ASHRAE Journal - November 2014 - 46

COLUMN BUILDING SCIENCES

a plastic "skirt" to the
FIGURE 4 Capillary Wicking. Without the slab edge protection water can wick dozPHOTO 2 Perimeter Slab Protection. The
ens of feet inward via capillarity.
perimeter form boards or
plastic vapor barrier is run over the top
of the form boards and trimmed later
running the plastic over
after the form boards are stripped.
Wood Baseboard
the top of the form boards
Wood Flooring
(Photo 2) and trimming the
Concrete
Slab
plastic later after the form
boards are stripped. Both
of these approaches are not
Sub-Slab Vapor
common but need to be in
Barrier
Perimeter Edge of
place if you are installing
Slab Not Protected
Below Grade
water sensitive flooring
like wood on a concrete
Ground Saturated
slab foundation. Without
With Water
the slab edge protection,
water can wick dozens of
feet inward via capillarity
PHOTO 3 Wood Flooring Problems. Discoloration,
(Figure 4) and then you end up with
day. They are here to stay so just deal
movement and decay.
flooring problems such as discolorwith it.
ation, movement and decay (Photo 3).
So let's say we separate the conIt is also a good idea to paint the
crete from the ground-both from
exposed edge of the slab above grade underneath and from the sides.
to keep it from getting wet from
Now what? We need dry concrete.
"splash back" (rainwater hitting the
What's the big deal? Just wait for the
ground and bouncing back up) and
concrete to dry before you install
enthusiastic over irrigation by occu- the wood flooring (Figure 6 and Photo
pants. The paint needs to be "liquid
5). Problem with this strategy is
water" closed and "vapor water"
that with "wet" concrete mixes this
open-hygrophobic and vapor
might mean many, many months.
raff is that "bleed water" typically
permeable-that would be typical
In new construction we might only
disappears when the w/c ratio is less
acrylic latex paint formulated for
have 90 days between placing the
than 0.6. Now, this is not a hard and
concrete substrates.
concrete and installing the flooring. fast rule. Mixing, fines, aggregate
It is a lot easier to handle a stem
To make this timeline work we need types all factor in-but it is a pretty
wall foundation where the edge of
concrete without much water in it.
reasonable "rule." Without bleed
the slab is wrapped and the slab is
We need low water-to-cement (w/c)
water you can easily place concrete
well elevated (Photo 4 and Figure 5).
ratio concrete. The good news is that on plastic vapor barriers or on fluted
So why not just use stem wall founto make good concrete you need a
metal decks and not mess up your
dations? Sometimes the ground
low w/c ratio. The lower the w/c ratio finishing by having water separate
conditions suck and you need to
the better the concrete. Period.
from the mix and float to the top
go post-tensioned. Sometimes the
How low do you need to go? A w/c
surface and pond. One of the dirty
‡ How do I
local trade base does not exist to go
ratio below 0.6 for sure.
secrets to finishing crappy concrete
know that? Well, one of the secret
there. Monolithic slab foundations
was to place it on a sand layer so that
are faster to construct and are easier numbers that old-time concrete mas- the sand layer would act like blotter
to construct. And that often wins the ters know but don't share with the riff paper and suck up the bleed water.§
‡Of course most people do not order concrete based on w/c. They order it based on "slump" and compressive strength ("psi"). For a w/c below 0.6 you typically need 3,500 psi (25 MPa) concrete with a
slump of less than 4.5. I typically specify a w/c below 0.5 to absolutely, positively, beyond a shadow of a doubt, stay out of trouble. Having said that, if you don't screw with the mix on the site, 0.6 will work.
The key is not letting anyone screw with the mix on the site. Good luck with that.
§I am repeatedly stunned at the practice of installing sand over polyethylene under concrete slab foundations in California. To me that is a sign of bad concrete. With a low w/c ratio you don't need the sand.
We were here a long time ago, "Concrete Floor Problems," ASHRAE Journal, January 2008.

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