Bob Morrison wrote:
In a previous post Matt Whiting says...

CF&SG is correct in that rebar and lower water content will improve
structural performance. However, only the latter will have any effect
on crack control.

Yes, but rebar will help prevent problems is there is a soft spot in the
ground under the slab that settles a little. It can also prevent cracks
from opening up or shifting vertically.




Matt:

Quite true. However, I tell the contractors to spend their time and
effort on subgrade preparation. It's easier to do than correctly
placing rebar, and with the current price of steel may even be cheaper.

A good unreinforced slab on properly prepared subgrade with adequate
crack control should not have any problems.

Yes, unfortunately, a soft spot can go quite deep and be sizeable. It
may not even be noticed by most excavators. Unless you dig down 6' and
fill with gravel, it is hard to be sure that you won't have problems in
the future. This depends greatly, obviously, on location and soil type.
Here in PA, it isn't unusual to have challenging soil conditions. I
figure a few bucks worth of rebar is cheap insurance. You still want
proper preparation, however, I like suspenders with my belt for things
that are hard to fix later. And cured concrete that has cracked and
heaved an inch isn't cheap to fix.

Matt

RicodJour wrote:

Bob Morrison wrote:

In a previous post Matt Whiting says...

CF&SG is correct in that rebar and lower water content will improve
structural performance. However, only the latter will have any effect
on crack control.

Yes, but rebar will help prevent problems is there is a soft spot in the
ground under the slab that settles a little. It can also prevent cracks
from opening up or shifting vertically.

Quite true. However, I tell the contractors to spend their time and
effort on subgrade preparation. It's easier to do than correctly
placing rebar, and with the current price of steel may even be cheaper.

A good unreinforced slab on properly prepared subgrade with adequate
crack control should not have any problems.


NY Code for concrete cover of reinforcing

TABLE 1907.7.1
MINIMUM CONCRETE COVER


MINIMUM

COVER

CONCRETE EXPOSURE inches



1. Concrete cast against and permanently exposed to earth 3

2. Concrete exposed to earth or weather

No. 6 through No. 18 bar 2

No. 5 bar, W31 or D31 wire, and smaller 1-1/2


If I'm reading that right, and you can figure out a way to cast the
driveway above grade and then lower it into place (!), you can go with
as little as a 3" slab. Otherwise you need 6" if it's cast in place
and reinforced.

Weird thing. If you _reinforce_ it, code requires a thicker slab!

Bob's got it right, the slab doesn't move unless something underneath
it does. Preparation - drainage, compaction, etc. - is the key.

Absolutely correct, but the problem is that you can't guarantee that
something won't move. Thus a little reinforcement is, IMO, cheap insurance.

Matt

RicodJour wrote:

Again with the math! You're such an engineer sometimes.

They allow some leeway with the coverage - I believe it's +/- 3/8".

Oh, and coming from a woodworking background, I half-lap my rebar so
it's only one layer thick.

And do you sand those nasty ridges off the rebar to make them smooth as
you would with wood? :-)


Matt

Matt Whiting wrote:
Rico

Bob's got it right, the slab doesn't move unless something underneath
it does. Preparation - drainage, compaction, etc. - is the key.

Absolutely correct, but the problem is that you can't guarantee that
something won't move. Thus a little reinforcement is, IMO, cheap insurance.

Much like the single guy with no dependents buying life insurance.
It's cheap, but is it necessary? Too many people think that it is a
panacea for all problems concrete. It's not. Just like painting, slab
on grade results are all in the preparation.

R

Matt Whiting wrote:
RicodJour wrote:

Again with the math! You're such an engineer sometimes.

They allow some leeway with the coverage - I believe it's +/- 3/8".

Oh, and coming from a woodworking background, I half-lap my rebar so
it's only one layer thick.

And do you sand those nasty ridges off the rebar to make them smooth as
you would with wood? :-)

Hell, no! That would take foever and give inferior results. I turn
them down on the lathe.

Where have you been Matt? I'm guessing the spouse-enforced two week
vacation.

R

RicodJour wrote:
Matt Whiting wrote:

Rico

Bob's got it right, the slab doesn't move unless something underneath
it does. Preparation - drainage, compaction, etc. - is the key.

Absolutely correct, but the problem is that you can't guarantee that
something won't move. Thus a little reinforcement is, IMO, cheap insurance.


Much like the single guy with no dependents buying life insurance.
It's cheap, but is it necessary? Too many people think that it is a
panacea for all problems concrete. It's not. Just like painting, slab
on grade results are all in the preparation.

Not a good analogy at all. I've seen many slabs crack and shift up to
an inch even with what appeared to be proper preparation. And there is
a major road nearby that hs shifted nearly 6' in one set of lanes of a
divided highway. I'm sure the construction company thought they had
prepared the soil properly also, but my point is that you can't always
be sure.


Matt

RicodJour wrote:

Matt Whiting wrote:

RicodJour wrote:

Again with the math! You're such an engineer sometimes.

They allow some leeway with the coverage - I believe it's +/- 3/8".

Oh, and coming from a woodworking background, I half-lap my rebar so
it's only one layer thick.

And do you sand those nasty ridges off the rebar to make them smooth as
you would with wood? :-)


Hell, no! That would take foever and give inferior results. I turn
them down on the lathe.

Where have you been Matt? I'm guessing the spouse-enforced two week
vacation.

Good guess. Yes, just returned from two weeks driving around the
southwest in NV, AZ, CO and UT. Nice country for a visit. A little
warm (I have a shot of the car thermometer at 108 or something
ridiculous like that), but with the low humidity I found it very
comfortable compared to PA with our 95 and 95 conditions.


Matt

Matt Whiting wrote:

Good guess. Yes, just returned from two weeks driving around the
southwest in NV, AZ, CO and UT. Nice country for a visit. A little
warm (I have a shot of the car thermometer at 108 or something
ridiculous like that), but with the low humidity I found it very
comfortable compared to PA with our 95 and 95 conditions.

I'm guessing Four Corners, Grand Canyon, Mesa Verde and maybe Bryce.
How much did you lose in Vegas? Oh, right, what happens in Vegas stays
in Vegas.

I rode my bicycle through that area one summer. Never got above 124 F
in the shade. I'll take the humidity.

R

RicodJour wrote:
Matt Whiting wrote:

Good guess. Yes, just returned from two weeks driving around the
southwest in NV, AZ, CO and UT. Nice country for a visit. A little
warm (I have a shot of the car thermometer at 108 or something
ridiculous like that), but with the low humidity I found it very
comfortable compared to PA with our 95 and 95 conditions.


I'm guessing Four Corners, Grand Canyon, Mesa Verde and maybe Bryce.
How much did you lose in Vegas? Oh, right, what happens in Vegas stays
in Vegas.

I rode my bicycle through that area one summer. Never got above 124 F
in the shade. I'll take the humidity.

Well, this is pretty fat OT already, but we hit the above and in
addition several things around Colorado Springs (Pikes Peak, Garden of
the Gods, Cave of the Winds, etc.), as well as Zion in addition to Bryce
and a few other things. It was a fun two weeks.

The temps stayed right around 100 in Vegas which was the warmest area.
Colorado Springs was nice with temps in the upper 80s to lower 90s. Did
have one whopper of a thunderstorm the afternoon that we ate lunch at
Solo's, which is a restaurant inside an old airplane (KC-97 tanker).
Man was it loud when the 1/2" hail began to hit the aluminum!

I'm not a gambler so I didn't even enter a casino in Vegas. The only
money I lost there was on souvenirs! :-)


Matt

On 30 Jun 2005 12:57:25 -0700, "RicodJour"
scribbled this interesting note:

Bob's got it right, the slab doesn't move unless something underneath
it does. Preparation - drainage, compaction, etc. - is the key.

In our area, with the soil we have, it is impossible to "correctly
prepare" what is underneath the slab. The black, clay rich soil we
have expands and contracts so much, I remember, as a child, having
soccer games cancelled because the cracks in the hard-as-a-rock dirt
were so large a kid could have broken an ankle or leg!

In this area, post tensioned slabs always move and crack. Any slab
that is built to FHA minimum standards will crack. Those standards
must be exceeded by a fair margin to insure the slab stays where you
want it, as you want it.

I wish this area had soil that was easier to build on. Doing any kind
of digging around here is a major chore.


--
John Willis
(Remove the Primes before e-mailing me)