Review of Fine Aggregate Angularity Requirements in · PDF fileReview of Fine Aggregate Angularity Requirements in Superpave ... and 9.5-nun coarse aggregate were used in the study.

Review of Fine Aggregate Angulari ty Requirements in Superpave

Chili-Jen Lee Changlin Pan

Thomas D. White Purdue University, West. Lafayette, Indilina, USA

Ab5tract

Superpave aggregate qualification includes the fine aggregate angularity (FAA). The numerical value of the FAA is the voids in the mineral aggregate of the loosely packed fine aggregate. Use of FAA has been predicated on the philosophy that higher and lower values of FAA represent fine aggregate that wiU exhibit high and low internal friction, respectively. The amount of friction depends on the aggregate particle shape and texture. Higher intemal friction is associated with increased rutting resistance.

Fine aggregate angularity levels used in the Superpave system are below 40, 40 to 45 and above 45. The higher values are spetified for layers near the surface and for higher traffic levels. Past and current experience shows that there are fine aggregates in mixtures pcrfonning well thai are below the specified levels. There are also aggregates above these levels in mixtures thai are not performing as desired. A study has been conducted utilizing the Purdue University laboratory wheel (PURWhee[) tracking device 10 develop performance-based data on mixtures with various fine aggregates.

A single asphalt (PO 64-22) and 9.5-nun coarse aggregate were used in the study. The coarse aggregate was selected to emphasis the fine aggregate performance. Six fine aggregates were used with FAA ranging from 39 to 49. In addition, two of these aggregates were blended in various proportions to produce blends with FAA values of 43, 45 and 46. Wheel track tests successfully delineated potential performance of the mixtures studied.

Test results show that FAA alone may not be adequate to evaluate the contribution of fine aggregate to the mixture performance. Other factors including gradation, absorption, affinity for asphalt, elc. would also affect mixture perfonnance.

Keywords: Asphalt mixtures, performance, Superp.ave, laboratory wheel tests, PUR Wheel, aggregates, fine aggregate angularity

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Acknowledgements

The authors would like 10 express thanks and appreciation to the Indiana Department of Transportation (INOOn, Federal Highway Administration (FHWA) and Indiana Mineral Aggregate Association for support of this work.

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Introduction

One of the material qualification requirements of the Superpave mix design process is the Fine Aggregate Angularity (FAA), lbe purpose of this requirement is to insure that fine aggregates utilized in Superpave mixtures have adequate internal frictional characteristics. Fine aggregates have significant effect on asphalt mixture rutting potential, i.e. high internal friction wi ll minimize rutting.

The FAA requirements genernlly provide a measure for delineating fine aggregate qualities. However, experience with current criteria indicates that there art anomalies where fine aggregates above and below the minimum FAA exhibit contrary performance. Also, rrom experience of implementing Superpave requirements during the: 1996 and 1997 construction seasons in Indiana, there are crushed fine aggregates that fail to meet the FAA criteria.

To address the problem a study was conducted to evaluate FAA requirements. Tilt Purdue University laboratory wheel (PURWheel) tracking device was used to generate performance data. Tests were conducted on mixtures with fine aggregate having values of FAA over a range of from 39 to 49. The fine aggregates included materials with original, single FAA values as well as three blends of two oflbe fine aggregates.

Materials

Asphalt mixtures have three major components: coarse and fine mineral aggregate and binder. Each component contributes to the perfOimance of the mixture. However, careful planning is required to quantifY the effect of only one component because the effects of the other two may confound the results. The problem can be explained by the approach in a concurrent research project to study the effect of additives and modifiers on the binder component.

In the concurrent study of mixtures with modified bindm, it was recognized that if both high quality coarse and fine aggregate were used then the difference in perfollllance of the mixtures would be minimal. In fact, mixtures with WlOlooified binders would perfonn as well as those with mooified asphalt. To emphasize the binder component in mixture performance, relatively poor quality aggregate components were utilized. Subsequent laboratory wheel track rutting tests have been effective in delineating the performance of various modified binders.

There is an analogy between the effect of modified binders and fme aggregate angularity on asphalt mixture performance. The analogy is that a very good coarse aggregate might mask differences in perfonnance of the fine aggregate. As a result, a study plan was developed that included use of a single asphalt binder (PG 64-22) and a single coarse aggregate (gravel). Subsequently, asphalt mixtures with fine aggregates having a range of FAA values would be prepared and tested.

In the initial proposal, the target range of FAA was from a low of 35 to 50+. However, the FAA of candidate materials identified jointly by the Indiana Department of

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Transportation and the Indiana Mineral Aggregate Association ranged from 39 to 49. Characteristics of the ~ and fine aggregates used in the study are given in Table 1. The fine aggregates include natural sand, crushed limestone: sand, dolomite sand, slag sand, and crushed gravel sand. Stockpile gradations for each material are given in Table \.

In addition \0 using the above stockpiles, two of the fine aggregates were blended. The two fine aggregates selected for blending were Indiana sowces #2497 and #2164. These are the natural sand (FAA '" 39), and crushed gravel (FAA '" 49), respectively. Target FAA values for blending were 43, 4S and 46. Several blends of these two aggregates were prepared and a curve developed so thaI the proportions could be selected to achieve these target FAA levels.

Mirlure Designs

Mixrure designs were conducted using Superpave volwnetric criteria. Proctdures and methods given in SP·2 (Asphalt Institute, 1996) were utilized. Numbers of gyration in the Supcrpave Gyratory Compactor (SGC) utilized were Niailial = 8, Ndcr;ip = 96 and N~ = 152, which correspond to a design traffic level of 3·10 million equivalent single axle loads (ESAL) and an average design high air temperature ofless than 3goC.

A separate mix design was conducted for each mixlUre. TotaJly, nine mixtures were designed. The initial mix design was conducted with fine aggregate having an FAA value as dose to 45 as possible. This was sowtt #231 1, a crushed limestone sand. The gradation resulting from this initial mix design was used as a target for the remaining mixtures. Figure 1 and Table 2 show the gradations for all of the mix designs. From Figure I, it can be seen that with the exception of the "S" gradation (#2314), the aggregate blends were close to the target gradation. The "S" gradation proved to be important in analysis of the results.

Table 3 shows a summary of the mix designs. There is variation from Supe:rpave volumetric criteria. This variation was accepted to keep from modifying the stockpiles. To do so would have required that the as received material gradations be modified. The voids in the mineral aggregate (VMA) for the natural sand mixlUre was lower than the criteria. Also, the air voids for aggregate sources 112478 (stag) and 112314 ("8" gradation) were higher than the criteria. In these latter mixlUreS, if the air void criteria had been held to four percent then the upper voids filled with asphalt (VFA) criterion of 75 percent would have been exceeded. Since the asphalt content was already high, a decision was made to maintain the voids filled at 75 percent and violate the air voids requirement. This resulted in a reduced asphalt content for these (wo mixtures.

Figures 2. 3 and 4 show the relationships of Asphalt Content vs. FAA, VMA vs. FAA and Asphalt Content vs. VMA for the nine mixtures in the study. In Figure 4, the relationship between Asphalt Content and VMA is almost linear and hence the scatter plots of Asphalt Content \IS. FAA (Figure 2) and VMA YS. FAA (Figure 3) look similar.

From Figure 2 and Figure J, a trend can be observed that both VMA and asphalt content increase with the iocreasing value of FAA. With the eXtep(ion of mixtures #2478 (slag) and 112314 ("8" gradation), the trend is bounded by a VMA of 16 and asphal t content of about 6.0.

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The fact that high VMA will resuU in high asphalt content for fixed air voids (A V) criteria is well known. This study will use performance test ing to examine the significance of the high asphalt content. Current French criteria (Aschcnbrener, 1992) uses a range of AV.

PURWheel Tests

A laboratory scale wheel test device was designed and fabricated at Purdue University. The device is shown in Figure 5. The PURwheeJ test device was designed and fabricated to evaluate stripping/rutting performance of bituminous mixtures (Pan, 1997). Two specimens can be tested simultaneously. The test environment can be either hot/wet or hot/dry. Typical test temperatures range from 55 to 60 degrees centigrade, although the test temperature can vary from room temperature to 65 degrees centigrade. In application, test specimens are compacted to 6-8 percent air voids with a laboratory linear compactor (Habennann, 1994). Specimen dimensions art typically 29.0 em wide and 31.0 em long. Specimen depth varies depending on the type of mixture being tested. For example, surface, binder and base mixture thicknesses in Indiana are 18 em, 5.1 cm and 7.6 em, respectively. Tests can be conducted with steel or pneumatic wheels. All tests in this study were conducted with the pneumatic wheel. The pneumatic wheel is loaded and tire presswe adjusted to achieve a gross contact pressure of about 620 kPa. The wheel velocity can be varied but is typically 33 ± 2 em/sec (1.1 ftlsec or I mph). Specimens are subjected to 20,000 wheel passes or until 20.0 mm of deformation develops. This deformation is downward relative to the original sample surface, i.e. uplift is not included.

In the FAA study, 6 percent air voids were targeted for the compacted samples. Sixteen samples were prepared for each mixture. Four samples each were tested wet and dry and at 45 and 60 degrees centigrade.

By testing four slabs. results were obtained over a range of air voids. Using this information, performance in tenns of nwnber of passes at a common percent of air voids can be determined. This is done by regression analysis on the wheel passes versus air voids of the four tes ted samples. Although the compaction level for each sample was targeted at 6 percent air voids, variation does occur. Since, from past experience, the rutting performance is sensitive to the air voids content, the normalization is done to ensure the mixtures can be compared on the same basis Air voids were calculated based on volume of the slab determined by measured slab dimensions (i.e. width, length and thickness).

Typical PURWheel test results are shown in Figures 6 and 7. The line aggregate in mixture #2497 is a natural sand (FAA=39). Performance of the mixture is poor as indicated by the early and significant rutting reflected in Figure 6. In contrast, the performance exhibi ted in Figure 7 by mixture #2311 prepared with a limestone sand (FAA=45) is good.

The number of passes versus asphalt content, VMA and FAA are shown in Figures 8, 9 and 10, respectively. From Figures 8 and 9, the rutting performance increases with increasing value of VMA and asphalt content. As in Figures 2 and 3, for this maximum

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aggregate size mixture, upper limits of VMA at 16 and asphalt content at 6.0 can be observed.

Mixture #2211 inc ludes dolomite sand (FAA=48). This mixture satisfies all of the volumetric design criteria. The PUR Wheel performance of this mixture is over 40,000 wheel passes. In conclusion, this mixnue is satisfactory. As a consequence, performance of mixture #2211 can be used as a benclunari: to evaluate performance of odler mixtures. The mixtures that exhibited poorer performance than mixture #2211 are mixture #2497 (natural sand, FAA=39), mixture #2478 (slag, FAA=47) and mixture #23 14 (limestone sand with "S" shaped gradation, FAA:44).

Film Thickness

Film thickness affects a 001 mix asphalt (HMA) durability and stability. However, durability and stability are somewhat contradictory characteristics. Thick binder films lend to protect the aggregate and produce more durable HMA. However, the truck film also acts as lubricant between aggregate particles and result in less stable mixtures. In contrnst, thin binder films produce less durable mixtures bUllhe aggregale skeleton rernruns a more stable structure when the mixture is subjected to external loads.

Campen, et al. presented a relationship for HMA ai r voids, swface area, film thickness and stability. On the basis of the data they analyzed, film thickness ranging from 6 to 8 microns was found to provide the most desirable pavement mixtures. Also, K.andhaI et aI. suggested a minimum average asphalt film thickness of 8 microns be used to ensure mix durability instead of minimum VMA. In the C1.1lTell1 study, the authors used average film thickness as a means to evaluate the mixture design results. Lower and upper film thickness limits of 8 10 10 microns were adopted for this pwpose. The film thickness of the nine mixtures incorporated in this study ranges from 7.6 microns 10 16.3 microns as shown in Figure II.

In Figure II , il can be seen that both of the mixtures with high asphalt contents, mixtures #2478 and N23 14, have very thick binder fi lms. Mixture #2497 has a relatively low film thickness. which reflects ease-of-compaction of the natural sand. The above three mixtures exhibited the poorest perfonnance in the PURW,het:i as compared to the other mixtures. It is believed that the poor performance of mixture #2314 and #2478 was due to the high asphalt contentlthick binder films. Thick binder ftIms tend 10 lower the stability of the mixtures. Mixture #2497 with natuJ1li sand would have poor internal frictional characteristics as indicated by its low FAA value. It appears that a range of film thickness could be a candidate criterion 10 balance the durability and stabilily ofHMA.

Results

A separate design was conducted for each of the nine mixtures in the study. The asphalt contents from these mix designs reflect the effect of particle shape, texture and gradation.

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Asphalt content tends to increase with FAA High FAA values reflect materials with increased resistance to compaction and higher VMA. Very high FAA values, FAA of 48 or higher, may reflect particle shape (slivered) such as the crushed gravel or texture such as the slag.

During compaction of the crushed gravel, the slivered particles will break or tum fial The result is that the VMA and associated asphalt demand arc reduced. However, this is not the case with slag aggregate. The slag mixture VMA and asphalt demand remains high. A gradation such as the "S" gradation can also result in a mixture with high VMA and associated high asphalt content. Therefore, both mixture #2478 (s lag) and mixtw'l: '2314 ('S" gradation) have bigh asphalt content.

Figure 9 shows the effect of increasing the natural sand in combination with the #2164 crushed gravel sand, Le. mixtures BI, 82 and B3. As shown in Table 2, mixtuJts BI , 82 and BJ have I O.9"~, 15.1% and 23.5% natwal sand, respectively. The associated fAA values arc 46, 45 and 43. Rutting increases with decreasing value of FAA.

CODdwioDS

Several conclusions can be drawn from this study: I. The PurwheeJ data on individual asphalt mixtures in Figw-e 9 shows the effect of

FAA on rutting perfonnance. These data include the effect of the associated YMA as reflected by the aggregate shape and texture and mixture gradation. All of these factors combine to determine the asphalt demand to satisfy the air voids criterion.

2. FAA alone may not be adequate to evaluate the contribution of fine aggregate to the mixture perfonnance. Other factors including surface texture, absorption, affinity for asphalt, etc. would also affect mixture perfonnance.

3. Performance is sensitive to the design asphalt content. This result is mirrortd in the VMA. VMA is critical because it is detennined by fme aggregate shape and texture and mixture gradation. Fine aggregate angularity becomes less critical if an upper limit is adopted for VMA in combination with the use of crushed sand.

4. It appears that an acceptable VMA for the 9.5-mm nominal mixture is in the range of from 14 to 16. The upper VMA limit would rtduce incidences of high asphalt contenl However, mixtures in this study within the 14 to 16 VMA range still have a relatively high film thickness of about 10 microns.

5. The PURWheellaboratory wheel test device was effective in showing the relative perfonnance of the mixtures tested in this study. This type of equipment can be utilized to test a large number of material and mixture variables in a relatively short period of time.

6. Relative evaluation using perfonnance of mixtw'e 1122H as a basis suggests that the FAA index test does not by itself fully capture potential perfonnancc of miKtures. Perfonnance tests resul ted indicate that there are crushed fine aggregates with FAA values lower than 45 that can be used in mixtures that will perfonn equal to or better than mixtures with fine aggregates having FAA values higher than 45.

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References

I. T. Aschenbrener, and K.. Stuart, "Description of the Demonstration of European Testing Equipment for HOI Mix Asphalt Pavement," Final Report, No. COOT-DTD­R -92-10, Colorado Department ofT ransportalion, October 1992.

2. Asphalt Institute, Superpave uvel I Mix Design, Superpave Series No. 2 (SP·2), Lexington. KY, 1996.

1 R.B. McGennis, R.M. Anderson, T.W. Kennedy, and M. Solaimanian, "8ackgrolU'ld ofSUPERPAVE Asphalt Mixture Design and Analysis," Publication No. FHWA-SA· 95'()()3, Federal Highway Administration, February 1995

4. H. Huang, and T.D. White., "Minimum Crushed Aggregate Requirements in Asphalt MixtW"tS," Joint Highway Research Project Draft Final Report, Purdue University, November 1996

5. c.L. Pan, "Analysis of Bituminous Mixtures StrippingIRutting Potential", A Thesis Submitted to the Faculty of Purdue University, August, 1997

6. 1.F. Campen, J.R. Smith, L.G. Erickson and L.R. Mertz, "The Relationship between Voids, Surface Area, Film Thickness and Stability in Bituminoll'j Paving Mixtures", Proceeding, AAPT, VoL 28, 1959

7. P.S. Kandhal and Sanjoy Chakraborty, "Effect of Asphalt Film Thickness on Short­and Long-Term Aging of Asphalt Paving Mixtwts," Transportation Research Record 1m , 1996

8. P.S. Kandhal, Kee Y. Foo, R.B. Mallick, ''Critical Review ofVMA Requirements in Superpave", Paper prepared for presentation in the 71" Annual Meeting of the T ransportalion Research Board .

9. F.L. Roberts, et al. , "Hot Mix Asphalt Materials, Mixture Design and Construction," III edition, National Center for Asphalt Technology

10. John Haddock, C.L. Pan, Aiwen Feng, T.D. White, Draft Interim Report of the National Pooled FWKI Study No. 176, 1998

II. R.C. Williams, G.R. Duncan, Jr., T.D. White, "Sources, Measurement and Effects of Segregated Hoi Mix Asph.alt Pavements," Final Report. Joint Highway Research Project, FHW A/lNI1HRP-96116, 1996

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Table I Properties or Slockpilts

C"'" fineAF~ A""".

",",u 12m 12311 12497 "". Illil 1241. "'14 N""'"

Logan~n, Indian-

Lo .. ~ W'" Huntin "" Indian-Source apolis, L<bo- "'". OIiClio, apori~ [N

IN pon, IN

"'" IN [N IN IN

Type of III Gnvel

.~ N.""" en.b<d Do"" S,", .~

(IM~% Gmd ,. Mmrill Crush CounI) "'" "'" "'" "'" "'" "'"

FAA 4J. U JI.11 41.97 4t 1l 44.91 44.1$

Ap_ 2.7307 2.7251 2.1111 2.1415 2.1546 2.8924 2.6854 SG

'SG 2.6091 2.6449 2.5990 2.6]11 2.7521 2.76]9 2.5917 Sieve

Pm:ent rwsin& by wtigbl S""~'

9.S 11.4% 1110" 1110" [00% 100% [110% [00%

4.75 21.5% '110% [00% [110% [00% [00% 1110" 2.36 10% 88.4% 82,3% 73.5% n.~% 17.4% 47.3%

l.lS [1" 54.7% 59.1% 42.1 44.4% 63.9 19.7%

0.' )2..6% HI% 25.65 24.8% 31.4% 9.'" OJ IIJ% 11.3% IH% 11 .9% 17.1% l.'"

0.15 9.7% 11% 1.1% S.'" 10.3% '-9% o.m Sj" IJ" HI" 2.6% 6.4% IJ"

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r.blt 2 Mil Design Gndatioru

SooreeiMix Number m i l 12497 12'" 12211 12471 '231. ., .2 ., FAA 4S " " .. " " .. " " CouI1e Apple % " " " " " 58.7 II II II

Fine Aggregate. % (12497 for BI.Sn " 40 40 40 .. 361 10.9 15.1 23.5

Fine Aggregate, % )1.1 26.9 IS.S 1'2164 for BI-Bl) MinenlJ Filler, % I , , , I l.l l l l Design Asphalt l .l ••• l .' l.O , .. l .l l .l l .l l .O

Content, %

Sitvt Sill: (mm) Ptn:ent Pwint

)2.S 100 100 100 100 100 100 100 100 100

' .l "' .. 91.0 91.0 9\.0 "' .. " .0 91 .2 911 91.2

4.75 60.' 61.4 61.4 61.4 60.' 74.1 '" 62.1 62.1

2.36 4lJ 40J 37.0 ]1.6 41.1 31.9 11.4 38.1 ]9.5

US 25.7 30.5 23.7 24.4 29.7 ". 25.4 26.1 27.5

0.' 15.9 19.1 16.& 16.2 IS.4 10.7 17.1 17.4 18.0

OJ ' .l 10.9 '" 11.1 OJ 1.7 11.4 11.3 10.9

O.IS l .l , .. I.' 1.4 l.1 l.l 1.4 71 , .. 0.075 lJ ., ••• ' .1 3.7 3.' ' .2 '.2 <.0

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Table 3 Mix Design Results

FAA Air Void(%) AC(%) MTSG VMA(%) VFA(%) OIA

12111 " 4.0 S.l 2.472 IH 13.6 0.'

"497 19 4.0 4.' 2.481 12.4' ,1.1 1.1

'''64 " 4.0 l .' 2.m 15.0 73.4 1.0

1i221 1 .. 4.0 l .O 2.m lS.3 73.3 1.0

12471 41 4.4 ' ' .9 2.474 li.I 7l 0.2

NlJI4 44 4.6' 7.l 2.402 18.6 7l 0.'

BI " ' .0 l .l 14" IS.G 73.4 0.9

Bl " ' .0 l.l 2.451 14.1' n .7 0.9

" 4J ' .0 l .O 2.441 14.4' l2.l 0.'

• These are items not meeting Superpave requirements

"" 90

'" • 70 • ~ '" • • '" • , • '" , • • ~

70

70

, , "

, , " (Sir/t Size) ....

Figure I Mix Design Gradations

""'*-2311

""*""2497 -+-21601

-+-221 1 -+-2478

-+-2314

-" -B2

..... "

12

IJ

AC%vs.FAA

50 I

I ! • ' 2164_,G vel Sand + #2211 Dolomite

Sa"" • W8 Slag Sand

'" 45 _ .. Ba~ /"

• BJ 12314 Stone Si po

-1-----' -.12~97 NJruraI Sand

40

, I

;

35 3 4 , 6 7 6

Figure 2 Asphalt Content vmus FAA

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VMAvsFAA

50

'12~ ..... "'" • • "'" I • ~478 Slag Sand I , .B1 ! , • . ...,. -, B2 • I I I ." 12314 SlOne S

45

- - --.12491 Natu 50'"

35 - -- -

30 10 12 14 " " 20

figure 3 VMA versus FAA

15

AC %vsVMA

2 0

9 --_. +12314

I 8 t ' r'i'--I - ._--

16 .. ---.

" l1t2311 f- . _ _ .... . - .12164 - -_. , , B1 +.3 B2

" . --

13 - - ----- ... __ .. _- - .-.

• 12497 12 l- .. - -

11

10 3 • 5 6 I 6

Figure 4 Asphalt Content versus VMA

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Figw-e lPURWheeI

~ ,-----,------r, ,--------i-----J/~_h.>< " r

" M!':...-j~--·

o - -o ,,.,.,

No. of Pllset ", ..

17

~-+-5· ... "-1 110- 5.04% Ai' vms

: )(- 1.08'110 Ai' Vo«II l

Figure 6 PUR Wheel Test Results for Mixture 1#2497, Dry at 60 Degrees Centigrade

" " E " E " f " . • 0 • , • •

• ,,.,., No. of PUJI'

, ...

r ;- 5~ 5J"AifVoidl l -II 6.66'10 ..... Vaicit I

.... 5.13'11. Ai' Voi:It I .M-7.on. ... ~J

Figure 7 PURWhetJ Test Results for Mixlwt #2] II, Dry at 60 Degrees Centigrade

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AC % vs. PassH@6%AV

140000 I ."'~ 1m"

120000 . -- ..

100000

---- I- .. , .82

. - - -83.12311 (St ioo ''''I '2211 (DoIom~

40000 . ",

, I • ~ I I ~2J14 I''''''' " - - - - . -- .---20000

•• 2497 ( ",,'" ,,, ) • ~4781'" "" ) o 3 4 5 , 7 8

Fi~ g Asphalt Content versus Passes at Six Percent Air Voids

,

19

140000 ,---,------r----,-----,------,

I I 120000

100000

• '2164 ~ Sand

---;--------+, ---j-------- -

-- - --f--, , -

---f------f----.. , •• 2

j 60000 t------+ -- ----.63 _12311 Stone Sand

. 22 I Dolomite .noo f-----~-----1--~.-~.--_+----

21lOOO -, 1-

• 1231~ Stone S ~

-----1--- 1- -• #20497 Nalu Saoo

• ~478 Slag Said O~--+---+---~--~~~~

10 12 16 18 20 v.,

Figure 9 VMA versus Passes at Six Percent Air Voids

,,.,. ,.,..

• .... , • j .... " . .,

•

-

-

--_._--

.

•• "

20

12~64 aw.ed QIlIvel • , sand

- . _.-

-. 82. al

_. . .83 12311 .lone sand

.~, - - .... .~ 141ton1 unci

11111uta1 unci

" ,. .,2411111g

50

Figure 10 FAA versus Passes at Six Percent Air Voids

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Purwhlll P,rfonnince (Ory/Hot)Y$. Film Thlc kn ••

...... - "1

• 91(1211W)

I 110CK0l 1---~ I ~ ~ .

t- ---• ~

:. 11l.2(Bl) • __ +--__ 1~) _ ___ _

:10.1(12311);" 10.3183)

~ IO.11r2211) - - -- -- _ .. - ----;--- ---- --• 16.3(123 4)

mo -

I 01.-_ -. _ _ _ • 1.6(12~97} .14,2(12418)

---'---+------o , • 6 • " " .. " " Film Thickness (micrOllll

Figurt II Film Thickness