Concrete has a few properties that might make it a superior material for BSF units:

Concrete is a terrible insulator - In most climates heat is a problem when culturing BSFL outdoors during the warm season. Concrete is a poor insulator which may make it a good tool for keeping a BSFL colony cool. Hypertufa is easy to work with, but it has good insulating properties which may detract from it's desirability as a material for this purpose.

Concrete wicks moisture - This property might help reduce the moisture in the castings, and it's probably another factor in cooling because of evaporation. We do need a fairly high moisture content (I believe around 70%) to maintain a healthy colony, but most people run into problems from too much accumulated liquid in the unit. I would rather have the issue of the waste being too dry than too wet because it's much easier to add liquid (water or high moisture waste) than it is to dry the waste out.

Concrete is cheap - Few materials are as inexpensive and readily available

Concrete is easily molded - Well, maybe not EASILY, but it's something a decent DIY'er as an alternative to working with existing plastic shapes.

I'll be working on a concrete unit soon and as always would appreciate any feedback.

There are a couple of designs using Hypertufa (link) which is lighter than concrete. IIRC these are similar to the original round BioPod. On the forum there's BSFL Bin Of Hypertufa & Plastic Tub (link) by Gourmet Bugs

This is an old post, but I wanted to add a note about Hypertufa: it won't hold up over time, esp in areas of freeze/thaw. The moisture from the inside of the bin will be absorbed by the hypertufa as long as you have larvae in it. If you leave the bin outdoors in winter, the moisture will freeze, expand and the hypertufa will start separating.

For those of you in warmer winter climates, it may not be as much of an issue.

Sue

Thanks for finding that BW. If I'm seeing this project correctly he's left the concrete inside of the (gray) drum. That would be very similiar to marinos' project here:



I think the mass of the concrete as well as the low cost is appealing in both projects, but by leaving the concrete encased in plastic (if that's what Winston did) you lose the wicking and evaporation that I think might be beneficial. They seem a lot easier to build than my effort though.

I think there's a couple of version's shown there but what caught my attention was how he had the drums at a slant to cast the ramps. That's similar to what Tarvus suggested for a storage tote here.

I like the simplicity of construction, but wish it was a concrete-only system.

Jerry: "I think the mass of the concrete as well as the low cost is appealing in both projects, but by leaving the concrete encased in plastic (if that's what Winston did) you lose the wicking and evaporation that I think might be beneficial... I like the simplicity of construction, but wish it was a concrete-only system."

I agree with you.

Also, concrete doesn't stick to plastic well. It might be possible to use that as an advantage.

Suppose you cut down the plastic that the ramp is formed in, to form a reusable 'ramp mold'? Cut the plastic down to where the top of the ramp will be; there would be no plastic projecting beyond the ramp, anywhere. Spray the inside of the form with water, then press a big X of HD sheet plastic strips (like 2 big bandaids) inside, smoothed to the bottom and walls, with good-sized 'handles' hanging over the edges (to be used to leverage the ramp out of the form later).

Make a fairly stiff Portland cement/sand or cement/sand/perlite or cement/perlite mix. Reddi-mix is not suitable for this project. No gravel, no rocks.

Form the concrete ramp by hand, be religious about cleaning the edge with a damp kitchen-type sponge so no concrete hangs over the edge, and let it sit for a day. Cover with plastic and mist it if it even starts to look dry. Then try to flex the plastic a bit, just enough to break the seal between it and the concrete. Grab the handle-flaps of the excess plastic wrap (4 hands might be handy here) and very gently, work the ramp loose and lift up out of the form.

Make your bin out of the same-sized plastic tub that you made the ramp mold from, but hand-form the concrete on the outside of the tub. Use toothpicks or bamboo shish kabob skewers to keep checking the thickness of the concrete (I'm thinking 1" should be enough if the bin isn't oversized, and you intend to cure it reasonably well.) The bottom of the tub should be smaller than the upper part for easy release and removal.

In the interest of keeping the weight down, I wouldn't have two bottoms; either put a bottom on the ramp when you make it, or put a bottom on the bin. I'm thinking it might be better to put the bottom on the ramp, both to make it more stable for when the concrete is fragile and you're trying to get it out of the mold, and it would probably be easier to remove the tub from the bin walls without it. But without actually doing it, I'm not sure.

ONCE YOU'VE FINISHED EACH SECTION, DON'T LET IT DRY OUT. Remember, concrete doesn't dry, it cures, it takes 28 days, and it won't cure properly without enough water. Have a spray bottle handy and keep it misted once it sets. When it's hard enough so you can't dent it easily with your fingernail, you can use the mister on a garden hose. In hot areas esp, do the work in a shady place, then cover it with plastic. Check under plastic and re-wet as needed -- probably daily in FL and TX!

When you set the ramp inside the bin walls, wet the whole thing down well (WET concrete will NOT stick to DRY concrete), then mix up a small amount of fresh concrete (same recipe as the ramp and walls) and wearing a latex or nitrile glove*, smear the concrete into the joint all around the ramp where it meets the inner wall so no larvae can get into the crack. Don't be stingy here, force as much concrete into that crack as you can. An easy way is to put the concrete in a Ziplock freezer bag and nip off a corner, use it like you're decorating a cake, then press it in well with your gloved finger.

If you built the bottom into the bin itself rather than the ramp, also mortar the seam around the bottom edge of the ramp, making a good seal, and forming a ( shape to the corner would be a good idea so you don't have a square corner there. You'll have to clean it someday, after all; plan ahead and make it easier.

If you built the bottom into the ramp, you will eventually have to mortar the bottom of the underside of the ramp to the bottom edge of the bin wall. Personally, I would wait at least a couple of weeks (a month would be better) before doing this, to let all the previous concrete work cure, as you'll have to tip the whole thing on its side and mortar the bottom and edge together (again, saturate all parts with water so the fresh concrete will stick). I doubt that the concrete filling the top edge of the ramp will be strong enough to tolerate much movement or standing it upside-down.

All of this is theory. If I can work it into my schedule to actually make one, I'll let you know. If you try it and hit something that looks wrong, I may have made a mistake somewhere. *gasp!* And even my first one will be containing perlite to make it lighter.

*Concrete is caustic (a pH of 12-14), so don't do much bare-handed; a little is okay, but wash it off right away (have a bucket of clean water handy). Some people are more sensitive than others, but don't beg for trouble, no matter what you've seen some guys doing with concrete.

Sue