Environmental Engineering Reference
In-Depth Information
-
electrical resistivity measurement
;
- dielectric constant
measurements with
capacitance probes
(frequency < 100
MHz, see Chapter 10, section 10.2.2) or by
TDR
(time domain reflectometry),
maximum frequency > 1 GHz;
- radar
: electromagnetic wave reflection analysis and interpretation of the
echoes observed;
- nuclear magnetic resonance
(NMR): analysis of the signal emitted by a
material after a magnetic excitation, which measures sensitivity to water and its
environment at the microscopic scale.
Among all these techniques, some are non-destructive: ultrasonic sounding and
radar, for instance. Others, like TDR and capacitance probe, are only slightly
intrusive since they involve the introduction of small probes into the stone. All
deliver measurements (velocity, attenuation rate, frequency, propagation time, phase
shift, amplitude), however, that are
correlated
with water content only. Therefore,
they should be calibrated. Sometimes the same calibration curve can be applied to a
class of materials of the same type but in most cases a specific calibration has to be
made. Moreover, parameters other than water content can influence these indirect
measurements:
− electrical resistivity, for example, is a decreasing function of water content but
also depends greatly on the salt concentration and on temperature;
− sound velocity is mainly determined by the stone's mechanical properties;
− radar and TDR “see” impedance contrasts;
− nuclear techniques are also density dependent, etc.
This is probably the reason why the universal water content sensor we were
talking about does not already exist and commercially available sensors must be
used carefully, taking account of their principles and limits.
9.2.2.
Water
−
stone interaction
9.2.2.1.
Wettability
We will now examine the exact nature of
capillarity
, which is actually the main
interaction mechanism between the stone solid phase and liquid water.
At the microscopic scale, water in the pores is bounded with solid surfaces by
the
wetting
phenomenon [DAI 96, GUE 92, JEA 97, LAU 98]. Contrary to what we
might think, a liquid like water does not always perfectly wet a solid surface. This
