United States Air and Energy Engineering

Environmental Protection Research Laboratory

Agency Research Triangle Park, NC 27711

E AP Project Summary

Research and Development EPA/600/SR-95/032 March 1995

The report gives results of a characterization of radon generation and transport 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 concentrations 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 residential concretes. The measured air permeability 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 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

Diffusion can be a significant mechanism for radon entry into dwellings. While

the diffusive flux of radon through conRadon Generation and

Transport in Aged Concrete

Vern C. Rogers, Kirk K. Nielson, and Rodger B. Holt

crete floors is much smaller than the advective 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 emanation 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 expression with a correlation coefficient of r

= 0.96. The data also indicate that compressive strength reaches about 90% of

its maximum value in about 5 to 10 years.

Only one specific study on the age effects of either D or K for concrete was

found in the literature. Measurements of

air permeability are reported for six concrete 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 concretes. Reported K values increased with

the applied air pressure. The initial K values 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 3.9x104

Pa. The data indicate that K reached its
2

maximum value at about 20 years and

that K increased to about 80% of its maximum 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 residential concretes from Florida. The Ra concentrations 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 5.2x10-7 m2

 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 obtained from residential floor slabs in Miami, 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 (generally 0.10 m long). The concrete densities ranged from 1.96x103

 to 2.12x103 kg

m-3. The extent of their alkali-aggregate

reaction was described and estimated from

visual observations of the samples. The

extent of observable alkali-aggregate reaction generally increased with age, and

should also depend on the type of aggregate.

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 estimate of the total porosity (pt

), and the

intrusion method gives an estimate of the

interconnected porosity (pi

). The interconnected porosity is more closely related to

the transport of radon through concrete.

To prepare the samples for the diffusion measurements, each cylinder was

epoxied into standard diffusion sample

holders using an epoxy that has negligibly

low radon diffusion and permeability coefficients. Air permeability was also measured 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 diffusion sample holders. The sample holder

was sealed closed on one end and evacuated using a vacuum pump. Air was then

introduced back into the sample, and the

volume of air needed to re-establish equilibrium 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 determined by extracting the free radon from

the sealed can into a Lucas cell and counting to determine the free radon-222 concentration.

Measurement Results

The D values ranged from 1.5x10-7 to

5.5x10-7 m2 s-1, 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 interval about the mean was 2.6x10-7 to 3.6x10-

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 deviation of 3.1. The 95% confidence interval about the mean was 1.6x10-16 to

4.4x10-16 m2

.

The Ra concentrations for the aged concrete 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 pi

 values

were all generally less than or equal to

the pt values, within measurement uncertainties. For the two exceptions, the pi

values exceeded the pt values by about

8%. The pi

 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 pi

 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 difference 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 variations. 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 generally 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 reaction 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 previously.

The effect of density on K can be reduced by again dividing the aged concrete values by the corresponding correlation estimate. The resulting ratios showed

no general trend with age. This is consistent with the previous results: K for concrete reaches about 80% of its maximum

value by about 12 years of age. Therefore, a decrease in K with density is a

density trend that is not otherwise associated with concrete age.

Radium, Emanation Coefficient,

Density, and Porosity

The average radium concentrations,

emanation coefficients, densities, and total 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 coefficients 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 concretes are about the same as those for

the new concretes.
3

Vern C. Rogers, Kirk K. Nielson, 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, 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

EPA/600/SR-95/032

Official Business

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