|
 Some
countertop contractors use other reinforcement materials,
such as expanded metal, and report good results. Some
experts, however, express concern related to the variable
properties of some of these materials, especially when
they are used for primary reinforcement.
Fibers are frequently used in concrete countertop mix
design, but reinforcing fibers should only be used as
secondary reinforcement. Most fibers are synthetic polypropylene
or nylon - and so they are physically unable to provide
significant, tensile reinforcement. What a matrix of
fibers in the mix can do is to help stabilize the wet
concrete as it cures. The fibers help by distributing
shrinkage stresses thereby minimizing or eliminating
or eliminating large cracks.
 But
Ralston cautions that fibers can be problematic if they
clump up and are not mixed or dispersed well.
Some fibers are structural, such as hooked steel fibers,
chopped carbon fibers, polyvinyl alcohol and chopped
alkali-resistant glass fibers. While these individual
fibers are strong, and they improve the mechanical properties
and tensile strength of concrete, Girard cautions they
are not a replacement for reinforcing steel.
Placing Reinforcement
Where you need to place reinforcement depends on your
countertop. In Girard's view, because countertops are
beams, they should all have the reinforcement placed
in the bottom. "There should he between 1/4 inch
and no more than 3/8 inch of concrete between the bottom
of the countertop and the reinforcement. And there should
be at least 1/2 inch of concrete between the reinforcement
and any edge," he explains.
 Ralston
has a different Lake. He believes that the plastic shrinkage
cracking begins at the top of the stab, thus the reinforcing
should be closer to the top to hold together any cracking
right from the onset.
What if your countertop is also acting as a cantilever?
You will definitely need reinforcement at the top. Girard
recommends placingg the top reinforcement between 1/4
inch and 3/8 inch from the finished top. The recommendation
remains the same even if you will be grinding the top.
"Usually exposed aggregate countertops are ground
down a maximum of 1/8 inch to expose the aggregate,
so the reinforcing would start out 3/8 inch hour the
top and end up
1/4 inch from the top after grinding is complete,' Girard
explains.
Because it is relatively flat and strong, Karmody likes
to use wire mesh in both bottom and top placemetals.
When he knows where a cantilevered section will go,
Buddy Rhodes, of Buddy Rhodes Studio, uses a two-layer
approach to beef up the countertop. "The two-layer
approach is to put steel reinforcing (ladder wire) near
the bottom of the slab to counteract the flexural forces
at the bottom of the countertop and carbon fiber grid
at the top to counteract the flexural forces produced
when the cantilevered section has weight on it All this
is done in the same pour. He explains that for a right-side-up
mold, you pour about a third of the thickness of the
slab and place the ladder wire, then pour another third
and place the grid, then top off the last third and
strike off, then trowel smooth. He says, conversely,
an upside-down mold would have the grid first and the
steel reinforcing in the top (bottom) of the slab.
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