Characterization of Failure and
Permanent Deformation
Behaviour of Asphalt Concrete
JINGANG WANGKarakterisering van het Faal‐ en
Blijvende Vervormingsgedrag van
Asfaltbeton
Proefschriftter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof.ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op woensdag 08 Juli 2015 om 10:00 uur Door
Jingang Wang
Master of Science in Material Science Wuhan University of Technology, China Geboren te Zaoyang in Hubei Province, China
Dit proefschrift is goedgekeurd door de promotor: Prof. dr. ir. A.A.A. Molenaar
Prof. S.P. Wu, BSc., MSc., PhD. Copromotor
Ir. M.F.C. van de Ven
Samenstelling promotiecommissie:
Rector Magnificus, Technische Universiteit Delft, voorzitter Prof. dr. ir. A.A.A. Molenaar, Technische Universiteit Delft, promotor Prof. S.P. Wu, BSc., MSc., PhD. Wuhan University of Technology, promotor Ir. M.F.C. van de Ven, Technische Universiteit Delft, copromotor Prof. G. Thenoux, BSc., MSc., PhD. Pontificia Universidad Católica de Chile Prof. K.J. Jenkins, BSc., MSc., PhD. Stellenbosch University
Prof. dr. A. Scarpas, Technische Universiteit Delft
Dr. ir. Z. Su, Icopal BV, Groningen
Prof. dr. ir. H.E.J.G. Schlangen, Technische Universiteit Delft, reserve lid
Published and distributed by:
Jingang Wang
Section of Road and Railway Engineering Faculty of Civil Engineering and Geosciences Delft University of Technology
P.O.Box 5048, 2600 GA Delft, the Netherlands
Email: Jingang.Wang@tudelft.nl; jingang1126@gmail.com ISBN 978-94-6203-863-9
Printing: Wohrmann Print Service, Zutphen, The Netherlands © 2015 by Jingang Wang
All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized 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.
I dedicate this dissertation to my parents
Acknowledgements
The research presented in this dissertation was conducted at the former Road and Railway Engineering (RRE) section of the Faculty of Civil Engineering and Geosciences of the Delft University of Technology. The research was mainly funded by the Chinese Scholarship Council (CSC). In addition, during the last year financial support came from the RRE section. This PhD project originated from the cooperation between the Delft University of Technology and Wuhan University of Technology. The initiators made great efforts to build an excellent cooperation between these two organizations. I wish to extend my gratitude to these organizations and individuals for their great support.
Looking back to the process of this PhD project, I would like to conclude that a journey of pursuing a PhD degree is like a marathon. Although knowing that finishing a PhD research abroad means an unprecedented great challenge, it is also irresistible for a passionate young man who wants to be different. This journey is fairly lengthy and full of uncertainties; it is not possible to complete it without the contribution and support from amazing people. Therefore I would like to express my sincere gratitude to all of them on this special occasion.
First of all, the greatest respect and the deepest appreciation go to my promotor, Prof.dr.ir. A.A.A. Molenaar. He, in my heart, is a very amiable friend in my life and a learned mentor in my study. He always encouraged me and provided me with valuable guidance throughout every stage of my PhD study. The respect to him is not only because of his academic attitudes and achievements but also because of his wise philosophy of life. I greatly appreciate his critical and thought‐provoking comments on the outputs of my study and his efforts on the revision of the draft thesis. In the meantime I also wish to extend my appreciation to my promotor Professor Shaopeng Wu for his valuable academic advice. He has shown me the promising research field and helped me grabbing the golden opportunity of studying abroad. Without his insightful guidance and constant encouragement, it would have been impossible for me to accomplish my Master and PhD study.
I am very grateful for working with my daily supervisor Associate Professor Martin van de Ven. He is a very supportive and thoughtful man. He always provided me with suggestions and help whenever needed. His patience and guidance to my study are deeply appreciated. I also would like to extend my sincere gratitude to Professor Kim Jonathan Jenkins from Stellenbosch University in South Africa. Much appreciation goes to him for his support and understanding for my research. The supports offered by Prof.dr. A.
Scarpas, Prof.dr.ir. S.M.J.G. Erkens and Ir. H.J.M. Lambert are also greatly appreciated.
I also would like to present my thanks to all the colleagues of the Road and Railway Engineering section. The extensive laboratory experimental works could not have been finished without the guidance and assistance provided by the laboratory staff Marco Poot, Jan‐Willem Bientjes, Dirk Doedens and Jan Moraal. Many thanks go to our secretaries, Jacqueline Barnhoorn and Sonja van de Bos for their daily assistance. Special thanks to Ing. W. Verwaal, P.M. Meijvogel and J.G. van Meel for their help in performing the CT scans and data preprocessing. Words of thanks are also extended to my PhD colleagues: Mo, Gang, Dongxin, Jian, Yue, Ning, Quantao, Mingliang, Pengpeng, Yuan, Dongya, Milliyon, Pungky, Diederik, Mohamad, Alemgena, Sadegh. Especially, thanks go to the Railway fellows: Xiangming, Haoyu, Shaoguang, Zilong, Zhen, Xiangyun, Lizuo, Chen, Chang, Zhiwei, Yuewei, Maider, Marija and Nico. I really enjoyed my time with them.
I also appreciate the time being with my dear friends: Hongbin, Liyuan, Yuguang, Bin, Qian, Jinlong, Fangliang, Zhuqing, Haoliang, Liangyue and so many others. With them, life during my PhD was much easier and joyful. To my parents I dedicate this dissertation as a token of appreciation for their great support and endless love. During those years when I studied abroad my parents always missed me so badly and only weekly long‐distance calls could make them feel better. Being their son, I understand their hearts and feel so sorry for such a long time without my company. I also would like to give my deepest thanks to the family of my sister for their understanding and support. Great gratitude goes to my sister and brother‐in‐law for their good cares of our parents and my little nephews who bring a lot of joy to our whole family. Much appreciation also goes to my relatives for their ongoing support. Jingang Wang 2015.03
Summary
Asphalt concrete is a viscoelastic material consisting of aggregates, filler and bitumen. The response of asphalt concrete is highly dependent on temperature, loading rate and confining pressure. Permanent deformation is one of the most important distresses developing during the flexible pavement service life. The total deformation which is visible at the pavement surface is the sum of the deformation that developed in each and every layer. In this thesis however attention will only be paid to permanent deformation of the asphalt layers. The main goal of this research was to investigate and better understand the permanent deformation behaviour of asphalt mixtures at 50°C which is a temperature that regularly occurs in asphalt wearing courses in the Netherlands and which therefore is applied in the Dutch standard for testing the resistance to permanent deformation of wearing courses.
The thesis is divided in two parts. The first part focuses on sample preparation, testing procedures and fundamental properties of dense asphalt concrete (DAC) and porous asphalt concrete (PAC) mixtures and skeletons. In the second part the focus is on the prediction of permanent deformation. In the first part of this research special attention was paid to the following aspects:
Effects of end constraints on test results
Friction between the ends of the specimen and the top and bottom loading platens introduces extra confinement at the top and bottom of the specimen. In this research an extensive monotonic compressive testing program was performed on DAC and PAC mixtures under two different end contact conditions being full friction and reduced friction. “Full friction” was achieved by gluing the specimen to the top and bottom loading platens. “Reduced friction” was obtained by using a sandwich‐shaped friction reduction system which consisted of two thin rubber sheets and vacuum grease in between. The results show that in the case of “full friction” the failure stress is overestimated and the displacement at failure is underestimated. The results also show that in the case of uniaxial testing without confinement and when using the friction reduction system, a deformation correction is needed to obtain the true deformation of the specimen. When confining pressure is applied deformations due to the friction reduction system can be ignored.
The stress‐strain behaviour of asphalt mixtures
The permanent deformation behaviour of asphalt mixtures is highly dependent on temperature, stress conditions and number of load repetitions. A better understanding of the stress‐strain behaviour of asphalt mixtures is
beneficial for a better understanding of the permanent deformation. Therefore an extensive monotonic compressive test program was conducted on DAC and PAC at 40°C and 50°C with 3 various confining pressures and 5 different loading rates. The test results showed that the stress‐strain behaviour of DAC significantly depends on temperature, strain rate and confinement. The results also showed that at high temperatures the PAC mixture behaves much alike a granular material with little cohesion. In this case the skeleton of PAC plays a significant role in the mechanical behaviour and this behaviour is highly dependent on the level of confinement.
Behaviour of aggregate skeletons
Permanent deformation develops at elevated temperatures. At elevated temperatures, the contribution of the aggregate skeleton becomes crucial. For this reason monotonic compressive tests were conducted on DAC and PAC skeletons at two strain rates and two confining pressure levels. The stress‐ strain behaviour of the DAC and PAC skeleton were compared with the stress‐strain behaviour of both mixtures. The results implied that the bituminous mastic in DAC acts as a binder and contributes to the behaviour of the DAC asphalt mixture. The results also showed that the PAC aggregate skeleton shows typical elastoplastic behaviour regardless of the strain rates. In the second part of this thesis repeated load triaxial tests to study the development of permanent deformation were performed on the DAC mixture at 50°C and the following questions related to permanent deformation of the DAC mixture were discussed.
Scatter observed in permanent deformation results
A power function was used to model the obtained permanent deformation. A large scatter in the model parameters was observed even at the same stress ratio for selected test specimens. The possible relation between the scatter on one hand and the air voids content and resilient modulus of specimens on the other was studied. CT scanning was used to investigate the internal structure of the intact and tested specimens. The results showed that there is no clear relationship between air voids and the parameters describing the development of the permanent deformation with increasing number of load repetitions. The results also showed that the model parameters were stress dependent and a strong relationship was found between the model parameters and the resilient modulus after 1000 load repetitions. The CT scan results showed that different failure modes took place in the permanent deformation tests and that the internal structure of specimens is important for the development of permanent deformation. It is believed that part of the scatter in the test results can be explained by the variation in internal structure.
Influence of loading pattern on permanent deformation
The influence of different loading pulses and rest times on the permanent deformation of DAC was also investigated by performing Triaxial Repeated Load Permanent Deformation (TRLPD) test. The tests were performed at two different loading patterns, being 0.2 s load + 1.8 s rest and 0.4 s load + 0.6 s rest, and at two confinement levels of 150 kPa with a stress ratio 0.43 and 100 kPa with a stress ratio 0.3. The results showed that the stress ratio has a significant influence on the permanent deformation while the loading time has little influence on the development of permanent deformation. At the same stress level it seems that the longer loading time does not result in larger permanent deformation.
Evolution of the resilient modulus in relation to the number of load repetitions
In order to obtain a better understanding of the relationship between permanent strain and resilient strain which is developed in the DAC test specimens during the repeated load triaxial tests, the evolution of the resilient modulus of DAC with the number of load repetitions was investigated and modelled. The results showed that the resilient modulus of DAC reduced during the first load repetitions and tended to take a constant value after 1000 load repetitions.
Evolution of Burgers’ model parameters in relation to the number of load repetitions
The measured total strain was decomposed into elastic strain, delayed elastic strain and viscous permanent strain and modelled by means of the Burgers’ model. The evolution of Burgers’ model parameters obtained at two different loading patterns was investigated in this study. It was found that the value of the parameter representing the dashpot in series, used for modelling the permanent deformation, increased with increasing number of load repetitions and tended to be constant after thousands of load repetitions. This value however strongly decreased when dilation of the specimen occurred. The loading pattern had a significant influence on the value of this viscous parameter
Shake down limit in permanent deformation of DAC
In order to explore the existence of a shakedown limit for the tested DAC mixture, five representative permanent deformation tests were analyzed. From this analysis it appeared that below a stress ratio of 0.3 (the ratio of applied vertical stress to vertical stress at failure at the same confinement level) only a limited amount of permanent deformation developed after a large number of load repetitions. The stress ratio of 0.3 is proposed to be the shake down limit at 50°C for the test conditions and DAC mixture used in this research.
Permanent deformation modeling based on Dissipated Energy Concept
Most of the permanent deformation prediction models are simply relating permanent deformation to stress conditions and the number of load repetitions. In this study a permanent deformation model was developed based on dissipated energy. It was shown that the initial dissipate energy, the applied stress level and the number of load repetitions explain very well the permanent deformation development.
Samenvatting
Asfalt is een visco‐elastich materiaal dat bestaat uit aggregaat (steen, zand, vulstof) en bitumen. De respons van asfalt is sterk afhankelijk van temperatuur, belastingtijd en steundruk. Permanente deformatie is een van de belangrijkste schadebeelden die zich kunnen ontwikkelen tijdens de levensduur van een flexibele verharding. De totale deformatie die zichtbaar is aan het oppervlak is de som van de blijvende deformatie die in elke laag optreedt. In dit proefschrift wordt echter alleen aandacht geschonken aan de blijvende vervorming van de asfaltlagen. Het hoofddoel van deze thesis is het onderzoeken en beter begrijpen van het permanente deformatiegedrag van asfaltmengsels bij een temperatuur van 50°C, omdat dit de temperatuur is die regelmatig voorkomt in asfaltdeklagen en daarom ook toegepast wordt in de Nederlandse Standaard voor onderzoek naar permanente deformatie van deklagen.
Deze thesis bestaat uit twee delen. In het eerste deel is de focus op proefstukbereiding, testprocedures en fundamentele eigenschappen van dichtasfaltbeton (DAB) en zeer open asfaltbeton (ZOAB), zowel voor het mengsel als het skelet. In het tweede deel is de focus op de voorspelling van permanente deformatie.
In het eerste deel is speciaal aandacht gegeven aan:
Effecten van de eindopsluiting op de proefresultaten
Wrijving tussen de uiteinden van het proefstuk en de belastingplaten boven en onder veroorzaken extra steundruk aan de boven‐ en onderkant van het proefstuk. De DAB‐ en ZOAB‐mengsels zijn beproefd onder twee verschillende contactcondities aan de uiteinden van de proefstukken, namelijk volledige wrijving en sterk gereduceerde wrijving. “Volledige wrijving” werd gerealiseerd door het proefstuk aan de boven‐ en onderplaat te lijmen. “Gereduceerde wrijving” werd verkregen met een sandwichachtig wrijvingsreductie systeem bestaande uit twee dunne rubberen vellen met een smeermiddel ertussen. Uit de resultaten blijkt dat in het geval van “volledige wrijving” de faalspanning wordt overschat en de verplaatsing bij falen wordt onderschat. In het geval van “gereduceerde wrijving” blijkt dat er bij uni‐ axiale proeven zonder steundruk een correctie nodig is om de effecten van de vervorming van het wrijvingsreductiesysteem te compenseren en de werkelijke verplaatsing van het proefstuk te verkrijgen. Als echter een steunspanning aanwezig is kunnen de effecten van verplaatsingen van het wrijvingsreductie systeem worden verwaarloosd.
Het spannings‐vervormingsgedrag van asfaltmengsels
Het permanente deformatie gedrag van asfaltmengsels is sterk afhankelijk van de temperatuur, spanningscondities en het aantal lastherhalingen. Kennis van het spannings‐vervormingsgedrag van asfaltmengsels is essentieel voor een beter begrip van permanente deformatie. Deze kennis is verkregen door monotone drukproeven uit te voeren op DAB en ZOAB bij 40°C en 50°C onder 3 verschillende steundrukken en 5 verschillende verplaatsings‐ snelheden. Uit de proefresultaten blijkt dat het spannings‐vervormingsgedrag van DAB sterk afhankelijk is van temperatuur, vervormingssnelheid en steundruk. Ook bleek dat bij hoge temperaturen het ZOAB mengsel zich gedraagt als een granulair materiaal met enige cohesie. Het aggregaatskelet van ZOAB bepaalt het mechanisch gedrag en zoals verwacht is dit gedrag sterk afhankelijk van de hoogte van de steundruk.
Het gedrag van het aggregaat skelet
Permanente deformatie treedt op bij hoge temperaturen. Bij hoge temperaturen wordt de bijdrage van het aggregaatskelet essentieel. Om dit te onderzoeken zijn monotone drukproeven uitgevoerd op de DAB en ZOAB aggregaatskeletten bij twee steundrukniveaus en twee vervormingssnelheden. Vervolgens is het spannings‐vervormingsgedrag van het DAB en ZOAB skelet vergeleken met het spannings‐vervormingsgedrag van beide mengsels. Uit de resultaten blijkt dat de mastiek in DAB functioneert als een bindmiddel en een belangrijke bijdrage levert. Ook bleek duidelijk dat het aggregaatskelet van ZOAB typisch elasto‐plastisch gedrag vertoont dat niet afhankelijk is van de vervormingssnelheid.
In het tweede deel van het onderzoek zijn tri‐axiaal proeven met herhaalde belasting uitgevoerd op het DAB mengsel bij 50°C om de ontwikkeling van de permanente deformatie te onderzoeken. Daarbij zijn de volgende aspecten gerelateerd aan het permanente vervormingsgedrag van het DAB mengsel onderzocht.
Waargenomen spreiding in de permanente deformatieresultaten
Een eenvoudige machtsfunctie is gebruikt om de gemeten permanente deformatie te beschrijven. Veel spreiding werd waargenomen in de modelparameters, zelfs bij herhaalde proeven met dezelfde spanningsverhouding. Mogelijke oorzaken voor de spreiding, zoals verschil in holle ruimte en stijfheid van de proefstukken, gaven geen uitsluitsel. CT scans zijn daarom gebruikt om de interne structuur van onbelaste en belaste proefstukken te analyseren . Hieruit is geconcludeerd dat de variatie in de interne structuur waarschijnlijk een grote invloed heeft gehad op de spreiding in de meetresultaten. Uit de resultaten bleek verder dat de modelparameters spanningsafhankelijk waren en een sterke relatie werd gevonden tussen de
modelparameters en de resilient (elastische) modulus na 1000 lastherhalingen. Uit de CT scans bleek dat verschillende bezwijkmechanismen optraden gedurende de permanente deformatie proeven en dat de initiële interne structuur van de proefstukken grote invloed heeft gehad op de ontwikkeling van de permanente deformatie.
De invloed van verschillende lastpulsen en rustperioden op de permanente vervorming
De invloed van verschillende lastpulsen en rustperioden op de permanente vervorming van DAB is ook onderzocht door middel van triaxiale permanente vervormingsproeven met herhaalde belasting. De proeven zijn uitgevoerd met twee verschillende belastingregimes, namelijk 0,2 s last + 1,8 s rust en 0,4 s last + 0,6 s rust, bij twee steunspanningsniveaus, 150 kPa met een spanningsverhouding 0,43 en 100 kPa met een spanningsverhouding 0,3. De proefresultaten laten zien dat de spanningsverhouding een significante invloed heeft op de permanente vervorming terwijl de belastingtijd weinig invloed heeft op de ontwikkeling van permanente vervorming. Het lijkt dat bij hetzelfde spanningsniveau de langere belastingtijd niet leidt tot grotere permanente vervorming.
Verloop van de resilient modulus in relatie tot het aantal lastherhalingen
Om een beter begrip te krijgen van de relatie tussen de permanente en elastische vervorming van het DAB mengsel in de tri‐axiale proef met herhaalde belasting, is de ontwikkeling van de resilient modulus van DAB in relatie tot het aantal lastherhalingen onderzocht en gemodelleerd. Uit de resultaten blijkt dat de resilient modulus van DAB afnam gedurende de eerste lastherhalingen en vervolgens constant werd na ongeveer 1000 lastherhalingen.
Verloop van de Burgers’ model parameters in relatie tot het aantal lastherhalingen
De totale gemeten vervorming is gesplitst in elastische, vertraagd elastische and visceuze permanente vervorming en gemodelleerd met het Burgers’ model. Het verloop van de parameters van het Burgers’ model voor twee verschillende belasting/ontlasting verhoudingen is in deze studie onderzocht. De waarde van de parameter die de demper in serie vertegenwoordigt, en dus de permanente vervorming beschrijft, bleek toe te nemen met het aantal lastherhalingen en werd min of meer constant na ca 1000 lastherhalingen. De waarde nam echter sterk af in die gevallen waar dilatatie in het proefstuk optrad. Het bleek ook dat het belastingspatroon een significante invloed heeft op deze visceuze parameter.
Shake down limiet voor de permanente deformatie van DAB
Vijf representatieve tri‐axiaal proeven waarop een herhaalde belasting was uitgevoerd, zijn geanalyseerd om na te gaan of een shakedown limiet kon
worden gevonden voor het DAB mengsel. Het bleek dat voor een spanningsverhouding van 0,3 en lager (dit is de verhouding tussen de toegepaste verticale spanning en verticale bezwijkspanning bij dezelfde steundruk) slechts een beperkte permanente deformatie werd waargenomen na een groot aantal lastherhalingen. De spanningsratio van 0,3 kan daarom worden gezien als de shake down limiet voor dit DAB mengsel bij 50°C voor de in dit onderzoek gebruikte proefcondities.
Modellering van permanente deformatie gebaseerd op gedissipeerde energie
De meeste voorspellingsmodellen voor permanente deformatie relateren de permanente deformatie aan de spanningscondities en het aantal last‐ herhalingen. In deze studie is een permanente deformatie model ontwikkeld dat gebaseerd is op gedissipeerde energie. Aangetoond is dat de initiële gedissipeerde energie, het toegepaste spanningsniveau en het aantal lastherhalingen samen zeer goed de ontwikkeling van de permanente deformatie verklaren.
List of Abbreviations and Symbols
Abbreviations
2D Two Dimensional 3D Three Dimensional AASHTO American Association of State Highway and Transportation Officials AGRAC Asphalt GRAnular Cement APT Accelerated Pavement Testing ASTM American Society for Testing and Materials BAM A Dutch construction company BISAR Bitumen Stress Analysis in Roads CCP Constant Confining Pressure CDRC Constant Displacement Rate Compression CROW Bureau for Contract Standardization and Research for Civil Infrastructure CT Computer Tomography DAC Dense Asphalt Concrete DE Dissipated Energy DER Dissipated Energy Ratio EN European Norm EPSILON Epsilon company in USA EVT Temperature at which the viscosity of bitumen is 170 cSt FRS Friction Reduction System HU Hounsfield Units IMACS Integrated Multi‐Axis Control System IPC An Australian company LINTRACK LINear TRACKing Apparatus LVDT Linear Variable Differential Transformer MEPDs Mechanistic‐Empirical Pavement Design system Mr Resilient Modulus MTCT Monotonic Triaxial Compression Test NMAS Nominal Maximum Aggregate Size OAC Open Asphalt Concrete PAC Porous Asphalt Concrete PD Permanent Deformation Q‐Q plot Quantile‐Quantile plot RAW Dutch standard specification for the civil engineering sector RLTT Repeated Load Triaxial Test RMITT Indirect Tension Test for Resilient Modulus ROI Region of InterestRRRL Road and Railway Research Laboratory RVE Representative Volume Element SBC Shear Box Compactor SD Standard Deviation SHRP Strategic Highway Research Program SMA Stone Mastic Asphalt STAC STone Asphalt Concrete TRLPD Triaxial Repeated Load Permanent Deformation UTM Universal Testing Machine VCP Variable Confining Pressure VEROAD Multilayer pavement design program VESYS ViscoElastic SYStem‐ a linear viscoelastic pavement design system VFA Voids Filled with Asphalt VICKERS An Australian company VMA Voids in Mineral Aggregate
Symbols
A, B power model parameters p permanent strain N number of load repetitions normal stress or standard deviation normal strain ratio of a circleʹs circumference to its diameter T period of haversine function t time Nf flow number S stiffness eq W number of standard wheel passes r resilient strain E complex stiffness modulus E elastic modulus i E elastic modulus of spring in Burgers’ model i viscosity of dashpot in Burgers’ model ratio of resilient strain to permanent strain or mean bulk stress R stress ratio & strain rate 0 L overall deformation measured by the actuator LVDT s L deformation of the steel specimen a L deformation of the asphalt specimen Poisson’s ratio diameter F force h thickness of the specimen in indirect tensile test d total recoverable horizontal deformation in indirect tensile test aggregate size density mb density of compacted mixture mix P property of asphalt mixture max P maximum values of the modelled property min P minimum values of the modelled property H activation energy c F failure strength in compression t F failure strength in tension C cohesion force angle of internal friction 2 R coefficient of determination 1 I the first stress invariant 2 J the second deviatoric stress invariant T temperature e elastic strain de delayed viscoelastic strain e W elastic energy h W dissipated energy (heat) p W dissipated energy (fatigue&PD) d W total dissipated energy i DE initial dissipated energy . accum DE accumulated dissipated energy IR infrared surface temperature hr time of day f frequency shear strain eff L effective length of the stress pulse eff f effective frequency of moving load vehicle speed shear stress
Table of Contents
Chapter 1 Introduction ... 1 Flexible pavements ... 1 1.1 Rutting in asphalt pavements ... 2 1.2 Issues in practice ... 5 1.3 Objectives of this research ... 7 1.4 Organization of the dissertation ... 8 1.5 References ... 10 Chapter 2 Literature Review and Research Program ... 11 Asphalt mixtures and mechanical characterization ... 11 2.1 Components of asphalt mixtures ... 14 2.1.1 Factors influencing mechanical properties ... 15 2.1.2 Effects of stress conditions, temperature and loading rate ... 19 2.2 2.2.1 Stresses in pavement layers ... 19 2.2.2 Confining pressure ... 22 2.2.3 Temperature and loading rate ... 23 Permanent Deformation of Asphalt Mixtures ... 24 2.3 2.3.1 Repeated load triaxial permanent deformation test ... 24 2.3.2 Permanent deformation models ... 26 2.3.3 Research on permanent deformation in the LINTRACK project ... 30 Summary and research program ... 33 2.4 Research program ... 36 References ... 36 Chapter 3 Materials and Experimental Program Design ... 39 Materials ... 39 3.1 3.1.1 Material properties ... 40 3.1.2 Material pretreatment ... 40 Mixture design... 41 3.2 Specimen preparation... 43 3.3 3.3.1 Mixture compaction ... 44 3.3.2 Air voids distribution in asphalt blocks ... 47 3.3.3 Sawing and coring plan ... 53 3.3.4 Selection of test specimens ... 55 Design of the test program ... 59 3.43.4.1 Determination of test conditions ... 59 3.4.2 Test Program ... 62 References ... 64 Chapter 4 Effects of end constraint conditions in compression ... 67 Influences on material characterization ... 68 4.1 4.1.1 Representative specimen size ... 69 4.1.2 Boundary conditions ... 72 4.1.3 Measurement devices ... 73 Setup for triaxial compression ... 78 4.2 Friction reduction system (FRS) ... 80 4.3 4.3.1 Selection of a friction reduction system ... 81 4.3.2 Deformation correction of FRS ... 84 4.3.3 Evaluation of rubber friction reduction system ... 87 Effects of end constraints on stress strain behaviour ... 90 4.4 4.4.1 Test with 0 kPa confining pressure ... 92 4.4.2 Test with 200 kPa confining pressure ... 98 Conclusions and recommendations ... 103 4.5 References ... 105
Chapter 5 Effects of strain rate and confining pressure on the stress‐strain behaviour of DAC and PAC at high temperatures ... 107
Experimental consideration ... 110 5.1
5.1.1 Monotonic compression test on asphalt mixtures ... 110 5.1.2 Indirect tension test for resilient modulus (RMITT) ... 111
Effect of strain rate, temperature and confinement on the compressive 5.2
behaviour of DAC and PAC ... 115
5.2.1 Effects of confinement, strain rate and temperature on the compressive behaviour of DAC ... 117 5.2.2 Effects of confinement, strain rate and temperature on the compressive behaviour of PAC ... 122 5.2.3 Time‐temperature superposition principle for large strains in uniaxial compression ... 128
Interaction between bituminous mastic and skeleton ... 137 5.3
5.3.1 Gradation of the DAC and PAC aggregate skeletons ... 137 5.3.2 Preparation of the specimen ... 137 5.3.3 Comparison of the stress‐strain behaviour of skeletons and asphalt mixtures ... 138
Influence of strain rate, temperature and confining pressure on cohesion 5.4
5.4.1 Mohr Coulomb failure criterion ... 140 5.4.2 Determination of a nonlinear yield surface ... 144 Conclusions and recommendations ... 151 5.5 References ... 153 Chapter 6 Permanent deformation of DAC ... 155 Materials and experimental program ... 156 6.1 Permanent deformation of DAC ... 159 6.2 6.2.1 Permanent deformation at various stress levels ... 159 6.2.2 Influences on permanent deformation parameters... 166 6.2.3 Volumetric properties ... 169 6.2.4 Permanent deformation using different loading patterns ... 178 Evolution of resilient modulus ... 182 6.3 6.3.1 Determination of the resilient modulus limit ... 183 6.3.2 Influence of confining pressure on resilient modulus ... 188 Burgers’ model parameters ... 189 6.4 6.4.1 Model parameters under the test conditions in program I in Table 6.1 ... 194 6.4.2 Model parameters under the test conditions in program II in Table 6.1 .. 196 Analysis of permanent deformation using dissipated energy concept 199 6.5 6.5.1 Dissipated energy concept ... 199 6.5.2 Dissipated energy concept in permanent deformation tests ... 202 6.5.3 Accumulated dissipated energy and permanent strain ... 206 Conclusions ... 213 6.6 References ... 215 Appendix: Cross section view and profile of specimens after test ... 217 Chapter 7 Conclusions and recommendations ... 223 Conclusions ... 223 7.1 7.1.1 Related to the literature review ... 223 7.1.2 Related to the materials tested and experimental program design ... 224 7.1.3 Related to the effects of end constraint conditions in compression ... 224 7.1.4 Related to the effects of strain rate and confining pressure on the behaviour of DAC and PAC ... 224 7.1.5 Related to permanent deformation ... 225 Recommendations ... 226 7.2 7.2.1 Related to deformation measurements on the specimen ... 226 7.2.2 Related to the yield surface ... 226 7.2.3 Related to the failure modes ... 226
7.2.4 Related to the relationship between dissipated energy and permanent deformation ... 226 Appendix A: Analysis of stress and strain in asphalt layers ... 227 Basic information of the LINTRACK pavement sections ... 228 A.1 Iteration procedure for DAC and PAC at 40°C ... 230 A.2 A.2.1 The initial inputs of material properties ... 230 A.2.2 Principle of determination of equivalent principal stress and strain ... 233 A.2.3 Equivalent principal stresses and equivalent length of loading pulse ... 235 A.2.4 Strain rate determination ... 239 A.2.5 Updating the resilient moduli ... 243 Iteration procedure for DAC and PAC at 50 °C ... 245 A.3 Stress ratios at mid depth of the top layers ... 251 A.4 A.4.1 Estimation of compressive and tensile strengths at unconfined conditions ... 252 A.4.2 Determination of cohesion and angle of internal friction ... 253 A.4.3 Compressive strength and stress ratios at different confinement levels .. 254 References ... 256 Appendix B: Basic properties of bitumen and asphalt mixtures ... 259 Properties of bitumen ... 260 B.1 Comparison with Muraya’s data ... 261 B.2 Resilient modulus ... 261 B.3 References ... 264
Chapter 1
Introduction
his chapter gives an introduction to flexible pavements and distresses observed in asphalt pavements. The focus is on the permanent deformation of asphalt layers. This chapter also presents the objectives and scope of this research. Finally the organization of this dissertation will be given as well.
Flexible pavements 1.1
A flexible pavement structure, just as its name implies, deforms when it is subjected to loading. A flexible pavement structure is typically composed of several layers of materials which are bitumen bounded, unbounded granular materials and the natural soil. The flexible pavement structure normally consists of a surface layer, a base course and a subbase course. Each layer carries the loads from the above layer, spreads them, and then passes on these loads to the next layer below. In order to take maximum advantage of this property, flexible pavement layers are usually arranged in order of descending load spreading capacity with the highest load spreading capacity material at the top and the lowest load bearing capacity material on the bottom.
In 1838 d’Erinas used stamped down asphalt to pave sidewalks in Paris and Philadelphia. The first Dutch road with a top layer of asphalt was constructed in Amsterdam in 1873. Nowadays the European road network consists of motorways, regional roads and local roads and the total length of this network sums up to more than 5 million kilometers of which 66,700 kilometers are classified as motorways. More than 90% of the European Road network is surfaced with asphalt [1].
Asphalt concrete is a mixture of gravel, sand and filler, bounded by bitumen. Bitumen is a viscoelastic material produced during the refinery of crude oil. At high temperature it is a water‐like fluid with a low viscosity while at low temperature it is solid exhibiting elasticity. At intermedium temperatures it behaves as a viscoelastic material which is loading rate dependent. The interlocking of aggregates in the bituminous mixture provides most of the
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compressive strength of asphalt materials while the bituminous mortar provides resistance to tension.
Rutting in asphalt pavements 1.2
The development of distress is an important consideration in pavement performance. Knowledge of the various types of distress and the analysis of the causes are of importance to the pavement designer. A good reference for identifying pavement distress is the “Highway Pavement Distress Identification Manual” published by the Federal Highway Administration in 1979 [2].
One of the most important distresses in asphalt pavements is rutting. Rutting is the surface depression in the wheel paths as shown in Figure 1.1. This deformation is the sum of the permanent deformation that develops in each of the individual layers. In this thesis however attention will only be paid to permanent deformation of the asphalt layers.
Figure 1.1 Rutting in the wheel path
Rutting develops rapidly during the first few years after construction and then levels off to a slower rate. Rutting results from permanent deformation caused by consolidation/densification or deformation due to shear of the materials due to traffic loads in any of the pavement layers or in the subgrade. Rutting can be caused by plastic movement of the asphalt mixture either in hot weather or from inadequate compaction during construction. Significant rutting can lead to major structure failures and can cause hydroplaning [3].
The deformation of asphalt concrete involves viscous flow, densification or compaction and shear deformation [4]. It is generally accepted that permanent deformation is influenced by the stress conditions, the properties of the materials used in the surface and other layers and the climatic conditions. The wheel load introduces non‐uniform vertical, longitudinal and lateral contact stresses. Typical patterns of those contact stresses are shown in Figure 1.2 [5]. The stress conditions in a particular point due to a moving wheel load are complicated. Also the rotation of principal stresses may have a significant influence on the development of permanent deformation.
The distribution of stresses in the pavement depends on the material properties of each layer, for instance stiffness and Poisson’s ratio, and the thickness of the pavement layers.
The stress conditions that can be applied in the laboratory may not be in line with the true stress conditions in the field. In reality e.g. rotation of principal stresses occurs which is very difficult to simulate in the laboratory.
Climatic conditions also have a significant impact on the development of permanent deformation. Since an asphalt mixture is viscoelastic its deformation response is highly dependent on the temperature and the loading rate.
The discussion above implies that special attention should be given to a careful design of simulation and testing programs including: employing of appropriate test set up, proper control of the climatic conditions to which test specimens are subjected, strict control over the contact conditions between the specimen and test set up as well as a reliable measuring and data acquisition system.
In the laboratory, the permanent deformation behaviour of asphalt mixture is usually evaluated by performing repeated load triaxial compression tests. A typical permanent deformation curve versus applied number of load repetitions can include three distinct phase as shown in Figure 1.3: (1) the primary stage with a decreasing strain rate during which permanent deformation accumulates rapidly; (2) the secondary stage with a constant strain rate; (3) the tertiary stage with an increasing strain rate during which permanent strain rapidly accumulates till failure is reached.
Figure 1.2 Example of vertical, longitudinal and lateral contact stresses at the pavement surface due to a wheel load (De Beer, 1997)
Many models were proposed to describe this permanent deformation behaviour. The most commonly used model is the power law model as expressed in Equation 1.1.
p ANB 1.1
Where, pis permanent strain, N is number of load repetitions and A, are B model parameters. Figure 1.3 Schematic plot of the permanent strain and three regimes with increasing loading repetitions Issues in practice 1.3
Although there is consensus that the repeated load triaxial test is very well suited for studying the resistance to permanent deformation of asphalt mixtures, and although this test has been used extensively, a number of questions still need to be answered which are related to sample preparation, testing procedures and analyzing the results of permanent deformation tests. Some of these issues are important for practice while others are of a fundamental nature. This thesis addresses a number of these questions and after ample consideration it was decided to pay attention to the following issues:
Minimum sample size in relation to grain size and other factors. This is an important issue since getting large samples from a pavement with a diameter of e.g. 100 mm and a height of 200 mm is problematic since asphalt layers have at best a layer thickness of about 60 to 70 mm. This 0 400 800 1200 1600 2000 2400 2800 3200 3600 0 5 10 15 20 Tertiary Primary Permanent Strain (%)
Number of load repetitions
Nf
limited layer thickness is making it impossible to obtain 200 mm high specimens and therefore an investigation into the minimum allowable specimen sizes is valuable.
Application of a friction reduction system. Friction between the top and bottom of the specimen with the loading platens will influence the test result. This is not so much of a problem as long as the amount of friction is known. Without taking special precautions when testing, the magnitude of the friction force is unknown. Therefore, two extreme solutions are considered: gluing the specimen to the top and bottom loading plate and ensuring “full friction” in this way, or use of an effective friction reduction system resulting in “full slip” conditions. The effects of both solutions on the test results need to be investigated. Also two solutions are considered with respect to displacement
measurements. The first one is using “on sample” transducers which measure the deformation over e.g. the mid third part of the specimen. This way of measuring displacement is almost inevitable when friction at the load platens is affecting the deformation of the specimen. Such measurements however are quite complicated when used in triaxial tests. A much simpler solution is measuring the displacement of the specimen over the entire height of the specimen. This way of measuring displacements seems only to be applicable in the case that “full slip” conditions occur at the loading platens. Measuring the deformation over the entire height however implies that one should take care of the displacements because of deformations in the friction reduction system. It is clear that this topic needs to be investigated in order to be able to provide guidelines to practice about how to perform a repeated load triaxial test. Ratio of duration of the loading pulse to the duration of the rest period. Several testing protocols are defining different ratios for the load : rest time duration. The question then is “what is the most appropriate ratio to simulate truck traffic during the hot season”. This question is of practical importance since shorter rest periods will reduce the duration of the entire test and for practical reasons, getting results in the shortest time possible is of importance.
Contribution of the aggregate skeleton to the resistance against permanent deformation. Permanent deformation mainly occurs at
viscosity of the bituminous mastic/mortar that glues the coarse particles together is low and a significant part of the resistance to permanent deformation needs to be generated by the stone skeleton. Only limited information is available about how high this contribution is, and therefore investigations on this topic are necessary.
Shake down limit. It is known that the permanent deformation of aggregate skeletons is limited when the stress conditions are below a certain threshold (shake down limit). It is worthwhile to investigate if such a limit also exist for asphalt mixtures since this would simplify mixture and pavement design.
Factors influencing permanent deformation. Most of the permanent deformation models are “simple” regression models which relate the permanent deformation to stress conditions and applied number of load repetitions. The question however is whether the regression constants can be related to material properties making the empirical models more meaningful. It is obvious that this is a useful research topic as well.
Burgers’ model parameters. Burgers’ model is often used to explain the immediate and delayed elastic response of asphalt mixtures as well as the viscous (permanent) deformation. The question is to what extent this model can be used to explain the permanent deformation development and how the model parameters are changing as a function of the number of load repetitions and stress conditions.
From the above it is clear that this thesis is addressing quite an array of topics, all of which are helping to better understand the formation of permanent deformation in asphalt mixtures and the factors influencing this development.
Objectives of this research 1.4
The behaviour of asphalt mixtures (for instance stiffness, strength and resistance to permanent deformation) depends on several factors namely: the stress conditions, the composition of the mixture and the climatic conditions. The effect of these influences, when characterizing the material properties in the laboratory, needs to be investigated. The influence of temperature, loading rate and confining pressure on the mechanical properties needs to be better understood. The evolution of asphalt material properties with the increase of number of load repetitions needs to be investigated to obtain a better understanding of permanent deformation as well. Last but not least the
influence of the microstructure on the permanent deformation, and the failure patterns inside the specimen, needs to be studied.
All in all the aim of this research work is to better understand the permanent deformation behaviour of an asphalt mixture under various test conditions. Based on this aim, the objectives of this research are defined as follows:
1. Compare the mechanical properties obtained at two extreme constraint conditions: the specimen is fully glued to the loading plates and a full friction reduction system is applied between specimen and loading plates. 2. Investigate the influence of loading rate and confining pressure on the stress‐strain behaviour of dense asphalt and porous asphalt concrete at high temperatures. 3. Characterize the permanent deformation of dense asphalt mixture 4. Determine whether a stress threshold value below which no significant permanent deformation exists.
5. Study the influence of different loading patterns on permanent deformation development.
6. Investigate the relationship between permanent deformation and dissipated energy
Organization of the dissertation 1.5
Chapter 2 provides a brief introduction of pavement design and asphalt materials and a literature review on permanent deformation. It also addresses factors influencing the results of mechanical testing such as the effects of stress conditions, temperature and loading rate. After that the literature review also focuses on permanent deformation testing. Permanent deformation research results obtained from Accelerate Road Tests are also discussed.
Chapter 3 is dedicated to materials and the experimental program design. Two typical wearing course mixtures are investigated in this study being dense asphalt course (DAC) and porous asphalt course (PAC). The mixtures are designed with the Marshall design method. To better simulate field compaction, a Shear Box Compactor was used to produce asphalt blocks with the designed volumetric properties. X‐Ray Computer Tomography technology was used to look into the distribution of the aggregates and air voids. Then the cutting and coring program is described which is based on CT scan results obtained in the produced asphalt blocks. At the end of the chapter the selected testing conditions are presented.
Chapter 4 presents the study related to effects of friction between the loading plates and the specimen on the mechanical characterization of asphalt
test setup is described. Then a comparison of the test results obtained using two different end constraint conditions is made.
Chapter 5 offers the results of the study on the effects of strain rate and confining pressure on the stress‐strain behaviour at high temperatures. Master curves of the stiffness modulus of DAC and PAC are constructed from the resilient modulus test results. By using the time‐temperature principle master curves for the failure strength of DAC and PAC mixtures are constructed as well. The stress‐strain curves of the asphalt mixtures and their aggregate skeletons are compared and discussed. This chapter concludes with a discussion of the influence of strain rate and temperature on the cohesion and angle of internal friction of the mixtures.
Chapter 6 presents the study on permanent deformation behaviour. The Burgers’ model was used to describe the observed permanent deformation behaviour and its parameters are analyzed and discussed. The influence of pulse duration, rest period and confinement on permanent deformation was also investigated. At the end of this chapter the deformation patterns that were obtained after completion of repeated triaxial loading are discussed. Following that the evolution of the resilient modulus of DAC and the Burgers’ model parameters are investigated and discussed. The dissipated energy concept is used to study the permanent deformation and a model based on initial dissipated energy is proposed.
Appendix A explains how the triaxial test conditions as used in this study were determined from the representative triaxial stresses and strain rates which occurred in the LINTRACK pavement sections
Appendix B provides the basic properties of bitumen and asphalt mixtures used in this study. Figure 1.4 shows the structure of the dissertation.
Figure 1.4 Structure of the dissertation
References
[1]. EAPA, European Asphalt Pavement Association. Available: http://www.eapa.org/asphalt.php, (Accessed Date: 2015.03.10).
[2]. Smith, R.E., Darter, M.I., and Herrin, S.M., Highway Pavement Distress
Identification Manual for Highway Condition and Quality of Highway Cosntruction Survey. Federal Highway Administraion, Contract No.
DOT-FH-11-9175/NCHRP 1-19, 1979.
[3]. Huang, Y.H., Pavement Analysis and Design. (2 ed.). New Jersey: Prentice
Hall, 1993.
[4]. Garba, R., Permanent Deformation Properties of Asphalt Concrete Mixtures.
Ph.D. dissertation, Department of Road and Railway Engineering, Norwegian University of Science and Technology, 2002.
[5]. De Beer, M., Fisher, C., and Jooste, F.J., Determination of Pneumatic
Tyre-pavement Interface Contact Stresses under Moving loads and Some Effects on Pavements with Thin Asphalt Surfacing Layers. Proceedings of the Eighth
International Conference on Asphalt Pavements (ICAP' 97). University of Washington, Seattle, Washington, USA, pp. 179-227, 1997.
[6]. Erkens, S.M.J.G., Asphalt Concrete Response (ACRe)-Determination, Modelling and Prediction. PhD dissertation, Delft University of Technology:
Delft, The Netherlands, 2002.
Chapter 1 Introduction
Chapter 2 Literature Review and Research Program
Chapter 3 Materials and Experimental Program Design
Chapter 4 Effects of End Constraint Conditions in
Compression
Chapter 5 Effects of Strain Rate and Confining Pressure on Stress-Strain Behaviour at High
Temperature
Chapter 6 Permanent Deformation of DAC
Chapter 2
Literature Review and Research Program
his chapter deals with a literature survey on the topics related to permanent deformation and compressive failure of asphalt mixtures. In this chapter, a general introduction on the composition of asphalt mixtures is presented first. Several concerns related to testing asphalt materials are offered as well. In the second part of this chapter a brief discussion on permanent deformation of asphalt mixtures is given. In addition to this, failure modes of asphalt specimen under compression are described and the used computation tomographic (CT) technique is introduced. Apart from this the objectives and methodology of this thesis are presented.
Asphalt mixtures and mechanical characterization 2.1
The use of bitumen on a large scale for pavement construction began in the late 1800s and grew rapidly with the emerging automobile industry [1]. Since then the empirical design of asphalt mixtures was developing rapidly. The efforts made in the middle of the 1900s in designing long‐lasting asphalt pavements using analytically‐based methods can be taken as the starting point of the mechanical‐empirical design methodology [2]. The outcomes of the first International Conference on Asphalt Pavement Design (University of Michigan in 1962) had a significant influence on the development of pavement design methods. Research done at that time in the United States, Europe, South Africa, and Australia contributed a lot to the entire asphalt pavement society both in terms of research and practical perspective.
In Figure 2.1 some significant developments in design and construction of long‐lasting asphalt pavements (after Monismith, C.L. 2004) [2] are presented. It shows that the current knowledge and construction technology in asphalt pavements is based on a systematic approach consisting of mechanical analysis, material characterization, in situ pavement testing, mechanical‐ empirical pavement design, and pavement management as well as construction practices. Among all of those key aspects, the characterization of asphalt materials itself plays a very important role. The mechanical characteristics of asphalt materials are serving as input to design and
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rehabilitation models. Thus the characterization of asphalt materials is not only of importance in the initial stage of the pavement design but also for the design of maintenance and rehabilitation strategies. As the development of mechanical design methods of asphalt pavements progressed also the requirements with respect to materials characterization increased.
Components of asphalt mixtures 2.1.1
Asphalt concrete is a composite material which is used in base and surface layers in pavements. It includes many types of mixtures which are made of well graded aggregates and a bituminous binder. Typically asphalt mixtures are divided into three mixture categories: dense graded asphalt, open graded asphalt and gap graded asphalt. Dense asphalt concrete (DAC) is produced with well or continuously graded aggregates and bitumen. The aggregates are “floating” in the mastic matrix (mixture of filler and bitumen) but are bonded together with interlocking action to obtain a low air void content and a sufficient deformation resistance. The open‐graded asphalt concrete is produced with relatively uniformly‐sized aggregates typified by an absence of intermediate‐sized particles. A typical open‐graded asphalt mixture is porous asphalt concrete (PAC) which requires 20% in air voids content to allow water to freely drain and to reduce noise produced by the tire‐ pavement surface interaction. Gap‐graded mixtures use an aggregate gradation with particles ranging from coarse to fine with some intermediate sizes missing or present in small amounts. Stone‐matrix asphalt (SMA) e.g. will be missing most intermediate sizes but does have a relatively high proportion of fines and bitumen. Figure 2.2 shows the macrostructures of DAC, PAC and SMA.
DAC PAC SMA
Figure 2.2 X‐ray tomography images of DAC, PAC and SMA
This figure clearly shows that the asphalt mixtures are composed of three phases: solid aggregates, mastic or mortar and air voids. The mechanical behaviour of asphalt mixtures is determined by the bituminous mastic, which is defined as the mixture of filler and bitumen, the aggregates as well as the air voids. The aggregate particles can be considered to be elastic and their high stiffness does not show temperature and loading rate dependency. The interlock action between the aggregate particles at the contact points provides a major portion of the shear resistance of asphalt mixtures. The mastic in asphalt mixtures mainly provides bonding and because of that
