Rooftop topping

[cris300]

Hi

I'm designing a toppping concrete (3 inches) on a slab (5 inches) but do not want that the topping crack that tips should I take the time to build

the slab is built
and the surface are floated

I have no snow effects

And whats would be cut widths if i have a topping of (3 inches)


thanks

 

[JedClampett]

Your question is kind of garbled. Could you restate it?

 

[cris300]

Sorry

Here again the explanation more Detailed

I have the rooftop of a parking building with a structural concrete slab (working in one direction) with 5 inches
of thickness and span of 10 feets, supported by beams 76x152 inches the total area is 25,000 square feet

I want to place over this slab a bonded whitetopping with 3 inches of thickness purpose of preventing water leakage
but I worry about the cracks retractions all the documentation I've found is about pavements and parking buildings with T prefabricated beams and doesn't help me

I think to place a steel mesh into de concrete of the whitetopping but i'm not sure whether it will be sufficient to prevent cracks and I am also undecided about the distances to make cuts

thanks

cris

 

[JedClampett]

Is the rooftop a parking area?

 

[msquared48]

I'd be more worried about the additional dead load of 35 to 40 psf, and the fact that this is REAL close to the probable live load of 50 psf for the parking.

Does your system have the capacity to support this without major modification or collapse?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[cris300]

I took load in the design from the beginning

Thanks for your concern

 

[cris300]

yes it is a parking area

 

[JedClampett]

I think whatever you do with a rigid topping is going to crack and leak. If you saw cut, you're introducing cracks that are likely to leak. Provide good drainage and well planned expansion joint details.
Why are you so worried about a leakproof surface in a parking garage? Cars are made to get wet.

 

[cris300]

Thanks for your answer

i'm worried about the leak and cracks becouse
the cars will be on the roof,
down there will be a supermarket

 

[hokie66]

You can't use a topping as waterproofing. Period. You have to have a waterproof membrane, and concrete is not waterproof. The membrane goes under the topping.

 

[shobroco]

Anybody heard about a place in Northern Ontario called Elliot Lake?

 

[Ron]

Agree with hokie66...you have to waterproof before you consider the topping. A membrane is absolutely essential. Don't try to stop the cracks in the topping....control them and seal them.

 

[cris300]

I will use a membrane as you recommend
but there is no way to ensure that the cracks do not appear?
because the supervisor is very strict with this

perhaps with fibers,
what do you think?

 

[JedClampett]

Rule No. 1: Concrete cracks.
Rule No. 2: Engineers can only pretend to do something about Rule No. 1.
Reinforce your concrete, put in joints on a tight grid (5 to 6 feet OC), water cure and prepare your boss for the worst.

 

[hokie66]

An amendment to Jed's rule...concrete shrinks, and if it is restrained, it cracks.

Therefore, a bonded topping will crack more than one which is free to shrink without restraint.

Reinforcement controls cracking, and the amount and size of the reinforcement determines the width of cracks.

 

[Ron]

....and I reiterate....control the inevitable cracks and seal them.

 

[Quanski]

An engineer should be hired to evaluate the structure. I would be less worried about cars getting wet and more about corrosion issues that can have major negative impacts on the structure. Hire a structural engineer with experience in structural rehab.

 

[hokie66]

This is a roof, as well as a floor, and a roof is not supposed to leak. But don't discount the problem of leakage onto cars...they aren't "made to get wet", at least from the water that comes through a concrete slab. I recently did a report on a structure where there was a "green roof", planters...that sort of thing, above an underground parking area. Leakage had been left for a period of time, and there was alleged damage to expensive automobiles in the high six figures.