[GSBN] Looking for experiences with AAC and Rastra

[Derek Roff]

Thanks for your comments, Bill.  If Autoclaved Aerated Concrete (AAC) 
blocks are subject to spalling, freeze/thaw cracking, and other 
moisture-related damage, use in a foundation or stem wall could be a 
significant problem.

I'm concerned about the embodied energy question, but reminding myself: 
at a density of 1/5th to 1/20th that of concrete, AAC can gather some 
greenie points where it replaces or reduces a greater mass of concrete 
used in a structure.

I was surprised to learn that AAC was invented and put into commercial 
production and use in Sweden, during the 1920's.  I guess it's been a 
bit slow to catch on in the USA.

Bill, in talking about FlexCrete, what did you mean when you wrote, 
"similar aerated concrete block which is not aerated"?

Thanks,
Derelict

Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm.edu


--On Wednesday, December 22, 2010 8:08 PM -0600 bill christensen 
<billc at greenbuilder.com> wrote:

I have no experience with Rastra other than looking at some finished
projects, but there are handful of houses in my neighborhood which
were built with AAC.  Here's one of them:
<http://FirstAustinLeedHome.com>.  Another can be seen at
http://greenhomesforsale.com/listing.php?id=19098  and
http://www.listedgreen.com/index.cfm/page/property-listing/adid/1324

AAC makes a nice building, though the material is a bit expensive and
like anything new, figure some extra labor for the learning curve.
They're quiet, solid, and well insulated.  It's relatively easy to
work with, but can be quite dusty too.

I've collected some of the leftover block from some of the projects,
and I've discovered that raw, unplastered AAC doesn't do well with
water and freeze/thaw cycles.  Some were stored below a shed roof for
a number of years which poured water into the center of the stack...
and we had some freezing.  When we went to move them the blocks in the
center of the stack had begun breaking down - lots of cracks and
fissures, some straight through the block.  Many broke.  Those near
the edge which had a chance to dry out were far more solid.

The second of the two homes above has a low wall also made of
plastered AAC as well, with a flat plastered top, and they experienced
a fair bit of spalling.  The builder said, while doing the repairs,
that it really shouldn't be done without a cap.

Regarding embodied energy, you might want to do some research on
<http://flex-crete.com/>, or similar aerated concrete block which is
not aerated.

I'll be happy to put you in touch with the builder if you'd like.

On Wed, Dec 22, 2010 at 11:09 AM, Derek Roff <derek at unm.edu> wrote:
> I've worked with Rastra a few times.  I don't like it.  I don't 
believe
> their insulation figures, even after they have scaled them back. 
Working
> with Rastra gives me a powerful reminder of some negative aspects of
> concrete and styrene, the component parts.  Cutting and shaping is 
possible,
> but it produces a lot of dust.  The dust is awful to be around.  I 
don't
> know if it is fair to say that the material is fragile, but it 
certainly
> surprised us multiple times with sudden breaks.  Working with 
ten-foot long,
> 150 pound blocks carries a significant injury risk to a small, 
amateur crew.
>
> I've worked with two different kinds of Autoclaved Aerated Concrete 
(AAC)
> blocks.  They are more pleasant to work with than Rastra, for me. The 
ones
> that I used were very consistent in size and shape, with crisp edges 
and
> corners, which could be chipped in handling.  A few blocks cracked 
when
> dropped, but they seemed fairly strong.  I have concerns about the 
embodied
> energy, the need for two different kinds of massive kilns to make 
them, and
> their insulation claims.  It seems like every company talks about the
> amazing properties of their 2 pounds per cubic foot blocks, but the 
ones
> they actually sell have a higher density, which they often don't 
list.  The
> ones that I have used are still pretty light, but very different from 
what
> the industry keeps promising.
>
> AAC blocks have a very rectilinear, industrial feel, which is the 
antithesis
> of the organic houses that I like.  But is seems reasonable to me to
> consider this option for foundation/stemwalls.
>
> Derelict
>
> Derek Roff
> Language Learning Center
> Ortega Hall 129, MSC03-2100
> University of New Mexico
> Albuquerque, NM 87131-0001
> 505/277-7368, fax 505/277-3885
> Internet: derek at unm.edu
>
>
> --On Wednesday, December 22, 2010 10:41 AM -0500 Chris Magwood
> <chris at chrismagwood.ca> wrote:
>
>> Hey all!
>>
>> I'm wondering if anyone out there has much or any experience working
>> with Autoclaved Aerated Concrete (AAC) blocks and/or Rastra blocks
>> for foundations?
>>
>> Neither product is readily available here in Canada, but I'm
>> preparing some info on insulated foundation options and would like to
>> include something about these products if I can get some info. I've
>> obviously web-researched both, but I'm looking for actual
>> experience... did they meet your cost expectations, were there
>> peculiarities about the install process, how would you rate them
>> environmentally, etc?
>>
>> Feel free to respond to me off-list, as I know this isn't exactly SB
>> stuff. However, I'm hoping that among the vast experience included on
>> this list would be some hands-on stories about these materials.
>>
>> Happy holidays!
>>
>> Chris
>
>
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