TRANSPORT RESEARCH LABORATORY

Rheological properties of polymer-modifiedbinders for use in rolled asphalt wearingcourse

Prepared for Quality Services (Civil Engineering)of the Highways Agency

D R Gershkoff, J Carswell and J C Nicholls

TRL REPORT 157

The information contained herein is the property of the TransportResearch Laboratory. This report has been produced by the TransportResearch Laboratory under a contract placed by the Department ofTransport. Any views expressed in it are not necessarily those of theDepartment. Whilst every effort has been made to ensure that thematter presented in this report is relevant, accurate and up-to-date atthe time of publication, the Transport Research Laboratory cannotaccept any liability for any error or omission.

Transport Research Foundation Group of Companies

Transport Research Foundation (a company limited by guarantee) trading as TransportResearch Laboratory. Registered in England, Number 3011746.

TRL Limited. Registered in England, Number 3142272.Registered Offices: Old Wokingham Road, Crowthorne, Berkshire, RG45 6AU.

First Published 1997ISSN 0968-4107

Copyright Transport Research Laboratory 1997. All rights reserved.

iii

CONTENTS

Executive Summary 1

1 Introduction 3

2 The binder test programme 3

2.1 Selection and preparation of binders 3

2.2 High-temperature viscosity 4

2.3 Empirical tests 4

2.4 Dynamic testing 5

3 Relationships between empirical and dynamic tests 6

3.1 Penetration 6

3.2 Ring and ball softening point 8

3.3 Fraass breaking point 8

3.4 Summary 9

4 Bituminous mixture test programme 9

4.1 Rolled asphalt mixtures 9

4.2 Design properties 9

4.3 Wheel-tracking test 10

4.4 Repeated load axial test (RLAT) 11

4.5 Yield strain 11

5 Relationships between binder propertiesand mixture deformation resistance 11

5.1 Wheel-tracking rate 11

5.1.1 Expected behaviour 11

5.1.2 Laboratory-derived relationships 13

5.2 Dynamic creep stiffness 15

6 Relationships between binder propertiesand resistance to cracking 15

6.1 Performance at low temperatures 15

6.2 Yield strain 16

7 In-situ deformation results 16

7.1 A38 Trial site 16

7.2 Validation from site 18

8 Conclusions 18

9 Acknowledgements 19

10 References 19

iv

Appendix A: Glossary of rheological terms 21

Appendix B: The viscoelastic response of bitumen 22

B.1 General 22

B.2 Viscosity 22

B.3 Stiffness and Van der Poel 22

B.4 Shell bitumen test data chart 22

B.5 Dynamic characterisation of bitumens 23

B.6 Definitions of dynamic parameters 23

B.7 Presentation of dynamic results - master curves 23

B.8 Creep characterisation of bitumens 26

B.9 References 27

Abstract 28

Related publications 28

1

This report describes a two-stage programme investigatingthe effect of polymer-modified binders on the deformationresistance of rolled asphalt wearing course mixtures. Thefirst stage examines a range of binders using a variety ofempirical binder test methods and fundamental rheologicalcharacterisation; an explanation of the concepts used todescribe viscoelastic properties of bitumens and a glossaryof some of the main rheological terms are given inappendices. The second stage examines the properties of arolled asphalt mixture with the same range of binders inorder to establish the relationship between binder propertiesand the deformation resistance of asphalt mixtures. Inaddition the resistance to cracking, as measured by theyield strain test, is also investigated. Deformation resultsafter 8 years trafficking for a number of modified rolledasphalt wearing courses from a full-scale road trial(A38, Staffordshire) are reported to validate the laboratoryinvestigation.

Empirical binder tests, such as penetration and softeningpoint, are often used for specifying binders and predictingthe likely performance of mixtures incorporating thesebinders; however, their use can result in misleadingpredictions when applied to polymer-modified binders. Theresearch shows that fundamental rheological testing hashelped to clarify which empirical tests are likely to proveuseful both for specifying modified binders and forpredicting wearing course performance.

The tests carried out on the rolled asphalt mixesconcentrated on the measurement of deformation resistance(wheel-tracking and dynamic creep) although otherperformance properties were also measured. Thedeformation properties were compared with thefundamental properties of the binders and encouragingrelationships were derived. The deformation properties

were also compared with the proposed SHRP deformationcriterion.

Both deformation parameters (wheel-tracking anddynamic creep) could be reliably predicted from themeasurement of the complex shear modulus of the binder(G*). In particular, a strong correlation was foundbetween wheel-tracking rate and the SHRP deformationcriterion G*/sinδ, where δ is the phase angle betweenstress and strain. However the SHRP criterion greatlyunderestimated the wheel-tracking rates of one modifiedbinder at both 45oC and 60oC. A better relationship wasestablished by choosing a longer loading time and usingjust the G* parameter (sinδ having little effect). A verystrong relationship was obtained between the creep(RLAT) and binder complex shear modulus results(G* at 1800 seconds).

In-situ deformation results from the full-scale trial werealso promising in that they confirmed the appropriatenessof the wheel-tracking rate and, by inference, dynamiccreep and binder shear modulus as predictors of rut-resistance. Thus, it should be possible to predictdeformation behaviour from measurements of binderrheology alone. In fact, a similar study examining binderproperties and mix stiffness for modified roadbasematerials has already indicated the usefulness of dynamicrheological testing for predictive purposes.

The research supports the concept of either specifyingor designing rolled asphalt mixtures for deformationresistance from properties of the binder for a givenaggregate skeleton. With the move towards performance-related specifications for rolled asphalt wearing coursesusing the wheel-tracking test, this should help suppliers toassess the appropriate binder to use with particularaggregate sources and grading to ensure compliance.

EXECUTIVE SUMMARY

2

3

rolled asphalt wearing course materials, particularly interms of deformation resistance. Previous studies haveexamined the modified binders used in these trials usingrelatively simple empirical tests. Therefore the HighwaysAgency commissioned TRL to carry out a laboratory-basedresearch programme to evaluate the effect of modifiers onbinder properties measured using the new generation ofrheometers. The programme involved a detailed rheologicalanalysis of a number of modified binders using dynamic(oscillatory) measuring techniques as well as the morefamiliar empirical test methods; this was followed by alaboratory evaluation of a standard rolled asphalt mixmanufactured using these same binders.

This report presents the results of these tests, discussesthe relationships between the empirical and dynamic binderproperties and considers the relationships between thesebinder properties and the deformation resistance of theresultant mixtures. A brief appraisal is made of theSHRP deformation criterion with regard to theserelationships. Also, the measured deformation of a full-scale road experiment, with a number of modified rolledasphalt mixtures, is discussed in relation to the laboratoryprogramme.

Appendix A gives a glossary of some of the mainrheological terms used throughout the report, andAppendix B gives a brief description of the viscoelasticresponse of bitumens. Readers who are not familiar withthe subject of bitumen rheology may wish to read theappendices before reading the main body of the report.

2 The binder test programme

Eleven different bituminous binders were studied; threeunmodified bitumens, seven laboratory-prepared polymer-modified bitumens and one proprietary polymer-modifiedbitumen. The polymers chosen were ethylene-vinyl acetate(EVA), styrene-butadiene-styrene (SBS) block co-polymerand polyethylene (PE). An extensive binder test programmewas carried out on the binders using both empirical testmethods and fundamental measuring techniques.

2.1 Selection and preparation of bindersThe conventional binder normally used in rolled asphaltwearing course mixtures is 50 pen bitumen. However, aspolymer modification often results in a stiffer binder,polymers are generally blended with softer grade bitumens,such as 70 pen bitumen. To study the effects of bitumentype on the properties of the resulting polymer-modifiedbitumens, two different 70 pen bitumens, from differentcrude sources designated A and B, were used. A 50 penbitumen from source A was used as a control. The sourceof the bitumen used in the proprietary SBS modified binderis not known. The PE, EVA and SBS-modified binders arecommonly used to improve the in-service performance ofrolled asphalt surfacings and the claimed improvedworkability of EVA-modified bitumens has led to their usein adverse weather conditions. The details of the bindersused are given in Table 1.

The laboratory blends were prepared by blending thepolymer granules into hot bitumen with a high shear mixer

1 Introduction

In recent years, the number of vehicles on UK roads hasrisen dramatically with larger and heavier trucks being usedfor road haulage. This is placing increased demands uponthe pavement structure and hence upon the materials thatconstitute that structure. As a result, there is a growinginterest in the development, use and specification ofimproved materials.

The binder has an important role to play in theperformance of bituminous materials, particularly in rolledasphalt where the mechanical properties of the binder havea strong influence upon the mechanical properties of themixture. The use of polymer additives to improve thebehaviour of bitumen is an increasingly popular route toachieving improved performance from bituminousmaterials, but there are many different types of polymermodifiers available with potentially different properties.

When polymers are added to bitumens, even in relativelysmall quantities, the rheological behaviour of the resultingbinder is often dramatically different from that of theunmodified bitumen. Empirical test methods, such aspenetration and softening point, do show some evidence ofthe presence of an additive but cannot be relied upon toquantify the changes that have been brought about. Thus, itis necessary to look towards other methods for specificationpurposes. Fundamental rheological testing can be used toaccurately define the rheological response of binders,including modified binders, over a very wide range oftemperatures and loading times. In particular, dynamictesting techniques are proving increasingly popular forstudying the effects that polymer modification have uponbitumens.

The concept of moving towards fundamental testing toaid the prediction of asphalt performance has beenrecognised for many years. In the USA, the StrategicHighways Research Program (SHRP) has proposed variousdynamically measured rheological parameters which areclaimed to predict mix performance. The SHRP proposalsfor binder properties are more fundamental than current UKbinder specifications and also include an examination of theageing properties of binders used. Perhaps the mostimportant documents are those relating to specifying binderproperties for bituminous mixes (Lytton et al, 1993). TheSHRP proposals for binders are primarily for asphalticconcrete type mixtures and cover three climatic zones. Oneof the outcomes of the SHRP research has been theproposal for a specification based on two fundamentalrheological properties which is designed to limitdeformation.

The SUPERPAVE (SUperior PERforming PAVEment)is the final product of the SHRP asphalt researchprogramme (Kennedy et al, 1994; Cominsky, 1994). It is aninnovative mixture design and analysis system to be used atthree different levels of traffic. Currently, validation of theSUPERPAVE analysis model is limited to Level 1 (roadscarrying less than 1 million equivalent standard axles)which involves volumetric design using a gyratorycompactor.

There are a number of on-going full-scale trials in theUK to evaluate the performance of modified materials in

4

to obtain a homogeneous mix. The SBS and PE modifierswere blended at 180oC for fifty minutes and the EVAmodifiers were blended at 160oC for thirty minutes. TheEVA and PE additives were blended at the five per centlevel; the SBS additive was blended at the seven per centlevel to allow direct comparison with the proprietarySBS-modified binder.

2.2 High-temperature viscosityViscosity measurements over the range 120oC to 180oC,corresponding to the conditions for mixing, laying andcompacting rolled asphalt wearing course mixtures, weremade on each binder using a Haake viscobalance, aviscometer which utilizes the principle of a falling sphere ina fluid. The equiviscous temperatures corresponding tomixing and compaction conditions for rolled asphaltwearing course materials (Nicholls & Daines, 1993) aregiven in Table 2.

For all the laboratory-prepared blends, the addition ofpolymer increased the viscosity of the binder over the entiremixing, laying and compaction range without alteringsignificantly the shape of the temperature/viscosity curve.The SBS modification had the greatest effect and resultedin a ten-fold increase in the viscosity. The PE modificationhad the next greatest effect, increasing the viscosity by afactor of five, while the EVA 33-25 and EVA 18-150additives increased the viscosity by approximately threeand two and a half times, respectively.

The very high mixing and compaction temperaturesindicated for several of the blends would, in practice, resultin increased energy production costs and, probably,unacceptable hardening of the binder. In practice, it hasbeen found that adequate mixing and compaction of EVAand SBS modified binders can, with care, be achieved withsignificantly lower temperatures. For example, the supplierof binder SBS(Pr) recommends a mixing temperature rangeof 160o to 170oC, some 30oC lower than the temperatureinferred from viscosity measurements. However, trials withpolyethylene (PE) modified materials have shown thathigher temperatures are required to produce adequatecompaction (Denning & Carswell, 1983).

2.3 Empirical testsThe penetration (IP 49), softening point (IP 58) and Fraassbreaking point (IP 80) of all the binders were measured,both before and after they were submitted to the rolling thinfilm oven test (RTFOT) (ASTM, 1974). The RTFOT isdesigned to simulate the hardening experienced during themixing, laying and compaction processes. In addition tomeasuring at the standard test temperature of 25oC,penetration was also measured at 15o and 35oC. The resultsfor all these tests are given in Table 3a and Table 3b. Thetables also includes the penetration index (PI) calculated asa best-fit through the three penetration results.

The penetration and softening point results for 50(A) areclose to the middle of the specification for 50 pen bitumenin BS 3690 (BSI, 1989). The two 70 pen bitumens,70(A) and 70(B), are close to the stiff and soft end of theirspecification limits respectively. In fact, the differences inproperties between 70(A) and 50(A) are far less than thosebetween 70(A) and 70(B).

Of all the binders tested, SBS(Pr) had the highestsoftening point, the lowest Fraass point and the highestpenetration at 25oC. Comparing the properties of thelaboratory-prepared blends to those of their base bitumens,several trends became apparent. For each of the threeadditives common to both binders, softening pointincreases were greater with the bitumen from crude sourceA than with the bitumen from crude B. SBS had thegreatest effect on the softening point followed by18-150 EVA and 33-25 EVA with PE having the leasteffect. The reverse was true of the effect on the penetrationat 25oC with the bitumen from crude B showing greaterreductions than the bitumen from crude A with all threeadditives. This was particularly noticeable for SBS whichreduced the penetration of 70(A) by 20mm/10 and 70(B) by35 mm/10. The addition of polymers had a variable effectupon the Fraass breaking point. Of the seven binders, onlySBS(Pr) showed any improvement; the Fraass breakingpoint for the rest either increased (that is, worsened) orremained the same.

The effect of RTFOT hardening on the three unmodifiedbitumens was to reduce their penetrations at 25oC bybetween 35 and 38 per cent with corresponding increases insoftening point and Fraass breaking point temperatures.This is in good agreement with the 65 per cent retainedpenetration rule-of-thumb generally found during themixing and compaction processes (Bell, 1988). The

Table 2 Equiviscous mixing and compactiontemperatures

Compaction

Binder Mixing Maximum Minimum

0.2 Pa.s 0.6 Pa.s 10 Pa.s

50(A) 163oC 137oC 92oC70(A) 155oC 132oC 89oC70(B) 145oC 124oC 83oCPE(A) 187oC 161oC 109oCSBS(Pr) 195oC 159oC 105oCSBS(A) 207oC 173oC 120oCSBS(B) 205oC 170oC 114oCEVA

18(A) 183oC 151oC 99oC

EVA18

(B) 170oC 142oC 90oCEVA

33(A) 185oC 155oC 102oC

EVA33

(B) 175oC 147oC 98oC

Table 1 Binders tested in the study

Binder Code Description

50(A) 50 pen (Crude source A)70(A) 70 pen (Crude source A)70(B) 70 pen (Crude source B)PE(A) 70(A) with 5 per cent polyethyleneSBS(Pr)* Pre-mixed proprietary binder with 7 per cent SBSSBS(A) 70(A) with 7 per cent Styrene-Butadiene-StyreneSBS(B) 70(B) with 7 per cent Styrene-Butadiene-StyreneEVA

18(A) 70(A) with 5 per cent Ethylene-Vinyl Acetate 18-150

EVA18

(B) 70(B) with 5 per cent Ethylene-Vinyl Acetate 18-150EVA

33(A) 70(A) with 5 per cent Ethylene-Vinyl Acetate 33-25

EVA33

(B) 70(B) with 5 per cent Ethylene-Vinyl Acetate 33-25

* Proprietary (Pr)

5

penetrations of the modified binders also fell after theRTFOT, but not as consistently. SBS(Pr) dropped by only22 per cent while the penetration of all the laboratory-prepared modified binders decreased by varying degrees to30 ±3 mm/10. The RTFOT increased the softening pointsof all but two of the modified binders by between 2oC and11oC; SBS(Pr) and SBS(B) showed slight reductions.Fraass breaking point temperatures were reduced for allbinders except SBS(A).

2.4 Dynamic testingDynamic measurements were made on all the binders afterthe RTFOT using a controlled stress rheometer using 4cmand 1cm diameter parallel plate geometries. Testing wascarried out at 80oC, 60oC, 40oC, 25oC, 15oC, 5oC, -5oC and-15oC over the frequency range 0.1 to 10 Hz with carebeing taken to ensure that the response of the binders waskept within the linear viscoelastic region. These testingconditions cover the range of temperatures and loadingtimes likely to be experienced in service.

The master curves of the logarithm of complex shearmodulus, log (G*), and phase angle, δ, at 25oC for the threeunmodified bitumens are shown in Figure 1.

Table 3a Empirical test results: Initial properties

PenetrationBinder R&B Fraass

(mm/10) PI(oC) (oC)

15oC 25oC 35oC

50(A) 54.2 -17 21 49 102 +1.070(A) 50.0 -20 27 63 146 +0.670(B) 46.6 -16 29 80 254 -1.1PE(A) 60.2 -15 17 39 97 +0.4SBS(Pr) 92.8 -22 34 95 180 +0.7SBS(A) 89.8 -15 19 43 74 +2.1SBS(B) 84.6 -13 19 45 80 +1.7EVA

18(A) 73.0 -15 22 39 81 +2.4

EVA18

(B) 66.2 -16 21 49 118 +0.4EVA

33(A) 61.4 -20 22 45 97 +1.5

EVA33

(B) 52.6 -20 23 59 143 +0.1

Table 3b Empirical test results: Properties after rolling thin-film oven test

PenetrationBinder R&B Fraass (mm/10) PI

(oC) (oC) 15oC 25oC 35oC

50(A) 61.8 -12 12 32 66 +0.570(A) 57.6 -16 14 39 86 +0.170(B) 51.6 -13 21 50 138 -0.2PE(A) 70.0 -13 11 27 62 +0.4SBS(Pr) 91.6 -21 24 74 161 -0.2SBS(A) 100.0 -17 15 32 58 +2.2SBS(B) 80.8 -12 14 29 67 +1.1EVA

18(A) 81.6 -12 13 33 57 +1.5

EVA18

(B) 68.2 -12 14 31 71 +0.9EVA

33(A) 72.2 -16 13 33 61 +1.2

EVA33

(B) 62.0 -16 15 32 80 +0.6

109

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102

101

I G*

I (P

a)

10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104 105 106 107 108

ωAT

(Rad.s-1)

90

80

70

60

50

40

30

20

10

0

δ (degrees)

50 (A)

70 (A)

70 (B)

IG*I δ

Figure 1 Master curves for the bitumens (RTFOT) at 25oC

6

The curves show characteristic bitumen behaviour,approaching viscous and elastic extremes at low and highfrequencies respectively. At angular frequencies below100rad/s, 50(A) has the highest modulus and the lowest δ,followed by 70(A) and then 70(B). At higher frequencies,the response of the two bitumens from crude A is nearlyidentical, the modulus and δ curves being flatter than thosefor 70(B).

Master curves for all of the modified binders except thetwo EVA

18 blends are shown in Figure 2 with the results for

50(A) superimposed to enable direct comparison to be made.The dynamic data for the EVA

18 has not been included because

the results were extremely inconsistent, varying quitesignificantly from test to test. This is thought to be due to theunstable nature of this particular polymer-bitumen blend, theEVA separating out from the bitumen on cooling, giving rise toa coarse dispersion rather than a uniform blend (Hoban, 1987).

The response of SBS(Pr) is dramatically different from that ofunmodified bitumens. The δ curve is bell shaped rather than thenormal sigmoid, peaking at around 72o at an angular frequencyof 10 rad/s. At low frequencies (and/or at high temperatures),SBS(Pr) exhibits soft elastic behaviour rather than purelyviscous behaviour. Over most of the frequency rangeinvestigated, the modulus level of SBS(Pr) is far lower than thatof 50(A). However, at low frequencies, the G* curve starts toflatten out and crosses the curve for 50(A) at around10-4 rad/s at 25oC. This type of rheological response has beenobserved by others (Vonk & Van Gooswilligen, 1989;Dickinson, 1981) and is referred to as networking because thepolymer has formed a network structure within the binder.

The results for the laboratory-prepared SBS blends donot show the same departure from the conventionalbitumen response, but the effects of modification are stillvery evident. The G* master curves are flatter than those oftheir base bitumens, being far stiffer at low frequencies andconsiderably less stiff at high frequencies. The δ curves arealso much flatter than those of their respective basebitumens, SBS(A) appearing to flatten out at lowfrequencies. The addition of SBS to the stiffer of the two70 pen bitumens, 70(A), has resulted in a binder, SBS(A),with a modulus far greater than of that of the 50 penbitumen at low and intermediate frequencies, and far lowerthan 50 pen at high frequencies. The failure of thelaboratory-prepared SBS blends to show the dramaticnetworked response of SBS(Pr) confirms the need to use acompatible bitumen for blending with SBS.

The 33-25 EVA-modified binders show signs ofmodification at low frequencies where modulus levels aremuch higher than their respective base binders, but not ashigh as those of the SBS blends. Modulus and δ values atvery high frequencies are very similar to those of theunmodified binders. The response of the PE-modifiedbinder is nearly identical to that of 50(A), modulus levelsbeing marginally greater across the whole frequency range.

The best-fit characteristic temperature (TS) values are

listed in Table 4 for all the binders; the higher the value, thegreater the dependence on temperature (see Appendix B).Of the three bitumens, 50(A) is the most sensitive tochanges in temperature, followed by 70(A) and then 70(B)whilst, of all the binders, SBS(Pr) has the least sensitivity totemperature. The T

S values for the laboratory-prepared

blends are very similar to those of the base bitumen, thepolymer appearing to have little or no effect upontemperature dependence. In general, a low value of Ts ispreferable although it is the overall response of the binder,which is governed by temperature and time dependency(shear susceptibility), that is most important.

3 Relationships between empirical anddynamic tests

3.1 PenetrationPrevious research has shown that, for unmodified bitumens,the penetration test relates well to the stiffness or modulusat a loading time of 0.4 seconds (Van Der Poel, 1954).Figure 3 shows the logarithm of G* at 0.4 seconds plottedagainst the logarithm of penetration for all the binders inthis study. The graph includes the penetrations(and corresponding G* values) at 15oC, 25oC and 35oC.

Linear regression analysis of the data produces Equation 1with a correlation of r2 = 0.98.

log (G*0.4 s

) = 8.74 - 1.91 x log (Pen) .... (1)where

G*0.4 s

is G* at a loading time of 0.4 seconds inPa; and

Pen is the penetration in mm/10 for 100g loadfor 5 seconds.

If results from studies of surface dressing binders(Gershkoff, 1991) and roadbase binders (Carswell &Gershkoff, 1993) are also included, Equation 1 is slightlymodified to Equation 2 with r2 = 0.97.

log (G*0.4 s

) = 8.80 - 1.95 x log (Pen) .... (2)

The high correlation shows that the relationship is validfor both modified and unmodified binders. The relationshipbetween the two parameters is such that halving thepenetration produces approximately a four fold increase inbinder stiffness at 0.4 seconds.

A short loading time of 0.4 seconds relates reasonablywell to traffic loading conditions. For binders with the sameshear susceptibility (time dependency), the penetrationresults will also correlate well with stiffness or viscosityvalues at other loading times. However, this is not true ofbinders with different shear susceptibilities.

Table 4 Characteristic temperature values

Binder TS

(oC) Binder TS (oC) Binder T

S (oC) Binder T

S (oC)

50(A) 48 - - - - - -70(A) 46 SBS(A) 49 EVA

33(A) 48 PE(A) 46

70(B) 43 SBS(B) 44 EVA33

(B) 43 - -- - SBS(Pr) 40 - - - -

7

Figure 2 Log (G*) and δ master curves at 25oC for the modified binders superimposed on the results for 50(A)

109

108

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103

102

101

10-6 10-4 10-2 100 102 104 106 108

ωaT

(rad/s)

G*

(Pa)

90

80

70

60

50

40

30

20

10

0

δ (degrees)

109

108

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106

105

104

103

102

101

G* (Pa)

90

80

70

60

50

40

30

20

10

0

δ (degrees)

10-6 10-4 10-2 100 102 104 106

ωaT

(rad/s)

108

SBS(Pr)SBS(A)SBS(B)

�∇

Hollow symbols δ Filled symbols G*

109

108

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106

105

104

103

102

101

G*

(Pa)

10-6 10-4 10-2 100 102 104 106 108

ωaT

(rad/s)

10-6 10-4 10-2 100 102 104 106 108

ωaT

(rad/s)

109

108

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106

105

104

103

102

101

G* (Pa)

90

80

70

60

50

40

30

20

10

0

EVA33(A)EVA33(B)

�

∇PE(A)

90

80

70

60

50

40

30

20

10

0

8

3.2 Ring and ball softening pointPenetration at the softening point is approximately 800mm/10for many, but not all, unmodified bitumens; the exact valuevaries with penetration index (PI) and wax content (Pfeiffer& Van Doormaal, 1936). Values of G*

0.4 s at the softening

point temperatures have been calculated from the mastercurves, along with predicted penetration calculated usingEquation 2. These values are shown in Table 5. Thesoftening point temperatures have been increased by 1.5oCto allow for differences between the stirred (IP58) andunstirred tests, as used in the ASTM and many otherstandard methods (Krom, 1950).

The predicted penetrations for the three unmodifiedbitumens range from 588 to 740mm/10, somewhat lowerthan the expected value of 800mm/10. However, given thatthe reproducibility of the softening point test is 2.5oC and

that the predicted penetration values are based on aconsiderable extrapolation of a tentative relationship, theagreement is reasonable. The stiffness and, therefore, thepredicted penetration values for the EVA

33- and

PE-modified bitumens are comparable with those of theunmodified bitumens. However, the three SBS-modifiedbitumens have far lower stiffness values at their softeningpoint. Therefore, plotting the softening point of anSBS-modified binder on a Shell Bitumen Test Data Chart(BTDC) (Heukelom, 1969) is misleading. This explainswhy the use of softening point values to predict the hightemperature deformation behaviour of SBS-modifiedbinders in bituminous mixes tends to overestimate theirperformance (Denning & Carswell, 1981).

3.3 Fraass breaking pointThe Fraass breaking point test is one of the few tests thatcan be used to investigate the behaviour of bituminousbinders at low temperature (as low as -30oC). It is used forspecification purposes in a number of countries whichexperience very low winter temperatures including Canada,Finland, Norway and Sweden. For conventional road gradebitumens, the Fraass breaking point has been found torepresent an equi-stiffness temperature at a loading time ofeleven seconds (the period of flexure in the Fraass test)with a stiffness of around 110 MPa (Van der Poel, 1954).However, the stiffness at failure may be far greater ataround 2100 MPa (Thenoux et al, 1987). These stiffnessvalues are equivalent to shear modulus values of 37 and700 MPa, respectively. The Fraass breaking point isrepresented on a BTDC by the temperature at which thebinder has a penetration of 1.25 mm/10.

Table 5 G*0.4 s

and predicted penetration at softeningpoint

Predicted PenetrationBinder Measured G*

0.4 s(using Equation 2)

(Pa) (mm/10)

50(A) 1660 73770(A) 1950 67870(B) 2570 589PE(A) 1413 800SBS(Pr) 380 1571SBS(A) 631 1211SBS(B) 646 1197EVA

33(A) 2754 568

EVA33

(B) 2291 624

Figure 3 Relationship between penetration and G* at 0.4s

107

106

105

104

10 20 30 40 50 60 70 80 90 100 200

Penetration (mm/10)

G*

(Pa)

9

Values for G* at loading times of 0.4 and 11 seconds atthe Fraass breaking point temperatures of the binders havebeen calculated and are shown in Table 6.

The G* values at 11 seconds for the unmodifiedbitumens and the laboratory-prepared blends ranged from41 to 89 MPa with a mean value of 68 MPa. This isreasonably close to the mean value of 37 MPa referred toabove, especially given the poor precision of the testmethod. The result for SBS(Pr) was somewhat higher, withG* equal to 138 MPa. The G* values at 0.4 seconds wereslightly less scattered with SBS(Pr) still the stiffest of thebinders. The predicted penetration values ranged from1.64 to 2.34 mm/10 with a mean of 1.95 mm/10, slightlyhigher than 1.25 mm/10. However, given that the G* valuesat 11 seconds were somewhat higher than expected, thepenetration values should be lower than 1.25 mm/10.Therefore, it is probable that this discrepancy is due to asmall inaccuracy in Equation 2 which has been magnifiedby extrapolation.

These findings confirm that, for unmodified bitumensand the laboratory-prepared blends, the Fraass temperaturerepresents, to a first approximation, an equi-stiffnesstemperature, corresponding to a G* of approximately70 MPa for a loading time of 11 seconds. The result forSBS(Pr) indicates that it is stiffer at its Fraass breakingpoint than the other binders.

3.4 Summary

� There is a strong correlation between G* at 0.4sand penetration for both unmodified andmodified binders.

� The relationship between ring and ball softeningpoint and binder modulus is less precise butSBS-modified binders have far lower stiffness valuesat their softening points.

� The Fraass breaking point represents, to a firstapproximation, an equi-stiffness temperature,corresponding to a G* of approximately 70 MPafor a loading time of 11 seconds.

4 Bituminous mixture test programme

The binder testing programme was complemented by alimited programme in which the same binders were used inrolled asphalt design mixtures. The properties of theresultant mixtures were tested to compare the relativeperformance of each binder.

4.1 Rolled asphalt mixturesEach of the binders tested and described in Section 2 wereused to manufacture a standard rolled asphalt mixture. Thecoarse aggregate from Bardon Hill (nominal 14mm) with amid-stability sand fine aggregate (Compton Bassett) andlimestone filler were combined to give the mid-point of theaggregate grading specified in Table 3, Column 3/2 ofBS 594: Part 1 (BSI, 1992) (Designation 30/14 rolledasphalt); the grading envelope is shown in Table 7. Todetermine the design properties of the rolled asphalts,samples were manufactured in accordance with BS 598:Part 107 (BSI, 1990a).

For the wheel-tracking, yield strain and creep testing,rolled asphalt samples (300 x 300 x 50mm and 300 x 300 x70mm) were manufactured in the computer-controlledroller compactor at TRL using the same aggregate sources,grading envelope and at the design binder content of7.7 per cent. Both the binder and aggregate were pre-heatedeither to the mixing temperatures given in Table 2 or to170oC, whichever was the lowest. A standard compactionregime of 20 passes at a load of 30kN was used in anattempt to compact the samples to the same density.

4.2 Design propertiesThe design binder content of 7.7 per cent for the mixtureswas determined with triplicate samples in accordance withBS 598: Part 107 (BSI, 1990a) using the 50pen bitumen(from crude A). It was assumed that this design bindercontent of 7.7 per cent would be the same for all the otherbinders. A limited testing programme at the design bindercontent and ±0.6 per cent (representing allowabletolerances in practice) was carried out and the results aregiven in Table 8. (Insufficient quantities of theEVA

18-modified binders were available to carry out this

part of the test programme.)Table 8 also shows the change in stability for each

material over the binder tolerance range of design±0.6 per cent. Given that the results are based on one set ofa restricted range, some interesting trends are observed.Most of the binders show a similar change in absolutestability values over the 1.2 per cent allowable range. Themain exceptions are for the PE modified material, where

Table 6 Modulus values at the Fraass breaking pointtemperature

Fraass G* at 11s G* at 0.4s PredictedTemperature Penetration

Binder (oC) (MPa) (MPa) (mm/10)

50(A) -12 55 120 2.3470(A) -16 85 182 1.8970(B) -13 71 195 1.83PE(A) -13 63 155 2.06SBS(Pr) -21 138 240 1.64SBS(A) -17 62 120 2.34SBS(B) -12 41 182 1.89EVA

33(A) -16 81 200 1.81

EVA33

(B) -16 89 209 1.78

Table 7 Aggregate gradings

Proportion ProportionBS Sieve passing BS Sieve passing(mm) (per cent) (µm) (per cent)

14 100 600 25 - 4710 90 - 100 212 5 - 306.3 35 - 70 75 4 - 82.36 35 - 47

10

the material appears relatively unaffected by small changesin binder content and for the laboratory-blended SBS(A)binder, where it is believed that separation occurred duringthe manufacture of the samples. Further, the 70(B) materialwhich had a low stability showed less change in stabilitywith binder content but the values were such that thismaterial would not be considered suitable for heavilytrafficked road sites. In fact, adding SBS or EVA

33 to 70(B)

still resulted in a marginal material (in terms of stability);the same could also be said of both the 70(A) and, even the50(A) material. As stated in Section 2.3 (above), the 70(A)bitumen was fairly similar to the 50(A) bitumen; this isconfirmed by the stability results shown in Table 8.

4.3 Wheel-tracking testA major requirement of a wearing course material is that itshould resist deformation under traffic loading. Rolledasphalt is particularly dependent on the stiffness of themortar because of its lack of aggregate interlock.Historically, the resistance to deformation was assessed bystability, but recent experience (Daines, 1992a) hasconfirmed previous concerns about the ability to controldeformation by stability (Szatkowski, 1979). Therefore, thewheel-tracking test, developed in order to provide amechanism to control deformation resistance, has beendefined in the UK by British Standard Draft forDevelopment test method DD184 (BSI, 1990b) althoughnot routinely called up in specifications as yet.

Samples of rolled asphalt were tracked in accordancewith the DD184 except that:

� each test consisted of a single determinationrather than the mean of six; and

� tests were carried out a temperature of 60oC aswell as at 45oC.

The additional, higher test temperature was chosen fortwo principal reasons; firstly, to discriminate better betweenmodified and unmodified materials and secondly, proposals

Table 8 Rolled asphalt mixture designs

Binder Stability Binder StabilityContent Stability Range* Content Stability Range*

Binder (M/M%) (kN) (kN) Binder (M/M%) (kN) (kN)

50(A) 6.0 5.7 50(A) (7.1) 6.5 +1.46.5 6.2 (7.7) 5.1 07.0 6.9 (8.3) 4.6 -0.57.5 5.7 70(B) 7.1 4.3 +0.38.0 4.9 7.7 4.0 0

70(A) 7.1 6.8 +1.7 8.3 3.5 -0.57.7 5.1 0 SBS(Pr) 7.1 7.3 +0.68.3 3.6 -1.5 7.7 6.7 0

PE(A) 7.1 8.7 0 8.3 5.1 -1.67.7 8.7 0 SBS(B) 7.1 10.7 +1.48.3 8.3 -0.4 7.7 9.3 0

SBS(A) 7.1 11.2 +1.5 8.3 7.0 -2.37.7 9.7 0 EVA

33(B) 7.1 6.4 +1.0

8.3 7.5 -2.2 7.7 5.4 0EVA

33(A) 7.1 7.5 +1.4 8.3 4.1 -0.7

7.7 6.1 08.3 4.9 -1.2

* The range of values of stability measured over the normal tolerance permitted on the binder content.

are being formulated which may include a 60oCwheel-tracking requirement for very heavily traffickedroads. The results of the wheel-tracking tests at both testtemperatures are given in Table 9.

At 45oC, the ranking of the materials starting with thelowest wheel-tracking rate is:

� SBS(Pr) and PE(A);

� SBS(B) and SBS(A);

� EVA33

(A) and EVA18

(A);

� EVA33

(B); and

� 50(A), 70(A) and 70(B).

At 60oC, the ranking was similar except that thedifferences between materials were much greater. Whilst itis difficult to interpret the results from a singledetermination on each material, some interestingobservations can be made:

� All the modified materials gave wheel-trackingrates less than 2mm/h at 45oC, indicatingsuitability (from a specifying viewpoint) for use onheavily trafficked roads.

� At 60oC differences emerge where, for the softerEVA grade (33/25) in both bitumen sources(A and B) and the EVA

18 grade in 70(A), high

tracking rates (over 20mm/h) are observed.

� Only the SBS blends, one of the EVA18

blendsand the PE blend gave fairly low wheel-trackingresults at 60oC. However, for the laboratoryblended SBS blends, there were concerns about thecompatibility/homogeneity of the resultant binders usedin the mixed material testing programme.

Based on this limited information, there would appear tobe a good case for setting a wheel-tracking requirement at60oC (say, less than 15mm/h) in order to screen outpotentially deformation prone materials at high in-serviceroad temperatures or, with transposition of temperature andloading time, under slow-moving traffic.

11

4.4 Repeated load axial test (RLAT)A larger mould, measuring 305mm x 305mm x 70 mm, wasused to make one slab of compacted material for eachbinder type from which five 100mm diameter cores werecut. These cores were used to measure the dynamic creepstiffness at 30oC±0.5oC of the mixtures by the repeat loadaxial test (RLAT) after being scanned in the TRL GammaRay Core Scanner to measure density.

RLAT is a dynamic load creep test in which a pulsedstress of 100 kPa is applied once every two seconds, for aduration of one second, to a cylindrical test specimenthrough a flat load platen. The deformation is measured bysumming the output from two diametrically opposed linearvariable displacement transducers (LVDTs) resting on theupper platen. The permanent strain measured at the end ofthe test (1800 load cycles) is used to calculate the creepstiffness, which is the maximum axial stress applied to thespecimen divided by the strain at the end of the test. Thetest is currently a British Standard Draft for Developmenttest method (BSI, 1993).

The mean creep stiffness results for each material aregiven in Table 10.

The unmodified mixtures generally had the lowest creepstiffness values with the effect of modification varying. Inparticular, EVA

33(A) had the greatest creep stiffness with a

value nearly three times that of EVA33

(B), the modifiedmixture with the lowest value. The 33/25 grade is arelatively soft grade of EVA and it is suspected that theblending of the EVA

33 in 70(A) has produced an unusually

stiff binder. Grouping the mixtures to produce a ranking,starting with the highest creep stiffness, gives:

� EVA33

(A);

� SBS(B), SBS(A) and PE(A);

� SBS(Pr), EVA33

(B) and 70(A); and

� 50(A) and 70(B).

4.5 Yield strainThe yield strain test, a development from research onreflection cracking in bituminous materials, gives ameasure of the ability of a mix to withstand tensile strains.The yield strain is the tensile strain at break in a samplesubjected to a constant tensile stress. In general, it isthought that materials with higher yield strain values areless likely to crack when subjected to the tensile strainscaused by thermal and other movements. In this project, thetest was used to examine whether the modified materials, inparticular the highly-elastic SBS binders, would provide ahigher yield strain than unmodified materials. Threesamples of approximately 100 × 60 × 50mm were cut fromeach of the untracked portion of the slabs used for thewheel-tracking tests for the yield strain test. Unfortunately,it was not possible to test either the 50(A) or 70(A) bindersbecause of damage to the specimens. A constant tensilestress of 103 N/mm was applied to the specimens at a testtemperature of 20oC (± 1oC) and the results are shown inTable 11.

The results for the laboratory-prepared blends rangefrom 1.6 per cent (for PE(A)) to 2.6 per cent (forEVA

33(B)). The unmodified binder 70(B) had a slightly

better result with a yield strain of 2.9 per cent. The bestresult was achieved by binder SBS(Pr) which had a yieldstrain of 4.0 per cent.

5 Relationships between binderproperties and mixture deformationresistance

5.1 Wheel-tracking rate5.1.1 Expected behaviourPermanent deformation in bituminous pavement layers iscaused by the accumulated viscous and plastic deformationin the mixture resulting from the repeated application oftraffic loading. This deformation can occur throughout thewhole of the pavement structure but, because bituminousmaterials have a low thermal conductivity, in practice themajority of deformation occurs in the wearing course wheretemperatures are higher, as well as the imposed load beingmore locally concentrated. Although it is widely recognisedthat the rutting tendencies of bituminous materials areprimarily influenced by mixture composition, the propertiesof the binder are also important.

The viscosity of the bitumen influences deformation

Table 9 Results of wheel-tracking tests

Wheel-tracking Wheel-trackingrate (mm/h) rate (mm/h)

Binder @ 45oC @ 60oC Binder @ 45oC @ 60oC

50(A) 4.4 45*70(A) 6.1 >100* 70(B) 15 57*PE(A) 0.6 12.8 SBS(Pr) 0.6 9.1SBS(A) 1.1 2.5 SBS(B) 1.0 7.5EVA

18(A) 1.4 30* EVA

18(B) - 12.7

EVA33

(A) 1.3 27* EVA33

(B) 1.9 80*

*Test stopped early due to excessive rutting of sample.

Table 10 Mean results of repeated load tests

RLAT Density RLAT DensityCreep (Core Creep (CoreStiffness scanner) Stiffness scanner)(@ 30oC) (@ 30oC)

Binder (MPa) (Mg/m3) Binder (MPa) (Mg/m3)

50(A) 1.82 2.33 70(B) 1.57 2.3570(A) 2.30 2.35 SBS(Pr) 2.67 2.34PE(A) 4.34 2.31 SBS(B) 4.73 2.34SBS(A) 4.68 2.30 EVA

33(B) 2.46 2.20

EVA33

(A) 7.41 2.29

12

resistance but, because polymer-modified binders tend toexhibit far greater time dependent or shear-thinningbehaviour than unmodified bitumens, there is someuncertainty as to what loading time should be used tocompare binder properties with mixture deformationresistance. This problem is illustrated in Figure 4, whichshows the complex viscosity (the complex modulus dividedby the angular frequency) for each of the binders at 45oCplotted against angular frequency.

At the lowest frequencies shown, the three bitumensreached a newtonian low shear-rate viscosity plateau,which would produce a gradient of unity for a plot oflog(G*) against log of frequency. As the frequencyincreases, the viscosity decreases but the ranking order forthe three bitumens remains unaltered with 50(A) being themost viscous and 70(B) the least viscous. The laboratory-prepared binders show varying degrees of shear thinning,although they all approach a limiting viscosity at lowfrequencies. PE(A) and EVA

33(B) have similar shear rate

dependencies to the unmodified binders. The other blendsshow increased shear rate dependency, as well assignificantly higher viscosities at low shear-rate. Theviscosity versus frequency plot for SBS(Pr) is ratherdifferent from those of the other binders with viscosity

increasing markedly at lower frequencies.The different shear rate dependencies of the modified

binders means that the relative viscosities (or moduli) of thebinders change with loading time. The Strategic HighwayResearch Program (SHRP) in the USA concluded that theresponse at short loading times governs the resistance todeformation (Kennedy et al, 1994) and hence havedeveloped specifications based around the behaviour at10 rad/s, equivalent to a loading time of 0.1 seconds.G*/sin δ is used in preference to G* in order to take intoaccount the increased elastic nature (lower δ values) ofmodified binders. However, there is a strong correlationbetween the wheel-tracking rate of rolled asphalt and ashear rate of 0.05 s-1, equivalent to a loading time of20 seconds, at 45oC for a variety of binders includingpolymer-modified blends (Denning & Carswell, 1981).Many pavement design methods also use the response ofthe material at long loading times to predict ruttingpotential (Powell et al, 1984; Nunn 1986).

Values of G* at 0.02, 0.1, 20 and 1000 seconds andG*/sin δ at 0.1 seconds, together with the modulus relativeto that of 50(A), are shown in Table 12. The results at1000 seconds have been obtained by extrapolation of thecomplex viscosity curves illustrated in Figure 4.

Comparing the relative G* values for the laboratoryblends at 0.02 seconds, EVA

33(A) is approximately 100 per

cent stiffer and PE(A) is 50 per cent stiffer than 50(A). Theother three blends are within plus or minus ten per cent of50(A). SBS(Pr) has by far the lowest modulus, one quarterthat of 50(A). At 0.1 seconds, the relative modulus valuesof the binders are similar to those at 0.02 seconds, althoughsome of the laboratory polymer blends are starting to showsome improvements; SBS(Pr) still has the lowest modulus.Division of G* by sin δ produced very little change, the

Table 11 Results of yield strain tests

Binder Yield Strain Binder Yield Strain

70(B) 2.9 per centPE(A) 2.3 per cent SBS(Pr) 4.0 per centSBS(A) 1.9 per cent SBS(B) 2.1 per centEVA

18(A) 2.2 per cent EVA

18(B) 1.7 per cent

EVA33

(A) 1.6 per cent EVA33

(B) 2.6 per cent

Table 12 Modulus values at 45oC

G* at 0.02 seconds G* at 0.1 seconds G*/sin δ at 0.1 secondsCriterion G* Rel’ve G* Rel’ve G*/sin δ Rel’veBinder (kPa) G* Ranking (kPa) G* Ranking (kPa) G*/sin δ Ranking

50(A) 242 1.00 4 77 1.00 4 90 1.00 570(A) 173 0.71 7 48 0.62 7 52 0.59 770(B) 117 0.48 8 26 0.34 8 27 0.30 8PE(A) 363 1.50 2 128 1.67 3 150 1.68 3SBS(Pr) 62 0.26 9 16 0.21 9 18 0.20 9SBS(A) 266 1.10 3 136 1.78 2 193 2.16 2SBS(B) 234 0.97 5 75 0.98 5 92 1.03 4EVA

33(A) 492 2.03 1 197 2.57 1 245 2.74 1

EVA33

(B) 217 0.90 6 64 0.83 6 72 0.80 6

G* at 20 seconds G* at 1000 secondsCriterion G* Rel’ve G* Rel’veBinder (kPa) G* Ranking (kPa) G* Ranking

50(A) 1.5 1.00 5 31 1.00 670(A) 0.6 0.36 8 11 0.35 870(B) 0.2 0.15 9 4.5 0.15 9PE(A) 2.0 1.32 4 52 1.68 5SBS(Pr) 0.7 0.44 7 400 12.9 1SBS(A) 7.1 4.69 2 400 12.9 1SBS(B) 2.2 1.48 3 70 2.26 4EVA

33(A) 7.6 5.03 1 300 9.68 3

EVA33

(B) 1.1 0.70 6 27 0.87 7

13

106

105

104

1030.001 0.01 0.1 1 10 100

Angular frequency (Rad.s -1)

Com

plex

vis

cosi

ty (

Pa.

s)

Figure 4 Complex viscosity versus frequency at 45oC

main effect being to slightly increase the relative moduli ofSBS(A) and EVA

33(A). At 20 seconds, EVA

33(A) and

SBS(A) are about five times stiffer than 50(A), the relativemodulus values for the other blends are similar to before.SBS(Pr) is still the least stiff of all the modified binders butit is stiffer than both the 70 pen bitumens.

As an alternative to the SHRP loading time, there is aview that deformation is associated with creep at longerloading times. At 1000 seconds, EVA

33(A), SBS(A) and

SBS(Pr) are an order of magnitude stiffer than 50(A) withSBS(B) and PE(A) having relative moduli of 2.26 and 1.68respectively. Comparing the ranking of the bindersdetermined from values of G*/sin δ at 0.1 seconds (theSHRP criterion) and G* at 1000 seconds, the SBS blendstend to be relatively stiffer at the longer loading time whilethe ranking of the other binders tended to remainunchanged. The most significant difference is for SBS(Pr),which has the lowest stiffness at 0.1 seconds and thehighest stiffness at 1000 seconds. Experimental evidenceobtained both in the laboratory and from full scale roadtrials with SBS(Pr) indicates that it is more deformationresistant than conventional 50 pen bitumen in rolled asphaltwearing course, which would suggest that the response ofthe binder at long loading times governs deformationresistance. Thus, for SBS-modified bitumens such asSBS(Pr) which exhibit unconventional time dependentbehaviour, the SHRP binder deformation test will greatlyunderestimate their potential to resist permanentdeformation.

5.1.2 Laboratory-derived relationshipsTo investigate the relationship between wheel-tracking rateand the SHRP binder deformation criterion, values ofG*/sin δ at 10 rad/s have been calculated at both 45oC and60oC and plotted against their equivalent wheel-trackingrates (using a double logarithmic scale) in Figure 5. Thelinear relationship between the two parameters for all of thebinders except SBS(Pr), given in Equation 3 with r2=0.79,is encouraging.

log (WT) = 2.60 - 1.14 x log (G*/sin δ) ....(3) where:

WT = wheel-tracking rate (mm/hr), andG*/sin δ = G*/sin δ (kPa) at 10 rad/s.

Nevertheless, as expected, Equation 3 with the SHRPcriterion greatly overestimates the wheel-tracking rates ofSBS(Pr) at both 45°C and 60°C.

As a check to see whether using a longer loading timewould improve the relationship between binder stiffnessand wheel-tracking rate, G* and G*/sin δ values werecalculated at a loading time of 100 seconds (0.01 rad/s).Although this had little effect on the scatter of therelationship for most of the binders, it had a significanteffect upon the results for SBS(Pr), effectively bringingthem into line with the other binders. Since the G*/sinδ andG* results were very similar, the relationship between G*and wheel-tracking rate has been plotted and is shown inFigure 6.

SBS(A)

SBS(B)

SBS(Pr)

PE(A)

EVA33(A)

EVA33(B)

50(A)

70(A)

70(B)∇∇∇∇∇

■■■■■

▲▲▲▲▲�

�

�

14

Figure 5 Relationship between wheel-tracking rate and the SHRP deformation criterion (G*/sin δ at 10 rad.s -1)

1000

100

10

1

0.11 100010010

G* / sin δ (kPa)

Whe

el-t

rack

rat

e (m

m/h

r)45oC60oCSBS(Pr)

1000

100

10

1

0.11 100010010

G* at 0.01 Rad/s (Pa)

Whe

el-t

rack

rat

e (m

m/h

r)

45oC

60oC

SBS(Pr)

10000

Figure 6 Relationship between G* (0.01 rad/s) and wheel-tracking rate

15

The relationship for all the binders, including SBS(Pr), isgiven as Equation 4 with r2 = 0.71.

log (WT) = 2.50 - 0.81 x log (G*) .... (4) where:

WT = wheel-tracking rate (mm/hr), andG* = G* (Pa) at 0.01 rad/s.

Although the correlation coefficient is slightly less forEquation 4 than for Equation 3, it is a more universalpredictor because it was derived from all binders whereasthe derivation of Equation 3 excluded SBS(Pr).

5.2 Dynamic creep stiffnessDifferences between binder and mix properties at 30oC arenot as pronounced as they are at either 45oC or 60oC.However, there is still a sufficient range of dynamic creep(RLAT) and binder stiffness results to investigate therelationship between the two measures. Because, in thedynamic creep test, 1800 one-second pulses are applied,G* values calculated at a loading time of 1800 secondswere chosen for comparison and the results are presented(using a double logarithmic scale) in Figure 7. Therelationship obtained (Equation 5) is very strong with acorrelation coefficient (r2) of 0.80. The only surprisingresult is 50(A) which had a somewhat lower creep stiffnessthan would have been expected from the binder results.

log (RLAT) = 0.367 x log (G*) - 0.542 .... (5)with a correlation coefficient, r2 = 0.80

where:RLAT = dynamic creep modulus (MPa), andG* = G* (Pa) at 1800s.

6 Relationships between binderproperties and resistance to cracking

6.1 Performance at low temperaturesThe use of very stiff low penetration grade bitumens canprovide bituminous mixtures with good load bearing abilityand good resistance to deformation. Unfortunately, thesebitumens tend to be stiffer over the entire servicetemperature range making them more susceptible to brittlefracture at low temperatures. Low temperature cracking ofthe wearing course of bituminous pavements is more of aproblem in the UK than previously thought. Although themechanisms are not yet fully understood, the probability offailure due to thermally induced stresses or traffic inducedstresses can be reduced by using binders which have a lowstiffness modulus at low temperatures (Hills & Brien, 1966).These materials tend to have improved ductility, makingthem better able to withstand tensile strains withoutfracturing. This type of failure does not usually occur untilthe binder has hardened due to oxidation and othermechanisms. Nevertheless, if a binder has a low modulus inthe early stages of its life, it is likely to reach critical failureconditions later than a binder that has a higher modulus inits early life.

Values of G* at -5oC obtained from the master curves atloading times of 0.02 seconds and 10,000 seconds,corresponding to traffic and thermally induced loadingrespectively, are shown in Table 13.

Under traffic loading conditions, the modulus values ofthe binders ranges from 78.6 to 195 MPa. Surprisingly,50(A) has the lowest stiffness of the unmodified bitumenswith 70(B) having the highest modulus of all the binderstested. The laboratory-prepared SBS binders have thelowest modulus values, considerably lower than those of

1098

7

6

5

4

3

2

1100 1000 10000

G* at 1800s (Pa)

RLA

T (

MP

a)

Figure 7 Relationship between dynamic creep and G* at 1800 seconds

16

Figure 8 Relationship between yield strain and G* at 7200s

4

3

2

110 000 100 000

G* (Pa)

Yie

ld s

trai

n (%

)

1000

Table 13 Modulus values at -5oC

0.02 s (Traffic Loading) 104 s (Thermal Loading)Binder G* (MPa) Ranking G* (kPa) Ranking

50(A) 120 3 794 670(A) 133 5 457 470(B) 195 9 232 2PE(A) 140 7 807 7SBS(Pr) 138 6 76 1SBS(A) 78.6 1 839 8SBS(B) 92.3 2 484 5EVA

33(A) 126 4 1078 9

EVA33

(B) 158 8 398 3

the other binders. The addition of EVA has had less effectupon modulus under these conditions, values being onlyslightly lower than those of the base 70 pen bitumens.

At 10,000 seconds, the range of modulus values isconsiderably greater. Under these conditions, SBS(Pr) hasby far the lowest stiffness making it considerably less proneto thermally induced cracking than any of the other binders.70(B) has the next lowest modulus. The polymer blends areall stiffer than their respective base 70 pen bitumens,although those blended with the softer bitumen from crudesource B are less stiff than those made with the crude Abitumen.

The results show that a low modulus under trafficloading conditions does not necessarily mean a lowmodulus under thermal loading conditions. Hence, theperformance of the binder at low temperature will dependupon which of the two mechanisms is more critical.

6.2 Yield strainTo investigate the relationship between yield strain andbinder properties, G* values at 20oC at a number of

different loading times were calculated and compared to theyield strain results. In general, there was a strong negativecorrelation between yield strain and G* but the bestcorrelation (r2=0.85) was obtained with G* valuescalculated at a loading time of 7200 seconds (2 hours)(Equation 6). The relationship obtained is shown in Figure 8.Since the yield strain test takes several hours to complete, itis not surprising that G* values at such a long loading timeshow the best correlation.

yield = 7.69 - 1.33 x log (G*) .... (6)with a correlation coefficient, r2 = 0.84

where:yield = yield strain (per cent), andG* = G* (Pa) at 7200s.

Although the relationship shown in Figure 8 ispromising, further research is required to determine theimportance of yield strain results and their application towearing course materials.

7 In-situ deformation results

7.1 A38 Trial siteIn 1987 a number of trial sections of rolled asphalts usingthree different aggregate gradings and eight binders (six ofwhich were modified) were laid on the southboundcarriageway south of the Clay Mills junction of the A38Burton bypass, Staffordshire. Full details of the rolledasphalts are given in RR323 (Daines, 1992b). A summaryof the rolled asphalt sections is shown in Table 14 and theirgradings in Table 15.

Four cores were taken from each section soon after the

17

laying of the rolled asphalt sections and these were wheel-tracked at 45oC. The averaged results are shown in Table 16together with density values for each material.

Deformation measurements have been made annually onthe rolled asphalt sections using a 2.0m straightedge beamand calibrated wedge, bridging the nearside wheel-path inthe nearside lane. Measurements were made at 10 locationsin each section and at each location, measurements of beamclearance were made to an accuracy of ±0.5mm at sixequidistant points along the beam. The method ofcalculation was that as given in RR323 and the mean results(together with their associated standard deviations) after8 years of trafficking are also given in Table 16.

All the modified materials show improvements in

deformation resistance (as measured by the wheel-trackingtest) when compared with their respective controls with theexception of the 100 pen Chemcrete modified material.This material, together with the 70 pen Chemcrete modifiedasphalt, relies on chemical curing of the modifier in orderto stiffen the mixture. However, the rate of cure isunpredictable and rutting is clearly evident at this site,particularly in the 100 pen modified asphalt. Other work(Nicholls, 1990) has also highlighted the variableperformance of Chemcrete modified asphalts; the modifierhas since been withdrawn from the UK market.

The wheel-tracking rate recorded for the 70 pen EVAmodified material is higher than expected; measurementsmade on a laboratory prepared sample gave a tracking rate of

Table 14 Rolled asphalt sections, A38

Target binderSection Aggregate contentNumber Binder Grading (per cent)

17 Control 3 - 70 pen bitumen 4 8.1 ± 0.69 100 pen bitumen + 2% U1-49 Chemcrete 4 8.1 ± 0.614 70 pen bitumen + 2% U1-49 Chemcrete 4 8.1 ± 0.615 70 pen bitumen + 5% UL 15019 EVA 4 8.1 ± 0.611 Control 1 - 50 pen bitumen (BS594: 1985, Col 9) 5 7.2 ± 0.68 50 pen bitumen + SR (synthetic rubber) 5 7.2 ± 0.610 Medium PI bitumen 5 7.2 ± 0.612 Cariphalte DM (SBS) 1 5 7.2 ± 0.616 Control 2 - 50 pen bitumen (BS594: 1985, Col 15) 6 6.9 ± 0.613 Cariphalte DM (SBS) 2 6 6.9 ± 0.6

Table 15 Aggregate gradings

Per cent by mass passingBS sieve Grading 4 Grading 5 Grading 6

20 mm 100 100 10014 mm 99 96 9910 mm 81 80 836.3 mm 67 70 702.36 mm 66 66 66600 mm 64 56 43212 mm 37 25 2475 mm 10 10 10

Table 16 Initial wheel-tracking rates and deformation results after 8 years, A38

Cored Samples (Initial) Measured deformation after 8 yearsW/T rate Density Mean Standard

Section (@ 45oC) deviationNo. Material (mm/h) (Mg/m3) (mm) (mm)

17 Control 3 - 70 pen 11 2.33 10.4 3.79 100 pen + Chemcrete 15 2.32 8.1 1.614 70 pen + Chemcrete 7.1 2.31 4.4 0.815 70 pen + EVA 6.1 2.31 2.1 0.811 Control 1 - 50 pen 4.5 2.35 4.0 1.58 50 pen + SR 2.1 2.33 3.0 1.110 Medium PI 3.1 2.34 2.4 1.312 Cariphalte DM 1 1.0 2.34 2.9 1.016 Control 2 - 50 pen 2.2 2.42 3.4 1.513 Cariphalte DM 2 1.1 2.38 2.1 0.9

18

1.1mm/h. In-situ deformation results indicate goodperformance to date. In fact, all four polymer modifiedasphalts and the medium PI asphalt (since namedMultiphalte) are showing the greatest deformation resistance,with all of them better than their respective controls.

7.2 Validation from siteFrom the deformation results obtained from the A38 site,Figure 9 has been constructed which shows the relationshipbetween the wheel-tracking rate measured on road coressoon after placement and the accumulated deformationmeasured in the nearside wheel-track after 8 yearstrafficking. The relationship is encouraging although itshould be noted that the wheel-tracking results were basedon the mean of four cores only.

8 Conclusions

The main conclusions reached from this study are asfollows:

1 Conventional empirical test methods are inadequate forthe characterisation of polymer-modified binders;rheological measurements are needed to provide a betterunderstanding of the effects of modification.

2 The rheological response of all the binders can bedescribed by two independent components, a timedependent component and a temperature dependentcomponent. The main effect of polymer modification isto alter (to differing extents) the time dependency ofbitumen without significantly changing the temperaturedependency.

Figure 9 Deformation on A38 against wheel-tracking rate

12

10

8

6

4

2

06420 12108 1614 18

Wheel-Tracking Rate (mm/hr)

Def

orm

atio

n af

ter

8 ye

ars

(mm

)

3 The G* values of polymer-modified binders can besignificantly lower than those of conventional bitumensat low pavement temperatures. Polymer modificationmay therefore offer the potential for reducing crackingdue to traffic and thermal loading.

4 The wheel-tracking rate for a particular aggregate sourceand grading can be predicted from G* of the binder, witha strong correlation between wheel-tracking rate and theSHRP criterion of G*/sinδ at 10rad/s for the laboratory-prepared binders but not for the pre-mixed binder; abetter relationship was established for all binders byselecting G* at a longer loading time whilst theparameter sinδ was not found to significantly affect therelationship.

5 Results from a full-scale road trial in the UK indicate thatthe in-situ deformation that will develop in a road can bepredicted by the wheel-tracking rate and, hence, by G*of the binder for a particular aggregate grading andsource.

6 There is a strong relationship between dynamic creepstiffness and G* at a loading time of 1800 seconds;dynamic creep stiffness is a measure of deformationresistance in a similar manner to wheel-tracking rate.

7 There is a strong relationship between yield strain andG* measured at a loading time of 7200 seconds(2 hours). However, the practical significance of theyield strain results has yet to be fully established.

19

9 Acknowledgements

The work described in this paper was carried out in theCivil Engineering Resource Centre (Resource CentreManager Mr P G Jordan) of the Transport ResearchLaboratory with Mr J C Nicholls as Project Manager andMr M E Nunn as Quality Audit and Review Officer.Mrs K Hayes and Mrs K J Lloyds carried out the samplepreparation and testing of the mixed materials.

10 References

Institute of Petroleum. Standard methods for analysis andtesting of petroleum and related products. Institute ofPetroleum, London.

IP 49/86: Penetration of bitumen (IP 49)IP 58/86: Softening point of bitumen - ring and ball (IP 58)IP 80/87: Breaking point of bitumen, Fraass method (IP 80)

Bell C.A. (1988). Introduction to bituminous binders.Bituminous Pavements: Materials, Design and Evaluation.University of Nottingham.

American Society for Testing and Materials (1974).Standard method of test for: effect of heat and air on amoving film of asphalt (rolling thin-film oven test). ASTMD 2872-70. American Society for Testing and Materials,Philadelphia.

British Standards Institution (1989). Bitumens forbuilding and civil engineering: specification for bitumensfor roads and other paved areas. British Standard BS 3690:Part 1: 1989. British Standards Institute, London.

British Standards Institution (1990a). Sampling andexamination of bituminous mixtures for roads and otherpaved areas: Part 107, Method of test for the determinationof the composition of design wearing course rolled asphalt.British Standard BS 598: Part107: 1990. British StandardsInstitution, London.

British Standards Institution (1990b). Method for thedetermination of the wheel tracking rate of cores ofbituminous wearing courses. Draft for developmentDD184: 1990. British Standards Institution, London.

British Standards Institution (1992). Hot rolled asphaltfor roads and other paved areas: Part 1, Specification forconstituent materials and asphalt mixtures. British StandardBS 594: Part 1: 1992. British Standards Institution,London.

British Standards Institution (1993). Methods forassessment of resistance to permanent deformation ofbitumen aggregate mixtures subject to unconfined uni-axialloading. Draft for development DD185: 1993. BritishStandards Institution, London.

Carswell J and D. R. Gershkoff (1993). The performanceof modified dense bitumen macadam roadbases.Department of Transport TRL Report RR 358. TransportResearch Laboratory, Crowthorne.

Cominsky R.J. (1994). The superpave mix design manualfor new construction and overlays. Strategic HighwayResearch Program SHRP-A-407. Washington D C.

Daines M.E. (1992a). The performance of hot rolledasphalt containing crushed rock fines, A303 Mere.Department of Transport TRL Report RR 298. Transportand Road Research Laboratory, Crowthorne.

Daines M.E. (1992b). Trials of porous asphalt and rolledasphalt on the A38 at Burton. Department of TransportTRRL Report RR323. Transport and Road ResearchLaboratory, Crowthorne.

Denning J.H. and Carswell J. (1981). Improvements inrolled asphalt surfacings by the addition of organicpolymers. Department of the Environment, Department ofTransport TRRL Report LR 989. Transport and RoadResearch Laboratory, Crowthorne.

Denning J.H. and Carswell J. (1983). Assessment of‘Novophalt’ as a binder for rolled asphalt wearing course.Department of the Environment, Department of TransportTRRL Report LR 1101. Transport and Road ResearchLaboratory, Crowthorne.

Dickinson E.J (1981). Assessment of the deformation andflow properties of polymer modified bitumens. AustralianRoad Research, Volume 11, Number 3, 1981.

Gershkoff D.R. (1991). A study of the rheologicalbehaviour of some surface dressing binders. MScdissertation. University of Nottingham.

Hoban T.W.S. (1987). Modified binders: anunderexploited road maintenance tool. Highways, Volume55, Number 1926, 1987.

Heukelom W. (1969). A bitumen test data chart forshowing the effect of temperature on the mechanicalbehaviour of asphaltic bitumens. Journal of the Institute ofPetroleum Technologists, Volume 55, 1969.

Hills J.F. and Brien D. (1966). The fracture of bitumensand asphalt mixes by temperature induced stresses.Proceedings of the Association of Asphalt PavingTechnologists, Volume 35, 1966.

Kennedy T.W., Huber G.A., Harrigan E.T., CominskyR.J., Hughes C.S., Von Quintus H. and Moulthrop J.S.(1994). Superior performing asphalt pavements (Superpave):the product of the SHRP asphalt research program. StrategicHighway Research Program SHRP-A-410. Washington D C.

20

Krom C.J. (1950). Determination of the ring and ballsoftening point of asphaltic bitumens with and withoutstirring. Journal of the Institute of Petroleum Technologists,Volume 36, 1950.

Lytton R.L., Uzan J., Fernando E.G., Roque R.,Hiltunen D. and Stoffels S.M. (1993). Development andvalidation of performance prediction models andspecifications for asphalt binders and paving mixes.Strategic Highway Research Program SHRP-A-357.Washington D C.

Nicholls J.C. (1990). A review of using Chemcrete inbituminous materials. Department of Transport TRLTechnical Report TR 271. Transport and Road ResearchLaboratory, Crowthorne.

Nicholls J.C. and Daines M.E. (1993). Acceptableweather conditions for laying bituminous materials.Department of Transport TRL Report PR13. TransportResearch Laboratory, Crowthorne.

Nunn M.E. (1986). Prediction of permanent deformation inbituminous pavement layers. Department of TransportTRRL Report RR26. Transport Research Laboratory,Crowthorne.

Pfeiffer J.P. and Van Doormaal P.M. (1936). Therheological properties of asphaltic bitumens. Journal of theInstitute of Petroleum Technologists, Volume 55, 1969.

Powell W.D., Potter J.F., Mayhew H.C. and Nunn M.E.(1984). The structural design of bituminous roads.Department of the Environment, Department of Transport,TRRL Report LR 1132. Transport and Road ResearchLaboratory, Crowthorne.

Szatkowski W.S. (1979). Rolled asphalt wearing courseswith high resistance to deformation. Rolled asphalt roadsurfacings. Institution of Civil Engineers, London.

Thenoux G., Lees G. and Bell C.A. (1987). Laboratoryinvestigation of the fraass brittle test. Asphalt Technology,Number 39, 1987.

Van der Poel C.J. (1954). A general system describing theviscoelastic properties of bitumen and its relation to routinetest data. Journal of Applied Chemistry, Volume 4, 1954.

Vonk W.C. and Van Gooswilligen G. (1989).Improvement of paving grade bitumens with SBSpolymers. Fourth Eurobitume Symposium, Volume 1.

21

Appendix A: Glossary of rheological terms

Apparent viscosity (η) The shear stress divided by rate of shear when this quotient is dependent on rateof shear. Also called viscosity and shear viscosity. Measured in Pa.s.

Characteristic temperature (TS) A single temperature value which, when used with the WLF equation, describes

the temperature dependenceof a bitumen.

Complex modulus (G*) The peak stress divided by the peak strain measured in a dynamic test.

Complex viscosity (η*) A viscosity determined in a dynamic oscillatory test, equal to the complex modulusdivided by angular frequency.

Dynamic testing This is the measurement of the response of a material under an applied sinusoidalstress or strain wave.

Elastic liquid A liquid showing elastic as well as viscous properties (also viscoelastic andelastico-viscous).

Elastic modulus Also known as stiffness or stiffness modulus, this is equal to the stress applied toa material divided by the strain of that material. With viscoelastic materials, this isdependent upon loading time.

Flow A deformation, of which at least part is non-recoverable.

Hookean behaviour Ideal elastic behaviour; the application of stress results in an instantaneous strain,fully recovered uponremoval of the stress, with the strain being directly proportional tothe applied stress.

Isochrone Measurements of stiffness, phase angle, etc., at the same frequency or loading time.

Isotherm Measurements of stiffness, phase angle, etc., at the same temperature.

Master curve A composite graph of rheological data covering a wide frequency range at asingle temperature, produced by shifting data obtained at different temperatures.

Newtonian behaviour Ideal, purely viscous behaviour; the viscosity of a material does not changewith rate of loading and no elasticity is exhibited by the material.

Phase angle (δ) The amount the stress leads the strain by in a dynamic test.

Shear rate The change of shear strain per unit time. Measure in units of reciprocal seconds.

Shear-thinning Also known as pseudo-plasticity, shear-thinning behaviour refers to a viscosity thatdecreases with the rate of shearing.

Shift factor (AT or a

T) A multiplication factor that is needed to shift rheological results from one

temperature to another. The factor is applied to the frequency (or loading time) data.

Simple shear A shear caused by the parallel relative displacement of parallel planes.

Static testing Otherwise known as creep, this is the measurement of the response of a materialunder a constant applied stress.

Strain The measurement of deformation relative to a reference configuration of length,area or volume. Also called relative deformation.

Stress A force per unit area.

Thixotropy A decrease in apparent viscosity under constant shear stress or shear rate. The effectis time dependent.

Viscosity The resistance to flow of a material.

WLF equation Williams, Landel, Ferry equation used to model the temperature dependence ofviscoelastic response.

22

Appendix B: The viscoelastic response ofbitumen

B.1 GeneralBitumens are highly complex materials whose rheologicalproperties are strongly dependent upon temperature andtime of loading. At high temperatures and/or long loadingtimes, bitumens can exhibit purely viscous (Newtonian)behaviour; at low temperatures and/or short loading times,they tend to exhibit elastic (Hookean) behaviour. Atintermediate temperatures and loading times, they exhibitviscoelastic behaviour which is a combination of the twoclassical extremes. It is this strong dependence upontemperature that enables bitumens to be fluid enough athigh temperatures for bituminous mixtures to be mixed,laid and compacted and yet stiff enough at servicetemperatures for those mixtures to resist traffic generatedstresses.

B.2 ViscosityViscosity is defined as the resistance to flow exhibited by amaterial. Mathematically, the shear viscosity, η, isexpressed by Equation B.1.

η = shear stress/shear rate .... (B.1)

A material is said to exhibit Newtonian behaviour if theviscosity is independent of the rate of shear. However, formost liquids, η is not a constant but rather varies with shearrate. Viscosity measurements for these materials are thenreferred to as apparent viscosity at a particular rate of shear.At the high temperatures encountered in asphalt mixingprocesses, bitumens tend to exhibit Newtonian behaviour.Most paving grade bitumens also exhibit Newtonian flowbehaviour at high pavement temperatures, say 60oC.However, at lower temperatures such as 25oC, mostbitumens do not exhibit Newtonian flow behaviour butshow shear-thinning (pseudo-plastic) behaviour.

B.3 Stiffness and Van der PoelThe stiffness modulus, S, for bitumens is essentially a timeand temperature dependent version of Young’s modulus(Van der Poel, 1954), where S is given by Equation B.2.

S = tensile stress/tensile strain .... (B.2)

Bitumens behave as an incompressible material and,therefore, the stiffness modulus is numerically equal tothree times the shear modulus (measured at the sameloading time and temperature). The stiffness moduli usingstatic (creep) and dynamic (oscillatory) measuringtechniques can be represented on a common time orfrequency scale for a wide range of bitumens with thestiffness measured in a creep test of duration t secondsbeing equivalent to that determined in a dynamic test at anangular frequency ω equal to 1/t.

Based on the concept of the penetration index (PI)(Pfeiffer & Van Doormaal, 1936), the stiffness of bitumens

can be estimated from the results of a few simple tests. Ifthe log of penetration is plotted against temperature, astraight line is obtained with gradient m; the PI is calculatedfrom m using Equation B.3.

PI = 20 . (1 - 25.m) .... (B.3) (1 + 50.m)

Rearranging Equation B.3 yields Equation B.4 forcalculating m from PI.

m = 20 - PI .... (B.4) 50 (10 + PI)

Equation B.3 was chosen so that a particular Mexican200 pen bitumen yielded a PI of zero. Bitumens with highPIs are less temperature susceptible (a PI of 20 wouldindicate a penetration independent of temperature), and lowPI binders are more temperature susceptible (a binder witha PI of -10 would be infinitely susceptible).

The penetration test relates to stiffness at a loading timeof 0.4 seconds (despite the five second duration of the test)and the softening point temperature is equivalent to thetemperature at which the penetration equalledapproximately 800 mm/10. If one plots stiffness at aparticular loading time against softening point minustemperature, bitumens with the same PI will lie on the sameline. In addition, the stiffness of all bitumens at lowtemperatures and short loading times approached 3 GPa.Van der Poel’s nomograph makes use of these relationshipsto determine the stiffness from PI and softening point data.Although empirically based, the nomograph is able topredict stiffnesses over a wide range of times andtemperatures to an acceptable level of accuracy and is usedwidely within the bituminous industry.

B.4 Shell bitumen test data chartThe Shell bitumen test data chart (BTDC) was developed inthe late 1960s (Heukelom, 1969). It allows penetration,softening point, Fraass and viscosity data to be plottedagainst temperature. The chart has a linear horizontal scalefor temperature and two vertical scales for the penetrationand viscosity. The penetration scale is logarithmic and theviscosity scale is pseudo-logarithmic, devised so thatpenetration grade bitumens with normal PIs give straight-line relationships. This chart has become a popular tool forrepresenting bitumen data and is particularly useful as itallows penetration and softening point results to beextrapolated to yield approximate mixing, laying andcompaction temperatures. The ring and ball softening andFraass breaking points are assumed to represent thetemperatures at which the bitumen possesses penetrationsof 800 and 1.25 mm/10 respectively.

The BTDC is also useful because it enables waxy andblown bitumens to be clearly identified. These types ofbitumens do not produce a single straight line plot over theentire temperature range but rather two different straightlines. More recently, the chart has become popular fordemonstrating the effects of polymer modification ofbitumens.

23

B.5 Dynamic characterisation of bitumensMuch of the recent rheological research has concentratedon measuring the dynamic oscillatory response ofbitumens. Testing can take place in controlled stress orcontrolled strain modes, the two techniques yieldingidentical results. In controlled stress measurements, asinusoidal stress is applied to the sample and the resultingstrain wave is measured. In controlled strain mode, thesample is forced to deform sinusoidally and the stressrequired to deform the sample is measured. Dynamictesting is usually carried out in shear although uni-axialtension/compression can also be used.

Regardless of the mode of testing employed, smallamplitude strains are used at low temperatures to ensurethat the bitumen remains in the linear viscoelastic range;this is less critical at high temperatures. In addition tomeasuring the amplitude of the stress and strain waves, thephase difference between the stress and strain is alsomeasured. This allows complex dynamic properties to bedetermined, thereby revealing further information about theresponse of the bitumen.

B.6 Definitions of dynamic parametersIn the case of a controlled stress measurement, the appliedsinusoidal stress wave causes the sample to deformsinusoidally. Generally, there will be a phase lag, δ (delta),between the stress and strain waves. The complex (shear)modulus, |G*| (more commonly known as G* or G star), isdefined as the peak stress divided by the peak strain. Thismodulus may be resolved vectorially into two components;G’ (G prime), in phase with the stress, and G” (G doubleprime), 90o out of phase with the stress. G’ is called thestorage modulus and is related to the amount of energystored per cycle. G” is known as the loss modulus and isrelated to the amount of energy lost per cycle throughviscous heating. This is conveniently described by thecomplex notation of Equations B.5, B.6 and B.7.

G* = G’ + iG” .... (B.5)where: i = -1

G’ = G* cos δ .... (B.6)G” = G* sin δ .... (B.7)

G’ is the real part and G” the imaginary part of G*.Equation B.8 defines the relationship between the phaseangle and the modulus components and it is possible todefine a complex viscosity in terms of the angularfrequency, ω radians per second, by Equation B.9.

tan δ = G”/G’ .... (B.8)η* = G*/ω .... (B.9)

A plot of η* versus ω closely resembles that of theapparent viscosity versus shear rate.

The relationship between G’, G”, G* and δ can beconveniently represented using an Argand diagram, anexample of which is shown in Figure B.1. The X axisrepresents the real (in-phase) part of the modulus and theY axis represents the imaginary (out of phase) componentof the modulus.

With a purely elastic material, there is no viscous (loss)component, δ equals 0o and G*=G’. For a purely viscousmaterial, there is no elastic (storage) component, δ equals90o and G*=G”. In general, for a viscoelastic material, δwill lie somewhere between 0 and 90o and Equation B.10can be used to determine the complex modulus.

[ ] [ ]G G G* ( ' ' ' )= +2 2

....(B.10)

The response of ideal viscous and elastic materials in adynamic experiment is shown diagrammatically inFigure B.2. When it is subjected to a sinusoidal stress wave,an elastic material will deform in sympathy and in phasewith the stress. The maximum strain therefore coincideswith the point of maximum stress (i.e. stress is proportionalto strain - Hooke’s law). With the purely viscous material,the shape of the stress wave is still sinusoidal and of thesame frequency as the applied stress wave but the stressleads the strain by 90o. At the point of maximum stressamplitude, the strain rate of a viscous material is at itsmaximum (i.e. stress is proportional to rate of strain -Newton’s law of viscosity).

B.7 Presentation of dynamic results - master curvesDynamic measurements on bitumens are generally carriedout over two or three decades of frequency and at five orsix different temperatures. Typical ranges are from 1 to 100Hertz and 0 to 60oC. Results can be presented in a varietyof ways although it is normal to plot isotherms of thedifferent moduli and δ against angular frequency. Theinter-relations between the different dynamic parametersmeans that any two parameters can be used to describecompletely a material’s response. Plots of log G* and δagainst log ω are the most commonly chosen because G*represents the stiffness of the binder while δ indicates thetype of response of the binder. Logarithmic scales are usedbecause of the wide variation in the parameters; theexception to this is the phase response δ which is usuallyplotted linearly.

The effect of temperature on the viscoelastic parametersis to shift them along the log ω axis without changing theirshape. This permits the reduction of isotherms of log G*and δ to a single master curve. This phenomenon is knownas thermo-rheological simplicity, time-temperaturesuperposition or the method of reduced variables and wasfirst observed with polymeric materials. This method is alsoused to shift complex modulus and other rheological dataobtained on bituminous mixes in the near-elastic responserange. Strictly, the method should be applied to viscoelasticdata that has been multiplied by density. However, thevertical shifts introduced by this are very small forbitumens and are usually ignored.

Figure B.3 shows log G* and δ isotherms and mastercurves for a 50 pen bitumen. These curves demonstrate thecharacteristic behaviour of bitumens. At low frequenciesand high temperatures, viscous behaviour is dominant: δapproaches 90o and the log G* curve approaches a gradientof unity. At high frequencies and low temperatures, theresponse approaches elastic behaviour asymptotically: δtends towards 0o and the log G* curve flattens out. Forconventional bitumens, the shape of the G* versus

24

Figure B.1 Argand diagram

Figure B.2 Dynamic response of 'ideal' materials

GI Real Part

GII

Imqa

gina

ry P

art

δ Phase Angle

IG*I Complex Modulus

AM

PLI

TU

DE Purely elastic

responseδ = 0o

Purely viscousresponseδ = 90o

TIME

Applied stresswave

�

�� �

�

�

25

frequency curve is an indicator of the rheological type andis related to the PI value of the bitumen. The moduluscurves for high PI bitumens tend to flatten off at highfrequencies more rapidly than low PI bitumens. Thus, atany given temperature, the modulus of a high PI bitumenwill be less dependent upon loading time than a low PIgrade. However, in rheology, it is the change of viscositywith loading time that is used to describe the timedependency of a material. Therefore, because viscosity isequal to modulus divided by frequency, low and high PIbitumens are referred to as having low and high shear-ratedependencies respectively.

The relation between the shift factors, aT, used to obtain

master curves and temperature, T, is conveniently describedby Equation B.11, generally known as the Williams, Landeland Ferry equation (WLF).

log (aT) = -C

1 . (T - T

S) .... (B.11)

C2 + T - T

S

where C1 and C

2 are constants and T

S is a reference

temperature (Ferry, 1961).If T

S is suitably chosen for each material, C

1 and C

2 can

be allotted the universal values of 8.86 and 101.6respectively; T

S is then referred to as the characteristic

temperature for that material. However, these values tend tooverestimate the shift factors required at low temperaturesand a second pair of constants (C

1 and C

2) are used below

TS (Dobson, 1969).Figure B.4 shows the fit of Equation B.11 to the results

obtained with the binders used in a recent DBM roadbasetrial (Carswell & Gershkoff, 1993). To enable all the data to

Figure B.3 Log G* and δ Isotherms and master curves (25oC) for 50 pen bitumen

1010

109

108

107

106

105

104

103

102

101

100

90

80

70

60

50

40

30

20

10

0

10-410-3 10-210-1100 101 102 103 104 105 106 107 10-410-3 10-2 10-1 100 101 102 103 104 105 106 107

Del

ta (

degr

ees)

IG*I

(P

a)

Temp (deg C)

-5

5

15

3545

6080

15

5

-5

8060

4535

Temp (deg C)

Master Curveat 25degrees C

Log ω AT

(Rad.s-1) Log ω AT

(Rad.s-1)

Figure B.4 Shift factor/temperature relationship showing WLF equation fit

8

7

6

5

4

3

2

1

-1

-2

-3

-4

-5

0

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90

LOG

10a T

T - Ts (oC)

DataWLF fit

26

be plotted on one scale, (T-TS) has been chosen as the

abscissa. Values for C1 and C

2 of 20.1 and 209.3

respectively below TS and 8.86 and 101.6 above T

S provide

an excellent fit.The production of master and shift factor curves can be

thought of as isolating the time and temperaturedependence of a bitumen’s behaviour. The log G* versuslog ω master curve indicates the shear susceptibility of thebitumen while the log a

T versus temperature curve indicates

the bitumen’s temperature dependence, the latterdependency being adequately described by the use of acharacteristic temperature and the modified version ofEquation B.11.

Although a free-volume theory has been developed toexplain the phenomenon of time-temperature superposition,it is still, in its generality, only qualitatively understood.Nevertheless, extensive testing of bitumens has shown thattheir behaviour can be represented in this way. Indeed, theextrapolation to very high frequencies has been verified at20,000 Hz using a torsion crystal arrangement (Brodnyan,1958).

B.8 Creep characterisation of bitumensAnother technique that is used for measuring theviscoelastic response of bitumens is the creep test.Generally in rheology, creep testing is used for measuring amaterial’s response at very long loading times whiledynamic testing is the preferred method at short loadingtimes, although either method can be used to cover verywide time scales. The phenomenon of time-temperaturesuperposition and the equivalence of transient and dynamicmoduli does reduce the need for performing both types oftest. Also, because creep testing is generally less precisethan dynamic testing, especially at high stiffnesses and atshort loading times, most research on bitumens hasconcentrated on dynamic properties. Nevertheless, there isstill quite a lot of interest in creep testing of bitumens. Twomajor reasons for this are:

� the equipment needed to perform the tests is usually lessexpensive than for dynamic tests and is more generallyavailable, and

� creep test results often show clearly the increasedelasticity imparted to the binder by the addition of certainpolymers.

Creep testing of bitumens is usually carried out in shear.The test involves the application of a low constant stress fora set period of time which may be followed by a period ofrelaxation. During both phases, the strain or deformation ofthe material is monitored and recorded as a function oftime. The compliance, J

(t), of the bitumen is given by

Equation B.12 in terms of the strain at time t, strain(t)

, whilethe stiffness modulus G

(t) is simply equal to the reciprocal

of J(t)

.

J(t)

= strain(t)

/ stress .... (B.12)

To explain the significance of creep testing, it is useful toconsider the response of ideal elastic and viscous materials.With an elastic material, the material deformsinstantaneously to a fixed strain. Upon removal of the

Figure B.5 Simple viscoelastic model

Spring

Dashpot

Voigt unit

stress, the material recovers, again instantaneously, to itsoriginal state. The strain is equal to the stress divided by themodulus. With a purely viscous material, the application ofstress results in a constant rate of deformation. When thestress is removed, the material remains permanentlydeformed. The gradient of the strain versus time curve isequal to the stress divided by the viscosity. The response ofan elastic and a viscous material can be analysed to that of aspring and a dashpot respectively. With a viscoelasticmaterial, the response in a creep test combines both thesetypes of behaviour and can be visualised by the modelshown in Figure B.5. This model has a spring, a dashpot,and a spring and a dashpot in parallel (known as a Voigtunit), all in series.

Figure B.6 shows the response of a viscoelastic materialin a creep test. When the stress is applied, the springdeforms instantaneously and then the Voigt unit deformsover a finite time governed by the spring stiffness anddashpot viscosity. Once these elastic and delayed elasticeffects have finished, the viscous response of the lonedashpot comes into play governing the material’s responseat long times. Upon removal of the stress the springrecovers instantaneously followed by the Voigt unit. Anyflow that has taken place in the viscous dashpot isirrecoverable.

With paving grade bitumens, the creep response at hightemperatures is purely viscous. As the temperature isdecreased, the viscosity of the bitumen increases and elasticand delayed elastic effects may be exhibited. The delayedelastic response shown by some bitumens can be modelledusing two or more Voigt units in series.

There is generally good agreement between creep anddynamic results for bitumens; creep results at 0oC atloading times of 1, 10, 102, 103 and 104 seconds anddynamic data show good agreement between transient andcomplex moduli, although the creep results after 1 secondwere different from dynamic results because creep is notreliable over short loading times (Pink et al, 1980).

27

Figure B.6 Viscoelastic creep response

Viscous response(dashpot)

Delayed elastic (Voigt unit)

Instantaneous recovery (spring)

Delayed elastic (Voigt init)

Instantaneous strain (spring)

Irrecoverabledue to viscousflow (dashpot)

Time

Str

ain

of C

ompl

ianc

e

B.9 References

Brodnyan J. G. (1958). Use of rheological and other datain asphalt engineering problems. Highway Research BoardBulletin Number 192, 1958.

Carswell J. and Gershkoff D.R. (1993). The performanceof modified dense bitumen macadam roadbases.Department of Transport TRL Report RR 358. TransportResearch Laboratory, Crowthorne.

Dobson G.R. (1969). The dynamic mechanical propertiesof bitumen. Proceedings of the Association of AsphaltPaving Technologists, Volume 38, 1969.

Ferry J.D. (1961). Viscoelastic properties of polymers.John Wiley & Sons, New York.

Heukelom W. (1969). A bitumen test data chart forshowing the effect of temperature on the mechanicalbehaviour of asphaltic bitumens. Journal of the Institute ofPetroleum Technologists, Volume 55, 1969.

Pfeiffer J.P. and Van Doomalaal P.M. (1936). Therheological properties of asphaltic bitumens. Journal of theInstitute of Petroleum Technologists, Volume 55, 1969.

Pink H.S., Merz R.E. and Bosniak D.S. (1980). Asphaltrheology: experimental determination of dynamic moduli atlow temperature. Proceedings of the Association of AsphaltPaving Technologists, Volume 49, 1980.

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Abstract

A series of fundamental and empirical tests were performed on eleven binders and bituminous mixes with thesame aggregate grading made with these binders. These consisted of three unmodified, seven laboratory-preparedmodified and one proprietary modified bitumen. The results showed that the empirical binder tests, such aspenetration and softening point, are deficient in predicting the performance of modified binders in asphalt mixes.The more fundamental dynamic tests are a better indicator of binder behaviour, but relationships need to bedeveloped between the measured dynamic properties and asphalt mix performance. By comparing the binder andmixture properties, relationships are derived for modified binders in relation to deformation resistance.Deformation results from a full-scale road trial using modified binders are also presented to validate thelaboratory testing programme.

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