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Fibre Reinforced Polymer (FRP) Composites


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The Evolution of Composites within Civil Engineering

For years, civil engineers have been in search for alternatives to steels and alloys to combat the high costs of repair and maintenance of structures damaged by corrosion and heavy use. For example, cost estimates for maintenance of highway bridge decks composed of steel-reinforced concrete are up to $90 billion/year. Since the 1940s, composite materials, formed by the combination of two or more distinct materials in a microscopic scale, have gained increasing popularity in the engineering field. Fiber Reinforced Polymer (FRP) is a relatively new class of composite material manufactured from fibers and resins (Figure 1) and has proven efficient and economical for the development and repair of new and deteriorating structures in civil engineering. The mechanical properties of FRPs make them ideal for widespread applications in construction worldwide.

FRP Laminate Structure

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FRPs are typically organized in a laminate structure, such that each lamina (or flat layer) contains an arrangement of unidirectional fibres or woven fibre fabrics embedded within a thin layer of light polymer matrix material (Figure 2). The fibres, typically composed of carbon or glass, provide the strength and stiffness. The matrix, commonly made of polyester, Epoxy or Nylon, binds and protects the fibers from damage, and transfers the stresses between fibers.

Suitability of FRP for Uses in Structural Engineering

The strength properties of FRPs collectively make up one of the primary reasons for which civil engineers select them in the design of structures. A material's strength is governed by its ability to sustain a load without excessive deformation or failure. When an FRP specimen is tested in axial tension, the applied force per unit cross-sectional area (stress) is proportional to the ratio of change in a specimen's length to its original length (strain). When the applied load is removed, FRP returns to its original shape or length. In other words, FRP responds linear-elastically to axial stress.

The response of FRP to axial compression is reliant on the relative proportion in volume of fibers, the properties of the fiber and resin, and the interface bond strength. FRP composite compression failure occurs when the fibers exhibit extreme (often sudden and dramatic) lateral or sides-way deflection called fiber buckling.

FRP's response to transverse tensile stress is very much dependent on the properties of the fiber and matrix, the interaction between the fiber and matrix, and the strength of the fiber-matrix interface. Generally, however, tensile strength in this direction is very poor.

Shear stress is induced in the plane of an area when external loads tend to cause two segments of a body to slide over one another. The shear strength of FRP is difficult to quantify. Generally, failure will occur within the matrix material parallel to the fibers.

Among FRP's high strength properties, the most relevant features include excellent durability and corrosion resistance. Furthermore, their high strength-to-weight ratio is of significant benefit; a member composed of FRP can support larger live loads since its dead weight does not contribute significantly to the loads that it must bear. Other features include ease of installation, versatility, anti-seismic behaviour, electromagnetic neutrality, excellent fatigue behaviour, and fire resistance.

However, like most structural materials, FRPs have a few drawbacks that would create some hesitancy in civil engineers to use it for all applications: high cost, brittle behaviour, susceptibility to deformation under long-term loads, UV degradation, photo-degradation (from exposure to light), temperature and moisture effects, lack of design codes, and most importantly, lack of awareness.

Applications of FRP Composites in Construction

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There are three broad divisions into which applications of FRP in civil engineering can be classified: applications for new construction, repair and rehabilitation applications, and architectural applications.
FRPs have been used widely by civil engineers in the design of new construction. Structures such as bridges and columns built completely out of FRP composites have demonstrated exceptional durability, and effective resistance to effects of environmental exposure. Pre-stressing tendons, reinforcing bars, grid reinforcement (Figure 3), and dowels are all examples of the many diverse applications of FRP in new structures.
One of the most common uses for FRP involves the repair and rehabilitation of damaged or deteriorating structures. Several companies across the world are beginning to wrap damaged bridge piers to prevent collapse and steel-reinforced columns to improve the structural integrity and to prevent buckling of the reinforcement.
Architects have also discovered the many applications for which FRP can be used. These include structures such as siding/cladding, roofing, flooring and partitions.
 
Intelligent Sensing for Innovative Structures (ISIS) Canada is a program that consists of collaborative research and development efforts of Canadian Universities in various engineering disciplines.  Its primary mission is in the development of innovative uses of FRPs in concrete structures.
 
In Canada, engineers have integrated fibre optic sensors into numerous FRP-reinforced systems to ensure that adequate supervision of the systems is provided.

 

Current Research on FRP

A serious matter relating to the use of FRPs in civil applications is the lack of design codes and specifications. For nearly a decade now, researchers from Canada, Europe, and Japan have been collaborating their efforts in hope of developing such documents to provide guidance for engineers designing FRP structures.

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Natalie Y.L. Chung, who started her career in structural engineering in 1997, completed her final year at Queen's University in Kingston in May 2002.  Relevant work experience includes the design of reinforced concrete, structural steel, masonry and timber structures. Over the years, she has developed many solid friendships with professionals and academics in the engineering industry.  Should you require additional information about FRP or Canadian codes or standards, please contact individuals on the following site http://www.civil.queensu.ca/research/structural.htm or contact Natalie by email.

Mechanics and Materials Website Link

Intelligent Sensing for Innovative Structures Website

Link to more information on FRP

Information on Fatigue Testing of FRP Composites

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