FIBRE REINFORCED PLASTICS – NEW AGE CONSTRUCTION MATERIALS

1 INTRODUCTION

With the increasing diversity of structures and the desire for new materials of

construction, many unconventional materials such as composites are being adopted.

During the past two decades the applications of composite materials have grown rapidly

and are now very common around the world.

2 COMPOSITE MATERIALS

Composite materials are formed by the combination of two or more materials that

retain their respective characteristics when combined together to achieve properties that

are superior to those of individual constituents. In generic terms, the idea of a composite

is analogous to that of reinforced concrete. However, composites present immense

opportunities for tailoring of the material to the specific requirements of the structure.

The main components of composites are reinforcing agents and matrix. The fibres,

particulates and whiskers act as the reinforcement and provide most of the stiffness and

strength. The matrix binds the reinforcement together thus effecting the load transfer

from matrix to reinforcement and adds to the performance merits of the material. Other

substances such as fillers are used to reduce the cost and improve process ability and

dimensional stability.

Today, the most common man made composites can be divided into three main

groups:

1. Polymer Matrix Composites (PMC’s): These are the most common and will be

the main area of discussion. These are also known as FRP- Fibre Reinforced

Polymers (or Plastics).

2. Metal Matrix Composites (MMC’s)

3. Ceramic Matrix Composites (CMC’s)

Fibre reinforced composites can be further divided into those containing

discontinuous or continuous fibres.

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The three most important requirements for materials which are to be used in

highly demanding applications are a high resistance to plastic deformations, resistance to

fracture and elastic stiffness.

The advantages of the use of FRP as a structural material are

1. Strong: high strength to weight ratio

2. Light weight: only two-thirds the weight of Aluminum and 20% of steel.

3. Durable: properly chosen it is resistant to atmospheric and chemical corrosion.

4. Non-conductive: excellent thermal and electrical insulating properties.

5. Maintenance: low to maintenance free; painting not required.

6. Easily workable.

3 CONSTITUENT MATERIALS

It is a well-known fact that FRP consists of high performance fibres and a matrix.

The various fibres are: glass fibres, ceramics (like silicon carbide, silicon nitride etc),

carbon fibres, tungsten, cold drawn wires etc. Among the newly explored possibilities are

those made from Coir, Hemp, Jute, Flax etc.

The matrix is usually a resin system which are usually an unsaturated polyester

resins to which additives, catalysts and pigments are added to achieve specific purposes

THE DESIRABLE PROPERTIES OF FIBRES AND MATRIX

3.1 Fibre

1. Should have a high tensile strength.

2. Variation of strength among individual fibres should be as small as possible.

3. Should belong to the same type, should have cross sectional diameter and surface area

uniform.

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Some of the Physical and Mechanical Properties of a few fibres are tabulated as below

Table 3.1 Physical and Mechanical Properties of Glass Fibre:

Fibre Type E-Glass S-Glass

(1). Diameter (m) 8-14 10

(2). Density (kg/m3) 2540 2490

(3). Tensile Modulus (GPa) 72.40 85.5

(4). Tensile Strength (Mpa) 2400 3450

(5). % Elongation 1.8-3.2 5-7

(6). Coefficient of thermal

expansion (/C)

5 5-6

(7). Specific Gravity 2.54 2.49

Table 3.2 Physical and Mechanical Properties of Carbon Fibre

Fibre Type Poly acrolonitrilie (PAN)

(1) Diameter(m) 10-11

(2) Density(kg/m3) 2020

(3) Tensile Modulus (GPa) 345

(4) Tensile Strength (MPa) 3500

(5) % Elongation 0.4-0.9

(6) Coefficient of thermal expansion (/C) -0.9 to 1.6

(7) Specific Gravity 2.02

Table 3.3 Physical and Mechanical Properties of Aramid Fibre

Fibre Type Kevlar Fibra

(1) Diameter (m) 12 38

(2) Density (kg/m3) 1440 970

(3) Tensile Modulus (GPa) 62 117

(4) Tensile Strength (Mpa) 2760 2580

(5) % Elongation 3-4 4-5

(6) Coefficient of thermal expansion (/C) -2 --

(7) Specific Gravity 1.44 0.97

3.1.1. Fibre Type Comparisons

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Comparing the properties of all of the fibre types with each other, shows that they

all have distinct advantages and disadvantages. This makes different fibre types more

suitable for some applications than others. The following table provides a basic

comparison between the main desirable features of generic fibre types. ‘A’ indicates a

feature where the fibre scores well, and ‘C’ indicates a feature where the fibre is not

so good.

Table 3.4 Fibre Type Comparisons

Property Aramid Carbon Glass

High Tensile Strength B A B

High Tensile Modulus B A C

High Compressive Strength C A B

High Compressive Modulus B A C

High Flexural Strength C A B

High Flexural Modulus B A C

High Impact Strength A C B

High Interlaminar Shear Strength B A A

High In-plane Shear Strength B A A

Low Density A B C

High Fatigue Resistance B A C

High Fire Resistance A C A

High Thermal Insulation A C B

High Electrical Insulation B C A

Low Thermal Expansion A A A

Low Cost C C A

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3.2 Matrix

1. It should transfer the stress to fibres efficiently by adhesion or friction.

2. It should not react chemically with the fibres.

3. It should bind the fibres and protect its surface from damage during handling,

fabrication etc.

Polymers used as matrix materials are commonly referred to as resins. There are

two basic classes of resins. They are:

1 Thermosets – when heated they undergo an irreversible chemical change called

curing. They chemically cross link and develop a network structure that sets them

in shape. If they are heated after they have been cured they do not melt.

2 Thermoplastics – These resins melts when heated and solidify when cooled. Once

they initially melt to form the composite, heating above the lower forming

temperature can reshape them.

Some of the Physical and Mechanical Properties of resins are listed in Table 3.4

Table 3.5 Physical And Mechanical Properties of resins

Fibre type Polyester Vinylester Epoxy

(1). Density (kg/m3) 1100-1500 970 1100-1400

(2). Tensile Modulus

(Gpa)

62 117 2-6

(3). Tensile Strength

(MPa)

2760 2580 35-150

(4). % Elongation 3-4 4-5 1-8.5

(5). Specific Gravity 1.1-1.5 0.97 1.10-1.14

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3.3 FIBRE MATRIX INTERFACE

A bundle of fibres by itself is useless as a load bearing structure, but embedding

the fibre in a resin matrix gives the necessary stiffness in shear and compression. The

fibre and the matrix are mutually reinforcing: the strong stiff fibre carries most of the

stress and the polymer matrix distributes the external load to all the fibres, while at the

same time protecting them.

This load sharing requires that stress be transferred across the interface between

the fibre and resin. There is no sharp well defined interface between the fibre and resin,

for the fibre is coated with a heterogeneous mixture forming the matrix of the composite

structure .The whole interfacial region is about 10-² mm thick or greater.

Stress can only be transferred across an interface between two materials if they are

in intimate molecular contact with each other, separated only by about the same distance

as the molecules inside the bulk materials. The materials are then said to be adhering to

each other. There need not be any chemical linking of the materials; they merely have to

be so close that the normal intermolecular forces are operative.

The plastic flow of the matrix under stress transfers the load to the fibres. As a

result a high modulus composite is obtained. A typical FRP reinforcement will consist of

60%to 65% of fibres and the remaining cross section is the matrix.

4 COMMON FABRICATION METHODS

Some of the common fabrication methods are:

A) Custom Contact Moulding

1) Hand Lay up process2) Spray Lay up process

B) Filament Winding C) PultrusionD) Resin Transfer Moulding

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5 COMPARISON WITH OTHER STRUCTURAL MATERIALS

There is a very large range of mechanical properties that can be achieved with

composite materials. Even when considering one fibre type on its own, the composite

properties can vary by a factor of 10 with the range of fibre contents and orientations that

are commonly achieved. The comparisons that follow therefore show a range of

mechanical properties for the composite materials. The lowest properties for each

material are associated with simple manufacturing process and material forms (e.g. spray

lay up glass fibre), and the higher properties are associated with higher technology

manufacture (e.g. autoclave moulding of unidirectional glass fibre), such as would be

found in the aerospace industry. The graph below shows the Tensile Strength of Common

Structural Materials.

Tensile Strength of Common Structural Materials

6 MECHANICAL BEHAVIOUR OF COMPOSITES

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The mechanical properties of the composites depend, to a large extent, on the

proportion of fibre, which they contain. The proportion of fibre is, in turn a matter of the

arrangement of fibres- the more closely can the fibres be packed and higher,

consequently, the maximum possible proportion of the fibre.

The first of the three categories into which composites may be divided is that in

which the fibres are randomly distributed within the plastic matrix. The fibres are

generally short and because they are randomly arranged the maximum proportion of fibre

is limited to about 50% by weight, though in practice there may be only 10% of fibre.

The principle examples of this category include laminates made with chopped strand

mats, dough moulding compounds and glass fibre reinforced thermoplastics.

The second category are those composites in which the fibres are arranged

orthogonally, generally in the plane of the laminate, by being woven into glass cloth or

woven rovings. Because of the more orderly arrangement of the fibres the maximum

percentage of glass can range between 40-65% by weight.

In the third category of composites the fibres are all laid in one direction. This is

the case with rods produced by pultrusion and certain laminates based on filament

winding. Since the fibres are all laid in one direction, the percentage of fibres is

maximum compared to the above two categories and is as high as 90.67% by volume in

theory. In practice however it ranges from 60-90% by weight.

Unidirectional

Uniaxial

Tensile strength Orthogonal

Random

Percentage of Glass by Weight

Tensile Strength v/s Glass Content by Weight

6.1 STRESS STRAIN CHARACTERISTICS

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Since Polymer Matrix Composites combine a resin system and reinforcing fibres

the properties of the resulting composite material will combine something of the

properties of the resin on its own with that of the fibres on their own.

Overall, the properties of the composite are determined by:

1) The properties of the fibre

2) The properties of the resin

3) The ratio of fibre to resin in the composite (Fibre Volume Fraction)

4) The geometry and orientation of the fibres in the composite.

6.2 STRENGTH AND FACTORS INFLUENCING IT

The amount of energy which glass reinforced plastics can absorb before they

break is a function not only of the maximum stress but also of the strain up to failure. In

fact, it is proportional to the product of the stress and strain i.e. to the area under the

curve in the stress –strain curve. When examined it is apparent that glass reinforced

plastics fails without much elongation making it a brittle material. In practice it has been

observed that its strength is sensitive to the presence of notches and other local stress

raisers.

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Environmental conditions such as temperature too have a bearing on its

performance. Increase in temperature decreases the stiffness and strength of glass

reinforced plastics composites until a limit of mechanical performance is reached in the

range of 200-300C with commonly used thermosetting resins. On the other hand,

operating temperatures below normal tend to be advantageous to glass reinforced plastics

which do not become as brittle as other materials at low temperatures. This makes them

attractive for cryogenic applications.

The above-mentioned mechanical properties of glass-reinforced plastics relate to

their behaviour under tensile loads. However, many of them are equally applicable to the

behaviour under compressive or shear loads.

There are however some important differences. In particular, under compressive

loading applied parallel to the direction of the fibres, the latter tend to become unstable

and buckle like long slender struts making them less effective under compression than

under tension.

The high glass content composites are also relatively less effective under shear

because the resistance to shear between the fibre layers depends predominantly on the

plastic matrix.

7 APPLICATIONS OF FRP IN CIVIL ENGINEERING

FRP have been used in many civil engineering applications, such as bridges,

buildings, off shore structures and retaining walls in Japan, Europe, Canada and USA.

The following describes some selected completed, in progress and future applications to

highlight the potential use of these materials.

7.1 SHORT SPAN BRIDGES

Many pedestrian bridges have been constructed using FRP .The span of these

bridges vary from 7 to 10 mtrs. Fibre reinforced plastic tendons have also been used as

reinforcement and to prestress concrete bridges to enhance the durability for severe

environmental conditions. In addition to the high strength and good fatigue properties, the

low young’s modulus of FRP tendons could also be an advantage to reduce the

prestressing losses. These characteristics greatly enhance the use of FRP as prestressing

tendons for short span bridges.

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7.2 LONG SPAN BRIDGES

As a result of the superior advantage of high –strength –to weight ratio of FRP

compared to conventional materials, FRP provide unique alternative to steel and concrete

materials to construct long span bridges, which can not be built by conventional

materials. FRP structural sections can be used effectively in combination with FRP cables

in producing unique bridge girder configurations to build very long bridges. Comparative

studies were undertaken to examine the feasibility of constructing bridges using steel,

glass fibre reinforced plastics (GFRP) and carbon fibre reinforced (CFRP) plastics, for

cable –stayed bridges and classical suspension types. The study concluded that the most

feasible design would be a cable-stayed bridge using CFRP.

The specific design loads versus the canter spans for the classical form of suspension

bridges made of steel are compared with those made of GFRP or CFRP. The comparison

shows the use of advanced composites would allow doubling or tripling of the limiting

span in comparison to steel structures.

7.3 COMPOSITE BRIDGE DECKS

FRP composite bridge decks are made of pultruded components that are bonded and

interlocked. They are placed transversely to the traffic and are supported by longitudinal

beams. The FRP decks comprise of double trapezoid composite connected with full depth

hexagons that provide mechanical interlock and an extensive bonding surface.

Construction of highway bridges with modular FRP decks requires the understanding of

the deck performance under traffic loads. Traffic loads include repetitive stress cycles on

bridge decks during the service life of the structure. The composite bridge decks are

modular in design and can be produced and can be produced in continuous lengths

because of inherent process adopted (pultrusion technique) and these lengths can be cut

to size depending on the users requirement. Hence, it furnishes greater flexibility in

fabrication of the composite bridge decks to suit various product dimensions.

Pultruted FRP Sections

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The first ever-recorded composite bridge is the Gindji Bridge in Bulgaria, which

was constructed in 1982 using the hand lay up technique. Over the last 5 years several

bridges have been constructed, both pedestrian and highway, such as the Aberfeldy Foot

Bridge in Scotland, the Bonds Mill Lift Bridge in England, the Kolding Bridge in

Denmark, etc. In most of these cases, the preferred method of construction is deck

supported by beams.

Composite bridge decks are being used for both permanent bridges for

state/national highways and over culverts in USA. The composite bridge decks installed

in USA were designed and tested by American Association of State Highway and

Transport Officials (AASHTO). The following data gives the some of the composite

bridge decks installed in USA.

51.2 m x 14.6 m FRP decks on W 36x81 steel girder installed at Salem

Avenue, Montgomery Country, Ohio in November 1999

6.1 m x 4.9 m FRP decks on FRP wide flange beams installed at Laurel

Lick bridge, Lewis country, West Virginia in May 1997

9.14 m x 6.6 m FRP decks installed at Wickwire run bridge, Taylor

County, West Virginia in September 1997

7.32 m x 3.74 m FRP decks installed at Shawnee Creek bridge, Xenia,

Ohio in October 1997

8.66 m x 10.04 m FRP decks installed at Laurel run road, Somerset

County, Pennsylvania in October 1998

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7.4 TUNNEL LINING

FRP grids could be extremely effective as reinforcement for tunnel lining using

shortcrete technique to form the skin surface. It has advantages over steel due to its high

corrosion –resistance and flexibility, which is convenient for, curved surfaces, in addition

to their excellent alkali, acid and chemical resisting properties. The material is very

lightweight, having approximately one fourth the specific gravity of steel, and may be cut

easily cut with a hacksaw.

7.5 MARINE STRUCTURES

Fibreglass boats, fibreglass pretensioned piles hard-core marine fender etc are the

common applications in the field of marine structures. The hard core marine fender is

composed of parts that are heavy-duty wear-resistant rubber and ultra high molecular

weight polythene face, which is thermoplastically bonded to the truss panel to resist

abrasion.

7.6 REPAIR OF STRUCTURES

A number of chimneys, columns, slabs and girders have been repaired and

strengthened with CFRP products due to earthquake damage and/or structural needs to

increase their capacity. Many products are currently available for this type of retrofitting.

The products are often unidirectional and produced in the form of fibre tapes, fibre

winding strands and fabrics.

The materials are effective for both flexural and shear strengthening of structures.

A retrofit process of a structure, such as chimney, begins with preparation of the concrete

surface, trowelling the surface with mortar or epoxy, followed by placement of the auto-

adhesive tapes in the longitudinal direction and confining of the outer surface in the

circumferential direction by winding small diameter carbon cables. To facilitate the cable

winding operation, an automatic winding machine is currently available. To satisfy the

fire resistance requirements, a fire resistant material, such as cement mortar, normally

covers the surface.

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Strengthening deteriorated steel and concrete bridges by bonding carbon fibre

reinforced epoxy laminates to the exterior of the structure has been studied. The study has

shown that the use of CFRP laminates in place of steel plates for such applications could

reduce the total cost by about 20%. Although more expensive, FRP materials are

lightweight and have better corrosion resistant properties that could result in significant

reduction in overall long-term costs.

7.8 SPECIAL APPLICATIONS

The non-magnetic neutrality of FRP makes it an attractive material to be used in

some special applications. For example: fibre glass cables were used in the rehabilitation

of the Marine d’Ivry subway station in Paris, where the non –magnetic nature of these

cables played an important role in the selection of this type of cables instead of high

stress steel cables.

8. MATERIALS AND DURABILITY

Although composites are often touted as having very good resistance to

environmental exposure, it must be stressed that this is highly dependent on the types of

fibre, fibre sizing and resin used. The inappropriate selection can result in premature

failure.

Most resins absorb moisture to a certain degree causing an increase in weight.

However, provided that the moisture does not react at the interfacial level the effects are

reversible after drying. In cases where it is known that the composite will be exposed to

significant moisture levels, it is recognised that the use of resin rich layers and gel coats

can be advantageous. Similarly it is expected that for primary structure bearing

significant loads the main reinforcing fibre will be graphite rather than glass, due to its

inertness in such an environment.

This underlines the need for the definition of proper safety factors and stress

levels as well as the recognition of fibre – material combinations.

The use of techniques associated with the external attachment of the composite

plates to the soffit of decks and then underside of beams is attractive due to factors

related to the ease of access and decreased need for extensive changes to the existing

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structures. Although the bonding of steel plates has been used for quite some time it has

been plagued by various problems ranging in difficulty in placement to concerns related

to overall durability.

Composite plates do not suffer from these deficiencies, due to the high stiffness

and strength to weight ratio, corrosion resistance and lightweight. However, an aspect

that needs to be kept in mind during the design of such schemes is that related to long-

term environmental durability of the composite and the durability and effectiveness of the

adhesive bond between the concrete and composites.

Failure due to material aspects of the assembly can generally be associated with

one or a combination of the following modes

1. Peel failure into concrete.

2. Interfacial failure between concrete and adhesive.

3. Cohesive failure in the adhesive.

4. Interfacial crack between the adhesive and concrete.

5. Alternating crack path between the two interfaces.

The first mode of failure occurs due to high peel stresses caused by mismatch and

concentration of shear stresses at the end of composite plates. Also it should be

remembered that the bond between the concrete is through an adhesive or the actual resin

itself and therefore one needs to pay attention to aspects such as moisture intrusion,

freeze-thaw etc.

9 SUMMARY

Composites have immense potential for use in civil infrastructure in applications

ranging from rehabilitation to new construction. However for these applications to be

successful in terms of economics and longevity proper attention needs to be paid to the

selection of materials and the processes used to fabricate and place these composites in

the field. Errors in this regard through the inappropriate use of a material system or

manufacturing technique should not be considered as flaws on the part of the technology,

but rather on the designer’s part. Composites present immense opportunities for tailoring

and for the integration of form and function, presenting the civil engineer a plethora of

design opportunities.

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REFERENCES:

Cahn R.W. and Harris.B.[1970], “Natures of composites” –Glass reinforced plastics –

Brian Parkyn, Butterworth Publications,England.

Carles W.Dolan , 1999,”FRP Prestressing in the USA”,Concrete International Vol.21,No

.10, pp.21-24

Norman R.H., Stom M.H. and W.C. Wake,[1970],“Resin Glass Interface”- Glass

reinforced plastics – Brian Parkyn. , Butterworth Publications,England.

Sami H. Rizkalla [Dec 18-20,1995], “FRP for the 21st Century”- International Conference

on Fibre Reinforced Structural Plastics in Civil Engineering, IIT Madras

Balaguru, Perumal Swamy. N, Chang and Peter, “ High Strength Composites For Repair

Rehabilitation and Strengthening of concrete Structures (ICI Journal : Jan- Mar 2003)

Balaguru and Shah, Text boon on “Fibre Reinforced Cement Composites”.

American Society For Metals, Source book on “Reinforced Plastics For Commercial

Composites”

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