2" non-structural topping mix

[WWTEng]

It's a 2" non-structural topping over pre-cast floor planks. I am specifying 3000 psi concrete with, W/C = 0.40 and slump 4 +/- 1. But I am not sure about the reinforcement (WWM) to be specified. What would be a suitable choice?

 

[BadgerPE]

For interior exposure I typically specify the minimum amount for T/S. I also like to use something with 12"x12" or 14"x14" (if readily available) spacing so that the workers can step through it and not on it.

 

[hawkaz]

Why is it non-structural? The topping slab can be used compositly to get more strength.

 

[Ron]

WWT...agree. Minimal steel. Keep in mind that a topping slab usually has a higher slump than you've specified. If you have a 0.40 w/c, you will not get a 4" slump without some plasticizer. Also, a 2-inch topping will require smaller coarse aggregate, thus increasing the cement content. That will run the strength up likely above 3ksi. Your topping slab should also be more compatible in strength with your pre-cast, which is likely 4 or 5 ksi.

 

[BadgerPE]

"Why is it non-structural? The topping slab can be used compositly to get more strength."

We typically don't design as composite unless necessary to reduce cost. I feel as though the increased labor cost to get the topping to act compositely with the precast often outweighs the small material increase. Also, often times on longer spans, the precast plank is cambered. When the non-structural topping is poured, its thickness often varies up to 1" from edge to center and therefore it is just more conservative to let the plank carry all of the loading.

 

[Archie264]

hawkaz,

How do you attach it to get composite action? By doweling in rebar or shear studs of some sort?

 

[hawkaz]

That's up to the precaster- but I've usually just seen it done with a roughened surface. No dowels or studs.
The camber note above is valid for longer spans. You can also have an issue at balconies or other sloped slabs where your topping slab thickness can get decreased.

 

[Archie264]

Hmm. Ok. I didn't realize a roughened surface would provide enough mechanical interaction to cause composite action. News to me.

 

[frv]

Roughened surface is what I've seen.

 

[Archie264]

For clarity, I wasn't doubting it, I was just admitting my ignorance. Thanks for the info. I can even think of a project where that might explain why a thin slab has held up as well as it has with a topping slab above it. I had considered the topping slab to be just dead weight but it sounds as though they could be acting in unison.

 

[hokie66]

I don't know why you need reinforcement in a non-structural topping. As the planks will restrain the topping, it will crack, reinforced or not.

 

[brotherson]

I would have thought that's exactly why you need reinforcement. Reinforcement will give you a degree of control over the restraint cracking, with the reinforcement holding the cracks together. You end up with a larger number of smaller cracks instead of a few huge ones. If the cracks need to be tightly controlled you can consider some fibre in the mix.