Omitting SOG Diamonds at Columns

[KootK]

I've got a client that wants to omit the diamond buck outs that we typically place in the slab on grade at columns to reduce unsightly slab cracking. They don't want to have to do a second pour. I've warned the client that there will be an increased risk of unsightly cracking and it seems that they're okay with that:

Questions:

1) Are there any truly structural implications? Will the bottoms of my columns get sheared off from their foundations?

2) Anyone have a sense for how bad the cracking could potentially be and the risk of it occurring?

3) Any ideas for one pour solutions that wouldn't add a lot of cost? Larger compressible joints? Low shrinkage concrete?

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.

 

[XR250]

I would be looking at larger compressible joints. Other options would seem to negate any cost benefit from doing the second pour (if cost is the reason).

 

[DaveAtkins]

I think you can bring the control joints right up to the column on all four sides, since you have forewarned the client about the potential for cracking.

Will there be a floor covering applied after the concrete has had a chance to shrink? If so, the cracking won't matter.

DaveAtkins

 

[KootK]

Thanks guys. It's parkade SOG. Running the joints to the columns means tooling rather than saw cutting though, right? We're also adverse to tooling. Have you guys had any success with pinwheel joints?

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.

 

[SlideRuleEra]

1. No, if there is adequate expansion material surrounding the column.

2. If the columns are square or rectangular, cracks starting at the reentrant corners of the floor. Length of the cracks depends on several factors: floor reinforcing steel, concrete mix design, concrete curing method and duration.
If the columns are circular, possibly no problems at all.

3. One or two short rebar (say, 24" long) close to each reentrant corner. Position the rebar perpendicular to the expected path of a crack. This will help limit extent of cracking.
Wet curing the concrete... but that could add to cost compared to curing compound or other short cuts.

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[BUGGAR]

Proper finishing and really, really good curing are my vote. The joints are there to hide or allow for the cracking. Go after the cracking, first.

 

[KootK]

Maybe rebar and good curing is the way to go.

I'm still curious about saw cutting versus tooling. If I take the control joints right up to the column, it would not be possible to saw cut them then, would it? They would need to be tooled?

Also, we've been mostly focused on on horizontal slab movement. There's also a vertical movement issue I believe. As weight is added to the columns and they settle, they will tend to drag the SOG downwards near the columns and create tension cracks in the top of the SOG near the columns. Are the compressible joints, by themselves, generally considered to be an adequate form of vertical isolation?

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.

 

[SlideRuleEra]

KootK - Why not tool the joints for 12" to 18" from the column, then saw cut up to the end of the tool joints? Saw cuts should be made as soon as practical after concrete placement. There would not be much lag time between placement and saw cutting for cracks to propagate from the end of the stubby tooled joints. I have not tried this combination, but it should work.

Expansion material will be fine for both horizontal and limited vertical isolation. A call to a manufacturer's representative will help select the best product: Link

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[darthsoilsguy2]

seems weird to me except for those pre-engineered metal buildings that get slabbed before steel shows up.

most of the time in this condition i've seen the slab is very very thick at the diamond compared to the slab and the column gets encased in concrete to slab elev. suppose you could backfill over the ftgs which hardly ever gets done to the degree of the wide open areas and have soil/stone against the column steel coated or not... diamond concrete is more/less part of the footing than the slab... having diamonds lets the contractor do better finishing technique on small areas... surely this could get bundled in with another concrete misc pour. will the steel be finished with no need to plumb when the slab is done?

 

[TLHS]

Reinforced slab and fibre/foam board or something to isolate the column from the other pour.

 

[KootK]

Why not tool the joints for 12" to 18"

Perhaps. I wonder if the transition would be aesthetically unappealing however.

will the steel be finished with no need to plumb when the slab is done?

Concrete columns this time around.

I pushed back a little regarding risk of cracking and the client caved. The diamonds are back in.






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.

 

[hokie66]

I like to have my joints intentionally miss the columns, with the columns located centrally to the panels and just treated as an opening like a drain. Reinforce around it with diagonal bars, and place a compressible filler against the column. With the joints continuous, you can just saw straight through.

Cracks in a slab on grade in a parking structure are just aesthetic. The joints are just straight cracks, and even then, sometimes you get crooked cracks.

 

[Ron]

Leave the diamonds in place. Form them with a key form and leave in place. Place all concrete at same time. Benefit of both the diamond and no separate placement.

The diamond is there to prevent re-entrant corners, obviously. It is also a "stand-off" termination of the saw-cut joints to allow the sawing operation to finish at the apex of the joint rather than getting close to the column and allowing the shrinkage crack to migrate to the column through the surface of the slab.

The owner will complain about the "random" cracking later, without regard to what he says now. Further, when he sells the building (which he will at some point), the cracks will cost him money in a reduced value of the building....not much, but something and it creates a negotiating point that doesn't need to be there.

 

[KootK]

I like to have my joints intentionally miss the columns, with the columns located centrally to the panels and just treated as an opening like a drain. Reinforce around it with diagonal bars, and place a compressible filler against the column. With the joints continuous, you can just saw straight through.

What a radical departure from standard practice in my area. It's eye opening to see how things taken as inviolate in one market are no big deal in another. One nice feature of your method, I think, would be that your CJ layout could be much more regular than your column layout.

Leave the diamonds in place. Form them with a key form and leave in place. Place all concrete at same time. Benefit of both the diamond and no separate placement.

What? I thought that a major reason for the diamond was to allow the slab to shrink without getting hung up on the columns. Every firm that I've worked with has had verbiage about not allowing the diamond to be cast for a certain number of days which I've assumed was for this reason. Is your understanding different?

With regard to the standoff sawcut terminations, I'm now starting to wonder: how is this done where control joints abut walls?



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.

 

[Ron]

kootk....that shrinkage stuff on the columns is crap. It will make absolutely no difference if you place all that concrete at the same time. That thinking comes from the old school thought that if you alternated placement of concrete in a checkerboard fashion with construction joints it would make the joints better. Rubbish. ACI finally came out with a replacement for that recommendation.

Where joints abut walls, the joint is sawn as close as practicable and allowed to crack on its own for the last few inches.

 

[MrHershey]

I've done it. Not ideal, but not like you *can't*.

Have typically done what hokie advocated. Provide corner bars (especially if you have control joints going to faces of column), and frame it out just like you would a drain or even like an opening in a suspended slab. Just make sure to provide building paper or expansion material or whatever you would normally provide around the columns for the differential movement.

 

[hokie66]

With respect, I think Ron's comment is the rubbish one. Restraint at the columns is a big contributor to cracking. That is the reason for planting the column in the centre of a panel...all the shrinkage is toward the column instead of away.

 

[Ron]

hokie66...I agree with your concept of shrinkage toward colums but disagree that similar result can't be controlled by diamonds. See attached sketch.

 

[hokie66]

My aversion to diamonds is partly because of differential vertical movement I have seen in places. And I just think the location away from columns is simpler and easier to accomplish in the field.

 

[KootK]

My aversion to diamonds is partly because of differential vertical movement I have seen in places.

In this regard, would your method not be nearly identical to Ron's in that they both rely upon vertical slip and compressible joint around the column? If anything, I would think that Ron's scheme might have a slight leg up in that you might luck out and see some of your circumferential cracking right at the diamond.

And I just think the location away from columns is simpler and easier to accomplish in the field.

No doubt about that.

Provide corner bars (especially if you have control joints going to faces of column), and frame it out just like you would a drain or even like an opening in a suspended slab.

I sketched this up while I was at a meeting. For any reasonable corner bar length, the bars would actually intersect across the control joints coming into the column. In a sense, isn't that kind of defeating the purpose of the control joints?

That thinking comes from the old school thought that if you alternated placement of concrete in a checkerboard fashion with construction joints it would make the joints better

I presume that you're referring to these statement from ACI 224.3/302:

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.