COMPOSITES - Materials for Civil and Construction Engineers - page 450

compete with, and in many cases are preferred over, conventional building
materials. Composites are used by civil engineers as structural shapes in
buildings and other structures and can replace steel and aluminum structural
shapes (Figure 11.5). Table 11.2 provides an example of physical properties of
fiber-reinforced composite round rods and bars.
Fiber-reinforced polymer (fiberglass) rebars can also be used for con-
crete reinforcement instead of steel rebars. Composites have been used for
tanks, industrial flooring, trusses and joists, walkways and platforms, waste
treatment plants, handrailings, plastic pipes, light poles, door and window
panels and frames, and electrical enclosures. Composites can also be used
to strengthen and wrap columns and bridge supports that are partially
damaged by earthquakes and other environmental factors (Grace 2002)
(See Figures 11.6 - 11.9).
Fiber-reinforced concrete is another composite material that has been
used by civil engineers in various structural applications. Different types of
fibers, such as separate fibers, chopped-strands, or rovings, can be used to re-
inforce the concrete. If separate fibers or chopped-strands are used, they are
mixed with the fresh concrete in a random order. In such a case, fibers hinder
or impede the progression of cracks in concrete. Figure 11.10 shows a scan-
ning electron micrograph of concrete mortar mixed with about 3-mm-long car-
bon fibers at a volume fraction of 12%. Fiber rovings, on the other hand, are
placed in the direction in which the tension is applied in the structural mem-
ber. In this case, fibers carry the tensile stresses. In general, fibers increase the
tensile and flexure strength of concrete so that a more efficient structural
member can be designed. Table 11.3 shows typical ranges of physical prop-
erties of glass fiber-reinforced concrete at 28 days. Research has shown that
glass fiber-reinforced concrete offers two to three times the flexural strength
of unreinforced concrete. Moreover, the material under increasing load does
not fail abruptly, but yields gradually. This gradual yielding occurs because
FIGURE 11.5 Structural
shapes made of fiberglass com-
posites. Courtesy of Creative
Pultrusions, Inc.