Info on Cob cistern and Cob Swauna (sweat:sauna)

[Vernon B. Johnston]

i'd appreciate any info on cob and cisterns. 

this is kind of an aside - maybe pat newberry might know best:  does 
anybody know why it is inappropriate to mix straw with clay if one is making 
bricks for a fired structure 'a la nader khalili'? i was wondering about 
methods for air-entrapment with ceramic structures - i thought maybe firing a 
cob structure might result in a strong, water-resistant but relatively 
light-weight semi-porous (ie. higher r-value) ceramic structure. or does the 
straw expand too much when it is burning, causing the clay/adobe to crack? 

the reason i mention this is that i was wondering whether the steam 
from the sweatlodge might saturate the cob with so much moisture that the 
building would slump after a while, reducing the strength of the dome. i've 
never built anything with cob, i was just curious whether you thought this was 
a valid consideration. i'm trying to get a feel for how the material behaves 
over time. i would imagine (based on no experience whatsoever) that cob in a 
humid environment would act like wax in warm environment - that it would 
basically melt, but really slowly. what do y'all think? 

thanks 
lars fields

We are very interested in your cob cirstern. When you are able, please post
more info on this!  Thank you for your post about the cob sweat, it was
quite informative.

Pete


Your Welcome!  I am enjoying the process of building with earth immensely.  It makes me feel good.  As for the cistern, I was considering various structures to collect the rain water in.  I don't believe they were overly pricey ($500 to $2000) considering the value that I place on water, but if I could come up with an alternative to plastic, concrete, or galvanized then that money could be spent on something else  -  something pleasurable like traveling to sunny warm climates in the winter, or dinner and a good movie, or...???   What I thought might work would be a cob cylinder raised off of the ground just like a cob home.  It seems to me that 12 inches or so thick would work just fine.  The concerns that I had were:  (1) would any severe freezing cause an ice expansion inside the container to break my tank, and (2)  how do I keep the water in the tank.  
    In my area the temperature may drop to 15 degrees F, and on occasion to zero.  I remember reading somewhere (when I find it again I'll post the reference) that with the mass of the water and with  the mass of the structure it should be all right.  Anybody else have any answers, experience, or ideas?  I am willing to bet that the straw which acts like little rebars will be sufficient, especially if I "knit" well with lots of long straw. 
     The other reason that I am willing to bet it will be fine is the synergy created by the use of cement mortar to line the inside of the tank.  Not only do I get a decent liner to keep my water in, I get increased tensile strength.  Anyway, that is what I decided to line the Cob tank with - about a half inch or so of cement mortar, either hung on chicken wire or metal lath.  I know that Lars did not want to use concrete, but perhaps this liner would be minimal enough to be acceptable.
    With those two concerns answered in my own head I did comfortable enough to build the periphery foundation for the cistern.  But... what really made me feel good about the project was a section in a book entitled "Ferrocement Water Tanks: and Their Construction" by S.B. Watt.  Don't worry!  This book has more to it than ferrocement.  It seems as though the Dogon people in Mali were in need of water storage for the water that they got from their roofs.  What they used were the old granary bins which had capacities to 10 cubic meters or more.  Before the used them for water storage they lined them with 1 cm or so thick of cement mortar mixed 1:3 (cement:sand by volume) over chicken mesh that was stapled to the inside of the bin.  
    These granary bins were right along side of their homes so their location was perfect.  Also...they were constructed of an adobe material with was reinforced with grass fibres.   Sounds like cob to me!!!
    When they build a new storage tank, the Dogons rounded  the bottom of the basin using a soil-cement mixture of about 1part cement to 10 parts soil.  What I did was after I built my rock foundation (up about a foot to keep run-off from penetrating the tank and to protect the cob from run-off and rain) I dug out a concave shape in the bottom of the structure.  I will then build it up smooth with cob and/or cement:soil mixture.
     Hope this answers some of your questions about my cob cistern. Now to the Cob Swauna.

    Lars, your concern is whether the cob structure will "melt" due to the action of the steam.  That is not a concern of mine.  The amount of time the Swauna will NOT be in use will allow a lot of airing out and drying time for the cob.   Also, I do plan on the cob to be fairly dry and tight before the first sweat.  Cob structures that have not been plastered, have an adequate roof ,and have been exposed to the elements seem to hold up just fine.  Sweats generally are not used on a daily basis, but if they were I feel the structure would be fine.  There is the possibility of plastering the inside, but I have not given that much consideration.  Thanks for the question.  It does get me thinking!
    By the way, when you start to build a cob sweat/sauna, don't start to corbel your wall in too soon.  I spent almost 6 hours readjusting my walls because I started to go in too soon.  We need room for the our backs and head, so after you start building sit in it to get a feel for what it would be like.  And... for sure, make sure that if you haven't been in a sweat before, do it before you build.

Thanks  -    Vernon      vajohnston at nas.com
    
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<DIV><EM>i'd appreciate any info on cob and cisterns. <BR><BR>this is kind of an 
aside - maybe pat newberry might know best:  does <BR>anybody know why it 
is inappropriate to mix straw with clay if one is making <BR>bricks for a fired 
structure 'a la nader khalili'? i was wondering about <BR>methods for 
air-entrapment with ceramic structures - i thought maybe firing a <BR>cob 
structure might result in a strong, water-resistant but relatively 
<BR>light-weight semi-porous (ie. higher r-value) ceramic structure. or does the 
<BR>straw expand too much when it is burning, causing the clay/adobe to crack? 
<BR><BR>the reason i mention this is that i was wondering whether the steam 
<BR>from the sweatlodge might saturate the cob with so much moisture that the 
<BR>building would slump after a while, reducing the strength of the dome. i've 
<BR>never built anything with cob, i was just curious whether you thought this 
was <BR>a valid consideration. i'm trying to get a feel for how the material 
behaves <BR>over time. i would imagine (based on no experience whatsoever) that 
cob in a <BR>humid environment would act like wax in warm environment - that it 
would <BR>basically melt, but really slowly. what do y'all think? 
</EM> </DIV>
<DIV><EM><BR>thanks <BR>lars fields</EM> </DIV>
<DIV><EM></EM> </DIV>
<DIV><EM>We are very interested in your cob cirstern. When you are able, please 
post<BR>more info on this!  Thank you for your post about the cob sweat, it 
was<BR>quite informative.<BR><BR>Pete</EM> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV><FONT color=#000000 size=2>Your Welcome!  I am enjoying the process of 
building with earth immensely.  It makes me feel good.  As for the 
cistern, I was considering various structures to collect the rain water 
in.  I don't believe they were overly pricey ($500 to $2000) considering 
the value that I place on water, but if I could come up with an alternative to 
plastic, concrete, or galvanized then that money could be spent on something 
else  -  something pleasurable like traveling to sunny warm climates 
in the winter, or dinner and a good movie, or...???   What I thought 
might work would be a cob cylinder raised off of the ground just like a cob 
home.  It seems to me that 12 inches or so thick would work just 
fine.  The concerns that I had were:  (1) would any severe freezing 
cause an ice expansion inside the container to break my tank, and (2)  how 
do I keep the water in the tank.  </FONT></DIV>
<DIV><FONT color=#000000 size=2>    In my area the temperature 
may drop to 15 degrees F, and on occasion to zero.  I remember reading 
somewhere (when I find it again I'll post the reference) that with the mass of 
the water and with  the mass of the structure it should be all right.  
Anybody else have any answers, experience, or ideas?  I am willing to bet 
that the straw which acts like little rebars will be sufficient, especially if I 
"knit" well with lots of long straw. </FONT></DIV>
<DIV><FONT color=#000000 size=2>     The other reason that I 
am willing to bet it will be fine is the synergy created by the use of cement 
mortar to line the inside of the tank.  Not only do I get a decent liner to 
keep my water in, I get increased tensile strength.  Anyway, that is what I 
decided to line the Cob tank with - about a half inch or so of cement mortar, 
either hung on chicken wire or metal lath.  I know that Lars did not want 
to use concrete, but perhaps this liner would be minimal enough to be 
acceptable.</FONT></DIV>
<DIV><FONT color=#000000 size=2>    <FONT color=#000000>With 
those two concerns answered in my own head I did comfortable enough to build the 
periphery foundation for the cistern.  But... what really made me feel good 
about the project was a section in a book entitled "Ferrocement Water 
Tanks: and Their Construction" by S.B. Watt.  Don't worry!  This 
book has more to it than ferrocement.  It seems as though the Dogon people 
in Mali were in need of water storage for the water that they got from their 
roofs.  What they used were the old <FONT color=#000000>granary </FONT>bins 
which had capacities to 10 cubic meters or more.  Before the used them for 
water storage they lined them with 1 cm or so thick of cement mortar mixed 1:3 
(cement:sand by volume) over chicken mesh that was stapled to the inside of the 
bin.  </FONT></FONT></DIV>
<DIV><FONT color=#000000 size=2><FONT color=#000000></FONT>    
These granary bins were right along side of their homes so their location was 
perfect.  Also...they were constructed of an adobe material with was 
reinforced with grass fibres.   Sounds like cob to 
me!!!</FONT> </DIV>
<DIV><FONT color=#000000 size=2>    When they build a new storage 
tank, the Dogons rounded  the bottom of the basin using a soil-cement 
mixture of about 1part cement to 10 parts soil.  What I did was after I 
built my rock foundation (up about a foot to keep run-off from penetrating the 
tank and to protect the cob from run-off and rain) I dug out a concave shape in 
the bottom of the structure.  I will then build it up smooth with cob 
and/or cement:soil mixture.</FONT> </DIV>
<DIV><FONT color=#000000 size=2></FONT><FONT size=2>     
Hope this answers some of your questions about my cob cistern. Now to the Cob 
Swauna.</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT color=#000000 size=2>    Lars, your concern is whether 
the cob structure will "melt" due to the action of the steam.  
That is not a concern of mine.  The amount of time the Swauna will NOT be 
in use will allow a lot of airing out and drying time for the cob.   
Also, I do plan on the cob to be fairly dry and tight before the first 
sweat.  Cob structures that have not been plastered, have an adequate roof 
,and have been exposed to the elements seem to hold up just fine.  Sweats 
generally are not used on a daily basis, but if they were I feel the structure 
would be fine.  There is the possibility of plastering the inside, but I 
have not given that much consideration.  Thanks for the question.  It 
does get me thinking!</FONT></DIV>
<DIV><FONT color=#000000 size=2></FONT>    <FONT size=2>By the 
way, when you start to build a cob sweat/sauna, don't start to corbel your wall 
in too soon.  I spent almost 6 hours readjusting my walls because I started 
to go in too soon.  We need room for the our backs and head, so after you 
start building sit in it to get a feel for what it would be like.  And... 
for sure, make sure that if you haven't been in a sweat before, do it before you 
build.</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>Thanks  -    
Vernon      <A 
href="mailto:vajohnston at nas.com">vajohnston at nas.com</A></FONT></DIV>
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