Evaluation of Microaggregates by Smith Triaxial Test

E G O N S TONS and G I L L E S G. R E N A U L T , Respectively, Assistant Professor of T r a n s ­portation Engineering, Department of C i v i l Engineering, Massachusetts Institute of Technology and Special Projects Engineer, The Warnock Hersey Company, Ltd .

Bituminous concrete mixes usually consist of binder, coarse aggregate, fine aggregate, and mineral f i l l e r . Because the function of the f i l l er i s not just to f i l l the voids but to a great extent also dictate the phys i ­ca l and other character i s t i c s of the mix, a more descriptive and p r e ­c ise name for a fU ler i s "microaggregate. "

The study included five microaggregates: limestone, hydrated l ime, hydrite (c lay) , asbestos, and quartz. The quartz microaggregate was further tested using three different gradations.

A l l microaggregates were incorporated in a standard Massachusetts dense-graded bituminous concrete mix. The coarse aggregate rhyolite and the fine aggregate (sand) were kept constant in a l l experiments. V a r ­ious asphalt contents were used.

The Smith T r i a x i a l ce l l was adopted in the strength evaluation of a l l mixes . The specimens were compacted in layers using akneading compactor to s i m ­ulate "ultimate" densification under t ra f f i c .

A l l mixes except those containing asbestos had relatively low void contents after the kneading compaction. These cases were accompanied by low strength. Mixes containing three quartz microaggregates showedhigher strength when the gradation of the quartz was close to F u l l e r ' s maximum density curve .

• M I N E R A L A G G R E G A T E S used in bituminous concrete mixes are often subdivided i n ­to three groups: coarse aggregate, fine aggregate, and mineral f i l l e r . In this paper the term "mineral f i l l e r " has been changed to microaggregate which the authors feel i s more descriptive for the following reasons:

1. A l l mineral part ic les used in bituminous concrete are usually pieces of rock dif­fering in s ize .

2. They a l l have a positive and active role in regulating the engineering properties of bituminous concrete. The term "f i l ler" suggests passivity.

3. The maximum diameter of minera l f i l l e r part ic les i s generally set at 74 u (passing 200 sieve) . Because part ic les smal l er than 100 ^ cannot be seen by the a v e r ­age eye without magnification (microscope), the term "micro" i s suitable.

Interest and r e s e a r c h in expansion of knowledge in bituminous concrete mater ia l s and design have been growing rapidly during the last five years . T h i s i s necessary not only because of present traff ic densities and loads but also to gather data for un­avoidable future developments. Because bituminous concrete is a composite mater ia l , changes in various components produce different engineering properties . Although r e ­s e a r c h work has been done on effects of coarse aggregate, fine aggregate, and m i c r o -aggregate, the function and potentialities of the m i c r o - s i z e s are not well defined. Therefore , the work described m this paper was p r i m a r i l y concentrated in the a r e a of microaggregates.

The purpose of this study, which i s s t i l l continuing, i s to determine the differences in Theological behavior of a given compacted mix in which the p r i m a r y variable is the microaggregate.

1̂ 8

49

The f i r s t r e s e a r c h phase described in this paper involves seven microaggregates in a standard Massachusetts C l a s s I mix. Kneading compaction simulating long-term traff ic densification on the road was employed to prepare tr iax ia l specimens at differing asphalt contents. T e s t resul ts include physical data on various components and mixes , and ultimate strength calculations using the logarithmic sp ira l method.

P R O P E R T I E S O F M I C R O A G G R E G A T E S E M P H A S I Z E D IN T E C H N I C A L L I T E R A T U T ? P

Considerable effort has been e^^ended in the past to determine the influential prop­ert ies of microaggregates in a bituminous concrete mix. The overwhelming majority of the investigations have been concerned with phys ical character i s t i c s of the par t i c l e s . The following factors have drawn attention:

1. P a r t i c l e - s i z e distribution; 2. Part i c l e shape; 3. Part ic le surface texture; 4. Unit surface area; and 5. Part ic le surface chemistry

Other factors , such as bulk density, fract ional voids, pore diameter, and p e r m e ­ability of packed f i l l e r s to f luids, have also been mentioned.

P a r t i c l e - S i z e Distribution

Although gradation or par t i c l e - s i ze distribution i s usually watched in selection of coarse and fine aggregate fract ions , there are very few agencies that specify the r e ­quired gradation of microaggregates. R e s e a r c h resul ts indicate that one-s ized m a ­ter ia l , just passing the 74-^ sieve i s infer ior to f iner and well-graded part i c l e s , even close to the colloidal s ize range (4, 11). The gradation might be especial ly important when the amount of fine and microaggregate exceeds a certain l imit where the coarse aggregate part ic les become embedded in the f iner aggregate and asphalt mass (9).

Part ic le Shape and Surface Texture

The angularity of aggregates under certain conditions affects the res istance to de­formation of a compacted mix (2, 8) . The effect of particle shape of microaggregates on the stability of mix appears to be greatest when the fine fraction (including m i c r o -aggregates) i s between 40 and 60 percent of the total weight of the mix (10).

Although the surface texture of l arger part ic les (rock and sand) have some physical significance (30) it i s not known what role the texture plays m part ic les below 74-fx s ize . Roughness of part ic les indicates larger surface a r e a of the aggregate. Tens i le strength might be superior with mixes containing aggregates with rough surfaces (40).

Surface A r e a and Chemistry

Surface a r e a per given weight (or volume) of microaggregate i s often assumed as one of the most important phys ical properties influencing the mechanical behavior of a mix (8). Increased surface a r e a can result in increased contact a r e a s between part ic les thus influencing the strength character i s t i c s of bituminous concrete (30).

Mineral aggregates used in bituminous concrete are not inert part ic les but chemi­cal ly active substances. T h e i r composition var i e s widely presenting a problem of s u r ­face chemistry different for every rock used in a mix. Microaggregates have large surface a r e a per unit mass and therefore their performance in a mix depends greatly on their chemical composition. Adhesion of asphalt and selective adsorption of some components from the asphalt appear to be determined to a great degree by the surface chemistry of the microaggregate (48).

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Miscellaneous Propert ies

Bes ides the considerations of bas ic properties of each microaggregate part ic le , attempts have been made to character ize their behavior in m a s s . F o r instance, the so-ca l led "fractional void" content princQ)le has been used to predict optimum binder content (6).

Several attenqits have been made to study aggregates and their effect on mix proper­ties without singling out any part icular aspect {5, 12, 13, 14). The most common con­clusion i s that various microaggregates can impart greatly different propert ies to the mix.

M I C R O A G G R E G A T E S U S E D I N S T U D Y

Altogether seven variet ies of microaggregates were used In this init ial study. Three of them were ground quartz, differing only in gradation. One was weathered quartz ob­tained f r o m sand, s i m i l a r in gradation to one of the fresh ly crushed quartz. In addition chrysoti le asbestos, hydrated l ime , and limestone m i c r o a ^ e g a t e s were tested.

F r e s h l y Ground Quartz

Ottawa sand was used to obtain coarse , medium, and fine gradations of this m i c r o -aggregate. Quartz i s a hard minera l composed of s i l i c a and oxygen. Its crys ta l s are usually pr i smat ic and the part ic les hexagonal.

A l l of the coarse ly ground quartz microaggregate passed sieve 200 and was retained on sieve 325. The medium fine quartz had a gradation between 5 and 74 n, and the finely ground quartz contained part ic les down to colloidal s i ze . The gradation curves are shown in Figure 1 and specif ic gravity and surface a r e a data are given in Table 1.

T A B L E 1

B A S I C D A T A ON V A R I O U S M I C R O A G G R E G A T E S

Microaggregate Code Let ter

Specific Gravity

Surface A r e a

(cmVg)

7M asbestos A 2.640 7,250 (Canadian chrysotile)

Limestone L S 2.730 2.500 Coarse quartz C Q 2.649 1,605 Medium gradation quartz MQ 2.648 1,670 Fine gradation quartz F Q 2.647 4,580 Sand dust (quartz) SQ 2.651 1,450 Hydrated l ime H L 2.617 1,810 7M asbestos, 50 percent Limestone, 50 percent A L S 4,875 7M asbestos, 50 percent Limestone, 50 percent A L S 4,875

Quartz Obtained from Sand Dust

The sand microaggregate was s i m i l a r in gradation to the medium gradation quartz . Under the microscope the part ic les appeared to have lost some of their sharp edges and looked rounded as compared to the crushed part ic les .

Limestone, Hydrated L i m e and Asbestos

The limestone microaggregate was a commerc ia l grade, containing p r i m a r i l y CaCOs (high calc ium limestone). The hydrated l ime used in this e:q)eriment was a commercia l ly available high calc ium l ime, chemical ly consisting p r i m a r i l y of Ca(OH)2.

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1 0 0

L E G E N D

C O - C o a r s e Quor tz

M Q - M e d i u m G r a d o l i o n Quor tz

o / / / . ' / ' S Q - S a n d F i n e s ( Q u o r t z )

- / / ^ L S •* ' / L S - L i n o e s t o n e

m ' / ' j/ H L - H y d r a t e d L ime

2 I / / S Q ~ ^ « / F Q - F i n e G r a d a t i o n Q u a r t z <r-2 UJ o IX UJ

0 0 0 5 0 01 0 0 5 0 1 P A R T I C L E S I Z E , MM

Figure 1. Particle-size distribution of various microaggregates as indicated by-hydrometer testj asbestos not included.

The asbestos was a fibrous-type mater ia l consisting of hydrous magnesium s i l icate . Because of its f ibrous structure it i s also cal led "chrysotile. " The diameter of each f ibre var ied between 7.06 x 10"* and 11.8 x 10"* in .

In a s t r i c t sense asbestos i s not a typical microaggregate and its par t i c l e - s i ze d i s ­tribution cannot be obtained by conventional methods. It i s usually c lass i f i ed according to the Quebec Standard T e s t (Canadian Chrysoti le Asbestos Class i f icat ion) . Pertinent data on asbestos are given in Table 1.

Gradations of the various microaggregates (except asbestos) were obtained by the hydrometer test ( A S T M Designation D422-54T) after each mater ia l was passed through a 200 sieve. The surface a r e a was determined by the a i r permeabil i ty method using Bowen apparatus.

Other Bituminous Concrete Components

Bes ides 5 percent of various microaggregates, the Massachusetts C l a s s I Top Mix had the following components (for aggregate gradation, see F i g . 2):

Coarse Aggregate. —Massachusetts rhyolite, a f ine-grained hard crushed rock with low absorption character i s t i c s and apparent specif ic gravity of 2.66.

Fine Aggregate. —Clean quartz sand with a s m a l l amount of mica , apparent specif ic gravity of 2 .63.

Asphalt. —85-100 penetration, 5. 5 to 7. 5 percent by weight of the total mix. M a s s a ­chusetts specification ca l l s for asphalt range between 6.0 and 7.0 percent.

T R I A X I A L E V A L U A T I O N T E S T

The Smith tr iax ia l test was selected for the comparitive strength tests described in this paper. T h i s i s the f i r s t step that wi l l be followed by tension studies using various temperatures and t imes of loading. The p r i m a r y reasons for selecting this test were

1. The test gives comparative angle of internal fr ict ion and cohesion values that can be used for shear strength calculations.

52

100

o UJ

80 4 0 20 SIEVE NUMBER

3/8 1/2

Figure 2. Average gradation curve of Massachusetts Class I Top Mix, used as basis i n investigation.

2. The test i s relatively simple and the apparatus was readily available.

The t r iax ia l ce l l (see F i g . 3) is de­scr ibed in detail in references (33) and (38)- "

The specimens for t r iax ia l testing were prepared one at a t ime. Fol low­ing is a brief outline of the procedures and the reasons for using them:

Heating and Mixing

Aggregates were f i r s t weighed out combining various sieved particle s i z e s . The total weight of aggregates for any specimen was kept constant while the asphalt content was var ied .

The aggregates and the 85-100 pene­tration asphalt were heated to 300 F and then combined by a mechanical m i x e r . The mixing time was kept at a constant 2 min.

I

Figure 3. Loading machine with Smith t r i a x i a l test c e l l in place.

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Compaction

The Smith t r iax ia l method cal ls for static compaction of the 8- in . specimens. To measure what happens to a Massachusetts C l a s s I Top Mix under heavy compaction ef­fort, as it might be experienced during the life of the pavement that is rol led over by frequent and heavy traff ic loads, the kneading compaction was chosen (1̂ , 30, 31).

The method of compaction for Hveem stability specimens was taken as a guide. E a c h 8- in. specimen was made in six layers of about equal thickness. The unit compaction foot pressure was taken as 500 p s i . The energy of densification used for Hveem's specimens amount to 150 applications of 500 psi pre s sure , and for the tr iax ia l spec i ­mens a total of 450 tampings at 500 ps i were used (because the tr iaxia l specimens have about three times as large a mass ) .

To keep the temperature of the specimen above 240 F during compaction, the molds and the tamping foot were heated (see F i g . 4). Thus by controlling the temperature, using 500 p s i unit pres sure , and layers of about 1 1 / 2 in . in thickness, homogeneous, well-compacted specimens were obtained.

After molding, a l l specimens were cooled, taken out of the mold, and placed in a special holder (to keep their shape) for 48 hr of curing.

Test ing

After curing at 77 F ( i 2), the specimens were placed in the tr iax ia l ce l l and sub­jected to a load. The equipment and procedures for the Smith tr iax ia l ce l l are de­scribed in references (33) and (38). B r i e f l y , this test i s a closed system tr iax ia l test where the vert ica l loads are applied by increments. The vert ica l pressure is plotted against the corresponding horizontal pressure and a tangent i s drawn to the curve. F r o m these data the angle of internal fr ict ion and cohesion i s obtained (see F i g . 5).

F r o m the tr iax ia l test, values of fr ict ion and cohesion are obtained, both of which contribute to the strength of bituminous concrete under load. To obtain comparative values of shear strength for the various mixes , the logarithmic s p i r a l method and u l ­timate strength concept were used (48). The lengthy calculations were done with an electronic computer.

R E S U L T S

The results of this investigation are summarized in various tables and f igures. These include mix data, tr iaxia l test measurements, and ultimate strength c a l ­culations. Following i s a discussion of various observations and measurements on the microaggregates and mixes in which they were incorporated:

Observations During Mixing and Compac­tion

Although the v isual appearance of the various compositions during and after mix­ing was not the same, the differences be­tween the mixes containing granular m i c r o -aggregates (at a given asphalt content) were generally minor. The mixes containing l ime seemed to have a lower m a s s v i s ­cosity after mixing. Asbestos mixes looked noticeably dryer than the rest , especial ly at 5. 5 percent binder content, when it be­came difficult to coat the coarse aggregate

Figure h. Manual kneading compactor with part ic les . Thi s was one of the reasons why assembled t r i a x i a l test mold and acces- asphalt contents below 5. 5 percent were not

sories for temperature control. used in the test s e r i e s .

54

400

300

250

< O

Q. a. <

200 h

I 50

1 1 1 1 /

-

/ /

/ //

// / / / / / /

- / / / / / / / / / / 1 1 1 1

20 40 L A T E R A L P R E S S U R E , PSi

60 80

Figure 5 . An example of t r i a x i a l test curve froir. which cohesion and f r i c t i on values are obtained.

At asphalt contents approaching 7. 5 percent a l l mixes with granular microaggregates showed flushing of the asphalt on the top of the surface . The 50-50 mixture of asbestos and limestone ( A L S ) showed only traces of flushing while the 5 percent asbestos mixes had none.

Void Contents of the Mixes

Amount of voids at various asphalt contents i s shown in Figure 6. The asbestos (A) has the highest void content with other curves being concentrated in a relatively narrow band. Because the bas ic mix proportions and compaction efforts are s i m i l a r in a l l cases , the indications are that mixes containing just asbestos (5 percent) as m i c r o -aggregate would require more compactive effort to obtain a certain density at a given asphalt content. A s soon as 50 percent of the asbestos was replaced by limestone, the void content was lowered close to other granular microaggregates. The straight l i m e ­stone mixes had very low voids.

The void volume in most mixes at asphalt contents of 6 percent or higher were near the 1 percent mark or below.

55

L E G E N D CO - C o a r s e Quar tz M Q - M e d i u m Grodotion Quortz S Q - S o n d F i n e s (Quor tz ) L S - L imes tone A - A s b e s t o s

A L S - A s b e s t o s o n d L i m e s t o n e ( 5 0 - 5 0 ) H L - H y d r o t e d L i m e F O - F i n e G r o d o t i o n Q u a r t z

6 5 7 0

A S P H A L T C O N T E N T , IN P E R C E N T

Figure 6. Voids vs asphalt content for various mixes.

o

< z

O

o z <

L E G E N D CO - C o o r s e Quartz M Q - M e d i u m Gradation Quartz SQ - S a n d F i n e s ( Q u a r t z ) L S - L imestone H L - H y d r a t e d L ime A - A s b e s t o s

A L S - A s b e s t o s and L imestone ( 5 0 - 5 0 1 FQ - F i n e G r a d a t i o n Q u a r t z

A S P H A L T C O N T E N T , I N P E R C E N T

Figure 7. Asphalt content vs angle of internal friction for a given mix with 5 percent of various microaggregates.

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Angle of Internal Fr i c t ion

The curves for angle of internal fr ict ion are shown in Figure 7. A s expected, the fr ict ional resistance of a mix decreases with increased asphalt content. L i m e ( H L ) , fine ground quartz ( F Q ) , and limestone (LS) microaggregates in general show the low­est fr ict ion angles. Asbestos i s character ized by a relatively gradual slope as com­pared to the others. F r o m these data it appears that f ine-graded granular m i c r o -aggregates with relatively large surface areas act more as viscosity boosters of the asphalt and do not help much to hold the angle of internal fr ict ion with increasing asphalt contents (compare H L , F Q , and L S in F i g s . 1 and 6). Microaggregates con­taining more coarse part ic les (close to 74 fi) appear to make the mix l e s s sensitive to asphalt changes. An exception here i s asbestos which has a large surface a r e a (and a f ibre-shaped part ic le ) . A l l asbestos mixes looked d r i e r during mixing and compaction as compared to the others. T h i s may be due both to the larger surface a r e a and to greater chemical affinity of asphalt for the asbestos surfaces .

The limestone microaggregate mix had a very high angle of internal fr ict ion at 5. 5 percent asphalt content. When a combination of asbestos and limestone was used, the frict ion angle s t i l l remained high and above that of pure asbestos at this low asphalt content. It decreased at a more rapid rate with higher asphalt percentages.

Cohesion

The cohesion of the mixes varied widely at the 5. 5 percent asphalt (see F i g . 8). At 6 percent and higher asphalt contents, the cohesion was close to or below 10 ps i except for asbestos (A) and asbestos-l imestone ( A L S ) .

3 0 L E G E N D CQ - Coarse Quar tz

MQ - Medium Gradation Quartz S Q - S a n d F i n e s (Quar tz ) L S - L i m e s t o n e A - A s b e s t o s

A L S - A s b e s t o s and L i m e s t o n e ( 5 0 - 5 0 ) H L - H y d r a t e d L i m e F Q - F i n e Grada t ion Q u a r t z

2 0

(/5

UJ X

o

6 5 7 0 A S P H A L T C O N T E N T , IN P E R C E N T

7 5

Figure 8. Asphalt content vs cohesion for a given mix with 5 percent of various micro-aggregates.

57

The high cohesive strength of the asbestos' mixes cannot be fully explained with the facts available. The f ibrous part i c l e s may act as a reinforcement in the asphalt. The affinity of asphalt for asbestos can resul t in excellent bonding, thus adding to the r e ­inforcement.

Satisfactory Mixes

Although the ultimate strength analys i s was used in this paper to combine the angle of internal fr ict ion and cohesion there are no agreed on c r i t e r i a available to define a sat isfactory mix by this method. To get another indication of which mixes are s a t i s ­factory for highway use, a chart f rom the Smith tr iax ia l design procedure in reference (33) was used. The resul ts of this comparison are given in Table 2. According to these c r i t e r i a , the asbestos mix i s satisfactory for a l l asphalt contents tested; the asbestos limestone mix for a l l but the 7. 5 percent; while the re s t of the mixes for 5. 5 o r 6.0 percent only.

T A B L E 2

S A T I S F A C T O R Y M I X E S USING A N G L E O F I N T E R N A L F R I C T I O N AND C O H E S I O N AS O U T L I N E D IN (33 P . 122)

Percent of Asphalt Microaggregate 5.5 6.0 6 .5 7.0 7 .5

Asbestos X X X X X Asbestos-l imestone X X X X Coarse quartz X X Medium fine quartz X X Sand dust (quartz) X X Limestone X L i m e X Fine groimd quartz X

Ultimate Strength of Var ious Mixes

The ultimate strength curves obtained f rom combining cohesion and frict ional strength are given in Figure 9. In this comparison most microaggregates had high strength at 5. 5 percent asphalt. T h i s strength, decreased as the asphalt content i n ­creased , but with different r ^ i d i t y . With limestone ( L S ) , l ime ( H L ) , and fine quartz ( F Q ) mixes the slopes of the curves are very steep dropping to 100-psi strength at 6. 5 percent asphalt content.

The l imestone-asbestos ( A L S ) mixes s t i l l had relatively high ultimate strength as compared with the others. Only at 7 .5 percent binder content, the curve joins other microaggregates. T h i s combination has reduced the slope of the curve and ra i sed the strengtn of the mix considerably.

Much of what has been sa id about the factors affecting the angle of internal fr ict ion and cohesion values can also be applied to the ultimate strength curves . The mixes containing finely graded microaggregates lose their strength very rapidly with increases in asphalt contents f rom 5. 5 to 7. 5 percent. The mixes containing asbestos also lose their strength but at a s lower rate. In addition, their initial strength at 5. 5 percent binder content i s relatively high.

Mixes containing just asbestos microaggregate had generally a higher void content (see F i g . 5). Although low voids are often associated with low strength of a compacted mix, the resu l t s show that strength i s not ruled by the volume of voids alone. F o r i n ­stance, asbestos (A) and limestone (LS) mixes at 5. 5 and 7. 5 percent asphalt content.

58

3000

a. I

C9

ill CE

CO

UJ

<

2 0 0 0

LEGEND- CO - Coarse Quartz MO - Medium Gradation Quartz SO - Sand Fines (Quartz) LS - Limestone A - Asbestos

ALS-Asbestos and Limestone (50-50) HL - Hydrated Lime FQ - Fine Gradation Quartz

V . . ^ C Q ^ V S Q : ^ >

MQ V

l O O O h

6 0 6 5 7 0 ASPHALT CONTENT, IN PERCENT

Figure 9. Ultimate strength of mixes containing various microaggregates and at various asphalt contents.

respectively, show reasonably high strength at about 0.5 percent'void content.

Poss ible T r e n d s of Ultimate Strength at Asphalt Contents Below 5. 5 Percent

The asphalt contents used in this investigation were taken f rom the specification, Massachusetts C l a s s I Top mix, extending the range one-half a percent on each side of the 6 to 7 percent l i m i t s . The aggregate in asbestos mixes at 5. 5 percent binder con­tent were difficult to coat and the mix looked dry . Also , the ultimate void content was over 5 percent. Therefore , it i s very doubtful that this mix could be compacted wel l on a road at 5. 5 percent o r lower.

The other mixes , especial ly the l ime and limestone, probably could be mixed and compacted by a kneading compactor at lower asphalt contents than reported in this paper. It i s not known whether on the road these mixes can be properly compacted to a durable pavement surface at asphalt contents below 5. 5 percent.

If the testing would be continued, however, at lower asphalt contents, a point of maximum ultimate strength would be reached.

P r a c t i c a l Implications

If the compaction technique and strength measurements in this work are close to actuality, the ultimate strength curves reveal a significant finding as f a r as the bas ic mix used in this study i s concerned (Massachusetts C l a s s I Top mix) . A l l mixes con­taining various microaggregates, except those of asbestos show very low ultimate strenght at 7 percent asphalt content, which i s allowed according to the specif ications. T h i s explains some case s of unstable pavements found in Massachusetts . The present specif ied maximum asphalt content of 7 percent i s too high for the mix with granular

59

microaggregates s i m i l a r to those used in the tests. Under heavy traff ic with the p a s ­sing of time such mixes can lose their strength.

The desirable maximum asphalt content v a r i e s with different microaggregates. F o r coarse and medium gradation quartz this l imi t may be 6. 5 percent or slightly below. F o r limestone, finely graded quartz, and hydrated l ime the maximum asphalt content should not exceed 6 percent. If asbestos i s added to the limestone microaggregate (50-50 mixture), the present 7 percent l imit seems reasonable. F o r straight asbestos microaggregate the maximum "allowable" asphalt content i s beyond the 7. 5 percent mark.

C O N C L U S I O N S

1. Kneading compaction, simulating ultimate densification of the mix by traff ic on the road, produced low void contents at the minimum specified (6 percent) amount of asphalt with a l l mixes containing granular microaggregates. Mixes with f ibrous (asbes­tos) microaggregate averaged a higher volume of voids at the asphalt content investiga­ted.

2. The angle of internal fr ict ion of various mixes did not differ greatly at the lowest asphalt content (5. 5 percent) . The decline of this angle with increasing asphalt content was l e s s rapid with the mix containing straight asbestos microaggregate.

3. The cohesion as measured by the Smith t r iax ia l test differed widely at the low (5. 5 percent) asphalt content. It decreased rapidly with increas ing asphalt content in the case of granular microaggregates but remained relatively high with the fibrous (asbestos) microaggregate mix.

4. The ultimate shear strength values (using logarithmic s p i r a l analysis) were high with a l l mixes at 5. 5 asphalt content. With increas ing asphalt contents, the mixes con­taining relatively f ine-graded microaggregates showed sharper decline in strength than mixes having coarse , more uniform microaggregate part ic les . The 5 percent asbestos mix showed the highest strength leaving the asbestos-l imestone mix in the second place.

5. It i s suggested that Massachusetts C l a s s I Top mix used for heavily traveled roads should be either redesigned or the maximum allowable asphalt content lowered to 6. 5 or even 6.0 percent when granular microaggregates (such as tested in this ser ies ) are used. When asbestos was added replacing 2 72 percent of the limestone microag­gregate, the mix had stUl f a i r ly high strength at asphalt content of 7.0 percent.

S imi lar ly , when the whole 5 percent of the microaggregate was asbestos, the u l t i ­mate strengtii even at 7. 5 percent of asphalt was relatively high.

Research work i s in progress to evaluate the various mixes at temperatures between -20 and 140 F and loading t imes of 0.001 sec to 1,000 h r .

A C K N O W L E D G M E N T S

The work described in this paper was initiated by GUles G . Renault and supported f r o m funds of the Joint Highway R e s e a r c h Projec t established at the Massachusetts I n ­stitute of Technology by a grant f rom the Massachusetts Department of Public Works for r e s e a r c h in the f ield of highway engineering.

Additional support for expansion of the project on microaggregates was given by the Johns-Manvil le Company. The help of numerous individuals in the Transportation M a ­ter ia l s R e s e a r c h Laboratory and other laboratories at M. I . T . i s gratefully acknowl­edged.

R E F E R E N C E S

1. Minor, C . E . , "Correlat ion of Hveem StabUometer and Cohesiometer T e s t R e ­sults and Kneading Compactor Densit ies with Service Records of Bituminous Pavements. " A S T M Si)ecial Tech . Publ . 252, Bituminous Paving Mater ia ls .

2. K e y s e r , J . H . , 'Influence des Proprietes des Melanges s u r le Comportement de Pavages Asphaltlques. " Shell Asphalt Paving Conference, Montreal (1959).

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60

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Appendix Basic Data Used to Obtain the Various Figures and Tables in Paper

Asphalt Specific Bulk Maximum Percent Friction Cohesion Content Gravity Density Theoret. Voids Angle (psi)

i$) Aggregate Specimen Density Voids (psi)

(a) Hydrated lime mixes (HL)

5.5 2.617 2.351 2.410 2.49 38.8 24.9 5.5 2.617 2.360 2.410 2,08 7.8 30.4 6.0 2.617 2.368 2.389 0.88 22.4 9.9 6.5 2.617 2.355 2.373 0. 72 18.2 4.7 6.5 2.617 2.360 2.374 0.35 16.6 4.3 7.0 2.617 2.342 2,355 0.65 12.0 3.5 7.5 2.617 2.352 2.351 0 5.2 1.4

(b) Limestone mixes (LS)

5. 5 2.653 2.425 2.435 0,41 51.5 9,5 5.5 2.653 2. 421 2.434 0.52 45.2 11,7 6.5 2.653 2.378 2. 399 0.41 18.2 5.3 6.5 2.653 2. 401 2. 398 0 16.0 7.4 7.5 2. 653 2.384 2.380 0 10,8 4.1

(c) Asbestos mixes (A)

5.5 2. 641 2. 298 2. 427 5.29 40.7 28,4 6.0 2.641 2. 307 2. 407 4.15 36.2 25.1 6.5 2. 641 2.313 2.390 3.22 37,2 22.3 7.0 2.641 2. 337 2,372 1.48 31,2 17,7 7.0 2. 641 2.348 2,373 1.05 36.4 21,5 7.5 2.641 2.351 2,356 0,21 30.2 16,0

(d) Asbestos-limestone mixes (ALS)

5.5 2. 646 2. 365 2.430 2,65 48,6 18,0 5.5 2.646 2. 335 2.431 3.96 42,0 20,7 6.5 2. 646 2.381 2,395 0.56 28,4 16,8 6.5 2. 646 2. 380 2,395 0.58 37,0 22,0 7.5 2. 646 2.354 2,360 0.25 22,0 3,2 7.5 2. 646 2.357 2. 360 0,10 17,0 4,4

(e) Medium graded quartz (MQ)

5.5 2.648 2. 365 2,433 2. 79 41,8 17,9 6.0 2.648 2.373 2. 413 1,66 40.4 9.3 6,5 2, 648 2.375 2.396 0.88 35.0 5.3 7.0 2.648 2.371 2.378 0.30 28.6 2.1 7.5 2.648 2.363 2, 363 0 30.0 1.4

62

63

Asphalt Content

Specific Gravity

Aggregate

Bulk Maximum percent Density "nieoret. ^^.^^

Specimen Density Friction Angle

Cohesion (psi)

(f) Coarse graded quartz (CQ)

5.5 6.5 7.0

2.650 2.650 2.650

2.365 2.434 2.84 2.383 2.391 0.59 2.369 2.379 0.42

40.6 36.0 20.6

30.5 7.7 4.3

(g) Fine graded quartz ( F Q )

5.5 6.0 6.5 7.0

2.648 2.648 2.648 2.648

2.414 2.434 0.81 2.397 2.413 0.66 2.385 2.395 0.42 2.367 2.378 0.46

39.4 33.0 19.4 7.5

20.0 10.0 7.3 5.2

(h) Sand dust (quartz) (SQ)

5.5 6.5 7.0

2.651 2.651 2.651

2.336 2.434 4.04 2.395 2.398 0.13 2.375 2.381 0.25

40.2 36.0 22.9

22.1 6.9 1.0