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*Superintending Engineer, M/o RT&H, Regional Office (C), Guwahati – 781 003 (India),email – sgarg70@rediffmail.com.

PERPETUAL FLEXIBLE PAVEMENTS: PAVEMENTS OF FUTURESanjay Garg*

ABSTRACT

Perpetual Pavements (PP) concept, attempted successfully in Europe and USA duringlast decade, is aimed to build more cost-effective superior pavements with enhanced design lifeby restricting all distresses to pavement surface only. Deep seated structural distresses such asbottom-up fatigue cracking and/or full-depth rutting are considered improbable, or if present,are very minimal. In PP, all pavement layers, except the renewable surface course, areconsidered as permanent and require no reconstruction. They are subjected only to periodicsurface maintenance and/or renewal in response to distresses confined within top layers ofpavement through the repair strategy of mill and replaces the surface layer. This repair strategyalso enabled the utilization of recycled materials retrieved from milled surface course and thus,providing further cost savings and environmental benefits. Another, inevitable benefit of PP isto ensure the minimum utilization of precious material resources, which are depleting rapidlyday by day and thus, imparting some contribution in environmental conservation.

A PP structure provides a durable, safe, smooth, long-lasting roadway withoutexpensive, time-consuming, traffic-disrupting reconstruction or major repair at short intervalsand aimed to minimize material consumption, lane closures, user delay cost and life-cycle costin optimized manner and thus, advantages of this superior pavement needs to be reaped out inhandle ever increasing traffic volume and loading including sporadic overloading in Indianscenario especially on NHDP projects and proposed expressways. The purpose of this paper isto introduced this noble concept in its entire ambit and provide a state-of-the-art regardingdevelopments of PP.

Key words: Endurance limit, fatigue, open graded friction course, perpetual pavements,rutting, stone mastic asphalt, and thick bituminous layered pavements.

1. BACKGROUND

For National Highway Development Projects and construction of proposed 18000 kmexpressways in India during forthcoming period, the construction of good quality, economicaland eco-friendly roads is the need of the day in order to meet the need and aspiration oftransportation industry and road users, in general. To achieve this goal, it is imperative for theengineers for suggesting the economical and future orientated road structures satisfying theengineering, social, industrial and environmental constraints/parameters. The cost of roadconstruction is increasing day by day due to non-availability of suitable aggregates. Haulingdistances are continuously increasing, while the ecological destruction that takes place duringthe mining of aggregates is less and less accepted. This result in the form of huge investments,loss of material, reconstruction tedium due to which country’s economy suffers to a greatextent. As the demand on existing pavements in India increases with potentially minimalfunding for expansion and rehabilitation, optimized economical design of new andrehabilitated sections is the need of hour.

In India, the flexible pavements on NH and Expressway are designed for a design lifeof 15 years and 20 years[1] only, respectively and therefore, need to be reconstructed after 15-20 years. An increase either in design life or in traffic volume will necessitate an increase in

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the pavement thickness. Further, increasing the design life to 50 years or more, pavementthickness increases substantially and may exceed even one meter psychological barrier.Current designed pavements cannot sustain structural distresses such as fatigue cracking andfull-depth rutting in pavements developed in early phases of service life due to ever increasingtraffic volume and loading coupled with poor construction practices and environmental effects,and thus, leading to pre-mature failure, increased maintenance cost, reduced maintenancefrequency, frequent road closures and inconvenience to road users, reduced residual life andincreased whole life cost of the pavement. For a truck, travelling between Delhi and Mumbai(distance–1400 km), delivery time and vehicle operating cost increased due to ongoingmaintenance activities in about 300 km of road length spread over many sections, which needsto be minimized to reduce product cost to end users. Availability of a safe and comfortableroadway without any hindrance or closure is the prime goal. Highway agencies are looking forpavements with long service life, minimal disruption of traffic, reduced user-delay and life-cycle cost, and fewer maintenance frequency for handling the ever increasing traffic volumesand loads on road networks.

Due to inherent problems of Indian metropolitan cities like geometric and drainageproblems arises as a consequence of repeated resurfacing, non-availability of a hot mix plant inthe vicinity of city and consequently more haulage distance for mix transportation, poorcompaction and reduced life for a conventional flexible pavement demands a long lifepavement structure with minimal and constant thickness in a metro city. Further, highexpectation of road users in terms of riding quality, safety, clean environment and easymobility with least disturbance to traffic during maintenance activities coupled with constraintsand restriction imposed on construction activities by civic authorities and economic concernsalso require a superior pavement.

It is from such need that the concept of “perpetual pavements” arises and it becomesthe need of present era to develop such pavement design that could address all these problemsand pavement must perform its intended function economically and optimally during longerdesign life. It is reported[2, 3, 4, 5 and 6] that “A well constructed flexible pavement, built above aminimum strength will remain structurally serviceable for a considerable period, providednonstructural deterioration, in the form of surface-initiated cracks and deformation, to bedetected and remedied before it begins to affect the structural integrity of the pavementstructure”. For a given level of material quality, the thickness of the pavement could notincrease indefinitely with increasing traffic. Increasing pavement thickness beyond a point forany traffic level is worthless and results only overuse of material resources and superfluouscost. Recent advances in design, construction, and materials technology further boosting thedevelopment of this noble concept.

2. THICK BITUMINOUS LAYERED FLEXIBLE PAVEMENTS

Full-depth bituminous layered pavements are constructed by placing one or more layersof dense graded bituminous layers directly on the sub-grade while deep-strength bituminouslayered pavements are placed on relatively thin granular base course (GSB) as shown in figure1. Both types of pavements have thick dense graded bituminous layers (DBM and/or BC),usually in the range of 150 to 400 mm or more and referred collectively as thick bituminouslayered flexible pavements. However, total pavement section is thin in comparison toconventional flexible pavement, generally used in India and thus, utilizes minimum quantity ofmaterial. Thick bituminous layered pavement is generally considered the most cost-effectivetype of flexible pavement for heavy traffic in USA and Europe and is quite popular in areaswhere local materials are not available due to procuring only one material (dense graded mix

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like DBM and BC) and thus, minimizing the haulage, administration and equipment costs.They have additional advantages[7] like (a) They have no permeable granular layers to entrapwater and impair performance, (b) Less construction time and extended construction period,and (c) Less affected by moisture variation or frost.

(a) Deep-strength flexible pavement (b) Full-depth flexible pavement

Figure 1. Typical cross-section of a Deep-strength and full-depth flexible pavement

It is typically assumed that for thicker bituminous layered pavements, material selectionand mixture design techniques are sufficient such that sub-grade stress/strain levels aregenerally within acceptable limits and sub-grade permanent deformations are not significant.Thus, thickness requirements of bituminous layers for high traffic volume are controlled bybituminous bottom layer fatigue cracking considerations only. By reducing the potential forfatigue cracking through adequate stiff and thick bituminous layers and confining cracking tothe upper removable/ replaceable layers, many of these pavements have far exceeded theirdesign life of 20 years with minimal rehabilitation; therefore, they are considered to besuperior pavements[5]. Interstate 90 in the state of Washington (USA) is one example to proveperformance of thick bituminous layered pavements comprise about 225 km length. It wasrevealed that none of the section had ever been rebuilt for structural reasons, despite pavementages ranged from 23 to 35 years, and average age of resurfacing was 12 to 20 years.

Table 1. Composition of pavement structures and their cost in Rs. Lakhs/km

A. Design details of conventional flexible pavement[1]

Cumulative traffic (or W18), , msa/Pavement layer thickness (mm)

50 100 150Unit rate of constitutive

layer per cum, Rs.BC 40 50 50 10000

DBM 140 150 170 9000Granular base (WMM) 250 250 250 1900

GSB 300 300 300 1800Pavement cost per km for 2-lane NH,

Rs. Lakhs187.3 200.6 213.2

B. Design details of full-depth and deep-strength bituminous pavements

SN 6.32 6.90 7.25

Bituminous layer thickness (mm) alone/cost per km (Rs. Lakhs)

365 /255.5

400 /280.0

420 /294.0

Full-depth bituminouspavement

Bituminous layer thickness (mm) with300 mm GSB /cost per km (Rs. Lakhs)

290 /240.8

325 /265.3

345 /279.3

Deep-strength bituminouspavement

To compare design details of thick bituminous layered pavement structures withconventional flexible pavement, the former is designed as per AASHTO pavement designguide (1993) with following input values and latter is designed as per IRC:37-2001[1] with

bituminous Surface, 150 to 400mm

Unbound Base, 250 to 400 mm

Compacted Sub-grade

Natural Sub-grade

bituminous Surface, 50 to 200mm

bituminous Base, 50 to 400 mm

Compacted Sub-grade

Natural Sub-grade

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fixed sub-grade CBR value of 5% for different traffic levels and details of pavementcomposition is indicated in table 1.

• Reliability, R = 95 %• Overall standard deviation, So = 0.45• Sub-grade Resilient Modulus, MR = 48.3 MPa (7000 psi )• Design serviceability index, Δ PSI = 4.2-2.2 = 2.0• Structural Number, SN = a1D1 + a2D2m2 + a3D3m3+...• Layer coefficients, a1 for HMA = 0.44 and a2 for GSB = 0.11• Drainage coefficients m2 = m3 = 1.0• W18 = predicted number of 80 kN (18,000 lb.) ESALs

Pavement cost per km for two lane NH (7m carriageway width) along with currenttypical unit rate of constituent pavement layers per cum considered are also shown in table 1. Itis noted from table 1 that for traffic level of 50 msa (million standard axles) to 150 msa, totalpavement thickness and pavement cost per km for conventional flexible pavement varies from730 mm to 770 mm and from Rs. 187.3 lakhs to Rs. 213.2 lakhs, respectively. For thickbituminous layered pavements, total thickness varies from 365 mm to 645 mm much less thanconventional pavements (hence, results less consumption of precious materials) and pavementcost per km varies from Rs. 240.8 lakhs to Rs. 294.0 lakhs, about 25-30% higher thanconventional pavements. This cost excess is compensated by less maintenance cost and thus,thick bituminous layered pavements are optimized and economical pavement structures.

3. PERPETUAL FLEXIBLE PAVEMENT

Experience gained from the superior performance of many thick bituminous layeredpavements and observed distress pattern as stated in previous section was motivated theresearchers and highways engineers in the search of a long lasting durable pavement withthinner sections and service life of 40 years or more as well as with least maintenance. As aresult, the concept of the perpetual pavement (PP) was evolved in which the traditional conceptof “ever increasing pavement thickness approach with increasing traffic or design life” wasdefied with the proposal of “endurance failure limits” and “limiting pavement thicknessapproach” explained latter on under clause 6. For the moment, it is stated that the concept ofPP is nothing but an engineered extension of thick bituminous layered pavements, in whichbituminous layers are divided in to three distinct layers for performing specific functions andplaying relevant roles. A perpetual pavement structure, indeed, will comprises a three-layeredbituminous pavement system – a rut-resistant, impermeable, wear-resistant and renewable topstructural bituminous (surface) layer; a rut-resistant and durable bituminous intermediate(binder) layer; and a fatigue-resistant and durable bottom bituminous (base) layer resting overa durable foundation (sub-base and sub-grade) as shown in figure 2[4]. Figure 2 also gives anidea about the layer thicknesses of top four layers.

A Perpetual flexible Pavement[8] (PP), also termed as long-life, long-lasting, heavyduty or maintenance free pavements, is “a type of pavement where no significant deteriorationwill develop in the foundations or the road base layers provided that correct surfacemaintenance is carried out”. This definition is objective oriented and is also applicable tocomposite and rigid pavements; the objective being the persistence of the structure apart fromrepair of surfacing. It implies that all pavement layers, except the renewable surface course, areconsidered as permanent and need no reconstruction.

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A Perpetual flexible Pavement[5] can also be defined as “an bituminous pavementdesigned and built to last longer than 50 years without requiring major structuralrehabilitation or reconstruction, and needing only periodic surface renewal (i.e. a repairstrategy of shave and pave or mill and replace the surface layer) in response to distressesconfined to the top of the pavement”. The basic premise of obtaining a long pavement life[5] isthat an adequately thick and stiff bituminous pavement placed on a stable foundation willpreclude structural deterioration that originate at the bottom bituminous layer and/or in thefoundation under the action of anticipated traffic loading and environmental influences and thateventually require expensive reconstruction to correct properly. Rather, deterioration seems toinitiate in the pavement surface within uppermost bituminous layer or layers as top downcracking, rutting and wear. If these surface-initiated defects can be detected and remediedbefore they impact the structural integrity of the pavement, the pavement design life could begreatly increased. Thus, experiencing only non-structural forms of deterioration in thesurfacing rather than structural deterioration deeper in the pavement structure is the maincharacteristic of a PP structure.

Figure 2. Perpetual Pavement Principles[4, 5 and 6]

Recent advances in materials selection and technology, appropriate material placementwithin the pavement structure, mixture design, pavement design and construction, andperformance testing offer us the knowledge and methodology needed to obtain PP structuresthrough limiting the potential for structural deterioration while periodically replacing thepavement surface in response to non-structural deterioration confined within top layers only.

A PP structure provides a durable, safe, smooth, long-lasting roadway withoutexpensive, time-consuming, traffic-disrupting reconstruction or major repair at short intervalsand aimed to minimize material consumption, lane closures, user delay cost and life-cycle costin optimized manner and thus, is a strong competitor to rigid pavements. Though the use of PPstructure is focused at high-volume traffic (such as NHDP corridors, expressways, arterialurban roads within a metropolitan and corridors connecting to major ports, airports where laneclosure or user-delay costs may be prohibitive), the justification may be made for medium- andlow- volume roads as well, where the possibility of future funding constraints may requiredeferred rehabilitation.

4. DETRIORATION MECHANISM

Renewable Surface course, 50-100mm

Layer 3, bitumen rich, fatigue resistant, base course, 100-200 mm

Layer 4, Treated or untreated sub-base course, 300 mm

Layer 5, Compacted sub-grade, min. CBR = 5%Foundation

Zone of highcompression

100 to 150 mm Binder course, 150-350 mmLayer 2

Layer 1

Bituminous layers

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PP structures also deteriorate gradually under the action of traffic loading andenvironment influences. Indicators of structural deterioration affecting the structural integrityof these pavements are rutting and fatigue cracking originating deep in the pavement structure,which needs to minimized or eliminated. Non-structural deteriorations, confined in uppermostbituminous layer or layers, are only allowed.4.1 Fatigue Distress

Fatigue can be defined as “the phenomenon of fracture under repeated or fluctuatingstress having maximum value generally less than the tensile strength of the material”. Fatigue(bottom-up) cracking appears in the locations of the road structure where the largest tensilestresses are generated under combined action of temperature and traffic loads. It is a long termeffect and therefore promoted by ageing. The development of fatigue cracking is gradual andcan be reduced by suitable design of the mixture and/or the design of the pavement structure.The fatigue criterion in mechanistic design approach is based on limiting the horizontal tensilestrain on the underside of the bituminous (bottom) layer due to repetitive loads on thepavement surface, if this strain is excessive, cracking (fatigue) of the layer will result and mayeventually propagate to the surface and affecting all layers of the pavement structure. It is welldocumented that limiting the horizontal strains at the bottom of the bituminous base can helpcontrol fatigue cracking, for which the following techniques suiting the given set of conditionscan be employed:

a. Improve fatigue resistance with highbitumen content mixes b. Minimize tensile strain by increasing

pavement thicknessFigure 3. Fatigue Resistant Bituminous Base[4,5, and 6]

a. Total thickness of bituminous layers in the pavement structure is increased enough suchthat the tensile strain at the bottom of the base layer is insignificant.

b. Effective bitumen content is increased by making bottom bituminous course – a bitumen-rich course to make it extra flexible, as it is well documented that “increasing the binder(bitumen) and/or filler content up to a point facilitates greater compaction for achievinghigher density and low air-voids, which improves fatigue resistance and durability of themix” as shown in figure 3.a. Combining the increased quantity of bitumen with anappropriate total thickness of bituminous layers will help to further reduce the magnitude oftensile strains and thus, diminish the potential against fatigue cracking from the bottomlayer as shown in figure 3.b.

c. Using a fine graded bituminous mixtures.d. Using a stiff base material either with hard graded unmodified bitumen or heavily modified

bitumen.

4.2 Rutting (permanent deformation) Distress

Log ε

Log N

Lowbitumencontent

High bitumencontent

Tension

Comp.

Tension

Comp.

Log N

Log ε

Endurancelimit

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Rutting in paving materials develops gradually with increasing numbers of loadapplications, usually appearing as longitudinal depressions in the wheel paths accompanied bysmall upheavals to the sides. It is enhanced by high temperatures and low traffic speeds. It iscaused by a combination of densification (decrease in volume and, hence, increase in density)and shear deformation and may occur in any or all pavement layers, including the sub-grade.However, for well compacted bituminous concrete, shear deformation rather than densificationis the primary rutting mechanism. Structural rutting occurs when the overall strength of thepavement structure is not enough to preclude the development of large permanent deformationeither in granular base or sub-grade beyond their structural capacity under the imposed trafficand environmental influences. It eventually leads to a break-up of the pavement structure.Permanent deformation is an important factor in flexible pavement design. It is welldocumented that a thick and stiff pavement structures tend to prevent structural rutting in thesub-grade by limiting the compressive strain induced at the top of the sub-grade and limitrutting to the surface layers of the pavement structure, as the traffic-induced strains in the sub-grade are too low to cause structural deformation. Surface rutting is confined within a depth ofabout 100 mm from surface, and do not have a serious effect on the structural integrity of thepavement. It can be remedied with removal and replacement of surface layer.

4.3 Surface Distresses

As stated earlier that perpetual pavements are subjected only to non-structuraldeficiency such as top down cracking, rutting, lack of smoothness and riding quality, lowfriction (poor skid resistance), surface defects, poor surface light reflectivity, induced trafficnoise etc. These deteriorations in wearing courses can be viewed to be caused by tire-pavementcontacting properties, thermal gradient, type of mix and mix properties, aging, and constructionfactors and needs to be rectified before they affecting the structural integrity of pavementstructure and thus, to ensure durability and perpetuity of the pavement.

5. COMPONENTS OF PERPETUAL PAVEMENT STRUCTURES

It is already stated that a perpetual pavement structure has two main components:(a) Foundation – comprising a stabilized or unstabilized granular sub-base and sub-grade.(b) Bituminous layers – comprising three bituminous layers namely top (surface),

intermediate (binder) and bottom (base) layer.

5.1 Foundation

A pavement foundation is comprised of either a compacted sub-grade or chemicallystabilized sub-grade, a capping layer (if provided) and/or stabilized or unstabilized granularsub-base course (GSB) constructed preferably from crushed aggregates. Regardless of the kindof material employed, a sub-grade with minimum resilient modulus of 50 MPa (or CBR value>5%) along with a layer of GSB of minimum thickness of 300 mm as the foundation isrecommended for a PP structure in India in order to prevent overstressing of the sub-grade,facilitate paving operations, and to ensure sufficient stability against the rutting throughoutconstruction as well as during the life of the pavement. It is preferable for greater performancethat hydraulically bound mixtures, comprising either cement, slag or fly ash binder; or afactory blend of these binders and crushed aggregates, should be used to get minimumfoundation surface modulus of more than 200 MPa for heavy traffic (more than 80 msa)[9].Depending upon site conditions and pavement design, chemical or mechanical stabilization ofsub-grade soils or sub-base course materials may be considered, if the soil CBR is less than8%. Beyond 8% CBR value, stabilization may be considered unnecessary.

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The design and construction of a strong, stable and consistent foundation is essential toperformance of a PP structure. It must have adequate stiffness and shear strength to carry theanticipated traffic loading during construction as well as in service. During pavementconstruction, the strength and material thickness of foundation at each level should besufficient to withstand the load of construction vehicles/equipments applied at that level andclimatic variations without damage. Further, the foundation should have sufficient stiffness forthe overlying pavement layers to be placed and adequately compacted. The performance of thefoundation will also depend on the design, construction and maintenance of the earthworks andassociated drainage system. It is essential that the drainage system ensures that there is noaccumulation of water in the pavement and foundation layers and that all excess moisture isallowed to disperse.

5.2 Bituminous Layers

Since the PP structure is tailored to resist specific distresses in each layer, the materialsselection, mix design, and performance testing need to be specialized for each layer material.The mixtures' characteristics need to be optimized to resist rutting or fatigue cracking,depending upon which layer is being considered, and durability will be a primary concern forall layers. Simply increasing pavement thickness is not a guarantee that the pavement will havea long service life. Although, it is accepted that increasing the thickness of a pavement willdecrease the tensile strain at the bottom of the bituminous layer, however, it is important toknow the magnitude by which this reduction occurs. Rate of reduction is mix dependent. Thus,it becomes important to specify the right mixture for the right application in the pavement.

5.2.1 Bituminous Base (Bottom) Course

Base course is perhaps the most important structural layer of the pavement, which isintended to effectively distribute traffic and environmental loading in such a way thatfoundation layers are not subjected to excessive stresses/strains. Base layer should be stiffenough as it is subjected to high compressive stresses, and exposed to wettest continuousconditions from the surrounding soil and underneath foundation layers. This is the bottommostbituminous layer designed specifically to resist fatigue (bottom-up) cracking, a primerequirement of PP structure, to eliminate the structural deterioration due to fatigue crackingoriginating in this layer. To achieve a base course exhibiting long-life characteristics i.e.enhanced fatigue resistance and adequate stiffness, one or more strategies as outlined insubclause 4.1 will be used.

Mix is designed as per Superpave approach, and the bitumen content in the base shouldbe defined as that which produces low in-place air voids (2-4%) or results in in-placemaximum density of 96% to 98%. It will result increased effective bitumen content, which iscritical to flexibility and durability. It was also thought that this bitumen-rich layer would alsohelp to preclude cracking during the early stages of the pavement's life. In consideration ofeconomy and performance, dense graded bitumen mix with nominal maximum aggregate size(NMAS) of 19 mm, 25 mm and 37.5 mm is recommended for use in both base andintermediate layer. Either fine graded or coarse graded mixtures (with precaution to avoidsegregation problem) can be used. In case of heavy traffic loading, the polymer modifiedbitumen in conjunction with crushed aggregates could be used to provide rutting resistance.Tack coat with polymer modified bitumen must be provided at the interface of all bituminouslayers to enhance the interface friction. The bitumen grade should have the same high-temperature characteristics of the overlying layers. However, the low-temperature grade for

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base as well as binder course could probably be relaxed one grade. If base layer is to be openedto traffic during construction, provisions should be made for rut testing the material.

Durability in this bottom bituminous layer can be increased by decreasing thepermeability of the mix. Similarly, to minimize seasonal movements in the pavement, anbituminous mixture with low permeability would be required. Either using a fine graded mix ora coarse graded mix with in-place air void content of 2-3% will result low permeability.Because this layer is the most likely to be in prolonged contact with water, moisturesusceptibility needs to be considered and tested as per AASHTO T 283 during the mix design.Performance testing for the material in this layer should include a fatigue or stiffness test aswell as a moisture susceptibility test, at a minimum.

5.2.2 Bituminous Binder (Intermediate) Course

The binder layer is designed to withstand the highest shear stresses induced by wheelloads, occurring about 50 – 100 mm below the bituminous surface, which may causing ruttingthrough shear failure. The binder course is therefore placed between the surface course andbase course to reduce rutting by combining qualities of stability and durability. Stability in thislayer can be obtained by achieving stone-on-stone contact in the coarse aggregates with NMASof 19 mm, 25 mm and 37.5 mm and using a stiff and/or modified bitumen with an appropriatehigh-temperature grading. For better results, mix should be designed by Superpave methodwith in-place air voids of 3-6%. Other mix design details are already given in sub-clause 5.2.1.Performance testing should include rut testing and moisture susceptibility.

5.2.3 Bituminous Surface or Wearing (Top) Course

The surface course constitutes the top layer of the pavement and should be able towithstand high traffic- and environmentally-induced stresses without exhibiting unsatisfactorycracking and rutting, in order to provide an even profile for the comfort of the user andadequate skid resistance. Depending on local conditions such as traffic conditions,environment, local experience, and economics, functional (performance) characteristics likegood surface friction (or skid resistance), riding comfort, mitigation of splash and spray,minimization of tire-pavement noise, and durability are often required for wearing courses andused to select appropriate surface course. In PP structure, a wide range of surface layerproducts can be used depending on specific requirements as listed below:

i. Asphaltic Concrete (AC) or bituminous concrete (BC) as per IRC:111-2009,ii. Stone Mastic (Matrix) Asphalt (SMA) as per IRC:SP:79-2008,iii. Open Graded Friction Course (OGFC),iv. Mastic Asphalt (MA) as per IRC:107-1992,v. Hot Rolled Asphalt (HRA),vi. Porous Asphalt (PA).

For regions of high truck traffic volumes and the need for rutting resistance, durability,impermeability and wear resistance would dictate the use of SMA. SMA with thickness of 50-100 mm is a rut-resistant, gap-graded hot bituminous mix that relies on stone-on-stone contact(stone skeleton) and the matrix (combination of bitumen and filler; obtained by using polymer-modified bitumen complying with IRC:SP:53-2010, with fibers of either mineral or cellulose,or in conjunction with specific mineral fillers) to provide strength and stiffness. A highbitumen content (6-7 % or more) and low in-place air voids (< 6%) should be used to ensuremix impermeability and durability. In instances traffic volume < 3000 CVD, the use of a well

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designed, dense-graded Superpave mixture (BC) with in-place air voids 3-6% might be moreappropriate and economical. On perpetual pavement subjected to heavy traffic loading as wellas heavy rainfall (>300 mm/year), at least 40 mm thick Open-graded friction courses (OGFC),designed to have 18-22% voids for allowing drainage of water from the roadway surface, canbe used over at least 50 mm thick SMA. Noise reduction could require the use of a double-layered porous asphalt that conflicts with the requirement of a very durable surface layer.

6. DESIGN ASPECTS

In Mechanistic–Empirical Method (M-E method) of pavement design, based on themechanics of materials, pavement response in terms of stresses, strains, and deflections underthe effects of wheel load and environmental influences will be obtained at pre-defined criticallocations within the pavement structure from mathematical modeling of pavement structures,generally presuming the pavement and sub-grade as a multi-layer linear elastic system, witheach layer characterized by its modulus of elasticity and Poisson’s ratio[1]. In general, twomajor distress modes of failure or pavement deterioration are assumed as permanentdeformation (considered by limiting compressive strains induced at the top of sub-grade), andfatigue cracking (considered by limiting tensile strains induced at the bottom of the bituminouslayers) induced by repeated traffic load and environmental influences.

Figure 4. Schematic representation of M-E design procedure

Traffic (volume, axle loads & forecasting)

No

Yes

Sub-gradeanalysis

Drainage condition &Reliability

Pavement materialsproperties

Environment (Temperature& moisture)Input

variables

Modify thestrategy

Pavement response models (σ, ε & δ)

Damageaccumulation

with time

Allowable loads, Ni

D > 1?D << 1?

Viable Strategy

Pavement performance model

Select trial pavementstrategies (h, E, μ)

Actual loads, n

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The relationship between critical pavement responses and presumed pavementperformance (at predefined modes of pavement deterioration level, measured physically) for agiven design life is correlated by empirically derived equations (called as transfer functions)based on experimental/field observations in order to compute the allowable number of loads topavement failure (Nf) for a given condition of loading and material properties. By summing upall incremental damage occurring in the pavement structure due to repeated loads on the basisof Minor’s hypothesis, the degree of damage (D) or damage index is defined as under:

1 ( )

mi

i f i

nD

N

……(1)

in which ni is the actual number of anticipated load repetitions for the ith load group,(Nf)i is the allowable number of load repetitions for the ith load group, and m is the number ofload groups. Failure of the pavement is assumed at a point where ‘D’ approaches to one i.e.D=1. The trial design is selected, if it satisfied the predefined performance criteria as describedabove and as summarized in the flowchart shown in figure 4, then the selected design(pavement thickness & composition) is adopted, otherwise, iterative procedure will befollowed to find the suitable solution. In above procedure of incremental damage accumulationconcept, it was assumed that cracking or structural rutting would eventually occur for eachload application. Therefore, increasing traffic volumes and/or design life necessitate thickerflexible pavement structures for sustaining a specified level of service to its users. Such designphilosophy also sometimes known as “ever increasing pavement thickness approach”prohibitive to adopt a longer design period, more than 20 years.

Revolutionary research carried out by Nunn et.al.[2] has concluded that this approachwas not borne out in fact for thicker bituminous layered pavements and “A well constructedflexible pavement, built above a defined threshold strength, will have a very long structural lifeprovided that non-structural distresses, in the form of surface-initiated cracks anddeformations, is detected and remedied before it begins to affect the structural deteriorationdue to fatigue or cracking of bituminous base, or deformation originating deep within thepavement structure exists in roads that conform with the criteria for long life.” This is becausethere is a bending strain level at the bottom of the bituminous layers below which fatiguedamage will not occur, and any additional thickness of bituminous layers to reduce strain willbe superfluous. This strain level is known as endurance limits or limiting response criteria andis the basis for designing a Perpetual Pavement. It is further reported by other researchers[3, 4, 5,

6, 10 and 12] during last decade that for a given level of material quality, the thickness of thepavement could not increase indefinitely with increasing traffic and /or design life and thus,defy the approach of “ever increasing pavement thickness”. There comes a point beyond whichthe thickness of the pavement is more than adequate for the heaviest loads expected and anyadditional pavement results in an overly-conservative cross section and an unnecessary addedcost, and overuse of material resources which does not fit within an environmentalsustainability framework. This can be referred as “limiting pavement thickness approach”.

The Fatigue Endurance Limit (FEL)can be defined as “a level of strain belowwhich there is no cumulative damage over aninfinite number of cycles” and thus, abituminous layer experiencing strain levelsless than the FEL should not fail due tofatigue. Significance of FEL is that “such alimit would provide a thickness limit for thepavement and increasing the thickness beyond

Perpetual Pavement

ConventionalPavement zone

Load repetitions, log Nf

Flex

ural

str

ain,

log

εm

icro

stra

in

Endurance limit,70 µstrain

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this limiting thickness would provide noincreased structural resistance to fatiguedamage and represent an unneeded expense.”

Figure 5. Strain–load relationship illustratingthe Fatigue Endurance Limit.

At FEL point, mixture’s traditional strain-Nf curve on logarithmic scale deviates fromthe typical straight line relationship and assumes a relatively flat slope as shown in figure 5.This flat slope indicates that lower strains produce extraordinarily long fatigue lives, oftenreferred to as “infinite” or “indefinite” life. Using this property and the following designphilosophy, a PP structure with basic concept as illustrated in figure 2 will be designed as perprocedure indicated in the flowchart depicted in figure 6[6]:

[a.] A Perpetual Pavements must have enough structural strength, integrity and thickness topreclude structural distresses such as bottom-up fatigue cracking and/or permanentdeformation, originating deep in the pavement structure,

[b.] It must be durable enough to resist damage from traffic forces (such as abrasion) andthe environment influences (e.g., temperature and moisture damage),

[c.] Endurance limits of strains are required to define such that below which structuraldamage does not accumulate. Practically, this means that structural damage isconsidered to be zero below this point in the M-E design process. If the pavement canbe designed so that the vast majority of loads expected produce stresses, strains, ordisplacements lower than those which would cause structural damage, then the designcan be said to be for a Perpetual Pavement. Although, endurance limits are assumed tobe function of bitumen and mixture characterizations under the effects of temperature,aging and crack propagation, however, most acceptable values for fatigue and ruttingendurance limits are 70 µstrains or με (µ/m) and 200 µstrains or με (µ/m), respectively.

[d.] PP structures are designed and constructed from the bottom-up so that all layers act inconcert to determine the useful life and failure mode of a pavement.

[e.] Structural integrity of PP should be considered intact during the entirety of thepavement’s life along with periodic resurfacing at the interval of 12 to 20 years forremoving and/or repairing the surface defects in functional surface characteristics.

Traffic (volume, axle loads & forecasting)

Yes

Materialinputs

Environmentinputs

Inputvariables

Select trial pavementstrategies (h, E, μ)

σ, ε, δ > limit ?

No

Analytical ModelModify thestrategy

Pavement response models (σ, ε & δ)

% responses over limits

% responses overlimits acceptable?

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Figure 6. Schematic Flowchart of M-E design procedure for Perpetual Pavement[6]

Therefore, to ensure the perpetual design, (a) horizontal tensile strain at the bottom ofthe lowest bituminous layer (εt ): ≤ 70 με (to prevent bottom-up fatigue cracking), and (b)vertical compressive strain on the top of subgrade ( εv): ≤ 200 με (to avoid full-depth rutting).A PP structure meeting these strain response criteria is considered to be structurally adequateboth in terms of fatigue cracking and structural rutting. Otherwise, the layer thicknesses andmaterial properties would need to be modified.

It is reported[10] that overloads in the range of 7 times the design axle load forapproximately 15 to 20 percent of the time did not negate the “no damage” performance whenstrains returned to a value below FEL. It is also reported that bituminous layers in a PPstructure can sustain “sporadic overloading” and return to “endurance limit” performanceprovided that subsequent strains in bituminous layers must lie below endurance limit.Although, this result needs further studies for its validation, however, it has significant role toplay in Indian scenario where many instances of overloading are found and as per one studyconducted recently by Ministry of Road Transport and Highways vide research scheme R 87, itwas concluded that 10% overloading increases the damage to the road by 1.50 times and 20%overloading increases the damage to the road by 2.25 times for conventional flexiblepavements. Thus, Perpetual Pavement should provide an optimal solution for sporadicoverloading problem and needs to be adopted in India for reducing the potential for prematurefailure and life cycle cost.

Table 2. Perpetual Pavement composition and cost per km in Rs. lakhs

Problemset

Layer 1 Layer 2 Layer 3 Layer 4 Layer 5 P% Pavement costMaterial type AC AC AC GSB Soil

1 Modulus, (MPa) 1380 2070 2070 138 48.3 92.24 340.83Thickness, mm 76 254 102 305 -

2 Modulus, (MPa) 1380 2070 2415 138 48.3 92.84 332.43Thickness, mm 76 242 102 305 -

3 Modulus, (MPa) 1695 2070 2415 138 48.3 92.16 328.23Thickness, mm 76 236 102 305 -

4 Modulus, (MPa) 1695 2415 2932 138 48.3 91.50 305.13Thickness, mm 76 203 102 305 -

5 Modulus, (MPa) 2070 2760 2932 138 48.3 90.01 293.23Thickness, mm 76 186 102 305 -

6 Modulus, (MPa) 2070 3105 3105 138 48.3 90.24 288.33Thickness, mm 76 179 102 305 -

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To illustrate the design of Perpetual Pavement Structure, PerRoad 3.2[11], a computeranalysis program based on M-E design philosophy described in figure 6 is used in whichlayered elastic analysis coupled with a statistical analysis procedure (Monte Carlo simulation)is performed to compute stresses and strains within a pavement structure using fatigue andrutting endurance limits of 70 µε and 200 µε, respectively and the following input data:

• Poisson’s ratio for AC is taken as 0.35, for GSB as 0.40 and soil sub-grade as 0.45.• Performance Grade, PG 70-22 binder as per Superpave is considered for AC mix.• Number of trucks in design lane and direction = 2250,• Number of axle groups per day = 6829,• With traffic growth rate of 4% and design life of 20 years, traffic works out to be 162 msa.

Season Summer Fall/spring WinterDuration, weeks 28 12 12Mean air temperature, 0C 35 25 10

• Stiffness modulus for dense graded AC used in layer 1 indicated in table 2 is at 35 0Cobtained from neat VG 30[1] in India and/or polymer modified bitumen.

• Resilient modulus for sub-grade considered in table 2 corresponds to CBR value of 5%.

It is observed that the percentage of load repetitions lower than limiting pavementresponses against rutting is always 100% for all designed PP structures. The percentage of loadrepetitions lower than the limiting pavement responses against fatigue cracking (P%) is shownin table 2, as evaluated on the basis of accumulated damage equal to 0.1 (Recall, D=1.0 isconsidered failure). It is generally recommended that the designer strive for a value of P equalto 90 percent or more on high volume roads. Pavement cost per km for two lanes NH (7mcarriageway width) is also shown in table 2 for presumed layer cost as per table 1. It is alsoseen from the table 2 that total pavement thickness varies from 662 mm to 737 mm andthickness of bituminous layers varies from 357 mm to 432 mm [slightly more than deep-strength pavement (345 mm)], while the pavement cost per km varies from Rs. 288.33 lakhs toRs. 340.83 lakhs depending upon the stiffness of mix in bituminous layers of PP structure aswell as resilient modulus of foundation. It is interesting to note that total pavement thicknessfor PP structure is less than that of conventional flexible pavement (770 mm) and cost is only30-40% extra, which is compensated by reduced maintenance cost, saving in user-delay costand nil reconstruction cost after each 15 years. Thus, PP structure is definitely economical ascompared to conventional flexible pavement or even rigid pavements and should be, therefore,adopted in India on proposed expressways and NHDP projects. Nunn et.al.[3] has concludedthat a pavement with bituminous layers thickness of 370 mm will sustain the traffic well inexcess of 5 msa per year. For accounting any increase in legal maximum axle load, thethickness of bituminous layer can be suitably enhanced, say 20 mm for increasing legal axlelimit from 10.5 tonnes to 11.5 tonnes. The above results are further in agreements to the studymade by Heemun Park et. al.[12]. For Foundation Surface Modulus of more than 200 MPa, UKDesign Manual for Roads and Bridges[9] has also specified that for the design traffic of morethan 80 msa, a flexible pavement structure with 320 mm thick bituminous layers may beprovided.

7. CONSTRUCTION AND PERFORMANCE GOALS

Construction of a Perpetual Pavement is almost similar to that of conventional flexiblepavements except optimized structural design and mix type selection, best construction

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practices and strict adherence to quality control employed in former to ensure its desiredperformance. The foundation layers must be well-compacted, smooth, and stiff enough tosupport construction traffic and provide resistance to rollers. In service, it is necessary tominimize volume changes in the foundation layer due to swelling soils or frost heave. Localexperience best dictates how to handle these situations. During construction of the bituminouslayers, more attention should be paid to the following parameters in order to ensureperformance and longevity of the pavement structure and to impart the desired characteristicsinto the pavement:

• Ensuring optimum design level of in-place density and permeability,• Proper material selection for the various bituminous layers of the pavement structure

with respect to the functions they must serve,• Proper mix design and volumetric control of the mixtures, production, placement and

handling of the mix material during manufacture, transport, and laydown for attaininggood uniformity and in turn good performance,

• Promoting interface layer bonding with tack coat at all lift interfaces of bituminouslayers,

• Optimizing the compacted lift thickness in laying of bituminous layers,• Optimize the compactive energy and improving the rolling passes and/or pattern for

attaining the requisite level of compaction quality for selected lift thickness,• Staggering of construction joints at every lift bituminous layer,• Strict adherence to the construction specifications and quality control test protocols

throughout the construction along with increased inspection frequency and fullyequipped and staffed quality control laboratory,

• Provide seal coat over bituminous binder or base course, if they are subject toprolonged exposure to traffic/environmental conditions prior to placement of topsurface layer.

The evaluation of pavement performance is an important part of pavement design,rehabilitation, and management. It includes the evaluation of the overall condition of pavementin terms of distress, roughness, friction, and structure strength and durability subjected totraffic, weather and drainage. Performance of flexible pavements is closely related to theperformance of bituminous layers, which depends upon mixture design, pavement design,construction, and rehabilitation. Performance testing for the mixtures will include fatigue orstiffness test, rut testing as well as a moisture susceptibility test, at a minimum. In-situ testingof the pavement will include dynamic cone penetrometer, falling weight deflectometer, groundpenetrating radar (GPR), Infra-red thermal imaging (IRTI), portable seismic analyzer, andprofile measurements should be considered for use in future PP construction projects asadditional construction QC test protocols. IRTI and GPR measurements (supplemented withcoring) were found to be ideal and effective non-destructive testing tools for constructionquality, QC monitoring and performance evaluation of the PP structures. They are successfullyutilized for bituminous mat temperature measurements, layer thickness uniformity andcompaction density measurements, and detection of subsurface anomalies such as densityvariations, localized voiding, vertical segregation, debonding, and moisture entrapment withinthe PP structures. This is particularly very critical in pavement maintenance programs for PPstructures and can beneficially lead to timely pre-treatment of the defects prior to severedeterioration.

8. MAINTENANCE ACTIVITIES

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The bituminous surface course of a new perpetual pavement structure should haveexcellent functional surface characteristics such as good and consistent surface profile(smoothness and riding quality), surface texture, high surface friction (skid resistance), nocracking and surface defects, lack of ruts, favourable pavement surface light reflectivity, andlow pavement induced traffic noise. Naturally, these characteristics will gradually deteriorateas a consequence of traffic loading and environmental effects. If any of the functionalcharacteristics deteriorate beyond an agreed limit (often referred to as an intervention level), anappropriate maintenance treatment, should be carried out. As a perpetual pavement structure issubjected to only non-structural deteriorations to any aforesaid functional surfacecharacteristics, a periodic surface renewal involving a repair strategy of shave and pave (ormill and replace) the surface layer either with same thickness or increased thickness should beprovided to avoid their accumulation and ensure the perpetual nature of the structure. Theperformance goal, however, is to minimize the amount of additional thickness required infuture overlays. A longer life surfacing could further reduce the maintenance costs (direct orindirect) thus extending the benefit of long-life pavements.

9. ADVANTAGES OF PERPETUAL PAVEMENTS

A perpetual pavement is subjected only to non-structural deteriorations, which requirerehabilitation at a interval of 12-20 years with mill and replace methodology and in turn resultsreduced cost of rehabilitation, prolonged maintenance cycle and reduced time to completemaintenance activities and hence, public inconvenience and user delay costs will also be less.Overall, other major benefits derived from perpetual pavements include the following:

a. provides a durable, safe, smooth roadway with longer (indefinite) service life (morethan 50 years),

b. theoretically eliminate the need for reconstruction of pavement forever,c. lowest life-cycle costs,d. Improved user safety, mobility and user satisfaction due to fewer premature pavement

failures and greater serviceability,e. Lower overall maintenance costs over time,f. high structural capacity for high traffic volume and heavy truck loads,g. can sustain sporadic overloading,h. reduced construction time and extended construction seasons,i. proven technology and knowledge to build them,j. Lesser aggregates consumption,k. milled bituminous material retrieved during repairing of surface course is reusable,

recyclable – providing further cost savings and environmental benefits,l. with superior surface course material, these pavements are offering reduced noise,

reduced splash and spray, and greater skid resistance, andm. competitive option to rigid pavements.

10. CONCLUSIONS

This paper briefly introduced the concept of perpetual pavement (PP) almost in eachaspect of pavement design and construction. Deterioration in well-designed and constructed PPstructure is not structural. Rather, non-structural deteriorations seem to initiate in pavementsurface within top bituminous layer or layers as top down cracking, rutting and wear. If thesesurface-initiated defects can be detected and remedied through mill and replace repair strategybefore they impact the structural integrity of the pavement, the pavement will remainserviceable for indefinite period. The knowledge and engineering capability to design andbuild such a structure exists. However, this information and the design procedure must be

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synthesized into a useful system to construct this structure through proper materials selection,mixture design, pavement design and construction, and performance testing. PP structureprovides a durable, safe, smooth, long-lasting roadway without expensive, time-consuming,traffic-disrupting reconstruction or major repair at short intervals and aimed to minimize laneclosures to traffic, user delay cost and life-cycle cost and is a strong competitor to rigidpavement. PP provides long-term solution to ever increasing traffic volumes with sporadicoverloading observed on Indian roads and therefore, it should be adopted in India on proposedexpressways and NHDP projects. On the basis of cost per kilometer for a service life of 50years or more, perpetual pavements definitely proves to be economical.

“Nothing will be attempted if all possible objections must first be overcome.”(Anonymous)

The opinion expressed in this paper is solely of the author and has no link with hisdepartment, where he is serving.

11. REFERENCES

[1.] IRC:37-2001, “Guidelines for the Design of Flexible Pavements”, Second Revision,The Indian Road Congress, New Delhi, 2001.

[2.] Nunn, M.E., A. Brown, D. Weston and J.C. Nicholls, “Design of Long-Life FlexiblePavements for Heavy Traffic”, Report No. 250, TRL, Berkshire, United Kingdom,1997.

[3.] Nunn, M.E., and B.W. Ferne, “Design and Assessment of Long-Life FlexiblePavements”, Transportation Research Circular Number 503, TRB, Washington, D.C.,pp 32-49, 2001.

[4.] Newcomb, D.E., M. Buncher, and I.J. Huddleston. “Concepts of Perpetual Pavements”,Transportation Research Circular No. 503, TRB, Washington, D.C., pp. 4-11, 2001.

[5.] Asphalt Pavement Alliance (APA). "Perpetual Pavements, A Synthesis", APA 101,Asphalt Pavement Alliance (APA), Lanham, MD, 2002.

[6.] Newcomb, D.E., R. Willis, and D.H. Timm, “Perpetual Asphalt Pavements – ASynthesis”, IM – 40, APA, 2010.

[7.] Huang, Yang H., “Pavement Analysis and Design”, Second Edition, PearsonEducation, Inc., USA, 2004.

[8.] FEHRL, “A guide to the use of long-life fully-flexible pavements – ELLPAG Phase I.”FEHRL Report 2004/01, Brussels, Belgium, 2004.

[9.] Design Manual for Roads and Bridges, The Highways Agency, U.K., Volume 7,Section 2, Part 3, HD 26/2006 – “PAVEMENT DESIGN”, 2006.

[10.] Thompson, M. R., and S. H. Carpenter, “Fatigue Design Principles for Long LastingHMA Pavements,” ISAP International Symposiun on Long-Lasting AsphaltPavements, Auburn University, June 7-8, 2004.

[11.] PerRoad 3.2 software can be downloaded for free from the APA athttp://asphaltroads.org/.

[12.] Heemun Park, Jewon Kim, Yeonbok Kim and Hyunjong Lee, “Determination of theLayer Thickness for Long-Life Asphalt Pavements”, Proceedings of the Eastern AsiaSociety for Transportation Studies, Vol. 5, pp. 791 – 802, 2005.