|  Ralston,
who casts-in-place Most Of tile time, uses another technique:
"If we have large cantilevers extending beyond
the countertop cabinet more than 10 inches, we use small
pieces of L bar to connect the countertop and then fasten
wire and rebar to that." if the counter-Lop extends
beyond 12 inches, Ralston sometimes recommends the use
of corbels.
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| Galvanized wire used for reinforcement
in a backsplash. |
One could argue that cast-in-place countertops are
not beams because there is plywood supporting them.
However, if you think about it, even in construction,
plywood isn't a structural component, it is flexible.
Also, one could point out that cast-in-place countertops
are not moved about like precast countertops. Girard
emphasizes that the same engineering principles apply
to cast-in-place, but he concedes reinforcement may
not be as critical - shrinkage control and mix design
are the dominant issues.
Dave Pettigrew, owner of Diamond D Co., cases countertops
in place, he stresses the importance of good mix design,
including the use of angular rock instead of pea gravel.
"Angular rock interlocks and provides strength,'
he says. Still, Pettigrew uses a variety of reinforcement
materials in his countertops.
How does reinforcement affect the final appearance?
Well, it shouldn't.
Some contractors are worried about rust forming the
reinforcement isn't galvanized or epoxy coated Karmody
says he actually doesn't mind a little rust because
it helps the concrete and steel create a better bond.
What about ghosting? This phenomenon is frequently
attributed to having the reinforcement too close to
the surface. Actually, reports Girard, ghosting (or
shadowing) occurs when the reinforcement is pushed down
into the concrete or the concrete is poured through
the reinforcement. "This pushes the aggregate (or
larger sand grains) aside and forms a trench that fills
in with the finer particles of the cement paste. The
result is that the physical composition of the concrete
above the reinforcing is different than the rest of
the concrete." Because those lines of concrete
are different in composition, they will cure differently
and take stain differently.
|
Countertops
as Beams: Where the Stresses Occur
A beam is a horizontal
structural member that spans some open space and
is supported near the ends. The beam can then
support some weight placed on top of it somewhere
between the end supports. A floor joist is a beam.
Concrete countertops are also beams.
When a beam has weight
placed on top of it, that weight causes the beam
to deflect (bend). Small weights on stiff beams
cause almost no deflection, while large weights
on flexible beams cause significant deflection.
The deflection in the beam causes two things to
happen: The top surface of the beam is compressed
and tries to get shorter, and the bottom surface
is in tension and tries to get longer.
Between the two, something
important occurs. Compression is the opposite
of tension, so as one progresses down the beam
from the top surface to the bottom, the compression
stress gradually decreases to zero and then the
stresses reverse, go into tension and gradually
increase towards the bottom of the beam.
If an unreinforced
beam has an asymmetrical cross-section (like a
rectangle), the stress switch occurs at the midpoint
between the upper and lower faces. This is important
because, given that there is no tension or compression
stress at the midpoint of a countertop, placing
reinforcing steel there does absolutely no good.
The point at which this switch occurs is called
the neutral axis, and can be thought of as an
imaginary line that runs parallel to the length
of the beam.
If a countertop is
made out of concrete (with no reinforcement),
any significant weight placed on top of it will
cause it to fail at the bottom of the countertop
because the tension stresses in the bottom of
the countertop will exceed the tensile strength
of the concrete. A crack will form at the bottom
and progress upward literally at the speed of
sound.
Some argue that because
concrete countertops usually actually span only
the width of a cabinet box (usually a maximum
of 36"), they are rather short beams, and
therefore the stresses involved are not that
high. This is true, but what about when an
8-foot long precast slab is picked up in the
shop and loaded onto a truck for transportation?
The largest stresses and biggest risk of cracking
occur in the shop. Once the slabs are installed,
only settling of the cabinets or building would
impart much stress. |
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