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Re: Separating slab on grade

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>From your description, it sounds like the slab is doing exactly what you
want it to do.  You don't mention any cracks EXCEPT at the control joints,
so I am assuming that is the case (i.e. only cracks at the control

If so, then the slab is doing what is intended.  It is shrinking toward
the middle of the slab pieces with cracks ONLY forming at the control
joints.  If so, then there is nothing really to repair in terms of the
slab-on-grade.  It would appear that designer/installed of the ceramic
tile floor goofed in that they should have designed the layout to
accomidate the cracking at the control joints.  About the only thing that
could be done with the slab would be to rip it out and replace it with a
new slab with MUCH smaller control joint spacing, which would result in
smaller cracks at the control joints (which the tile maybe better able to

About the worst that can be said about the existing design is that SOMEONE
should have known that there would be significant sized cracks at the
control joints (if the slab worked as designed and did not crack between
the control joints) and then accomidate those cracks in the layout of the
tile floor (i.e. place a joint in the tile floor directly above the
concrete slab control joint and fill that joint with some filler material
that could accomidate the movement/crack).

The end result is that the intent of a reinforced slab-on-grade is so that
when shrinkage occurs (and it will), all the cracking should occur at the
control joints and this appears to be the case from your description.  As
someone else pointed out, the control joints should have been a little
closer together (i.e. in the range of 12' or so instead of the 17' that
appears to have been used).  But, assuming the slab worked as intented,
you would have STILL had all your crackings at the joints, but they would
have been slightly smaller cracks.


Ypsilanti, MI

On Wed, 16 Jul 2003, Andy Richardson wrote:

> 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.
> Andy Richardson, PE
> Bluffton, SC
>  <mailto:arichardson(--nospam--at)> arichardson(--nospam--at)
>  <>

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