Concrete Cover? 1

[SteelPE]

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?

 

[JedClampett]

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.

 

[SteelPE]

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.

 

[SrVaro]

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.

 

[SteelPE]

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.

 

[Lion06]

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.

 

[HeavyCivil]

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

 

[ron9876]

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.

 

[SteelPE]

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.

 

[JedClampett]

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.

 

[SteelPE]

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.

 

[ron9876]

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.

 

[SteelPE]

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.

 

[HeavyCivil]

While it doesn't look like he'll be successful, maybe he's looking for one of these:

 

[Lion06]

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.

 

[hokie66]

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.

 

[SteelPE]

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.

 

[a2mfk]

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?

 

[SteelPE]

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.

 

[hokie66]

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