New floor has cracked too

[ct03911]

Last fall I had a seven year old mess of a floor removed and poured a 6' 4000psi floor, 26'x28'.

We went round and round on control joints because I was going to put in a lift, maybe two, but the exact post positions were hard to figure.

The contractor did not feel they were necessary anyways. Well, it appears they were.

I'm thinking of repairing the cracks with a product like a few of the vendors on here sell and THEN adding control cuts.

I know, most say if the control joints are not in the first 48hrs for sure they don't really work but I don't want to repair the floor, then epoxy it to then discover more cracks down the road.

How could cutting control joints now do anything but possibly help guard against future stress cracks? They won't hurt and the contractor has suggested he will do them for free and perhaps pay for some of the floor finishing product. He knows I do not want to look at a crack after paying him thousands to take my floor out asand pour another one.

Is repairing the crack I have now first and then doing control joints then coating the way to go?

When you epoxy finish a floor, do you fill the control joints with any type of product to go for a smooth flat finish - not like the hard products for crack repair - but an expandable crack filler, before putting down the epoxy finish?

Thanks

 

[akcooper9]

There are 3 guarantees with concrete:

1. It will get hard
2. It will crack
3. No one will steal it

IMHO as long as its not lifting or pulling apart, don't worry about the crack.

 

[DougWil]

Last fall I had a seven year old mess of a floor removed and poured a 6' 3000psi floor, 26'x28'.

We went round and round on control joints because I was going to put in a lift, maybe two, but the exact post positions were hard to figure.

The contractor did not feel they were necessary anyways. Well, it appears they were.

Of course they were, 2-2.5x thickness. You needed cuts every 12'- 15'.
The slab sawn in 1/4s would have been fine at 13 and 14'.

Please tell us you have rebar in that slab and not just fiber.

 

[Chevy-SS]

I apologize for being somewhat critical, but I can't believe you guys didn't do control cuts. Why would you go through all this trouble and then neglect the cuts? Concrete typically cracks.......

Anyway, best of luck. Hopefully you find a satisfactory remedy.

 

[ct03911]

Regarding repairing and epoxy coating, would you:

A: repair the crack, then do control cuts and then epoxy coat the floor?

Or,

B: control cuts now, repair the crack and then coat?

The cuts were not done because the position of the lift was uncertain. Does it seem dumb in retrospect? Well, a lot of stuff does when you get smarter or have a bad experience.

The cuts could not go down the middle, ie make four quarters in the garage - because of the lift columns. They could also not saw half way into the garage because we did not know how deep into the garage the lift would go.

So, you'd have to do the floor in third's each direction, making nine squares about 8'x9'.

If I was to do it today, that's what I'd do. But today the lift is sitting there and now it is easier to figure it all out. When I did not know where the lift was going it was not so clear, trust me and nine 9' blocks seemed crazy.

Anyways, it cracked, yes, even with rebar.

The question is, fix it, cut it and paint it, or what? Even if cutting it now is all but useless, if it can't hurt I'll do it in case it saves another crack later.

 

[Radix2]

just fix it and move on - the shrinking stress has been relieved at this point.

As an aside - Where did it crack vs. where you would cut it now? Obviously any new cut near a current crack would be doubly useless.

 

[Armorpoxy]

We had had customers cut, and then repair the cracks with our Crack Repair Epoxy Putty. It's stronger than concrete.

You can then epoxy over the floor and fill the joints with flexible joint filler, or just leave them be.

 

[DougWil]

The cuts could not go down the middle, ie make four quarters in the garage - because of the lift columns. They could also not saw half way into the garage because we did not know how deep into the garage the lift would go.

So, you'd have to do the floor in third's each direction, making nine squares about 8'x9'.

If I was to do it today, that's what I'd do. But today the lift is sitting there and now it is easier to figure it all out. When I did not know where the lift was going it was not so clear, trust me and nine 9' blocks seemed crazy.

Anyways, it cracked, yes, even with rebar.

The question is, fix it, cut it and paint it, or what? Even if cutting it now is all but useless, if it can't hurt I'll do it in case it saves another crack later.

The saw cuts or grooved joints are for initial slab shrinkage while curing.
Your concrete should be long past that if it was kept wet or with a curing agent.
In short all the major shrinkage has taken place, saw cuts now won't do anything.

The recommended depth of saw cut is 1/4 the slab thickness. In this case 1-1/2".
But in interior slabs w/o wild temp swings you can get by with less depth.
The rebar should extend across the saw cut or if you are making separate pours dowels should extend across the joint.

Doing so and assuming good base compaction you really don't have a weak spot where a lift with a large baseplate couldn't be placed.
If an anchor is close to the joint, I would use an epoxy and not the expansion type.

The rebar is for primary reinforcement and will not prevent shrinkage cracks, but it will prevent cracks from opening excessively or separating into broken separate slabs.

The shrinkage cracks you have should be very small, I either wouldn't worry about them or would epoxy the floor and let them fill them.

 

[ct03911]

Thanks all. Let me see what the contractor comes back with after speaking to his supplier.

I suppose I'm thinking just repair the cracks and coat the floor. I just wonder if the cracking is really done yet. It just started, to my eye anyways, maybe three weeks ago and I can tell they have opened some over that time. When the crack is really done moving is probably a guess.

I suppose I could do a repair, wait until the fall and see if I've had any others and coat the floor then.

I do still wonder why, if a crack has now developed, why wouldn't a saw cut a month ago have given the floor a place to relieve itself.

And by that thought process, why wouldn't saw cutting now keep any future stress relief in those saw cuts.

I get that doing the cuts right after the pour is better. I don't get why doing them now is useless, or if it is really.

Thanks.

 

[DougWil]

I do still wonder why, if a crack has now developed, why wouldn't a saw cut a month ago have given the floor a place to relieve itself.

And by that thought process, why wouldn't saw cutting now keep any future stress relief in those saw cuts.

I get that doing the cuts right after the pour is better. I don't get why doing them now is useless, or if it is really.

Thanks.

Because they are shrinkage cracks or we ASSUME they are.

Was this a load bearing slab?
Or a free floating slab that only supports vehicles etc?
Do you think the soil and base below was properly compacted?
Any chance of frost heaves or water getting the base soft or expanding it (clay soils)?
^Because those may not be shrinkage cracks but tension cracks.

But back to shrinkage cracks, they occur as the water content of the concrete changes.
Lots of shrinking initially and decreases with time. And is basically complete in 12-18 months in the field, 6 months under lab conditions.
Shrinkage or shortening also occurs as the slab temp drops.

The shrinkage cracks you see are completely through the slab, they aren't just at the surface. So the concrete at the crack has zero tension strength. The rebar has stretched and it carrying the tension.
Whereas concrete saw cut near that crack still has tension capacity because it is uncracked below the sawcut.
So the new sawcut isn't the weak spot, the existing crack is.


So exactly how old is your slab?

 

[ct03911]

The slab is six months old. It is on mostly sandy soils that have been basically undisturbed since the original floor went in 8 years ago. I doubt there is any soil settling or water pressure under it in the winter. Good drainage area.

All these guys did was remove the old floor, basically some light surface grading and stone added and set up for the new floor.

I understand this new crack is the weak spot now and goes all the way through.

My last floor cracked all over the place which was probably attributable to rapid drying. Because of that experience I'm just gun shy that if I repair this crack and paint the floor or don't do a cut maybe another could form.

The garage has full frost walls poured and the floor is a separate pour inside that.

Let me ask it this way: could doing saw cuts now harm anything?

 

[DougWil]

Let me ask it this way: could doing saw cuts now harm anything?

No, saw away.

 

[Falcon67]

I apologize for being somewhat critical, but I can't believe you guys didn't do control cuts. Why would you go through all this trouble and then neglect the cuts? Concrete typically cracks.......

Anyway, best of luck. Hopefully you find a satisfactory remedy.

I didn't saw cut and no major cracks, just minor stuff. The slab on the next lot is 40x60 and has no saw cuts either. Same deal, just small shrinkage cracks. House next door with 92 yards in the foundation - no saw cuts in that either.

 

[HemiRamOn22s]

I would fix the old crack and still do saw cuts. It will still crack in the future. You can fill the saw cut with caulk. My house and shop has a combination of things. The basement was metal keyway and zip strip. The garage floor was the same. The front porch was keyway. The back porch was sawcut then filled with gray caulk. The shop floor was zip strip and saw cut. The outsite patio under the lean to was keyway. All of the floors cracked exactly where we wanted. Everyway situation is different. By the sound of your floor you should of done keyway and zip strip with the small size it is. The floor will crack where the keyway is then you fill the cracks and epoxy over.

 

[LegacyIndustrial]

How does it go?

Some concrete will crack and the rest will too?

 

[Firebird 1]

There are 3 guarantees with concrete:

1. It will get hard
2. It will crack
3. No one will steal it

IMHO as long as its not lifting or pulling apart, don't worry about the crack.

I will have to add the third to my list. I have always told customers. I guarantee concrete to 1. get hard, and 2. to crack!
The trick is trying to persuade the concrete where it wants to crack.

 

[tn_popo]

Once a concrete floor cracks how long should I wait before repairing the cracks? How long til it has cracked all it's going to?

 

[DougWil]

Once a concrete floor cracks how long should I wait before repairing the cracks? How long til it has cracked all it's going to?

Concrete shrinks from curing for up to 18 months.
If the slab is cracking for other reasons it could crack forever.

 

[tn_popo]

Thanks