Bulletin of the Transilvania University of BraşovCIBv 2015 • Vol. 8 (57) Special Issue No. 1 - 2015

ASPHALT PAVEMENT RECYCLING

E-L. PLEŞCAN1 C. PLEŞCAN2

Abstract: These paper presents the specific methods of asphalt pavementrecycling, considering the benefits of the recycling like cost reduction,cconservation of aggregate and binders, preservation of the existingpavement geometrics, reservation of the environment, conservation ofenergy, less user delay. After the introduction in the asphalt pavementrecycling is presented a guide for selection of recycling method and theApplicability for the use of different recycling techniques. In the followingpart are considered: the recycling technology in Romania and the structuraldesign of recycled pavement.

Key words: road pavements, recycling, structural design

1 Civil Engineering Department, Transilvania University of Braşov.2 Civil Engineering Department, Transilvania University of Braşov.

1. Introduction

Recycling or reuse of pavement materialis a very simple but powerful concept.Recycling of existing pavement materialsleads to new pavement materials thatconsiderable saving material, money, andenergy [1]. At the same time, recycling ofexisting material also helps to solvedisposal problems. Because of the reuse ofexisting material, pavement geometricsand thickness can also be maintainedduring construction. In some cases, trafficdescription is less than that for otherrehabilitation techniques. The specificbenefits of recycling can be summarized asfollows:

reduced costs of construction; conservation of aggregate and

binders; preservation of the existing

pavement geometrics; preservation of the environment;

conservation of energy; less user delay.

2. Recycling as a RehabilitationAlternative

Recycling is only one of the severalrehabilitation alternatives available forasphalt pavements [2]. The choice ofrehabilitation alternative depends onobserved pavement distress, laboratory andfield evaluation of existing material, anddesign parameters [3]. Also, maintenanceof geometrics and original thickness ofpavements, especially in underpasses,influence the choice of rehabilitationmethod. However, recycling has someunique advantages which are not availablewith other types of rehabilitationtechniques. For example, recycling canresult in savings, help in conservation ofnatural resources, and can maintainpavement geometrics as well as thickness.

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It is well recognized that a soundinfrastructure, including roadways, isrequired for a good economy with anadequate level of growth. Studies haveindicated that if a roadway is maintained,at an acceptable level of service, it willultimately cost less.

Different recycling methods are nowavailable to address specific pavementdistress and structural needs. Allpavements deteriorate over time due totraffic and environmental factors [1].

Rehabilitation is needed to maintain thepavement at a certain condition.Rehabilitation may be required in case of,inadequate ride quality, because ofexcessive pavement distress. It also may berequired when the coefficient of frictionbetween tire and pavement is reduced, orwhen there is low structural capacity orinadequate capacity to carry projectedtraffic volumes.

3. Recycling Methods and Processes

The Asphalt Recycling and ReclaimingAssociation [5] define different types ofrecycling methods as follows [1]:

Hot mix asphalt recyclingHot in-place recyclingCold in-place recyclingFull depth recyclingHot mix asphalt recycling is the process

in which reclaimed asphalt pavementmaterials are combined with newmaterials, sometimes along with arecycling agent, to produce hot mix asphalt(HMA) mixtures. Both batch and drumtype hot mix plants are used to producerecycled mix. The reclaimed asphaltpavement material can be obtained bymilling or ripping and crushing operation.The mix placement and compactionequipment and procedures are the same asfor regular HMA.

Fig. 2. Hot mix asphalt recycling [1]

Fig. 1 Plot of pavement condition versus time[4]

E-L. PLESCAN & C.PLESCAN: Asphalt pavement recycling 267

Hot in-place recycling consists of amethod in which the existing pavement isheated and softened, and thenscarified/milled to a specified depth.

Cold in-place recycling involves reuseof the existing pavement material withoutthe application of heat.

Except for any recycling agent, notransportation of materials is usuallyrequired, and aggregate can be added,therefore hauling cost is very low.Normally, an asphalt emulsion is added asa recycling agent or binder.

Fig. 5. Cold in-place recycling [10]

Full depth recycling has been defined asa recycling method where all of the asphaltpavement section and a predeterminedamount of underlying base material aretreated to produce a stabilized base course.It is basically a cold mix recycling processin which different types of additives suchas asphalt emulsions and chemical agentssuch as calcium chloride, Portland cement,fly ash, and lime, are added to obtain animproved base.

4. Selection of a Recycling Method

If recycling is chosen as a rehabilitationalternative, there is a variety of recyclingmethods available for rehabilitation ofHMA pavements.

These recycling methods offer a numberof advantages, which include the following[5]: allow the use of existing material with

the elimination of disposal problems. the asphalt mix may be improved

through changes to the aggregateand/or asphalt binder.

the pavement profile may be correctedand the ride improved.

cost reductions may be achieved overconventional rehabilitation methods.

In Table 1 from below it is presented theguide for the selection of the methodaccording to the literature.

5. Structural Design of RecycledPavements

Structural design of pavements takes intoconsideration those aspects of designwhich provide required strength orstiffness to the pavement structure. Thedesign method has evolved from theapplication of engineering judgment topredominantly mechanistic or semi-mechanistic procedures. Pavementmaterials can be characterized by resilientmodulus and fatigue characteristics.

Fig. 4. Hot in-place recycling [9]

Fig. 6. Full depth recycling

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Guide for selection of the recycling method [5] Table 1

Pavement materials with differentstrength and structure can be considered byappropriate “structural numbers”.

Recycled asphalt materials can providepavements similar or even better thanpavements constructed with conventionalhot mix asphalt. However, the wide rangeof properties of recycled mixes, resultingfrom variation in material and constructionmethods must be considered duringstructural design of recycled pavements[7].

On an average, the coefficients for bothrecycled surface and recycled base coursesare found to be greater than thecoefficients for respective conventional

mixes determined in the AASHTO RoadTest. The AASHTO guide indicates that inessence there is no difference between hotrecycled and virgin HMA material, andrecommends the structural rehabilitationanalysis method (for conventional mix) fordesign of recycled pavements as well.However, it also cautions that since long-term performance data is not available forrecycled mixes, engineering judgmentshould always be applied for design ofsuch mixes.

The AASHTO guide [7] proposed amethod of overlay design based primarilyon structural number, thickness ofunderlying layers, and drainage

Type of Pavement Distress HotRecycling

Hot In-Place Cold In-Place Full Depth

Recycling Recycling Reclamation

Surface DefectsRaveling x xBleeding x xSlipperiness x xDeformationCorrugations x xRutting - shallow x xRutting - deep x xCracking/Load AssociatedAlligator x x xLongitudinal - wheel path x x x xPavement edge x x xSlippage x xCracking/Non-Load

AssociatedBlock (shrinkage) x x xLongitudinal-joint x xTransverse (thermal) x x xReflection x x xMaintenance PatchingPothole x x xProblem Base/Subgrade

(Soft, Wet)x

Ride Quality/RoughnessGeneral unevenness x xDepressions (settlement) x x xHigh spots (heaving) x x x

E-L. PLESCAN & C.PLESCAN: Asphalt pavement recycling 269

coefficients. Basically, a monograph isused to calculate a combined totalstructural number for the whole pavementsection, based on performance period,traffic, and change in PresentServiceability Index (PSI). The structuralnumber can be represented by acombination of product of depth, structuralnumber, and drainage coefficients for eachof the pavement layers. The structuralnumber of the recycled layer required iscalculated by subtracting the effectivestructural number of the existing pavementfrom the structural number required by the“new pavement,” which includes therecycled layer. The effective structuralnumber of the existing pavement ismodified by a remaining life factor for theexisting pavement. The equation is asfollows:

SNOL = SNY - (FRL X Snxeff)

where:SNOL = structural number of the required

overlaySNY = structural number required for a

“new” pavement to carry the estimatedfuture traffic for the prevailing roadbedsoil support conditions

FRL = remaining life factorStructural Design of Recycled PavementsSnxeff = effective structural number of the

existing pavement at the time the overlayis placed

Structural number (SN) is defined asfollows:

SN = a1D1 + a2D2m2 + a3D3m3

where:a1, a2, a3 = layer coefficients

representative of surface, base, andsubbase courses, respectively

D1, D2, D3 = actual thickness (in mm,inch) of surface, base and subbase courses,respectively

m2, m3 = drainage coefficients for

untreated base and subbase layers,respectively

One important feature of the designmethod is the inclusion of reliability factorin traffic and performance prediction.

According Asphalt Instituterecommendations [8] for hot-mix recyclingthe design procedure shows the same as forconventional mixes. The parametersrequired for designing the pavementthickness include the following: the equivalent 18-kip single-axle load

(ESAL) applications. The designESAL is calculated by the summationof the products of number of vehiclesand the corresponding truck andgrowth factors.

the Resilient Modulus, MR of thesubgrade. This can be determined bytesting or through correlations withCBR or R-value.

the type of surface and base. The totalrequired pavement thickness can becalculated by entering the designtraffic and MR values in the designcharts.

In the comparison of properties ofrecycled materials to those of newmaterials, the recycled materials have beenconsidered equivalent to the conventionalmix.

The overlay design procedure can also beused for thickness design. The overlaythickness is calculated as the differencebetween the thickness required by a newpavement to the design traffic ESAL andthe effective thickness of the existingpavement.

The effective thickness of the existingpavement can be determined based on thePresent Serviceability Index, andequivalency factors for converting variouspavement materials to equivalentthicknesses of asphalt concrete are used.Also the conversion factors for eachpavement layer (based on the condition ofeach layer prior to overlay) can be used to

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convert each layer to an equivalentthickness of asphalt concrete.

The effective thickness of the existingpavement should be subtracted from thethickness of the recycled layer.

6. Conclusions

The Romanian existing roadwayinfrastructure has aged, and a significantnumber of roadways are nearing the end oftheir service life. Limited funding anddemands on existing resources have shiftedthe emphasis from new construction topreservation and extending the service lifeof the existing roadways.

The implementation of more timely orproactive preventative maintenance andrehabilitation treatments is being used as ameans of preserving the existing roadwayinfrastructure. Therefore the application ofasphalt recycling technologies are moreoften use in our country.

According Romanian technical norm [8],hot in place recycling technology ofbituminous road pavements can be appliedin the following alternatives, workingprinciple:

a) by cold milling of the existingbituminous layers, followed by recyclingitself, made by mixing of added material ina mobile plant.

b) by infrared heating of the existingbituminous layers, followed by theirscarification and mixing with addedmaterials over the sub base.

Compacting operation is performedunder the same conditions as forconventional asphalt mixes.

The bituminous layers made by hotrecycling may serve as wearing or basecourse layers.

Hot recycling road pavements areconstructed during the hot seasons,between April 15 and October 15, whenthe atmospheric temperature is at least+5°C.

When the bearing capacity of roadstructure is appropriate, the hot recycledroad layers will be covered with wearingcourse.

References

1. Pavement Recycling Guidelines forState and Local Governments, U.S.Department of Transportation FederalHighway Administration, 1997

2. J.A. Epps, D.N. Little, R.J.Holmgreen, and R.L. Terrel.Guidelines for Recycling PavementMaterials, NCHRP Report 224, TRB.National Research Council,Washington, DC, 1980.

3. J.A. Epps, R.L. Terrel, and D.N. Little.“Recycling Pavement Materials,”Rural and Urban Roads, May 1978.

4. Asphalt Recycling and ReclaimingAssociation. Hot In-Place Recycling,Presented by the Hot In-PlaceRecycling Technical Committee,ARRA Convention, 1996.

5. An Overview of Recycling andReclamation Methods for AsphaltPavement Rehabilitation. AsphaltRecycling and Reclaiming Association,Annapolis, MD, 1992.

6. American Association of StateHighway and Transportation Officials(AASHTO). AASHTO Guide forDesign of Pavement Structures,Washington, DC, 1986.

7. Asphalt Institute. Asphalt Hot-MixRecycling, Manual Series No. 20 (MS-20), College Park, MD, 1986

8. Normativ privind reciclarea la cald aimbracamintei rutiere bituminoaseAND 575-2002 (Standard regardingthe recycling of hot mix asphalt AND575-2002) AND, 2002

9. http://www.equipmentworld.com/asphalt-recycling-section-3/

10. http://www.ecopavesystems.com/hotinplacerecycling.htm