Ad Hoc Construction Joint in Waffle Slab?

[TarikHKJ]

Hi,

I'm not sure if this is going to sound a bit stupid but here's the situation:

I work at an 8-story parking garage building project, we already completed the first waffle slab and now the second slab is on its way, awaiting confirmation for the time to pour concrete,

but one of the management has decided (NOW!!) that this building is too big and needs a construction Joint to split the slab in half to prevent shrinkage, etc.

I'm not sure this is possible now, and I came here for help.

this is an EXTREMELY rough sketch and I will post the structural and other CADs later, :

https://www.dropbox.com/s/gqwb9xwqwd75383/waffle slab problem.dwg?dl=0

PDF if you dont have ACAD at the moment:

https://www.dropbox.com/s/t7s9t1urlkrsr3h/waffle slab problem.pdf?dl=0

not sure if this is possible for my case

http://www.concretenetwork.com/concrete-joints/construction-joints.html

http://www.sefindia.org/forum/files/construction_joints_119.pdf

Much appreciated!

 

[Ingenuity]

I assumed this waffle slab is PT.

I assume you meant NOT PT.

 

[hokie66]

Yep, I left out the key word. Thanks.

 

[TarikHKJ]

Its a dire situation, the seniors are convinced that we must place a construction joint at the span in the middle of the slab, solely to assist with the problem of thermal expansion of concrete. Opposite to what hokie said in his second post that a CJ will not materially assist with that.

Kindest regards,
T. K.

 

[hokie66]

I don't know why it is dire. The joint won't hurt if properly constructed, whether made for good reasons or not.

 

[TarikHKJ]

They want to do something different, to pour areas 1 and 2 and leave a 1 meter strip (area 3) to be poured 1 week later...
The 1 meter strip they will pour later is at third of span in the middle span of the slab

Reference sketch :

http://s19.postimg.org/ogsh5p0n7/screenshot_41.jpg

do you think this will help with thermal expansion at all?


Kindest regards,
T. K.

 

[KootK]

do you think this will help with thermal expansion at all?

It won't help with thermal but it will help with shrinkage which is probably the primary concern. In my neck of the woods, we'd call this a "pour strip" and I think that it's an excellent solution here. If I were the contractor here, I think that I'd want to create the construction joint by removing a line of waffle slab forming pans anyhow. That way you'd make the joint in solid (not ribbed) concrete and, I think, have a much easier time installing the bulkheads and creating a shear key(s).

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]

Yes, shrinkage is an issue, not expansion. And a delayed pour strip is helpful, but a week is not long enough to make a big difference. Agree with KootK that removing a line of pans would make the construction joint much easier to build.

Hard to tell from your plan how much restraint to shrinkage you have. Are there perimeter walls, or are the shaded parts just balustrades?

 

[TarikHKJ]

There are perimeter walls from 3 sides.

 

[hokie66]

Then shrinkage is certainly a big issue. Those long side walls will restrain the slab from shrinkage. I hope you have a lot of reinforcement to constrol the size of cracks.

 

[KootK]

I imagine that's the two short side and one of the long sides. As such, a delay strip across the short dimension will probably make pretty good sense.

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.

 

[TarikHKJ]

The two long sides are perimeter walls, and the fourth shorter side does not have walls to give access to the interfloor ramp.

 

[rapt]

Agree with Hokie66, a one week delay will have little effect on shortening restraint. And with the walls along the 2 long sides, you will need a lot of reinforcement for crack control. Australian code would say at least .6% and possibly more.

 

[hokie66]

I know it is too late now, but this structure should have been designed so that restraint is minimized, rather than provide a lot of crack control reinforcement. With adequate crack control reinforcement, there is still a lot of cracking, but the cracks are hopefully not very wide.

There will also be restraint cracking perpendicular to the shorter end wall.

 

[TarikHKJ]

The latest in joint technology.

https://goo.gl/photos/o6Nno2Q8D1Q8xNeE9

 

[hokie66]

Good to see you have a sense of humour.

 

[TarikHKJ]

is this acceptable?
not sure, but maybe people with more expertise than mine can say their opinion?

Kindest regards,
T. K.

 

[calvinandhobbes10]

If it is truly a construction joint (or delay strip), the entire cross-section of the concrete waffle slab needs wood bulkheads. The ribs, with wood cut to fit tightly against the pans; and the top slab itself, also cut tightly. You're probably seeing why it was suggested to remove one of the rows of waffle pans completely---a little more concrete but a more straightforward joint.

 

[TarikHKJ]

The bulk heads they made did not hold up to the concrete weight, and they ended up cancelling it,
Follow up question, waffle slab formwork removal is 28 or 14 days ?

 

[jayrod12]

It should be in the contract drawings. It's typically expressed in a length of time or a percentage of the design strength.