Encyclopedia
Please note this is a comparison between Version 1 by Paweł Kossakowski and Version 3 by Lindsay Dong.
Fiber-reinforced polymer (FRP) composites are two-phase materials consisting of a base material and filler material. Base material is referred as a matrix or a binder material. This is a polymer (plastic), either thermoset or thermoplastic. Polymer matrices are natural or synthetic. The latter kind is petrochemical-based and includes polyester, polypropylene (PP), polyethylene (PE) and epoxy. Due to its specific mechanical properties, a polymer matrix needs to be reinforced by filler material. FRP composites consist of fibers or other reinforcing material, which provide sufficient strength in one or more directions.
- composites
- bridges
- fiber-reinforced polymer (FRP)
- fibers
- novel structural materials
- polymers
1. Introduction
The development and modernization of the bridge industry, which has been continually observed since ancient times, is closely linked to the materials used for bridge construction. For a long time, wood and stone were the basic construction materials. Later on, brick became common in construction. It was not until the late 18th century when bridges began to be made of other materials. The first modern material was cast iron, which was used to erect the Iron Bridge in England in 1779. Later, in the 19th century, steel was used to build metal bridges. It is worth mentioning at this point that iron materials were used earlier for bridge building in China and India. According to Plowden [1], cited by Kanji Ono [2], the first iron bridge, Jihong Bridge, may have been erected in China in AD 56 in the Eastern Han era, and another iron structure was possibly built around 200 BC. At the end of the 19th century, reinforced concrete was introduced into the bridge industry. It is believed that the first bridge to be built using reinforced concrete is the 50 ft (15 m) Homersfield Bridge over the Norfolk/Suffolk border in 1870 [3]. In the 20th century, this material was improved in the form of prestressed concrete. It revolutionized concrete bridges, allowing for much longer structures. The 20th century was a period of development and bringing into general use of modern structural steels in bridge construction but also a period of application of aluminum alloys [4][5][6][4,5,6]. It is also interesting to note the use of stainless steels for bridge structures [7]. On the other hand, the turn of the 21st century is a period of using fiber composites in the bridge industry [8]. Bearing in mind the new possibilities offered by fiber composites, mainly due to their favorable mechanical parameters, especially the weight–strength–stiffness relation, further applications in the bridge industry should be observed with interest.
Among a group of composites, fiber-reinforced polymer composites are one of the most widely used novel materials. Their range of applications covers civil, mechanical, automobile, aerospace, marine and biomedical industry [9]. Although the best results are achieved using synthetic fibers [10][11][12][13][14][10,11,12,13,14], very interesting effects can be obtained using natural fibers [15][16][15,16]. A separate stream of research is in the field of development of nano-composites [17][18][19][17,18,19]. It can be said with certainty that fibrous composites are novel materials of the future. Due to their properties and the possibility of use, composites were used in building engineering quite early. This applies to architecture, construction and bridge engineering. The latter branch was most interested in the possibilities of using composites. Two general fields of application of fiber-reinforced polymer (FRP) composites are repair and strengthening possibilities and methods [20][21][20,21], as well as research and solutions to make parts, in addition to the entire supporting structure of bridges [22][23][24][25][26][27][22,23,24,25,26,27]. In this respect, the issues of the response of bridge structures under static and dynamic load are important, which is also the subject of the latest intensive research [28][29][30][28,29,30].
2. Materials
In FRP composites, most of the load is carried by the fibers. Composites with glass, carbon or aramid fibers are most often used in building structures.
Due to the relatively low cost, the most widely used are glass fibers. Their other advantages are hardness, high corrosion resistance and a small influence of temperature (in the scope in which the bridges operate) on mechanical properties. Their most important disadvantages include a low modulus of longitudinal elasticity and sensitivity to moisture.
From a constructional point of view, it is preferable to use carbon fibers that have a high modulus of elasticity. Their further advantages are very high fatigue strength and high creep resistance. However, these are expensive materials.
Aramid fibers are characterized by high fatigue strength, but due to their creep susceptibility, high cost and complicated production technology, they are rarely used in construction.
In recent years, basalt or boron fibers have been used more and more often [8].
The resin primarily acts as a binder for the fibers. The choice of the type of resin depends on the adopted technology for the composite production and the expected properties of the material. Elements of building structures usually use polyester, epoxy and vinyl ester resins.
The vast majority of elements in the construction industry are made with the use of polyester resins. Composites based on polyester resins are characterized by lightness, high strength and good chemical resistance. These are therefore advantages that are particularly desirable in bridge construction.
The use of epoxy resins ensures high strength and high chemical resistance, but the problem with bridge structures is their low resistance to UV radiation.
Vinyl ester resins are characterized by relatively high elongation, as well as good impact strength and fatigue strength. The main disadvantages include high shrinkage [8].
3. Benefits of FRP in Bridges
As with other FRP composite applications, they are increasingly used for bridges. Decisive in this regard are four fundamental benefits [9]:- (a)
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excellent mechanical parameters, which enables a reduction in weight due to a positive strength/stiffness–density relation;
- (b)
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high corrosion resistance, increasing the durability;
- (c)
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low maintenance requirements;
- (d)
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possibility to form varied complex geometries and shapes of bridge elements and structure.
3.1. Mechanical Parameters
Generally, every modern FRP composite has a better tensile strength in comparison to structural steels used in bridges, as well as in civil engineering. For deformations corresponding to the range of live loads typical for bridges, the differences in the strength are significant, which is shown schematically in Figure 1. Typical structural steels used in bridges, i.e., S355, have much lower strength compared with basically every kind of FRP composite [31][32][32,33].
Figure 1. Schematic comparison of stress–strain characteristic for FRP composites and steel, based on [31].Schematic comparison of stress–strain characteristic for FRP composites and steel, based on [32].3.2. Fatigue Resistance
FRP composites have high fatigue strength, which allows them to be used in bridge engineering. Most importantly, their application in bridges meets the high durability requirements according to the EN-1990 standard for anticipated traffic [28][29][30][28,29,30]. Composites used to strengthen bridge elements and their structures also increase the fatigue life of both reinforced concrete and steel structures. This phenomenon is also observed with regard to the reinforcement of joints.3.3. Low Weight
A favorable strength/stiffness–density ratio results in FRP profiles that are significantly lighter than those of steel and especially of concrete. CFR profiles can achieve the same great strength as steel, thanks only a quarter of the density. Thus, bridge structures made of FRP material have a noticeably lower weight [23][24][25][26][23,24,25,26]. It is also possible to reduce structure mass in comparison to aluminum structures as a result of 30% lower density of FRP structures.3.4. Corrosion-Free
Modern FRP materials are highly resistant to aggressive environment effects. This is especially true of corrosion attack, which is fundamental in the case of bridges. They are extremely susceptible to risk of corrosion and reduction in their durability and load-carrying capacity. FRP profiles are resistant to aggressive chemicals, liquids and alkalis [23].3.5. Minimal Maintenance
The high durability of FRP structures, including bridges, ensures a long life, even in demanding conditions. This is important for structures located in different world regions, where atmospheric actions are extreme. The corrosion-free property of FRP structures results in low maintenance, which is very important for long-span bridges with limited access [8][20][22][8,20,22].3.6. Free Formability
The manufacturing technologies of FRP material allow for production and forming of different, complex and sometimes custom shapes of bridge components. This is important for composite decks, which often have quite complicated cross-sections made as one element (module), and FRP profiles used as a main load-carrying members. The free formability of composites allows for the formation of practically any shape of formworks dedicated to particular bridge elements [8].3.7. Competitive Life Cycle Cost
High corrosion resistance, low maintenance requirements and the long overall service life of FRP bridges reduce their costs. In the long run, one should hope that FRP bridges will be cost-competitive with traditional bridges made of steel or reinforced concrete [8].3.8. Electrical and Thermal Insulation
High electrical insulation is another favorable parameter of FRP composites. Application of these materials minimizes the complexity of earthing. It allows for a reduction in the costs of both installation and future inspections. FRP composites provide a significantly lower heat distribution gradient than metals, i.e., steel and aluminum. This is important, considering that temperature gradients causes additional internal forces in the bridge structure during the summer and winter.4. FRP Composites in the Bridge Industry
Although composite materials came into common use relatively long ago, their application as a basic material in construction was very limited and late. The first use of composites took place in boatbuilding, with the first FRP composite boats built in the 1940s. It is estimated that currently about 90% of boats are built from composite materials. In the next decade, the automotive industry began to use composites to make car bodies. This material was also used in later years for the construction of cabs and fairings for trucks. The 1970s saw the use of composites in the aviation industry for the construction of aircraft fuselages. These materials began to be the basic construction material in many other applications, such as the green energy industry for the construction of windmill components, as well as in the chemical industry for pipelines, tanks and many other purposes. The first applications of FRP composites in the building industry were related to the reinforcement of existing structural elements or as architectural elements. In the first case, the reinforcements included mainly reinforced concrete structures, as well as masonry, steel and wooden structures. It was only since the 1980s that the first attempts were made to construct building objects with the main load-bearing structure made of composites. Bridges were the first construction objects to use composite as the basic construction material. Generally, the scope of application of FRP composites in the bridge industry is very wide. They are used as material to strengthen bridges, as well to build their structure. FRP composites are applied to repair deteriorated bridges and make them useful in terms of actual standard requirements, which are being updated or, in cases of necessity, to increase load-carrying capacity. The second field of application includes new construction. FRP composites are used to form bridge decks, as well as reinforcement in concrete decks instead steel bars. These materials are also used for composite columns or piers. In suspension bridges, the cables are made of FRP materials. Another application of these composites is in stay-in-place formworks.5. Structural Elements of Bridges Made of FRP Composites
5.1. FRP Deck Panels
The widest application of FRP composites in bridge engineering involves construction of deck slabs. This is due to high durability of FRP compared to traditional materials, especially concrete. The issues of durability and resistance to environmental conditions are of particular importance here [33][41] because the deck slab is the most exposed to destructive factors, such as aggressive rainwater, concentrated loads from vehicle wheels, temperature changes and freezing/defrosting cycles, among others. FRP slabs can be divided into three main groups: decks composed of pultruded profiles; sandwich panels; and hybrid decks, most often composed of FRP composite and concrete [34][42].5.2. FRP Beams and Girders
Regardless of the structure of the deck slabs, attempts are made to use FRP beams in bridge engineering, but designers and constructors face limitations of low stiffness of these elements, which is a serious barrier to the construction of long spans. Despite this inconvenience, many bridges with FRP beams have been erected, usually with small or medium spans. The first implementations of this type used box-section beams manufactured by the traditional method of manual lamination. The main disadvantage of FRP beams is their low stiffness. This limitation is less severe in the case of footbridges due to the lower values of the loads. For this reason, composite beam and slab structures are currently used primarily in footbridges. The dynamic development of this type of structure that has taken place in recent years has introduced many system solutions for composite girders. An example is the EcoSafe system of the Dutch company Lightweight Structure [8][35][8,53].6. Difficulties and Limitations
Regardless of the numerous advantages of FRP composites mentioned above, there is a number of problems and limitations that make their use in bridge structures difficult. The basic limitation mentioned in the literature is the high cost of composite structures—higher than in the case of traditional materials [36][38]. This is due, firstly, to the cost of the material itself and, secondly, to the production technology of composite elements. Technologies used in industry allow for achievement of relatively low costs for repetitive elements produced in large series (e.g., in the aviation industry). In the case of bridge structures, individually designed elements dedicated to a specific design solution are usually used, which increases the cost. Another limitation is the characteristics of the material itself and, more precisely, the lack of a plastic scope of work [37][62], which is particularly desirable in bridge structures, as it enables the redistribution of internal forces. More importantly, such a property of the composite implies a brittle failure mechanism with no visible plastic deformation, so the possible failure of a structural element is sudden. Moreover, the behavior of FRP under the influence of temperature changes is a major engineering problem. First, the thermal expansion coefficient of typical composites differs significantly from that of steel and concrete, which makes it difficult for these elements to work together after bonding. However, a particular problem for designers is the low resistance of composites to high temperatures. For example, in the case of composites with a polyester matrix, a decrease in strength is already observed at 80 °C [36][38]. It is therefore a significant problem not only in fire situations, but even in normal use, especially in the case of specific renovation work. Regardless of the above, the factor limiting the development of FRP composites bridges is the lack of engineer experience, as well as the lack of national standards for the design of this type of structure. Each design case is considered individually; the design process is supported by research, which extends the entire process and generates additional costs. The lack of standardized design methods mainly concerns connections that have been “borrowed” from metal structures without being specifically adapted to a new type of material. Among the limitations of the use of FRP composites in bridge engineering, some other issues are also discussed in the literature, namely the creep susceptibility of the material and low resistance to the impact of loads, which, when applied directly to the composite element, easily lead to its failure.
