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Assessment of cracks detection in pavement by a
distributed ber optic sensing technology
Xavier Chapeleau, Juliette Blanc, Pierre Hornych, Jean-Luc Gautier, Jean
Carroget
To cite this version:
Xavier Chapeleau, Juliette Blanc, Pierre Hornych, Jean-Luc Gautier, Jean Carroget. Assessment of
cracks detection in pavement by a distributed ber optic sensing technology. Journal of Civil Structural
Health Monitoring, Springer, 2017, 7 (4), pp.459-470. �10.1007/s13349-017-0236-5�. �hal-01582525�
Noname manuscript No.
(will be inserted by the editor )
Assessment of cracks detection in pavement by a
distributed fiber optic sensing technology
X. Chapeleau · J. Blanc · P. Hornych ·
J-L. Gautier · J. Carroget
Received: date / Accepted: date
Abstract This paper presents the feasibility of damage detection in asphalt
pavements by emb ed d ed fiber optics as a new non-destructive inspection tech-
nique. The distributed fiber optic sensing technology based on the Rayleigh
scattering was used in this study. The main advantage of this te chnique is that
it allows to measu r e strains over a long lengt h of fiber optic with a high spatial
resolution, less than 1 cm. By comparing strain profiles measured at different
times, an attempt was made to link strain changes with the appearance of
damage (cracking) in the pavement. This non-destructive method was evalu-
ated on accelerated pavement testing facility, in a bituminous pavement. In our
experimentation, t h e optical fibers were placed ne ar the bottom of the asphalt
layer. The application of 728 000 heavy vehicle loads (65 kN dual wheel loads)
was simulated in the experiment. Optical fiber measur em ents were made at
regular intervals and surface cracking of the pavement was surveyed. After
some traffic, a significant increase of strains was detected by the optical fibers
at different point s in the pavement structure, before any damage was visible.
Later, cracking developed in the zone s where the strain p rofi l es were modified,
IFSTTAR, COSYS, SII / Inria, I4S
Route de la Bouaye, 44340 Bouguenais, France
E-mail: xavier.chap el eau@ if sttar. fr
IFSTTAR, MAST, LAMES
Route de Bouaye, 44340 Bouguenais, France
E-mail: juliette.blanc@ifsttar.fr
IFSTTAR, MAST, LAMES
Route de Bouaye, 44340 Bouguenais, France
E-mail: pierre.hornych@ifsttar.fr
COLAS
Campus Scientifique et Technique, 4 rue Jean Mermoz, 78772 Magny les Hameaux, France
COLAS
Campus Scientifique et Technique, 4 rue Jean Mermoz, 78772 Magny les Hameaux, France
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2 X. Chapeleau et al.
thus indicating a clear relationship between the increased strains and crack ini-
tiation. These first tests demonstrate that distributed fiber optic sensors based
on Rayleigh scatte r in g can be used to detect crack initiation and propagation
in pavements, by monitoring strain profiles in the bituminous layers.
Keywords Distributed fiber optic sensor · Asphalt concrete pavement ·
Cracks · Non-destructive inspection
1 Introduction
Due to the constant increase of traffic, roads suffer from cracks, rutti ng and
surface wear more frequently. In t he same time, users expect roads th at are
more and more safe and reliable in the meaning of permanently open. With
increasing budget constraints, road managers have the difficult task to main-
tain roads in good condition. Pavement management systems can help them to
make decisi on s for the planning of maintenance operations. One essential com-
ponent of these systems is pavement monitoring, which includes visual surface
inspection, traffic and weather obs er vations and measurements on t h e pave-
ment. Usually, this monitoring is made by an operator from a moving vehicle,
or using automated sensors mounted to a vehicle. For more accurate diagnos-
tics, sensors can be embedded in the pavement. Since several past decades, a
wide variety of se ns ors was developed to measure strain and stress distributions
in pavement structures. The data obtained by different in-situ measurements
(stress, strain , displacement etc) are essential for a better understanding of
pavement behaviour and identification of the main failure mechanism which
is complex to determine due to pavement variability, temperature sensitive-
ness and viscoelasticity of pavement materials. By combining thes e data with
numerical models [1], [2] pavement damage can be predicted more reliably.
Moreover, embedded sensors offer an additional advantage. They make it pos-
sible to detect damage earlier than visual inspection. Since it is less expensive
to keep a road in good condition than to repair it once it has deteri orat ed , the
early detection of damage by in-situ sensors allows road managers to optimize
their maintenance plan and to save money.
Pavements are multilayer structures, consisting of layers of granular and
bituminous materials. The sensors used for pavement instrumentation must
be compatible with the heterogeneous natur e, and mechanical properties of
pavement materials. First, th e sensors should be as small as possible so that
they are not too intrusive in the bit u mi n ous layers. Secondly for strain mea-
surements, stiffness of the sensors has to match that of the asphalt mi x t ur e in
order to measu re correctly the mechanical propertie s of the pavement. More-
over, the embedded sensors must withstand the str es se s experienced during
the pavement con st r uc t i on process (high temperature and compression). Af-
ter that, if a long term monitoring is considered, the sen sor should be resistant
to corrosion and to thermo-mechanical fatigue conditions.
Differents kinds of sensors can be used to monitor pavements. They can be
classified in two categories: electri c al sensors and optical fiber sensors. Classi-
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Title Suppressed Due to Excessive Length 3
cal el ec t ri c al sensors used for paveme nt instr um entation inc l ud e displacement
sensors, used to measure vertical displacements, strain gages, used to measure
horizontal strains in bound pavement layers, temperature probes, and pres-
sure cells, used to mesure vertical stresses or pressures in unbound pavement
layers. Typical examples of pavement instrumentation used on the IFSTTAR
(The French institute of science and technology for transport, development
and networks) accelerated pavement testing facility can be fou nd in [3]. Con-
trary to electrical sen sor s, fiber optic sensor s are technologies which are a little
less mature but promising. In civil engineering structures, there is a growing
interest in fiber optic sensors because they offer attractive benefit s when com-
pared wi t h traditional sensors [4], [5]. Their main advantages are small size,
electrically passive operation, electromagnetic immunity, flexibility, corrosion
resistance, etc. Moreover, fiber optic se ns or s can be used to perform local or
distributed measurements with precision in a wide range of strain and tem-
perature. Several fib e r optic sensor technologies have already been used for
experimental investigation of pavement behavior [6], [7] and pavement moni-
toring [8], [9] with positive results. The most tested is certainly the fiber Bragg
grating technology. A fiber Bragg grating is a small portion of an optical fiber
several millimeters or centimeters long in w hi ch a diffraction grating is written
by UV exposure. The optical property of t h is grating is to reflect a narrow opti-
cal b and (around a center wavelength called Bragg wavelength) of the incident
spectrum. Fiber Bragg gratings have the intrinsic quality to be very sensi ti ve
to thermal and mechanical stimuli. The Bragg wavelength is proportional to
the temperature and/or strain variation. Since, this sensor is very brittle, it
needs to be packaged. Many tests of packagings with steel [10], polymer [11],
geotextile [12] and so on were reported in literature. It is worth to mention
that optical fiber sensors based on Fabry-Perot interferometry technologies
were also tested [13] successfull y. Since fiber optic sensor technologies (Fiber
Bragg grating and Fabry-Perot) allow to perform dynamic measurements at
a sampling rate of at least of 0.5-1kHz (for the standard interrogators), th ey
are investigated particularly for the development of traffic cl as si fic at i on and
weigh-in-motion systems [14], [15].
Fiber Bragg grating and Fabry-Perot technologies [16] deliver a strain mea-
surement like an electrical strai n gage. It is a l ocal measurement. Despite their
high sensitivity and accuracy, they are not suitable for detection of cracks or
damage. Indeed, due to their relat i vely small dimensions compared to those of
a pavement, a crack can be detected only if it propagates in the vicinity of the
sensor. A much more promi si n g technology for damage detection is distributed
sensing technique because in one acquisition, a lot of measurement points can
be obtained along a long length of fiber optic which can be placed longitu-
dinally or transversely in the pavement. Two distributed fiber opt i c sen si ng
techniques bas ed on the Brillouin scattering and the Rayl e igh scaterring are
available [17]. These two phenomenons result from the interactions of the light
with the optical medium (the core of optical fiber). Rayleigh scattering is char-
acterized as a quasi-elastic scattering where the scattered photons maintain
the same freq u en cy as the incident light. This penomenon is due to impuri-
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4 X. Chapeleau et al.
ties or density fluctuations of the refractive index on a scale smaller than the
optical wavelength. Brillouin scattering is due to the interaction of the light
with acoustic phonons. It is an inelastic phenomenon that lead to two spec-
tral lines at Stokes and Anti-Stokes frequencies. These two distributed sensing
techniques are sensit i ve to strain and temperature variations and the main
difference between them is the spatial resolution. The Rayleigh scattering has
the highest spatial resolution. Typically, it is 1 cm for Rayleigh and 0.5-1 m
(25 cm for the best equipment) for Brillouin scatterings. This high spatial res-
olution of measurements is an asset for crack detection. Another parameter
to take into account is the duration of measurement, less than 1 minute for
the standard interrogators based on Rayleigh scattering and up to 10-15 min-
utes (depending of th e length range and resolution chosen) for those based on
Brillouin scattering. For both distributed sensing techniques and with stan-
dard interrogators, no dynamic measurements can be obtained. Nevertheless,
it should be not a limitation for our study since cracking is a long term process
of damage.
The study pr es ented in this paper aims at assessing the use of the dis-
tributed fibe r optic sensing technology based on Rayleigh scattering combined
with the use of embedded fiber optics in pavements for early d et ec t ion and
localization of cracks. To be as close as possible to real condi t ion s of pavement
construction and traffic, this study was realized on a full scale demonstra-
tor (circular pavement) and tested with t h e IFSTTAR accelerated pavement
testing facility. The implementation of the fiber optics in the pavement layers
represented a real challenge, due to high temperature and stress levels applied
during construction.
The first part of the paper presents the accelerated pavement testing fa-
cility and the implementation of the distribu t ed sensing technology based on
Rayleigh scattering which is used for cracks detection. Then, the pavement
structures and the experimental program are described. A total of 728000
load cycles have been applied in the experiment, until a significant level of
damage of the pavements was at t ai ne d. In t he l ast part of the paper, resu lt s
of the monitoring of the pavements with the fib e r op t i c sensors are presented
and discussed.
2 Description of the test and measurement facilities
2.1 Accelerated pavement testing facility
The IFSTTAR accelerated pavement testing facility (Figure 1), in Nantes
(France), is an outdoor circular carou se l dedicated to full-scale pavement ex-
periments, carried out with public and /or private partners. The carousel con-
sists of a central tower and four arms (each 20 m long) equipped with wheels,
running on a circular test track. The experimental circular pavement has a
mean radius of 17.5 m and a width of 6 m, and thus a total length of ap-
proximately 110 m. The position of the loading modules can be ad ju st e d for
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