Water proofing admixtures 1

[PatrickT]

I'm building our home with a flat concrete roof. The home over looks a lake so we want the roof to be a loftly patio. While visiting the World of Concrete in Las Vegas, I saw a few concrete water proofing methods. Some where placed on the finished slab and other as an Admixture. I belive one was Xypex? The material is suposed to fill small cracks via a crystine formation method. The slab is approx. 3500 sq-ft and will have expansion joints over structural walls where the steel bar joists rest. The expansion joints will be covered with a flexable roofing material.

Any good/bad news on water proofing admixtures?

Patrick T

 

[civilperson]

Regular concrete is water resistant. Put a nominal slope on your roof, (2-3%),and the water will flow away. The water proofing crystaline forming solutions that I am familiar with are applied to a cured surface. Concrete placed with attention to details and crack control reinforcing work as roofs in airport terminals and other architectural significant structures. Stay-in-place metal decking with concrete composite deck over joists is one water proof system.

 

[PatrickT]

CivilPerson,

Thanks for the quick reply. The floor/roof system I'm using is Hambro,

http://www.hambro.ws

The system uses steel joists spaced such that the decking is re-usable plywood. It drops below after setting.

Patrick T

 

[JKW05]

Patrick,

While I agree with civilperson that concrete can be designed and constructed to be watertight, my experience has been that these "patios" over finished spaces are potential trouble spots. I respectfully suggest that it needs to be looked at much more closely than he may be suggesting.

Is the top of the concrete slab the exposed traffic surface? All of the projects I have been involved in have detailed conditions like this as a roof, complete with insulation and a membrane, and pavers as a wearing surface.

The Hambro system is probably using a relatively thin slab. This creates much less of a barrier than is usually found in concrete tanks, which generally have walls at least 12 inches thick.

I think the consensus would be that regardless of how well concrete is detailed and placed, it will probably crack. And while we all strive to minimize the size and number of these cracks, in this particular application, even a minor like will ruin architectural finishes below. Is this project in a location that will be subject to freezing and snow? Ice and snow may not flow away immediately, and as it melts may have ample time to migrate through small cracks.

Where will the insulation be? Will condensation form on the underside of the system in a concealed attic space, where it may then drip onto the drywall ceiling?

The crystalline water proofing system you mentioned (Xypex) can be applied as a coating, but it is more economical to use the admixture (no labor required for prep and installation) and it gets dispersed through-out the thickness of the concrete. I personally like the Xypex product, and have specified it for the concrete tanks that I have designed, both as an admixture, and as a repair coating. Many others feel that a good mix that is properly placed in a properly detailed structure is sufficient for water-tightness. To me, given the variables that we can't control (weather, transport time, the skill of the concrete crew, etc.) the admix is justified added protection where it is critical for a structure to be watertight, especially since we know the concrete *will* exhibit some cracking. However, I would not rely on just the Xypex in a relatively thin slab as a complete roofing system.

Don't gloss over these details, now. Do it right the first time. It will be much more difficult and costly to fix it after the roof / floor leaks and ruins the interior of this "Lake Home". And the owner will NOT be happy when that happens.

 

[PatrickT]

JKW05,

The slab will be 4" thick. There will be 14" of expanded foam below the slab and in direct contact. Yes drywall will hang from below the steel joists, but not untill water proofing has been confirmed!

I'm leaving open the option to use a thin coat of elastimeric coating. This product is designed for flat concrete roofs. Also, Southern Indiana, snow and rain and more rain...

Thank you for taking the time to provide a good over view

Patrick T

 

[Zambo]

As stated by JKW05 it is not wise to rely purely on a 4" concrete slab to provide a waterproof roof.

Xypex as with other products can provide additional protection either as an admixture or applied to the cured concrete surface.

I do not recommend metal decking permanent formwork as the solution as it is expensive and then provieds the additional problem of condensation depending on the weather conditions and aircon environment.

At the end of the day it all comes down to cost and detailing. The detailing includes the slope and the edge detail as well as any joints you might incorporate.

The cost depends on the type of proprietry waterproofing system that you can find locally. I think a bitumen tack coat and then bitumenous sheeting torch applied would be economic. But watch out for additional costs which might be required to get your as cast roof slab finish up to the requirement.

 

[Speedycrete]

You might want to consider 'fibremesh' in the concrete mix as well. This, in addition to other prudent measures (w/c ratio, and attention to the gradation of the finer fractions of the aggregate) will greatly reduce the cracking potential.