Constituent Materials and Properties - Strengthening Design of Reinforced Concrete with FRP - page 44

FIGURE 3.3 Carbon fiber yarn and carbon and glass fiber sheets used in strengthening.

despite the considerable drop in their prices, because of the high demand for these
fibers (Reinhart 1990), as seen in Figure 3.3. Aramid polymeric fibers, which also
have the trade name of Kevlar ® , are used in structural applications as well. Aramid
has approximately half the density of glass with very high strength, toughness, duc-
tility, and impact resistance (Gibson 1994).
Boron fibers are composites made from coating a substrate of carbon or tungsten with
boron. They are as heavy as glass, as demonstrated in Table 3.1 (Gibson 1994), and
expensive to produce.

3.3 MATRIX
The matrix in a composite plays various roles such as holding the fibers into the
composite part shape, protecting fibers from direct exposure to the environment,
transferring the stresses through the fiber-matrix interface to the fibers, and resist-
ing some of the applied load, especially transverse normal stresses and interlaminar
shear stresses (Barbero 2011). The application of a composite is limited by the prop-
erties of its matrix. The thermal stability and useable service temperature as well as
chemical resistance, moisture resistance, and abrasion resistance are all dependent
on the matrix and its properties. Certain conditions, such as moisture, act on lower-
ing the glass transition temperature ( T g ) of the polymer matrix, thus significantly
degrading the composite when the operational temperature exceeds T g .
The matrix transitions from its operational state, where it is stiff and glassy, to
a soft rubbery state once T g is exceeded (Hyer 1998). In general, matrix materials
can be made of polymers or resins, metals, or ceramics. The polymer matrix is the
most common among matrix materials because of the ease of manufacturing com-
plex components and relatively inexpensive tooling (Barbero 2011). This text focuses