Civil Engineering Reference
In-Depth Information
CHAPTER 7
Fibers
Fibers have been used in construction materials for many
centuries. The last three decades have seen a growing
interest in the use of fibers in ready-mixed concrete, pre-
cast concrete, and shotcrete. Fibers made from steel,
plastic, glass, and natural materials (such as wood cellu-
lose) are available in a variety of shapes, sizes, and thick-
nesses; they may be round, flat, crimped, and deformed
with typical lengths of 6 mm to 150 mm (0.25 in. to 6 in.)
and thicknesses ranging from 0.005 mm to 0.75 mm
(0.0002 in. to 0.03 in.) (Fig. 7-1). They are added to concrete
during mixing. The main factors that control the perform-
ance of the composite material are:
1. Physical properties of fibers and matrix
2. Strength of bond between fibers and matrix
Although the basic governing principles are the same,
there are several characteristic differences between con-
ventional reinforcement and fiber systems:
1. Fibers are generally distributed throughout a given
cross section whereas reinforcing bars or wires are
placed only where required
2. Most fibers are relatively short and closely spaced as
compared with continuous reinforcing bars or wires
3. It is generally not possible to achieve the same area of
reinforcement to area of concrete using fibers as com-
pared to using a network of reinforcing bars or wires
Fibers are typically added to concrete in low volume
dosages (often less than 1%), and have been shown to be
effective in reducing plastic shrinkage cracking.
Fibers typically do not significantly alter free
shrinkage of concrete, however at high enough dosages
they can increase the resistance to cracking and decrease
crack width (
Shah, Weiss, and Yang 1998
).
ADVANTAGES AND DISADVANTAGES
OF USING FIBERS
Fibers are generally distributed throughout the concrete
cross section. Therefore, many fibers are inefficiently
located for resisting tensile stresses resulting from applied
loads. Depending on fabrication method, random orienta-
tion of fibers may be either two-dimensional (2-D) or
three-dimensional (3-D). Typically, the spray-up fabrica-
tion method has a 2-D random fiber orientation where as
the premix (or batch) fabrication method typically has a
3-D random fiber orientation. Also, many fibers are
observed to extend across cracks at angles other than 90º
or may have less than the required embedment length for
development of adequate bond. Therefore, only a small
percentage of the fiber content may be efficient in resisting
tensile or flexural stresses. “Efficiency factors” can be as
low as 0.4 for 2-D random orientation and 0.25 for 3-D
random orientation. The efficiency factor depends on fiber
length and critical embedment length. From a conceptual
point of view, reinforcing with fibers is not a highly effi-
cient method of obtaining composite strength.
Fiber concretes are best suited for thin section shapes
where correct placement of conventional reinforcement
would be extremely difficult. In addition, spraying of fiber
concrete accommodates the fabrication of irregularly
shaped products. A substantial weight savings can be real-
ized using relatively thin fiber concrete sections having
Fig. 7-1. Steel, glass, synthetic and natural fibers with dif-
ferent lengths and shapes can be used in concrete. (69965)
