Formwork removal of slab embedded beams?

[TarikHKJ]

Hi,

In a two-way ribbed slab with embedded beams, this table from ACI code for formwork removal says 14 days for slabs and 21 days for beams,

Does that mean the contractor should not remove the formwork of the beams until the 21 days have passed, or are the beams part of the slab and their formwork can be removed with the slab at 14 days?


Thanks!

 

[Once20036]

There's a difference between the formwork and the "props".
The formwork is the plywood that holds the wet concrete in place. This is supports by posts/props/reshores. The top category indicates when the concrete is hardened sufficiently to remove the formwork.

As the formwork is being removed, posts/props/reshores are installed. Even after the concrete is set it takes 28 days to reach its design strength/stiffness and these reshores hold everything in place until that occurs. This is asfter 14-21 days depending on your application.

 

[TarikHKJ]

So the embedded beams might have different time to remove the props than the slab?

 

[Once20036]

I`m not familiar with your terminology / embedded beams.
If you have beams spanning more than 6m, timing to remove those props (21 days) is different that the time to remove them from the slab (14 days)

 

[TarikHKJ]

I used this term to describe beams that are not drop beams, which the contractor claims that only drop beams' props should be removed at 21 days, and the beams within the slab are part of the slab itself.

 

[jayrod12]

Are you saying you have a flat plate floor system? With just concentrated rebar making up "beams"?

 

[TarikHKJ]

Its a two-way ribbed slab, with 1.2m wide beams, and I just came here to find if the contractor's claim has merit.

 

[jayrod12]

What does the concrete testing say? what kind of strength have you reached.

 

[TarikHKJ]

at 7 days it has reached 70% of the 30 MPa design strength

 

[SlideRuleEra]

TarikHKJ - IMHO, whether the props are removed at 14 days, 21 days, or based on concrete test cylinder results, all the props should be removed at the same time and in the right order: Slab props, then beam props.

Say the beam props were removed but the slab props remain in place. Since the beams are integral with the slab, the dead load of the unsupported beams will try to deflect, but will not... the slab props will pick up the dead load and prevent significant deflection. I doubt the concrete slab was designed to support the beams.

On the other hand, removing the slab props first just simulates what the beams are supposed to do - support the slab.

My interpretation of the ACI table is that the removal dates are for either slabs or beams, not a combination of both.

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