United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-94/053
June 1994
EPA Project Summary
Laboratory Assessment of the
Permeability and Diffusion
Characteristics of Florida
Concretes: Phase I. Methods
Development and Testing
R. Snoddy
The ability of concrete to permit air
flow under pressure (permeability) and
to permit the passage of radon without
any pressure difference (diffusivity) has
not been well determined. To establish
a standard concrete mix and its maxi-
mum radon-resistant placement, these
parameters needed to be quantified and
their relationship to concrete's physical
properties evaluated. The concrete test-
ing consisted of separate permeability
and diffusivity measurements and a set
of preliminary measurements to deter-
mine the size, weight, and porosity of
each sample. Ten concrete samples were
tested. Cylinders represented one of the
four general types of concretes manu-
factured in Florida. Permeability was
measured with a device developed for
the project using custom software. The
diffusion coefficient was determined with
a system developed by and purchased
from Rogers and Associates Engineer-
ing Corporation. Two of the samples
had measured permeabilities 100 times
greater than the other samples due to
defects in the concrete. All of the corre-
lations of the various physical param-
eters were investigated, but there were
insufficient data to confidently determine
any correlations. The most significant
fault in this phase of the research was
the lack of unbiased, representative con-
crete slab samples.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Much of Florida's natural soil and the
sand recovered from the phosphate min-
ing/beneficiation process there contain sig-
nificant quantities of radium. Buildings con-
structed on these high-radium soils have
been found to contain elevated radon lev-
els.
To decrease elevated radon levels,
Florida's legislature instructed its Depart-
ment of Community Affairs (DCA) to de-
velop new construction standards for
radon-resistant buildings, primarily
slab-on-grade constructions.
It is well known that concrete slab is the
primary barrier to radon entry. But the
extent of its ability to permit air flow under
pressure (permeability) and to permit the
passage of radon without any pressure
difference (diffusivity) has not been well
determined. To establish a standard con-
crete mix and its maximum radon-resis-
tant placement, these parameters needed
to be quantified, and their relationship to
concrete's physical properties evaluated.
Concrete testing consists of separate
permeability and diffusivity measurements
and a set of preliminary measurements to
determine the size, weight, and porosity
of each sample. After preliminary tests
are completed, each sample is mounted
in a 4-in.* long section of 4-in. schedule
80 wrought steel pipe. The concrete re-
mains in this pipe for both tests.
Ten concrete samples were tested and
divided into two groups. The first group
11 in. = 2.54 cm.
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consisted of two Rogers and Associates
Engineering Corporation (RAECORP)
samples that were cored from two differ-
ent compression test cylinders. The re-
maining eight samples (the second group)
were cored from four compression test
cylinders (two samples from each cylin-
der) from the Florida Concrete and Prod-
ucts Association (FC&PA). Each compres-
sion test cylinder represented one of the
four general types of concretes manufac-
tured in Florida. The four compression
test cylinders were made in Jacksonville,
Tampa, Orlando, and Miami.
After the cylinders were received, they
were logged in on a sample custody form.
Entries included identifying marks. The
RAECORP samples had already been re-
duced to the nominal testing size, 4-in.
diameter by 2-in. thick. The three, larger,
FC&PA cylinders required coring and slic-
ing to produce the nominal sample size;
the smaller, 4- by 8-in. cylinder only re-
quired slicing. Two of the cylinders were
cored on site. The remainder of the coring
and slicing was performed by Lipscomb
Concrete Cutting Company of Raleigh, NC.
Sample permeability was measured with
a device developed for this project. The
sample holder's open end was sealed air-
tight into the permeability test fixture us-
ing Mortite, a non-hardening, clay-type
sealant. Atop plate kept the sample holder
sealed to the fixture. Compressed air at
25 psi* (nominal) pressurized the space
enclosed by the test fixture and the bot-
tom side of the concrete. The pressurizing
valve was closed, and a pressure-sending
unit measured the pressure in the sealed
volume. As the air escaped through the
concrete, the pressure decreased.
The diffusion coefficient was determined
with a system developed by and purchased
from RAECORP. The method uses ura-
nium mill tailings as a strong emitter of
radon gas. The tailings are in a 30-gal.**
drum with a fitting built into the lid, which
accommodates the sample holder. The
sample holder is mounted in the fitting,
and the detector assembly is mounted on
top of the holder. After the background
count rate is measured, the valve is
opened between the drum of radon gas
and the bottom surface of the concrete
sample. The scalar ratemeter counts and
produces a paper record of the number of
counts per interval. When the count rate
stops increasing, the radon in the drum
and in the space above the concrete has
reached equilibrium. The valve is closed,
and the sample holder and detector appa-
ratus are disassembled.
The permeability time-versus-pressure
data were analyzed by software written
for this permeability determination method.
The software also provided the automatic
data collection system. Usually, data were
collected from the pressure sender every
10 sec. Then, six of the data points were
averaged to produce a raw data point
every 1 min. This could be varied; for
some of the high permeability concrete
samples, raw data points were saved each
second with no averaging. The sampling
technique allowed more data to be col-
lected and improved the standard devia-
tion.
The physical parameters of the test (air
temperature, sample thickness, sample
diameter, and volume under pressure)
were used to calculate the permeability
coefficient. The software requested the
data as measured, then converted them
to the appropriate units. The errors in each
parameter were used to determine an es-
timated standard deviation for the calcu-
lated permeability coefficient. The results
were written to a data file in a format
suitable for printing.
RAECORP software was used to deter-
mine the diffusion coefficient. The soft-
ware uses 10 pairs of data points from the
breakthrough region of the alpha activity
data. For this work, 10% offsets from the
baseline and the equilibrium level were
used to determine the breakthrough re-
gion.
Within this region, 10 data points, evenly
spaced in log time, were selected. The
next highest adjacent data point was used
as the second data point of the pair. The
first data point of the pair was used to
calculate the diffusion coefficient. The sec-
ond data point was used to estimate the
standard deviation of the diffusion coeffi-
cient.
The permeability and diffusion coeffi-
cients are graphically illustrated in Figures
1 and 2. The permeability coefficients for
Samples 2a and 2b are very high be-
cause of defects in the concrete.
The air seemed to flow primarily from a
set of pores near the center. It is possible
that only a few through-connected pores
were responsible for the increased per-
meability, and the remainder of the sample
had a much lower permeability. Even if
that were the case, a distribution of such
pores in a slab would significantly alter
the slab's overall effective permeability.
But Sample 2 does not deviate from the
range of the other samples in the diffusivity
graphs. Sample 4, however, is higher in
both the pore space and bulk coefficients.
The data quality for this project met all
the data quality objectives listed in an
EPA-approved Quality Assurance Project
Plan (QAPP). All 10 samples were col-
lected and analyzed (except for the den-
sity on samples COOO and C001); the re-
sults are acceptable, within the limitations
of this project. The limitations of this re-
search were that these measurements of
concrete had not been performed before
on this scale, and that the permeability
1E-13
* 1 psi = 6.89 kPa.
** 1 gal. = 0.0038 m3.
1E-17
00 01 2a 2b 3a 3b 4a 4b 5a 5b
Sample ID
Figure 1. Permeability coefficients.
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00 01 2a 2b 3a 3b 4a 4b 5a 5b
Sample ID
Figure 2. Bulk diffusion coefficients.
test system was a new method developed
for this project.
Because the permeability test was a
new method (there were no available data
on the permeability of concrete, and there
was no test standard for calibration pur-
poses), it is not possible to calculate the
percent difference between the actual and
measured permeability values. The data
collected in these tests compare favorably
(order of magnitude) with the expected
permeability coefficients. The limiting fac-
tor in the permeability test is the system
leak rate. This leak rate imposes a mini-
mum detectable limit of 10-20m2, which is a
factor of 10,000 lower than the concrete
samples measured.
The objectives of Phase I of this project
have been met. The permeability and dif-
fusion coefficients of concrete can now be
measured. Compression test cylinders of
concrete from the four representative ar-
eas of Florida have been processed and
analyzed. The correlations of the various
physical parameters were investigated, al-
though there were insufficient data to con-
fidently determine any correlations.
The most significant fault in this phase
of the research was the lack of unbiased,
representative concrete slab samples. Al-
though the compression test cylinders were
intended to represent the actual concrete
mix from which the sample was drawn (in
accordance with the applicable ASTM stan-
dard), a compression test cylinder will
never accurately represent an as-poured
slab because it does not include the ef-
fects of on-site water addition, finishing
and curing practices, and other proce-
dures.
The Phase I method of slicing cores
into 2-in. thick sections further removed
the samples from representing typical slab
concrete. The correlation plots do indicate
some trends in the data, but the lack of
reliability and representativeness of the
concrete cylinders does not allow defini-
tive correlations to be determined. That
the various physical parameters should
correlate to the coefficients is evident and
reasonable, but not significantly so, based
on these data. The cause and meaning of
the Sample 2 and 4 outliers have not
been determined.
Further analysis of unsliced concrete
cores from real slabs would help to obtain
reliable data on the mix design. Mixing
design data and on-site practice data
should help meet the ultimate goals of
this project.
A larger number of samples also would
provide better, more significant data. The
recommended minimum sample is two
cores from four slabs, from each of the
four major areas in Florida with the mix
design data and, if possible, the on-site
treatment. Additionally, one old concrete
core from each of the four areas could be
collected and included in the data base as
historical data. Nationwide samples could
be considered for additional support for
the data base.
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R. Snoddy is with Acurex Environmental Corp., Research Triangle Park, NC27709.
David C. Sanchez is the EPA Project Officer (see below).
The complete report, entitled "Laboratory Assessment of the Permeability and
Diffusion Characteristics of Florida Concretes: Phase I. Methods Development
and Testing," (Order No. PB94-162781; Cost: $27.00, 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
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EPA/600/SR-94/053
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