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Concrete Cover?

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?
 

RE: Concrete Cover?

I hate to admit this, but the contractor might have a point. I don't think ACI considers tie wires in their clearance requirements.  And I've seen many cases of corrosion starting at tie wires, continuing to the main reinforcing and causing spalling. Usually, there was some other mitigating factor, but that was the starting point.
I'm not going to suggest you drop your reinforcing, but you might want to coat the tie wires.

RE: Concrete Cover?

(OP)
Jed,

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?

Remember the clearance given is the minimum allowed, not a requirement.  Then there are tolerances in the field for actual placement.  For this reason I have never used less than 1" of cover and usaully more on slabs.  I have not see ties on my slabs but have on tilt walls with the 1" 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?

(OP)
SrVaro,

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?

If you have the WWR at 3/4" clear, they are going to cut through them when they sawcut the control joints.  

We show 1-1/2" clear at S-O-G's for this reason.

RE: Concrete Cover?

Lino06 has a good point about cutting through bar at CJ's. Assuming this isn't something you want to do since its a structural slab.

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?

It is the contractor's responsibility to bend wire ties down. I have never heard of this creating a problem (I am in South Florida where the condo lawyers will attack anything even if it isn't a problem). If you want 3/4" cover stick to it. Does this mean no 3/4" cover for the bottom of a slab. I have seen this used many times on exterior balcony bottoms. No problems.

RE: Concrete Cover?

(OP)
VTEIT

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.

RE: Concrete Cover?

I'm not sure if in my cases the bars were bent into the slab or not. I was in the damaged concrete phase of the project.

RE: Concrete Cover?

(OP)
I'm in the "We are getting ready to pour and we don't like what you have done but there is not really much you can do to change your design we just want to go on record saying that anything that goes wrong is your fault." phase of the project.

RE: Concrete Cover?

I would ask for his justification for concerns about spalling since there seems to no reasonable answer. Also why can't they bend down the ties like on all other projects. In my opinion there is no problem here.

RE: Concrete Cover?

(OP)
ron9876

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.  

RE: Concrete Cover?

The thin wire ties that tie the bars together?  I would not even give them a second thought.  Bending the ties down to make sure they don't stick out of the slab is his means and methods and should have nothing to do with your design criteria.

RE: Concrete Cover?

Thorough inspection will assure that the ties don't project into the cover.  You will do that, won't you?  The contractor is probably having trouble finding the right size chairs to get the top steel to where you need it.

RE: Concrete Cover?

(OP)
Lion06

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?

Though I've seen that picture of the boat before, I find new anger welling inside of me. It'd be a shame if he ran it into the dock and it sank...

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?

(OP)
Well, it ended up that they were concerned with spalling of the concrete due to the fact that the bar only had 3/4" of cover.  They didn't believe 3/4" of cover was enough.

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?

a2mfk,
It is not a SOG.  It is a suspended slab, so the effective depth to the top bars matters a lot.

RE: Concrete Cover?

It is the contractor's responsibility to bend ends of tie wires away from the finished surface.  CRSI Placing Reinforcing Bars (page 10-33) states "Ends of finished ties should be kept clear of the concrete surface."  ACI 301-10 3.3.2.3 states, in part, "Position tie wire ends away from concrete surfaces."  ACI 301 and 318 allow 3/4" _specified_ minimum cover and allow tolerance as permitted by ACI 117.  I do not have my copy of ACI 117 (tolerances) handy, but if my notes are correct, the tolerance on slab cover is +/- 3/4", up to 1/3 of required 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.


--
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?

You might get surface cracking at the rebar locations.  It will probably make bull floating difficult.  Can you increase the slab thickness? What is the slab thickness? If there is any doubt I'd thicken the slab if possible or lower the mat.  You should assume the contractor knows what he's talking about.  What does the client have to say about it?  Any wires poking through can be dealt with while finishing the slab. Rebar near the surface is kind of tough to deal with.

RE: Concrete Cover?

Surface cracking at rebar locations? I've never heard of such a thing....

Why would running a bull float be difficult?

 

RE: Concrete Cover?

pendejo,
"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?

If your cover is less than the aggregate size, the bull float will ride up on the aggregate over the bar.  To push it down it has to push it aside.  Then you have an irregularity and will probably get more water bleeding up there also.  I don't know for sure that it'll crack, but I've never poured a slab with so little cover.  I have however poured stem walls with only 1" cover over the top bar.  It cracks.  

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.   

RE: Concrete Cover?

While the wire tie issue may be a distraction, I agree with the contractor on top clear cover.
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?

Well, each to his own, but I've designed and supervised hundreds of floors with 3/4" or 20mm top cover.  The only time cracks mirror the bars are when consolidation is inadequate.

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