Fibre optic sensors and smart composites for

Fibre-optic sensors and smart composites for
concrete applications
K.-T. LauÃ
Hong Kong Polytechnic University
Many extensive researches in the area of utilising fibre-reinforced plastic (FRP) materials for retrofitting and
repairing existing damaged concrete structures have increasingly been made in recent years. The FRP can improve
the overall flexural and compressive properties of the structures by using externally bonded FRP sheets. The
conventional non-destructive inspection technologies such as strain gauge, and acoustic emission become inap-
propriate for structures after being repaired by externally bonded FRP materials. Optical fibre sensors have
attracted considerable interest recently as non-destructive structural health monitoring devices for infrastructure
elements. This paper gives a brief discussion of the principles of the fibre-optic technology for concrete structure
assessment and its potential use in future engineering applications. Due to the increasing use of the FRP in most
civil infrastructure elements, the conceptual idea of ‘smart composites’, which can be used as reinforcements as
well as real-time structural health monitoring devices for concrete structures, is also discussed.
Introduction
In recent years, the demand for the development of
new materials to strengthen, upgrade and retrofit exist-
ing aged and deteriorated concrete structures has in-
creased rapidly. The continuing deterioration and
functional deficiency of existing civil infrastructure
elements represents one of the most significance chal-
lenges facing the world’s construction and civil en-
gineers.
1,2
Deficiencies in existing concrete structures
caused by initial flawed design due to insufficient de-
tailing at the time of construction, aggressive chemical
attacks and ageing of structural elements enhance an
urgent need of finding an effective means to improve
the performance of these structures without additionally
increasing the overall weight, maintenance cost and
time. In the last 50 years, a large number of civil
concrete structures have been built; many of these
structures, particularly in off-shore regions
3
have now
deteriorated and require repair in a short period of time.
Moreover, the increase of traffic volume and population
in many developing countries is causing the demand to
upgrade existing concrete structures to increase. The
damage of reinforced concrete (RC) structures through
reinforcement corrosion and residual capacity are the
most important issues that concern engineers.
4
These
problems occur not only in constructed concrete struc-
tures but also in structures strengthened by externally-
bonded steel reinforcements.
In the past, the external steel plate bonding method
has been used to improve strength in the tensile region
of concrete structures with an epoxy adhesive and has
proved to be successful over a period of 20 years.
5
However, the use of steel reinforced plates and rebars
has its disadvantages including high corrosion rates,
which could adversely affect the bond strength and
cause surface spalling of the concrete, due to volu-
metric change in the corroded steel reinforcements.
Since the early 1980s, fibre-reinforced plastic
(FRP) materials have been used as a replacement for
conventional steel materials for concrete strengthening
applications. In recent years, the interest in utilising
FRP materials in the civil concrete industry in forms of
rods, plates, grid and jacket has grown increasingly.
6,7
When an FRP plate with high tensile strength proper-
ties bonds on the concrete surface, it can strengthen the
structure with minimum changes to its weight and
dimensions. FRP offers substantial improvement in
solving many practical problems that conventional
Magazine of Concrete Research, 2003, 55, No. 1, February, 19–34
19
0024-9831 # 2003 Thomas Telford Ltd
Ã Department of Mechanical Engineering, The Hong Kong Polytech-
nic University, Hung Hom, Kowloon, Hong Kong.
(MCR 950) Paper received 4 June 2001; last revised 8 October 2001;
accepted 12 February 2002

materials cannot solve to provide a satisfactory service
life of the structure. Unlike the conventional steel ma-
terials, FRP is corrosion resistant. The beneficial
characteristics of using the FRP in concrete construc-
tion include its high strength-to-weight ratio, low la-
bour requirement, ease of application, reduced traffic
interruption during repair, cost reductions in both trans-
portation and in situ maintenance for a long-term strat-
egy. Its high damping characteristic also attracts more
structural engineers to use these materials for seismic
retrofitting.
8–10
Due to the increasing use of FRP-plate bonding
techniques in strengthening civil concrete structures,
the interest in finding a suitable means of monitoring
the structural health conditions of these strengthened
structures has therefore increased substantially. Since
strengthened structures are covered by the FRP plates,
the mechanical properties of the concrete may not be
measured or detected easily through conventional non-
destructive evaluation (NDE) methods, such as strain
measurements using surface mounted strain gauges or
extensometers, radiography, thermography and acoustic
emission methods, particularly in areas with micro-
cracks and debonds underneath the externally-bonded
plate. Besides, these structural inspection technologies,
in certain cases, require special surface preparations or
a high degree of flatness in the concrete surface.
These requirements may be hard to achieve, particu-
larly for an area that is exposed to a harsh environ-
ment.
During the 1990s, a multi-disciplinary field of engi-
neering known as ‘Smart Structures’ has developed as
one of the most important research topics in the field.
The structure is formed by a marriage of engineering
materials with structurally-integrated sensor systems.
The system is capable of assessing damage and warn-
ing of impending weakness in the structural integrity of
the structure. Fibre-optic sensor technology is a most
attractive device currently used in the aerospace and
aircraft industry for on-line monitoring of large-scale
FRP structures. The development of distributed fibre-
optic sensors, which provides information on a large
number of continuously distribution parameters such as
strain and temperature is of great interest in most
engineering applications.
11,12
The sensors are em-
bedded into a structure to form a novel self-strain-
monitoring system, i.e. the system can self-detect its
health status and send response signals to operators
during any marginal situation during service. The em-
bedding sensor, due to its extremely small physical
size, can provide the information to a high accuracy
and resolution without influencing the dimension and
mechanical properties of the structure. Fibre-optic sen-
sors present a number of advantages over the conven-
tional strain measuring devices: (a) providing an
absolute measurement that is sensitive to fluctuation in
irradiance of the illuminating source; (b) enabling the
measurement of the strain in different locations in only
one single optical fibre by using multiplexing techni-
ques; (c) having a low manufacturing cost for mass
production; and (d) its ability to be embedded inside a
structure without influencing the mechanical properties
of the host material.
This paper presents a brief review of the principles
of the fibre-optic strain sensing technologies including
localised, multiplexed and distributed array systems for
concrete structures. The potential applications of fibre-
optic sensors and FRP materials with integrated optical
fibre sensors, known as ‘Smart Composites’ for civil
infrastructure elements are also discussed.
Optical fibre sensors for concrete
applications
A new development of ‘Smart materials and struc-
tures’ was driven by a strong demand for high perform-
ance over recent years. A system integrated into
structures and being able to monitor its host’s physical
and mechanical properties such as temperature and
strain, during service is appreciated as a ‘Smart struc-
tural health monitoring system’. The term smart materi-
al and structure is widely used to describe the unique
marriage of material and structural engineering by
using fibre-optic sensors and actuation control tech-
nology.
13
The smart structure is constructed of materi-
als that can continuously monitor their own mechanical
and physical properties, and thereby, be capable of
assessing damage and warning of impending weakness
in structural integrity. This design concept results in
improved safety and economic concerns regarding the
weight saving and avoidance of over-designing of the
structure in the long run. In Fig. 1, a schematic illustra-
tion of the structure’s possibilities created by the con-
fluence of the four disciplines is shown. In the figure, a
structure invested with actuating, sensing and neutral
networking systems to form a new class of adaptive
structures is shown. A structure with integrated sensor
or actuator systems is able to provide a self-structural
health monitoring or actuating response, respectively. If
both systems are integrated together into a structure,
the sensor and actuators can act as nervous and muscu-
lar systems, like a human body, to sense the conditions
such as mechanical strain and temperature of the struc-
ture (a smart structure) and to provide control of such
changes of stiffness, shape and vibration mode (a con-
trolled structure). The combination of these two sys-
tems into one structure is called a ‘Smart adaptive
structure’. This structure with a built-in neural net-
working system, like a brain, is then able to self-
evaluate the conditions, which are based on changes of
structural parameters, thermal conditions and ambient
environments to give an appropriate mechanical adjust-
ment. This structure is commonly called an ‘Intelligent
adaptive structure’.
Recent research has shown a great interest in the use
K.-T. Lau
20 Magazine of Concrete Research, 2003, 55, No. 1

of fibre-optic sensors as embedded sensors for the
measurement of temperature variation and strain profile
during the manufacturing and loading processes of FRP
materials.
14
The characteristics desired for the ideal
optical fibre as intrinsic or extrinsic strain or tempera-
ture sensors in civil structure applications would in-
clude the following: (a) reliable; (b) the ability to
provide localised or field measurements; (c) adequate
sensitivity and dynamic range; (d) insensitive to ther-
mal fluctuations; (e) immune to power interruption; (f)
able to multiplex; (g) easy to mass produce; and (h)
durability for the life of the structure. Moreover, the
fibre must be able to withstand a high alkaline content
environment as well as the physical abuse caused by
installation in concrete.
Fibre-optic sensors
Fibre optics were developed for long-distance data
transmission in the telecommunication industry more
than 30 years ago. However, in their earliest applica-
tion, fibre optics were conceived of as a medium for
the transmission of light in medical endoscopy. The use
of fibre optics for applications in the telecommunication
industry actually started in the mid-1960s, and ever
since has seen tremendous growth. The development of
fibre-optic sensors started in earnest in 1977, though
some related demonstrations preceded this date.
15–18
The increased use of advanced composites in aeronau-
tics instigated the need for new damage detection tech-
niques, which could monitor the integrity of structural
components during service periods. Green et al.
19
and
Chan et al.
20
have successfully demonstrated that em-
bedded fibre-optic sensors can be used to measure
strains in advanced composites to a high accuracy and
resolution. Du et al.
21
and Chan et al.
20
have also
demonstrated that fibre-optic Bragg grating sensors
could be multiplexed to measure strains at different
locations within a composite plate through one single
optical fibre.
The use of fibre-optic sensors in concrete was first
suggested by Me´ndez et al.
22
Subsequently, several
research groups in the USA and Europe have reported
on a variety of fibre-optic sensors embedded in or
attached to RC structures; most of the studies have
mainly focused on laboratory demonstrations of their
suitability for strain, vibration and temperature meas-
urements. In recent years, multiplexed grating sensors
have been installed along a pre-stressed girder in the
Taylor Bridge to monitor strain responses of different
positions during service.
23
Fig. 2 shows the installation
process of the grating sensors on the girder surface.
Maaskant et al.
24
have demonstrated the static strain
measurements from the Beddington Trail Bridge sensor
array as illustrated in Fig. 3. The sensor array was
made by multiplexed grating sensors in one single-
mode optical fibre. At that period, the accuracy of
strain measurement was about Æ 40 ìå by using a
wavelength division multiplexing (WDM) technique.
The sensors could also be used for monitoring curing
of the concrete. Since the girders have been post-ten-
sioned, the strain measurements during the process re-
vealed the conditions of stress relaxation in the tendons
from the combined effects of destressing, concrete
shrinkage and creep, the dead loading of the bridge
deck, and the post-tensioning applied across the two
spans. The sensors, however remained in the bridge as
smart sensor for the health monitoring of the bridge.
Figure 4 illustrates the structure of a typical optical
fibre made for strain and temperature monitoring.
In general, the optical fibre consists of a silica glass
core surrounded by a glass cladding material, which
has a slightly less refractive index than the core
(ncladding=ncore % 0:99) in order to produce a total re-
flection in the core region. Since the fibre core and
Fig. 1. Structures’ possibilities created by the confluence of
four disciplines
Fig. 2. Installation process of the grating sensors on the
girder surface
23
Fibre-optic sensors and smart composites for concrete applications
Magazine of Concrete Research, 2003, 55, No. 1 21

cladding are made of the same material, the fibre core
and cladding are commonly referred as a ‘core’ region
in most theoretical and numerical studies.
25–27
The out-
er shell of the optical fibre is made of a rubber-like
material, which is use to protect the core from being
flawed by abrasion, particularly in the presence of
moisture.
In an alkaline concentrated environment such as in
the concrete (pH . 13 in most cases), the coating made
by a polymeric material can protect the core deteriora-
tion by hydroxide ions (OHÀ
). These ions would attack
the primary component of silica and cause a break
down of molecules into Si-O-Si single bond forming
the glass molecular structure. Eventually, the reduction
in strength and formation of cracks may result. The
properties of several common types of coating material
are shown in Table 1. The chemical properties of these
materials have been studied by submerging optical
fibres in concrete with a pH level of about 12·4 for 28
days as well as by soaking coated fibres in alkaline
solutions (pH 7, 11, 12·4 and 14) for 21 days. Habel et
al.
28
have provided detail studies on the survivability
and durability of fibre coating materials in aggressive
chemical environments.
In the fibre-optic sensors, external perturbations such
as strain, pressure or temperature variations induce
changes in the phase, intensity, or wavelength of light
waves propagating through the optical fibres. The
changes in one or more of the properties of light can
then be related to the parameter being measured. Opti-
cal fibres are geometrically versatile and can be config-
ured to arbitrary shapes. The smart structure concept
takes advantage of the geometric adaptability of optical
fibres. In this technology, fibre-optic sensors are em-
bedded within the structural material or bonded on the
structure surface for the purpose of real-time damage
assessment. The most attractive feature of fibre-optic
sensors is their inherent ability to serve as both the
sensing element and the signal transmission medium,
allowing the electronic instrumentation to be located
remotely from the measurement site. This is especially
useful for remote monitoring of the condition of
bridges. Moreover, the advantages of using embedded
fibre-optic sensors in composite materials are dimen-
sional and material compatibility. The fibres do not
degrade during curing, they do not corrode, and bond
strongly to the matrix. Incorporation of the fibres dur-
ing the processing stage also offers the opportunity to
monitor the condition of structural elements during
fabrication.
29–31
The success of fibre-optic sensor technology in the
condition monitoring of composite materials led to a
limited number of research and development activities
in the civil engineering discipline. A number of re-
searchers realised that this emerging field of technol-
ogy could have an impact on the condition monitoring
of civil structures, so that durability, safety and effi-
Bragg gratings
Pre-stressing tendon
Deformed shape
Duct for post-
tension cables
Pre-stressing
tendons
Sensor
Fibre-optic strain sensor location
(bridge elevation)
Cross-section view of bulb-T girder
Fig. 3. Schematic illustration of strain measurements from the Beddington Trail Bridge sensor array
Fibre core
7–20 µm
Cladding
125 µm
Protective coating
250 µm
Light in
Cladding
Fibre-core
Fig. 4. The structure of a typical optical fibre
Table 1. Properties of fibre-optic polymer coating
Polyimide Acrylate Silicone
Elastic modulus (MPa)1
2400 700 2
Tensile strength (MPa)1
130 26 5·5
Static fatigue resistant1
Yes No No
Chemical durability1
Poor Good –
Mechanical protection2
Very poor Very poor Moderate
Transmissibility of
strain2
Excellent Moderate Poor
Price2
High Low Moderate
1
Based on results
28
2
Based on results
69
K.-T. Lau

ciency of the infrastructure system could be improved.
Proper application of fibre-optic sensors to the concrete
structure requires an understanding of certain funda-
mental methodologies pertaining to sensor mechanisms
as well as sensor multiplexing strategies.
Currently, three different types of fibre-optic sensor
arrangements have been developed in real-life applica-
tions; they include localised, multiplexed and distribu-
ted sensor systems. The following subsections will give
detail introduction for each type of the sensors, which
could be applied in civil engineering applications.
Localised sensors. Localised fibre-optic sensors
determine the measurement over a specific segment
in an optical fibre, and are similar in that sense to
conventional strain or temperature gauges. Sensing
based on intensity modulation pertains to light inten-
sity losses that are associated with bending or micro-
bending of optical fibres along any portion of their
length is appreciated as microbend strain sensor.
32
In
Fig. 5, a fibre-optic microbend sensor with elastic
and enclosed diaphragm is shown. Sensors taking
advantage of this phenomenon are termed intensity-
or amplitude-type sensors. The advantages of inten-
sity-type sensors are the simplicity of construction,
and compatibility with multi-mode fibre technology.
Phase sensors cover a broad range of optical phenom-
ena for sensing purposes. The microbend sensors can
be attached to the civil infrastructure to measure
displacement, strain and vibration with appropriate
calibrations. This is also of particular interest in de-
veloping fibre-optic microbend sensors for distributed
sensing application along a long bridge or construc-
tion building elements for detecting vibration modes
and frequencies under a strong wind condition.
33
A fibre optic Bragg grating (FBG) type sensor is one
of the most exciting technologies in the field of fibre-
optic sensors in recent years. The FBG sensor appears
to be ideally suited for structural health monitoring of
composite materials and civil engineering applic-
ations.
34–35
The technology of the FBG sensor was
discovered by Hill et al. in 1978.
24
They found that the
reflective grating could be photorefractively formed in
the core of the gramanium doped silicate fibres. The
schematic illustration of FBG strain measuring system
is shown in Fig. 6. The FBG technology is defined as
the change of the core refractive index (ncore) of an
optical fibre in specified point (grating) and acting as a
mirror for reflecting a light signal emitted from the
source. The reflective wavelength of the light from the
grating depends on the variation of the core refractive
index, which is caused by changing the physical dimen-
sion or thermal deformation at the grating region. For
the structural strain monitoring, load is directly trans-
ferred from the host to the fibre core by shear. This
changes the length in the grating region and thereby,
the resultant refractive index at the core section is
varied in due course. The mechanical properties of the
structure are simply determined by measuring the re-
flective wavelength change from the system due to this
transformation of the refractive index. Meissner et al.
36
and Masskant et al.
24
have demonstrated two different
methods of using grating sensors for concrete bridge
strain monitoring as shown in Figs 7 and 8. A sensor
carrier has been designed to protect the sensor during
concrete pouring process. The loop section could
increase bonding resistance between the carrier and
concrete.
Interferometric sensors are highly sensitive for meas-
uring strains. However, they require the interference of
light from two identical single-mode fibres, one of
which is used as a reference arm while another is an
actual sensor. The sensor, which is widely used to
measure strain and temperature in concrete, is called
white light fibre-optic interferometer.
37–39
Fig. 9 shows
an embedded fibre-optic white light Michelson inter-
ferometric strain sensor system for measuring crack-tip
strain in a concrete beam. An exception to a two-arm
interferometric sensor is a single-fibre Fabry–Perot type
sensor.
40
In the Fabry–Perot type sensor, the fibre is
manipulated in such a way so as form two parallel
reflectors (mirrors), perpendicular to the axis of the
fibre. The interference of the reflected signals, which
are formed in the cavity by the two partial mirrors,
creates the interference pattern.
A Fabryl–Perot sensor is only capable of providing
localised measurements at the cavity formed by the two
mirrors. The interference pattern generated at the out-
put end of the phase sensors is sinusoidal in shape and
is directly related to the intensity of the applied strain
field. The period of this waveform constitutes a fringe
and, if properly calibrated, it relates the optical signal
to the magnitude of the measurand,
40–43
i.e. strain.
In Fig. 10, schematic illustrations of an intrinsic and
extrinsic Fabry–Perot interferometer sensors are shown.
The extrinsic Fabry–Perot interferometer (EFPI) is
formed by inserting two fibres into a large-diameter
tube. The advantage of the EFPI sensor is its low
thermal sensitivity due to the fact that the cavity is in
air with a low thermal coefficient of expansion. How-
ever, the structural discontinuity along the fibre may
Concrete surface
Deformed concrete surface
Microbend strain sensor
Light in Light out
Fig. 5. The fibre-optic microbend sensor

cause stress concentration when the sensor is embedded
into a structural material.
Since the embedded optical fibre is easily damaged
in harsh environments, particularly in situ during con-
Cladding
Light in, 1
Light reflected, 1
Light
transmitted, 1
Spatial pitch = Λ
Fibre core
Wavelength shift
Grating
Coating
Broadband source
(LED)
Optical spectrum
analyser
2 × 2 fibre
coupler
Tension Tension Compression
Original wavelength spectrum
Final wavelength spectrum
Sensing FBG array
FBG 1 FBG 2 FBG n
λ1
λ1 λ2 λn λ, light wavelength
λ2 λn
Fig. 6. The fibre-optic Bragg grating strain measuring system
Fig. 7. Sensor carrier developed by Meissner et al.
36
Steel bar
Cross-ribLoop
section
Position of grating
Optical carrier
Sensor carrier
Measuredstrain,ε
Length
To light source and
signal receiver
Pre-stressed tendon
Multiplexed FBG sensors
Concrete structure
Fig. 8. Quasi-distributed sensing using a series of point sen-
sors
K.-T. Lau

struction or the concrete mix pouring process,
44
and the
coating materials of the fibre easily crinkle, peel off
and generate several micro-cracks when they are em-
bedded into cement mortar,
28
a new sensor based on
the fibre-optic technology is necessary. Recently, Yuan
et al.
45
have developed a new type of sensor named a
fibre-optic pre-embedded concrete bar (PECB) sensor.
A tiny optical fibre sensor is pre-embedded into a
matrix made by epoxy or cement base materials to form
a bar. The outer shape of the PECB is designed in a
corrugated pattern in order to improve the bonding
properties between the PECB and the host concrete
material. The designs of the PECB sensors for civil
concrete application with minimising the risk of dam-
age from rough working environments are shown in
Fig. 11.
Multiplexed sensors. Multiplexed sensors are
usually constructed by combining a number of indivi-
dual sensors for measurement of perturbations over a
large structure. Theoretically, it is possible to use
optical switching and other innovative ideas for this
purpose. A number of researchers have developed
innovative methods for the development of multi-
plexed fibre-optic sensors. A most widely employed
multiplexed sensing technique is based on measure-
ment of propagation time delays of travelling light in
the fibre based on the measurement-induced change
in the transmission of light. An optical time-domain
reflectometer (OTDR) is mainly used for this
purpose.
46,47
A pulsed light signal is transmitted into
one end of the fibre, and light signals reflected from
a number of partial reflectors (splices) along the fibre
length are recovered from the same fibre end as
shown in Fig. 12. By using this concept, it is possible
to determine the distance to the strain field, d, by a
way of the two-way propagation time delay, 2t,
through the simple relationship (relating velocity and
distance): d ¼ 2t 3 v, where v is velocity of light in
the fibre, and 2t is the time required for the two-way
travel of the signal between individual reflectors.
External applied load
Crack Location of an embedded
optical fibre coil
Concrete beam
Reference fibre
Measurement fibre
Mirrors
Delay line
Amplifier
PD
Data aquisition Analyser
Coupler
Coupler
Source
Light source
(LED)
Fig. 9. The fibre-optic white light Michelson interferometric strain sensor system
Fibre-core
In-fibre reflective splices
Optical signal
Reflected light signals
Bond or fusion weld
Intrinsic Fabry-Perot
interferometer
Extrinsic Fabry-Perot
interferometer
In-line fibre etalon
Reflecting
fibre
Outer 'alignment tube'
Cavity
'Hollow-core'
fibre section Fusion weld
Fig. 10. Schematic illustrations of an intrinsic and extrinsic
Fabry–Perot interferometer sensors

Since the velocity of light is known, the OTDR is
capable of detecting the location of the strain fields
through measurement of reflected time signals.
A promising technique for the civil engineering ap-
plication is based on wavelength division multiplexing
(WDM) by using Bragg gratings.
48,49
In this technique,
a broadband light source, defined as light containing a
number of wavelengths within a certain region of the
spectrum, is employed for scanning a number of Bragg
grating type sensors in series and/or in parallel. The
reflectance wavelength of each Bragg grating is slightly
different from the others. In this way, wavelength shifts
of individual sensors are recognised, detected and then
related to the magnitude of strain at specific sensor
locations. Lau et al.
50
demonstrated the use of the
multiplexed FBG sensors as embedded sensors for plain
concrete structures and FRP-repaired concrete struc-
tures. They found that the embedded sensors were able
to measure the internal strain of the concrete precisely
and to indicate premature failures that surface mounted
strain gauges could not measure accurately in concrete
and bond interface between the concrete and externally
bonded reinforcements. In Fig. 13, a multiplexed FBG
sensor array system for strain measurement of an FRP-
strengthened concrete beam is shown. Fig. 13(b) shows
the frequency spectrum captured from the experiment.
Cement or epoxy
surrounding materials
Fibre-optic sensor
Corrugated PECB sensor
Necked PECB sensor
Grooved PECB sensor
Optical fibre
To light emission and
signal demodulating system
Fig. 11. PECB sensors for civil concrete applications
Pulse laser
source
Processor Detector
Launch-end
reflection
Reflection from
mid-fibre splice
Far-end
reflection
Rayleigh scattering
from fibre 1
Rayleigh scattering
from fibre 2
Logofreceived
lightintensity
Fibre end
(Freanel reflection)
Slice
Slice
Fibre 1
Fibre 1
Time: Equivalent distance
Fig. 12. Concept of the basic optical time-domain reflectometer
K.-T. Lau

Distributed sensors. Distributed sensors make full
use of optical fibres, in that each element of the
optical fibre is used for both measurement and data
transmission purposes. The purpose of making meas-
urements by distributed or multiplexed optical fibres
is to determine locations and values of measurements
along the entire length of the fibre. These sensors
are most appropriate for application to large struc-
tures owing to their multi-point measurement cap-
abilities. A distributed sensor permits measurement
of a desired parameter as a function of length along
the fibre. One way is based on Michelson interfero-
metric technique by using white light (low coherence
light) as the interferometer source. The multiplexed
strain sensors in series configured by using switch
and in parallel by 1 3 N star coupler are demon-
strated.
2,48
In Fig. 14, the distribution of an optical
fibre sensor system is shown. The distributed sensors
can be used for a concrete bridges to monitor the
mechanical behaviour of cables during construction,
under traffic loads and due to daily and seasonal
fluctuations.
Diaz-Carrillo et al.
51
used FBG sensors with a com-
bination of multiplexed and distributed techniques for
strain and deformation measurements of a long span
supported concrete structure. They have developed
a system based on a tunable Fabry–Perot filter, that
allows fast strain measurements (up to 50 Hz) with
enough accuracy (, 30ìå). Fortunately, the latest
equipment can measure dynamic strain measurements
for multiplexed FBG sensors to an accuracy of 1 ìå,
this limitation is dependent on the scanning power of
the tunable filter and the number of multiplexed FBG
sensors used. Fig. 15 shows a Bragg grating demodula-
Broadband
source
Intensity
modulator
Signal
generator
Voltage
control
oscillator Tunable
optical
filter
Computer
control
system
Coupler
Electrical spectrum
analyser
Mixer
Amp
G1
G2G3
Crack
Glass fibre
composite laminates
Rectangular concrete beam
Freq OFS 0 Hz
RBV 300 Hz
VBV 1 Hz
SVP 500 ms
Start 20·0 kHz Stop 158·8 kHz
fbeat1
50 kHz
fbeat2
75 kHz
fbeat3
105 kHz
(b)
(a)
Fig. 13. The multiplexed FBG sensor array system for strain measurement of an FRP-strengthened concrete beam

tion arrangement for multiplexed and distributed FBG
sensors.
Other applications
The previous sections reported the functions of fibre-
optic sensors for civil concrete applications being
mainly focused on two categories: strain and deforma-
tion measurements. In fact, the embedded sensors are
not only restricted into these two areas, in which the
sensing philosophy relies on the physical change in the
fibre materials. Ansari
52
has developed a fibre-optic
sensor for determination of the air content in freshly
mixed concrete. The sensor was based on the measure-
ment of the reflected intensity of light through the fibre
end (tip) of an optical fibre, which is in contact with
fresh concrete, since the reflective light intensity re-
ceived from the optical fibre is dependent on constitu-
ent materials including cement, aggregate, water and
void, which come into contact with the fibre end. The
reflectivity at the interface between the air and the
optical fibre gives high magnitude with the other con-
stituents. In Fig. 16, the refractive indices for the light
emitted from the optical fibre to other materials are
shown.
Lo and Xiao
53
and Panova et al.
54
have developed a
single pitch Bragg grating corrosion and pH-sensitive
sensors, respectively. A thin copper shell is coated onto
a pre-strained optical fibre at the grating region to form
a corrosion sensor (Fig. 17). The principle of the sensor
is that environment corrosion would change the thick-
ness of the coating, and eventually cause the changes
of residual strain inside the grating region. A para-
meter, which corresponds to the corrosion rate, can be
measured according to the reflected wavelength change
from the grating. This sensor is able to measure the
curing condition of the concrete and may be also used
to measure the penetration rate of the chemical sub-
stances in the concrete, which is situated in a highly
aggressive environment.
Application to concrete structures
The advantages of using FRP materials in civil con-
LED
PIN
FC
GRIN lens
Measuring arm
FC
2 × 2
coupler
1 × N star
coupler
Motor-driven
scanning mirror
Sensor 1
Sensor 2
Sensor 3
Sensor 4
Sensor 5
CFRP cables
Position of
distributed
sensors
Sensor5
Sensor 1
Fig. 14. The distribution of fibre-optic sensor system
Arm 1
Arm 2
Arm n
Reference arm
FBG 1 FBG 2 FBG n
Temperature compensation
Coupler
Data manipulation
Photo
receiver
TunableF-P
filter
Light source
(LED)
Controller
Isolator
Fig. 15. The Bragg grating demodulation arrangement for multiplexed and distributed FBG sensors
K.-T. Lau

crete applications are addressed above. Many publica-
tions address the importance of the use of FRP materi-
als with embedded fibre-optic sensors in aerospace,
automobile and large structural industries. Hence, the
use of this technology for civil concrete applications
can lead to reductions in weight, inspection intervals
and maintenance cost of the structures. Du et al.
21
and
Chan et al.
55
have used embedded FBG sensors, which
were located at the interlayer of FRP laminates, to
measure strains of the FRP plate. They found that
the embedded FBG sensors could provide a strain
distribution along the structure without influ-
encing the mechanical properties of host material.
Dewyntes-Marty
56
has embedded FBG sensors into a
glass fibre/epoxy laminate to monitor thermal and resi-
dual strain properties during curing process in auto-
clave. The temperature measured from the sensor gave
a good agreement with that measured by autoclave
temperature probes. The FBG sensor could sustain
compressive strain up to 3100 ìå strain without fail-
ure.
50
Kalamkarov
57
found that the FBG sensor em-
bedded in the carbon fibre reinforced composite plastic
Freshly mixed air
entrapped concrete
Freshly mixed air
entrapped concrete
Silica fibre
n = 1·46
Reflective index in concrete
Air = 1·0
Water = 1·33
Aggregate = 1·55 to 2·0
Cement paste > 1·33
Position along the concrete
Direction of sensor travels
Position 2
Position 1
Position 3 Position 4 Position 5
Optical fibre
Laser diode
To photo detector
Reflectedlight
intensity:Volts
Fig. 16. The refractive indices for the light emitted from the optical fibre to other materials
Loss of passive
layer Carbonation zone
Carbon dioxide
Carbon dioxide
Rust
Bragg grating fibre
Pre-loaded grating fibre
Copper coating grating fibre
Copper coating
Corrode copper coating
Strain release due to
coating thickness change
Embedded corrosion sensor for concreteSteel corroded due to carbonation
Fig. 17. Corrosion sensor for concrete applications

(CFRP) rod is unaffected by fatigue load and creep
effects.
Since the use of FRP materials in concrete rehabilita-
tion, retrofit and repair has increased rapidly in the past
few years, applying these materials with an embedded
fibre-optic sensor, as structural health monitoring de-
vice in concrete structures would definitely benefit the
community through long-term strategy. The experiences
from the aerospace and aircraft industry have proved
that this technology would be used with confidence.
The multiplexed FBG sensors have been used in civil
construction industry only in recent years. The con-
ventional strain gauge, for strain measurements in
concrete, consists of multi-layers protection against dif-
fusing moisture, which itself influences the accuracy of
measurement. However, the fibre-optic strain sensor is
protected by a thin layer of a special alkali-resistant
adhesive varnish and therefore, the precision of strain
and temperature measurements can be achieved.
The first use of FBG sensors as structural health
monitoring device in bridge structure was demonstrated
in 1994.
58
An array of FBG sensors was adhered on the
surface of carbon fibre composite tendon to measure
strain and deformation of the structure. Saouma et al.
59
and Lau et al.
25,60
have used embedded FBG sensors to
monitor strains of laboratory sized concrete beams.
They found that the results gave good agreement with
those measured by externally bonded electric strain
gauges. In Figs 18 and 19, a laboratory sized concrete
structure with an embedded of FBG sensor and a
micrographic diagram of the structure at the sensor
embedding region are shown. The Naval Research La-
boratory in the USA
61
performed an experimental study
for a bridge, which is in 1/4 scale of real structure with
the embedment of 60 FBG sensors to measure strain.
They found that the FBG sensors could measure the
strains in real time. However, due to the limit of the
scanning speed of the Fabry–Perot tunable filter used
for wavelength-shift measurement, only static strain
could be measured. In Canada, multiplexing FBG sen-
sors were embedded during construction along pre-
stressed girders for a bridge which was made of FRP
materials to monitor strain response at different posi-
tions during service.
23
Lau et al.
62
and Davis et al.
63
have installed multi-
plexed FBG sensors at the bond interface between the
damaged concrete surface and externally-bonded FRP
reinforcements for FRP-strengthened concrete struc-
tures. One sensor was attached on the concrete surface
and just in front of a crack-tip to measure strain. An
externally bonded strain gauge was also used on the
surface of the reinforcement to compare strain meas-
urement. The experimental results showed that the FBG
sensor often gave high strain value compare with the
strain gauge, particularly for the use of a thick reinfor-
cement. Since the sensor was located on the concrete
surface and at the crack-tip, which was suspected as a
high stress concentration region,
64
the sensor was
highly susceptible to micro-cracks on the concrete sur-
face and the occurrence of debond at the bond inter-
face. This phenomenon could not be detected
accurately through the use of surface mounted strain
gauges. Fig. 20 shows the strain measured from the
embedded FBG sensor and externally bonded strain
gauge and the debond failure at the bond interface
where the sensor was located for an FRP-strengthened
concrete structure.
Intelligent and integrated bridge systems
The term ‘Smart Composites’ implies that the com-
posites have an ability to detect damage and impending
failure at certain marginal conditions.
12
The sensor, like
a nervous system in the human body, can sense or
Fig. 18. The laboratory sized concrete structure with an em-
bedded of FBG sensor
Fig. 19. Micrographic diagram of the structure at the sensor
embedding region
K.-T. Lau

forecast hidden trouble for global and local deforma-
tions, and damage in the structures. The latest publi-
cations have addressed the use of fibre-optic sensors
in association with other actuators such as piezoelectric
and shape memory actuators, which could build a
‘live’ structure,
65
i.e. the structure can sense, manipu-
late and response to any undesired structural deforma-
tion or damage in order to recover strength of the
structure.
2,66–68
The applications of fibre-optic sensors in the civil
engineering industry have been discussed independently
in the previous sections. However, the use of the smart
composite in civil concrete structures is not yet adopted
in all real-life infrastructure elements. It has been
proved that the use of the FRP can substantially in-
crease the strength of the concrete structures by using a
simple composite plate bonding or hand lay-up techni-
ques. The fibre-optic sensors, however, also provide the
ability to measure strain remotely and precisely without
imposing any strength degradation on the structure.
Thereby, the use the smart composites for concrete
strengthening as well as structural health monitoring
devices can definitely improve the durability and safety
of the structure.
The technology of smart composites, in which the
optical fibre sensors are integrated into the composites
to form a single part compound, has maturely devel-
oped. The smart composites can be used as reinforce-
ment as well as sensors for repairing and strengthening
damaged concrete structures as shown in Fig. 21. The
composite can be formed as a patch to bond on the
concrete surface to improve the tension properties of
the concrete structure. The sensors are integrated into
the composite, which results in reducing the risk of
damage of the fibre, and they can be manufactured in-
house without affecting the in situ environment.
Problems in applications
Embedded fibre-optic sensors have been well estab-
lished as intrinsic strain and temperature-measuring
devices to assess structural conditions. However, the
alkaline attack to the fibre core is still a problem for
embedded strain sensors; since the coating at the meas-
uring region is generally required to be removed, it
allows a direct contact between bare fibre and sur-
rounding materials.
Fibre-optic sensors in concrete use a variety of meas-
urement methodologies. The sensors are able to be
attached or embedded into existing concrete structures,
even after being strengthening by FRP materials, and to
evaluate the crack growth rate of a damaged concrete
structure. However, more importantly, the measurement
philosophy used in the laboratory and real-world appli-
cation are very different. In the factual environment,
damage of the fibre may happen easily from weath-
ering, rough handling by labourers and the concrete
pouring process.
Although many successful research achievements
have been reported in recent years, the utilisation of the
fibre-optic sensor technology in real-life civil engineer-
ing applications still needs more effort in solving sev-
eral real practical problems.
Conclusion
Fibre-optic sensors exhibit considerable potential for
real-life application. Due to the physical size of the
optical fibre, sensors are relatively small compared
with infrastructure elements, embedding the sensors
into the structure does not degrade the mechanical and
geometrical properties of the structure. Smart compo-
sites have been widely adopted in most high technology
engineering applications such as a smart wing in aero-
space and aircraft industry.
12
The results from all re-
Fig. 20. (a) Strain measurements from the embedded FBG
sensor; (b) and debond failure at the bond interface
45
40
35
30
25
20
15
10
5
0
Load:kN
0 1000 2000 3000 4000
Strain: µε
Strain gauge
Embedded fibre-optic sensor
(a)

search achievements show that the smart composite
could be used as primary structure as well as sensor to
monitor strain or temperature of the structure. However,
this innovative concept is not yet adopted in all civil
engineering applications.
In the future, the development of smart materials and
structures will pay a major role in all engineering dis-
ciplines. The optical fibre sensors will be one of the
most structural health monitoring devices in achieving
this goal. The use of a smart composite as an externally-
bonded reinforcement as well as real-time structural
health monitoring device can greatly improve durability
and substantially improve the safety of the structure.
Acknowledgements
This work has been funded by The Hong Kong Poly-
technic University (G-YW60). The author would like to
thank Professor L. B. Yuan, Department of Physics,
Harbin Engineering University, for his advice and con-
structive suggestions.
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Discussion contributions on this paper should reach the editor by
1 August 2003
K.-T. Lau

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