CONC. Relief Cut Spacing
On our 2100 sq ft., slab-on-grade house well have conc. as the finished floor and I am wondering if there is a standard spacing that I should add to the specification for the relief cuts in the Conc. I think previously we discussed that they should be about 1/4 depth of the conc. (Conc is 4″), but what spacing should I put in the plan?
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It's good practice to go no more than ~125 square ft in the spacing. I'd choose dimensions that equally divided the slab into sections for ease of the concrete crews, even if it's smaller than that. So if it came out that it was easier to make 90-100 square ft sections, and going up would be over 125, I'd keep it there and accept a few more cut lines. They can be sealed up following with a color matched joint filler which should minimize their appearance.
To add to what Kyle said: Place your saw cuts under interior partitions. For the few rooms where they will need to be exposed - Master Bedroom, Great Room - design the cuts as part of the architecture. So perhaps a cut each way centered on the room.
Another approach is to use them decoratively everywhere. My downstairs slab is saw cut on 3 ft centers, and the cuts grouted. The floor looks like big concrete tiles.
An area that always cracks is at inside corners, so make sure to detail these areas well. I would recommend 2 rebars placed diagonally at each inside corner, and you may also want to have a relief cut heading straight out from your inside corners, this cut would be at 45 degrees to your exterior walls, and perpendicular to the rebars.
I agree with the advice above, but I'll add that relief cuts are typically not required by residential codes and that I've had better luck with damp-curing reinforced slabs than by cutting control joints on a typical slab. Damp-curing usually means keeping the slab damp for seven days to allow strength to develop. I still add diagonal rebar at penetrations and inside corners, and also spec a rebar grid and extra-fine micro-fibers for slabs that will be exposed as finished floors. Another tip is, perhaps counter-intuitively, it's better to keep the compressive strength on the low side because high-strength mixes are often more prone to shrinkage, resulting in cracking.
Thanks all. I will specify that relief cuts be used with no more the 90-100 ft^2 of area between cuts; with sealant applied later. Also, the placement of rebar as noted above at inside corners.
Are "inside corners" where the point of the corner in is directed/pointed into the dwelling space?
Use "saw cuts" or trowel type cuts? I think I've seen both but I am not sure if one is better than the other.
I'll likely lean away from using them as a specific design feature though. I don't have an architect on staff to make sure that layout turns out right.
An inside or outside corner really just means there’s an abrupt discontinuity in the edge of the concrete. All cracks seem to start at these locations as the stress there is highest anywhere in the slab. A good concrete crew will recognize them and make a cut along the diagonal, bisecting the 90 degree corner.
They can be minimized with cuts, rebar placement, and good curing as mentioned above.
In terms of joints, saw cuts are best. They’re narrower, flatter, and well, better than a grooved joint. They can use a green saw and cut them in at the end of their day.
They usually make the cuts with a circular saw mounted on a little trolley. If the cuts will be visible, make sure they snap chalk lines and follow them; I've seen way too many crooked cuts.