Sub-surface control joints in concrete??

[Mike Lamb]

This basement floor had a bunch of relatively straight cracks at various locations. The builder told me that they were control joints and the method used is to lay a strip of metal – Or something like that –beneath the concrete surface and an intentional crack will develop there. I never heard of this and my research came up with nothing to corroborate. He said it was a relatively new thing. I don't think it's a problem but is the fellow full of malarkey? Thoughts?

[Jerry Simon]

No, he's not.   Google concrete zip strips.

Those have been around for at least the past 30 years or so, at least around here.  If you get down close, you can usually seen the strips of metal (or mostly white plastic is what I've seen), inside the cracks.

That said, the diagonal one in the pict is a bit odd and may be a typical shrinkage crack.

Edited February 21 by Jerry Simon

[hausdok]

I used to see that every once in a while. The builders said that they'd placed strips of plastic and that allowed the surface of the concrete to crack in line with the plastic. That way, when the concrete cured and shrank, and curing cracks developed, it would be in a straight line instead of random.

[Bill Kibbel]

The deeper the strip be, the more jagged be the crack.

[Les]

we have that in 99% of new builds for the past 30years.  the diagonal may be too deeply set.  

 

CRACK INDUCER

This PVC rigid profile will provide an extended weakened plane joint when placed directly under the surface control joint (ZipStrip or saw cut). Crack Inducer will successfully crack an oversized, thick on grade slab from the top of the slab to the bottom. (NO. 21-CI-1.5)

[Jim Katen]

I used the metal versions 40+ years ago. In the early 2000s, I placed a garage slab at my own house and found that they had been changed to plastic somewhere along the way. The metal ones pretty much stayed put as the concrete was placed, but the plastic ones tended to bow outward when concrete was placed on one side, causing the finished joint to have a slight curve. 

Frankly, I'm amazed that these aren't used everywhere. I rarely see a concrete slab without control joints around here.