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Frequently Asked Questions
Cement & Concrete
Technology Home > FAQs > Fibers
Q: Can fibers replace wire mesh or welded wire fabric in a concrete slab
on ground?
 A:
To answer this question, it is first necessary to establish why
wire mesh or fibers would be used in a concrete slab. One of the
main reasons is for crack control. PCA, however, states that cracks
in slabs on ground can be controlled with uniform subgrade support
and proper joint spacing (as a function of slab thickness). See
Table 6-3 from PCA’s
Concrete Floors on Ground (EB075).
Table 6-3. Reinforcement for Floors on Ground
| Is reinforcement necessary? |
| NO |
With uniform support and short joint spacing |
| YES |
When long joint spacing is required
When joints are unacceptable in floor use |
 Although
short joint spacings alleviate the need for reinforcement, wire
mesh will allow for increased distance between joints if correctly
placed in the upper portion of the slab, at least 2 in. below the
surface. In these slabs with longer joint spacings, the purpose
of wire mesh is to hold random intermediate cracks tight.
Plastic fibers should not be expected to replace wire mesh in a
slab on ground. However, although not affecting joint spacing, plastic
fibers are used to reduce plastic shrinkage cracking.
Plastic shrinkage cracks are those that occur immediately following
concrete placement, before the concrete has hardened. Plastic fibers
are commonly dosed at 0.1% by volume for slabs on grade; this is
equivalent to 0.9 kg/m3 (1.5 lb/yd3). Polypropylene
fibers are among the most common for controlling plastic shrinkage
cracking. There are many types of fibers in addition to polypropylene,
including other plastic materials, such as nylon. Fibers serve different
purposes based on the characteristics of the material from which
they are made. For instance, steel fibers provide high flexural
strengths and impact resistance and are found in heavy-duty industrial
floors.
For additional information on fibers for use in concrete, see Fibers(IS532)
and Fiber Reinforced
Concrete
(SP039)
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