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Geotechnical News • December 2014
41
GEOTECHNICAL INSTRUMENTATION NEWS
provide and maintain a specified VOC
vapor concentration in the lower test
chamber in accordance with Henry’s
Law. A granular activated carbon
(GAC) filter is attached to the upper
test chamber to absorb VOCs that
diffuse across the membrane or con-
crete core
.
Both the upper and lower
chambers are vented to the atmosphere
to prevent the development of a pres-
sure differential between the upper
and lower chamber. Typical results
obtained for one solvent (tetrachloro-
ethylene or PCE) are shown in Table
1.
Although the diffusion coefficients
measured for the concrete core
samples are higher than those of the
membrane samples (i.e. the VOCs
can diffuse more readily through the
concrete), the intact concrete would
actually provide a higher overall
level of resistance to diffusion of the
VOCs due to its greater thickness. The
resistance to diffusive transmission,
or impedance, is represented by the
thickness of the barrier divided by its
diffusion coefficient for the compound
in question. Based upon a typical
4-inch floor slab thickness, the relative
impedance of the materials outlined
previously (normalized to 60-mil
HDPE) would be as shown in Table 2.
Accordingly, even low strength con-
crete (when intact) can provide sig-
nificant resistance to the transmission
of VOCs to the interior of a building.
While a concrete floor slab can gener-
ally not be relied upon to function as
a vapor barrier due to the potential for
cracks to form within that material,
the effects of the concrete floor slab on
vapor probe monitoring results must
be considered if the slab is in good
condition.
One such example involved a former
dry cleaning facility in San Diego,
California where a 4-inch thick floor
slab constructed of 2,500 psi concrete
was present above a 60-mil spray-
applied vapor barrier. PCE vapors
were measured at a concentration of
5,000 ppm in a gas probe installed
below the vapor barrier, and at a con-
centration of 350 ppm in a gas probe
above the vapor barrier. The local
regulatory agency initially concluded
the vapor barrier was not functioning
properly due to the elevated VOC lev-
els measured above the barrier. Upon
investigating the condition of the floor
slab, it was found that it was in good
condition with some minor localized
cracking. The total area of the open
cracks was found to be 0.018% of the
area of the floor slab. Based upon that
ratio and the testing results described
previously, the impedance of the con-
crete floor slab was calculated to be
8% of that of the vapor barrier. It was
shown that the PCE vapor concentra-
tion above the barrier, assuming the
barrier was intact and functioning as
intended, should be 350 ppm under
that condition. This was consistent
with the measured value and the bar-
rier was approved by the regulatory
agency.
Both of the cases involve common
engineering monitoring problems
where there are (or were) widespread
misconceptions regarding the trans-
mission of liquids or vapors
across
relatively impermeable barriers. In
both instances, modeling and simula-
tion of the barrier systems provided a
means of understanding and quantify-
ing the behavior and performance of
those systems.
Glenn D. Tofani
Principal Engineer
GeoKinetics
77 Bunsen, Irvine, California 92618
Tel: (949) 502-5353
email: glenn@geokinetics.org
Table 1
Material
PCE Vapor
Concentration
Diffusion
Coefficient
Concrete (2,500 psi)
10,000 mg/m
3
1.4 x 10
-8
m
2
/day
Concrete (5,000 psi)
10,000 mg/m
3
3.0 x 10
-9
m
2
/day
60-mil HDPE
6,000 mg/m
3
1.1 x 10
-9
m
2
/day
60-mil Spray-Applied
Membrane
6,000 mg/m
3
2.4 x 10
-9
m
2
/day
Table 2
Material
Relative Impedance
Concrete (2,500 psi)
5.2
Concrete (5,000 psi)
24
60-mil HDPE
1.0
60-mil Spray-Applied Membrane
0.6