If I were building a driveway, I would pour it 8 inches thick instead of
6 or 4 and put two layers of rebar in; one an inch below the top of the
slab and the other an inch above the bottom of the slab.
The reason why drywall is as strong and rigid as it is (considering what
it's made of) is because paper is very strong in tension. Try folding
up a sheet of paper so that it's 11 1/2 inches long an 1 inch wide and
pulling it apart by straight tension (rather than by tearing it) and
you'll see how strong paper actually is.
In order for drywall to bend, the paper on one side of the gypsum or the
other has to stretch to conform to the new shape, and it's the strenth
of the paper on each side of the drywall and it's resistance to
stretching that gives drywall it's strength and rigidity.
Reinforced concrete is designed exactly the same way. "Reinforced"
concrete consists of ordinary concrete with rebar inside it. If you
imagine a slab of concrete with two layers of rebar inside the concrete;
one near the top of the concrete and one near the bottom, then for that
slab to bend, the rebar on either side would have to stretch to conform
to the new shape.
But, since steel is very strong in tension, it's the steel's resistance
to stretching that would prevents the slab from bending, and therefore
cracking. That is, the slab could support very much MORE weight or
force on it before it bent far enough for the concrete to crack. The
steel rebar carries the load so that the reinforced concrete slab
DOESN'T bend far enough for the concrete to crack, and that's why
reinforced concrete is so much stronger than unreinforced concrete.
It's exactly the same reason why drywall is considerably stronger than
the gypsum core itself.
The problem is that when they pour a typical garage pad or drive way,
THEY DON'T put the rebar where it's needed, which is at the top and
bottom surfaces of the slab so it's in the proper location to resist
bending of the slab. Instead, they just put a layer of rebar in the
MIDDLE of the slab; equidistant from the top and bottom of the slab
where it's unable to prevent the slab from bending. So, the slab bends
easily to the point where the concrete cracks, and for $50 more for a
second layer of rebar and $100 more in labour to install that second
layer of rebar before pouring the concrete, you could have prevented
that driveway slab from cracking.
Really, when they put a layer of rebar in the middle of a slab, the
purpose is NOT to strengthen the concrete like it would if you had two
layers. The purpose of that single layer of rebar in the middle of the
slab is simply to hold the pieces of slab together AFTER it cracks. The
idea is just to prevent any cracks from spreading. TWO layers of rebar
would also prevent any cracks from spreading, but two layers properly
located would go a far sight further to prevent the slab from cracking
in the first place.
If you made drywall the same way they pour your average driveway, what
you'd have is a layer of paper sandwiched between two 1/4 inch thick
layers of gypsum. Something like that is gonna break if you breathe on
it too hard. Putting the paper on the outsides makes the drywall strong
and rigid because it takes a lot of force to stretch paper. Ditto for
steel, and putting steel on the outsides of a concrete slab would make
for a very much stronger and more rigid slab of concrete; one much less
likely to bend to the point where it cracks.