United States National Risk Management

Environmental Protection Research Laboratory

Agency Research Triangle Park, NC 27711

Research and Development EPA/600/SR-95/103 July 1995

Laboratory Assessment of the

Permeability and Diffusion

Characteristics of Florida

Concretes: Phase II. Field

Samples and Analyses

Project Summary

The report gives results of a study to

establish the capability to measure

concrete’s permeability and diffusivity;

to measure these parameters in a small

sampling of the typical types of Florida

concrete; and, if possible, to correlate

the physical parameters of the concrete

(mix design, porosity, etc.) to the measured diffusion and permeability coefficients.

Sample permeability was measured

and analyzed with a device developed

for the project. The diffusion coefficient

was determined using a system also

developed for the project.

The statistical analysis consisted

of 1) a stepwise search using an

Efromyson-type search methodology,

2) a standard linear-modeling, leastsquares approach to evaluate individual variables and sets (up to sets

of three), and 3) robust or least-median-square methods to further evaluate possible correlations and examine

the dataset for outliers.

For both permeability and diffusivity,

the amount of water added to the mix

at the site was directly and positively

related and identified as a possible

major factor. The data indicated that

the total amount of sand and stone in

the mix was possibly correlated to the

permeability. A third possible correlation involved the amount of fly ash or

the cement to fly ash ratio.

This Project Summary was developed

by EPA’s National Risk Management

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).

Overview

Much of Florida’s natural soil, as well

as the disturbed overburden and the sand

tailings from the phosphate mining/

beneficiation process, contains significant

quantities of radium. Buildings constructed

on these high-radium soils contain elevated

radon gas levels. Elevated indoor radon

gas levels can cause lung cancer in humans.

To decrease elevated radon gas levels,

Florida’s legislature instructed its Department of Community Affairs to develop new

construction standards for radon-resistant

buildings, primarily slab-on-grade construction.

It is well-known that the concrete slab is

the primary barrier to radon entry. However, the extent of its ability to permit air

flow under pressure (permeability) and to

permit the passage of radon gas without

any pressure difference (diffusivity) has

not been well-determined. To establish a

standard concrete mix and its maximum

radon-resistant placement, these parameters needed to be quantified and their

R. Snoddy
2

relationship to concrete’s physical properties evaluated.

Primary research objectives were to establish the capability to measure concrete’s

permeability and diffusivity; to measure

these parameters in a small sampling of

the typical types of Florida concrete; and,

if possible, to correlate the physical parameters of the concrete (mix design, porosity, surface finish, etc.) to the measured

diffusion and permeability coefficients.

Sample permeability was measured with

a device developed for this project. The

sample holder’s open end was sealed airtight into the permeability test fixture using Mortite, a non-hardening, clay-type

sealant. A top plate kept the sample holder

sealed to the fixture. Compressed air at

25 psi (172 kPa) (nominal) pressurized

the space enclosed by the test fixture and

the bottom side of the concrete. The pressurizing valve was shut, and a pressuresending 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 Rogers and Associates Engineering

Corporation. The method uses uranium

mill tailings as a strong emitter of radon

gas. The tailings are in a 30-gal (114 L)

drum the lid of which has a fitting into

which the sample holder is mounted, and

the detector assembly is mounted on top

of the holder. After the background count

rate is measured, the valve between the

drum of radon gas and the bottom surface

of the concrete sample is opened. The

scalar/ratemeter counts and produces a

paper record of the number of counts per

interval. When the count rate stops increasing, the radon gas in the drum and

in the space above the concrete has

reached equilibrium. The valve is shut,

and the sample holder and detector apparatus 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 s. 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 collected and improved the standard deviation.

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 it to

the appropriate units. The errors in each

parameter were used to determine an estimated standard deviation for the calculated permeability coefficient. The results

were written to a data file in a format

suitable for printing.

Rogers software was used to determine

the diffusion coefficient. The software 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 region.

Within this region, 10 data points spaced

evenly 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 second

data point was used to estimate the standard deviation of the diffusion coefficient.

Proper function of the diffusion test

system was verified by testing four materials with different diffusion coefficients:

air, 4 mm glass beads, coarse sand,

and fine sand. 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 purposes. 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 factor in

the permeability test is the system leak

rate. This leak rate imposes a minimum

detectable limit of 10-20 m2 which is a

factor of 10,000 lower than the concrete

samples measured.

A statistical analysis of the data was

performed using the S-Plus statistical

analysis package for desktop computers.

A three-step process was used to evaluate possible correlations of the mix design

and other factors to the permeability and

diffusion coefficients. The first step used a

stepwise search of the possible explanatory variables using an Efromyson-type

search methodology. The second step

used a standard linear-modeling, leastsquares approach to evaluate individual

explanatory variables and sets of explanatory variables (up to sets of three). The

third step was to use robust or least-median-square methods to further evaluate

possible correlations and examine the

dataset for outliers. The correlation analysis has provided some indications of the

components of mix design and placement

which may have the greatest effect on the

permeability and diffusivity of the concrete.

The primary correlation to the diffusion

coefficient was a direct relationship to the

amount of water added to the mix at the

site before the concrete was placed. Other

possible correlations were the cement to

fly ash ratio and the total amount of fly

ash in the mix. The permeability coefficient was most correlated to the total

amount of aggregates (sand and stone) in

the mix. Other possible correlations to the

permeability coefficient were the amount

of water added at the site, the ratio of the

total amount of aggregates to the total

amount of cementitious materials, and the

amount of fly ash in the mix.

The objectives of Phase II of this project

have been met. Concrete samples from

typical residential slabs from throughout

Florida have been tested. The mix design

and placement data have been examined

for correlations to the permeability and

diffusion coefficients.

For both permeability and diffusivity, the

amount of water added to the mix at the

site was directly and positively related and

identified as a possible major factor. The

data indicated that the total amount of

sand and stone in the mix was possibly

correlated to the permeability. A third possible correlation involved the amount of fly

ash or the cement to fly ash ratio. Fly ash

is commonly used as a cementitious material in addition to cement powder. It has

good properties as a binding agent but is

more porous than cement powder. This

porosity may be negated somewhat by

the small size of the fly ash particles and

the ability of the cement powder to seal

these small porosities and reduce their

effect. The data for the admixtures were

not included in the correlations since they

tended to be unavailable or unreliable.

It is also interesting to note that samples

c002a, c002b, and c018, which had defects, all have high permeability and average diffusivity. This indicates that small

defects with high tortuosity have small effects on the diffusivity but are very good

channels for air flow.

First and foremost, great care should

be taken to ensure that the concrete slab

remains a fully intact barrier. The effects

of a very tight concrete mix are nullified

by cracking and unsealed penetrations.

Proper grading/compaction will help to

eliminate settlement cracking. Drying

shrinkage cracks can be reduced by ensuring that the slab wet cures for an extended time to allow the concrete to gain

strength before stresses due to drying are
3

placed on the slab. Reducing the amount

of water in the mix may also reduce the

amount of drying shrinkage cracks. Increasing the amount of cement in the mix

will reduce radon transport and produce a

better quality of concrete but will also increase the cost.

Additional barrier measures should also

be considered. A plastic sheet or layer of

asphaltic cement will reduce both transport and the impact of defects in the concrete. Coating the top of the slab (e.g.,

painting) could also be used to seal imperfections and small cracks in the slab. It

is important that cracks are sealed to depth

to prevent the soil gas from bypassing a

thin surface seal. Resealing could be done

when the floor coverings are replaced.

All of these options support the primary

goal: to provide an intact primary barrier

with reduced transport properties.
4

United States

Environmental Protection Agency

National Risk Management

Research Laboratory (G-72)

Cincinnati, OH 45268

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT No. G-35

R. Snoddy is with Acurex Environmental Corp., Research Triangle Park, NC 27709.

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 II. Field Samples and

Analyses,” (Order No. PB95-243168 Cost: $19.50, 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

National Risk Management Research Laboratory

(formerly Air and Energy Engineering Research Laboratory)

U.S. Environmental Protection Agency

Research Triangle Park, NC 27711

Official Business

Penalty for Private Use

$300

EPA/600/SR-95/103