nate different properties in different directions,
Industrial applications of
rendering it anisotropic. Commonly used fibers
FRP composites
There has been a rapid growth in the use of FRP
are glass, carbon, and aramid. Commonly used
materials in civil engineering applications during
matrices are polyester, vinyl ester, epoxy, or phe-
the last several years. When this project was con-
nolic.
ceived several years ago, the trend of progressive
A comparison of various composites vs. struc-
replacement of carbon steel and aluminum as
tural steel is presented in Table 1. Many compos-
structural materials in many applications was
ites are in fact stronger than structural steel, but
obvious. Today the composite industry produces
their weight could be only a fraction of that of
more than 1.5 billion kg (3.3 billion lb) of compos-
steel. Their modulus of elasticity is also lower than
ites in a wide array of products (Busel 1995). The
steel. Also, as mentioned before, composites do
potential of composites in construction applica-
not usually exhibit yield, and they break at a strain
tions, including repair and rehabilitation of civil
much lower than steel. Creep is practically negli-
infrastructure, has been fully understood. The use
gible for FRP composites, although Kevlar com-
of composites has recently been widely demon-
posites may show noticeable creep over long time
strated in building systems, marine/waterfront
periods. Composites are also well known for their
structures, repair and rehabilitation, corrosion
fatigue resistance. Fiberglass composites, which
reduction, and structural alternatives for utilities
are a common choice for general applications in
(Busel 1995).
structures because of its relatively low cost, do
The recent trend in the increasing use of FRP
not have a fatigue limit comparable to steel. The
composites has been spurred by the need to over-
impact resistance of composites is difficult to de-
come the effects of corrosion on traditional mate-
fine and cannot be compared well with conven-
rials, such as steel rebars. The use of resin impreg-
tional materials, as the failure mechanism in com-
nated fiberglass, carbon fiber, or aramid rods has
posites is complex and totally different from steel,
been studied extensively both within the U.S. and
as discussed before. Carbon composites may have
abroad (Meier and Kaiser 1991, Kim and Meier
a negative thermal expansion coefficient; in gen-
1991, Rizkalla and Abdelrahman 1995). Notable
eral, the thermal expansion coefficients are prima-
s u c c e s s e s of these applications have been
rily controlled by the fiber architecture (layering)
reported. Lately, "all-composite" bridge struc-
of the composite.
tures have been designed and built at several
places.
Disadvantages of composites as
structural construction materials
At the present time, the primary obstacle to the
Differences between FRP and
wide application of composites is higher material
traditional materials
Most traditional construction materials are ho-
cost. Moreover, the fabrication techniques do not
mogeneous and isotropic, which means they have
follow any well-established civil engineering or
the same properties in all directions. As stated
construction practice; rather, they are constantly
before, composites are made of fibers embedded
evolving. The anisotropy of the material proper-
in a matrix and commonly placed in layers to form
ties, often leading to unfamiliar analysis tech-
laminates. The fibers may be oriented in different
niques for civil engineers, has added to the prob-
directions in different layers. This gives the lami-
lem. In general, the lower values of Young's
Table 1. Representative properties of FRP composites.
Characteristics
Materials
Matrix
Epoxy
Polyester
Vinyl ester
Vinyl ester
Phenolic
Epoxy
Steel
Reinforcement
Graphite
Graphite
Glass roving
Chopped glass
Glass fiber
Kevlar
Fiber vol. fraction (%)
62
62
75
28
62
62
Tensile strength (psi)
278,000
220,000
55,000
13,000
44,000
72,000
50,000
Elongation (%)
1.4
1.1
0.8
12
Flexural strength (psi)
254,000
272,000
111,000
28,000
66,000
58,000
40,000
Flexural modulus
17.1
17.7
3.8
1.6
3.5
3.3
30
( 106 psi)
SI conversion factor: 1 psi = 6.89 kPa
7
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