Given the warm weather we have had lately (all our snow is gone and it has consistently been above 0 degrees celcius), we are planning the next stage of our building project. If the weather stays warm we hope to start building again too!
The main tire wall was finished last fall. The next step is placing a bond beam on top of this wall. This beam ties the walls and roof of the building together and increases the stability of the walls.
We have decided to pour a concrete bond beam as detailed in Volume III of the Eartship books, as opposed to using the laminated 2×12 bond beam described in volume I. We chose this route as we would prefer to minimize any wood that ultimately might be back filled. Also, the sad reality is that I am trained as an engineer; both solutions seem adequate, but concrete just leaves me feeling that much better about the situation. This decision does mean more time and cost.
The Bond Beam
Our bond beam will be 8″ wide by 8″ high with two continuous runs of 1/2″ rebar. The beam will be anchored to the tire wall by; excavating every second tire, and driving a 3′ length of 1/2″ rebar into each excavation such that approximately 6″ of rebar remains above the top of the tire and will be buried in the concrete poured for the beam.
The perimeter of the back tire wall of the earthship is (I simply took this from the plans):
12′ + 08′ + 12′ + 16′ + 28′ + 11′ + 15′+ 11′ + 11′ + 12′ + 28′ (7×4′) + 18′ = 182′
So, our bond beam will have an approximate length of 182′.
Figuring out the concrete required for this bond beam:
concrete in beam = 2/3′ x 2/3′ x 182′ = 81 cubic feet = 3 cubic yards
This does not allow for the holes excavated in the tops of the tire or variations in tire heights that mean more concrete is used. So … rough estimate of 4 cubic yards.
I figure on the following materials for the bond beam:
- 24 bags portland cement (4 cubic yards of concrete)
- engineered fibres (how much?)
- 550 lineal feet 1/2″ rebar (2 runs x 182′ + 40 tires x 3′ + min. 18″ lapping = 550 ?)
- hay wire (to wire the lapped rebar)
- embedded fasteners in the concrete to secure the beams (not sure what these are yet)
The Bond Beam Form
A form needs to be built to hold the concrete after it is poured, until it sets up. I figure I have two options here…
Option 1: A Pop Can and Concrete Form for the Bond Beam
As outlined in Volume III many earthship builders use a pop can and concrete form instead of wood. This is done primarily (I believe) to reduce the amount of wood used in the project.
Some rough calculations …
- A pop can is basically 5″ long x 2.5″ in diameter.
- This means each can has a volume of 3.14 x (2.5/2)^2 x 5″ = 24.54 cubic inches = 0.0142 cubic ft.
- A lineal foot of can wall will use at a minimum 3 cans in each horizontal course (total of 9 cans for 1 sqft of wall).
- A lineal foot of the bond beam form wall will use a total of 6 cans as it is only 8″ high. Ignoring these 6 cans, a lineal foot of the form wall needs 5″ x 8″ x12″ = 480 cubic inches of concrete. Subtracting the volume of the tin cans this works out to 332.76 cubic inches of concrete or 0.00713 cubic yards of concrete per running foot of can wall for the bond beam form
So, for our 182′ bond beam the form would need the following materials:
- volume of concrete = 2 x 182′ x 0.00713 cu.yards/’ = 2.6 cubic yards
- number of cans = 2 x 182′ x 6 cans/’ = 2184 cans
- fasteners to hold rebar in place during the pour
I am guessing we have accumulated enough cans to form this beam but I will not have many left over for other walls in the building. Also, there is a material cost in portland cement to mix the concrete form.
Option 2: A Lumber Form for the Bond Beam: My rough numbers
Assuming I use lumber for my forms, I need the following materials:
- fasteners to hold the two sides of the form together while concrete is poured,
- fasteners to hold rebar in place during the pour,
- lumber fasteners for the lumber,
- lumber for the forms
I believe I have gathered enough pop cans to consider the first option for forming the beam. However, I would probably use all the pop cans I have accumulated to date, and be left looking for more later on.
I actually have access to low grade lumber suitable for forming the beam. It makes more sense to me to use lumber for this form; the material is available to me, and I think it will be easier to finish around the roof beams down the road. The simple reality is that if I do not use the lumber for this job, it will need to be re-worked for some other use.
Using lumber will probably not save me much money, as I will require fasteners for the form which I currently do not have. I do hope that this decision will save us time and labour.