United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-93/169 November 1993
&EPA Project Summary
Measurement of the Surface
Permeability of Basement
Concretes
Arthur Scott
Wall/floor joints are often sealed to
reduce the entry rate of
radon-containing soli gas into build-
ings. This practice Is generally inef-
fective because of one or more of the
following:
(1) not all openings are sealed;
(2) the sealants do not block soil
gas entry because of inadequate
coverage or adhesion; and
(3) the permeability of the concrete
Is high enough (>1015 m2) to al-
low significant soil gas entry
rates through the concrete itself.
The problems with unsealed
openings or poor sealants can be
overcome by better seal designs,
improved training, and quality con-
trol; but if the construction materi-
als have high permeability, then
soil-gas-resistant foundations can-
not be produced regardless of de-
sign or workmanship. The purpose
of this study was to develop a method
and apparatus to test the surface per-
meability of concrete sections in-sitis,
and to perform field measurements to
determine whether surface permeability
is generally so high that standard seal-
ing techniques will be unsuccessful at
curtailing the entry of radon-containing
soil gas.
Modern sealant materials have very
low air permeability when set; thus,
seal performance is determined by the
permeability of the surfaces bridged
by the sealant. Concrete basement
walls and floors are poured separately
and are not interlocked; the joint be-
tween them presents the largest poten-
tial opening for soil gas and radon
entry. Radon resistant foundations
must seal this entry path. Horizontal
floor surfaces are usually trowelled
or floated smooth for appearance,
which works cement paste to the sur-
face and eliminates pores from the
surface layer. In contrast, walls are
poured into vertical forms, and their
surface is untouched until the con-
crete sets. The surface consists of
cement paste and the smallest aggre-
gate fragments. During the setting
process, water bleeds to the surface
and tends to drain down between the
form and the face, producing vertical
channels in this surface layer. A wall/
floor joint sealant contacts both sur-
faces, and may be rendered ineffec-
tive if the pores and channels in the
vertical wall surface layer are large
enough to act as a bypass for soil
gas.
A portable surface permeameter,
suitable for field use, was devel-
oped, tested, and used to mea-
sure surface permeability of
concrete in new houses. The oper-
ating principle is based on measure-
ment of airflow induced by a pressure
difference across a temporary test
seal applied to a surface. The feasi-
bility of the equipment and the test
procedure was demonstrated and de-
veloped by laboratory tests. The
equipment can measure surface per-
meability as low as 1016 m2, the nomi-
nal bulk permeability of solid
concrete.
Printed on Recycled Paper
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Measurements were made on con-
crete basement walls and floors of
houses under construction. Areas se-
lected to be free of obvious surface
defects such as air bubbles were found
to have permeability in the range 10-"
to 10'16m2. High resistance seals can
be produced between surfaces with this
low permeability.
However, air bubbles and other sur-
face defects were found to be very com-
mon on vertical concrete surfaces.
These defects caused the permeability
of areas chosen at random to be
>10-12 m2, too high for standard sealing
details to produce effective seals be-
tween vertical concrete sections.
As seal performance depends on sur-
face permeability, sealing will be a prac-
tical passive radon exclusion measure
only if there are low cost surface prepa-
ration methods, sealing details, and pro-
cedures to produce high-resistance
seals even when there are surface de-
fects.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that Is fully docu-
mented In a separate report of the same
title (see Project Report ordering Infor-
mation at back).
Introduction
Radium in the soil is a continual source
of radon, which enters the air spaces be-
tween the soil grains and is carried into
the house by air movement through the
soil. The flow rate into the house is set by
the pressure difference between the house
and the soil, and the series airflow resis-
tance of the soil and the house founda-
tion.
The resistance of a concrete house foun-
dation is set by the resistance of the floor
and walls, and the resistance of the joints
and openings between and in them. In
normal basement construction there are
so many open joints that foundation resis-
tance is only a small fraction of the soil
resistance. The report identifies a perfor-
mance criterion for radon resistant hous-
ing, suggesting that a "radon-resistant"
foundation should have an average soil
gas entry rate below 40 L/h (1x10"* m3s'1).
This standard of soil gas exclusion will
ensure low house radon concentrations
regardless of soil conditions.
The report identifies the air permeability
of bulk concrete as 10'15 to 1O'18 m2, too
low to allow significant soil gas flows
through concrete. Most of the soil gas and
radon enters the building through
low-resistance joints and openings be-
tween the concrete sections, such as the
wall/floor joint, shrinkage, or settlement
cracks.
Radon Resistant Construction
The report identifies the characteristics
of a radon resistant foundation as:
Soil gas entry rate < 1x10* m^1,
• Total basement flow resistance
>4x108Pa.s.m-3,
Concrete resistance >4x1O8 Pa.s.rrr3
perm2-
• Joint resistance >1.6x108 Pa.s.rrr3
perm,
• Surface resistance in contact with
sealant >3.2x108 Pa.s.rrr3 perm
(implies surface permeability
<10-13m2), and
• Length of unsealed joints <100 mm.
Obstacles to passive radon resistance
include:
not all openings through the base-
ment walls and floor were sealed
during construction,
the effective resistance of the joints
and openings was less than
4x106 Pa.s.m-3, despite the applica-
tion of sealants, and
the bulk permeability of the con-
crete used in the foundation was
much higher than 10'15 m2
If the surface cement layer contains a
channel 5 mm long, and 1 mm in diam-
eter, the resistance of this tube is
4x10* Pa.s.m'3. Only four such tubes by-
passing the sealant are required to re-
duce the resistance of a "perfect" radon
resistant basement to 10* Pa.s.m'3, too low
to be fully radon resistant. Thus the con-
nected porosity of the concrete surface
layer is of major importance in determin-
ing sealant performance for soil-gas ex-
clusion.
Permeameter Design
A surface permeameter was devel-
oped, tested in the laboratory, and then
used to measure the surface permeabil-
ity of basement walls and floors in newly
constructed houses. The operation prin-
ciple is that a temporary seal is placed
on a concrete surface, a pressure dif-
ferential produced across the seal, and
the resulting airflow measured. A
permeameter chamber is sealed to the
surface on one side of the seal, and a
second chamber is sealed to the sur-
face on the other side of the seal. There
is no connection between the chambers
except that both touch the same con-
crete surface. One chamber is depres-
surized. Air drawn through the concrete
beneath the temporary seal draws air out
of the second chamber, and the volume
removed is measured by the displace-
ment of an oil slug in a capillary tube
attached to the chamber.
Each chamber is semicircular in cross
section, 0.35 m long, 80 mm wide, and
40 mm high. The length of concrete sur
face under test is 0.3 m. At a seal linear
resistance of 1x1010 Pa.s.m^ (100 times
higher than the required value for an ef
fective radon resistant seal), an
underpressure of 1000 Pa in the first cham
ber will give a flow of 3x1 Q-8 m3s'1 (3 cm3'
min) out of the second chamber. This is
equivalent to a 0.2 m/min displacement
rate in a 3 mm tube, which is readily de
tectable. A detailed description of the ap
paratus and its use are included in the
report.
Conclusions
The maximum surface permeability ac-
ceptable for "radon-resistant" seals is
~10'13 m2, which is larger than most mea-
surements on smooth defect-free concrete
Good seals can be produced between se
lected or prepared concrete surfaces. How
ever, when wall areas are chosen at
random, and hence include subsurface
defects, the effective permeability mea
sured is >5x1012 m2. This shows that high
resistance seals cannot be guaranteed for
unprepared concrete surfaces, due to the
connected air bubble pores providing low
resistance subsurface paths through the
near-surface concrete layer. These
by-passes limit sealant performance, no
matter how good a bond the sealant makes
with the surface.
Inspections of 10 houses found that the
vertical concrete walls had a cement sur
face skin of 1 to 5 mm thickness, contain
ing many small air bubbles and large pits,
depressions, and wormtracks caused by
air bubbles trapped between the concrete
and the form. The surface finish depended
on form preparation, not concrete mix, for
different surface textures were found on
adjacent vertical form sections of the same
wall. Some forms even removed the sur
face skin when they were stripped from
the concrete, leaving uneven and rough
patches with many small surface pits
caused by air bubbles entrained in the
mix.
The surface cement paste layer on con
crete sections has a permeability much
lower than the 10'13 m2 needed to produce
good seals. There are no fundamental
reasons to prevent high resistance seals
for joints and openings in concrete foun
dations. However, seals have to be ap
plied to unfinished vertical surfaces that
are rough, uneven, pitted, covered with
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loosely adherent material, contain con-
nected subsurface channels, and are diffi-
cult to reach. These defects either prevent
the sealant from contacting solid concrete
or provide subsurface by-pass paths
around the seal, increasing the effective
permeability to >5x10*12m2. Good seals
between vertical concrete sections cannot
be guaranteed unless these surface de-
fects and by-passes are removed. This
requires removal of the surface cement
layer. Application of a caulk bead in the
vicinity of an unprepared wall/floor joint
will not achieve a good seal.
&U.S. GOVERNMENT PRINTING OFFICE: 1993 - 5SO-M7/80I22
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A. Scon is with Arthur Scott and Associates, Mississauga, Ontario, Canada
L5L 1K2.
Timothy M. Dyess is the EPA Project Officer (see below).
The complete report, entitled "Measurement of the Surface Permeability of Base-
ment Concretes," (Order No. PB93-232114; Cost: $17.50; subject to change) will
be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/SR-93/169
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