Concrete Cover?
Concrete Cover?
(OP)
I have a question regarding concrete cover (I thought I posted this already but I guess I didn't).
I have a structural slab on grade that is supported by grade beams and piles. I show a concrete cover of ¾" to the top reinforcing in accordance with ACI 318. This is an interior slab in a warehouse and it is not subjected a corrosive environment.
The contractor has raised some concern about the wire ties used to hold the reinforcing in place and spalling of the concrete that may happen. He is concerned that ¾" is not enough cover and that the wire ties may end up protruding from the slab. He would like to drop the reinforcing mat in order to avoid this problem. I believe he is raising concerns about spalling as a way to reinforce his argument about the wire ties.
We have followed the recommended requirements from ACI with regards to cover (ACI 318 section 7.7) and bar spacing (ACI 318 10.6.4). From what I can tell, there should be no concern about spalling since the reinforcing will be protected from the elements. In fact, most of the concern about spalling has to deal with the placement/finish of the concrete slab itself.
Is there anything that I am missing with his argument?
I have a structural slab on grade that is supported by grade beams and piles. I show a concrete cover of ¾" to the top reinforcing in accordance with ACI 318. This is an interior slab in a warehouse and it is not subjected a corrosive environment.
The contractor has raised some concern about the wire ties used to hold the reinforcing in place and spalling of the concrete that may happen. He is concerned that ¾" is not enough cover and that the wire ties may end up protruding from the slab. He would like to drop the reinforcing mat in order to avoid this problem. I believe he is raising concerns about spalling as a way to reinforce his argument about the wire ties.
We have followed the recommended requirements from ACI with regards to cover (ACI 318 section 7.7) and bar spacing (ACI 318 10.6.4). From what I can tell, there should be no concern about spalling since the reinforcing will be protected from the elements. In fact, most of the concern about spalling has to deal with the placement/finish of the concrete slab itself.
Is there anything that I am missing with his argument?






RE: Concrete Cover?
I'm not going to suggest you drop your reinforcing, but you might want to coat the tie wires.
RE: Concrete Cover?
I don't understand why they just can't bend the wires over into the slab. It's not like we are talking about a #4 bar. It's just a little piece of wire. I know it's a PITA to make sure they are bent over, especially in a larger warehouse.
RE: Concrete Cover?
I would say you have a good contractor who is looking out for the best results (there actually are a few out there). After all he could have said "I did what was on the drawings, not my fault".
Since it appears to be a structural slab it might not be an option to drop the reinf. But it might be worth exploring.
RE: Concrete Cover?
I can accommodate a 1/4" drop in the reinforcing location (to 1" cover) but any more and I will be in violation of ACI 318 section 10.6.4 with my bar spacing.
RE: Concrete Cover?
We show 1-1/2" clear at S-O-G's for this reason.
RE: Concrete Cover?
SteelPE - you are correct about being able to bend the wires down. Are his Ironworkers using 'twist ties' or cutting wire to length? Most union Ironworkers I see cut wire to length and twist it manually, whereas non-union workers seam to always use twisters. With a twister, its super easy to twist the wire and push it bellow the top layer in one fell swoop.
Go figure why the union ones prefer the more time consuming method. Although with large diameter bars and complex cages its not much of an option
RE: Concrete Cover?
RE: Concrete Cover?
You are correct about no wanting CJ's as this is a structural element. The reinforcing is pretty extensive so cracking should be limited.... and this is a union contract. It just seems like someone is not wanting to do their job.
ron9876
This is for a structural SOG so we are casting the concrete directly on the ground. So we have a larger cover on the bottom reinforcing than we do on the top. I am willing to give a little on the cover but I can't give that much.
I had a simple site visit scheduled tomorrow to take a look at the placement of the reinforcing. Now they want to have a meeting to go over their concerns... ugh.
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I will find out tomorrow. I just don't want to be blind sided by someone giving me a code requirement as to why this is an issue. I'm sure they will have some excuse. Whether it is legitimate or not has yet to be seen.
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Yes, I am talking about the little tiny wire ties that hold the bars together during the concrete pour.
hokie66
This is why I was scheduled to be on site tomorrow, to see how the forming and tying is going. As far as the chairs go, I believe these are custom formed chairs.... at least they were shown on the rebar shop drawings.
RE: Concrete Cover?
I am wondering how to take this contractor's comment, is this something he has had problems with before and is truly concerned with spalling; OR, is he just covering his butt. Having used the wire ties before and tied bars, though doing hundreds is no fun they are very easy to bend even with no tool. I don't see a problem with other than a few extra seconds to make sure the ties are not projecting upwards.
I'm no concrete guru, what is the problem with a little more cover on the top of the concrete SOG?
RE: Concrete Cover?
They were also concerned since this is a warehouse and they have racks being anchored into the slab. They wanted me to drop the top mat more than what it was shown on the drawings so they could easily install the anchors. That request was denied.
Some of the concern is coming from the fact that the contractors PM has never constructed a slab of this type (pile foundation/structural slab). So there was no concern in dealing with previous jobs/problems. They just didn't believe it would work. I informed him that the design was in compliance with ACI requirements and that nothing was done out of the ordinary.
As far as the wire ties are concerned, I just told them to bend the ties over and that there was nothing I could do about them.
RE: Concrete Cover?
It is not a SOG. It is a suspended slab, so the effective depth to the top bars matters a lot.
RE: Concrete Cover?
There should be little problem with spalling if the concrete is of good quality and finished properly, unless there is excessive stress in the reinforcing. (Assuming you have designed the laps with correct length for the cover and observed minimum requirements in 318 12.2.2.) Also keep in mind that if your cover minus tolerance is only 1/2", but your aggregate is larger than 1/4-3/8", the contractor will have paste puddling on top of the bar without adequate aggregate coverage. That can create a condition which will increase risk of spalling (no aggregate interlock and a localized paste concentration with resulting high water-cement ratio.) This is one reason you will tend to have ghosting over the reinforcement and will frequently develop cracks therein.
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My notes say:
ACI 117
Members over 12" depth: +/- 1/2"
Not to exceed 1/3 of required cover
Slab on grade: +/- 3/4"
ACI 318
Tolerance on d: +/- 1/2" (d>8")
Tolerance on cover: - 1/2"
John Turner CSP PE
CRSI Greater Southwestern Regional Manager
RE: Concrete Cover?
RE: Concrete Cover?
Why would running a bull float be difficult?
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"You should assume the contractor knows what he's talking about." Indeed? Your contractors must be different from mine.
And as for surface cracking along rebars, that only occurs due to settlement of the plastic concrete in thick slabs and footings, and must be closed by revibrating or finishing techniques.
RE: Concrete Cover?
You think this contractor is pouring his first slab? Doubt it. I suggest pouring a sample with the above conditions before pouring the whole thing.
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3/4" clear cover for walls and slabs is a bad idea. Too much variation in bar placement (actual and permitted by ACI 117), especially slabs with rebar chairs in soil.
We use 1" top and side clear cover for interior slabs and walls.
Better to be fat 1/4". Hopefully your design is not that sensitive to 1/4" variation.
RE: Concrete Cover?