Light weight fill
My house structure consists of a number of vaults. It will be earth sheltered, so one possibility would be to fill those voids with 100 lb/cubic foot earth… That earth would hold water (or ice) and get a lot heavier or swell and cause me all sorts of problems. Instead, I wanted a relatively smooth top across the vaults, so I needed to fill them with something. I discovered “flowable fill”, this is a very light weight concrete (as low as 30 lb/cf) that can be poured into a void to fill it. The secrete ingredient is very tiny bubbles that are actually too small to pop. They are mixed with the concrete and fluff it up, reducing its density. Then the mixture is poured and finished much like regular cement. It can support more than enough pressure, but does not have a very hard surface (it can be punctured or dug with a shovel), so I will need to add a 2 inch topper layer of concrete, which is fine because that will form a very strong concrete triangle across each vault.
I did a lot of research and found the best aircrete product is from a company called “Cellular Concrete Technologies”. The problem is they don’t have any distributors in my area. I called them up and they were nice enough to tell me about one of their competitors in my area that I could work with. The competitors product was not as good, but I contacted a local crew and got bad service, followed eventually by a ridiculous price… A price high enough for me to buy my own equipment and do it my self and still have thousands of dollars left over. I called CCT back and they offered me a distributorship for Michigan. Part of me really liked that idea. I am an MBA student and I am pretty sure I could kick my competitions butts just by returning phone calls and a few other basic customer service or IT related things.
However… I am also in the middle of a complex build and I already have a good job, so I started to think about simpler alternatives that wouldn’t require me to purchase and ship equipment across the country and possibly even setup a distributorship. And also wouldn’t require me to commit to an elaborate, precise and yet, probably, messy process of pouring this fluffy concrete.
I worked out that if I pour 75 yards of this 30 lb/cf cellular concrete stuff, I can do it for about 90$ a yard… What other light weigh fillers could I get for that amount? Well, I can buy a rigid insulation, such as Formular 150, for $15.70 for a 1 inch thick board. Do the math and that is 158$ per cubic yard… Formular 250 is a bit pricier at 34.51 for a 2 inch board, which comes to 174.71$ per cubic yard. But how about ordinary (cheap) polystyene foam boards? I can get that stuff for 78.64$ per yard regular price at Home Depot… Maybe cheaper when I buy in bulk? Here is a table.
Comparing other properties, you can see that the aircrete can support much more load than these lighter fill materials, but the required load bearing capability is only a few PSI, so any of these options is sufficient. The advertised R value of the cellular concrete is pretty good, but it is tough to beat the light weight of the Polystyrene foam. And it would be even lighter if I left voids.
As far as final cost goes, I will have lots of garbage polystyrene left over from my quad-deck floor and can easily get much more, so I won’t actually need to purchase all the polystyrene that I will need. I can also install this pretty easily myself… Even my kids could help. Although I will still need a couple inches of concrete topper to seal it all in.
The tricky part will be figuring out how to place the rigid insulation so that I still get a good base for the concrete topper.
Anyway, I am still thinking about it. I have lots of other fun news. Meetings with contractors, progress on the virtual build, even a stock pile of eye candy, but I will save that for another time.