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Posted by Bob Morrison on November 3, 2006, 10:26 am
In a previous post carolyn wrote...
> Can you tell us why you don't like rebar in a thin slab? My understanding
> is that concrete has great compression strength, but snaps easily when
> pulled apart. The weight of a car on the top of a slab pushes down on the
> top of the slab, applying compression strength to the top, while the bottom
> of the pad is being pulled apart. Enough weight, and the cracks form in
> the bottom of the pad and tear through the top. Put the rebar in the lower
> half of the slab, and it will hold the bottom together and prevent that
> tear through.
> I don't know how think it needs to be for what weights, and 5" may be enough
> for cars, but what about the 18 wheeler delivery truck, dropping off a new
> table saw? (Yeah, my saw and drill press were delivered in an 18 wheeler.
In theory this is true if your subgrade is so poor that it won't support the
weight of the vehicle above it. I can tell you that NOTHING will prevent
shrinkage cracks in a slab. If you get one truck a year then a 5-inch slab
on a good base will be fine. If you get one truck per month then up the
thickness to 6 inches. If you expect frequent heavy traffic then make the
slab thicker, but you can still leave the rebar out. Put the money into
proper subgrade preparation and into proper crack control joints.
> 5" and rebar may be over minimum code for a driveway - but the worst that
> will happen is that it will be too strong and last much longer than it
> otherwise would.
I regularly specify unreinforced slabs on grade. The American Concrete
Institute has a whole book on this subject. Most city streets and much of
the interstate highway system are unreinforced slabs on grade and are
usually between 8 and 10 inches thick. My favorite example is an 18-inch
unreinforced slab I specified for a log handling yard. It sits on a 24-
inch deep compacted gravel base, which makes the total pavement thickness
42 inches. That pavement is subject to 130,000 pound axle loads and except
for some surface scarring has held up very well.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com
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Posted by Glenn on November 3, 2006, 11:17 am
I don't disagree with no steel but if it is for me, I still want
#4 rebar, 24" oc, 100% tied and on chairs
> In a previous post carolyn wrote...
> > Can you tell us why you don't like rebar in a thin
> > slab? My understanding is that concrete has great
> > compression strength, but snaps easily when pulled
> > apart. The weight of a car on the top of a slab pushes
> > down on the top of the slab, applying compression
> > strength to the top, while the bottom of the pad is
> > being pulled apart. Enough weight, and the cracks form
> > in the bottom of the pad and tear through the top. Put
> > the rebar in the lower half of the slab, and it will
> > hold the bottom together and prevent that tear through.
>
> > I don't know how think it needs to be for what weights,
> > and 5" may be enough for cars, but what about the 18
> > wheeler delivery truck, dropping off a new table saw?
> > (Yeah, my saw and drill press were delivered in an 18
> > wheeler.
>
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Posted by Bob Morrison on November 3, 2006, 11:57 am
In a previous post Glenn wrote...
> I don't disagree with no steel but if it is for me, I still want
> #4 rebar, 24" oc, 100% tied and on chairs
>
Max code bar spacing is 18" o/c.
BTW, if you assume 3000 psi concrete, grade 40 bars and a span of 12",
this slab will support a wheel design load of 4000 pounds. Doesn't sound
to me like the rebar is worth the effort for such a thin slab unless you
want to simply pour the concrete on unprepared rough ground. And, as I
mentioned in another post you need at least 6-1/2" of concrete to meet
minimum cover requirements.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com
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Posted by Matt Whiting on November 3, 2006, 5:41 pm
Bob Morrison wrote:
> In a previous post Glenn wrote...
>
>>I don't disagree with no steel but if it is for me, I still want
>>#4 rebar, 24" oc, 100% tied and on chairs
>>
>
>
> Max code bar spacing is 18" o/c.
>
> BTW, if you assume 3000 psi concrete, grade 40 bars and a span of 12",
> this slab will support a wheel design load of 4000 pounds. Doesn't sound
> to me like the rebar is worth the effort for such a thin slab unless you
> want to simply pour the concrete on unprepared rough ground. And, as I
> mentioned in another post you need at least 6-1/2" of concrete to meet
> minimum cover requirements.
>
Bob, what is the design wheel load if there is no rebar in a slab of the
same thickness?
Matt
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Posted by Bob Morrison on November 3, 2006, 7:14 pm
In a previous post Matt Whiting wrote...
> Bob, what is the design wheel load if there is no rebar in a slab of the
> same thickness?
>
Using a modulus of rupture (bending stress) for concrete of 2*SQRT(f'c)
and f'c=3000 psi, a span of 12" and an slab thickness of 5"
M = PL/4 = Fb * Sx = 2*SQRT(3000)*[(12)(5^2)/6] = 5500 #-in
For L = 12" P = 1835# or about one light pickup truck wheel load
If you increase the slab to 6-1/2" then P = 3100#
Compare the unreinforced slab to the 6-1/2" slab with #4 @ 18" o/c, which
gave a P = 4000#
If you increase the concrete strength to 4000 psi, then
P = 3100# * SQRT(4000/3000) = 3600#
Pretty simple calculation isn't it?
Of course it can be made more complex if you factor in the footprint of
the tire. Using this sort of uniform load (32-35 psi depending on tire
pressure) will either increase the design span or the allowable wheel
load. This sort of calculation is not solvable by normal algebraic
manipulation because there are too many unknowns and only a limited number
of equations.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com
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