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Re: Slabs on Ground
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Slabs on Ground
- From: chuckuc <chuckuc(--nospam--at)pacbell.net>
- Date: Fri, 18 Jul 2003 18:43:54 -0700
The biggest dispute seems to be over where to locate the vapor barrier.
A couple of years back I heard a talk by Tom Butt who served (chaired?)
on ASTM E 1745. Their
conclusion was that if moisture coming up through the slab would be a
problem (vinyl, wood floors,etc.) Then put the barrier on top where
you could verify it's integrity before the pour & then insist on a
wet cure for the slab. Tom does a lot of forensic work & has seen
a lot of damage from crummy, perforated, visqueen barriers. PCA
wants the barrier under the "blotter" layer which makes life easier for
the finishers but increases the risks of later moisture problems. I
don't do a lot of slabs but I try to discuss the issues with the owner
& contractor early on.
C. Utzman, P.E.
Some miscellaneous comments:
One of the biggest challenges in slab on ground construction is the
wide range of construction it encompasses. What is appropriate for a
driveway or sidewalk is not necessarily appropriate for a materials
handling facility with forklift traffic 24 hours a day.
Even within a particular type of construction, there are so many
variables that any "design procedure" has to be very simplified. In
addition, many of the variables are not really under the control of the
designer, they are a function of the job site conditions. The designer
can attempt to enforce certain requirements during construction, but my
experience has been that the "engineering representative" stationed on
site to do construction administration is usually someone that
graduated from college two months before, with absolutely no
understanding of the project requirements.
Since slabs on ground are not considered structural elements unless
they are supporting the building load there has been very little
research done. Most of the work has been sponsored by someone trying
to sell something. Such as fibers.
In addition, many engineers seem to feel that slabs on ground are
simple enough that they do not need any expertise to either design them
or recommend repairs on someone else's faulty design. As a result,
over the years a lot of bad practices and misconceptions have
The "WRI method" does not determine the amount of reinforcing, it
determines slab thickness for single wheel axle loads and uniform loads
with aisles. The "subgrade drag" formula is an equation for
determining amount of reinforcing but is generally considered to result
in an amount of reinforcement that is so low that it is useless.
There is a modified subgrade drag equation that comes up with about
four times as much reinforcement for the same criteria.
The four basic references for slab on ground construction are ACI 302,
ACI 360, the PCA Slabs on Ground book and the Ringo/Anderson book. To
design (or evaluate) a slab simply based on what you have read in a
book is asking for trouble though. All four of these books contain
good information but in my opinion none of them present the information
in a way that is easy to understand, or in some cases even logical.
The most recent edition of the PCA book contains a number of errors.
The ACI documents are written by volunteers. Some of the information
is outdated, even in recent publications, because no one took it upon
themselves to check it. ACI 360 for example has a lot of discussion of
testing of admixtures from 1964 - none of the admixtures are even
produced anymore. Some of the information may just be one person's
opinion that no one else felt strongly enough about to challenge.
The Ringo/Anderson book focuses on post-tensioned slabs on
expansive/compressive soils; alot of the information is not applicable
for other types of construction.
ASTM E 1745 specifies three classes of vapor retarder, based on
puncture resistance and tensile strength. In situations where it is
necessary to have a vapor retarder, it is probably worthwhile to
specify class A (the strongest), and make sure it is installed
correctly. Otherwise, don't bother specifying a vapor retarder.
My personal opinion is that all engineering requires expertise that
can only be gained by working with someone that has the expertise.
Reading books isn't going to help you with the answers if you don't
know the questions.
The Truth is rarely pure and never simple.