Fibre Reinforce Polymer Composites in Bridge Structures - Term Paper Example

? Fibre Reinforce Polymer Composites in Bridge Structures Fiber reinforced polymer (FRP) composite as a new construction material has gradually gained acceptance in the construction industry especially bridge construction. FRP composites have primarily been developed for use in the defense industries and the aerospace industries but can as well be utilized in civil infrastructure. The first ever all-composite super-bridge was constructed in Miyum, China in 1982. Since then, they have been utilized for various applications in the construction industry, such as strips and sheets for making the existing bridges stronger. They have also gained prominence as concrete reinforcing bars, and some engineers prefer them to steel. In addition, FRP composites have also replaced traditional bridge decks, girders, and stay cables. This paper aims at investigating the specific use of thermoset polymer material with specific focus on polymer related issues.To achieve this, the paper will investigate the use of Fibre Reinforced Polymer composite in bridge engineering by first presentingan introduction on Fibre Reinforced Polymer including definitions, component description general application areas and the mechanical properties. The focus will then be shifted to FRP composites as a material for the manufacture of structural elements, describing its properties, the process of manufacturing that are relevant to bridge engineering, and assortment of tendons, cables, bridge deck systems, and structural profiles. In addition, this paper will focus on the properties of Fibre Reinforced Polymer composites and compare them with those traditional materials. Introduction Mechanically, a composite is a separable combination of two or more materials that are not the same at the molecular level but purposefully mixed to come up with a new material that has optimal and superior properties, which are different from those of its components (123). These materials have been utilized over the years in construction. An ancient way was the use of straw for reinforcement in clay and mud bricks (4). In the last century, in several structural systems, combination of reinforcing concrete and steel has been used for construction. However, a new crop of composite material has since been largely accepted by most engineers for construction and the rehabilitation of new structures and existing facilities. This class consists of Fibre Reinforced Polymer composites, which were in the earlier days developed specifically for defense and aerospace. Fibre Reinforced Polymer (FRP) are a combination of polymeric resins which acts as binders or matrices that has stiff and strong fibre assemblies which act as the reinforcing phase. The combination of a reinforcing phase with the matrix phase produces a material system that is analogous to steel reinforced concrete and is new. However, these concentrations may significantly vary; generally, the reinforced concrete rarely has more than 5% reinforcement, whereas Fibre reinforced polymer composites contains more than 30% reinforcements. Components of a Fibre Reinforced Polymer Composite Fibre This is a material that has been made into a long filament. A single fibre normally has up to 15µm. When the diameter is bigger, the probability of surface defects increases. The aspect ratio of diameter and length in continuous fibres can range from thousands to infinity. They usually occupy 50% of the weight and over 30% of the volume. Fibres are mainly meant for providing fitness, carrying the load, providing thermal stability, strength, and other structural properties to Fibre reinforced polymers (FRP) (2). The fibres in FRP composite need to have very high ultimate strength, high modulus of elasticity, low variation strength, high uniformity of diameter, high stability, and high uniformity of surface dimension among fibres. There are various forms of fibre used as reinforcements for polymer composite reinforcement exists. The manufacturer of materials made from FRP composites normally present different reinforcement techniques in design/specification guides. For example, Fibre line composites. Two forms of reinforcements fabrics and rovings exists. Examples of roving as a one-dimensional FRP composites are: Interlaced roving – bundle of filaments are longitudinally arranged with interlaced fibres in a loop that connect neighboring rovings. Smooth roving - bundle of filaments are longitudinally arranged in a free manner. Stapled fibre – these are filaments that are short are meant for cutting smooth roving; Tangled roving – are filaments that are longitudinally arranged and mutually interlaced to provide better co-operation of filaments that are neighboring in a single roving. Minced fibres – very short filaments that are obtained by sifting and milling stapled fibres. In order to ensure that surface elements are well strengthened in more than a single direction of reinforcement, the following forms are also used: Interlaced roving fabrics – this connect fabrics Smooth roving fabrics – made from interlaced roving Mats – made of random, discontinuous fibres However, unidirectional surface reinforcements are also produced. Fibre Reinforced Polymer (FRP) composites are classified based on the type of fibres used as the reinforcement. Three types of fibres; carbon, glass, and aramid fibres currently dominate the civil engineering industry. The table above represents the properties of various fibres. Glass fibre is made from glass, which is made of several oxides most of which is silica oxide, and other different raw materials such as fluorspar, limestone, clay, and boric acid. These are manufactured by putting the melted oxides into the filaments that range from 3 to 24 Mm. five different forms of glass fibres are used for the reinforcement of the matrix material; chopped strands, fibres, strand mats, surface tissues, and woven fabrics. The woven fabric and glass fibre strands are the most used in construction engineering applications because of their low costs. However, glass fibres have shortcomings such as low humidity, relatively low Young’s Modulus, low long-term strength due to stress rupture as well as alkaline resistance. Carbon fibres are among the high performance fibre available in the industry. These are manufactured by controlled crystallization and pyrolysis of organic precursors at very high temperatures above 2000?C. During this process, carbon crystallites are oriented and produced along the length of the fibre. The manufacturing process of carbon fibres uses three choices of precursors: polyacrlonitrile (PAN), rayon, and pitch precursors. The most used precursor in the commercial manufacture of carbon is the PAN precursors. It yields approximately 50% of the original mass of fibre. Pitch precursors also yield high carbon at lower costs. Compared to glass fibres, carbon fibres have high fatigue strength and elastic modulus, which is also higher than those of reinforced polymers are. The disadvantages of carbon fibres include comparatively high-energy requirements, reduced radial strength (inherent anisotropy) in production. Their costs are also relatively low. On the other hand, just as carbon fibres, aromatic or aramid polyamide fibre is also a high-performance fibre that civil engineers use. It is manufacture through the extrusion of a solution, aromatic polyamide, at very high temperatures of about -50?C to -80?C into a 200?C very hot cylinder. There are fibres that are left from the evaporation, which are then drawn and stretched to increase their stiffness and strength. In this process, the molecules of aramid become longitudinally oriented. These fibres are having high dynamic, static fatigue, and impact strengths. The disadvantages of aramid fibres includes reduced long-term strength, low compressive pressure of about 500-1000 MPa as well as sensitivity to Ultra Violet radiation. In addition, aramid fibres are difficult for machining and cutting. Mechanical Properties of Fibre Reinforced Polymer Composites The properties of fibre reinforced polymer composites are dependent on the properties of the components that make it, the fibre orientation in the matrix, properties of the fibre-matrix bond, and the volume ratios of the components. Almost all composite materials depict some common properties, which results from their composite reinforcement and nature. These properties include low density, mechanical and physical properties of the composite that is dependent on its components respective proportions, anisotropy that is dependent on the reinforcement type, high resistance to oxidation and corrosion, ability to form complex shapes and relatively high mechanical properties. To improve the properties of FRP composites, two or more different types of fibres are normally combined in the same array. For instance, a material, which is composed of carbon and glass fibres, has high impact resistance (a property that CFRP does not have when they are not combine with glass fibre), and high tensile strength. They can also be produced at very low costs. One of the major advantages of Fibre Reinforced Polymer is its low density that brings with it advantages such as ease of assembly and handling, ease of transportation, and reduction of loads on the elements that are being supported. Due to these, costs are greatly reduced. For various fibre types, the density of the composites varies between 0.9 – 2.3g/cm3. However, it varies between 1.2 and 1.8 g/cm3 in most cases. When compared to metals such as steel whose density is about 7.8 g/cm3, FRP composite delivers the highest levels of specific strength and specific stiffness. A simple rule that bases the volume fraction of each component is applied when there is need to determine the density of material made from resin and fibres of known properties: Where: ?m – density of matrix material; ?c – density of composite ?f – the density of fibre material Vm – volume fraction of matrix; Vf - Volume fraction of fibres Another mechanical property of FRP composites that is significantly affected by type fibres orientation and the reinforcing composite material is modulus. The table below represents some examples of three different types of composite materials, the variation of their shear modulus, transverse modulus, longitudinal modulus, and Poisson’s ratio for unidirectionally reinforced fibre reinforced polymer (FRP) composites. These kinds of composite materials have parallel and straight fibres. They are considered orthotropic materials in nature since they have two orthogonal planes of planes. Composite fibre/resin ELongitudinal ETransverse G V GPa GPa GPa - Carbon/epoxy 181.00 10.30 7.17 0.30 Glass/polyester 54.10 14.05 5.44 0.25 Aramid/epoxy 75.86 5.45 2.28 0.34 Table 2: typical modulus values of unidirectional FRP composites As depicted in the above table, materials with the highest stiffness are those that are formed by epoxy resin and carbon fibres. However, composite materials with higher transverse Young’s modulus are those that are composed of polyester resin and glass fibres. This makes them very useful materials for those elements that are subject to heavy loads in both longitudinal and transverse directions. Aramid based composite materials only have good properties in fibre direction. It is worth noting that, the volume fraction of fibres affects the modulus values of the final composite significantly in both transverse and longitudinal directions. The transverse and longitudinal modulus for unidirectionally reinforced polymers can be estimated using the following formulas: Where: Ef - Modulus of the fibres; EL – Longitudinal modulus in fibre direction; Em – Modulus of the matrix; Vf - Volume fraction of fibres; Vm – Volume fraction of matrix; ET – Transverse modulus of the composite in a perpendicular direction to the fibres; The orientation of fibres also affects the modulus of fibre reinforced polymer composites. Transverse and longitudinal modulus varies with the inclination angle of the fibres as illustrated in the figure below. Fig. 1:Transverse and longitudinal modulus as a function of inclination angle of the fibres As depicted by the diagram above, when the inclination angle of the fibres equals 00, that is, in composites that are unidirectional reinforced, the longitudinal modulus reaches it maximum. However, when the inclination angle of the fibres reaches or equals 90?, the transverse modulus reaches its maximum. Estimation of Young’s modulus becomes more complex when it comes to estimating Young’s modulus for irregular orientation of the reinforcement and not only depends on the inclination angles between the fibres but on their lengths and diameters. For composite materials, the Poisson’s ratio may considerably vary depending on fibre orientation. Poisson’s ratio values are usually similar to metals ranging from 0.25 to 0.35 when the inclination angle between fibres and the load direction equals to 0?. Poisson’s ratio can significantly vary from 0.02 to 0.05 for the 90? angle depending on different orientation of fibres. For composite materials, Poisson’s ration can be estimated/calculated using the analogical formula: v = vf.vf + vm .vm Where: vf –Poisson's ratio of fibre material; vm - Poisson's ratio of matrix material; v - Poisson's ratio of composite; Vm – Volume fraction of matrix; Vf - Volume fraction of fibre material The following figure shows how the Poisson’s ratio varies with the inclination angle of the fibres. Fig. 2: Poisson?s ratio as a function of inclination angle of the fibres The Manufacturing Process of Fibre Reinforce Polymer Before the process of manufacturing an FRP composite, the right technology needs to be chosen. This is done by putting into consideration certain factors such as the number of elements, their dimensions, and the shapes of the elements to be produced. Other requirements that pertain to issues such as the Young’s modulus, tensile strength, and other properties such as surface quality, accuracy of dimensions, etc. need to be considered. For the purposes of obtaining proper compatibility between the matrix and the reinforcement, properties such as the thermal expansion coefficient should be carefully selected. Several methods of manufacturing FRP composites exist including manual and fully automated techniques. This study will look at several of this methods such as spray-up, resin transfer moulding, hand lay-up, pultrusion, and filament winding. Spray-up and hand lay-up are examples of manual methods. Wet lay-up or hand lay-up is among the oldest FRP manufacturing techniques. This method is very labor intensive. In this method, fibre reinforcement is manually placed on top of the moulding once liquid resin has been applied to it. Afterwards, a metal laminating roller is used to remove any trapped air and impregnate the fibre with resin. These steps are severally repeated until a suitable thickness is achieved. However, this method has its own shortcomings, which include low fibre volume fraction, inconsistency in the quality of the parts produced, and health and environmental concerns of emitted styrene. Fig. 2.0: Hand lay-up process, image obtained from www.ale.nl Similar to the hand lay-up process is the spray-up process. However, the spray-up process is less expensive and much faster as opposed to the hand lay-up process. A spray gun is utilized in this process to apply chopped reinforcement and resin to the mould. Usually, glass fibres that have been chopped to the length of about 10 – 40 mm are used as reinforcements. This process is however, most suitable for the manufacturing of non-structural parts that do not need high tensile strength. One disadvantage of this process is that, it is very difficult to control the fibre thickness and volume fraction. This process is also highly dependent on skilled labor and is thus not suitable for use in manufacturing parts that require dimensional accuracy. Fig. 3.0: Spray-up process, image obtained from www.ale.nl An example of a semi-automated process of manufacturing FRP composites is resin infusion that is done under a flexible tooling process. This method is mostly used to retrofit Carbon Fibre Reinforce Polymer (CFRP) to cast iron, steel, and concrete bridges. In this method, the material transported to the site is that of preformed fibre in a mould. The preform is then fixed on to the retrofitted structure and then cover by vacuum bagging system and a supply of resin. Injection of resin into the preform is then done forming both an adhesive bond between the structure and the composite and the composite material itself. Fibre volume fraction as high as 55%, is the resultant yield of this process. An example of a fully automated method of FRP composite manufacture is pultrusion. This process enables the continuous production of FRP composite profiles with constant material properties and cross sections manufactured for very specific purposes. This is the only known method according to several sources that ensures continuous sufficient and consistent quality. This process has been in use for over 60 years now. Pultrusion involves the continuous pulling of reinforced material through a guide where there are fibres precisely placed in relation to the cross section of the profile. These fibres are then led through processing equipment and then they are impregnated with matrix material. The combined mixture is then pulled through heated equipment and cured into its final geometry. The fully cured profile is then cut into defined lengths by pulling it forward to a floating suspended saw. Fig. 4: The Pultrusion Process The number and type of continuous fibres, the dimensions and type of complex mats and weaves are carefully arranged in a way that makes it possible for one to visually check when the mats and fibres are in a profile. The quality and properties of the finished product solely depends on the positioning of the mats and fibres in relation to the cross section of the profile. The matrix is added through injection when the injection is pushed into the processing equipment. This kind of pultrusion known as pultrusion by injection is good when there is need for checking and controlling the reinforcement, the speed of changing from profile to profile. It also eases the changes in matrix during the process. Another decisive factor that affects the properties of a finished product is the degree of impregnation of the fibre. Reinforcement in traditional pultrusion is led through an opening in the mat containing the matrix. Solvent evaporation is kept at a minimum in the pultrusion process since the injection process takes place in fully enclosed processing equipment. The whole product moves to the next zone once all the fibres have been impregnated with the injected matrix. The next zone is where the heating process occurs and the process of curing the profiles is accelerated. Final curing is done in the last section of the equipment. Pullers that are outside the equipment, provides the pulling power, which overwhelms the friction thereby proving the process’ driving force. Either reciprocal pullers or belts can be used for pulling. At the final phase of the process, a saw mounted to move at the same speed as the profiles, shortens the profiles thus ensuring a continuous process. Another method of manufacturing FRP composites is filament winding. This method entails wounding of resin-impregnated fibres over a rotating mandrel at a given angle. The starting materials in this process include continuous aramid, carbon, or glass fibres. In this process, the liquid thermoset resins used include polyester, epoxy, and vinyl ester. From the mandrel, a composite unit is removed. This unit is then placed in an enclosed oven at about 60?C for approximately 8 hours for curing. Filament winding as a process of manufacturing FRP composite is usually preferred and used to fabricate pipes and tubular structures. The use of low costs tooling and low cost material explains why this process is low cost. Despite these advantages, this process or method is suitable for the manufacture and production of convex and closed structures. It also gives very low volume fraction of fibres. Fig.5. The Process of Filament Winding /image obtained from www.ale.nl/ The process of moulding where resin is transferred, referred to as ‘resin transfer moulding,’ is where fabrics, sometimes prepressed to the shape of the mould, are put down as a dry stack and are together held into place by a binder. This allows these preforms to be easily laid down into the moulding tool. Afterwards, a second moulding tool is clamped on top of the first moulding tool. Then, thermoset resin mixture that is highly pressurized, color, a catalyst, filler, etc. is pushed into the vat using dispensing equipment so that structural parts can be formed. The inlets of the resin are all closed once all the fabric wet out and the laminate is allowed to cure slowly. Curing and injection can both take part at either elevated temperatures or ambient temperatures. Fig.6. Resin transfer moulding process /image obtained from www.ale.nl/ Resin transfer moulding is an ideal method for manufacturing of small and medium volume qualities and in medium-sized structure. Complex parts can be obtained at intermediate volumes rate using resin transfer moulding, thus allowing for limited, cost-effective production. Through resin transfer moulding, fibre volumes fraction of over 60% can be easily achieved. This method also has several limitations including the very high costs of the tooling equipment, which are more complex when compared to the spray-up or the hand lay-up processes. Again, when compared to the pultrusion method, the adherence to dimensional tolerance is much lower. The fact that resins needs to have very low viscosities compromises the mechanical properties of the finished product. It also worth noting that, the rein transfer moulding method has different varieties in the sense of how the liquid resin is introduced in the mould cavity to the reinforcement. These variations and differences range from vacuum assisted (VARTM) to vacuum infusion resin transfer moulding (VIRTM). Fig. 7: VARTM process / image obtained from www.ale.nl/ Applications of Fibre Reinforced Polymer in ride Engineering There are several assortments of Fibre Reinforced Polymer composite elements that are used in bridge engineering including: Structural profiles–these profiles that are used in bridge engineering and their production is primarily through the pultrusion method/process. Mostly, the forms are made from cross sections of steel but in innovative ways, they can be adapted to Fibre Reinforced polymer composite properties. Cables and Tendons – these are FRPs that are in form of wires mostly Carbon Fibre Reinforced Polymer (CFRP). They have interesting properties such as high fatigue resistance, tensile strength, excellent chemical resistance, low weight, as well as high tensile strength. Decks – these are multilayered fibre reinforced polymer decks also known as sandwiches. Their mass is mostly concentrated in low-density core and the surface layers. They have high resistance to pressure and bending and high stiffness to the ratio of weight. They are generally light in weight compared to the weight of concrete slab, have high corrosion resistance, and fatigue strength. Therefore, they can quickly and easily be applied in bridge engineering. Normally, bridge decks are made of polyester, fibre glass or vinyl resin. These are the most used FRP elements in bridge engineering to achieve dead load savings. Disadvantages of Fibre Reinforced Polymer Despite the numerous advantages of FRP over traditional construction methods that have been hugely mentioned in this study, the adoption and acceptance of FRP composites in the engineering industry especially in bridge engineering has been very slow. This can be attributed to several reasons such as: Costs –compared to traditional materials, the costs of fabrication and materials of FRP in civil engineering are still very high. This is because FRP composites are cheaply produced if the production is large scale, which is not the case with civil engineering where different projects require different designs thus rendering mass production a less cost-effective approach. However, manufacture of modular lightweight components from FRP can greatly reduce the cost of construction. This is because there will be ease of installation or erection, and transportation. Uncertain durability – Various research and test that have been taken to ascertain the durability of FRP materials in different macro- and micro-climates, but no standard way that can be utilized to ascertain a standard method of manufacturing these materials in such a way that they can withstand different climatic conditions and answer the critical question of the FRP materials durability. Lack of Ductility – FRP materials do not exhibit definite yield strengths like steel materials. With ductile materials, redistribution of internal forces favorably is possible thus increasing safety. Therefore, the fact that FRP materials lack ductility is a problem in the sense that designers will have some problems during the design process. Lack of fire resistance – fire resistance is a very important property that any material used for construction of bridges need to exhibit. FRP materials in this case have very low fire resistance and are thus easily combustible with unhealthy gases. Lack of design standards Conclusion Fibre reinforced polymer (FRP) composite is a material that is two-phased and is made by combining several constituent materials, normally two. As a material that can be utilized for construction, its greatest strengths are its mechanical properties such as durability, which can be predetermined when it is being manufactured. As noted in the discussion above, several manufacturing processes that exist can be used for the production of structural elements that can be used in civil engineering applications. A fibre-reinforced polymer composite is a potential material that can be utilized by civil engineers in the construction of bridges. This is because of it superiority over traditional materials which include its potential resistance to chemical and environmental damages, and its high strength to weight ratio. Despite this, its adoption and acceptance into the civil engineering industry especially in bridge engineering is quite slow. This can be attributed to issues such as structural performance, durability, code specifications, costs, and the nature of the Fibre Reinforced Polymer composite industry. Therefore, for civil engineers to fully accept and adopt fibre reinforced polymer composites as a more reliable and practical construction material, intense projects involving the study of FRP composite need to be carried out to ascertain the in-service durability and the long-term cost-saving properties of FRP composites. Reference Bank L. C. (2006). Composites for Construction - Structural Design with FRP Materials, John Wiley & Sons, Inc. Davis, D., and Porter, M. L. (1999). ‘‘Glass fiber reinforced polymer dowel bars for transverse pavement joints.’’ Proc., 4th Int. Symposium,Fiber Reinforced Polymer Reinforcement for Reinforced ConcreteStructures, C. W. Dolan, S. H. Rizkalla, and A. Nanni, eds., SP-188, American Concrete Institute, Farmington Hills, Mich., 297–304. Deitz, D. H., Harik, I. E., and Gesund, H. ~1999!. ‘‘One-way slabs reinforced with glass fiber reinforced polymer reinforcing bars.’’ Proc.,4th Int. Symposium, Fiber Reinforced Polymer Reinforcement for ReinforcedConcrete Structures, C. W. Dolan, S. H. Rizkalla, and A. Keller T. Use of Fibre Reinforced Polymers in Bridge Construction, IABSEStructural Engineering Documents No 7. 2003 Zhou, A., Lesko, J. (2006).Introduction to FRP Composites, Showcase on Virginia Fiber-Reinforced Polymer Composites: Materials, Design, and Construction, Bristol, Virginia. Read More