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Advanced fibre polymer composite
materials and their properties for
bridge engineering
L. C. Hollaway University of Surrey
This chapter will introduce the polymers, fibres and advanced polymer composites
used in bridge engineering, from the point of view of the basic components of
composites, the manufacturing techniques of composites and the mechanical and
in-service properties of the composite materials.
Introduction
The legacy of a lack of investment in research and development in the construction industry for more than 30 years
following the end of the Second World War is clearly
illustrated by the lack of progress in construction methods
and in construction materials. Consequently, no interest
was shown by potential material investors in civil engineering and the technological revolution in materials and their
processing techniques utilised in other sectors of the manufacturing industry bypassed the construction industry. The
Latham Report was published in 1994 and regarded the
construction industry as low technology, low skilled and
labour intensive compared with most other industries. It
related the mismatch between research investment and
construction expenditure to an inadequate understanding
of many aspects of civil engineering, such as the deterioration mechanism for structures, and this has resulted in a
lack of due allowance being made for practical repair and
maintenance. However, notwithstanding this report, over
the past 20 years there is evidence of a transition, by the
construction industry, from the conventional civil engineering materials to the more advanced materials.
One of the promising advanced materials to enter the civil
engineering industry some 20 years ago was the advanced
polymer composite (APC). The developments associated
with the APC material, during this period, have been
considerable and the requirements that have initiated this
state have revolutionised the manufacturing and fabrication techniques for the production of the APC materials.
The aerospace and defence industries have been utilising
the APC materials for some considerable time and the
manufacturing techniques which are currently used in
construction have been developed from those used in
the two industries. It will be demonstrated in this chapter
that one area of the civil engineering industry which is
realising great benefits from the use of APCs is that of
bridge engineering.
Before discussing the subject of APC materials associated
with bridge engineering it is essential for the reader to have
doi: 10.1680/mobe.34525.0485
CONTENTS
Introduction
485
Reinforcement mechanism
of fibre-reinforced polymer
composites
486
Advanced polymer
composites
493
Adhesives
500
References
500
a clear understanding of the meaning of that material.
Therefore, the definition which was adopted, in 1989, by
the Study Group (on Advanced Polymer Composites) of
the Institution of Structural Engineers, UK, will be given
here. It was developed specifically for the construction
industry from that produced by the British Plastics Federation for general polymer composites. The definition is as
follows:
Composite materials consist normally of two discrete
phases, a continuous matrix which is often a resin, surrounding a fibrous reinforcing structure. The reinforcement
has high strength and stiffness whilst the matrix binds the
fibres together, allowing stress to be transferred from one
fibre to another producing consolidated structures. In
advanced or high performance composites, high strength
and stiffness fibres are used in relatively high volume fractions whilst the orientation of the fibres is controlled to
enable high mechanical stresses to be carried safely. In
the anisotropic nature of these materials lies their major
advantage. The reinforcement can be tailored and orientated to follow the stress patterns in the component leading
to much greater design economy than can be achieved with
traditional isotropic materials. The reinforcements are
typically glass, carbon or aramid fibres in the form of continuous filament, tow or woven fabrics. The resins which
confer distinctive properties such as heat or chemical resistance may be chosen from a wide spectrum of thermosetting or thermoplastic synthetic materials, but those
commonly used are the thermoset resins, the epoxy, the
phenolic and sometimes the polyester. More advanced
heat resisting types such as bismaleimides are gaining
useages in high performance applications and advanced
carbon fibre/thermoplastic composites are well into a
market development phase.
It will be seen later in this chapter that the main fibres
used in bridge engineering are the carbon, glass and
aramid fibres but these names are generic fibre names and
it is vitally necessary, when referring to these materials, to
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Advanced fibre polymer composite materials and their properties for bridge engineering
define precisely which category of fibre is being used/
described when discussing these materials. This will
become clear in the section on Fibres where the various
fibres and their forms are discussed. Likewise, the manufacturing techniques for composites should be mentioned or
their properties should be given when discussing/describing
these materials as the various methods of production
will produce different forms of composite and therefore
different mechanical and in-service properties; this
also will become clear in the section on Advanced polymer
composites.
The term ‘polymer composite material’ encompasses a
wide range of fibre–matrix materials each with their own
unique characteristics. It is not the intention of this chapter
to discuss the whole family of polymer composites; reference should be made to Hollaway (1993), Matthews and
Rawlings (2000), and Hollaway and Head (2001) for
further information on this topic. In this chapter the
relevant advanced polymer composites and adhesive
materials used in bridge management and their specific
manufacturing techniques will be discussed, thus enabling
the most efficient and cost-effective solution to the design
problem to be made. It will then continue by describing
the utilisation of the material in conjunction with the
more conventional civil engineering materials used in
bridge structures and as ‘all-composite’ bridge constructions. Currently there are no Eurocodes for the design of
fibre-reinforced polymer composites in civil engineering;
these are being considered by the European Commission
Joint Research Centre. There are UK design guides and
specifications, European design guides, the ACI design
codes USA and Canadian and Japanese design codes for
APCs; these are given in the Appendix at the end of this
chapter.
Reinforcement mechanism of fibrereinforced polymer composites
Introduction
Advanced composite materials are usually referred to as
fibre-reinforced polymer composites (FRPs) and this term
will now be used in all subsequent discussions. Fibrereinforced composite materials are made by the controlled
distribution of two materials: these are (1) the continuous
matrix phase (phase 1), (2) the fibre reinforcement phase
(phase 2). In addition there is the boundary between the
matrix and the reinforcement (the interface area), which
controls the properties of the given pair of materials.
The continuous matrix phase (the
polymer)
The matrix phase (the polymer) is an organic material composed of molecules which are repeats of similar but simpler
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units called the monomer. The requirements of the matrix
phase are (1) to bind and maintain the fibres in position,
(2) to protect the surfaces of the fibres from external
influences, environmental degradation and abrasion, and
(3) to transfer stresses to the fibres by adhesion and/or
friction; the adhesion to the fibres must be coupled with
adequate matrix shear strength. In addition, the matrix
must maintain chemical and thermal compatibility with
the fibre over the life span of the composite. Furthermore,
during the manufacturing process complete wet-out of the
fibres by the matrix must be achieved.
The three major types of matrices (polymers), which are
used in bridge engineering, are the thermosetting, the
thermoplastic and the elastomeric; each requires different
procedures for their manufacture. Their mechanical and
in-service properties will be different and these are discussed
in sections on Thermosetting polymers and The mechanical
and in-service properties of the thermosetting polymers.
The three types of polymer are composed of long chain
molecules and will be either (1) amorphous, which implies
a random structure with a high concentration of molecular
entanglement, or (2) crystalline, which has a high degree
of molecular order or alignment. The random structure of
the amorphous polymer, when heated, will become disentangled and the material will then be changed from a
solid to a viscous liquid, whereas heating the crystalline
polymer will change the material to an amorphous viscous
liquid.
It is noted that the matrix polymer material requires two
components, namely the resin and curing agent (hardener).
In bridge engineering there are two types of curing procedures: one uses a cold cure resin and the other uses a
hot cure resin. The former would be used in situ in the
field to form either the composite and/or adhesive, when
this latter is used. The cure temperature of the site would
determine the length of the polymerisation period. It is
advisable when using a cold cure resin to post cure the
material at an elevated temperature. The length of time
over which this cure takes place will depend upon the
value of the elevated temperature; the higher the postcure temperature the lower will be the cure period. The
hot cured resin products would be manufactured by automated techniques such as the pultrusion or the fibre
pre-impregnation in a factory under controlled conditions
of temperature and pressure; the manufacturing and cure
procedures are discussed in the section on The method of
manufacture of the composite.
Thermoplastic polymers
Thermoplastic polymers are composed of long chain
molecules which are held together by relatively weak Van
der Waals forces but the chemical valency bond along the
chains is extremely strong; they are generally of a semicrystalline form. Their strength and stiffness will be derived
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Advanced fibre polymer composite materials and their properties for bridge engineering
from the very high molecular weight and from the inherent
properties of the monomer units.
The polyolefins and polyesters are the main thermoplastics polymers which are utilised in bridge engineering
for geotechnical applications. These materials are formed
into geotextiles, geo-linear elements and geomembranes;
geosynthetics are not discussed in this chapter but references can be found in Ingold and Miller (1988), Hollaway
and Head (2001) and Fannin (2004). If the thermoplastic
polymer, polyacrylonitrile (PAN), is placed under a tensile
load the material behaves visco-elastically with a gradually
decreasing slope of the stress–strain relationship. At a limiting load value the material will be unable to support further
load and will continue to elongate excessively; the slope of
the stress–strain relationship will become zero. During this
elongation the molecules will align themselves with the
direction of the applied load – the greater the load the
greater the elongation – and in so doing a fibre is formed.
This fibre is the precursor for the manufacture of carbon
fibres (see section on Carbon fibres).
Thermosetting polymers
The thermosetting polymer consists of long chain
molecules, which are cross-linked in a curing reaction
producing covalent bonds between them; this chemical
reaction is known as polycondensation, polymerisation or
curing. This network, and the length and the density of
the molecular units, are a function of the chemicals used
in the manufacture of the polymers and the cross-linking
is a function of the degree of cure of the polymer. Both
the network and the cross-linking will have an influence
on the mechanical and in-service properties of the material.
Furthermore, the degree of cure is a function of the
temperature and the length of the polymerisation period.
The main thermosetting polymers used for structural
components in bridge engineering are the epoxies, the
vinylesters, and occasionally, the unsaturated polyesters.
The elastomer
A third member of the polymer family of interest to the
bridge engineer is the elastomer, of similar form to that of
rubber; the elastomeric material can be either reinforced
or unreinforced. The material is composed of long chain
molecules which are coiled and twisted in a random
manner, similar in form to that of the thermoplastic polymer and due to the molecular flexibility of the material it
is able to undergo very large deformations. The molecules
are cross-linked by a curing process known as vulcanisation; it is a similar procedure to the curing of the thermosetting polymer. The main use of this polymer in bridge
engineering is for elastomeric bridge bearings; further
discussions are given in the section on Elastomeric bridge
bearings in the next chapter.
Thermosetting polymers
Epoxies
The large family of epoxy resins represent some of the
highest-performance resins of those available at this time.
Epoxies generally out-perform most other resin types in
terms of mechanical properties and resistance to environmental degradation; this has led to their almost exclusive
use in aircraft components and a sizeable use in the
construction industry. The most important epoxy resins
for the bridge engineer are the low molecular weight polymers (oligomers), which are produced from the reaction
of bisphenol A and epichlorohydrin. They range from the
medium viscosity liquids through to high melting solids.
Epoxies used in bridge engineering possess the following
properties:
n They have the capability of being used for different composite
manufacturing techniques, where polymerisation can take
place at room temperature (cold cure resins) or at elevated
temperatures (hot cure resins); thus two different resin and
curing systems (i.e. the cold or the hot cure systems) will be
required for these manufacturing methods. Consideration
must be given to the glass transition temperature (Tg ) of cold
cure epoxy polymers (see the section on In-service properties).
n They have high specific strengths and dimensional stability.
n They have higher operating temperatures compared to that of
the polyester due to their superior toughness.
n They have low shrinkage during polymerisation and good
adhesion to many substrates. This allows mouldings to be
manufactured to a high quality and with good dimensional
tolerance.
n They use hot cured polymer systems which have a high temperature resistance and can be used at temperatures up to
2008C. Some epoxies have a maximum temperature range up
to 3168C.
Since the amine molecules ‘co-react’ with the epoxy
molecules in a fixed ratio, it is essential that the correct
mix ratio is obtained between the epoxy resin and the
curing agent to ensure that a complete reaction does take
place. If amine and epoxy are not mixed in the correct
ratios, unreacted resin or curing agent will remain within
the matrix, which will affect the final properties after
cure. To assist with the accurate mixing of the resin and
curing agent (particularly for site fabrication), manufacturers usually formulate the two component parts to give
a simple mix ratio that is readily achieved by measuring
out the two components by weight or volume. Alternatively, the resin and curing agent are provided, by the
manufacturers, in exact amounts in two separate containers; the resin container would be of sufficient volume to
pour the curing agent into it and to mix by mechanical
paddle.
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Advanced fibre polymer composite materials and their properties for bridge engineering
Vinlyesters
The vinylester is a hybrid form of polyester resin which has
been toughened with epoxy molecules within the main
molecular structure. Its molecule features fewer ester
groups compared to the polyester resin and as these ester
groups are susceptible to water degradation by hydrolysis,
vinylesters exhibit better resistance to water and many
other chemicals than do their polyester counterparts.
Furthermore, as vinylesters have unsaturated esters of
epoxy resins, they have similar mechanical and in-service
properties to those of the epoxies, but they have similar
processing techniques to those of the polyesters. Generally,
the vinylesters have:
n a processing method at both room temperature (cold cure
resins) and elevated temperatures (hot cure resins); as for
epoxy polymers, consideration must be given to the glass
transition temperature (Tg ) of cold cure resins (see the section
on In-service properties)
n good wetting characteristics and bond well to glass fibres
n good resistance to strong acids and strong alkalis
n reduced water absorption and shrinkage properties, as well
as enhanced chemical resistance, compared to those of the
polyester.
Unsaturated polyesters
Polyesters are not widely used for the manufacture of
bridge structural components but they have been largely
replaced by the vinylester polymers. However, they are
used on occasions in the production of automated pultrusion components (see section on Automated processes).
They are also readily processed and cured at ambient temperatures using a wet lay-up procedure (see section on
Manual techniques); these ambient cured polymers must
be post-cured. Styrene, in amounts of up to 50%, is the
most commonly used diluent. However, it should be
noted that during the wet lay-up (hand laminated) production method large amounts of styrene and trace quantities
of styrene-7,8-oxide (SO) are released, which will cause a
health risk unless safety precautions are taken.
There are three types of commonly used polyester resins:
the orthophthalic, the isophthalic and the bisphenol A. The
first resin mentioned has low thermal stability and chemical
resistance. The isophthalic resins, which contain isophthalic
acids as an essential ingredient, are of superior quality with
better thermal and chemical properties. The bisphenol A
resin is of a quality superior to both the orthophthalic
and isophthalic resins with a higher degree of chemical,
thermal and hardness resistance and has some degree
flame resistance. The shrinkage at cure of the polyester
resin tends to be high, particularly if styrene in monomeric
form is used as a reactive diluent in the resin. This will cause
(1) an increase in hydrophobicity, thereby effectively
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decreasing the level of moisture absorption, and (2) an
increase in shrinkage up to levels of 5–19% by volume.
This can result in significant micro-cracking in resin-rich
areas and high residual stresses in composites having high
fibre volume fractions.
The polymerisation of thermosetting polymers must be
completed before the polymer (or the FRP composite) is
used in practice as the long-term performance properties
of the composite will be influenced by the degree of cure
of the resin, particularly for ambient (cold) cured systems.
The section on the Method of manufacture of the composite
discusses the various methods for the manufacture of
composites and it will be seen that materials which are
manually fabricated in the field, together with adhesive
polymers (if these are employed during fabrication), use
cold cure systems at ambient temperatures. Composites,
which are manufactured under factory conditions, use hot
curing polymers and are cured at the operating temperature
of the manufacturing process. The production temperature
values of these processes are of the order of 120–1358C; the
actual value will depend upon which system is being used.
Ideally all cold cure adhesive polymers and polymer
composites should be post cured at an elevated temperature
for a certain length of time; the higher the temperature the
shorter is the post-cure period. The initial cure of these
polymers will reach only about 90% of the full polymerisation value after some ten days (depending upon the polymer
system and ambient cure temperature). Cain et al. (2006)
investigated the post-cure effects on E-glass/vinyl-ester
fibre-reinforced polymer composites manufactured using
the vacuum-assisted resin transfer moulding (VARTM)
method (see the section on Method of manufacture of the
composite). The composites were differentiated by varying
levels of post-cure temperature and duration, and examined
for the effects of advancing cure at various points in the
time after post-cure. In parallel, the matrix polymer was
inspected using Fourier transform infrared spectroscopy
to directly assess the degree of conversion. The results suggested the degree of conversion is limited to 80% for the
vinylester oligomer and 90–95% for styrene following a
post-cure of 938C. It was observed that after 300 days of
ambient temperature storage, the non-post-cured samples
approached the degree of conversion exhibited by those
post-cured at 938C. The research team concluded that
resin-dominated quasi-static properties are greatly affected
by the degree of cure whereas fibre-dominated properties
are not affected so much. As mentioned above, this
under-cure will affect the long-term mechanical property
of the polymer with the result that its exposure to a variety
of adverse and sometimes harsh environmental conditions
encountered in construction will degrade the polymer and
the FRP composite material, and thus will alter their
mechanical performance. A further concern regarding the
ambient-cured systems is that their relatively low cure
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Advanced fibre polymer composite materials and their properties for bridge engineering
temperature will affect their glass transition temperature
(Tg ) (see the section on In-service properties), which will
generally be about 15–208C above the cure temperature.
This point is discussed in the section on Dimensional
stability.
The mechanical and in-service
properties of the thermosetting polymers
The mechanical properties
The important mechanical properties of the three polymers
that are used in bridge engineering are given in Table 1.
Although these mechanical properties are important to
the bridge engineer, it should be remembered that the
polymer composite derives its mechanical characteristics
wholly from those of the fibre. The most important properties of the polymer are its in-service properties as given in
the section below.
The in-service properties
The most important in-service properties of polymers are
listed below.
The stiffness of thermosetting polymers
As stated earlier, the stiffness is influenced by the degree of
cross-linking of the polymer. The process of cross-linking
produces a tightly bound three-dimensional network of
polymer chains. This network, and the length and the
density of the molecular units, are a function of the
chemicals used in the manufacture of the polymer, and
the cross-linking is a function of the degree of cure of the
polymer.
The strength of thermosetting polymers
The short-term strength of a thermosetting polymer is
dependent upon the bonding, length, density and the
degree of cross-linking of the molecular structure of the
polymer. It will also depend upon the type of loading
applied to that polymer. The long-term strength of the polymer will be dependent upon its durability in the environment into which it is placed. The section on Moisture,
aqueous and chemical solutions discusses the durability of
Material
Specific
strength
Ultimate
tensile
strength:
MPa
Modulus of
elasticity in
tension:
GPa
Coefficient
of linear
expansion:
10ÿ6 /8C
Polyester
1.28
45–90
2.5–4.0
100–110
Vinylester
1.07
90
4.0
80
Epoxy
1.03
90–110
3.5
45–65
Table 1 Typical mechanical properties of the three thermosetting
polymers used in bridge engineering
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composites which are largely dependent upon the durability
of the polymer.
Thermal conductivity of all polymers
The thermal conductivity of polymers is low and consequently they are good heat insulators. The thermal
conductivity of a polymer can be reduced by incorporating
metallic fillers at the time of manufacturing the polymer.
Coefficient of thermal expansion of thermosetting
polymers
This characteristic is extremely important in design when
considering joining FRP composites to a dissimilar
material. The coefficient of thermal expansion of a polymer
is an order higher than that of the more conventional civil
engineering materials. In bridge engineering practice the
only time that this situation occurs is when an adhesive
material is used to join two dissimilar materials, e.g. when
FRP composite material is bonded to reinforced concrete,
timber or to steel bridge structural members to strengthen/
stiffen them. This will be discussed in the sections on
Adhesive bonding of polymer composites to steel adherends
and Rehabilitating reinforced-concrete bridge engineering
in the next chapter.
Toughness of a thermosetting polymer
The toughness of a brittle thermosetting polymer can be
improved by blend filling or co-polymerising it with a
tough but lower stiffness one. However, an increase in the
toughness of the polymer will tend to decrease its stiffness
and, consequently, a compromise between the strength
and stiffness of a thermosetting polymer must be made.
Permeability
Polymers with high crystallinity/density or a high degree of
cross-linking will generally have low permeability, thus
gasses and other small particles will not readily permeate
through it. Haque and Shamsuzzoha (2003), Liu et al.
(2005) and Hackman and Hollaway (2006) have shown
that the ingress of moisture will permeate through polymers
over time, particularly if the polymer (composite) is permanently immersed in water or salt solution, or is exposed to
de-icing salt solutions.
Chemical resistance
The chemical resistance of a thermosetting polymer
depends upon the chemical composition and bonding in
the monomer; generally, the resistance is good.
Creep characteristics of polymers
Polymers are classified as visco-elastic materials which
indicate that they have both elastic solids and viscous
fluids characteristics. In assessing the creep of a polymer
material it is important to know the following:
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Advanced fibre polymer composite materials and their properties for bridge engineering
Stress
Strain
Stress
Stress increase
For 100 s duration
100 s
0.2
Log time
Strain
Log time
(a)
(b)
(c)
Note: For fibre–matrix composite materials
As the fibre–matrix composite contains a visco-elastic polymer, this material will creep, however, the fibres do not creep and
therefore stabilise the polymer thus causing the composite to creep less than that of the pure polymer but similar forms of
creep curves to those above for polymers will be formed for composites.
Figure 1 Creep curves: isochronous stress–strain curves (adapted from BS 4618 1970): (a) isostress creep curve for polymers (experimental/field values);
(b) 100 s isochronous stress–strain curve for polymers (values derived from (a)); (c) isostrain creep curve for polymers (values derived from (a))
n the time-dependent nature of the micro-damage in the
composite material subjected to stress.
To minimise creep it is necessary to ensure that the service
temperatures do not approach the glass transition temperature of the polymer. Figure 1 illustrates a family of
creep curves consisting of isostress creep curve (Figure
1(a)), 100 s isochronous stress–strain creep curve (Figure
1(b)) and the isostrain creep curve (Figure 1(c)). These
latter curves have been produced by cross-plotting, from
the isostress creep curves at constant times.
To produce isochronous stress–strain curves, BS 4618
(1970), ISO 899-1 (1993) specification requires that the
constant load tests are carried out under controlled
conditions for the following durations 60 s, 100 s, 1 h, 2 h,
100 h, 1 year, 10 years and 80 years. This method yields a
family of stress–strain curves, each relevant to a particular
time of loading. From Figure 1(b), it will be seen that a
family of creep curves, for any material, may be obtained
by varying the stress. Isochronous stress–strain curves will
correspond to a specific loading direction. Thus a 100 s
isochronous stress–strain curve is a plot of the total strain
(at the end of 100 s) against the corresponding stress level.
The slope at any specific point on this curve will then
define the creep modulus of the material at that particular
stress level. The creep modulus will vary for every stress
level.
A sophisticated approach, to obtain creep characteristics of
polymers, such as the time–temperature superposition principle (TTSP) has been developed by Aklonis and MacKnight
(1983). By conducting a series of tests at different temperatures (but the temperatures should not approach the
Tg of the polymer), activation energy may be determined
and through a kinetic approach of temperature–time
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The glass transition temperature of polymers
The glass transition temperature (Tg ) of an amorphous
material (the polymer) is the temperature at which it
changes from (or to) a brittle or vitreous state to (or
from) a plastic state; this change takes place over a temperature range and the mid-point of that range is taken as
the Tg ; this is shown on Figure 2. The Tg depends upon
the detailed chemical structure of the polymer which in
turn depends upon the cross-linking of their molecules for
strength. Crystalline polymers do not have a glass transition
temperature but have a melting point, and possess some
degree of amorphous structure; this portion usually
makes up 40–70% of the polymer sample. Thus, a polymer
can have both a glass transition temperature (the amorphous portion) and a melting temperature (the crystalline
portion). The effect of crystallinity on properties is observed
Volume change
n the loading histories and the nature of the applied load
superposition plots, a master curve can be deduced and
predictions made; this master curve has been discussed in
Hollaway (1993). The time, applied stress superposition
principle (TSSP) (Cessna, 1971) can also be used for
polymers and polymer composite materials.
Modulus of elasticity
n the temperature and moisture environments into which it is
placed
Tg
Temperature increase
Tg
Temperature increase
Figure 2 Change in properties at the glass transition temperature of
thermosetting polymers
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Advanced fibre polymer composite materials and their properties for bridge engineering
when the polymer is at a temperature between the Tg and
the crystalline melting point. All polymers below the Tg ,
whether crystalline or not, are rigid and frequently brittle;
they therefore have both stiffness and strength. Above the
Tg , the amorphous polymers are soft elastomers or viscous
liquids, consequently they have no stiffness or strength,
but the crystalline polymers will range in properties from
a soft to a rigid solid depending upon the degree of crystallinity. The epoxies used in construction would generally be
in the amorphous state, with a small amount of crystalline
structure. The numerical value of the Tg may be quoted
with slightly different values depending upon the testing
technique used; these methods are dynamic mechanical
thermal analysis (DMTA) and differential scanning calorimetry (DSC). The drastic changes which any polymer will
experience at the Tg is shown in Figure 2 as typical mechanical/physical properties. Modulus of elasticity (modulus of
elasticity plotted against temperature) is shown in Figure
2(a); hardness (volume plotted against temperature) is
shown in Figure 2(b).
From the above discussion it will be clear that polymer
composite structural members should not be exposed to
temperatures above the Tg value of the matrix material.
Hot cured thermosetting polymers would normally have a
higher Tg value than the room temperature cured thermosetting materials; their actual values would generally be
about 15–208C above the curing temperature. The Tg of
some low-temperature (ambient cured) moulded composites
can be increased in value by further post-curing the polymer
at a higher temperature than that of the original cure but
there is a maximum value of the Tg of all polymers
irrespective of the post-cure temperature value.
Dimensional stability
The thermosetting polymers used in bridge engineering are
semi-crystalline, semi-rigid and frequently brittle in nature
and will lose their dimensional stability above the glass
transition temperature (Tg ), therefore the strength and
stiffness will be much reduced. If polymers with a high
crystallinity are used they will have a region of acceptable
dimensional stability above the Tg . The post-curing
procedures of thermosetting polymers are vitally important, particularly the ambient cured ones as in this case
the polymerisation will only be about 90% complete after
about seven to ten days, depending upon the ambient
temperature during the initial cure.
The fibres
The fibres that are mainly used in bridge engineering for
structural applications are carbon, glass and aramid fibres
and for geotechnical applications are those made from
one of the polyolefin family of thermoplastic polymers,
(see Aboutaha, undated; Hollaway and Head, 2001;
Chia-Nan Liu, 2003).
Glass fibres
Glass is the common name given to a number of mutually
soluble oxides which can be supercooled without crystallising; they are all clear amorphous solids when cooled.
Several grades of glass fibre are produced commercially,
of which the most important ones used in bridge engineering are as follows:
n E-glass fibre. This has a low alkali content of the order of 2%.
It is used for general purpose structural applications and is the
major fibre used in the construction industry. It also has good
heat and electrical resistance.
n S-glass fibre. This is a stronger (typically 40% greater strength
at room temperature) and stiffer fibre with a greater corrosion
resistance than the E-glass fibre. It has good heat resistance.
The S-2-glass fibre has the same glass composition as S-glass
but differs in its coating. The S-2-glass fibre has good resistance
to acids such as hydrochloric, nitric and sulphuric acids.
n E-CR-glass fibre. This has good resistance to acids and bases
and has chemical stability in chemically corrosive environments.
n R-glass fibre. This fibre has a higher tensile strength and tensile
modulus and greater resistance to fatigue, aging and temperature corrosion than does E-glass.
n Cemfil (or AR-glass) fibre. This is specifically employed for
resisting the alkali in Portland cement and is used as the
reinforcement for glass-fibre-reinforced cement (GFRC).
Table 2 shows the chemical composition of E-glass and
S-glass.
The commercial manufacturing technique for glass fibres
is undertaken by drawing swiftly and continuously fine filaments from the molten glass which is at a temperature of
1200–14008C. These series of filaments have exceptionally
high specific stiffness and strength and range in diameter
from 3 to 24 mm. During this production stage strands,
each consisting of 200 individual filaments, are produced
and a surface treatment or sizing is applied before the
fibres are gathered into strands and wound onto a drum.
The manufacturing technique has been discussed in
Hollaway and Head (2001). The mechanical properties of
the various types of glass fibre are given in Table 3.
Carbon fibres
The two high-performance carbon fibres used in Europe,
North America and Japan for bridge engineering are
Oxides used
CaO
Al2 O3
For E-glass
17.5
14.0
For S-glass
–
25
MgO
B2 O 3
SiO2
4.5
10
54
–
65
10
Table 2 The chemical composition of E-glass and S-glass (after Phillips,
1989)
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Advanced fibre polymer composite materials and their properties for bridge engineering
Material
Fibre
Glass fibre
E
A
S-2
Carbon fibre
PAN-based fibres:
Hysol Grafil Apollo
Elastic
modulus:
GPa
72.4
72.4
88.0
Tensile
strength:
MPa
Ultimate
strain:
%
2400
3030
4600
3.5
3.5
5.7
IM
HM†
HS‡
300
450
260
5200
3500
5020
1.73
0.77
2.00
BASF Celion
G-40-700
Gy 80
300
572
4960
1860
1.66
0.33
Torayca
T-300
234
3530
1.51
Pitch-based fibres:
Hysol Union
carbide
T-300
T-400
T-500
T-600
T-700
227.5
–
241.3
241.3
248.2
2758.0
–
3447.5
4137.0
4550.7
1.76
–
1.79
1.80
1.81
Aramid fibre
49
29
125
83
2760
2750
2.40
4.00
High modulus (intermediate modulus); † ultra-high modulus (high modulus); ‡ high
strain.
Table 3 Typical mechanical properties of glass, carbon and aramid
fibres (adapted from Hollaway and Head, 2001)
defined as the high modulus (H-M) carbon and the ultrahigh modulus (UH-M) fibres, (in North America, Japan
and some Asian countries these two definitions are referred
to as intermediate-modulus and high-modulus carbon
fibres, respectively). The basic manufacturing techniques
for the H-M and the UH-M fibres are the same but the
heat treatment temperature will be greater the higher the
modulus of the fibres, thus a more highly orientated fibre
of crystallites will be formed for the UH-M fibre. Typical
mechanical properties of carbon fibres are given in
Table 3. Hollaway and Head (2001) have stated that
carbon fibres are manufactured by controlled pyrolysis
and crystallisation of certain organic precursors and the
manufacturing process consists of sequences of procedures
which include (1) stabilisation, (2) carbonisation and (3)
graphitisation and finally (4) surface treatment; the manufacturing technique of the carbon fibre is illustrated in
Figure 3.
The precursor fibres that are used for the production of
carbon fibres, are (1) the polyacrylonitrile (PAN) and (2)
the pitch fibres. The first fibre precursor is manufactured
by spinning to produce a round cross-section fibre; the
yield is only 50% of the original precursor fibre. The
PAN precursor carbon fibres can also be manufactured
by a melt-assisted extrusion as part of the spinning operation. I-type and X-type rectangular cross-section carbon
fibre composites are produced with a closer fibre packing
in the composite. The aerospace industry uses only these
types of carbon fibres and they are also mainly used in
bridge engineering. The pitch precursor fibres are derived
from petroleum, asphalt, coal tar and PVC; the carbon
yield is high but the uniformity of the fibre cross-sections
is not constant from batch to batch. This non-uniformity
(a) Precursor (polyacrylonitrile)
Either
or
T = 200–300°C
(To achieve
dimensional stability)
T > 800°C
(Carbon crystallites
formed)
T > 1200°C
(Highly orientated fibres
of crystallite formed)
Oxygen is being
absorbed
Inert
atmosphere
Inert gas
Stabilisation
Carbonation
Graphitisation
(b) Pitch fibre
Melt spinning
(by-product obtained by the
destructive distillation of coal)
Pick-up spool
Surface treatment
Precursors to the carbon fibre used in civil engineering.
(a) Polyacrylonitrile (PAN) fibre (used in the manufacture of high-modulus carbon fibres)
(b) Pitch fibre produced from the distillation of coal (used for the manufacture of ultra-high-modulus carbon fibres)
Figure 3
492
Manufacture of carbon fibre (adapted from Hollaway and Head, 2001)
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Advanced fibre polymer composite materials and their properties for bridge engineering
Aramid fibres
Aramid fibres are synthesised from the monomers 1,4phenyl-diamine ( para-phenylenediamine) and terephthaloyl
chloride. The result is a polymeric aromatic amide
(aramid) with alternating benzene rings and amide
groups; these polymer strands, when produced, are randomly orientated. To manufacture aramid fibres the polymer strands are dissolved and spun at a temperature of
between ÿ508C and ÿ808C and are then extruded into a
hot cylinder which is at a temperature of 2008C; this
causes the solvent to evaporate and the resulting fibre is
wound onto a bobbin. The fibre then undergoes a stretching
and drawing process to increase its strength and stiffness
properties. The resulting commercial fibre has a high
strength, is heat resistant and the value of its strength is
unaffected by immersion in water. Two grades of stiffness
are generally available: the first has a modulus of elasticity
of about 60 GPa and is used in applications such as cables
and body armour; the second has a modulus of elasticity
of 130 GPa and is used as reinforcement in polymer composites in construction.
The aramid fibre is an anisotropic material and, as is the
case with all fibres, gives higher strength and modulus
values in its longitudinal direction compared with its
transverse direction. The value of the longitudinal strength
is 2.7 GPa and the fibre is heat and cut resistant. It retains
its mechanical properties from cryogenic temperatures up
to 4008C and is resistant to fatigue, both static and
dynamic, and is elastic in tension but behaves non-linearly
Ultra-high-modulus carbon fibre
High-modulus carbon fibre
Aramid fibre
Glass fibre
Tensile stress: GPa
is acceptable to the construction industry and the pitch fibre
precursor is invariably used when the ultra-high stiffness
carbon fibres are required. The carbon fibres made from
the pitch precursor tend to be cheaper than the PAN precursor. As the modulus of elasticity of the pitch fibres is
invariably much higher than that of the PAN carbon
fibres the strain to failure will be lower.
Typically the diameter of carbon fibre filaments are 5 and
8 mm and are combined into tows containing 5000–12 000
filaments. A common size of untwisted carbon fibre ‘tow’
is called 12K which contains 12 000 filaments of carbon
and is sold as (1) high-modulus fibres (intermediate-modulus fibres; stiffness 250–300 GPa), (2) ultra-high-modulus
fibres (high-modulus fibres; stiffness 300–1000 GPa); the
value of stiffness of the UH-M carbon fibre used in bridge
engineering is generally about 500 GPa. The tows can be
twisted into yarns and woven into fabrics similar to that
for glass fibre.
In addition to strong and high-stiffness fibres, bridge
engineering requires in-service resistance to withstand
high temperature and aggressive environmental conditions;
carbon fibres, in general, are not affected by moisture, solvents, bases and weak acids. Table 3 provides general and
physical properties of epoxy polymers.
Slopes of lines indicate
modulus of elasticity
0
1
2
3
4
Tensile strain: %
Figure 4 Typical comparative stress–strain characteristics of glass,
aramid and carbon fibres
in compression and in addition has a ductile compressive
characteristic. The fibre possesses good toughness and
damage tolerance properties. It does not rust or corrode
and its strength is not affected by immersion in water.
The main weakness of aramid fibre is that it decomposes
under alkaline or chlorine environments. Typical mechanical properties of aramid fibres and their stiffness characteristics are given in Table 3 and Figure 4 respectively.
There are various types of aramid fibres on the market,
including Kevlar1, which is made in the USA, Technora1,
produced in Japan by Teijin, who also produce Twaron1.
Advanced polymer composites
The advanced polymer composites (APCs) are manufactured by the impregnation of the fibre into the matrix
material to form a composite system and in so doing to
obtain a wide range of composites with varying mechanical
properties. The fibre–matrix composite can have the properties of anisotropy or isotropy by virtue of the arrangement
and direction of the fibres in the matrix.
Mechanical properties of composites
The mechanical properties of the final fibre–polymer
composite will be dependent upon the following:
n the method of manufacture
n the mechanical properties of the component parts (i.e. carbon,
glass or aramid fibres and the polymer)
n the relative proportions of the polymer and fibre (fibre volume
fraction)
n the orientation of the fibre – that is, unidirectional (anisotropic
composite), bidirectional aligned (orthotropic composite) or
randomly orientated (quasi-isotropic composite)
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Advanced fibre polymer composite materials and their properties for bridge engineering
n the long-term durability of the composite system; this will
generally involve reduction in property values of the composite
system from those of its pristine condition
n the creep of the composite.
Method of manufacture of the
composite
2 The CFRP sheet cut to size is adhered to the concrete
using the first laminating layer of polymer; it is pressed
down on the surface using a roller to expel entrapped
air between the fibres and the polymer.
3 After the paper backing is removed from the carbon
fibre sheet the second layer of impregnating polymer is
applied.
4 Steps 2 and 3 are repeated in the case of multiple plies.
5 The system is then allowed to cure.
There are a number of techniques available for the manufacture of advance polymer composites, all of which have
an influence on the mechanical properties of the final composite. As would be expected, the automated fabrication
method tends to give higher values of strength and stiffness
than the manual fabricated techniques because of the
greater degree of process control, compaction and curing
in the former techniques, thus providing a higher fibre
volume fraction.
The various manufacturing processes given here are all
relevant to bridge engineering and can be divided into
three major divisions: (1) the manual production process,
(2) the semi-automated process, and (3) the automated
process. A short discussion of these processes and an
indication when specific applications are used in practice
now follows.
The Dupont method is a system using Kevlar fibres which is
marketed as a repair system for concrete structures. The
application of the material to the surface to be retrofitted
is similar to the above.
The Tonen Forca method is an unidirectional carbon fibre
sheet in an epoxy laminated system marketed in the UK by
Kyokuto Boeki Kaisha Ltd. The system was originally
developed by Mitsubishi Chemical Corporation and is
therefore similar to the RePlark system.
These methods have been used in bridge engineering to confine columns or for strengthening/stiffening bridge beams
(see the section on Rehabilitating reinforced-concrete bridge
structures in the next chapter). Further details on these
methods may be obtained from Hollaway and Head (2001).
Manual techniques
Semi-automated processes
The manual processes used currently are a variation of the
general wet lay-up method. The commercial companies
which manufacture APCs by this process are:
The semi-automated processes used currently are:
n the REPLARK method
n the resin transfer moulding (RTM) process
n the Dupont method
n the hot-melt factory-made prepreg.
n the Tonen Forca method.
The XXsys Technologies method was developed in the USA
by XXsys Technologies, Inc., San Diego, California, as a
semi-automated process for seismic retrofitting and
strength restoration of concrete columns using continuous
carbon fibre. The XXsys carbon fibre composite jackets
are installed with a fully automated machine called RoboWrapper2 and portable oven for curing. The technology
associated with the technique is based upon the filament
winding of prepreg carbon fibre tows around the structural
unit thus forming a carbon fibre jacket; currently, the structural unit to be upgraded would be a column. The polymer
is then cured by a controlled elevated temperature oven and
can, if desired, be coated with a resin to match the existing
structure. An advantage of this automated process is that
the carbon fibre prepreg is impregnated with the polymer
under factory-controlled conditions, providing good quality
control and as a consequence a high strength-to-weight ratio.
The equipment is erected on site with minimum disturbance
to traffic and the whole operation is undertaken in minimum
time; the latter will, however, depend upon the size of the job.
The carbon fibre jacket which is eventually formed around
the column will increase the shear capacity of the column
These techniques are basically the same with minor variations. The wet lay-up technique consists of in situ wetting
of dry fibres in the form of sheets or fabrics impregnated
in situ with a polymer. The sheets or fabrics are wrapped
around the structural member or placed on the mould
and their size will depend upon the size of the member or
mould, but fibre reinforcements are generally of widths
varying between 150 mm to 1500 mm.
The REPLARK method was developed by the Mitsubishi
Chemical Corporation, and is a pre-impregnated (prepreg)
carbon fibre sheet where the matrix material is an
epoxy resin (Epotherm) and the fibres are Mitsubishimanufactured fibres, unidirectionally orientated. The
sheet has a paper backing, which serves to keep the fibres
in position; the paper backing is removed when the sheet
is placed in position on the mould structure. The procedure
employed to apply the carbon-fibre-reinforced polymer
(CFRP) sheet is as follows:
1 The first layer of impregnating polymer is applied to the
prepared surface of the mould or structural member.
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n the resin infusion under flexible tooling (RIFT) process
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Advanced fibre polymer composite materials and their properties for bridge engineering
and will confine the concrete and greatly enhance its ductility
in the flexural plastic hinge region. Furthermore, it will
provide lap splice clamping and prevent local buckling of
the vertical reinforcement. For corrosion-damaged columns,
the jacket restores shear capacity and will prevent spalling of
the cover concrete.
Resin transfer moulding is a low-pressure, closed-mould
semi-mechanised process. In RTM process, several layers
of dry continuous strand mat, woven roving or cloth are
placed in the bottom half of a two-part closed mould and
a low-viscosity catalysed resin is injected under pressure
into the mould cavity, and cured. Flat reinforcing layers,
such as a continuous strand mat, or a ‘preform’ that has
already been shaped to the desired product, can be used
as the starting material in the RTM process. The potential
advantages of RTM are the rapid manufacture of large,
complex, high-performance structures with good surface
finish on both sides, design flexibility and the capability of
integrating a large number of components into one part.
This method can be employed to form large components
for all composite bridge units but it is seldom used.
The hot-melt factory-made pre-impregnated fibre (prepreg) is cured in either the factory, for the production of
precast plates, or on site if the prepreg composite is to be
fabricated on to a bridge member. In the latter case a compatible film adhesive is used and the adhesive and the prepreg components are cured in one operation under an
elevated temperature of 658C applied for 16 h or 808C
applied for 4 h; a vacuum-assisted pressure of 1 bar is applied
for simultaneous compaction of the composite and the film
adhesive (see Hollaway et al., 2006). It is estimated that
this method will be used increasingly for strengthening/stiffening degraded bridges (see the section on Rehabilitation
of steel structural members in the next chapter).
In the UK the manufacturing specialist in the production
of hot-melt factory-made pre-impregnated fibre for the
construction industry is ACG in Derbyshire.
Automated processes
The automated processes which are available to the
construction industry are:
n the pultrusion technique
n the filament winding technique.
The pultrusion technique is used quite extensively in the
construction industry and a large percentage of this use is
associated with bridges. Sections are manufactured in a
factory using a hot cure resin, the dies operating at temperatures between 1208C and 1358C. The pultrusion technique is
associated with bridges (see the section on Rehabilitating
reinforced concrete bridge structures). The sections produced
will generally be fully cured but this does depend upon their
size; the large sizes of the sections may require post-curing.
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Flat plates and various geometrical cross-sections can be
produced; they are generally straight in the longitudinal
direction although products can be manufactured which
are curved in plan. Care must be taken to ensure that (1)
the fibres are well compacted into any bends in the crosssection (thus preventing voids forming), (2) there is complete
wetting of the fibres in the pultruded unit (again, preventing
voids forming), and (3) the fibres are well distributed in all
cross-sections. It is not usual to have fibre weight fractions
(f.w.f.) greater than 60% although uncomplicated sections,
such as small-diameter rods, have been manufactured with
70% f.w.f. To provide resistance to hostile environments, a
resin-rich exterior surface to the pultruded section can be
fabricated using a surface veil, which is incorporated into
the structural component at the time of manufacture. The
limiting factors on the size of the unit and the complexity
of the cross-section is the pull force required to draw the pultruded section through the die; the more complex the section
the greater will be the force due to friction of the unit within
the die. Hydraulic pulling force systems up to 50 t on sections
of 2.5 m 275 mm thick are currently in operation in the
USA. Carbon, aramid and glass fibres and epoxy, vinylester
and polyester materials have all been used for the production
of pultruded units. The epoxy polymers are probably the
most difficult to pultrude but do have low shrinkage
during polymerisation (3–4%). The vinylester polymers
have a shrinkage value of 6–10% and the polyester
polymers have a large shrinkage during polymerisation of
12–19%. Further information may be obtained from Starr
(2000) and Hollaway and Head (2001). Figure 5 illustrates
the pultrusion technique.
Some companies providing details of geometries and
mechanical properties of pultruded profiles are Fiberline
(Europe), Creative Putrusions, Strongwell and Bedford
(USA).
The pultrusion technique is utilised in bridge engineering
to upgrade and retrofit structural beams, the manufacture
of ‘all-composite’ bridges, bridge decks, near-surfacemounted rods, internal reinforcement to concrete and
bridge enclosures and fairings.
The filament winding technique is used to manufacture
pressure pipes and to undertake wrapping of columns (see
the section on FRP confining of concrete in the next chapter). There are two different winding methods: (1) wet winding, and (2) pre-impregnated winding, The wet winding
method consists of continuous strands or roving of dry
fibres from a series of creels which are passed through a
bath of ‘cold cure’ resin, cold curing agent, pigments and
UV retardants on to a ‘pay-out eye’ which is mounted on
a moving carriage along the length of a constant-speed
rotating mandrel. The roving fibre delivery system reciprocates along the length of the mandrel and is controlled
relative to the rotation of the mandrel to give the required
fibre orientation. The speed of reciprocation and rotation
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Advanced fibre polymer composite materials and their properties for bridge engineering
Heated die
Puller
Alternatively, fibres can be impregnated with resin by injection
through portholes in the heated die as the fibres pass through
Resin bath
Creels of fibre
(carbon and/or glass)
Figure 5
A diagrammatic representation of the pultrusion machine (adapted from Hollaway and Head, 2001)
are synchronised to hold a preset winding angle typically
between 7–908. The machine has the ability to lay fibres
in any direction and to employ as many permutations of
movements as is required by the structural design. After
winding, the filament-wound mandrel is subjected to
curing and post-curing operations during which the mandrel is continuously rotated to maintain uniformity of
resin content around the circumference. After curing, the
product is removed from the mandrel either by hydraulic
or mechanical extraction. The pre-impregnated winding
consists of passing pre-impregnated fibres over a hot
roller until tacky to the touch and they are then wound
on to the rotating mandrel. Wet winding is the method
usually used and has several advantages over pre-impregnated winding; these are low material cost, short winding
time, and the resin formulation can be readily varied to
meet specific requirements. Figure 6 shows a schematic
representation of the filament winding technique. The
XXSys Technology method, which is a site procedure for
wrapping concrete bridge columns, is a filament winding
technique; an example has been illustrated in Figure 7.
Resin transfer moulding is a low-pressure, closed-mould
semi-mechanised process. The process enables fabrication
of simple low- to high-performance articles in varied sizes
and profiles. The RTM process has been successfully used
in moulding of complex three-dimensional shapes. In
RTM, several layers of dry continuous strand mat, woven
roving or cloth are placed in the bottom half of a twopart closed mould and a low-viscosity catalysed liquid
resin is injected under pressure into the mould cavity,
which is subsequently cured. Instead of using flat reinforcing layers such as a continuous strand mat, the starting
material in the RTM process can be a ‘preform’ that
already has the shape of the desired product. The potential
advantages of RTM can be summarised as rapid manufacture of complex, high-performance structures with good
Traversing resin bath
Resin bath
Rovings
Gear box
Drive motor
Rotating mandrel
Figure 6 Diagrammatic representation of a filament winding machine
(adapted from Hollaway and Head, 2001)
496
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Figure 7 Column wrapping using XXsys technique (by kind permission
of XXsys Technologies, Inc., San Diego, CA, USA)
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Advanced fibre polymer composite materials and their properties for bridge engineering
E-glass
Specific
weight
1.90
Tensile
strength:
MPa
Tensile
modulus:
GPa
Flexural
strength:
MPa
Flexural
modulus:
GPa
760–1030
41.0
1448
41.0
S-2 glass
1.80
1690.0
52.0
–
–
Aramid 58
1.45
1150–1380
70–107
–
–
Carbon (PAN)
1.60
2689–1930
130–172
1593
110.0
Carbon (Pitch)
1.80
1380–1480
331–440
–
–
Table 4 Typical tensile mechanical properties of long directionally
aligned fibre-reinforced composites (fibre weight fraction 65%)
manufactured by automated process (the matrix material is epoxy)
(adapted from Hollaway and Head, 2001)
surface finish on both sides, design flexibility and capability
of integrating a large number of components into one part.
The technique is sometimes used to manufacture bridge
street furniture.
There are three main industrial methods, all of which are a
variation of the general RTM method; these are the Seemann
composites resin infusion manufacturing (SCRIMP)
method, the VA-RTM (vacuum-assisted RTM) and the
TERTM2 method.
Randomly orientated laminate
90°
75°
Cross-ply laminate
60°
Strength or stiffness
Material
45°
30°
15°
Strength or stiffness
Unidirectional
laminate
Minor axis of
laminate
90°
Plane of
strengh
0°
Major axis of laminate
Figure 8 The relationship between the stiffness–strength at angle to the
major axes (08 direction) for the different types of polymer–fibre
composites
latter being equivalent to the matrix stiffness. Conversely,
the randomly orientated fibre composite gives equal properties in all directions from 08 to 908 axes direction.
The long-term durability of the
composite
The civil engineering environments which cause the major
concern for the durability of FRP composites are:
Mechanical properties of the component
parts of the composite
n moisture and aqueous solutions (alkaline environments)
The properties of the composite are highly dependent upon
the type of fibre used. The three fibres used in bridge
engineering, namely carbon, aramid and glass fibre, all
have different stiffness values and the higher the stiffness
of the fibre (Table 3) the greater will be the stiffness of the
composite; the orientation of the fibre also plays a large
part in the stiffness of the composite (see the section on
the Orientation of the fibre).
n thermal effects
The relative proportions of the fibre and
matrix
Moisture, aqueous and chemical solutions
The fibre is the load-carrying component of the composite
material, therefore the greater the fibre volume fraction
the stronger the composite.
The orientation of the fibre
The direction of the fibres – that is, either unidirectional
(anisotropic composite properties) or bidirectional (orthotropic (i.e. special type of anisotropy) composite properties)
angle ply or randomly orientated fibres (quasi-isotropic) –
will determine the strength and stiffness of the composite.
Figure 8 shows the relationship between the stiffness at
angle to the major axes (08 direction) for the different
types of polymer–fibre composites. It will be observed
that for the unidirectional fibre composite the major axes
(08 direction) of the composite give the greatest stiffness
and the 908 axes direction gives the least stiffness, this
n fire
n ultraviolet radiation.
Furthermore, the long-term loading conditions in which the
material may have to function are:
n fatigue
n creep.
Durability of FRP composites is one area which does
require further research in order to gain a greater understanding of the characteristics of the material over long
periods of time. All engineering materials are sensitive to
environmental changes in different ways which may lead
to their degradation over time. FRP materials are no
exception to this rule but they do offer some significant
durability advantages over the more conventional construction materials. One such advantage is associated with their
use as external reinforcement for reinforced concrete (RC)
or metallic bridge members as they have good resistance
to corrosion. A further example is the use of FRP rebars
in RC members. One of the problems in acquiring data
relating to the durability properties of any material is the
length of time involved in gathering the relevant information. This is particularly difficult with respect to construction polymer composites as there are many different
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Advanced fibre polymer composite materials and their properties for bridge engineering
polymers on the market and some of these have been
modified by chemists over the years to improve their performance and as a consequence it would be hoped their
durability performance as well. Moreover, modern structural FRP composites have been used in bridge engineering
construction for some 15 years only. The majority of those
that are used will have additives incorporated into them to
reduce the length of cure time or to improve some specific
mechanical or physical property. In effect, the addition of
additives to composites is equivalent to adding impurities
to the polymer; these additives tend to reduce their mechanical properties. Consequently, many of the FRP composite
bridge components that are being, or have recently been,
erected are monitored for any changes in their stiffness
characteristics or any signs of degradation. Indeed, bridge
structures are visually inspected at regular intervals and
any degradation which has occurred will be reported and
rectified accordingly.
To obtain the durability characteristics of a composite as
quickly as possible, sometimes test specimens are exposed
to an accelerated test regime which generally involves the
specimens being subjected to an environment many times
more severe than that which would be experienced in
practice. In addition, these test samples are exposed to an
elevated temperature to further increase degradation. This
method of accelerating the degradation of polymer composites should be used with caution. High temperatures are
not relevant for most civil engineering polymer composites
that operate at normal environmental conditions; furthermore, the degradation mechanisms in FRP materials at
high temperatures are different to those under the lower
practical temperatures. It will also be realised that as the
environmental temperature rises towards the glass transition temperature of the polymer, the latter will lose some
of its stiffness and strength with the result that the
investigation will not be analysing the original material.
The accelerated test procedures for obtaining durability
data under one environmental situation will generally not
be equivalent or relevant to the more gradual degradation
effect had the environment been applied in a less rigorous
manner. Moreover, materials used in construction would
normally be exposed to many different environments
acting simultaneously but in a less harsh way, and each
environment possibly having an effect upon the other.
Monitored field tests are the most relevant tests to establish the durability characteristics of polymer composites
used in bridge construction but the disadvantage of this
method of obtaining information is the length of time
involved. Hollaway (2007) (and see also Hollaway (2008a,
b) where the following discussion has been reported due
to the importance the author attaches to the understanding
and interpretation of the accelerated test results) has discussed many field surveys of bridges and other structures
which have been and are being undertaken throughout
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the world. The results from these tests have revealed or
are revealing interesting discoveries regarding the resistance
of FRP composites to the natural or specific environments.
One interesting example is associated with the degradation
of glass-fibre-reinforced polymer (GFRP) rebars in concrete. Accelerated laboratory test results of GFRP in a
simulated concrete pore water solution of high pH values
and at elevated temperatures up to 808C have indicated
that there is a decrease in the tensile, shear and bond
strengths for that material (Bank and Gentry, 1995; Bank
et al., 1998; Sen et al., 2002). Uomoto (2000) has concluded
that these results would suggest that there is a case for not
using GFRP rebars in concrete. However, Tomosawa and
Nakatsuji (1997) have shown that after 12 months’ exposure to alkaline solutions at a temperature between 20
and 308C, and Clarke and Sheard (1998), likewise, after
two years’ exposure to a tropical climate on a test platform
off the Japanese coast, have reported that there had been no
material or physical deterioration to the GFRP composite.
Furthermore, Sheard et al. (1997) reported that the overall
conclusions of the work of the EUROCRETE project were
that GFRP is suitable in a concrete environment. In the
section on Internal reinforcement to concrete members in
the next chapter, a discussion is given of the tests undertaken
by ISIS, Canada Research Network of Centres of Excellence,
on the reliability of five GFRP RC structures to provide
information on the reliability of GFRP materials.
Methods to improve the durability of FRP composite
materials in the civil infrastructure have been discussed in
Hollaway (2007), and will not be repeated here.
Fire behaviour of polymer composites
A major concern of the engineer using polymers in the
construction industry is the problem associated with fire.
The polymer component of the composite is an organic
material and is composed of carbon, hydrogen and nitrogen
atoms; they are flammable to varying degrees. It is possible,
however, to incorporate additives into the resin formulations, to add nano-clay particles to the pristine polymer,
Hackman and Hollaway (2006), or to alter the chemical
structure of the polymer, thereby modifying the burning
behaviour and producing a composite with a much
enhanced fire property. Nevertheless, fire can damage composites and indeed all civil engineering materials. Many
composites used in bridge engineering have high fibre
volume fractions therefore the progress of the fire is reduced
considerably as the fibre does not burn and the penetration
progress of the fire into the interior of the composite
through the burning of the polymer is much reduced.
FRP composites are widely used in the rehabilitation of
bridge beams (see sections on Adhesive bonding of polymer
composites to steel adherends to Seismic retrofit of RC
structures in the next chapter) and bridge columns (see
section on FRP confining of concrete in the next chapter)
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Advanced fibre polymer composite materials and their properties for bridge engineering
which are constructed from either reinforced concrete or
metallic materials. The potentially harmful effects of fire
and high temperature on FRP-strengthened reinforced
concrete and metallic bridge structures are well recognised
in the literature, although few studies have been undertaken on this topic. The first researchers to investigate
FRP composites bonded to RC beams were Deuring
(1994) and Blontrock et al. (2001) and limited laboratory
experimental work has been undertaken on wrapped
cylindrical columns after exposure to elevated temperatures (see Saafi and Romine, 2002). Bisby et al. (2004)
undertook a numerical and experimental programme to
investigate the fire performance of FRP-wrapped fullscale reinforced-concrete columns in order to provide firedesign guidance. The numerical modelling is capable of
predicting the thermal and structural response of an FRPwrapped concrete column under exposure to a standard
fire.
Thermal effects
Coefficient of thermal expansion
The glass, aramid and carbon fibre components of fibre–
matrix composites have a much lower coefficient of thermal
expansion in the longitudinal direction than the matrix
material and will be in the region of 10 10ÿ6 /8C,
ÿ2 10ÿ6 /8C, and ÿ0:9 10ÿ6 /8C respectively, depending
upon the type of specific fibre; the value for the matrices are
given in Table 1. The fibres stabilise the composite system
and reduce its coefficient of thermal expansion to a value
near to that of the conventional material; the actual value
will depend upon the fibre volume fraction and fibre
array. In addition, the value of the coefficient of thermal
expansion will vary with the environmental temperature
range into which the composite is placed. Furthermore,
the degree of cross-linking of the polymer will also influence
the rate of thermal expansion.
Thermal conductivity
This property is particularly important when FRP composites are used in the production of FRP bridge decks
(see the section on FRP bridge decks in the next chapter).
The thermal conductivity of polymers is low and consequently they are good heat insulators (see the section on
In-service properties). Thermal-stress analyses have been
undertaken by Alnahhal et al. (2006), utilising the finite-element method to predict the failure mechanisms and the ‘fire
resistance limit’ of a superstructure of a bridge under
extreme thermal loading conditions. The results were verified by undertaking field results provided by the New
York State Department of Transportation. In addition,
damage simulations of the FRP deck as a result of snow
and ice clearance by snowplough were performed to investigate any possibility of bridge failure after damage had
occurred. Thermal simulations showed that FRP bridge
decks are highly sensitive to the effect of elevated temperatures. The FRP deck approached the fire resistance limit at
an early stage of the fire incident under all cases of fire
scenarios. The damage simulations due to a snowplough
showed minimal bridge failure under the worst-case
damage scenario. These results provide an insight into the
safety and reliability of the FRP systems after the stipulated
damage scenarios are considered. The paper provides discussions concerning the recommended immediate actions
necessary to repair the damaged region of the FRP deck
panels.
The creep of the composite
The mechanics of creep in fibre/polymer composite materials
are related to:
1 the deformation characteristics of the creep of the viscoelastic matrix (see the section on Creep characteristics of
polymers)
2 the near zero creep of the fibres
3 the stabilising effect of the fibres on the polymer as a
result of items 1 and 2
4 the progressive changes in the internal balance of forces
within the materials resulting from the behaviour of the
matrix, the fibre, the adhesion and the load transfer at
the fibre–matrix interface.
The deformational behaviour of an advanced polymer
composite is a function of the resin type, the fibre type
and its architecture, fibre volume fraction, direction of heat
flow, and service temperature. The mechanical behaviour
of composite materials can be established by applying constant loads over long periods of time; these investigations
may be defined as creep tests. The tests produce curves of
elongation against time at different stress levels and although
they are not able to produce data that may be converted
directly into stress–strain curves, constant time sections
through families of such creep curves have been used to
produce isochronous stress–strain curves (see the section
on Creep characteristics of polymers).
Tensile and compressive properties of
polymer composites
Tensile properties of composites
Table 4 gives typical properties of composites manufactured using long directionally aligned fibre reinforcement
of glass, aramid and carbon fibres with a fibre/matrix
ratio by weight of 65%. Typical tensile mechanical properties of glass fibre composites manufactured by different
techniques are given in Table 5; the effects that the methods
of fabrication have on the properties is clearly shown. The
variation of the composite tensile properties when the fibre/
matrix ratio is changed but the method of manufacture
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Advanced fibre polymer composite materials and their properties for bridge engineering
Flexural
strength:
MPa
Flexural
modulus:
GPa
Tensile
strength:
MPa
Tensile
modulus:
GPa
Wet lay-up
62–344
4–31
110–550
Spray-up
35–124
6–12
83–190
5–9
RTM
138–193
3–10
207–310
8–15
Filament winding
550–1380
30–50
690–1725
34–48
Pultrusion
275–1240
21–41
517–14 448
21–41
Method of
manufacture
6–28
Table 5 Typical tensile mechanical properties of glass fibre composites
manufactured by different fabrication methods (adapted from Hollaway
and Head, 2001)
and component parts of the composite remain constant is
shown in Table 6.
Composites under compressive loads
The integrity of both component parts of the composite is
far more important under a compressive load than under
a tensile load. Furthermore, local resin and interface
damage caused by compressive loading leads to fibre
instability which is more severe than the fibre isolation
mode which occurs in tensile loading. The mode of failure
for FRP composites subjected to longitudinal compression
will depend upon the type of fibre, the fibre volume fraction, the type of resin and may include fibre micro-buckling,
transverse tensile failure or shear failure. The compressive
strengths of CFRP and GFRP composite materials increase
as the tensile strengths increase, but the aramid fibres in
aramid-fibre-reinforced polymer (AFRP) composites
exhibit non-linear behaviour in compression at a relatively
low level of stress.
The value of the compressive modulus of elasticity of
FRP materials is generally lower than their tensile value.
Test samples containing 55–60% weight fraction of
continuous fibres in a vinyl-ester resin have compressive
modulus of elasticity values of approximately 80%, 100%
(but the compressive strength is very low) and 85% of the
tensile value for GFRP, AFRP and CFRP respectively.
Subramaniyan et al. (2003) have shown that by adding
nanoclays to the polymer the compressive strengths of
GFRP composites do increase.
Flexural
strength:
MPa
Flexural
modulus:
GPa
Tensile
strength:
MPa
Tensile
modulus:
GPa
Fibre/matrix
ratio:
%
Specific
weight
67
1.84–1.90
483
17.9
269
19.3
65
1.75
406
15.1
214
15.8
50
1.80
332
15.3
166
15.8
Table 6 Typical tensile mechanical properties of glass fibre/vinylester
polymer (compression moulding – randomly orientated fibres) (adapted
from Hollaway and Head, 2001)
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Adhesives
Structural adhesives used in bridge engineering are usually
epoxy systems and can be (1) a two-part cold cure, (2) a
two-part hot cure or (3) an adhesive film; (2) and (3) require
heat for polymerisation. A considerable amount of
bonding, particularly when upgrading bridge structures, is
required to be undertaken on site and therefore cold cure
adhesives are generally used; the adhesive should be postcured to a temperature of at least 508C. The Tg (see the
section on In-service properties) is then about 658C. If it is
not post-cured the Tg value will only be about 158C
above the ambient temperature of cure. If the bonding
operation is undertaken in a factory environment the hot
cure adhesive system is used and is cured under heat; the
Tg is then dependent upon the heat of polymerisation.
The cross-linked chemical structure of the adhesive renders
the polymer insoluble and infusible and these characteristics greatly reduce the creep of the adhesive (see the section
on Creep characteristics of polymers). To increase the
toughness of an adhesive polymer it is possible to blend,
fill or co-polymerise it with a tough polymer. This will
cause a reduction in strength of the adhesive and in this
situation it is necessary to compromise between strength
and stiffness. In the case of bonding of the FRP to
metals, or when bonding FRP materials to the top surface
of a bridge deck that is to receive hot bituminous surfacing,
an adhesive with a high Tg must be used (The Concrete
Society, 2000, 2003). Further information on adhesive bonding may be obtained from Mays and Hutchinson (1992).
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