Monday, October 17, 2011

RE: cantilever balcony

These are good points and that is why prequalification of the contractors is important.  The creep issue is pretty well understood these days by the better contractors.  VSL is a good source for bonded encapsulated PT.  GTI is another player, but I have more experience with VSL.  The PT suppliers are a good source for appropriate contractors also. 
With properly designed PT, you can design the concrete to remain in compression.  PT greatly reduces the size and the width of concrete cracks.  Generally, the cracks are eliminated in a cantilvered condition.  As you pointed out, excessive post tensioning can cause an upward camber due to creep and shrinkage.  Again proper design anticipates this.  The fully encapsulated bonded PT system encapsulates the strand in plastic.  The plastic sheath is corrugated and when the space between the strand and the sheath is grouted, but strand is bonded to the concrete, but protected from any corrosives that may penetrate the concrete. 
The long term creep is more of an issue when you try to go too thin in the concrete slab.  That is why I prefer to use a ribbed slab.  The section allows for a draped PT strand and the void area between the ribs reduces weight. 
See below for specific responses.

Regards, Harold Sprague
> Date: Mon, 17 Oct 2011 13:01:29 -0400
> Subject: re: cantilever balcony
> From:
> To:
> I don't know a lot about PT slabs, but maybe Harold knows about this
> phenomenon. I was speaking with a contractor this weekend, and though
> this project was a while ago, they had some major issues with upwards
> curling of cantilevered balcony slabs on a project in S Florida. They
> had issues with the water draining towards and into the units because
> when the slab was tensioned the balconies curled upwards. Not to
> mention the corrosion problems this may cause if you are near the
> coast at that joint. I imagine this is an understood issue now and
> they maybe place the slabs with a slope so they level out when the
> cables are tensioned?
Agreed.  I think I covered most of these issues above. 

> Given the location and the construction type, this sounds like
> high-end construction. Perhaps the best method of protection of the
> slab and the joint will come from your architect. Many high end
> balconies get tiled, and you could provide a step down of an inch or
> two from the main living area, and then provide a waterproof membrane
> over the slab, then a sloped thick-bed mortar tile system over the
> top. There are balcony specific waterproof membrane systems on the
> market that have very specific details and if followed correctly they
> offer guarantees. Then maybe some type of counter-flashing to shed the
> water from the wall away from the balcony slab to wall joint. Also, in
> high end condos in Florida, they now commonly install sliding glass
> doors with very deep tracks, like two inches or more, to prevent water
> infiltration during storm events. Not exactly a big issue in Malibu.
> Harold- say you have a typical flat slab with cantilevered balconies,
> I see these all of the time in Florida on the coast. The top steel
> will keep the cracks relatively tight, and then you can have them
> epoxy injected prior to placing floor finishes. After that, would you
> anticipate much movement other than very slight from long-term creep
> of the steel? (Which is why you recommended PT, along with the fact
> the strands are encased?)
Cracking should not be a problem for a properly designed cantilevered PT slab.  The suggestion of SCC concrete would provide added protection from potential contaminates by a dense concrete and make consolidation easy. 

> Agree with Harold, do not depend on critical items to be properly
> maintained. Especially near the coast. It is a very unforgiving
> environment for steel and steel reinforced structures. There are
> construction companies in Florida where all they do is rehab concrete
> buildings on the coast.
> HTH,
> Andrew Kester, PE
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