flexible pavement design in india

8/18/2019 Flexible Pavement Design in India

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* Superintending Engineer & Regional officer, M/o RT&H, Regional Office (C),

Bengaluru (India) –560001. Email – [email protected], [email protected]

FLEXIBLE PAVEMENT DESIGN IN INDIA: PAST, PRESENT AND FUTURE

Sanjay Garg*

Abstract

In India, new flexible pavements during twentieth century were designed by

California Bearing Ratio method, an empirical method, based on subgrade strength

measured in terms of CBR value which was, with the advent of twenty first century, taken

over by Mechanistic-Empirical methods. Continued improvement in traffic

characterization, material characterization and quality, mix designs, pavement design

approaches and construction methodologies, performance prediction models and laboratory

testing procedures, and maintenance approaches will call for a review of the present state-

of-the art pavement design approaches. In this paper, a review of the flexible pavement

design in India till 2012 is briefly outlined along with some suggestions/future steps to be

taken up to further refine the current flexible pavement design method in view of current

developments in order to optimize the pavement structure and its performance, and toevolve a sustainable pavement structure.

1. BACKGROUND:

Figure 1 Conventional flexible

pavement used in India

Broadly, there are three types of pavements; flexible

pavements, rigid pavements and composite

pavements. In this paper, discussion is limited to

flexible pavements only. In flexible pavements,

wheel loads stresses are transferred by grain-to-grain

contact of the aggregate through the granular

structure which acts like a flexible sheet due to less

flexural strength. The wheel load acting on thepavement will be distributed to a wider area, and the

stresses decreases with the depth. Taking advantage

of this distinct stress distribution characteristic,

flexible pavement normally has many layers in which

material quality deceases from top to bottom.

In India on all National Highways, a conventional flexible pavement consists

usually five layers – 40 mm or 50 mm thick surface or wearing course (BC), 50 to 200 mm

thick binder course (DBM), 250 mm thick unbound granular base course (WMM or

WBM), 200-350 mm thick granular sub-base course (GSB) and 500 mm thick compacted

subgrade over natural subgrade as shown in figure 1. If combined thickness of allbituminous layers (surfacing and binder course) is about 75 mm or less, then it is termed as

thin bituminous pavement while a thick bituminous pavement usually have combined

thickness of all bituminous layers equal to 150 mm or more.

Unlike other civil engineering structures, the structural design of a pavement

structure is, practically, a complex and daunting task due to uncertainty, variability and

approximations of everything associated with the design of new and rehabilitated

pavements. Traffic loading is a heterogeneous mix of vehicles, axle types, and axle loads

with distributions that vary with time throughout the day, from season to season, and over

the pavement design life. Traffic forecasting is very difficult. Pavement materials respond

to 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


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significant measure of variability. Pavements as a function of time and maintenance

strategies 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 a

pavement structure during twentieth century. Pavement design consisted basically of

defining thicknesses of layered materials that would provide strength and protection to a

soft and weak subgrade. In an empirical pavement design approach, the relationship

between design inputs (e.g., traffic loads, materials, layer configurations and environment)

and pavement (performance) failure were arrived through empirical correlations between

required pavement thickness and soil classification or simple strength tests of subgrade

materials using the data of past experience (based on successes and failures of previous

projects), experiments or a combination of both. Index-value-based characterizations of

material properties (layer coefficients, R-value, California Bearing Ratio etc.), and

engineering 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 soil

classification and subgrade shear strength were developed. First empirical method for

flexible 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 which

relates the subgrade material’s shear resistance evaluated by CBR value to the required

thickness of overlaid layer (cover). The thickness computed was defined for the standard

crushed stone used in definition of the CBR test.

The empirical AASHTO method (1993), based on the pavement performance data

collected during American Association of State Highway Officials (AASHO) Road Test

carried out in 1960s, was mainly used in USA and Canada. The AASHTO design equation

were developed through regression models to link the performance data with design inputs

and represent a relationship between the number of load cycles, pavement structural

capacity, and performance, measured in terms of serviceability loss. The concept of

serviceability, based on surface distresses commonly found in pavements, was introduced

in 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 exhibited

good accuracy, however, they were valid only for the local conditions [like materialselection, traffic (type, volume and axle loading), climatic conditions, drainage measures,

and construction techniques etc.] in which they were developed. As an empirical procedure

relies entirely on past observations of field performance, therefore, these methods could not

be 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 in

the multilayered pavement systems. Hence, such empirical approaches are considered to

possess only limited capabilities. The AASHTO method, for example, was adjusted several

times over the years to incorporate extensive modifications based on theory and experience

that allowed the design equation to be used under conditions other than those of the

AASHO Road Test. CBR method was also improved consistently and became the most

popular design method around the world. In India also, CBR method was used for flexiblepavement design till 2001.


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1.2 ERA of MECHANISTIC-EMPIRICAL PAVEMENT DESIGN METHOD:

During last decades of twentieth century, traffic volume and loading have increased

and new materials started to be used in pavement structures that provided better subgrade

protection, but with their own failure modes which also bring changes in the design

criterion. Besides providing subgrade support, it became equally important to evaluate

pavement performance through ride quality that governs the rate and/or extent ofdeterioration of pavement structures. Performance became the focus point of pavement

designs. Initially, empirical methods, such as AASHTO design guide, 1993, based on

serviceability (an index of the pavement service quality) loss were developed.

Later on, classical theories of mechanics were used to evaluate the pavement

structural responses in terms of stresses, strains, and deflections at critical locations within

the pavement structure under the effects of traffic loading from structural (mathematical)

modeling of pavement structures, generally presuming the pavement structure and

subgrade 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. In

Empirical part, these (critical) pavement responses were correlated with pavementperformance indicators in the form of pre-defined pavement distress modes for a given

design life by empirically derived equations known as distress models or transfer functions

derived from the performance prediction models based on past experiences, field

observations and laboratory results that compute the number of repetitive loading cycles to

specified pavement failure. Initially, two classical failure modes namely bottom-up fatigue

cracking at the bottom of bituminous layers and permanent deformation in subgrade layer

were considered in the performance models. Based on this Mechanistic-Empirical (M-E)

two-step (hybrid) approach, Asphalt Institute method (Asphalt Institute, 1982, 1991) and

the Shell method (1977, 1982) besides other methods were developed. With slight

modifications in these two methods based on in-house research results and feedback on the

performance of the pavement designs in India during twentieth century, flexible pavementdesign based on Mechanistic-Empirical pavement design approach (MEPDA) were

formalized as IRC:37–2001 and adopted in India. Later on, with appropriate modifications

in transfer function as depicted in IRC:37–2012, this approach was used to design the

composite flexible pavements also which laid the foundation for new era in the history of

pavement design in India.

Mechanistic-Empirical (M-E) pavement design approach provides the capability to

determine the required layer thicknesses so that the pavement would last for specified

design life without exceeding predetermined distress levels. This approach represents a

major 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 transfer

functions, material characterization and their variations in relation to environmental

influences over time, and the characterization of traffic. The accuracy of structural response

model and performance prediction model are a function of quality of the input variables

and the calibration of empirical distress models to observed field performance. It remains

difficult to quantify pavement distresses and performance predictions using the concepts of

mechanics and to relate them with pavement responses. This is the reason, why in

performance prediction models used so far, empirical formulas are used to predict

pavement distresses from the pavement responses. It is also a reality that a fully

mechanistic 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 of

local highway agencies, quite diverse practices for pavement design were prevalent


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in India. In 1970, an empirical method, “CBR method” based on CBR design

curves evolved by the Road Research laboratory, United Kingdom was introduced

via IRC:37-1970 by IRC as a unified approach for flexible pavement design in

entire country. The thickness of different layers of sub-base, base and surfacing

were determined by repeated use of these design curves subject to specified

minimum thicknesses for constituent layers. These design curves were applicable

for single axle loads of 8200 kg and tandem axle load of 14500 kg. Beyond these

values of axle loads, pavement thickness was increased appropriately.

2.2. For subgrade soil, CBR value was calculated from the sample prepared at optimum

moisture content corresponding to Proctor compaction and soaked in water for a

period of four days prior to testing. Traffic was considered in units of heavy

commercial vehicles per day (CVD) with a laden weight of 3 tonnes or more in

both directions (irrespective of whether the design is for a two lane or a dual

carriageway), divided into seven categories as indicated in the table 1.

Table 1 Traffic classification as per IRC:37–1970

Design Trafficvolume, CVD 0-15 15 -45 45 -150 150 -450 450 -1500 1500 -4500 >4500 and allexpressways

CBR design curve,

applicableA 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 per equation (1) and then, was used to

determine the applicable CBR curve from table 1 and which in turn used to assess

the total pavement thickness.

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), and

n = 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) volume expected at the

end of design life = 1501 CVD. Design the flexible pavement.

(b) Design flexible pavement with subgrade CBR of 5% and AD equal to 4501 CVD.

Solution:(a) For given input data, using design curve ‘F’, total pavement thickness from

IRC:37–1970 comes out to be 475 mm. Let, sub-base, GSB = 150 mm and base

course will comprise, WBM = 250 mm, provided in three layers of 100 mm 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) with surfacing of 20 mm thick

open-graded premix carpet (PC) or surface dressing will be provided. This

pavement design was applicable for the design traffic volume ranging from 1501 to

4500 CVD. Provided thickness = 150+250+50*1.5=475 mm. O.K.

(b) For given input data, using design curve ‘G’, total pavement thickness from

IRC:37–1970 comes out to be 530 mm. Let, sub-base, GSB = 200 mm and

base course will comprise, WBM = 250 mm, provided in three layers of 100 mm of

WBM Grade I + 75 mm each of WBM Grade II and WBM Grade III.


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50 mm thick bound base course (like bituminous macadam, BM) with surfacing of

25 mm thick semi-dense carpet (SDC) will be provided. This pavement design was

applicable for the design traffic volume more than 4500 CVD and all expressways.

Provided thickness = 200+250+50*1.5+25*1.5=562.5 mm > 530 mm. O.K.

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

:

3.1. The empirical design method for flexible pavement, proposed in 1970, was

continued for design traffic volume up to 1500 CVD. However, the modified CBR

curves for 10.2 tones single axle legal limits were used instead of 8.16 tones and

consequently, the pavement thickness was increased by 10 to 20%.

3.2. Recognizing the fact that the structural damage caused by a vehicle depends on the

axle load it imposed on the road, the equivalent axle load concept was introduced

in India also similar to other countries of the world to handle the large spectrum of

axle loads actually applied to a pavement. Design traffic (Nx) carried by pavement

during its design life was considered in terms of cumulative number of standard

axles in the lane carrying 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 for different subgrade strengths (assessed in terms of CBR value) were

evolved. Pavement composition (thickness of component layers) was therefore

might be decided by the designer subject to the minimum thickness as determined

from the thickness combination block given in the IRC:37–1984.

Example 2: Given that, subgrade CBR = 5%, and traffic after construction, A = 730

CVD. Design flexible pavement for 10 years for two lane NH in plain

terrain 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 get

Nx = [365 x (730*0.75) * {(1+.075)10

– 1}* 2.75/0.075] = 7.78 msa

therefore, T = 540 mm from IRC:37–1984.

As per IRC:37–1984, surfacing should be 25 mm SDC or BC with binder

course 75 mm DBM while base course should have a minimum thickness of

250 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 layers

or 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.


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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 volume

after construction varies from 730 CVD to 2183 CVD while as per IRC:37–1984, it

varies from 540 mm to 635 mm making the pavement design more responsive to

applied traffic volume (loading).

4. STATE-OF-THE-ART FLEXIBLE PAVEMENT DESIGN IN INDIA –

IRC:37–2001[5]

: To overcome limitations and empiricism in pavement design as

discussed in paragraphs 1.1, 2 and 3, attempts were made under the patronage of

Ministry of Road Transport and Highways (MORT&H), Government of India via

Research Schemes R-6, R-19 and R-56 which gave birth to IRC:37–2001 and thus,

laid down the foundation for Mechanistic-Empirical Pavement Design Method

( MEPDM ) for flexible pavement designs in India and open a new chapter in the

history of pavement designs 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 has

been considered for pavement design, which has been modeled as a three layer

structure 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 or

WMM) as layer 2, and compacted subgrade as layer 3. After taking

(i) a typical fixed value of elastic modulus (E1) at average annual pavement

temperature of 350C and Poisson’s ratio (μ1) of 0.50 for bituminous layers having

DBM/BC constructed with 60/70 grade bitumen,

(ii) μ2 = 0.40 for granular layers and a restricted composite elastic modulus of sub-base

and base course (E2) determined by the empirical equation 3(a) 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 flexible pavement structures were analyzed by ‘FPAVE’ software.

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 compressing

strain (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 layers – to avoid the

bottom-up fatigue cracking] at pre-defined locations, have been computed using the

linear elastic model “FPAVE” developed under MORT&H’s Research Scheme R-

56 “Analytical Design of Flexible Pavements”. Rutting within the bituminous

layer(s) was avoided or controlled by meeting 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 of

two classical modes of structural distresses namely bottom-up fatigue (alligator)

cracking and rutting in subgrade layer resulting from repeated (cyclic) application

of 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 develop

the following fatigue cracking failure criterion which relates allowable number of


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load repetitions (the fatigue life of the pavement) to horizontal tensile strain at the

bottom of the bituminous layer (εt) for a pre-defined performance level (as

considered in the form of fatigue cracking in 20% of the design lane area).

ε εε ε

−−−−

====

3 . 8 9 0 . 8 5 4

4 1 12 . 2 1 1 0

f

t

N x E

…(4)

in which, Nf is the allowable number of load repetitions to control fatigue cracking

and E is the effective elastic modulus of all bituminous layers.

4.3.2 Rutting Criteria: Similarly, for limiting the permanent deformation in

subgrade layer up to 20 mm, the rutting failure criterion relates allowable number of

cumulative standard axles (N r ) to vertical compressive strain (εc) at the top of the

subgrade layer as:

ε εε ε

−−−−

====

4 . 5 3 3 7

8 14 .1 6 5 6 1 0

r

c

N x …(5)

4.4. Consequently, the pavement design tables or catalogues for the conventional

standard 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% and

c. an average annual pavement temperature of 350C.

4.5. Cumulative traffic for 20 years design period and two lane highways with vehicle

damage factor of 4.5 and traffic growth rate of 7.5% becomes 150.3 msa for initial

traffic volume after project construction of 2825 CVD only. It becomes difficult for

a designer to design the pavement structure for an expressways which will carrytraffic volume certainly more than 3000 CVD as IRC:37–2001 is applicable only

for cumulative design traffic up to 150 msa. And, the approach suggested in

IRC:37–2001 for dealing traffic more than 150 msa needs to be reviewed as

IRC:81–1997 was based on empirical method which has very limited applicability

due to changes in the conditions (such as pavement structure, traffic volume and

loading, construction methods, material quality, and climatic conditions etc.) in

which it was developed. Similarly, for most of the projects constructed under BOT

model or PPP model or any similar financing model with usual range of

concessioner period of 25 to 30 years, the current IRC guidelines are unable to

provide an optimal pavement design which commensurate with service life. Design

life for flexible pavements needs to be enhanced to 30 to 40 years in line withpractices in USA and Europe.

4.6. Pavement design catalogue as outlined in IRC:37–2001 provide one of the easiest

method in the world to design the flexible pavement on the basis of the

Mechanistic-Empirical Pavement Design philosophy. However, these design

catalogue or tables were applicable only for a fixed set of conditions namely a

standard flexible pavement structure as shown in figure 1, material properties of

bituminous mixture, design (failure) criterion, and annual average pavement

temperature (350C) as pointed out also in para 4.1. Neither FPAVE nor the analysis

and design approach is available in public domain either for free or some cost.

Therefore, it is not known to the designer what will happen or in what way will heanalysis and design the pavement structure, if any of these variables will vary?

Absence of any pavement design software is the biggest difficulty as a designer is

unable to perform the analysis and design the pavement structure with user-defined


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(or project-specific) input variables and thus, to optimize the design. Further, for a

given set of traffic volume and subgrade strength, it gives one feasible solution only

leaving no scope for the designer to optimize the pavement structure economically

or in terms of material consumption and/or quality as available at site.

Example 3: Given that, subgrade CBR = 5%, traffic growth rate, r = 7.5% and traffic after

construction, 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 F N

r

+ −

= ,

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

Design details are given in table 2, from which it is clearly evident that pavement

thickness for a pavement structure designed as per IRC:37–2001 increased by 13% to

23.7% over the pavement design as per IRC:37–1984 primarily to account for the increased

share of heavy axle loads and ensuring some certainty in pavement performance against

two classical modes of pavement failure i.e. bottom-up fatigue cracking and subgrade

rutting. However, in view of current developments it is questionable whether such

enhancement in pavement thickness is justifiable? Will it lead to overdesign? For a given

pavement design, how much level of pavement performance or service life can be ensured?

Table 2 Pavement design details for Example 3

Design

Life

Pavement Design as per

IRC:37–1984 (F-2.75)

Pavement Design as per

IRC:37–2001 (F-4.50)

10 years • A = 225 CVD, then

Nx = 2.40 msa and T = 460 mm.

•

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

• A = 2183 CVD, then

Nx = 23.25 msa and T = 635 mm.


Nx = 3.92 msa and T = 553 mm.

•

A = 730 CVD, thenNx = 12.72 msa and T = 668 mm.


Nx = 38.04 msa and T = 718 mm.

15 years • A = 225 CVD, then

Nx = 4.42 msa and T = 500 mm.


Nx = 14.35 msa and T = 585 mm.


Nx = 42.92 msa and T = 665 mm.


Nx = 7.24 msa and T = 616 mm.


Nx = 23.49 msa and T = 697 mm.


Nx = 70.24 msa and T = 738 mm.

5. IRC:37–2012[6]

: Tentative Guidelines – A way ahead:

5.1. This guidelines can be used for the design of a flexible pavement on any

highway (excluding low volume roads), in which bituminous surfacing is

provided over (a) granular base and granular subbase, (b) a cementitious base and

cemented subbase with a crack relief layer of aggregate interlayer over cementitious

base layer, (c) a cementitious base and cementitious subbase with SAMI layer between

bituminous and base layer, (d) Reclaimed Asphalt Pavement (RAP) with or without

addition of fresh aggregates treated with foamed bitumen/bitumen emulsion, and (e)

bituminous base and granular subbase using the concept of long life deep strength

bituminous pavements. Last four options are new additions over 2001 revision. Thereis no change in pavement design approach or in modeling of the pavement structure.

5.2. A flexible pavement as implied in the guidelines consists of different layers ofmaterials as shown in Figure 2. The base layer may consist of either unbound


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granular layer(s) such as wet mix macadam, water bound macadam or granular

materials treated with cement or bitumen emulsion. In case of pavements with

cemented base layer, crack relief layer either of crushed aggregates of thickness 75-

150 mm or of SAMI (Stress Absorbing Membrane Interlayer) is provided at the

interface of the bituminous layer and cemented base layer which is believed to delay

the propagation of the reflection cracks in the overlaid bituminous layers.

Reclaimed asphalt pavements with or without addition of fresh aggregates treated

with (foamed bitumen)/(bitumen emulsion) having the required indirect tensile

strength can preferably be used as base layers due to their obvious benefits.

5.3. To avoid frequent maintenance, concept of reliability is now introducedimplicitly. A reliability level of 90% is recommended for high volume roadshaving a design traffic exceeding 30 msa. In IRC:37-2001, a reliability level of80% was assumed.

5.4. Load associated failures namely bottom-up fatigue cracking and rutting insubgrade layer resulted from repeated application of traffic loads are only considered as the mode of failure in the design of traditional flexiblepavements as done in IRC:37-2001. Environment effect is taken care of incalibration of rutting and fatigue transfer equations. Fatigue criteria given byequation 4 in IRC:37-2001 is modified to account for the increasedreliability level of 90% and mix volumetric properties (air voids -Va &volume of bitumen - Vb) as defined by equation 6. Similarly, rutting criteriagiven by equation 5 is also modified to account for the increased reliabilitylevel of 90% as defined by equation 7. In general, fatigue failure criteria willgovern the pavement design for traditional bituminous flexible pavements.

3 . 8 9 0 . 8 5 4

4 1 10 .5 1 6 1 1 0

f

t

N x C xE ε εε ε

−−−−

====

…(6)

where, C = 10M

, = 4.84

−0.!" …(6.a)4 . 5 3 3 7

8 11 . 4 1 1 0

f

C

N x ε εε ε

−−−−

====

…(7)

5.5. Bituminous mix design and pavement design are now integrated to get an optimumpavement design. As mix volumetric properties have profound effect on thepavement service life, therefore, bituminous mix design and construction ofbituminous layers at work site becomes two other vital prerequisites besidespavement composition and layer thickness for ensuring a performing anddurable pavement structure. Selection of bitumen, bituminous mixtures andthickness of bituminous surfacing depends upon the climate, traffic and the

composition of the underlying layer. Since most places on plains of India havemaximum air temperatures equal to 40ºC or higher, VG40 bitumen is

recommended for both bituminous layers namely BC and DBM to carry

higher traffic over 2000 commercial vehicles per day. For traffic up to 30 msa,

Figure 2 Different layers of a bituminous pavement as per IRC:37-2012

Subbase layer (unbound/bound)

subgrade

Base layer (unbound/bound)

Bituminous layer


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VG30 bitumen can be used in BC and DBM. In general, the thickness

requirement for a pavement structure is less when either stiffer binder is usedor bottom bituminous layer will be made fatigue resistant by increasing the

binder content by 0.5% to 0.6%.

5.6. In case of comparatively weak embankment soil below the 500 mm thick

compacted subgrade, effective strength of the subgrade will be consideredfor pavement design after duly account for the strength of the weakembankment soil.

5.7. Close and coarse graded granular materials should be used in the subbaselayer. It should be composed of two layers, the lower layer forms theseparation/filter layer to prevent intrusion of subgrade soil into the pavementand the upper GSB forms the drainage layer to drain away any water thatmay enter through surface cracks. The drainage layer should be tested forpermeability and gradation may be altered to ensure the requiredpermeability. Filter and drainage layers can be designed as per IRC:SP:42-1994 and IRC:SP:52-1999.

5.8.

For cumulative design traffic more than 200 msa, perpetual pavements shouldbe provided. However, its complete design procedure and details are notgiven. Due to which, it is difficult to get a rational design of these pavements.

5.9. Now, a designer can either use the design catalogues with specified set of input

variables or use ‘IITPAVE’ software to analyse any flexible pavement structure for

his chosen set of input variables and carry out the sensitivity analysis also to

optimize the pavement design as per laid down design approach. Thus, it removes

two biggest difficulties experienced by a pavement designer earlier with IRC:37-

2001 as pointed out in paragraphs 4.5 and 4.6.

5.10. For two lane NH in plain terrain, table 3 illustrates the design of tradition

bituminous pavement structure over a subgrade with 8% CBR or resilient modulusof 65 MPa as per IRC:37-2012 as well as presents a comparison of pavement design

as per IRC:37-2001. 250 mm thick Granular Subbase (GSB) and 250 mm Wet Mix

Mecadam (WMM) are laid over subgrade, which have composite modulus of 213

MPa with Poisson’s ratio (µ) of 0.35. Over WMM, dense bituminous concrete

(DBM) having thickness varying from 135 mm to 190 mm is provided along with

bituminous surfacing layer of 50 mm thick Bituminous concrete (BC), both having

resilient modulus of 1700 MPa and µ = 0.35. Column 2 and 3 in the table 3 gives

the thickness details of DBM required to sustain the various traffic volumes, as per

IRC:37-2001 and IRC:37-2012, respectively.

Table 3 Pavement design details as per IRC:37-2001 and IRC:37-2012Single axle with dual wheel load assembly exerting tyre pressure of 0.72

MPa is used in the analysis by IITPAVE software. Bituminous mix used in

DBM has 3.50% air voids and 12% bitumen volume. VG30 bitumen is used.

Design Traffic,

N (msa)

DBM thickness (mm)

as per IRC:37–2001

DBM thickness (mm)

as per IRC:37–2012

50 115 135

75 130 150

100 140 160

125 150 170

150 155 180175 165 190

200 170 195


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From table 3, it is clearly evident that the thickness of bituminous layers in a

traditional bituminous pavement structure designed on the basis of IRC:37–2012 is

increased by 15% or more to that of IRC:37–2001 to ensure an increased reliability of 90%

assumed in IRC:37–2012. However, billion rupees question is that this increase in the

thickness of the pavement structure is really required, justifiable and worthy? Will it simply

lead to overdesign? It is already well documented that the pavement design thickness as

obtained from AASHTO 1993 is already proved to be on higher side when compared to

pavement thickness designed on the basis of MEPDG 2008. It is also worthy to mention

that an increase of just 10 mm in bituminous layer (DBM) thickness will increase the

pavement cost by about 3% and costs about Rs. 300 crores per year to the country

considering the construction of 5000 km long 2 lane National Highway per year (pavement

cost per km is about Rs. 2.0 crore). Further, for a given pavement design, it is very difficult

to predict the extent/level of pavement performance or service life. A single reliability level

of 90% is not sufficient to ensure an adequate service level over service life for highly

trafficked and overloaded highways usually observed in India. In the absence of adequate

field studies in the country and any concrete feedback on the experience gained from

NHDP, we are unable to reply all these questions.

6. WHERE WE ARE:

Although the design method for flexible pavements as stipulated in IRC:37–2001 or

in IRC:37-2012 was a major step forward it still has many crucial issues at stake in the

design aspects mainly due to so much simplifications and assumptions either in the

determination of input design variables, mathematical modeling of the pavement structure

or in the application of versatile Mechanistic-Empirical pavement design (MEPD)

approach which otherwise has huge potential to design and optimize the pavement

structure. Succeeding paragraphs presents a detailed discussion on some of these essential

issues which need to be considered to further refine the pavement design process asdepicted in IRC:37.

6.1. At present, pavement design is explicitly based on the traffic loading and the effect

of environmental influences is considered implicitly in the derivation of transfer

functions. Rationally, both, traffic loading and environmental influences need to be

considered explicitly in the pavement design process to optimize the pavement

design.

6.2. Current IRC pavement design guidelines (IRC:37) can be used only to design new

flexible pavement structures. Therefore, pavement strengthening or structural

overlay design still remains based on empirical approach which has very limited

applicability for the thick bituminous layered pavement structures constructed now-

a-days and carry traffic volume and loading well beyond their tested domain.

Development of a unified pavement design method for all types of new or

reconstructed pavements as well as rehabilitated pavements is the need of the hour.

6.3. Characterization of Traffic: At present in India, the concept of equivalency

factors are used to characterize traffic in which different axle types are converted

into 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, the

concept of relative damage is not quantifiable as it is based on the results of

AASHO Road Test, wherein it was concluded that the pavement damage increaseswith axle weight raise to fourth power. Besides, LEF (or VDF) depends on the

specific set of conditions that include the axle loading, axle configuration, pavement

type and thickness, tire type, tire inflation pressure, environment, distress mode, and


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terminal serviceability. Because of this, it is difficult to calculate ESALs for (a) new

types of vehicles, (b) axle multiplicity, change in axle loading and configurations,

(c) change in tire type and tire pressure, and (d) change in pavement failure modes.

This difficulty and inbuilt empiricism are the major reasons for moving towards

other ways for traffic characterization such as axle load spectra approach as used in

India for the design of rigid pavements (IRC:58) and described in para 7.2.1. The

lateral traffic wander has not considered in the current design practice, which needs

to be included in design process as it influences the number of load applications

over a point and hence, affects prediction of fatigue cracking and permanent

deformation.

6.4. Characterization of pavement materials: For a successful and effective pavement

design, characterization should be based on material properties that accurately

capture the material response which influenced by construction quality, applied

traffic loading and environmental conditions varies over design life. At present, no

such consideration in material characterization is taken care of.

6.5. Location of critical pavement responses: Currently, the critical stresses and/or

strains 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 is

not correct for multi-axle loads as the critical location is a function of the wheel

load magnitude, axle configuration and the pavement structure. To evaluate the

maximum principal (design) strains under single or multi-axle loadings, pavement

response should be evaluated at several locations and corresponding pavement

damages (distresses) will be calculated for each location. Location of maximum

damage will be the critical location and should be made part of design process.

6.6. 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 dense

mix graded bituminous layers having thickness from 125 mm to 225 mm. For such

thick bituminous layered pavements, other structural distress modes like top-down

fatigue (longitudinal) cracking and thermal fatigue (transverse) cracking may also

play vital role in predicting their performance. Besides, rutting in bituminous layers

as well as in unbound base/sub-base layers is equally important. In cold regions like

Jammu 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 and

should be made part of design process. Functional distresses like surface roughness

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 were

developed through two research schemes R-6 and R-19 mainly for the thin

bituminous flexible pavements. Further, the transfer function for fatigue failure

criteria was calibrated for one specific condition (at 350C for BC surfacing having

80/100 bitumen) only. There is no published study or reports which tell us how

these transfer functions are sensitive to varied conditions of traffic, climate,

material quality, mix designs, pavement constructions, and maintenance practices

etc. as per actual field conditions found in India? It is, however, of paramount

importance that these transfer functions should accurately reflect the actual

performance of pavements under the expected conditions, because extrapolating

them beyond their tested bounds can result in over-designed or under-designed


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pavement sections. This is the weakest part of the current design process and

exhaustive field/lab studies should be carried to address this issue.

(c) In current IRC:37-2012 (or IRC:37-2001) pavement design practice, a fixed distress

criteria in terms of fatigue cracking in an area of more than 20% and/or a rutting

depth of 20 mm or more is considered to define the failure mode of the pavement.

Variability in type and extent for all failure modes to specify a performance(distress) level is required to be left for the designer to decide it as per importance

and need of the project, and desired level of pavement performance and adopted

maintenance strategies.

6.7.

Reliability concept: Implicit introduction of the reliability concept in IRC:37-2012

is a welcome step as the design of flexible pavements is associated with many

factors (like traffic prediction, material characterization and behavior modeling,

environmental conditions, construction quality, and maintenance practices etc.) that

introduce a substantial measure of variability and uncertainties. Explicit

incorporation of the reliability concept as an input variable may provide a choice to

the designer to incorporate the desired degree of certainty into the design process

and to ensure that the various design alternatives will survive for the analysis periodwithout reaching to unacceptable condition of pavement performance.

6.8. For an efficient and economical pavement structure design, use of planned stage

construction approach as suggested either in IRC:37-2001 or IRC:SP:84-2009 may

recall a review as the current pavement design is based on cumulative damage

approach either against fatigue and/or rutting which involves highly nonlinear

relationship of the design inputs with pavement responses and pavement damages.

6.9. There is still no guideline or direction in India to design the thick bituminous

pavements[7]

such as full-depth bituminous pavements (constructed by placing one

or more layers of dense graded bituminous layers directly over the sub-grade) and

deep-strength bituminous pavements (in which dense graded bituminous layers areplaced on relatively thin granular base course). These bituminous pavements along

with perpetual pavements[7]

are relatively more useful alternatives to handle heavy

traffic volume and loads observed generally on Indian Highways besides (a) less

construction time and extended construction period, (b) less affected by moisture

variation or frost and (c) using only one material (dense graded mix like DBM and

BC) and thus, minimizing the haulage, administration and equipment costs.

Additional benefits include less consumption of materials and relatively less

maintenance. Hence, their designs are also required to be included in the

forthcoming IRC guidelines.

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

The dilemma is that pavement materials do not exhibit the simple behavior assumed

in isotropic linear-elastic theory. Loading rate, time and temperature dependency,

nonlinearities, and anisotropy are some examples of complicated features often observed in

pavement materials. However, continued improvement in material characterization, and

constitutive models make it possible to incorporate nonlinearities, loading rate effects, and

other realistic features of material behavior. Determination and/or prediction with sufficient

accuracy of some prime input parameters such as climate, traffic and the quality of the

materials as laid and the variation therein remains an issue to be addressed. Large databases

now exist for traffic characterization, site climate conditions, pavement material properties,

and historical performance of in-service pavement sections coupled with improved

modeling of pavement structure provide the technical infrastructure that made possible the

refinement in structural analysis of pavement responses.


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Pavement performance models should be extended and refined further with

inclusion of more distress modes of pavement failure, calibrated with comprehensive

testing and characterization of materials in bituminous, bound/unbound base layers, and

subgrade soils and validated with actual field observations/testing under varied conditions

of material quality, mix designs, pavement constructions and maintenance practices. It is

also possible to model pavements structures as accurate as possible using non linear elasto-

visco-plastic models and using advanced finite element techniques formulated on the

concepts of either classical mechanics or damage mechanics or fracture mechanics that

allow damage initiation and progression to be taken into account as well as the effects of

stress 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, these

methods also allow to analyze the effects of moving loads which implies that inertia and

damping effects can be taken into account.

Finally, Mechanistic-Empirical pavement design approach, so evolved, will provide

more rational and realistic methodology to account for uncertainty, variations andapproximations in structural modeling, traffic loading, environmental effects, material

characterization, and performance models. Further, it will provide a much better insight in

why pavements behave like they do and provide a good estimate of pavement performance.

Later on, pavement design process may be integrated with in-service maintenance

needs/decisions and desired performance levels to evolve optimal pavement management.

However, they involve advanced testing and analyses techniques, some are already

developed and others are under evolution stages.

All these refinements are experiencing from last 20 years in abroad specifically in

USA under 20 years Long-Term Pavement Performance Program (1989-2009) as a part of

the Strategic Highway Research Program (SHRP) and to realize them in India also, similarquantum of dedicated research works and field efforts are required. Research should be

large scale spread over the geographical breadth of the country; long term and integrated;

all-inclusive and result-oriented which blends the lab tests and field tests. It should include

all components of pavement design process like traffic characterization, material selection

and characterization, pavement modeling and design techniques, mix design and

construction methodology, maintenance approach, performance assessment and distress

predictions etc. To measure the resilient modulus, moisture susceptibility, permanent

deformation, and fatigue cracking properties of the bound and unbound materials within

the pavement structure and to quantify the quality and effectiveness of the pavement

construction and treatment practices, improved Accelerated Pavement Testing (APT), and

Non-destructive testing (NDT) technologies should be developed. In the research, all majorstake holders should be included with their respective roles and contributions. Besides

others, at least the following objectives may be part of this research in India:

i. Identify the reasons for development of the distresses in early phases of pavement

service 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.


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Findings must be documented and well circulated among the field experts of the

country for comments and further refinements. Finally, research reports should be

published and made available in public domain to learn the lessons and/or gain experience

to further improve the pavement design, construction and maintenance process. It is most

essential to generate the field performance data bank based on actual lab and field tests

rather than visual observations and enter the era of more rational and realistic quality

control practices besides nurturing the research industry in India as well as upgrading the

level of our research institutes. Otherwise, we always remain dependent upon the findings

of foreign institutions and will be waiting for their technology transfer besides wasting our

huge indigenous talent in nonproductive works. It is the high time we will wake up and

understand the importance of result-oriented research activities as said many times by the

Hon’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 must

develop and provide a conducive environment for the research activities which will benefit

us and our forthcoming generations.

Figure 2 Overall design process for flexible pavements (NCHRP[8]

)

Mechanistic-Empirical Pavement Design Guide (MEPDG, 2008)[8, 9 and 10]

was

developed in USA to address the shortcomings in the current pavement design methods. It

is applicable for new as well as rehabilitation of flexible, composite or rigid pavements.

Basically, it incorporates axle load spectra approach, variation of material properties with

climate and traffic loading, consideration of all major modes of structural distresses plus

one functional distress (smoothness), incremental distress computation approach tosimulate how pavement damage occurs in nature (field) and transfer functions developed

after comprehensive simulative lab testing coupled with field observations during LTPP

under varied climate and traffic conditions. Figure 2 is showing an overview of the Flexible

Pavement Design Process used in MEPDG, 2008[8]

.

With this background and based on MEPDG (2008)[9]

, pavement design method in

India should also involve the following major steps besides other refinements to eliminate

or obviate the aforesaid shortcomings in the state-of-practice pavement design process:

7.1. 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,


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d. it should be constructible and use available local material to extent possible, and

e. it should result an optimal and sustainable pavement structure – optimize the

pavement thickness and material consumption, minimize the initial cost as well as

life-cycle cost, and improve the pavement sustainability.

7.2.

Design inputs: Based on the criticality of the project and the available resources, it

is recommended to employ the hierarchical approach (similar to MEPDG, 2008[8]) in the

selection of design inputs with regard to traffic, materials, and environmental parameters.

All design input variables should be clearly defined as explained below:

7.2.1. Characterization of Traffic: Traffic is the most important design input variable, as

a pavement structure is designed to carry traffic. To eliminate the empiricism in the

concepts of ESAL and VDF, a more direct and rational approach like axle load spectra

method similar to IRC:58 should be used to quantify the characteristics of traffic loads

carried by a flexible pavement structure as it allows mixed traffic to be analyzed directly

and thus, enhances pavement design process. The approach estimates the effects of actual

traffic on pavement response and distress. Additional advantages of the load spectra

approach include: the possibility of special vehicle analyses, analysis of the impact ofoverloaded trucks on pavement performance, and analysis of weight limits during critical

climate conditions. Load spectra are simply the collective axle weight distributions grouped

by axle type for a given traffic stream 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 axle applications within each load interval for single, tandem,

tridem, and quad axles. Vehicle class distributions, daily traffic volume, and axle load

distributions define the number of repetitions of each axle load group at each load level.

For a given load group, the damage caused by each load, on each axle type, and under each

climate condition during the year is simulated over the design life of the pavement.

7.2.2.

Characterization of Pavement Materials: Effective characterization of pavementmaterials is a key requirement for a successful and effective pavement design. The state

and characterization of the different materials forming the pavement layers changes with

variation in temperature and moisture condition which in turn affected the structural

response of the pavement structure subjected to traffic loading. An effective analytical

model should account for all of these factors in analysis leading to a performance-based

design. Instead of index properties, fundamental engineering properties of material like

dynamic modulus of bituminous materials and the resilient modulus of unbound materials

(granular materials or native soils) as a function of time and environmental influences over

the entire design period and duly account for the variation in applied stress state, pavement

depth etc. will be considered to compute the pavement responses.

7.2.3.

Environmental condition: Moisture level and temperature changes are the two

main environmental variables which can significantly affect the pavement material’s

properties and, hence, impact the strength, durability, load carrying capacity, service life

and serviceability of the flexible pavements. The resilient modulus of bituminous materials

can increase during winter months by as much as 20 times its value during hot summer

months. Excessive moisture can drastically lead to stripping of bituminous mixture.

Similarly, resilient modulus of unbound materials at freezing temperatures exhibits high

values compared to thawing months. The moisture content affects the state of stress of

unbound materials and it breaks up the cementation between soil particles. Increased

moisture contents lower the modulus of unbound materials. Appropriate climatic model to

simulate changes in the behaviour and characteristics of pavement and subgrade materialswill be developed that concur with climatic conditions over the design period. The model

computes and predicts the modulus adjustment factors, pore water pressure, water content,

frost and thaw depths, frost heave and drainage performance in case of granular or


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subgrade layers. For the bitumen bound layers, the model evaluates the changes in

temperature as a function of time to allow for the calculation of the dynamic modulus and

thermal cracking. The model provides varying moduli values in the computation of critical

pavement response parameters and damage at various points within the pavement system.

7.2.4. Distress Prediction Model and Design Criterion: Pavement distress prediction

model 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 the

structural and functional performance of the pavement structure at the end of the design

period. Design criteria for each distress should be pre-defined as indicated in table 4 and

will be compared with respective accumulated distress at the end of analysis or design

period by the designer to check the adequacy and validity of the design.

Table 4 Distresses for flexible pavement with their design criterion

Structural/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 allof the pavement layers and subgrade,

10 to 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

7.2.5. Reliability concept is explicitly made part of the design process.

7.3. Pavement Structure and its Mathematical Modeling: A pavement structure,

flexible, composite or rigid, is composed of one or more layers constructed with different

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 material

response may be linear or nonlinear, viscous or non-viscous, and elastic or plastic or

viscoelastic and accordingly, structural analysis model will be chosen. Due to simplicity

and computational speed, layered elastic model is the most commonly used structural

model for a flexible pavement structure. To consider the effect of temperature and traffic

load rate variation on bituminous layers and moisture changes on unbound granular layers,

these layers should be divided into sub-layers. To account for traffic wander and various

types of axles in the traffic mix, appropriate number of analysis points (critical locations) in

each sub-layer should be considered primarily to determine the following critical pavement

responses 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), and

b. 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.

7.4. Pavement Performance and its Prediction: The concept of pavement

performance includes consideration of functional performance, structural performance, and

safety. Pavement performance is affected by several factors namely traffic, soil and

pavement materials, environment, drainage condition, and construction and maintenance

practices. The structural performance of a pavement relates to its physical condition, or

other conditions that would adversely affect the load-carrying capability of the pavement

structure or would require maintenance. Structural distress indicators includes fatigue

(load-induced and thermal) cracking and rutting (in all layers) for flexible pavements, and

joint faulting, and slab cracking for jointed plain concrete pavements. The functional


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performance 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 dominant

characteristic of functional performance. Riding comfort is quantified in terms of

smoothness as express by International Roughness Index (IRI) which combines the effects

of initial pavement/subgrade condition with the distresses developed over time.

7.5.

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 accumulate

damages over the entire design period. Attempts will be made to simulate how pavement

damage occurs in nature, incrementally, load by load, over continuous time periods. To

achieve this goal, design life is divided into shorter design analysis periods or increments

beginning with the traffic opening month. Within each increment (or analysis period), all

factors (traffic and material characterization) that affect pavement responses and damage

are held constant for simplification and computational speed. Critical pavement responses

(stress and/or strain values) for each distress type are determined for each analysis

increment 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 all

increments and output at the end of each analysis period is used by the designer to compute

the accumulated distress and later on, to compare them with respective design criteria for

each distress.

7.6. Adaptability to New Developments: The analysis and design philosophy should

be capable to adapt the latest developments in pavement engineering. Therefore, design of

promising perpetual pavement structure[7]

may be elaborated along with inclusion of the

design procedure for other thick bituminous pavements in forthcoming Indian Guidelines

as they provides a durable, safe, smooth, long-lasting roadway without expensive, time-

consuming, traffic-disrupting reconstruction or major repair at short intervals and aimed to

minimize material consumption, lane closures, user delay cost and life-cycle cost besides

handling ever increasing traffic volume and loading including sporadic overloading in

Indian scenario especially on NHDP projects and proposed expressways.

7.7. 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 by

analyzing initial costs and discounted future cost, such as pavement construction,

maintenance, rehabilitation, and reconstruction cost as well as administrative cost to

perform all these activities over the useful service life of pavement structure, and salvage

value (all are grouped under Agency Costs); and User Costs (includes vehicle operating

costs (VOC), crash costs, and user delay costs). In view of huge investment made in India

in highway infrastructures during forthcoming period and to ensure the best value ofinvested public money, LCCA is becoming the most inevitable component of pavement

design process and should therefore, be made part of design process in India also. LCCA

can also be used to evaluate the overall long-term economic efficiency between competing

alternative investment options. Either of the economic decision tool such as Benefit/Cost

Ratios, Internal Rate of Return, Net Present Value, and Equivalent Uniform Annual Costs

can be used in LCCA.

8. CONCLUSION AND RECOMMENDATIONS:

With the advent of twenty first century, IRC:37–2001 introduced the Mechanistic-

Empirical Method instead of empirical methods used during last century to design theflexible pavements in India which provides the capability to a designer to determine the

required layer thicknesses so that the pavement would last for selected design life without

exceeding predetermined distress levels. IRC:37–2012 includes more options of flexible


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pavements and evolve optimum pavement design with sub-standard or recyclable materials

also. It also introduced the design of perpetual pavements which need to be elaborated

along with inclusion of the design procedure for other thick bituminous pavements.

Continued improvement in traffic characterization, material quality and characterization,

mix designs, pavement construction methodologies and maintenance approaches,

performance prediction models, and laboratory/field testing procedures etc. necessitates

further refinement of the IRC pavement design guidelines which, however, need all-

inclusive, extensive, integrated and long term result-oriented research works and field

efforts financed with annual dedicated budget which should be undertaken without any

further delay. It is, however, safely stated that the refinements in current pavement design

method as suggested in this paper or else will definitely result into reduction of pavement

thickness and better pavement management practices which simply translates an annual

saving of more than ` 1000 crores per year in view of current pace of highway development

programs in the country.

Development of unified and improved pavement design method for all types of new

or reconstructed pavements as well as rehabilitated pavements is the need of the hour.Dream for an optimum and sustainable pavement structure cannot be visualized without

including the analysis and design of composite and perpetual pavements based on

comprehensive pavement design approach as discussed in this paper. Development of

integrated and comprehensive design software is the essence of pavement design without

which it is almost impossible to consider the variability of input parameters and thus, to

optimize the pavement design based on LCCA and other considerations. Consequently, it

will be possible to design an optimal and sustainable pavement structure based on

indigenously developed design method which must not only be structurally and

functionally adequate during entire design life but also survive the pre-defined performance

level at the end of its design life as per designer’s chosen reliability level with minimum

life-cycle cost. Improved design methodologies and treatment strategies for the

preservation, repair and rehabilitation of existing pavements may be evolved. A National

long-term pavement performance and maintenance database should be established which

will 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 the

views, if any, of Ministry of Road Transport and Highways, of which the author is an

employee.

9. 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, Pearson

Education, 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”, Second

Revision, The Indian Road Congress, New Delhi, July, 2001.

6.

IRC:37–2012, “Tentative Guidelines for the Design of Flexible Pavements”, TheIndian Road Congress, New Delhi, July, 2012.

7. Garg, Sanjay, “Perpetual Flexible Pavements: Pavements of Future”, Journal of the

Indian Road Congress, Indian Roads Congress, Vol.73-1, 2012.


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8. NCHRP, “Mechanistic-Empirical Design of New and Rehabilitated Pavement

Structures”, National Cooperative Highway Research Program, NCHRP Project 1-

37A, National Research Council, Washington, D.C., 2004.

9. AASHTO, “Mechanistic-Empirical Pavement Design Guide, Interim Edition: A

Manual of Practice”, American Association of State Highway and Transportation

Officials, Washington, D.C., 2008.

10.

Nicholas J. Garber and Lester A. Hoel, “Traffic and Highway Engineering”, Fourth

Edition, Cengage Learning, Toronto (Canada), 2009.

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flexible pavement design in india

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