United States Air and Energy Engineering

Environmental Protection Research Laboratory

Agency Research Triangle Park, NC 27711

Research and Development EPA/600/SR-94/175 November 1994

E AP

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 Environmental 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 under the FRRP are presented in several

technical reports. This report summarizes a project that examined radon generation and transport through Florida

residential concretes. The concretes

are characterized by radium concentrations, radon emanation coefficients, 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 documented in a separate report of the same

title (see Project Report ordering information 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 negative indoor pressure compared with that in

the soil and the outdoor atmosphere. Radon generated in the concrete floor and

radon diffusion from the soil through the

concrete floor have generally been ignored.

Recently, attention has been directed toward the importance of diffusion as a significant 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 predominance 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 concretes used in dwelling floors to better

assess this mode of radon entry. It also is

instructive to characterize the relative importance of radon generated within the

concrete to determine whether aggregates

or other concrete components may contribute significantly to indoor radon concentrations. Very little relevant concrete

data exist in the general literature.

This report characterizes the radon generating properties of Florida concretes. The

work was conducted by Rogers & Associates 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 constituents. The report also identifies the main

properties of concrete that influence radon migration from the subsoil into dwellings. The parameters characterizing radon

transport through concrete are diffusion

coefficient, porosity, and permeability coefficient. The report then examines the

relation of the measured properties to other

physical properties of the concretes. Finally, it examines the relative importance

of the concrete properties, including radium concentrations, to radon entry into

dwellings. The radon entry correlations are

based on laboratory data, on a simple

Project Summary
2

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 pozzolan 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 2.19 g cm-3. The measured

diffusion coefficients ranged from 1.8x10-4

to about 4.6x10-3 cm2 s-1. Uncertainties associated 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 addition to transmitting radon from the underlying soils into the dwelling. The importance

of the concrete floor and walls as an indoor 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 constituents.

The Ra and E measurements were

made on some of the Florida concrete

samples used for the D and K measurements. 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 values for the mixed concrete. The concrete

mixes were sieved to separate the aggregate, 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 elevated in radium. Phosphogypsum was

selected as an additive to concrete constituents to investigate this effect.

Six phosphogypsum concrete samples

were tested to determine the radon diffusion coefficient of the concrete. The results for D from the phosphogypsum

concrete fall within the range of measurements on regular Florida concretes. The

phosphogypsum does not appear to have

a significant impact on the concrete’s ability to hinder radon migration via diffusion.

The results for the permeability coefficient 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 correlations 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 = 1.5x10-6 exp (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 scatter 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 generation and transport through soil and concrete into dwellings is complex and

involves multidimensional models such as

RAETRAD. However, for Florida concretes, 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 studied 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 concrete slab (pCi s-1)

As = area of concrete slab (m2)

28 = units conversion factor and

constants (m2 sg-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 approximation of the indoor radon balance equation.

The approximation assumes that all indoor radon enters via the concrete foundation area, and that the indoor volume is

uniformly diluted with clean air having an

insignificant radon concentration.

For a simple slab-on-grade house geometry typical of Florida construction, the

expression is

Cin = [15.5 Ra + 0.22 Css]

/ (1000λv

)

where

Cin = steady-state indoor radon

concentration (pCi L-1)

Css = subslab radon concentration

(pCi L-1)

λv = ventilation rate of indoor volume (ach-1).*

_________________________

* ach = air changes per hour.
3

Summary and Conclusions

The Florida concretes tested generally

have Ra concentrations less than 3 pCi

g-1, and emanation coefficients usually less

than 0.08. Ra concentrations over 1 pCi

g-1 may either be due to the Ra in either

the cement or the aggregate. However,

the aggregate has very low E values, rendering its Ra less important than Ra in the

cement component.

The correlations for D, K, and Qc

 are

very useful and provide sufficient accuracy for general scoping studies. Concretes

with Ra content less than about 2 pCi g-1

generally contribute less than 10% of the

total radon entry into the example dwelling.
4

United States

Environmental Protection Agency

Center for Environmental Research Information

Cincinnati, OH 45268

Official Business

Penalty for Private Use

$300

EPA/600/SR-94/175

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT No. G-35

V. Rogers, K. Nielson, 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, 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