Control Joints - Cutting them in long after the pour

[drivinhard]

Probably through our house/shop build over the last 2 yrs, I've learned as much about concrete as I have anything (A lot of it post fact, but hey that's part of the game). Vapor barriers, mixes, finishing, control joints, etc.

I have a (roughly) 31x32' shop (pics here: http://www.garagejournal.com/forum/showthread.php?t=14932) I will be coating soon. It's 5+ inches thick, has 16" deep 3x4' piers under where the lift posts will be, and is sitting on dirt that is anywhere from 5 ft, to 12 ft dug down. It's very hard, no fill, on a good gravel base. What little I've learned is that concrete cracks, period, and with control joints you're basically telling it where to crack, in smaller sections so that you don't end up (or minimize) rouge cracks. They seem to love inside corners.

I have no control joints in the shop floor, and the floor has been poured for approx 14 months. I have no visible cracks now (but have found several rouge ones in the driveway, and 1 in the carport, and a couple in our poured walls). I'm a bit worried about a pour that large with no control joints, holding up over time. I don't want to coat it, and then have rouge cracks mess with the floor coating down the road.

1.) Am I right to think that a ~ 30x31 slab is a bit large to have no control joints, regardless of thickness or good hard base?

2.) If you have no cracks (visable) after 1+ yr on a pour, will cutting in control joints still be effective to relieve any stress should it/when it cracks? I know it's supposed to be done 24-48 hrs obviously, but if it hasn't cracked yet...???

3. Would cutting hurt anything, and if so, how many squares should I divide it into, 4 equal ones, or more?

I was noticing how Bill's floor had so many control joints, very small sections of concrete squares.

http://www.garagejournal.com/forum/showthread.php?t=20722

thoughts/recomendations?

 

[bigtorque]

Youre right , its the nature of concrete to crack due to movement / settling / tempature changes , etc... Im surprised it hasnt cracked yet. Good mix whoever poured it for you.

Theres probably some sort of rule of thumb as to where and how many joints youre supposed to have per sq footage. Im not sure what that is. Heres something I found : http://www.concretenetwork.com/concrete/slabs/controljoints.htm

You wont hurt anything by cutting it. You`ll need some kind of diamond cutting wheel on a grinder or a bigger one on a gas saw. Figure out where you want the joints and snap some chalk lines on the floor. If you cut it dry , be prepared for a TON of dust. Its nasty. Or cut it wet to keep the dust down but makes a slurry mess everywhere.

Youre think right that you should cut joints or you could get some random diagonal cracks that reflects up through your nice epoxy floor.

 

[rsanter]

there is two kinds of concrete
the kind that is cracked and the kind that will crack

bob

 

[premierguy]

A 5” deep slab is setting the standard for normal 4” deep slabs…

Georgia weather isn’t as extreme to temperature change as the upper states…

Medium size quartz gravel (non-paved road size) for bedding (tamped) before a good gravel base would set a new standard…


I think you already know the pier support -for lifting- will dictate your decision-making coupled with the stress load on the building perimeter for joints distanced from the walls.


Irrigation (run-off) for the ensuing rains…???