Slab Shrinkage Reinforcement Spacing 1

[XR250]

Why are we limited to 18” O.C. spacing for this? If it was installed 36” O.C., is the concrete really going to crack between bars? Seems with Gr 60, #4 bars and a 4” slab, you could place 'em 36” O.C. and meet the minimum area requirements and save a bunch or labor and materials.

 

[GeorgeTheCivilEngineer]

Short answer is yes, the cracking would occur between the large bars. For anti-crack it's better to have more, smaller bars to achieve the area required.

This make intuitive sense as the steel is restraining the concrete from shrinkage and trying to stop the cracks with fewer large bars - which will have a higher restraining effect - is making the situation worse.

 

[phamENG]

I don't have any first hand experience with wider spacing not working, but that's possibly because I've always kept it tighter. I was taught to limit it to 12" on center for typical slabs on grade in conjunction with joints limited in spacing to t/30.

 

[XR250]

@George,I wonder if there has been any testing to validate that requirement?
FWIW, on my personal projects, I have always placed #4 @ 36 O.C. in 4" slabs without any cracking -even when using soupy pump mixes.

 

[GeorgeTheCivilEngineer]

Not something I've personally tested but then 99% of the code requirements I haven't tested myself!

The rules are certainly conservative and based on worst case conditions - so I don't think you can't draw conclusions from your personal experience. Arguably in a 4" slab, the shrinkage forces will be considerably less than a thicker slab anyway.

The only time I really think hard about crack prevention is when specifying water retaining concrete as you're worried about near-microscopic cracking.

 

[Settingsun]

On ground with contraction joints at close spacing?

 

[XR250]

I can see that being an issue for water retaining structures.
For a typical 4" slab, the requirements seem excessive.
I feel like i could get a lot more builders to be on-board with rebar in slabs if it was't spaced 18" O.C.
Their WWF ends up at the bottom in most cases.

Another question...
If code does not require reinforcing in a typical residential slab, would I automatically be required to specify 18" O.C.if I was going to show any rebar?

 

[GeorgeTheCivilEngineer]

In the UK we almost always specify reinforcement mesh (WWF). In fact, I can't think of any time I've specified reinforcement bars for something that's less than 200mm thick. The builders I've dealt with don't have issues putting mesh on cover spacers. Only need 5 or 6 per mesh panel.

If it was UK I'd say drop the bars and only spec WWF for crack control.

 

[RPMG]

Say you used the #4 @ 36" in lieu of #3 @ 18". Both can be walked through, have similar amounts of material cost, and either way, there is labor associated with laying the mat and pouring concrete. So, was there really significant cost savings, or does it just look like it should be cheaper? Also, would the cost estimator even catch the savings? I would assume not, but now we have a slab with #4 @ 36" which may or may not be more likely to propagate cracks. Probably not. 18" was probably pulled out of a hat, and material scientists didn't test wider spacings. But 18" is wide when you compare it to steel fibers, which essentially have zero spacing and zero cracks.

Technically, if this is a slab on grade, ACI says it's not part of the structure, and the minimum spacing doesn't apply. Or, it's designed as plain concrete with rebars added. From ACI's perspective, they haven't told you not to use 36" spacing. However, is it effective and is the cost savings justify using the larger spacing. I would say no, the #3 @ 18" is cheap enough.

 

[dik]

First, I don't like using WWM for slab reinforcing... I find it generally ends up in the bottom of the slab. Rebar spaced at 18" on proper chairs... could be closer... the closer spacing makes shrinkage crack width smaller. Closer spacing of rebar makes it more difficult to walk on when placing concrete and reinforcing. You can step between bars... more difficult with 12" spacing. 16" or 18" is a good compromise.

Dik

 

[XR250]

Thanks for everyone's insights.
As someone who lays and ties his own rebar, installing 1/2 the number of bars is a big time saver - even if they are almost twice as heavy.
Sounds like I am covered since the minimum spacing does not apply.
On commercial jobs, I will stick with standard practice.

 

[r13]

I wonder if there has been any testing to validate that requirement?

There should have some testing and good references on ACI350, concerning crack width and crack control, although it is focused on hydraulic structures, the concept is the same - smaller bars in a closer spacing yields finer crack that large bars placed farther apart.

For 4" slab, I would avoid rebar, but provide wire mesh instead.

 

[phamENG]

For everyone who complains about WWR at the bottom of the slab - are you not spec'ing and enforcing chairs? I've held up more than one pour because the contractor was going to "pull it up" while they poured or put the "chairs" so far apart the wire was on the bottom. I didn't approve the reinforcing until they were up on proper chairs or blocks without excessive sag in the mats and it was all tied to prevent shifting.

Does it make people mad? Sure. But I've gotten pretty good at explaining why the other methods are worthless in lay terms, and most architects and GCs accept my statements over the concrete sub (though some are still a bit skeptical, I admit).

 

[rapt]

I would say you are not covered. Most codes would limit it to 12" for crack control to be effective. Even 18" is too much.

 

[dik]

Here we can use 5x the slab thickness for non structural slabs.

Dik

 

[dik]

Pulling up should never be allowed... even with chairs, due to the flexibility of the chairs, they often get 'kicked' over.

Dik

 

[Tomfh]

We find 100mm x 100mm mesh performs better than 200mm x 200mm mesh of the same area.

 

[Agent666]

Keep in mind that you should ideally also outline to the client the expectations around degree of cracking expected.

I have seen one job where owner had interpreted expectations of zero cracking, and well that didn't pan out on the designers end with cracking in some locations away from sawcuts and in general what you might call a light level of reinforcement based on the degree of restraint and trafficking by light forklifts.

One saw cut was inadvertently left off the plans by the drafters and not noticed by the checking engineer. Several meters long in a crucial location. Slab naturally cracks there... Then that one missing line on the drawings cost them several hundred thousand dollars when they tried to fight it in court and lost.

 

[Tomfh]

I have seen one job where owner had interpreted expectations of zero cracking,

Then that one missing line on the drawings cost them several hundred thousand dollars when they tried to fight it in court and lost.

How did the owner come to have that expectation? And how was the engineer hung on a slab cracking? What had they agreed to?

 

[XR250]

My experience with WWF echos Phams - the contractors want to pull it up as they go. Just about every residential slab I have seen demo’ed verifies that this does not occur.
Unless an owner expresses that they want crack-free concrete, i give them what seems reasonable for the budget of the structure (residential).
Most contractors around here stick to the philosophy that there are two type of concrete - Concrete that has cracked and concrete that is going to crack!