Replacing Gym Floor with Concrete

Midway through the pandemic I decided to turn the old shed behind my house into the gym. The floor always seemed a bit creaky but I decided to think nothing of it and continue dropping stuff on it. Unfortunately this attitude, coupled with a 200lb sandbag and trying to clean too much weight, has resulted in 2 huge holes in the floor.

I recently started pulling up the carpeting (neighbor told me the pervious owner had someone living in the shed) and found that the flooring is on a really poorly made frame with some particle board and plywood over top.

Now I need to figure out what/how to replace this floor so that it will survive. Originally I was thinking of just adding additional braces to the existing frame, but I worry this won’t quite hold up. I want to instead put concrete down but have a couple questions:

  1. Can I pour concrete around a wooden frame? (see picture)
  2. Can I use old broken concrete as a filler so I don’t need to buy/mix as much? (there’s a ton of broken concrete on the side of the shed)
  3. If I put stall mats over the concrete should I put something between the concrete and mats?

Any advice would be greatly appreciated.

Sorry, no pic visible.

Not a great idea, especially for uniform strength and thickness.

Stall mats are good.

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Is there a gap between the floor and the ground? Your description would imply that is the case.

Pouring concrete over a weak frame elevated from the ground sounds like a very bad idea to me.

Yes you can pour concrete on the broken concrete to reduce volume needed. The problem will be removing air pockets and ensuring it flows into all crevices. You can half-ass it by just using a stick of rebar to poke around a bunch, or 2/3ass it and get a concrete vibrator tool. You’ll also want a less viscous mix.

This doesn’t need to be perfect. A rotted, poorly built wooden frame floor help up fine for a while. A shitty concrete floor with a layer of plywood then horse mats will be more than fine. Who cares if it cracks? It’s covered with horse mats. This is a fairly stop-gap fix, right? You don’t need it to last for 25 years.

That said, do not underestimate how much goddamn concrete you’ll need to mix. It’s a real PITA. If you have a concrete buddy, ask them to swing by next time they have some left over in their truck at the end of the day.

Dig out 3" of dirt. Throw 3" of 5/8 minus base rock in there and compact. Fill the rest with concrete mixed in a wheelbarrow.

It can be ugly. It doesn’t need to be perfect. Don’t overthink it.

How long do you plan on using this shed for lifting and dropping barbells/sandbags?

Personally, I’d throw a couple more 2x4s in, fill with rock, compact to level, and throw down plywood and horse mats. Replace a sheet of splintered plywood every two years or more. Simple. Ymmv.

Yes it does! Just like welds that will never be seen!


@Californiagrown The old lady doesn’t like the look of the shed, so I doubt I will be leaving it to my prodigy. She hates my gym being on our patio even more though, so I need to get this floor in place and move everything back in before she leaves me…JK…kind of.

I like your idea of adding extra 2X4’s and filling in the space with small rocks. I’ll post some pictures of the leftover concrete on the side of the house. I think this concrete might actually have come from whatever was were the shed is now. I would like to some how incorporate it so that I can get rid of it and not have to spend too much on concrete.

@SkyzykS I like your idea about doing this properly for a longevity, unfortunately I’m kind of under the gun to get this job so this will have to be a rush job. As long as it can support dropping weights for the next 2-3 years I should be good.

I will post some more pictures as this project moves along. Hopefully I don’t end up on r/diWHY

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Laying concrete was a monumentally crappy task. I learned a lot about concrete and even more about myself. I learned that:

A) Concrete sucks
B) I hate concrete

I’ve attached a picture, please don’t laugh at my terrible craftsmanship.

My plan is to put something that will cushion it over the concrete (maybe even just put the carpet back) then build a lifting platform over top of that.

These two things should give me the stability and strength I need to throw sandbags around.

Can someone please help me understand why my concrete didn’t turn out smooth? I added the required amount of water and even used an electric mixer, yet for some reason the cement part didn’t seem to “come alive” and instead I was left with very granular concrete.

It needs to be floated. That’s when you take a tool called a float- and with very light and even pressure, glide the float across the top.

This creates a capillary action between the wet surface of the concrete and the float which pulls the cream to the top while simultaneously pushing the aggregate down a little.

Then for like a glass or finished surface you can trowel it just before its cured. Or brushing for that nice neat textured surface.

Concrete is an art, and a tough one at that. I did it for a while. When you put it down and it’s nice, it’s nice. But when it sucks, it sucks for ever.

Looks like you needed a jitterbug.

Thanks for the advice about the float. I was trying to get the same effect with a 2x4, didn’t work.

It also seems like all the concrete they use in promotional youtube video’s had a different consistency than mine. By about bag 13 I found out how to get it a little more runny while still using the required amount of water, but never quite got it to the porridge like consistency I saw online.

Putting down this gym floor was supposed to be a trial run for a much bigger project of next to my driveway. I’m starting to think the next project is better left to the professionals

I finished the floor. Is it great? No. Is it good? Also No. Is it functional? Define functional.

The original pour left so many dents, gaps, and holes to the point where the floor felt like walking on the moon. I decided to cover it up with a cement sand mix. This mix was much easier to work with ONLY after I added twice the amount of water it required.

Once the floor was done I put the carpet back and made a crappy platform out of some old plywood (I can’t afford $65 for a piece of plywood…no way).

Part of me wants to try another project involving cement, a way to stick it to the cement (pun intended?). The other part of me is like “nah”.

I’m still confused about the required water. The required amount according to the packaging is no where near enough, thoughts?


Yeah. You can mix concrete pretty wet and it will still work and maintain the desired properties.

It shouldn’t be splashy/sloppy wet, but more like oatmeal, less like peanut butter.

When we poured driveways and steps on steep hills we would pour it “tight” almost like peanut butter. It’s harder to work, but stays in place. On flatter applications we’d pour it loose which makes it easier to make flat and finish, but it can be overworked (too much cement rises to the top) and the finish will spall (shatter and break off) under any significant impact. It’s also weaker when over worked because the aggregate settles on bottom and isn’t properly bonded.

Working it- we’d pour or place into the forms, then screed it out using a 2×4. This is the first step to flatten and smooth it, roughly. Next came the bull float, which further flattens and smooths, then hand float and various finishes.

It’s a process of packing down the aggregate and separating a little bit of the cement (hand floating actually has a suctioning effect) to make a nice finish.

Edit: Not Bad! Certainly better than where it started.

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