MOISTURE
Early stage implementation of a
moisture mitigation strategy helps
to save time and can reduce costs.
The Rise and Fall of Moisture-Related
Flooring Problems
New materials and approaches are helping to make moisture-related
flooring and coating problems a thing of the past.
By Peter Craig
Moisture-related
problems with floor covering and
coating installations over concrete
floor slabs have been a serious and
extremely costly issue for many years.
While moisture has always been a
concern for flooring installations,
such problems became far more
common over the past 25 years.
Much has been written about
where potentially damaging moisture
comes from, how moisture migrates,
how long it takes concrete to dry, and
how moisture acting either alone, or
as an initiator, can lead to serious
flooring and coating problems. The
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focus of this article is not to go back
over the same ground but rather to
discuss the evolution of strategies
used to mitigate unacceptably high
slab moisture conditions and how
today’s approaches are reliably
bringing an end to such problems.
The evolution of
moisture mitigation
The need for vapor retarders and
their location below concrete floor
slabs had for many years been one
of great debate and controversy.
Those whose primary focus was the
behavior of the slab itself cited a
number of valid concerns that led
to recommendations to sandwich
a layer of granular fill between the
vapor retarder and the underside
of the slab. That approach did help
to mitigate many of the concreterelated concerns. However, flooring
failures directly related to that detail
ultimately led design, construction,
and flooring professionals to agree
that to provide the highest level of
protection to moisture-sensitive
flooring materials and products, the
vapor retarder/barrier needed to be
placed directly in contact with the
underside of the slab. In ACI 302.1R-
COncrete Construction January-February 2016
37
MOISTURE
15; “Guide to Concrete Floor and Slab
Construction,” the vapor retarder/
barrier decision-guiding flow chart
supports this position.
Over the years we’ve also come
to understand and appreciate that
the relative humidity in the ground
below a structure will approximate
100% regardless of where in the
country a project is being built or
how far the water table is below the
structure. With the surface of the
ground covered by a structure, the
water table can be hundreds of feet
below the surface of the earth and the
relative humidity beneath the building
will still approximate 100%.
Too often, those involved with
projects designed and constructed in
arid regions of the country mistakenly
did not include below-slab moisture
protection on the belief that moisture
from the ground would not be an
issue. Many of these projects and
the participants, have paid a serious
price for this error in judgment. With
mandates from organizations within
the flooring industry, and from the
manufacturers of flooring materials,
the installation of an effective,
low-permeance vapor retarder
placed directly below the slab is not
optional when a moisture-sensibtive
flooring material is being installed
over a concrete slab on ground. With
what we now know about moisture,
moisture migration and its potential
affects on flooring installations, and
products, in the litigious world in
which we live, it would be a serious
error for a design team, owner, or
contractor to omit installing an ASTM
The End of
Moisture Problems
The following measures, along with
proper installation and good construction
practices, are proving helpful in putting
a cost-effective end to moisture-related
flooring and coating problems on new
construction projects.
1. Determine ahead of time if moisture
mitigation will be necessary. Review the
installation requirements for the flooring
or coating material to be installed and
determine whether there is sufficient
time, and favorable enough conditions,
for the slab to dry naturally to the levels
required. Consider that the drying of
conventional concrete does not begin
until the surface of the slab is not
exposed to topical sources of water and
the surface is open enough for internal
moisture to escape. If it is determined,
or if there is any doubt, that there is not
enough time or favorable conditions for
the slabs to dry naturally to the levels
required, a moisture mitigation strategy
will be necessary. An approach that
allows implemention at the beginning
of the construction process should be
considered.
2. Take the ground completely out of play
with an ASTM E1745-compliant, Class
A vapor retarder with the permeance
requirement lowered to 0.01 perms
after conditioning.
3. Place the vapor retarder directly
beneath the slab and incorporate
design and construction measures
necessary to minimize any potentially
adverse effects of placing concrete
directly in contact with the underside of
the slab (such as curling, cracking, or
dominant joint activity).
4. Calculate in advance the amount of
deflection anticipated for elevated slab
placements and provide ample provision
in the budget for the amount of leveling
material that will be necessary to bring
the floor levelness to what’s specified.
(Above) New generation vapor retarders not only have extremely low
permeance, but are also capable of
standing up to construction traffic.
5. When moisture mitigation and floor
leveling is undertaken early in the
construction process, the slab should
be protected from freezing, topical
sources of water, traffic damage, and
contamination throughout the remaining
construction process.
(Left) Rapid-drying concrete is an
early stage answer to the mitigation
of moisture in concrete slabs.
38
January-February 2015 COncrete Construction
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MOISTURE
E1745 Class A-compliant vapor
retarder directly below the slab.
Standing up to traffic
Over the past 20 years significant
improvements to vapor retarder
materials have resulted in products
with after-conditioning permeance
levels of 0.01 perms or lower. The
best of these newer generation
materials are also capable of standing
up to the direct traffic of transport
and placement equipment over the
material. Using an extremely lowpermeance vapor retarder positioned
directly below the slab is an important
step in making moisture-related
flooring and coating problems a thing
of the past.
At the time when the frequency of
moisture-related flooring problems
was beginning to increase it was
generally assumed that a concrete
floor slab on a new project would
dry naturally to an acceptable level.
Little attention was given to the slab’s
moisture condition until late-stage
testing of the slab determined that the
moisture levels were unacceptable.
By that time the interior walls were
up, many trades were at work, and
no provision had been made for what
would be done to mitigate a high slabmoisture condition or who would pay
for it. Disputes over responsibility and
cost began to escalate.
As unacceptable slab moisture
conditions became more common,
the potential need for a moisture
mitigation strategy on many projects
became part of the thought process
during design and a mitigation
system was bid as a contingency item.
With this approach, if the slabs had
not dried to an acceptable level, the
system to mitigate the condition had
already been selected, bid out, and
funds appropriated.
If, however, the slabs had dried
to an acceptable level, the mitigation
system was not installed and the
money was not spent. With the
contingency approach the decision
40
January-February 2016 COncrete Construction
whether the mitigation system was
needed was in the 23rd hour, such
that when mitigation was deemed
necessary, implementation was
difficult, disruptive, and costly.
While selecting a mitigation
system, and bidding it as a
contingency item made sense, and is
still an approach some follow today,
the number of projects where the
slabs did not dry naturally to an
acceptable level and mitigation was
needed ultimately led many design
firms, contractors, and owners to
view moisture mitigation not as
a contingency but as a necessity.
The search then became how to
implement a moisture mitigation
strategy at the most opportune time
and at the lowest cost.
Early-stage approaches
Historically, even when moisture
mitigation was deemed necessary
and included in the project budget
from the beginning, it was a process
undertaken at the 23rd hour when
interior walls were framed and other
trades were at work. Installing a
mitigation system at that point was
not only disruptive to the project
schedule and other trades, but
surface preparation and application
was challenging. It was the most
expensive means of mitigating
moisture in a concrete floor slab.
The search for easier, more costeffective solutions to mitigating slab
moisture on new construction projects
has led to early-stage approaches
such as Rapid-Drying Concrete and
early application of two-part, no-limit,
hybrid epoxy coatings that today are
helping to reliably make moisturerelated flooring problems a thing of
the past.
Early-stage approaches address
the issue of slab moisture and floor
leveling when the building is wide
open and activity from other trades is
minimal. Not only does this approach
help lower costs, but it also minimizes
disruption of the project schedule and
Quality in Concrete Slabs
Luncheon and Forum:
The Nuisance and Necessity
of Placing Concrete Directly
on a Plastic Vapor Retarder
Wednesday, Feb. 3,
at World of Concrete
• Speakers: Peter Craig (Concrete
Constructives), Scott Tarr (North
S.Tarr Consulting), Steve Lloyd (Lloyd
Concrete Services), and Rick Smith
(Structural Services Inc.)
• How do concrete slabs placed directly on plastic vapor retarders behave?
A four-person panel of slab experts
led by Peter Craig will explore these
issues and respond to audience comments and questions.
• Sponsored by Stego Industries and
Wagner Meters
the activity of other trades that comes
with late-stage implementation. The
ability to mitigate moisture and level
the slabs shortly after they are placed
provides the lowest possible cost and
disruption to the project schedule and
other activity.
Today, the use of new materials
and approaches is helping bring
the era of moisture-related flooring
failures to an end. CC
Peter Craig is an independent concrete
floor consultant with the firm Concrete
Constructives. He is one of the nation’s
foremost experts on moisture-related
slab issues. Visit www.FloorWorks3.com
for more information.
www.concreteconstruction.net