Shear Susceptibility of Asphalts in Relation to Pavement Performance

8/9/2019 Shear Susceptibility of Asphalts in Relation to Pavement Performance

1/27

SHEAR SUSCEPTIBILITY OF ASPHALTS IN RELATIONTO PAVEMENT PERFORMANCE

PRITHVI S. KANDHAL, LEO D. SANDVIG, andMONROE E. WENGER

ABSTRACT

The rate of change of viscosity with rate of shear is referred to asshear susceptibility and, like initial viscosity, can be considered as anintrinsic property of the asphalts. Many researchers have indicatedthat absolute viscosity alone can not specify the complete rheologicalbehavior of paving asphalts, and other parameters like shear suscepti-bility and/or temperature susceptibility are needed. Hence, it has beenthe intent to study the shear susceptibility parameter in relation to theperformance of the in-service pavements.

Six viscosity graded asphalts from different sources were usedduring 1964 in the construction of wearing course on Legislative Route219 in Clinton County, Pennsylvania. In the construction of these sixtest pavements, the only significant variable is the asphalt type. Teststo determine the properties of the original asphalts as well as the as-phalt recovered from time to time during the last 8 years, have beenconducted. Pavement performance evaluation has been carried out by ateam of five engineers.

Very good correlation was observed between aging indices andshear susceptibility values of the aged asphalts. Aging index-shearsusceptibility relationship appears to determine the pavement perform-ance, Control of gain in shear susceptibility and aging index seems tobe a necessary specification requirement for paving asphalts. It is in-dicated that the ratio of Asphaltenes to Nitrogen Bases affects the shearsusceptibility of asphalts.

INTRODUCTIONSeveral factors are involved in the performance and durability of as-phalt pavements. Studies relating to more durable pavements mustconsider the engineering properties of the pavement structure as wellas the physical and chemical properties of the asphalt component of thepavement, be it only the minor component volumetrically. The func-tions of the asphalt binder film are to effect cohesion of the aggregatecomponents, to waterproof the pavement, and to effectively transmit theenergy resulting from transient or sustained loads, without unduly de-forming the structure or fracturing the asphalt film (1).

Recent research (l), (2), (3), (4), (5),(6) has indicated that absoluteviscosity alone can not specify the complete rheological behavior of paving grade asphalts, and other parameters like shear susceptibilityand/or temperature susceptibility are needed. The rate of change of

viscosity with rate of shear is referred to as shear susceptibility andlike initial viscosity, can be considered as an intrinsic property of the

Bituminous Testing and Research Engineer, Director and Materials Engineer,respectively, Bureau of Materials, Testing and Research, Pennsylvania Department of Transportation. The oral presentation was made by Mr. Sandvig.

99

Paper published in the Journal of the Association of AsphaltPaving Technologists, Asphalt Paving Technology Volume 42,

1973


2/27

100 KANDHAL, SANDVIG AND WENGER

materials. The purpose of this report based on data from a researchproject, is to determine the effect of shear susceptibility parameter onpavement durability and performance.

RESEARCH DATA AND DISCUSSION OF RESULTS

Six test pavements, totaling 3.67 miles in length, were constructedin Clinton County on Legislative Route 219 (US 220) between Mill Halland Beech Creek, Pennsylvania. This study is limited to an evaluationof six experimental sections of ID-2 wearing course surfaces, eachcontaining a different type of asphalt. Average daily traffic on this roadis 4200 vehicles.

Type and gradation of the aggregates used (Table 1) in the sixwearing course mixes was held consistent. Optimum asphalt content of

Ta b le 1 . G ra da t i on o f Aggtegate fo r ID-2 Wea r ing Cour se

Limestone Natural Limestone SelectedScreenings Sand 1 B Aggregate Design

Percent usedPercent passing l/2Percent passing 3/8Percent passing 4Percent passing 8Percent passing 16Percent passing 30Percent passing 50Percent passing 100Percent passing 200

41.4

100.0 199.472 .o45.030.020.016.0

9.8

20.8

LOO.0

96.280.070.057.220.0

4. 02. 0

37.8100.0

91.020.0

4. 02.92.42. 01.81.4

100.0100.0

96.666.348.035.126.113.1

7.64.6

6.4 percent was also held consistent. Marshall test data on the fieldmixtures are given in Table 2. Properties of the six asphalts used inthe project are shown in Table 3. It will be observed from the data thatfive asphalts conform to Pennsylvania AC-2000 (AASHO AC-20) asphaltcements, since for project samples permissible viscosity is 2000 600poises. Asphalt 4 conforms to AASHO AC-l0. The initial air voids andsubsequent decreasing trend of the air voids, with time under traffic, isgraphically illustrated in Figure 1.

Thus, in this closely controlled research project, the only signifi-cant variable is the asphalt type. Since construction of these six pave-ments, periodic core samples have been obtained in order to determine

Tab le 2 . Mar sha l l Tes t Da t a on F i e ld Mix tu r e s

Asphalt Stability, Flow,Ty p e Pounds Units

Air Voids Plant Initial Air Voids in theCompacted Specimens, Constructed Pavement,

Percent

1 2048 9 5.5 9.52 1722 9 5.3 10.43 1813 8 5.2 9.64 1842 12 5.3 10.25 1910 9 5.0 10.96 1990 10 4.8 11.2


3/27

SHEAR SUSCEPTIBILITY TO PERFORMANCE 101

Tab le 3 . Asp h a l t Tes t P rope r t i e s (P ro j ec t Samp le s )

Asphalt Type 1 2 3 4 5 6

Viscosity

Viscosity at 140 F.,poises

Viscosity at 215 F.,

Viscosity at 39 F., at0.05

Viscosity at F., at0.05

Viscosity at 115 F., at0.05

Shear Susceptibility at39.2 F.

Susceptibility atF.

Shear Susceptibility at115 F.

Penetration

Penetration at 39.2 F.,g., 5 sec.

Penetration at 71 F.,100 g., 5 sec.

Standard Ductility at39.2 F. 1 cm.per cm.

1613 1447 1544(AC-20) (AC-20) (AC-20)

339.6 475.4 343.0

1.19 4.22 10 2.65

3.05 4.83 1.06

2.09 104 1.15 104 1.54 104

0.20 0.35 0.20

0.05 0.11 0.02

0.02 0.04 0.03

966

318.5

9.50

9.15 105

1.15

0.45

0.06

0.03

9 28 11 19

62 149 92 114

14.0 101.0 53.3 23.5

Composition Analysis)

(A), percent 16.30Nitrogen Bases

percent 25.65First

percent 18.62Acidaffinspercent 26.51

percent 12.92Coefficient 1.123

Ratio A/N 0.64

26.80

15.90

14.70

30.9011.700.7181.69

of Asphalt After Mixing in

Penetration at F..100 g., 5 sec.

Penetration. retained3658

98

Viscosity F., poises 3645 2971 Viscosity ratio at 140 F. 2.27 2.06

After MixingBased on Viscosity at11 F., 0.05 sec.-Shear Rate 3.3 2.5

Viscosity at F., 0.05determined fromAging Indices,

69

25051.62

665820782.16

697334631.57

607647701.80

1.9 3.3 2.1 1.9

1.01 10 1.21 10 2.01 3.02 2.77 x 3.52

18.74 19.98

24.56 20.63

15.8414.32

26.78 21.9114.08 17.100.988 0.7750.76 0.97

2200(AC-20)

509.4

1.68

1.32

2.19

0.32

0.12

0.04

15

94

68.3

25.05

24.08

21.11

20.968.801.5181.04

2649(AC-201

556.7

2.57

1.85

2.80 104

0.32

0.09

0.02

12

80

21.9

26.18

21.75

20.23

21.8410.001.3181.20

*No data available. These were determined from Figure 4 of the paper by and(ASTM, STP. No. 309, which gives relation between Percent of Original Penetration

Viscosity Aging Index for Thin Film Test.


4/27


2 -

5 I I 24I 30 I 36 I 42 54 781

Fig. 1. Air Void vs. Time in Months.

the percent air voids in the pavements and the rheological properties of the aged asphalts. The last core sampling was done 78 months after theconstruction.

Evaluation of Pavement PerformanceOlsen et al (7) have suggested a rating method for pavement evalu-

ation to evaluate the effect of asphalt aging on pavement condition. Thisrating method was used as a guideline to accomplish the visual pave-ment condition survey by a team of five engineers. Details of ratingforms used are given in reference (8). The overall ratings obtained onindividual pavements, in increasing order, are as follows:

Asphalt 1 19 (Poorest) Asphalt 2 22 Asphalt 6 23 Asphalt 4 Asphalt 3 Asphalt 5 31 (Best)

An ideal pavement according to this performance evaluation wouldrate 33, each individual observation being assigned a number. Brief details of the pavement condition after 80 months in service are givenin Table 4.


5/27


Tab l e 4 . P a vemen t C on d i t i on Af t e r 80 Mon ths (Br i e f De t a i l s )

Item Observed Asphalt 1 Asphalt 2 Asphalt 3 Asphalt 4 Asphalt 5 Asphalt 6

Riding Quality Fair Good to Goodgood

Raveling Severe Moderate Slight Slight tomoderate

of Matrix Severe severe Slight Slight

Rutting (inch)Cracking (shrinkage) Slight NoneSurface Texture Average Closed Average

to open

Good togood

Slight to

Slight to

NoneClosed

Good

Moderate

Moderate

Average

Rheological Properties of Asphalts

The following tests were carried out on the asphalts recovered

periodically from the test pavements:(1) Viscosities at 39.2 F., 7 7 F. and 115 F. over a range of shear

rates, using the sliding plate microviscometer designed byShell Oil Company (both glass and stainless steel plates wereused for these tests), and

(2) Viscosity at 140 F. using Cannon-Manning Vacuum Viscometer

Unless specifically mentioned otherwise, viscosity data at 77 F.implies 0.05 shear rate. The raw data on viscosities at 39.2 F.,115 F. and 140 F. obtained on the asphalts recovered from time to timeare available elsewhere (8). The data on viscosities and shear suscep-tibility values at 77 F. on recovered asphalts are given in Table 5.Plots of viscosity at 77 F. versus time in months are given in Figure 2.The data on viscosities were further analyzed to obtain aging indices of

the recovered asphalts.

Aging Index .

Aging indices were determined considering the changes in viscosityat 77 F. (both 0.05 and 0.001 shear rates and at constant shearstress) and viscosity at 140 F., as follows:

Aging Index = Viscosity after aging Viscosity before aging

The use of this index (ratio) eliminated the variability caused bydifference in the viscosities of the original asphalts and gives a clearerpicture of the hardening rate (9).

Values of these aging indices versus time in months were plotted(8). Out of these, only the aging indices based on viscosity at 77 F.(0.05 shear rate) conform exactly to the pavement performanceratings and thus seem to be more meaningful to indicate comparative


6/27


7/27


8/27


9/27


involved a precise location for such a dividing line will be extremelydifficult to determine.

A similar trend can be observed in Figure 6 in spite of poor corre-lation between viscosity and shear susceptibility values of all the as-phalts. The exception is Asphalt 2 which in spite of lower viscositiesperformed better than Asphalt 1 only. It seems that higher shear sus-ceptibility at relatively lower viscosities (at F.) can cause initialcracking inducing early failure of the pavement. Cracking was also ob-served in the test section using Asphalt 2. This, again, emphasizesthat aging index-shear susceptibility relationship is perhaps morecritical.

ASPHALT 2 - - - - - - - - -ASPHALT 3 - - - - -ASPHALT 4ASPHALT 5

TIME MONTHS

Fig. 4. Shear Susceptibility at 77 F., vs. Time in Months..

Unfortunately, no data are available on shear susceptibility valuesfor these six asphalts just after mixing in the or for TFO resi-due. However, it appears from Figure 4 that, except for Asphalt 1, theshear susceptibility values for Asphalts 2 through 6 were probably lessthan 0.20 just after mixing. It is expected that Asphalts 2 through 6 willstill be in serviceable condition for almost three more years and thuswill have at least 10 years service life before failure. Therefore,based on this study, it is believed that a pavement will perhaps be rea-sonably durable if after mixing the shear susceptibility (at 77 F.)


10/27


o42 MO.

2+

LOG Y 0.579 +CORR. COEFF. 0.893

Fig. 5. Aging Index (Based on Viscosity at 77 F.,0.05 vs. Shear Susceptibility.

is less than 0.20 at an aging-index of less than 4 and iscompacted under traffic during the first 18 24 months to the extentthat it will have no more than 5 percent air voids.

It seems apparent that the control of gain in shear susceptibilityand aging-index is a necessary specification requirement. Shear sus-ceptibility requirements for thin-film residue can be substituted inplace of the ductility test since a good correlation between these twoproperties was found by Welborn et al (3). Significance of ductility inrelation to pavement performance has been investigated in the labora-tory and field, and results are reported in the literature (12).

Also, a maximum aging-index based on viscosity at 77 F. for TFO resi-due should be incorporated in the specifications for paving asphalts.

Effect of Temperature on Shear Susceptibility

Data on temperature susceptibility of the asphalts reported else-where (8) show that this parameter has a decreasing trend with agingin the 39.2 to 140 F. temperature range. It appears that the shear sus-ceptibility which is increasing with aging predominates the temperaturesusceptibility for the six asphalts used in this study. This is also in-dicated by diminishing ratio of shear susceptibility at 77 F. and 39.2 F.


11/27


with aging (Figure 7). After 78 months aging, this ratio is minimumfor Asphalt 1 which is poorest, and maximum for Asphalt 3 which isvery good in performance. It seems that the temperature susceptibilityparameter may have more significance in case of low viscosity gradeasphalts. This is further confirmed by the absence of any crack pat-tern on these test pavements, which can be attributed to temperaturesusceptibility.

T.!

2 -

l o -

:

5 -

83 -

2 - +

CORR. COEFF. 0.671

0 0.2 0.3 0.5 0.7SHEAR SUSCEPTIBILITY AT

Fig. 6. Shear Susceptibility vs. Viscosity at77 F., 0.05 Sec.-l.

Effect of Asphalt Composition on Shear Susceptibility

Shear susceptibility of asphalts had been reported to be affected bytheir chemical composition (13). Skog (14) had found that the shearsusceptibility of asphalts correlates roughly with the ratio of tenes (A) to Nitrogen Bases the component responsible for consist-ency and the component functioning as a peptizing agent. Chemicalcomposition of the six asphalts used is given in Table 1. Plots of A/N

vs. shear susceptibility are made in Figure 8. The correlation coeffi-cient is 0.724 which is fair. This relationship seems logical and in ac-cord with the definitions and the functions of the fractional componentsas postulated by Rostler. While comparing six asphalts of differentconsistency, containing components of different molecular weights or


12/27

110 KANDHAL, SANDVIG WENGER

differing in volatile matter, these other variables have also to be takeninto account. In broad sense, shear susceptibility appear to be an indi-cation of an internal phase relationship of the asphaltic constituentswhich in turn have an important bearing on the serviceability factorsof the asphalt.

CONCLUSIONS

Based upon the rheological properties of the six asphalts studiedand the preceding discussions, the following conclusions can be drawn:

1. Aging indices based on viscosity at 77 F. conform to the pavementperformance ratings and thus seem to be more meaningful to indi-cate comparative aging and life expectancy of the test pavements.

2. Very good correlation was observed between aging indices and shearsusceptibility values of the aged asphalts. Thus shear susceptibilityis one of the important factors affecting the pavement performance.The aging index-shear susceptibility relationship seems to deter-mine the pavement durability and performance.

ASPHALT 3 - - - - -

ASPHALT 4ASPHALT 5

0 IO 20 30 40 50 60 70 80TIME MONTHS

F ig. 7. Shear Susceptibility Ratio 39.2177vs. Time in Months.


13/27


0.06

Y= -0.0034 + 0.076X

0.5 0.7 0.9 1.3 1.5 1.7N BASES

Fig. 8. Ratio Asphaltenes /N--Bases vs. Shear Susceptibility.

3. Control of gain in shear susceptibility and aging index appears to bea necessary specification requirement for paving asphalts.

4. Shear susceptibility seems to be an indicator of an internal phaserelationship of the asphaltic constituents which in turn have an

bearing on the serviceability factors of the asphalts.5. Further research is needed to establish the critical aging

shear susceptibility relationship for asphalts produced from variouscrude sources.

6. The significance of some of these parameters affecting pavementperformance will perhaps be more clear with the continued evalua-tion of these test pavements until failure.

ACKNOWLEDGMENTS

Thanks are due to Messrs. David Streby and Richard Basso for allthe viscosity test data, to Edward for preparing the illustrations

to June Viozzi for compiling the test data.The opinions, findings and conclusions expressed in this paper are

those of the authors and not necessarily those of Pennsylvania Depart-

ment of Transportation.


14/27


LITERATURE CITED

1. F. C. Gzemski, Properties of Paving Asphalt, American Society for Testing and Materials, Special Technical Publication No. 328(1962).

2. F. N. Hveem, E. Zube, and J. Skog, Proposed New Tests and Specifica-tions for Paving Grade Asphalts, Proceedings of the Association of Asphalt Paving Technologists, Vol. 32 (1963).

3. J. Y. Welborn, E. R. Oglio, and J. A. Zenewitz, Study of Graded Asphalt Cements, Proceedings of the Association of Asphalt Paving Technologists, Vol. 35 (1966).

4. John Skog, Setting and Durability Studies on Paving Grade Asphalts,of the Association of Asphalt Paving Technologists, Vol. 36 (1967).

5. C. H. Chipperfield, J. L. Duthie, and R. B. Girdler, Asphalt Characteris-tics in Relation to Road Performance, Proceedings of the Association of Asphalt Paving Technologists, Vol. 39 (1970).

6. V. P. Puzinauskas, of Properties of Asphalt Cements withEmphasis on Consistencies at Low Temperatures, Proceedings of the As- sociation of Asphalt Paving Technologists, Vol. 36 (1967).

7. R. E. Olsen, J. Y. Welborn, and B. A. Vallerga, Pavement Evaluation-toEvaluate the Effect of Asphalt Aging on Pavement Condition, Proceedingsof the Association of Asphalt Paving Technologists, Vol. 38 (1969).

8. P. S. Kandhal, L. D. Sandvig, W. C. Koehler, and M. E. Wenger, Viscosity Related Properties of In-Service Pavements in Pennsylvania, Bureau of Materials, Testing and Research, Pennsylvania Department of Transportation, Research Report (February, 1972).

9. R. N. Traxler, Changes in Asphalt Cements During Preparation, Layingand Service of Bituminous Pavements, Proceedings of the Association of

Asphalt Paving Technologists, Vol. 36 (1967).10. P. C. Doyle, Yardstick for Guidance in Evaluating Quality of Asphalt

Cement, Highway Research Board Record No. 24 (1963).11. Study of Bituminous Concrete Pavements in Ohio, Public Roads, (Aug-

ust, 1941).12. J. T. Pauls and J. Y. Welborn, of Hardening Properties of

Materials, Public Roads, (August, 1953).13. R. S. Winniford, The Influence of-Asphalt Composition on Its Rheology,

American Society for Testing and Materials, Special Technical PublicationNo. 294 (1960).

14. R. M. White, Written Discussion on Paper, Final Report on the Asphalt Test Road, Proceedings of the Association of Asphalt

Paving Technologists, Vol. 38 (1969).

D i s c u s s i o n

PROF. H. E. SCHWEYER: We certainly are pleased to see thispaper because in a paper that we are presenting on Wednesday we haveshear susceptibility as one of the parameters that we studied in ourfield program. I would like to point this out as a comment and if the

blackboard was available Id give a lecture. The shear susceptibility asmeasured in this paper differs a little bit from the commonly usedshear susceptibility definition as will be shown for the analysis of shearstress versus rate of shear data.


15/27


In the ordinary plot log of shear stress, versus log shear rate,with the power law valid, the following applies

=

where and may be any reference coordinates and C is the slopeof the log-log plot. If a value of apparent viscosity, = ismeasured at any desired the results for versus may also beplotted on logarithmic coordinates to give a straight line (neglectingcertain corrections). This relation may be expressed as:

=

where the slope of the line is C-l with C being the same C as in Eq.Thus, in the authors paper the negative value of shear suscepti-

bility is mathematically correct (for C less than 1) and can be recon-ciled with the value normally used.

MR. W. I have a question regarding initial force andthe ultimate force as shown in one of these graphs. This gives me theimpression that the initial compaction was less than, say, 95 percent of the final volume weight. In this respect, I would like to remark that inour country (Netherlands), there is a requirement for a minimum of 95percent of the volume weight that can be obtained by laboratory com-paction for the first base course layer, while for the subsequent layersthe minimum is 98 percent. I guess that in our country it is quite im-possible to have such an enormous deformation after compaction of thepavement as you have shown here.

MR. L. D. SANDVIG: We have similar requirements in Pennsyl-vania now. During 1964, when this research project was undertaken,we allowed 2 to 8 percent air voids in the Marshall design and the re-quired compaction was 95 percent of the design. This got us into thissituation with the high voids in the placed mat to the extent of 10.3 per-cent. Today it would be substantially less. We were rather new in thecompaction procedures of bituminous surfaces during the early 1960s.It was from this data we learned that we should have something better.

MR. L. W. CORBETT: If I understand you correctly you are favor-ing the use of low shear susceptibility asphalt, is that right?

MR. SANDVIG: Yes.

MR. CORBETT: Also you are looking at aging index as a quality

criterion?MR. SANDVIG: Yes.

MR. CORBETT: Theres one thing that bothers me in this re-spect and that is, most specifications tend towards trying to define as-phalt cements with good temperature susceptibility or high shear


16/27

114 SANDVIG AND WENGER

susceptibility. Now this seems a little anomolous, because in one casewe are trying to define high shear susceptible asphalts and then we find,as a result to your work, that maybe the low shear as-phalts are preferable. Would you comment?

MR. SANDVIG: I think most of you know my feelings on existingspecifications. They dont exactly, in my judgment, measure the qual-ity of the asphalt. I think that is an old story. Right now Im not par-ticularly pushing this sophisticated test for a means of controlling as-phalt as a production. I think this is good data which we need to get toin developing the specifications of the future. Frankly, the shear testsare lab tools at the present time. However, we have to get some of thiscriterion which describes these attributes so that we get out of theshear testing by some means that can be used routinely in the labora-tories today where we control asphalts like ductility at low tempera-

tures after the TFO test and the viscosity ratio, etc. as part of thedevelopment in the new viscosity graded specifications.

PROF. T. W. THOMAS: Every thing which I have been able tolearn about the weathering of asphalt pavements indicates that the mostimportant single variable determining the degree of weathering is thevoid content of the initial pavement. Would not this variation in voidcontent tend to obscure the effect of other variables studied?

MR. SANDVIG: I agree with you on this. I think Ive already ex-plained that we have changed our specifications in Pennsylvania in re-gard to air voids. But in retrospect being a researcher I think we aregetting results more promptly because we did have high air voids ini-tially. It may not be related particularly to the actual pavements werebuilding today, but at least were gathering some data.

MR. F. ROSTLER: was certainly for the life of thepavement that the void contents were high, but if it is incorrect that thevoid contents were alike in all test sections, it was for theexperiment, because it causes to show the results much faster. Werethe void contents alike in all these test sections?

MR. SANDVIG: Im sorry I dont understand the question.

MR. ROSTLER: Did you say that the voids content was the same inall sections?

MR. SANDVIG: Substantially, yes, they averaged rather high bytodays standards of accepting pavements in Pennsylvania. They aver-aged around 10 percent because at that time we had a Marshall accept-ance criteria of 2 to 8 percent air voids. Our compaction requirement

was 95 percent of Marshall.MR. ROSTLER: This is, of course, very fortunate for the experi-

ment because it magnified the phenomena explored and speeded up ob-taining results.


17/27


MR. SANDVIG: Right, were looking backwards toward that. Itdoesnt relate to the pavements were building today because weve gotless air voids. We still would be reporting on this 5 years from now.

MR. ROSTLER: It is very unfortunate, that, as you said, you dontknow the original shear susceptibility of the asphalt before it was used.

At the last meeting we had a paper stressing the necessity toreally identify the specimens used in a study. If this had been done andif we would now know the shear susceptibility or calculate it from re-corded data, this could answer the question as to change in shear sus-ceptibility in service. If you are going to do some additional work inthis area, as you indicated, I suggest very strongly that the asphaltspecimen should be well defined before testing, so that you can latercome back and compare changes in original properties.

MR. SANDVIG: Ten years ago we didnt know what we know now.MR. V. PUZINAUSKAS: It seems to me like Figure 4, at zero

months there is a value for shear susceptibility so you have the shearsusceptibility for the original material,

MR. SANDVIG: No shear susceptibility values are available afterthe TFO test or after mixing. We are starting on the X axis there withthe original asphalts before mixing through this TFO test.

DR. G. LEES: I would like to ask you this. It would appear per-haps that for prevention of cracking of the surfacing you should bechoosing a softer bitumen to start with and perhaps more of it, in orderto have a lower initial air voids content. But on looking at Table 4, therutting has been rather slower with Asphalt 1 compared with some of those asphalts which have apparently performed better in respect of resistance to cracking. My question is twofold really. If you were toinclude a higher initial binder content, in order to reduce the voids,and also adopt a softer bitumen both with the object of improved re-sistance to cracking, arent you likely to suffer too much from rutting.This would also probably reduce your surface texture which at themoment is average to open with Asphalt 1 and rather close with someof the mixes which showed better performance in resisting cracking.There was no reference to skid resistance contained in your evaluationof quality. Would you anticipate that although it was cracking badly,

Asphalt 1 would show the better skid resistance at high speed. Isnt theanswer to the problem to design a dense mix to begin with which haslow voids due to a well designed dense aggregate grading, but whichwill not shove or rut due to the closeness of the aggregate interlock?Then it would be desirable for this to possess an open surface texture,for dispersal of water under the tire, which could be maintained be-cause of the high resistance to deformation of the mix. That is the ap-proach which we are following at Birmingham University.

MR. SANDVIG: As far as the skid resistance is concerned, wedidnt consider this as part of the project. Because of emphasis on


18/27


19/27


gave some control of these properties. We therefore looked forward tothis paper with keen anticipation.

However, we were quite disappointed to find that the paper is notin fact concerned with the effect of shear susceptibility on rheologicalbehaviour (and hence on mechanical properties) of asphalt. Instead itis concerned with the relationships between shear susceptibility, agingindices and visual assessments of pavement condition.

Furthermore, we do not accept the conclusion of the authors re-garding the necessity of controlling gain (i.e., increase) of shear sus-ceptibility via specifications at the present time. Our views in thisrespect are based on our own findings together with dissatisfaction withevidence given in the paper, as follows:

1. We do not feel that the authors are justified in drawing such ageneral conclusion from results on only one aggregate mix.

2. The consistencies of the original bitumens varied considerably:the penetration ranged from 62 to 149 and the viscosities at 140 F.varied between 966 and 2,649 poise. Thus, at the outset it was likelythat performance would be affected by a variable other than the oneunder scrutiny. Although we accept that the use of aging indices, byeliminating differences in the viscosities of the original bitumens, ishelpful in studying hardening rates, we do not accept that variation of original viscosity can be assumed not to affect pavement performance.

3. The asphalt mixes used all had relatively high initial air voids(about 10 percent) which reduced to about 2.5-4-O percent after 24months, with one exception (Asphalt No. 1) which still had over 7 per-cent air voids after 24 months.

4. Although it is not entirely clear from the paper, it would seemthat no filler was added to the mixes and that the natural filler contentwas only about 4.5 percent. This contrasts with a total filler content of about 10 percent widely employed in dense surfacings in Europe, andmay help to explain the high voids contents. It also suggests the pos-sibility that, even after compaction under traffic to less than 4 percentair voids, the surfacings may be permeable to air and water.

5. The variations in air voids, and in particular the continuing highvoids content for Asphalt No. 1, appear to be primarily due to the vari-ation of consistency of the bitumens after mixing in the asshown in Figure A of this contribution.

6. In such open mixes it is to be expected that considerable hard-ening will occur in service and indeed the authors found that it did asshown by their Figures 2 and 3. Comparison of Figures 1 and 3 of thepaper shows that the rates of hardening generally followed the changesin air voids.

7. In our experience, consistency and shear susceptibility arehighly correlated and it is no surprise at all to find that the shear sus-ceptibility of the bitumens increased as they hardened in service asclearly shown in Figures 5 and 6 of the paper.

8. Figure B of this contribution contains data from the paperplotted to show the pavement performance ratings and air voids contents


20/27


21/27


serviceability ratings given in the text in our Figure C. Clearly, thecorrelation is good and the performance ratings are highly dependenton the aging indices, but it is an overstatement to claim that the con-formity is exact.

11. We feel that a figure showing the relationship between shearsusceptibility and pavement performance rating is desirable in a paperentitled Shear Susceptibility of Asphalts in Relation to Pavement

We attach such a relationship for the data in the paper asour Figure D, although, as stated already, we feel that the final shearsusceptibility values were consequent upon the aging rather than thecause of it.

12. Although there is an undeniable trend shown in this figure, therelationship shown is not sufficiently good to be the basis for a specifi-cation, even if it could be shown that increase of shear susceptibility

was a cause rather than an effect. For example, Asphalt No. 5 had thehighest serviceability rating, 31, the second lowest shear susceptibil-ity, 0.33, and was placed in Group 3 (best), while Asphalt No. 2 withonly a slightly higher shear susceptibility, 0.36, had the second lowestserviceability rating, 22, and was placed in Group 2.

\

\\

A F T E R M O N T H S

I I I I I I

2 3 5 6F I N A L A I R V O I D S C O N T E N T

Fig. B. Relationship between Pavement Serviceability Ratings*and Final Air Voids Content of Asphalts.


22/27


13. Since the authors have shown that hardening and shear suscep-tibility are highly correlated with each other there does not seem to beanything to be gained by including both properties in a specification. If hardening during service in the pavement is to be limited by specifica-tion, yet another test would be required to predict such hardening, overand above that which occurs during mixing and laying. Apart from thefact that the degree of hardening to be predicted will vary according tothe permeability of the mix, the severity of the laboratory test condi-tions that would be required to produce the degree of hardening experi-enced by the authors would be such as to confer a very high uncertaintyfactor on the relationship between predicted and observed bitumenperformance.

A G E I N G I N D E XV I S C O S I T Y AT 7 7 A F T E R A G E I N G S E R V I C E )V I S C O S I T Y AT 7 7 B E F O R E M I X I N G

I I I I I I I

6 11 2 0 2 2A G E I N G I N D E X

Fig. C. Relationship between Pavement Serviceability Ratingsand Aging Indices of Bitumens* After 80 Months in Service.

14. The time available for the preparation of this contribution doesnot allow for a full presentation of our own data concerning the effect of shear susceptibility on pavement performance or for the translation of our shear susceptibility parameters into the parameter used by theauthors. Nevertheless, we feel that the following evidence shows thatthe rate of hardening in dense, impermeable asphalt surfacings, and theconsequential increase in shear susceptibility, are negligible compared

with that experienced by the authors in their work.Figure E of this contribution is reproduced from and relates tofour of the eight bitumens used in a comprehensive trial of 110 testsurfacings laid in southern France in May 1963. The shear susceptibil-ity is a function of the parameter b and it should be noted that shear


23/27

SHEAR SUSCEPTIBILITY OF PERFORMANCE

I O I O

S H E A R S U S C E P T I B I L I T Y O F B I T U M E N

T A N G E N T O F A N G L E O F l o g S H E A R R AT El o g V I S C O S I T Y A T

121

Fig. D. Relationship between Pavement Serviceability Ratings andShear Susceptibility of Bitumens After 80 Months in Service.

susceptibility increases as b decreases. It can be seen that for allfour bitumens, considerable hardening took place during the mixing andlaying operations, as indicated by the increases in , 25 (Newtonianviscosity at 25 C., 77 F.). There was a corresponding increase inshear susceptibility (reduction in b value) during this period. Howeverthe further hardening that took place over 78 months under traffic, andthe accompanying increase in shear susceptibility, were insignificant.This absence of significant hardening is attributed to the impermeabil-ity of the asphalt mixes used (see below). After 78 months, the service-ability of all these sections was at least satisfactory. The main reasonsfor ratings less than good were reduction of surface texture anda slight tendency towards excess binder.

In not one case was there any significant raveling, rutting, loss of matrix or cracking and the riding quality was excellent. All these sur-facings are still in service after almost 10 years.

It should be noted that all the binders used in this trial werenominally of penetration. (The values for the original bindersactually ranged from 74 to 96.)


24/27

122 WENGER

A further factor, which we consider to be highly significant, is that75 percent of the test sections, incorporating all the test bitumens, hadinitial air voids of only 4.4 to 4.7 percent which reduced to 2.2 to 2.4percent under traffic. The remaining 25 percent of test sections weremade with a coarse basalt aggregate and did not compact so well. Forthese, the mean values of the initial and final air voids for all the bitu-mens were 8.1 and 5.0 percent respectively. Nevertheless tests indi-cated (2) that even these sections were impermeable.

In this trial, such minor differences as were noted between testsections were attributable to aggregate quality and mix design varia-tions rather than to binder properties.

15. A road trial (4) to investigate the relationships between theshear susceptibility of bitumen and the skid resistance of asphalt isbeing carried out, in Belgium, by the Belgian Centre de Recherches

and BP.

/ - - - - - - _ _ _,

, 2

B I T U M E NC O D E N o

24 -9 - - - - - -

_ - - - -_ - - - -- - - - - -- - - - - -

- -- - 99

44

11I I I

2 0 4 0T I M E I N S E R V I C E - M O N T H S

Fig. E. Changes in Rheological Characteristics of FourBitumens Used in Large Scale Road Trial.


25/27

SHEAR SUSCEPTIBILITY TO PERFORMANCE

S H E A R S U S C E P T I B I L I T Y =A P P A R E N T V I S C O S I T Y AT

A P PA R E N T V I S C O S I T Y AT

06 0

2 . 2S H E A R S U S C E P T I B I L I T Y O F B I T U M E N

1

- 7 6

i i

872

- 7 0

- 6 6

6 0

5 6

123

Fig. F. Relationship between Skid Resistance of Asphalt(After 4 Years Service) and Shear Susceptibility*

of Original Bitumens.

Figure F of this contribution shows the most recent relationshipsobtained in this trial between skid resistance measured at 50 and 80kph and the original shear susceptibility of six different bitumens in acommon dense aggregate mix. These skid resistance values are Side-ways Force Coefficients, measured after 4 years trafficking, by amethod technically equivalent to one given in Reference (5).

The shear susceptibilities in this case are expressed in terms of

where:= viscosity at 25 C. (77 F.) and shear stress of

10 dynes/cm Apparent viscosity at 25 C. (77 F.) and shear stress of

3 X


26/27

124 SANDVIG AND WENGER

Although the relationships are not very good, a clear trend is evi-dent, particularly at the more critical higher speed. As with the as-phalts discussed under 14 (above), voids contents are low, hardening inservice has been minimal, and after 4 years trafficking the test sectionsare in very good condition.

These results, which will be included in a further publication onthis subject, have been quoted in an attempt to offset the impressiongiven by the authors that high shear susceptibility is undesirable.

16. From a rheological/mechanical point of view, high shear sus-ceptibility can convey definite advantages, one of which, indicated inearlier publications is a reduction of the change of asphaltstiffness with temperature and loading time. This property can be em-ployed to reduce both the tendencies of asphalt to crack at very lowtemperatures and to deform at high temperatures.

17. It is pertinent to record that the effect of bitumen hardening inservice (i.e., increase in Newtonian viscosity) on increase of stiffnessat thermal and traffic loading times is very much reduced by the asso-ciated increase in shear susceptibility of the bitumen. (Study of Fig-ures 2 and 3 of Reference (1) will make this clear.) Thus, far frombeing undesirable, the increase of shear susceptibility helps to alleviatethe situation created by the hardening of the bitumen.

In conclusion, we feel that from the results of their trial the authorsare not justified in attributing differences in road performance to in-creases in shear susceptibility.

Literature Cited

1. J. L. Duthie, Proposed Bitumen Specifications Derived from FundamentalParameters, Proceedings of the Association of Asphalt Paving Technolo-gists, Vol. 41 pp. 70-117.

2. E. H. Chipperfield, J. L. Duthie, and R. B. Girdler, Characteristicsin Relation to Road Performance Proceedings of the Association of Asphalt Paving Technologists, Vol. 39 pp. 575-613.

3. J. Huet, J. Reichert, and P. Outer, en Asphaltiques-Bitumes et Technique Rout&e, (Brussels), Vol.14, No. 2 pp. 37-40.

4. J. Huet, Enseignements des Routes en Asphaltiques (Grand Meise, Hoeilaart), La(Brussels), Vol. 17, No. 1 pp. 3-28.

5. Road Research Laboratory, Research on Road Safety. London: Her Ma- jestys Stationery Office, 1963.

6. E. H. Chipperfield and T. R. J. Fabb, Development of More Durable RoadSurfacings, Proceedings of the Canadian Technical Asphalt Association

pp. 157-180.

AUTHORS CLOSURE: It should be emphasized, as mentioned inthe paper also, that shear susceptibility is not the only factor affectingthe pavement performance. There seems to be a critical agingshear susceptibility curve (Figure 5) above which higher shear suscep-tibility would be a potential cause of poor service and below which shear


27/27


susceptibility would not be a significant factor affecting pavement dura-bility and performance.

The shear susceptibility (or shear index) value as employed in thisstudy is the tangent of the angle of log shear rate (X axis) versus logviscosity (Y axis) determined during performance of the viscositytest using the microviscometer as the asphalt ages, this angle increasesindicating an increased shear susceptibility value.

In regard to Mr. Fosters comments, we agree that the shear sus-ceptibility of the original asphalt or the aging index after TFO is not of much help in predicting the pavement performance. We do need a lab-oratory durability test which can simulate the hardening which takesplace in an actual pavement. The data, collected and reported in thispaper, on changing shear susceptibility and aging index values in actualpavements, would be helpful then to predict the pavement performance.

Date post:	29-May-2018
Category:	Documents
Upload:	prof-prithvi-singh-kandhal
View:	215 times
Download:	0 times

Shear Susceptibility of Asphalts in Relation to Pavement Performance

Documents