Performance related
characterisation of the mechanical
behaviour of asphalt mixtures
Road and Hydraulic Engineering Institute
Performance related
characterisation of the mechanical
behaviour of asphalt mixtures
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft
op gezag van de Rector Magnificus prof. dr. ir. J.T. Fokkema, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op 20 januari te 10:30 uur
door
Jacobus Michaël Maria MOLENAAR
metaalkundig ingenieur geboren te Edam
Dit proefschrift is goedgekeurd door de promotor: Prof. dr. ir. A.A.A. Molenaar
Samenstelling promotiecommissie:
Rector Magnificus, Technische Universiteit Delft, voorzitter Prof. dr. ir. A.A.A. Molenaar Technische Universiteit Delft, promotor Prof. dr. ir. Ch.F. Hendriks Technische Universiteit Delft
Prof. dr. R.L. Lytton Texas A&M University, Verenigde Staten Prof. dr. U. Isacsson Royal Institute of Technology, Zweden
Dr. ir. J. Zuidema Technische Universiteit Delft
Dr. P.C. Hopman Netherlands Pavement Consultants
Ir. L.A. Bosch Rijkswaterstaat, Dienst Weg- en
Waterbouwkunde
Prof. dr. ir. F. Molenkamp Technische Universiteit Delft, reservelid
Dit onderzoek is gedeeltelijk in dienstverband bij de Dienst Weg- en Waterbouwkunde van Rijkswaterstaat uitgevoerd. De auteur is de DWW zeer erkentelijk voor de geboden mogelijkheid.
ISBN 90 369 5556 4 DWW-2003-129 Published by:
Dienst Weg- en Waterbouwkunde, Rijkswaterstaat Road and Hydraulic Engineering Institute
P.O. Box 5044 2600 GA Delft The Netherlands
© Jaap Molenaar, Zoetermeer, 2003
All rights reserved. No part of the material protected by this copyright may be reproduced or utilised in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written consent from the publisher.
To Miente, Lieuwke, Niels, Beppeke, and Willemijn
Keywords:
Asphalt mixture, performance related characterisation, mechanical property, creep, permanent deformation, crack-growth, fracture, constitutive modelling.
Foreword
At this place I want to thank Cos van Teylingen, former head of the department “Realisation and Maintenance Infrastructure” I am working in, for stimulating me to write this thesis, Peter Hoogweg, director of the Road and Hydraulic Engineering Division, for giving his consent, and André Molenaar, professor at the Road and Rail Road Laboratory of the Delft University of Technology, for his willingness to be my promotor. I want to thank also my former colleagues and project leaders in the Scientific Asphalt Research Project (Technisch Wetenschappelijk Asfalt Onderzoek, TWAO), Harry Verburg and Rutger Krans. I want to thank Jos van der Heide, representative of the Dutch Asphalt Producer’s and Contractor’s Association, VBW-Asfalt, for his sharpening views from the “other side” in the project team. I want to thank all those who contributed directly or indirectly to the realisation of this thesis, those who participated in or contributed to the TWAO-project teams, Arthur van Dommelen (RHEI), Bernard Eckmann (first Exxon, later Nynas), Louis Francken (Belgian Road Research Centre), professor André Gastmans (first Exxon, later Nynas), Piet Hopman (first TU-Delft, later NPC), André Houtepen (KOAC), Maarten Jacobs (first TU-Delft, later KOAC), Jasper van der Kooij (RHEI), Piet Kunst (NPC), Hans Nugteren (RHEI), Ad Pronk (RHEI), Carl Robertus (Shell), Theo Terlouw (Shell), Fedde Tolman (NPC), Kees Valkering (Shell), Martin van de Ven (NPC), Ann Vanelstraete (Belgian Road Research Centre), and Gerrit Westera (KOAC). I want to thank Jan Zuidema (TU-Delft) for his cooperation and his graduate students, Carel Kleemans, Jan Boone, Johan Schulte, and Marlies Arbouw who were willing to do their Master of Science thesis on crack-growth of asphalt mixture. I thank Ad Pronk for his critical reading and comments on the analysis in appendix 1. And I want to thank Tom Dingjan for his help with the illustrations.
The TWAO-project started in 1990. It was started because there was a wide spread desire to develop so-called “functional”, or performance related requirements for asphalt pavements and asphalt mixtures. The contractors eagerly developed new products. However, the application of new products was, and is, hindered by the difficulty that the traditional requirements are not applicable to newly developed products. It was felt that a fundamental approach to the characterisation of asphalt mixtures was necessary to help find an answer to this difficulty. The TWAO-project was intended to be scientific, to improve the knowledge and expertise of the RHEI, but also to show that the RHEI is a road authority which cooperates with the road building industry to enhance innovation. The ambitious goal of the project meant among other things that new tests and
test protocols had to be developed from scratch. Research into the mechanical behaviour of asphalt mixture is particularly difficult for a number of reasons. The behaviour is time dependent and stress dependent. The material is heterogeneous, and the test specimens are relatively small, compared to the material heterogeneity – a necessity from a practical viewpoint, but a disaster from a scientific (interpretative) viewpoint – which means that the interpretation of the test results is troubled by size effects of the specimens. Scientific research can grow mature if conditions are favourable. Looking back, the whole project was a big adventure, because the research infrastructure had to be developed, and, more importantly, expectations among those from whom support of the project was needed were diverse. In spite of that, management took the visionary decision to support the project. Meanwhile, the 150 million dollar United States’ Strategic Highway Research Project was finished by the end of 1993, leaving results that were received with mixed enthusiasm in both the United States and in Europe. In Europe, Technical Committees of CEN were struggling to achieve consensus about the testing conditions of performance related tests for asphalt mixture. The whole atmosphere around performance related specification of bituminous materials sometimes breathed an air of disappointment.
The TWAO-project ended 1997. After I had written a 70 page summary report about the activities and results, Cos asked me if I was interested to write a dissertation about the subject. Although I had great doubts as to whether that would be wise, I finally agreed to his proposition, expecting that the effort already invested would be to my advantage. It turned out that I was to learn that a scientific treatment of a subject demands a greater depth of interpreting test results, and consequently a greater investment in time than is considered acceptable in a customer-oriented organisation. The cost of science is sometimes considered a burden rather than an investment in the future. I hope this study will help to prove that the opposite is true. I finished this study out of the conviction that it pays a contribution to indispensable innovation in road building. I believe this study shows it to be possible to evaluate or judge the cost-effectiveness and the risk of failure of a newly developed paving material, based on a characterisation based on constitutive models. Thus, I believe that the acceptation of newly developed paving materials is feasible based on laboratory testing and some accelerated pavement testing but can do without a monitoring of its behaviour during the service-life. Then, this study will ultimately prove to give value for money by its contribution to the enhancement of innovation of paving materials.
I remember a discussion I had with Cos, saying how little requirements for asphalt mixtures had changed over thirty years time, and that something had to be done to draw attention to innovation of paving materials. After
all, durable paving materials are an asset to our nation’s transport infrastructure, economy, and prosperity.
Last but not least, I want to thank my wife and children for the countless hours invested in this study instead of in our family life.
Zoetermeer, December, 2002
Samenvatting
Het onderzoek is gedaan ter ondersteuning van innovaties op het gebied van asfaltverhardingsontwerp en materiaalkeuze, om het van risico van falen en de kosteneffectiviteit van nieuw ontwikkelde verhardings-materialen aantoonbaar te maken, en daarmee die verhardings-materialen toepasbaar te maken. Om het risico van falen en de kosteneffectiviteit te bepalen is informatie nodig over de kwaliteit van de verharding. Om de kwaliteit van de verharding te definiëren is het nodig het gedrag van de toegepaste materialen te kennen. Om het gedrag van de materialen te kennen is het nodig de eigenschappen van de materialen te kennen, die voor het gedrag in de weg relevant zijn. De volgende aspecten van het mechanisch gedrag van asfaltmengsels werden onderzocht, omdat op basis van ervaring bekend is dat het de bepalende fenomenen zijn: het viscoëlastisch en viscoplastisch spanning-rekgedrag, en het scheurgroeigedrag. Daarbij werden analytische en numerieke methoden toegepast.
Er zijn testmethoden ontwikkeld die voor gebruik in een praktische context geschikt zijn om de stijfheidsmodulus, de weerstand tegen permanente vervorming, en de weerstand tegen vermoeiing en scheurgroei te bepalen.
Het onderzoek leidt tot de conclusie dat de volgende proeven geschikt zijn voor de karakterisering van het mechanisch gedrag van asfaltmengsels in een praktische context: een vierpunts buig frequency sweep test, ter karakterisering van het lineair dynamisch viscoëlastisch spanning-rekgedrag, een dynamische triaxiale kruipproef, ter karakterisering van het niet-lineair dynamisch elasto-viscoplastisch spanning-rekgedrag, een trekproef, ter karakterisering van de weerstand tegen scheurgroei, en een breuktaaiheidsproef, ter karakterisering van de weerstand tegen breuk. De genoemde eigenschappen zijn van belang voor de functionaliteit van de verhardingsconstructie, die wordt uitgedrukt in draagvermogen, oppervlakkenmerken en lange-termijngedrag.
Verder kan worden geconcludeerd dat het op basis van de genoemde proeven mogelijk is, een systeem van gedragsgerelateerde specificaties te ontwikkelen, op basis waarvan asfaltmengsels relatief op toepasbaarheid kunnen worden getoetst, dat wil zeggen in vergelijking met standaard-asfaltmengsels waarvan het gedrag bekend is.
Met analytische methoden kan sneller dan met empirische methoden worden bepaald of verbeteringen van verhardingsmaterialen nut hebben, en is het mogelijk sneller over de informatie te beschikken die nodig is om het gedrag van een nieuw ontwikkeld, niet-gestandaardiseerd materiaal op risico van falen en kosteneffectiviteit te beoordelen. Daarom zullen analytische methoden de toepasbaarheid van innovatieve, niet-gestandaar-diseerde verhardingsmaterialen versnellen.
Summary
The investigation was undertaken to support innovations in the field of asphalt pavement design and material selection, and to be able to evaluate or judge the risk of failure and cost-effectiveness of newly developed paving materials in order to justify their application. To be able to determine the risk of failure and cost-effectiveness, information is needed about the quality of the pavement. In order to define the quality of the pavement, it is necessary to know the behaviour of the applied materials. In order to know the behaviour of the materials it is necessary to know the properties that are relevant for the behaviour of the material in the pavement. The following aspects of the mechanical behaviour of asphalt mixture were investigated, because it is known based on experience that these are the important phenomena: the viscoelastic and viscoplastic stress strain behaviour, and the crack-growth behaviour. Both analytical and numerical approaches were followed.
Test methods were developed that are suitable for use in a practical context for the determination of the stiffness modulus, the resistance to permanent deformation, and the resistance to fatigue and crack-growth. It is concluded that the following tests are suitable for the characterisation of the mechanical behaviour of asphalt mixtures in a practical context: a four point bending frequency sweep test, to characterise the linear dynamic viscoelastic stress strain behaviour, a dynamic triaxial creep test, to characterise the nonlinear dynamic elasto-viscoplastic stress strain behaviour, a tensile test, to characterise the resistance to crack-growth, and a fracture toughness test, to characterise the resistance to fracture.
Those properties are important to the functionality of the pavement structure that is defined in terms of bearing capacity, surface characteristics, and long-term behaviour.
It is concluded that it is possible, based on the tests mentioned, to develop a set of performance related specifications, which will allow newly developed asphalt mixtures to be tested for applicability relative to standardised asphalt mixtures for which the behaviour is known.
Analytical methods will allow one to determine useful improvements to paving materials faster than empirical methods, and to obtain the information required to judge a newly developed and non-standardised paving material for its risk of failure and cost-effectiveness. Therefore, the use of analytical methods will facilitate the acceptation for application of innovative, non-standardised, paving materials.
About the author
The author was born in Edam, December 9, 1952. He attended the Waterlant College in Amsterdam, studied chemistry and physics at the University of Amsterdam, and received a diploma Kandidaatsexamen Chemistry and Physics in 1973.
He then went to study metallurgy at Delft Technical University and graduated as a Metallurgical Engineer, MSc, in January 1980. He served the Royal Navy as a marine corrosion specialist, and returned to Delft University as a research associate in September 1981. There he worked in a research project aimed at refining recycled aluminium alloys by means of rapid solidification, and specialised in the stirred solidification of semi-solid Al-Cu alloy.
In September 1986, he joined the Road and Hydraulic Engineering Institute and worked as a research project leader in the field of bituminous construction materials. From 1989 until 1997 he was project leader of the Scientific Asphalt Research Project aimed at developing performance related test methods for asphalt mixtures. He started with the present study in May 1998.
Contents
Samenvatting x
Summary xi
About the author xii
List of symbols xxi
List of units xxiv
List of abbreviations xxv
Chapter 1: Introduction
1 Motive for this study 11.1 Socio-economic developments 1
1.1.1 Control of development of transport infrastructure 2
1.1.2 The growth of road traffic 4
1.1.3 Sustained use of materials and energy 5
1.1.4 Design-Build-Maintain contracts 7
1.1.5 Road building, changing road authorities, and the knowledge economy 7
1.2 Innovation and product quality – The need for a rationalised quality control methodology 9
2 Scope of this study 12
2.1 Introduction 12
2.2 This study’s subject 14
3 Aim of this study 15
3.1 General objective 15
3.2 Practical goal 15
3.3 Research goal 15
4 Selected topics 16
5 Outcome of this study 16
5.1 General objective 16
5.2 Practical goal 17
5.3 Research goal 18
6 Importance of this study to the current practice 18
Chapter 2: The current pavement design methodology
1 Introduction 212 The current methodology of asphalt pavement design, material selection, and asphalt mixture design 21
2.1 The pavement design method 21
2.1.1 Bearing capacity 22
2.1.3 Two functional requirements for pavements:
longitudinal evenness and skid resistance 24
2.1.4 Summary 24
2.2 Material selection 24
2.2.1 Asphalt concrete 25
2.2.2 Porous asphalt 28
2.2.3 Stone mastic asphalt 31
2.3 The asphalt mixture design method 31
3 Discussion and conclusions 33
Chapter 3: Characterisation of viscoelastic and viscoplastic
behaviour of asphalt mixture
1 Aim 372 Methodology 37
3 Theory 38
3.1 Linear viscoelastic creep susceptibility of asphalt mixture 38
3.2 Viscoplastic creep susceptibility of asphalt mixture 42
4 Experimental details 42
4.1 Test methods 43
4.2 Test set-up and testing conditions 43
4.2.1 Four point bending test 43
4.2.2 Static uniaxial compression creep test 44
4.2.3 Dynamic uniaxial compression creep test 45
4.2.4 Dynamic triaxial compression creep test 45
4.2.5 Modified friction reduction system 48
4.3 Materials 49
5 Results of frequency sweep tests – Complex modulus 52
6 Results of static creep tests in the absence of confinement – Dependence of the creep of asphalt concrete on time, temperature, stress, and mixture composition 58
6.1 Dense graded asphalt concrete 0/16, unmodified 59
6.2 Dense graded asphalt concrete 0/16, polymer modified 61
6.3 Comparison of static creep of DAC 0/16, unmodified and DAC 0/16, polymer modified 63
7 Results of dynamic creep tests in the absence of confinement – Dependence of the creep of asphalt concrete on time, Temperature, stress, and mixture composition 65
7.1 Dense graded asphalt concrete 0/16, unmodified, using a block-wave form of applied stress 66
7.1.1 Time dependence of creep properties – constant rest-time 66
7.1.2 Time dependence of creep properties – constant loading time 76
7.2 Dense graded asphalt concrete 0/16, polymer modified, using a
block-wave form of applied stress 80
7.2.1 Time dependence of creep properties 80 7.2.2 Temperature and stress dependence of creep properties 97 7.3 Dense graded asphalt concrete 0/16, unmodified, using a
sinusoidal applied stress 97
7.3.1 Time dependence of creep properties 97 7.3.2 Temperature and stress dependence of creep properties 102 7.4 Dense graded asphalt concrete 0/16, polymer modified, using a
sinusoidal applied stress 103
7.4.1 Time dependence of creep properties 103 7.4.2 Temperature and stress dependence of creep properties 109 7.5 Block-wave form of applied stress versus sinusoidal applied stress 109 7.5.1 Time dependence of creep properties 109 7.5.2 Temperature and stress dependence of creep properties 111 7.6 Comparison of dynamic creep to static creep 112 7.7 The interrelatedness of J10 and zˆ of equation (21) and the
parallelism of the creep curves 115
7.8 Change of volume of the specimen 117
7.9 Barrelling 119
8 Results of dynamic creep tests in the presence of confinement – Dependence of creep properties on time, stress,
specimen height, and mixture composition 120
8.1 Creep susceptibility of dense graded asphalt concrete – Transition of creep according to eq. (15) to creep according to eq. (16) 120 8.2 Creep susceptibility of dense graded asphalt concrete and
porous asphalt – Time and stress dependence 122
8.3 Change of volume of the specimen 131
8.4 Dependence on the specimen height 132
9 Results and interpretation of finite element computations of the creep of a heterogeneous viscoelastic-viscoplastic creep
specimen 134
10 Modelling the time dependence of linear viscoelastic stress strain
behaviour 137
10.1 Linear viscoelastic stress strain behaviour in the Burgers model 138
10.2 Static creep 139
10.3 Dynamic creep using a block-wave form of applied stress 139 10.4 Dynamic viscoelastic stress strain behaviour, using a
sinusoidal wave-form of applied stress 141 10.5 Complex modulus and linear viscoelastic creep susceptibility
of asphalt mixture 143
10.6 Discussion 144
11 Discussion I – Dependence of creep properties on the
specimen geometry (height) 146
12 Discussion II – Relatedness of dynamic viscoelastic and elasto-viscoplastic (creep) properties of asphalt mixture 149
12.1 Relatedness of complex modulus and creep properties 149
12.2 Creep of asphalt mixture in the absence of confinement 149
12.3 Creep of asphalt mixture in the presence of confinement 150
13 Discussion III – Physical meaningfulness and predictive value of viscoelastic and elasto-viscoplastic properties of asphalt mixture 151 14 Conclusions 153
Chapter 4: Characterisation of the resistance to crack-growth
and the resistance to fracture of asphalt mixture
1 Aim 1552 Methodology 155
3 Theory 158
3.1 The stress intensity factor 158
3.2 ASTM minimum size requirements 161
3.3 Fatigue crack-growth and creep crack-growth 161
3.4 Indirect methods of characterising the resistance to crack-growth 163
3.5 Definitions of tensile strength 163
4 Experimental details 164
4.1 Test methods 164
4.2 Test set-up and testing conditions – Influences of stress condition, shape of the waveform of applied stress, and specimen geometry 166
4.2.1 Crack-growth test using the centre-cracked tensile (CCT) specimen 166
4.2.2 Crack-growth test using the four point bending (4PB) specimen 170
4.2.3 Determination of the fracture toughness test using the semi-circular bending (SCB) specimen 170
4.2.3.1 The ASTM-method for metals 172
4.2.3.2 A modified ASTM-method for asphalt mixture 174
4.2.4 Three tensile tests 175
4.2.4.1 Uniaxial tensile (UT) test 177
4.2.4.2 Semi-circular bending (SCB) test 179
4.2.4.3 Indirect tensile (IT) test 179
4.3 Materials – Influence of the material composition 179
4.3.1 Sand asphalt test in the CCT-tests and in the 4PB-tests 179
4.3.2 Asphalt mixtures tested in the SCB fracture toughness test and indirect tensile test 179 4.3.3 Asphalt mixtures used in the SCB tensile tests,
uniaxial tensile test and indirect tensile test 180 4.3.4 Asphalt concrete mixtures used in the indirect tensile test 180
5 Results and analysis of crack-growth tests 181
5.1 Obtaining a graph of da /dN versus ∆K from raw test data 181 5.2 Crack-growth test using the CCT-specimen – Influence of
frequency, applied stress, and specimen thickness 186 5.2.1 Constant load amplitude tests 187
5.2.2 Constant ∆K test 193
5.2.3 Creep crack-growth tests 195
5.3 Crack-growth test using the 4PB-specimen – Influence of
frequency and specimen thickness 195
5.4 Analysis of the parameters A and n of the Paris equation 195
5.5 Discussion 208
5.6 Prediction of dynamic crack-growth based on static
creep crack-growth 212
5.7 Discussion 214
5.8 Critical stress intensity factor 216
6 Results and analysis of fracture toughness tests
using the SCB-specimen 219
6.1 Dependence of the fracture toughness on the specimen size 219 6.2 Dependence of the fracture toughness on the displacement rate 219 6.3 Influence of the material composition 221
6.4 Analysis of the results 221
6.4.1 Dependence of the fracture toughness on the specimen size 221 6.4.2 Dependence of the apparent fracture toughness on the
specimen size at 15ºC 228
6.4.3 Validity of the fracture toughness 228 6.4.4 Dependence of the apparent fracture toughness on the
displacement rate 231
7 Results and analysis of tensile strength using the uniaxial tensile (UT) specimen, the semi-circular bending (SCB)
specimen, and the indirect tensile (IT) specimen 233
7.1 Case 1: Fine porous asphalt – Uniaxial tensile strength and bending tensile strength – Influence of temperature
and diplacement rate 233
7.2 Case 2: Porous asphalt – Uniaxial tensile strength and bending tensile strength – Influence of mixture grading, bitumen
content, and type of bitumen 234
7.3 Case 3: Various asphalt concrete and porous asphalt mixtures – Uniaxial tensile strength, bending tensile strength, and indirect tensile strength – Influence of
temperature and displacement rate 236
7.4 Case 4: Asphalt concrete – Indirect tensile strength – Influence of specimen size, loading strip, temperature,
and mixture composition 240
7.5 Analysis of the results 244
7.5.1 Uniaxial tensile strength and bending tensile strength
at low temperature (0ºC and lower) 246 7.5.2 Uniaxial tensile strength and bending tensile strength
at high temperature (15ºC and higher) 249 7.5.3 Indirect tensile strength at low temperature
(0ºC and lower) 250
7.5.4 Indirect tensile strength at high temperature
(15ºC and higher) 253
7.5.5 Summary 253
7.6 Relationship between fracture toughness and bending tensile
strength in the SCB-test 254
8 Finite element model of the SCB-specimen 256
8.1 Specimen geometry 256
8.2 Definition of damage 257
8.3 Material 257
8.4 Results of computations 258
8.5 Discussion and conclusions of computational results 261
9 General discussion 262
10 Conclusions 264
Chapter 5: Performance judgement of asphalt mixtures
1 Introduction 267
2 Composition-relatedness: An impediment to innovation in
road building 268
3 Property-related requirements for asphalt mixture – The key
to enable innovation in road building 271
3.1 Definition of performance relatedness 271 3.2 Physical meaningfulness and predictive value of constitutive
model variables and parameters 272
3.3 Controlling pavement performance 273
3.3.1 Cost-effectiveness and risk of failure 273
3.3.2 Pavement performance 281
3.3.3 Performance related asphalt mixture properties 281
3.4 Summary 283
4 Asphalt mixture: The complicating factor in the development
of performance related requirements 284
4.1 Dependence of asphalt mixture properties on specimen size
and shape 284
4.2 The stress state of the specimen 285
4.3 Relative (qualitative) predictive value 288
5 Interrelatedness of constitutive model variables and parameters 289
5.1 Complex modulus 289
5.2 Linear viscoelastic creep 291
5.3 Creep of asphalt mixture 294
5.4 Crack-growth in asphalt mixture 298
5.5 Fracture toughness and tensile strength 300
5.6 Summary 300
6 Discussion: Importance of the interrelatedness of constitutive model variables and parameters 301
7 Conclusions 302
Chapter 6: General discussion
1 Introduction 3032 General objective 303
3 Practical goal 304
4 Research goal 306
Chapter 7: General conclusions
1 Introduction 3092 General objective 309
3 Practical goal 310
4 Research goal 313
5 Property-related requirements versus composition-related requirements for asphalt mixture 313
Appendix 1: Influence of the shape of the wave-form of the
applied stress on the response strain of the Burgers model
1 Introduction 3152 Analysis 316
2.1 Constant applied stress 317
2.2 Sinusoidal applied stress 320
2.3 Unidirectional sinusoidal applied stress (haversine) 320
2.4 Alternating block-wave of applied stress 321
2.5 Unidirectional block-wave of applied stress 322
2.6 Half sine waveform of applied stress 323
2.7 Influence of the shape of the waveform of the applied stress on the creep strain rate 325
3 Detailed solutions of the Burgers equation 326
Case 1 327
Case 2 329
4 Conclusion 333
Appendix 2: Influence of the friction reduction system
1 Introduction 335
2 RHEI investigation into friction reduction 335
3 Discussion 336
Appendix 3: List of creep models
339Appendix 4: The material model of the FEMMASSE finite
element model of a heterogeneous viscoelastic-viscoplastic
creep specimen
1 FEMMASSE finite element code 347
2 The aggregate grains 347
3 The bituminous matrix 348
Appendix 5: The Asphalt Concrete Response (ACRe) Model
1 The material model 351
1.1 Definition of damage 351
1.2 Plastic flow criterion 351
1.3 Simulation of the hardening process 353 1.4 Simulation of the degradation process 354
1.5 Simulation of crack-growth 355
2 Determination of material parameters 356
Appendix 6: Computer programme to compute the
K
dN
da
/
−
∆
-relationship
359List of symbols
a crack-length
A constant (coefficient of the Paris equation)
A0 constant
B specimen thickness
BTS bending tensile strength
C constant
D specimen diameter
e base of the natural logarithm
E elasticity modulus
E1, E2 spring constants of rheological model
f frequency
Fm Marshall flow
I constant
I1 first stress invariant
ITS indirect tensile strength
J compliance J* complex compliance ′ J storage compliance ′′ J loss compliance
J0 constant (instantaneous elastic compliance at t = 0
J1 compliance at time t = 1 s
′
J2 second deviatoric stress invariant
k constant
K constant
stress intensity factor
Kc critical stress intensity factor
KI mode I stress intensity factor
KIc critical mode I stress intensity factor, or fracture toughness KIQ apparent fracture toughness (before test of validity)
m slope of the complex compliance on log-log scale
n constant (exponent of the Paris equation)
N number of load repetitions
Nf fatigue-life P0 pertinent force Pm Marshall stability Qm Marshall quotient R stress ratio S stiffness modulus * S complex modulus
S′ storage modulus
S′′ loss modulus
S− stiffness modulus in compression
S+ stiffness modulus in tension
Smix mixture stiffness t time
tl loading time tr rest-time
T temperature
TR reference temperature UTS uniaxial tensile strength
W specimen width
z slope of the creep compliance on semi-ln scale
z slope of the creep compliance on semi-log scale
z~ slope of the creep compliance on ln-ln scale or log-log scale
zˆ slope of the creep compliance versus ln t on ln-ln scale, or
the creep compliance versus log t on log-log scale
α constant
β constant
&
γ shear rate
Γ Gamma-function, fracture energy δ loss angle
δ& deformation rate
∆K difference between minimum and maximum stress intensity factor, ∆K = Kmax − Kmin
ε strain $ ε strain amplitude & ε strain rate &
ε0 minimum permanent strain rate &
εperm permanent strain rate
$
ε− strain amplitude in compression
$
ε+ strain amplitude in tension
η1, η2 dashpot viscosities of rheological model ς reference time
time dependent analogue of z ς time dependent analogue of z ς~ time dependent analogue of z~
λ relaxation time of rheological model, λi =ηi /Ei
ν Poisson’s ratio, kinematic viscosity
π 3.14159..
σ stress $ σ stress amplitude 0 σ pertinent stress σ1 axial stress σ3 radial stress σR rupture strength ys σ yield strength Σ sum τ shear stress ω angular frequency cos cosine exp exponential sec secans (1/cos)
tan tangent
tanh tangent hyperbolicus ∝ is proportional to
List of units
%m/m mass percent °C degrees Celsius g gram K Kelvin m meter N Newton Pa Pascal, 1 Pa = 1 N/m2 rad radian s secondPrefixes
G giga, 1 giga = 109 k kilo, 1 kilo = 103 m milli, 1 milli = 10-3 M mega, 1 mega = 106List of abbreviations
App. appendixASTM American Society for Testing Materials BTS bending tensile strength
Ch. Chapter
Chs. Chapters
CROW Bureau for Contract Standardisation and Research for Civil Infrastructure
CGAC crushed gravel asphalt concrete DAC dense graded asphalt concrete
eq. equation
eqs. equations fig. figure figs. figures
GAC gravel asphalt concrete ITS indirect tensile strength
ln logarithmus naturalis, natural logarithm log logarithm
LVDT linearly variable displacement transducer OAC open graded asphalt concrete
PA porous asphalt SMA stone mastic asphalt
RHEI Road and Hydraulic Engineering Institute, Ministry of Transport, Public Works, and Water Management SAL standard axle loads
SAL100 equivalent 100 kN standard axle loads
sec. section tc traffic class
1
Introduction
1 Motive for this study
The commonly used road paving materials are asphalt and cement concrete. In The Netherlands, asphalt pavements make up approximately 95% of the main road network. There is a need for a different approach to control the quality of asphalt pavements and asphalt mixtures than the traditional one. This need arises from a variety of socio-economic developments.
1.1 Socio-economic developments
By virtue of the constitution, the State protects the prosperity and welfare of its citizens. Constitutional rights are, for example, the protection of the population’s means of living, the nation-wide distribution of prosperity, the enhancement of room for living and employment, and the right of health-care and education. Important for the fulfilment of constitutional rights is the competitiveness of the national economy. The greater the productivity, the lower the unit cost of production and the greater the competitiveness of the economy. The competitiveness of the economy is enhanced by economic growth and innovation, which, in turn, are necessary for the preservation of employment, and, by that, for the preservation of the population’s means of living and prosperity.
Figure 1. Use of the private car and public transport between 1950 and 1995. [Data from: National Bureau of Statistics].
1.1.1 Control of development of transport infrastructure
Important pillars of the economy are the different transport infrastructure networks; the road network, the waterways network, the rail network, and the air and seaports. The economy strongly depends on an efficient goods transport sector. An indication of the quality of the national transport infrastructure, in comparison to surrounding countries can be obtained from the World Competitiveness Report of IMD/World Economic Forum. In this yearly study, an international panel of entrepreneurs give a rating of different sorts of infrastructure. The rating of the Dutch road infrastructure has decreased during the last few years, and is clearly lower than that of neighbouring countries. A similar trend can be observed in the rating of the Dutch rail infrastructure. The ratings of the air and sea ports have been better than those for the surrounding countries.
In 1996, it was recognised that the execution of the Second Traffic and Transport Framework Programme 19881, FPTT II (MT 1988) needed intensification. This condensed in the bill Beating Congestion2 (MT 1996a) and the action plan Balancing Transport3 (MT 1996b). The main policies mentioned in the Balancing Transport plan are:
. the enhancement of the competitiveness of sustained transport, in particular by rail, canal, and short sea,
. the reduction of the burden on the environment caused by road traffic, . improved access to economic areas for road transport of goods.
1
Tweede Structuurschema Verkeer en Vervoer, SVV II. 2
Samen Werken aan Bereikbaarheid, SWAB. 3
Transport in Balans, TiB. 0 20 40 60 80 100 120 140 160 1950 1960 1970 1980 1990 2000 year tr a ve lle r k m ( b illi o n k m ) private car public transport
Economic growth has as an effect a growth of transport of persons and goods. The development of the use of private cars as compared to public transport since 1950 is shown in figure 1. The Second Traffic and Transport Framework Programme forecasted an auto mobility increase by 72% from 1986 until 2010. In FPTT II and the National Environmental Programme 19884, NEP (MH 1988), it was targeted to limit this increase to 35%. However, this was realised as early as 1993. In view of the enormous increase in congestion (70% increase of vehicle-loss hours since 1986), it is clear that the road infrastructure is incapable of accommo-dating the extrapolated auto mobility in the next fifty years.
Over the past decades, the developments in traffic and transport have been largely autonomous, being the resultant of decisions by individuals in an individualised society. The national authority saw its influence reduced to control the utilisation of the different modalities. Extension of the road infrastructure has not provided an adequate answer to the developing congestion. Typically, the supplied road capacity suffices during most of the daytime, and is insufficient during rush hours. The average utilisation of the private car during rush hours has remained constant to just 1.2 persons, despite measures to stimulate drivers to use different modalities. The average utilisation of trucks has decreased since the mid eighties until approximately 50% at present (as a result of external factors). Thus, the existent road infrastructure is not utilised optimally, whilst further extension requires a lot of money, time and green space, and in fact creates additional over-capacity.
The property of the different types of transport infrastructure is in the hands of the national authority. The infrastructure networks are a national collective good, in the sense that the construction and maintenance has been financed collectively. The road user is not charged directly in relation to his use of the road infrastructure. As a result, there is at present no relationship between the user’s use of road infrastructure and his compensation for this use. This yields abundance and temporary scantiness of road capacity uncontrolled. With the National Traffic and Transport Framework Programme 20015, NTTFP (MT 2001), the original aims of FPTT II and NEP have been reconsidered. The growing need for mobility is acknowledged. The aim is to make possible that further growth of road traffic can be accommodated. To manage the growing demand, three main goals have been formulated in NTTFP:
. to improve utilisation of infrastructure,
. to enhance capacity (by “smart” utilisation as well as physical extension), . road pricing.
It is anticipated that these goals are within reach thanks to advances in information and communication technology.
4
Nationaal Milieubeleidsplan, NMP. 5
Figure 2. Domestic road transport of goods. [Data from: National Bureau of Statistics].
Figure 3. International road transport of goods. D = Germany, B/L = Belgium and Luxembourg, F = France, I = Italy, SP = Spain.
[Data from: National Bureau of Statistics].
1.1.2 The growth of road traffic
The tremendous increase in the use of the private car over the past fifty years has had mainly environmental implications, such as loss of green space, noise emission, and air pollution. However, the damaging effect of the private car to the road pavement is practically negligible, in comparison to that of the truck. Therefore, if the growth of traffic is considered in relation to design and maintenance of roads, then it suffices to consider only truck traffic. Truck traffic also has been growing beyond
0 5 10 15 20 25 1975 1980 1985 1990 1995 2000 year dom es ti c road t rans port of goods (billion t onk m) 0 10 20 30 40 50 60 1984 1986 1988 1990 1992 1994 1996 1998 2000 year int e rn. road t rans port of goods (m ln t on) D B/L F I SP
Figure 4. Axle load spectrum from 1968 to 1999. The median axle load, indicated by the arrows, increases steadily with time. [Data from RHEI6]. expectations both in number and weight. Figure 2 shows the domestic road transport of goods from 1980 to 1998. Figure 3 shows the international road transport of goods from 1986 to 1998. In figure 4, the axle load spectrum shows a gradual increase of the median with time. Thus, it follows that in particular the combination of number and weight of trucks causes a significant increase of the traffic load of the road network. Figure 5 shows the increase of the percentage of trucks equipped with super singles. The super single has a greater tyre pressure and a smaller tyre/pavement contact area than the traditional dual wheel configuration. Therefore, the damaging effect on the pavement is greater, in comparison to the dual wheel. This effect may increase further, should the European Community decide to allow a higher maximum axle load without limiting simultaneously the tyre pressure. This trend is supported by economic and environmental advantages, because bigger trucks are more efficient in fuel consumption, and mean fewer trucks, fewer wasted tyres and lesser congestion.
1.1.3 Sustained use of materials and energy
Sustained Development is an embedded policy, which compels economic use of materials and energy to protect the environment. It means many things. It means a continuous effort to re-use materials to save energy of
6
The curve for 1999 represents the cumulative result of axle load measurements performed over the period between 1-1-1993 and 1-1-2000. Data from RHEI 2001.
0 20 40 60 80 100 120 0 50 100 150 200 axle load (kN) c u m u lat iv e perc ent age 1968 1979 1986 1993 1999
Figure 5. Top (a): Super single and dual wheel. Bottom (b): Increase in the use of super singles between 1980 and 2000. [Photographs from RHEI; data from RHEI 1996].
production and reduce emission of green house gasses. It means that new materials are developed, as well as production and maintenance techniques, to make possible a more efficient use of materials and energy. In the road building industry, important contributions to this can be realised by re-use of building materials in road bases, and re-use of old asphalt. Hot recycling of asphalt is developed by stimulating increased recycling percentages, and re-use of old asphalt in the original application (e.g. porous asphalt in porous asphalt) if possible. Cold re-use of old asphalt is developed in various ways, for example, by means of polymer modified bitumen emulsions, and foam bitumen. Further contributions can be realised by developing new, more durable paving materials.
0 20 40 60 80 100 1980 1985 1990 1995 2000 2005 year pe rc ent a ge s u per s ingl e s
A continuous effort is developed to control traffic noise emission, by the development of “silent” running surfaces. A second generation of silent running surface is made of two-layered porous asphalt, which consists of a top-layer of fine-graded polymer modified porous asphalt, on top of a lower layer of standard porous asphalt. Thus, up to 6-7 dB(A) noise reduction can be achieved instead of the normal 3 dB(A).
1.1.4 Design-Build-Maintain contracts
A recent development in road building is the development of Design-Build-Maintain contracts. In these contracts, the contractor is offered the opportunity to submit a tender incorporating not only the building but also the design and maintenance of a road or pavement for a long period, e.g. a substantial portion of the pavement’s service-life, the entire service-life, or even a longer period. This type of contract aims among other things to stimulate innovation in road building. The incorporation of design, build and maintenance in a single contract implies that the contractor takes responsibility for the design including the material selection, and the consequences of that for maintenance. This type of contract causes responsibilities to shift from the client (the road authority) to the contractor. This, in turn, demands from the contractor an in-depth analysis of the project risks involved, not only from a contractual viewpoint, but also from an insurance point of view.
1.1.5 Road building, changing authorities, and the knowledge economy7
In modern society, the national road authorities are no longer the sole organisations where the knowledge to build roads is concentrated. Relevant knowledge is more and more distributed in the economy. National authorities, not just road authorities, have indicated they wish to be more efficient. One way to achieve this is to make use of relevant knowledge available in the private economy, rather than to develop relevant knowledge inside the public organisation. It seems, authorities prefer to lose intentionally capacity of developing new knowledge in favour of developing a capacity of arranging knowledge available in the private economy. Real innovating power lies in enterprises, where the development costs of innovations can be regarded as an investment with a chance to become profitable. It is important to permit room for innovative developments, also in road building and maintenance, in order to keep up
7
A knowledge economy is an economy in which the production factors labour and capital are strongly aimed at the development and application of new technology. Romer (1986, 1990) has proposed a change to the neo-classical model of economics by seeing technology (and the knowledge on which it is based) as an intrinsic part rather than an exogeneous factor of the economic system. In Romer’s theory, knowledge is the basic form of capital. Ecomic growth is driven by the accumulation of knowledge.
with socio-economic development in general and to hold the cost of building and maintenance of the road network on an acceptable level. To achieve that, it is necessary to put available knowledge to work anywhere in the economy in an efficient manner. How can that be achieved? It requires more than one thing to change. One thing necessary is to remove restrictive systems that inhibit the application of knowledge available in the economy. Imagine businesses and road authorities come together to communicate about the performance and the price of objects of public infrastructure. The current system of contractual requirements and technical specifications works satisfactorily as long as it is operated within its framework of standardised technology. With any new development the road authority asks if current requirements are applicable, and if not, to develop new requirements. This question unfolds the current system’s restriction. The restriction lies in its empirical character. This causes the limited applicability of the current requirements and specifications to newly developed products, and the long time needed to evaluate the performance of new products, and, because of that, also a long time to develop new requirements. The time needed to develop new requirements let alone the time needed to develop the knowledge to be able to develop a more fundamental approach, causes the implementation of innovative techniques and materials to stay at a low pace, until a system is developed which permits development of more generally applicable requirements. That the pace of innovation is low can be observed from the rate at which new standardised materials and test methods have been introduced in standard regulations and requirements (CROW 2000) over a period of say thirty years. Thus, the present restrictive system of requirements and specifications prevents economic parties from communicating over the performance quality of innovative asphalt pavements and asphalt mixtures.
To improve this situation, to make possible that available knowledge in the economy can be put to work in an efficient manner, it is necessary to modernise pavement design concepts and asphalt mixture design concepts. It is necessary to replace current requirements by requirements that permit economic parties to communicate over performance quality of new products. It seems that requirements have to be more generally applicable, not just to standardised technology but to new technology as well. The question may be posed which form such requirements should take. When businesses and road authorities communicate over performance quality, they discuss many things including road comfort, traffic safety, environment, and health and safety of road workers. If the performance quality of the object of infrastructure itself is considered, the essence of what both parties are concerned with is the cost-effectiveness and the risk of failure. The cost-effectiveness and the risk of failure of standardised technology, asphalt pavements, paving materials, production technology,
and paving technology, are known, based on experience, laid down in the empirical requirements and technical specifications. With a new development, these certainties fall away as soon as normal requirements and technical specifications are not applicable. Suddenly, in order to be able to judge its cost-effectiveness and risk of failure, there is the need to predict the pavement’s behaviour. Technicians start building computer models to predict the pavement’s response to loading. That is where a significant change takes place in comparison to the current pavement design methodology. The current system with its empirical test methods to determine optimal mixture compositions is inadequate. Typical examples of empirical methods are the Marshall test, to some extent the fatigue test, the wheel tracking test, various ravelling tests; the empirical parameters – a Marshall stiffness, a fatigue-life as commonly reported, a permanent deformation in the wheel-tracking test, a mass-loss by abrasion in a ravelling test – are not the sort of properties required to predict the pavement’s behaviour. In a functional or performance related approach concerned with the prediction of pavement behaviour, and the evaluation of the cost-effectiveness and risk of failure, the material composition is irrelevant; relevant are only the properties needed to predict or judge the cost-effectiveness and the risk of failure.
As long as the road network is a national asset and a public interest, and thus the responsibility of a road building authority, it is the responsibility of that authority to impose (performance related) requirements. This means that the answers to questions regarding the facilitation of innovation cannot come from the responsible authority if that has lost its knowledge to judge the cost-effectiveness and risk of failure of pavements and the applied paving materials.
1.2 Innovation and product quality - The need for a rationalised quality control methodology8
A challenge of the future is to unify economical and environmental goals in improving road infrastructure utilisation and enhancing its capacity. It means, for example:
1 a further growth of traffic has to be accommodated, while noise levels and pollution must be reduced,
2 more durable pavements must be developed, so that maintenance frequencies are reduced, to avoid congestion by maintenance and the negative effects of congestion on road safety,
3 new materials, production techniques, or maintenance techniques have to be developed, which save materials and energy.
8
The word methodology is used to indicate a body of methods. The quality control methodology covers an asphalt production control method, an asphalt pavement design method, a material selection method, an asphalt mixture design method, and a mixture constituents selection method.
Asphalt production control method pavement design method material selection method mixture design method mixture constituents selection method
Figure 6. Scheme, illustrating the total quality control methodology (light) and the pavement design methodology (dark).
The realisation of these goals requires a technological innovation, which has to be developed at a higher pace than in the past. Considering the growth of road traffic over the past five decades, it is probable that roads are now built for future traffic loads that are far beyond our present experience. The developments in traffic and transport make it probable, that we will have to use pavement structures and materials with which we have presently no experience.
Apart from new products, new instruments for quality judgement of products are needed. The present study is concerned with the instrument to judge the quality of the asphalt mixtures. Figure 6 illustrates the asphalt production quality control methodology and the pavement design methodology. The asphalt production quality control methodology can be considered to consist of five methods: a method for the selection of the asphalt mixture constituents, an asphalt mixture design method to design an optimised mixture composition, a material selection method to select the asphalt mixtures to be applied in a pavement design, and an asphalt production control method to control the production quality (which includes the quality of the asphalt after paving, i.e. after compaction). Not considered in the present study are the method for the selection of the asphalt mixture constituents , and the asphalt production quality control
method. The remaining three methods, the pavement design method, the material selection method, and the asphalt mixture design method (the dark section of figure 6), is conveniently called the pavement design methodology. Considering the methods indicated in figure 6, it can be observed that currently all five methods are almost entirely or entirely empirical. The pavement design method is the only method, which uses more or less “fundamental” design criteria, but apart from that is mainly empirical. The asphalt production control method, the material selection method, the mixture design method, and the mixture constituents selection method, are based on the compositions of the asphalt mixtures. The fact that the methods are composition-based causes the methods to be empirical. What this means is explained in Intermezzo 1.
An empirical methodology requires renewal of empirical reference data, based on practical experience. To gain practical experience with a new pavement design, or a new type of asphalt mixture, requires monitoring of the nominal service-life, to gather reference data, and to verify the performance (cost-effectiveness with respect to standard pavement designs, respectively asphalt mixtures). This leads to a delay of innovation that is no longer acceptable. Thus, to date, the current empirical design methods have been of very little value for the development and acceptation of new types of asphalt pavement and asphalt mixture.
The alternative of the empirical method that relies on practical experience is the fundamental method, which relies on theory. However, one may wonder what can be the added value of a fundamental method since it is not possible to predict pavement performance quantitatively, i.e. to predict
Intermezzo 1
A characteristic of an empirical method is that it relies on practical experience rather than theories. This makes an empirical method descriptive rather than explaining. An empirical law can describe a phenomenon without providing an understanding, although the empirical law itself could be considered a sort of “understanding”; yet, this differs from an understanding in terms of fundamental principles, which have more general predictive value. An empirical law is predictive merely in its own reference system. The following example can illustrate this. An empirical law could be, for example, the moon moving from the east to the west across the south in the northern hemisphere. This law is predictive in the northern hemisphere, but not in the southern hemisphere, where the moon moves from the east to the west across the north. To design a similar law which is predictive for both hemispheres, a deeper understanding of the system of the moon and the two hemispheres is required.
the type and amount of a specific type of damage as function of the time during the pavement’s service-life. The main reason is the complexity of the road system and the unpredictability of a number of influence factors, such as the traffic, the climate (during paving and service), the variability of mixture constituents, the variability of production and paving, and the spill of chemical agents (leaking motor oil, solvents). Furthermore, one should realise that apart from the complexity of the road system and the unpredictability of influence factors, the available theories are in fact oversimplifications of the reality. This is caused by the following assumptions:
. homogeneous and isotropic pavement material,
. linear elastic stress strain behaviour, instead of nonlinear viscoelasto- plastic stress strain behaviour
. simplified dynamic loading by traffic,
. a one-dimensional uniform contact pressure distribution, instead of a three-dimensional nonlinear distribution,
. a simplified temperature distribution in the pavement (mean annual asphalt temperature).
A fundamental approach requires that:
1 materials are characterised by means of true material properties, 2 tests are available to determine those properties.
By definition, a true material property is a property which is independent of the geometry (size and shape) of the specimen, and which is not influenced by the measurement itself. In a popular way of saying, a true material property is reproducible in different tests. The behaviour of an asphalt mixture is really too complicated to be described in detail by any available fundamental model. It is shown in this study that fundamental models exist, in which the material is assumed to be homogeneous and isotropic, and that these models can be used as approximate models when applied to an asphalt mixture. Thus, a truly fundamental method to control the quality of asphalt is not feasible to date. What is feasible, is a rational approach, i.e. an approach based on reason instead of belief. It is not meant by this, that the traditional empirical method is irrational. However, it is based on empirical fact rather than reason. A new method can be more adaptable to new developments, if reason gains importance as one of its pillars relative to (historical) fact.
2 Scope of this study
2.1 IntroductionA pavement’s main functions are: bearing capacity, surface characteristics, and long-term performance. Long-term performance can be defined as the gradual loss with time of the pavement’s bearing capacity and surface
characteristics. The total pavement response9 to loading can be regarded as the resultant of the actual response to loading, and the long-term performance. In terms of response to loading, the long-term performance can be defined as the gradual decline of the actual response as a function of the time during the pavement’s service-life. The pavement’s main functions are controlled by the pavement’s functional properties. These are indicated in table 1.
Table 1. Functional properties of asphalt pavements.
Functional properties of an asphalt pavement
bearing capacity Surface characteristics Long-term performance
stiffness resistance to fatigue (crack-growth from bottom of pavement upwards) evenness resistance to skidding slant noise emission hydraulic conductivity light reflectivity resistance to: * ravelling * rutting * surface cracking stiffness resistance to: * permanent deformation * fatigue * surface cracking * ravelling * disbonding (stripping) * ageing
Bold-faced properties are called functional properties of asphalt mixtures.
The pavement’s functional properties are controlled by the functional properties of the applied materials. In the case that the applied materials are asphalt mixtures, the functional pavement properties are controlled by means of the functional asphalt mixture properties. In the following, table 1 is explained in more detail.
The pavement’s bearing capacity is controlled by the pavement’s stiffness, which, in turn, is controlled by the thickness of the asphalt layers and the stiffness of the applied asphalt mixtures. It is important that the pavement stays intact, and that no cracks form. Therefore, it is important that the pavement has resistance to fatigue.
The pavement’s surface characteristics can be divided into two types: 1 the characteristics at the very pavement surface, which the pavement derives from the properties of the asphalt mixture’s constituents at the pavement surface,
9
The word response is used to indicate the mechanical reaction to mechanical loading. The total pavement behaviour might be more generally defined as the resultant of the actual behaviour and the long-term behaviour, whilst the word behaviour is used to indicate any type of reaction (mechanical, thermal, or chemical), to the corresponding type of loading. However, the scope of this study is limited to the mechanical behaviour of asphalt mixtures.
2 the characteristics determined by the mechanical properties of the asphalt mixture lying in the top pavement layer.
For a full performance related approach to pavement design it is necessary to consider the pavement’s long-term performance. Important for the pavement’s long-term performance is the characterisation of the asphalt mixture’s physiochemistry, i.e. the adhesion and disbonding of mineral aggregate and bitumen, and the ageing of bitumen10. The asphalt mixture’s long-term performance depends on the adhesive properties of the mixture constituents, how these are influenced in the presence of an agent11, and the ageing properties of the asphalt mixture and the mixture constituents12.
2.2 This study’s subject
This study is concerned with the mechanical properties of the asphalt mixtures, which are relevant to the actual pavement response. The asphalt mixture’s mechanical properties are bold-faced in table 1. A reason for the selection of this subject can be given as follows:
1 mechanical test methods are needed to characterise both an asphalt mixture’s actual response and long-term performance,
2 assuming that the asphalt mixture’s resistance to fatigue, resistance to fatigue crack-growth, and resistance to ravelling (i.e. the mechanical aspect of ravelling) are a part of the characterisation of the mixture’s actual response, then the characterisation of this actual response requires mechanical testing protocols,
3 assuming that the asphalt mixture’s resistance to disbonding and stripping, and its resistance to ageing, are a part of the characterisation of the mixture’s long-term performance, then the characterisation of this performance requires testing protocols to test disbonding and stripping properties, and ageing properties, in addition to mechanical testing protocols.
10
These aspects have been, and are still investigated: cf. Elphingstone (1997), Groenendijk (1998), Voskuilen et al. (1996), Kuppens (1997), Mes (2003). 11
Agents can be additives to improve adhesive properties, or can be agents which affect the adhesion in a negative sense: water, ice, de-icers, oil-spill, chemical solvents, clay in mineral aggregate, and other contaminations.
12
Ageing is thought to have a physical component (e.g. time-hardening, loss of volatiles from the binder, bitumen), and a chemical component (e.g. oxidation by the air, interaction with ultra-violet radiation in day-light).
Thus, the characterisation of the actual response can be considered critical to the development of a performance related asphalt mixture design method. That is, if mechanical tests to characterise a mixture’s actual response are lacking, then a performance related mixture design method is not feasible, and a performance related pavement design methodology is
also not feasible. Testing protocols for characterisation of adhesion, disbonding, and ageing can always be embedded later into a performance related quality control methodology.
3 Aim of this study
3.1 General objectiveThe ultimate aim is to facilitate the acceptance of new and non-standardised paving materials that are needed to enhance the durability of our heavily trafficked main road network.
The general objective is limited to the actual response to loading, i.e. the mechanical behaviour, of an asphalt mixture.
3.2 Practical goal
3.3 Research goal
Mechanical testing of an asphalt mixture is particularly difficult, because of the combination of the following features:
. the material heterogeneity,
. the different mechanical properties of the constituent phases,
. the time dependence and stress dependence of the mechanical behaviour, . the use of a specimen which is relatively small in comparison to the material heterogeneity (i.e. the maximum grain size).
A relatively small specimen is convenient from a practical viewpoint, but complicates the testing for the reason that the interaction of the material heterogeneity and size and shape effects of the specimen may influence the measured property. Therefore,
The ultimate aim is to limit the empirical character of the methods used for pavement design and asphalt mixture design (type testing), and to find methods for validation of methods other than practical experience.
The general objective of this study is to make a characterisation of the mechanical behaviour of an asphalt mixture possible in as much as that is relevant to the pavement’s main functions, bearing capacity, surface characteristics, and long-term performance.
The practical goal of this study can be formulated as: to make possible a characterisation of an asphalt mixture’s mechanical behaviour relevant to the pavement’s main functions, bearing capacity, surface characteristics, and long-term performance, allowing the use of tests that are suitable for the practical purposes of material selection in pavement design, asphalt mixture design (type testing), and production quality control.
The research goal of this study can be formulated as: to develop a method for the validation of simple tests for practical purposes.
4 Selected topics
The following topics are discussed: In chapter 2, the current pavement design method, including the material selection method, and the asphalt mixture design method are discussed. Chapter 3 contains the experimen- tal evidence and an analysis of the viscoelastic properties and creep or viscoplastic properties of asphalt mixture. Chapter 4 contains the experimental evidence and an analysis of the crack-growth and fracture properties of asphalt mixture. Chapter 5 discusses elements of a method for the evaluation or judgement of the performance of an asphalt mixture in a pavement in relation to cost-effectiveness and risk of failure. This thesis ends with a general discussion in chapter 6, and the general conclusions in chapter 7.
The topics in chapter 3 and chapter 4 were selected because permanent deformation (rutting in the pavement) and crack-growth are important elements of the method discussed in chapter 5, which is needed to quantify the bold-faced properties in table 1. These properties can be quantified by quantifying the following aspects of the mechanical behaviour:
. linear viscoelastic stress strain behaviour, . time dependence of the stress strain behaviour, . stress dependence of the stress strain behaviour, . fatigue and crack-growth behaviour.
5 Outcome of this study
5.1 General objectiveIn general terms, the outcome of this study is that it is possible to characterise the mechanical behaviour of asphalt mixture by means of constitutive models. Constitutive model parameters define mechanical properties of an asphalt mixture. Constitutive models serve to attribute a physical meaning to an asphalt mixture’s mechanical properties. It is important to be able to attribute a physical meaning to constitutive model parameters. This is important for the predictive value of the model and its parameters. A predictive value is indispensable for a quantification of the cost-effectiveness and the risk of failure.
However, mechanical properties of asphalt mixture, determined in the laboratory, lack quantitative or absolute predictive value for the behaviour of asphalt mixture in the pavement. Two reasons can be given for this: 1 The dependence of, for example, a creep property on the specimen geometry is an intrinsic property of a granular bituminous material that exhibits nonlinear stress strain behaviour. This means, that this dependence cannot be avoided or eliminated.
2 The stress strain behaviour depends on the shape of the waveform of the applied stress. The shape of the waveform of the stress induced in the
