Civil Engineering Reference
In-Depth Information
7.6.5 Air permeability
Swiss Standard SIA 262:2003 “Concrete Construction” states: “The imper-
meability of the cover concrete shall be checked by means of permeability
tests (e.g. air permeability measurements) on the structure or on core sam-
ples taken from the structure”. The PermeaTORR, developed by Roberto
Torrent, is an instrument designed precisely to measure air permeability of
the cover concrete on site. The method serves to measure the coefficient of air-
permeability of the cover concrete on site, in a non-destructive manner, and
operates as described next (also see Figure 7.15). Vacuum is created inside the
two-chamber vacuum cell, which is sealed onto the concrete surface by means
of a pair of soft rings, creating two separate concentric chambers. At a time
between 35 and 60 seconds (with a vacuum of about 5 to 50 mbar, depending
on the concrete, instrument, etc.), Valve 2 is closed and the pneumatic system
of the inner chamber is isolated from the pump. The air in the pores of the
material flows through the cover concrete into the inner chamber, raising its
pressure (P
i
). The rate of pressure rise (ΔPi;
i
; measurement starts at t
o
= 60 s)
is directly linked to the coefficient of air permeability of the cover concrete.
A pressure regulator maintains the pressure of the external chamber per-
manently balanced with that of the inner chamber Pi).
e
= P
i
). Thus, a controlled
unidirectional flow into the inner chamber is ensured (Figure 7.15) and the
coefficient of permeability to air kT (m²) can be calculated as described in
the following equation. A microprocessor stores the information and auto-
matically calculates the air permeability coefficient value kT (m
2
) that is
displayed at the end of the test. The end of the test occurs when ΔPi
i
rises by
20 mbar or, in cases of highly permeable concrete, after 6 or 12 minutes,
depending on the instrument's brand, from the initiation of the test.
Consequently, depending on the concrete permeability, the test may take
from 2 to 6 minutes (12 minutes for one product). The microprocessor is capa-
ble of storing a great deal of test data and the information can be transferred
to a PC for further analysis and filing. The function of valve 1 is to restore the
system for a new test by ventilating it with air at atmospheric pressure.
Since the geometry of air flow is well defined, it is possible to calcu-
late the coefficient of permeability with the following equation, derived in
Torrent (20 09).
2
ln
PP
PP
+∆
−∆
−
a
i
2
=
V
A2P
µ
ε
c
a
i
kT
t
t
a
f
o
where
kT = Coefficient of air-permeability (m²)
V
c
= Volume of inner cell system (m³)
A = Cross-sectional area of inner cell (m²)
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