Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006

PREDICTION OF RADON CONCENTRATIONS AT CONCRETING
IN CONSTRUCTION SITES WITH LIMITED VENTILATION
Konstantin Kovler
National Building Research Institute - Faculty of Civil and Environmental Engineering
Technion - Israel Institute of Technology, Haifa 32000, Israel

INTRODUCTION
It is known that radon exhalation rate can achieve extremely high values during cement
hydration. Radon contained in trace amounts in hardening material, as other inert gases,
does not interact with the surrounding media chemically, and hence does not interfere
with chemical reactions accompanying cement hydration. Therefore, the unique
properties of radon as a noble gas can be used for monitoring cement hydration and
microstructural transformations in cementitious system.
The first attempt to measure radon exhalation in hardening cementitious materials and
use radon as an indicator of microstructural changes occurring during cement hydration
was made in Czechoslovakia by Balek and Donhhlek (1983). Balek and Donhhlek
attributed the changes in radon exhalation rate, mainly, to the changes in surface area and
microporosity occurring in the hardening material during hydration. The authors
suggested labeling the sample of cement powder before hydration by the parent isotopes
of radon. They impregnated cement powder with a non-a ueous (acetone) solution
containing 2 2 8 ~or
h 2 2 4 ~ina the concentration of 1011 Bq m.' The radionuclides were
adsorbed on the surfaces of cement particles and the non-aqueous solution was
evaporated. The specific radioactivity of the labeled cement was 106 - 107 Bq kg", i.e.
higher by 5-6 orders of magnitude than that before labeling, therefore the sensitivity of
the method was tremendously enhanced.
The disadvantage of the labeling is that working with very high radioactivity
concentrations requires specific safety precautions. In addition, labeling cement particles
for a certain depth, which is independent of cement particle size, leads to uncertainty with
the interpretation of the results, especially when the effect of cement fineness is studied.
The more detailed review of the publications on using radon exhalation method for
monitoring cement hydration and microstructural transformations in hardening
cementitious materials is available in the State-of-the-Art Report of RILEM
(International Union of Laboratories and Experts in Construction Materials, Systems, and
Structures) Technical Committee TC-185 ATC "Advanced Testing of Cement-Based
Materials during Setting and Hardening" (Kovler, 2005).

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Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006
Potentially, high radon exhalation rates can be a concern for construction workers
employed on daily basis at concreting in sites with limited ventilation. The goal of the
present work was to measure the kinetics of radon exhalation rate during Portland cement
hydration in the laboratory and to estimate generated radon concentrations, which can
develop in construction sites with different ventilation rates.

METHODOLOGY
In general, radon concentration C(t) (Bq m'3) generated in the given time t (s) in the
hermetically closed chamber can be determined as

where
C, - initial radon concentration in the chamber (Bq m3);
Co) - background radon concentration in the room (Bq m'3);
E - radon exhalation rate of the source, per unit surface area or mass (Bq m'2 s" or
Bq kgm's-I);
S - characteristic of the radon source, which can be either mass M (kg) of the specimen in
the test chamber or its surface area A (m2);
Veq- equivalent volume occupied by air in the test chamber (volume of the chamber plus
volume of the circuit pipes of the radon monitor instrument connected to the chamber
minus volume of the specimen, m3);
q radon leakage (m3 s-I). Note: in a hermitically closed chamber, this term is zero.
1.q - equivalent radon decay constant (s"'), which is a sum of radon decay constant
~ back diffusion coefficient AM (s") and ventilation rate I.\*eni(s"'):
(2.1 1o " s"'),

-

We have to emphasize that the concentration is not a characteristic of the material,
because it increases with time until achieving a constant value Ccocorresponding to the
saturation condition. At infinite time t Ñ <x>, assuming that Co = Ci = 0, we obtain that
maximum radon concentration in the hermetically closed space under saturation Coo(Bq
m'3) is:

Radon concentration C(t) at a given time is related to maximum radon concentration Coo
as follows:
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Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006

The method of determination of radon exhalation rate of the building material via radon
concentration measured in the hermetically closed chamber has been described by several
authors, and in particular, by Kovler et al. (2005). It is based on measuring the radon
concentration at a given time C(ij and calculating Cm, for determination of the radon
exhalation rate E:

However, the described method is valid only under assumption of constant radon
exhalation rate of the sample in time. For materials, which change their properties in
time, the radon exhalation rate E is time-dependent, so the formula is modified:

EXPERIMENTAL RESULTS AND DISCUSSION

*

Concentration of radium ^ ~ ain Portland cement used in the current study was 64.2 2
Bq kg-'. It was determined in cement powder sample by gamma-ray spectroscopy, using
Nal detector (NPP Doza). The sample was dried at 105OC for 24hours before testing.
Cement was measured as a sample of 200 cm3 volume in sealed cylindrical polyethylene
container after 30 days, to achieve secular equilibrium of the ^ ~ aprogeny.
Cement paste specimens of 5 kg mass were mixed in a pan mixer, cast in the glass
desiccator chamber of 6 liter. This experimental arrangement (the specimen mass to
volume of the chamber ratio) represented a reasonable compromise to fulfill both
requirements: to have acceptably small back diffusion and small experimental scatter
(Kovler, 2006). The desiccator was hermetically closed and the radon concentrations
generated in the chamber were measured by means of continuous radon gas monitor
RAD-7, Durridge Company Inc. Such a small chamber enabled to obtain high radon
concentrations within a short time of the test. The duration of each test was a few days.
The radon exhalation of unhydrated cement was compared with that of hydrating cement
paste, made with water to cement ratio of 0.33. The radon concentrations were measured
at 20°CIn order to compare the measurements data, the relative radon concentrations
(per 1 kg of cement) were calculated. These values are presented on a logarithm scale in
Figure 1. The low-value range is a characteristic of unhydrated cement. The radon
concentration curve for hydrating cement paste starts from zero (background)
concentrations, similarly to unhydrated cement. However, the radon concentrations
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Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006
developed at the end of the test in the desiccator containing cement paste were
significantly higher than those of unhydrated cement. This fact proves the importance of
microstructural transformations taking place in the process of cement hydration, in
comparison with the radon emanation from cement particle, which is a time-stable

10
Time, hours

- * - Cement *Cement

Paste

Figure 1 - Dependence of relative radon concentration in the desiccator (in Bq m'3 per 1
kg of cement) vs. time for unhydrated cement and hydrating cement paste
To explain the enormous difference in the amount of radon atoms exhaling from the
materials containing the same number of radium atoms, we have to take into account
several important mechanisms occurring in the course of cement hydration. For this
reason, it would be convenient to distinguish between three main stages, which are
readily seen in Figure 1 and correspond with stages in cement hydration and
microstructural development: stage I (dormant period), stage I1 responsible for setting,
when intensive microstructural transformations occur, and stage 111 (the densification of
the structure and drying).

PREDICTION OF RADON CONCENTRATIONS IN CONSTRUCTION SITE
The dramatic increase of the radon exhalation rate up to the maximum observed in a few
hours after mixing with water is one of the main findings reported and analyzed in the
previous section. Our experiments show that the maximum value of radon exhalation rate
observed for hydrating cement pastes made with different water to cement ratios, with
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Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006
and without different chemical and mineral admixtures, can reach 0.6 and sometimes
exceeds 1.0 mBq kg-' s-I. Such extremely high values of radon exhalation rate
significantly exceed all E-values known from the previous literature dealing with radon
exhalation from cementitious materials. Potentially, enormous radon exhalation rate
when concrete sets may lead to the development of high radon concentrations in
construction sites at closed spaces. This phenomenon may be of radiological concern for
construction workers, which are employed in casting concrete in a routine form (by daily
or weekly basis), and consequently are exposed to enhanced radon concentrations in
work places, especially where ventilation is poor.
Assuming that the time dependence of radon exhalation rate in-situ remains similar to
that determined in the laboratory, the radiological consequences of the extremely high
values of radon exhalation rate developing a few. hours after mixing cement with water,
should be evaluated in terms of predicted radon concentrations for construction site. The
result of this analysis is reported hereafter.
The ratio MJV for the real construction conditions is different from those tested in the lab.
Let us consider the case of casting concrete floor each square meter of which contains 50
kg of cement paste. The rest of concrete composition are aggregates, entrained and
entrapped air and admixtures, which almost do not contribute in radon exhalation of
concrete floor. Let us assume that the height of air column above each square meter of
concrete floor in the space, where concreting is executed, is 2.5 m. It means that M/V
ratio for this case is 5012.5 = 20 kg m'3 only. In addition, the space should have some air
exchange; that is why the back diffusion can be neglected for the calculations of radon
concentrations in situ.
For deriving the expression for radon concentrations developing during concreting in
closed spaces, equation (6) is solved as "First order linear differential equation":

The results of the calculation by formula (7), assuming that the radon exhalation rate
develops according to the curve obtained in the laboratory (the case of W =439 kg m"
3), but in the smoothed and approximated form, makes possible to predict the
development of radon concentrations on construction site. The curves of radon
concentration calculated for different ventilation conditions (from the space closed
hermetically to the cases of air exchange rate of 0.5 hr") are shown in Figure 2.

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Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006
Concrete Floor, M/V = 20 kg m1

+vent. rate 0.1 llhr
+vent. rate 0.2 1/hr +vent. rate 0.3 1/hr
vent. rate 0.4 1/hr

Time, hours

-

Figure 2 Predicted radon concentrations at casting concrete floor under different
ventilation conditions (hermetically closed space and air exchange rate of 0.1 - 0.5 hr")
Let us assume that the action level in dwellings is 150 Bq m'3, according to the EPA
(USA). This limit is shown by dashed line parallel to the time axis. It can be seen that in
the cases of extremely poor ventilation (&en, = 0.1 h r and less) the radon concentrations
exceed 150 Bq m"3. Moreover, the peak concentrations can increase, when taking into
account the background in the given place. In this case, the curves shown in Figure 2 will
shift up. However, from the radiological point of view, such shifting is also not of any
concern, because the local maximum of radon concentration is observed only when
concrete sets (4-10 hours after casting). Then the concentrations drop down to the level,
which depends on the background and usual radon entry rate from hardened building
materials and other possible sources.
= 0.1 hr" to 0.2 hr" only,
It can be seen that increasing ventilation a little, from
improves significantly the situation. The difference in radon concentrations between Lenl
= 0.1 h f and A,& = 0.2 hr" is rather impressive. In any case, the minimum ventilation
rate accepted in the design practice is 0.5 hr", which guarantees that the concentrations in
most cases will not exceed the action level and that they are not of any radiological
concern for construction workers employed in concreting in closed spaces.

This conclusion is drawn with a high safety factor. The elevated radon concentrations
developing in a few hours after mixing cement with water, are very short-lived, so
comparing with an action level which is meant to be applied to average long-term
concentrations is fraught with danger. In addition, construction sites are usually very
draughty, and have much activity going on which will increase air exchange, so the true
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Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006
values are likely to be on the lower side. Also, the workers move from one construction
site to another, so it is unlikely that they are always exposed to the low-air-exchange
conditions.

CONCLUSIONS
1. The maximum value of radon exhalation rate observed in h drating cementitious
material can reach 0.6 and sometimes exceeds 1.0 mBq kg' 7s". These values are
extremely high and exceed significantly those known before about cementitious
materials, both unhydrated and hardened (well-hydrated).
2. At the same time, the elevated radon concentrations developing in a few hours
after mixing cement with water are short-lived. The minimum ventilation rate
accepted in the design practice (0.5 hr"), guarantees that the concentrations
expected to develop at concreting in close spaces will not exceed the action level
in most of the cases and that they are not of any radiological concern for
construction workers employed in routine daily concreting in poorly ventilated
sites.

REFERENCES
Kovler, K., Perevalov, A., Steiner, V. and Metzger, L.A., 2005. 'Radon exhalation of
cementitious materials made with coal fly ash: Part 1 - scientific background and testing
of the cement and fly ash emanation', J. of Environmental Radioactivity, 82,321-334.
Balek, V. and Donhhlek, J., 1983. 'A new method for investigating cement hydration by
radioactive indicators', Cem. & Concrete Res. 13, 1-6.
Balek, V. and Donhhlek, J., 1992. 'Radiometric emanation method for monitoring
morphology and porosity changes during radwaste cementation', Cem. & Concrete Res.
22,459-464.
Tanner, A.B., 1964. 'Radon migration in the ground: a review', in 'Natural Radiation
Environment', Eds. J.A.S. Adams and W.M. Lowder (Chicago, Univ. of Chicago Press)
161-190.
Stranden, E., Kolstad, A.K. and Lind, B., 1984. 'Radon exhalation: moisture and
temperature dependence', Health Physics 47,480-484.
Kovler, K., 2005. Chapter 7 'Using Radon Exhalation Method for Monitoring Cement
Hydration', RILEM Report 3 1 'Advanced Testing of Cement-Based Materials during
Setting and Hardening', Edited by H.W.Reinhardt and C.U. Grosse, RILEM Publications
S.A.R.L., Bagneux, France, pp. 297-309.

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Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006
Kovler, K., 2006. 'Radon exhalation of hardening concrete: Monitoring cement hydration
and prediction of radon concentration in construction site, J. of Environmental
Radioactivity 86,354-366.

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