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Re: Concrete slab topping

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Richard,

The kind of damage that you are describing sounds like what I have seen
happen to slabs that have been damaged by mineral-containing moisture rising
to the surface of the slab and evaporating.  The process sometimes leaves
deposits of salts (efflorescence), the presence of which would be a clue
that this is the problem.

In concrete that is not dense, the process can cause crystals of salt to
form in fine pores at the surface of sound material; the crystals develop
enough pressure to break up the adjacent material.  The process progresses
slowly into the concrete (often in annual cycles of wetting and drying),
always occurring at the surface of sound material.  Each wetting/drying
cycle produces a little more damage.  I've also seen the wetting/drying
cycle process produce defoliation of concrete, breaking loose layers in
large areas that are 1/8" to 1/4" in thickness.  I haven't learned the
process that causes the defoliation effects, but it also seems to be related
to mineral-bearing moisture entering the concrete.

If the process that I've described is at work, you should find sound
material someplace down there that can become the base for the repairs.  The
repairs probably will not be of permanent value if the source of moisture is
not found and eliminated.  Sometimes site grading, or gutters and downspouts
that direct stormwater water away from the building are all that are needed.

Once you are down to sound material, a trowel-applied topping may be
appropriate.  ACI 546.1R Repair of Bridge Superstructures has some guidance
on preparation of surface, recipes and slump for the topping; also ACI
201.2R Guide to Durable Concrete.

Nels Roselund
Structural Engineer


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