Cold Joint in Pad Footings 1

I say make him pull it out and do it correctly. This is inexcusable, no matter what ACI says.

Vertical cold joints in concrete members are usually OK in areas of high moment (presuming the reinforcement is carried through the joint and the joint is perpendicular to the reinforcement).

You usually avoid cold, vertical joints in areas of high shear.

For a spread footing with a column/baseplate in the center, and a joint across the pad, it seems that the moment in one direction would be OK - perpendicular to the "span".
And it would seem that the moment in the other direction is unaffected.

Shear for a two-way slab is one way and also punching - both may not be too affected by the joint as it doesn't occur at the critical sections.

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I have seen similar before. Usually we specify a standard detail for a cast-in-place control joint which can be used as pour-stop in this condition.

I would think the 'blob' would likely cause an unsightly finished joint and cause surface cracking and breaks. You may need them to sawcut a clean line down without cutting through rebar and demo a vertical-ish surface and then rely on aggregate interlock/friction

EDIT: I read that as in a slab. So pardon if not applicable,

My initial thought without a sketch or pic:

Less than ideal and the moment is max near the halfway point if the column is centered. BUT, concrete flexural design assumes that it's the section is cracked anyway, so what's the difference.

And generally speaking a rough surface is GOOD for a cold joint.

Are there anchor bolts affected?

hokie66, you posted as I was typing - I would agree that this is questionable and I've never seen this done before - in fact we require all joints terminating single concrete placement to have the prior approval of us before they do it so if there is some kind of language like that in the OP's original specs or notes then they probably could insist on it.

One thing I didn't think about was the anchor bolts - I would think that a cold joint near an embedded anchor might be problematic per Appendix D*





*Appendix D s%#ks



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I would say pull it out. If you let them away with this, what will the contractor want to talk you into next .... Time to set the tone at the footing stage so that all construction above can be completed according to good construction standards.

In my opinion, if they want to save the blob of concrete, the surface demo for a clean aggregate surface, water or sandblasting to clean the surfaces and application of bonding agent which I would require would far outweigh the costs of yanking the existing blob out with a loader and starting over. Not to mention the additional time you would have to deal with detailing/explaining all this to the contractor and likely all the back and forth that will likely accompany it.

The contractor should have called for a cleanup and he didn't. Make him do it right.

Mike McCann, PE, SE (WA)

Prep the interface vertical and rough, or pull it out. Left alone, the concrete angle of repose essentially introduces an artificial shear crack at the worst possible location.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

The main reason I gave a short answer is that the builders I work with would have known the answer before asking. The owner is paying for a footing, not two tenuously connected blobs of concrete.

KootK has beaten me to the issue of shear, as the moment and shear are both maximum in cantilevers.

In addition, the first pour of concrete as described cannot be well compacted, as there was nothing to restrain the side with the "rough, lumped over blob".

Truly excellent point regarding the compaction.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

I probably wouldn't allow it either, but to be fair that cold joint isn't going to be a full on shear crack initially. You'll have adhesion between the two pours. Given that footings are often fairly low stress situations, it might calc out.

That doesn't make it good workmanship or acceptable, though.

On the other hand, if the footing isn't overly thick and there's steel top and bottom and the interface isn't too close to one side, this could actually edge towards acceptable. Stupid, but maybe acceptable.

KootK's sketch shows a diagonal shear crack off the center concentrated load. With a vertical, roughened construction joint the shear crack and joint really wouldn't interact in the way I'm perceiving it.

Think about this - there are vertical and roughened or keyed construction joints placed in elevated beams and slabs all the time - usually placed intentionally near points of maximum moment but also kept away from high shear areas.

The vertical joint in the footing - directly at a centerline - would not be a high shear area as long as the concentrated load is spread out a little - either with a base plate or via a concrete column.



But all this above is under ideal conditions so a "blob" of concrete and messed up subgrade would certainly be a concern.

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TLHS said:

I probably wouldn't allow it either, but to be fair that cold joint isn't going to be a full on shear crack initially. You'll have adhesion between the two pours.

Click to expand...

I agree, there would be some capacity. But how would one calc it out? You would basically be relying on the tensile stress capacity perpendicular to that very cold, rough, diagonal joint to resist diagonal tension. What does that take from a preparation and verification standpoint? Is there some kind of ASTM pull off test that would be appropriate?

TLHS said:

Given that footings are often fairly low stress situations, it might calc out

Click to expand...

While this would likely be true for one way shear, the two way shear demand in many pad footings leaves little reserve in the tank. And this cold joint pseudo shear crack may well be 1/4 of what might become the critical punching shear perimeter.

JAE said:

KootK's sketch shows a diagonal shear crack off the center concentrated load. With a vertical, roughened construction joint the shear crack and joint really wouldn't interact in the way I'm perceiving it.

Click to expand...

You may not have interpreted my sketch as intended JAE. In my sketch, the diagonal line is not a shear crack but, rather, the cold joint itself that has the potential to become a critical shear crack. Bigmig has yet to clarify this but I've assume that the as built interface is a sloping surface, not a vertical one. I've also assumed that the joint is near centre but not necessarily at centre. As I mentioned in my last post, if the interface is vertical, or is made vertical, I'm fine with it as a shear friction joint that will benefit from high moment if not, simultaneously, low shear.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

You all are making more of this than it is. Just sit down and watch Steve McQueen's first starring role in 1958 in "The Blob". All your problems will come into perspective.

Mike McCann, PE, SE (WA)

KootK - got it - I didn't read your note close enough.

If the joint is vertical and "half way" through the middle as the OP suggests, then I see no problem.

If it is a sloping "blob" then per your sketch shear might be an issue.

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Mike,

If we use "The Blob" analogy wouldn't the foundation shrink in cold weather?

From a liability and economics point of view there is no way I'm spending an hour trying to get this mistake to work by calculating and trying to persuade myself about shear planes. The condition that the owner expects, and has paid for is one that is 100% correct and not a dime under. Why would I risk thousands of dollars in insurance claims and legal costs over one of the most simple details on the entire project....and for what ? $100 in concrete and rebar? Like others pointed out, trying to accomodate every mistake sends an implied message...."build however you want because the engineer will find a way to make it work".

The problem is the concrete, which goes into compression at this mystery joint when that pad loads. If there are voids or anomalies of any type at the contact point between pours, they will collapse or slip to a point of equilibrium. I'm going to be directing them to pull this pad Monday.

Good choice in my opinion.

Mike McCann, PE, SE (WA)

Koot, an unprepared cold joint can take a bit more than 1 MPa of tension and maybe 1.5MPa of shear from adhesion. I've read tests over the years on it. If anything, this should do slightly better if it's nice and rough.

You may be able to justify shear capacity with the Canadian code (although maybe not, because if you don't some reinforcement through it the equation will kind of explode because of the way it's set up and whether you'd want to without bars holding back shrinkage is another question) but pretty sure you couldn't with ACI. You wouldn't be able to justify tensile capacity by code.

From a realistic standpoint, there's capacity along that joint. From an engineering standpoint, like I said before, I wouldn't allow it.