Help with what concrete to use for an usu ( aka a giant mortar)?

I've never worked with concrete before but I have a burning passion to make an usu which is more or less a giant mortar about 1 1/2 foot deep and 2 feet wide in the inside diameter that uses giant mallets as a pestles to pound sweet rice into mochi. What I need to know is what kind concrete will be able to hold up to hot steaming rice and constant impact with wooden mallets without cracking or chipping. My idea is to cut a garbage can down to the height I need and to use a styrofoam mold that are the diamentions of the hole I need. Do I need some kind of release agent or will the water evaporating from the concrete shrink it enough to allow the styrofoam to be taken out?

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ryanrall105 years ago
The trash can will work & so will the foam . But make sure the walls of the usu are at least 4" thick all the way around & on the bottom. It would be a very good idea to use some rebar also, 3/8 diameter will be sufficient . The other issue is the foam is lighter than the concrete &will immediately float up out of the trash can when pouring the "mud" (concrete) . To counteract the floating, get some wood like a piece of 2x4 only needs to be the length of the trash can. And put it on the bottom of the trash can on the outside, next poke at least two holes on the bottom of trash can & run some tie wire (baling wire,rebar wire etc. ) around the 2x4 through the holes and up & over the foam blank and back down again through the holes to the 2x4 . Also put some duct tape on top of the foam where the wire crosses over so as to keep the wires from cutting through the foam like a knife. Just buy a couple bags of premixed concrete (6$ a piece) mix it up in a wheel barrow or on a piece of plywood out in an open area if you don't have access to a wheel barrow. Mix the mud to where it's a pourable thick paste . You don't want soupy concrete as this will dilute & weaken the concrete & you don't want super thick sludgy mud either it's to hard to work with. Just start out w/ a little water and keep adding it in till it's right, someone once said you can always add water but you can't take it out :^) Pour the mud in & make sure you tap the outside w/ a hammer or piece wood or something solid , this will consolidate the concrete and remove any air pockets or voids. Let it sit overnight it'll be rockhard in the morning.You'll More than likely end up destroying the trash can stripping it away from the concrete usu, (so make sure it's a plastic trash can) You can take a little bit gasoline & pour it on the foam and it'll dissolve right away or just use a claw hammer and dig out the foam since it is soft enough it'll come out . :^)
Mr. E Meat5 years ago
Please don't forget to take pictures when you make your usu! I'd really like to see the finished product!
Re-design5 years ago
It should shrink enough to get the foam out and if it doesn't then it's still easy to just dig it out.

Another suggestion is to keep the conc. moist for up to 28 days.  Conc. get stronger and harder for 28 days if kept wet.  Once dry the hardening process stops.
ssbookyu (author)  Re-design5 years ago
How do you keep it moist and should I worry about mold?
Cover it with sawdust, towels or what ever. You can also trickle a garden sprinkler over it. It shouldn't be a problem with mold.
rickharris5 years ago
A standard mix of 1:2:3 cement - sand and aggregate should be OK