I finally figured out why my control joints kept cracking early

Last month I poured a 400 square foot patio slab outside Boise and the control joints started spiderwebbing within two days. I was using the old-school method of cutting them at 8 hours with a wet saw, same as I always have. Turns out the issue was my timing, the temps dropped from 85 to 50 overnight and the slab was still too green when I cut. I switched to waiting 14 hours and using a groover instead, plus I added more saw cuts at half the depth I normally do. The last slab I poured last Tuesday has zero cracks so far, which is a first for me with this weather. Has anyone else had trouble with early joint cutting in cooler fall nights, or do you adjust your timing based on the forecast too?

2 comments

2 Comments

casey34221h ago

...and honestly I think the bag of ice in the mix is what saved me more than anything. My buddy down in Twin Falls swears by waiting until the sun hits the slab just right and the moisture beads off like sweat on a cold beer can. He cuts everything with a rented groover that's older than his truck and it works every time. I tried that whole 14 hour window you mentioned and ended up chasing shadows with a flashlight at 3 AM because the forecast lied about the dew point. The real trick for me was dumping a couple of cheap thermometers around the pour site and checking them every few hours like a paranoid dad with a newborn. Now I only cut when the surface temp matches the air temp within five degrees, no matter what the clock says. My last patio has more control joints than a spider's web but not a single crack, so I'll take the ugly look over the busted concrete any day.

cora81319h ago

Temps matching is good, but nobody talks about the wind. @casey342, if the breeze kicks up during that window, your surface dries faster than the mix underneath. I started wetting burlap and draping it over the slab after the float. Keeps the edges from setting up uneven, so the cut line don't wander. Cheap and saves the headache.