Testing - Concrete Mix Design, Quality Control and Specification - page 181

µ = Viscosity of air (= 2.0. 10 -5 Ns/m²)
ε = Estimated porosity of the cover concrete (assumed = 0.15)
P a = Atmospheric pressure (N/m²)
ΔP i = Pressure rise in the inner cell at the end of the test (N/m²)
t f = Time (s) at the end of the test (2 to 6 or 12 minutes depending on
the instrument brand)
t o = Time (s) at the beginning of the test (= 60 s)

The knowledge of kT allows the estimation of concrete depth affected
by the test (typically between 10 and 50 mm), which is also indicated by
the device. The air permeability kT is very sensitive to the “covercrete”
microstructure, covering some 6 orders of magnitude (0.001.10 -16 to
100.10 -16 m 2 ). Table 7.3 shows the classification of concrete permeability
(ages from 28 to 180 days) as a function of kT.
The original Figg tests originated in the United Kingdom but have subse-
quently been neatly combined into a single instrument by James Instruments
in the United States. A hole is drilled into the concrete (which may be in
situ concrete or a test specimen) and a plastic plug inserted to create a cell
below the surface of the concrete. A hypodermic needle is inserted through
the plug to provide access. The first test involves applying a suction to the
cell so as to draw in air through the surrounding concrete. The (very small)
volume of air is measured by the movement of mercury in a tube through
which the suction is applied. The second involves filling the cell with water
and using movement in the same tube (but in the opposite direction) to
measure the rate at which water is absorbed into the surrounding concrete.
The Wexham variant identifies two problems sometimes encountered
with the aforementioned test. One is that air permeability is substantially
affected by moisture content. The other is that air may be entering via
defects in the concrete or a leaking plug rather than via permeable concrete.
These two potential problems are solved, first by using a slightly larger
diameter hole and including an instrument to measure humidity in the hole.
Second, pressure rather than suction is employed so that any leaks can be
detected by bubbles in a soapy water film on the surface.
An additional advantage of these kinds of in situ tests is that they can
be used to measure the adequacy of curing (which has a large effect on

Table 7.3 Concrete permeability classes
Class

kT (10 -16 m 2 ) Permeability

PK1 <0.01 Very low

PK2 0.01-0.10 Low

PK3 0.10-1.0 Moderate

PK4 1.0-10 High

PK5 >10 Very high