1. science
2. medicine
3. physiology

Development of a Morphology-based Analysis
Framework for Asphalt Pavements
IBRAHIM ONIFADE
Licentiate Thesis
Stockholm, Sweden 2015
TRITA-BYMA 2015:1
ISSN 0349-5752
KTH Division of Building Materials
SE-100 44 Stockholm
SWEDEN
Akademisk uppsats som med tillstånd av Kungl Tekniska högskolan framlägges till
offentlig granskning för avläggande av teknisk licentiatexamen fredag den 8 maj
2015 klockan 10.00 i sal B26, KTH, Brinellvägen 23, Stockholm.
© Ibrahim Onifade, May 2015
Tryck: Universitetsservice US AB
Abstract
The morphology of asphalt mixtures plays a vital role in their properties and behaviour. The work in this thesis is aimed at developing a fundamental understanding of the effect of the asphalt morphology on the strength
properties and deformation mechanisms for development of morphology-based
analysis framework for long-term response prediction. Experimental and computational methods are used to establish the relationship between the mixture
morphology and response. Micromechanical modeling is employed to understand the complex interplay between the asphalt mixture constituents resulting
in strain localization and stress concentrations which are precursors to damage
initiation and accumulation. Based on data from actual asphalt field cores,
morphology-based material models which considers the influence of the morphology on the long-term material properties with respect to damage resistance,
healing and ageing are developed. The morphology-based material models are
implemented in a hot-mix asphalt (HMA) fracture mechanics framework for
pavement performance prediction. The framework is able to predict top-down
cracking initiation to a reasonable extent considering the variability of the input
parameters. A thermodynamic based model for damage and fracture is proposed. The results from the study show that the morphology is an important
factor which should be taken into consideration for determining the short- and
long-term response of asphalt mixtures. Further understanding of the influence of the morphology will lead to the development of fundamental analytical
techniques in design to establish the material properties and response to loads.
This will reduce the empiricism associated with pavement design, reduce need
for extensive calibration and validation, increase the prediction capability of
pavement design tools, and advance pavement design to a new level science and
engineering.
Keywords: Morphology, damage, X-ray computed tomography, top-down cracking, fracture
i
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Sammanfattning
Asfaltblandningars morfologi har en avgörande betydelse för deras egenskaper och beteenden. Arbetet i denna avhandling syftar till att utveckla en
grundläggande förståelse för effekten av asfaltsmorfologin för deras hållfasthetsegenskaper och deformationsmekanismer och utveckling av ramverksanalysmorfologi baserat på långsiktig förutsägelse. Experimentella beräkningsmetoder används för att fastställa sambandet mellan blandningens morfologi och
respons. Mikromekanisk modellering används för att förstå det komplexa samspelet mellan asfaltmassans beståndsdelar som resulterar i spänningslokalisering och spänningskoncentrationer som är föregångare till initiering av skador och ackumulation. Morfologibaserade materialmodeller beaktar påverkan
av morfologin på de långsiktiga materialegenskaperna med avseende på skademotstånd, helande samt åldrande, och är utvecklade från data hos verkliga asfaltsfältskärnor. Morfologinbaserade materialmodeller är implementerade
i en varmblandad asfalt-(HMA)-brottmekanik-ramverk för förutsägelse av beläggningsprestanda. Ramverket kan i rimlig utsträckning förutspå variationen i
ingångsparametrarna ’top-down’ sprickbildningsinitiering. En termodynamiskbaserat ramverk föreslås för skador och brott. Resultaten från studien visar
att morfologin är en viktig faktor som bör beaktas för att bestämma respons
av asfaltblandningar på kort och lång sikt. Ytterligare förståelse av inverkan
av morfologin kommer att leda till utvecklingen av grundläggande analytiska tekniker i design för fastställning av materialegenskaper och belastningars
respons. Detta kommer att minska empirism som förknippas med beläggningskonstruktionen, minska behovet av omfattande kalibrering och validering, öka
förutsägelseförmågan av designverktyg för beläggningen, samt avancera beläggningsdesign till en ny vetenskaplig nivå och ingenjörskonst.
Nyckelord : Morfologi, skador, röntgendatortomografi, ’top-down’ sprickbildning, fraktur
Preface
The work presented in this thesis has been carried out partly at the Division of
Highway and Railway Engineering and the Division of Building Materials at KTH
Royal Institute of Technology, Stockholm. I would like to express my gratitude to my
supervisors Prof. Björn Birgisson, Associate Professor Nicole Kringos and Assistant
Professor Denis Jelagin for their assistance during the course of the work. I will also
like to appreciate the financial support of the Swedish Transport Administration
(Trafikverket). My appreciation also goes to Gerald Huber and Bill Pine of the
Heritage group, Indianapolis for their insights and discussions with respect to the
work in this thesis. I also appreciate the support of my colleagues at the department
for providing an enabling and conducive environment.
My sincere gratitude also goes to my wife Busola Odubonojo for her understanding, love, care and support.
Ibrahim Onifade
Stockholm, Sweden.
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List of appended papers
Paper I
Dinegdae, Y., Onifade, I., Jelagin, D., Birgisson, B. (2015). Mechanics-based Topdown Fatigue Cracking Initiation Prediction Framework for Asphaltic Pavements.
Submitted to Road Materials and Pavement Design.
Paper II
Onifade, I., Jelagin, D., Birgisson, B., Kringos, N. (2015). Towards Asphalt Mixture
Morphology Evaluation with the Virtual Specimen Approach. Submitted to EATA
2015 conference for publication in Special Edition of Road Materials and Pavement
Design Journal.
Paper III
Onifade, I., Balieu, R., Birgisson, B. (2015). Energy-Based Damage and Fracture
Framework for Viscoelastic Asphalt Concrete. Submitted to the Journal of Engineering Fracture Mechanics.
In addition to the appended papers, the work has resulted in the
following conference publications and presentations:
Onifade, I., Jelagin, D., Guarin, A., Birgisson, B., Kringos, N. (2013). Asphalt Internal Structure Characterization with X-Ray Computed Tomography and Digital
Image Processing, in: Kringos, N., Birgisson, B., Frost, D., Wang, L. (Eds.), MultiScale Modeling and Characterization of Infrastructure Materials, RILEM Bookseries. Springer Netherlands, pp. 139-158.
Onifade, I., Jelagin, D., Guarin, A., Birgisson, B., Kringos, N. (2014). Effect of
Micro-scale Morphological Parameters on Meso-scale Response of Asphalt Concrete,
in: Asphalt Pavements. CRC Press, pp. 1775-1784.
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Contents
Abstract
i
Sammanfattning
iii
Preface
v
List of appended papers
vii
Contents
ix
1 Introduction
1
2 Methods
2.1 Asphalt Morphology Framework . . . . . . . . . . . . . . . . . . . .
2.2 Multi-scale modeling approach . . . . . . . . . . . . . . . . . . . . .
5
5
7
3 Results and Discussion
3.1 Paper I - Mechanics-based Top-down Fatigue Cracking Initiation Prediction Framework for Asphaltic Pavements . . . . . . . . . . . . . .
3.2 Paper II - Towards Asphalt Mixture Morphology Evaluation with the
Virtual Specimen Approach . . . . . . . . . . . . . . . . . . . . . . .
3.3 Paper III - Energy-Based Damage and Fracture Framework for Viscoelastic Asphalt Concrete . . . . . . . . . . . . . . . . . . . . . . . .
9
9
12
16
4 Summary and Conclusions
19
References
21
Appended Papers
23
ix
Chapter 1
Introduction
Asphalt concrete is a composite material made up of aggregates, bitumen binder
and air voids. The morphology of the asphalt mixtures may be defined as a set of parameters describing the geometrical characteristics of the constituent phases, their
relative proportions as well as spatial arrangement in the mixture. In particular,
at the meso-scale, the morphology of asphalt mixtures is of considerable practical
importance, as it has been shown in several studies that the deficient internal structure of the material results in compromised performance in the field, e.g. Epps
et al. (2002). In order to ensure the adequate internal structure of the material, the
asphalt mixture design methods put requirements on aggregate size distribution,
their angularity and texture as well as binder, voids in mineral aggregates (VMA)
and target air void content, e.g. E-C124 Transportation Research Board (2007).
These requirements are however formulated primarily based empirical observations
and also considers the contribution of the individual morphology component, and as
a result, they cannot be used to fully optimize the internal structure of the mixture.
The effect of the asphalt mixture internal structure on its performance received
considerable attention in the literature. Recent attempts in the field are focused
on combining experimental investigations with numerical modeling. For instance,
Souza et al. (2012) studied the effects of aggregate angularity and binder content
on bituminous mixture and related these effects to materials fracture resistance. It
was concluded that the fracture energy is increased as the aggregate angularity is
decreased and the binder content increased. In Chen et al. (2005), the effect of
coarse aggregate shape on rutting performance of asphalt concrete mixtures has
been evaluated. The authors proposed a measure of the combined effect of the
particle shape, angularity, and surface texture referred to as Particle Index (PI)
was used in the study to define the stability of an aggregate in the mix. The air
voids content, and bitumen content and voids in mineral aggregates (VMA) have
also been related to mixture performance, e.g. Epps et al. (2002); Kandhal and
Chakraborty (1996b). The VMA has been related to the durability of mixtures and
its ability to resist changes in the hot mix asphalt (HMA) properties. Inadequate
VMA can result to rapid oxidization of the asphalt which could make the pavement
1
2
CHAPTER 1. INTRODUCTION
too brittle, e.g. Chadbourn et al. (2000). Inadequate air voids content can have
an adverse effect on the mixture performance and is mainly manifested in asphalt
pavements as bleeding and rutting of the asphalt pavement, e.g. Epps et al. (2002).
The effect of asphalt binder film thickness on mixture performance was investigated
in Kandhal and Chakraborty (1996a). It was found that there is a correlation
between the asphalt binder film thickness and the mixture tensile strength, tensile
strain, resilient modulus and ageing susceptibility. Mixtures with low asphalt binder
film thickness have higher strength and stiffness properties.
Recently, a new morphology framework was developed by Lira et al. (2012);
Yideti et al. (2013) which can be used to characterize the internal structure of unbound granular and asphalt concrete materials. The framework considers the size
gradation and distribution of the stones, the distribution of the bitumen and the
distribution of the air voids in the asphalt mixture matrix. It allows for the determination of a morphological parameter called "Primary Structure" (PS) coating
thickness (tps ) which is the characteristic average value of the thickness of the mastic
coating around the main load carrying structure. Asphalt mixtures with different
gradation and volumetric properties have distinct tps values. Das et al. (2013) have
studied the influence of the changing morphological parameter on the performance
of asphalt mixtures. It was found that there exist a relationship between the morphology parameter and the resilient modulus, creep compliance, and fracture energy
of the mixtures.
The characterization and influence of the morphology on the long-term material response is not an integral part of existing mechanistic-empirical (ME) design
tools. These ME tools are used to predict the damage in an asphalt pavement as
a function of age or accumulated traffic loads. The measure of damage usually
considered in these tools are fatigue cracking (bottom-up), permanent deformation
and thermally induced cracking. However, advances in the use of non-destructive
X-ray Computed Tomography (CT) techniques provide the means to digitally capture, quantify and interactively modify the internal structure of asphalt. X-Ray CT
and micromechanical modeling techniques has been used to investigate the mesoscale response of asphalt mixtures to different loading conditions, e.g. Bažant et al.
(1990); Onifade et al. (2013); Tashman et al. (2002); You et al. (2012, 2008).
The objective of the work in this thesis is to investigate the influence of the
morphology on the short-term and long-term behaviour of asphalt concrete using
experimental and computational modeling tools. The morphology framework developed by Lira et al. (2012) and Yideti et al. (2013) is used for the characterization of
the internal structure. Relationships between the morphology and the key mixture
properties for the determination of the long-term material response are established.
These relationships are implemented in an analysis framework to predict the topdown crack initiation time in asphalt pavements. Micromechanical modeling is used
to develop more fundamental understanding of the influence of morphology on the
strength and deformation mechanism in the mixture. A first step for the development of a morphology-based homogenized macro-scale model for damage and
fracture characterization in asphalt pavements is developed and proposed.
3
The thesis is divided into three parts. The first part is focused on the implementation of morphology-based material models in an analysis framework for
predicting top-down cracking initiation. Relationships between morphological parameter and key mixture properties are developed using data from asphalt field
cores. The second part is focused on the investigation of the influence of the asphalt morphology on the strength and degradation mechanisms in asphalt mixtures
using micro-mechanical modeling techniques. The third part is focused on the development of a thermodynamic framework for damage and fracture characterization
based on the understanding from the previous parts.
Chapter 2
Methods
A multi-scale modeling approach and the asphalt morphology framework are employed in this thesis to develop an understanding of the influence of morphology
on mixture performance. The asphalt morphology framework is used to characterize the internal structure of the asphalt mixture and micro-mechanical modeling
technique is used to investigate the response to load at the meso-scale. Continuum
damage mechanics and the thermodynamics of irreversible processes using internal
state variables are used for the development of a macro-scale damage and fracture
model. The relationships between the multi-scale model characterization and the
mixture morphology is relied on for the development of morphology-based macroscale model for asphalt pavements.
2.1
Asphalt Morphology Framework
In this thesis, the asphalt concrete morphology is characterized using a morphology
framework presented in Das et al. (2013); Lira et al. (2012); Yideti et al. (2013).
The morphology framework can be used to categorize the aggregates in the asphalt
mixture into four different groups namely, the primary structure, the secondary
structure, the oversized particles, and the filler particles. The primary structure
is the main load carrying structure in the mix and it is made up of interacting
aggregate particle sizes. The main idea is that there is a range of particle sizes that
interacts to form the main load carrying structure in the mixture. The secondary
structure consists of particle sizes smaller than the primary load carrying structures
and fills the voids inside the primary structure. The secondary structure can help
in stabilizing the primary structure but too much of the secondary structure can
result in disruption of the load carrying structures. The oversized particles are
those particles with sizes greater than the primary load carrying structure and
do not contribute to the load carrying capacity. An interaction check between
consecutive sieve sizes beginning with the largest sieve sizes is used to check if
the particle sizes retained on the consecutive sieve sizes are interacting with each
other to transfer load in the mixture. Once the four different structures have been
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CHAPTER 2. METHODS
identified, another term called the "Primary Structure" coating thickness (tps ) can
be defined. The Primary Structure coating thickness is average value of the coating
of the secondary structure, the mineral filler and the bitumen all mixed together
around the primary structure. The "Primary Structure" coating thickness is referred
to as the "mastic coating thickness" in this thesis. Figure 2.1 shows the condition
for the identification of the different groups in the aggregate structure.
The relationship for the mastic coating thickness and the porosity of the primary
structure is shown in Equation 2.1.
tps = 0.95(ηps )1.28 × dp /2
(2.1)
Where:
tps : is the mastic coating thickness
ηps : is the porosity of the primary structure
dp : is the weighted average diameter of the primary structure particles
Figure 2.1: Identification of different groups based on aggregate gradation Das et al.
(2013)
2.2. MULTI-SCALE MODELING APPROACH
2.2
7
Multi-scale modeling approach
Asphalt mixtures are heterogeneous materials at several length scales. At mesoscale, the asphalt mixture is considered as a heterogeneous material with the aggregates, mastic and air voids modeled separately and assigned appropriate material
properties. Meso-mechanical analysis provides information about the stress and
strain fields due to the interaction of the constituents in the mix. It takes into
account the influence of the interaction at the interface between the aggregate and
the mastic on the overall mixture performance. Adhesive damage due to breaking
of the bond between the mastic and the stones as well as cohesive damage which is
due to the lose of integrity of the mastic phase can be captured as well.
At the macro-scale, the asphalt mixture is considered a homogeneous material
and modeled with effective material properties. The macro-scale models do not
account for the distribution of the constituents, stress concentration and strain
localization as well as the behaviour at the stone-mastic interface. A great deal of
details are not accounted for in the macro-scale models.
The characterization of the morphology as well as understanding of its influence
on material response using micromechanical modeling techniques provide a way
to systematically take into account meso-scale morphological parameters in macroscale models. This provides the possibility to develop morphology-based macro-scale
models to scale up from the meso-scale to the macro-scale. The morphology-based
macro-scale models will then take into account the influence of the morphology on
the short-term and long-term material response. Figure 2.2 shows a schematic of
the scaling up approach from meso-scale to macro-scale based on the morphology.
8
CHAPTER 2. METHODS
Figure 2.2: Scaling up from the meso-scale to the macro-scale using morphologybased models
Chapter 3
Results and Discussion
3.1
Paper I - Mechanics-based Top-down Fatigue
Cracking Initiation Prediction Framework for
Asphaltic Pavements
The paper presents a morphology-based top-down fatigue cracking analysis framework for asphalt pavements. Top-down cracking has been the predominant failure
mode observed in core samples evaluation in many parts of the world including
Japan, UK, Europe and the United States, e.g. Gerritsen et al. (1987); Uhlmeyer
et al. (2000). It was also reported that over 90% of the cracking in asphalt pavements in the state of Florida was in the form of top-down fatigue cracking, e.g.
Uhlmeyer et al. (2000). Existing mechanistic-empirical ME design tools have not
been optimized for the prediction of top-down cracking.
The morphology-based top-down fatigue cracking analysis framework is based
on the hot-mix asphalt (HMA) fracture mechanics. The HMA fracture mechanics
identifies the existence of a fundamental mixture property i.e. Dissipated Creep
Strain Energy (DCSE), below which any damage induced in the material is healable. Relationships between the morphology and the key mixture properties such
as the DCSE and healing potential are established to predict the changes in key
mixture properties over the pavement service life. These established relationships
(morphology-based sub-models) are used in the framework to predict the variations in the material properties with time. Figure 3.1 shows the description of the
top-down cracking initiation prediction framework.
Twenty-eight different pavement sections which include state roads, turnpikes
and interstates were selected for the calibration and validation of the framework.
The relevant information needed as input in the analysis framework include asphalt
mixture gradation and volumetrics, binder type, cross-sectional properties and dimensions, traffic, and temperature profile. The observed crack initiation time of the
pavement sections were obtained from the FDOT database, Florida department of
transportation (FDOT) (2013). Sixteen pavement sections were selected for model
calibration and categorized into 2 different groups. Pavement sections with an an9
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CHAPTER 3. RESULTS AND DISCUSSION
Inputs Module
Mixture, environmental and
cross- sectional properties
Traffic
Material-property sub-models
Pavement response sub-model
Damage accumulation and
recovery sub-model
Crack initiation
prediction sub-model
Figure 3.1: Top-down cracking initiation prediction framework
nual traffic volume of 100,000 ESALS or less are referred to as "low traffic volume
group" while those with annual traffic volume of more than 100,000 ESALS are
referred to as "high volume group".
Observed
Predicted
CI time (year)
15
10
5
0
1A
5-
I7
1B
5-
I7
3
5-
I7
5-
I7
2
80
SR
-1
80
SR
-2
Pavement section
5S
I-7
B
I-7
5S
B2
I-1
B
0E
SB
01
U
3
S-
Figure 3.2: Predicted vs observed results for medium to high volume roads
3.1. PAPER I - MECHANICS-BASED TOP-DOWN FATIGUE CRACKING
INITIATION PREDICTION FRAMEWORK FOR ASPHALTIC PAVEMENTS11
Observed
CI time (years)
15 Predicted
10 5 0 SR222
SR 16-6
US 19-2
SR16-4
SR89
SR18
Pavement sections
Figure 3.3: Predicted vs observed results for low volume roads
Due to the variations in the input factors that influence the long term performance of asphalt pavement sections, a crack initiation time of ±3 years from the
observed crack initiation time in the field is considered a "good" prediction. Figures
3.2 and 3.3 show the calibration results for the medium to high volume roads and
the low volume roads respectively. For the "medium to high volume roads", it can
be seen that the predicted crack initiation time is consistent with the observed crack
initiation time except for SR80-1 that deviates from the observed value by 3.7 years.
For the "low volume roads", the predicted crack initiation time is also in the range
of acceptable prediction except for US19-2 and SR89 that deviated by more than 3
years. Twelve pavement sections are used for the validation of the model. Figure
3.4 shows the result of the model validation. The result of the validation shows
that the framework is capable of predicting the top-down crack initiation time to a
reasonable extent.
CI time (year)
15 Observed
Predicted
10 5 0 K
TP
2
39
NW
1
39
NW
2
3
48
SR
3
56
SR
US
41
3
82
SR
US
27
A
A1
60
SR
US
1
46
SR
Pavement section
Figure 3.4: Model validation result using 12 different pavement sections
12
CHAPTER 3. RESULTS AND DISCUSSION
With the consideration of the influence of the morphology on the key mixture
properties, the framework is able to predict the top-down crack initiation time with
reasonable accuracy. The model has accounted for the fundamental mechanism of
pavement degradation on which further study will be based. More accurate traffic
characterization will further improve the prediction capability of the framework.
3.2
Paper II - Towards Asphalt Mixture Morphology
Evaluation with the Virtual Specimen Approach
The paper presents the study of the effect of different internal structures on the
strength properties and deformation mechanisms of asphalt mixtures. Using Xray computed tomography (CT), the internal structure with the distribution of the
different constituents in an asphalt concrete sample is captured. Image processing
technique is used to identify, segment and quantify the constituent into air void
phase, mastic phase and aggregate phase. The morphology framework is used to
characterize the internal structure of the mixture. The framework is also used to
characterize the three dimensional (3D) distribution of the average mastic thickness
around the aggregate main load carrying structure. This mastic thickness is referred
to as the "mastic coating thickness". The mastic coating thickness is used as a
morphological parameter to characterize the morphology of the asphalt sample.
Figures 3.5a and 3.5b show the scanned asphalt concrete sample and a typical
slice from the X-ray CT scan. The image processed asphalt concrete sample and
the segmentation of the different phases is shown in Figure 3.6.
(a)
(b)
Figure 3.5: a)Asphalt concrete sample, b) X-Ray CT slice from the scanned asphalt
concrete sample.
3.2. PAPER II - TOWARDS ASPHALT MIXTURE MORPHOLOGY
EVALUATION WITH THE VIRTUAL SPECIMEN APPROACH
(a) Asphalt concrete filtered image
(b) Air-void phase
(c) Mastic phase
(d) Aggregate phase
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Figure 3.6: Image processed asphalt concrete sample and segmentation results
The morphology of the scanned asphalt concrete sample is then virtually modified using erosion morphological operation tool to make the 12.5mm and 9.5mm aggregate sizes finer. The scanned asphalt concrete sample is referred to as "Structure
1". The Erosion morphological operation is used to modify the aggregate gradation
inside the mixture. The 12.5mm particle size in "Structure 1" is eroded by one pixel
to obtain "Structure 2" while the 9.5mm particle size in "Structure 2" is eroded by
one pixel to obtain "Structure 3". The percentage air voids is dilated to compensate
for the increase in percentage binder when the 12.5mm and 9.5mm aggregate sizes
are made finer. In this way, the three different resulting structures have relatively
the same amount of binder but different morphological structures. The effect of
the varying morphological structure to mechanical loading at 0o C is studied using
the finite element method by subjecting the three structures to the same boundary
conditions. The stones are modeled as isotropic linear elastic materials and the
14
CHAPTER 3. RESULTS AND DISCUSSION
mastic modeled as a viscoelastic material using the generalized Maxwell’s model.
The mastic coating thickness and the primary structure range for the structures are
shown in Table 3.1. Figure 3.7 show the three different morphological structures
obtained after the modification.
Table 3.1: Mastic coating thickness and primary load carrying structure range for
the three different morphology structures
Mastic coating thickness (mm)
PS Range (mm)
Aggregate proportion in
Primary Structure (%)
(a) Structure 1
Structure 1
2.13
12.5 - 4.75
Structure 2
2.47
9.5 - 4.75
Structure 3
2.48
9.5 - 4.75
75.5
56.0
54.8
(b) Structure 2
(c) Structure 3
Figure 3.7: The three different morphological structures with cropped region shown
in Figure 3.6a
The results show that the structure with the lowest mastic coating thickness
has better stress distribution patterns with less stress concentrations in the mastic
regions. High strain localizations can be observed in the structure with the highest
mastic coating thickness. It can be observed that the structure with the lowest mastic coating thickness has the highest effective modulus and the modulus decreases
as the mastic coating thickness increases. Figure 3.8 shows the effective relaxation
modulus and the stress-strain response for the three different structures. Figure
3.9 show the finite element mesh and the first principal stress streamlines at the
aggregate boundaries.
3.2. PAPER II - TOWARDS ASPHALT MIXTURE MORPHOLOGY
EVALUATION WITH THE VIRTUAL SPECIMEN APPROACH
3.5
Structure 1
Structure 2
Structure 3
6
3
5
4
3
2
1.5
2
1
1
0.5
0
0
Structure 1
Structure 2
Structure 3
2.5
stress [MPa]
Effective relaxaion modulus [GPa]
7
15
10
20
30
40
50
60
70
80
time [s]
(a)
90
100
0
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
strain [%]
(b)
Figure 3.8: a)Effective relaxation modulus, b) stress-strain response.
(a) Finite element mesh
(b) Structure 1
(c) Structure 2
(d) Structure 3
Figure 3.9: FE mesh and First principal stress streamlines at aggregate boundaries
after application of a uniaxial tensile stress of 2MPa
16
CHAPTER 3. RESULTS AND DISCUSSION
Micromechanical modeling provides fundamental understanding of the influence
of the internal structure on the material response. The work is a step towards the
implementation of a digital specimen approach for optimization of mix designs to
meet desired functional requirements. The work also highlights the importance of
the morphology of the asphalt mixture in the characterization of mixture performance. The morphology parameter can be used in homogenized asphalt concrete
models to take into account the effect of the morphology on short-term and longterm material response.
3.3
Paper III - Energy-Based Damage and Fracture
Framework for Viscoelastic Asphalt Concrete
The paper is focused on the development of a unified damage and fracture framework for asphalt concrete mixtures with particular attention paid to micro-crack
initiation and accumulation. The paper provides a step towards a more fundamental pavement response prediction. The work suggests the development of two
different potential-based models to accurately characterize the material response
at low and high temperatures respectively. One of the models will focus on the
characterization of cracking at low temperatures, while the other will be used to
characterize the plastic deformation at high temperatures. Both models can then be
coupled to characterize the material behaviour at intermediate temperature. The
integrated model will provide improved prediction of material response at extreme
temperatures i.e. low and high temperatures, while minimizing material prediction
errors at intermediate temperatures.
The proposed damage model is based on continuum damage mechanics and
the thermodynamics of irreversible processes using internal state variables. The
internal state variable is used to characterize the distributed damage in viscoelastic
asphalt materials in the form of micro-crack initiation and accumulation. At low
temperatures and high deformation rates, micro-cracking is considered as the source
of non-linearity in the material response and thus the cause of deviation from linear
viscoelastic response. Using a non-associated evolution law, a damage initiation
criterion is used to identify the instance of micro-crack initiation while another
criterion is used to derive the micro-crack evolution by means of restrictions imposed
by the second law of thermodynamics.
The Superpave IDT test is used to characterize the performance of six different
asphalt concrete mixtures used in this study. The Superpave IDT test procedure
consists of three different tests (resilient modulus test, creep test and strength test)
which can be used to characterize the samples nondestructively and destructively.
In this study, the Superpave IDT tests are carried out at three different temperature (−20o C, −10o C and 0o C) for each test setup. The low temperature range is
considered to minimize the effect of plasticity. The linear viscoelastic response and
the strength characteristics of the mixtures are obtained from the Superpave IDT
test results.
3.3. PAPER III - ENERGY-BASED DAMAGE AND FRACTURE
FRAMEWORK FOR VISCOELASTIC ASPHALT CONCRETE
17
A micro-crack initiation threshold is identified below which the material response
is purely linear viscoelastic. Temperature coupling is introduced to predict the damage parameters and critical micro-crack initiation threshold at other temperatures
not tested for. It was observed that the critical micro-crack damage threshold increases as the temperature increases. This results in an increase in resistance to
micro-crack formation at higher temperatures. The proposed damage model shows
the capability to characterize the damage in both conventional and unconventional
asphalt mixtures. Figure 3.10 shows the evolution of the micro-cracking damage
initiation threshold for AG1-mixtures from −40o C to 40o C
Figure 3.10: Evolution of micro-cracking damage initiation threshold for AG1mixtures from −40o C to 40o C
Internal scalar damage variable is used to estimate the distributed damage in
the material. The damage variable D is estimated from the Superpave IDT strength
test as a function of the experimental observed stress and the theoretical predicted
linear viscoelastic stress response. Figures 3.11 and 3.12 show the experimental
damage evolution and the model predicted damage evolution for the conventional
mixtures, AG1-0 and AG2-0 mixtures.
18
CHAPTER 3. RESULTS AND DISCUSSION
Damage evolution vs strain
0.25
experimental
model prediction
damage (D)
0.2
0.15
0.1
0.05
0
0
200
400
600
800
1000
1200
1400
1600
1800
2000
strain (microstrain)
Figure 3.11: Micro-crack damage evolution: model and experiment for AG1-O
Damage evolution vs strain
0.25
experimental
model prediction
damage (D)
0.2
0.15
0.1
0.05
0
0
200
400
600
800
1000
1200
1400
1600
1800
strain (microstrain)
Figure 3.12: Micro-crack damage evolution: model and experiment for AG2-O
The proposed damage evolution law is consistent with the estimated damage
evolution in the Superpave indirect tensile (IDT) strength test with a good accuracy
at the temperatures considered. The energy-based formulation enables the extension
of the model to a wide range of temperature and easy incorporation of the healing
and ageing phenomena.
Chapter 4
Summary and Conclusions
The thesis is focused on developing fundamental understanding of the influence of
asphalt morphology on its response. Experimental investigation of field cores is
used to determine the effect of the morphology on resistance to damage and other
key mixture properties. Micromechanical modeling is used to study the complex
interaction between the asphalt mixture constituents at the meso-scale. It was
found that the morphology doesn’t only have effect on the instantaneous effective
properties but also, it determines the load distribution pattern and deformation
mechanisms within the mixture which influences the long-term performance.
The understanding of the deformation mechanism and interaction between constituent composition over a wide range of temperature provided the motivation for
the development of a thermodynamic based damage and fracture model. The model
presented focuses more on the characterization of micro-crack initiation and accumulation. The model identifies the existence of a critical micro-crack damage initiation threshold below which the response is purely linear viscoelastic. Temperature
coupling is integrated into the model to characterize the material at temperatures
not tested for.
The results highlight the importance of the morphology for improved material
characterization. Fundamental understanding of the influence of the morphology
on the short- and long term bulk material response will enable the development
of new morphology-based models for improved pavement performance analysis and
prediction. The models will provide the basis for more robust analytical techniques
in the design and analysis of asphalt pavements with which mixture morphology can
be optimized to meet certain functional requirements. This development will present
the opportunity for the realization of performance-based specifications for asphalt
mix design and as well reduce or totally eliminate the black boxes in pavement
design. This will as well reduce the empiricism associated with pavement design,
reduce need for extensive calibration and validation, and advance pavement design
to a new level science and engineering.
19
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Appended Papers
Contribution of the work in the appended papers:
Paper I
Dinegdae, Y., Onifade, I., Jelagin, D., Birgisson, B. (2015), Mechanics-based Topdown Fatigue Cracking Initiation Prediction Framework for Asphaltic Pavements.
Submitted to Road Materials and Pavement Design.
Onifade and Dinegdae were involved in the development of the top-down fatigue
cracking initiation prediction framework and analysis of the data. Jelagin and Birgisson provided guidance during the work. Dinegdae wrote the paper.
Paper II
Onifade, I., Jelagin, D., Birgisson, B., Kringos, N., (2015). Towards Asphalt Mixture Morphology Evaluation with the Virtual Specimen Approach. Submitted to
EATA 2015 conference for publication in Special Edition of Road Materials and
Pavement Design Journal.
Onifade was responsible for the data analysis and writing of the paper. Jelagin
took part in writing the paper. Jelagin, Kringos and Birgisson provided guidance
during the work.
Paper III
Onifade, I., Balieu, R., Birgisson, B. (2015), Energy-Based Damage and Fracture
Framework for Viscoelastic Asphalt Concrete. Submitted to the Journal of Engineering Fracture Mechanics.
Onifade was responsible for the development of the framework, writing the paper
and the data analysis. Balieu and Birgisson provided guidance during the work.
23