I am currently working with a non-profit organization called Community Rebuilds in Crested Butte, Colorado. We are building two, three-story duplex straw bale homes for lower income families as a solution to affordable housing. The foundation was supposed to get poured last fall, however the concrete company in the area was behind and instead we have gotten to be a part of it so I thought I would blog about the process.
1. Concrete Form
Our first job was to build the form for the concrete.This was a very tedious process because you have to get the whole thing level and in the correct position. The way you do that is by using survey points. Survey points are legal points that are put in by surveyors and we are not allowed to move them. This is because if the house ended up being over the property line then we wouldn’t be liable.
So initially we set up all of the walls that created the form using long pieces of wood. We would find the survey points and create our corners then drive stakes in behind the wood so that they would stay in place. Then once the shape of the house was created, we would move it to get it exactly where it needed to be according to the blueprints. Once the form was in the correct place, we then had to level it. This required a laser level. We had to walk around the form and add or take away dirt to get it all the exact height. Once that was completed, we had to again fix the position of the form and get it where it needed to be. This took us several days to accomplish.
Once that was complete the plumber came in to lay in the piping, only a few of us got to go up and help him with that.
2. Radon Pipe
Finally once the forms were set we had to lay our radon pipe. The radon pipe was a black perforated tube that is laid into the ground and surrounded the entire foundation area.
Note: Radon comes from the uranium that is in the ground. You usually have to deal with radon in mountainous areas. Radon is atomic so it can come through the smallest of spaces. You have lay several layers and special tubing in the foundation to redirect it out of the house. The legal limit of radon is 4 and it is the 2nd most common cause of lung cancer with smoking being the first. Because the tube is obviously hollow, the radon will go into it because it will take the path of least resistance. The pipe will lead out of the house with a fan at the end of it to suck out any radon that has entered. Therefore preventing radon from entering the home.
Once our radon tube is laid we added gravel.The purpose of the gravel is for drainage and it should cover the radon tubing. The gravel also had to be leveled evenly.
Next was a layer of plastic.The purpose of the plastic is to create an extra barrier for the radon to have to travel through.
After that we laid a layer of blue board foam for insulation. It’s important in places with climates like Crested Butte to have lots of insulation in your foundation to hold heat energy.
Note: Looking at the blueprints there were certain spots that we had to leave open with no gravel or blue board. That is where there will be main support for the roof and to create the strongest structure possible it needs to be directly on concrete and compacted earth with no gravel or blue board underneath.
You'll notice how hard we work...
6. Rebar Grid
After the insulation was added we created a rebar grid. Concrete isn’t strong enough on it’s own to withstand pressure so you have to add rebar to create stability and structure. Our rebar grid was 16”x16” squares. This was another tedious process of getting the rebar laid correctly. We had to cut the pieces to the correct length with an angle grinder and a rebar bender.
Then we had to lay them out and pull out our tape measures and tie them with rebar ties. The north and east walls had to have “L” shaped rebar connecting to them that stood vertical so that we would have something to connect to retaining wall to. We have to build a retaining wall because of the height of the dirt behind the house. However you can’t build the retaining wall until the bottom slab is finished and since concrete dries chemically and not mechanically, you have to have a way to connect the two masses. (Something that dries chemically can never go back to it’s original state, something that dries mechanically, like cob, once dried can have moisture added to it to become soft and shapeable again)
7. Radiant Heating
Once the grid was created the plumber came in and laid radiant heating that will heat the concrete and floor of the house. Radiant heating is amore efficient way of heating because it heats the mass rather than trying to heat the air.
8. Pour concrete, add anchor bolts, smooth
Once all of that is completed then the concrete is ready to be poured. They brought in one concrete truck at a time and began the pour. The process was to pour, smooth/level, clean the tube, change trucks and repeat. They began by filling the outside first. Once the outside was filled up level with the form they used that to scree off of and fill into the middle. They then smooth it out with magnesium trowels and a really long float called a bull float. After the bull float has been used they place anchor bolts where they need to go. Anchor bolts are how you connect the house to the foundation. They stick a few inches out of the concrete and when you go to frame, you drill holes into the bottom of boards and connect them to the anchor bolts. The blueprints mark where they should be placed. You want to make sure they don’t get put in doorways and you want them to be within 12” of corners and buried 7” into the concrete spaced 48” on center. Once the anchor bolts are in place and everything has been smoothed they use a power trowel or finishing machine that almost looks like a push lawn mower with a big fan that spins on the bottom, with each fan blade being a trowel. After that someone will put on trowel skis so that they can crawl out and smooth out any bumps that were left from the power trowel.
Note: The wood that connects to the anchor bolts has to be pressure treated or it would rot being on the concrete.
Now all you have to do is wait for the slab to set!
10. Control Joints
The very last thing that gets done once the concrete sets is someone has to come in with a concrete saw and add control joints. Control joints are lines that are put into the slab so that if the slab was to move due to movement in the earth then it would crack at the joint and not spider web throughout the slab.
Here are some pictures of the process of the grid and form for the retaining wall going up as well as the entire finished product.
Now that the process of a foundation is explained, I want to talk a little bit about concrete.
I didn't think concrete was a green product...
Since we are doing natural building I'm sure you are curious about the choice to use concrete.. Obviously as most people know concrete foundations aren’t exactly “green”. A concrete foundation has a pretty high embodied energy. Embodied energy being the amount of energy associated with the mining and processing of materials, manufacturing, transport of product and production. In our situation we have to follow codes so we didn’t really have many other options, however there are a few to choose from if you are building on a smaller scale. First though, let’s discuss the pro’s of a concrete foundation.
Pro's of Concrete...
So the positive side of a concrete foundation with the insulation we added to it is that because it is a monolithic slab it has a huge amount of mass, and mass holds energy. So although it has high embodied energy to begin with, the amount of heat energy it saves can potentially make it more sustainable in the long run because you won’t have to heat your home nearly as often. Your mass will hold heat rather than letting it get lost. So you are using less fossil fuels long term and in the end is much more efficient. Other benefits are that it has the potential to last longer and is more structurally sound.
Here are some points to be aware of with foundations we learned from our construction supervisor Dusty Sylvanson
- -Get below the frost line
- -Eliminate soil moisture
- -Look at capillary breaks (a barrier so moisture can’t get into building)
You must consider…
- -Site conditions
- -Building design materials
- -Energy and material efficiency
- -Long term or short term structure
- -Code jurisdiction requirements
A good foundation is…
- -Low embodied energy
- -Well insulated
- -Well drained
Foundations for Straw bale…
- -Wide platform
- -Capillary break
- -Minimum 6” from grade (grade being the ground surrounding the house)
- -Plaster stops
- And always remember that bales should never be below grade because they will rot!
Here is a picture of the blueprint model of what we did. At the very bottom the hatches are the compacted soil, then you see the next layer is black diamonds which is the gravel. Then lines which is blue board. Then you see the long black lines with the dots, that represents the rebar. The bottom is obviously the bottom slab and the vertical one is the retaining wall.
So lastly let's discuss the alternatives to concrete foundations. There is another form of conventional concrete foundation that we could have used that includes a crawl space and is less concrete, however it doesn't have as much mass and therefore doesn't hold as much heat energy.
So... the alternatives
Here is a picture of a power point Dusty showed us. These are some of the alternatives that include:
- a shipping pallet on a gravel pad
- straw bale directly on gravel pad
- sandbags topped with cob or concrete
- framed box filled with stone and concrete
- stabilized adobe, rammed earth or field stone
- the Earthship approach which is used tires with rammed earth
- railroad ties and gravel
- rock-filled gabion
So while these can all be acceptable foundations in certain situations depending on climate and terrain, there are still several issues you could potentially face. If you are having to deal with codes and regulations you could be very limited on your options. Also, many of these options aren't good for load bearing which means you are limited to the size of your structure. So if you just want something small one of them could be perfect for you. They also may not last quite as long, unfortunately these options aren't guaranteed to last.
Sadly there isn't another material out there yet that works as well for a foundation as concrete. Some people have mentioned hempcrete but it is not nearly as structurally sound. However, there has recently been a new product that has been found that apparently is stronger than concrete, but they haven't found a way to mass produce it yet. Below is the attached video if you are interested.
Hopefully in the future we will have better alternatives to concrete that will be able to be up to code for neighborhood regulations. Until then, this is the foundation we created and we all learned an extensive amount in the process. Thank you for taking the time to read my blog, I hope you found it to be informative.