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proper placement of vapor retarder below slab-on-grade (redux)

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This subject has already been beat to death on this forum, so I figured
why not beat it some more.

I've been vacillating on my opinion as to the need to install a 4" thick
"blotter" layer of compacted granular fill over vapor retarders below
slabs-on-grade. I have been designing building structures for over 20
years, have never seen or specified such blotter layers below slabs on
grade, and have never had any problems with curling (or anything else)
on my slabs.  Some engineers I've spoken to insist that the blotter
layer over the vapor retarder is necessary and should be standard
practice.

I was reading ACI 302.1R - 96 today and upon reading the addendum that
was added at the very end of that publication (perhaps as a last minute
addition before going to print) there was a cautionary note that seems
to indicate that these blotter layers may cause problems with vapor
sensitive floor coverings.  A flow chart was provided in this addendum
that basically said the following:

Exterior slabs-on-grade: no vapor retarder

All slabs-on-grade with moisture sensitive floor coverings: Install
vapor retarder directly below the slab.

Slabs-on-grade that will be constructed inside a building below a roof
where the roofing system has been installed:  Install blotter layer over
vapor retarder.

All other slabs on grade: Install vapor retarder directly below slab.

It seems that ACI 302.1R recommends that the vapor retarder (when
needed) should always be installed directly below the slab except in the
(rare) case where both of the following conditions are met:

Slab is constructed below the protective cover of a roof.

Vapor sensitive floor coverings will not be installed on the floor slab.

Am I wrong with in my understanding that the purpose of the blotter
layer is solely to prevent curling of the slab-on-grade?

Couldn't curling be prevented by properly curing the freshly placed
concrete?

Are we engineers attempting to prevent a curling problem that might be
caused due to some contractors' inattention to properly curing the
concrete, only to create other (potential) problems related to floor
coverings and flooring adhesive failures due to vapor transmission
through the slab (for which fingers will be rapidly pointed in our
direction)?

Instead of going to all the trouble of installing the blotter layer over
the vapor retarder, why not just make the contractor properly cure the
slab - like they are suppose to do anyway?

Maybe the need for blotter layers is more important in certain parts of
the country where there is very low humidity year round and the moisture
rapidly wicks out the top of the slab - but it does not seem to be as
much of an issue in the Mid-Atlantic States where most of our projects
are located.

Like I said before - 20 years, no blotter layers, no problems.  Why
start now?

Thanks in advance for any comments, opinions, etc. I sincerely
appreciate all of the knowledge that is so freely shared on this forum.

Clifford Schwinger, P.E.


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