Best way to make 24" diameter concrete disks?

Interest Groups Trebuchet
#1

For trebuchet weights, we need to make 18 concrete disks, each disk is 24" diameter and 1.625" thick (about 0.44 cubic feet per weight which is 1 $3 60lb bag) and will have a section of steel pipe in the middle with some rebar welded to it. Total amount of concrete used will be ~8 cubic feet of concrete.

Anyone have an idea on the best way to make these?

Could we find some 24" sonotube somewhere for cheap? Would it be worth renting a concrete mixer?

The entire depth of the weight on each side needs to be less than 15", which is why the diameter is so large for the concrete. Using steel as weight instead of concrete would be >3x more dense, but also >10x more expensive, $500 vs $50 for 1,000 pounds.

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#2

Maybe rebar reinforcement might be considered. Afaik, concrete is weak under tensile loads like what will be experienced under deceleration. Alternatively: get some large diameter pipe, plasma cut and weld side plates, and fill with weight of choice.

#3

Could you use old bench or squat plates, from gym equipment? You might be able to find those on craigslist, and they sound like the would be close to the size you need.

#4

How about using expanded metal & perhaps burlap fibers in association with the rebar to keep the surface area greater. For the concrete you can buy it ready mixed say from A-rentco. They do small batches.

#5

I don’t think concrete is a good material choice here, they’ll likely break in half.

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#6

Especially for the thinness that Op is looking for: 1.625" thick? Eeek.

He’ll really need some of that fiber reinforcement material mixed in and it’ll still shed chunks as it’s being flopped around under use.

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#7

We can buy cardboard concrete caissons (Home Depot or equivalent) to use as the form. Does it have to be 24"? I think they might come in 18" dia. This would force the disk to be thicker to achieve the same weight.

#8

How much total weight are you going for?

I know I can donate some steel weights to the cause. I’m sure there are others that have these sitting around unused. Could also throw in the metal bar you could cut down to size.

#9

Initial arbitrary goal is 1,000 pounds, which is how I estimated the amount of concrete and size of each disk, idea of keeping each disk narrow was to make it easy to handle, but could double thickness and then each disk would be in 120-150 pound range

Was also planning on ordering 140 feet of 1/2 inch rebar to put ~8 feet inside each disk

I assumed we wouldn’t be able to source enough free weights for free to get to 1,000 pounds. How many people have Olympic size(2" center) 100 pound weights?

#10

How about optional of making steel as encasemeant technique. In other words make a piece on thinish plate then weld the inner & outer sections on to the base. Then leaving enough space to add a cap of steel after filling with concrete. I have enough 1/8" that I’d give to the cause. We do have the plasma cutter to make them quickly.

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#11

That is a good idea (build forms out of steel to increase weight and strength), but I think it would take $200 worth of sheet metal (not necessarily out of the question, would still be in budget)

If we made each disk 3" thick, would need 10 of them, strips around the sides would be 3" x 63’ of metal, requiring one 4’x4’ sheet. Then to make 10 2’ diameter circles would require 3 more 4’x4’ sheets.

A 4’x4’ 16 gauge sheet would be ~$45 x 4 sheets = ~$180

#12

I assume that after done this will be dismantled.

If so, I know I’d be willing to “sponsor” a disk and get metal back afterwards.

Also, there are probably lots of 25# disks that could be encased in concrete out there. Not all of us went for the He-Man size disks!

#13

You could easily get remnant steel from the steel yard. I have picked plenty up from yards before. The yard I frequent has like 1/2"plate 24-30" diameter circles often.

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#14

Also. I think I have a half stick of 2 3/8" pipe, you are welcome to it.

#15

Modifying idea above here is what I am thinking, there would be no steel circles on the faces, just a sheet steel strip around outside. This takes care of the need for buying a separate form and protects the weights from chipping around the edge. I’m assuming just setting it on cardboard or thin plastic drop cloth would work while pouring them

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#16

That rebar positioning isn’t going to help you much. A concentric square with overlapping ends or a hoop would be more effective. Make sure the reinforcement is as close to center as possible. You’ll want to tie the overlaps of the bars too. Make sure you get deformed bars, not smooth bars.

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#17

Yeah, ~$0.70 per pound is about the cheapest they were going for on CL, so unless several someones at garage sales don’t know what they have … not a cheap proposition.

#18

Two thoughts on concrete… Excuse me if they don’t fit the project. New boy.

A) #4 rebar (1/2") seems like a heck of a big bar for such a small pour. I would think that welded wire mesh would be much more appropriate. I think the squares are around 5 inches. Probably a 1/4" wire. Give or take.

B) with regards to a concrete form, you might consider buying a plastic trash can. Cut it up in whatever diameter you desire. Maybe cheaper than sonotube. Maybe not.

Give it a thought…

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#19

The rebar is definitely overkill, it is there just as much to add mass and to make it easy to make the sheet metal form around the outside. Was thinking of putting in wire mesh as well.

#20

I have two on the front of my tractor. Got 'em off Craigslist.