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Self-Healing Technology for Asphalt Pavements

Authors:
  • TNO; Delft University of Technology; University College Dublin

Abstract and Figures

Self-healing technology is a new field within material technology. It represents a revolution in materials engineering and is changing the way that materials behave. Incorporating self-healing technology into the road design process has the potential to transform road construction and maintenance processes by increasing the lifespan of roads and eliminating the need for road maintenance. By decreasing the unnecessary premature ageing of asphalt pavements, self-healing asphalt can reduce the amount of natural resources used to maintain road networks, decrease the traffic disruption caused by road maintenance processes, decrease CO2 emissions during the road maintenance process and increase road safety. In addition to environmental savings, self-healing materials have the potential to deliver significant cost savings for road network maintenance across the EU. There are three main self-healing technologies available for asphalt pavement design: nanoparticles, induction heating and rejuvenation. This chapter reviews all three options and outlines the future development of self-healing asphalt technology.
Content may be subject to copyright.
Adv Polym Sci
DOI: 10.1007/12_2015_335
©Springer International Publishing Switzerland 2015
Self-Healing Technology for Asphalt
Pavements
Amir Tabakovic
´and Erik Schlangen
Abstract Self-healing technology is a new field within material technology. It
represents a revolution in materials engineering and is changing the way that
materials behave. Incorporating self-healing technology into the road design pro-
cess has the potential to transform road construction and maintenance processes by
increasing the lifespan of roads and eliminating the need for road maintenance. By
decreasing the unnecessary premature ageing of asphalt pavements, self-healing
asphalt can reduce the amount of natural resources used to maintain road networks,
decrease the traffic disruption caused by road maintenance processes, decrease CO
2
emissions during the road maintenance process and increase road safety. In addition
to environmental savings, self-healing materials have the potential to deliver
significant cost savings for road network maintenance across the EU. There are
three main self-healing technologies available for asphalt pavement design:
nanoparticles, induction heating and rejuvenation. This chapter reviews all three
options and outlines the future development of self-healing asphalt technology.
Keywords Asphalt pavements Induction heating Microcapsule Nanoparticles
Rejuvenation Self-healing
Contents
1 Introduction
1.1 Self-Healing Properties of Asphalt Pavement
2 Examples of Self-Healing Technology for Asphalt Pavements
2.1 Nanoparticles
2.2 Induction Heating
2.3 Binder Healing Agent (Rejuvenation)
A. Tabakovic
´(*) and E. Schlangen
Materials and Environment, Faculty of Civil Engineering and Geosciences, Delft University of
Technology, Delft, The Netherlands
e-mail: A.Tabakovic@tudelft.nl;Erik.Schlangen@tudelft.nl
3 Towards New Generations of Self-Healing Asphalt Pavements
4 Cost and Environmental Benefits of Self-Healing Technology for Asphalt Pavement
Design
5 Concluding Remarks
References
1 Introduction
The global road network spans 16.3 million kilometres [1], of which 5 million
kilometres is in the EU, 4.4 million kilometres in the USA and 3.1 million
kilometres in China [2]. Road networks fulfil a major economic and social goal
by facilitating the movement of goods and people. The operational health of the
road network is of the utmost importance for national and regional economic and
social life. As a result, governments invest heavily in the development and main-
tenance of national and regional road networks. In 2009, EU governments invested
42% (4.5 billion) of the EU transport network fund (10.74 billion) into the
development and maintenance of EU road networks [2]. The development and
maintenance of the EU road network is crucial for the growth and competitiveness
of the EU economy.
A typical modern road system comprises double or triple asphalt layers [3] with
an expected lifespan of 20–40 years [4]. Recent research highlights the importance
of developing long-life or perpetual pavements and has called for innovation to
prolong pavement lifespan and reduce maintenance [5,6]. The development of self-
healing asphalt and its use in road paving is an innovation that could potentially
double road lifespan to between 40 and 80 years and could significantly reduce road
maintenance activity. In comparison with current maintenance processes, self-
healing asphalt has the potential to improve traffic flow, reduce demand for fresh
aggregate, reduce CO
2
emissions and enhance road safety. The excellent durability
of self-healing materials does not arise from the classical approach of minimizing
damage but from the novel approach of designing materials with “self-healing”
capabilities.
The objective of self-healing technology is to enable/assist material systems to
heal after damage. It aims to reduce the level of damage and to extend or renew the
functionality and lifetime of the damaged part [7]. Fisher defines the self-healing
and self-repair of a material or system as “the ability to substantially return to an
initial, proper operating state or condition prior exposure to a dynamic environment
by making the necessary adjustments to restore to normality and/or the ability to
resist the formation of irregularities and/or defects” [7].
The repair is, in principle, an automatic initiated response to damage or failure.
To perform repair, any self-healing system must be capable of identifying and
repairing damage. Fisher classifies repair into two categories:
Attributive repair: restoring the attributes of the system to its original state
(i.e. to full capacity).
A. Tabakovic
´and E. Schlangen
Functional repair: restoring the function of the system. If full functionality
cannot be restored, the remaining available resources are employed/used to maxi-
mize the available functionality [7].
Attributive repair is the optimal solution. If the attempt to restore the system to
its complete original condition fails, there is still significant benefit to be gained, in
most instances, if the system continues to operate, even with reduced functionality.
Living organisms possess intrinsic self-healing properties, enabling them to recover
from damage or injuries. This repair or healing occurs with no external intervention.
Some natural self-healing composite systems, such as bones, go beyond simple
healing to continuous remodelling and strengthening [8]. Over the past decade, self-
healing technology has entered the field of materials engineering [7]. Self-healing
technology represents a revolution in materials engineering. Examples of engineer-
ing materials to which self-healing technology has been successfully applied are
presented in Table 1.
1.1 Self-Healing Properties of Asphalt Pavement
Asphalt pavement is a self-healing material. When subjected to rest periods, asphalt
pavement has the potential to restore its stiffness and strength by closing the micro-
cracks that occur when the pavement is subjected to traffic loads. Research to date
has focused on its autogenous healing properties (see Table 2).
Crack repair in an asphalt pavement system occurs as a result of the wetting and
interdiffusion of material between the two faces of a micro-crack, to regain the
properties of the original material [7,33]. The three primary steps in the autono-
mous asphalt self-healing process are as follows:
Table 1 Materials to which self-healing technology has been successfully applied
Material Healing mechanism Reference Year
Polymer Healing agent encapsulation Jin et al. [9] 2012
Concrete Bacteria Jonkers and Schlangen [10] 2009
Hollow fibres Dry [11]; cited in [12] 1996
Microencapsulation Boh and S
ˇumiga [13]; cited in
[12]
2008
Expansive agents and mineral
admixtures
Kishi et al. [14]; cited in [12] 2007
Asphalt Nanoparticles Tabatabee and Shafiee [15] 2012
Steel fibres – induction heating Garcia et al. [16] 2011
Rejuvenator encapsulation Su et al. [17] 2013
Coatings Healing agent (resin) encapsulation Wilson and Magnus [18] 2011
Composites Memory alloys Kirkby et al. [19] 2008
Metals and
alloys
Press and sinter powder metallurgy Lumley [20] 2007
Self-Healing Technology for Asphalt Pavements
1. Wetting of the two faces of a micro-crack
2. Diffusion of molecules from one face to the other
3. Randomization of the diffused molecules to reach the level of strength of the
original material
Binder is key to the self-healing process in an asphalt pavement. Self-healing
takes place on a molecular level, when broken (non-associated) molecules are
available to form links and chains via hydrogen bonds [7]. The process is termed
“reversible hydrogen bonding” [34] and is achieved by bringing together the
associated molecules to form both chains and crosslinks via hydrogen bonds
[7]. A repaired molecular network can be formed by linking ditopic and tritopic
molecules [35], which are able to associate with each other (see Fig. 1). This
system, when broken or cut, can be simply repaired by bringing two fractured
surfaces into contact [3436].
Qiu et al. [36] reported that self-healing in an asphalt pavement system is a
viscosity-driven process, dependent on time (rest periods) and temperature. Qiu
et al. [36] also demonstrated the self-healing time and temperature dependency of
bituminous materials (see Fig. 2). A longer healing time and increased healing
temperature lead to better healing [36]. Shorter time to heal results in the formation
of fewer bridges across the interface and the development of a weaker bond across
the break. However, if broken bonds are not healed immediately (i.e. if the fractured
surfaces are not brought into contact with each other), the number of non-associated
groups available for linking decreases (i.e. healing efficiency decreases) [35]. This
is because, immediately after breakage, the free (non-associated) groups begin to
seek other free groups within the broken part to link with [35].
Qiu et al. [36] successfully modelled the time–temperature dependence of the
self- healing process in asphalt mastic using a time–temperature superposition
principle, which can be expressed by the following formulation:
Table 2 Factors influencing the autogenous healing properties of asphalt pavement
Factors influencing healing Reference Year
Bitumen properties Bitumen type van Gooswiligen et al. [21] 1994
Viscoelastic properties Kim and Little [22] 1991
Surface free energy Lytton et al. [23] 1993
Ageing Edward [24] 2006
Diffusion Bhasin et al. [25] 2011
Modifiers Lee et al. [26] 2000
Asphalt mix composition Bitumen content van Gooswiligen et al. [21] 1994
Aggregate structure Kim and Roque [27] 2006
Gradation ABO-Qudais and Suleiman [28] 2005
Thickness Theyse et al. [29] 1996
Environment Temperature Verstraeten [30] 1991
Loading history Castro and Sanchez [31] 2006
Rest period Bhasin et al. [25] 2007
Water/moisture Hefer and Little [32] 2005
A. Tabakovic
´and E. Schlangen
Fig. 1 Supramolecular network: Scheme of a reversible network formed by mixtures of ditopic
(blue) and tritopic (red) molecules associating by directional interactions (represented by dotted
lines)[35]
Fig. 2 Self-healing results of PBmas (asphalt mastic mix containing limestone filler and 70/100
pen bitumen) and SBSmas (asphalt mastic mix containing limestone filler and bitumen modified
with styrene-butadiene-styrene polymer) [36]
Self-Healing Technology for Asphalt Pavements
Ht;TðÞ¼100 1þm
tαT

log 2
n
"#
n
log 2
ð1Þ
where the time–temperature superposition shift factor in Eq. (1) is based on the
Arrhenius equation and is calculated using the following formulation:
log αTTðÞ¼ ΔEa
2:303R
1
T1
T0
 ð2Þ
where α
T
is the time–temperature superposition shift factor; m,nare the model
parameters; ΔE
a
is the apparent activation energy, (J/mol) and Ris the universal gas
constant, 8.314 J/(mol K).
The healing of an asphalt pavement at high temperatures is governed by the
so-called thixotropic effect, which describes the transformation of asphalt binder
from a solid to gel state, allowing recovery from structural damage [30]. Wu
reported that visible in-situ cracks within asphalt pavements disappear during
periods of warm weather, only to reappear during cold weather [37]. At high
temperatures, surface cracks close, but high temperatures dissipate quickly through-
out the pavement depth [38], meaning that cracks 20–30 mm below the pavement
surface do not heal and re-appear at lower temperatures or as a result of heavy
traffic loading. Results presented in Fig. 2 show that the healing progress of two
different asphalt mixtures (PBmas and SBSmas) is only 10% after healing at 10C.
The results from Qiu et al. [36] show that temperatures below 20C are insufficient
to initiate full recovery of asphalt pavements. However, the self-healing properties
of asphalt can be enhanced either by heating the asphalt material or by adding
modifiers or a healing agent (i.e. rejuvenator), as shown in Fig. 2.
2 Examples of Self-Healing Technology for Asphalt
Pavements
The asphalt pavement design standards focus on enhancing asphalt pavement
performance, that is, they aim to increase its durability and improve its load-
carrying capability [39]. However, the authors consider that future of asphalt
pavement design lies not with the enhancement of asphalt pavement properties,
but in allowing it to repair itself to its original state.
As in nature, the self-healing performance of an asphalt pavement can be
improved, for example, by introducing modifiers and additives to the asphalt mix
to upgrade its self-healing properties. To function in an asphalt pavement system,
the self-healing technology must have the capacity to survive the harsh conditions
that prevail during asphalt pavement construction (mixing and compaction) and
service life (traffic loading and environmental conditions such as rain, ice, snow,
A. Tabakovic
´and E. Schlangen
high temperatures, etc.). Qiu et al. [34] outlined five essential conditions for self-
healing agents to be included into asphalt pavement design:
1. Good compatibility with bitumen
2. High temperature stability
3. Ability to survive mixing and construction conditions
4. Healing temperature between 30C and 40C
5. Capable of continuous/multi-time healing
In this section, a number of existing self-healing approaches for asphalt pave-
ment design are presented and critically analysed with respect to their functional
design and performance.
Three self-healing methods for asphalt pavements have been reported to date,
they are:
1. Incorporation of nanoparticles
2. Induction heating
3. Rejuvenation
2.1 Nanoparticles
2.1.1 Nanoclay
Nanoclay materials are used in asphalt pavement design to improve the ageing,
rheological and thermal properties of asphalt mixtures [40]. However, they also
have the potential to repair micro-cracks in asphalt [34]. The nanoparticles tend to
move towards the tip of the crack, driven by the high surface energy, and thus stop
crack propagation and heal damaged asphalt material [34]. Tabatabee and Shafiee
[15] studied the effect of rest periods on the fatigue life of organoclay-modified
asphalt mixes and found that introduced rest periods at 3% and 5% strain level
increased the fatigue resistance of these mixes. This finding demonstrates that
nanoclay material can be used for improving the self-healing properties of asphalt
mixtures. However, the nanoclay self-healing technique has not been researched in
great detail to date and there is insufficient data available on the long-term effect of
nanoclay particles on the performance of self-healing asphalt mixtures. This is an
interesting area for future research.
2.1.2 Nanorubber
As with nanoclay materials, polymer and rubber modifiers are used in the bitumen
mix to improve the physical and mechanical properties of the binders and, as such,
to improve in-situ performance of an asphalt pavement [4143]. Rubber modifiers
in the form of nanoparticles have also been used to improve the healing properties
Self-Healing Technology for Asphalt Pavements
of asphalt mastic [34]. Qiu et al. [34] studied the repair of asphalt mastic with
nanorubber-modified binder (70/100 pen binder).
1
They conducted ductility self-
healing tests to assess the self-healing capacity of asphalt mastics containing two
different types of nanorubber modifier (NanoA and NanoB) and varying percent-
ages of modifier (0–5%), where 0% was a control mix. The tests were performed on
dog bone test specimens. The test specimens were cut at the centre, joined and left
to heal for 4 h at room temperature (20–22C). The test results showed good self-
healing for non-modified asphalt mastic mixes, up to 70%. Recovery of the
modified asphalt mixtures varied from 15% to 90%, depending on the type of
nanorubber modifier and the amount added [34]. Unfortunately, the authors did
not present the composition of the nanorubber modifier, so it is difficult to deter-
mine what type of healing mechanism activated the healing process and to under-
stand the variability in the healing performance. Nevertheless, the study showed
that nanorubber modifiers can be used to improve the self-healing properties of
asphalt mixtures. Future studies should indicate the properties of the nanorubber
modifiers used, that is, whether they are styrene-butadiene-rubber (SBR), styrene-
isoprene-styrene (SIS) or olefinic rubber. This would enable determination of the
type of healing mechanism that had occurred (i.e. whether it was reverse hydrogen
bonding). The clear advantage of nanorubber as a modifier is its double role; it can
improve asphalt mix durability and also act as self-healing modifier in the mix.
However, the disadvantage of polymer-based modifiers is their thermodynamic
incompatibility with asphalt binder as a result of the large differences in material
density, polarity, molecular weight and solubility between the polymer and the
asphalt [40]. This can result in delamination of the composite during thermal
storage, which is not readily apparent and adversely affects the asphalt mix when
it is used [40]. The thermodynamic incompatibility of nanomodifiers needs to be
studied, because at high temperatures the separation of nanorubber modifier and
binder could take place, resulting in deterioration of asphalt pavement performance.
The nano-effect and reverse hydrogen and ionic bonding are known for their
multi-healing abilities [34], which is a benefit of this type of self-healing mecha-
nism. However, the effect of healing time and temperature need to be further
investigated to determine what effect time has on the healing of the asphalt mix.
If ineffective at lower temperatures and without improvements in healing times
relative to autonomous bitumen healing, the technology would be unsuitable for the
self-healing of asphalt pavements.
Although nanoparticle self-healing technology has demonstrated its potential in
asphalt pavement mix design, more substantial evidence of its performance must be
demonstrated before it becomes acceptable as a viable self-healing technology.
1
“pen” stands for penetration. Bitumen penetration test (EN 1426:2015) indicates the hardness of
bitumen, lower penetration indicating a harder bitumen. Specifications for penetration graded
bitumen normally state the penetration range for a grade, e.g. 70/100.
A. Tabakovic
´and E. Schlangen
2.2 Induction Heating
Induction heating in asphalt pavement design was pioneered by Minsk [44,45]. He
developed and patented the first electrically conductive asphalt pavement using
graphite as a conductive medium for the purpose of melting snow and ice on
roadway surfaces by induction heating. More recently, induction heating has
regained popularity in asphalt pavement research for to improving self-healing in
asphalt pavements [4649]. Electrically conductive fibres and fillers (carbon fibres,
graphite, steel fibres, steel wool and the conductive polymer polyaniline) were
added to study the electrical conductivity in asphalt pavement. Results showed
that the electrical resistivity significantly varied with the type, shape and size of
fibres and fillers. Wu et al. [46] studied induction heating in asphalt pavement using
conductive carbon fibres, carbon black and graphite as conductive media and
demonstrated that adding conductive fibres to the mixture increases conductivity
more effectively than adding conductive filler. Subsequent research by Garzia
et al. [47] and Liu et al. [48] initiated the development of a self-healing asphalt
pavement mix by inclusion of electrically conductive steel and wool fibres into the
asphalt mix and activation of self-healing by induction heating.
The induction process operates by sending an alternating current through the coil
and generating an alternating electromagnetic field. When the conductive asphalt
specimen is placed under the coil, the electromagnetic field induces currents
flowing along the conductive loops formed by the steel fibres [50]. This method
can be repeated if damage returns. A schematic diagram of induction healing is
illustrated in Fig. 3.
The major healing mechanism in induction healing is the capillary flow and
diffusion of the asphalt binder (bitumen) at high temperatures. Garcı
´a[50] verified
this healing mechanism using bitumen capillary flow tests. Liu [51] studied the
induction healing effect of steel fibres and steel wool and characterized asphalt
healing via the following equation:
HI ¼C2
C1
ð3Þ
where HI is the healing index (%), 100% indicating complete healing of damage
and 0% indicating no healing at all; C
1
is the number of loading cycles for the first
loading; and C
2
is the number of loading cycles for the second loading.
Fig. 3 Induction healing in porous asphalt [51]
Self-Healing Technology for Asphalt Pavements
Figure 4gives the results of a fatigue test performed by Liu [51], showing values
of C
1
and C
2
for two loading cycles. The loading value C
2
illustrates the effect of
induction healing.
Liu et al. [48] investigated the effect of long steel wool type 000 versus short
steel fibre. The results showed that long steel wool is better than short steel fibre for
making porous asphalt concrete electrically conductive.
Liu et al. [48] demonstrated that the addition of steel fibres reinforces the mastic
(bitumen, filler and sand) of porous asphalt concrete, which can delay the ravelling
effect in asphalt pavement. Liu et al. also demonstrated that the inclusion of steel
fibres into the asphalt pavement mix prevents the drainage of bitumen from the
surface of the asphalt pavement. The advantage of this is that it achieves a better
bond between the large aggregates (stones) in the pavement.
Although induction healing can enhance the self-healing capacity of asphalt
pavement, an adverse effect is that heating the asphalt mix ages the bitumen.
Furthermore, overheating (>110C) the asphalt mix can cause binder swelling
and drainage, which adversely affects pavement performance. Liu et al. [48]
suggested that the optimal heating temperature for a porous asphalt mix is 85C.
Liu [51] demonstrated that the degree of damage incurred by overheating also
affects the healing ratio, (i.e. the heating should not be applied too early or too late).
If applied too early (when resilient modulus is >80% of the original asphalt
pavement stiffness value), the asphalt pavement can heal itself and heating is not
required. However, if the asphalt is heated too late (when resilient modulus value is
<80% of the original asphalt pavement stiffness value), the healing effect is poor,
because structural damage such as permanent deformation or stone aggregate
cracking can occur and is beyond the healing capacity of the induction healing
process. Liu [51] further concluded that the best healing results were achieved when
induction heating was combined with a rest period. This study demonstrated that
healing can be improved by 15% if the autogenous healing process is aided by
induction heating.
In December 2010, researchers from TU Delft in cooperation with the Dutch
National Roads Authority resurfaced a road (the A58 in Netherlands) using an
Fig. 4 Fatigue recovery of
porous asphalt concrete
cylinder for resilient
modulus reduction to 80%
[51]. C
1
and C
2
are loading
cycles
A. Tabakovic
´and E. Schlangen
induction self-healing asphalt mix (i.e. an asphalt mix containing 1 cm long steel
fibres) [52]. Testing of the self-healing process within the A58 motorway is ongoing
at present [52].
Inductive heating is the most progressive self-healing technology for asphalt
pavements reported to date. This technology has transitioned from laboratory to site
in a short period of time (3 years). Although its ageing effect can be compensated
for by the healing effect, a problem that has not been addressed by research is the
loss of conductivity via oxidation (corrosion) of the steel wool and fibres. However,
this should not be an insurmountable problem, as steel could be replaced by carbon
fibres and/or conductive polymer [49,53]. In addition, the piezoresistivity of
conductive asphalt, which refers to the change in electrical resistivity with applied
mechanical pressure, can be used for self-sensing of strain [54]. Self-sensing of
damage for evaluating pavement distress is possible if there is a relationship
between the electrical property and internal damage. Moreover, some conductive
additives can improve the durability of asphalt concrete, thereby increasing the
service life of the pavement system [48,55,56]. This aspect of self-healing
induction technology needs to be further developed for the initiation of healing
processes and also for self-health assessment of asphalt pavement.
2.3 Binder Healing Agent (Rejuvenation)
During the service life of a pavement, the volatile components of bitumen evapo-
rate, and oxidation and polymerization can occur [57]. As a result, the bitumen ages
and loses some of its viscoelastic properties. Asphalt binder is a combination of
asphaltenes and maltenes (resins and oils). Asphaltenes are more viscous than either
resins or oils and play a major role in determining asphalt viscosity [58,59]. The
oxidation of aged asphalt binder during construction and service causes the binder
oils to convert to resins and the resins to convert to asphaltenes, resulting in
age-hardening and a higher viscosity than for fresh binder [59,60]. Although this
process is irreversible, the viscoelastic state of the asphalt mix can be recovered
through the addition of either bitumen with a high penetration value or a rejuve-
nating agent such as a cationic emulsion [6163].
A rejuvenator is an engineered cationic emulsion containing maltenes and
saturates. The primary purpose of a rejuvenator is to reduce the stiffness of the
oxidized asphalt binder and to flux the binder to extend the pavement life by
adjusting the properties of the asphalt mix [62]. Some commercially available
rejuvenating agents are Reclamite, Paxole 1009, Cyclepave and ACF Iterlene
1000. A recent study by Su et al. [64] demonstrated that a by-product of waste
cooking oil (WCO) can also be used as binder rejuvenator.
Self-Healing Technology for Asphalt Pavements
2.3.1 Self-Assembled Monolayers
When cracks within the surface layer of an asphalt pavement are still in an early
phase, it is possible to apply a rejuvenator to the wearing course to prevent further
crack propagation and pavement failure [65]. By applying the rejuvenator to the
surface course, the lifespan of the asphalt pavement can be extended by several
years; however, this only applies to the top few centimetres of the asphalt pave-
ment. Shen et al. [66] reported the use of three different types of rejuvenators and
found that none could penetrate further than 20 mm into asphalt concrete. A further
issue encountered when applying these materials is the need for road closures for a
period of time after the application. The rejuvenators can also cause significant
reduction in the surface friction of the pavement and could also be harmful to the
environment. Microencapsulation of the rejuvenators represents a potential means
of overcoming these problems.
2.3.2 Microcapsulation Technique
The inclusion of a rejuvenator into the asphalt mix via microcapsules to restore the
original binder properties is a self-healing method that has been studied by Su
et al. [64,67], Su and Schlangen [68] and Garcı
´a et al. [69]. The principle behind
this approach is that when micro-cracks begin to form within the pavement system,
they encounter a capsule in the propagation path. The fracture energy at the tip of
the crack opens the capsule and releases the healing agent. The healing agent then
mixes with the asphalt binder to seal the crack, thus preventing further propagation.
This healing process is illustrated in Fig. 5. The process prevents the formation of
micro-cracks within the pavement mix and prevents complete failure of the pave-
ment system. Su and Schlangen [68] and Garcı
´a et al. [69] demonstrated that
Fig. 5 Rejuvenator Encapsulation — A Self-healing Mechanism in Asphalt Mix
A. Tabakovic
´and E. Schlangen
various types of capsules containing rejuvenator can be produced and that these
capsules are sufficiently thermally and mechanically stable to survive the asphalt
production process.
To date, the most successful microcapsule shells have been made of a
prepolymer of melamine–formaldehyde modified by methanol (solid content
78.0%) and the rejuvenator was an oily product [17]. Figure 6illustrates the
fabrication process of double-shell microcapsules containing rejuvenator by
two-step coacervation (TSC). The efficiency of microcapsule fabrication is mea-
sured by amount of rejuvenator retained within the microcapsule [68]. The highest
efficiency achieved was 70% [67] using the following production conditions: core/
shell ratio of 1:3, stirring speed 3,000 revolutions/min and 2.0–2.5% by weight
styrene maleic anhydride (SMA) copolymer dispersant. Figure 7shows the SEM
morphology of dried microcapsules with a core/shell ratio of 1:3 and an average
shell diameter of 25 μm.
The asphalt mortar films between aggregate particles in an asphalt pavement are
found to be 50 μm thick [67]. To avoid being squeezed or pulverized during the
asphalt pavement mixing and compaction processes, the size of the capsule needs to
be less than 50 μm. However, Su et al. [67] stated that microcapsules of 10 μm and
smaller are unsuitable for self-healing as they do not contain sufficient rejuvenator
to rejuvenate the aged binder. The size of the capsule can be controlled by
regulating the core/shell ratio (weight ratio between core and shell material). This
can be achieved by modifying the prepolymer and the emulsion stirring rate
[67]. Figure 8shows the morphology of bitumen containing varying capsule
content (10–30% of total binder volume) and capsules of varying size (10–
Fig. 6 Fabrication process for double-shell microcapsules containing rejuvenator by a two-step
coacervation (TSC) method: (a) chemical structure of SMA alternating copolymer and hydrolysis
polymer, (be) the first step coacervation, (fh) the second step coacervation, and (i) microstruc-
ture of microcapsules produced by TSC method [68]
Self-Healing Technology for Asphalt Pavements
30 μm). Figure 8a
1
,a
2
, shows that microcapsules with a mean size below 10 μm
tend to congregate/attract as a result of the electrostatic attraction of tiny particles.
In Figure 8b
1
,b
2
, microcapsules of 20 μm size have a homogeneously distributed,
preventing from hard agglomeration of the capsules in the bitumen. Figure 8c
1
,c
2
shows dispersion of larger microcapsules (30 μm size) with content of 10% and
30% in bitumen. Figure 8c
2
shows that large micro capsules will occupy more
space, where the capsule content in the bitumen is 30%. This phenomenon may lead
to the interface separation between microcapsules and binder and may result in the
formation of microcracks or other structural defects [67,70]. This could result in
Fig. 7 SEM morphologies of microcapsules containing rejuvenator [67]
Fig. 8 Fluorescence microscope images of morphology of bitumen samples with 10% (a
1
,b
1
,c
1
)
or 30% (a
2
,b
2
,c
2
) microcapsule content and diameters of <10 μm(a
1
,a
2
), 20 μm(b
1
,b
2
)or
30 μm(c
1
,c
2
)[67]
A. Tabakovic
´and E. Schlangen
poor mechanical performance of the asphalt pavement and premature pavement
failure. In conclusion, the optimum content of microcapsules in bitumen must be no
more than 30% of overall bitumen volume within an asphalt mix [67].
More recently Su et al. [64] demonstrated that rejuvenators produced from
recycled waste cooking oil (WCO) can be encapsulated and used in the asphalt
self-healing technology. Capsules with high capsule shell strength of 1.0–1.52 GPa,
core/shell ratio of 1:3 and good thermal stability (melting point 180C) can survive
the harsh asphalt production process. This research demonstrated that
microencapsulated WCO rejuvenator can successfully penetrate and rejuvenate
aged standard bitumen binder of 80/100 pen, and defuse it at low temperature
(0C) (see Table 3).
The microcapsule approach is favourable for asphalt self-healing in that it allows
the rejuvenation of aged binder (i.e. returns it to its original physical and mechan-
ical properties). However, the downside to this approach is that it works only once
(i.e. once the healing material is released from the microcapsule it cannot be
replenished) [34]. Nevertheless, this self-healing process is still in its early devel-
opment stage and its full potential will be demonstrated in coming years. Methods
for introducing a reasonable dose of microcapsules into the asphalt mix to achieve
appropriate dispersion of capsules throughout the asphalt mix and enhancement of
the multiphase self healing process need to be the focus of future research work in
this field.
3 Towards New Generations of Self-Healing Asphalt
Pavements
The key aim of research on self-healing asphalt pavements is to develop asphalt
pavement material that is capable of healing itself without external intervention.
Therefore, the ultimate goal for road designers is to develop an asphalt pavement
material that can mimic nature itself. To achieve this, the self-healing processes
Table 3 Properties of virgin bitumen, aged bitumen and aged bitumen after rejuvenation with
waste cooking oil [64]
Material
property
Original bitumen
(80/100 pen)
Aged bitumen
(40/50 pen)
Rejuvenated aged bitumen with
microWCO (wt%)
24681012
Penetration at
25C (mm)
86 43 52 61 74 82 84 88
Softening point
(C)
46.7 53.5 52.0 51.2 50.3 48.4 46.4 45.0
Viscocity at
135C (mPa s)
325 578 570 520 470 390 333 310
Diffusion of microencapsulated waste cooking oil (microWCO) was carried out at 0C. Penetra-
tion, softening point and viscosity tests were conducted following standard procedures
Self-Healing Technology for Asphalt Pavements
embedded within the asphalt pavement system should be capable of self-
assessment. This would enable the material to assess its structural and material
health and to trigger a response to initiate self-healing where necessary [7].
To develop this new generation of self-healing asphalt pavements, based on
findings of currently available self-healing technologies (presented in Sect. 2), three
specific working areas are identified that need particular effort:
1. Development/design of damage sensing and repair triggering elements. These
elements are incorporated within pavement systems to give the capacity to
trigger the self-healing process (i.e. signalling activation of the healing mecha-
nism). This means that the sensory function should be enhanced and extended
with an active learning functionality that is able to differentiate and detect
damage, interpret the obtained information and trigger/stimulate the healing
action on demand. These sensor elements should ideally be a structural compo-
nent of the pavement system and not deteriorate the general functionality of the
pavement system. Development of a sensory mechanism within the pavement
system will allow healing-on-demand. Such an action could be triggered by a fall
in current/resistivity in the pavement system or by a concentration of stress,
which would initiate the repair action while activating an initiator (healing agent
or heating).
2. Development of multiple self-healing processes. To date, only a limited number
of self-healing mechanisms for asphalt pavements have been developed, such as
induction heating [16,48] and rejuvenator encapsulation [17,64,68]. To explore
the additional potential of self-healing asphalt technology, new self-healing
mechanisms must be developed to respond to a broader range of performance
demands, such as healing/rest time. All three self-healing mechanisms presented
in Sect. 2require at least 4 h of rest time in order for the asphalt pavement to
achieve full recovery. On roads with high traffic flow, this is difficult to achieve.
Perhaps technology can improve the repair times for self-healing asphalt pave-
ments. Another, essential, part of the self-healing mechanism in asphalt pave-
ments is multiphase self-healing. If a self-healing mechanism is “once off”, it is
vulnerable to cracking after the first repair. This ultimately leads to asphalt
pavement failure. This requirement of a self-healing mechanism is directly
linked to the sensory/triggering mechanism. If a healing-on-demand technology
can be achieved within asphalt pavements, the repair action can be re-activated
and thus make self-healing asphalt more efficient.
3. Development of self-healing assessment mechanisms. Such mechanisms are
necessary to achieve self-assessment of the asphalt pavement system and to
quantify the success of the self-healing process. To date there has been only
limited understanding of how to quantify the success of self-healing, mostly
done by measurement of mechanical performance, such as material strength
[63]. This requires in-situ pavement evaluation, or a laboratory material evalu-
ation of test samples obtained from site. Such evaluation requires traffic control
and potential traffic delays, which increases the cost and reduces the benefit of
self-healing asphalt pavements. A mechanism for the autonomous self-
A. Tabakovic
´and E. Schlangen
assessment of asphalt pavement system health and assessment of the self-healing
process should be a focus for future research in this field.
If these developments in the self-healing process are accomplished, then it will
be possible to create a truly smart asphalt pavement system that senses its internal
state and external environment and responds in an appropriate manner to this
information. The primary advantage of moving towards smart/self-healing technol-
ogy is the potential cost benefit of condition-based maintenance strategies and the
prospective long lifespan that can be achieved for asphalt pavement materials
through in-situ health management.
The potential benefits of self-healing asphalt technology in material performance
and environmental and social benefits will undoubtedly stimulate interest in the
wider use of self-healing technology in asphalt pavement design and construction.
However, for self-healing technology to become accepted as the industry standard,
its superiority in the construction and maintenance of asphalt pavements must be
demonstrated from functional, economic and environmental perspectives.
4 Cost and Environmental Benefits of Self-Healing
Technology for Asphalt Pavement Design
To accurately assess the cost reductions that can accrue from self-healing materials,
it would be best to compare the change (increase) in material costs with the change
(decrease) in maintenance costs. Depending on the application, other costs such as
operating costs, disposal costs and environmental costs could be factored in the
cost–benefit analysis. It is expected that periods between road maintenance will
extend when self-healing asphalt is employed, resulting in a decrease in traffic
congestion and associated costs. For example, in the Netherlands the combined
annual savings related to major repairs and traffic jam costs are approximately 65
million for an asphalt lifespan extension of 25%, and over 100 million for a
lifespan extension of 50% for the entire porous asphalt pavement area in the
Netherlands [71]. Even if the price of self-healing asphalt was double that of
standard bitumen, the Netherlands would save approximately 90 million annually
by investing in self-healing asphalt with a 50% extended lifespan, compared with
traditional porous asphalt. The Netherlands is a fairly small country by European
standards and represents only 3% of total European asphalt production, and only
one third of this asphalt (1%) is used in surface layers [52]. If we extend the
potential savings in the Netherlands to the EU as a whole, the potential savings
could total 9 billion [52].
These figures outline the clear financial benefits to be accrued from self-healing
technology in asphalt pavement design. However, the full potential benefits of self-
healing technology in asphalt pavement design can only be understood when the
full life cycle costs of asphalt pavements are known (financial, environmental and
societal). Butt et al. [72] studied the effect of self-healing on the lifetime, energy
Self-Healing Technology for Asphalt Pavements
and environment of asphalt pavements. Using a life cycle analysis (LCA) frame-
work, performed in conjunction with a numerical model that simulates the self-
healing capacity of asphalt pavements [73], they determined that self-healing
asphalt pavements increased the lifetime of the pavement by 10% (from 20 years
to 22 years) compared with asphalt pavements without any self-healing capacity.
This increase in lifetime would result in a reduction in energy consumption of 3%
(22 GJ) and CO
2
emissions of 3% (1.5 Tonne). If the increased lifetime of an
asphalt pavement is projected to 100% (from 20 years to 40 years, based on the
assumption that self-healing technology can double asphalt pavement lifespan), the
benefits in terms of reduced cost and reductions in energy consumption and CO
2
emission would increase accordingly.
A greater insight into the potential of self-healing technology for the asphalt
pavement industry can be achieved with full scale in-situ implementation of self-
healing technology in asphalt pavement design, as in the A58 road in the Nether-
lands [52]. However, the clear benefits of self-healing asphalt materials, in the form
of an extended lifespan of the asphalt pavement and reduced maintenance costs,
only become apparent over time. Steyn explains [65]: “The important point to take
from reality is that for any innovation to provide real benefit in the pavement
engineering field, there has to be a real positive benefit/cost ratio.”
It is unlikely that decisions regarding implementation of self-healing technology
in asphalt pavement design will be taken at a local level because of the high initial
costs and long timeframe for savings. It requires regional (European) leadership,
informed by a sound evidence base (i.e. research), to convince all that these
technological advances are worth considering at a national and European level.
Initial research results are positive, as shown in Sect. 2, and there is optimism that
this technology has strong potential in the design of asphalt pavements.
5 Concluding Remarks
Roads . . . [are] the most ancient of human monuments, surpassing by many tens of
centuries the oldest thing of stone that man has reared to mark his passing. The tread of
time. . . has beaten only to a more enduring hardness the pathways that have been made
throughout the world [74].
Throughout the centuries, a civilisations prosperity and economic development
was measured and ensured by the quality of its road network, which was used for
economic, social and military purposes. In its latest report Road Infrastructure – the
backbone of transport system [75], the European Commission states: “Transport
infrastructure influences both economic growth and social cohesion. A region
cannot be competitive without an efficient transport network.”
Today, roads designs are sophisticated engineering creations. Despite this, the
materials used in asphalt mixes have remained largely unchanged for the past
100 years. The main ingredient of a modern road is the bitumen. It is a
co-product of crude oil, whose production is in decline [76], meaning that the
A. Tabakovic
´and E. Schlangen
financial and environmental costs of bitumen are on the rise [76,77], which will
result in an increase in the cost of road/asphalt pavements. Unless investment levels
keep apace of increased costs, road networks of poorer standard could result.
Incorporating self-healing technology into asphalt pavement design presents a
solution for some of the difficulties facing asphalt. Currently available self-healing
road technologies are paving the way for the evolution of road design. Existing
technologies have demonstrated their potential in repairing distressed asphalt
pavements. They offer great opportunities for increased durability and reliability,
reduced maintenance and lower overall cost of asphalt pavements. This includes a
reduction in the material resources needed, because the usual over-design of
materials is no longer required. The repair of an asphalt pavement is addressed in
situ by its internal self-healing system at the very position of first appearance of
damage, eliminating the need for classical in-situ maintenance processes.
However, the key objective of self-healing technology for asphalt pavement
design is the development of a truly smart asphalt pavement system, capable of self-
assessment and automatic response. Despite the progress made in the development
of self-healing asphalt technology, further work is required to achieve truly smart
asphalt pavements. Future work needs to focus on:
1. Damage sensing and repair triggering elements
2. Development of multiple self-healing processes
3. Development of self-healing assessment mechanisms
The development of such areas of self-healing technology for asphalt pavements
will truly revolutionize asphalt pavement design. This will also lead to another
evolutionary step in road construction and design and bring the idea of self-healing
roads from science fiction to reality.
Acknowledgements This research has been conducted under the Marie Curie IEF research
funding, research project Self-Healing Asphalt for Road Pavements (SHARP), project number
622863.
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Many road pavements using asphalt mixtures in Indonesia are found to be not durable, especially heavy traffic roads. Even road damage often occurs early before the service life is reached. Many efforts have been made to improve the strength and durability of the mixture, but have not been effectively. For example, the use of fillers can increase the strength and durability of the mixture significantly, but its performance at low temperatures has an impact on the risk of hardening and cracking. The research objective is to propose the idea of implementing nanotechnology for palm oil waste materials to increase the durability of road pavement materials based on the research roadmap developed. The method used is through five approaches, namely: (i) review of research results on the durability of road pavement materials, (ii) review of research results on the implementation of nanotechnology in road pavement materials, (iii) research gaps, (iv) research ideas, and, (v) proposed research roadmap. A research roadmap for the implementation of nanotechnology to improve the durability of road pavement materials has been prepared. The substance of the research roadmap proposes three ideas, namely maximizing the function of nanomaterials as: (i) anti-aging agent, (ii) protecting water infiltration into the body of the asphalt mixture, and (iii) bonding agent between asphalt-aggregate.
... Various investigations aimed at its use have allowed its use in civil construction, especially light ash in the manufacture of pozzolanic Portland cement [43][44][45]. This procedure began in the mid-1930s, when the ashes began to be available in significant quantities, occupying a large space in industries and requiring specialized infrastructure to restrict environmental legislation [46][47][48] The use of alternative materials aims to improve the properties of asphalt mixtures to reduce the defects to which a pavement is subjected [49][50][51]. The permanent deformation in the wheel track of the support layer is one of the most important defects, since this type of defect, in addition to providing an accelerated degradation of the pavement structure, considerably reduces the comfort and safety of the user, thus increasing operating costs [52,53]. ...
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The current study presents the evaluation of the mechanical behavior of an asphalt mixture using the alternative aggregate boiler coke ash, an element that originates in nickel processing. Hereby, we have focused the research on the runways for military purposes, which marks a great difference to the existing commercial runways in the Western Brazilian Amazon. This area suffers extreme heat, with temperatures oscillating up to 80 ◦C on the corresponding asphalts. This leads to deformations that are the main aim of the present investigation and the main consideration of fatigue damage. The main property of the alternative aggregate, whose granulometry composes the fine elements of the asphalt mix, is the pozzolanity that acts as a cement in the putty of the mix. Based on our experimental approaches, there is a significant improvement in the results of the tests standardized by DNIT, ABNT and DIRENG, allowing the technical and economic evaluation of the used mixture. Another fundamental aspect is the reduction of the volume of waste disposed of in nickel processing plants in Brazil.
... These techniques have been used to control damage levels, reduce viscosity, and accelerate the flow, resulting in a better recovery capability of the asphalt (Ayar et al., 2016). In the encapsulation technique, rejuvenators are stored in capsules or hollow fibers and then placed in the bulk of asphalt (Shu et al., 2019;Tabaković and Schlangen, 2015). If the asphalt cracks, the crack breaks capsules, releasing the rejuvenator into the crack. ...
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Bio-based and nature-inspired solutions have been investigated recently to develop sustainable, resilient, and durable construction including but not limited to roadway infrastructures. This paper reviews state-of-the-art studies on self-healing, self-cleaning and self-rejuvenating asphalt, and concrete construction. This review draws three conclusions. (1) Self-healing construction materials have the potential to significantly extend the service life of construction elements. Urban and industrial wastes such as food waste, biomass, metals have been used to create self-healing construction materials that are more environmentally friendly. (2) Self-cleaning construction materials not only remove pollution by repelling water on their superhydrophobic surface, but also cut building and infrastructure maintenance costs, while improving cities’ air quality by degrading pollutants such as NOx. Pavement engineers have exploited self-cleaning characteristic to facilitate the de-icing of pavements and lengthening the service life of pavements. (3) Self-rejuvenating materials including bio-oils can revitalize materials and delay aging; bio-oils can also be used to make bio-binders, thereby reducing the need for petroleum-based binders. The optimum concentration of bio-oil for asphalt modification depends on the chemical structure of oils. Still, regardless of dosage, self-rejuvenating binders improve asphalt workability and performance at low temperatures and increase the resistance of the asphalt mix to fatigue and cracking. This review also identified critical research gaps, including (1) the lack of a reliable, unified, and standard method to accurately measure construction materials’ self-healing, self-cleaning and self-rejuvenating properties; (2) the lack of long-term field performance data to conduct comprehensive life cycle assessment and life cycle analysis; (3) the lack of accurate technoeconomic analysis to facilitate market entry of abovementioned solutions. Addressing these gaps and determining contribution of nature-inspired and bio-based technologies to a carbon neutral economy along with issuing carbon certificates can facilitate the widespread application of these technologies while promoting resource conservation and sustainability.
... Several authors have proposed to extend the service life of pavements by recovering their mechanical properties after a process called "self-healing" [12][13][14][15]. This process consists of sealing the micro-cracks and fissures in cracked asphalt pavements through a binder flowing through the crack and the application of an energy source (heat or electricity) [16][17][18] or the targeted incorporation of rejuvenating additives [19]. ...
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The environmental and service conditions exposed to asphalt pavements affect their rheological and performance properties, making them brittle material susceptible to cracking. One way to partially re-establish the mechanical properties of the cracked pavements is through binder runoff through the cracks in a capillary healing process. It is possible to incorporate conductive materials (electrical or thermal) in asphalt mixtures as additives or partial substitutes for the natural aggregate to maximize the efficiency of the self-healing process and induce a pseudo-Newtonian flow behavior in the binder. The present study evaluated the self-healing performance by UV radiation of asphalt mixtures incorporating copper slag (CS) in different sizes (2.00 mm, 0.25 mm, < 0.063 mm) as a conductive material (heat diffuser) in partial replacement of the natural aggregate. Likewise, the degree of ageing caused by the self-healing process was evaluated through the rheological characterization of the binder extracted after healing. All the CS mixes obtained self-healing rates in mechanical strength higher than 40%, in particular, the 2.00 mm CS mixes with self-healing rates close to 60%. The optimum self-healing temperatures for mixtures with CS sizes with higher thermal inertia are between 95°C and 105°C corresponding to 18000 s and 30000 s of healing, while for mixtures with CS sizes with lower thermal inertia the optimum self-healing temperatures are between 105° and110°C corresponding to self-healing times of 18000 s to 25000 s. It was not possible to determine an accelerated ageing effect in the asphalt mixtures due to the self-healing processes by UV radiation.
Chapter
Self-healing materials are those that have a potential to repair mechanical damages and cracks, without the need for human interference and restore their original set of properties. Incorporating self healing property of bitumen into road design process helps to extend the life of asphalt pavements. Traffic load repetition and ageing causes microcracks in the pavement which gradually leads to pavement failure. This necessitates frequent pavement maintenance procedures. Self healing property of bitumen can automatically repair damage and recover strength. By reducing premature ageing of asphalt pavements, self healing asphalt can reduce amount of natural resources used in road maintenance and reduce environmental pollution by decreasing CO2 emission in maintenance process. Addition of rejuvenator to asphalt material is the widely used method for achieving self healing of bitumen. A comprehensive understanding of the self healing properties of bitumen such as its ability to heal, factors affecting healing property can considerably help in the development of durable and sustainable asphalt pavements. ABAQUS is 3D finite element analysis software used for predicting mechanical behavior and pavement performance subjected to various traffic factors. In this study, modeling of bituminous material modified with rejuvenator is done using ABAQUS software in which model dimensions, element types and meshing strategies are taken by successive trial and error to achieve desired accuracy and convergence of the study. Analysis of self healing asphalt modified with microcapsule containing rejuvenator shows that there is a reduction of stress around the microcapsule region.
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The global road network spans 64.3million km and is of huge significance for the social and economic development. The level of investment in road construction and maintenance is high, e.g. EU €44billion/year (2019), China €614.7billion/year (2019) and US €94billion/year (2019). Despite the level of investment, there has been minimal investment in the development of new asphalt technologies, particularly when compared with R&D investment in other industries, such as the automotive industry. Despite the limited investment, there have been some innovations in asphalt technology. For the past 20 years, researchers have developed bio-inspired asphalt technology, self-healing and bio-binders and have applied them to asphalt pavements. This research has emerged as a response to global warming and the need to reduce both carbon emissions and reliance on oil in asphalt technology. This paper charts the development of two bio-inspired technologies and considers their significance in relation to the need to reduce carbon emissions and oil dependence (in line with the UN strategic goals, specifically: SDG 9, 11 and 12). This paper considers the potential benefits of bio-inspired technologies and outlines the current barriers to their further development. This paper aims to begin a conversation with stakeholders on how to speed up the acceptance of bio-inspired asphalt technologies and their adoption in road design, construction and maintenance. Or is it the case that we have reached the end of the road for bio-inspired road construction materials?
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By the repetition of damaging-healing cycles, the healing potential of asphalt mixtures declines, and some parameters, such as the strain level, damage level, healing method, and temperature can also affect the healing performance of asphalt mixtures. This study aimed to investigate the effect of Waste Steel Shaving (WSS) additive, healing method (with induction heating or at room temperature), strain amplitude, and damage level on the healing performance of asphalt mixtures. The semi-circular bending (SCB) fracture test was used to assess the impact of WSS, and the four-point bending beam fatigue test, which was carried out at various damaging-healing cycles, was used to explore the impact of strain level, damage level, and healing technique. The findings revealed that although adding WSS improved the heating rate, it had a detrimental impact on the heating distribution in the mixture, causing the specimen to heat up unevenly and to overheat in certain areas. The initial healing cycle's healing performance was improved by raising the strain level. However, by the repetition of damaging-healing cycles, the positive effect of the strain level decreased and in some cases this parameter affected the healing performance adversely. Increasing the damage level reduced the healing potential, and the effect of damage level on the healing potential was greater than heating during the rest time. Therefore, for a better healing performance, it is recommended to control the damage level when the heating equipment is not available.
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A historical overview of the South African mechanistic pavement design method, from its development in the early 1970s to the present, is presented. Material characterization, structural analysis, and pavement life prediction are discussed, and stiffness values are suggested for a range of materials in the absence of measured values. The modes of failure for these material types include the fatigue of asphalt material, deformation of granular material, crushing and effective fatigue of lightly cemented material, and deformation of selected and subgrade material. The critical parameters and transfer functions for these material types and modes of failure are discussed and included in the pavement life prediction process.
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Although repeated traffic loading causes damage to accumulate in asphalt pavements, the damage heals during rest periods (time between traffic loadings). Consequently, this healing enhances the fatigue life of the pavement. A method was developed to determine the healing rate of asphalt mixtures in terms of recovered dissipated creep strain energy (DCSE) per unit time. The healing properties of four different asphalt mixtures were evaluated with this approach. The test procedure consists of a repeated loading test and periodic resilient modulus tests. A normalized healing rate in terms of DCSE/DCSEapplied was defined to evaluate the healing properties independent of the amount of damage incurred in the mixture. From the test results, it was determined that the healing rates of the asphalt mixtures tested increased dramatically above 10°C and were more affected by the aggregate structural characteristics (i.e., aggregate interlock, film thickness, voids in mineral aggregate) of the mixtures than by polymer modification. Although styrene-butadiene-styrene polymer modification reduced the rate of damage accumulation, it had no effect on the healing rate of the mixtures tested.
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Self-healing materials are man-made materials which have the built-in capability to repair damage. Failure in materials is often caused by the occurrence of small microcracks throughout the material. In self-healing materials phenomena are triggered to counteract these microcracks. These processes are ideally triggered by the occurrence of damage itself. Thus far, the self-healing capacity of cement-based materials has been considered as something "extra". This could be called passive self-healing, since it was not a designed feature of the material, but an inherent property of it. Centuries-old buildings have been said to have survived these centuries because of the inherent self-healing capacity of the binders used for cementing building blocks together. In this State-of-the-Art Report a closer look is taken at self-healing phenomena in cement-based materials. It is shown what options are available to design for this effect rather than have it occur as a "coincidental extra".
Chapter
This state of the art report discusses self-healing concepts in cement based materials. Cement based materials have a self-healing property by nature. This autogenic self-healing ability of these materials is known for years and also investigated and proven my many authors, as described in the previous chapters. The autonomic self-healing ability, in which the material is designed to be selfhealing, is rather new.
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Self-healing polymers have emerged as a new class of smart materials that have the capability of repairing themselves after damage without the need for detection or repair by manual intervention. Self-healing coating systems are being developed for extended corrosion protection of steel substrates, based on the latest advancements in the area. A self-healing based on this technology will extend the lifetime of the corrosion protection system and reduce the cost of labor associated with corrective and preventive maintenance. One such self-healing polymer system is based on the Grubbs' catalyst-initiated ring opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD) in the site of damage, healing the damage and restoring structural continuity. Polydimethylsiloxane (PDMS)-based chemistries of such self-healing polymer systems are being applied in the development of self-healing coatings for heavy-duty industrial and marine applications.
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The addition of polymers to bitumen allows the modification of certain physical properties, such as softening point, brittleness and ductility, of the bitumen. Polymer modified bitumen: Properties and characterisation provides a valuable and in-depth coverage of the science and technology of polymer modified bitumen. After an initial introduction to bitumen and polymer modified bitumen, the book is divided into two parts. Chapters in part one focus on the preparation and properties of a range of polymer modified bitumen, including polymer bitumen emulsions, modification of bitumen with poly (urethanes), waste rubber and plastic and polypropylene fibres. Part two addresses the characterisation and properties of polymer modified bitumen. Chapter topics covered include rheology, simulated and actual long term ageing studies; the solubility of bituminous binders in fuels and the use of Fourier transform infrared spectroscopy to study ageing/oxidation of polymer modified bitumen. Polymer modified bitumen is an essential reference for scientists and engineers, from both academia and the civil engineering and transport industries, interested in the properties and characterisation of polymer modified bitumen.
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The effects of artificial heating on the temperature and rejuvenation of aged asphalt during hot in-place recycling of asphalt pavements have been investigated through finite element modeling and experimental study. The major conclusions are: the temperature rise that results from heating dissipates very quickly along the depth of the pavement (1.6°C to 2.8°C per mm), a very high surface temperature does not ensure a desirable temperature of plus 100°C below the surface, and a more uniform temperature profile (along the depth) is achieved by using hot air, compared to radiation only; for the radiation levels that are desirable with respect to maximum surface temperatures of 180°C, and for conventional heating time periods, an effective increase in temperature in the pavement to plus 100°C can only be possible within the first 30 to 50 mm of the surface; the extent of rejuvenation depends on the temperature, the time of mixing, as well as the viscosity of the rejuvenator; there exists a gradient of rejuvenation across the thickness of the film of the Reclaimed Asphalt Pavement (RAP) asphalt binder. For example, for a mixing time of 90s, for a rejuvenator with a viscosity of 1/4th the viscosity of RAP binder, the extent of rejuvenation ranges from a maximum of 35 percent at the surface of the RAP binder to zero percent at 1 micrometer, for mixing at 60°C; 70 percent at the surface to zero percent at 8 micrometer, for mixing at 150°C. The percentage of rejuvenation is higher for a longer mixing time and a rejuvenator with lower viscosity. The selection of the appropriate recycling agent or rejuvenator, and for hot recycling should be made on the basis of consideration of temperature and time of mixing.