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Separating slab on grade

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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.