Cob RE: spans of thin shell concrete slabs + eucalyptus ++

[Bj örn Brandberg]

Just for anybody´s interest.
The pantheon dome in Rome was built in non reinforced concrete 1300 years ago and is still standing. It has a span of 44 m (144 ft). 
     At the other end of the technology of domes: We have  built some two hundred thousand dome shaped latrines slabs in Mozambique with non reinforced concrete. Thickness around 4 cm. Test load 6 people. Mix 1+2+4 volumes of cement sand and 12 mm crushed stone. Local variation in aggregate quality may as for some testing. We cast them  on top of a heap of sand with a peripheral mould of sheet metal. We found non reinforced slabs to perform better than slabs reinforced, probably due to better compaction. A ring reinforcement of the periphery was abandoned as the slabs were strong enough without it but is used in areas with aggregate problems.
The method i spreading over Africa more or less spontaneously through various organizations.
Bjorn Brandberg

PS
Does anybody have any experience or contacts lists or web-sites for 
1.   how to build wooden houses with eucalyptus
2.   low cost environment friendly treatment against flying wood eating insects
3.   low cost environment friendly termite protection
4.   suitable chemicals for dip treatment of wood indoors and out doors.



Patrick Newberry wrote 30/7/98 10:04 

>With all this talk of ferro-cement and thin shelled domes etc, I have 
>a real life questions with I am to deal with.
>
>I made the (for better or worse) decision to use a multi-layered 
>chicken wire and cement roof. Some sections will  use my carpet 
>cement as well. Any way I am laying down my vegas (pine, cut and 
>peeled by the maukly crew). What I am trying to decide is what span 
>should I look at for these vegas (logs used as roof joists)
>I have two type sections to deal with. One is a 22 foot dome with a 
>bamboo and pine sapling frame in place. Two is a some what low  
>sloped roof that extends from the dome to an outer wall (a earthbag 
>and strawbale wall combo)
>
>THe procedure I'm looking at is :
>On top of the vegas or the bamboo shell of the dome:
>1. Lay a layer of 1 inch chicken wire down.
>2. lay a layer of tar paper down.
>3. lay some type of insulation (rigid?) (suggestions?)
>4. More layers of tar paper
>5. then multiple layers of chicken wire and then cement this. 
>   (or discarded carpet soaked in cement)
>
>First question I face is what distance can I span with this method?
>
>things to think about. One is that I live in a very mild winter 
>climate so heating is not hard to deal with. (as in small number of 
>days does it get below freezing, but it can get to the teens (F), 
>but only maybe once every couple of years and then only for a day or 
>so.. Long hot humid summers. 
>
>The underside of this (remember the 1 layer chicken wire and the tar 
>paper) I'll plaster with a white gypsum type plaster (a ceiling!).
>
>Money is a consideration:
>
>I could add some 1 x 4's across the vegas but between the "ceiling" 
>wire and tar paper to help with span. 
>
>It would be nice to be able to walk at least gently on this for 
>repair, inspection etc. 
>
>
>I also am thinking that I can minimize the up front cost by adding 
>only a thin layer of ridged insulation here and latter adding a 
>second shell over this at a latter date.  this gets me dry space at a 
>lower cost, but allow me to improve the efficiency at a latter date. 
>Air flow would occur between the two shells.
>
>Can the ridged insulation sit on the vegas with the thin shelled 
>cement roof on top of it and not crush the insulation. 
>
>and as always, Open to other suggestions.
>
>I realize a photo would help, and my last photos got lost in the bit 
>bucket, but I found a fellow in MS whom will scan them for me for a 
>buck a piece, I'll have them in them mail Tomorrow and he claims 
>three working days after he gets them and I'll have them scanned... 
>so cross your fingers. 
>
>
>Pat (busy, tired but happy)
>and the rest of the Maukly crew.
>http://www.gnat.net/~goshawk
>Mauk Georgia
>
> 
>