Pouring the Yurt's Foundation
Rebar, concrete and lazers -- what fun :-)
For the last few days we've been focused on working out answers to questions about how we're going to handle various roof details. For example, we're determined to have a round skylight for the center of the yurt, and it took some searching to find one that we felt would best serve our needs. It's very important to have critical hardware on hand before construction starts so that everything can be built to fit.
We've also been exploring roof treatments since we want to have a fire-resistant metal roof since when you build in a forested place, flamability is something you have to take into account. We also want to use a roofing system that will allow us to create a traditional yurt effect at the edges -- we're going for as "hobbitish" an effect as we can since the whole point of these structures is to create a cozy, fun place to camp-in for the summer.
So, after being distracted for a few days by other aspects of the project, it's time to get started on the task of pouring the yurt's circular, inverted "T" foundation. No matter how many times I do this sort of work, I always get a thrill out of mixing concrete and rebar to create something that last and serve for years to come.
The first step was bend twenty-foot lengths of rebar into the appropriate arcs. The outer circumference of the trench is thirty-six feet, so two sticks of rebar cover the distance nicely with an adequate overlap.
Once the two circles are in place, they're cross-connected by nine inch long pieces of rebar that will serve to keep the rebar in place while the cement's being poured into the trench. They also serve as attachment points for other nine inch sections of rebar that will rise to connect the lower rebar with the third circle of rebar that will re-enforce the narrow portion of foundation that rises to support the wall structure.
Todd checks level with the laser
The next step was to put in place a series of leveling stakes in order to insure that the concrete footing, when poured, would be level front to back and side to side. If the top of the base footing is level, then the rest of the structure will go up level too-- and if not, there's be a world of headache ahead as you have to shim and trim to make it so.
The first step is to set up the rotary laser level in the center of the yurt space. The top of the level has a ruby laser that spins in a circle establishing a level plane which makes it simple to take a measuring stick to any point in the foundation trench and see how much space there is between the bottom of the trench and the laser mark on the stick, thereby determining the high and low point of the trench. I'm pleased to report that over the trench's thirty-six feet of length, there was only an inch variance over all.
So we split the difference between high and low and drove the first stake into the ground so that the footing would be five and a half inches thick on the high side, and six and a half inches thick on the low side. After that it was just a matter of driving additional stakes at two foot intervals around the circle to create plenty of reference guides as we mix and pour concrete into the trench.
Rebar and level stakes in place
The morning was overcast, damp and chilly -- perfect weather for pouring the base of the yurt's foundation. At thirty-six feet in circumference, six inches deep and twelve inches wide, we were looking at needing some eighteen cubic feet of cement -- not a trivial pour, but well within our experience and capacity.
We use five gallons of crushed rock, three gallons of sand and a gallon and a half of portland cement per mix, all of which adds up to a bit more than a cubic foot per mix, and over the years we've worked out a dependable routine for mixing our own concrete with little stress and strain, and a key part of that involves working out of the back of the work truck.
One of the joys and dangers of what we do involves the chance to use a wide range of tools and techniques, a variety which goes a long way towards making this a fun, sustainable lifestyle, but there is the down side that we can hurt ourselves because we're not physically used to the demands of certain tasks. Mixing concrete is a prime example of that because of the weight of the ingredients which have to be hoisted shoulder high and poured into the mixer.
Rebar and level stakes in place
It's easy to strain or even tear a shoulder or elbow tendon while lifting and pouring eighty pounds of rocks, so over the years we've developed techniques that allow us to do with work and end up with only the ordinary aches and pains the come with hard work of this sort.
We start with the buckets of rock and sand already three feet off the ground in the back of the pickup. By sliding the five gallon bucket to the edge of the tailgate, we're able to bend our knees, hold the bucket to our chest, and lift it the rest of the way using the power of our legs rather than the more risky power of our lower backs.
Standing perpendicular to the mixer, we're then able to rotate the pucket over our left arm so that the rocks fall into the mixer's barrel. Because the arm that's holding the weight remains tucked snug against the body, its tendons and ligaments aren't stressed, something which is very important when you're going to repeat the procedure a dozen times.
If we're only going to need a batch or two such as when we're cementing a railroad tie fence post in place, then we just go ahead and mix the concrete right there in the wheelbarrow since that's easier to just do it by hand than to have to deal with setting up and then cleaning the power mixer.
Walt dumps the mixed concrete while Todd catches
But it's in pours such as this one that the power mixer is especially handy since it allows us to create a relatively "dry" mix, i.e. a mix that's dry enough to not flow around too much. Making a wet mix in a wheelbarrow is easy enough since there's enough water to allow the ingredients to mix readily, but to get the same degree of mixing with a dry mix takes a lot more effort.
Having a dry mix for the foundation is important here because a soupy mix doesn't set up as strong as a drier mix will, but also because we were pouring into a trench, and needed some stiffness in order to pile the concrete up to the desired heighth and have it stay there while we mixed the next batch.
After the second or third batch, the work settles into a comfortable routine in which perhaps the hardest thing is to remember to raise the vertical rebar sections so that they stick up out of the concrete to serve as the basis for a connection with third ring of rebar that's going to strengthen the vertical portion of the "Inverted T" footing.
After checking that the surface of the poured concrete is flush with the top of the leveling stakes, they're pulled, and the concrete jiggled to fill in the holes they left.
Adding another batch to the footing trench
As we finish off the concrete from the last mix -- took sixteen mixes all together -- Gina stops by to congradulate Todd and I on the work done. One of the most rewarding parts of the work we do is the knowledge that the other members of the team see and appreciate the efforts we all make to move this project forward, to make Windward a better place. It's a "return on our investment" that can't be measured in dollars and sense, but it sure feels good to have one's work appreciated by those you care about and who care about you.
Gina stops to offer a few words of support and appreciation
And so I am pleased to report that the first pour of the season was a complete success. For the record, our criteria for success is that:
- The job was completed without anyone getting hurt.
- None of the equipment was damaged.
- The established criteria for the work was met.
- And we had a good time doing it.
The next step will be to pour the six inch high, six inch wide riser part of the "inverted T" foundation, the part that the walls will rest on, and to do that we need a form that curves to match the outside circumference of the yurt.
Concrete forms are held together with pins and shoes -- more on that later. The key point here is that the hardware presumes that you're making the form out of three-quarter inch plywood; the problem there is that it's a bit hard to bend plywood that thick into the tight radius we need here. And so, the trick is to glue two layers of three-eighth's inch plywood into the right arc, let it set up, and away you go. Note: this radius was tight enough that it was pushing the ability of the three-eighth's plywood to make the curve without breaking -- next time we make a form for this tight a radius, we'll use three layers of quarter inch.
In order to define the arc, we used two of the plywood arcs that will form the bottom of the walls (it's green because it's treated plywood -- any time you have wood in contact with concrete, you have to use treated wood to prevent rot.) By bending the plywood form to the arcs, we'll get a form that will create a wall that exactly fits.
glue-lammed curved concrete form
As you can see in the pic, there's no such thing as having too many clamps in a woodshop -- the more the better, and part of what made this work out so well was that I was able to use the anchor bolts (the silvery thingies protruding from the middle of the form) as the primary way of cinching the two glue-lamed forms to the arcs -- worked like a charm.
And while I'm here, a word of appreciation for the new wood working space. It was really nice to have plenty of indoor room to work on the form and not have to worry about the intermittent rain showers that have been with us most days this spring. There are a number of projects that have been waiting on this space to get operational, and right now it's a toss up between finishing the space and getting started using it. Have to just keep doing a bit of both.
Concrete has incredible compressive strength, but very little tension strength -- in other words, it can bear a lot of weight, but will crack if pulled on. The reason for the use of rebar is that it provides the needed tensile strength. In this case it also provides something to tie the anchor bolts to.
The weather continues to remain perfect for pouring concrete, and we're not having to worry about the concrete drying out too much before the next pour. Next yurt we'll start pouring the riser wall the day after pouring the footing, but for now, the cool weather is saving us from having to worry about such things.
one more pass of rebar to strengthen the riser wall
Spent much of today in the shop working on completing the dual fuel set up for our work truck. The goal is to be able to run the truck on fuel produced here at Windward, but one of the interim steps in that process is to get the engine running on both liquid fuels like gasoline and gaseous fuels like propane.
Once the truck's able to run on both liquid and gaseous fuels, the next step will be to add the ability to store useful amounts of wood gas (a mixture of hydrogen and carbon monoxide).
mounting the propane carb assembly
Also spent time working on the gasifier that we'll be using to fire the traditional stone oven that we're bringing on-line as part of "Project Pizza."
It's a design known as a fluidized bed in which compressed air is let in at the bottom of a cylinder filled with wood chips. The chips undergo partial combustion from the top down, enough to generate significant quantities of combustible gas which is then piped into the oven's burner.
welding a gas input port onto the bottom of the gasifier
The next step on the yurt will be to pour the six inch high, six inch wide riser part of the foundation wall using a glue-lamed form, and in order to keep the form from sticking to the concrete (so that we can remove it, reset it and keep on pouring as we proceed around the circle) the form needed to be coated with a release agent.
The release agent we use is used motor oil which soaks into the wood nicely and serves the purpose at a very good price :-)
coating the forms with used motor oil as a release agent
Before the forms are ready to pour, it's necessary to span the form with a yoke and some bracing so that it will remain true to the desired arc.
The next step is to insert the pins into the inward side of the mold and tighten them down with pin shoes. Then the outer half of the form is mounted on the other end of the form pins, and shoed tight.
the form in place with the yoke and two braces attached
Since we're going to be pouring this riser in sequential steps, it's necessary to install a dam at each end of the form, something that's a bit tricky since the dam has to be assembled around the horizontal run of rebar.
After that it's time to wire an anchor bolt to the first and last pins so that we'll be able to bolt the walls securely to the foundation.
the form with an end dam installed
At that point it was time to crank up the mixer again and turn out another three cubic feet of concrete.
the first eight feet of riser poured
After setting for a day, the concrete is strong enough to allow the form to be removed and reset. The first pin hole in the form is mounted on the last pin of the previous pour, and then new pins are set in the remaining three holes. The end dam is replaced and we're ready to pour the next six feet of riser.
the form moved forward six feet
and attached to the previous pour
the first fourteen feet of the riser is done
the first twenty feet is done
Don't know how cool this is looking on the web, but in person, it's really fun to see the circle taking shape since it makes it much easier to visualize the yurt.
Other matters are going to be taking me away from Windward for a couple of days, and Todd's down south to dismantling and bringing back another greenhouse, so it won't be until next week that we get the rest of the foundation poured.
another day and that makes twenty-six feet poured
Had to take a couple of days away to attend to some vehicle repair after the Volvo got into a fight with a rock and came away with a cracked oil pan. In spite of their reputation for being tough, it turns out that Volvo oil pans are cast aluminum, and to make matter even more fun, it turns out that you have to remove what's called the engine's sub-frame in order to remove and replace the oil pan. <sigh>
Finally got back on track with getting ready to pour the next section of the yurt foundation, but wasn't able to back the outside form away enough to clear the pins, so as you can see in the pic, I had to bring the cutting torch up and cut off the offending pins. Note: next time, dig away more dirt on the outside to leave more clearance for form removal.
next to last pour in place
And one more pour to close the circle. The final hole in the form was three inches off from the first pin, which isn't bad -- three inches off in thirty-six feet of circular foundation -- so I just twised off the pins and used pipe clamps to secure the form to the first pour, and poured an "extra" three inches of concrete -- concrete is very forgiving that way :-)
next to last pour in place
Notes From Windward - Index - Vol. 66