We Need PavementsThe need for safe, durable andsmooth riding pavements onhighways and in airports cannot becontended. Construction andmaintenance of pavements costmoney which is mostly providedthrough taxes that are collected fromthe citizens. The money is spent onemploying workers, quarrying andmanufacturing usable products,transporting them, and laying downand compacting the materials. Inthis whole process, which getsrepeated to different extents duringthe maintenance and rehabilitationcycles, man-made material is laiddown to cover virgin (most likelyarable) ground (making it un-arablefrom that point onwards), asignificant amount of naturalresources is uti l ized, (nonrenewable) energy is consumed,landfills are created, and differenttypes of chemicals (in the form ofgases) are emitted to theenvironment (all of these processes

Pavement Engineering

are irreversible). Thus, pavementengineering is a destructive andhigh impact activity. But rememberthe first sentence of this paragraph and herein lies the dichotomy ofmodern civilization you buildsomething that you need but in theprocess you affect the earth in anirreversible harmful way. So what isthe solution?

There is most likely not asolution, but multiple ones. In fact,they are more like tools that existin the pavement engineers toolbox,which could be used, as the needarises. It is the intent of this articleto describe a few of such importanttools and their impacts.

First ConsiderationFirst and foremost, the pavementinfrastructure should be viewed aspart of a larger system, and notviewed alone. Buildings, pavementsand natural ground/water bodies(nature) form the built environmentin which we live and breathe.Hence, pavements need to be

compatible and if possible,complementary to the other twocomponents buildings and nature.This means construction andmaintenance of pavements shouldnot be taxing the natural resources,energy util ization should beminimized, byproducts in the formof emissions should not be harmfulto the environment, and the layoutof the pavement should not besignificantly impacting the naturalenvironment. Broadly speaking, thisincludes layout and type, designand construction of pavement. It isonly through this process we canensure that our future generationswill have no/less problem ofcontinuing the work of building ormaintaining/rehabilitating thepavement infrastructure. And, this isthe very essence of sustainability.True sustainability consists of threeaspects - social, economic andenvironmental sustainability. Thediscussions in the followingparagraphs refer to either one or

A. Veeraragavan,Professor, Indian

Institute of Technology,Madras (IIT-M),

Chennai

Rajib B. Mallick, AssociateProfessor, Civil andEnvironmental EngineeringDepartment, WorcesterPolytechnic Institute (WPI),Massachusetts, USA,

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Figure 1: Elements of a sustainable pavementinfrastructure system (Courtesy: American Concrete

Pavement Association)

more of these three different typesof sustainability.

Design of Layout ofPavementsSome of the more important issueswith the layout/geometric design ofa pavement can be summarized asfollows:1. Protection of forests: forests

provide a home to many speciesof plants and animals and at thesame time serve as a source ofeconomic products such astimber and enjoyment for tourists.Roads should be preferablydesigned away from forests, or ina way that creates minimumimpact on them.

2. Protection of wild l ife:Considerations must be made toprovide features that wouldminimize the effect of roadconstruction on animals andsubsequent vehicle-animalcollisions (such as protectedcross overs).

3. Consideration of uniquegeological conditions, such assteep slopes with vegetation orcaves/landforms should be made.Precaution should be taken toavoid such areas where a slightdisturbance of the landscapewould lead to a significantirreversible disturbance of thelocal environment.

4. Context-sensitive design shouldbe adopted: the layout of the roadshould conform to the values ofthe communities through which itis passing, by making sure that itis integrated with the localcharacter such as flora andfauna in rural areas.

5. Access to critical areas, such ashospitals and police stationsshould be provided, such that theroad helps the local communitiesand in return the people valuethe road.

6. Consideration of the impact ofdiversion routes on the localenvironment, during the

construction of new pavementshould be made. Properforecasting methods andmitigation procedures should beadopted.

7. Consideration of traffic growthand providing adequate room forfurther growth should be made,such that traffic saturation doesnot lead to a drastic change inthe character of the communityand significant construction activitycould be avoided for a longerperiod of time.

The issues listed above canbe represented in terms ofproblems such as deforestation,noise pollution, crossing of habitatareas, flooding/storm watermanagement, creation of wastage,urban heat island effect, drinkingwater pollution, air pollution, trafficjams, soil erosion, and use ofnatural resources and energy.

The above problems could besolved with the use of one or moreof a variety of methods, whichinclude habitat connection acrossroadways through underpasses oroverpasses, fencing along the roadfor animal protection, amphibianrescue fences, storm water pollutionprevention plan, pedestrian, bicycle,and transit access, noise barriers,buffer zones, or specially plantedslopes/embankments to reducenoise pollution, intersections withinterchanges, use of pervioussurfaces in pavement or shoulders,runoff treatment practices to cleanrunoff water, avoidingsensitive aquifers,making the locations ofservice connectionseasily accessible,providing easy access toemergency services, anddefining animplementation strategyto be followed duringconstruction.

Some of thesedesirable features areshown in Figure 1.

Construction ofPavementsFor construction of asphaltpavements the key factors that needto be considered are using localmaterials (to reduce transportationrelated energy expenses), loweringthe need for new materials,reducing production andconstruction temperature, usingmore environment-friendly products(such as bio-asphalt), andenvironment-friendly layers/mixes(such as those that would allowthe percolation of water into thesoil layers and hence facilitaterecharging of aquifer, and those thathave reduced noise levels), utilizingmixes that would allow recycling orreuse of discarded materials, andlonger lasting pavements, andconducting a proper life cycleanalysis.

The favorable impact of usingsustainable pavements is farreaching. For example, if a recyclingprocedure is used to rehabilitatean existing pavement, then thefollowing benefits can be ensured:1. Less use of new natural

resources such as mineralaggregates

2. Significant reduction intransportation cost of newmaterials

3. Less amount of energy use inobtaining and processing newmaterials (aggregates)

4. Less amount of energy use in

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Figure 2: Pervious pavement cross section (Courtesy: Richard Bradbury, US FederalHighway Administration)

manufacturing new materials(asphalt binder, cement)

5. Less emission and pollution ofthe environment

6. Reduction or avoiding of fillingup precious landfill space

7. Significant reduction in overalltransportation energy, specificallyif an in-place recycling process isutilized.

There is a variety of recyclingtechniques that are available today,each having its applicability underspecific conditions, as shown inTable 1.

A pervious layer in roads wouldallow the percolation of rain waterto the lower layers, and offer thefollowing benefits, that are relatedto both groundwater as well asstorm water/flood control:1. Recharge of ground water, and

hence a reduction in the potentialof lowering of groundwater andthe related problems.

2. Detention of rain water and hencea lowering of the storm watervolume that ends up in roadsewer system;

3. Filtering of storm water that iscontaminated by different methodson the road; studies show that asignificant amount of suspendedsolids, phosphorous, zinc andhydrocarbons are removed.

For asphalt pavements, a typicalpervious pavement is obtained byusing asphalt treated permeablebase over a layer of pervious stonelayer over the subgrade (with filtermaterial) as shown in Figure 2. Forconcrete pervious pavements, there

is the option of using interlocking/segmented concrete pavers. Evenif not used in mainline, storm watertree trenches can allow thedetention of a significant amount ofstorm water for sufficient timebefore releasing it into the sewersystem (Figure 3).

Low energy mixes, particularlywarm mix asphalt are becomingmore common these days, becauseof their inherent advantage of lowerenergy cost and lower emissionpotential. Another key factor inconsidering warm mix asphalt isthe fact that because of lowertemperature, the asphalt binderundergoes less aging, and hencea better pavement mix is obtainedduring construction whichtranslates to longer life and henceless frequent rehabilitation cycles.There are different types of WMAtechniques available, such aszeolites, wax, emulsion and foam -with newer products and techniques

being developed, and one or moresuitable methods can easily beadopted for regular use by theindustry. The selection of thespecific method should be madeon the basis of the availability ofthe material, expertise andexperience with that specifictechnology.

The use of bindersmanufactured from bio-resources isbeing researched and conducted indifferent parts of the world. Thesebinders are essentially derived frombiomass agricultural and forestresidue with the help of biochemicalor thermochemical processes. Onesuch example of bio binder whichis fully derived from vegetable isVegecol from COLAS. Such binderscan be utilized for modifying asphalt(< 10% replacement), extendingasphalt binder (>25% replacement)or totally replacing asphalt. Differentgrades of such binders could alsobe used for cleaning roadequipment or partially replacingfluxing agents. The advantages ofusing such binders are significantreduction in pollution no SOx andmajor reduction in NOx emissions,and manufacturing from 100%renewable agricultural products.Furthermore, the temperature of mixproduction could be reducedsignificantly.

One of the most excitingtechnologies that are beingdeveloped in pavement engineering

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Figure 3: Stormwater tree trench concept applied in Green Street project inPhiladelphia (Courtesy: Glen J. Abrams, AICP, Watersheds Planning Manager,

City of Philadelphia)

Figure 4: High RAP content recycling in Germany(Courtesy: Gerhard Riebesehl, Storimpex Asphal Tec

Matthias Nlting, Sasol Wax GmbH)

is the combined use of the recyclingand warm mix concepts to producegood performing and long lastingpavements. Binder from renewablesource could also be used in thisprocess. Using sophisticatedmaterials and plant/process controltechnology, it is now possible toeffectively recycle close to 100%reclaimed asphalt pavement (RAP)material, at a reduced temperature(for example below 130oC asopposed to 150oC) to meetemission requirements, and stillobtain a pavement which hasexcellent performance under bothhigh and low temperatures, andwhich can be opened to trafficquickly after construction, even at atemperature which is higher thanthat at which conventionalpavements could be opened.Overall, this translates to savingsin money and time, materials andnatural resources and energy, and

significant reduction in emission.Example of such a project (inGermany) is shown in Figure 4.

Recycling of discarded tires inthe form of crumb rubber in asphaltpavement mixes is now an acceptedpractice in many parts of the world.Apart from allowing the user to avoidthe filling up of precious landfillspace with hazardous waste, thisprocess allows the userto:1. Incorporate more

asphalt binder (thickerfilm) in mixes to make,them longer lasting anddurable

2. Use a coarsergradation for specificuses such as drainageof water.

3. Reduce noise levelsfrom pavementssignificantly.

The key to successful use ofrubber modified asphalt binder issuccessful incorporation andretention of the rubber into thebinder/mix. As is the case for manytechnologies, this is heavilydependent on available plant andconstruction equipment. Forexample, appropriate technologymust exist to uniformly blend thecrumb rubber mechanically orchemically with the binder, ensurethat the binder remains in that stateduring transportation, and for properquality control test methods to testand prevent the separation of therubber and the binder prior tolaydown and compaction. In termsof recycling of other materials,artificial aggregates such as blastfurnace slag, generated from steeland iron production could besuccessfully incorporated in pavingmixes.

The production of Portlandcement, the key constituent inconcrete pavements and clinkerrequired for manufacturing ofcement, are a major source of CO2emission in the modern world. Theuse of limestone and supplementalcementitious materials (SCMs, suchas fly ash from thermal powerplants using coal, and rice huskash) provide attractive options ofreducing the use of cement andthereby cutting down its CO2footprint. Blended cement,consisting of Portland cement andSCMs are becoming popular. Formaintenance of concrete

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pavements, modern techniquessuch as dowel bar retrofits, crossstitching, partial-depth repairs, jointand crack resealing, slabstabilization, diamond grinding couldbe used to improve the conditionsof concrete roads and at the sametime avoid or reduce the need forenergy intensive major repairs/rehabilitation work.

The concept of perpetualasphalt pavement has been recentlyproposed as a sustainabletechnology. The objective of thistechnique is to provide thick andappropriately constructed (withspecific mixes for different layers)layers such that the crit icalresponses (such as tensile strain)in the pavement are kept very low,and as a result no major structuralfailure could be expected for a longperiod of time. The onlydeterioration that could be expectedwill be in the form of surfacedistresses, which could be relativelyeasily and economically addressedby less involved rehabilitationprocedure, such as in place surfacerecycling. Obviously, thesepavements would cost more thanconventional pavements initially butif constructed properly, they allowthe users and the highwayagencies to enjoy a better qualityand durable pavement withsignificantly reduced cycle ofrehabilitation (the pavements needto be maintained properly), andhence significant reduction inactivities that could negatively impactthe environment.

In terms of longevity, proponentsof the concrete pavement industryargue that the longer life of concretepavements translates to reducedneeds for new natural resources,energy and waste disposal. Anotherimportant factor is that because oflower deflection in concretepavements and hence lower rollingresistance, the mileage isincreased by about 3.85% inconcrete pavements over asphaltpavements (with similar

smoothness) which translates tolower amount of fuel spent andhence lower amount of greenhousegas emission. Furthermore,because the surface of a concretepavement reflects a higher amountof solar energy than asphaltpavement, concrete pavement helpsin reducing the indirect contributionof pavements towards the urbanheat island effect. And at nighttime,the lighter color of the concretepavement surface helps in asignificant reduction in the amountof energy that is needed forillumination. However, note thatsmoothness has a significantpositive effect on gas mileage,asphalt pavements are in generalsmoother than concrete pavements,and the reflectivity of asphaltpavements can be improved byusing surface gritting or chip sealswith light colored aggregates,colorless synthetic binders andsurface paints, and the surfacetemperature could be reduced withthe help of open graded frictioncourse.

Life cycle analysis (LCA) inpavement engineering is anestablished concept, and is utilizedin many, if not most, high costprojects. However, traditionally, thecost is evaluated in terms ofmaterials/construction/salvage costonly. For sustainable pavementengineering, the environmental andsocial cost must also be evaluated.Furthermore, the cost of extendingthe life of a pavement (and thuslowering the life cycle cost) mustbe evaluated in the context of theamount of natural resources andenergy that are spent on producingthe premium materials that areused to extend its life. The impactof the pavement construction/maintenance and rehabilitationshould be evaluated in terms of allthree components of sustainability social, economic andenvironmental. Examples of suchframework and components offramework are the Sustainability Life

Cycle Assessment (SLCA), the IRFGreenhouse Gas Calculator, andPaLATE (Pavement Life-cycleAssessment Tool for Environmentaland Economic Effects).

WorkersHundreds of thousands of humanbeings men, women and in manycases even children work inpavement related projects all overthe world. In many cases, this workgoes on under extreme conditionsof weather, in areas that are remotefrom basic facilities such as schoolor hospitals, and under conditionsof emissions/pollution fromconstruction conditions (smoke/gas/particulate emission). This makesthe entire realm of pavementengineering socially unsustainable.Conditions must be improved suchthat environmental and economicjustice is prevalent among thepavement workers. While there aremany things that need to be done,first and foremost, it is theresponsibil ity of pavementengineers to select materials andmethods such that workingconditions are improved. Forexample, switching to warm mixasphalt instead of hot mix asphaltwill reduce harmful emissionssignificantly (Figure 5). Secondly,engineers and authorities mustenforce strict safety regulations andprovide for the basic necessitiessuch as protective body and eyewears.

Pavement-Building-Nature-SymbiosisAs pointed out earlier, pavementsneed to be considered as part ofthe built environment going onestep forward, in the future;pavements need to be consideredas part of a symbiotic system thatalso includes buildings and thenature. Imagine a day when we willhave smart pavements, that will beable to self control temperaturesand adapt to changingenvironmental conditions to sustain

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loads without deforming or cracking,will be able to harvest heat energyfrom the solar radiation falling onthem, and supply this to buildingsfor various uses, and in return, thebuildings will be keeping thepavements below a crit icaltemperature, thus extending theirlives, and also reducing the amountof heat that is radiated back intothe environment, and henceindirectly reducing the urban heatisland effect. Multidisciplinaryresearch, as indicated in Figure 6,needs to be conducted to makethis vision a reality.

Regulatory Bodiesand Impetus forSustainabilityAdopting sustainable practices canreduce many of the environmentalimpacts of pavement construction.However, both impetus andregulations (carrots and stick)need to be in place to acceleratethe adoption of sustainablepractices. Impetus in the form ofprestigious certif ication, firstvoluntary, and then gradually,mandatory, should be introduced.Example of such certification is theUS Green Building CouncilsLeadership in Energy andEnvironmental Design (LEED) in the

Figure 5: Range of reduction in emission through the use of Warm Mix Asphalt

United States, which awards silver,gold and platinum certifications forprojects in increasing order ofcontribution towards improvingperformance across all the metricsthat matter most: energy savings,water efficiency, CO2 emissionsreduction, improved indoorenvironmental quality, andstewardship of resources andsensitivity to their impacts.Regulatory bodies such as theEnvironmental Protection Agency(EPA) in the US enforce strictemission standards to prevent theemission of harmful gases andparticulates into the atmosphere.

Figure 6: Multidisciplinary approach to consider pavement infrastructure as partof the built environment

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Human FactorPavements are engineeredsystems that need to be planned,designed, constructed,maintained and rehabilitated byknowledgeable and skil ledpeople. However, for achievingsustainable pavementengineering, we need sociallyresponsible engineers, withbroad vision and compassion forhumanity and respect for nature.

Therefore, the preparation forsustainability starts today in ourColleges and Universities withthe students who will be joiningthe profession tomorrow.Professors, administrators andthe tax paying general publicneed to be educated on theimpacts of pavement engineeringand the different ways tominimize such impacts. This canbe achieved by establishingmultidisciplinary centers ofexcellence in sustainablepavement engineering in majorcolleges and Universities inconjunction with the industry andthe government in the differentparts of the country. While

education on existing and newconcepts of sustainability can betransmitted by this method, thecenters should also focus ondeveloping, through research,new sustainable concepts andpractices. And in that research, itis vital that engineers look at thefuture innovative, sophisticatedmaterials and techniques, aswell as into the past todetermine what has survived overcenturies without harmfullyaffecting our environment.

This has been a fairly longarticle, and it is time for theauthors (and perhaps the readersas well) to relax and sip a cup oftea preferably in an Indian claycup have you ever wonderedwhat a marvel of sustainableengineering the Indian clay cupis? It comes from the earth,serves its purpose in the mostefficient way even adds flavorto the tea, and then goes backcompletely into the earth, withoutleaving any lasting impact on theenvironment. We, pavementengineers, have a lot to learn fromit!