Introduction to FRP

ISIS Educational Module 2:

An Introduction to FRP Composites for Construction Prepared by ISIS Canada A Canadian Network of Centres of Excellence www.isiscanada.com Principal Contributor: L.A. Bisby, Ph.D., P.Eng. Department of Civil Engineering, Queens University Contributor: J. Fitzwilliam March 2006 ISIS Education Committee: N. Banthia, University of British Columbia L. Bisby, Queens University R. Cheng, University of Alberta R. El-Hacha, University of Calgary G. Fallis, Vector Construction Group R. Hutchinson, Red River College A. Mufti, University of Manitoba K.W. Neale, Universit de Sherbrooke J. Newhook, Dalhousie University K. Soudki, University of Waterloo L. Wegner, University of Saskatchewan

ISIS Canada Educational Module No. 2: FRP Composites for Construction

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Objectives of This Module The objective of this module is to provide students with an overall awareness of the properties, behaviour, and application of fibre reinforced polymer (FRP) materials in civil engineering construction applications. This document is one of a series of modules on innovative FRP technologies available from ISIS Canada. Further information on FRP materials and on the use of FRPs in a variety of innovative applications can be found on the internet at www.isiscanada.com. While research into the use of FRP materials in a number of structural applications is ongoing, an overall knowledge of currently available FRP materials is essential for the new generation of civil engineers. Experience has shown that the problems of the future cannot generally be solved with the materials and

methodologies of the past, and FRPs are rapidly emerging as key materials for use in durable and sustainable infrastructure.

The primary objectives of this manual can be summarized as follows: 1. to provide civil engineering students with a general

awareness of the properties and behaviour of FRP materials;

2. to provide information on some of the potential uses of FRPs in civil engineering applications;

3. to facilitate the use of FRP materials in the construction industry; and

4. to provide guidance for students seeking additional information on this topic.


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Additional ISIS Educational Modules Available from ISIS Canada (www.isiscanada.com) Module 1 Mechanics Examples Incorporating FRP Materials Nineteen worked mechanics of materials problems are presented which incorporate FRP materials. These examples could be used in lectures to demonstrate various mechanics concepts, or could be assigned for assignment or exam problems. This module seeks to expose first and second year undergraduates to FRP materials at the introductory level. Mechanics topics covered at the elementary level include: equilibrium, stress, strain and deformation, elasticity, plasticity, determinacy, thermal stress and strain, flexure and shear in beams, torsion, composite beams, and deflections. Module 3 Introduction to FRP-Reinforced Concrete The use of FRP bars, rods, and tendons as internal tensile reinforcement for new concrete structures is presented and discussed in detail. Included are discussions of FRP materials relevant to these applications, flexural design guidelines, serviceability criteria, deformability, bar spacing, and various additional considerations. A number of case studies are also discussed. A series of worked example problems, a suggested assignment with solutions, and a suggested laboratory incorporating FRP-reinforced concrete beams are all included. Module 4 Introduction to FRP-Strengthening of Concrete Structures The use of externally-bonded FRP reinforcement for strengthening concrete structures is discussed in detail. FRP materials relevant to these applications are first presented, followed by detailed discussions of FRP-strengthening of concrete structures in flexure, shear, and axial compression. A series of worked examples are presented, case studies are outlined, and additional, more specialized, applications are introduced. A suggested assignment is provided with worked solutions, and a potential laboratory for strengthening concrete beams in flexure with externally-bonded FRP sheets is outlined. Module 5 Introduction to Structural Health Monitoring The overall motivation behind, and the benefits, design, application, and use of, structural health monitoring (SHM) systems for infrastructure are presented and discussed at the introductory level. The motivation and goals of SHM are first presented and discussed, followed by descriptions of the various components, categories, and classifications of SHM systems. Typical SHM methodologies are outlined, innovative fibre optic sensor technology is briefly covered, and types of tests which can be carried out using SHM are explained. Finally, a series of SHM case studies is provided to demonstrate four field applications of SHM systems in Canada.

Module 6 Application & Handling of FRP Reinforcements for Concrete Important considerations in the handling and application of FRP materials for both reinforcement and strengthening of reinforced concrete structures are presented in detail. Introductory information on FRP materials, their mechanical properties, and their applications in civil engineering applications is provided. Handling and application of FRP materials as internal reinforcement for concrete structures is treated in detail, including discussions on: grades, sizes, and bar identification, handling and storage, placement and assembly, quality control (QC) and quality assurance (QA), and safety precautions. This is followed by information on handling and application of FRP repair materials for concrete structures, including: handling and storage, installation, QC, QA, safety, and maintenance and repair of FRP systems. Module 7 Introduction to Life Cycle Engineering & Costing for Innovative Infrastructure Life cycle costing (LCC) is a well-recognized means of guiding design, rehabilitation and on-going management decisions involving infrastructure systems. LCC can be employed to enable and encourage the use of fibre reinforced polymers (FRPs) and fibre optic sensor (FOS) technologies across a broad range of infrastructure applications and circumstances, even where the initial costs of innovations exceed those of conventional alternatives. The objective of this module is to provide undergraduate engineering students with a general awareness of the principles of LCC, particularly as it applies to the use of fibre reinforced polymers (FRPs) and structural health monitoring (SHM) in civil engineering applications. Module 8 Durability of FRP Composites for Construction Fibre reinforced polymers (FRPs), like all engineering materials, are potentially susceptible to a variety of environmental factors that may influence their long-term durability. It is thus important, when contemplating the use of FRP materials in a specific application, that allowance be made for potentially harmful environments and conditions. It is shown in this module that modern FRP materials are extremely durable and that they have tremendous promise in infrastructure applications. The objective of this module is to provide engineering students with an overall awareness and understanding of the various environmental factors that are currently considered significant with respect to the durability of fibre reinforced polymer (FRP) materials in civil engineering applications.


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Section 1

Introduction and Overview BACKGROUND The construction industry has historically been dominated by four traditional materials: stone, timber, concrete, and steel. Up until a few hundred years ago, stone and timber were the primary materials used to build structures. In the past two hundred years or so, structural steel and reinforced concrete have emerged as leading construction materials, and most modern urban landscapes are now defined largely by these two materials. Steel and concrete have served the civil engineering community well, and have enabled the construction of the elaborate world-wide systems of infrastructure that have greatly contributed to the economic health and prosperity of the developed world. However, steel and concrete both suffer from various forms of degradation, and after decades of neglect and overuse, our aging infrastructure systems are crumbling (Fig. 1-1).

In an effort to slow and/or prevent infrastructure deterioration, engineers are looking for new materials that can be used to prolong and extend the service lives of existing structures while also enabling the design and construction of durable new structures. Fibre reinforced polymers (FRPs), a relatively new class of non-corrosive, high-strength, and lightweight materials, have, over the past 15 years or so, emerged as practical materials for a number of structural engineering applications.

FRPs have been used in the automotive and aerospace industries for more than 50 years, in applications where their high strength and light weight can be used to greatest advantage. As their name suggests, these materials are composed of high-strength fibres embedded in a polymer matrix. The fibres are extremely strong and stiff, and the matrix binds them and enables them to work together as a composite material.

The focus in the present discussion is on those FRP materials that are currently used in structural engineering applications. It is important to remain cognizant of the fact that many different material combinations (combinations of fibre and matrix) are possible, and that 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 also available for FRP materials, but that only those most relevant to structural engineering are discussed in this document. More complete discussions of FRP materials are available in various composite materials texts.

The rapid increase in the use of FRP materials for structural engineering applications that has occurred over the past 15 years can be attributed to continuing reductions in cost, and to the numerous advantages of FRPs as compared with conventional materials such as concrete and

steel. Some of the commonly cited advantages of FRP materials over more conventional materials like steel include: high strength-to-weight ratios; outstanding durability in a variety of environments; ease and speed of installation, flexibility, and

application techniques; electromagnetic neutrality, which can be important in

certain special structures such as magnetic imaging facilities;

the ability to tailor mechanical properties by appropriate choice and direction of fibres;

outstanding fatigue characteristics (carbon FRP); and low thermal conductivity.

Fig. 1-1. Severely corroded reinforcing steel in these bridge columns has resulted in spalling of the concrete cover and exposure of the steel reinforcement. New repair techniques for these types of members are now available which make use of FRP materials.

However, FRP materials also have a number of potential disadvantages. Foremost among these disadvantages is the initial material cost of FRPs, which can be several times that of steel. However, when the cost of a structure is considered over its entire life cycle, the improved durability offered by FRP materials can make them the most cost-effective material in many cases.


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Fig. 1-2. An example of a flexible carbon FRP sheet that can be used for repair of concrete structures. FRPS IN CONSTRUCTION Since the early 1990s, interest in the use of FRP materials for structures has increased steadily, and there are currently hundreds of field applications of FRPs in structures around the world. Some of the more common FRP applications in civil engineering structures are described in detail in Section 5 of this document, and include: Externally-bonded FRP plates, sheets, and wraps for

strengthening of reinforced concrete, steel, aluminum, and timber structural members (Fig. 1-2);

FRP bars, rods, and tendons for internal reinforcement of concrete (Figs. 1-3 and 1-4);

all-FRP structures; and FRP hybrid structures.

This module presents an introduction into the properties and uses of FRP materials in civil engineering structures, with a particular emphasis on their use for reinforcement and strengthening of structural concrete.

Fig. 1-3. Examples of currently available glass fibre FRP reinforcing bars for concrete.

Fig. 1-4. Examples of currently available carbon FRP reinforcing bars for concrete.

Section 2

Fibre Reinforced Polymer (FRP) GENERAL FRPs are a subgroup of the class of materials referred to more generally as composites. Composites are defined as materials created by the combination of two or more materials, on a macroscopic scale, to form a new and useful material with enhanced properties that are superior to those of the individual constituents alone.

When most people think of composite materials, they tend to consider one of a number of advanced material systems developed in the modern era. However, many

composites have been in use in civil engineering for hundreds of years (e.g. concrete, a composite material composed primarily of gravel, sand, and cement paste). Indeed, organic composite materials also exist throughout nature. Wood and bone are both examples of natural composite materials of tremendous strength. Bone, for example, is composed of fibres of the protein collagen, bound together by a crystalline calcium compound called apatite.


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Fig. 2-1. Basic material components that are combined to create an FRP composite.

An FRP is a specific type of two-component composite material consisting of high strength fibres embedded in a polymer matrix. The study of FRPs is complicated by the innumerable combinations of materials that can be used to create an FRP composite. This is both an advantage and a disadvantage for FRPs as engineering materials. For instance, FRPs can be tailored to suit virtually any application; however, this versatility leads to a wide range in possible properties, making it difficult in many cases to arrive at generalizations with respect to FRP behaviour.

Because FRPs are composed of two distinct materials, overall FRP material properties depend primarily on those of the individual constituents. It is thus instructive to examine the role and properties of each of the component materials, the fibres and the matrix, separately, before discussing the properties of the FRP composite as a whole.

MATRIX The matrix is the binder of the FRP and plays many important roles. Some of the more critical functions played by the matrix are: to bind the fibres together; to protect the fibres from abrasion and environmental

degradation; to separate and disperse fibres within the composite; to transfer force between the individual fibres; and to be chemically and thermally compatible with the

fibres. A major selection criterion for matrix materials is that

they have a low density, usually considerably less than the fibres, such that the overall weight of the composite is minimized.

While the fibres provide the strength and stiffness of an FRP, the matrix is essential to transfer forces between the individual fibres. This force transfer is accomplished through shear stresses that develop in the matrix between the individual fibres. Obviously, the quality of the bond

between the fibres and the matrix is thus a key factor in obtaining good mechanical properties.

A polymer matrix is an organic compound comprised of long-chain molecules consisting of smaller repeated units called monomers. Although an enormous variety of polymer matrix materials exist for the manufacture of FRP materials, the focus herein is on FRPs used in infrastructure applications, and thus only a few specific matrix materials are discussed.

Matrix materials for FRPs can be grouped into two broad categories: thermoplastics and thermosetting resins. Thermoplastics include such polymer compounds as polyethylene, nylon, and polyamides, while thermosetting materials include epoxies and vinylesters.

Thermoplastics are polymers composed of long-chain molecules that are held together by relatively weak Van der Waals forces, but that have extremely strong bonds within individual molecules. In these materials, the molecules are free to slide over one another at elevated temperatures, and so thermoplastics can be repeatedly softened and hardened by heating and cooling without significantly changing their molecular structure. Thermosetting polymers are also composed of long-chain molecules built from monomers, but for these materials the molecular chains are cross-linked through primary chemical bonds. Thus, thermosets cannot be reversibly softened and will deteriorate irreversibly at elevated temperatures.

Almost exclusively, thermosets are currently used in structural engineering applications. These polymers generally have good thermal stability at service temperatures, good chemical resistance, and display low creep and relaxation properties in comparison with most thermoplastics. However, because it is difficult to reversibly soften thermosets, FRP components made from thermoset matrices must be bent or formed during the manufacturing process. This can become a problem in some specific applications. For instance, FRP reinforcing bars for concrete that incorporate thermosetting polymer resins cannot be bent on site, and research is currently underway to develop satisfactory thermoplastic matrices for these specialized applications.

Three specific types of thermosetting resins are commonly used in the manufacture of infrastructure composites: polyesters, vinylesters, and epoxies. Polyesters Polyesters are the most widely used polymers in the manufacture of FRP components for infrastructure applications due to their relatively low cost and ease of processing (these resins cure at ambient temperatures). Numerous specific types of polyesters are available for use, with varying degrees of thermal and chemical stability, moisture absorption, and shrinkage during curing.

+ =

FIBRES POLYMER MATRIX

FRP


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Vinylesters Vinylesters are often identified as a class of polyesters because of their similar processing procedures. However, in chemical structure they are essentially unsaturated epoxides, and so their properties are more akin to epoxies. Vinylesters are resistant to strong acids and alkalis, which is one reason that they are commonly used in the manufacture of FRP reinforcing bars for concrete (the environment inside concrete is highly alkaline). Vinylesters also offer reduced moisture absorption and shrinkage as compared with polyesters. Vinylesters cost slightly more than polyesters. Epoxies Epoxies are often used in wet lay-up applications of FRP plates and sheets (discussed in detail later) because of their ability to cure well at room temperature and owing to their outstanding adhesion (bonding) characteristics. Epoxies have high strength, good dimensional stability, relatively good high-temperature properties, strong resistance to chemicals (except acids), and superior toughness. Epoxies, however, cost significantly more than polyesters or vinylesters. FIBRES The fibres provide the strength and stiffness of an FRP. Because the fibres used in most structural FRP applications are continuous and are oriented in specified directions, FRPs are orthotropic, and they are much stronger and stiffer in the fibre direction(s). Fibres are generally selected to have: high stiffness; high ultimate strength; low variation of strength between individual fibres; stability during handling; and uniform diameter.

For structural engineering applications, fibres are also characterized by extremely large length-to-diameter ratios (they are considered continuous) and by extremely small diameters (as small as 5-10 microns across, refer to Figure 2-2). The small diameter of the fibres is significant, in that the molecular structure of the material is aligned along the length of the fibres giving them high tensile strength. Also since the probability of a sample of material containing a flaw large enough to cause brittle failure decreases with its volume, microscopic fibres have fewer defects than the bulk fibre material, and hence higher strengths. In the event of a single fibre break within the FRP, force transfer to adjacent fibres, through shear stresses that develop in the polymer matrix, prevents failure of the overall FRP composite. It is important to note that the force transfer required to prevent overall failure of the FRP depends primarily on the shear strength of the matrix.

Many different types of fibres are available for use, and all have their respective advantages and disadvantages. In

civil engineering applications, the three most commonly used fibre types are glass, carbon (graphite), and to a lesser extent, aramid (KevlarTM). The suitability of the various fibres for specific applications depends on a number of factors including the required strength, the stiffness, durability considerations, cost constraints, and the availability of component materials. Figure 2-3 shows typical stress-strain curves for various currently available fibres. Note that these curves are for the pure fibres only, and they do not include the effects of the polymer matrix.

Fig. 2-2. Scanning electron micrograph showing microscopic carbon fibres used in FRP fabrication. Glass Fibres Glass fibres are commonly produced by a process called direct melt, wherein fibres with a diameter of 3 to 25 microns are formed by rapid and continuous drawing from a glass melt. Glass fibres are the most inexpensive, and consequently the most commonly used, fibres in structural engineering applications. There are several different grades available, but the most common are E-glass and the more expensive, but stronger, R-glass. Glass fibres are characterized by their high strength, moderate modulus of elasticity and density, and by their low thermal conductivity. Glass fibres are often chosen for structural applications that are not weight critical (glass FRPs are heavier than carbon or aramid) and that can tolerate the larger deflections resulting from the comparatively low elastic modulus of the glass fibres. Glass fibres are often used in the manufacture of FRP reinforcing bars, pultruded FRP structural sections, FRP wraps for seismic upgrade, and filament wound FRP tubes. Carbon Fibres Carbon fibres are produced by a process called controlled pyrolysis, wherein one of three potential precursor fibres is subjected to a complex series of heat treatments (stabilization, carbonization, graphitization, and surface treatment) to produce carbon filaments with diameters in the rage of 5-8 microns. The resulting fibres can have properties that vary widely, and so several classes of carbon

50 m


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fibres are available, differentiated based on their elastic moduli: standard, 250-300 GPa intermediate, 300-350 GPa high, 350-550 GPa ultra-high, 550-1000 GPa

Although considerably more expensive than glass fibres, carbon fibres are beginning to see widespread use in structural engineering applications such as prestressing tendons for concrete and structural FRP wraps for repair and strengthening of reinforced concrete beams, columns, and slabs. Their steadily increasing use can be attributed to their steadily decreasing cost, their high elastic moduli and available strengths, their low density (low weight), and their outstanding resistance to thermal, chemical, and environmental effects. Carbon fibres are an ideal choice for structures which are weight and/or deflection sensitive.

Strain (%)0 1 2 3 4 5

Stre

ss (M

Pa)

0

1000

2000

3000

4000

5000

6000E-GlassAramid-49Standard CarbonHigh-Modulus CarbonUltra High-ModulusReinforcing Steel

Fig. 2-3. Stress-strain properties of typical fibres.

Aramid Fibres Aramid fibres are manufactured from a synthetic compound called aromatic polyamide in a process called extrusion and spinning. Two stiffness grades are readily available: 60 GPa and 120 GPa. Aramid fibres are characterized by high strength, moderate elastic modulus, and low density. In addition, FRPs manufactured from aramid fibres have low compressive and shear strengths as a consequence of the unique anisotropic properties of the fibres. Aramid fibres are also susceptible to degradation from exposure to ultraviolet radiation and/or moisture. FRPs Although the strength and stiffness of an FRP are governed by the fibres, the overall material properties depend also on the mechanical properties of the matrix, the fibre volume fraction (the volume of fibres per unit volume of FRP), the fibre cross-sectional area, the orientation of the fibres within the matrix, and the method of manufacturing. It is the interaction between the fibres and the matrix that gives

FRPs their unique physical and mechanical characteristics. The orientation of the fibres within the matrix is a key consideration in the design and use of FRP materials. In the present discussion we will focus on unidirectional FRPs, or on FRPs in which the fibres are all aligned in a single direction. Unidirectional FRPs are commonly used for FRP reinforcing rods and tendons, FRP wraps for concrete rehabilitation, and pultruded FRP structural sections (all discussed in detail later in this document).

Figure 2-4 shows various FRP products currently used for reinforcement or rehabilitation of concrete structures, and Figure 2-5 shows a number of pultruded FRP structural sections.

Fig. 2-4. Assorted FRP products currently used for reinforcement or rehabilitation of concrete structures. In North America, glass and carbon are the two most commonly used fibres, and matrices are generally epoxies or vinylesters. Aramid fibres and polyester resins are also used very occasionally. Glass is widely used because of its comparatively low cost, and because there is historically much more experience with it. However, glass fibres have demonstrated certain significant disadvantages, such as a relatively low elastic modulus and some durability concerns in alkaline environments. These disadvantages have made carbon FRPs, with elastic moduli that can compare more closely with steel, more attractive, even given their considerably higher cost. The primary concerns associated with aramid FRPs are that they are sensitive to creep and have displayed poor durability characteristics resulting from their propensity for moisture absorption. Aramid fibres also perform poorly at high temperature. Table 3-3 provides a comparison of various types of FRPs based on a number of important criteria.


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MANUFACTURING TECHNIQUES As stated earlier, there is a wide variety of techniques by which FRP components can be manufactured. However, only those manufacturing methods of immediate interest to the structural engineer are included herein. Pultrusion, wet lay-up, and filament winding are all discussed in some detail, while other techniques such as pull-winding, resin transfer molding, vacuum bag molding, and injection molding are left to specialized composite materials texts. Pultrusion A manufacturing process called pultrusion is commonly used to produce FRP bars, rods, tendons, plates, and structural sections. The technique is fully automated and is thus highly economical. It is similar to the extrusion process by which many metal sections are fabricated. Illustrated in Figures 2-6 and 2-7, the pultrusion process is accomplished by pulling raw fibres through a resin bath and then through a heated die. As the resin-impregnated fibres pass through the die the polymer matrix hardens into the shape of the die, thus producing a structural component. The FRP component is pulled from the cured end. This process is continuous and has the advantage that FRP components of virtually any length can be fabricated. The reader will note that all of the fibres in a pultruded element are aligned along the length of the component, thus creating a unidirectional FRP.

Fig. 2-5. Various available pultruded FRP structural sections. Wet Lay-Up (Hand Lay-Up) Wet lay-up, sometimes referred to as hand lay-up or contact molding, is an FRP manufacturing technique often used in structural rehabilitation applications, where FRP sheets or fabrics are bonded to the exterior of reinforced concrete, steel, aluminum, or timber members. In this technique, a rigid mould is covered with resin and a roller is used to press the fibres (usually in the form of a sheet or fabric of

raw fibres) into the resin. In some cases, additional resin is added to the outer surface of the fibres to ensure that they are fully impregnated. Additional layers of FRP can be added on top of each other to achieve any desired thickness of FRP. In structural rehabilitation applications, the mould is simply the existing structural member to be strengthened, and the FRP remains bonded to the mould after curing (which is normally accomplished at ambient temperature). This technique has the advantage that it is easily and rapidly performed in the field, providing significant financial advantages over conventional structural rehabilitation techniques such as external plating with steel. However, quality control is extremely important in this procedure, and skilled labour is often required. Wet lay-up for structural rehabilitation of a concrete column is illustrated in Figure 2-8.

Fig. 2-6. Schematic showing the pultrusion manufacturing process.

Fig. 2-7. Glass fibres being drawn off of creels and used in the fabrication of pultruded glass FRP reinforcing bars for concrete. Filament Winding Many innovative applications of FRPs in structural engineering, such as stay-in-place formwork for concrete piles (discussed later) make use of hollow FRP poles, pipes, and tubes. These members are commonly produced using a manufacturing process called filament winding. In this automated process, illustrated in Figures 2-9 and 2-10, raw fibres are drawn off spools, through a resin bath, and wound onto a rotating mandrel. The placement of the fibres on the mandrel is controlled by a computer, allowing for the fibres to be placed with extreme precision and with various desired

resin tank

shaping and heating die puller

creel


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orientations. By varying the fibre orientation, filament wound members can be created with a variety of mechanical properties tailored for specific applications.

Fig. 2-8. Glass FRP sheets being applied to strengthen a reinforced concrete column using the wet lay-up (hand lay-up) procedure.

Fig. 2-9. A schematic showing the filament winding manufacturing process.

Fig. 2-10. Glass fibres being wound onto a triangular mandrel during the fabrication of triangular filament would glass FRP tubes.

Section 3

Mechanical Properties of FRPs The performance of any engineering material in a specific application is dependent on its mechanical properties, durability, and cost. This section focuses on the mechanical properties of FRPs, including the stress-strain response, and other properties such as creep, fatigue, fracture, and bond. GENERAL The mechanical properties of an FRP depend on a number of factors including: the relative proportions of fibre and matrix; the mechanical properties of the constituent materials

(fibre, matrix, and any additives); the orientation of the fibres within the matrix; and the method of manufacture.

Figure 3-1 shows typical stress-strain curves for several unidirectional FRP materials. Also included in Figure 3-1 is a stress-strain curve for reinforcing steel. Some commonly available FRPs used in concrete reinforcing applications, and their respective properties, are listed in Tables 3-1 and 3-2.

Strain [%]

0 1 2 3

Stre

ss [M

Pa]

0

500

1000

1500

2000

2500SteelISOROD CFRPISOROD GFRPNEFMAC GFRPNEFMAC CFRPNEFMAC AFRPLeadlineTM CFRP

Fig. 3-1. Typical tensile stress-strain curves for various currently available FRP concrete reinforcing products. Table 3-3 provides a comparison between various types of FRPs and conventional reinforcing materials for concrete.

Mobile Resin Bath

Motor

Fibre Roving

Rotating Mandrel


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From this data it is evident that both glass and aramid FRPs have moduli that are considerably less than steel in the pre-yield zone, but that carbon FRPs have moduli that are

comparable to, or even higher than, steel in some cases. Also evident from the data is the fact that FRPs have ultimate strengths that can be many times greater than steel.

Table 3-1. Selected Properties of Typical Currently Available FRP Reinforcing Products

Reinforcement Type Designation Diameter [mm] Area [mm2]

Tensile Strength [MPa]

Elastic Modulus [GPa]

Deformed Steel #10 11.3 100 400* 200 V-ROD CFRP Rod 3/8 9.5 71 1431 120 V-ROD GFRP Rod 3/8 9.5 71 765 43 NEFMAC GFRP Grid G10 N/A 79 600 30 NEFMAC CFRP Grid C16 N/A 100 1200 100 NEFMAC AFRP Grid A16 N/A 92 1300 54 LEADLINETM CFRP Rod Round 12 113 2255 147

* specified yield strength Table 3-2. Selected Properties of Typical Currently Available FRP Strengthening Systems*

FRP System Fiber Type Weight [g/m2] Thickness

[mm] Tensile

Strength [MPa] Tensile Elastic Modulus [GPa]

Strain at Failure [%]

Fyfe Co. LLC [www.fyfeco.com] Tyfo SEH-51 Glass 930 1.3 575 26.1 2.2 Tyfo SCH-35 Carbon -- 0.89 991 78.6 1.3

Mitsubishi [www.mitsubishichemical.com] Replark 20 Carbon 200 0.11 3400 230 1.5 Replark 30 Carbon 300 0.17 3400 230 1.5 Replark MM Carbon -- 0.17 2900 390 0.7 Replark HM Carbon 200 0.14 1900 640 0.3

Sika [www.sika.com] Hex 100G Glass 913 1.0 600 26.1 2.2 Hex 103C Carbon 618 1.0 960 73.1 1.3 CarboDur S Carbon 2240 1.2-1.4 2800 165 1.7 CarboDur M Carbon 2240 1.2 2400 210 1.2 CarboDur H Carbon 2240 1.2 1300 300 0.5

Degussa Building Systems [www.wabocorp.com] MBrace EG 900 Glass 900 0.35 1517 72.4 2.1 MBrace CF 530 Carbon 300 0.17 3500 373 0.94 MBrace AK 60 Aramid 600 0.28 2000 120 1.6

* Additional information can be obtained from the specific FRP manufacturers Table 3-3. Comparison of Typical Approximate Properties for Reinforcing Materials for Concrete

Property Steel Rebar Steel

Tendon GFRP Rebar

CFRP Tendon

AFRP Tendon

Tensile Strength (MPa) 483-690 1379-1862 517-1207 1200-2410 1200-2068 Yield Strength (MPa) 276-414 1034-1396 N/A N/A N/A Tensile Elastic Modulus (GPa) 200 186-200 30-55 147-165 50-74 Ultimate Elongation (%) >10 >4 2-4.5 1-1.5 2-2.6 Compressive Strength (MPa) 276-414 N/A 310-482 N/A N/A CTE* (10-6/C) 11.7 11.7 9.9 0 -1--0.5 Specific Gravity 7.9 7.9 1.5-2.0 1.5-1.6 1.25

FRP materials are continually being developed with better properties. The properties given are circa 2000. * coefficient of thermal expansion (CTE)


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Table 3-4. A qualitative comparison of the three main types of FRPs Fibre Type Criterion

Carbon Aramid Glass Tensile Strength Very Good Very Good Very Good Modulus of Elasticity Very Good Good Adequate Long Term Behaviour Very Good Good Adequate Fatigue Behaviour Excellent Good Adequate Bulk Density Good Excellent Adequate Alkaline Resistance Very Good Good Adequate Price Adequate Adequate Very Good

Modulus of Elasticity For unidirectional FRP materials, the greatest strength and stiffness are achieved when the composite is loaded in tension in the direction of the fibres. In this case, the elastic modulus of the FRP, Efrp, can be approximately expressed in terms of the elastic moduli of the component materials, Em for the matrix and Ef for the fibres, and their respective volume fractions, Vm and Vf. This is done through an equation known as the rule of mixtures: ( ) mfmfffmmfrp EVEEVEVEE +=+= (Eq. 3-1) The above expression is valid only in the direction of the fibres for unidirectional composites, and the modulus of elasticity perpendicular to the fibres is generally very much lower. Refer to Tables 3-1 and 3-2 for listings of elastic properties for a variety of unidirectional FRP materials in tension. The compressive elastic modulus is generally less than that achieved in tension. Values of the compressive elastic modulus are typically about 50-80% of those determined from tensile testing, depending on the type of FRP under consideration. Strength The strength of unidirectional FRP materials when loaded in the fibre direction is dependent on whether the applied load is tensile or compressive, with most FRPs being vastly more effective in tension (hence their common use as tensile reinforcement for concrete).

The response of an FRP material in tension is dependent largely on the failure strains of the two component materials, and two possible cases of behaviour demand consideration. Figures 3-2 and 3-3 show the potential scenarios for failure strains of the fibres and matrix, and provide insight into the failure behaviour of FRP materials.

If the failure strain of the matrix, m,ult, is less than the failure strain of the fibres, f,ult, as shown in Figure 3-2, and the fibre volume fraction, Vf, is small (say less than about 0.10), then failure of the FRP is governed by the matrix. This condition is described by the following approximate

expression, which gives the tensile strength of the FRP in terms of the strengths of the fibres and the matrix: ( )fultmffultfrp VV += 1' ,, (Eq. 3-2) However, if the fibre volume fraction is large, then the fibres carry the vast majority of the load and failure of the matrix is not critical. In this case, load is transferred to the fibres, which continue to carry the load, until their failure strain is reached. This condition is described by:

fultfultfrp V,, = (Eq. 3-3) If the failure strain of the matrix is greater than the failure strain of the fibres, as shown in Figure 3-3, and the fibre volume fraction is small, then failure of the FRP is prevented when the fibres fail and the ultimate strength of the FRP is described by: ( )fultmultfrp V= 1,, (Eq. 3-4) However, if the fibre volume fraction is large, then the transfer of load from the fibres to the matrix at initial fibre fracture is large and the FRP fails. This condition is approximately described by: ( )fmfultfultfrp VV += 1',, (Eq. 3-5) For most applications of FRPs in civil engineering applications, the fibre volume fraction, Vf, is greater than about 0.1 and is considered large.

The tensile strength perpendicular to the fibres is much less than that in the fibre direction and depends on a range of factors. Since FRPs are rarely loaded in this manner in civil engineering applications, no further discussion of transverse loading is included here.

When loaded in compression, the FRPs ultimate strength is less than that achieved in tension, and depends on a number of factors including the fibre type, the matrix properties, and the matrix-fibre interface strength. The ultimate compressive strength of FRPs can be reached due to fibre micro-buckling, transverse tensile failure in the


11

matrix, or shear failure. Axial compressive strengths for uniaxial FRP materials loaded in the fibre direction are typically about 55%, 20%, and 78% of the axial tensile strength for glass, aramid, and carbon FRPs respectively. Aramid fibres perform particularly poorly in compression, and, as a consequence, FRP strength is commonly neglected when acting in compression when used as reinforcement for concrete. In some cases however, as in the case of pultruded FRP structural sections in compression or bending, the compressive strength of FRPs is relied upon to carry load.

Fig. 3-2. Failure strains of FRP component materials when the failure strain of the matrix is less than that of the fibre.

Fig. 3-3. Failure strains of FRP component materials when the failure strain of the matrix is more than that of the fibre. Fatigue Fatigue refers to the degradation or failure of a structural material or element after repeated cycles of loading and unloading. Most unidirectional FRP materials used in civil engineering applications display good fatigue behaviour in comparison with steel, although some FRP materials (carbon FRPs in particular) display superior fatigue characteristics.

Research into the fatigue behaviour of FRP composites is ongoing, but the following general comments can be made. Carbon FRPs display outstanding fatigue behaviour. This has been attributed to the fact that carbon fibres have a very high stiffness, which limits the strains experienced by the polymer matrix component, and prevents matrix cracking and breakdown of the matrix-fibre interface bond, thus preventing failure. Tensile fatigue tests conducted on unidirectional carbon/epoxy FRP strands have indicated that CFRP can sustain much greater mean stresses and stress amplitudes than steel. Glass fibres are considerably less stiff, and so glass FRP matrices experience larger strains during load cycling which lead to more matrix cracking and can eventually lead to failure. While aramid fibres have stiffness that is intermediate between glass and carbon, so we might expect that they display intermediate fatigue behaviour, aramid fibres themselves are innately sensitive to fatigue damage through a process called de-fibrillation, which can lead eventually to fatigue failure. Figure 3-4 shows typical fatigue life curves for carbon and glass FRPs, where the superior fatigue behaviour of carbon FRPs is evident, as is the effect of fibre modulus on fatigue life.

Number of Cycles to Failure

Max

imum

Cyc

lic S

tress

/ Te

nsile

Stre

ss

0

20

40

60

80

100

120

High Modulus Carbon FRPIntermediate Modulus Carbon FRPLow Modulus Carbon FRPE-Glass FRP

101 102 103 104 105 106 107 1080

Fig. 3-4. Fatigue-life curves for FRPs with different fibre types. Creep Creep is a phenomenon exhibited to varying degrees of severity by virtually all engineering materials. Creep refers to a condition of increasing strain under a sustained (constant) level of stress. Although carbon, glass, and aramid fibres display comparatively little creep themselves under most ambient conditions, FRP matrix materials are visco-elastic (they display properties of both elastic solids and viscous fluids) and so FRP materials will creep under sustained load. The amount of creep exhibited by a particular FRP will depend primarily on the fibre volume fraction and the orientation of the fibres with respect to the applied loads. In addition, both temperature and moisture

Fibre

Matrix

m,ult f,ult

m,ult m

f,ult Stress,

Strain,

Fibre

Matrix

f,ult m,ult

m,ult

f

f,ult Stress,

Strain,


12

can have significant effects on the creep behaviour of polymers. However, for most unidirectional FRP materials used in civil engineering applications creep is not normally a significant concern, provided that the sustained stress in the FRP is limited. Thus, ISIS Canada conservatively recommends that the sustained stress levels in unidirectional FRP structural components be limited to the following percentages of the FRPs design ultimate strength: Glass FRP, 20% Aramid FRP, 30% Carbon FRP, 50%

Creep Rupture (Stress-Corrosion) Some types of fibres (glass in particular) are susceptible to a failure mode known as creep rupture (sometimes called stress-corrosion). In this mode of failure, the fibres fracture under sustained load levels that are much less than the failure stress of the composite observed under static testing. Because of the susceptibility of glass fibres, and hence glass FRPs, to creep rupture, stress levels in glass FRPs are often severely limited under sustained loads, to less than 20 or 25% of the static tensile strength (see the stress limits quoted previously).

Section 4

Environmental Durability of FRPs The mechanical properties of engineering materials are obviously of paramount importance to structural designers contemplating their use. However, equally important in the examination of potential materials and systems for use in infrastructure applications are environmental and durability considerations. Exposure to a variety of adverse conditions can significantly alter the mechanical performance of FRP materials, and failure to consider the effects of factors such as temperature, moisture, ultra-violet radiation, assorted chemicals, and fire can lead to unsatisfactory performance. This section briefly examines a number of important factors which have the potential to influence the durability of FRP materials used in construction. It is important to remain cognizant of the fact that all engineering materials are sensitive to different environments in different ways. The factors listed in this section should in no way be construed as issues being unique to FRPs. In fact, FRPs offer significant durability advantages over conventional materials such as steel in many cases.

The durability of FRP reinforcing bars in concrete is a complex topic and research in this area is ongoing. Readers seeking additional information on the durability of FRP materials are encouraged to consult ISIS Educational Module #8, also available from ISIS Canada at www.isiscanada.com.

Temperature Temperature is an extremely important factor in the design and use of FRP materials for infrastructure. At elevated temperatures, polymer materials will decompose, or in some cases, burn. The operating temperature to which an FRP component is subjected in service is therefore limited to about 20C less than the glass transition temperature (GTT), for an epoxy resin, or the heat distortion temperature (HDT), for a vinylester or polyester. At these temperatures, major changes are observed in the mechanical properties of the polymer matrix materials which lead to a rapid deterioration

of the mechanical properties of FRP components. Elevated temperatures can also have important effects on the long-term durability of FRP materials, as discussed below. Low temperatures are not generally a concern for polymer matrix FRPs in most structural applications, except in those rare instances where extremely low (cryogenic) temperatures result in embrittlement of polymer matrix materials.

Temperature effects on FRPs are varied and complex, and research into the effects of temperature on FRP materials is ongoing. As such, an exhaustive discussion of this topic is avoided here. However, the following is a list of issues of which the reader should have a general awareness when considering the use of FRPs in a potential structural application: high temperatures will increase the rate of creep for

FRP materials; higher temperatures will increase the rate of FRP

degradation due to chemical attack or moisture ingress; differential thermal expansion (between the fibres and

the matrix, or between the FRP and the substrate in cases where FRP is bonded to concrete, steel, or timber) may lead to the development of thermal stresses which could damage the matrix-fibre interface or the interface between the FRP and the substrate. This is because the coefficient of thermal expansion (CTE) of FRPs vary depending on type and may be significantly different than the substrate materials (refer to Table 3-2); and

thermal cycling, and the resulting repeated thermal stresses that are induced, can cause damage to FRP materials through matrix cracking and fibre fracture, which can exacerbate problems due to moisture ingress and/or chemical attack.

Moisture Almost all polymers, if placed in a wet environment, will absorb moisture from their surroundings until their saturation point is reached. The amount of moisture


13

absorbed and the effects of this absorption on the mechanical performance of the composite will vary depending on the FRPs composition and properties. It is currently believed that moisture ingress in FRP composites generally occurs in the matrix region close to the fibres (by capillary action) and subsequently degrades the properties of the matrix while also damaging the surface of the fibres. Thus, for unidirectional composites, the tensile strength of the FRP, which is less dependent on matrix properties, is relatively unaffected by moisture uptake, while the matrix dominated properties, such as shear and compressive strength, can be severely degraded. Aramid fibres are particularly sensitive to the effects of moisture since the fibres themselves are known to absorb moisture and swell, subsequently causing matrix cracking and resulting in the development of internal stresses. Thus, aramid fibres have recently fallen out of favour in applications where moisture is a potential concern.

It is worth noting that degradation of the mechanical properties of FRPs due to moisture will reach some maximum level when the polymer matrix reaches its saturation point. Once this point is reached, no further reduction in mechanical properties is expected. This behaviour is fundamentally different than the deterioration of steel due to moisture, where corrosion will continue until the metal has corroded completely. UV Radiation Ultra-violet (UV) radiation can degrade FRP materials. Aramid fibres are known to be particularly sensitive to UV radiation, while both carbon and glass fibres are resistant to UV light. In addition, most polymer matrices will degrade slightly due to UV radiation.

Degradation due to UV light can be prevented through the use of various matrix additives, the application of a pigmented gel coat on the exterior of an FRP, or by painting the FRP with an opaque paint. For instance, in most field applications of concrete strengthening by externally-bonded FRP plates or sheets, the FRP is finished with a UV resistant

paint which matches the substrate concrete and effectively hides the repair material.

Alkali Effects FRP materials are increasingly being used as internal reinforcement for concrete structures in an attempt to address the corrosion problems that are commonly encountered when concrete is reinforced with conventional steel rebars. The environment inside healthy concrete is highly alkaline (the pH level is generally between 12 and 13.5), and this can be a concern for glass fibre materials, which suffer from reduced toughness and strength through alkalinity-induced embrittlement. As a result, GFRP rebars are often manufactured using alkali-resistant polymer matrices. While there remains some uncertainty as to the effects of alkalis on glass FRP rebars, it is generally agreed that GFRP can be used for reinforcement of concrete provided that sustained (service) stresses are limited (as mentioned previously) to account for potential degradation. Fire All polymer resins will burn when subjected to sufficiently high temperatures, and thus fire can be a serious concern for FRP materials and for structures which incorporate them. Polymers commonly used as matrices in infrastructure FRPs are all combustible and generally release large quantities of dense, black, and sometimes toxic and/or corrosive smoke. Thus, the potential consequences of fire must be considered during the design process for any structure incorporating FRP materials. Various options exist for fire protection of FRP materials through the use of intumescent coatings, fire insulation, matrix additives, and ceramic paint, although all involve trade-offs in terms of cost, ease of application, and effects on the mechanical properties of the FRP. Research is ongoing in this area.

Section 5

Applications of FRPs in Civil Engineering Because of the tremendous variety in types, shapes, and properties of FRP materials, there is an enormous variety of applications in which FRPs can be effectively used in structural engineering and infrastructure applications. This section briefly outlines some of the more common applications of FRPs in the civil infrastructure. The reader should remember that the use of FRP materials in structures is a rapidly evolving discipline, and many new applications

appear each year. For detailed and current information, the reader is referred to www.isiscanada.com. ALL-FRP STRUCTURES The most obvious potential use of FRPs in structures is to fabricate entire structures, or specific structural components, out of FRP. This is most easily and inexpensively


14

accomplished using pultruded FRP structural sections which can be manufactured relatively easily and inexpensively from glass FRP. All-FRP structures are becoming more common for small-scale structures such as pedestrian bridges, utility poles, parking garage stairwells, and platforms in marine and offshore structures. Figure 5-1 shows a short-span all-FRP road bridge in the United States.

In addition to all-FRP structures, FRPs have also been used to construct specific structural components such as bridge deck panels (Figure 5-2) and girders, cable-stayed bridge support cables, blast panels, space trusses, modular residential building systems, marine sheet piling, and ground anchors.

Fig. 5-1. A short-span all-FRP road bridge.

Fig. 5-2. A section of a glass FRP bridge deck panel being tested under a simulated vehicle wheel-load in a structural engineering research laboratory. FRP-REINFORCED CONCRETE Because FRP materials will not corrode electrochemically, FRP bars, rods, and tendons are increasingly being used in lieu of conventional reinforcing steel for internal reinforcement of concrete. Both glass and carbon FRP rebars and reinforcing grids have been used successfully as internal reinforcement in concrete beams and slabs, as have various hybrid FRP grids composed of both glass and carbon fibres. Research and field applications of FRP rebars in concrete bridge decks have indicated that these materials perform well in the harsh Canadian climate. The

major design issues which require consideration in the design of FRP-reinforced concrete members include: the fact that FRPs are linear-elastic to failure, unlike steel which exhibits a well-defined yield plateau, and the fact that FRP reinforcements generally have elastic moduli that are less than steel, so serviceability requirements often govern the design. Figure 5-3 shows glass FRP reinforcement installed in a concrete bridge deck in Quebec (shown just prior to placement of the concrete).

Fig. 5-3. Glass FRP reinforcing bars placed in a concrete bridge deck immediately prior to placement of the deck concrete.

FRP tendons have also been successfully used as both internal and external prestressed reinforcement for concrete beams, slabs, and bridge decks. However, glass FRP should not be used as prestressed reinforcement because of its susceptibility to creep-rupture, nor should aramid FRP because of its sensitivity to moisture-induced swelling.

A complete discussion of the design and analysis of concrete members incorporating FRP reinforcing materials can be found in ISIS EC Module 3, which is readily available from ISIS Canada. REPAIR AND REHABILITATION As stated earlier, after decades of neglect and overuse, the North American infrastructure is crumbling. Many agencies have recently suggested that expenditures in the trillions of dollars are now required to bring our infrastructure up to an acceptable level. There are many factors contributing to the unsatisfactory state of our infrastructure, including: deterioration due to corrosion; environmental degradation; greater traffic volume and increased vehicle weights; updated design loads and seismic requirements; and vehicle collision, fire, and vandalism.

During the past fifteen years or so, a number of preservation, rehabilitation, and strengthening techniques which use FRP materials have emerged for use with a


15

variety of concrete, steel, aluminum, masonry and timber structures. Several are discussed below. Concrete Structures FRP materials are becoming increasingly popular for repair and strengthening of reinforced concrete structures, and FRPs are now materials of choice for flexural, shear, and axial strengthening of reinforced concrete members. In this application, FRP plates or sheets are bonded to the exterior of reinforced concrete members using the wet lay-up procedure with an epoxy resin/adhesive. The FRP sheets or plates are generally bonded to the tension faces of flexural elements to increase their bending capacity, or to their side faces to increase the shear capacity (Figure 5-4). In addition, FRP sheets can be applied circumferentially around reinforced concrete columns to provide confining reinforcement which has been shown to increase both their strength and ductility (Figure 5-5). A much more complete discussion into the use of FRPs for strengthening concrete structures is provided in ISIS EC Module 4, which is also readily available from ISIS Canada.

Fig. 5-4. This concrete bridge girder has been strengthened in shear with externally-bonded carbon FRP sheets. The FRP was subsequently painted with a camouflaging UV-resistant paint. Metallic Structures Many metallic structures such as bridge girders, cranes, hydroelectric structures, and overhead signs are also currently in need of structural upgrading, and FRP materials are beginning to see use in these applications due largely to the ultra high-modulus fibres that are now available. By externally bonding FRP sheets or wraps to the exterior of metallic structures, their flexural, shear, axial, and joint strengths can be significantly enhanced. Some specific applications have included glass FRP wraps for the repair of damaged welds in aluminum overhead signs (Figure 5-6), and the use of ultra-high modulus carbon FRP sheets for increasing the flexural capacity of steel bridge girders.

Fig. 5-5. A circular concrete column being strengthened with carbon FRP sheets.

Fig. 5-6. Repair of welded joints in an overhead tubular aluminum sign standard using glass FRP sheets. Masonry Structures Many aging masonry structures, built long before current design loads and guidelines were formulated, are now considered structurally inadequate in a number of respects (commonly with respect to seismic loading). Externally-bonded FRP reinforcements can be used to increase both the strength and ductility of masonry walls and columns for both in-plane and out-of-plane shear and flexural behaviour. An example of a typical FRP strengthening application on a masonry wall is shown in Figure 5-7, where transparent glass FRP sheets have been used to improve the in-plane behaviour of a traditional clay brick masonry wall.


16

Timber Structures FRPs have successfully been used to strengthen existing timber structures such as historic buildings and covered timber bridges (Figure 5-8). This is commonly done to increase the flexural capacity of a beam or girder, and can be accomplished by external bonding or near surface mounting (NSM). In external bonding, FRP plates or sheets are bonded to the exterior of the timber member using a structural adhesive (normally an epoxy or phenolic resin), as shown in Figure 5-9. This application is similar to concrete or steel strengthening applications of FRPs. In the NSM technique, small grooves or troughs are cut into the tension face of the member and an FRP bar or plate is inserted into the groove along with a structural adhesive grout. NSM is particularly attractive in applications where it is desired that the aesthetics of the original structure be maintained.

Fig. 5-7. This traditional clay brick masonry wall has been strengthened (in a laboratory setting) with externally-bonded glass FRP sheets for improved in-plane behaviour.

Fig. 5-8. This historic timber bridge which has been strengthened with carbon FRP materials.

Seismic and Blast Retrofit In addition to repair and strengthening applications incorporating FRPs as outlined in the previous section, externally bonded FRP wraps, plates, and sheets have been successfully used to improve the performance of reinforced concrete and masonry structures subject to the dynamic loads resulting from seismic and/or blast loading. These advanced applications involve similar techniques as those discussed previously for repair and strengthening. The most common seismic and blast strengthening applications involve FRP plating or wrapping of concrete or masonry walls and columns to increase both load carrying capacity and ductility.

Fig. 5-9. A historic timber bridge which has been strengthened with carbon FRP materials applied to the underside of selected beams. HYBRID FRP/CONCRETE MEMBERS Recently, a number of hybrid FRP/concrete structural systems have been developed for use as structural members. Many variations on this general theme have been proposed, although most of these systems involve concrete and FRPs in combination and are fabricated in such a way as to place the concrete in the compressive region of the cross-section while the FRP is concentrated in the tension region. These innovative structural systems can thus be a highly efficient use of materials; they can be very light, and are presumably maintenance-free since no corrosion is expected to occur. Hybrid members can be used as supporting elements in buildings and as girders for bridges, as well as for concrete-filled FRP piles for bridge and marine structures (Figure 5-10).

Another interesting application of FRPs which results in a hybrid member is FRP stay-in-place formwork. In these applications the concrete formwork is fabricated from FRP and remains in place after the concrete has cured. If the FRP can be made to act in a composite manner with the hardened concrete, then the FRP can be used as the tensile reinforcement for concrete slabs and beams. Concrete-filled FRP tubes are an example of the stay-in-place formwork


17

concept, where the concrete is placed inside a precured FRP tube (usually fabricated using the filament winding process). Once the concrete has hardened, the FRP tube formwork provides both tensile and confining reinforcement to the concrete. An application of concrete-filled FRP tubes is shown in Figure 5-10.

Fig. 5-10. An example of hybrid FRP/concrete members, concrete-filled FRP tubes act dually as foundation piles and bridge piers in this application. THE FUTURE The future holds unlimited promise for the use of FRPs in structural engineering applications. One of the most exciting recent advances is the development of smart

materials and smart structures. Smart structures are those in which sensors are installed to continuously monitor the performance of the structure throughout its lifetime. Recently, FRP materials have been developed which include fibre-optic sensors (FOS) as part of their internal structure. These FOS can be used to measure variations in strain and temperature within the structure itself, and can provide information to engineers on its short and long-term performance. These materials can be considered an emerging technology, although several smart structures have already been built in Canada and are currently under observation. Smart structures and materials will undoubtedly become more important and widespread in the future. Figure 5-11 gives an example of a smart structure in Canada: the Taylor Bridge near Winnipeg. More information on smart structures is available from the ISIS Canada website (www.isiscanada.com).

Fig. 5-11. The Taylor Bridge near Winnipeg, Manitoba, seen here under construction, is one of Canadas first smart structures.

Section 10

References and Additional Information Additional information on the use of FRP materials can be obtained in various documents available from ISIS Canada: ISIS Design Manual No. 3: Reinforcing Concrete Structures with Fiber Reinforced Polymers. ISIS Design Manual No. 4: Strengthening Reinforced Concrete Structures with Externally-Bonded Fiber Reinforced

Polymers. ISIS Design Manual No. 5: Prestressing Concrete Structures with FRPs. ISIS Canada Specifications for Product Certification of Fibre Reinforced Polymers (FRPs) as Internal Reinforcement in

Concrete Structures ISIS Educational Module 1: Mechanics Examples Incorporating FRP Materials. ISIS Educational Module 3: An Introduction to FRP-Reinforced Concrete. ISIS Educational Module 4: An Introduction to FRP-Strengthening of Concrete Structures. ISIS Educational Module 6: Application and Handling of FRP Reinforcements for Concrete.


18

ISIS Educational Module 8: Durability of FRP Composites for Construction. The following publications have been used in the preparation of this module and can be consulted for a more complete discussion of the various topics presented herein: CSA 2002. CAN/CSA-S806-02: Design and Construction of Building Components with Fibre Reinforced-Polymers.

Canadian Standards Association, Ottawa, ON. CSA 2005. CAN/CSA-S6-05: The Canadian Highway Bridge Design Code (CHBDC). Canadian Standards Association,

Ottawa, ON. ACI 2003. ACI 440.1R-03: Guide for the design and construction of concrete reinforced with FRP bars. American

Concrete Institute, Farmington Hills, MI. ACI 2002. ACI 440.2R-02: Guide for the design and construction of externally bonded FRP systems for strengthening

concrete structures. American Concrete Institute, Farmington Hills, MI. ACI 1996. ACI 440R-96: State-of-the-art report on fiber reinforced plastic reinforcement for concrete structures.

American Concrete Institute, Farmington Hills, MI. Teng, J.G., Chen, J.F., Smith, S.T., and Lam, L. 2002. FRP strengthened concrete structures. Wiley. Hollaway, L.C., and Head, P.R. 2001. Advanced polymer composites and polymers in the civil infrastructure. Elsevier. Hollaway, L.C. 1990. Polymers and polymer composites in construction. Thomas Telford Ltd., London, UK. Chawla, K.K. 1998. Composite materials: Science and engineering. Springer. ICE 2001. FRP composites: Life extension and strengthening of metallic structures. Institution of Civil Engineers,

Design and practice guides. Thomas Telford Ltd., London, UK. Further information on field applications of FRPs in various types of structures is available from a number of sources, including: ACI Special Publication SP-215-9. Field Applications of FRP Reinforcement: Case Studies. Published by the American

Concrete Institute, 2003. ASCE Journal of Composites for Construction. Published by the American Society of Civil Engineering, 1997-2004.

Notation

Ef Elastic modulus of the fibres (MPa)

Efrp Elastic modulus of the FRP (MPa)

Em Elastic modulus of the matrix (MPa)

Vf fibre volume fraction

Vm matrix volume fraction

f,ult failure strain of the fibres

m,ult failure strain of the matrix

frp,ult ultimate tensile strength of a unidirectional FRP in the direction of the fibres (MPa)

f stress in the fibres at failure (MPa)

m stress in the matrix at failure (MPa)

f,ult ultimate tensile strength of the fibres (MPa)

m,ult ultimate tensile strength of the matrix (MPa)


19

Appendix A: Suggested Laboratory The following laboratory procedure is given as an example of a materials laboratory that could be given in conjunction with an undergraduate course on engineering materials or mechanics of materials, and that includes tests on both steel and FRP materials. Given the wide variety of laboratory and testing facilities available at various Canadian universities, this laboratory is given primarily as an example for professors of what can be done using FRP materials to increase laboratory impact and student understanding of important materials concepts.

Inclusion of FRP materials into traditional strength of materials laboratories is advantageous for a number of reasons, including: it introduces students to a new and innovative material

which is gaining acceptance within the civil engineering industry;

it increases student understanding of the fundamental materials concepts and assumptions used in structural design and analysis;

it forces students to consider and understand important mechanics concepts such as elasticity, plasticity, and ductility;

it vividly illustrates the concept of ductile versus brittle materials, and demonstrates the need for thoughtful consideration of materials behaviour during engineering design; and

it exposes students to the state-of-the-art in civil engineering materials and thus increases student enthusiasm for the course content, subsequently, in many cases, increasing student participation and effort. The laboratory presented herein suggests several

possible options for tensile test configurations for FRP materials. It is important to recognize that the laboratory procedures can be adapted to include the use of any specific type of FRP reinforcement, and the specific configurations suggested herein have been used only as an example.

Caution: FRP Materials FRPs are linear elastic materials. As such, these materials do not display the yielding behaviour observed when testing steel and they provide little warning prior to failure. It is extremely important that instructors, students, laboratory demonstrators, and technical staff be made aware of the

specific failure modes to be expected when testing FRP materials, and that appropriate safety precautions be taken in addition to those precautions that are normally enforced.


20

A Comparative Study of Fibre Reinforced Polymers (FRPs) and Steel under Axial Tension OVERVIEW This laboratory is intended to increase students understanding of the behaviour of both steel and fibre reinforced polymers (FRPs) in tension. During the laboratory, tensile tests will be carried out on both steel and FRP test specimens in order to ascertain their characteristic stress-strain behaviour and material properties. This laboratory illustrates the following important concepts: 1. the overall tensile behaviour of both steel and FRP; 2. concepts of stress, strain, and elastic modulus; 3. concepts of elastic and inelastic material behaviour; 4. use of the elastic modulus to calculate stresses and

strains for linear-elastic materials; 5. yield strength, plasticity, and the behaviour of steel

before and after yielding; 6. ultimate strength of different engineering materials;

and 7. characteristic brittle failures observed for FRP

materials versus the more ductile failures commonly observed with steel.

The class will be divided into groups of four students each, each group being responsible for the analysis their own data obtained during two tensile tests, one on FRP and the other on steel, and for the submission of a single laboratory report. All data obtained during the testing performed by each group will be made available to all other groups for use in writing their laboratory reports. Laboratory Report The laboratory report should consist of the following: 1. A title page giving the group name and number. 2. An abstract, briefly stating the purpose and procedure

of the lab and the major conclusions drawn. 3. An introduction providing information on the

materials used, testing setup, instrumentation, procedures, etc.

4. A calculations and analysis section detailing all calculations performed for the laboratory. Where a calculation has been performed more than once only a sample calculation should be provided.

5. An experimental results and discussion section, summarizing the test results obtained for all specimens tested. This section should include photographs (where available) and plots showing specimen behaviour along with a thorough comparison of

theoretical and observed results, and a comparison of the behaviour of the data obtained by the other lab groups.

6. A conclusion in which the major points of interest from the above sections are highlighted. The focus in the conclusion should be on the consequences of the observed behaviour on the practical design of engineering structures.

7. A list of references. All tests referenced during the course of the laboratory project should be listed using an accepted referencing format.

OBJECTIVES The objective of this laboratory can be summarized as follows: 1. To observe and develop stress-strain relationships for

both steel and FRPs in tension. 2. To determine the elastic modulus of both steel and FRP

materials. 3. To define the proportional limit and the yield strength

of steel. 4. To define the ultimate stress and strain for both steel

and FRP in tension. 5. To observe the effects of inelastic behaviour. 6. To compare the overall stress-strain behaviour and

failure modes of ductile materials versus those of brittle materials.

Apparatus The following are required for this laboratory: Universal Testing Machine Axial Extensometer X-Y plotter Digital Calipers/Digital micrometer Gauge Block Standard ASTM Steel Specimen Standard ASTM FRP Specimen (or alternative FRP

specimen depending on testing apparatus being used) PROCEDURES The Universal Testing Machine will be used to perform the tensile tests on the specimens (refer to Figure A.1 below). A controlled tensile load will be applied to the specimens by


21

means of a movable crosshead. During the test, tensile load and elongation in the test specimens will be recorded using a load cell and an axial extensometer, which will measure the elongation of the specimens over a specific gauge length. Either a constant rate of change for load or a constant rate of change for displacement may be selected. Constant rate of change for displacement will be used herein. Electronic signals from the load cell and extensometer will be input into an X-Y plotter and a computer data-logging system in order to allow construction of a load-deformation curve and to prepare data for analysis by the students.

Figure A.1: Schematic of the Dynamic Testing Machine Materials Steel A common structural steel grade, such as ASTM A572, CSA G40-21 350 W, or other available construction-grade steel will be tested. These materials are generally carbon

steels with minimal alloy elements, and are commonly used in different structural applications. Typical material properties for construction-grade carbon steel are given in Table A.1. The steel specimen for this laboratory can consist of a machined bar (in accordance with ASTM Standard E8-91) or a section of conventional reinforcing steel (in accordance with ASTM Standard A370-97), depending on the availability of testing equipment and specimen grips (refer to Figure A.2). Fibre Reinforced Polymer (FRP) Unidirectional carbon, glass, and aramid fibre reinforced polymers are gaining acceptance as structural materials in a variety infrastructure applications. These materials demonstrate outstanding strength and stiffness characteristics when loaded longitudinally in the direction of the fibres. Glass FRPs are relatively inexpensive and are therefore the most commonly used. Recently, glass FRPs have emerged as cost-effective, non-corrosive reinforcing materials for concrete. Carbon fibres, which are more costly but have a higher modulus of elasticity and slightly better durability and fatigue characteristics, are emerging as prestressing materials for concrete. Both carbon and glass FRPs are currently being used in externally-bonded FRP strengthening applications for rehabilitation and strengthening of reinforced concrete structures. This laboratory can be performed either with FRP rebar specimens or with FRP coupon specimens, depending on the availability of the required grips in the testing laboratory (refer to Figure A.2). FRP bar and coupon specimens are shown schematically in Figure A.2. Coupon specimens should be prepared in accordance with ASTM D3039-98. FRP bar specimens are extremely weak in the transverse direction, and hence they cannot be tested using standard steel rebar testing grips. FRP bar specimens should be prepared by anchoring the ends of the bar specimen into steel tubes using a high strength concrete grout. The steel tubes can then be effectively gripped using standard reinforcing steel testing grips. ASTM standards for this test method do not currently exist. Additional guidance on the preparation of FRP bar tensile specimens can be obtained from ISIS Canada or Committee 440 of the American Concrete Institute. Typical material properties for glass and carbon FRP materials are given in Table A.1.

Table A.1 Typical mechanical properties of steel and FRP materials Material Yield Stress

(MPa) Ultimate Stress

Elastic Modulus

Strain at Failure (%)

Carbon Steel 250-450 450-700 MPa 200 GPa 7-15 Glass FRP Bar N/A 517-1207 MPa 30-55 GPa 2-4.5 Carbon FRP Bar N/A 1200-2410 MPa 100-165 GPa 1-1.5 Glass FRP Coupon N/A 530-750 N/mm width 25-35 kN/mm width 2.1-2.2 Carbon FRP Coupon N/A 595-3920 N/mm width 70-360 kN/mm width 0.3-1.7

Movable Crosshead

Load Frame Test

Specimen

Stationary Crosshead

Hydraulic Cylinder


22

Figure A.2: Typical Test Specimens

Testing Procedure 1. Using the digital calipers, measure the specimens to

determine the cross-sectional area at 3 locations within the gauge section. Record the average area of each specimen in units of mm2 to an accuracy of +/- 0.01mm2. If FRP coupons are being tested (as opposed to FRP bars) measure the width only, since properties for these materials are generally quoted in N/mm of sheet width.

2. Using the gauge block, lightly punch two marks on the steel specimen. These will be used to measure the elongation of the gauge length manually after the axial extensometer is removed. Measure the exact gauge length of the steel using the digital calipers, and record this measurement on your data sheet.

3. Using the material properties given for the specific steel and FRP specimens being tested (consult the laboratory demonstrator): a. Calculate the expected yielding load and the

corresponding deformation for the steel specimen. Values for 50% and 80% of the yielding load are also required.

b. Calculate the expected ultimate load and the corresponding deformation for the FRP specimen. A value for 50% of the ultimate load is also required.

Steel Specimen 4. With the help of the instructor, mount the steel

specimen in the load frame and affix the extensometer to the specimen. Loading of the machine, and setting of scales and the rate of loading/deformation should be done by the laboratory instructor or by an experienced technician. Record the loading/deformation rate for the specimen.

5. With the help of the instructor or technician, ZERO the load applied to the specimen by adjusting the position of the Dynamic Testing Machines hydraulic ram. Also, adjust the X-Y plotter so that it indicates zero force and deformation in the test specimen.

6. Load the specimen in tension up to 80% of the expected yield load and reverse the direction of loading.

7. Reduce the load to zero, and then reload the specimen in tension until yielding is observed. Continue loading beyond yield to ascertain that the yield point has been exceeded (or if no true yielding occurs, that the proportional limit has been exceeded).

8. It is now required to put the specimen through an unload-reload cycle. Reduce the tensile load to about 50% of the yielding load. Reload the specimen again to the previously observed yielding load. Allow a little more deformation to occur beyond this point to

Machined Steel Bar FRP Coupon with

Anchorage Tabs

Steel Rebar

OR OR

FRP Bar with Steel Pipe Anchor Ends

End Tabs

Steel Pipe

Cement grout

FRP Bar


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establish the shape of the curve. Stop loading, turn off the X-Y recorder input and carefully remove the extensometer from the specimen.

9. The steel specimen will now be loaded to tensile failure. Take length measurements between the gauge marks using the digital calipers at deformation increments of approximately 1 mm or about every 15-30 seconds. Record these length measurements and the corresponding load values. Watch the digital display closely to determine the maximum load reached. At that point, a caliper measurement of the length between gauge marks is to be recorded.

10. After fracture, stop the test by having the instructor turn off the controller and remove the fractured pieces of the specimen from the machine. Re-align the pieces and measure the final length between the gauge marks. Also measure the final diameter at the location of fracture. Save the pieces for sketching the mode of failure.

11. Remove the plot from the X-Y plotter and turn off the plotter.

FRP Specimen 12. Repeat steps 4 and 5 for the FRP specimen. Turn on the

X-Y plotter. 13. Load the specimen in tension up to 50% of the expected

ultimate load and reverse the direction of loading. 14. Reduce the load to zero, and then reload the specimen in

tension up to 50% of the expected ultimate load. Stop loading, turn off the X-Y recorder input and carefully remove the extensometer from the specimen.

15. The FRP specimen will now be loaded to rupture. Watch the digital display closely to determine the maximum load reached. Failure of the FRP specimen may be sudden and violent, with little warning. Maintain a safe distance from the testing machine.

16. After fracture, stop the test by having the instructor turn off the controller and then carefully remove the remainder of the specimen from the machine. Save the fragments for sketching the mode of failure.

17. Remove the plot from the X-Y plotter and turn off the plotter.

CALCULATIONS A graph with the appropriate titles and units should be prepared, showing the stress-strain profiles for both the steel and FRP specimens on the same axes. The LOAD vs. DEFORMATION plot obtained from the X-Y plotter and the recorded data from the caliper measurements should be used to prepare this graph. For steel, be sure to clearly label the proportional limit, yield point, ultimate strength, and point of rupture. For FRP, be sure to clearly label the point of rupture. Use the data in this plot to produce a plot of stress versus strain for both materials. Using the formulae provided, determine the following

values/parameters for the steel specimen: elastic modulus, Es stress at proportional limit, p stress at yield point (if apparent) or at 0.2% offset

strain, y ultimate tensile strength, s,ult stress at fracture, s,f percentage deformation at fracture, s,f percentage reduction of cross-sectional area at fracture type and character of fracture Using the formulae provided, determine the following values/parameters for the FRP specimen: elastic modulus, Efrp ultimate tensile strength, frp,ult estimated percentage deformation at ultimate/fracture,

frp,ult type and character of failure A separate tabular summary of the values for the various quantities listed above should be made for both materials. REQUIRED DISCUSSION The discussion portion of your lab report should address the following key topics: 1. Compare and contrast the elastic modulus of the two

materials tested. 2. Compare the yield strengths and ultimate tensile

strengths of the two materials tested. Did both materials yield?

3. Compare the elastic modulus, yield strengths, and ultimate tensile strengths for both materials with typical values published in material handbooks or texts (provide your reference source). Attempt to explain any observed differences.

4. Discuss the general shape of the stress-strain curve for steel in the region beyond the yield point, especially during the unloadreload cycles.

5. Explain why the stress-strain plot for steel follows the path it does after it has reached ultimate stress.

6. Using the plots for both materials; explain the differences between linear, nonlinear, elastic and inelastic behaviour.

7. Explain the difference between brittle and ductile failure. In which of the tested materials were these failures observed?

8. Discuss the fracture surfaces observed. How do they relate to the ductility of the materials? (Sketches are helpful.)

Show all calculations and include original data sheets, sketches, and notes in an appendix. USEFUL FORMULAE


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Stress: AP=

Strain: oL =

Hooke's law: E= (elastic region only)

NOTATION A cross-sectional area of specimen (mm2) d diameter of test specimen (mm) E elastic modulus (MPa) Efrp elastic modulus of FRP (MPa) Es elastic modulus of steel (MPa) Lo gauge length of extensometer (mm) P axial load in (N) t thickness of test specimen (mm) w width of test specimen (mm) axial deformation of over gauge length (mm) strain in specimen (mm/mm) frp,ult percentage deformation at fracture for FRP s,f percentage deformation at fracture for steel axial stress in specimen (MPa) frp,ult ultimate tensile strength of FRP (MPa) p proportional limit (MPa) s,f stress at fracture of steel (MPa) s,ult ultimate tensile strength of steel (MPa) y yield stress (MPa) REFERENCES 1. American Society for Testing and Materials (ASTM),

1998. D3039 Standard Test Method for Tensile Properties of Fibre-Resin Composites

2. American Society for Testing and Materials (ASTM), 1998. E8M-03 Standard Test Methods for Tension Testing of Metallic Materials

3. American Society for Testing and Materials (ASTM), 1997b. A370-97a Standard Test Methods and Definitions for Mechanical Testing of Steel Products.

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Introduction to FRP