Constituent Materials and Properties - Strengthening Design of Reinforced Concrete with FRP - page 41

3 Constituent Materials

and Properties

3.1 OVERVIEW
Fiber-reinforced polymer (FRP) is composed of two material phases: fiber and
polymer matrix. Fibers are impregnated into the polymer matrix to form a macro-
scopically orthotropic layer of material with distinctly higher mechanical properties
along the fiber direction compared to the transverse directions. The advantages of
using FRP are the high strength and stiffness-to-weight ratio along the fiber direc-
tion, ease of application in construction due to its light weight, corrosion resistance,
electromagnetic inertness, and design versatility in which high strength and stiff-
ness (fibers) may be oriented where needed in design. Continuous fibers become
extremely strong and stiff as fiber diameter becomes smaller due to the reduction and
sometimes elimination of defects in the microstructure, as illustrated in Figure 3.1.
On the other hand, small-diameter fibers are not capable of carrying axial compres-
sion or shear stresses due to the lack of shear transfer medium between them. Thus,
the fibers are embedded into a polymeric matrix that binds them together and allows
load transfer by shear among the fibers. Additional specifics about fibers and matrix
are described in the following sections.

3.2 FIBERS
Fibers are typically made of glass, carbon, and aramid. Other synthetic fibers are
made of polymers which are not used in structural applications due to their low
mechanical properties. Glass fibers are primarily composed of silicon dioxide with
some modifying agents (Gibson 1994). E-glass (electrical glass) accounts for the
largest production of glass fibers in industry due to its low cost despite its mechani-
cal properties that are lower than other grades of glass fibers, as shown in Figure 3.2.
On the other hand, S-glass (structural glass) is more expensive to produce, but it
has significantly higher strength and slightly higher modulus, as shown in Table 3.1
(Gibson 1994) and Table 3.2 (Hyer 1998). C-glass (chemical glass) has an improved
durability against alkali and acid attacks.
Carbon fibers or graphite fibers are the most widely used fibers in industry due to
their high stiffness and strength as well as environmental stability. Carbon fibers con-
tain less than 95% carbon, while graphite fibers have at least 99% carbon (Schwartz
1984). Graphite and carbon fibers are still the most expensive fibers on the market,

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