United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-94/175 November 1994
EPA Project Summary
Radon Generation and Transport
Through Concrete Foundations
Vern C. Rogers, Kirk K. Nielson, Michael A. Lehto, and Rodger B. Holt
The Florida Radon Research Program
(FRRP), sponsored by the Environmen-
tal Protection Agency and the Florida
Department of Community Affairs, is
developing the technical basis for a
radon-control construction standard.
Results of the research conducted un-
der the FRRP are presented in several
technical reports. This report summa-
rizes a project that examined radon gen-
eration and transport through Florida
residential concretes. The concretes
are characterized by radium concen-
trations, radon emanation coeffi-
cients, radon diffusion coefficients,
and permeability coefficients.
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
Indoor radon entry has been modeled
most commonly as advective transport by
pressure-driven air flow from the soil
through foundation openings or cracks.
The flow is caused by the typically nega-
tive indoor pressure compared with that in
the soil and the outdoor atmosphere. Ra-
don generated in the concrete floor and
radon diffusion from the soil through the
concrete floor have generally been ignored.
Recently, attention has been directed to-
ward the importance of diffusion as a sig-
nificant mechanism for radon entry. While
the diffusive radon flux through concrete
floors is much smaller than the advective
flux through cracks in the floor, the pre-
dominance of the intact floor area over
the crack area may compensate for the
difference in fluxes. Thus, it is desirable to
examine the diffusive properties of con-
cretes used in dwelling floors to better
assess this mode of radon entry. It also is
instructive to characterize the relative im-
portance of radon generated within the
concrete to determine whether aggregates
or other concrete components may con-
tribute significantly to indoor radon con-
centrations. Very little relevant concrete
data exist in the general literature.
This report characterizes the radon gen-
erating properties of Florida concretes. The
work was conducted by Rogers & Associ-
ates Engineering Corporation as part of
the Florida Radon Research Program
(FRRP) cosponsored by the Florida State
Department of Community Affairs and the
U.S. Environmental Protection Agency.
The parameters measured are the radium
concentrations and emanation coefficients
of Florida concretes and their constitu-
ents. The report also identifies the main
properties of concrete that influence ra-
don migration from the subsoil into dwell-
ings. The parameters characterizing radon
transport through concrete are diffusion
coefficient, porosity, and permeability co-
efficient. The report then examines the
relation of the measured properties to other
physical properties of the concretes. Fi-
nally, it examines the relative importance
of the concrete properties, including ra-
dium concentrations, to radon entry into
dwellings. The radon entry correlations are
based on laboratory data, on a simple
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indoor radon balance equation, and on a
complete numerical analysis of combined
diffusive-advective radon entry.
Laboratory Tests of Radon
Transport Properties
Radon diffusion (D) and permeability
(K) coefficient tests were performed on 17
samples from Florida. Two samples were
made from a concrete mix from Florida
and included a processed gypsum poz-
zolan additive. The processed gypsum
comprises 8 and 15 weight percent of the
mix, respectively.
The water/cement ratios (W/C) ranged
from about 0.52 to 0.67. Densities ranged
from 1.94 to 2.19gcrrr3. The measured
diffusion coefficients ranged from 1.8x10~4
to about 4.6x10"3 cm2 s~1. Uncertainties as-
sociated with the measurements range
from 20 to 30%.
In general, the Florida concrete samples
had very low permeabilities. None had a
value greater than 7x10~12 cm2.
Related Diffusion and
Permeability Coefficient
Measurements
The D and K measurements by R.
Snoddy, also part of the FRRP, were made
with equipment similar to that used in the
measurements reported here. In general,
Snoddy's values for D and K are within a
factor of two of the present results, which
is within the experimental uncertainties of
the measurements.
Laboratory Measurements of
Radium and Radon Emanation
Concrete floors in buildings generate
radon that can enter the dwelling in addi-
tion to transmitting radon from the under-
lying soils into the dwelling. The importance
of the concrete floor and walls as an in-
door radon source depends mainly on the
radium concentration (Ra) and the radon
emanation coefficient (E) in the concrete.
This section gives the results of Ra-226
measurements and E measurements in
Florida concretes and in concrete con-
stituents.
The Ra and E measurements were
made on some of the Florida concrete
samples used for the D and K measure-
ments. The Ra ranges from 1.0 to about
2.4 pCi g-1. The E values average 0.062.
Dry concrete mixes were obtained from
manufacturing facilities in the Jacksonville,
Lakeland, Tampa, and Pensacola areas
in order to measure the Ra and E of the
constituents and compare them to the val-
ues for the mixed concrete. The concrete
mixes were sieved to separate the aggre-
gate, sand, and cement components. In
addition, water was added to the samples
to form solid concrete samples with water/
cement ratios of 0.50. Radium and radon
emanation measurements were then made
on these samples. The Ra and E values
are similar to the values from the intact
concrete samples.
Radon Transport Properties of
Concrete Containing
Phosphogypsum
One of the project's objectives is to
determine the properties of and impacts
from concretes that have constituents el-
evated in radium. Phosphogypsum was
selected as an additive to concrete con-
stituents to investigate this effect.
Six phosphogypsum concrete samples
were tested to determine the radon diffu-
sion coefficient of the concrete. The re-
sults for D from the phosphogypsum
concrete fall within the range of measure-
ments on regular Florida concretes. The
phosphogypsum does not appear to have
a significant impact on the concrete's abil-
ity to hinder radon migration via diffusion.
The results for the permeability coeffi-
cient of the phosphogypsum concrete
samples range from the upper end of the
range of the previous tests to a factor of
five greater than the upper end of the
range.
Data Interpretation and
Modeling
Several correlations and simple models
can be obtained from application of the
measured data. This section identifies cor-
relations for the water/cement ratio, the
diffusion and permeability coefficients, and
the radon entry from concrete floors into
structures. The radon entry correlation is
compared to radon entry from a concrete
floor as calculated with the RAETRAD
code.
The correlation between the measured
values of D for concrete and the W/C ratio
is
D = 1.5x10-6exp(11.4 W/C),
where
W/C = water/cement ratio.
The correlation coefficient is r=0.82.
The permeability data do not exhibit the
same definite trends with W/C as do the
diffusion coefficient data. Much of the scat-
ter in the data is due to experimental
errors and uncertainties. A slightly better
fit is obtained for the correlation between
K and d. This expression is
K = 0.22 exp (-12.4 d),
where
d = bulk dry density of concrete
(g cm-3)
The associated correlation coefficient is
0.80.
Indoor Radon Entry from
Florida Concretes
In general, the calculation of radon gen-
eration and transport through soil and con-
crete into dwellings is complex and
involves multidimensional models such as
RAETRAD. However, for Florida con-
cretes, advection through the concrete is
negligible, and the total radon generation
rate per unit area is small compared to
the radon generation rate per unit area in
the subsoil. Under these conditions, the
radon flux from the concrete floor can be
estimated separately and can be added to
the diffusive indoor flux from the subsoil.
For the range of Florida concretes stud-
ied in the present work, the indoor entry
of radon generated in the concrete can be
estimated by
QC = RaAs/28,
where
Qc = radon entry rate from con-
crete slab (pCi s'1)
As = area of concrete slab (m2)
28 = units conversion factor and
constants (m2 s g~1).
For Ra = 2.31 pCi g-1, and a house area
of 141 m2, the radon entry rate from radon
generation in the concrete is 12 pCi s~1.
For comparison, comprehensive RAETRAD
calculations yield an entry rate of 13 pCi s~1
for radon generated in the concrete slab.
This value is about 6% of the radon entry
rate from the subsoil, where the subsoil is
a loamy sand with a radium concentration
of 2 pCi g-1.
The significance of indoor radon entry
by diffusion through concrete floors can
be estimated from a simplified approxima-
tion of the indoor radon balance equation.
The approximation assumes that all in-
door radon enters via the concrete foun-
dation area, and that the indoor volume is
uniformly diluted with clean air having an
insignificant radon concentration.
For a simple slab-on-grade house ge-
ometry typical of Florida construction, the
expression is
C = [15.5 Ra + 0.22 C ]
/(1000X)
where
Cin = steady-state indoor radon
concentration (pCi L1)
C = subslab radon concentration
(pCi L1)
Xv = ventilation rate of indoor vol-
ume (aclr1).*
* ach = air changes per hour.
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Summary and Conclusions g"1 rnay either be due to the Ra in either The correlations for D, K, and Qc are
the cement or the aggregate. However, very useful and provide sufficient accu-
The Florida concretes tested generally the aggregate has very low E values, ren- racy for general scoping studies. Concretes
have Ra concentrations less than 3 pCi dering its Ra less important than Ra in the with Ra content less than about 2 pCi g~1
g~1, and emanation coefficients usually less cement component. generally contribute less than 10% of the
than 0.08. Ra concentrations over 1 pCi total radon entry into the example dwell-
ing.
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V. Rogers, K. Me/son, M. Lehto, and R. 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 Through Concrete
Foundations," (Order No. PB95-101218; 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
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