Concrete control joints not controlling anything

[** Frank **]

Contractors and DIY use groovers or saw cut control lines so that the concrete will break at the joints. That's the plan but more lightly the concrete slab will crack everywhere except at the joint lines. For the concrete slabs I saw on patios, driveways and sidewalks, it seems pointless to have control joints. Expansion joints excepted, off course.

So the question is, when concrete cracks then how do you make it break at the control joints more often than elsewhere?

[Malcolm Hoar]

It really depends on the specific situation but in general terms:

1. Prepare a proper foundation for the pour. If you pour concrete on dirt, it'll likely crack all over the place.
2. Add steel mesh and/or rebar to each slab.
3. Use the appropriate mix and provide it with an opportunity to cure properly (pay attention to temperature and moisture levels).
4. Agitate and otherwise ensure a complete lack of large air or water bubbles within the concrete itself (esp, around any steel reinforcement).

If you do those things right, correctly sized slabs will not crack.

[gfretwell]

If you aren't using steel the best chance is if you cut the joints when the concrete is still green, certainly within 24 hours on a cool day.. Stresses get relieved earlier while the concrete is still a little elastic, but this still will not do anything if the substrate you poured on is bad. Once it sets, if the ground shifts, the concrete cracks. Steel will tend to pull that stress to a controil joint but YMMV.

[BobK207]

Depends the slab thickness but for a 5" slab......place cuts such that you have 150 sq ft max panel size (try to keep the panels somewhat squarish) & cut depth 1/4 to 1/3 of slab thickness, cut as soon as concrete can be worked upon without damaging surface.

Too much water in the mix can cause shrinkage cracks that cuts will not totally help avoid.

This should work out pretty well.

cheers Bob

[DanG]

I don't know that I agree with Malcolm.

Good subgrade prep and compaction - yes. Nothing wrong with the right kind of dirt, select fill preferred. No sod, no loam,

Mesh does not reinforce concrete. Rebar in a 4" slab does not help, actually hurts.

Avoid re-entrant corners. Concrete wants to be square, not rectangular. Never exceed 12' in any direction without at least a contraction joint, closer preferred. Cut all contraction joints a minimum of T/4, that is, 1/4 or more of the slab thickness. Most lumberyard jointers cut 1/4" deep. Make sure the keel is at least 1" or you're wasting your time, I prefer 1 1/2. If the contraction joints are sawed into the slab (my preference) they must be sawed the same day as the pour as soon as the concrete doesn't ravel under the blade. You CANNOT wait and cut them the next day. Curing the concrete is crucial. If you use a curing compound, make sure it is compatible with potential floor finishes/adhesives. A 3 day minimum full wet cure is the best.

[** Frank **]

I'm about to redo a 4" slab, 3' wide sidewalk with rebar. If rebar (and mesh as well) is not good, then what should be used?

Thanks for the pointer as I thought you could come back next morning and saw away.

[** Frank **]

Wow! Thanks to all, great information.

[trader4]

What exactly is the problem with coming back the next day and sawing the control joints? Stamped concrete contractor did exactly that with my patio and it worked. Only place I have a crack is in one section where he should have placed another cut, but left the span too long. One the second day, the concrete is only partially cured, and I don't see how it could have shrunk enough to cause cracking.