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32'x42' concrete slab, 4"-5" thick, on foam and stable sand base: Do I need saw cuts?

Just poured the garage and had it sealed with a cure and seal compound. Slab ended up 5" in places (nominal 4"). I sits on 1 inch of foam and Ampex radiant panels (3 inch foam equivalent total). It's a floating slab, with 2 inches of foam separating it from the frost walls. The whole thing is on 4' of compacted sand, that sat for a year before the pour. Slab is 32 Mpa concrete with microfiber, no steel reinforcement.

My question is, do I really need the saw cuts? I would be willing to deal with the odd crack. I figure it would be a lot easier to keep the floor clean without the saw cuts.

Asked by Mai Tai
Posted May 17, 2018 6:44 AM ET
Edited May 17, 2018 9:10 AM ET


7 Answers

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Mai Tai,
My own advice is: Yes, you need the saw cuts, especially if your contractor didn't install metal strips as control joints. Eventually, the slab will crack. Straight control joints are more attractive than random cracks.

Answered by Martin Holladay
Posted May 17, 2018 7:00 AM ET


You should have had them groove the concrete when it was wet. Much better than cuts that accumulate dirt and junk.


Answered by user-6872033
Posted May 17, 2018 9:22 AM ET


saw cut are for concrete shrinkage as the water slowly leaves the slab


Answered by tech1234
Posted May 17, 2018 10:09 AM ET


With no reinforcement and no microfiber, your slab will likely develop cracks, even with a curing compound. In theory, control joints are supposed to hide the cracks, but they often don't work as intended. I don't like the look of control joints and don't spec them on my jobs. (But I do spec reinforcement of various sorts.) Sawcut control joints are much less noticeable than wet-troweled control joints. If you don't want them, don't use them. When the concrete cracks, you can fill the cracks with polyurethane sealant or with concrete slurry. The sealant will flex if the crack grows, but the slurry will blend in better.

Answered by Michael Maines
Posted May 17, 2018 10:59 AM ET


You could make the saw cuts and then immediately fill them with sealant (to keep the dirt out).

For others, steel is useful for keeping cracks/cuts small.

Answered by Jon R
Posted May 17, 2018 11:14 AM ET


Mai Tai,

Control joints are for both initial concrete shrinkage and differential settling over time. The idea is to make one area weaker so if cracks occur it is at the thinner (weaker) area under the control joints.

You don't have to leave the joints open. You can grout them and they will still work, or use colour-matched caulking. I use a pattern that enhances the architecture of each space. My own house, being quite small, has joints every four feet. The grey-green grout I used makes the concrete look like large tiles. In terms of comfort and maintenance it is the same as a floor without joints.

Spider cracks, even very small ones, look like hell and are almost impossible to remediate.

Answered by Malcolm Taylor
Posted May 17, 2018 11:19 AM ET


Thanks to all that responded. Both house and garage slabs have micro-fiber. I actually speced and insisted on structural 2" polypropylene fiber, but like many other things I got what was convenient, not what I asked for.

I finally got the slab and front porch cut. They did a good job, it all looks ok. You can see where the machine rubbed off the sealer in the garage on both side of the cuts, but I guess that was never a permanent product. Also the sealer (Diamond Clear 350) did a number on the exposed foam(patially melted), and had the highest VOC content known to man. I don't think I would use this product again. Hopefully it helped me get a bit better surface hardness on the garage slab.

I already have 2 cracks on the garage slab, but it's pretty obvious why. The originate at the man door cutouts. Basically the slab is floating, except for the 2 man door "tabs" where it was resting on the foundation. If I had known better i would have insisted they groove it wet, in a straight line at the tab. I'm just hoping now the (2) 16' garage door tabs will hold...Hopefully the cuts a bit further up will help with that part of the settling/shrinking.

No cracks on the house yet, but it's still early. House does not have "tabs" but it does have a mid plane concrete wall that has an opening in the middle. Dollars to doughnuts this will be the place it cracks first, if it does. No saw cuts in the house per my wife's request, so we are really rolling the dice. Worst case we can always tile the first floor if it's too unsightly.

Answered by Mai Tai
Posted May 17, 2018 9:17 PM ET
Edited May 17, 2018 9:30 PM ET.

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