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I mixed 92 pounds of mix by hand with shovel and trowel in a big plastic tub. Total added water was 4.5 quarts.

Hand mixing is slow. I didn't time it, but let's say it took 30 minutes total to mix and then weight out two buckets of 45 pounds of wet concrete each.

Then I poured it into two 16 inch diameter circle molds. (I'm making DIY weight lifting plates.) I tamped down the mix to level it with a wood block and tapped the mold sides.

There was a small amount of water bleed on top and bottom edge of molds which were not attached to a plastic sheet underneath. I sponged off the bleed water with a paper towel just one time a few minutes after leveling the mix in the mold. Then I covered it with a plastic sheet.

The concrete to water ratio seemed good. I didn't go over the recommended max, not much bleed, the mix leveled off with a smooth top. Actually, I improvised a 12 inch slump cone while mixing, and that seemed good. It held up with only an inch or two of slump.

I figure I spent about one hour, no more that 90 minutes, starting the mixing to finally leaving the mix in the molds to set. Would that amount of time be considered overworking? At what point in time toward setting will mixing the concrete again ruin its strength.

Part of the reason for my concern is I used left over mix to make a 1.5 x 7 inch disk using a metal can with the top and bottom cut out. The day after all these molds set I removed the 7 inch mold first as a test. The concrete stuck to the metal mold, which I had oiled, so I tried tapping it out with a mallet. I got it out without breaking, but later while holding it in my hand, it just broken in half from its own weight. It had been setting for 12 hours.

I removed the 16 inch molds with no trouble at all, but waited a full 24 hours of setting time before pulling the molds. Both 16 inch plates are now pond curing in a tub of water.

I'm not sure if I did a bad mixing job or just treated my 7 inch mold too roughly while it was still soft and uncured.

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  • Concrete reaches full strength after 28 days so taking it out of the mold after 12 hours was likely your downfall.
    – MonkeyZeus
    Jul 28, 2021 at 17:00
  • Maybe, but 28 days to full cure isn't the same as time to remove molds. My real concern is the 16" pieces which haven't had any rough handling out of their molds. Should I expect them to reach full strength?
    – Michael Curtis
    Jul 28, 2021 at 17:09
  • @MonkeyZeus, that's an overly specific rule of thumb that's not particularly useful. It's also wrong 99% of the time. I'm not saying that your general point is wrong, but....
    – isherwood
    Jul 28, 2021 at 18:25
  • Was the "leftover mix" wet or dry prior to adding water? How long the leftover mix was left there and with or without protection?
    – r13
    Jul 28, 2021 at 19:30
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    I was pointing to the leftover that seems to have a problem. The 16" pours should be good, just need time to cure, by water or cover with a tarp or plastic sheet. The mold can be removed the day after the pour, not to wait too long, as the form of the concrete has already stabilized but is still fragile and prone to damage. The concrete will start to harden about 48 hours after the pour.
    – r13
    Jul 29, 2021 at 22:31

3 Answers 3

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I think your timeframe is fine. Overworking usually refers to 1) working the finish of concrete too long, allowing it to dry out and cure poorly on the thin top layer, or 2) adding water to "retemper" the mix, which can weaken it throughout.

MonkeyZeus is right--12 hours is a very short cure interval. Concrete is very "green" and soft at that point. When building homes we would sometimes work on basement slabs that soon, and we had to be very careful not to scuff the surface or drop tools, which can cause divots. We could drive standard wire nails through wall plates into the slab by hand.

Next time, wait at least two days (48 hours). Three to 5 would be better, with a little watering in the meantime. At that point you should reach 50-75% final strength, which is usually adequate for gentle handling. (Keep in mind that standard concrete at 1½" thick is always fragile; it has high compression resistance but fairly weak tension strength.)

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  • Is the 48 hour wait time before removing from molds?
    – Michael Curtis
    Jul 28, 2021 at 18:41
  • From any handling, really. Any jarring can crack it. The one didn't break in your hands. It broke when you tapped it, but just didn't fall apart at that time. It's little more than cookie dough until the chemical bonds develop.
    – isherwood
    Jul 28, 2021 at 18:43
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Because this question is so far out there I want to make sure future readers understand.

  1. Your cure time before it is strong enough to just move around on something like plates is literally 4-5 days at least.

  2. As a lifter and former weightlifting coach I commend you for the DIY. But for your sake and anyone reading this... Creating your own concrete plates to seriously use has to be one of the dumbest and riskiest ideas I have ever heard. Concrete cracks. Concrete cracks faster when there is hole in the middle of it. It is very likely that this will do fine for X amount of time. For your sake I hope you are not benching as a weight cracking and falling off could mean death. If you are squatting it, you might go to the ER. If you are overhead pressing... well lots of things could happen, most not good.

Just helping run a gym and getting leftover plates from the 70s (metal)... a few broke, thank god during things like deadlifts or hang cleans. We threw these out plus any like them. Making concrete plates and using them to lift is akin to blindly rewiring your house.

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    Making concrete plates is a popular DIY thing lately. You can even buy molds for them.
    – Michael Curtis
    Jul 29, 2021 at 13:35
  • @MichaelCurtis - I get it. But lots of popular things are dumb. Also if they don't even break the weight of the plates are so important. Those same 70s plates we had the 45s were actually 44.2s. And everyone knew. I had some guys try to find them and use them to bench for the mental boost. Anyway if this is a craft project then fine. If this is for serious lifting (safety) then it is a joke.
    – DMoore
    Jul 29, 2021 at 19:59
  • Yeah just to stress this answers, concrete weights will likely not survive being dropped even once.
    – whatsisname
    Jul 29, 2021 at 20:18
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    @DMoore, where in my question - which was about over working concrete not powerlifting - did you conclude that I will be deadlifting 400 lbs, dropping weights, labeling the weight wrong for an ego boost? I appreciate the safety advice, but you're describing a scenario that doesn't apply to me.
    – Michael Curtis
    Jul 29, 2021 at 22:01
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    @MichaelCurtis - I wrote the answer for the community, not necessarily you. I understand that you think its a good idea. I want others to understand how dangerous this is. Also I am not sure why that was your particular comment. Out of all the lifts we do the bench press is for sure the most dangerous and it actually kills more people a year than sharks... One of these crack when you are benching and you could be dead. Good luck.
    – DMoore
    Jul 30, 2021 at 3:30
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Per previous answers on curing time vs overworked - it needs at least 4-5 days before moving, the full 30 days to reach full strength before using. The concrete is not over-worked and causing the concrete to fail.

Not to go into whether or not it's a good idea or not to build your own concrete weights for lifting, your description of how you build your weights is lacking in a lot in how to properly build the weights.

  1. Don't use just any type of concrete. Use a high-end concrete, i.e., High Strength Crack Resistant concrete
  2. Insert a wire lath/mesh to increase the structural strength and decrease the chance of the weight from breaking apart
  3. Add fiberglass fiber to the mix
  4. Find a way to shake/vibrate the mode to remove air bubbles
  5. Use a tool with a curve to round the top edge, which will help to reduce or prevent chipping on the outside edge
  6. Use proper drying techniques, discussed in other answers
  7. Spray with one of the various coating products to minimize chippings, like flex-rubber or truck bed liners spray/coating.

Watch some of the various YouTube videos on how to do this to get an idea of the whole process.

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  • Thanks for the bullet list. I skipped some detail, because I was mostly concerned if I had over worked the mix. Yes, I did add in wire mesh for reinforcement, and also after 28 days of cure I plan to coat them with truck bed liner. I didn't power vibrate the molds, but tapped and shook the sides. I watched loads of YouTube videos. The funny things is all of them remove, paint, and use the weights to demo much too soon by the advice here.
    – Michael Curtis
    Jul 29, 2021 at 21:51
  • I wish I knew the 4-5 day wait sooner. But I only moved them once, very gently, to put them in a tub of water to cure for a week. After that will be another 3 weeks to dry and cure before coating. ...I used the all purpose Quikrete concrete mix 4000 PSI rated.
    – Michael Curtis
    Jul 29, 2021 at 21:54
  • You are probably ok with the larger concrete weights based on your new information. The smaller weight would be prone to failure, because normal concrete has to be a minimum of 2", plus you didn't allow enough time to cure before removing from your DIY mold. For you main weights, I would use the high strength, with fibre, or the ones specified as crack-resistance. On the video's, you really don't know the cure time the authors used.
    – Programmer66
    Jul 30, 2021 at 1:19

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