Hot Mix Bituminius Paving Manual 014716

7/30/2019 Hot Mix Bituminius Paving Manual 014716

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Hot-Mix BituminousPaving Manual

MARCH 1985 FHWA-EM8428


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Federal Highway MministrationOffice of Highway OperationsConstruction and Maintenance DivisionGeotechnical and Materials Branch

WC~I985


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Acknowledgement

This manual was developed by the Geotechnical andMaterials Branch of the Federal Highway Administrationbased on information and materials prcwided by M r . W.H. Reynolds, of the FHW Arkansas Division. Specialappreciation is expressed to the FI1WA Region 6 Office,The Arkansas Division Office, and to Mr. Reynolds fortheir support in the sutmission of the originalinformation and in the assembly and refinement of themanual. Appreciation is also expressed to Ms. AnnieHmner and Ms. Angie Gregory for the cooperation andassistance in typing and editirq the manual.


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TABLE OF CONTENTSB i tu m i no u s Pa vi ng I n t r o d u c t i o n . . . . . . . . . . . . . . . . . . 1-1Pavement Distresses . . . . . . . . . . . . . . . . . . . . . . -2Ma te r i a l s a n d M ix D e s ig n . . . . . . . . . . . . . . . . . . . . . -1A s p h a l t . . . . . . . ... . . . . . . . . . . . . . . . . . . -1Aggregates ........ . . . . . . . . . . . . . . . . . . . -9Marsha l l M ix Des ign Method . . . . . . . . . . . . . . . . . . . -15HveemMix Des ignMethod . . . . . . . . . . . . . . . . . . . . . -16Dens i ty and Vo ids Ana lys is . . . . . . . . . . . . . . . . . . . -1 8M ix D e f i c i e n c i e s . . . . . . . . . . . . . . . . . . . . . . . . -22H o t - M i x p l a n t s . . . . . . . . . . . . . . . . . . . . . . . . . . -1S t o c k p i l i n g . . . . . . . . . . . . . . . . . . . . . . . . . 3 - 1B a tc h P la n t s . . . . . . . . . . . . . . . . . . . . . . . . . . -3. . . . . . . . . . . . . . . . . . . . . . . .r u m Mi x P l a n t s - 3 - 9

Comparisons/Contrasts . . . . . . . . . . . . . . . . . . . . . . -18Placing Hot.Mix . . . . . . . . . . . . . . . . . . . . . . . . . . -1Pr im ing and Tack ing. . . . . . . . . . . . . . . . . . . . . . . - 1. . . . . . . . . . . . . . . . . . . . . . . .r u c k L o a d i n g - 4 - 3. . . . . . . . . . . . . . .. . . . . .aver Opera t ion . . . 4-5J o i n t C o n s t ru c t io n . . . . . . . . . . . . . . . . . . . . . . . -1 4P l acement Problems . . . . . . . . . . . . . . . . . . . . . . . -1 9Compaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . -1. . . .ompactive Forces . . . . . . . . . . . . . . . . . . . -1. . . . . . . . . . . . . . . . . . . . . . . .t ag es o f R o l l i n g 5-4S tee l Whee l Ro l le rs. . . . . . . . . . . . . . . . . . . . . . . -5. . . . . . . . . . . . . . . . .ubber -Ti red IPneumat ic R o l l e r s 5 -7V i b r a t o r y R o l l e r s . . . . . . . . . . . . . . . . . . . . . . . . -9Compac t ion P r inc ip les . . . . . . . . . . . . . . . . . . . . -1 4Heat Checking. . . . . . . . . . . . . . . . . . . . . . . . . . -1 6T e s t S t r i p C o n st r u c ti o n. . . . . . . . . . . . . . . . . . . . . -17De ns i t y Measurements . . . . . . . . . . . . . . . . . . . . . -2 2Asphal t Pavement Performance . . . . . . . . . . . . . . . . . . . - 1R a v e l l i n g. . . . . . . . . . . . . . . . . . . . . . . . . . . 6 - 1B le e d in g . . . . . . . . . . . . . . . . . . . . . . . . . . . . -2R u t t i n g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -3Shoving . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 - 6Tender Mixes . . . . . . . . . . . . . . . . . . . . . . . . . . -7

S t r i p p i n g . . . . . . . . . . . . . . . . . . . . . . . . . . . . -8Crack ing . . . . . . . . . . . . . . . . . . . . . . . . . . . . -1 5. . . . . . . . . . . . . . . . . . . . . . . . . . . .eferences R - 1


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BITUMINOUS PAVING INTRODUCTION

This training package has beendeve loped t o add re ss the a rea o faspha l t pavemen t construc tion . I tincludes as an in t roduc t ion , theidentification of som e majorpavemen t d is tresses . I t thenaddresses the major phases ofmat erials , design, prod uct ion,p lacement , and compact ion , thenconcludes with a more de ta i ledlook at aspha l t pavemen tperformance.

Many people, when thinking ofasphalt pavements, tend to onlythink of the major phasesindependently. This presentation isintended to show the properapproach and considerations ineach phase and also theinterrelationships between thephases.

There are several sources ofliterature on these major phases.Two key sources are The AsphaltInstitute publications and theNational Asphalt PavementAssociation publications.

Once one gets involved and studiesthese publications one realizes thatthere are many factors affectingeach major phase.


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Major Factors Affect ingPerf ~ r m a n c ea. Aspha lt Viscosity: Low visc ositymay cont r ibu te to ru t t ing and4hoving, w her eas high viscosityp a y cont r ibu te t o c racking.b. Mix Design : In mix design we@re oncerned with obta ining a@roper gradation and asph altcon ten t to a s sure suf f ic ien tGtrength an d a proper air voidssys tem in the compac tedpavement.c. Compaction: The degre e ofcompaction on the roadwayafter rolling should produce amat wi th 6% t o 8% voids.

Underdensif icat on couldresult in rutting, cracking, andst ipping. Overden sif ica tio ncan lead to rutting, shoving,and bleeding.

These th r e e ma jor f ac tor s mus t bematched t o the roadwayconditions, and tra ff i c andenvironmen tal conditions.Before we discuss the variousphases of mix design andconstruction of asphalt pavements,perhaps we should look at some ofthe distresses. W e will then beab le to r e la te each e lement of th edesign and construction phases tot h e e f f e c t it may have on thepavement 's performance.


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,8. Bleeding: This is the presence offree asphalt on the surface of apavement. I t wil l result inextremely slippery pavementsurfaces when wet.

Rutting: This is the longidutinaldeformation that develops in anasphalt pavement under the actionof channelized loadings (traffic ).

Shovina is the horizontal dis~lace-

increased.Stripping: Stripping is the breakingof the adhesive bond between theaggregate surface and the asphaltcement. Stripping occurs whenwater gets between the asphaltfilm and the aggregate surface.The asphalt is then displaced bythe water or water vapor.Stripping may also result from theemulsification of the asphalt dueto chemical inbalances.Stripping does not start at thepavement surface and proceeddown into the pavement structure,

1-3 bu t normally starts i n an interlayerof the pavement structure.


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Cracking of a n asphal t pavemen t i sconsidered a normal fai luremechanism. Prem ature crackingmay indicate a mix design problemo r a cons truct ion problem.

Unfqrtunately our mix design andcons truct ion proc edures of ten buildweaKnesses into our pavements.Segregation, as shown here, can bea major factor in causingprem ature pav ement d is t resses.

Havlng examined some of themajdr distresses of an asphal tpavqment , th e presentat ion wi llnow go on to address the majorphasps as described previously.The p resen ta t i on wi l l a t t empt t oshow how the major phases arein t e r e l a t ed and fu r ther , t he i rr e l a i o n s h i p t o t h e p av emen tperfb r mance.


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MATERIALS AND M IX DESIGN

This slide shows a typical crude oildisti l lation with the disti l lationtemperatures of the variousproducts. The gas oils are removedin the temperature range of 7 0 0 ' ~to 9 0 0 ~ ~nd asphalts remain. Thetwo processes by which asphalt canbe produced from a crudepetroleum are:a. The vacuum dist il la tionprocess where heat andvacuum are applied, andb. The solvent ex trac tion processwhere the gas oils areremoved with a solventleaving a residual asphalt.A specific grade of asphalt isusually produced from a particularcrude source or blend of crudesources, or by blending di fferen tgrades o f ref ined asphalt.


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Crude petroleum sources vary f ro mheavy crudes to li gh t crudes. Asshown in this slide, the percent o fasphalt that can be produced variesf r o d a high percentage from theheavy crudes to very little or noasphalt fr om the lig ht crude. I nsome refineries asphalt is asubsbantial part of their productiona n d therefore, they pre fer theheavy crudes.The following is a list of most ofthe tests conducted on-asphalt.a. Penetr ation a t 77Fb. Viscosity a t 140:~c. Viscosity a t 275 Fd. Flash Po in te. Solubilityf. Duct i l i tyg. Loss on Heatingh. Penetration of Aged Materiali. Duc til i t y of Aged MaterialAnother valuable test not shown isthe viscosity o f aged material.

6. Asphalt cements are gradedaccording to three differentsystems. The system most widelyused is based on asphalt viscosity.I n this system the flow of asphaltat 1400F is measured and theasphalt is placed in a particulargrade based on tha t flow.


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Several western States gradeasphalts according to theirviscosities after undergoing anaging process.

The t hi rd method of grading -is bythe penetration test.

7. The penetration aparatus, as shownhere, is used to determine thepenetration value of the asphalt.

8. The penetration test is run on asample of asphalt at 77'~. I tmeasures the penetration (in unitsof 0.1 mm) of a standard needlewith a 100 gram weight into theasphalt after 5 seconds.


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The v iscos i ty test a t 1 4 0 ' ~ uses acap i l l a ry t ube v i s come te r , ac a l i b r a t e d g l a s s t u b e t o m e a s u r eth e f low o f a spha l t. The ca l i b r a t edtubes a r e m oun ted i n at e m p e r a t u r e - c o n t r o l l e d w a t e rba th , p r ehea t ed t o 1 4 0 ~ ~ . h ecap i l l a ry t ube has ca l i b r a t edmarks . As t he a spha l t beg ins t of low, i t s p rog re s s f rom one marko n t h e t u b e t o t h e n e x t i s c a r e f u ll yt imed . Th i s measu red t im e is t henu s e d t o d e t e r m i n e t h e a b s o l u t eviscos ity of th e asphal t . Bec ausea sp h al t a t 1 4 0 ' ~ i s to o v i sc ou s t of low r ead i ly t h rough the t ube , apa r t i a l vacuum i s app li ed t o t hesma l l end of t he t ube t o d r aw th easph al t through.

11. The v iscos i ty tesh a t 2 7 5 ' ~ iss i m i l ia r t o t h e 1 4 0 F test, e x c e p tfor th e fo llowing:a. Cle a r o i l is used i n t he ba thins t ead o f wa te r because o fth e high temperatur:.b. T h e a s p h a lt a t 2 75 F is f luidenough to f l ow th rough thev i s c o m e t e r t u b e w i t h o u t t h eas s i s t anc e of a vacuum.c. Because g rav i t y and no t a

vacuum is used to induce f low,the v iscos i ty i s a k inema t i cviscosi ty.


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A th i rd g rad ing sys t em used bys e v e r a l of t h e w e s t e r n s t a t e sd e s c r i b e s t h e a s p h a l t s ac c o r d in g t othe i r v i scos i ty a f t e r ag ing . As tanda rd ag ing exposure test in al abora to ry oven i s u sed to s im ula t eag ing in t he a spha l t p l an t du r ingm ix ing. The a spha l t r e s idue th a tr e m a i n s a f t e r a g i n g i s g r a d e daccord ing to i t s v i scosi ty in po i sesat 140 '~ . The idea o f t h i s g rad ings y s t e m i s t o i d e n t i f y w h a t t h ev i scos i ty cha rac t e r i s t i c s w i l l bea f t e r t h e a s p h a l t i s p l a c ed in t h epavement .

T h e a s p h a l t g r a d e s d e s i gn a t e d a sI fAR" which s tands for "AgedResidue ," are AR-10 t h roughAR-160. An AR-10 asp hal t wi th aviscos i ty of 1 ,000 poises i s re fe r re dto a s so f t a spha l t w h i l e an A R-160a s p h a l t w i t h a viscosisty of 16,000p o is e s i s r e f e r r e d t o a s a h a r dasphal t .

15. T e m p e r a t u r e S u s c e p ti b il i tyE a c h p a r t i c u l a r a s p h a l t h a s i t s o w ntemperature-viscos i ty re la t ionship .Th i s r e l a t i onsh ip i s som et im esd e sc r ib e d a s t e m p e r a t u r esuscept ib i l i ty . Cha nges in cru desources o r b l ends o f d i f f e r en tc rude sources w i l l have the i r ow npar t i cu l a r t em pera tu re -v i scos i tyre l a t ionsh ip cu rve . These cu rv esa r e ve ry use fu l in de t e rm in ing th eo p t i m u m m i x i n g t e m p e r a t u r e a n do p t i m u m c o m p a c t i o n t e m p e r a t u r eof a par t icular asphal t .


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16. Opt imum M ix ing Te mpe r a tu r e a ndCornpiaction TempertatureT h e ~ p t i m u m mix ing t e mpe r a tu r eis thait corresponding to a viscosityof 170+ 20 cent is tokes . Theo p t i m u m c o m p a c ti o n t e m p e r a t u r ei s t ha t c o r re s pond ing to a viscosityof 280+ 30 centis tokes. Thete mpbr a tc re - v i s cos i ty c u r ve c a n beused , t o d e t e r m i n e - t h e o p t im u mmixir ig tempera ture range and theop t i r hum c ompa c t ion t e mpe r a tu r erange.

Tem pera tu re suscept ib i li ty is oneof the aspha lt ' s m os t im por ta n tchartbc te r is t ics . How ever i t i simpor ta n t to unde r s tand a ndr e c o g n i z e t h a t t e m p e r a t u r esuscgptibili ty in asphalts var iesbe tw~e e n d i f f e r e n t pe t r o leumsourges , even i f the aspha l ts a reidenlically graded. In thi s s lideAspha l t A and Aspha l t 6 a r epe ne t r at ion g r a de d a s pha l t s w i thi d en t ic al p e n et r at i on s a t 7 7 O ~ , u ta r e f r om d i f f e r e n t c r ude s ou r c e s .No te the d i f f e r e nc e in the i rviscosities a t the h ighe rtempera tures . Aspha l t A will bemore f lu id than Aspha l t 6. Thism e a n s t h a t t h e t e m p e r a t u r erequlired to make Asphalt A f luidenough to proper ly coa t a l l o f theaggn'egate par tic les in the mix islowe r tha n the t e mpe r a tu r e ne e de dt o g e t t h e s a m e r e s u l ts f r o mAsphal t 6. T h e s a m e i s t r u e f o rcomlpaction tem peratu re. I t willp robab ly be ne c e s s a r y to c o mpa c ta rqix with Asphalt A a t a lowe rt e m p e r a t u r e t h a n t h e s a m e m ixwith Aspha l t 6.


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18. Same Viscosi tyThe same is true for viscositygraded asphalts. Here Asphalt Aand Asphalt B have the sameviscosity a t 1 40~, however, a t a l lother temperatures theirviscosities differ. As wi th thepenetration graded asphalts,Asphalt A would probably require alower mixing and compactiontemperature than Asphalt B.Knowing the temperaturesusceptibility of an asphalt beingused in a paving mixture isimportant because i t indicates theproper temperature at which tomix the asphalt with aggregate andthe proper temperature a t which tocompact the mixture on theroadway.During the past 10 years a lot ofconstruction and performanceproblems have been blamed on theasphalt. This may be true bu tbefore we place al l of the blame onthe asphalts, we need to look a tthe changes that have taken placeduring the past 10 years. Up un ti labout 10 years ago al l of the greatmajority of the refineries had fixedcrude petroleum sources.Therefore, a particular grade ofasphalt from a given refineryprobably had very nearly the samepenetration, viscosity, andtemperature susceptibility. We i nthe highway industry learned to usethese asphalts from fixed crudepetro leum sources. Dur ing thepast 10 years refineries no longerhave fixed crude sources. Theyhave new sources and are blendingsources. The temperaturesusceptibility of asphalts from newand blended crude sources may be


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quite different from the asphaltsmade from the fixed sources webecarhe accustomed to 10 yearsago.A l l agphalts tend t o age or hardenin Qhe paving mix ture duringconsQruction and i n the pavementitse lf. The hardening is causedprimbrily by oxidation, when theasphdlt combines wi th oxygen, aprocgss th at occurs most readily a thigh temperatures and when theasphalt is in a thin film. The mostsevere oxidation and hardening ofthe asphalt occurs i n a batch plantprocess i n the pugmil l mixing whenthe asphalt is both at a hightemperature and in a thin f i lm as i tcoats the aggregate particles. Theasphalt can also undergo severeoxidation and hardening in a drummixer under certain conditionswhich w i l l be discussed la te r i n thesection on asphalt plants. Asindicated here asphalts fromdifferent sources age differentlywhen subjected to a standardlaboratory aging test.Arkansas AsphaltsThere is no direct correlationbetween a viscosity graded asphaltand a penetration graded asphalt.Those states switching from thepenetration grading system to the.viscosity grading system may wantto keep a maximum penetration ora penetration range to insure thatthey continue to get an asphaltwith the hardness they areaccustomed to working with. Forexample, the Arkansas StateHighway and TransportationDepartment in switching to theviscosity grading system has triedto maintain an asphalt with a 60-70penetration range. I n order to


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maintain this approximatepenetration value, their viscositygraded asphalts vary from anAC-20 to an AC-40 grade asphaltdepending on the particularrefin ery used. No te the differencein the temperature susceptibilityof the asphalts from these variousrefineries as indicated by th edifferences in the optimum mixingand optimum compactiontemp erature ranges.


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Aggregate cleanliness is usuallyconsidered by limiting the typesand amount of soft or deleteriousparticles. Aggregates must behard enough to resist abrasion anddegradation during constructionand under tra ff ic . The Los AngelesAbrasion Test provides a measureof an aggregate's resistance towear and abrasion.

The shape of aggregate particles

and mi x particles.


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For a description of processedaggregate the terms as shown onth is s lide are used.

Some of the effects of aggregategradation on the mixture have beendetermined by experience asfollows:a. Too much coarse aggregateproduces a harsh mix whichtends to segregate easily.

b. Too much sand produces a sof tmix which becomes too tenderto properly compact.

c. Too much f il ler produces aharsh stiff mix.

d. Too l i t t l e fi ll er produces a mi xwi th low cohesion.

e. Crushed aggregate produces amix with high interparticlefriction.

P. Smooth aggregate produces amix with lower interparticlefriction.

I n this gradation chart developed inthe early 1960's by the Bureau ofPub lic Roads, the sieve sizes havebeen raised to the .45 power. Onthese charts the maximum densityline is determined by drawing astraightline from the bottom leftcorner of the chart through the95% passing point or the f ir st sievecontaining material. The maximumdensity line is used to analyze thegradation of a particular mix.Usually a gradation following themaximum density line or very nearthe maximum density line produceshigh stabili ty but low void mixes.


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31. The re al purpo se in establ ishin gand con ro l li ng agg rega t eg rada t i on i s t o p rov i de andmainta in a proper void content int h e aggrega t e . Th i s i s ca l l ed vo i dsi n t h e m i n e r a l a g g r e g a t e or VMAand i s i l lus t ra ted in th i s s lide.

The VMA mus t be l a rge enough toa l l ow each pa r t i c l e t o be coa t edw i t h a spha l t and s t i l l ma i n t a i n aprope r a i r vo id con t en t i n t h e f i na lcom pac ted mix. Minimum VMAr equ i r emen t s va ry d i r ec t l y w i t ht op agg rega t e s i ze.

Airmix,unde

T he

vo i ds i n t he f i na l compac t eda f t e r a b o u t t h r e e s u m m e r sr traffic, should be 3-5Yo.

max i mum d ens i t y li ne can a l sobe used as a gu i de t o ad j us t t heminera l voids and the a i r voids oft he mix . The more t h e s l ope of t hemi x g r ada t i on va r i e s f r om t hes l ope o f t he max i mum dens i ty l i net h e m o r e m i n e r a l a g g r e g a t e v o i d sa r e provided in th e mix. In so memater ia l s , especia l ly l imes tonemixes, ga p grading may beneces sa ry t o p rov i de adequa t evoids. In veri f icat i on andinves t iga t ion of th e gradat ioncha r t du r i ng i t s deve l opmen t , aser ies of gradat ions and mixesw ere made up and t e s t ed . N o t e int h i s s l i de t ha t cu rve 3 of themax i mum dens it y l i ne p rov i des t h elowes t voids in the minera l

2-12 aggregate whi le curves 1 , 5, and 6provide the highest voids.


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35. N o t e o n t h is c h a r t t h a t c u r v e 1 0 i sc lo s e s t to the ma x imum de ns i tyLine and provid es t he lo we st voidsand h ighes t s tab i l i ty whi le curve 7provides the h ighes t vo ids butlowes t s tab i l i ty .

36. In these f iner mixes , no te tha tc u r ve s 15, 16, a nd 1 7 ve ry ne a r lyfo l low the maximum dens i ty l ine toprovide the lowes t vo ids andhighes t s tab i l i ty whi le curves 13 ,and 1 4 provide h igh voids but lows tab i l i ty .

In these coarser mixes ,s a m e i s t r u e w i t h r e s p evoids and s tabili ty.n o t e, c t t o t h et h e


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Having d iscussed the mainingredients of a mix, we will nowr e d i r e c t the d i s c us s ion to themixture i tse l f and i tsc ha r a c te ri s t i c s. The s e f ou rc ha r a c te r i s t i c s wi ll be addressed asthe y a r e mos t c r i t i c a l to the mix ' sper formance in the pavement .Dens i ty or un i t weight isde te r mine d to e sta b li s h t a r g e tcompact ion va lues in the f ie ld andto ensure a proper air void systemin th e mix,An air void 's determination isc onduc te d to a s s u r e s pa c e f o rthermal expans ion and addi t iona lin-place densif ication undert r a f f i c ,T h e VMA i s de te r mine d aspreviously described.The a s pha l t c on te n t i s e s ta b l i s he df o r the mix to provide suf f ic ien tcoa t ing of aggre ga te par t ic les .A n o t h e r c r i t i c a l mix ch a rac t e r -i s t i c , a l though not shown ont h i s s l i d e , i s moisture sus-c e p t i b i l i t y . The e f f e c t ofmois ture on some asphal t /


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a g g r e g a t e c o m b i n a t i o n s i se x tr em e s t r i p p i n g . T h i s effects h o u l d be e v a l u a t e d as parto f t h e mix d e s i q n p r o c e d u r e s

procedures, a density and voidsanalysis of samples containingvarious proportions of asphalt andaggregate, and a measurement ofthe stability and flow underspeci fic loading conditions.Test specimen compactionrequirements and design criteriafor each mix is determined basedon the traffic volume on thepavement.

Specimens are then made up andcompacted as shown,


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The results of the testing are thenplotted on charts as shown and anoptimum asphalt content selected.


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The s pec imens a re t hen t es t ed i n aHv eem s t ab i i ome t e r as s hown t ode t e rm ine t he s pec imen's s t a b i l i t yvalue. A s we l l t es t i s a l s oc o n du c te d t o d e t e r m i n e t h eamoun t o f wa t e r a s pec imen c anabs orb and t he amoun t o f s we l l i ngth is abs orpt ion causes.

53. The op t im um asphal t con tent i st hen s e lec t ed us ing a py ram idchar t , the tes t resu lt s , and a v i s ua lex am ina t i on o f t he t es t s pecimens .


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Regardless of the mix des ignprocedure used, i t i s e s sen t i a l t h a tt he dens i t y and voids analysis beunderstood, Den si t ies or uni tweights a re used dur ingcompac t ion t o con t ro l t he ro l l i ngopera t ions and m ore impor t an t ly t oassure a proper a i r voids sys tem inth e mat . The dens i ty of th elaboratory specimens i s determinedand a i r voids ca lcula ted. Thisrequi res a determinat ion of themaximum theoret ica l dens i ty ofth e mix.

The method used in determiningthe maximum theore t i ca l dens i t yof t h e mix tu re can have as igni f icant ef fec t on the dens i ty ofthe m a t in t he f ie ld , I t i s e s sen t i a lthat we obta in a t r u e 6-8% voids int h e m a t a f t e r c o m p a c t i o n a n dbefore i t i s open t o t r a ff i c . S inceth e roadway dens i ty i s expressed asa percen t age of t he maximumtheoret ica l dens i ty 9r spec i f i cgravi ty of the mixture , thede t e rmina t i on o f t he t heore t i ca lmaximum densi ty or speci f icgrav i t y i s t he key t o de t e rmin ingthe t rue a i r vo id con t en t o f t hecompa c ed mat . Most highwayagenc i es de t e rmine t he maximumtheoret ica l speci f ic gravi ty of themixture by determining thespeci f ic gravi ty of the coarseaggrega t e , f i ne aggrega t e , mine ra lf i l l e r and the asphal t , then usethese speci f ic gravi t ies in amathemat ica l formula , based onpercentage of each ingredient int h e m i x tu r e t o c o m p u t e t h emaximum theore i ca l spec i f icgravi ty.


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T h e R i c e M e t h o d (AASHTO T209)u t i l i ze s vacuum sa tu ra t ion tod e t e r m i n e t h e m a x i m u mt h e o r e t i c a l s p e c i f i c g r a v i ty of t h ecom bined m ix tu re .

a g g r e g a t e r e p r e s e n t s t h e a i r v o i d s .The w hi t e band su r round ing thea g g r e g a t e r e p r e s e n t s t h e a s p h a l t .The hashed r ing a round thea g g r e g a t e r e p r e s e n t s t h e a b s o r b e dasphal t. T h e s p e c k l e d r i n grep rese n t s t he vo ids f i l led by w a te rbut not f i l led wi th asphal t . Andf i n a l l y t h e d a r k c e n t e r c o r er e p r e s e n t s t h e s o l i d a g g r e g a t evolume. Th e voids in th e mine ra la g g r e t a t e , VMA, r e p r e s e n t s t h ev o id s b et w e e n t h e a g g r e g a t epa r t i c l e s and depend ing on t h em e t h o d u s ed t o d e t e r m i n e t h espec i f i c g rav i ty , t he vo ids w i th int h e a g g r e g a t e p a r t ic l e s . T h e a i rv oi d c o n t e n t i s t h a t a r e a o f t h eVMA no t f i l led wi th asphal t . I f th ea p p a r e n t s p e c i f i c g r a v i t y i s u s e d ,a l l vo ids f i l l ed w i th w a te r a r eaccou n ted fo r. H ow ever , t he vo idsf i l l ed w i th w a te r w i l l no t a l l bef i l l ed w i th absorbed a spha l t andthe m ix tu re in t he f i e ld cou ld be al i t t l e h igh in a spha l t con ten t andlow in a i r voids . If th e ef fe c t iv espec i f i c g rav i ty i s u sed , t h eaggrega te vo ids absorb ing a spha l t /a r e a c c o u n t e d f o r. T h i s m e t h o dshou ld accoun t fo r t he t rue vo idcon ten t o f t h e m ix tu re in t h e f ie ld .M any peop le be l i eve the R ic e


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Method most near ly makes thi sdeterminat ion. If the bulk speci f icgrav i t y i s used , t hen ne i t he r t hevoids absorbing asphal t nor thesmal ler voids absorbing water areacc oun ted for. If asp hal t isabsorbed i n to t he aggrega t e , t henthe mixture in the f ie ld would tendto be low in asphal t content andhigh in air voids. The refo re, i t isimpor t an t that one know whichmethod i s being used fo rde t e rmin ing t he spec i f i c g rav i t yand wha t t ha t pa r t i cu l a r spec i f i cgrav i t y means i n t e rms o fmaximum theore i ca l spec i f i cgravi ty or dens i ty , VMA, and a i rvoids. I t i s be l ieved, tha t th e RiceMethod (AASHTO T209) i s t h em o s t a c c u r a t e m e t h o d o fde t e rmin ing t he maximumtheore t i ca l spec i f i c g rav i t y o f abi tuminous mixture and in turn thet ru e a i r voids in th e m ixture .

weight o f t h e t o t a l m i x t u re i s 5.93.The asphal t absorbed by theaggrega t e pa r t i c l e s i s 0.8, and t hespeci f ic gravi ty of the asphal tcem en t i s 1 .011. No te th e chang ein the void propert ies of thec o m p a c t e d m i x t u r e f o r t h e 6di f f e r en t methods . T he pe rcen t a i rvo ids va ry f rom a t heore t i ca l-0.73% t o 4.85%.


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58. Effe cts of Mix Te mp era ture ofCompact ionI t i s im por t an t t ha t t he m ix ing andcom pac t ion be done at t h e p r o p e rt em pera tu res . The t em pera tu re o fthe mix cont ro ls the v iscosi ty ofth e aspha l t. The aspha l t m us t bein a liquid state during mixing andcompact ion. In th is g raph th e voidcon ten t in m ixes com pac ted by th e50 blow Marshall method atd i f f e r e n t t e m p e r a t u r e s i sc o m p a r e d t o t h e v o i d c o n t e n t oft he m ix com pac ted at 2750F. Withonly a var i a ti on in t h e com pac t iont e m p e r a t u r e n o t e the d i f f e rence invoids of the compacted mixes.Voids of 5% are a t t a i n e d at 2750F,bu t t he vo id s have increased to15% at 175oF. When th e mix isc o m p a c t e d at 2000F the vo idcon ten t i s 12%. In th is par t icu la rmix , i f th e voids ar e to be under10% i t i s necessa ry to com pac t t h i smix before it r eaches atem pe rat ure of 200-2250F.In considering the densi ty/voidsrelat ionship, i t is im por t an t t or e m e m b e r t h e g o a l for com pac t ionof the pavement dur ing the ro l l ingoperat ions . The pr imary goals a r el isted on thi s slide.

Sta bility vs. D ensityIn addi t ion to making a p a v e m e n timpermeable , another pr imaryreason fo r com pac t ion i s t o g iveth e mix s tab i l i ty . As th e densi tyof a m ix decreases f rom l abcom pac ted dens i ty , t he s t ab i l i t ydecreases a lso. Th is e f f ec t can beseen in th is s l ide for a given mix.A compact ion level o f 97% of labdensi ty resu l t s in a Marshalls tab i l i ty of ab out 40% of th es tab i l i ty of the mix whencom pacte d t o 100% lab density .


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Void RangeWe a t t e m p t t o d e si gn a highp e r f o r m a n c e m i x w i t h 3 t o 5p e r c e n t v o id s a s sh o w n in t h e d e s i gnv oid r a n g e a r e a . We a t t e m p t t oc o m p a c t t h e m i x t ur e t o a t r u e 6 t o8 percen t vo ids on the roadw ay .This wi l l a l low for addi t ionalc o m p a c t i o n u n d e r t r a f f i c w h ic h w i llr e d u c e t h e v oid c o n t e n t o f t h e m a tb u t k e e p i t o u t o f t h e 3 p e r c e n t o rl e s s v o id s a s s ho w n in t h e a r e a o nth e r igh t s ide of th e s l ide . Whent h e r o a d w a y p a v e m e n t g e t s t o 3percen t o r l e s s vo ids , ru t t i ngand /o r b l eed ing can be expec ted .L i ke w is e w e w a n t t o s t a y o u t o ft h e z o n e w h i ch i s a b o v e 10 p e r c e n ta i r voids . This ar ea would bec o n s i d e re d u n d e r- c o m p a c t e d a n dwil l resul t in addi t ionalconso l idat ion unde r t r a f f i c . Th iscould a lso resul t in h igh voidc o n t e n t wh ic h a c c e l e r a t e sox ida t ion o f t he a sph a l t , a l l ow s th ee n t r y o f w a t e r i n t o t h e p a v e m e n ts t r u c t u r e , a n d m a y c o n t r i b u t e t os t r ipping.A n o t h e r f a c t o r t o co n s i d e r in t h ev o i d s c o n t e n t is i t s r e l a t i o n s h i p t oa s p h a l t c o n t e n t . M os tspec i f i ca t ions inc lude a t o l e r a n c er a n g e f o r a s p h a l t c o n t e n t . W hent h e o p t i m u m a s p h a l t c o n t e n t i sse l ec t ed dur ing m ix des ign , t hea l low ab le to l e rance shou ld bea d d e d t o i t a n d t h e c o r re s p on d i nga i r vo ids de t e rm ined . Th is a i r vo idc o n t e n t sh ou ld b e a t l e a s t 3%. Asshow n in th e s l ide , t h e s lope o f t h e


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I n s umma r y , th i s s e c t ion ha si l lu s t r a te d the ne c e s s i ty o fc ons ide r ing the r oa dwa y type a ndtra f f ic in the mix des ign process .I t h a s i l l u s tr a t e d t h e i m p o r t a n c e ofs e le c t ing the p r ope r ma te r ia l s a ndp r opo r t ion ing the m c o r r e c t ly toobta in a su i ta b le mix . Thepresenta t ion wil l cont inue to showth e impor t ance of unders tandingth e c on c e p t s a nd p r inc ip le s o f e a c hphase in order to assure as a t i s f a c to r y pa ve me n t .


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HOT MIX PLANTS

Plan t Operations: I n this sectionwe wi l l discuss the operations ofthe typical batch plant and thedrum mixer plant. This wi l linclude a comparison of the batchplant and the drum mixer plant andsome of the key factors inproducing a high quality mix.

2. Schematics of Batch Plant andDrum Mi x Plan t operations: Thetop sketch is a schematic of atyp ica l batch plant. The aggregatetravels through the cold feed bins,through the drier, up the hotelevator, over screens and in to theho t bins. I t is then weighed in theweigh hopper, dropped into thepugmill for mixing, and mixture isdeposited into a truck or a storagesilo. The bo tt om sketch representsa typ ica l drum mix plant. Themajor components are theaggregate stockpiles, the co ld feedbins and conveyor, the drying andmixing drum, and a surge orstorage silo.

3. Segregated Stockpiles:Stockpiling is critical to both typeplants. Proper stockpili ngtechniques wi l l ensure tha t unif ormmaterial is being fed into the hotmix plant. The construction ofstockpiles in a cone shape or withsteep sides can result in severesegregation. As shown in thisslide, the large particles haverolled to the outside of thestockpile which will causesegregated material to be fed into


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the plant. Although the batchplant with its screening operationis more able to "even outw thissegregation, i t will result in annonuniform operation and producea variable material.Correct Stockpile Construction:The stockpiles should beconstructed in horizontal layers.Trucks or front end loaders can beused to make small piles over theentire area of the proposedstockpile. The tops should beleveled of f and a ramp provided forthe hauling equipment to start asecond layer, and so forth on up.

Stockpile Layers:This slide shows a stockpile beingconstructed in layers with aconveyor system or with trucks byusing a dozer to spread themateria l i n layers. The operationof the dozer should be monitoredclosely to ensure i t is not causingdegredation or otherwise damagingthe gradation o f the m ater ial beingstockpiled. Also, the dozer shouldnot push mater ia l down the slope.The end-result should be stockpilesthat are not segregated, that aresufficiently separated, and that areplaced on a clean and stablesurface. Normal ly a front e6dloader is used to remove materialfrom the stockpile and take i t tothe cold feed system of the plant.In removing the material from thestockpile, the loader operatorshould pass the loader bucketvertically up the face of thestockpile. I n this method somematerial from each layer in thestockpile is blended i n the removalprocess.


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7. Our next subject of discussion inthis section wi ll be the batch plant.It's operation will be covered step-by-step with emphases on keyitems i n the control process.

8. Picture of Cold Feed Bins: A l lplants require co ld feed bins wherethe in it ia l proportioning of theaggregate takes place. The binsshould have dividers to precludeany spillover into the adjacentbins.

Each plant is equipped with anaggregate cold feed system. Thequantity of material leaving eachbin is regulated by the size of thegate opening, the speed of the belt,or a combination of the two.


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Batch Plant Cutaway: Theaggregate then continues to thehot elevator, through the screensinto the hot bins. I t is releasedinto the weigh hopper, droppedinto the pugmill for mixing withasphalt, and the mixture droppedinto a waiting truck or conveyed toa storage silo.

design aggregate.

3-4


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screen. The screens should not beblinded by aggregate particlesstuck in the openings.

18. The aggregate is then deposited inthe weigh hopper one bin a t a time.The coarse aggregate is placed onthe bottom so that when thematerials are dropped into thepugmill the fine material wil l notbe trapped out of the paddle'sreach.


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The aggregates are then droppedinto the pugmill. Af te r a short drymixing, the asphalt is added andmixed. The action of the pugmillpaddles and the aggregate shearsthe asphalt into a very thin filmand forcibly coats the aggregate.

broken should be replaced.A t maximum operating efficiencythe paddle tips should be barelyvisible at the surface of thematerial during mixing. I f thelevel of materia l is too high, theuppermost mate rial tends to flo atabove the paddles. Also, i f thelevel is too low the tips of thepaddles rake through the materialwithout actually mixing it. Eitherof these problems can be avoidedby following the manufacturer'spugmill batch ratingrecommendations.


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The control varthe equipmentthe mechanicalplant. The ag!shown here withproper aggregatdesign mix ture.

I or trai lerneeded tofunctionsgregate scapointers se;e weights

hocorofle st atfo r

Jsest ro lthearethethe


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27. Hot Bin Contr ol Panel: Thecontrol panel of most batch plantscontain indicators of the amount o fmaterial stored in each hot bin. Aconstant indication of high or lowlevel is generally caused byimproper cold feed proportioning.

28. Temperature Monitors: Themonitor on the right shows thetemperature of the aggregate. Asa ru le of thumb, the stacktemperature and the aggregatetemperature should not vary bymore than 2 5 ' ~ to 40F. I f theplant is equipped with a baghouse,i t wi l l also have controls. Themonitor on the left side of theslide is the baghouse control. Theblack needle indicates thetemperature i n the baghouse. Thered needle is set around 400 '~ tocontrol the maximum temperatureof the baghouse.

29. Temperature Recorder: The batchplant control room should have achart recording the temperature ofthe aggregate leaving the dryerand/or the temperature of theasphalt. I f the specificationsrequire that these charts be keptas a par t of the records, the t im eof the day should be proper ly set.


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Mixing Times: The mixing cycle iscontrolled by a timer. I t wi l lcontrol the wet mixing cycle anddry mixing cycle. The dry mixingcycle, if equired, should be only afew seconds. I t can be harmful increating too much fines. I t can bebenefical i f you are working with acoated aggregate by helping toremove the coating.The wet mix time must be longenough to produce a uniformilycoated homogeneous mixture. Butbecause most of the aging of theasphalt occurs in the pugmillmixing cycle, the mixing cycleshould be no longer than necessary.The Ross Count or AASHTO T 195,"Determining Degree of ParticleCoating of Bituminous -AggregateMixture," can be used to set themixing time. This system basesthe degree of mixing on thepercentage of coarse part icles tha tare completely coated w it h asphaltand correla tes i t wit h mixing time.The drum mixer is a relativelysimple process for producing hotmix. The key difference betweenthe drum mix plant and a batchplant is that the drum mix plantnot only dries and heats theaggregate, but also coats theaggregate with asphalt within thedrum.As shown in this schematic, themajor components of a drum mixerplan t are: the aggregate stockpilesand co ld feed bins, the conveyorand aggregate weighing system,the drum mixer, the dust collectionsystem, the ho t mix conveyor, thesurge bi n or silo, the asphaltstorage tank, and no t shown thecon trol van.


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The aggregate stockpiles areprobably the most importantcomponent of a successful drummixer process. The stockpi lingconcerns discussed earlier aremuch more important in the drummix plant operation than in thebatch plant operation, as wi l l beshown.

Schematic of Screens and H o t Bins:Unli ke the batch plant, the drummixer plants does not have thescreens, hot bins, and a weighhopper. Therefore, the aggregatemust be sized and proportionedprior to it s entry into the drum.Generally, to provide the sameleve l of m ix control, the number ofstockpiles in a drum mix operationand the number of ho t bins in abatch mi x operation must be equal.

This may require a change in thetypical crushing and screeningoperations to provide the necessarynumber of stockpiles as requiredand the required particle sizes.The stockpiles must be properlygraded and split into at least asmany different sized fractions asrequired. Normally 4 stockpiles fo ra binder course or minus 314" orlarger surface course; 3 stockpilesfo r a minus surface course; and2 stockpiles fo r a fr ic ti on course.Once sized, the aggregate must beproperly stockpiled and handled t oavoid segregation. Segregatedstockpiles cannot be usedsuccessfully.


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Also, i t is important that themin imum size stockpiles, such as a5 day production quantity bemaintained. This is needed fo radequate testing and aging of thefreshly crushed aggregate.

Cold Feed Bin Calibration: Eachbin must be calibrated separately.The calibration consists ofcollecting and weighing a timedaggregate sample for severaldifferent speeds of the bin belt.The calibration chart is a plot oftons/hours vs. amperage, orfrequency, which ever contro ls thebe lt speed. The bins must becalibrated each time a differentmaterial or aggregate size i s used.Once the bins have beencalibrated, the required proportionf ro m each bin is determined. Thecalibration charts are used to setthe speed of each bin belt toproduce the correct proportionsfr om each bin. Af ter the speed fo reach be lt i s set, a sample should betaken to check the gradation o f thecombined aggregate mixture.


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T h e i n d i v i d u a l b i n c o n t r o l s a r ei n te r l o ck e d t o t h e m a s t e r c o l d f e e dc o n t ro l . T h u s t h e t o t a l t o n a g e ofa g g r e g a t e g o i n g i n t o t h e d r u mm i x e r c a n b e i n c r e a s e d o rd e c r e a s e d w i t h o u t c h a n g i n g t h ep r o p o r t i o n s c o m i n g f r o m e a c h b in .Each co ld feed b in should bee q u i p p ed w i t h a d e v i c e t o m o n i t o rt h e f l ow o f m a t e r i a l f r o m t h e b ina n d s h u t d o w n t h e o p e r a t i o n i f t h ec o l d f e e d f l o w i s i n t e r r u p t e d .Ag gr eg a te Weigh Br idge : Th ed r u m m i x e r p l a n t r e q u i r e s ac o n t i n u o u s w e i g h i n g s y s t e m o n t h ec o l d f e e d c o n v e y o r b e l t. O n e o ft h e c o n v e y o r i d l e r s i s m o u n t e d o nt h e p i v o t e d s c a l e c a r r i a g e a n d i sd e s i g n a t e d t h e w e i gh i d l e r o r w e ig hb r id g e . A s t h e m a t e r i a l p a s s e so v e r t h i s i d l e r , t h e w e i g h t i st r a n s l a t e d i n t o TONS PER HOURand v i sual ly d i sp layed a t thec o n t r o l c e n t e r .T h i s r e a d i n g i s n e c e s s a r y f o r t h ea g g r e g a t e l a s p h a l t b l en d in g s y s t e m .D u r i n g t h e c a l i b r a t i o n o f t h e c o l df e e d b in s, t h e a c c u r a c y of t h eweigh br idge should be checked .Th is should be done by hanging as e r i e s of w e i g h t s o n t h e i d l e r f o rs t a t i c c a l i b r a t io n a n d t h e n a l s oa c t u a l l y w e i g h i n g t h e a g g r e g a t et h a t c o m e s o f f t h e b e l t in a g i v e nper iod of t ime .I t s h ou ld b e r e m e m b e r e d t h a t t h ew e i g h b r i d g e w e i g h s a l l t h em a t e r i a l p a s s in g o v e r i t. T h i su s u a l l y i n c l u d e s a g g r e g a t e a n dm o i s t u re . S i n c e t h e t o t a l w e i g h td e t e r m i n e s t h e q u a n t it y o f a s p h a l ta d d ed , t h e m o i s t u r e c o n t e n t of t h ea g g r e g a t e m u s t b e d e t e r m i n e d a n da cor r ec t io n made . A var ia t ion int h e m o i s t u r e c o n t e n t d u r i n g t h eo p e r a t i o n c a n h a v e a s i g n i f i c a n te f f e c t o n t h e a s p h a l t c o n t e n t o ft h e m ix .

3-12


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Dru m Mixer Cutaway:In the drum mixer the aggregate ischarged into the drum at theburner end so the hot gases andaggregate move through the drumin the same direction. The asphaltwhich is introduced about themidpoint of the drum has to beprotected f rom the harmful effectsof oxidation and direct contactwi th the burner flame. This isaccomplished by using theevaporating moisture and the veilof aggregate. Some drum mixerplants utilize a heat shield toprotect the asphalt.FLIGHT DESIGN: Dif fer ent fl ig htdesigns are used in the drum mixerto accommodate an environmentwhere both aggregate and asphaltare present. To achieve a un if ormdrum loading, spiral flights areused at the charging end to conveythe aggregate away from the inlet.Tapered l if t i ng flights a t thebeginning of the drum elevate anddischarge the aggregate in acontrolled manner to form auniform veil without smotheringthe burner flame. Fli ght sthroughout the remainder of thedrum provide a uniform veil acrossthe drum to protect the asphaltfro m the burner flame.A spray pipe is used to introducethe asphalt in to the drum. Thepoint at which the asphalt isinjected varies, but is usually inthe middle 1 /3 of the drum length.Aggregate Veil: I n most drummixers the flights pick up theaggregate and showers i t acrossthe fu ll width of the drum. Thisveil protects the asphalt from theburner flame. Therefore, i t is verycritical that the aggregate showerproduce a uni for m veil. Overshowering or under showering willresult in an overheated or burnedasphalt.The addition, delet ion orreplacement of flights may berequired to correct this.


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44. Another vi ta l component of thedrum mixer plant is an asphaltsystem capable of providingaccurate continuous proportioningof asphalt. Drum mi x plants areequipped with an asphalt meteringsystem such as a positivedisplacement pump or flow meterand adjustment. This meteringsystem should be calibrated using ameasured container and weightscales to determine the actual f lowof asphalt at various settings. Thesystem should be recalibrated i fthere is a change in the asphaltsupply, temperature, or specificgravity of the asphalt.

45. Mixing Process: Dur ing the smoothcontinous flow of material, certainevents occur in phases wit hin fa ir lywell delineated zones of activityinside the drum. One of the majordifferences between the batchplant process and the drum mixerprocess is the manner in which theaggregate is coated w it h asphalt.Phase I s the early heating phase.The aggregate has entered thedrum mixer, and the surface andfree moisture begins to leave theaggregate as the temperaturerises.


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Ph as e 11: In th is ph as e moh e a t r i se o c c u r s a n d t h e at e m p e r a t u r e r e a c h e sm at ely 1 7 0 ' ~ t o 1 8 0 ~ ~ .t h i s p h a s e t h e m a j o r i t ym o i s t u r e i s d ri v e n o f f a n dof i nc rea se in t he at e m p e r a t u r e l e v e ls o f f.

1st o f t h e, g g r e g a t eapproxi -O u r n gof t het h e rate~ g g r e g a t e

P h a s e 111: As t h e a g g r e g a t et e m e r a t u r e r e a c h es b e t w e e n8180 F and 2 0 0 ~ ~h e a s p h a l t i si n t r o d u c e d t h r o u g h t h e s p r a y p i p aT h e f r e e a n d s u r f a c e m o i s t u r ed r i v en o f f n o w c a u s e s t h e a s p h a l tt o foa m s l igh t ly . Th i s foam inga c t i o n o c c u r s r a pi d ly a n d c a u s e st h e v o l um e o f t h e a s p h a l t t o b egrea t ly i nc reased . Th i s ac t i one n t r a p s d u st a s w e l l a s t h e l a r g e rp a r t i c l e s , a n d r ap i d ly c o a t s t h ea g g r e g a t e .

Phase IV: In th i s phase, t he f ina lt e m p e r a t u r e of t h e m i x t u r e i sachieved.T h e f i n a l p o rt i o n o f t h i s p h a s e a n dt h e s i lo s t o r a g e p h a s e i s of s o m econce rn , e spec i a l ly w hen them i x t u r e i s h e a t e d t o a hight e m p e r a t u r e . T h e c o n c e r n i s w i t hth e poss ibi li ty of i n t e r na l m oi s tu r eb e in g d r iv e n f r o m t h e a g g r e g a t ea f t e r i t i s c o a t e d . If in t e r n a lm oi s tu re i s d r iven ou t o f t hea g g r e g a t e d u r i n g t h i s t i m e , i t m a yi m p a i r t h e a s p h a l t l a g g r e g a t ecoa t ing .


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49. Storage Silo: Since the drum ....xerplant is designed to operate in asmooth continuous operation, i t isnecessary to transfer the mix to asurge or storage silo. This silo maybe a source of segregation.

50. The method of charging the silohas been shown to be critical inmin imizing segregation. One ofthe methods proven to cause lesssegregation is the rotating turned-down shute.

51. Another method which has beenproven acceptable is the batcher.When a top batcher i s used, i t isimportant tha t the ma teria l dropsto the center of the silo and thatthe batcher fills each time beforerelease into the silo.

52. Two major factors in obtaining aquality uniform mixture from adrum mix plant is calibration andcont rol of the cold feeds. Thisslide depicts several of the stepsinvolved in the cold feedcalculations.


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The point of sampling of aggregatei n the drum-mix operation is at thestockpile. Obtaining a representa-tive sample from an aggregatestockpile is difficult at best.Samples should be taken fromseveral locations over the stockpileand combined and then split i fnecessary. Aggregate samplingalso can sometimes be done off theindividual cold feed belts.

percentage of material neededfrom each cold feed bin iscalculated.

56. In order to set and control thesupply of material from each cold-pi,z:F?feedT I ,L , bin, the b in opening and bel t" ' ' '$peed must be calibrated. The

''A i, * u * d .feeder gate openings for each binshould be varied and the materialflow determined for each settingas shown, with the feed beltsrunning at a fixed median speed.The gate openings should then beadjusted to the opening size thatwi l l supply apprpximately theweight of mate rial needed fromeach cold feed bin.


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T h e n e x t s t e p i n t h e s e t -u p a n dc a l i br a t i o n p r o c e d u r e is t o v a r y t h eb e l t s p e ed a n d t o d e t e r m i n e t h ec o r re s p on d i ng r a t e o f m a t e r i a lf low . A f t e r p roduc t ion begins t h es l igh t ad jus tm en t s necessa ry w i l lusua l ly be m ade by va ry ing thes p e e d o f t h e c o l d f e e d b e lt s.

58. T h e a s p h a l t m e t e r i n g a n d d e l i v e r ys y s t e m m u s t b e i n t e r l o c k e d w i t ht h e a g g r e g a t e w e i g h i n g a n dd e l i v e r y s y s t e m t o a s s u r e t h e m i xc o n t a i n s t h e p r o p e r a s p h a l tc o n t e n t .

59. T h e c o n t r o l s f o r t h e d r u m m i xp lan t are g e n e r a l l y l e s s e l a b o r a t et h a n a ba tch p l an t and usua l lyr e q u i r e l i t t l e a c t i o n by t h e p l a n to p e r a t o r o t h e r t h a n m o n i t o r i n gf e e d rates a n d t e m p e r a t u r e .

60 . Compar ison of Batch vs . DrumP r o ce s s: T o u n d e r s ta n d t h ed i f f e rences in t he tw o p rocesses ,t he fo l low ing com par i sons a remade:a . T h e t e m p e r a t u r e v i s c o s i t yre l a t ionsh ip i s l e s s im por t an tin t he d rum m ix as will bed i scussed in t h e ne x t s l i de .


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b. Design and Test Procedures -The current test procedureswere developed to duplicatethe batch plant process.Laboratory design specimensare prepared wit h clean, hot,dry aggregate and the mixingis by a mechanical shearingaction, which is very similarto the batch plant pugmill.The current test procedures donot duplicate the drumprocess,

c. Aggregate Conditons - Thegradation and moisturecontent of the aggregate inthe stockpiles is not nearly ascr i t ical in the batch process asi t is i n the drum process. Thedryer, hot elevator, screens,ho t bins, proportioning, anddry m ixing phases of the batchprocess help to removeaggregate coating and interna lmoisture, to produce a clean,hot, dry aggregate for mixing.I n the drum process, thegradation, moisture, andaggregate condition have t o becontrolled a t the stockpile.

d. Addi tional Propor tion ing - Thescreening, ho t bin storage, andproportioning phases of thebatch process are the keyelements in controlling thegradation and aggregate-asphalt proportions of the mix.I n the drum process, theaggregate gradation iscontrolled at the stockpilesand the proportioning iscontrolled at the cold feedbins. The aggregate passingover the weigh bridge musthave a constant moisturecontent and gradation, i f theasphalt content is to beconstant.


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e. Hot Aggregate Storage - Theaggregate is stored for aperiod after heating in thebatch plant operation. Thisallows time for any trappedmoisture to escape.

process. The aggregate is hot, dry,clean, and starting to cool whenthe asphalt is introduced and themixing process takes place.I n the drum process, the aggregateis coated immediately w it hfoaming asphalt. This is an asphaltcoating process rather than amixing process. The operatingtemperatures of the plant is notdependent on the temperatureviscosity relationship of theasp ha1 . The operatingtemperature needs to be highenough to remove all but about 1percent excess moisture. The 1percent excess moisture is neededto allow foaming of the asphaltcement. Excess moisture in theaggregate delays foaming ofasphalt, and may result ininadequate coating of theaggregate and heating of the mix.Insufficient moisture in theaggregate may result in a lack offoaming, resulting in inadequatecoating.Storage silo - The storage or surgesilo, which is required in the drummi x plant, can lead to segregationproblems i f not properly operated.


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Coo l ing Rate: Because of themoisture in the drum mix, thecool ing rate may be faster than abatch mix because of theevaporat ion. This af fe ct s theal lowable t ime to achievecompact ion before &he mixtemperature cools to 17 5 F.

62. Adv ert ised Propert ies: The in tro -duct ion and acceptance of thedrum mixers b rought w i th i t someadvert ised benef i ts on theproper t ies of the mixes produced inthis type plant. Some of thosewere:

1. The lower temperature mixesresul ted in energy and fuelsavings. Ty pica l ly, mix eswere be ing grod uce d in therange of 200 F to 250'~. Thiswas mgc h less than th e 2750Ft o 325 F temperature range ofthe convent ional batch p lantprocess. These lo w ertemperatures, however,a l lowed less t ime betweenprodu ct ion and compact ion.2. The lack of age hardening ofthe aspha lt i n t he drum mixerprocess compared t o th e batchplant process was verynot icable. The asphal ts i n thedischarged mixes werepract ical ly as sof t as theorig ina l asphalts. Thereduct ion in age hardening wasso not icable there werediscussions of using a hard ergrade o f asphalt.3. The presence of moisture inthe mixtu re was thought toac t as a l ubr icant and a id inthe compact ion of these lowertemperature mixes.


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However, diff icul ty in obtainingadequa t e compac t i on of t he sel ow er t empera t u r e mi xes becamet he ru l e r a t he r t han t h e excep ti on.A s a r e su l t , t he mi x t empera t u r ewas increased, which many t imeswa s accomplished by a lowering ofth e product ion rate. Today, manydrum mixers a re d i schargingm ix tu re s a t 2 7 5 ' ~ t o 3 0 0 ~ ~ .hus,t h e s e m i x t e m p e r a t u r e s a r e t h esame a s t hose i n t he ba t ch p l an tprocess.This combinat ion of higher mixtempera tures and lower product ionra t e i n some d rum p l an t s r educedt h e a g g r e g a t e v e i l t h a t p r o t e c t e dthe asphal t f rom being overheated .These burned or severe ly agedasphal t s had the fo l lowingcharac ter i s t i cs :1. Very bri t t le2. N ot s t icky t o t he t ouch3. Very high absolute viscosi tyvalues4. Very low penetrat ion values5. Very low duc t i l i ty values

original absolute viscosi ty. Ofconcern i s the recovered asphal t ofa mixture be ing di scharged f rom aplant , and/or of roadway coresf rom new or re la t ive ly newpavements, which has a highabsolute viscosi ty value, very lowpenet ra t ion va lues and a lmost noduct i l i ty.


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65. Common Factors of Aged Asphalt:There appears to be some commonfactors associated with theaccelerated aging problem.These common factors are:1. High temperature mixes ofapproximately 3 0 0 ~ ~ .2. Mixes produced in a drummixer which relies solely on anaggregate veil to protect theasphalt from the burner flame.3. When the drum mixer isoperated at less than about70% of i t s capacity.I t does not appear that the problemoccurs in lower temperature mixeswhen the drum mixer is operatingin excess of about 70% of i tscapacity, or when the drum mixeris equipped with a heat shield orsimilar method to protect theasphalt from the burner flame.

66. Possible Corrections: Where over-heating or accelerated aging of theasphalt occurs, several correctiveactions can be taken as shown onthe slide.

67, Water in the Mix: A mixtureproduced in a drum mixer processwill probably contain somemoisture. I n 1972 and 1973 theNorth Dakota Highway Departmentallowed the use of drum mixers onseveral projects while closelymonitoring and recording themix ture properties. One of theitems recorded was the moisturecontained in the mixture duringvarious phases of the operation,This chart shows the results ofthe ir findings,


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I f a moisture correction is notmade in an ext raction sample, orthe sample is not dried, themoisture will show up as asphalt i nthe test results. Also, whenrunning an extraction test on adrum mixer mixture, or anymixture where the baghouse finesare used, an ash correction shouldbe determined for each plant andeach mix type/aggregatecombination. I f the ash contentincreases and a proper correctionis not made, the excess ash may bepaid for as asphalt.Further, i f the Rice Method,AASHTO T 209, is used todetermine the Maximum Theo-retical Specific Gravity of a loosemixture produced i n a drum mixer,the results may need to be adjustedbecause of the moisture.

~emperature Drop: Finally, theevaporation of the moisture wi l lincrease the rate of cooling of themixture. Theref ore, especially incool weather, the Combinedhauling, laydown, and compactiontime may become very cr it ical toobtaining satisfactory roadwaydensities.Test results and field experienceindicate that lower temperaturemixes do require more compactiveeffort to obtain densities equal tomixes of more conventionaltemperatures.


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Batch Plant/Drum Mixer Sketch.I n summary, i t should beremembered tha t the screens, hotbins and weighing are the keyelements responsible for thequality con trol i n the batch plant.But in a drum mix operation thekey elements are the stockpilingand cold feed operations.Therefore, to maintain the qua litycontrol in a drum mixer processthe screening must take placebefore the aggregate is stockpiled;the stockpiles have to replace theho t bins; and the co ld feedoperation replaces theproportioning.Until about 10 years ago, mostmixes were produced in batchplants. The batch plants had beenused for years and we hadexperience and knowledge of thebatch plant process. However,many of the contractors have sinceswitched to the drum process. Thedrum process produces mixes whichusually have properties andcharacteristics different frommixes produced i n a batch plant.But i f properly set-up andcontrolled, ei ther process canproduce a satisfactory high qualityuniforrn bituminous mixture.


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PLACING HOT MIX

W hethe r w e a re p l ac ing ho t -m ix ona subgrade o r a s an ove r l ay , p rope rp r e p a r a ti o n o f t h e s u r f a c e isesse ntial . If we are going toc o n s t r u c t a s t rong durab le a spha l tpavem ent , i t m us t be p l aced on aw e l l p repa red f i rm foundat ion . A l lweak and yie lding spots should ber e m o v e d o r r e p l a c e d p r i o r t oplac ing t h e a s p h a l t m a t . J u s t p r io rt o p la c in g t h e a s p h a l t , t h e s u r f a c eshould be swe pt c lean of loose andfo re ign m a te r i a l .

a spha l t m a t i s t he app l i ca t ion o f ap r i m e c o a t o r t a c k c o a t . G e n e r a l l yi t i s r e co m m e n d e d t h a t a t a c k c o a tbe app l ied p r io r t o p l ac ing e achpav eme nt course . Whenc o n s t r u c t i n g s u c c e s s i v e a s p h a l tc o u r s e s w i t h l i t t l e o r n o d e l a y , at a c k c o a t m a y n o t b e n e ed ed .


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5. Types of TrucksThere are several types of trucksused to transport the hot mix fromthe plant to the paving site.Probably the most common is thetandem axle dump truck carryingapproximately 15 tons of mix.Using the proper number of trucksis a key factor in providing asteady flow of material to thepaver. This is dependent upon thesize of the project, and the lengthof haul.

Picture of NAPA Truck DrivingTechniquesThe National Asphalt PavementAssociation publishes a bookletcalled "Truck Dr iv ing Techniques."I t provides a good review of propertechniques for everyone who is apart of the asphalt paving team.


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7. A Truck Being Prepared forLoadingRegard less of the type haul ing uni tu se d, i t i s impor ta n t t ha t the t r uc kbe kep t in good, sa fe opera t ingcondition. A non-petroleum

I use of diesel should not be allowedas a re lease agent .

8. The truck bed should be welldra ined pr ior to the hot mixloading operations.

Imprope r Truck LoadingImproper t ruck loading is a veryl ikely cause of segrega t ion . Due t othe rap id t ruck loading , dr iverstend to pul l under the b in and loadwithout moving the ir t rucks . Asshown in th i s s lide , when a t r u c k i sloa ded wi thou t mov ing , t he c oa r s ema te r ia l w i l l r o l l to the f r on t a ndr e a r o f the t r uc k as well as e a c hside.


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A s t r u c k s a r e d u m p ed , t h e l a s tm a t e r i a l f r o m a s e g r e g a t e d t r u c kl o ad a n d t h e f i r s t c o a r s e m a t e r i a lon t h e n e x t t r u c k a r e c o m b in e d t oi n c r e a s e t h e a m o u n t o f s e g r e g a t e dm a t e ri a l. T h e c o a r s e m a t e r i a lc o m i n g f r o m t h e s i d e s of t h e t r u c kis usua l ly then depos i ted in thew i n g a r e a o f t h e p a v e r h o p p e r a n dr e m a i n s t h e r e u n t i l t h e h o p p e r s a r edumped. If th e hoppers a red u m p e d b e t w e e n t r u c k l o a d s , t h i sa d d s t o t h e v o l u m e o f s e g r e g a t e dmat er ia l . Th is opera t ion usua llyp r o d uc e s a s e g r e g a t e d s p o t a t t h ee n d of e a c h l o ad a n d i s us u al ly a t av e ry c o n s t a n t i n t e r va l w h e r e t h et r u c k l o a d s are o f t h e s a m e s i ze .

P r o p e r T r u c k L o a d i ng P r o c e d u r e sT h i s s l i d e s h o w s t h e p r o p e rp r o c e d u r e s f o r l o a d i n g a t ruck.T h e f i r s t d r o p s ho ul d b e m a d e a tt h e f r o n t of t h e t r u c k be d n e a r t h ecab. Th e second drop should bem a d e n e a r t h e r e a r o f t h e t r uc k .T h e f i n a l d r o p s ho u ld b e m a d e int h e c e n t e r of t h e t r u c k . T h i sp r o c e d u r e r e q u i r e s t h ec o o r d i n a t io n a n d e f f o r t o f b o t h t h et r u c k d r i v e r a n d t h e p l a n to p e r a t o r .


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Keys to Pavement L i f eI n order for an asphalt pavement t operform properly, severalobjectives must be met during thelaydown operations. Those keygoals are:1. A smooth mat surface whicheliminates the detrimental

effects of dynamic loadingfr om a rough surface.2. A uniform mat thicknesswhich wi ll have a unifo rm

rolldown.3. A uniform ma t density whichwil l result in uniformdensif cati on under tra ffi c.

Paving TrainI f the paving equipment is to do itsjob i t is important that everyonewho has a responsibility fo r placingthe hot mix understand theprinciples behind the design andoperating characteristics of thepaver.

14. The paver consists of two units,the*ractor un it and the screed.The tract or uni t supplies al l thepower needs of the paver. I tmoves the paver down the road a ta uni form speed, receives the mix,and ihoves the mix to the screed.Screw augers located at the rear ofthe tractor unit distribute thematerial lateral ly so that acontrolled amount of material iskept in front of the screed at alltimes.


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15. The Screed Un i tThe screed unit is towed by thetractor unit. I t consists of thescreed plate, vibrators or tamperbars, thickness controls, crowncontrols, and screed heaters.

16. Screed ForcesThe screed is connected to thetractor unit by a single pinconnection on either side. A l lpavers operate on the sameprincipal as to leveling and controlof thickness of the mat. As thescreed is pulled into the material,i t automatically seeks the level atwhich al l forces acting on i t are inbalance. This slide illustrates theforces acti ng on the screed. Basicaction of the full floating self-leveling screed may be comparedto a water skier who is pulled alongthe water surface wi th skies tippedup just enough to support theweight.

17. Vibrating ScreedI n addit ion to str iking off the mixat the proper elevation andmaintaining the correct matthickness, the screed also con-solidates the mix and imparts thein it ia l compaction. The amount ofinitial compaction developed bythe screed varies with theproperties of the particular mixbeing placed but is usually about80% o f the target density. A l lpavers employ one of two methodsto consolidate the mix. Thevib rat ing screed, as pic tured here,is the method used by most pavers.The vibration of the entire screedconsolidates the mix and impartsthe in it ia l compaction.


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Tamper B ar ScreedThe other method of consol idat ionfound on some pavers is known asthe tamper bar screed. Instead ofvibrat ing the ent i re screed, thetamper bar sl ides up and down theleading edge of the screed tocompact the mix just enough sothat the screed can fo l lowsmoothly over the tamped mix.The tamper bar should extend 1/6+of an inch below the screeda t t he bo t tom o f t he s troke.

19. Screed Crow nThe crown adjustment of thescreed is impor tant for propertextu re behind the paver. Bo th theleading and t rai l ing edges can becrowned . i a w ~ d e n t l y . The.leading edge of the screed shouldhave s l igh t l y more c rown than thetra i l in g edge. As pic tur ed here,too m uch c rown in the le ading edgeof the screed w i l l produce an opentextu re in the m at a long the edges.Too l i t t l e c rown in the l ead ingedge w i l l cause an open tex turedown th e center.

Screed Hea terThe screed uni t is also equippedw it h a fan-augmented heater .He at ing of the screed prevents themix f rom s t i ck ing to the sc reedplate. The heaters are only to beused to heat the screed p late atthe sta r t o f the paving operations.However, the heaters should neverbe used to a t tempt to add heat t othe m ix be ing de l i vered to thepaver.


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Screed Too HotThe bottom of the screed must besmooth and true to place the mixproperly. I n addition to normaland/or uneven wear the screed canbe overheated which may warp thescreed by causing i t to buckle downor up.

Don't Stop the PaverIn addition to the paver beingproperly adjusted and in goodoperating condition, there aresome basic operating guidelineswhich must be followed i f we areto bu ild good, smooth pavements.The first and probably mostimportan t operating rule is to keepthe paver moving at a constantspeed, and don't stop. A t everylocation where the paver stops, thescreed leaves an imprint in thesurface of the roadway that cannot be rolled out.

capacity. Therefore there is noreason to start-stop, start-stop.Another common occurrance is thetruck backing into the paver.When the truck bumps the paver, 'the floating screed is knocked intothe mat. The truck should alwaysstop in front of the paver and bepicked up as the paver movesforward.


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Auger UnderloadedThe failure to deliver a constantsupply of mix to the spreadingaugers is another poor practicethat w i l l cause waves, tex tureproblems, and rough pavements.Too little mix ahead of the screedwi ll cause the mat density todecrease under the front of thescreed. This slight decrease i n matdensity will slightly reduce thebearing capacity of the mix as thefull screed moves unto it . As aresult the screed wi ll sink into themix producing a thin mat.Auger OverloadedToo much mix ahead of the screedwill cause an increase in the matdensity just under the front of thescreed. This w i l l result i n a slightincrease in the bearing capacity ofthe mix causing the fu ll screed torise and produce a thicker mat.

Auger DistributionAn uneven distribution of materialcan likewise eff ect the density o fmate rial under the front o f thescreed and can produce an unevenma t thickness. Therefore i t isimportant to keep the augerdistribution of the material as evenas possible. Mos t properly adjustedpaving machines w i l l automat icallykeep a steady supply of ma terial infront o f the screed.Paver SpeedFailure to hold the forward speedof the paver constant is anotherimportant factor which can causevariations in the mat smoothnessand surface texture. The tamperor vibrator speed is generally fi xedand as the paver speed changes theenergy imparted to the matchanges. This change incompactive effort wil l result inchanges in the ma t thickness.


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28. Faster Paver SpeedFor example, i f the paver speed isincreased, the tamps or vibrationsare spaced farther apart. Thisimparts less compactive effort tothe mat and lowers the density andbearing capacity under the f ront o fthe screed. Therefore the ma tbecomes thinner.

paver could be slowed to match thecontrolling plant or deliverycapacity and eliminiate manyunhecessary star -stop operationsand variations i n the forward speedof the paver.


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Paving Speed Versus M at ThicknessThis slide shows curves from whichpaver speeds can be determined forvarious mat thicknesses andvarious plant production or truckdelivery rates in tons per hour.For example, i f a hot mix plantproducing 250 tons per hoursupplying a paver placing a mat 3"thick and 12' wide, the paver speedmust be 2[r per minute to balancethe laydown rate with theproduction rate at the plant. I nsome cases the delivery capacityof the trucks is the controllingfactor in which the delivery ra teshould be substituted for the plantproduction rate. Similiar curvesare available i n publications by TheAsphalt Institute and other suchagencies.

32. Grade Cont ro lMost States specify that elec-tronic grade controlled pavers beused. This equipment makes i tpossible to keep all layers moreuni form in thickness thus providinga smoother pavement.

I 33. Tow Point RAMBy means of the grade controlsystem, the angle of attack of thescreed can be varied.


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35. Ski ControlThe wire is usually mounted on anadjustable length ski as shown inthis slide.

Thickened MatAs qn example of the operation ofthe sensor, a paver enters ashallow spot in the road, the gradesensor detects the fact that thetow point has dropped and signalsfor the m at t o be thickened.

Results of Thickened Ma tThe sensor readings aretransmitted to the hydrauliccontrol of the tow point ram. Byraising the tow points, the angle ofthe screed is altered to bring aboutthe necessary change in matthickness.


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38. Cross-Slope ControlThe cross-slope of the pavement iscontrolled by the movement of thetow-point ram on the side of thepaver opposite the grade sensorside. A pendelum mounted on abeam senses the difference inelevation between the beam ends.This cross-slope controller signalsthe tow point ram so that thescreed may be maintained at thedesired cross slope.

39. Superelevation and Wedge SectionsIn addition t o making i t possible toplace pavement to a precise grade,automated screed controls havegreatly simplified the incorpora-tion of superelevation in curves,where the superelevation is to beaccomplished with an asphalt layerof varying thickness.

40. When greatly increasing thesuperelevation of a curve, thbrecommended practice is to placesuccessive wedges which arethicker on the outside edge than onthe inside edge. This slide showsthe proper construct ion method.An important consideration is theroll-down o f the wedge layers. Theamount of roll-down should beconsidered and incorporated intothe laydown thickness.


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41. R u t t e d S e c t io n a n d L e v e l U pTh e rolldmwn dur ing co mp act io nm us t be cons ide red w hen pav ingover a ru t t ed sec t ion . A s show n i sth i s s l i de, a 14" over l ay of a ru t t edsec t ion w i l l p rov ide approx im a te ly1" m a t t h i ck n e s s b e t w e en t h ew hee l pa ths and 2" in t h e w hee lpa ths . The pave r s c reedc o n s o l i d a t e s t h e m a t e r i a l a n di ni ti al ly c o m p a c t s t h e m a t t o a b o u t80% of th e t a rg e t densi ty . Th eo t h e r 20% is obta ine d by th ecom pac t ion rol lers . This 20%rol ldown of th e addi t ion al incht h i c k n e s s i n t h e r u t t e d w h e e l p a t hsect ion wi l l resul t in a 3/16" r u ti m m e d i a te l y a f t e r c o m p a c ti o n.The re fo re , a l eve l-up cour se shou ldbe used on seve r ly ru t t ed sec t ionso r a n y p l a c e w h e r e v a r i a b l e m a tth i cknesses w i ll occur . M any t im e st h e c o n t r a c t o r s w a n t t o p l a c e t h el eve l -up cour se a s a por t ion o f t heregu la r ove r lay . Th i s shou ld no t beal lowed, as on ly success ive l i f t sw i ll e l i m i n a t e t h e r u t t e d o r u n l ev e lsec t ions . A l so no t e t h a t leve l-upc o u r s e s on a r u t t e d s e c t i o n m u s t b erol led wi th a r u b b e r t i r e d r o l l e r t oc o m p a c t t h e r u t t e d s e ct io n . T h es t e e l w hee l ro l l e r w i l l b r idge ove r ,and no t p rope r ly com pac t , ru t t edsec t ions . H ow ever , t h i s add i t i ona lc o m p a c t io n in t h e r u t t e d s e c t i o n sw i ll p robab ly occ ur soon a f t e r t h es e c t i o n s a r e o p e n t o t r a f f i c .A g ai n, t h e r u b b e r t i r e r o l l e r i s t h eo nl y t y p e r o l l e r t h a t i s c a p a b l e ofcom pac t ing a m a t w i th a va r i ab l ethickness , such as a r u t t e d s e c t io n ,o r level-up courses .

42. JointsA cold longi tudinal jo int usual lyre su l t s i n a c r ack w hich a l low s thee n t r a n c e of w a t e r i n to t h e b a s e o rsubgrade . Where poss ible andfeas ible , the cold longi tudinal jo intshou ld be e l im ina ted .


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On some projects, especially largeprojects on new construction, twoor more pavers may be used andoperated one just behind the otherto place the full width pavement.This procedure eliminates the coldlongitudinal joint.

43. Fu l l Width PavingThere are pavers on the markettoday that are capable of pavingtwo or three lane highways fullwidth. However, this type pavingoperation can only be used on new

I not have to be maintained.

44. Paving Under Tr af fi cBut on many small jobs and jobswhere traffic must be maintained,only one paver can be justified,making a cold longitudinal join tnecessary.

Cold joint construction willgenerally result in a lower densityin the mat adjacent to the joint inthe lane placed first. This lowerdensity area is generally subject t oearlier distress because of the highvoids and greater permeabili ty.


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Jo in t C ons t ruc t ionT h e r e a r e t w o m e t h o d s c o m m o n l yused to cons t ruc t l ong i tud ina ljoints. The t radi t iona l metho d i sto build a near ly ver t ica l jo int . Anew method i s to bui ld a slopedjoint wi th about a 4 t o 1 hor i zon t a lto ve rt ic al s lope. This joint isc o m p a c t e d w i t h a rubber-t i rerol ler to r e s u l t in t h e s a m e d e n s i tyas t h e ad j acen t ma t . T h is s l opea l l ows t r a f f i c to c ross t he j o in teas i e r on t h i cke r l if ts . P roper c a r eand co r r ec t t echn iques in p repa r ingand cons t ruc t i ng e i t he r o f t heselongi tudinal jo ints wi l l he lp ensurea durab l e ma in t enance f r ee j o in t .T he amo unt o f ov e r l ap shou ld no te x c e e d 2". Note t ha t t he unro l l edm a t s h o ul d b e a b o u t K t h i c k e r t h a nt h e r ol le d m a t t o a ll ow f o r t h erolldown.

th e sloped-joint should be bu il t onlyby crowding th e over lap.

48. Jo in t T r immingA t r immed long i tud ina l isc o n s t r u c t e d by removing a l l thef reshly lapped mater ia l . Thisma te r i a l shou ld no t be b roadcas tove r t h e su r f a ce of t h e unro ll edmat .


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50. St a t i c Ro l le r on t he Jo i n tBecause of t he need t o ge t as gooda longi tudinal joint as possible,during compact ion this joint shouldbe ro l led f i rs t . However , th eproper joint rol l ing techniquevar ies s igni f i cant ly wi th the typeroll er being used. As shown in thissl ide, if a s t at ic rol ler is being usedi t shoul d r un on t he c om pac t ed m a twi th approximate ly a 6" l ap on t h ehot uncompacted mat .

Vibra tory Rol le r on Jo in tIf a vibratory rol ler i$ being used,i t s hou ld be r un m os tl y on t h e ho tm a t wi t h app r ox i m a t e ly 6" ove r l apon t h e c o m p a c t e d m a t . I t i s veryi m p o r t a n t t o r e m e m b e r t h a t t h evibra tory compactor should neverbe ope r a t ed on t he com p ac t ed m a t .I n a l m os t a l l c a se s t he v i b r a t o r yrol le r wi l l cause decompact ion ofthe previously placed mat .


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the previously compacted mat.


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Straight Edge JointsRegardless of the method used toconstruct the transverse joint, i tmust be carefully checked with astraight edge. I f i t is not true andsmooth, the full thickness of themat should be removed far enoughback to make corrections. Thenaft er ini tia l compaction, the jointshould again be checked with astraight edge. Any deficienciesshould be corrected immediately.The paving train should not moveon un ti l the transverse joint iscorrect.Good equipment and it s properoperation is essential to transportand place asphalt hot mix.Construction techniques areconstantly changing and as a resultwe often see segregation in thepavements. Segregation can occurin the stockpiling and handling ofthe aggregates, in the plantoperations, or i n the laydown.Regardless of the source, theresult will be seen immediatelybehind the paver. Two of the mostcommon types of segregation canbe seen in this slide as linearsegregation and spot segregation.


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On a paving project normally thedemand on one side of the paver isgreater than the other. As a resu ltmaterial w i l l remain in one side ofthe hopper while the other sideruns empty. Then i f materialwhich is segregated in the truck isadded to the hopper i t naturallygoes to the empty side of thehopper. Therefore, the spotsegregation will usually occur onone part icu lar side of the paver.While i t is not the most commoncause, a large imbalance in thepaver demand on one side canresu lt in the spot segregation. Thiscan be noted when one side of thehopper runs empty while materialis heaped up on the other side.When one side of the hopper isbeing pulled empty, the materialmoves in a swirling eddy typeaction and segregation can readilybe seen. This overdemand ofmaterial on one side of the paverusually occurs when the pavingwidth is extended on only one sideinstead of being balanced on eachside and/or when the m at thicknessis not unifo rm across the pavementwidth. As shown in this picture,the wing area of the hoppers on theolder model laydown machines usedto be sloped. By being sloped theyprovided a degree of self-cleaning.Pict ure o f Laydown MachineIn efforts to increase the storagecapacity of the hopper, mostmanufacturers of laydownequipment have flattened the wingarea. Note the wide flat ledge onthe hopper of this new laydownmachine. The hopper wings of thisnew machine consists of a widef la t ledge. These f l a t wings arenot self-cleaning. The mat eri al isremoved only when the wings are


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dumped. Since the hopper is widerthan a truck, mat eri al beingunloaded from a truck only coversabout the middle two thirds of thehopper width in more or less a coneshape. As the laydown machine isbeing operated, vibra tion occurs.As a result, the coarse mixparticles trickle down the side ofthe cone and are deposited i n theflat areas of the hopper wings.This segregated material remainsin the hopper wings until they aredumped.

61. Dumping the Hopper WingsThe paver is equipped wi th two sla tconveyers to move the hot mixmaterial from the hopper to theaugers at the rear of the tractor.These slat conveyors operateindependently to meet the demandfor material on their particularside of the paver. Also there is a

bed, the hopper wings can only bedumped between truck loads. Aspreviously discussed, unless thematerial demand for each side ofthe paver is perfectly matched,one side of the hopper wi l l runempty while mat eria l remains i nthe other side. I f one side of thehopper is empty and the segregatedmaterial in the hopper wings isdumped, almost al l of the mate ria lin the hopper wings is eitherdumped directly into or tumblesinto the empty side of the hopper.


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D u m p in g t h e T r u c kI f t h e m a t e r i a l i n t h e f r o n t a n dback o f t h e t rucks i s s eg rega ted ,t h e l a s t m a t e r i a l t o b e d u m pe df r o m t h e p r e v i o u s t r u c k a n d t h ef i r s t m a t

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Hot Mix Bituminius Paving Manual 014716