So what are we building? Part 2

Those that know me will not be surprised I’ve already read through and taken notes on all the relevant information from the back issues of The Owner Builder magazine. One thing has been confirmed in my mind, and that is building with earth makes more sense than anything else.

The question is now which method is going to provide the benefits we want. 
The Methods
At the risk of insulting those who make earth building their lives, there are perhaps 4 methods of earth building, although there is considerable overlap between all of them. Again, without offending those that make this their passion, here is my oversimplified version of how things work.

Mudbrick-  60% of the world lives in mudbrick dwellings. Unfired, sun dried clay bricks is what they are. In Australia they are now typically stabilised with cement, lime or organic emulsions mixed into the clay slurry to improve weathering resistance and improve their strength for load bearing applications.

Mudbricks can be puddled, requiring lots of space, plenty of molds and incredible stamina to mix and pour the bricks into the molds. The brick mix for puddling is extremely wet, allowing for a slow cure that produces a very strong brick.

Pressed bricks use less water but some sort of  pressure to activate the clay and stabilising agents in the brick to produce a brick that could be used immediately, or left to dry just as the puddling bricks are.
Both bricks may have straw incorporated into them to improve insulating efficiency and also to lighten the bricks weight. A traditional mudbrick made in Australia would weigh around 20 kilograms.

Mudbrick is also known as adobe which in my world means bloated software. I believe the word is a derivation of Spanish/Arabic for brick.

Rammed Earth- Large form works are bolted into place and filled about 600mm at a time with a mostly sand mix stabilised with concrete or lime before being compacted inside the form work.

Poured Earth – smaller individual forms more like molds are used and a wet mix, again mostly sand with some clay and cement or lime to stabilise, is poured into the mold. The pour is allowed to set overnight, the mold removed and the next layer of bricks is mixed and poured.

Light Earth- A clay slurry is mixed with straw or wood fibres until all the straw is coated in the clay. This is then pressed into wall frames often made from woven timber strips, or as a newer and less time consuming alternative, over light steel mesh. The clay protects the straw, the straw/fibres acts as both insulation and filler again reducing the weight

Costing each method
I don’t enjoy paying more for something than I see value in. This often leads me down the DIY path, and thankfully for our bank balance and the wife’s sanity I am not one of those “Tool Time Tim Taylor” DIY tragics. Therefore I would consider trying anything, but only after research, thinking, research, planning, research, consulting, research and research. My figures here are accurate for the quantities involved, I even added 10% to cover incidentals. There are ALWAYS incidentals.

Mudbrick – I have no problem with digging a dirty big hole, taking the soil and mixing it with a stabiliser before using an hydraulic press to press a brick.
Cost approximations –
1 x 40kg bag of cement per 40 bricks. 4660 bricks = 116 bags of cement @ $12.50 = $1450
$150 to weld 2 molds, 1 working, 1 spare.
$420 for a 20 ton hydraulic press and a lightweight steel mesh safety shield. 
$9000 for a shed to keep the entire process out of the weather.
So a realistic cost for doing mudbricks, made by ourselves, is $3, 000 without the shed. (I won’t get a shed with any of the other options below either. Realistically any of these methods need somewhere to securely store machinery out of the weather but for comaprisons sake lets ignore the shed).

Interestingly to have the bricks made and delivered would cost around $17,200!

Rammed Earth - Specialist work requiring a LOT of experienced labour and a LOT of expensive machinery. We would also need to import a lot of sharp sand to cut back the heavier clay soil we have on the property. I have not honestly costed this option, I can do rough sums in my head and know that 2x bobcats at $100 an hour for a fortnight and 4-6 labourers on site is more than I am prepared to spend on hired hands.

Poured Earth - This one is tricky. We have plenty of soil that will need cutting back with some sharp sand to produce a better sand to clay mix more suitable for this. The volume of walls we will need to make is 58 cubic metres ( not allowing for any window or door openings), lets do these figure on needing to import the full raw material on to site.
Steel for brick molds – $400
Ply for brick molds – $450
52 cubic metres of sharp sand – $2240 (yes only 52, there will be roughly 6 cubic metres of cement)
Cement, 161 bags @ $12.50 = $2020
Realistic cost for this would be somewhere around $5600.

This would let us form 40 bricks 600x300x300mm per day, or around 24 metres of wall. Hell it wouldn’t take long nor be expensive to build a poured earth shed would it? Edit – No, it wouldn’t, $2000 for walling materials.

Light Earth – I’m still trying to figure out why you would bother with this method. It requires a framed wall, lots of steel mesh and amazing amount of patience as you literally take a handful of clay and straw and force it into the mesh. I don’t like the method nor do I particularly like the finished result. Straw can be expensive, the material needs to be screeded, it takes forever with manual or mechanical labour to turn clay into a smooth slurry and the biggest downside for me is I doubt you’d get a load bearing wall made from this method without extensive frame work behind it.

Speed of build
Time is money is time. I and the wife are self employed, if we are working on the house we are not earning any money. Quite simple really. The quicker we can get our building to lockup the better.

Mudbrick – I’ve seen believable accounts (read that as not on the site of the manufacturer of these presses and molds) from people who were able to make 15 per hour with 2 people mixing, forming and pressing the bricks. So we would also need 310 hours, and I’m guessing an 8 hour shift will be physically demanding enough so around 40 days of really motivated effort (ahem) to produce all the bricks. Lets be real and say 60 days or 2 months.

The longer the bricks are allowed to dry the more dimensionally stable they will become and the less shrinkage and possible cracking will occur in the walls when built, so lets allow 3 months drying time.

The bricks will weigh 20-25kg each making for a lot of really hard labour when it comes to laying the little blighters. Accounts from experienced mudbrickers suggest 100 a day for 2 people, including mixing the earth mortar and laying and pointing the bricks. Add in incidentals like moving scaffolding as you go, hot and/or rainy days, your body telling you to GFY and I’d expect 2-3 months as a realistic time frame.

That adds up to 8 months, 5 of which can be expected to be back breaking labour.

Poured Earth – I own a petrol powered mixer that can comfortably mix .099 cubic metres per load. (The real amount a mixer can mix is not the full bowl capacity, which is what you buy them by. That load works out to 3.5 cubic feet and the mixer is rated as 5.5 cubic feet.)

Some stats based on the verified capacity of that mixer:
It would take 15 loads to fill each set of molds.
It takes 15 minutes to thoroughly mix each load, maybe a minute to shovel each load into the molds.
The perimeter wall of our house is 72 metres, we would need 9 courses in height. I have worked this out to be 20 days @ 5 hours per day of work allowing for window and door openings.

I would need about a day to create the mold system after having all ply and steel parts precision cut so they only need to be bolted and screwed or welded together. This is where eager retired parent and parents-in-law come in handy. Especially when they also have their own welders and know how to weld. I’ll rabbit on about the mold system and the design specs and manufacturing in another post, if and when it becomes a reality.

So we would need 3 weeks to a month to get the poured earth walls built. Here we have the luxury of doubling the mold system for relatively small cost. Either way that will double the capacity or turn our mix and pour days from half to full, halving the days required.

Rammed Earth and Light Earth – I have read rammed earth is still time consuming and I can’t even begin to guess how much walling can be made by a team of labourers in a day. Light earth, well as above why would you want to? Certainly not for me but I’m sure it has its time and place.

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