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Fatigue Testing of Bituminous Binderswith a Dynamic Shear Rheometer

by

Hilde SoenenBernard Eckmann

AB NYNAS PetroleumGroup Competence Center

Nynashamn, Sweden

Technical Session : Performance Testing and Specifications for Binder and MixKeywords : Fatigue, Testing, Binder

Abstract

In literature, fatigue properties are most often measured on asphalt mixes, and little attention has

been given to a possible fatigue property of the binder itself. At the Nynas Group CompetenceCenter (GCC) a method to study the fatigue resistance of bituminous binders with a dynamic shear rheometer is under development. Experimental evidence shows that repetitive shear oscillations can

generate a fatigue failure provided the stiffness is above a certain value, or indirectly that thetemperature is low enough. Crack formation and growth is seen as a steadily decrease in modulus

with increasing number of oscillations. Measurements have mostly been done under controlledstress conditions, with initial strains varying from 0.004 to 0.015. Due to the fact that the fatigue

phenomenon is only observed at higher stiffness values (e.g. 20 to 50 MPa), one must however ensure that the measurements are not biased by the compliance of the equipment.

As is well known for many other materials, the number of oscillations needed to produce failureshows a linear dependency to the applied initial strain in a log-log plot. However, the slope and

position of these lines is binder specific, and changes with polymer modification or oxidative

ageing treatments. Up to now, no correlation with the SHRP fatigue parameter could be obtained(binders with similar G*sinδ values may show very significantly different fatigue lines). An

intrinsic behaviour could be evidenced in the case of straight-run pure bitumens of the same crudeorigin. When testing different penetration grades (in this case, ranging from 15 dmm to 90 dmm) at

a fixed stiffness level (thus at different temperatures), one obtains approximately a unique fatigue

line !

So far, only a limited number of direct comparisons between binder and asphalt mix fatigue lines isavailable. These results show a good agreement in the case of pure bitumens but which is less

satisfactory in the case of polymer modified binders. The planned continuation for these investigationsis to evaluate the potential impact of rest periods.

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Fatigue Testing of Bituminous Binderswith a Dynamic Shear Rheometer

by

Hilde SoenenBernard Eckmann

AB NYNAS PetroleumGroup Competence Center

Nynashamn, Sweden

This paper presents and evaluates a method to measure the fatigue behavior of bituminous binders,using a dynamic shear rheometer. In literature, fatigue is mostly investigated on bituminous mixes,and several experimental test methods are available (2). On the other hand, fatigue measurement on

bitumen itself only recently got some attention (4-6). One of the reasons is certainly the lack of a

convenient fatigue test for bitumen binders and the lack of experimental evidence showing itsimpact on mix fatigue behavior. Fatigue tests on the bitumen binder however, provide theadvantage that no elaborate mix slabs need to be prepared, and that the influence of different mixdesigns, including their statistical variations in compaction, is absent.

Fatigue failure, as considered herein, is a form of cracking resulting from repeated loading.Engineering materials, like polymers, steel, ceramics, have been observed to fail as a result of the

repeated application of stresses at levels considerably below that required for immediate fracture. Therepeated stresses that are insufficient in magnitude to produce failure in one cycle nonetheless induce

damage in the material with every cycle. This damage accumulates and ultimately leads to failure.Most often, S-N diagrams in which the logarithm of the magnitude of the alternating stress, or strain, is

plotted versus the logarithm of the number of cycles to failure, is used to represent fatigue test results.

In this paper we present experimental evidence that a fatigue induced cracking can take place in

bituminous binders, within certain limits of strain and stiffness, and show a means of measuring thisfatigue life on the binder directly. In addition, experimental difficulties and limitations of this methodwill be discussed. In the course of this investigation, we have focused on the following parameters on

the fatigue life; the influence of bitumen stiffness (penetration grade), differences in fatigue behavior upon changing from straight-run bitumen towards slightly and severely oxidized products, and the

influence of a polymer modification. At this time, the evaluation of the impact of the binder fatigue properties on the fatigue behavior of asphalt mixes is still ongoing, but first directions can beaddressed.

Experimental approach

Sample Selection:Four straight run bitumen, coming from the same crude origin and differing only in penetration grade,

were selected. In addition, one slightly and one severely oxidized product was used. Sample properties are shown in Table 1. One of the straight run samples was also used after aging. The aging

procedure was chosen according to SHRP: RTFOT followed by PAV at 100C. In addition, differentPMB`s have been used and their composition will be given when the test results are presented.

Rheology:

A fatigue cracking was induced by applying continuous oscillatory shear loading using a RheologicaStress Tech rheometer. Plate-plate geometry (4mm radius) was chosen, and the measurements were

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Table 1. Properties of the different bituminous samples.

Sample ID Description Pen (mm/10) R&B ( °C)

B15 Straight-run 17 62

B65 Straight-run 70 47

B90 Straight-run 94 45B180 Straight-run 192 37.9

SOx35 Slightly oxidised 35 55Ox20 Severely-oxidised 20 66.5

performed with a 2 mm gap setting, the reason for this will be discussed together with the experimental

difficulties. In order to observe fatigue cracking, the stiffness of the binder needs to be high enough.For example the sample B90, showed a fatigue cracking at stiffness levels between 10 MPa and 50MPa. In this range, repeated sinusoidal oscillations, controlled stress as well as controlled strain

deformations, lead after a certain number of loadings to an abrupt decrease in modulus. In case of constant stress measurements, the sample completely breaks, and lower and upper plates are separated

after the test. In the case of controlled strain measurements, the decrease in modulus levels off at amuch lower value as the initial stiffness level. In these tests the frequencies were between 10Hz and50Hz. Since the stiffness of the binders needs to be high, the different penetration grades could not be

compared at a constant temperature. Instead, they were compared at a constant value of G*. Thus, thetest temperatures were adapted.

Microscopy:The microscopic photographs presented in this paper were made by Peter Westerlund, Bergström

Instrument AB, Solna, Sweden.

Time, (s)

1e+1 1e+2 1e+3 1e+4

G * ,

( P a )

1e+7

1e+8

B90, 6.5oC

B15, 20oC

B65 + 3.5% SBS, 8.5oC

B90 + 5% EVA, 8.5oC

Figure 1: Time sweeps for different binders at the same initial strain levels and with the same SHRP fatigue parameter.

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Results and discussion

Fatigue observation:Figure 1 represents an example of a time sweep resulting in fatigue damage. In this figure four binders

were evaluated, and a constant stress mode was used. The fatigue of the sample is observed as a

complete breakage, upper and lower plate are no longer connected. In constant stress tests, the time or the number of oscillations needed to break the sample is taken as the fatigue time. In a similar way, a

constant strain can be applied to the sample and in this case, the stress will decrease as the stiffnessdecreases. The rate of damage slows down as the stress becomes low. In controlled strain, the fatigue

time is taken as that time at which the stiffness is reduced by 50% of its initial value, in analogy withfatigue mix tests. During fatigue damage, hairline cracks start to grow from the edge towards themiddle of the sample. A microscopic picture of the bitumen surface at bottom, respectively upper plate

is shown in figure 2A and 2B. If the sample is left at rest for a longer period of time, the cracks willslowly heal and a smooth surface will be formed, figure 2C. The rate at which the cracks disappear

strongly depends on the bitumen stiffness and thus indirectly on the temperature.

In figure 3, the fatigue time is plotted as a function of strain. As strain increases, fewer oscillations arenecessary to produce fatigue damage, and this relation is linear in a logarithmic plot. This fatiguedamage occurs independent whether the initial strain is inside or outside the linear viscoelastic domain.

The measurements in figure 3 were performed using constant strain tests, but similar graphs areobtained using constant stress tests, as will be shown later. In figure 3 the fatigue of a polymer modified sample with a low polymer modification is also shown. In this case, the influence of the

polymer on the fatigue line is strain dependent and rather small for the higher strain levels tested here.

Figure 2A. B15, bottom plate after fatigueat 50Hz, 0.01 strain.

Figure 2B. B15, upper plate after fatigue at 50Hz, 0.01 strain.

Figure 2C. B15, after fatigue damage, left at room temperature for 3 days

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Experimental considerations:Since the stiffness of these samples is high, compliance errors may effect the measurements. To

investigate this, stiffness measurements for the same sample were made at different gap settings, asshown in figure 4. As the gap decreases, compliance effects influence the measurements, and the

measured stiffness deviates from the true stiffness. The measurement at a gap of 2 mm is still similar

to the measurements made at higher gaps, for the stiffness range investigated here. This indicates thatcompliance errors are inapplicable.

In order to improve the repeatability of these measurements, some other experimental factors had to becontrolled carefully: a good adhesion of the sample to both plates was assured by heating the samples

in the rheometer to a high temperature (60°C-80°C, depending on the samples stiffness) beforetrimming and starting the measurement. Trimming had to be done very carefully in order to get aconstant outward bulge of the sample, and temperature gradients inside the sample had to be kept as

small as possible (<0.2°C). This has required certain modifications to the temperature control of therheometer.

Evaluation of different parameters:

The different binders listed in table 1, were not compared at a constant temperature but at a constantvalue of G*. So the test temperatures were adapted. After some initial testing under differentconditions, the final measurement conditions for these samples were set at 50Hz for the frequency,

and 20 MPa for G*, and the tests were performed under constant stress. In figure 5, themeasurement points and regression lines are shown. These bitumen differ only in penetration. Thetest temperature for each sample is indicated in the graph. As one can see, all measurements seem

to be similar within the precision of this test, indicating that fatigue resistance is fairly independentof the penetration grade as such, at least as long as the same crude origin and manufacturing processis used. However, the stiffer samples will exhibit fatigue cracking at already much higher

temperatures than the softer grades. This independence of penetration grade was also found for measurements under constant strain (not shown).

In figure 6, the measurements for straight-run together with oxidized samples are shown. Uponoxidation the slopes of the lines become steeper, oxidized samples are more resistant to fatigue atlow levels of strain, but at high strain levels oxidized samples are very prone to fatigue failure.

For practical purposes, this would indicate that for roads with heavy traffic and thin layers, whichhave high stresses and strains, highly oxidized products may show more fatigue cracking. On the

contrary, roads with thick layers and only light traffic, oxidized products may even become beneficial regarding fatigue resistance. Furthermore, upon oxidation the temperatures at whichfatigue occurs in our test, are also higher than for straight run samples.

In this study, also aged and polymer modified samples were included. Aging has an effect similar to oxidation. The fatigue line becomes steeper and more strain dependent. As a result of increasing

stiffness, the test temperature for the aged sample is higher compared to the non-aged product. For Pmb samples, the polymer modification can have a positive effect on the fatigue behavior, as seenin figures 1 and 3, but this positive effect was not always observed. Especially low volumes of

added polymers showed only little improvement.

Comparison with the SHRP fatigue parameter:Finally, we have compared this fatigue failure with the SHRP parameter, G*.sin(phase) for fatigue. Acorrelation could only be found for the 4 straight run, non modified samples, since their SHRP

parameters and fatigue lines were very alike. However, for (all) the oxidized and modified samplesused, no correlation could be found. An example is shown in figure 1, since these four binders were

actually evaluated at temperatures where they had the same SHRP parameter.

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Initial Strain

0,01

T i m e t o f a i l u r e ,

( s )

1e+3

1e+4

1e+5

Figure 3: Repeatability test on fatigue measurements.

B65, 15oC

B65 + 3,5% SBS, 15oC

Phase, degree

20 30 40 50 60

G * ,

( P a )

1e+7

1e+8

3.0 mm gap2.5 mm

2.0 mm1.5 mm1.0 mm

0.47 mm0.35 mmcone-plate

Figure 4: Black curves measured at different gap settings.

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Initial Strain

0,001 0,01 0,1

T i m e t o f a i l u r e ,

( s )

100

1000

10000

B15, 33.0oC

Regr.

B65, 20.5oC

Regr.

B90, 18.0oC

Regr.

B180, 12.5oC

Regr.

Figure 5: Time-initial strain plot for four bitumenhaving different penetrations.

Initial Strain

0,001 0,01 0,1

T i m e

t o f a i l u r e ,

( s )

100

1000

10000 Straight-run

Regr

Sox35, 24oC

Regr

Ox20, 29oC

Regr

Figure 6: Time-initial strain plot for straight-run and oxidized samples.

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Comparison with fatigue mix testing:So far, only a limited number of direct comparisons between binder and asphalt mix fatigue lines were

made. In a first study, 5 straight-run and 5 polymer modified binders were compared regarding binder-and mix-related fatigue properties. The test conditions such as temperature, frequency and in this case

the use of controlled stress, were exactly the same in both cases. These results showed a good

agreement in the case of pure bitumens; the position as well as the slopes of the fatigue lines were verywell alike. For the PMB’s however, a less satisfactory agreement was found.

In a second comparative study, the same “intrinsic” fatigue behaviour as shown in Figure 5 could beverified under controlled strain conditions for straight-run bitumens of different penetration grades.

When tested under equi-stiffness conditions (hence at different temperatures), the fatigue lines of the binders were confounded. The same result was obtained for the fatigue lines measured on a mixat the corresponding temperatures. Again, however, this agreement was not found when

contemplating polymer-modified binders. Additional tests are planned to clarify this issue.

On interpreting these fatigue lines, and their impact on fatigue cracking in asphalt mixes, it is clear that the strain level, which the bitumen actually is exposed to in the asphalt mix, is crucial. Fatigue

damage increases with increasing strain, especially for oxidized and aged samples. Recent research(3), has indicated that the actual strain levels in asphalt mixes is much higher, about 10 to 100times, as the bulk strains to which mixtures are subjected. Because of the large difference in

stiffness between the binder and the aggregates, the bitumen domains will take most of thedeformation. And because of the non-uniformity of the binder domains, it is expected that there is acomplex strain distribution over these domains. In our comparison between binder and mix fatigue

life times, we also found a strain difference between the binder and the mix of about 100 times, inorder to obtain the same life time.

Conclusions

q Below a certain temperature, bitumen binders exhibit fatigue cracking, which can be measured by applying continuous cyclic shear loadings, as for example in a dynamic shear rheometer.

q Different penetration grades have the same fatigues line if tested at a constant value for G*.

q Upon oxidation and after aging, fatigue lines become more strain dependent.q Polymer modification can have a positive effect on the fatigue life, but this is not general.

q No correlation with the SHRP fatigue parameter could be established.q So far, our results suggest a good correlation with fatigue mix tests for non-modified binders, less

correlation for PMB`s

References

(1) D.A. Anderson, D.W. Christensen, H.U. Bahia, R. Dongre, M.G. Sharma, J.J. Button, Binder Characterization and evaluation, Volume 3, Physical characterization, SHRP-A-369 Report, The StrategicHighway Research program, National research Council, Washington, D.C., 1994

(2) S.C.S. Rao Tangella, J. Craus, J.A. Deacon, C.L. Monismith, Summary report on fatigue response of asphalt mixtures, SHRP-A/IR-90-011-Report, The Strategic Highway Research program, Nationalresearch Council, Washington, D.C., 1990

(3) H.U. Bahia, H. Zhai, K. Bonnetti, S. Kosi, Non-linear viscoelastic and fatigue properties of asphalt binders, a paper submitted for presentation at the 1999 Annual Meeting of the Association of AsphaltPaving Technologists, July 1998

(4) V. Potscha, H. Schmidt, Fatigue life of polymer modified bitumen, Proc. Eurobitume Congress, Vol IA,16-18 June, Stockholm, 1993

(5) M.C. Phillips, Multi-step Models for Fatigue and Healing, and Binder Properties Involved in Healing,Eurobitume Workshop 99 – Performance Related Properties for Bituminous Binders, paper No. 115(6) H. Soenen, B. Eckmann, Binder-related Fatigue Properties studied by Rheology, V International

Conference Durable and Safe Road Pavements, Kielce 11-12 May 99

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