FRP Materials

FRPs are composite materials made of a polymer matrix reinforced with fibers. In comparison to concrete (that is also a composite material), the fibers may carry and transfer both compressive and tensile stresses. The polymer matrix bonds these fibers together, prevents budding of the fibers in compression, transfers stresses between discontinuous fibers, protects the fibers from environmental impact, and maintains the overall form of the resulting composite material. 

Polymer matrix materials are categorized into thermoplastics and thermosets. Thermoplastics soften and melt above a specific temperature and become solid when cooled. They can be formed by repeated heating and cooling. In contrast, thermosets normally cure by irreversible chemical reaction (between two components, a resin and a hardener, for example, for epoxy (EP)) and chemical bonds are formed during the curing process. This means that a thermoset material cannot be melted and reshaped once it is cured. Thermosets are the most common matrix materials used for FRP composites in construction nowadays. The most common thermosets are unsaturated polyester (UP), EP, and vinylester (VE) [9]. Because of their organic material nature, aU of these matrix materials are sensitive to elevated temperatures and fire. 

Property E-glass fibers Carbon fibers Aramid fibers 

FRP composites, as a combination of fibers and polymer matrix, show also lightweight and high strength. In addition, because of the polymer matrix, they present high corrosion resistance and low thermal conductivity. Table 1.2 shows a comparison of basic material physical properties of FRP composites and other common constructive materials [11]. 

In comparison to steel and steel reinforced concrete, a distinction of FRP composites is their usually ortho tropic mechanical behavior. The strongest direction is always in parallel to that of the fiber direction. Strength and stiffness of a FRP component depend on the orientation of the fibers and quantity of fibers oriented in each direction. Bundles of parallel fibers are called roving. Different textiles 

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FRP materials are made up of the polymer and reinforcing fibers. The polymer is typically a thermoset polymer thermoplastics can be used as well. Some typical thermoset polymers used are epoxy resins, unsaturated polyester resins, epoxy vinyl ester resins, phenolic resins, and high performance aerospace resins such as cyanate esters, polyimides, and bismaleimides. These resins... 

The different methods of obtaining the fire resistance of the polymers have been discussed in previous chapters (Chapters 4 through 13). Fire codes and fire tests relevant to FRPs have also been discussed in previous chapters (Chapters 14 through 16 and 20 through 22), but there will be some discussions of tests solely relevant to composites in this chapter. This chapter will focus primarily on some methods for preparing FRPs, some of the factors that have to be considered when designing an FRP part and typical applications where fire retardant FRP materials are used. 

This chapter will mainly focus on designing thermoset resin FRP materials for fire performance. Many of the chemical and additive methods for making these resins fire retardant have been discussed previously in Chapters 4 through 13 and the details will not be included in this chapter except where appropriate to describe the designing of the FRP material for fire performance. 

To conclude, there are many applications where fire retardant FRP can be used and demand for FRP materials is likely to continue to increase due to the superior properties that FRP materials can have. Again, many factors need to be evaluated when determining whether FRP can be used and what resin and reinforcement system needs to be used to meet the requirements. When a flame retardant FRP is designed, the following factors must be considered ... 

With this information the correct resin and reinforcing fibers can be chosen to obtain a successful FRP material. 

The focus in the present discussion is on those FRP materials that are currently used in structural engineering applications. That is, although many different material combinations (combinations of fibre and matrix) are possible, only a very small sample of the almost infinite number of possibilities is presented herein. The reader should also keep in mind that several different manufacturing techniques, component shapes and end-use applications are available for FRP materials, but that only those most relevant to structural engineering are discussed. More comprehensive discussions of FRP materials are available in various composite materials texts see Eckold (1994), Herakovich (1998), Bank (2006) and GangaRao et al. (2007). 

The individual fibres within unidirectional composites have few defects, and are consequently resistant to crack initiation. Additionally, any crack that does form travels through the matrix and is not transmitted through adjacent fibres. These toughness and crack-arresting properties contribute to good fatigue performance in FRP materials. 

Bond properties at elevated temperatures. The bond, which relies heavily on the mechanical (shear) properties of the polymer matrix or adhesive, can be expected to be severely reduced at temperatures exceeding the glass transition temperature, Tg, of the matrix or the adhesive. Essentially no information is currently available on the specific behaviour of the bond between unprotected externally bonded FRP materials and concrete or masonry at high temperature. For example, in the case of insulated FRP systems, it is not clear exactly how long the bond between the externally bonded FRPs and the substrate can be maintained during a fire. 

Many types and shapes of FRP materials are now available in the construction industry. For the purposes of tensile reinforcement of concrete, the currently available reinforcing products include unidirectional FRP bars. 

Another frequently cited potential disadvantage of FRP materials is their relatively low elastic modulus as compared with steel. This means that FRP-reinforced concrete members are often controlled by serviceability (deflection) considerations, rather than strength requirements. 

The design of FRP-reinforced concrete members in flexure is analogous to the design of steel-reinforced concrete members. Flexural capacity can be calculated based on assumptions similar to those made for members reinforced with steel bars. The design of members reinforced with FRP bars should take into account the mechanical behaviour of the FRP materials (ACI-440.1R-03 2003). 

Bank, L. C. (2006). Composites for Construction Structural Design with FRP Materials. Hoboken, NJ, John Wiley Sons. 

In order for these materials to be fuUy exploited for applications in engineering structures, one challenge is to understand and predict the behavior of FRP materials and structures under elevated temperatures and fire. The fire requirements for structural members are an important and indispensable part in building specifications and standards. 

Kinetic theory was formulated to model the conversion degree of a material from one state to another. At each temperature, a FRP material can be considered as a mixture of materials in different states, with changing mechanical properties. The content of each state varies with temperature, thus the composite material shows temperature-dependent properties. If the quantity of material in each state is known and a probabilistic distribution function accounting the contribution from each material state to the effective properties of the mixture is available, the mechanical properties of the mixture can be estimated over the whole temperature range. 

Modeling of temperature-dependent mechanical properties for FRP materials was started in the 1980s. In many of the suggested models [1-5], -modulus were described as stepped functions of temperature achieved by coimecting experimentally gathered key points, such as the glass-transition temperature, Tg, and the decomposition temperature, T. -modulus values at different temperatures were obtained by DMA. 

Complex reactions are involved in the changes of material state of FRP materials under elevated and high temperature. For simplification, it is convenient to describe this process in four stages [24] ... 

Thermomechanical models for FRP materials were first developed in the 1980s. One of the first thermomechanical models for FRP materials was introduced by Springer in 1984 [1], where the degradation of mechanical properties was empirically related to the mass loss. In 1985, Chen et al. [2] added a mechanical model to the thermochemical model presented by Griffis et al. in 1981 [3] mechanical properties at several specified temperature points were assembled into a finite element formulation. Griffis et al. [4] introduced an updated version of Chen s model in 1986, whereby an extrapolation process was used to obtain the data in a higher temperature range. 

The time-to-failure of a structure or its components is an important issue for structural safety considerations in fire. On the basis of the strength degradation models for FRP materials under elevated and high temperatures developed in Chapter 5, the time-to-failure is predicted for GFRP tubes and laminates under both thermal and mechanical loading in compression. Temperature responses were again calculated using the thermal response model presented in Chapter 6. 

A model for predicting the compressive strength degradation of FRP materials in fire was applied as proposed in Chapter 5. Similarly to the modeling for stiffness degradation, it assumes that an FRP material at a certain temperature can be... 

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