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Concrete slab question spebby_92 06-29-2007
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Posted by on June 29, 2007, 12:32 am
I am building a garage that will be attached to the house. The slab
size is 24' X 31'. The slab will have a 14" X 18" footer on all 4
sides with rebar. My question: I have received 2 bids for the job,
one with rebar in the 4" slab and one with no rebar in the 4" slab.
Both bids have rebar in the footer. The bid without rebar uses 3500
concrete with HP fiber. Is 3500 concrete with HP fiber equivalent to
3500 with rebar? Should the grarage floor have re-bar regardless of
type concrete? There is $1,100 difference in the two bids. Both
bidders have provided good references. The re-bar bid is the higher
one. I don't mind paying the difference if re-bar is the better
option. The one with re-bar has the re-bar 18" o.c. Any comments will
be appreciated.


Posted by Dave on June 29, 2007, 12:54 am
>I am building a garage that will be attached to the house. The slab
> size is 24' X 31'. The slab will have a 14" X 18" footer on all 4
> sides with rebar. My question: I have received 2 bids for the job,
> one with rebar in the 4" slab and one with no rebar in the 4" slab.
> Both bids have rebar in the footer. The bid without rebar uses 3500
> concrete with HP fiber. Is 3500 concrete with HP fiber equivalent to
> 3500 with rebar? Should the grarage floor have re-bar regardless of
> type concrete? There is $1,100 difference in the two bids. Both
> bidders have provided good references. The re-bar bid is the higher
> one. I don't mind paying the difference if re-bar is the better
> option. The one with re-bar has the re-bar 18" o.c. Any comments will
> be appreciated.
>

How is the garage slab attached to the house?

Do you need footers? vice floating slab with beams on the perimeter?

Any substantial weight in the garage when done?
Dave



Posted by Chuck on June 29, 2007, 2:55 am

>I am building a garage that will be attached to the house. The slab
> size is 24' X 31'. The slab will have a 14" X 18" footer on all 4
> sides with rebar. My question: I have received 2 bids for the job,
> one with rebar in the 4" slab and one with no rebar in the 4" slab.
> Both bids have rebar in the footer. The bid without rebar uses 3500
> concrete with HP fiber. Is 3500 concrete with HP fiber equivalent to
> 3500 with rebar? Should the grarage floor have re-bar regardless of
> type concrete? There is $1,100 difference in the two bids. Both
> bidders have provided good references. The re-bar bid is the higher
> one. I don't mind paying the difference if re-bar is the better
> option. The one with re-bar has the re-bar 18" o.c. Any comments will
> be appreciated.
>

A 4 inch slab with 3500 psi concrete with HP fiber is good basically for
cracking. Normally minimum rebar is used in 4 inch slab with either welded
wire fabric or #4 bars 18 inch on center each way. I would go with 4000 psi
concrete with the HP fiber additive mainly for cost. I would also break up
the slab with control joints into four pieces.
For the footers, where are you located? Sounds like the footing is 14" wide
and 18 inch deep. Do you have a frost level for footings? Some places call
for bottom of footings at 18 inches below finish grade and that is OK. If
frost level is not a problem you can just use dropped edges for footing. 8
inch deep 12" wide with a 45 degree angle toward the inside from the bottom
of the footing to the slab. Put (2) #4 bars continuous in the dropped edge
of the slab. Hope this helps.



Posted by on June 29, 2007, 1:14 pm
Thanks Dave and Chuck, I appreciate the comments.

The slab will be poured against the existing house slab. Should the
new slab be "attached" to the existing slab in any way? For example,
drilling holes in the existing slab and inserting re-bar. Neither
bidder suggested "attaching" the new slab to the existing slab.

The footer will be 14" wide and 18" deep. I am located in central
Oklahoma and normally 18" is sufficient for frost. The slab will be
poured as a monolithic slab. Will these footers be sufficient?

Nothing heavier than a pick-up truck and car will be in the garage.

The slab will be cut into 4 sections. Both bidders indicated the slab
would be cut the same day it is poured.

Chuck, from your comments it certainly appears I should go with the
bid specifing #4 rebar 18 o.c. tied and on chairs. The slab will
have a 4" sand base.


Posted by on June 29, 2007, 1:30 pm
Thanks RicodJour for your comments. Although I am a little confused
now.


RicodJour from your comments it appears a well constructed base is
sufficient. Of course, with the cuts in the slab at an appropriate
distance. I think I will get a third bid.

One other question: My water well will be in the new garage. Should
the slab be isolated from the well's casing? I was thinking building
a form around the casing and filling the void between the casing and
concrete with pea gravel might be a good solution.




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