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Re: W/C ratio, was:purposely permeable concrete

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Thanks for the replies

The soils report says...

" to further reduce the potential for moisture to migrate through the
slabs-on-grade as a vapor, as well as to reduce concrete shrinkage, we
recommend that the concrete mix specified for the slab on grade floors have
a water/cement ratio of .45 or less."

This is a non-profit low income housing project, in many places there is
sheet vinyl flooring. The site soils are expansive and the top foot or so is
disturbed, so the site work includes removing several feet of soil, bringing
in new soil and raising the elevation of the entire site due to some
civil/road/grading issues. I have been told that certain admixtures and
plasticizers  yield unpredictable finish results because the top hydrates at
a different rate than the center. Given the same cost, I would prefer adding
cement rather then admixtures.

Jeff Smith
-----Original Message-----
From: Harold Sprague <hsprague(--nospam--at)>
To: 'seaoc(--nospam--at)' <seaoc(--nospam--at)>
Date: Monday, June 08, 1998 11:30 AM
Subject: RE: W/C ratio, was:purposely permeable concrete


Why is the soils engineer specifying w/c ratio?

The old theory is that if you lower the w/c ratio you will have less
shrinkage.  A lower w/c ratio will reduce bleed water and reduce
permeability, but has not much effect on shrinkage.

Harold Sprague

-----Original Message-----
From: Jeff Smith [SMTP:smthengr(--nospam--at)]
Sent: Monday, June 08, 1998 11:14 AM
To: seaoc(--nospam--at)
Subject: W/C ratio, was:purposely permeable concrete

On a similar subject, I am working on a job where the soils engineer
required a concrete water cement ratio of .45 for slabs on. Basically this
means you end up with a 5000 psi or more mix unless you substitute additives
in lieu of more cement. Locally, the additives costs at least as much as the
cement. The slabs only need to be 3000 psi for structural reasons. I believe
the w/c requirement is for permeability reasons. Has anyone had a similar
experience with an economical work around?

Jeff Smith