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Flexible Pavement Design: Transitioning fr omEmpirical to Mechanistic-Based Design Methods..................................................................................................................................................................................................................................

By : Engr. Ahmad Kamil bin Arshad M.I.E.M., P.Eng.

INTRODUCTIONA typical flexible pavement structure inMalaysia consists of asphaltic concretewearing and binder course, unboundgranular base and sub-base overlying thesubgrade(Figure 1). Presently there is ashift in the approach of flexible pavements t ructural design from empirical tomechanistic methods. The A A S H TO"2002 Guide for the Design of New andRehabilitated Pavement Stru c t u re s "( c u r rently under review) is based onmechanistic method rather thanempirical method, which is the basis forthe 1993 and earlier editions of theAASHTO Pavement Design Guide. Twomajor organizations, Shell in 1978 andAsphalt Institute in 1981 have producedmechanistic design procedures for thedesign of flexible pavements.Worldwide, it has been reported thatSouth Africa has long used themechanistic design method for theirpavement design (since late 1970’s).Australia in 2004 released their pavementdesign manual that adopted mechanisticdesign procedures. Jabatan Kerja RayaMalaysia (JKR) has released in late 2006 adraft flexible pavement design manualbased on mechanistic design procedures.

EMPIRICAL PAVEMENT DESIGNMETHODSEmpirical pro c e d u res are derived fro mexperience or observation alone, oftenwithout due re g a rd for system behaviour orpavement theory. The basis for theempirical design approach are theempirically derived relationships betweenperformance, load and pavement thicknessfor a given geographic location and climaticcondition. From these relationships, there q u i red pavement thickness, the numberof load applications or the occurrence ofd i s t resses is a function of factors such aspavement material properties, subgradetype, traffic loading and climate. TheArahan Teknik (Jalan) 5/85 – Manual onPavement Design, A A S H TO 1986/93

Pavement Design Guide and Road Note 29design methods are empirically-deriveddesign pro c e d u res based on the observedfield performance. There are a number oflimitations to their continued use forpavement design in this country. A m o n gthe limitations include the following:• The AASHTO Guide was based on the

1950s AASHO Road Test data, w h e reby the conditions are site-specific and is applicable only to those a reas with the same characteristics such as soil type, pavement materialtype and climate. There f o re, its applicability for Malaysian conditions is questionable as Malaysia has atotally diff e rent soil and material characteristics and diff e rent type of climate.

• In the AASHO Road Test, traffic loadings of about 1,114,000 axle loads had been applied. Extrapolation, to say,10 million ESALs for pavement designcalculations is unrealistic given the high number of loading repetitions andthe different conditions assumed such as different subgrade value.

MECHANISTIC-BASEDPAVEMENT DESIGN METHODSThe mechanistic-empirical method ofdesign is based on the mechanics ofmaterials that relates an input, such aswheel load, to an output (or pavementresponse), such as stress or strain. The

response values are used to pre d i c tpavement distress such asrutting/permanent deformation andfatigue cracking (Figures 2a and 2b)based on laboratory stress and fieldperformance data.

Mechanistic design methods offer manyadvantages over the empirical methodsuch as the following [1]:i. It allows an evaluation of changes in

vehicle loading on pavement performance;

Figure 1: Typical flexible pavement cross-section in Malaysia

Figure 2a: Fatigue cracking

Figure 2b: Rutting

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ii. New materials can be evaluatedthrough their design properties;

iii. The impact of variability inconstruction can be assessed;

iv. Actual engineering properties areassigned to the materials used in thepavement;

v. Pavement responses related to actualmodes of pavement failure are evaluated;

vi. Databases of materials used as input in pavement design can be developedand updated as information becomesavailable.

The biggest advantage to mechanisticdesign is that it allows a rapid analysis ofthe impact of changes in input items suchas changes in materials and traffic [1].

FRAMEWORK FORMECHANISTIC DESIGNMETHODFigure 3 shows the components of themechanistic design process [2]. The threemain components of the process areinput parameters, structural model andtransfer functions.

a) Input ParametersInput parameters consist of pavementconfiguration, material properties foreach pavement layer and expected traffic.Pavement configuration includes thenumber of layers, the thickness of eachlayer and type of material for each layer.Material properties that are required as

inputs include resilient modulus andPoisson’s ratio of each pavementcomponent layer (Table 1). The materialp roperties are modified to take intoaccount of the climate such astemperature and moisture from rainfall.Traffic loadings include axle loads, thenumber of load repetitions, tyre pressure

and contact area andvehicle speed. Loadrepetitions are normallye x p ressed in terms ofequivalent 80 kN (18,000lb) single axle load withdual tyres (ESAL),a p p roximated by twoc i rcular loaded are a s .Recent advances inw e i g h - i n - m o t i o nequipment have madepossible the measure m e n tof the actual load spectraof vehicles, but presentlyavailable methods usethe ESAL method.

b) Structural ModelA pavement stru c t u re can bemodeled as a multi-layere d

elastic system or as a finite element meshre p resentation (Figures 4 & 5). Regard l e s sof whether a layered elastic program or a

Finite Element program is used to model apavement, what is important is that themodel, while not perfect, fairly re p re s e n t sthe behaviour of pavement in field. Mostc u r rent mechanistic design methods usel a y e red elastic analysis for evaluatingpavement responses (stress, strain anddeflection). The analysis is carried out usingcomputer software such as KENPAVE (byHuang), SW-1 (by Asphalt Institute),BISAR/SPDM (by Shell) and CIRCLY ( b yMINCAD Systems Pty Ltd, A u s t r a l i a ) .

The critical stress or strain is themaximum value of that stress or strainthat occurs in the pavement systemunder the given loading conditions. It iscrucial to know where the location andwhat is the maximum value of the criticals t ress and strain so that materials ofadequate strength and thickness can bespecified to prevent pavement failureunder the actual loading conditions. Fora typical flexible pavement consisting ofthe asphaltic concrete, crushed aggregatebase, granular sub-base which issupported by the subgrade, the locationsof critical stress/strain are shown inFigure 6.

Two particular locations of concernare tensile strain at the bottom of theasphalt concrete layer and compressivestrain at the top of the subgrade. The

Figure 3: Components of Mechanistic Design Process [2]

Material Resilient Modulus (MPa) Poisson’s Ratio

Asphaltic Concrete (21oC) 3,500 0.35Crushed Stone 150-300 0.40Clayey Soils 35-100 0.45

Table 1: Typical Resilient Modulus and Poisson’s Ratio Value [1]

Figure 4: Multilayer elastic model [3]

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tensile stress at the bottom of theasphaltic concrete layer under repeatedt r a ffic loading will cause cracks top ropagate upwards leading to theformation of fatigue cracks. Pavementrutting results from repeated verticalstress on the top of the subgrade. Twoother areas of concern are thecompressive strain at top of the asphalticconcrete layer and the compressive strainat the top of the crushed aggregate basewhich leads to the rutting of thepavement structure.

c) Transfer FunctionsThe analysis output of stresses andstrains are then used in transferfunctions to relate them to the pre d i c t e dpavement performance. A t r a n s f e rfunction is an equation that is used top redict pavement life in terms ofnumber of axles repetitions to failurefor major pavement distresses such

fatigue cracking of theasphaltic concrete layer orpermanent deformation ofthe subgrade.

Asphalt Institute use thefollowing transfer functionswhich relates the strainscalculated to the number oft r a ffic load repetitions tof a i l u re for a particular typeof pavement distress [5]:

i ) Fatigue Cracking (20% of a rea cracked): Nf =0.0796 (εt) - 3 . 2 9 1 | E * |

- 0 . 8 5 4

…(1) w h e re :Nf = allowable number of

load repetitions toc o n t rol fatiguec r a c k i n g

εt = horizontal tensilestrain at the bottomof the asphalticc o n c rete layer

| E * |= dynamic modulus ofthe asphaltic concrete mix

i i ) Subgrade Permanent Deformation (0.5 in rutdepth):

Nd = 1.365 x10- 9 (εc) - 4 . 4 7 7

… ( 2 )w h e re :Nd = allowable number of load

repetitions to control permanent d e f o r m a t i o n

εc = vertical compressive strain on topof the subgrade layer

Design charts (Figures 7 and 8) based onmechanistic design method have beendeveloped by Asphalt Institute and Shellto simplify the calculations re q u i red forthe design of flexible pavements.

IMPLEMENTATION PROBLEMSAlthough mechanistic design approachappears to be promising as a suitable andrational design approach for flexiblepavements, there are several issues thatneeds to be resolved before the approachcan be implemented. Materialscharacterisation of subgrade, diff e re n ttypes of sub-base and base and asphalticc o n c rete re q u i re the determination of

physical properties such as re s i l i e n tmodulus (MR) and Poisson’s ratio (υ).The acquisition of equipment is veryexpensive and personnel have to betrained to operate the equipmentproperly in order to obtain consistent andreliable data results.

Tr a ffic loadings re q u i re more thanjust traffic count currently beingpracticed in this country. Actual trafficaxle spectrum is required for a morerealistic estimate of traffic loading. Thishowever re q u i res special equipmentsuch as WIM and additional weighingstations and re q u i res more effort inclassifying and collecting the actualtraffic loading data.

Transfer functions that relate pavementd i s t ress (permanent deformation andfatigue cracking) to the number of axlerepetitions to failure must be tested andverified in the actual pavement operatingconditions. The Asphalt Institute’s or Shell‘sf a i l u re criterions are at best only estimatesunless properly verified. Ve r i f i c a t i o nre q u i res effort in collecting data for there p resentative test road sections identifiedfor the study. Data collection includess e r v i c e a b i l i t y, traffic, subgrade support,pavement cross-section and enviro n m e n t a lconditions necessary for the study.

S o f t w a re availability should beconsidered before the implementation ofmechanistic design appro a c h .Developing a customized software istime-consuming and re q u i res a lot ofeffort. However, there are some reliableand affordable software to users such asS W-1 by Asphalt Institute. Also, theKENPAVE software comes free with thepurchase of the accompanying pavementdesign textbook [3].

F i n a l l y, there are not many experiencedengineers exposed in the mechanistic designof pavements in this country. Seminars andcourses should be conducted to expose andtrain engineers in mechanistic design ofpavements. Guidelines for mechanisticdesign should also be developed. As astarting point, guidelines from othercountries experienced in using mechanisticdesign such as Australia, France and SouthAfrica should be considered as a basis fordeveloping a guideline in this country.Further refinements can then be madet h rough local re s e a rch and increase data asthey become available in the future .

Figure 5: Finite element model [4]

Note:1 C o m p ressive strain on top of asphaltic concrete surface – rutting2 Tensile strain at bottom of asphaltic concrete – fatigue cracking3 Compressive strain on top of granular base – permanent

deformation4 C o m p ressive strain on top of subgrade – permanent deformation

Figure 6: Critical stress/strain locations in a conventional flexiblepavement

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CONCLUSIONMechanistic design of flexible pavement hasbeen discussed in the above sections.P re s e n t l y, the trend worldwide is to use themechanistic design approach as it provides am o re rational and logical approach to

pavement design.The advent ofp e r s o n a lcomputers and theavailability ofc o m m e r c i a ls o f t w a re for multi-l a y e red analysisand total pavementdesign packageshave furthersimplified ther i g o r o u sc a l c u l a t i o n sre q u i red formechanistic design.In order not to beleft out, Malaysiamust initiate it’sown mechanistic

design pro c e d u res sothat pavement design pro c e d u res are morein line with other advanced countries.R e s e a rch into local materialscharacterisation must also be done toreflect the actual properties of materials tobe used as data input in the design. ■

REFERENCES

[1] Newcomb, David E and Timm,David H. (2001,September/October). MechanisticPavement Design: The NextWave. Hot Mix AsphaltTechnology, 49-51.[2]

[2] Thompson, M.R. (1992). Illi-PaveBased Conventional FlexiblePavement Design Procedure. InProceedings of the 7thInternational Conference onAsphalt Pavements (pp. 318-333).London : ISAP.

[3] Huang, Yang H. (2004), PavementAnalysis and Design (2nd ed.),Englewood Cliffs: Prentice-Hall.

[4] Harichandran, Ronald S. (2001)MICH-Pave Users Manual. EastLansing: Michigan StateUniversity.

[5] Asphalt Institute (1981), ThicknessDesign Manual (MS-1), 9th ed.,Lexington: Asphalt Institute.

[6] Shell (1978). Shell PavementDesign Manual. London: ShellInternational Petroleum Co.Ltd.

Figure 7: Example of Asphalt Institute’s design chart [5]

Figure 8: Example of Shell pavement design chart [6]