Flexible Pavement Design in India_Past, Present and Future

*Superintending Engineer, M/o RT&H, Regional Office (C), Guwahati (India) – 781 003,Email – [email protected], [email protected].

FLEXIBLE PAVEMENT DESIGN IN INDIA: PAST, PRESENT AND FUTURE

Sanjay Garg*

Abstract

In India, during twentieth century new flexible pavements were designed byCalifornia Bearing Ratio method, an empirical method, based on subgrade strengthmeasured in terms of CBR value which was, with the advent of twenty first century, takenover by Mechanistic-Empirical methods. Continued improvement in trafficcharacterization, material characterization and quality, mix designs, pavement designapproaches and construction methodologies, performance prediction models and laboratorytesting procedures, and maintenance approaches will call for a review of the present state-of-the art pavement design approaches. In this paper, journey of the flexible pavementdesign in India is briefly outlined along with the shortcomings of the current flexiblepavement design method. Future steps to be taken up for its further refinement andimprovement are also discussed in view of current developments in order to optimize thepavement structure and its performance, and to evolve a sustainable pavement structure.

1. BACKGROUND:

Figure 1 Conventional flexiblepavement used in India

Broadly, there are three types of pavements; flexiblepavements, rigid pavements and compositepavements. In this paper, discussion is limited toflexible pavements only. In flexible pavements,wheel loads stresses are transferred by grain-to-graincontact of the aggregate through the granularstructure which acts like a flexible sheet due to lessflexural strength. The wheel load acting on thepavement will be distributed to a wider area, and thestresses decreases with the depth. Taking advantageof this distinct stress distribution characteristic,flexible pavement normally has many layers in whichmaterial quality deceases from top to bottom.

In India on all National Highways, a conventional flexible pavement consistsusually five layers – surface or wearing course (BC), binder course (DBM), granular basecourse (WMM or WBM), granular sub-base course (GSB) and compacted subgrade overnatural subgrade as shown in figure 1. If combined thickness of all bituminous layers(surfacing and binder course) is about 75 mm or less, then it is termed as thin bituminouspavement while a thick bituminous pavement usually have combined thickness of allbituminous layers equal to 150 mm or more.

Unlike other civil engineering structures, the structural design of a pavementstructure is, practically, a complex and daunting task due to uncertainty, variability andapproximations of everything associated with the design of new and rehabilitatedpavements. Traffic loading is a heterogeneous mix of vehicles, axle types, and axle loadswith distributions that vary with time throughout the day, from season to season, and overthe pavement design life. Traffic forecasting is very difficult. Pavement materials respondto traffic loading in complex ways influenced by stress state and magnitude, temperature,moisture, time, loading rate, and other factors. Pavement construction also introduces a

Surface course – BC

Binder course – DBM

Granular Base course– WMM or WBM

Granular Sub-basecourse – GSB

Compacted subgrade

Natural subgrade

mailto:[email protected]

mailto:[email protected]

2

significant measure of variability. Pavements as a function of time and maintenancestrategies exhibit significant variation in condition over its design life and therefore,performance predictions and its relation to input variables add further complications.

1.1 ERA OF EMPIRICAL METHODS:

Due to all these complexities, empirical methods were resorted to design apavement structure during twentieth century. Pavement design consisted basically ofdefining thicknesses of layered materials that would provide strength and protection to asoft and weak subgrade. In an empirical pavement design approach, the relationshipbetween design inputs (e.g., traffic loads, materials, layer configurations and environment)and pavement (performance) failure were arrived through empirical correlations betweenrequired pavement thickness and soil classification or simple strength tests of subgradematerials using the data of past experience (based on successes and failures of previousprojects), experiments or a combination of both. Index-value-based characterizations ofmaterial properties (layer coefficients, R-value, California Bearing Ratio etc.), andengineering judgment with failure criteria of limiting shear failure or deflections insubgrade layer or serviceability loss were used for pavement designs.

As experience evolved, several pavement design methods based on soilclassification and subgrade shear strength were developed. First empirical method forflexible pavement design was based on the soil classifications developed during 1920s,which lead to Group Index Method. In 1929, California Bearing Ratio (CBR) method[1, 2]

was developed by the California Highway Department using the CBR strength test whichrelates the subgrade material’s shear resistance evaluated by CBR value to the requiredthickness of overlaid layer (cover). The thickness computed was defined for the standardcrushed stone used in definition of the CBR test.

The empirical AASHTO method (1993) was, based on the pavement performancedata collected during American Association of State Highway Officials (AASHO) RoadTest carried out in 1960s, mainly used in USA and Canada. The AASHTO design equationwere developed through regression models to link the performance data with design inputsand represent a relationship between the number of load cycles, pavement structuralcapacity, and performance, measured in terms of serviceability loss. The concept ofserviceability, based on surface distresses commonly found in pavements, was introducedin the AASHTO method as an indirect measure of the pavement’s ride quality.

Although all these empirical methods were used for over fifty years and exhibitgood accuracy, however, they were valid only for the local circumstances [like materialselection, traffic (type, volume and axle loading), climatic conditions, drainage measures,and construction techniques etc.] in which they were developed. As an empirical procedurerelies entirely on past observations of field performance, therefore, these methods could notbe used for traffic load levels and in environments well beyond their observational domain.In other words, it allows no extrapolation beyond the range of these observations. Further,these index and empirical models do not include[2] effects of multidimensional geometry,loading, material behavior and spatial distribution of displacements, stresses and strains inthe multilayered pavement systems. Hence, such empirical approaches are considered topossess only limited capabilities. The AASHTO method, for example, was adjusted severaltimes over the years to incorporate extensive modifications based on theory and experiencethat allowed the design equation to be used under conditions other than those of theAASHO Road Test. CBR method was also improved consistently and became the mostpopular design method around the world. In India also, CBR method was used for flexiblepavement design till 2001.

3

1.2 ERA of MECHANISTIC-EMPIRICAL PAVEMENT DESIGN METHOD:

During last decades of twentieth century, traffic volume and loading have increasedand new materials started to be used in pavement structures that provided better subgradeprotection, but with their own failure modes which also bring changes in the designcriterion. Besides providing subgrade support, it became equally important to evaluatepavement performance through ride quality that governs the rate and/or extent ofdeterioration of pavement structures. Performance became the focus point of pavementdesigns. Initially, empirical methods, such as AASHTO design guide, 1993, based onserviceability (an index of the pavement service quality) loss were developed.

Later on, classical theories of mechanics were used to evaluate the pavementstructural responses in terms of stresses, strains, and deflections at critical locations withinthe pavement structure under the effects of traffic loading from structural (mathematical)modeling of pavement structures, generally presuming the pavement structure andsubgrade as a multi-layer linear elastic system: each layer characterized by its thickness,modulus of elasticity and Poisson’s ratio. This step is termed as Mechanistic part. InEmpirical part, these (critical) pavement responses were correlated with pavementperformance indicators in the form of pre-defined pavement distress modes for a givendesign life by empirically derived equations known as distress models or transfer functionsderived from the performance prediction models based on past experiences, fieldobservations and laboratory results that compute the number of repetitive loading cycles tospecified pavement failure. Initially, two classical failure modes namely bottom-up fatiguecracking at the bottom of bituminous concrete and permanent deformation in subgradelayer were considered in the performance models. Based on this Mechanistic-Empiricaltwo-step (hybrid) approach, Asphalt Institute method (Asphalt Institute, 1982, 1991) andthe Shell method (1977, 1982) besides other methods were developed. With slightmodifications in these two methods based on in-house research results and feedback on theperformance of the pavement designs in India during twentieth century, current flexiblepavement design as depicted in IRC:37–2001 came into existence.

Mechanistic-Empirical (M-E) pavement design approach provides the capability todetermine the required layer thicknesses so that the pavement would last for specifieddesign life without exceeding predetermined distress levels. This approach represents amajor improvement over empirical methods due to its accuracy and reliability. The biggest“empirical” part (also termed as weakest links) of M-E pavement designs are the transferfunctions, material characterization and their variations in relation to environmentalinfluences over time, and the characterization of traffic. The accuracy of structural responsemodel and performance prediction model are a function of quality of the input variablesand the calibration of empirical distress models to observed field performance. It remainsdifficult to quantify pavement distresses and performance predictions using the concepts ofmechanics and to relate them with pavement responses. This is the reason, why inperformance prediction models used so far, empirical formulas are used to predictpavement distresses from the pavement responses. It is also a reality that a fullymechanistic method for practical pavement design is still a goal to be achieved.

2. INTRODUCTORY PHASE OF PAVEMENT DESIGN – IRC:37–1970[3]:

2.1. Before 1970, on the basis of limited and localized experiences and judgments oflocal highway agencies, quite diverse practices for pavement design were prevalentin India. In 1970, an empirical method, CBR method based on CBR design curvesevolved by the Road Research laboratory, United Kingdom was introduced viaIRC:37-1970 by IRC as a unified approach for flexible pavement design in entirecountry. The thickness of different layers of sub-base, base and surfacing weredetermined by repeated use of these design curves subject to specified minimum

4

thicknesses for constituent layers. These design curves were applicable for singleaxle loads of 8200 kg and tandem axle load of 14500 kg. Beyond these values ofaxle loads, pavement thickness was increased appropriately.

2.2. For subgrade soil, CBR value was calculated from the sample prepared at optimummoisture content corresponding to Protector Compaction and soaked in water for aperiod of four days prior to testing. Traffic was considered in units of heavycommercial vehicles per day (CVD) with a laden weight of 3 tonnes or more inboth directions (irrespective of whether the design is for a two lane or a dualcarriageway), divided into seven categories as indicated in the table 1.

Table 1 Traffic classification as per IRC:37–1970Design Trafficvolume, CVD

0-15 15 -45

45 -150

150 -450

450 -1500

1500 -4500

>4500 and allexpressways

CBR design curve,applicable

A B C D E F G

2.3. Pavement was designed for the traffic volume expected at the end of design life(taken as 10 years), which was determined as:

AD = P (1+r)n+10 …(1)

where, AD = number of commercial vehicles per day (CVD) for design,P = number of commercial vehicles per day at last count,r = annual rate of increase in the number of commercial vehicles,

(taken as 7.50% in case authentic data is not available), andn = number of years between the last count and the year of completion

of construction.

Example 1: (a) Given that, subgrade CBR = 5%, and design traffic (AD) volumeexpected at the end of design life = 1501 CVD. Design pavement.

(b) Design the flexible pavement with data of part (a) with design traffic(AD) volume expected at the end of design life = 4501 CVD.

Solution:(a) For given input data, total pavement thickness from IRC:37–1970 comes out

to be 475 mm. Let, sub-base, GSB = 150 mm andbase course will comprise, WBM = 250 mm, provided in three layers of 100mm of WBM Grade I + 75 mm each of WBM Grade II and WBM Grade III.50 mm thick bound base course (like bituminous macadam, BM) withsurfacing of 20 mm thick open-graded premix carpet (PC) or surfacedressing will be provided. This pavement design was applicable for thedesign traffic volume ranging from 1501 to 4500 CVD.Provided thickness = 150+250+50*1.5=475 mm. O.K.

(b) For given input data, total pavement thickness from IRC:37–1970 comes outto be 530 mm. Let, sub-base, GSB = 200 mm andbase course will comprise, WBM = 250 mm, provided in three layers of 100mm of WBM Grade I + 75 mm each of WBM Grade II and WBM Grade III.50 mm thick bound base course (like bituminous macadam, BM) withsurfacing of 25 mm thick semi-dense carpet (SDC) will be provided. Thispavement design was applicable for the design traffic volume more than4500 CVD and all expressways.Provided thickness = 200+250+50*1.5+25*1.5=562.5 mm > 530 mm. O.K.

5

3. FINAL PHASE OF EMPIRICAL METHOD IN INDIA – IRC:37–1984[4]:

3.1. The empirical design method for flexible pavement, as proposed in 1970, wascontinued for design traffic volume up to 1500 CVD. However, the modified CBRcurves for 10.2 tonn single axle legal limits were used instead of 8.16 tonn andconsequently, the pavement thickness was increased by 10 to 20%.

3.2. Recognizing the fact that the structural damage caused by a vehicle depends on theaxle load it imposed on the road, the equivalent axle load concept was introduced inIndia also similar to abroad to handle the large spectrum of axle loads actuallyapplied to a pavement. Design traffic (Nx) carried by pavement during its design lifewas considered in terms of cumulative number of standard axles in the lanecarrying maximum traffic and evaluated as under:= ∗ ∗ [( ) ]∗

…(2)where, Nx = cumulative number of standard axles to be catered for design,

(expressed in terms of million (106) standard axles or msa)A = initial traffic, in the year of completion of construction in CVD, as

modified for lane distribution,r = annual growth rate of commercial traffic, taken as 7.5%,x = design life in years, taken as 10 to 15 years.F = vehicle damage factor.

3.3. Design curves relating pavement thickness to the cumulative number of standardaxles (8160 kg) for different subgrade strengths (assessed in terms of CBR value)were evolved. Pavement composition (thickness of component layers) was thereforemight be decided by the designer subject to the minimum thickness as determinedfrom the thickness combination block given in the IRC:37–1984.

Example 2: Given that, subgrade CBR = 5%, and traffic after construction, A = 730CVD. Design flexible pavement for 10 years for two lane NH in plainterrain as per (a) IRC:37–1970 and (b) IRC:37–1984.

Solution: Let, total pavement thickness = T, mm(a) Pavement design as per IRC:37–1970, and using equation (1), we get

AD = 730 (1+.075)10 = 1505, andtherefore, T = 475 mm from IRC:37–1970.This thickness is applicable for AD varies from 1501 CVD to 4500 CVD.

(b) Pavement design as per IRC:37–1984, and using equation (2), we getNx = [365 x (730*0.75) * {(1+.075)10 – 1}* 2.75/0.075] = 7.78 msatherefore, T = 540 mm from IRC:37–1984.As per IRC:37–1984, surfacing should be 25 mm SDC or BC with bindercourse 75 mm DBM while base course should have a minimum thickness of250 mm with material of 100% CBR. Therefore,thickness of sub-base = 540 – (25+75+250) = 190 mm > 150 mm, O.K.Provide sub-base of 200 mm with material of 30 % CBR.In base course, either (1 x 100 + 2 x 75 = 250 mm) of WBM in three layersor two layers each of 75 mm thick of WBM + one layer of 75 mm thick BM(2x75+75*1.5=262.5 mm) can be provided. Later option is preferable.

Comments: As per IRC:37–1984, for A = 2183 CVD, Nx = 23.25 msa and T =635 mm. As per IRC:37–1970, pavement thickness is constant for traffic volumeafter construction varies from 730 CVD to 2183 CVD while as per IRC:37–1984, itvaries from 540 mm to 635 mm making the pavement design more responsive toapplied traffic volume (loading).

6

4. STATE-OF-THE-ART FLEXIBLE PAVEMENT DESIGN IN INDIA –IRC:37–2001[5]: To overcome limitations and empiricism in pavement design asdiscussed in para 1.1, attempts were made under the patronage of Ministry of RoadTransport and Highways (MORT&H), Government of India via research schemesR-6, R-19 and R-56 which gave birth to IRC:37–2001 and thus, laid down thefoundation for Mechanistic-Empirical (M-E) Pavement Design Method for flexiblepavement designs in India and open a new chapter in the history of pavementdesigns with ample scopes for further improvements and refinements in future.Salient features of the IRC:37–2001 are briefly described below:

4.1. Only conventional standard flexible pavement structure as shown in figure 1 hasbeen considered for pavement design, which has been modeled as a three layerstructure consisting of binder layer (BM or DBM) plus surface layer (PC, MSS,SDBC, or BC) as layer 1, granular sub-base layer (GSB) plus base layer (WBM orWMM) as layer 2, and compacted subgrade as layer 3. After taking

(i) a typical fixed value of elastic modulus (E1) at average annual pavementtemperature of 35 0C and Poisson’s ratio (μ1) of 0.50 for bituminous layers havingDBM/BC constructed with 60/70 grade bitumen,

(ii) μ2 = 0.40 for granular layers and a restricted composite elastic modulus of sub-baseand base course (E2) determined by the empirical equation 3(a) and, and

(iii) μ3 = 0.40 for subgrade layer and elastic modulus of subgrade (E3 ) determinedempirically from the index property, CBR value through equation 3(b) and 3(c),

the pavement structures were analyzed.

E2 (MPa) = E3 * 0.20 * h0.45, …(3a)where, h = thickness of granular layers, mm

E3 (MPa) = 10 * CBR for CBR ≤ 5, and …(3b)= 17.6 * (CBR)0.64 for CBR > 5 …(3c)

4.2. The pavement responses, in terms of the critical strains [(a) vertical compressingstrain (εc) at the top of the subgrade – to avoid excessive strain and hence,permanent deformation (or rutting) in subgrade layer during design life), and (b)horizontal tensile strain (εt) at the bottom of the bituminous layer – to avoid thebottom-up fatigue cracking at the bottom of bituminous layers] at pre-definedlocations, have been computed using the linear elastic model “FPAVE” developedunder MORT&H’s Research Scheme R-56 “Analytical Design of FlexiblePavements”. Rutting within the bituminous layer(s) was avoided or controlled bymeeting the mix design requirements as per the MORT&H’s Specifications.

4.3. These strains were then, used to predict the performance level as defined in terms oftwo classical modes of structural distresses namely bottom-up fatigue (alligator)cracking and rutting in subgrade layer resulting from repeated (cyclic) applicationof traffic loads as per the following two failure criterions which ensure a specifiedlevel of pavement performance at the end of design life.

4.3.1 Fatigue Criteria: The distress prediction model was calibrated to developthe following fatigue cracking failure criterion which relates allowable number ofload repetitions (the fatigue life of the pavement) to horizontal tensile strain at thebottom of the bituminous layer (εt) for a pre-defined performance level (asconsidered in the form of fatigue cracking in 20% of the design lane area).

3 . 8 9 0 . 8 5 44 1 1

2 . 2 1 1 0ft

N xE

…(4)

in which, Nf is the allowable number of load repetitions to control fatigue crackingand E is the effective elastic modulus of all bituminous layers.

7

4.3.2 Rutting Criteria: Similarly, for limiting the permanent deformation insubgrade layer up to 20 mm, the rutting failure criterion relates allowable number ofcumulative standard axles (Nr) to vertical compressive strain (εc) at the top of thesubgrade layer as:

4 . 5 3 3 7

8 14 . 1 6 5 6 1 0r

c

N x …(5)

4.4. Consequently, the pavement design tables or catalogues for the conventionalstandard flexible pavement structure in terms of total pavement thickness andconstituent layer thickness were developed to cater for:a. design traffic (evaluated as before by equation 2 except with slight modification

in vehicle damage factor) ranging from 1 msa to 150 msa,b. sub-grade material characterized as before in terms of index property, CBR

value ranging from 2% to 10% andc. an average annual pavement temperature of 35 0C.

4.5. Pavement design catalogue as outlined in IRC:37–2001 provide an easiest methodto design the flexible pavement on the basis of the Mechanistic-Empirical PavementDesign philosophy. Table 2 provides the details for thickness determination ofpavement structure and constituent layers.

Table 2 Total pavement and layer thickness based on subgrade CBR value, mmTraffic (Nx), msa Subgrade strength as measured in terms of CBR value

2% 3% 4% 5% 6% 7% 8% 9% 10%10 850 760 700 660 615 580 550 540 54020 880 790 730 690 640 610 575 570 56530 900 810 750 710 655 630 590 585 58050 925 830 780 730 675 650 610 605 600

100 955 860 800 750 700 675 640 635 630150 975 890 820 770 720 695 660 655 650

Base course A constant thickness of 250 mm is considered.Sub-base course 460 380 330 300 260 230 200@ 200@ 200@DBM thickness* 100 -

21590 -210

80 -190

70 -170

65 -160

60 -165

60 -160

50 -155

50 -150

BC thickness 40 mm for traffic ≤ 50 msa and 50 mm for traffic > 50 msa.

@ Minimum thickness of GSB is 200 mm for design traffic more than 10 msa.* Binder course for all design traffic levels more than 10 msa is DBM. Thicknessof DBM varies from 50 mm to 215 mm to ensure structural adequacy of thepavement structure for varying subgrade strengths in terms of CBR value from 10%to 2% and cumulative design traffic varying from 10 msa to 150 msa.

Example 3: Given that, subgrade CBR = 5%, traffic growth rate, r = 7.5% and traffic afterconstruction, A = 225 CVD, 730 CVD and 2183 CVD. Design flexible pavement for 10and 15 years for two lane NH in plain terrain as per IRC:37-1984 and IRC:37-2001.

Solution: Let, Total pavement thickness = T, mm

Design traffic (in msa),( ){ }365* * 1 1 * *

x

x

A r D FN

r

+ −= ,

where, lane distribution factor, D = 0.75, for a two lane NH/SH.

Design details are given in table 3, from which it is clearly evident that pavementthickness for a pavement structure designed as per IRC:37–2001 increased by 13% to23.7% over the pavement design as per IRC:37–1984 primarily to account for the increasedshare of heavy axle loads and ensuring some certainty in pavement performance against

8

two classical modes of pavement failure i.e. bottom-up fatigue cracking and subgraderutting. However, in view of current developments it is questionable whether suchenhancement in pavement thickness is justifiable. Will it lead to overdesign?

Table 3 Pavement design for Example 3Design

LifePavement Design as perIRC:37–1984 (F-2.75)

Pavement Design as perIRC:37–2001 (F-4.50)

10 years • A = 225 CVD, thenNx = 2.40 msa and T = 460 mm.

• A = 730 CVD, thenNx = 7.78 msa and T = 540 mm.





15 years • A = 225 CVD, thenNx = 4.42 msa and T = 500 mm.


• A = 2183 CVD, thenNx = 42.92 msa and T > 665 mm.




5. WHERE WE ARE? – SHORTCOMINGS OF IRC:37-2001:

Although the IRC:37–2001 design method was a major step forward it still hasmany drawbacks due to partial application of the versatile Mechanistic-Empirical pavementdesign approach due to so much simplifications and assumptions which otherwise has hugepotential to provide an optimize pavement design. Succeeding paragraphs presents adetailed discussion on the shortcomings of the current flexible pavement design based onIRC:37–2001 in India.

5.1. The biggest drawback of these design guidelines is that pavement design is basedon a catalogue applicable only for a fixed set of conditions namely a standardflexible pavement structure as shown in figure 1, material properties of bituminousmixture, design (failure) criterion, and annual average pavement temperature (350C). Neither FPAVE nor the analysis and design approach is available in publicdomain either for free or some cost. Therefore, it is not known to the designer whatwill happen or in what way will he analysis and design the pavement structure, ifany of these variables will vary? Absence of any pavement design software is thebiggest hurdle as a designer is unable to perform the analysis and design thepavement structure with user-defined (or project-specific) input variables and thus,to optimize the design. Further, for a given set of traffic volume and subgradestrength, it gives one feasible solution only leaving no scope for the designer tooptimize the pavement structure economically or in terms of material consumptionand/or quality as available at site.

5.2. Second biggest drawback is that the pavement design was based solely on thetraffic loading. The effect of environmental influences was not considered.

5.3. Another main drawback is that IRC:37–2001 is unable to evaluate the effect ofvariations in materials on pavement performance and therefore, better utilization ofavailable materials cannot be ensured and the potential benefits of new types ofmaterials cannot be evaluated. Furthermore, if base or sub-base layer will be boundwith bitumen, cement, lime, puzzolana, fly-ash, soil-cement etc., then these designguidelines cannot be used. In case of bituminous layers constructed with recycledmaterials, these guidelines are unable to design pavement satisfactorily.

9

5.4. These guidelines can be used only to design new standard flexible pavementstructure. Therefore, pavement strengthening or structural overlay design remainsbased on empirical approach which has very limited applicability for the thickbituminous layered pavement structures constructed now-a-days and carry trafficvolume and loading beyond tested domain.

5.5. Characterization of Traffic: At present in India, the concept of equivalencyfactors are used to characterize traffic in which different axle types are convertedinto equivalent single axle loads (ESALs) through load equivalency factors (LEF)or vehicle damage factor (VDF) calculated on the basis of fourth-power law.Although, use of ESALs concept simplifies the design process, however, theconcept of relative damage is not quantifiable as it is based on empirical results ofthe AASHO Road Test, wherein it was concluded that the pavement damageincreases with axle weight raise to fourth power. Besides, LEF (or VDF) dependson the specific set of conditions that include the axle loading, axle configuration,pavement type and thickness, tire type, tire inflation pressure, environment, distressmode, and terminal serviceability. Because of this, it is difficult to calculate ESALsfor (a) new types of vehicles, (b) axle multiplicity, change in axle loading andconfigurations, (c) change in tire type and tire pressure, and (d) change in pavementfailure modes. This difficulty and inbuilt empiricism are the major reasons formoving towards other ways for traffic characterization such as axle load spectradescribed in para 6.2.1. The lateral traffic wander has not considered in currentdesign practice, which needs to be included in design process as it influences thenumber of load applications over a point and hence, affects prediction of fatiguecracking and permanent deformation.

5.6. Characterization of pavement materials: For a successful and effective pavementdesign, characterization should be based on material properties that accuratelycapture the material response which influenced by construction quality, appliedtraffic loading and environmental conditions varies over design life. At present, nosuch consideration in material characterization is taken care of.

5.7. Location of critical pavement responses: Currently, the critical stresses and/orstrains are computed at only two locations namely directly beneath the center of thetire and at the centre of dual tire for a single axle with dual tires. This approach isnot correct for multi-axle loads as the critical location is a function of the wheelload/axle configuration and the pavement structure. To evaluate the maximumprincipal (design) strains under single or multi-axle loadings, pavement responseshould be evaluated at several locations and corresponding pavement damages(distresses) will be calculated for each location. Location of maximum damage willbe the critical location and should be made part of design evaluation process.

5.8. Distress Prediction and Failure Criterion:(a) After the year 2000, flexible pavements on all major National Highways (specially

under NHDP programme) and some State Highways developed either under PPPmodels or as externally aided projects have been constructed in India with densemix graded bituminous layers having thickness from 125 mm to 225 mm. For suchthick bituminous layered pavements, other structural distress modes like top-downfatigue (longitudinal) cracking and thermal fatigue (transverse) cracking may alsoplay vital role in predicting their performance. Besides, rutting in bituminous layersas well as in unbound base/sub-base layers is equally important. In cold regions likeJammu and Kashmir, Himachal Pradesh, Arunachal Pradesh and Uttarakhand, low-temperature cracking also becomes important performance indicator. Therefore,critical strains for all these structural distress modes need to be evaluated andshould be made part of design process. Functional distresses like surface roughness

10

and friction may also be included as failure criterion and transfer function mayaccordingly be developed to ensure functional serviceability of the pavement.

(b) Transfer functions (empirical equations) for the two classical distress modes weredeveloped through two research schemes R-6 and R-19 mainly for the thinbituminous flexible pavements. Further, the transfer function for fatigue failurecriteria was calibrated for one specific condition (at 35 0C for BC surfacing having80/100 bitumen) only. There is no published study or reports which tell us howthese transfer functions are sensitive to varied conditions of traffic, climate,material quality, mix designs, pavement constructions, and maintenance practicesetc. as per actual field conditions? It is, however, of paramount importance thatthese transfer functions accurately reflect the actual performance of pavementsunder the expected conditions, because extrapolating them beyond their testedbounds can result in over-designed or under-designed pavement sections.

(c) In current IRC:37-2001 design practice, a fixed distress criteria in terms of fatiguecracking in an area of more than 20% and/or a rutting depth of 20 mm or more isconsidered to define the failure mode of the pavement. Variability in type andextent for all failure modes to specify a performance (distress) level is required tobe left for the designer to decide it as per importance and need of the project, anddesired level of pavement performance and adopted maintenance strategies.

5.9. Absence of Reliability concept: The design of flexible pavements is associatedwith many factors that introduce a substantial measure of variability anduncertainties. These factors include traffic prediction, material characterization andbehavior modeling, environmental conditions, construction quality, andmaintenance practices etc. Therefore, reliability concept is required to be introducedin the new IRC design practice as a means of incorporating desired degree ofcertainty into the design process and to ensure that the various design alternativeswill survive for the analysis period without reaching to unacceptable condition ofpavement performance.

5.10. Cumulative traffic for 20 years design period and two lane highways with vehicledamage factor of 4.5 and traffic growth rate of 7.5% becomes 150.3 msa for initialtraffic volume after project construction of 2825 CVD only. It is not understoodhow one can design the pavement structure for the forthcoming expressways whichwill carry traffic volume certainly more than 3000 CVD as IRC:37–2001 isapplicable only for cumulative design traffic up to 150 msa and the approachsuggested in IRC:37–2001 for dealing traffic more than 150 msa is not feasible asIRC:81–1997 was based on empirical method which has very limited applicabilitydue to changes in the conditions (such as pavement structure, traffic volume andloading, construction methods, material quality, and climatic conditions etc.) inwhich it was developed. Similarly, for all projects constructed under BOT model orPPP model or any similar financing model with usual range of concessioner periodof 25 to 30 years, the current guidelines are unable to provide an optimal pavementdesign for commensurate service life. Design life for flexible pavements need to beenhanced to 30 to 40 years in line with practice in abroad.

5.11. It is also questionable to use planned stage construction approach as suggestedeither in IRC:37-2001 or IRC:SP:84-2009 due to reasons as stated in sub-paras 5.4and 5.10 or if the pavement design will be based on cumulative damage approacheither against fatigue and/or rutting etc. which involves highly nonlinearrelationship of the design inputs with pavement responses.

5.12. There is still no guideline or direction in India to design the thick bituminouspavements such as full-depth bituminous pavements (constructed by placing one ormore layers of dense graded bituminous layers directly on the sub-grade) and deep-strength bituminous pavements (in which dense graded bituminous layers are placed

11

on relatively thin granular base course). In view of the author, these bituminouspavements[6] are relatively more useful alternatives to handle heavy traffic volumeand loads observed generally on Indian Highways besides (a) less construction timeand extended construction period, (b) less affected by moisture variation or frostand (c) using only one material (dense graded mix like DBM and BC) and thus,minimizing the haulage, administration and equipment costs. Additional benefitsinclude less consumption of materials and relatively less maintenance. Hence, theirdesign needs to be included in IRC guidelines.

6. WHAT WILL WE DO? – FUTURE OF PAVEMENT DESIGN:

The dilemma is that pavement materials do not exhibit the simple behavior assumedin isotropic linear-elastic theory. Loading rate, time and temperature dependency,nonlinearities, and anisotropy are some examples of complicated features often observed inpavement materials. However, continued improvement in material characterization, andconstitutive models make it possible to incorporate nonlinearities, loading rate effects, andother realistic features of material behavior. Determination and/or prediction with sufficientaccuracy of some prime input parameters such as climate, traffic and the quality of thematerials as laid and the variation therein remains an issue to be addressed. Large databasesnow exist for traffic characterization, site climate conditions, pavement material properties,and historical performance of in-service pavement sections coupled with improvedmodeling of pavement structure provide the technical infrastructure that made possible therefinement in structural analysis of pavement responses.

Pavement performance models should be extended and refined further withinclusion of more distress modes of pavement failure, calibrated with comprehensivetesting and characterization of materials in bituminous, bound/unbound base layers, andsubgrade soils and validated with actual field observations/testing under varied conditionsof material quality, mix designs, pavement constructions and maintenance practices. It isalso possible to model pavements structures as accurate as possible using non linear elasto-visco-plastic models and using advanced finite element techniques formulated on theconcepts of either classical mechanics or damage mechanics or fracture mechanics thatallow damage initiation and progression to be taken into account as well as the effects ofstress re-distribution as a result of that. Also such methods allow the effects of joints,cracks and other geometry related issues to be taken into account. Furthermore, thesemethods also allow to analyze the effects of moving loads which implies that inertia anddamping effects can be taken into account.

Finally, Mechanistic-Empirical pavement design approach, so evolved, will providemore rational and realistic methodology to account for uncertainty, variations andapproximations in structural modeling, traffic loading, environmental effects, materialcharacterization, and performance models. Further, it will provide a much better insight inwhy pavements behave like they do and provide a good estimate of pavement performance.Later on, pavement design process may be integrated with in-service maintenanceneeds/decisions and desired performance levels to evolve optimal pavement management.However, they involve advanced testing and analyses techniques, some are alreadydeveloped and others are under evolution stages.

All these refinements are experiencing from last 20 years in abroad specifically inUSA under 20 years Long-Term Pavement Performance Program (1989-2009) as a part ofthe Strategic Highway Research Program (SHRP) and to realize them in India also, similarquantum of dedicated research and field efforts are required. Research should be large scalespread over the geographical breadth of the country; long term and integrated; all-inclusive

12

and result-oriented which blends the lab tests and field tests. It should include allcomponents of pavement design process like traffic characterization, material selection andcharacterization, pavement modeling and design techniques, mix design and constructionmethodology, maintenance approach, performance assessment and distress predictions etc.To measure the resilient modulus, moisture susceptibility, permanent deformation, andfatigue cracking properties of the bound and unbound materials within the pavementstructure and to quantify the quality and effectiveness of the pavement construction andtreatment practices, improved Accelerated Pavement Testing (APT), and Non-destructivetesting (NDT) technologies should be developed. In the research, all major stake holdersshould be included with their respective roles and contributions. Besides others, at least thefollowing objectives may be part of this research in India:

i. Identify the reasons for development of the distresses in early phases of pavementservice life based on lab/field testing.

ii. Determine the effects of loading, environment, material properties and variability,construction quality, and maintenance levels on pavement distress and performance.

iii. Determine the effects of specific design features on pavement performance.iv. Evaluate the existing design methods and pavement performance.v. Develop unified and improved design methods/equations for all types of new or

reconstructed pavements as well as rehabilitated pavements.vi. Develop improved design methodologies and strategies for the preservation, repair

and rehabilitation of existing pavements.vii. Establish a national long-term pavement performance and maintenance database.

Findings must be documented and well circulated among the field experts of thecountry for comments and further refinements. Finally, research reports should bepublished and made available in public domain to learn the lessons and/or gain experienceto further improve the pavement design, construction and maintenance process. It is mostessential to generate the field performance data bank based on actual lab and field testsrather than visual observations and enter the era of more rational and realistic qualitycontrol practices besides nurturing the research industry in India as well as upgrading thelevel of our research institutes. Otherwise, we always remain dependent upon the findingsof foreign institutions and will be waiting for their technology transfer besides wasting ourhuge indigenous talent in nonproductive works. It is the high time we will wake up andunderstand the importance of result-oriented research activities as said many times by theHon’ble Prime Minister in the Indian Science Congress and other platforms. It is, however,emphasized that there is no dearth of funds in the country. The only need is that we mustdevelop and provide a conducive environment for the research activities which will benefitus and our forthcoming generations.

Mechanistic-Empirical Pavement Design Guide (MEPDG, 2008)[7, 8 and 9] wasdeveloped in USA to address the shortcomings in the current pavement design methods. Itis applicable for new as well as rehabilitation of flexible, composite or rigid pavements.Basically, it incorporates axle load spectra approach, variation of material properties withclimate and traffic loading, consideration of all major modes of structural distresses plusone functional distress (smoothness), incremental distress computation approach tosimulate how pavement damage occurs in nature (field) and transfer functions developedafter comprehensive simulative lab testing coupled with field observations during LTPPunder varied climate and traffic conditions. Figure 2[7] is showing an overview of theFlexible Pavement Design Process used in MEPDG, 2008.

13

Figure 2 Overall design process for flexible pavements (NCHRP[7])

With this background and based on MEPDG (2008)[7], pavement design method inIndia should also involve the following major steps besides other refinements to eliminateor obviate the aforesaid shortcomings in the state-of-practice pavement design process:

6.1. Objective statement: A pavement structure should be designed so that:

a. it must be structurally and functionally adequate during entire design life,b. it must survive the pre-defined performance level at the end of its design life as per

designer’s chosen certainty (reliability) level,c. it should integrate design process with maintenance strategies,d. it should be constructible and use available local material to extent possible, ande. it should result an optimal and sustainable pavement structure – optimize the

pavement thickness and material consumption, minimize the initial cost as well aslife-cycle cost, and improve the pavement sustainability.

6.2. Design inputs: Based on the criticality of the project and the available resources, itis recommended to employ the hierarchical approach (similar to MEPDG, 2008) in theselection of design inputs with regard to traffic, materials, and environmental parameters.All design input variables should be clearly defined as explained below:

6.2.1. Characterization of Traffic: Traffic is the most important design input variable, asa pavement structure is designed to carry traffic. To eliminate the empiricism in theconcepts of ESAL and VDF, a more direct and rational approach like axle load spectramethod should be used to quantify the characteristics of traffic loads carried by a pavementstructure as it allows mixed traffic to be analyzed directly and thus, enhances pavementdesign process. The approach estimates the effects of actual traffic on pavement responseand distress. Additional advantages of the load spectra approach include: the possibility ofspecial vehicle analyses, analysis of the impact of overloaded trucks on pavementperformance, and analysis of weight limits during critical climate conditions. Load spectraare simply the collective axle weight distributions grouped by axle type for a given trafficstream which can be easily determined from the axle weight data obtained from weigh-in-motion (WIM) station or else. These spectra represent the percentage of the total axleapplications within each load interval for single, tandem, tridem, and quad axles. Vehicleclass distributions, daily traffic volume, and axle load distributions define the number ofrepetitions of each axle load group at each load level. For a given load group, the damagecaused by each load, on each axle type, and under each climate condition during the year issimulated through the life of the pavement.

14

6.2.2. Characterization of Pavement Materials: Effective characterization of pavementmaterials is a key requirement for a successful and effective pavement design. The stateand characterization of the different materials forming the pavement layers changes withvariation in temperature and moisture condition which in turn affected the structuralresponse of the pavement structure subjected to traffic loading. An effective analyticalmodel should account for all of these factors in analysis leading to a performance-baseddesign. Instead of index properties, fundamental engineering properties of material likedynamic modulus of bituminous materials and the resilient modulus of unbound materials(granular materials or native soils) as a function of time and environmental influences overthe entire design period and duly account for the variation in applied stress state, pavementdepth etc. are considered to compute the pavement responses.

6.2.3. Environmental condition: Moisture level and temperature changes are the twomain environmental variables which can significantly affect the pavement material’sproperties and, hence, impact the strength, durability, load carrying capacity, service lifeand serviceability of the flexible pavements. The resilient modulus of bituminous materialscan increase during winter months by as much as 20 times its value during hot summermonths. Excessive moisture can drastically lead to stripping of bituminous mixture.Similarly, resilient modulus of unbound materials at freezing temperatures exhibits highvalues compared to thawing months. The moisture content affects the state of stress ofunbound materials and it breaks up the cementation between soil particles. Increasedmoisture contents lower the modulus of unbound materials. Appropriate climatic model tosimulate changes in the behaviour and characteristics of pavement and subgrade materialswill be developed that concur with climatic conditions over the design period. The modelcomputes and predicts the modulus adjustment factors, pore water pressure, water content,frost and thaw depths, frost heave and drainage performance in case of granular orsubgrade layers. For the bitumen bound layers, the model evaluates the changes intemperature as a function of time to allow for the calculation of the dynamic modulus andthermal cracking. The model provides varying moduli values in the computation of criticalpavement response parameters and damage at various points within the pavement system.

6.2.4. Distress Prediction Model and Design Criterion: Pavement distress predictionmodel are typically derived through statistically based correlations of pavement responsewith observed performance of laboratory test specimens, full-scale road test experiments,or by both methods. A distress model can include a variety of structural (load-associated)distress as well as functional distresses as depicted in table 4 to assess and predict thestructural and functional performance of the pavement structure at the end of the designperiod. Design criteria for each distress should be pre-defined as indicated in table 4 andwill be compared with respective accumulated distress at the end of analysis or designperiod by the designer to check the adequacy and validity of the design.

Table 4 Distresses for flexible pavement with their design criterionStructural/functional distresses to be predicted Design Criterion

(i) Bottom-up fatigue (or alligator) cracking, 10 to 20% area of design lane(ii) Surface-down fatigue (or longitudinal) cracking, 100 m to 150 m/km(iii) Permanent deformation (or rutting) in any or all

of the pavement layers and subgrade,20 mm

(iv) Thermal fatigue (transverse) cracking, 100 m to 150 m/km(v) Surface roughness as measured in terms of

International Roughness Index (IRI).2.5 to 3.2 m/km

6.2.5. Reliability concept must be made part of the design process as stated in subpara 5.9.

6.3. Pavement Structure and its Mathematical Modeling: A pavement structure,flexible, composite or rigid, is composed of one or more layers constructed with different

15

materials placed on the prepared soil or subgrade. Each layer in a mathematical model willbe structurally defined by its modulus or stiffness, Poisson’s ratio and layer thickness.Under the action of traffic loading and environmental influences, pavement materialresponse may be linear or nonlinear, viscous or non-viscous, and elastic or plastic orviscoelastic and accordingly, structural analysis model will be chosen. Due to simplicityand computational speed, layered elastic model is the most commonly used structuralmodel for a pavement structure. To consider the effect of temperature and traffic load ratevariation on bituminous layers and moisture changes on unbound granular layers, theselayers should be divided into sub-layers. To account for traffic wander and various types ofaxles in the traffic mix, appropriate number of analysis points (critical locations) in eachsub-layer should be considered primarily to determine the following critical pavementresponses for distress calculation.a. Tensile horizontal strain at the bottom or top of the bituminous layer and at the

bottom of stabilized base/sub-base layer – to account for fatigue cracking), andb. Compressive vertical stresses and strains within the bituminous layer, within the base

and subbase layers and at the top of the subgrade layer(s) – for rutting.

6.4. Pavement Performance and its Prediction: The concept of pavementperformance includes consideration of functional performance, structural performance, andsafety. Pavement performance is affected by several factors, which are traffic, soil andpavement materials, environment, drainage condition, and construction and maintenancepractices. The structural performance of a pavement relates to its physical condition, orother conditions that would adversely affect the load-carrying capability of the pavementstructure or would require maintenance. Structural distress indicators includes fatigue(load-induced and thermal) cracking and rutting (in all layers) for flexible pavements, andjoint faulting, and slab cracking for jointed plain concrete pavements. The functionalperformance of a pavement concerns how well the pavement serves the highway user.Riding comfort or ride quality and skid resistance (or surface friction) are the two dominantcharacteristic of functional performance. Riding comfort is quantified in terms ofsmoothness as express by International Roughness Index (IRI) which combines the effectsof initial pavement/subgrade condition with the distresses developed over time.

6.5. Incremental Damage Accumulation Procedure: The design and analysis of agiven pavement structure is based upon the accumulation of damage as a function of time,traffic and climate. The design procedure should have the capability to accumulatedamages over the entire design period. Attempts will be made to simulate how pavementdamage occurs in nature, incrementally, load by load, over continuous time periods. Toachieve this goal, design life is divided into shorter design analysis periods or incrementsbeginning with the traffic opening month. Within each increment (or analysis period), allfactors (traffic and material characterization) that affect pavement responses and damageare held constant for simplification and computational speed. Critical pavement responses(stress and/or strain values) for each distress type are determined for each analysisincrement and thereafter, are converted into incremental distresses either in absolute terms(e.g., incremental rut depth) or in terms of a damage index (e.g., fatigue cracking) by thedistress prediction model. Incremental distresses and/or damage are summed over allincrements and output at the end of each analysis period is used by the designer to computethe accumulated distress and later on, to compare them with respective design criteria foreach distress.

6.6. Adaptability to New Developments: The analysis and design philosophy shouldbe capable to adapt the latest developments in pavement engineering. Therefore, design ofpromising perpetual pavement structure[6] may also be included in Indian Guidelines asthey provides a durable, safe, smooth, long-lasting roadway without expensive, time-consuming, traffic-disrupting reconstruction or major repair at short intervals and aimed to

16

minimize material consumption, lane closures, user delay cost and life-cycle cost besideshandling ever increasing traffic volume and loading including sporadic overloading inIndian scenario especially on NHDP projects and proposed expressways.

6.7. Life cycle cost analysis (LCCA): LCCA is a tool to determine the most cost-effective and feasible pavement design alternatives to build and maintain them byanalyzing initial costs and discounted future cost, such as pavement construction,maintenance, rehabilitation, and reconstruction cost as well as administrative cost toperform all these activities over the useful service life of pavement structure, and salvagevalue (all are grouped under Agency Costs); and User Costs (includes vehicle operatingcosts (VOC), crash costs, and user delay costs). In view of huge investment made in Indiain highway infrastructures during forthcoming period and to ensure the best value ofinvested public money, LCCA is becoming the most inevitable component of pavementdesign process and should therefore, be made part of design process in India also. LCCAcan also be used to evaluate the overall long-term economic efficiency between competingalternative investment options. Either of the economic decision tool such as Benefit/CostRatios, Internal Rate of Return, Net Present Value, and Equivalent Uniform Annual Costscan be used in LCCA.

7. CONCLUSION AND RECOMMENDATIONS:

Although in India with the advent of twenty first century, Mechanistic-EmpiricalMethod via IRC:37–2001 was started to design the flexible pavements which provides thecapability to a designer to determine the required layer thicknesses so that the pavementwould last for selected design life without exceeding predetermined distress levels.However, due to enormous limitations of IRC:37–2001 as spelt out, it needs to beimproved in view of continued improvement in traffic characterization, material qualityand characterization, mix designs, pavement construction methodologies and maintenanceapproaches, performance prediction models, laboratory/field testing procedures etc.Refinement in all these parameters need all-inclusive, extensive, integrated and long termresearch financed with annual dedicated budget which should be undertaken without anyfurther delay. It is, however, safely stated that further refinement in current pavementdesign method will definitely result into reduction of pavement thickness and betterpavement management practices which simply translates an annual saving of more than `1000 crores per year in view of current pace of highway development programs.

Development of unified and improved pavement design method for all types of newor reconstructed pavements as well as rehabilitated pavements is the need of the hour.Dream for an optimum and sustainable pavement structure cannot be visualized withoutincluding the analysis and design of perpetual pavements. Development of design softwareis the essence of pavement design without which it is almost impossible to consider thevariability of input parameters and thus, to optimize the pavement design. Consequently,one may be able to design an optimal and sustainable pavement structure based onindigenously developed design method which must not only be structurally andfunctionally adequate during entire design life but also survive the pre-defined performancelevel at the end of its design life as per designer’s chosen reliability level with minimumlife-cycle cost. Improved design methodologies and treatment strategies for thepreservation, repair and rehabilitation of existing pavements may be evolved. A nationallong-term pavement performance and maintenance database should be established whichwill act as a knowledge base for future refinements of pavement design process.

The opinion expressed in this paper is solely of the author and has no link with theviews, if any, of Ministry of Road Transport and Highways, of which the author is anemployee.

17

8. REFERENCES:

1. Yoder, E.J. and Witczak, M.W. , “Principles of Pavement Design”, Second Edition,John Wiley & Sons, Inc., USA, New York, 1975.

2. Huang, Yang H., “Pavement Analysis and Design”, Second Edition, PearsonEducation, Inc., USA, New Jersey, 2004.

3. IRC:37–1970, “Guidelines for the Design of Flexible Pavements”, First published,The Indian Road Congress, New Delhi, September, 1970.

4. IRC: 37–1984, “Guidelines for the Design of Flexible Pavements”, First Revision,The Indian Road Congress, New Delhi, December, 1984.

5. IRC: 37–2001, “Guidelines for the Design of Flexible Pavements”, SecondRevision, The Indian Road Congress, New Delhi, July, 2001.

6. Garg, Sanjay, “Perpetual Flexible Pavements: Highways of Future”, a papersubmitted to Indian Road Congress, New Delhi, yet to be published.

7. NCHRP, “Mechanistic-Empirical Design of New and Rehabilitated PavementStructures”, National Cooperative Highway Research Program, NCHRP Project 1-37A, National Research Council, Washington, D.C., 2004.

8. AASHTO, “Mechanistic-Empirical Pavement Design Guide, Interim Edition: AManual of Practice”, American Association of State Highway and TransportationOfficials, Washington, D.C., 2008.

9. Nicholas J. Garber and Lester A. Hoel, “Traffic and Highway Engineering”, FourthEdition, Cengage Learning, Toronto (Canada), 2009.

************** END **************

Documents

Flexible Pavement Design in India_Past, Present and Future