AN OVERVIEW OF RECYCLING AND RECLAMATION METHODSFOR ASPHALT PAVEMENT REHABILITATION

Introduction

A number of recycling or reclamation methods are available for the rehabilitation orrestoration of asphalt pavements. This includes cold planing and several methods of recyclingwhich involves the reuse, after processing, of the existing pavement materials.

The primary objective of this Asphalt Recycling and Reclaiming Association (ARRA)publication is to familiarize governmental agency and consulting engineers and others involvedwith pavement rehabilitation with cold planing and recycling and their appropriate applications.

Reclamation Methods

The methods of asphalt pavement rehabilitation to be reviewed are cold planing, hotrecycling, hot in-place recycling, cold in-place recycling and full depth reclamation. Thesemethods offer a number of advantages which include the following.

1. Allow the use of existing materials with the elimination of disposal problems.2. The asphalt mix may be improved through changes to the aggregate and/or asphalt

binder.3. The pavement profile may be corrected and the ride improved.4. Cost reductions may be achieved over conventional rehabilitation methods.

Some of these methods are being used together on rehabilitation projects. A hotrecycled surface course can be placed on cold in-place recycled base or binder course. Onan Interstate project in Kansas, the three recycling methods of hot recycling, cold in-placerecycling and full depth reclamation were used for a complete reconstruction of the pavement.

A description of these reclamation methods is provided below.

A. Cold Planing

This method is the automatically controlled removal of asphalt pavement to a desireddepth and with restoration of the surface to a desired grade and slope and free of humps, rutsand other imperfections. Cold planing may be used for the roughening or texturing of apavement to eliminate slipperiness and restore skid resistance. The pavement removal ormilling is performed with a self-propelled rotary drum cold planing machine with the reclaimedasphalt pavement (RAP) transferred to trucks for removal from the job site. The resultingpavement can be used immediately by regular traffic and overlaid at some future time or leftas a textured surface.

B. Hot Recycling

With this method, RAP is combined with new aggregate and an asphalt cement orrecycling agent to produce hot mix asphalt (HMA). Both batch and drum type hot mix plantsare used to produce recycled mix. The RAP generally is obtained by pavement milling witha rotary drum cold planing machine. The RAP also may be from a ripping/crushing operation.The mix placement and compaction equipment and procedures are the same as for regularHMA.

The ratio or blend of RAP to new aggregate will vary with this dependent on therecycled mix properties desired and the type of hot mix plant. RAP/aggregate blends typicallyhave been from 10:90 to 30:70 with a maximum of 50:50 (drum plant). However, newtechnology using microwave heating may allow the use of up to 100% RAP without theoccurrence of "blue smoke" problems.

C. Hot In-Place Recycling

This method is performed on-site, in-place with the existing pavement typicallyprocessed to a depth of 3/4 to 1 1/2 inches. The deteriorated asphalt pavement is heated andsoftened to allow it to be scarified or hot rotary mixed to a specified depth. As required, newhot mix material and/or recycling agent or other liquid additive is added to the reclaimedmaterial.

Hot in-place recycling is performed as either a single or multiple pass operation. Witha single pass, the restored existing pavement material is combined with new/virgin material.

For multiple passes, the restored material is recompacted and the application of a new wearing '\course then follows after a prescribed interval or delay. -.....b

D. Cold In-Place Recycling

Cold in-place recycling is an asphalt pavement rehabilitation technique that reusesexisting pavement materials. All work is completed while on the pavement being recycled andnormally not required is transportation of materials except for the additive being used. Thedepth of treatment is typically from 3 to 4 inches. In some cases, virgin aggregate may beadded to the recycled material to change/improve the RAP characteristics. An asphalt emul­sion is normally the additive for the process but asphalt rejuvenating agents also are used.

This technique involves pulverizing the existing pavement, sizing of the RAP,incorporation of an additive and the placement and compaction of the recycled mix. A morerecent innovation in cold in-place recycling has been the development of the recycling train.This train includes a large cold milling machine which pulls a screening and crushing unitfollowed by a mixing device. The processed material is deposited in a windrow from themixing device where it is picked up and placed and compacted with conventional hot mixasphalt laydown and rolling equipment.

E. Full Depth Reclamation

Full depth reclamation is a recycling method where all of the asphalt pavement section

and a predetermined amount of underlying materials are treated to produce a stabilized base '\course. Additives are used for an improved base which include imported materials, asphalt ~emulsions and chemical agents such as calcium chloride, portland cement, fly ash and lime.

The full depth process consists of the five basic steps of pulverization, introduction ofadditive, shaping, compaction and application of a surface or wearing course. On someprojects, the amount of on-site material may not be adequate to provide the desired depth oftreated base but new materials may be imported for increasing the thickness.

Pavement Condition

An essential part of the process of selecting a rehabilitation method is the determinationof the existing pavement condition. The types, amount and in some instances, the depth ofdefects or distress needs to be determined by completing a pavement condition survey. Aninadequate evaluation of the pavement may result in the improper use of a recycling orreclamation method and less than expected performance.

The primary types of pavement distress for asphalt or flexible pavements are (1)surface defects, (2) deformation, (3) cracking, (4) maintenance patching, (5) base/subgradeproblems and (6) poor ride/roughness.

A. Surface Defects

This type of distress includes raveling, bleeding (flushing) and slipperiness (skid hazard).

Raveling: This is the progressive deterioration of the pavement surface caused by the lossof aggregate particles. Initially fine particles are lost followed by coarse aggregate becomingdislodged as the condition becomes more severe. Early raveling is usually due to a poorquality mixture with later raveling occurring as the asphalt binder hardens.

Bleeding: A film of asphalt appears on the pavement surface. This film may be from theupward movement of asphalt from a rich or over-asphalted or a low air voids mixture. Also,it may develop with chip seals where the asphalt application rate was too high or coveraggregate loss has occurred.

Slipperiness: This condition is the lack of good friction and is usually due to a smoothpavement surface. It almost always occurs when a pavement is wet and a film of waterexists. In some instances, the condition may be so severe as to result in the serious skidhazard of aquaplaning.

B. Deformation

Asphalt pavements may exhibit permanent deformation (distortion) which includescorrugations (washboarding), wheelpath rutting and shoving.

Corrugations: This defect is a form of plastic movement typified by ripples laterally acrossthe asphalt surface. This condition mostly occurs where traffic starts and stops or on hillswhere vehicles brake downgrade. The mixture usually lacks stability which may be causedby being too rich in asphalt, having a high sand content, containing too much smooth orrounded aggregate or the asphalt being too soft.

Shoving: This deformation is also a form of plastic movement and is caused by a mixturelacking stability. Shoving appears as localized bulging of the pavement surface.

Wheel path Rutting: This condition occurs as longitudinal surface depressions/channels in the ~wheel paths. The rutting may be due to the lateral movement or consolidation under traffic .'?"

of one or more of the underlying courses of pavement. It also may be caused by pavementwear or loss due to the abrasive action of traffic. Rutting may be due to an insufficientthickness of pavement, an unstable asphalt mixture, insufficient or over compaction duringconstruction and more recently, very heavy wheel loading and high tire pressures.

C. Cracking

There are primarily two types of cracking, load associated and non-load associated.However, one type of cracking, reflection, could be placed in both types since a combinationof stresses produces this cracking. Load associated cracking is due to the repeatedapplication of wheel loads with these types of cracks being alligator (fatigue), longitudinal­wheelpath, pavement edge and slippage. The non-load type cracking includes block(shrinkage), longitudinal-joint and transverse (thermal).

Alligator Cracks: These are interconnected cracks that create a series of small piecesresembling an alligator's skin or chicken wire. In most cases, alligator cracking is caused byexcessive deflection of the pavement surface over an unstable subgrade or lower courses ofthe pavement. However, in some instances, it may be due to repeated loads that exceed theload-carrying capability of the pavement.

Wheel path Cracks: This cracking appears as fractures or separations within the wheelpaths.The cracking begins as single or multiple longitudinal cracks and progresses with time to apattern resembling alligator cracking.

Pavement Edge Cracks: This distress results in longitudinal cracks within 1 to 2 feet of theouter edge of the pavement. This cracking is due to the lack of lateral (shoulder) support,base weakness, frost action and inadequate drainage. Also, the crack may appear at the jointbetween a pavement and widening.

Slippage Cracks: These are typically crescent or half-moon shaped cracks. They areproduced when vehicles brake or turn which cause the pavement surface to slide or push.This condition usually occurs when the asphalt mixture is of low strength or there is poor bondbetween the surface course and the layer below.

Block Cracks: This cracking occurs as interconnected cracks forming a series of largeblocks, usually with sharp corners or angles. It is sometimes difficult to determine whether thecracking is caused by volume changes in the asphalt mix or in the base or subgrade.Frequently, they are due to shrinkage of fine aggregate asphalt mixes where a low penetrationasphalt was used or the asphalt has hardened.

Longitudinal-Joint Cracks: This defect develops in the construction joint between laydownpasses of the asphalt paver. A poorly constructed paving joint accelerates the developmentof this cracking.

Transverse Cracks: These cracks extend across the pavement approximately perpendicularto the pavement centerline and usually initially occur as a single crack. This cracking isfrequently due to shrinkage from temperature changes and/or hardening of the asphalt.

Reflection Cracks: These are cracks in asphalt overlays which reflect the crack pattern inthe pavement structure underneath. Vertical and horizontal movements in the pavementbeneath the overlay produce stresses which create this condition.

D. Maintenance Patching

A patch is an area of distressed pavement which has been repaired with new materialand normally is considered a defect no matter how well it is performing. The patching maybe completed using cold or hot mix asphalt. The repair may involve the removal of existingpavement materials and their replacement with new mixture (deep or pothole patching) or theplacement of a thin overlay (skin patching). Also, the patching may be only a seal coatingof the distressed area (spray patching).

E. Problem Base/Subgrade

Severe cracking, settlement and rutting of an asphalt pavement often is the result ofa poor base or subgrade, particularly for thinner sections. A wet, soft base or subgrade is aproblem due to very low strength and load carrying capacity and the possible pumping/dis­placement of material to the surface through cracks in the pavement structure.

F. Ride Quality/Roughness

Ride quality should be evaluated to establish the severity of roughness with thefollowing factors included.

General Unevenness: This is the overall ride quality with consideration mostly to the generalsmoothness of the pavement (isolated, small areas of depression and high spots not givenprimary consideration).

Depressions: These are low areas which were created during the paving operation or arethe result of settlement of the pavement.

High Spots: These are high areas in the pavement surface which were caused by the pavingoperation or are due to frost heaving or the swelling of the subgrade soil.

Selection of Recycling Method

Table 1 provides guidance for the selection of recycling or reclamation methods for therehabilitation of asphalt or flexible pavements. For this table, cold planing is only the removalof material or surface texturing with a cold planing machine. However, cold planing may becombined with the placement of a new overlay or hot recycling with other defects than thoseindicated then treatable using this process.

For the two cold recycling methods, cold in-place recycling and full depth reclamation,and the multiple pass method of hot in-place recycling, a new surfacing will be required. Thetype of surfacing selected will depend on the amount of daily traffic (ADT) and the axle loads(ESAL's). For light traffic, an asphalt surface treatment may only be required, such as a chipseal, slurry seal or cape seal. For heavier traffic, an asphalt overlay is recommended with hotmix asphalt (HMA) or an asphalt emulsion cold mix such as an open-graded type (OGEM).

An essential part of the selection of a recycling or reclamation method is anassessment of the cost of the various options. Initial cost is an important factor but the long­term performance aspects, Le., cost effectiveness or life cycle cost, are of great importancein determining the reclamation method.

References

1. Guideline Specifications for Cold Planing, Asphalt Recycling and Reclaiming Association.

2. Proven Guidelines for Hot-Mix Recycling, Asphalt Recycling and Reclaiming Association.

3. Guideline Specifications for Hot In-Place Recycling, Asphalt Recycling and ReclaimingAssociation.

4. Guidelines for Cold In-Place Recycling, Asphalt Recycling and Reclaiming Association.

5. Guideline Specifications for Full Depth Reclamation, Asphalt Recycling and ReclaimingAssociation.

6. Asphalt in Pavement Maintenance, Manual Series No. 16 (MS-16), Asphalt Institute, P.O.Box 14052, Lexington, Kentucky 40512-4052.

7. Pavement Recycling with Asphalt Emulsions, Asphalt Emulsion Manufacturers Association(AEMA), #3 Church Circle, Suite 250, Annapolis, Maryland 21401.

8. Distress Identification Manual, SHRP-LTPP/FR-90-001, Available from the TransportationResearch Board (TRB), National Research Council, 2101 Constitution Avenue, N.W.,Washington, D.C. 20418.

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TABLE 1

GUIDE FOR SELECTION OF REHABILITATION METHODFULL DEPTH' OR AGGREGATE BASE ASPHALT PAVEMENTS

-,

Type of PavementDistress

ColdPlaning

HotRecycling

HotIn-PlaceRecycling

ColdIn-PlaceRecycling

Full

DepthReclamation

===============================================================================================================

Surface DefectsRavelingBleeding (Flushing)Slipperiness

xxx

xxx

x2

x5

x2------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

DeformationCorrugations (Washboarding)Rutting - Shallow3Rutting - Deep·

x5

x5

xxx

x5

x5

x6 X6•7

--------------------------------- .•.------------------------------------------------------------------------------- ..---------------------------------------------------------------- ..-------------

Cracking/Load AssociatedAlligator

xxx

Longitudinal - Wheel Path

xx8x x

Pavement Edge

xxx

Slippage

xxg

-------------------------------------------------------- .•.---------------------------------------------------------------------------------------------------------------------------------------

Cracking/Non-Load AssociatedBlock (Shrinkage)Longitudinal-jointTransverse (Thermal)

xxx

x'O

x

x

x

x------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Reflection x x x------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Maintenance Patchingx"Spray

X11xSkin

x"X11xPothole

xxx

Deep (Hot Mix)

xxx

--------------------------------------------------------------------------------------------- .•. ---------- .•.---------------------------------------------------------------------------------------

Problem Base/Subgrade (Soft, Wet) x-------------------------------------------------------------------------------------------------------- .•.---------------------------------------------------------------------------------------

TABLE 1

GUIDE FOR SELECTION OF REHABILITATION METHODFULL DEPTH1 OR AGGREGATE BASE ASPHALT PAVEMENTS

Type of PavementDistress

ColdPlaning

HotRecycling

HotIn-PlaceRecycling

ColdIn-PlaceRecycling

FullDepth

Reclamation===============================================================================================================

Ride Quality/RoughnessGeneral UnevennessDepressions (Settlement)High Spots (Heaving)

X

X12

X12

X

X12

X12

X

X12

X12

X13

X14

--------------------------------------------- -------- --- -- -------- -------- --- ..---- -------- ------ ..- -------- -------- -------- -------- -------- --- ----- --- ------- ..----------- --- --- ..--- ---- .. ---

IA pavement in which asphalt mixtures are used for all courses above the subgrade or an improved subgrade having portland cement, lime,lime-fly ash, fly ash or calcium chloride modification.

2Applicable if the surface course thickness does not exceed 1 1/2".3Rutting is limited to the upper portion of the pavement structure (top 1 1/2 - 2 inches).4Rutting originating from the lower portion of the pavement (below surface course and includes base and subgrade).5May be a temporary correction if entire layer affected not removed or treated by the addition of special asphalt mixtures.'The addition of new aggregate may be required for unstable mixes.7The chemical stabilization of the subgrade may be required if the soil is soft, wet.8Applicable if the cracking is limited to the surface course of the pavement.gApplicable if treatment is to a depth below the layer where the slippage is occurring.IDApplicable if the cracking is limited to the surface course of the pavement.11In some instances, spray and skin patches may be removed by cold planing prior to these treatments (considered if very asphalt rich,

bleeding).12May be only a temporary correction if the distress related to a subgrade problem.13Used if depressions due to a soft, wet subgrade condition.14Used if the high spots caused by frost heave or swelling of an expansive subgrade soil.

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ASPHAL T RECYCLING &RECLAIMING ASSOCIA TION

The Asphalt Recycling & Reclaiming Association (ARRA) is a non-profit interna­tional trade association of contractors, equipment manufacturers, suppliers,public officials and engineers, engaged in the recycling and reclaiming ofasphalt, working to build a stronger and safer network of highways, streets androads across the country and around the world.

A primary purpose of ARRA is to develop and maintain professional guidelinesfor the recycling industry. An Overview of Recycling and Reclamation Methodsfor Asphalt Pavement Rehabilitation was developed by the ARRA Committee onRecycling Education and was done so based on the collective experience andknowledge of its participating members.

These guidelines may be amended from time to time to reflect the latest instate-of-the-art technology.

Additional copies of this and other ARRA publications can be obtained bycontacting the Association headquarters.

Asphalt Recycling & Reclaiming Association#3 Church Circle, Suite 250Annapolis, Maryland 21401

USA(410) 267-0023

ARRA is for people in the recycling business!

Approved by ARRA - August 1992