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Archive for May, 2011

We’ve heard a lot about UV damage to polypropylene bags. I was interested in seeing for myself just exactly how badly they degrade so I set up a little experiment. I filled a bag with gravel and stuck it aside. Unfortunately I soon after forgot about it so I’m not sure exactly how long it takes to progress to this level of damage but one thing is for certain- this bag is toast.

I dragged a single finger through a patch in the bag that was splitting and you can see how badly it held up. So- protect your bags as you build. Anything from a light plaster to paint will work, just so long as the bag is protected from sunlight. And just a note- you do have to get on top of it pretty quickly. While not an experiment per se, I did think I had more time to get a light plaster up than I did. While the bags didn’t degrade like this one, they did loosen up slightly making plastering a bit more difficult. The smoother and tighter the bags, the easier the plaster goes up. Lesson learned.

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I haven’t had much time for updates recently. Between planting trees, amending soil, building and planting the gardens, and site preparation things have been a leeet’le busy around here. We did finally get started building on Saturday though, so the season is officially ON and we have nothing to slow us down but mother nature herself.

We’ve been asked a number of times what kind of mix we’re using in our bags. We’ve gone with a stabilized clay-soil/gravel mixture. The clay-soil is from our property, we were fortunate enough that the previous owners had dug giant holes on the property (to bury garbage if you can believe it) and they left massive hills of clay next to the holes. We get cheap gravel reject from a nearby pit and we use the smallest amount of cement to stabilize the mix.

sifting station

The first step is sifting the clay. It’s a bit of a time consuming process (although Shane’s got it down to a bit of an art) but necessary for the most consistent results. We’ve set up a salvaged expanded metal screen between two barrels. We set the wheelbarrow beneath the screen, toss shovelfuls of clay-soil onto the screen, and sift away. Shane can fill a wheelbarrow in about ten minutes- I won’t say how long it takes me.

work station

The work station is where all of the mixing happens. In this photo you can see the pile of gravel reject, the wheelbarrows full of sifted clay, the cement mixer, and the buckets that we use to haul the mix. You can also see the cats who’ve taken to using my gravel pile as a litter box- not impressive. I’m thinking of putting down some snow fence just to keep them off the pile. It’s either that or I’m shipping them off to the city. There’s nothing quite like the smell of ammonia to make a long day seem even longer.

very little cement

I use an old bucket to measure my quantities. I’m going for a 70:30 gravel:clay ratio so I use three and a half buckets of gravel and one and a half buckets of clay-soil. Every new load of gravel is slightly different, as is the clay content in the soil when we switch to a new part of the hill, so I constantly ‘re-check’ the texture of my mixes. I’ve handled enough mixes to be able to judge by touch whether I have the right ratio. The amount of cement added is only very small. You can see in this photo how high the cement content is compared to the other ingredients.

gravel & cement first

I load my gravel and cement into the mixer first, give the cement a little toss with my hand to premix it, and then add water and start the mixer. I like the gravel and cement fully mixed before I start adding clay. Clay prefers to stick to itself and I’ve found there’s way less clumping if I toss it in once the gravel is prepared, and the cement is better distributed.

hand mixing

I reach in and pull the mix from the back of the mixer while it’s turning just to make sure no dry materials are sitting at the back. No OH&S on this site I’m afraid. I add a very little bit more water if required at this stage. It’s important not to over-saturate your mix though, so only add a small amount at a time in between checking the texture.

checking mix

A good mix will form a ball when squeezed in your hand but will break if squeezed too hard. If it continues to compress under pressure, there’s too much water. You don’t want an overly wet mix- it’s insane to try tamping and takes forever to dry. Our mix tamps really well and forms a solid brick in no time.

one completed mix

One load gives me four buckets of mix. It actually takes six buckets (about 175 lbs) to fill a single bag so I’m only getting two bags for every three mixes that I prepare. I have (several times) tried to load more into the mixer but, like many of my attempted shortcuts, ┬áit’s not a time saver. I get the most consistent results from a smaller load and consistency is imperative for the best earthbag bricks. When Shane and I are working together, we can produce about twenty six bags (thirty nine mixes) a day. I do about half that working alone.

So that’s our mix process. As always, I recommend you do your own testing. Clay-soil differs at every location and the gravel reject you use may have a different clay or sand content. Experiment a little before you settle on the mix that is just right for you. Happy building!

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