[GSBN] Looking for experiences with AAC and Rastra

[RT]

Chris Magwood wrote:

> I'm wondering if anyone out there has much or any experience workingwith  
> Autoclaved Aerated Concrete (AAC) blocks and/or Rastra blocks for 
> foundations?

[snip]

Bruce King wrote:

> I worked with Rastra once, wouldn't again.  Generally agree with
> Derick about both Rastra and AAC.  They just don't interest me any
> more as first, second or third world applications.  Maybe some other
> world somewhere.

[snip]

(for full text of this thread, see:
      http://greenbuilder.com/pipermail/gsbn/2010q4/001125.html     )



Unlike the King of Sausalito, I've never had the displeasure of working  
with Rastra or any foamed plast-eccchh! ICFs.

Just looking at foamed plast-eccchh! ICF systems was enough to tell me  
that:

  (1) They are bass-ackwards approaches to thermal and structural design.

      (a) Thermal: They isolate the high embodied-energy concrete and steel  
behind thin layers of thermal insulation thereby negating any significant  
contribution that it otherwise might have had as beneficial thermal mass  
in moderating temperature swings for the enclosed living space.

       (b) Structural: Placing the structural element (ie the concrete) at  
the core is the least effective placement possible. It is so useless at  
that spot (ie the neutral axis of the cross section) that it needs a  
$#!+-load of even higher embodied-energy reinforcing steel embedded in the  
already high embodied-energy concrete just so the stoopit thing won't  
collapse with the first good gust of wind.

And of course, all that ultra-high embodied-energy embedded reinforcing  
steel is also placed at the neutral axis so that it isn't being utilised  
to anywhere near its potential.

That is to say, all of that resource-intensive/expensive material is being  
wasted by being used inefficiently.

And to add insult to injury, at the end of the service life of the  
building, all of that high embodied-energy steel and concrete cannot be  
reasonably deconstructed for re-use.

and

(2) Although foamed plast-ecchhh! ICFs are ostensibly intended to simplify  
wall building to the point that novice owner-builders should feel  
comfortable in taking on the wall erection process themselves (ie ICFs are  
for people who don't know how to lay up masonry),  the reality seems to be  
that very few owner-builders end up doing so.

In fact, of the foamed plast-echhh! ICF projects that I've monitored, they  
have all required more paid man-hours
and longer time frames to complete. (ie Higher labour costs)

Another common thread with foamed plast-eccchhh! ICF projects in my area  
seems to be that that are all grossly over-sized, upwards of 3000 sf.. Why  
? It seems that the owners are usually suckered by the "superior  
insulating qualities" claim so they feel that it's okay to make the  
interior volume bigger because the building will be energy efficient.  
(Wrong and wrong.) Two thin layers of foamed plast-eccchh! does not "well  
insulated wall" make.

When people ask me about foamed plast-ecchhh! ICF systems, I usually  
mention the above in the hopes of dissauding them but far too often they  
already have their hearts set on ICF walls so I usually end up pointing  
them to Durisol ICFs as a more Green and better alternative to foamed  
plast-ecchhh! ICFS ... and mention that a core-insulated, double-wythe CMU  
wall would be a far better approach, not only because the latter would  
provide a better wall thermally,structurally and economically, it provides  
the capability of easy deconstruction/re-use of the high embodied-energy  
concrete (ie CMUs) at the end of the service life of the building.

Not only that, here in Canada where relatively Green "Roxul" brand mineral  
wool insulation is readily accessible, it can be used as the core  
insulation, thereby completely eliminating foamed plast-eccchhh! from the  
picture.

					=== * ===

re: AAC blocks

I share the Derelict's and the King's disdain and disinterest in AAC block.

Their mortar joints make them useless structurally from a (out-of-plane)  
lateral resistance point of view so that they have to be re-inforced with  
a $#!+-load of steel. Same structural stupidity as with foamed  
plast-eccchhh! ICFs.

If used for foundations (bad application) the mix would have to be at the  
high end of the density (hence low end of thermal resistance so that the  
point of using them in the first place (ie insulating quality) becomes  
somewhat redundant.

The Derelict mentioned that one might acquire som

e Greenie Points in using AAC blocks since their lower density would mean  
less concrete and hence, less cement consumed.

I very much doubt that there would any reduction in cement consumption  
since the particle sizes of the aggregate used in AAC block are (I'm  
guessing) likely 3/8 inch and smaller ... whereas with concrete, the  
coarse aggregate which makes up the bulk of the volume of the concrete  
would likely consist of particle sizes 5/8" and larger.
ie less surface area to coat with cement paste = less cement consumed.

I remember WatJohn (aka Dr. John Straube) mentioning (a long, long time  
ago) that a wall made of AAC block would have a higher embodied-energy  
than the same wall made of solid fire clay brick.  I never crunched the  
numbers to verify that claim but I don't doubt that it is reasonably  
accurate.

If an analogy were made between wall systems and vehicles,  with bicycles  
at the Green end of the spectrum, I'd venture that AAC block systems would  
be "jet aircraft" in that spectrum.



-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at  Y a h o o  dot  c a >
manually winnow the chaff from my edress if you hit "reply"
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