construction joint question 1

[AskTooMuch]

Contractor is requesting to put construction joint on a slab on grade with grade beams. The c.j. will be located about 6" below top of beam (this is the thickness of slab). They want to pour grade beam 1st upto the slab bottom, then 2nd pour the entire slab including the top portion of grade beam.
Say they follow proper c.j. preparation (roughing and wetting). Will this be ok?

 

[JoelTXCive]

Are there stirrups in the beams? If so, I don't think you will be able to develop the stirrup legs in the 6" slab.

 

[AskTooMuch]

Yes there are stirrups.

 

[AskTooMuch]

No rebar will be cut due to construction joint

 

[jayrod12]

I do this quite often honestly. As long as the reinforcing isn't cut, I usually don't have an issue. Around here, closed ties are the norm.

 

[AskTooMuch]

Jayrod12, do you have them prepare the CJ? Cuz it's a lot of work to roughen top of beam of 1st pour

 

[jayrod12]

They just don't finish the top of beam smooth. They more than meet the 1/4" amplitude required. They have to clean the joint before they do the slab pour, but otherwise there's no special treatment. It's no different than a slab or beam vertical construction joint.

 

[chucklesNOLA]

Be mindful of any differential shrinkage potential in the grade beams vs the slab pour. I've seen a couple of these where the aspect ratio was very high, and there was a long delay between placement of the grade beams and placement of the slabs. Quite a bit of cracking occurred, and I suspect it was from restrained shrinkage of the slab. The grade beams had already experienced most of their shrinkage, fresh slab went on top, and, as it tried to shrink, the grade beams didn't move.

 

[AskTooMuch]

It seems contractor just want to pour grade beam first then leave it for a week before pouring the slab. They just dont want to pump out water and messing up the grade beam rebar when it rains.

They want pour grade beam, then do something else, then pour the slab above grade beam.

 

[AskTooMuch]

jayrod12, how long do you wait between pours (see chucklesNOLA reply)?
Contractor is asking for at least 1 week gap between pours.

 

[KootK]

Like Jayrod, I've seen and done a fair bit of this. The composite action part is a non-issue if you've got any reasonable amount of rebar crossing the joint. As for the restraint cracking in the slab, you might deal with that with some strategically placed rebar placed in the slab beside the beam to keep crack widths small. Shrinkage strain is front loaded temporally and I wouldn't want to find myself in the position of having a lot ride on whether or not a contractor will be back for the second pour within a certain number of hours/days,

 

[jayrod12]

The contractor pours at his convenience. I've honestly never batted an eye at it..

 

[Agent666]

It's pretty normal in this part of the world as well for grade beams and even elevated structures, especially where you are required to cast the first half of a beam to enable seating of precast flooring and the like.