Rammed Earth Construction vs Ferrocement

Surely a very green form of dwelling construction - building with Earth! Most of the material is soil, and you just use a small amount of cement to stabalise it.

However, the walls have to be really thick - so I actually wonder how it compares to something like Ferrocement which is at the other end of the spectrum - non-eco friendly but very thin. Ferrocement uses traditional sand and cement, and steel rods and mesh to form a super strong composite, which allows you to form thin-shell curvy structures.

Rammed Earth typical wall:
6% by volume, but material is typically rammed to 50% it's original form thus raising cement volume to 12%
18" thick minimum wall thickness

Ferrocement
33% cement (66% sharp sand) for the concrete, but 20% is steel of the final composite, thus giving 26% cement content
1" thick typical wall thickness

In fairness I do not know if 18" vs 1" yields equiverlent structural strength, so this may not be a fair comparison. I know that for multi-storey rammed earth dwellings your base walls need to in excess of 2ft thick and more for taller structures. I also know that boats can be made from 5/8" thick ferrocement and perform very well when faced with the particulary hazardous environment of waves crashing aginst the hull, and the predicament of the entire vessel being supported from just one point in stormy conditions. From a ferrocement manual, the construction of naval structures is a much more sensitive task - and that generally land structures can be much weaker. I thus conclude that both 18" rammed earth and 1" ferrocement construction would both be structurally sound for a single storey dwelling.

Thus for a 1 square metre wall I calculate the following volume of cement used:

Rammed Earth:
0.45 cubic metres in total, of which 12% is cement
= 0.054 cubic metres of cement

Ferrocement
0.025 cubic metres in total of which 26% is cement
= 0.0065 cubic metres of cement

This is not particulary favourable towards rammed earth, with it using more than 10 times the amount of cement! With a square structure of 10x10m in wall area (dismissing the issue of roof and foundation), and 2m high, we require 80 such 1sqm blocks. Thus simple maths gives us:

Rammed Earth: 4.32 cubic metres of cement
Ferrocement: 0.52 cubic metres of cement

With cement @ 1.44 tonnes per cubic metre, we can then work out the CO2 emmissions on a 1:1 ratio:

CO2
Rammed Earth: 6.22 tonnes
Ferrocement: 0.75 tonnes

Some other comparisons:

Rammed Earth
- heavier, so bigger foundation required (more cement?)
- Several rammed earth manuals suggest having to use reinforced concrete structures anyway for areas of a structure particulary under heavy loading.
- requires sealing against water
- lots of material means lots of work and effort and more time required, though construction is fairly simple - just shovel and pack material
- care must be taken to ensure soil is suitable for rammed earth constructoin
- theoretically good natural insulation purely from the thickness of the walls

Ferrocement
- dwelling need not be square... indeed the nature of construction favours curvy more organic structures
- naturally water-impervious (can make large boat hulls from it!)
- less material required overall meaning less labour intensive, and materials are simply purchased from a suitable store - however erection of steel wire framework requires thought and could be fiddly
- presumably poor insulative qualities requiring additional expenditure on an insulation solution

NOTE!

One thing I have missed totally is the inclusion of steel in the ferrocement. This clearly decreases it's apparant advantage in the above comparison, though it's difficult to calculate how much by. In weaker structures though, the steel can simply be less than half a dozen layers of chicken wire - so I find it unlikely that this would be enough to swing things in favour of rammed earth entirely. As I understand it, it's more feasible to produce steel in a green manner and from renewable energy sources and recycled materials, than it is cement and new types of "eco-cement".

I havn't really considered roofs or foundations either. I already believe rammed earth would require a more sizeable foundation because of it's sheer extra mass. A rammed earth structure would probably have a wooden framed roof. A ferrocement dwelling could have a wooden frame roof but would be structurally more capable if the roof moulded into the outer walls.

Summary

This discussion is more theoretical than fact, but does question the validity of supposed green solutions to our needs. By comparing two methods of dwelling construction - one considered green, the other more commonly associated with industry - it has shown that we should question the final impact of a solution rather than draw conclusions from the initial evidence provided.

Indeed, this discussion was originalyl fuelled by the fact I had overlooked ferrocement construction because of it's association with industry and high content of non-green materials. It wasn't until I saw it on a "mother earth" style website about ship building that I begun to research it further.

Should mention this post is not to discount the advantages of rammed earth - indeed it may well be suitable in many locations where the surrounding environment makes it more suitable.

Feedback Invited!

Again, most of this is theory so would welcome any feedback or thoughts on this and if my maths are anywhere near correct. It's a whole can of worms, the wall thickness, the amount of materal compression in the rammed earth type. I have also found people experimenting with papercrete, which is a sort of paper/cement/steel composite, and then people avoiding cement altogether and experimenting with just compressed paper to form a type of wood. This ultimatly brings us round to wood, which IMO is far greener than all of the above mentioned technqiues.

PS: forgive the rambling, I got carried away!

 

Anything about insulation values?

 

Alas no - but from what I've read about rammed earth it's not that great... the idea of it being cool in the summer and warm in the winter seems to be a bit of a myth compared to modern insulation materials. Except in warm climates, I believe both types would benefit from having proper insulation rather than relying on the structure alone. The foil backed bubble wrap type seems most space efficient and good value, whilst being moderatly eco-friendly.

Straw bale construction, wood, cob, peat, combinations of these would easily beat both rammed earth and ferrocement for green construction, insulative properties, etc. but that's a whole separate discussion I guess!

 

As with any alternatives, one must look to the environment within which the endeavor takes place to see what is most practical - often the monetary considerations have the final say.
As well, building codes often make building techniques much more cumbersome than they need be - but unless you want trouble from the social environment one finds themselves within....
Nomadic structures often seem to make the most sense, as usually they defeat building code rules being not applicable as permanent structures and not needing certificates of occupancy... - And of course I be speaking from my own environs not yours as to rules of regulations - which always need to be researched to find out what can be done within the limitations the gov. man puts on us.

I have found that nomadic structures by-pass permanent structures as to ease, time needed, costs and regulations- with the drawbacks being a wee bit more hardship - but that should not be contra-indicative for most drawn to alternative intentional communities.

Communication being by far one of the best things that proceed and precede and follow after communities.... brainstorming and getting peoples experiences and feedback becomes extremely valuable for all of us.
Thank you

 

I agree with all of that.

 

Ever heard about aerth bag building?

 

I presume you mean Earth bag? I had glossed over it previously thinking it was yet another permutation of rammed earth, but I like the idea of using the bag as a sort of stabaliser, performing the role of the cement. There doesn't seem to be that information on the subject - quite a few websites said you still need cement in some cases because the bags don't hold there shape. Some say you can use volcanic rock instead, but we don't have much of that around here.

I ordered a book about it ages ago but still havn't recevied it.