Durability of Non-Structural Topping Slab in Cold Climate 1

[KootK]

A client has specified the following for a non-structural patio topping slab:

1) Concrete thickness = 2"
2) Reinforcement = none
3) Air = 5-8%
4) Climate = bloody cold

How would you feel about this from a durability perspective? What is the minimum thickness / reinforcing that you would accept for such an element?



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

Non-structural equals "let the client figure it out" in my book :>

 

[KootK]

Yeah, me too. The flat-work contractor actually came back and expressed concern. Once that happens, let loose the dogs of conservatism!

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

Exactly. Maybe a 4" slab w/ #4 @ 18" o.c. each way would be enough for them to re-phrase their question.

 

[jayrod12]

Agreed wit XR. Give it a C-1 classification as well to deal with the salts the owner wants to use to keep it clean in the winter.

 

[KootK]

Agreed wit XR.

So you like 4" w/18" o/c? I thought XR was being a bit facetious.

I've done a few 1.5-2.0" toppings on above grade wood decks that seem to have fared alright. I dislike durability stuff. There never seems to be any equations yielding binary pass/fail results.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

I'd be comfortable with a 4" thick topping. I'd probably go with 10m at 16" to be honest.

But I think we were both being facetious. I'd have to see what we've done for exterior topping slabs.

 

[mike20793]

I've done 2" with wire reinforcement in sheets and it performs well as a nonstructural topping. Then again, it wasn't in the extreme cold you're probably referring to.

 

[Teguci]

2" topping without rebar sounds good to me. I'd be more concerned with the volume change differential between the existing reinforced slab and the new topping slab, but rebar isn't going to help that. Make sure they prepare the existing surface properly to get the best adhesion between surfaces. Due to salt, I like the idea of not having reinforcement in the topping slab. Just make sure they match the control joints in the new topping.

I'm assuming this slab will be cast during non bloody cold conditions.

 

[KootK]

Thanks for the advice so far gentlemen.

I've been torn about whether or not to include reinforcement in a 2" topping as well. On the one hand, it would provide some degree of crack control. On the other, it's steel subject to corrosion in a setup where cover will be inadequate. I suppose that one could use some manner of reinforcement other than steel.

One important feature that I should have mentioned is that the topping slab is seperated from the main, structural slab by a layer of insulation and waterproofing membrane.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Teguci]

For the top wearing element of a plaza system, you can go for a paver type system where the concrete control joints are 4 ft oc or so and allow these pavers to shrink and move a little under flexure, or you can go for a stiff solid concrete slab that will shear against the supporting layers under flexure. If you want to provide a stiff system of concrete, then you want to reinforce it properly (including cover)and detail a slip plane within your drainage and waterproofing membrane. You do not want the waterproofing membrane to become the slip plane.

 

[Brad805]

What are the soil conditions? If this is on rock or sands I would be less concerned than if you have any potential for swelling.

 

[KootK]

then you want to reinforce it properly (including cover)and detail a slip plane within your drainage and waterproofing membrane. You do not want the waterproofing membrane to become the slip plane.

Good point Teguci. Can you comment on what a suitable slip plane construction might be? In this particular case, the affected areas are pretty small in plan.

What are the soil conditions? If this is on rock or sands I would be less concerned than if you have any potential for swelling.

Gravels and sands, no swelling potential. This is, however, a suspended slab condition.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Teguci]

If you have a small area that doesn't deflect much, I'd say don't worry about it. For large areas, the slip plane is created between a thick plastic sheet (like mylar) and the rigid insulation.

 

[Brad805]

I should have figured that out. I agree with Teguci. Seems close to being a design component that one might consider "by arch"

 

[FoxSE14]

Seems as though this is a place that may need to remain aesthetically pleasing for resale value?? By architect, by architect, by architect. When adjusters/lawyers start circling because half of the patio slabs are cracked, you'll want to be on the sidelines (as much as possible).

Initial shrinkage and thermal movements are helped greatly with a slip sheet/visqueen layer below the floating slab...it will crack though without some amount of control jointing (not to mention when someone drops their bowling ball on it).

Cannot recall absolute specifics, but in a similar situation where this was our baby to deal with: we had 3 or 4" floating/plaza slab with welded-wire fabric reinforcing, and tooled joints at something like 4' or 6' o/c. Low W/C ratio (Chicago - heavy de-icing salt use) Went with tooled joints, and they are a lot nicer looking than saw cuts. Final product looked nice with only a couple of corner cracks on something like 20-25 patios. I never heard of any issues in the years since.

...FWIW, I would be raising the same concerns if I was the contractor. If the architect asks your opinion on the record, I recommend you get on record noting that some measures could be made to improve durability and life of his overlay.

 

[ajk1]

To Kootk: why not use stainless steel rebar to protect against the ravages of corrosive chloride deicing chemicals? Yes they are about 5 times the cost of black rebar, but if the patio is not too big an area, it might not cost that much, and then you can sleep nights.

 

[KootK]

o Kootk: why not use stainless steel rebar to protect against the ravages of corrosive chloride deicing chemicals?

I like it. How would we feel about a 3" topping with galvanized WWF at mid-depth?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.