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7 chapter two Fiber-reinForced Polymer materials FRP is categorized as a composite material consisting of fibers and a matrix resin. The fibers are respon- sible for providing the composite with strength and modulus, whereas the resin protects the fibers from physical or environmental deterioration. Fibers may be surface-treated to improve the bond against a polymeric resin. The direction of the fibers controls the mechanical properties of FRP composites, because the strength and modulus of the reinforcing fibers are much greater than those of the binding resins. Because the resin transfers stresses between the fibers, a bond of these two constituents is crucial when the composite material carries structural load. The properties of FRP are dependent on the char- acteristics of fibers embedded in the resin, such as their amounts (volume fraction), orientation angles, and types. Therefore, it is important that the designers write a product-specific performance specifica- tion. For highway infrastructure application, three types of FRP composites have been traditionally used: CFRP, GFRP, and aramid FRP (AFRP). The potential of basalt FRP (BFRP) is under evaluation in the research community, and field demonstration with BFRP is as yet limitedly reported. resins Polymeric resins are broadly used for FRP composites, which are based on the long molecular chains of complexly entangled chemical substances. The degree of hardness in a polymer is determined by the density of cohesive energy (Hu 2013). In general, polymeric resins can be classified into two groups: thermosets and thermoplastics. Thermoset resins comprise multiple monomers with long cross-linked molecular chains. Because of this complicated polymeric conformation, the deformation of thermosetting resins is irreversible when subjected to elevated temperatures and, consequently, their degraded engineering properties and shapes are not fully recoverable. By contrast, thermoplastic resins are based on molecules weakly tied by Van der Waals forces; accordingly, the dislocation of the molecules is less restrictive: the softening and hardening of thermoplastics can be reversible with a temperature change, while maintaining their polymeric structure. As far as FRP composites in high- way infrastructure are concerned, thermoset resins are broadly chosen to bind high-strength fibers (in most cases, thermoset resins are liquid before curing). It is well recognized that thermoset-based FRP products include marginal moisture absorption, satisfactory chemical resistance, and dimensional stability at service temperatures (Hollaway 2010). The following is a summary of thermosetting resins frequently used in manufacturing FRP for infrastructure applications, with typical engineering proper- ties listed in Table 2: â¢ Epoxy: this type of resin is predominantly employed when an existing structure is strengthened with FRP. Dry fiber fabrics are impregnated with an epoxy on site (sometimes, premanufac- tured FRP laminates are used) and the compound is bonded to a substrate of the structure, which is called âwet lay-upâ or âhand lay-up.â Epoxies are cured with a catalyst or a hardener (e.g., low-viscosity anhydrides), including exothermic reactions. Although the use of epoxies encom- passes many advantages such as good adhesion, dimensional stability with negligible shrink- age, durability, and favorable mechanical properties, research reports that its performance may be degraded by ultraviolet radiation, which can be minimized with sacrificial coatings. â¢ Polyester: polyester resins are available at affordable prices and used for producing FRP com- posites. Unsaturated polyester resins, made by organic acids and polyhydric alcohols, have a high-density, cross-linking structure after a hardening process has occurred and, consequently, provide good resistance to temperature-induced distress. Various admixtures can be blended with polyesters to modify the physical and chemical properties.
8 â¢ Vinylester: vinylester is an alternative resin to epoxies or polyesters and is typically used for manufacturing GFRP-reinforcing bars or bridge decks. A chemical reaction called the esteri- fication of epoxies with organic compounds produces vinylester. The durability of vinylester resins is favorable in alkaline and corrosive environments. Its engineering properties can be enhanced by thermal curing at elevated temperatures. â¢ Other types: although the resins discussed previously are frequently selected for FRP manu- facturing, phenolic and polyurethane resins may be considered because of several advantages such as toughness, impact strength, and thermal resistance, in addition to high glass transition temperatures over 392Â°F (200Â°C). Fibers Unlike short fibers in mechanical components or leisure equipment, long continuous synthetic fibers are used for highway infrastructure. Their aspect ratio (diameter divided by length) is significantly low, because the diameter of most fibers varies from 5 to 10 microns, as shown in Figure 1. The orientation of fibers (unidirectional or multidirectional angles) is determined by their usage. For a two-dimensional laminate structure, multiple fiber layers at various angles are stacked on top of each other and saturated with a resin. The effective properties of this composite material in the longitudinal and transverse direc- tions may be calculated in accordance with classical laminate theory (Hyer 1998). The selection of fibers is dependent on the requirement of respective applications; for instance, performance durability in a certain circumstance, strength and stiffness, and budgetary considerations. Fiber types with their properties (Table 3) are as follows: â¢ Aramid fibers: aramid fibers are a synthetic aromatic-polyamide material with multiple molecules having amide bonds. Because of a repeated molecular chain structure, aramid fibers Type Density lb/in.3 (g/cm3) Tensile Strength ksi (MPa) Tensile Modulus ksi (GPa) Elongation at Break (%) Epoxy 0.043 (1.2) 13.1 (90) 440 (3) 8.0 Polyester 0.043 (1.2) 9.4 (65) 580 (4) 2.5 Vinylester 0.041 (1.12) 11.9 (82) 510 (3.5) 6.0 Based on Bank (2006). TABlE 2 TyPICAl MECHAnICAl PRoPERTIEs oF PolyMERIC REsIns FIGURE 1 Microscopic image of CFRP composite (used by permission from Yail J. Kim).
9 have been traditionally utilized for the cases that require large energy dissipation or tough- ness, such as bulletproof vests. Aramid fibers are lighter than carbon and glass fibers, whereas they provide comparable mechanical properties (approximately between those of carbon and glass fibers). Aramid fibers may more likely be used for prestressed concrete members than for reinforced concrete structures. â¢ Basalt fibers: basalt fibers are a mineral-based inorganic product [melted basalt rock at 2,730Â°F (1,500Â°C)], with a diameter ranging from 0.0004 in. to 0.0008 in. (10 Âµm to 20 Âµm) and were recently introduced to the structural engineering community. Basalt fibers are chemically inert and demonstrate good acidic and thermal resistance. The properties and costs of basalt fibers are similar to those of glass fibers. Their costs are, however, much lower than the costs of car- bon fibers. Hybrid applications with other fiber types may soon be available that take benefits from both material properties and expenses. â¢ Carbon fibers: carbon fibers are manufactured by various heat treatment processes such as carbonization and graphitizing at approximately 3,600Â°F (2,000Â°C) and 5,400Â°F (3,000Â°C), respectively. several factors can affect the properties of carbon fibers: treatment tempera- tures, microstructure conformation, and precursors. In civil structural applications, high- strength and high-modulus carbon fibers are available. The former has a tensile strength of up to 730 ksi (5,000 MPa) along with intermediate moduli ranging between 29,000 ksi and 36,300 ksi (200 GPa and 250 GPa), whereas the latter demonstrates a modulus of more than 116,000 ksi (800 GPa) with a tensile capacity of up to 360 ksi (2,500 MPa). Polyacrylonitrile- and pitch-based carbon fibers can provide high strength and high modulus, respectively. Carbon fibers are durable in terms of thermal, moisture, and chemical resistance. The fibers are, how- ever, electrically conductive, so that galvanic cells may form when contacting a metallic substrate because of a difference in electric potential. â¢ Glass fibers: glass fibers are manufactured by extruding silica dioxide or similar compounds and have a diameter of approximately 0.001 in. (25 Âµm) or less. A number of glass fibers are avail- able; namely, C-glass (alkaliâlime glass with boron oxide) for corrosion resistance, E-glass (borosilicate glass) for electrical resistance, R-glass (alumino silicate glass without magnesium oxide and calcium oxide) for mechanical components, and s-glass (alumino silicate) for high- strength application. When forming an FRP product, the surface of glass fibers is coated with silane in many cases to improve displacement compatibility with a resin matrix. E-glass fibers are com- monly used for FRP bars reinforcing structural concrete with a typical tensile strength of 510 ksi (3,500 MPa) and corresponding modulus of 11,600 ksi (80 GPa). Glass fibers exhibit favorable mechanical properties; however, they are vulnerable to creep- and moisture-induced damage. test methods to determine material ProPerties The determination of reliable engineering properties in FRP composites is important, because they control the performance of FRP-based structures for both FRP-reinforced and strengthened members. Bank (2006) compiled a number of FRP-related test methods (Table 4) suggested by several standards organizations; namely, AsTM, American Concrete Institute (ACI), European Committee for standard- ization (CEn), Canadian standard Association (CsA), and Japan society of Civil Engineers (JsCE). nCHRP Project 10-55 (NCHRP Research Results Digest 282: Fiber-Reinforced Polymer Composites for Concrete Bridge Deck Reinforcement) discussed several test methods. ACI440.9R-15 (ACI 2015b) is a recently published guide dedicated to the durability assessment of FRP materials. This document elaborates on the background of accelerated conditioning protocols, test methods for FRP-reinforcing Type Density lb/in.3 (g/cm3) Tensile Strength ksi (MPa) Tensile Modulus ksi (GPa) Elongation at Break (%) Aramid 0.052 (1.45) 525â533 (3,600â3,620) 18,000â19,000 (127â131) 2.5â2.8 Carbon 0.064â0.078 (1.77â2.16) 275â640 (1,900â4,410) 32,000â110,000 (220â758) 0.32â2.0 Glass 0.09â0.092 (2.49â2.54) 500â625 (3,450â4,300) 10,500â12,600 (72.4â86.9) 4.8â5.0 Based on ACI (2007). TABlE 3 TyPICAl MECHAnICAl PRoPERTIEs oF REInFoRCInG FIBERs
10 sdohteM tseT noitazinagrO AASHTO MP22-13 Fiber-reinforced polymer composite materials for highway and bridge structures ASTM C1557 Standard test methods for tensile strength and Youngâs modulus of fibers ASTM D149 Standard test methods for dielectric breakdown voltage and dielectric strength of solid electrical insulating materials at commercial power frequencies ASTM D256 Standard test methods for determining the Izod pendulum impact resistance of plastics ASTM D570 Standard test methods for water absorption of plastics ASTM D635 Standard test methods for rate of burning and/or extent and time of burning of plastics in a horizontal position ASTM D638 Standard test methods for tensile properties of plastics ASTM D648 Standard test methods for deflection temperature of plastics under flexural load in the edgewise position ASTM D695 Standard test methods for compressive properties of rigid plastics ASTM D696 Standard test methods for coefficient of linear thermal expansion of plastics between â30oC and 30oC with a vitreous silica dilatometer ASTM D790 Standard test methods for flexural properties of unreinforced and reinforced plastics and electrical insulating materials ASTM D792 Standard test methods for density and specific gravity (relative density) of plastics by displacement ASTM D953 Standard test methods for bearing strength of plastics ASTM D1929 Standard test methods for determining ignition temperature of plastics ASTM D2343 Standard test methods for tensile properties for glass fiber strands, yarns, and roving used in reinforced plastics ASTM D2344 Standard test methods for short-beam strength of polymer matrix composite materials and their laminates ASTM D2583 Standard test methods for indentation hardness of rigid plastics by means of a barcol impressor ASTM D2584 Standard test methods for ignition loss of cured reinforced resins ASTM D3039 Standard test methods for tensile properties of polymer matrix composite materials ASTM D3171 Standard test methods for constituent content of composite materials ASTM D3379 Standard test methods for tensile strength and Youngâs modulus of high-modulus single-filament materials ASTM D3410 Standard test methods for compressive properties of polymer matrix composite materials with unsupported gage section by shear loading ASTM D3846 Standard test methods for in-plane shear strength of reinforced plastics ASTM D3916 Standard test methods for tensile properties of pultruded glass fiber-reinforced plastic rod ASTM D4018 Standard test methods for properties of continuous filament carbon and graphite fiber tows ASTM D4475 Standard test methods for apparent horizontal shear strength of pultruded-reinforced plastic rods by the short-beam method ASTM D4476 Standard test methods for flexural properties of fiber-reinforced pultruded plastic rods ASTM D5083 Standard test methods for tensile properties of reinforced thermosetting plastics using straight-sided specimens ASTM D5379 Standard test methods for shear properties of composite materials by the V-notched beam method ASTM D5961 Standard test methods for bearing response of polymer matrix composite laminates ASTM D6856 Standard guide for testing fabric-reinforced textile composite materials ASTM E84 Standard test methods for surface burning characteristics of building materials ASTM E662 Standard test methods for specific optical density of smoke generated by solid materials ASTM E831 Standard test methods for linear thermal expansion of solid materials by thermomechanical analysis ASTM E1356 Standard test methods for assignment of the glass transition temperatures by differential scanning calorimetry or differential thermal analysis ASTM E1640 Standard test methods for assignment of the glass transition temperature by dynamic mechanical analysis ASTM E2092 Standard test methods for distortion temperature in three-point bending by thermal mechanical analysis ACI B.1 Test method for cross-sectional properties of FRP bars ACI B.2 Test method for longitudinal tensile properties of FRP bars ACI B.3 Test method for bond strength of FRP bars by pullout testing ACI B.4 Test method for transverse shear strength of FRP bars ACI B.5 Test method for strength of FRP bent bars and stirrups at bend locations ACI B.6 Accelerated test method for alkali resistance of FRP bars ACI B.7 Test method for tensile fatigue of FRP bars ACI B.8 Test method for creep rupture of FRP bars ACI B.9 Test method for long-term relaxation of FRP bars ACI B.10 Test method for performance of anchorages of FRP bars TABlE 4 PRoPosED TEsT METHoDs FoR FRP-RElATED MATERIAls
11 bars and sheets subjected to aggressive service environments, and a bond test approach (pull-off of FRP sheets adhered to a concrete substrate). A comparative discussion with AsTM test methods is provided as well. There is no single test globally accepted, and each test method has its own advantages and dis- advantages. As such, manufacturers or product users are responsible for deciding the suitable test approaches that can most efficiently represent their engineering needs. As in the case of other con- struction materials, test information about FRP composites (e.g., preparation, specimen details, data acquisition, and technical results) is required for a report or data sheets. sdohteM tseT noitazinagrO ACI B.11 Test method for tensile properties of deflected FRP bars ACI B.12 Test method for determining the effect of corner radius on tensile strength of FRP bars ACI L.1 Test method for direct tension pull-off test ACI L.2 Test method for tension test of flat specimen ACI L.3 Test method for overlap splice tension test CEN 13706 Reinforced plastics composites: specifications for pultruded profiles CSA Annex A Determination of cross-sectional area of FRP reinforcement CSA Annex B Anchor for testing FRP specimens under monotonic, sustained, and cyclic tension CSA Annex C Test method for tensile properties of FRP reinforcements CSA Annex D Test method for development length of FRP reinforcement CSA Annex E Test method for FRP bent bars and stirrups CSA Annex F Test method for direct tension pull-off test CSA Annex G Test method for tension test of flat specimens CSA Annex H Test method for bond strength of FRP rods by pullout testing CSA Annex J Test method for creep of FRP rods CSA Annex K Test method for long-term relaxation of FRP rods CSA Annex L Test method for tensile fatigue of FRP rods CSA Annex M Test method for coefficient of thermal expansion of FRP rods CSA Annex N Test method for shear properties of FRP rods CSA Annex O Test method for alkali resistance of FRP rods CSA Annex P Test method for bond strength of FRP sheet bonded to concrete CSA Annex Q Test method for overlap splice in tension JSCE E131 Quality specification for continuous fiber reinforcing materials JSCE E531 Test method for tensile properties of continuous fiber reinforcing materials JSCE E532 Test method for flexural tensile properties of continuous fiber reinforcing materials JSCE E533 Test method for creep of continuous fiber reinforcing materials JSCE E534 Test method for long-term relaxation of continuous fiber reinforcing materials JSCE E535 Test method for tensile fatigue of continuous fiber reinforcing materials JSCE E536 Test method for coefficient of thermal expansion of continuous fiber-reinforcing materials by thermos mechanical analysis JSCE E537 Test method for performance of anchors and couplers in prestressed concrete using continuous fiber-reinforcing materials JSCE E538 Test method for alkali resistance of continuous fiber reinforcing materials JSCE E539 Test method for bond strength of continuous fiber reinforcing materials JSCE E540 Test method for shear properties of continuous fiber reinforcing materials by double plane shear JSCE E541 Test method for tensile properties of continuous fiber sheets JSCE E542 Test method for overlap splice strength of continuous fiber sheets JSCE E543 Test method for bond properties of continuous fiber sheets to concrete JSCE E544 Test method for bond strength of continuous fiber sheets to steel plate JSCE E545 Test method for direct pull-off strength of continuous fiber sheets with concrete JSCE E546 Test method for tensile fatigue of continuous fiber sheets JSCE E547 Test method for accelerated artificial exposure of continuous fiber sheets JSCE E548 Test method for freeze-thaw resistance of continuous fiber sheets JSCE E549 Test method for water, acid, and alkali resistance of continuous fiber sheets Compiled based on Bank (2006). TABlE 4 (continued)