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?

 

[TXStructural]

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

 

[Inlander]

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.

 

[HeavyCivil]

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

Why would running a bull float be difficult?

 

[hokie66]

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.

 

[Inlander]

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.

 

[ATSE]

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.

 

[hokie66]

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.