traffic consistentboth directions? If no, design fordirection with heaviest traffic.)

o Lane Factor – for multilane pavementsdesign the thickness based on the lanethat carries the greatest number of

trucks.




• Traffic Loading ODOT Pavement Design Guide Sec. 200

B-ESALs = ADT * %T 24 * %D * %LF * %B * CFC-ESALs = ADT * %T 24 * %D * %LF * %C * CF

B-ESALs + C-ESALs = Total Daily ESALs

ADT = Average Daily Traffic%T 24 = 24-hour truck percentage of ADT%D = Directional Distribution%LF = Lane Factor (percent trucks in the design lane)%B,C = % B (tractor trailer) or %C (straight body) trucks of

the total trucksCF = Truck conversion factor (ESALs per truck) based on

Functional Classification of the Roadway






















AASHTO Eqn. Thickness Design Inputs• Performance Period (years)• Soil Strength• Traffic Loading (accumulated ESALs)

• Climate Conditions – the effect ofseasonal changes on the pavementfoundation performance and strength. Forinstance, addressing frost-susceptible soilsand accounting for soil support during thaw.

Over Performance Period




AASHTO Eqn. Thickness Design Inputs• Design Period (years)• Soil Strength• Traffic Loading (accumulated ESALs)

• Climate Conditions• Loss of Serviceability ( Δ PSI) – the

amount of serviceability (riding comfort) theagency will tolerate losing beforerehabilitation is needed.





Terminal Serviceability(PSI 2.5) TOO ROUGH !

SMOOTH

Initial Serviceability(PSI 4.5 for HMA, 4.2 for PCC)


Δ PSI = P o - P t




TerminalServiceability

Level (P t)

Percent of PeopleStating

Unacceptable3.0 12

2.5 552.0 85

Reference: AASHTO Guide



Overview of Asphalt Concrete Pavement DesignReference: ODOT Pavement Design Guide

SERVICEABILITY FACTORSRIGID/COMPOSITE FLEXIBLE

Initial Serviceability 4.2 4.5Terminal Serviceability 2.5 2.5Design Serviceability Loss 1.7 2.0




• Climate Conditions• Loss of Serviceability ( Δ PSI)• Reliability “R” (safety factor) – Absent a

reliability factor the probability that the pavementwill provide useful service over its intended life is50 percent . Increasing the reliability has theeffect of increasing the traffic which in turnincreases pavement thickness and the probability

of meeting the intended life.





Overview of Asphalt Concrete Pavement DesignReference: AASHTO Guide Table 2.2, Suggested levels of

reliability for various functional classificationsFunctional Classification Recommended Level ofReliability (%)Urban Rural

Interstate and other freeways 85 – 99.9 80 – 99.9Principal Arterials 80 – 99 75 – 95Collectors 80 – 95 75 – 95Local 50 – 80 50 – 80




Reference: ODOT Pavement Design Guide, Plate 201-1

Functional Classification Recommended Level ofReliability (%)Urban Rural

Interstate and freeway 95 90Principal and Minor Arterials 90 85Collectors 90 85Local 80 80




• Climate Conditions• Loss of Serviceability ( Δ PSI)• Reliability “R” (safety factor) • Overall Standard Deviation (variability) –

accounts for the chance variation in thetraffic prediction and chance variation inactual performance.







OVERALL STANDARD DEVIATION ( δ O)Flexible Pavement 0.49Rigid Pavement 0.39



DETERMINE Structural Number (SN) using

AASHTO Equation/nomograph

SN is an abstract number (SN) that representsthe structural strength required for a pavementto perform in accordance with the designcriteria.






SN = 4.5






“LAYERED DESIGN ANALYSIS”

DETERMINE THE MAXIMUM ALLOWABLETHICKNESS OF EACH UNBOUND LAYER (i.e.aggregate layer), AND DETERMINE THEMINIMUM THICKNESS OF THE ASPHALTLAYER.

REASON: WE WANT TO AVOID OVER-BENDING THE ASPHALT LAYER – CAUSING ITTO FATIGUE -AND AVOID PUTTING THE

AGGREGATE LAYER INTO TENSION (WHY? Thestrength of the aggregate base layer comes

only when it is in compression.)



Determine the SN over the aggregate layer.



SNasphaltmin = 2.75

SNaggmax

= 4.5 - 2.75 = 1.75





SN = a 1*D1 + a 2*D2*m 2 + a 3*D3*m 3 + … 4.5 = SN asphaltmin + SN aggmax

4.5 = 2.75 + 1.75 (ensures min. and max. liftthickness requirements are met)

a = layer coefficient represents the structural

strengths (per inch) of a material in thebuildup.D = the thickness of the materialm = drainage coefficient (assumed to be 1.0

since drainage is “always” provided.





4.5 = 2.75 + 1.75 (ensures min. and max. liftthickness requirements are met)

When selecting appropriate values for the layerthicknesses, it is necessary to consider their… • Cost effectiveness• Construction constraints• Maintenance constraints





ASPHALT CONCRETE STRUCTURAL COEFFICIENTS (a i)Items 424, 442, 443, 446, 448, 826, 857, 859, 874 –

AC Surface0.43

Items 442, 443, 446, 448, 826, 857 – AC Intermed. 0.43Items 301, 302 AC Base Course. 0.36Item 304 – Aggregate Base 0.14Item 320 – Rubblized Concrete 0.14





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Overview of Asphalt Pavement Design

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