Minimum cover from top of footing/slab to bottom dot bars of a wall

[acwooten]

See attached picture. Is there a minimum cover from the top of a footing or slab to the bottom transverse bars of a wall, just above the construction joint?

Thanks in advance!

 

[JAE]

Technically ACI 318 does not address cover between bar and construction joint.
I would typically use at least 1" clear for vertical distances based on what ACI mentions about stacked bars in beams.
This allows full paste and aggregate to get below the bar.

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[JAE]

See ACI 318 7.6.2

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[dik]

Are you looking at cover or spacing to first bars... usually spacing/2.

Dik

 

[acwooten]

JAE,
I agree that you should have something. It just doesn't seem to make sense that you would place the rebar directly on the joint, but as you said, ACI doesn't address this case. Section 7.6.2 that you referenced is specifically for spacing between bars. I guess it's to your point that you want to be able to get some paste with aggregate between bars, but you could argue that that paste and aggregate is already there in all of the concrete below the joint for this particular case. There just happens to be a joint in the middle of that paste.

dik,
You can't really call it spacing because it is the first bar, so cover seems a more appropriate term. I'm looking at the required distance up from the construction joint, to the first layer of dot bars (transverse wall bars going in and out of the page).

 

[JAE]

acwooten - not sure I agree with your statement: [blue]"you could argue that that paste and aggregate is already there in all of the concrete below the joint for this particular case. There just happens to be a joint in the middle of that paste."[/blue]

The lower concrete (below the joint) is presumed to be already in place prior to the second concrete placement.

Therefore, whether it is another bar, or a solid surface of concrete, doesn't matter to the bar and to the aggregate/paste coming down from above and trying to get around and under the bar.

Because of that, I think the 7.6.2 section would be the section in 318 that "most" applies to this situation.

The concerns of placing the bar direct on the first, lower section of concrete, vs. some designated "cover" are:
1. There could be water infiltration through the joint and this might start bar deterioration.
2. The bar would lose some degree of bond (development) along its length due to limited concrete contact
3. The concrete of the second placement wouldn't get below and around the bar (my concern).
4. The bar's effectiveness in limiting shrinkage cracking would be diminished (due to item 2 above perhaps)




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[acwooten]

JAE,
I guess my point in saying that was to say that, assuming there is some sort of dowels coming up from the already poured footing into the wall, and you roughen the surface and pour your wall, maybe they will act as one system enough for the concrete below the joint to act as the cover for the dot bars.

As to your concerns:
1. I agree. There would have to be a waterstop on each face, outside of the bars, for this to even be a conversation. Otherwise, water would come in through the joint and the lack of cover wouldn't stop it from reaching the bars.
2. To my point above about the dowels, would the two systems acting as one make this a moot point?
3. See 1 and 2
4. That's an interesting point. I wouldn't really know how to approach that.

Some background on this, I am studying for the PE and the review course I am taking had a problem with this exact situation. Their solution assumed the dot bars were placed directly on the already poured footing, which I knew in practicality wouldn't be the case. They then defended their stance when I reached out to tell them I didn't agree. However, it was a construction question on taking off rebar quantity, but it still had structural implications. I just didn't have code to back me up! Though I'm not sure that would have helped as they literally referenced ACI for which cover to use, but were wrong in what cover they told us to use. Oh well.

 

[acwooten]

Here is the answer from ACI SP-66, which is the reinforcement detailing manual.