A Review of Fibre Reinforced Polymer Structures

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


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 material, which has been used in buildings and bridges for over 50 years. FRP use in marine, automotive and aerospace industries dates back to the 1930s [1,2]; it has also been used in rail, sport, and wind turbines. Construction uses about a quarter of globally produced FRPs [3][4][5]. Figure 1 shows the market share of FRPs by applications. FRP composites have fibres encased in a polymer resin. For structural use, glass, carbon, or aramid fibres are usually embedded in polyester, vinylester or epoxy resins. The fibres give strength and stiffness, whilst the resin glues the fibres together; it also protects the fibres and transfers forces between them [1,2].  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 currently copied from steel design practice, which mostly leads to oversized FRP compo-  [4,5]. Lightweight, high strength, corrosion resistance and expected durability over their lifetime are the key benefits of FRPs [6][7][8]. Glass or carbon fibre reinforced polymer (GFRP or CFRP) shapes are used in structural applications. GFRP is more common due to its electrical 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 sudden 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 transverse 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 currently 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][13][14][15][16][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 disturbing 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][20][21][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 connections 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 reviewed in any other review papers in [23][24][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 manufacturing 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 configurations 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.

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][28][29][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 composite piles; and it was built just in two weeks [32,33], see Figure 2d. The Startlink test house does not exist anymore. It has been taken down; and the author is not aware if it has been constructed anywhere else in the UK.  [33]; (e) Pultruded FRP cooling tower [12]; (f) Duracomposites railway platforms [34].
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 composite piles; and it was built just in two weeks [32,33], see Figure 2d. The Startlink test house does not exist anymore. It has been taken down; and the author is not aware if it has been constructed anywhere else in the UK.
FRPs are also used in cooling tower industry. Cooling towers are used for heating, cooling, ventilation and industrial purposes. GFRP profiles can resist corrosion and exposure to water often encountered in cooling towers. Manufactures [9,12] supply bespoke and standard FRP elements for cooling towers, see Figure 2e. GFRP railway platforms are also becoming popular due to speed of construction and ease of assembly [9,13,34], see Figure 2f. FRP composites are also used in secondary structures; these include insulated ladders, floor gratings, stairways with handrails, working platforms and walkways and building façade panels [1]. Bridge engineering applications of FRP are presented in Figure  3 and are discussed next.  [12]; (b) Five-storey Eyecatcher FRP demountable building Switzerland [31]; (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].
FRPs are also used in cooling tower industry. Cooling towers are used for heating, cooling, ventilation and industrial purposes. GFRP profiles can resist corrosion and exposure to water often encountered in cooling towers. Manufactures [9,12] supply bespoke and standard FRP elements for cooling towers, see Figure 2e. GFRP railway platforms are also becoming popular due to speed of construction and ease of assembly [9,13,34], see Figure 2f. FRP composites are also used in secondary structures; these include insulated ladders, floor gratings, stairways with handrails, working platforms and walkways and building façade panels [1]. Bridge engineering applications of FRP are presented in Figure 3 and are discussed next.
Corrosion and fatigue resistance, high strength-to-weight ratio and formability are some of the desirable properties of FRP for bridges [3,41]. FRPs have been used to repair, replace, or retrofit existing steel or concrete bridges. FRP is used in new footbridge and highway bridges. Critical elements in bridges are generally hybrids-FRP and traditional materials. Global developments in FRP bridges are reviewed in a recent paper [3]. FRP bridges in the UK are reviewed in [42], the US in [43,44] and the Netherlands in [39]. FRP bridges are fabricated using standard or bespoke FRP elements [45]. Aberfeldy, Scotland 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 attached to GFRP A-frames. It carried pedestrians and golf buggies on a golf course, see Figure 3a. Only foundations were constructed from concrete [36,42,45]. Corrosion and fatigue resistance, high strength-to-weight ratio and formability are some of the desirable properties of FRP for bridges [3,41]. FRPs have been used to repair, replace, or retrofit existing steel or concrete bridges. FRP is used in new footbridge and highway bridges. Critical elements in bridges are generally hybrids-FRP and traditional materials. Global developments in FRP bridges are reviewed in a recent paper [3]. FRP bridges in the UK are reviewed in [42], the US in [43,44] and the Netherlands in [39]. FRP bridges are fabricated using standard or bespoke FRP elements [45]. Aberfeldy, Scotland 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 attached to GFRP A-frames. It carried pedestrians and golf buggies on a golf course, see Figure 3a. Only foundations were constructed from concrete [36,42,45].
The first ever FRP road bridge is the Bonds Mill Lift bridge near Gloucester, UK (Figure 3b); it was constructed over a canal in 1994. The bridge had a multi-cellular box girder filled with structural foam to resist local bending from wheel loads [15,36,42,45]. The lightweight bridge elements helped in mechanical lifting system [45]. The first FRP composite bridge in the USA was constructed over no-name Creek, Kensas in 1996. The bridge used decks with glass FRP laminated skin and corrugated core [15]. Similarly, in Kolding, Denmark, an all-FRP cable stayed footbridge was constructed in 1997, see Figure 3c. It was 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]. The first ever FRP road bridge is the Bonds Mill Lift bridge near Gloucester, UK ( Figure 3b); it was constructed over a canal in 1994. The bridge had a multi-cellular box girder filled with structural foam to resist local bending from wheel loads [15,36,42,45]. The lightweight bridge elements helped in mechanical lifting system [45]. The first FRP composite bridge in the USA was constructed over no-name Creek, Kensas in 1996. The bridge used decks with glass FRP laminated skin and corrugated core [15]. Similarly, in Kolding, Denmark, an all-FRP cable stayed footbridge was constructed in 1997, see Figure 3c. It was 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].
The best example of combining carbon and glass FRPs is the Pont y Ddraig or the Dragons bridge ( Figure 3d  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 concerns 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 will support glass fibre reinforced polymer (GFRP) deck. Structural health monitoring equipment will also be installed on the bridge for research [40].  The best example of combining carbon and glass FRPs is the Pont y Ddraig or the Dragons bridge ( Figure 3d) at Rhyl Harbour, North Wales, built in 2013. This double bascule footbridge uses the freedom in geometry and lightweight of FRP materials. It had two mirroring, 32 m long decks hinged on a central caisson. The decks can be lifted for navigation by cables running up to a central stainless steel tower. They are made from 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 concerns 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 will support glass fibre reinforced polymer (GFRP) deck. Structural health monitoring equipment will also be installed on the bridge for research [40].

FRP as Reinforcement
FRP reinforcements have been used in structural engineering since 1950s. Today, FRP rebars, grids, fabrics, strips, prestressing tendons, and formwork are commercially available [2,48]. FRP reinforcements are suitable in aggressive conditions, such as alkaline, corrosive and chemical environments. Lightweight and electromagnetic neutrality are other benefits of FRP rebars, especially glass FRPs. Carbon, glass, aramid fibre reinforced polymer bars are commonly used. Research in FRP reinforcement is quite developed compared to all-FRP pultruded structures. Several design guides for FRP rebars are available too. In Europe, the task group 5.1 (formerly 9.3) produced technical report fib 40 [48] for FRP reinforcement in concrete structures. Also, The Concrete Society has its TR55 [49] design guide for strengthening applications including FRP rebars [50]. In the USA, ACI 440.1R-15 [51] deals with design of concrete members with FRP bars. FRP reinforcements and their applications are shown in Figure 5. Design of bridge beams prestressed with CFRP bars or cables is given in NCHRP research report 907 [52].

FRP in Strengthening Applications
Research, design and practice are well advanced for FRP use as strengthening material. FRP can be used for repair and strengthening of existing structures. Externally bonded reinforcements can be used to reinforce concrete, timber, steel and masonry structures [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-

FRP as Reinforcement
FRP reinforcements have been used in structural engineering since 1950s. Today, FRP rebars, grids, fabrics, strips, prestressing tendons, and formwork are commercially available [2,48]. FRP reinforcements are suitable in aggressive conditions, such as alkaline, corrosive and chemical environments. Lightweight and electromagnetic neutrality are other benefits of FRP rebars, especially glass FRPs. Carbon, glass, aramid fibre reinforced polymer bars are commonly used. Research in FRP reinforcement is quite developed compared to all-FRP pultruded structures. Several design guides for FRP rebars are available too. In Europe, the task group 5.1 (formerly 9.3) produced technical report fib 40 [48] for FRP reinforcement in concrete structures. Also, The Concrete Society has its TR55 [49] design guide for strengthening applications including FRP rebars [50]. In the USA, ACI 440.1R-15 [51] deals with design of concrete members with FRP bars. FRP reinforcements and their applications are shown in Figure 5. Design of bridge beams prestressed with CFRP bars or cables is given in NCHRP research report 907 [52].

FRP in Strengthening Applications
Research, design and practice are well advanced for FRP use as strengthening material. FRP can be used for repair and strengthening of existing structures. Externally bonded reinforcements can be used to reinforce concrete, timber, steel and masonry structures [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]). Various other guidelines have also been produced in USA, Japan and Italy [56][57][58][59][60][61][62][63][64] for FRP strengthening applications. Different design guides for FRP strengthening are compared in [65]. Environmental actions, poor design, lack of maintenance or accidental events cause deterioration to existing structures [54]. Strengthening of these structures with FRP systems not only restores them, but enhances their strength, too. FRP for strengthening is available as strips, sheets and fabrics.
FRP strengthening can be applied in-situ using hand layup or can be prefabricated off-site in a factory. Hand or wet layup consists of applying epoxy resin to woven fabric sheets or flexible fibre sheets to produce FRP sheets bonded to concrete members. The prefabrication method involves pultrusion for FRP plates bonded to beams and slabs or filament winding for making FRP shells for confining columns [67]. Pultrusion and filament 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 monitoring for this bridge. They used CFRP unbonded prestressed plates with wireless 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]. FRP strengthening can be applied in-situ using hand layup or can be prefabricated off-site in a factory. Hand or wet layup consists of applying epoxy resin to woven fabric sheets or flexible fibre sheets to produce FRP sheets bonded to concrete members. The prefabrication method involves pultrusion for FRP plates bonded to beams and slabs or filament winding for making FRP shells for confining columns [67]. Pultrusion and filament 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 monitoring for this bridge. They used CFRP unbonded prestressed plates with wireless 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].
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 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 in the joint area but there was some cracking close to the beam side. Strengthening of concrete T-beams using U-jacketing method with externally bonded CFRP sheets is studied in [72]. The authors used T-shaped shear-critical RC beams under four-point bending. CFRP sheets were used as external transverse reinforcement. CFRP strengthened beams showed enhanced shear capacity. But debonding of CFRP from concrete surface could not be prevented. The authors applied the mechanical anchorage system to U-jacketing, which delayed debonding resulting in about 70% increase in shear capacity. Application of three plies of CFRP sheets led to a 72% increase in shear capacity compared to the control specimen. The authors also compared their experimental results with various American and European design codes.

Materials and Manufacturing
This section is about raw materials and manufacturing processes for making FRP shapes-bars, profiles and sheets.

Fibres
Glass, carbon and aramid are the most common synthetic fibres. Synthetic fibres are man-made, usually formed by chemical processes. Their properties are given in Table 1. Glass fibres are used to make FRP profiles, rebars and sheets. They come in four different grades: • E-glass (electrical glass); • A-glass (window glass); • C-glass (corrosion resistant, also known as AR-glass or alkali-resistant glass);

Materials and Manufacturing
This section is about raw materials and manufacturing processes for making FRP shapes-bars, profiles and sheets.

Fibres
Glass, carbon and aramid are the most common synthetic fibres. Synthetic fibres are man-made, usually formed by chemical processes. Their properties are given in Table 1. Glass fibres are used to make FRP profiles, rebars and sheets. They come in four different grades: • E-glass (electrical glass); • A-glass (window glass); • C-glass (corrosion resistant, also known as AR-glass or alkali-resistant glass); • S-glass (Structural or high-strength glass). 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]. Carbon fibres are generally used for strengthening applications: CFRP strips, sheets, rebars 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: 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 [  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 protective 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][77][78].

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].

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].

Pultrusion
Pultrusion is the cost-effective way of producing FRP bars, profiles and strips [79,80]. It is the only automatic method for making constant section FRP shapes. Pultrusion is used to produce I-beam, wide-fanged, channel and multi-cellular profiles. Pultrusion is divided into two phases: fibre system and matrix system. In fibre system, different reinforcements (fibre bundles, mats and surface veil) are fed through the guide plate that shape the profile [1,2,81]. The fibre bundles are called rovings for glass fibres and tows for carbon fibres [82]. Schematic diagram of pultrusion process is shown in Figure 7. The unidirectional rovings or tows provide the strength along the length of the profile. While, the continuous filament or strand mat, woven roving or stitched fabric give the strength across the width of the profile. Polyester surface veils are also added for surface finishing and protection. These resin-rich veils also provide corrosion and ultraviolet resistance. In matrix systems, dry fibres are impregnated with resin and allowed to cure (solidify) in a heated mould. The FRP material is then pulled to give it tensile strength [1,2,81]. Pultrusion has six stages [1]: (1) set of spools stacked on creels for fibre reinforcement handling; (2) preforming guides; (3) resin impregnation bath; (4) forming and curing die; (5) pulling system; (6) cutting system.

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].

Pultrusion
Pultrusion is the cost-effective way of producing FRP bars, profiles and strips [79,80]. It is the only automatic method for making constant section FRP shapes. Pultrusion is used to produce I-beam, wide-fanged, channel and multi-cellular profiles. Pultrusion is divided into two phases: fibre system and matrix system. In fibre system, different reinforcements (fibre bundles, mats and surface veil) are fed through the guide plate that shape the profile [1,2,81]. The fibre bundles are called rovings for glass fibres and tows for carbon fibres [82]. Schematic diagram of pultrusion process is shown in Figure 7. The unidirectional rovings or tows provide the strength along the length of the profile. While, the continuous filament or strand mat, woven roving or stitched fabric give the strength across the width of the profile. Polyester surface veils are also added for surface finishing and protection. These resin-rich veils also provide corrosion and ultraviolet resistance. In matrix systems, dry fibres are impregnated with resin and allowed to cure (solidify) in a heated mould. The FRP material is then pulled to give it tensile strength [1,2,81]. Pultrusion has six stages [1]: (1) set of spools stacked on creels for fibre reinforcement handling; (2) preforming guides; (3) resin impregnation bath; (4) forming and curing die; (5) pulling system; (6) cutting system. A typical pultruded FRP profile has a middle layer sandwiched between two outer layers. The middle layer uses unidirectional roving bundles running in the direction of 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 to the outer layers [2,23,81]. Typically, the fibre volume in pultruded FRP profiles ranges from 35% to 50% [82]. For FRP bars and strips the fibre percentage ranges from 50% to 60% of the volume of FRP shape [2]. Typical mechanical properties of glass reinforced wide-flanged profiles with vinylester resin are given in Table 2. A comparison of steel and FRP (glass, carbon, aramid FRP) rebars' tensile properties is given in Table 3, as per fib 40 [48]. Other mechanical properties of FRP rebars are adapted from [48,83,84] and presented in Table 4. Mechanical properties of FRP sheets, strengthening strips and fabrics are not given in this paper. Typical material properties of commercially produced FRP sheets, strengthening strips and fabrics for strengthening purpose can be found on page 29-30  A typical pultruded FRP profile has a middle layer sandwiched between two outer layers. The middle layer uses unidirectional roving bundles running in the direction of 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 to the outer layers [2,23,81]. Typically, the fibre volume in pultruded FRP profiles ranges from 35% to 50% [82]. For FRP bars and strips the fibre percentage ranges from 50% to 60% of the volume of FRP shape [2]. Typical mechanical properties of glass reinforced wide-flanged profiles with vinylester resin are given in Table 2. A comparison of steel and FRP (glass, carbon, aramid FRP) rebars' tensile properties is given in Table 3, as per fib 40 [48]. Other mechanical properties of FRP rebars are adapted from [48,83,84] and presented in Table 4. Mechanical properties of FRP sheets, strengthening strips and fabrics are not given in this paper. Typical material properties of commercially produced FRP sheets, strengthening strips and fabrics for strengthening purpose can be found on page 29-30 Table 1.2 and  Table 1.3 in L. C Bank's book [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 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.

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].

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 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]. 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.

FRP Plate-to-Plate Bolted Connections
Pultruded FRP plate-to-plate bolted connections depend on various parameters: geometry, 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 connections 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.

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.

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][88][89][90][91][92]. Bearing failure has been extensively researched in [87][88][89][93][94][95][96]. Bearing failure can be ensured by sizing the connection geometry properly (generally e 1 /d 0 > 3 and w/d 0 ≥ 4 or higher) [2,23,96].

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, e 2 is small.

Shear-Out Failure
Shear-out is another tension-type failure. This happens when the end distance is small (e 1 /d 0 ≤ 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].

Cleavage Failure
Cleavage failure is a combination of net-tension and shear-out failures. It is a tensiontype failure that initiates at the end of the plate instead of the bolt vicinity. Cleavage failure happens when both end, e 1 and edge, e 2 distances are short. Also, it occurs when percentage of fibres in the longitudinal direction of FRP plate is high [23,97,101].

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 (d 0 ), end and edge distances (e 1 and e 2 ), and pitch and gauge length  Figure 8a. Past research is reviewed, and main findings are given in Table 5. Seminal research work on geometric parameters for singlebolted double-lap shear connections is presented by Rosner and Rizkalla [93,102,103]. Later, Turvey [104][105][106] carried out research on effects of width and end distance. Following general conclusions can be drawn from research described in Table 5

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 • .

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

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 multibolted 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;

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.

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 generalised 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.

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 reasonable 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

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.

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; 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.