TM 5-822-7/AFM 88-6, Chap. 8
C-3. Fibers. Fiber-reinforced concrete has been
C-5. Testing. Fiber-reinforced concrete requires
made with fibers of many different types of material.
some modification in testing. The flexural strength
should include the results of loading past the forma-
However, only steel fibers have been used in pavement
tion of the initial crack. The conventional slump test
applications. These steel fibers are typically either
is not an effective evaluation of workability for
cut from wire or sheared from sheets. Typical fibers
fiber-reinforced concrete. Either Vebe or inverted
are 0.01 to 0.02 inch in diameter and 1 to 2 inches
long. The fibers may be straight or the ends maybe
cone tests are superior to the conventional slump
tests. ACI 544.2R should be consulted whenever
deformed. Fibers with deformed ends are said to
provide better anchorage. Ultimate tensile strength
tests for fiber-reinforced concrete are planned.
of available fibers varies from 50,000 to 300,000 psi.
C-6. Construction. Before vibration, steel-fiber-
The maximum fiber aspect ratio (length divided by
reinforced concrete mixtures appear harsh and
diameter) should usually not be over 100 to avoid
unworkable, and a common mistake is to add water
mixing problems. Also, volume contents of over 2
in a vain attempt to obtain workability. Conventional
percent are often difficult to mix.
form riding and slipform paving equipment is ade-
C-4. Fiber clumps. If fiber clumps occur in the mix, it
quate to place and level fiber-reinforced concrete.
is usually due to fibers being added too fast at some
Use of a jitterbug, rollerbug, or a vibrating screed
point in the mixing operation so that they are already
will help embed fibers into the concrete, but there is
clumped by the time they get into the mixture. Usually
danger of over-using these techniques and floating
if the fibers get into the mixture without clumps, no
paste to the surface with very adverse effects on
clumps will form. Fibers should not be allowed to pile
durability. At least one agency has successfully
up anywhere in the operation, and the mixer should
specified a maximum of 18 exposed fibers per square
disperse the fibers into the mixture as rapidly as they
yard of pavement, and required the contractor to
are added. Other potential causes of fiber clumping
demonstrate the ability to meet this in trial sections.
are mixtures with too many fibers, using fibers with
A broom finish is generally used for texturing. A
too large of an aspect ratio, adding fibers too fast to a
burlap drag should be avoided to prevent fibers from
harsh mix, adding fibers first to the mixer, using
catching in the burlap and tearing the surface.
equipment with worn out mixing blades, overmixing,
Contraction joints should be saw cut 1/3 to 1/2 the
and using a mixture with too much coarse aggregate.
depth of the slab to assure proper fracturing of the
A common method of adding fibers to a mix is to add
joint. Recommended joint spacings for fiber-reinforced
the fibers through a shaker or hopper onto the fine
concrete are shown in Table C-2. Other than these
aggregate on a conveyor belt. Some fibers are also
points, normal pavement construction and curing
collated in bundles of about 30 fibers with a water-
techniques are adequate for steel-fiber-reinforced
soluble glue. These may be added to a mix as the last
step in the mixing process.