Salt (more properly, chloride ion from whatever source, i.e. sodium chloride
or calcium chloride) used for deicing is the primary factor causing the
failure of concrete structures on highways. Also, you'll find contractors
who'll want to add calcium chloride to the mix to accelerate strength gain
so that they can get the finishing over with and move on to the next job.
Except in emergency repair situations, DOT's generally won't allow the use
of calcium chloride in concrete. There are accelerators that don't cause
corrosion, but they're more expensive than calcium chloride.

In a normal concrete mix, reinforcing steel will quickly form a passive
coating and no rusting will occur for many, many years. If you apply
chloride deicers, the chloride ions will gradually work their way down to
the reinforcing steel. When the concentration of chloride ion gets high
enough at the surface of the steel, the passive coating breaks down and
rusting ensues. Since the rust takes up considerably more space than the
steel it was produced from, the pressures can become high enough to rupture
the concrete overlaying the steel (much like a tree root opening a crack in
a rock). Concrete bridge decks fail at the highest rate since they have so
much steel and more deicer is applied to them because ice forms there first
but the rest of the bridge suffers also from splash and seepage.

Concrete slabs will crack eventually and it will happen more quickly if you
haven't added sufficient expansion joints. Since you can't totally prevent
the cracks, your best bet is to use reinforcing steel so that the cracks
won't go anywhere once they form. If you put really heavy loads on the
driveway, you'll have to include load transfer steel at each expansion
joint.

If you absolutely must use large amounts of chloride deicers, then consider
reinforcing with epoxy coated rebar. If you use epoxy coated rebar, you'll
have to be on your toes to ensure that the epoxy coating isn't damaged
during placement of the steel and you'll have to inspect the epoxy coating
before placement to be sure it wasn't damaged (chipped, cracked) during
shipping/handling. Damaged epoxy coating can sometimes cause more problems
than not having epoxy coating at all.

Harry

"BobK207" wrote in message
oups.com...

pe wrote:
On Sun, 07 Jan 2007 23:30:41 GMT, "George E. Cawthon"
wrote:

wrote:
On Sun, 7 Jan 2007 13:10:56 -0600, "Steve Barker"

wrote:

Rebar, lots and lots of rebar. And concrete 6" thick. Did i mention
rebar?

Except lots of rebar can also mean lots of spalling. Rust requires
more room than
steel and can explode the concrete.


I think you are joking. If not, the rebar should
be in the lower third of layer. How do you
explain the lack of spalling on exposed concrete

Do a little net search.
Spalling is an IMMENSE problem in concrete structures.

pillars, e.g., bridge supports, basement supports,
etc. since they have lots of rebar.

They generally know what they are doing.

In Quebec recently an overpass collapsed killing people. Spalling is the
probable
cause.

Spalling in concrete on the ground can be very bad due to the moisture.
There is such
a thing as TOO much rebar.



Do a little net search.
Spalling is an IMMENSE problem in concrete structures.

Rebar corrosion....one of the causes of spalling

is strongly dependent on the thickness of the cover (amount of concrete
over the rebar), soundness of the concrete & the local environment.

a marine environment is probably one of the worst as is one where snow
removal chemicals are used

general cover suggestions

air 2"
soil 3"
marine environment a lot more

another cause of spalling is freeze thaw but I doubt we're concerned
with this on the OP's driveway

plus to add "too much rebar" to a slab it would have to be quite a
bit....I'm sure the OP's driveway project will get less than minimum
temperature (if it get's any)

btw I'd wait until some investigation work has been done before
suggesting a reason for the Quebec overpass collapse you mention.

This isn't the first collapse.

cheers
Bob