control joints elevated slabs

[mannypons]

there seems to be a consensus that control joints are not needed/required in elevated concrete floor slabs (composite floor, non-composite floor etc.). is there any written article about this subject to defend myself from an engineer's report that the cracks in the floor is due to the absence of control joints. mannypons :-(

 

[JAE]

I don't know of any documents that say "you don't need control joints in structural slabs". But there also aren't any documents that say "you need to put grapes in your concrete mix right after the addition of bananas".

In other words, could you turn the tables on the engineer and ask him/her to produce documentation that shows that control joints are needed in structural slabs? I don't think they'd find any.

 

[structuresguy]

Are you talking about slabs of metal deck, or concrete flat slabs? A control joint in a concrete slab (not on metal deck) would create a weak point in the slab, which may affect its structural integrity.

However, if you are talking about saw cut contraction joints in slabs on composite metal deck, then these constraction joints become "highly desired" to control cracking in the slab. I have personal experience where the contractor did not put in the saw cut joints as specified, and the slab had extensive uncontroled cracking due to shrinkage, and negative moments over girder supports.

 

[JCWilson]

To overstate the obvious, control joints are only meant to control the location of the cracks, not the quantity. Is the reviewing engineer only stating this "deficiency" for the record, or is this a finished slab where the cracks look ugly as opposed to neat?

The final finish on the slab should dictate whether the cracks are acceptable or not. Their presence alone does not necessarily indicate a problem. As strucuresguy alluded to, negative moment or variable loading conditions could have an affect.

What additional information do you have on slab geometry, loading, etc.? Perhaps you want to post this message in the concrete forum too...

 

[JAE]

structuresguy - I guess I just don't see the difference between "control joints" and sawed joints....both are an attempt to control the location of cracks in a slab and are used for slabs-on-grade. I've never seen a requirement for, or a need for, cut or formed joints in elevated structural slabs.

The cracks you experienced in your past project were of course not desired, but can be controlled with proper mix design and keeping the contractor from adding water to increase workability of the concrete for finishing. Cracks over negative moment areas should be controlled by proper reinforcing in the slab.

But sawed joints in an elevated slab?? I've never used them.

 

[ishvaaag]

Certainy I neither have seen sawed joints in elevated floors in my 26 years long career. Buildings have control joints, you plan and place them.

What the asking party is demanding is to say the least weird, if the interpretation above is the correct one. Crack control in the reinforced structures that are elevated floors or slabs is gained by different means, and it is quite likely that any unsightly shrinkage cracks there are the product of the classicals in the game: heat, wind, bad cure, high diameter rebar, irregular layout of the rebar, concreting against too stiff hardened parts without consideration of shrinkage etc

Other question if you have followed some specified layout in the concreting process. To diminish shrinkage effects, closing bands or checkered concreting and akin procedures are established, and not proceeding in accord with the instructions of the director of the works or specifications, in more than being criticable and cause of liability may lead to some bad results, this could be the case.

 

[mfrad]

Composite metal deck as a form is continuous for typically 3 spans or more. Later, when the concrete hardens with the concrete fill it is composite but is typically designed as a simple span slab and therefore is expected to crack over steel beam supports. These are later filled by the contractor, or ignored, depending on the architects flooring finish or the engineers spec requirements. Therefore control joints are not required in elevated composite metal deck slabs. If you have a garage floor and want continuity you would add mild reinforcement at the supports to make it a reinforced slab. Rememeber the deck is the positive reinforcement for the slab if it is galvanized.