Seeking advice on how to fill cracks in shop floor

[Ned Otter]

Hi everyone --

My shop is an old shed from around 1870 or so, and it's resting on concrete over stone. The concrete is not too thick, has developed cracks, and they are large enough to complicate moving equipment.

About a decade ago, I cleared out one of the wider sections and filled it in with new concrete, though I'm no mason. I could do that for the wider one pictured below, but for the narrower crack, there's nothing to really clean out (except sawdust). I thought of epoxy, but I don't think that will work, as it would just seep into the crack and fall to the stones underneath.

Please let me know your thoughts.

Thanks,

Ned

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[Charlie Velasquez]

My previous shop developed two intersecting cracks that subtended the floor into four rectangles. The crack meandered back and forth over about a 12” swath. The problem was there was about an 1” to 1.5” difference in height between the sections, that always made moving machines and benches a chore and an adventure.
The shop was midway near the bottom of a gully, and the soil was always damp. Not good during freeze-thaw cycles in Iowa. Concrete patch would just crack again, and wouldn’t address the different heights.

My solution was to get a diamond blade and cut out about a 24” path, removing the cracked concrete completely. I backfilled with tamped paver sand to about 2.25” of the floor surface, place treated sleepers on the sand, then covered with treated ply, shimmed and screwed down to be flush with each section of the floor.

Worked well. 5 years later an-edge had formed along one joint. I removed the screws, pulled that panel and added more sand to be flush again.

That 1.5” elevation change was easily managed by my rolling things over the 2’ wide transition panels.

[Bill Dufour]

That was built when concrete was a new invention so it probably varies from modern concrete. It has no steel reinforcement to prevent cracking, too old for that.
Location? does it freeze or get dry enough for the ground to crack/heave.
Bill D

[Frank Pratt]

I don't believe there is anything you can do with the concrete that won't just break up again. How about sleepers, shims & plywood or OSB over top of the old concrete?

[Ned Otter]

Thanks to all who have replied -- location is in the northeast - upstate NY.

Will consider options discussed so far

[Jim Dwight]

Thinset mortar would work better than concrete to try and fill the cracks. Latex modified is sticky and it's thin with no agregate. I'd try it before more aggresive solutions.

[Ole Anderson]

Just yesterday in the local hardware store there was a product (sorry I don't know the name) that looked like mortar with fine sand but was an elastomeric compound that filled even wide cracks. Not going to work on skinny ones though. Hmm, not a lot of help, am I?

[glenn bradley]

I live in earthquake country so maybe my causes are different than yours. I followed the directions and used an anchoring epoxy (https://www.homedepot.com/p/Quikrete-8-6-oz-High-Strength-Anchoring-Epoxy-862031/203604182) as I wanted a repair that was hard, not flexible. I over filled just a bit and then ground it smooth with a diamond wheel since I thought I would be rolling machines across it. Turned out I went another direction with the shop. Regardless, it has been a year (and many shakers) and it is still perfect. I used about 5 tubes.

[Frank Pratt]

In the OP's case though, it's clear that there is a poor sub-base under that old concrete. Have a look at the pic of the wide crack he previously repaired. The patch material has broken up to the point where it's just compacted rubble. That indicates movement. Any kind of rigid patch will fail in a similar fashion. That's why I think sleepers & plywood is the solution.

[glenn bradley]

Too true. I should have looked closer before opening my mouth ;-) I don't see any patch surviving that situation.

[Daniel O'Neill]

Ned,
Epoxy sinking would have more to do with the viscosity of it rather than anything else. If you get a thick enough epoxy it will fill the void, especially if you can do it in lifts. That however would be expensive. I might try some hydraulic patching. Or you could try some self leveling concrete but that is very runny and will likely take a lot to fill the cracks. There is a cement product used for road & bridge deck repair that would work. It has a high early PSI and depending on the product can go pretty thin to feather edge. I would skip the big box and talk to contractor suppliers. The brand names will vary by area but the bridge deck repair stuff will do the trick and you don't have to worry about sinking. That said when we do road patching, even if the road isn't up to snuff the patch is. Meaning clear out your crack to at least 4" and fill that with concrete. If there is space you could put some dowels into the slab on each side so that even if cracks reappear you shouldn't have too much elevation change. If there are no control joints in the slab it would be worth adding some.

On the other hand. If the slab is thin it might be worth pouring a new slab on top (4" thick or more). JMHO
Have a great day
Daniel

[Ned Otter]

Thanks very much to everyone for their feedback.

Just one point to clarify -- the larger "channel" (compacted rubble) in one of the photos I posted was not formerly repaired by me (it was another one).

I think I'll mull over the advice given, try a couple of things, and report back.

[Bill Dufour]

Since there is no steel in your concrete, to old for that, each slab will move relative to the others at the cracks. I wonder if the local engineering school would be interested in looking at the job and offering advice for the historical value?
Bill D.

https://en.wikipedia.org/wiki/Alvord_Lake_Bridge

[Tony Joyce]

About 10 years ago I worked at lumber plant where we did slab table tops.
We used epoxy to fill knot holes, before the river craze.
Anyway, I usually had epoxy left over and decided to pour it in the divots and holes in the concrete floor. I would overfill and sand flush.
Worked like a charm, so I did some in my own shop.
Very pleased with the durability, both these are in high traffic areas where carts with metal wheels are used. These are fills are about six years old.
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Tony

[Tom M King]

I like Jim Dwight's method the best in the thread so far. The other choice is a 20 pound sledge hammer to start. If the sections want to move later, relative to each other, it won't matter much what is in the cracks. Thinset if easy to work with. Wet both sides of the joint well to start with, or the concrete will suck the water out of the mix too fast, have no strength, as well as set too fast for you to work with it, and probably even crack. Follow the directions on the package. Thinset calls for slightly different mixing procedure than concrete.

[Ned Otter]

So far, I'm leaning towards some type of epoxy solution for the narrow cracks, even though they are long and there will be some expense to that. I've contacted the folks at West System for their advice, and have ordered materials that will allow me to test in these low-ish temps.

I cleaned out the cracks with a vacuum cleaner and a very narrow drill bit, which is about 6" long. I probed the length of some parts of the crack, and I could put the entire length of the 6" drill bit in the crack without hitting anything. So this will be a very expensive epoxy repair unless you also use filler and change the consistency of the epoxy. I'm not so concerned about bonding the crack, only weight from above as I move heavy machinery. My first test would be to see if the epoxy/filler combination is thick enough to hold weight, but not fall into the gap, a consistency sort of like peanut butter. The filler will turn the epoxy an off-white, which I'm not crazy about, but could live with. You can use a dye to make it more "concrete-like", but with multiple small batches, I doubt you'd get color consistency between batches.

For the wider area that looks like it was in an earthquake (2nd photo from my original post), I removed most of the loose concrete, only to find that in fact there is a long piece of wood underneath (not sure how thick it is). The total thickness of the concrete is no more than 1/2", and thinner in some parts - you can see stones directly underneath the concrete in some of the photos I've attached. The concrete I removed didn't really match what's on either side of the wood, so I'm guessing is from an older attempt to fill that void.

@Tony Joyce - do you remember what kind of epoxy you used in your shop? I'd like to stay with clear epoxy, but would need a way to prevent it from flowing into the cracks, which look similar to yours.

Thanks again to all who responded - will report back here when I've gone further.

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[Tom M King]

I would get that wood out of there, and just fill it with concrete, slick finishing the top. The narrower gaps, I'd use type S mortar, and add some plain Portland cement to it, so the top can be slick finished. You don't have to add a lot of Portland to it, and if you add too much, it will actually be weaker-just enough so you can float some "fat" to the surface to finish-sorry I don't have a formula.

The only tools you would need would be a 2" margin trowel (for getting it out of the bucket, and in place, a hand float, and a concrete finishing trowel.

Saturate both sides of the crack until it won't take any more water, before you add your mix to the crack.

Epoxy wouldn't be bad for the small cracks, if they have a bottom, but I wouldn't use it for the big one.

[Ron Selzer]

At some point it could be same cost, more/less just to replace the floor
With new concrete. Could do it in sections or all at once.
Another option would be pressure treated sleepers with 3/4 t&g plywood subfloor.
Would definitely look at all options, even ones way out of the box before starting any repairs
Something to consider once you get a handle on what all the epoxy costs.
good luck
Ron

[Tony Joyce]

Although this is labeled as table top it is very versatile and stable for cracks.
http://www.uscomposites.com/kk121.html
You can use sand to fill the bottom of the crack then pour the epoxy on top for the last 1/2" or so of crack.
I do not do multiple pour on cracks like mine.
You can also use cement(small amounts,dry) as a filler in the epoxy to color.
I over filled my cracks and sanded with a belt sander. I know it sounds crazy, but it works well.

Tony