fibers

Review
A Review of Fibre Reinforced Polymer Structures
Jawed Qureshi

School of Architecture, Computing and Engineering (ACE), University of East London, 4-6 University Way,
Beckton, London E16 2RD, UK; j.qureshi@uel.ac.uk; Tel.: +44-020-8223-2363

Abstract: This paper reviews Fibre Reinforced Polymer (FRP) composites in Civil Engineering
applications. Three FRP types are used in Structural Engineering: FRP profiles for new construction,
FRP rebars and FRP strengthening systems. Basic materials (fibres and resins), manufacturing
processes and material properties are discussed. The focus of the paper is on all-FRP new-build
structures and their joints. All-FRP structures use pultruded FRP profiles. Their connections and
joints use bolting, bonding or a combination of both. For plate-to-pate connections, effects of geometry,
fibre direction, type and rate of loading, bolt torque and bolt hole clearance, and washers on failure
modes and strength are reviewed. FRP beam-columns joints are also reviewed. The joints are divided
into five categories: web cleated, web and flange cleated, high strength, plate bolted and box profile
joints. The effect of both static and cyclic loading on joints is studied. The joints’ failure modes are
also discussed.

Keywords: FRP connections; hybrid joints; all-FRP structures; pultruded FRP structures; bolted
joints; plate-to-plate connections; beam-to-column joints; failure modes

1. Introduction



Masonry, timber, steel and concrete are traditional materials that have been used in


construction for the last 100 years. Fibre Reinforced Polymer (FRP) is a relatively new
Citation: Qureshi, J. A Review of
material, which has been used in buildings and bridges for over 50 years. FRP use in marine,
Fibre Reinforced Polymer Structures.
Fibers 2022, 10, 27. https://doi.org/
automotive and aerospace industries dates back to the 1930s [1,2]; it has also been used
10.3390/fib10030027
in rail, sport, and wind turbines. Construction uses about a quarter of globally produced
FRPs [3–5]. Figure 1 shows the market share of FRPs by applications. FRP composites have
Academic Editor: Constantin fibres encased in a polymer resin. For structural use, glass, carbon, or aramid fibres are
Chalioris
Fibers 2022, 10, x FOR PEER REVIEW usually embedded in polyester, vinylester or epoxy resins. The fibres give strength and 2 of 30
Received: 20 December 2021 stiffness, whilst the resin glues the fibres together; it also protects the fibres and transfers
Accepted: 25 February 2022 forces between them [1,2].
Published: 8 March 2022
Miscellaneous,
Appliances, 8%
Publisher’s Note: MDPI stays neutral 4%
with regard to jurisdictional claims in Consumer
goods, 8%
published maps and institutional affil-
Construction, 26%
iations.

Electronic, 10%

Marine, 12%
Copyright: © 2022 by the author.
Licensee MDPI, Basel, Switzerland. Automotive , 31%

This article is an open access article

distributed under the terms and
Aerospace, 1%
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
Figure 1. Market share of Fibre Reinforced Polymer (FRP) by application [4,5].
4.0/).
Figure 1. Market share of Fibre Reinforced Polymer (FRP) by application [4,5].

There is a good amount of research on members in all-FRP structures. The challenge

lies in dealing with the connections and joints for FRP members.
Fibers 2022, 10, 27. https://doi.org/10.3390/fib10030027
FRP joint details are cur-
https://www.mdpi.com/journal/fibers
rently copied from steel design practice, which mostly leads to oversized FRP compo-
Fibers 2022, 10, 27 2 of 29

Lightweight, high strength, corrosion resistance and expected durability over their
lifetime are the key benefits of FRPs [6–8]. Glass or carbon fibre reinforced polymer (GFRP
or CFRP) shapes are used in structural applications. GFRP is more common due to its elec-
trical insulation and electromagnetic transparency, whereas CFRP is electro-conductive [9].
GFRP is also less energy intensive to produce than CFRP. FRPs generally have linear-elastic
stress–strain behaviour up to failure. This is described as a brittle failure, a type of sud-
den failure without enough warning. Due to brittle nature of FRPs, concepts of stress
redistribution and plasticity are often discarded [10]. FRPs also have relatively poor trans-
verse or shear strength [11]. There are also concerns about behaviour of FRP at elevated
temperatures and exposure to fire [6]. There are three types of FRPs in Civil Engineering:
(1) All-FRP structures for new-build; (2) FRP rebars; (3) FRP strengthening systems. This
paper reviews the applications, materials, and manufacturing. The focus is on all-FRP
structures, especially their connections and joints.
There is a good amount of research on members in all-FRP structures. The challenge
lies in dealing with the connections and joints for FRP members. FRP joint details are cur-
rently copied from steel design practice, which mostly leads to oversized FRP components.
Due to no plastic redistribution, stress concentrations around bolt holes are higher than
ductile streel material, whilst anisotropy and low transverse properties of FRPs add yet
more complexity [1]. Whilst there are no agreed design codes for all-FRP structures, there
are design guides though and manufacturers’ manuals [9,12–17]; however, they have no
legal standing. The designers usually rely on the design guides produced by manufacturers.
FRP is a heterogenous material, which has lengthwise strength comparable with structural
steel and transverse strength about a third of the longitudinal value. However, the material
capacity of FRPs is rarely utilised, as the design is often controlled by deflections rather
than strength.
Joints between FRP members can be bolted, bonded or hybrid—combining bolting
and bonding [7]. Bolted joints are useful for demountable structures, but bolt holes create
stress concentrations due to discontinuity of fibres and can lead to moisture ingress [18].
Adhesively bonded joints use the maximum strength and stiffness of fibres without dis-
turbing fibres, but they result in sudden failure and are affected by humidity and high
temperatures [10]. Manufacturers [9,12,13] recommend using bolted or hybrid joints, and
discourage use of adhesive bonding alone. Combining bolting and bonding may not be
needed, as the load is mainly taken by the adhesive due to stiff load path, but there are
benefits of hybrid joints in some situations. Adhesive bonding is good at taking shear loads,
while bolting is the best at transferring direct tension and transverse loads. Fatigue life and
fire performance of hybrid joints is better than bolted or bonded joints alone. Hybrid joints
resist load in all directions too; they can be used in high temperature environment, if the
extra cost of fabrication is justified [19–22].
Major review papers on FRP joints in buildings have been written by Mottram [23,24]
and Turvey [25] with a review period from 1980s to 2014. These papers discuss testing
arrangement, joint configurations and moment-rotation response of pultruded FRP con-
nections and joints in detail. The present paper gives a wider perspective of FRPs in Civil
Engineering with a focus on FRP plate-to-plate connections and member joints, presenting a
good state-of-the-art review of FRPs in Structural Engineering from 1980s to 2021. It is vital
to create awareness about structural use of FRP in structural engineering community and
academia. The paper also provides a reasonable database for typical material properties,
applications, manufacturing processes and current design guidelines. Over 160 documents
have been reviewed from 21 different countries. The documents per country are identified
as: Japan: 1, China: 3, Canada: 8, Switzerland: 11, USA: 36, Saudi Arabia: 2, South Africa: 1,
Belgium: 1, Portugal: 6, UK: 66, Denmark: 2, Germany: 1, Italy: 9, Sweden: 3, Norway: 1,
Russia: 1, Netherlands: 2, Ireland: 1, Korea: 1, Egypt: 5, Greece 2 and Australia: 3.
A comprehensive review of monotonic and cyclic response of beam–column joints
together with FRP plate-to-plate connections is presented in this paper. Substantial research
on cyclic behaviour of FRP joints has emerged in past five years, which has not been
Fibers 2022, 10, 27 3 of 29

reviewed in any other review papers in [23–25]. This paper is novel in a sense that it
reviews a good number of publications on cyclic performance of FRP joints published
in last five years. The main emphasis of the paper is on experimental studies on FRP
connections and joints. The studies on numerical modelling of FRP joints are not included
in this review paper; moreover, this paper presents the key findings from past papers in a
tabular format for easy understanding of the readers by identifying the major knowledge
gaps in FRP joints and the need for future research in those areas. The paper is beneficial
for structural engineers and researchers for quick and easy access to main conclusions from
research on FRP connection and joints in last 40 years.
The objective of this paper is to review research conducted on all-FRP connections and
joints subjected to monotonic and cyclic loading, in addition to providing a wider context
of FRP’s use as reinforcing bars in concrete and use in repair and rehabilitation of existing
structures. To the author’s knowledge, no review paper exists on cyclic performance of
all-FRP joints. This paper addresses the gap in knowledge. Main findings from various
papers are presented as bullet points for identifying key research and development areas for
future. Most past review papers focus on niche research areas of FRP in Civil Engineering.
This paper not only provides a review of all-FRP joints but also discusses broader use of
FRP in other Civil Engineering applications; this makes it a key reference paper for both
structural engineers and academics for the state-of-the-art research in FRP.
Structural Engineering applications of FRP composites are discussed in Section 2.
All-FRP structures for new-build including buildings and bridges are briefly reviewed.
Then, FRP rebars, grids, prestressing tendons, and formwork for use in concrete structures
are reviewed. The section also highlights use of FRP sheets, plates, strips and fabrics for
repair and rehabilitation of existing structures. Section 3 is about materials and manufac-
turing of FRP composites. Different fibres and resins are discussed. Health issues and
mitigation measures related to polymer resins are also discussed. Manufacturing processes,
such as pultrusion, hand layup and other methods are described. Major research on FRP
plate-to-plate connections subjected to in-plane forces is reviewed in Section 4. The effects
of geometry, lateral restraint, fastener parameters, fibre orientation and multi-bolted con-
figurations are reviewed. Typical failure modes, such as net-tension, shear-out, cleavage
and bearing are reviewed. Section 5 deals with FRP frame joints between members. The
joints subjected to both monotonic and cyclic loading are reviewed. The main findings
from research in FRP connections and joints are presented in a tabular format. Section 6 is
about setbacks and future of FRP composites in Civil Engineering. Finally, conclusions and
research needs are presented in Section 7.

2. FRP Applications
2.1. All-FRP New-Build Structurs
All-FRP new-build structures mainly use pultruded fibre reinforced shapes. Pultrusion
is an automatic process for producing constant section profiles on a mass scale (details at
Section 3.2.1). The FRP shapes look like structural steel sections but behave similarly to
wood [26]. The standard profiles are produced as I, H, C, leg-angle and tubular sections,
see Figure 2a. FRP elements have been used in building systems, bridges, cooling towers,
chemical and food processing plants, railway platforms and marine structures [7,8,27–30].
The first mobile five-storey FRP building Eyecatcher (Figure 2b) was exhibited in 1999
at Swiss Building Fair. Later, it was relocated to another location in Basel, where it still
exists as an office building. The building had three adhesively bonded parallel frames with
wooden decks. Bolted joints were only used where needed for dismantling [26,31].
Startlink Lightweight Building System (SLBS) was introduced in the UK in 2012,
resulting in construction of a pultruded FRP test house at the developer’s site in Bourne,
Lincolnshire, UK. The prototype/concept modular FRP profiles are shown in Figure 2c.
However, these concept profiles and their snap-fit connections were not pursued further.
Much simpler and fewer pultruded FRP profiles, with easy to assemble connections, were
used in the construction of the actual test house. This all-FRP test house was supported on
Fibers 2022, 10, 27 4 of 29

composite piles; and it was built just in two weeks [32,33], see Figure 2d. The Startlink test
Fibers 2022, 10, x FOR PEER REVIEW
house does not exist anymore. It has been taken down; and the author is not aware if4itofhas30

been constructed anywhere else in the UK.

(a) (b) (c)

(d) (e) (f)

Figure
Figure 2.2. Applications
ApplicationsofofFRP
FRPininbuildings
buildingsand other
and structures:
other (a) (a)
structures: Glass fibre
Glass reinforced
fibre polymer
reinforced poly-
structural shapes [12]; (b) Five-storey Eyecatcher FRP demountable building Switzerland [31]; (c)
mer structural shapes [12]; (b) Five-storey Eyecatcher FRP demountable building Switzerland [31];
Startlink test house modular pultruded FRP concept profiles [32]; (d) Startlink—Fully composite
(c) Startlink test house modular pultruded FRP concept profiles [32]; (d) Startlink—Fully composite
test house [33]; (e) Pultruded FRP cooling tower [12]; (f) Duracomposites railway platforms [34].
test house [33]; (e) Pultruded FRP cooling tower [12]; (f) Duracomposites railway platforms [34].
Startlink Lightweight Building System (SLBS) was introduced in the UK in 2012, re-
FRPs are also used in cooling tower industry. Cooling towers are used for heating,
sulting in construction of a pultruded FRP test house at the developer’s site in Bourne,
cooling, ventilation and industrial purposes. GFRP profiles can resist corrosion and expo-
Lincolnshire, UK. The prototype/concept modular FRP profiles are shown in Figure 2c.
sure to water often encountered in cooling towers. Manufactures [9,12] supply bespoke
However, these concept profiles and their snap-fit connections were not pursued further.
and standard FRP elements for cooling towers, see Figure 2e. GFRP railway platforms are
Much simpler and
also becoming fewerdue
popular pultruded
to speed FRPof profiles, with and
construction easyease
to assemble connections,
of assembly [9,13,34],were
see
used in the construction of the actual test house. This all-FRP test house
Figure 2f. FRP composites are also used in secondary structures; these include insulated was supported
on composite
ladders, floor piles; andstairways
gratings, it was built justhandrails,
with in two weeks [32,33],
working see Figure
platforms and2d. The Startlink
walkways and
test house does not exist anymore. It has been taken down; and
building façade panels [1]. Bridge engineering applications of FRP are presentedthe author is not
in aware
Figure if3
it
andhasarebeen constructed
discussed next. anywhere else in the UK.
FRPs
Corrosion andused
are also in cooling
fatigue tower
resistance, industry.
high Cooling towers
strength-to-weight ratioare
andused for heating,
formability are
cooling,
some of the desirable properties of FRP for bridges [3,41]. FRPs have been usedand
ventilation and industrial purposes. GFRP profiles can resist corrosion expo-
to repair,
sure to water
replace, oftenexisting
or retrofit encountered in concrete
steel or cooling towers.
bridges.Manufactures
FRP is used in [9,12]
newsupply
footbridgebespoke
and
and standard FRP elements for cooling towers, see Figure 2e. GFRP railway
highway bridges. Critical elements in bridges are generally hybrids-FRP and traditional platforms are
also becoming popular due to speed of construction and ease of assembly
materials. Global developments in FRP bridges are reviewed in a recent paper [3]. FRP [9,13,34], see
Figure 2f. FRP composites are also used in secondary structures; these
bridges in the UK are reviewed in [42], the US in [43,44] and the Netherlands in [39]. FRPinclude insulated
ladders,
bridges are floor gratings,using
fabricated stairways withorhandrails,
standard working
bespoke FRP platforms
elements and walkways
[45]. Aberfeldy, and
Scotland
building façade panels [1]. Bridge engineering applications of FRP are presented in Figure
3 and are discussed next.
Fibers 2022, 10, 27 5 of 29

cable stayed bridge was the first major FRP composite footbridge completed in 1992. The
113 m long bridge had pultruded GFRP composite deck supported by aramid cables
Fibers 2022, 10, x FOR PEER REVIEWattached to GFRP A-frames. It carried pedestrians and golf buggies on a golf course,
5 ofsee
30

Figure 3a. Only foundations were constructed from concrete [36,42,45].

(a) (b) (c)

(d) (e) (f)

Figure
Figure 3.3. Applications
ApplicationsofofFRP
FRPinin
bridges: (a)(a)
bridges: Aberfeldy Scotland
Aberfeldy footbridge
Scotland [35];[35];
footbridge (b) Bonds Mill Mill
(b) Bonds lift-
ing bridge [36]; (c) Kolding Denmark FRP pedestrian bridge, 1997 [9,37]; (d) The Pont y Ddraig, the
lifting bridge [36]; (c) Kolding Denmark FRP pedestrian bridge, 1997 [9,37]; (d) The Pont y Ddraig,
Dragon bascule bridge in Wales, 2013 [38]; (e) Fly-over Waarderpolder bridge in Netherland with
the Dragon bascule bridge in Wales, 2013 [38]; (e) Fly-over Waarderpolder bridge in Netherland with
FRP edge elements [39]; (f) Emersons Green Cycle Footbridge, Bristol UK [40].
FRP edge elements [39]; (f) Emersons Green Cycle Footbridge, Bristol UK [40].
Corrosion
The first everand FRP
fatigue
roadresistance,
bridge ishigh strength-to-weight
the Bonds Mill Lift bridge rationear
and Gloucester,
formability UK are
some of the desirable properties of FRP for bridges [3,41]. FRPs have
(Figure 3b); it was constructed over a canal in 1994. The bridge had a multi-cellular box been used to repair,
replace, or retrofit
girder filled existing steel
with structural foam ortoconcrete bridges.
resist local FRP from
bending is used in new
wheel footbridge
loads and
[15,36,42,45].
highway bridges. Critical elements in bridges are generally hybrids-FRP
The lightweight bridge elements helped in mechanical lifting system [45]. The first FRP and traditional
materials.
compositeGlobalbridge developments
in the USA was inconstructed
FRP bridgesover are no-name
reviewedCreek, in a recent
Kensas paper [3]. FRP
in 1996. The
bridges in the UK are reviewed in [42], the US in [43,44] and the
bridge used decks with glass FRP laminated skin and corrugated core [15]. Similarly,Netherlands in [39]. FRP
bridges are fabricated
in Kolding, Denmark,using standard
an all-FRP or bespoke
cable FRP elements
stayed footbridge was [45]. Aberfeldy,
constructed Scotland
in 1997, see
cable
Figurestayed bridge
3c. It was the was
first the
FRPfirst major
bridge FRP
over composite
a busy railwayfootbridge completed
line in Europe. in 1992.
The bridge The
girders
113
andm long bridge
pylons had pultruded
were pultruded GFRP. GFRP
Onlycomposite
bolts anddeck supported
abutments werebystainless
aramid cables at-
steel [37].
tached to GFRP A-frames. It carried pedestrians and golf buggies
In 2011, a new all-FRP bridge was constructed at Dawlish rail station in Exeter, UK to on a golf course, see
Figure
replace3a. Only foundations
a rusty steel bridge.were The constructed
new FRP bridge from concrete
mimicked [36,42,45].
the shape of the old steel
bridge, see Figure 4. The new FRP bridge survived 2014 storms;near
The first ever FRP road bridge is the Bonds Mill Lift bridge Gloucester,
it remained UK (Fig-
undamaged
ure
and3b); it was constructed
corrosion free [46,47]. over a canal in 1994. The bridge had a multi-cellular box girder
filledThe
withbest
structural
example foam to resist local
of combining bending
carbon andfrom
glasswheel
FRPsloads
is the[15,36,42,45].
Pont y Ddraig Theorlight-
the
weight
Dragons bridge
bridgeelements
(Figurehelped
3d) at in mechanical
Rhyl Harbour,lifting
North system
Wales,[45]. The
built infirst
2013.FRP composite
This double
bridge
basculeinfootbridge
the USA was usesconstructed
the freedomover no-nameand
in geometry Creek, Kensas inof1996.
lightweight FRP The bridgeItused
materials. had
decks with glass FRP laminated skin and corrugated core [15]. Similarly,
two mirroring, 32 m long decks hinged on a central caisson. The decks can be lifted for in Kolding, Den-
mark, an all-FRP
navigation cablerunning
by cables stayed footbridge was constructed
up to a central in 1997,
stainless steel tower. seeThey
Figureare3c.made
It wasfrom
the
first FRP bridge over a busy railway line in Europe. The bridge girders and pylons were
pultruded GFRP. Only bolts and abutments were stainless steel [37]. In 2011, a new all-
FRP bridge was constructed at Dawlish rail station in Exeter, UK to replace a rusty steel
bridge. The new FRP bridge mimicked the shape of the old steel bridge, see Figure 4. The
new FRP bridge survived 2014 storms; it remained undamaged and corrosion free [46,47].
 Fibers 2022, 10, 27 6 of 29

resin-infused FRPs [15,38]. FRP edge elements were used in the fly-over Waarderpolder
in Haarlem, Netherlands (Figure 3e). Use of FRP edge elements removes durability con-
cerns inherent to steel and concrete edge elements, and give more freedom and choice
in geometry [39]; moreover, the FRP edge elements are also aesthetically pleasing. The
proposed Emersons Green East FRP cycle footbridge will be constructed in Bristol, UK,
see Figure 3f. The bridge will have carbon fibre reinforced polymer (CFRP) arch ribs that
Fibers 2022, 10, x FOR PEER REVIEW 6 of 30
will support glass fibre reinforced polymer (GFRP) deck. Structural health monitoring
equipment will also be installed on the bridge for research [40].

(a) (b)
Figure 4. Dawlish
Figure pedestrian
4. Dawlish bridge
pedestrian Exeter
bridge UK:UK:
Exeter (a) (a)
OldOld
rusty steel
rusty bridge,
steel 1937
bridge, [47];
1937 (b)(b)
[47]; New FRP
New FRP
bridge, 2011 [47].
bridge, 2011 [47].

TheFRP
2.2. bestasexample of combining carbon and glass FRPs is the Pont y Ddraig or the
Reinforcement
Dragons FRP
bridge (Figure 3d) athave
reinforcements RhylbeenHarbour,
used North Wales, built
in structural in 2013.since
engineering This double bas-
1950s. Today,
culeFRP
footbridge uses the freedom in geometry and lightweight of FRP materials.
rebars, grids, fabrics, strips, prestressing tendons, and formwork are commercially It had
two available
mirroring, 32 m long decks hinged on a central caisson. The decks can be
[2,48]. FRP reinforcements are suitable in aggressive conditions, such as alkaline, lifted for
navigation
corrosive byand
cables running
chemical up to a central
environments. stainless
Lightweight andsteel tower. Theyneutrality
electromagnetic are made arefrom
other
resin-infused
benefits ofFRPs [15,38].
FRP rebars, FRP edge
especially elements
glass were used
FRPs. Carbon, inaramid
glass, the fly-over Waarderpolder
fibre reinforced polymer
bars are commonly
in Haarlem, Netherlands used. Research
(Figure 3e).in FRP
Use ofreinforcement is quite removes
FRP edge elements developeddurability
comparedcon- to all-
FRP pultruded structures. Several design guides for FRP rebars are available
cerns inherent to steel and concrete edge elements, and give more freedom and choice in too. In Europe,
the task
geometry group
[39]; 5.1 (formerly
moreover, 9.3) edge
the FRP produced technical
elements report
are also fib 40 [48] for
aesthetically FRP reinforcement
pleasing. The pro-
in Emersons
posed concrete structures.
Green EastAlso, The Concrete
FRP cycle footbridge Society
will behasconstructed
its TR55 [49] design guide
in Bristol, UK, see for
strengthening applications including FRP rebars [50]. In the USA, ACI
Figure 3f. The bridge will have carbon fibre reinforced polymer (CFRP) arch ribs that will 440.1R-15 [51] deals
with glass
support design of concrete
fibre members
reinforced polymer with(GFRP)
FRP bars. FRP
deck. reinforcements
Structural healthand their applications
monitoring equip-
are shown in Figure 5. Design of bridge beams
ment will also be installed on the bridge for research [40]. prestressed with CFRP bars or cables is
given in NCHRP research report 907 [52].
2.2. FRP as Reinforcement
2.3. FRP in Strengthening Applications
FRPResearch,
reinforcements
design have been used
and practice are in
wellstructural
advanced engineering
for FRP usesince 1950s. Today,
as strengthening FRP
material.
rebars, grids, fabrics, strips, prestressing tendons, and formwork are commercially
FRP can be used for repair and strengthening of existing structures. Externally bonded avail-
ablereinforcements
[2,48]. FRP reinforcements
can be used to are suitableconcrete,
reinforce in aggressive
timber,conditions, such as alkaline,
steel and masonry structurescor-
[53].
rosive and chemical
Design guidelines environments.
for externallyLightweight
bonded FRPand electromagnetic
systems neutrality
concrete structures areare other
available
benefits of FRP(CEB-FIP
in Europe rebars, especially
fib bulletinglass FRPs.
14 [54]) andCarbon,
Americaglass,
(ACI aramid fibre[55]).
440.2R-17 reinforced
Variouspoly-
other
mer guidelines
bars are commonly used.
have also been Research
produced in FRP
in USA, reinforcement
Japan is quite
and Italy [56–64] for developed com-
FRP strengthening
pared to all-FRP pultruded
applications. structures.
Different design guidesSeveral
for FRPdesign guides for
strengthening are FRP rebarsinare
compared available
[65]. Environ-
too. mental actions,
In Europe, the poor design,5.1
task group lack of maintenance
(formerly or accidental
9.3) produced events
technical cause
report fibdeterioration
40 [48] for
FRPtoreinforcement
existing structures [54]. Strengthening
in concrete structures. Also, of these
The structures with FRP
Concrete Society systems
has its TR55not[49]
only
restores
design guidethem, but enhances their
for strengthening strength, including
applications too. FRP for strengthening
FRP rebars [50].isInavailable
the USA, as ACI
strips,
sheets [51]
440.1R-15 and deals
fabrics.
with design of concrete members with FRP bars. FRP reinforcements
FRP strengthening
and their applications are shown can be in
applied
Figurein-situ usingofhand
5. Design layup
bridge or can
beams be prefabricated
prestressed with
off-site in a factory. Hand or wet layup consists
CFRP bars or cables is given in NCHRP research report 907 [52]. of applying epoxy resin to woven fabric
sheets or flexible fibre sheets to produce FRP sheets bonded to concrete members. The
prefabrication
2.3. FRP method
in Strengthening involves pultrusion for FRP plates bonded to beams and slabs or
Applications
filament winding for making FRP shells for confining columns [67]. Pultrusion and filament
Research, design and practice are well advanced for FRP use as strengthening mate-
rial. FRP can be used for repair and strengthening of existing structures. Externally
bonded reinforcements can be used to reinforce concrete, timber, steel and masonry struc-
tures [53]. Design guidelines for externally bonded FRP systems concrete structures are
available in Europe (CEB-FIP fib bulletin 14 [54]) and America (ACI 440.2R-17 [55]). Vari-
 Fibers 2022, 10, 27 7 of 29

winding manufacturing processes are discussed later in Section 3. FRP strengthening of

concrete slabs, columns and beams [68] using FRP sheets is shown in Figure 6a–c. Flexural
strengthening of beams with FRP plates and FRP wrapping of concrete column [66] is
presented in Figure 6d. Next shown in Figure 6e is the 120 years old Münchenstein Railway
Bridge in Switzerland; this is a metallic rivetted bridge. Traditional strengthening solution
using steel plates or bonded CFRP plates were discarded due to unsmooth rivetted surfaces.
Ghafoori et al. [69] came up with innovative ideas of fatigue strengthening and wireless
Fibers 2022, 10, x FOR PEER REVIEW monitoring for this bridge. They used CFRP unbonded prestressed plates with wireless7 of 30
sensors to strengthen fatigue prone areas of the bridge [69], as shown in Figure 6f. Other
FRP strengthening techniques for bridges are discussed in [70].

(a) (b)

(c) (d)
Figure 5. FRP
Figure 5. FRP as
asaareinforcement:
reinforcement:(a)(a)
FRPFRP rebars
rebars [48,66];
[48,66]; (b) FRP
(b) FRP fabrics
fabrics [66];
[66]; (c) (c)bars
FRP FRPinbars
1995in 1995
Fidgett Footbridge Chalgrove-Oxfordshire, UK [66]; (d) Completed Fidgett Footbridge
Fidgett Footbridge Chalgrove-Oxfordshire, UK [66]; (d) Completed Fidgett Footbridge [66]. [66].

FRP
CFRPstrengthening
ropes have been can be as
used applied in-situ using
a strengthening methodhand layup orstrength,
to improve can be stiffness
prefabricated
and seismic
off-site responseHand
in a factory. of reinforced
or wet concrete beam-to-column
layup consists of applying joints in a recent
epoxy resin topaper
woven[71].fabric
The authors used X-shaped CFRP ropes to strengthen both sides of exterior
sheets or flexible fibre sheets to produce FRP sheets bonded to concrete members. The beam–column
joints. The six specimens
prefabrication were subjected
method involves to reverse
pultrusion cyclicplates
for FRP loading. Hysteretic
bonded curves,
to beams andload
slabs or
capacities, failure modes, stiffness and energy dissipation were determined to compare
filament winding for making FRP shells for confining columns [67]. Pultrusion and fila-
reinforced and non-reinforced joints. CFRP ropes significantly increased strength and
ment winding manufacturing processes are discussed later in Section 3. FRP strengthen-
seismic performance of the joints. The cracking in strengthened specimens did not appear
ing of concrete
in the joint areaslabs, columns
but there and beams
was some cracking [68] using
close FRP
to the beamsheets is shown
side. in Figure
Strengthening of 6a–c.
Flexural strengthening of beams with FRP plates and FRP wrapping
concrete T-beams using U-jacketing method with externally bonded CFRP sheets is studied of concrete column
[66] is presented in Figure 6d. Next shown in Figure 6e is the 120 years
in [72]. The authors used T-shaped shear-critical RC beams under four-point bending. old Münchenstein
Railway
CFRP sheetsBridge in used
were Switzerland; this
as external is a metallic
transverse rivetted bridge.
reinforcement. CFRPTraditional
strengthened strengthening
beams
solution using steelshear
showed enhanced plates or bonded
capacity. CFRP plates
But debonding of were
CFRPdiscarded due surface
from concrete to unsmoothcould rivet-
not be prevented. The authors applied the mechanical anchorage system
ted surfaces. Ghafoori et al. [69] came up with innovative ideas of fatigue strengtheningto U-jacketing,
which
and delayedmonitoring
wireless debonding resulting
for this in about 70%
bridge. Theyincrease in shear
used CFRP capacity. Application
unbonded prestressed of plates
three plies of CFRP sheets led to a 72% increase in shear capacity compared
with wireless sensors to strengthen fatigue prone areas of the bridge [69], as shown in to the control
specimen. The authors also compared their experimental results with various American
Figure 6f. Other FRP strengthening techniques for bridges are discussed in [70].
and European design codes.
CFRP ropes have been used as a strengthening method to improve strength, stiffness
and seismic response of reinforced concrete beam-to-column joints in a recent paper [71].
The authors used X-shaped CFRP ropes to strengthen both sides of exterior beam–column
joints. The six specimens were subjected to reverse cyclic loading. Hysteretic curves, load
capacities, failure modes, stiffness and energy dissipation were determined to compare
reinforced and non-reinforced joints. CFRP ropes significantly increased strength and
seismic performance of the joints. The cracking in strengthened specimens did not appear
 Fibers2022,
Fibers 2022,10,
10,27
x FOR PEER REVIEW 8 of 3029
8 of

(a) (b)

(c) (d)

(e) (f)
Figure 6.
Figure 6. FRP
FRP in
in strengthening
strengthening applications:
applications:(a)
(a)Flexural
Flexuralstrengthening
strengtheningofofslab
slab[68];
[68];(b)
(b)Wrapping
Wrapping
FRP fabrics around concrete columns [68]; (c) CFRP strengthening of beams [68]; (d) FRP plates
FRP fabrics around concrete columns [68]; (c) CFRP strengthening of beams [68]; (d) FRP plates
bonded to beams [66]; (e) Münchenstein Railway Bridge (120 years old) Switzerland [69]; (f) CFRP
bonded to beams [66]; (e) Münchenstein Railway Bridge (120 years old) Switzerland [69]; (f) CFRP
strengthening of Münchenstein bridge with sensors to monitor long-term prestress level [69].
strengthening of Münchenstein bridge with sensors to monitor long-term prestress level [69].
3. Materials
3. Materials and
and Manufacturing
Manufacturing
This section
This section is
is about
about raw
raw materials
materials and
and manufacturing
manufacturingprocesses
processesfor
formaking
makingFRP
FRP
shapes—bars, profiles and sheets.
shapes—bars, profiles and sheets.

3.1. Constituent
3.1. Constituent Materials
Materials
3.1.1.
3.1.1. Fibres
Glass, carbon
Glass, carbon and
andaramid
aramidare
arethe most
the mostcommon
common synthetic fibres.
synthetic Synthetic
fibres. fibresfibres
Synthetic are
are man-made, usually formed by chemical processes. Their properties are given1.in
man-made, usually formed by chemical processes. Their properties are given in Table
Glass 1.
Table fibres arefibres
Glass used to
aremake
usedFRP profiles,
to make FRPrebars and sheets.
profiles, rebarsThey
and come
sheets.in four
Theydifferent
come in
grades:
four different grades:
•• E-glass (electrical glass);
E-glass
•• A-glass (window glass);
A-glass
•• C-glass (corrosion resistant, also known
C-glass known as
as AR-glass
AR-glassor
oralkali-
alkali-resistant
resistantglass);
glass);
• S-glass (Structural or high-strength glass).
Fibers 2022, 10, 27 9 of 29

Glass fibre is an isotropic material. It has a bright white colour. E-glass is mostly used
for structural shapes due to its electrical insulation. A and C grades are used in specialized
structural products. Due to high strength, S-glass is used in the aerospace industry [1,2].
S-glass is 3–4 times more expensive than E-glass. E-glass fibres benefit from high strength
and relatively low cost. Some disadvantages of E-glass fibres include low modulus, low
humidity and alkaline resistances and reduced long-term rupture strength [73,74].

Table 1. Properties of fibres and thermosetting polymer resins [1,2].

Glass
Tensile Tensile Max
Density Fibre Transition
Material Grade Modulus Strength Elongation
(g/cm3 ) Architecture Temperature
(GPa) (MPa) (%)
(◦ C)
E 2.57 72.5 3400 2.5
A 2.46 73.0 2760 2.5 -
Glass Isotropic
C 2.46 74.0 2350 2.5
S 2.47 88.0 4600 3.0
Fibre Standard 1.70 250.0 3700 1.2
High strength 1.80 250.0 4800 1.4 -
Carbon Anisotropic
High modulus 1.90 500.0 3000 0.5
Ultrahigh modulus 2.10 800.0 2400 0.2
Aramid - 1.40 70.0–190.0 2800–4100 2.0–2.4 Anisotropic
Polyester - 1.20 4.0 65 2.5 - 70–120
Epoxy - 1.20 3.0 90 8.0 - 100–270
Polymer Vinylester - 1.12 3.5 82 6.0 - 102–150
Resin Phenolic - 1.24 2.5 40 1.8 - 260
Polyurethane - varies 2.9 71 5.9 - 135–140 [75]

Carbon fibres are generally used for strengthening applications: CFRP strips, sheets, re-
bars and prestressing tendons. Carbon fibres have high tensile, creep and fatigue strengths.
Their tensile modulus is higher than glass and aramid fibres; they have excellent chemical
resistance and low moisture absorption. Anisotropy, high production cost and thermal
conductivity are their drawbacks. Carbon fibres have a charcoal-black colour. Carbon fibre
strands are known as tow, and they are produced in four grades:
• Standard modulus (SM);
• Intermediate modulus (IM);
• High strength (HS);
• Ultrahigh modulus (UHM).
Aramid or Kevlar fibres are not common in structural engineering; yet, they are
still used in FRP bars and tendons. Relatively low compressive strength (500–1000 MPa)
and moisture absorption and high price make aramid fibres less suitable for structural
Engineering. Aramid fibres have high energy absorption due to their high toughness
properties. Their applications include bulletproof vests, helmets and automotive crash
attenuators [1,2,73]. Glass fibre is good all-rounder, carbon fibre has high stiffness and
aramid withstands impact [9]. Stress–strain behaviour of all fibre types is linear-elastic.
Fibres are used in various forms [73]:
• Rovings—parallel bundles of continuous untwisted filaments;
• Yarn—bundles of twisted filaments;
• Fibre mats with chopped or continuous fibres;
• Woven and non-woven fabrics;
• Stitched fabrics, grid, mesh and fleece;
• Carbon fibre tows.

3.1.2. Polymer Resins

Resin or matrix acts as a binder in the FRP composites. Resin protects fibres and en-
sures load transfer between them; it also stops buckling of fibres under compression. There
Fibers 2022, 10, 27 10 of 29

are two types of polymer resins—thermosetting and thermoplastic resins. Thermosetting

resins have cross-linked molecular structure; their shape does not change once set or cured.
Due to flowy nature and good adhesive properties, it is easy to place fibres in thermoset
resins [1,73]. There are five types of thermoset resins: polyester, epoxy, vinylester, phenolic
and polyurethane. Typical properties of thermoset resins are given in Table 1.
Thermoplastic resins are not cross-linked; they do not set and remain plastic, softening
on heating with the ability to change to any other shape. They can be recycled and
reprocessed, and it is hard to insert fibres though in a thermoplastic resin due to its gluey
nature and poor binding ability. Thermoplastic resins include polypropylene, polyamide,
polyethylene and polybutylene [1]. Thermoplastic resins are less strong and stiff than
thermoset resins. Polyphenylene and polyimide thermoplastic composites are used in
aerospace industry. Thermoplastic resins are rarely used in structural Engineering FRP
products. FRP structural shapes mainly use thermoset resins [2]. All resins are prone to UV
radiation. They require additives and surface fleece/veil for protection. Resins are isotropic
and show nonlinear viscoelastic stress–strain behaviour [1,73].
Epoxy resins are used as a matrix for FRP composites or as adhesives to connect
FRP shapes. Epoxy is mainly used for strengthening applications (sheets and strips).
Their use in FRP tendons and cables is also common. Due to high cost and difficulty in
processing, epoxy is not used in structural profiles. Polyester and vinylester resins are
widely used to make FRP profiles and bars. Due to its corrosion resistance and durability,
most FRP bars use vinylester matrix. Vinylester is more expensive than polyester. Most
manufacturers [9,12,13] produce identical FRP profiles in both polyester and vinylester
matrices [2]. Phenolic resins are the oldest and have superior fire resistance; their cost is
similar to polyester, and their use in FRP parts is very limited. Phenolic resin is used in
FRP gratings and strips. Polyurethane resin has high toughness, producing high tensile
and impact strength FRP composite when used with glass fibres. Polyurethane costs about
the same as vinylester.
There are health issues related to resins in construction industry. Limited research
exists on health effects of resins. Several health concerns associated with use of epoxy resins
are documented in a report by the Health and Safety Executive UK [76]. Skin contact with
epoxy causes allergic contact dermatitis (eczema), often known as skin sensitisation [77].
Dust or sprays generated by use of epoxy can also cause respiratory irritation. Some protec-
tive measures to reduce skin sensitisation effects include the use of protective equipment,
such as gloves, clothing and goggles. Instruction for workers about potential risks of using
epoxy could also reduce sensitisation. Barrier creams and protective spray coatings against
epoxy resins can also be good control measures [76–78].

3.1.3. Other Materials—Additives and Fillers

Fillers are used in resins to reduce the cost and improve properties of FRP shapes.
Fillers reduce shrinkage, improve fire rating and prevent cracking in resins. Fillers can
improve hardness and creep performance, and fatigue and chemical resistance. The typical
filler content ranges from 10–30% of the resin weight. While the filler can improve some
properties, it reduces mechanical properties and durability. Additives are used for variety
of reasons: UV protection, fire resistance, shrinkage reduction, pigments for colour, mould
removal, and electrical and thermal insulation. Additives are used in small amount, less
than 1% of the resin content. The physical and mechanical properties of FRP are affected by
additives [1].

3.2. Manufacturing Process

There are two FRP manufacturing processes for structural use: pultrusion (automatic)
and hand layup or wet layup (manual method). Pultrusion is used for FRP rebars, FRP
strips for external strengthening and FRP profiles. Hand layup is used for FRP sheets on
site for strengthening of existing structures [1,2].
 Fibers 2022, 10, x FOR PEER REVIEW 11 of 30

3.2. Manufacturing Process

Fibers 2022, 10, 27 There are two FRP manufacturing processes for structural use: pultrusion (auto- 11 of 29
matic) and hand layup or wet layup (manual method). Pultrusion is used for FRP rebars,
FRP strips for external strengthening and FRP profiles. Hand layup is used for FRP sheets
on site for strengthening of existing structures [1,2].
3.2.1. Pultrusion
3.2.1.Pultrusion
Pultrusion is the cost-effective way of producing FRP bars, profiles and strips [79,80].
It is the only automatic
Pultrusion methodway
is the cost-effective for of
making
producingconstant
FRP bars, section FRP
profiles andshapes. Pultrusion is
strips [79,80].
used to only
It is the produce I-beam,
automatic wide-fanged,
method for makingchannel
constantand multi-cellular
section FRP shapes. profiles.
PultrusionPultrusion
is
isused to produce
divided into twoI-beam, wide-fanged,
phases: channeland
fibre system andmatrix
multi-cellular
system. profiles.
In fibrePultrusion
system,isdifferent
divided into two(fibre
reinforcements phases: fibre system
bundles, matsand andmatrix
surface system. In fibre
veil) are fed system,
throughdifferent
the guiderein-
plate that
forcements
shape (fibre bundles,
the profile [1,2,81].matsThe and
fibresurface
bundles veil)are
arecalled
fed through
rovingsthe forguide
glassplate that
fibres and tows
shape
for the profile
carbon fibres[1,2,81]. The fibre bundles
[82]. Schematic diagram areofcalled rovings process
pultrusion for glass is
fibres
shown and in
tows for 7. The
Figure
carbon fibres [82]. Schematic diagram of pultrusion process is shown in Figure 7. The uni-
unidirectional rovings or tows provide the strength along the length of the profile. While,
directional rovings or tows provide the strength along the length of the profile. While, the
the continuous filament or strand mat, woven roving or stitched fabric give the strength
continuous filament or strand mat, woven roving or stitched fabric give the strength
across the width
across the widthofofthe the profile.
profile. Polyester
Polyester surface
surface veils veils
are alsoareadded
also added for surface
for surface finishingfinishing
and protection. These resin-rich veils also provide corrosion and
and protection. These resin-rich veils also provide corrosion and ultraviolet resistance. In ultraviolet resistance.
In
matrix systems, dry fibres are impregnated with resin and allowed to cure (solidify) in a(solidify)
matrix systems, dry fibres are impregnated with resin and allowed to cure
in a heated
heated mould.mould.
The FRPThe FRP is
material material is then
then pulled pulled
to give to give
it tensile it tensile
strength strength
[1,2,81]. Pultru- [1,2,81].
Pultrusion has six[1]:stages
sion has six stages (1) set[1]: (1) set
of spools of spools
stacked stacked
on creels on reinforcement
for fibre creels for fibre reinforcement
handling;
(2) preforming guides; (3) resin impregnation bath; (4) forming and curing
handling; (2) preforming guides; (3) resin impregnation bath; (4) forming and curing die; die; (5) pulling
system;
(5) (6) cutting
pulling system;system.
(6) cutting system.

Figure 7.
Figure 7. Schematic
Schematicdiagram
diagramof pultrusion process
of pultrusion (Courtesy
process of Strongwell
(Courtesy [12]). [12]).
of Strongwell

A typical
A typicalpultruded
pultruded FRP profile
FRP profilehas has
a middle layerlayer
a middle sandwiched between
sandwiched two outer
between two outer
layers. The middle layer uses unidirectional roving bundles running
layers. The middle layer uses unidirectional roving bundles running in the directionin the direction of of
pultrusion. The two outer layers use fibre mats, either continuous filament mat (CFM),
pultrusion. The two outer layers use fibre mats, either continuous filament mat (CFM),
chopped strand mat (CSM) or woven fabrics. Two polyester surface veils are also added
chopped strand mat (CSM) or woven fabrics. Two polyester surface veils are also added to
to the outer layers [2,23,81]. Typically, the fibre volume in pultruded FRP profiles ranges
the outer layers [2,23,81]. Typically, the fibre volume in pultruded FRP profiles ranges from
from 35% to 50% [82]. For FRP bars and strips the fibre percentage ranges from 50% to
35%
60% toof 50% [82]. For
the volume ofFRP
FRPbars and
shape [2].strips themechanical
Typical fibre percentage ranges
properties fromreinforced
of glass 50% to 60% of the
volume of FRP
wide-flanged shapewith
profiles [2].vinylester
Typical mechanical
resin are givenproperties
in Table 2.ofA glass reinforced
comparison wide-flanged
of steel and
profiles with
FRP (glass, vinylester
carbon, aramidresin
FRP) are given
rebars’ in Table
tensile 2. A iscomparison
properties given in Tableof 3,
steel and
as per fibFRP
40 (glass,
carbon, aramid
[48]. Other FRP) properties
mechanical rebars’ tensile
of FRPproperties is givenfrom
rebars are adapted in Table 3, asand
[48,83,84] fib 40 [48]. Other
perpresented
mechanical propertiesproperties
in Table 4. Mechanical of FRP rebars
of FRPare adapted
sheets, from [48,83,84]
strengthening andfabrics
strips and presented in Table 4.
are not
given in thisproperties
Mechanical paper. Typical material
of FRP sheets, properties of commercially
strengthening strips andproduced
fabrics areFRP sheets,
not given in this
strengthening
paper. Typicalstrips and fabrics
material for strengthening
properties of commerciallypurpose can be found
produced on pagestrengthening
FRP sheets, 29–30
Table 1.2
strips and and Table for
fabrics 1.3 in L. C Bank’s book
strengthening [2].
purpose can be found on page 29–30 Table 1.2 and
Table 1.3 in L. C Bank’s book [2].

3.2.2. Hand or Wet Layup

Hand layup is commonly used for FRP strengthening sheets and fabrics; it is a manual
method that involves stacking layers of fibres in the resin system. After curing, the solid
FRP part takes the form of the mould. This method is also known as laminating or wet
layup. Hand layup can be used onsite or off-site; when used in a factory, FRP parts are
produced in a mould. The FRP part is removed from the mould after curing. FRP sandwich
panels for bridge applications use hand layup in an off-site plant; however, many structural
Fibers 2022, 10, 27 12 of 29

engineering applications, strengthening for example, require onsite production. In that

case, a proper connection must exist between FRP elements and structural part to be
strengthened; thus, epoxy resins, which have high adhesive properties, are used with
carbon or glass fibres [1,2]. Carbon fibre tow sheet is used for strengthening applications.
It contains dry carbon fibre aligned lengthwise, glass fibre scrim cloth aligned at 45 degrees
and epoxy adhesive. The thickness of the sheet is about 0.3 mm and is available in
30 to 100 cm widths. Woven or stitched glass or carbon fibre fabrics can also be used
for strengthening purposes.

Table 2. Mechanical properties of pultruded FRP wide-flanged profiles (Glass reinforced Vinylester
shapes 6–13 mm thick) [2,6,7].

Estimated Fibre Volume 25–40%

Fibre architecture Roving and mat
Longitudinal 207–317
Tensile
Transverse 48–83
Longitudinal 207–359
Compressive
Transverse 110–138
In-plane 31–48
Strength (MPa) Shear
Out-of-plane 27–31
Longitudinal 207–338
Flexural
Transverse 69–131
Longitudinal 207–269
Bearing
Transverse 179–234
Longitudinal 18–28
Tensile
Transverse 6–10
Longitudinal 18–26
Compressive
Modulus (GPa) Transverse 7–13
Shear In-plane 3.0–3.4
Longitudinal 11–14
Flexural
Transverse 6–12
Poisson’s ratio Longitudinal 0.33–0.35

Table 3. Comparion of tensile properties of steel and FRP rebars (with volume fraction of fibre from
50 to 75%) [48].

Material
Property
Steel GFRP CFRP AFRP
Longitudinal modulus (GPa) 200 35 to 60 100 to 580 40 to 125
Longitudinal tensile strength (MPa) 450 to 700 450 to 1600 600 to 3500 1000 to 2500
Ultimate tensile strain (%) 5 to 20 1.2 to 3.7 0.5 to 1.7 1.9 to 4.4

3.2.3. Other Manufacturing Processes

Other processes produce single unit instead of continuous production in pultrusion [2].
They include [1]: (1) filament winding; (2) centrifugation; (3) resin transfer moulding
(RTM); (4) resin infusion moulding (RIM) and (5) vacuum-assisted resin transfer moulding
(VARTM). FRP tubular sections and piles are produced through filament winding. In
this process, resin-saturated fibre roving or tow is wound around a cylindrical mandrel.
A mandrel is a tapered cylinder against which material can be forged or shaped. After
curing, the part is removed from the mandrel. RTM, RIM and VARTM methods are used
for producing FRP bridge decks and jackets for column strengthening [1,2].

3.3. Sustainabilty of FRP Materials

Performance and economy used to be the main criteria for the material choice in the
past. Sustainability approach requires engineers to select materials based on environmental
Fibers 2022, 10, 27 13 of 29

factors, energy consumption, social and economic factors, and performance criterion.
Sustainability also accounts for whole life cycle assessment from extraction, production and
use to disposal/recycling [85]. Polymer matrices require triple the amount of energy for
production as compared with steel. Glass fibres are less energy intensive to produce than
carbon fibres. The light weight of FRP reduces energy input for transportation. The carbon
footprint for transportation of steel and concrete is much higher than FRP composites. The
lightweight and speed of construction reduces the environmental impact of FRPs [86].

Table 4. Typical mechanical properties of GFRP, CFRP and AFRP reinforcing bars [48,83,84].

GFRP CFRP AFRP

Property
E-Glass/Epoxy Carbon/Epoxy Kevlar 49/Epoxy
Fibre volume fraction 0.55 0.65 0.60
Density (kg/m3 ) 2100 1600 1380
Longitudinal modulus (GPa) 39 177 87
Transverse modulus (GPa) 8.6 10.8 5.5
In-plane shear modulus (GPa) 3.8 7.6 2.2
Major Poisson’s ratio 0.28 0.27 0.34
Minor Poisson’s ratio 0.06 0.02 0.02
Longitudinal tensile strength (MPa) 1080 2860 1280
Transverse tensile strength (MPa) 39 49 30
In-plane shear strength (MPa) 89 83 49
Ultimate longitudinal tensile strain (%) 2.8 1.6 1.5
Ultimate transverse tensile strain (%) 0.5 0.5 0.5
Longitudinal compressive strength (MPa) 620 1875 335
Transverse compressive strength (MPa) 128 246 158

For quantifying ecological impact of materials, embodied energy related to greenhouse

gases must be known. The energy needed for extraction, processing, manufacturing and
transportation is termed as embodied energy; it quantifies the impact at the beginning of
the material’s life span instead of the whole life cycle [86]. Due to corrosion and chemical
resistance and less maintenance, the expected life span of FRPs is considered long; however,
FRPs structures have not been in existence long enough to quantify their life span. FRP
materials have lower embodied energy than traditional materials, such as steel and concrete.
In the context of sustainability, the embodied energy of any material should consider
durability, local availability, decomposition, recycling and waste management. While
FRPs meet many of these aspects, recycling remains the toughest challenge, hindering its
sustainable use. Unlike steel and timber, recycling of FRPs is limited. Only 1% of FRP
composites can be recycled [86], which is not financially viable considering the cost of
recycling process.

4. FRP Plate-to-Plate Bolted Connections

Pultruded FRP plate-to-plate bolted connections depend on various parameters: geom-
etry, bolts, material properties, pultrusion direction, and lateral restraint. Plate-to-plate FRP
connections are used in trusses or bracing. The load transfer between members and connec-
tions takes place through in-plane forces parallel to the member axis. The members can be
single or double overlapping, replicated as single-lap or double-lap shear plate-to-plate
connections [2]. Generally, tensile load is applied to these connections.

4.1. Failure Modes

The failure modes of a single-bolt FRP plate-to-plate connection are identified as:
bearing, net-tension, shear-out and cleavage. The plate-to-plate connection geometry and
the failure modes are shown in Figure 8. Bolt shear failure does not happen as bolts are
generally stronger than FRP plates. Stainless steel bolts are used in practice to deal with
environmental effects.
Fibers 2022, 10, 27 14 of 29

Figure 8. Geometry and failure modes of FRP plate-to-plate connections [23]; (a) Connection
geometry; (b) Bearing failure; (c) Net-tension failure; (d) Shear-out failure; (e) Cleavage failure.

4.1.1. Bearing Failure

Bearing failure is the localised compression failure in the FRP plate near the bolt. It is
the most desirable and the only failure that is less brittle. Bearing is a damage tolerant failure
that happens when plate width-to-hole diameter ratio is high. It relies on lateral restraint
that can delay delamination cracking. The failure is progressive in nature, indicated by
local buckling of fibres and crushing of resin. A less common name of the bearing failure is
the longitudinal shear failure [23,87–92]. Bearing failure has been extensively researched
in [87–89,93–96]. Bearing failure can be ensured by sizing the connection geometry properly
(generally e1 /d0 > 3 and w/d0 ≥ 4 or higher) [2,23,96].

4.1.2. Net-Tension Failure

Net-tension failure happens when bolt diameter is large and plate width small. Hart-
Smith [97] was the pioneer in establishing theoretical basis for this failure; it happens in
multi-row connections near the first row [23,98]. Net-tension is a brittle failure and should
be avoided; sudden cracking transverse to the load direction occurs in this failure, and it is
likely to happen when the edge distance, e2 is small.

4.1.3. Shear-Out Failure

Shear-out is another tension-type failure. This happens when the end distance is small
(e1 /d0 ≤ 4). Mostly, it is a consequence of bearing failure with short end distance. The
shear-out and net-tension are in-plane (2D) failure modes, whereas bearing is a 3D failure
mode. Shear-out is a brittle failure mode [23,97,99]. Theoretical models for shear-out failure
are given in the seminal paper by Hollmann [100].

4.1.4. Cleavage Failure

Cleavage failure is a combination of net-tension and shear-out failures. It is a tension-
type failure that initiates at the end of the plate instead of the bolt vicinity. Cleavage failure
happens when both end, e1 and edge, e2 distances are short. Also, it occurs when percentage
of fibres in the longitudinal direction of FRP plate is high [23,97,101].

4.2. Effect of Geometry

The connection geometry is the single most important criterion controlling failure
modes and strength. The geometric parameters include plate width and thickness (w and t),
hole diameter (d0 ), end and edge distances (e1 and e2 ), and pitch and gauge length
Fibers 2022, 10, 27 15 of 29

(p1 and p2 ). The parameters are shown in Figure 8a. Past research is reviewed, and main
findings are given in Table 5. Seminal research work on geometric parameters for single-
bolted double-lap shear connections is presented by Rosner and Rizkalla [93,102,103]. Later,
Turvey [104–106] carried out research on effects of width and end distance. Following
general conclusions can be drawn from research described in Table 5:
• Bearing failure is a pseudo-ductile failure giving us warning before failure;
• Connections should be designed for bearing failure, if practically possible;
• Bearing failure is enforced if e1 /d0 > 3 and w/d0 ≥ 4;
• Shear-out failure happened when e1 /d0 ≤ 4;
• Net-tension failure happened when w/d0 ≤ 3;
• Net-tension and cleavage are brittle failures and should be avoided;
• Increase in plate thickness and width increases connection resistance;
• For values beyond e1 /d0 > 2.5 and w/d0 > 4, there is no change in connection resistance;
• Bolt-diameter-to-plate-thickness ratio should be in the range of 1.0 ≤ (d/t) ≤ 1.5 for
ensuring the ductile bearing failure mode.

4.3. Effect of Fibre Orientation

The influence of angle between tensile load and fibre direction is important. The
connection strength is maximum when the tensile load is aligned with pultrusion direction.
The fibre orientation affects failure modes too. Following conclusions are drawn from
various research papers presented in Table 5:
• Strength and stiffness decrease when pultrusion angle changes from 0◦ to 90◦ ;
• Bearing failure happens for off-axis pultrusion angle lower than 45◦ ;
• Net-tension failure occurs for off-axis pultrusion angle greater than 45◦ .

4.4. Effect of Fastener Parameters and Lateral Restraint

Bolt material, bolt tightening, threaded and plain pins, clearance hole and lateral
restraint affect connection resistance. Following conclusions can be drawn from the papers
reviewed in Table 5:
• Connection strength with FRP bolt is about half the strength using steel bolt;
• Bolt thread reduces bearing strength; threaded pin-bearing strength is 0.6 of the plain
pin-bearing strength;
• Pin-bearing strength is reduced by 20–30% by hot-wet aging
• Clearance hole of 1.6 and 6.4 mm leads to 2% and 9% reduction, respectively, in
connection resistance compared with no-clearance condition;
• Bolt clearance hole of 1.6 to 2 mm is acceptable for ease in fabrication;
• Lightly clamped (3 Nm) and fully clamped (30 Nm) connections showed a 45% and 80%
increase in load compared to pin-bearing state (0 bolt torque with no lateral restraint);
• Bolt torque increases connection resistance;
• Bolt tightening cannot be relied on due to viscoelastic nature of FRP;
• Connection strength increases with confinement area.

4.5. Multi-Bolted Connections

Practical pultruded FRP connections use several bolts. The analysis procedures are
mainly developed from single-bolted connections. Designing multi-bolted FRP connection
based on data from single-bolted tests could be unsafe. Bearing failure in single-bolted
connection can easily turn into net-tension failure in multi-bolted configuration. In multi-
bolted connections, first row transfers more load than other rows [92]. Abd-El-Naby and
Hollaway [107] performed main research using two-bolted FRP connections. The main
points from this and other papers are summarised in Table 5. Following conclusions can
be drawn:
• The strongest connection that can fail in bearing has only single row of bolts;
• The failure in multi-bolted connections is either net-tension or cleavage;
Fibers 2022, 10, 27 16 of 29

• Connection resistance may not be sum of load per bolt;

• First row transfers more load than the other rows;
• Connection strength depends on bolts numbers but may not be directly proportional;
• Only 25% increase in strength is achieved by adding a second row with two bolts;
• Resin injected bolted connections are suitable for FRP bridges;
• Basalt FRP bolts can replace steel bolts.

Table 5. Main findings and test parameters for FRP plate-to-plate connections.

Parameter Researcher Set Up e1 /d0 w/d0 Main Findings

Rosner, • Strength increases up to e1 /d0 = 5; strength increases with

Double-lap 0.9–10 1.2–12.2 plate thickness
Rizkalla [93,102,103]
• For e1 /d0 ≤ 1: Cleavage failure: e1 /d0 ≥ 4: Bearing failure
• Bearing failure cannot be achieved with low shear strength
Abd-El-Naby and Double-lap: of FRP
Hollaway [108] high fibre volume • Critical end distance (after that no strength increase) depends
on width
• Criterion for bearing failure:
Turvey and Cooper [104] Double-lap 2–8 2–8 • For wide plates (w/d0 ≥ 7): e1 /d0 = 6; For other plates
(4 < w/d0 < 6): e1 /d0 > 6
Geometry
• e1 /d0 > 1.5 had little effect on strength; increase in w/d0
Bearing load via increases strength
Wang [90] steel pin and no 1–5 2–8 • Bearing strength is decreased when hole size is enlarged
lateral restraints • Bearing failure happens if e1 /d0 ≥ 1.5 and w/d0 ≥ 4 in
longitudinal direction and net-tension failure in
transverse direction
Turvey [105,106] Single-lap 1.5–4 • For values beyond e1 /d0 > 2.5 and w/d0 > 4 no change
in strength
• e1 /d0 = 2 shear-out failure and e1 /d0 ≥ 2 bearing failure;
Lee [109] Double-lap 2–7 5–7 e1 /d0 ≥ 4 no load increase
• w/d0 = 3 net-tension failure, w/d0 > 5 recommended
Rosner [102,103] Double-lap • Strength in 0◦ greater than 45◦ and 90◦ fibre direction
Double-lap • For e1 /d0 > 2.5 connection strength is steady and bearing
Turvey, Cooper [101,110] Fibre: 0◦ , 30◦ , 2–6 4–10 failure dominates
Fibre 45◦ , 90◦ • Strength and stiffness decrease with the-off-axis-angle.
orientation
Double-lap, • Pultruded FRP nine-layer flat sheet
Yuan and Liu [111] Fibre: 0◦ , 15◦ , 30◦ , 3 7 • Strength decreases as pultrusion angle changes from 0◦ to 90◦ .
45◦ , 60◦ , 75◦ , 90◦ • Bearing failure: angle lower than 45◦ and net-tension: angle
more than 45◦
• Connection strength with FRP rod is half the strength with
Erki [112] Double-lap steel rods
• Strong bolt—FRP plate fails and weak bolt—bolt fails
• Connection strength decreases with increase in clearance hole
Fastener Yuan et al. [113] Double-lap • 1.6 mm clearance leads to 2% reduction and 6.4 mm results in
parameters
9% reduction
Semi-notched FRP • Threaded pin-bearing strength is 0.6 of the plain, conforms to
Mottram [87,89,94,114] ASCE [14]
samples with pin
• Pin-bearing strength reduced by 20–30% by hot-wet aging
Abd-El-Naby [108] Double-lap • Strength increased with confinement area by tight bolts or plates
• Lightly clamped (3 Nm) and fully clamped (30 Nm) connections
Lateral Cooper Turvey [101] Double-lap showed 45% and 80% increase in failure load, respectively,
restraint compared with pin-bearing condition
Khashaba [115] • Bearing strength increased with tightening torque
Yuan and Liu [111] Double-lap • Strength increases with the level of bolt torque (varied 0–34 Nm)
Hart-Smith [97] Theoretical model • Seminal paper—proposed semi-empirical formulae for
multi-bolt joint
• Two bolts in series and aim—mechanism of load transfer and
Abd-El-Naby [107] Double-lap failure mode
• Load per bolt is equal to load taken by single-bolted connection
• Strength of two bolts in series (2 × 1) or parallel (1 × 2)
Double-lap is identical.
Multi-bolted (Row × bolts):
Prabhakaran [116,117] • Connections failed either due to block shear or net-tension.
connections (2 × 1), (1 × 2) • Strength depends on the number of bolts but may not be
and (2 × 2) directly proportional
• Bolt tightening had little influence on bolt load distribution
Double-lap • Failure affected by plate width, end distance and pultrusion axis
(2 × 1), (1 × 2) • Load shared equally in bolts placed in series (1 × 2) and (1 × 3)
Hassan [118,119] (3 × 1), (1 × 3) 2–5 9.9–14.8 • Only 25% increase in strength by adding a second row with
and (2 × 2) Fibre: 0, two bolts
45, 90 • Proposed a model for strength of multi-bolt connection
Fibers 2022, 10, 27 17 of 29

Table 5. Cont.

Parameter Researcher Set Up e1 /d0 w/d0 Main Findings

Ascione [120] Double-lap 9 bolts • Nine bolts in 3 rows used, middle row took 26% and outer
rows 37%
Mottram [98] Theoretical • Used Hart-Smith [97] method to predict net-tension strength
• The predicted strengths agree with experimental strengths
• BFRP bolts can replace steel bolts
Multi-bolted Abdelkerim [121,122] Double-lap BFRP • Joining methods: bonded/bolted, resin injected and
connections bi-directional BFRP layers increased loading capacity by 30–60%.
Qureshi [123–126] Double-lap • Resin injected bolted connections resist fatigue and slip in
FRP bridges
• Multi-row connections have net-tension or cleavage failure
Mottram, Turvey [88,127] Review papers in general
• Structural integrity of PFRP connections is unknown

5. FRP Bolted Frame Joints

In plate-to-plate connection configuration, typically in a truss, the connections transfer
only axial forces. Connecting elements and bolts are aligned with the member axis. The load
path in the plate-to-plate connections has in-plane forces only. In contrast, the members in
frame joints are usually connected at right angles. Joining parts and bolts are not aligned
with the centroid of the members. This generates moment due to out-of-plane forces. The
moment leads to prying forces at the top and compressive forces at the bottom of the
joint. The forces transferred by frame joints can be bending moment, shear, axial forces
and torsion [2,23]. Different beam-column joint test arrangements used in past research
are shown in Figure 9. Findings from joints subjected to monotonic and cyclic loads are
presented in Tables 6 and 7, respectively. Pros and cons of various test set ups are given in
Table 8.

Figure 9. Test arrangement for beam-to-column joints; (a) Direct Compression [25]; (b) Simply
supported beam [25]; (c) Double cantilever beam [27]; (d) Single cantilever beam [7,8,128].
Fibers 2022, 10, 27 18 of 29

Table 6. Main findings for FRP frame joints subjected to monotonic loading.

Researcher Set Up Sizes Joint Configuration Main Findings

 Beam-column joint • For economical design, semi-rigid analysis recommended
 Members: 203 × 203 × 9.5 mm WF  Flange-web (FW) cleated • Combined bolting and bonding recommended
Bank Direct  Cleats: FRP 152 × 152 × 12.7 mm angles  FW with column angle stiffener • Observed failure modes were tensile tearing of column web-flange junction and angle cracking
[129–131] Compression  Bolts: 19 mm FRP threaded rods tightened to  FW with built-up top part • Moment increased by adding angle stiffener to the column
41 Nm bolt torque  FW with built-up top and • A prototype joint with built-up part using T-flange, gusset plate and tubular stiffeners presented
bottom part • Wrapped angle FRP flange cleat was also used

 Members: 203 × 203 × 9.5 mm and 254 × 254 ×

 Beam-column joint
•
•
Top cleat was identified as the main weakness and bespoke L-shaped pre-preg cleat proposed
Steel cleat or cleat with different fibre architecture and manufacturing process proposed
Mottram Double

12.7 mm WF
 FW cleated • To exploit semi-rigid action bolting, bonding or combination must be tried
[24,132–134] cantilever Cleats: FRP or steel 152 × 152 × 12.7 mm and
 FW cleated with bonding • First failure or damage onset introduced: Damage at which fibres exposed to allow water ingress

102 × 102 × 12.7
Bolts: 16 mm steel with 23 and 100 Nm torque
 Pre-preg cleat piece • First failure is associated with prying due to hogging moments
• Adhesive bonding leads to brittle failure and cannot be used on its own
 Members: 102 × 102 × 6.3 mm WF

Mosallam Direct  Cleats: FRP 75 × 75 × 9.5 mm

Beam-column joint • Universal connector (UC) developed to eliminate delamination cracking of top cleat
[135,136] compression  Bolts: 12.7 mm pultruded threaded rods
Flange cleated with threaded
FRP rods
•
•
UC connector improved the strength and stiffness of joints
Failure modes: flange separation from web (tension), punching of web into flange (compression)
brightened to 40 Nm bolt torque
• The main aim was to establish serviceability deflection limits
 Members: 203 × 203 × 9.5 mm and 254 × 254 × • Deflection limit for FRP beams is span/340, which is not far from deflection limit (span/360) for steel
9.5 mm WF  Beam-column joint beams with brittle finishes
Qureshi Double
 Cleats: FRP or steel 75 × 75 × 10 mm or 100 ×  FW cleated • Failure was delamination cracking with FRP cleats and tensile tearing of column with steel cleats
[27–29,137] cantilever
100 × 10 mm  Steel and FRP cleats • No bolt clearance used (about 0.1–0.3 mm)
 M16 steel bolts finger tight • First failure or damage onset happened at about half of the joint’s moment capacity
• Middle bolt found redundant; joint performed equally well with two bolts
• Thicker cleats will attract more initial stiffness
 Beam-column joint • Stainless steel cleats did not show any sign of yielding
Turvey Double
 Varies  FW cleated • Failure was due to shear-out of bolts in beam web and failure of tension flange of beam
[138–142] cantilever
 Steel and FRP cleats • Tensile strength of FRP angles 76 × 6.4 mm and 76 × 9.5 mm is tested in [141] and 102 × 6.4 mm in [142]
• These leg-angles possess a tensile strength (tying resistance) in excess of 4 kN [141,142]
Qureshi Tension  Beam: 254 × 254 × 9.53 mm and 203 × 203 × • Seminal paper to determine tying resistance of pultruded FRP joints
[80] pull test 9.53 mm WF; Cleats: FRP 100 × 9.53 mm and
75 × 9.53 mm equal leg-angles
 Beam pulled against stiff base •
•
FRP web cleats possess tying resistance in excess of 4.5 kN required by ASCE Pre-Standard [14]
An expression for tying resistance proposed that predicted strengths within 5% of experiments
Smith Direct  Members: 102 × 51 × 6.35 mm I-beam and  FRP I-beam and box sections • Proposed a monolithic cuff connection unit that required no bolting
[143–145] Compression box-section column  FW cleated • Box-beam joints with cuff connection performed better than I-beam joints with seated angles

Zafari Single  Bespoke floor beams and stud columns

 Beam-column dowel
•
•
Two physical tests on joints in STARTLINK house [27] portal frame
Hybrid joint with dowels and bonding was rigid; and the joint with dowels only was semi-rigid
connections bolted and bonded
[33,146] cantilever developed for STARTLINK house [27]
 Portal frame with rigid joints
•
•
Hybrid and dowel joints had three and two times more moment than ULS moment, respectively
Bracing suggested for more flexible joint with tight-fitting dowel connections

 Members: 152 × 43 × 9.5 mm two “C” profiles

• Flexural strength of FRP plate joints was 20 times higher than flange or web cleated joints.
Russo
Simply
joined with plate 78 × 152 × 15 mm.  Beam-column joints with
• Shear-out and bearing failure was dominant; net-tension and cleavage failure did not occur
[147–149]
supported
 Gusset plate: 230 × 362 × 15 pultruded FRP plates
• Global joint behaviour is not affected by the stiffness of pultruded profiles
beam
 14 mm steel bolts; torque: 20 Nm
•
•
Global behaviour of beam–column joint is influenced by progressive damage in FRP plates
Both symmetric and asymmetric multi-bolted FRP plates are tested
Fibers 2022, 10, 27 19 of 29

Table 6. Cont.

Researcher Set Up Sizes Joint Configuration Main Findings

• Bonded sleeve connections developed for joining FRP box beams and columns
Zhang Single  Members: FRP SHS 102 × 102 × 9.5
 Beam-column joints between • Sleeve connector was made by welding a steel tube to a steel endplate
[150] cantilever  Steel tube and flange plate sleeve connector
FRP box profiles using a steel
sleeve connector
•
•
All joints were classified as semi-rigid
Longer bond length specimens failed by yielding of plate and shorter bond length by adhesive failure
• Thickness of endplate and bond length were two main criteria for establishing failure
• Developed a steel connecting system for joints between box profiles
Single
 Members: GFRP I-section 150 × 75 × 8 mm
 • Bolt edge distance is the governing criterion for either shear-out or bearing failure modes
Martins
[151–153]
cantilever
and full  Stainless steel cleats 3, 6 and 8 mm with stainless
I-profiles and box profiles
beam–column joints
• Joints between I-profiles also tested and initial strength and stiffness prediction model proposed
frame steel rods and bolts • Joints with thin steel cleats failed by yielding of steel and thick cleats by tensile failure of web-flange
junction of the connecting column
• Use of CFRP fabric wrap for seat angles changed failure from brittle to pseudo-ductile mode
Ascione
[18,154]
Single
cantilever
 Members: 200 × 100 × 10 mm I-sections and  I-profiles and box profiles • Bonded joints showed similar strength as bolted joints
50 × 50 × 6 mm leg-angles beam–column joints • In bonded joints, failure happened in column’s web-flange junction similar to bolted joints
• Strengthening column with stiffener plate and leg-angles can increase its strength by 40%
Mosallam
[155]
Web-flange
junction tests  Members: 200 × 100 × 10 and 160 × 80 × 8 mm
 Tension pull tests on web-flange
•
•
Web-flange junction strength depends on location pull; end point pull about 2/3 of midpoint
Cracking appeared in the form of inverted V leading to complete separation of web from flange
junction of WF sections
• The paper highlighted inherent weakness of web-flange junction of off-the-shelf FRP profiles
Fibers 2022, 10, 27 20 of 29

Table 7. Main findings for FRP frame joints subjected to cyclic loading.

Researcher Set Up Sizes Joint Configuration Main Findings

Simply  Members: 203 × 101 × 9.53 mm

 Beam–column joint
• The first paper on cyclic response of rigid beam–column joints; bonded and bolted joints used
Bruneau and
Walker [156]
support
beam 
WF FRP
Joints: various angles and
 Web and flanged cleated
•
•
Weakness of web-flange junction of I-shaped profiles led to only 35% of the predicted capacity
Steel-like joint detailing can be inefficient for use in FRP
with column stiffener plates
T-Stub • Alternative manufacturing process suggested for seismic worthiness of FRP joints
 Members: 102 × 102 × 6.3 mm
• The authors developed “Universal” Connector to connect PFRP members

WF
 Beam–column joint • Both bolted and hybrid—combined bolted/bonded joints used
Mosallam [135]
Direct
compression 
Cleats: FRP 75 × 75 × 9.5 mm
Bolts: 12.7 mm pultruded
 Flange cleated with • Rotational stiffness of hybrid joints was five times higher than bolted only joints
threaded FRP rods • Low dissipated energy exhibited with no ductility and linear moment-rotation response
threaded rods brightened to
• Some ductility shown in the testing was attributed to use of composite rods
40 Nm bolt torque
• Monolithic composite cuff connections using VARTM manufacturing process, developed to connect PFRP box profiles
Smith [144,145],
Singamsethi [157],
Direct
compression  100 × 50 mm box FRP sections
 Beam–column connected by • Adhesive bonding used to connect cuff with box sections
Carrion [158] bonded cuff system • Thicker cuffs showed brittle behaviour and thinner cuffs showed ductile response
• Energy dissipation not discussed; papers focused on cuff connector instead of cyclic response of the joints
• Sleeve connector produced by welding a steel tube to a steel endplate
Zhang, Qiu
Single
cantilever or  Members: 102 × 102 × 9.5 mm
 Bonded sleeve connection • Failure modes observed were progressive cohesive failure at FRP beam-steel tube interface, yielding of steel endplate
[150,159,160] beam splice joining beam-beam or and rupture of the web-flange junction of the beams
box FRP sections
connection beam–column • Number of bolts had marginal effect on moment-rotation response; joints classified as semi-rigid
• Splice connections should be designed in a way that failure is governed by steel plate yielding
• Sleeve connection with one, two and four bolts were tried
Martins [161,162]
Single
cantilever
 Members: 120 × 120 × 10 mm  Beam–column sleeve system • Tests with sleeve using two bolts gave the best overall performance
box FRP sections for box FRP profiles • Cyclic sway with infill walls also tested; walls had major effect on frame behaviour
• Infilled frame showed more strength, stiffness and energy dissipation compared to unfilled frame
• Bonded joint between a tubular FRP profile and built-up beam composed of channel sections

Razaqpur [163]
Single
cantilever
 Members: 102 × 102 × 9.5 mm
 Beam–column joint
•
•
Strength, stiffness and fatigue studied under static and cyclic loads
Hybrid joints (bolted and bonded) had 82% more loading capacity than bolted only joints
box FRP sections
• Bonded joint failure moment was about a third of moment capacity of the beam
• Bonded joint could sustain about 200 loading and unloading cycles
 Members: 150 × 100 × 10 mm • Shear-out, debonding, and delamination cracking failure modes observed
Single FRP I-beam and 150 × 100 • Bonding delayed start of cracking in FRP cleats and members
Qureshi [8,128]
cantilever

steel column
50 × 50 × 6 mm steel or FRP
 Beam–column joint •
•
Hybrid joints showed twice as much stiffness as bolted joints
Flange cleated and flange/web cleated joints showed similar behaviour
angles • Dissipated energy of hybrid joints was about 75% higher than bolted joints
Fibers 2022, 10, 27 21 of 29

Table 8. Advantages and limitations of different test set ups and joint configurations.

Joint
Test Set Up Advantages and Findings Limitations
Configuration
• Easy to set up in a universal testing machine • Both stub members subjected to
Direct • Tensile or compressive force can be applied at free ends axial loads (tension or compression)
-
compression • Joint rotation can be determined by either displacement transducers • In real frames, axial forces are
or clinometers unlikely to exist in beams
Simply • One half of the beam bolted to column and the other rests on
• Two halves of the beam can be
supported - simple supports
misaligned with respect to the
beam • It produces two identical bolted joints
central column
• Joints are loaded by pulling the stub column
Double • Column-double cantilever beam is the most realistic and common test up
• The test arrangement is somewhat
cantilever - • It loads the beams in a similar manner to real frames
complicated than the other test set
beam • The locations of load points correspond to points of contraflexure in real
ups
beams with uniformly distributed load
Single • A single cantilever beam is connected with single column fixed at both ends • It requires more fixtures than double
cantilever - • It represents edge beams in real frame cantilever beam
beam • Rotations are measured via LVDTs or inclinometers at webs of beam • The column needs to be fixed
and column properly as it takes bending moment
Full scale • It is based on testing the real frame structure • It is the most complex and expensive
-
frame • Cyclic sway and gravity loads can be applied in this setup simultaneously of all test arrangements
• Both flange and web cleated joints can be used with FRP or steel equal • Delamination cracking can happen
leg-angles in FRP web cleats
- Cleated joints • It can give pinned or semi-rigid joints • Unwanted outward flexural
• Bolted or hybrid (bolted and bonded) joints can be used deformation in FRP column can
• These joints are mainly used with I-shaped beams and columns occur due to use of steel cleats
• Bonded or bolted cuff/sleeve joints are useful for connecting tubular beams
• The sleeve/cuff joint configuration is
Sleeve/ and columns
- only limited to use with tubular
cuff joints • Buckling is eliminated by use of box sections
sections
• Semi-rigid moment-rotation behaviour can be achieved with
• Test set up can be complex
this configuration

5.1. Joints Subjected to Monotonic Loading

Research in pultruded FRP member joints is mainly focused on tests using steel-like
joint detailing. FRP has no plasticity and adhesive is not like welding in steel. Bolting is the
main fastening method preferred by manufactures. It may lead to local stress concentrations
due to discontinuity of fibres. Adhesive bonding alone does not resist out-of-plane or prying
forces. It may not be suitable in high temperature environments. The effort since 1980s
has been towards the development of various FRP connectors. These connectors have a
different fibre architecture and are produced through other methods than pultrusion. Most
research is on joints subjected to static loading.
Joints subjected to static monotonic loading are reviewed in Table 6. Following gener-
alised conclusions can be drawn from past research:
• Steel-like joint detailing is not suitable for FRP joints;
• Use of FRP cleats leads to delamination cracking at the heel of the angle;
• Use of steel cleats results in tensile tearing of column flange from web;
• Adhesive bonding on its own is not suitable for FRP joints;
• Hybrid joints combining bolting and bonding are suggested for fail-safe mechanism;
• Semi-rigid analysis is suggested due to limited commercially available FRP profiles;
• Top cleat is the main weakness in FRP joints for I-shaped sections;
• Connectors using different fibre architecture are proposed to replace top cleat;
• First failure defined as start of hairline cracking or audible acoustic emissions;
• First failure or damage onset is related to prying due to hogging moments;
• Cuff/sleeve connectors with steel tube and plate are useful for joining box sections;
• Serviceability deflection limit for FRP beams is span/340;
• FRP web cleats possess sufficient tying resistance for robustness;
• The flexural strength of beam–column joints using pultruded FRP plate sandwiched
between built-up channel sections is about 20 times higher than the conventional
beam–column joints with web/flange cleats;
• In a three bolted web cleated joint, the middle bolt is unnecessary.
Fibers 2022, 10, 27 22 of 29

5.2. Joints Subjected to Cyclic Loading

Research on FRP joints subjected to cyclic loading is limited. There is some research
on joints between FRP box profiles, but it mainly deals with creating prototype connectors
for box sections instead of joints’ cyclic behaviour. In the absence of any design code for
FRP cyclic testing, cyclic loading has mainly been established from steel codes. Energy
dissipation has been a key performance indicator for the cyclic response. Research papers
are reviewed in Table 7 and main points are summarised. It is hard to generalise from
very limited empirical data available, but the author has tried to come to some reason-
able conclusions from review of past papers. More research is needed to validate some
generalisations made here. Following conclusions can be drawn from the review:
• Steel-like joints are inefficient in resisting cyclic loading;
• Web-flange junction of I-shaped profiles is the weakest spot;
• Alternative manufacturing suggested for producing cleat pieces;
• Most joint details produced low dissipated energy and almost no ductility.
• FRP frames with infill walls showed better cyclic performance;
• Hybrid joints produced higher dissipated energy than bolted only joints;
• Bonded sleeve joints and joints between tubular profile and built-up channel sections
showed promising cyclic behaviour;
• Flange cleated or web and flanged cleated combined showed same cyclic response
and therefore, flange cleats are redundant as they make no difference to cyclic joint
behaviour;
• Only steel connecting components are used to join tubular sections; no effort made to
try FRP connecting elements;
• A guide for cyclic testing of FRP joints should be developed; presently FRP testing
relies heavily on testing procedures from steel structures;
• Several cyclic loading protocols are dependent on yielding of steel, which is absent in
FRP

6. Setbacks and Future

The major setbacks to wider application of FRP in Civil Engineering are lack of
legal design codes, steel-like detailing for all-FRP structures [164], lack of ductility, scarce
information about fire and durability performance and lack of simplified FRP design
books for structural engineers. At present, some evolving design guides are available:
Eurocomp [17], ASCE Pre-Standard for pultruded FRP [14], FRP bridges [15] and the
Italian guide for pultruded FRP elements [16]; however, these guides have no legal status. A
comparison of these guides is presented elsewhere [23]. The main benefits of FRPs are their
high strength, lightweight and corrosion resistance. While in the repair and rehabilitation
market, FRPs have shown good progress over last three decades, all-FRP structures still
need their fair place in construction industry. This will be possible if whole life cost of FRP
assets is considered and legally binding design codes are produced.

7. Conclusions and Research Growth Areas

The paper presents a review on FRP structures. Material properties, applications,
manufacturing processes, and connections and joints are reviewed. FRP composites present
a unique opportunity for structural engineers to adopt an environmentally friendly material.
With an ecological impact of about a third of traditional materials [165], glass FRP can lead
to low carbon construction. The global market for glass FRP is valued at 9.7 billion USD
in 2021 [166] and it is set to grow in next decade. We are best placed now than ever to
exploit the competitive properties of FRP composites.
Detailed conclusions relevant to FRP connections and joints are presented in Sections 4 and 5.
The review leads to following general conclusions:
• Joint detailing from steel structures is not suitable for FRP structures;
• There is no design code for FRP beam-to-column joints;
• Available formulae for beam–column joints are taken from plate-plate connections;
Fibers 2022, 10, 27 23 of 29

• Design of FRP members is controlled by serviceability deflections;

• Tubular members are better suited for FRP structures due to high buckling resistance;
• Use of FRP cleats leads to delamination cracking;
• Use of steel cleats results in outward flexural deformation in I-shaped FRP columns;
• Choice of FRP section sizes is limited; semi-rigid analysis may help in better economy
• Bearing is the most desirable and ductile failure in plate-plate connections;
• Connection strength depends on geometry, lateral restraint, fastener parameters, fibre
orientation and number of bolt rows.
Further research is needed in following areas:
• FE modelling with progressive failure can be useful to estimate joints’ behaviour;
• Environmental considerations, fire performance and durability should be studied;
• A comparison between single-lap and double-lap plate-to-plate connections;
• Extreme loading conditions, such as blast, earthquake, dynamic and impact loads.
• Robustness and disproportionate collapse of all-FRP structures;
• Plate thickness, pitch and gauge distance, staggered bolts should be investigated;
• More research is needed on serviceability deflection limits for different joint detailing.

Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data used in this paper is available in public domain. All data sources
have been cited.
Conflicts of Interest: The author declares no conflict of interest.

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