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RE: Separating slab on grade

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Sounds like plain ol' shrinkage to me. Without steel reinforcement and a fairly low drag coefficient for the subgrade, there isn't much to hold it back. If you had two unreinforced slabs 17 ft long placed together, a gap of 1/8 to 3/16 represents free shrinkage of about 600 to 900 microstrains, which is not out of the ordinary, especially if the mix was rich (hot) or had a high w/c ratio to start with or poor quality control of the curing process. The mesh reduces permeability, reduces microcrack widths, and helps combat plastic shrinkage cracking, but doesn't begin to replace reinforcement in terms of structural strength or overall shrinkage control. I've mostly stopped using it and think it has been way oversold.
-----Original Message-----
From: Andy Richardson [mailto:arichardson(--nospam--at)]
Sent: Wednesday, July 16, 2003 4:42 AM
To: seaint(--nospam--at)
Subject: Separating slab on grade

We had a structural evaluation where the slab on grade is separating about 1/8” to 3/16” along the control joint and causing problems with the ceramic tile floor.  The plans call out for 4” reinforced concrete slab over 6 mil poly over 4” sandy fill.  The subgrade is assumed predominately noncohesive soil.  It is apparent that the foundation sub interpreted “reinforced” as fiberglass fibermesh reinforced.  There is no other reinforcement in the slab.  The slab is about 34’x34’ with control joints in the center.  Any ideas what may be causing this excessive cracking (assuming this is excessive), and how to rectify the situation.  My primary concern is that the floor will continue to separate after the floor is repaired, and hence more cracks in the tile.    


Another interesting point is that 6 months ago another homeowner called about his house which was also placed on a similar system.  A similar crack in the slab caused termite damage in his hardwood floors.