United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-95/032
March 1995
EPA Project Summary
Radon Generation and
Transport in Aged Concrete
Vern C. Rogers, Kirk K. Nielson, and Rodger B. Holt
The report gives results of a charac-
terization of radon generation and trans-
port in Florida concretes sampled from
12- to 45-year-old residential slabs. It
also compares measurements from the
aged concrete samples to previous
measurements on newly poured Florida
residential concretes. Radon generation
in the aged slabs is characterized in
terms of concrete radium concentra-
tions and radon emanation coefficients,
and radon transport is characterized
by radon diffusion coefficients and air
permeability coefficients. The radium
concentrations and radon emanation
coefficients (0.11 + 0.04) of the aged
concretes in this study are about the
same as those measured previously for
newly poured residential concrete
samples. The measured radon diffusion
coefficients ranged from 1.5x10'7 to
5.5x10-7 m2 s'1. On the average, these
values are about a factor of 2 higher
than average values for new residen-
tial concretes. The measured air per-
meability coefficients also average
about a factor of 2 higher than those
for new concretes.
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
Diffusion can be a significant mecha-
nism for radon entry into dwellings. While
the diffusive flux of radon through con-
crete floors is much smaller than the ad-
vective or diffusive flux of radon through
cracks in the floor, the predominance of
the intact floor area over the crack area
may compensate for the difference in the
fluxes. Thus, it is desirable to examine the
radon transport properties of concrete used
in floor slabs to better assess radon entry
into dwellings.
Radon generation and transport data
from scientific literature have been reported
for radium concentration (Ra), radon ema-
nation coefficient (E), diffusion coefficient
(D), and air permeability coefficient (K) of
concretes. The literature also references
the effects of aging on strength-related
properties of concrete. For example, the
compressive strength, flexural strength,
and modulus of elasticity are given for
concrete samples up to 20 years old. The
compressive strength data increase with
age and fit a least squares quadratic ex-
pression with a correlation coefficient of r
= 0.96. The data also indicate that com-
pressive strength reaches about 90% of
its maximum value in about 5 to 10 years.
Only one specific study on the age ef-
fects of either D or K for concrete was
found in the literature. Measurements of
air permeability are reported for six con-
crete slabs made over a period of 20
years. The slabs were constructed with
identical compositions and had a
water-to-cement ratio of 0.37, which is
much less than for typical residential con-
cretes. Reported K values increased with
the applied air pressure. The initial K val-
ues ranged from 2.4x10'14 to 2.0x10'13 m2,
with an average of 9.2x10'14 m2 for the
lowest pressure tested, which was 3.9x10*
Pa. The data indicate that K reached its
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maximum value at about 20 years and
that K increased to about 80% of its maxi-
mum value at 12 years. Thus, most of the
increase in K occurred in the first 10 to 12
years.
Measurements on New Florida
Concretes
Radon generation and transport were
measured on 25 samples of new residen-
tial concretes from Florida. The Ra con-
centrations in the new concrete samples
ranged from 0.5 to 2.4 pCi g-1, with an
average of 1.2 pCi g~1 and a standard
deviation of 0.6 pCi g-1. The E values
ranged from 0.02 to 0.17, with an average
of 0.08 and a standard deviation of 0.04.
The D values for these samples ranged
from 2.1x10'8 to 5.2x1Q-7m2 s'1, with an
arithmetic mean of 1.9x10~7 m2 s~1 and a
standard deviation of 1.4x10~7 m2 s~1.
Measurements on Aged Florida
Concretes
Twenty-two concrete samples were ob-
tained from residential floor slabs in Mi-
ami, Boca Raton, Pompano Beach, and
Delray Beach. The slab ages ranged from
12 to 45 years. Duplicate samples were
obtained at 11 separate locations. Each
sample consisted of a 0.09-m-diameter
cylinder core-drilled through the slab (gen-
erally 0.10m long). The concrete densi-
ties ranged from 1.96x103 to 2.12x103kg
nr3. The extent of their alkali-aggregate
reaction was described and estimated from
visual observations of the samples. The
extent of observable alkali-aggregate re-
action generally increased with age, and
should also depend on the type of aggre-
gate.
D, K, Ra, and E were measured with
the same equipment and procedures used
earlier. The porosity of the concrete
samples was determined from both the
measured dry density and an air intrusion
method. The density method gives an es-
timate of the total porosity (pt), and the
intrusion method gives an estimate of the
interconnected porosity (p). The intercon-
nected porosity is more closely related to
the transport of radon through concrete.
To prepare the samples for the diffu-
sion measurements, each cylinder was
epoxied into standard diffusion sample
holders using an epoxy that has negligibly
low radon diffusion and permeability coef-
ficients. Air permeability was also mea-
sured in the same diffusion sample holder
to minimize disruptive handling of the
samples.
The air intrusion method was used to
measure the interconnected porosity with
the concrete samples in the same diffu-
sion sample holders. The sample holder
was sealed closed on one end and evacu-
ated using a vacuum pump. Air was then
introduced back into the sample, and the
volume of air needed to re-establish equi-
librium with the ambient pressure was
measured with a bubble-burette system.
Ra concentration was measured using
the sealed-can, gamma-counting method.
The emanation coefficients were deter-
mined by extracting the free radon from
the sealed can into a Lucas cell and count-
ing to determine the free radon-222 con-
centration.
Measurement Results
The D values ranged from 1.5x10'7 to
5.5x10~7 m2 s'\ with an arithmetic mean of
3.1x10~7 m2 s'1 and a standard deviation of
1.1x10"7 m2 s"1. The 95% confidence inter-
val about the mean was 2.6x1 a7 to 3.6x1 Or
7 m2 s-1.
The K values ranged from 5.3x10'17 to
4.7x10'15 m2, with a geometric mean of
2.7x10'16 m2 and a geometric standard de-
viation of 3.1. The 95% confidence inter-
val about the mean was 1.6x10~16 to
4.4x10-16m2.
The Ra concentrations for the aged con-
crete samples ranged from 0.3 to 2.2 pCi
g-1, with an arithmetic mean of 1.3 pCi g~1
and a standard deviation of 0.6 pCi g-1.
The E values ranged from 0.03 to 0.19,
with an arithmetic mean of 0.11 and a
standard deviation of 0.04.
Except for two samples, the p: values
were all generally less than or equal to
the pt values, within measurement uncer-
tainties. For the two exceptions, the p:
values exceeded the pt values by about
8%. The p: values ranged from 0.12 to
0.25, with an arithmetic mean of 0.19, and
the pt values ranged from 0.16 to 0.24
with a mean of 0.21. Thus, the ratio of the
average p: to pt was 0.88.
The relative uncertainties associated
with the duplicate measurements were
21% for the D data, 37% for K, 15% for
the radium concentrations, and 30% for
the radon emanation coefficients.
Diffusion Coefficients
The D values for the aged concretes
averaged about a factor of 1.6 greater
than for the new concretes. This differ-
ence in the means is significant at the
95% level of confidence. However, since
D varies with density, this difference may
be attributed to differences in concrete
density. The densities and total porosities
of the new and aged concrete samples
were equivalent within the measured varia-
tions. Their means differed by only a few
percent. Thus, different density values
should not account for the differences in
the D values between the new and aged
concrete samples.
The lack of a trend in D/Dcor with time
for periods greater than 10 years gener-
ally is consistent with the trend in the
literature for the change in K and the
change in compressive strength with age,
as previously discussed. However, other
unmeasured or unknown parameters may
also influence D for aged concrete. As
mentioned earlier, the alkali-aggregate re-
action may occur over time in concrete.
While this may contribute to an increase
in D, there is no significant evidence in
the present data to confirm this.
Air Permeability
The variation of K with age was similar
to the variation with D. On the average,
the K value for the aged concrete was
about 2.2 times greater than the K value
for new concrete, and the range extended
about 6 times higher than the range of K
for new concrete. Both the aged and the
new concrete K values were significantly
less than the K values reported previ-
ously.
The effect of density on K can be re-
duced by again dividing the aged con-
crete values by the corresponding corre-
lation estimate. The resulting ratios showed
no general trend with age. This is consis-
tent with the previous results: K for con-
crete reaches about 80% of its maximum
value by about 12 years of age. There-
fore, a decrease in K with density is a
density trend that is not otherwise associ-
ated with concrete age.
Radium, Emanation Coefficient,
Density, and Porosity
The average radium concentrations,
emanation coefficients, densities, and to-
tal porosities were the same for the aged
Florida concretes as for the new concretes,
within the measurement uncertainties.
These variables also do not show any
significant trends with age.
Conclusions
Radon diffusion and air permeability co-
efficients have been measured for Florida
residential concretes ranging from 12 to
45 years old. In general, the D values for
the aged concrete average about 1.6 times
the values for the newly poured Florida
concretes but are within the range of D
values for the new concretes. The aged K
values also average about a factor of 2
higher than those for the new concretes,
but the range of K values increases by
over a factor of 6 for the aged concretes.
The Ra-226 concentrations and radon
emanation coefficients for the aged con-
cretes are about the same as those for
the new concretes.
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Vern C. Rogers, KirkK. Me/son, and Rodger B. Holt are with Rogers and Associates
Engineering Corp., Salt Lake City, UT 84110-0330.
David C. Sanchez is the EPA Project Officer (see below).
The complete report, entitled "Radon Generation and Transport in Aged Concrete,"
(Order No. PB95-181590; Cost: $17.50, subjectto 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-95/032
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