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Materials 2021, 14, 7826. https://doi.org/10.3390/ma14247826 www.mdpi.com/journal/materials
Article
Maintenance and Inspection of Fiber-Reinforced Polymer
(FRP) Bridges: A Review of Methods
Long Tang
School of Human Settlements and Civil Engineering, Xi’an Jiaotong University, 28 Xianning West Road,
Xi’an 710049, China; tanglong@xjtu.edu.cn
Abstract: Fiber-reinforced polymers (FRPs) are materials that comprise high-strength continuous
fibers and resin polymer, and the resins comprise a matrix in which the fibers are embedded. As the
technique of FRP production has advanced, FRPs have attained many incomparable advantages
over traditional building materials such as concrete and steel, and thus they play a significant role
in the strengthening and retrofitting of concrete structures. Bridges that are built out of FRPs have
been widely used in overpasses of highways, railways and streets. However, damages in FRP
bridges are inevitable due to long-term static and dynamic loads. The health of these bridges is
important. Here, we review the maintenance and inspection methods for FRP structures of bridges
and analyze the advantages, shortcomings and costs of these methods. The results show that two
categories of methods should be used sequentially. First, simple methods such as visual inspection,
knock and dragging-chain methods are used to determine the potential damage, and then radiation,
modal analysis and load experiments are used to determine the damage mode and degree. The ap-
plication of FRP is far beyond the refurbishment, consolidation and construction of bridges, and
these methods should be effective to maintain and inspect the other FRP structures.
Keywords: bridge monitoring; debonding; fiber-reinforced polymers; FRP; maintenance;
non-destructive inspection
1. Introduction
Fiber-reinforced polymers (abbreviation FRP) are compound materials [1–4]. Gener-
ally, FRP is composed of resin and fibers with high strength. The fibers are continuous
and embedded in the resin. Thus, the resin is a matrix and plays the role of binder to hold
and protects the fibers. The fibers act as reinforcing components, and the loads are trans-
ferred between the fibers through the resin. The intimal range of FRP application is evi-
dent in the aerospace, electric-car, high-speed-train and robot industries, for example. The
technology of FRP manufacturing has advanced in recent years [4–8]. In particular, the
pultrusion process has been developed to manufacture continuous and long FRPs. Such
FRPs can effectively resist ultraviolet rays, chemical corrosion, the freeze-thaw cycle, the
dry-wet cycle, high temperatures and humidity, and are light and high-strength [4–8]. In
this way, FRPs are widely used in the field of architecture. According to the type of fiber,
the FRPs applied in civil engineering are often aramid-FRP, basalt-FRP, carbon-FRP and
glass-FRP. The resin often consists of vinyl ester, epoxy and polyester [4–8].
FRPs are now intensively used in the refurbishment and construction of bridges [4–
8]. The decrease of the dead-weight of bridges and the increase of the load of bridges are
important due to the demand for the increase of bridge span. Notably, when the span of
a bridge is very large and the bridge is made of reinforced concrete, about eighty-five
percent of the bridge load is dead-weight. The decrease of bridge’s dead-weight depends
on light, high-strength and durabile materials. FRPs are an indispensable part of modern
bridge structures [4–8]. FRP bridges are relatively light and constitute 30% to 60% of the
Citation:
Tang, L. Maintenance, and
Inspection of
Fiber-reinforced
Polymer (Frp) Bridges: A Review of
Methods.
Materials 2021, 14, 7826.
https://doi.org/10.3390/ma14247826
Academic Editor:
José A.F.O.
Correia
Received: 10 November 2021
Accepted: 15 December 2021
Published:
17 December 2021
Publisher’s Note:
MDPI stays neu-
tral with regard to jurisdictional
claims in published maps and insti-
tutional affiliations.
C
opyright: © 2021 by the author. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
con-
ditions of the Creative Commons At-
tribution (CC BY) license (http://crea-
tivecommons.org/licenses/by/4.0/).
Materials 2021, 14, 7826 2 of 10
weight of traditional structures. The advantages of FRP bridges are their strong tolerance
to environmental corrosion, ease of construction and low cost of repair. Therefore, FRP
bridges can be widely used as overpasses of highways, railways and streets. At the same
time, FRP bridges also have important applications in rapid responses to emergencies,
with significant economic and social benefits [4–8].
Some studies have reviewed the types of FRP applications in reinforced concrete
structures [8–12]. Flexural application means that the boards, strips and fabrics of FRPs
are pasted on a simply supported beam. Shear application is a technique including near-
surface-mounted FRP plates and side bonding of FRP sheets. The technologies of torsion
application are similar to those of flexural and shear application. The technology of axial
application involves FRP composites that are spirally wrapped around concrete columns.
The combined loading application is the system of near-surface-mounted FRP composite.
Bridges are often affected by factors such as climate, oxidation, corrosion and static
and dynamic loads. Thus, damage to bridges is inevitable. The initial damage is the
debonding of the surface layer of FRP reinforcement because the surface layer is the first
to take the stress transmitted by the anchorage [6,7,10,11]. Then, the damage accumulates
at the microstructure level of FRP reinforcement over time [6,7,10,11]. When the level of
damages is higher than a certain threshold, the FRP reinforcement fails [6,7,10,11]. The
damages of FRP reinforcement are joint failure, such as resin crack, resin transverse crack,
interface debonding, delamination, fiber fracture and so on [6,7,10,11].
The components of bridges gradually age over time, threatening traffic safety. There-
fore, it is necessary to maintain and manage bridges and take various technical measures
to prolong their service life. The evaluation of material properties and the inspection of
FRP structures have received a great deal of attention [12–18]. Here, the techniques for the
maintenance and inspection of FRP bridges are reviewed. The aim is to summarize the
details of techniques, in particular, to compare the cost, equipment and accuracy of these
techniques of inspection. Thus, the proper techniques can be chosen easily.
2. Maintenance of FRP Structures
Bridge maintenance refers to regular maintenance, repair or reconstruction work.
These measures can maintain the normal functions of bridges and their appendages, re-
pair bridge damage and improve the service of bridges. According to the current, “Stand-
ard for maintenance of highway bridge and culvert” [19], bridge maintenance includes
minor repairs, medium repairs, major repairs, reconstruction and special projects. The mi-
nor repair (daily maintenance) of a bridge refers to preventive maintenance of bridges and
their affiliated structures and slight damage repairs. Medium repair refers to small engi-
neering projects, including the regular repair and reinforcement of the wear and damage
in bridges and affiliated structures. Major repair refers to the periodical and comprehen-
sive repair of considerable damage in bridges and affiliated structures to recover the
health level of the design standard, or the improvement of parts and addition of more
structures within the scope of the technical grade to improve the capacity of bridge. Re-
construction refers to the larger engineering projects that improve the technical grade, or
reconstruction of some parts to improve the capacity, load and flood discharge.
The service of a bridge exceeds the required level during operation, and the cost of
maintenance and the impact on the environment are minimized, which is the goal of
bridge maintenance [20–29]. Specifically, the bridge is clean; the deck is solid and flat; the
cross slope is moderate; the connection of the bridge head is smooth; the drainage is
smooth; the structure is complete; and auxiliary facilities such as signs, markings and
lamps are intact [20–29]. The maintenance of the FRP structure mainly includes five cate-
gories (Table 1).
Materials 2021, 14, 7826 3 of 10
Table 1. The Maintenance of FRP Bridge.
Name
Aim
Method
Sunscreen and waterproof Reducing the rate of aging
Painting anti-corrosion mate-
rials on the surface
Repairing surface damage Protecting FRP facilities
Pasting fiber cloth on facili-
ties by resin
Fire prevention
Eliminating accidents
Cleaning bridge
Repairing debonding parts
and cracks
Reinforcing and protecting
the area in which stress con-
centrates
Replacing damaged materi-
als with new ones
2.1. Sunscreen and Waterproof
The surface of the FRP will lose luster and color after long-term exposure. In addition,
after long-term rain erosion, water can penetrate into the material along the microscopic
chink at the interface between the fiber and resin, which weakens the bonding force be-
tween fiber and resin, leads to material aging, and thus damages the mechanical proper-
ties of the material. Therefore, it is necessary to paint anti-corrosion materials on the sur-
face of FRP components and check them frequently when FRP bridges are in hot and hu-
mid environments [20–25].
2.2. Repairing Surface Damage
The resin layer on the surface of the FRP component is thin, and resin peeling, deep
scratching, fiber exposure and other damage will occur after collision, impact and friction.
Therefore, it is necessary to frequently inspect the surface of FRP components and repair
the damage in time. Otherwise, rainwater will infiltrate, expand pores and accelerate ma-
terial damage. In addition, the ancillary facilities (e.g., parapet, sidewalk and lamp) that
are made of FRP materials are often bumped, so it is necessary to set anti-collision devices
to protect these FRP facilities.
The local damage of the FRP bridge can be repaired with fiber cloth and resin
[20,21,26,27]. The procedure is as follows: (a) remove the loose resin and fiber from the
damaged part, and then clean and dry the surface; (b) cut the fiber cloth according to the
size of the surface; (c) blend the resin; and (d) apply the resin to paste fiber cloth.
2.3. Fire Prevention
FRP materials are thermosetting and may be vitrified under high temperatures and
fire. Such vitrification will degrade the properties of materials and seriously endanger the
stabilization of bridge structures. Therefore, it is necessary to clean up the combustible
materials on and around the bridge in time [6,7,10,11].
2.4. Repairing the Debonding Part and Crack
Due to stress concentration and adhesive layer aging, delamination and adhesive
layer debonding often occur between the FRP layers, FRP members and the interface be-
tween the FRP members and concrete. These changes can reduce the integrity of struc-
tures.
FRPs are brittle, so cracks usually appear in the area where stress is concentrated,
such as bolt holes and corner joints of the profile. When the material cracks, the damage
will deteriorate from the crack position.
Therefore, it is necessary to periodically check the delamination and debonding of
FRP bridges, check whether there are cracks in the area of stress concentration and make
repairs on time [10,11,28,29]. There are many check-related methods, and an important
part of this review is nondestructive inspections of FRP bridges.
Materials 2021, 14, 7826 4 of 10
3. Nondestructive Inspections of FRP Bridges
Compared with metal, the advantages of FRP materials are their high strength, stiff-
ness, radiation resistance and easy operation. However, due to the mechanical character-
istics of FRP bridges, there are pores, impurities and delamination [6,7]. Thus, a monitor-
ing system is crucial to locating and identifying damage as well as diagnosing structures.
Furthermore, the sensors that receive and convert signals in the monitoring system must
be appropriate, and the data analyses for judging the abnormal signal must be correct.
Previous studies have shown that the integrity of an FRP structure is strongly af-
fected by the connection between the fiber and adhesive layer and the connection between
the composite and impregnated resin. At present, a variety of nondestructive inspection
techniques have been used to evaluate and identify the performance of materials, compo-
nents and structures. In addition, FRP materials have been used in aircraft and ship struc-
tures for a long time, so the nondestructive inspections used in the fields of aviation and
navigation can be used for the inspections of FRP bridges. Some methods, such as visual
inspection, dragging-chain and knock methods, do not need to use special equipment.
Other methods, such as thermal imaging, acoustic emission, ultrasonic waves, radiation,
modal analysis and load tests, are more complex and require specific equipment [6,7,12–
18]. According to the analysis of previous research results [30–53], the scope of the appli-
cations and functions are different among these methods (Table 2).
Table 2. Characteristics of Nondestructive Inspections of FRP Bridges.
Name
Equipment
Cost
Accuracy
Visual inspection [30]
Flashlight, mirror and ruler
Low
Low
Knock [31,32]
Coins, hammers, electronic
hammer and iron chain
Low Low
Thermal imaging
[33–36]
Natural or artificial heat source,
thermal imaging camera
Low
Natural heat source, low;
artificial one, medium
Acoustic emission
[37,38]
Piezoelectric sensor, multichan-
nel data receiver
Me-
dium
Medium
Ultrasonic wave [39–
41]
Pulse generator, wave ampli-
fier, and screen
Me-
dium
Medium
Radiation [42,43] Radiation source, screen
Me-
dium
Low (still in its experi-
mental stages, immature)
Ground radar wave
[38]
Ground penetrating radar, re-
ceiver
Me-
dium
Medium
Microwave [44–46]
Electromagnetic wave transmit-
ter, wave amplifier
High High
Optical fiber sense
[38,47–52]
Optical fiber sensor High High
Modal analysis and
load experiment
[31,53]
Strain gauge, accelerometer High High
3.1. Visual Inspection
Visual inspection is the simplest and most widely used method. Visual inspection
combined with some simple tools, such as flashlights, rulers and mirrors, can quickly de-
tect damage in bridge structures. The disadvantage of visual inspection is that it can only
detect the damage on the surface; however, it can neither quantify the damage nor detect
the internal defects of the structures. Therefore, when cracks, delamination, discoloration,
holes and deformation are found by visual inspection, further inspections through precise
methods are necessary [30].
Materials 2021, 14, 7826 5 of 10
3.2. Knock
Inspectors use coins, hammers or electronic hammering units to strike the surface of
FRP members, and they then detect delamination and holes by identifying the sounds.
Inspectors also drag the iron chain on the bridge deck and locate the potential delamina-
tion through the change of sound. Knock combined with visual inspection can effectively
identify most damage in FRP bridges. Alampelli [31] knocked the bottom of the bridge
with a rubber hammer and found holes in the bottom of the FRP slab bridge deck. Rosen-
boom [32] found that visual inspection and knock can effectively identify the bond quality
between FRP materials and concrete.
3.3. Thermal Imaging
The mechanism of thermal imaging is that the defects under the surface of the FRP
component will affect the thermal flow in the material. The heat source is set on one side
of the bridge deck, and the temperature is measured by the thermal imaging camera on
the other side. According to the change in temperature, defects such as delamination,
debonding, impact damage, moisture absorption and holes are determined. Thermal im-
aging can be natural or artificial.
The heat source of passive thermal imaging is the natural temperature. This method
only allows for qualitative detection and can identify potential defects by detecting abnor-
mal temperatures. The heat source of active thermal imaging is external and uniform.
Debonding, cracking, impact damage, water accumulation and other defects can affect the
thermal properties of materials. The area without defects can conduct heat more effec-
tively than the area with defects. The absorption or reflection of heat can create a thermal
gradient, and this gradient can be observed by a thermal imager. Because a thermal gra-
dient can be caused by nonuniform heating, the heat source of active thermal imaging
should be implemented on a surface that can be heated evenly.
Hag-Elsafi et al. [33] and Taillade et al. [34] successfully applied thermal imaging to
detect the bond between FRP and concrete. Halabe et al. [35] used a digital thermal imager
to detect defects under the surface of an FRP bridge deck and debonding between the
wearing layer and bridge deck. The internal temperature of the bridge can be measured
by a thermal sensor. Teng et al. [36] installed thermal sensors on the glass fiber-reinforced
polymer (GFRP) concrete columns of expressway bridges to monitor the temperature dif-
ference between FRP and concrete and found possible debonding damage.
At present, thermal imaging is often used for the qualitative detection of damage in
FRP structures. The results of laboratory tests and field tests show that thermal imaging
is effective in the nondestructive testing of FRP strengthened concrete members and FRP
bridge decks. This method can also be used to detect the quality of FRP members in the
pultrusion process, installation process and service process. Notably, thermal imaging is
probably ineffective in detecting thick FRP members.
3.4. Acoustic Emission
Acoustic emission is nondestructive and is used to detect the integrity of FRP struc-
tures. The mechanism of this method is the change in the intensity of the sound signal.
Due to the rapid release of energy, a stress wave is generated when the material is loaded.
The stress wave radiates from the wave source and is then recorded by the sensor ar-
ranged on the surface of the material. Usually, piezoelectric sensors are used to detect
acoustic emission. When the stress wave is transmitted to the sensor, the crystal will pro-
duce an output signal due to the pressure. When the intensity of the signal is higher than
the threshold that is set, the instantaneous signal is recorded as an impact. In the FRP
structure, acoustic emission can be caused by matrix cracking, fiber debonding, delami-
nation, fiber pullout and fiber fracture. The waves generated by fiber fracture can release
high energy, and thus, the intensity of the acoustic emission signal is high. In contrast, the
Materials 2021, 14, 7826 6 of 10
acoustic emission signal produced by matrix cracking and fiber matrix debonding is weak.
The duration of the acoustic emission signal is longer, and the intensity is moderate.
The equipment for this inspection consists of a piezoelectric sensor, a coupling agent,
multichannel data acquisition equipment and highly integrated analysis and data acqui-
sition software. Gostautas et al. [37] successfully completed the evaluation and detection
of six GFRP bridge decks that have various full-scale cross sections.
Some acoustic emission signals are false, so it is important to judge the authenticity
of the signal. There are many reasons for false signals, such as mechanical friction, leakage,
liquid flow, vibration, wind-induced vibration, rain, snow, hail and thermal expansion
under sunlight. Additionally, it is also very common for bearing sliding to produce false
signals.
3.5. Ultrasonic Waves
Ultrasonic wave inspection introduces high-frequency stress waves into the structure
to detect defects or changes in material properties. Pulse reflection is the most commonly
used method, and the energy of sound is introduced into the material as waves. When the
waves encounter discontinuities (such as cracks) in the transmission path, the partial en-
ergy will be reflected back from the defects. The reflected wave is recorded by the sensor,
converted into an electronic signal and then displayed on the screen. The location and size
of the defect can be determined. This method is easy to operate, and common ultrasonic
instruments can be used.
The equipment for ultrasonic waves is a converter, a pulse generator, a receiver/am-
plifier and a screen. This method has been used to detect debonding failure of concrete
members strengthened by FRP [39] and the failure of iron bridges by strengthened FRP
[40]. Muhmouda et al. [41] used surface acoustic waves (SAWs) to detect the interface
degradation of concrete members strengthened by carbon fiber-reinforced polymer
(CFRP) and suggested that this method be used to monitor the structures of concrete
bridges strengthened by FRP. The operator should master professional cognition to carry
out the inspection and interpret the test data. In addition, the cost of this method is high.
3.6. Radiation
Operators use X-rays or gamma rays to penetrate a component to detect defects. The
radiation source is arranged on one side of the component, and the screen is arranged on
the other side. The ability to absorb rays varies between defects in FRP bridge decks, such
as delamination, and healthy parts. The location of the defect can be displayed on a radi-
ographic film or a computer screen.
This method cannot capture the three-dimensional characteristics of defects. Alter-
natively, when the relative positions of defects, radiation source and film are appropriate,
the method can provide high-resolution images of defect planes and can detect delamina-
tion and debonding. Therefore, it is necessary to adjust the relative positions of defects,
radiation sources and films.
This method has been applied to the defect detection of composite laminates and
sandwich plates [42,43]. The disadvantages of this method are the threat of radiation to
the health of operators and the high cost. This method should be further improved before
it is applied in civil engineering.
3.7. Ground Radar Wave
Ground penetrating radar projects electromagnetic waves into materials. When an
electromagnetic wave encounters discontinuous defects, the waves produce reflected
pulses. Discontinuous defects can be the boundary of materials, the interface of different
media and delamination or debonding below the surface of materials. The position of the
discontinuity can be determined by the amplitude of the reflection wave and the corre-
sponding reflection time.
Materials 2021, 14, 7826 7 of 10
This method is effective in detecting damage under the surface of laminated materi-
als (such as FRP composites), such as debonding between the wearing layer and FRP
bridge deck and delamination of the flange of the FRP bridge deck. Compared with ther-
mal imaging, ultrasonic waves and ground-penetrating radar waves have a stronger pen-
etration ability, so they can detect deep defects and the degradation of concrete.
3.8. Microwaves
The frequency of electromagnetic energy ranges from several hundred MHz to sev-
eral hundred GHz. Such high-frequency waves can penetrate thick FRP materials to de-
termine the transmission and distribution of electromagnetic waves.
This method has been used to detect debonding, delamination and holes, in FRP con-
crete members. Important scale information, such as the spatial resolution, location and
size of the debonding area, can be determined by the microwave method. Li and Liu [44]
and Buyukozturk et al. [45] used microwaves to detect holes between FRP and concrete.
Aboukhousa and Qaddoumi used electromagnetic waves [46] to inspect a composite and
found that reflected waves can indicate debris under the surface of a composite plate with
five layers and 45.6 mm thickness.
3.9. Optical Fiber Sensing
Optical fiber sensing is widely used to detect the damage of intelligent structures in
compound materials. This method is nondestructive, and the equipment is small and
highly sensitive. An optical fiber is composed of hollow quartz glass, a photoconductive
coating and a plastic outer protective layer. According to the law of refraction, light will
only be reflected in the hollow glass. There are various optical fiber sensors, such as in-
tensity sensors, spectrum sensors and interference sensors.
Researchers embed optical fiber sensors in FRP bars so that FRP materials are intelli-
gent and can complete the detection or act as sensors by themselves. Kaamkarov et al. [47]
embedded Fabry Perot and Bragg grating fiber optic sensors in pultruded GFRP and
CFRP tendons. Thus, the whole FRP bar is the sensor. This method can be used to detect
the health of prestressing cables. Sim et al. [48] studied the tensile strength and pullout
performance of Bragg grating fiber optic sensors in GFRP bars and found that hybrid
GFRP bars can be reinforcement materials for concrete structures and can perform intelli-
gent monitoring.
An optical fiber is pasted or buried on the interface between layers of composite.
Thus, the optical fiber is placed in the composite laminate, and the epoxy glue can protect
the optical fiber. The strain of the material in the bonding surface of FRP concrete can be
measured by this method, and the unloading recorded by the sensor can indicate the
debonding of the surface between FRP and concrete bonding. Bonfigliol and Pascal [49]
used optical fiber sensors to measure the strain on the concrete surface in the debonding
area of the interface between FRP and concrete. Zhu et al. [50] arranged a sensor on the
surface between an FRP pipe and concrete to measure the strain and cracks in the concrete.
Optical fiber sensors can be embedded into materials or interfaces between different
materials during material production and bridge construction, and it is easy to combine
optical fiber sensors with composite materials. Laylor and Kachlakev [51] used Bragg grat-
ing sensors to detect the durability of concrete beams strengthened by FRP in a Horsetail
Falls bridge, Oregon, USA. Watkins et al. [52] used an optical fiber network composed of
interference sensors to monitor the static and dynamic strains of a concrete bridge
strengthened with FRP in Missouri, USA. The expectations of the design, finite element
calculation results and measurement results are consistent. Fiber optic sensors are also
widely used to monitor the construction of new FRP bridges and the reinforcement of FRP
bridges in Canada.
Materials 2021, 14, 7826 8 of 10
3.10. Modal Analysis and Load Experiment
The mechanism of this method is the difference in the response to vibration or load.
When an FRP bridge ages, the overall stiffness changes, resulting in the response to vibra-
tion or load changes. This change is used to evaluate the health of the bridge. Several
acceleration sensors are arranged on the bridge, and then a predetermined load is applied
on the bridge to determine the modality and vibration of the structure. The difference in
information between the measured modal and initial theoretical modalities can be used
to evaluate the degradation or damage of FRP bridges. Alampelli [31] used modal analysis
to detect the health of the first FRP bridge in New York state. Guan et al. [53] arranged
acceleration sensors on the FRP viaduct to collect vibration information and carried out
modal analysis to determine the long-term performance of bridges.
When the load experiment is carried out, a strain gauge, accelerometer and displace-
ment meter are set on the bridge structure to apply the preset load, and then the strain
and displacement are measured. The severity of strain and displacement are used to eval-
uate the performance of the bridge. Load experiments should be conducted every period
(such as 1–2 years) to detect the damage of the bridge structure over time.
4. Conclusion and Further Studies
The damages in the bridges with FRP structures are inevitable. For visual inspection,
knock and dragging-chain methods, the equipment is simple, and the required profes-
sional skill is moderate. Alternatively, for thermal imaging, acoustic emission, radiation,
modal analysis and field load tests; special and expensive test equipment; and more com-
plex skills are needed. Therefore, the two groups of methods can be combined. First, a
simple method is used to determine the potential damage, and then radiation, modal anal-
ysis and load experiments are used to determine the damage mode and degree. FRP ma-
terials have been a universal part of modern structures and can contribute to prolonging
the service life of old structures, such as seismic reconstruction, the reinforcement of con-
crete structures, the maintenance of metal and wood beams and the repair of historical
sites. The methods involved in actions related to inspection of FRP bridges can be applied
to inspecting those structures with FRP materials. I share the view of Naser et al. that the
application of sensing devices will provide insight into the long-term behavior of concrete
structure strengthened by FRP materials [8], which should be the main method for in-
specting FRP in the future with the progress of sensing device manufacturing technology
and the reduction of price.
The maintenance and inspection of FRP bridges is a large and important specializa-
tion, and it has received a great deal of attention. However, although many studies have
focused on the techniques of maintenance and inspection, few of them have demonstrated
the conditions for the applications of these techniques. In particular, do the characteristics
of bridges (such as span, height), uses (such as cross-sea and cross-railway bridges for
automobiles, cross- street bridge for pedestrian passage), and environments where
bridges are sited (such as temperature, humidity) influence the accuracy of techniques?
What frequency and interval are proper for the maintenance and inspection of FRP
bridges? Moreover, is it necessary to implement inspection during the construction of FRP
bridges? Comparing the accuracy of various detection methods and linking machine
learning with the mechanism of the detection is crucial for developing the intelligent in-
spection of FRP structure damage.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data sharing not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
Materials 2021, 14, 7826 9 of 10
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