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Posted by Chuck on July 2, 2007, 3:12 am
>>
>>
>>
>>
>> > On Jun 29, 12:32 am, spebby...@hotmail.com wrote:
>> >> 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.
>>
>> > If the sub-grade is prepared correctly - drainage issues addressed,
>> > organic soil removed, gravel and insulation added as required,
>> > compaction - the rebar adds little if any benefit. Concrete cracks -
>> > all concrete cracks. The rebar will not prevent cracking, but it will
>> > prevent gross movement between sections on either side of the crack.
>> > If there's enough movement between sections of the slab that the rebar
>> > is called upon to do work, the slab has already failed.
>>
>> > The fiberglass additive is designed to limit cracking on a small
>> > scale. As Chuck mentioned, creating control joints to induce the slab
>> > to crack where you want it to crack is a good idea. I'd space the
>> > control joints every eight or ten feet both ways.
>>
>> > BTW, unless that rebar is epoxy coated it's a bad idea to install
>> > rebar in a 4" slab. The IRC requires 3" of concrete coverage between
>> > the soil and the rebar.
>>
>> > R
>>
>> This depends on the local authority regarding the slab on grade. The 3
>> inch
>> clear space is for a footing or slab placed in soil without form work and
>> directly in soil. I have experienced building departments that allow
>> the 4
>> inch slab with rebar at the mid depth if the slab is placed on 4inch to 6
>> inch gravel sub base and vapor barrier. It depends on how the building
>> codes is interpolated.
>
> No doubt there will be substantial variance between building
> departments' interpretations of the code, and the code will also vary
> from place to place. I'm not arguing code requirements - arguing with
> a building inspector is often an exercise in futility. I just don't
> consider code - generally the minimum acceptable construction quality
> - to be the be all and end all of construction.
>
> Rebar in thin slabs has several fatal flaws. The standard practice is
> far from the recommended practice - typically rebar is placed without
> any attempt at raising it above the bottom of the slab, then when the
> concrete is placed, the workers pull up on the rebar in an attempt to
> position it in the middle of the slab. This is not a good practice.
> Other times it is raised up on brick pieces - also not a good
> practice.
>
> My major objection to rebar in a thin slab is that it can't do what
> people think it is doing when they spec it and install it. Rebar does
> essentially no work when place in the middle of the slab - the neutral
> axis. Rebar is meant to take the tension that concrete can't. That
> requires the rebar to be near the tension side of the slab/beam, not
> in the center. Placed in the center of the slab rebar only does work
> if the slab has already failed. In other words, rebar in a thin slab
> is planning on having the slab fail. That's backwards.
>
> Slab failures can be entirely prevented by proper sub-grade and site
> preparation. That preparation is a better way to spend the money than
> on rebar.
I was always understood that when you are using 'Ultimate Strength Design"
the concrete has already cracked and the rebar is yielding. Isn't this
considered as concrete failure???? The plastic neutral axis is not the
middle of the slab and is just be low the top of the slab ( ' a'/.85 of the
compression block ) and the location of rebar 'd' is located at mid depth.
You are saying that it doesn't work even though compression equals tension
and the moment capacity exceeds the factored moment required.
This is the same way you would design a steel composite slab considering
compression block in the slab portion to take the compression and the steel
beam to take the tension. Are you saying this process is incorrect?
Even so, the slab on grade rebar is minimum at mid depth primarily
temperature reinforcing to minimize cracking. The slab on grade needs to be
placed on a sub base of gravel with vapor barriers if required.
CID...
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