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Re: Lt wt. conc topping for exterior decks

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GM-

All the comments you've gotten so far make a lot of sense.  

IMO the deck will leak & on way or another moisture will to get into
the framing.

Drainage & ventilation are important to maintaining integrity of your system.

On the topic of "lightweight" concrete.   I had the opportunity to
attend a one day seminar about 10 years ago at CalTrans on lightweight
 concrete that was classed as structural.

There was a guy from back east (Maryland?) that had extensive
experience with the stuff.  He showed a filed study of a bridge  where
the north bound deck was standard weight & the S/B deck was
lightweight.  It had been in service for years and the L/W had
performed fine.

In addition there were some SE's from Sweden & they showed a bunch of
bridges they had built, some huge, spanning fiords.  They used a combo
of L/W and standard weight concrete to get the dead loads to work for
them in balancing the spans.

All the lightweight mixes talked about were structural, 4000psi+.  

I think the agg was "puffed" shale.  According the agg rep who was
there, this was the key.  If you don't have structural lightweight agg
available in the area then you won't be able get L/W "real" structural
concrete.  My recollection the densities were in the 110 to 125 range.


These structural L/W's are totally different from the gyp-crete crap,
good for sound & vibration reduction but not much else other than
driving up the lateral demand.

I think you're right, the architect (w/ your consultation) should work
this out.
Done poorly, this could really be a problem for the owner downstream

Concrete over plywood in an exterior application is not a great
system, just ask CSULB.

cheers
Bob

On 6/14/05, chuck utzman <chuckuc(--nospam--at)pacbell.net> wrote:
>  In addition, I always make sure the plywood substrate is sloped to drain.
> Water will always penetrate the concrete & you don't want it to pond on the
> w.p. membrane. But eventually the membrane will leak, so marine grade ply. &
> venting are highly recommended as well.
>  Chuck Utzman, P.E.
> 
>  
>  BCainse(--nospam--at)aol.com wrote: 
>  
>  
>  
> Gerard- 
> Others have weighed in on the concrete slab issue so I won't but one thing
> you don't mention is ventilation of the space under the slab.  I have seen
> numerous instances of joists completely rotten through because the joists
> spaces were not well ventilated. Concrete does crack allowing water
> penetration (even with "waterproofing").  A continuous line vent at each
> edge seems to work well by allowing air flow and a place for any trapped
> moisture from direct penetration and/or condensation to rapidly exit. 
>   
> Regards, 
> Bill Cain, SE 
> Berkeley CA 
>   
> In a message dated 6/13/2005 5:13:34 P.M. Pacific Daylight Time,
> gmadden(--nospam--at)maddengine.com writes: 
>  
>  
> 
> I'm designing a house overlooking Napa valley for my best client (he bought
> the house and is doing a massive renovation). Part of the project includes
> about 2000 square feet of exterior decks and terraces over two levels. 
> 
>   
> 
> He wants to use concrete topping slab over plywood sheathing over wood
> framing. I have designed everything for a 2" light weight concrete topping
> slab. 
> 
>   
> 
> He is specifying waterproofing over plywood and 5/8" gyp bd. w/ skim coat of
> stucco on the underside. 
> 
>   
> 
> He asked me about using a fibermesh type of topping slab and I told him that
> my experience with that has been with metal deck filled slabs on the
> interiors of steel framed buildings. 
> 
>   
> 
> Does anyone have any comments, warnings, etc… on the above assembly. I know
> two people on this list aren't great believers in this stuff and I greatly
> respect their opinions, so I'm cautious about saying go for it. 
> 
>   
> 
> He obviously wants minimal shrinkage cracking and is concerned about the sun
> beating down on the finish surface topping slab. Maybe a sealer over the
> slab is a better solution? 
> 
>   
> 
> I hope that made sense, I'm not too sure…hopefully someone can share their
> past experiences good or bad 
> 
>   
> 
> Thanks. 
> 
> -gm 
> 
>   
> 
>
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