Slab-on-grade maximum pour size

[ajk1]

Two questions:

1) Is the ACI recommendation for slab-on-grade pour size 9000 SF? If so, in which ACI document is this?

2) Is there a maximum recommended length of pour?

 

[Ron]

ajk1...have not seen a limitation given by floor size; however, the amount of area to be placed is limited by crew size, concrete delivery and other variables, not an arbitrary size limitation. The same is true of the length of placement...depends on crew size, resources and concrete delivery.

 

[ajk1]

Ron:

If the pours are too long, then the construction joints between pours may open excessivly, when the concrete dries and shrinks; the sawcut control joints don't necessarily break open and accommodate the shrinkage movement, particularly if placed on a low friction surface (we have had this problem many yeras ago), so I think it is prudent to have a maximum length of pour.

I had thought that one of the ACI Standards had a suggestd limit in maximum area...I expect it would be the Standard on construction...I should also check books on slab-on-grade.

 

[jayrod12]

If you have ample crew, concrete supply and good scheduling then you could in theory pour forever with no cold joints. but you would need the concrete layers, finishers and sawcutters all lined up perfectly to make it work. You can't put a number on it, the contractor should be able to say how much their crew can successfully pour in one go. I feel the engineer is not in charge of the ways and means (i.e. pour in 1 go instead of 4 little ones, etc) as long as the finished product performs as designed.

 

[msquared48]

ajk1:

You seem to be talking apples and oranges here. Ron is talking capacity and you seem to be talking control joint spacing or design.

The 9000 square foot figure would be more related to capacity, but, as Ron says, that is very arbitrary and not in any code document I know of.

However, relating to construction joint spacing, areas of 400 to 900 square feet have been regularly used to control crack width from shrinkage.

Mike McCann
MMC Engineering

 

[ajk1]

To clarify the apples and oranges issue, which has arisen due to my lack of care in framing the original question:

I am not concerned with how much a construction crew can place in a day or how many crews may be needed. I am also not asking about control joint spacing (for that we conform to the 4.5 m maximum spacing and not to exceed a certain multiple of the slab thickness depending on exposure conditions i.e inside heated building envelope or not; low shrinkage mix, etc.).

My question might perhaps be most clearly put as follows:

What is the maxium length of pour between construction joints that can safely be used (when sawcut control joints are made at the appropraite timing and spacing in accordanc with good practice) without causing performance issues with the slab? I generally take about 20 m, although this may depend on whether low shrinkage concrete mix is used (a relatively expensive thing to do) but I would be interested in hearing your views. One of my concerns is that the construction joints will open excessively due to shrinkage, if the distance between them is too great, irrespective of how well the sawcut control joints are done.

 

[hokie66]

If the sawcuts work, your concern about distance between construction joints is not warranted. Both sawcuts and construction joints are cracks...straight ones.

 

[Splitrings]

The width of the opening at the control joint is directly related to the spacing of the control joints, not the length of the total placement. Shouldn't the sum of the parts (opening widths at each joint) be equal to the whole(total shrinkage of the total slab). A 1 mile length of concrete pavement should shrink, on a per length basis, exactly the same as a 1 foot placement.

Maybe I am nissing something here.

 

[EngineeringEric]

As others have said, the gap opening at the control joint would only get larger if the slab never cracks at the saw cuts. If total shrinkage is say X% then we distribute that shrinkage to each Sawcut so the CJ does not get X% of 21m but only X% of 4.5m (using your values of spacing). So the CJ separation is equal to that of any sawcut... Did i make that more complicated than it needed to be

 

[hope9010]

Thanks for the comments. As I stated in my 10:41 post to Ron yesterday, we have found that the sawcut control joints do not necessarily all crack open in every case. I also mentioned our experience that when the slab is placed on low friction material, there is not enough friction to generate a force of sufficient magnitude to break all the sawcut joints open. We found that in this case (the slab was placed on polystyrene insulation so the friction would be expected to be low), much of the movement accumulated at the end of the pour and the gap that opened was quite pronounced and unacceptable.

It seems from the responses that the general feeling is that our experience is not typical of slab placed on the usual subgrade, even if there is a polyethylene sheet below. That is interesting and I will attribute our experience to the particularities of that job.

However, it leaves me wondering why ACI 223 limits the area of a pour. Why do you suppose that this would be?

 

[ajk1]

..one more thought: if there is welded wire mesh in the slab, as some engineers (not me) use, then the concrete between sawcut control joints will be pulled along. So the notion that the sawcut control joints eliminate the need for a maximum pore length is incorrect in that case. Similarly, as I have noted above, if not all sawcuts break open (as was our experience when the concrete was placed on a s low friction surface like styrofoam). So my opinion is that limiting the maximum length of pour (as ACI recommends) is soundly based, although the magnitude of the limit is open to question.

 

[Splitrings]

Seems like increasing the depth of the saw cut would help. I suppose you could attempt to calculate how deep the cut needed to be based on the tensile strength of the remaining slab and an assumed friction factor.

 

[ajk1]

As you imply, that would reduce the shear teansferred across the joint by aggregate interlock. I am not sure I would be comfortable doing that.

 

[hokie66]

Don't you discontinue some of the wires or bars at the sawcuts in order to ensure that the slab cracks?

 

[ajk1]

No, as far as I am aware that is not done for slab-on-grade mesh. That would be a bit of a chore. I can double check with our head of field inspection department. But like I say, we don't generally put mesh in slab on grade, although perhaps some engineers still do. It is a different situation of course for walls with weakness joints, where 50% of the rebar is stopped at the weakness joint.

 

[ajk1]

just to add to that: the mesh almost always ends up in the bottom of the slab, so it does not do much good.

 

[hokie66]

I have ALWAYS specified half the reinforcement in slabs on grade to be stopped at joints, and think that most of the time it happened. Not much of a chore at all, just requires a straight line and some bolt cutters or something similar. I agree that small mesh doesn't do much good, but large mesh does. And the location is not critical.

 

[ajk1]

Interesting. Cutting all the wires manually with a manual bolt cutter on large slab-on-grade would seem like a bit of a chore to me, but you have seen it done and I respect your opinion on that. I have learned something new. Thanks.

Have you found that is the North American Practice in your experience?

I have not seen that specified or done around here but perhaps I am wrong because it has been about 30 or 40 years since I was on site while a slab on grade with mesh was being placed, but I will check with my colleagues when I get into the office. Maybe others on this forum can comment. Like I said earlier, we and many other firms generally stopped specifying mesh in slab on grade about 30 or 40 years ago.

Have you gone to the site to check that it is done?

What do you mean by "large mesh"? For the usual 6"x6" 6/6 mesh, it usually ends up on the bottom of the slab, as many articles have noted and as we have proven by cores. That is why they developed the much wider spaced mesh that the workers can theoretically step between the wires, but which I have not seen used in this area of the continent (Toronto area).

 

[EngineeringEric]

In my part of the US, for regular commercial project slabs we will still specify WWF sometimes. And we specify cutting or discontinuing the reinforcement at the saw cuts. If you are concerned about aggregate interlock, are you providing a shear keys at construction joints or just rough surfaces?

Now i have not done an entire slab on foam so i cannot comment on increasing the saw cut depth but that would be my first thought as well.

 

[racookpe1978]

15 May 14 21:07
Don't you discontinue some of the wires or bars at the sawcuts in order to ensure that the slab cracks?

Think about the different problems the OP is combining as well: He has mentioned both expansion/contrsction (temperature movement) and settlement: the difference between say a 6 kilometer long uniform road expanding/contracting in the 6 km direction but bending easily as the earth moves up and down under each 5 meter length, and a uniform 45 m x 45 m slab inside a large temperature controlled building on top of a "slippery" insulation.

In the first case, the cut cracks will occur across the road at each 5 cut even if the mesh or rebar isn't cut at the 5 meter joints: There is a lot of resistance between concrete and ground, and the mesh is going to be "flexible" the long way. So the concrete cuts (or joints) at 5 meters are going to work: each 5 meter section will probably not break within the 5 meter, and each section will move independently of its neighbor. Even more so if each lane is separated from adjacent lanes also.

But the big building slab on a well-compacted dirt/fill/prepped area has little reason to "break" at the small 4.5 meter distances, and a lot of reason to behave like was mentioned: the concrete stays together as it contracts (slides over the slippery insulation underneath). So, in the big building, you need to cut much, much more rebar (all of it ?) at each joint to prevent all of the little squares from moving sideways at the same time as the concrete contracts/expands.

Could you put in deeper keyways into the insulation so all of the joints don't have to have cut rebar to support rolling lods like forklifts or trucks? I don't that would work - the keys would be stress risers instead. Pour a fancy (more expensive!) (much slower!) overlapping joint?