Casting a concrete beam at half.

[rcm87]

Hi. At work I'm currently preparing for casting a slab, and the following doubt arised.

One of the beams is a 1m tall beam which starts from the slab and goes up. The slab is a "waffle slab" of 20cm thickness.

The engineer which made the structural design, told us that it is OK to first cast the 20cm of the slab, and the next day we can proceed with casting the other 80cm of the beam.

He sustains this because the beam has continuous stirrups along all the length.

Is this a correct procedure?

Thanks. And sorry if my english isn't the best. I would gladly re-post anything that might not be clear.

 

[Ron]

The beam-slab interface will probably crack at some point anyway, so the engineer has likely considered this with respect to sizing and spacing the stirrups. If the design engineer is happy with it, yield to his discretion unless there is some compelling reason to do otherwise.

 

[rapt]

rcm87

As long as he has checked longitudinal shear and it is ok, then no problem.

It is very hard to get the formwork right for an upturn beam to be poured at the same time as the slab. The inner face of the form is very hard to brace. Much better to have the day old slab to brace it against.

 

[rcm87]

Thanks for the answers.

I've been trying to find in the ACI Manuals something about this. And have been unable to find it.

I'm trusting the engineer who designed the structure. But my civil engineer side wants to wants to learn more about this.

For example, how many stirrups should you use? Won't the joint of the old concrete with the new one, supossing the area were they join is in compression, tend to fail since the beam is not monolithic?

One other thing I found online was that this procedure shouldn't be done in seismic areas, which kind of makes sense.

But I'm still trying to find some design criteria for when this happens.

Thanks for any info.

 

[Ron]

The number of stirrups will depend on how the engineer designed the beam and the loads on it.

As for the compression issue, your interface is on the tension side of the stress block, or depending on loads and reinforcement, near the neutral axis. I doubt that compression is an issue.

As rapt noted, longitudinal shear (shear flow) must be accommodated.

 

[hokie66]

Casting upturned beams in this way is standard practice. There have to be joints in concrete structures. Not everything can be cast monolithically. The joint should be clean and rough. One issue is that the slab will have done a lot of shrinking, then when the beam is cast later, it wants to shrink as well, which will cause vertical cracking in the stem. Depending on the exposure, these cracks may need to be controlled with additional horizontal reinforcement just above the joint.

 

[graybeach]

Check out ACI 318 Section 11.7 on shear friction. You use different mus for different casting situations.

 

[csd72]

Good practice would be to scour the top surface of the first pour to provide a key for the second pour though if the engineer has not requested this then it is most likely not required.

If you are worried about it then this would be the way to go.