Environmental Engineering Reference
In-Depth Information
conductivity measurement is then correlated with the water content of the stone or
its soluble salt content [SCH 96b]. We can also use these conductivity
measurements to control desalination work [LAZ 90] as was done, for example, on
Notre-Dame La Grande church at Poitiers [VIE 96].
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Colorimetric methods
are based on evolution of the stone surface color, the
color being determined by three parameters: chroma, lightness and hue. The system
L* (lightness), a* (represents the green-red axis), b* (represents the blue-yellow
axis) is one of the most frequently used [BIS 94, DUR 94]. The change in color at
the stone surface is frequently linked to soiling. The measurement of variation of the
three parameters named above with a colorimeter as a function of time makes it
possible to evaluate this soiling and may be very useful for determining previous
cleaning interventions [VER 94b]. Color change of the surface may also be due to
biological colonization (see section 8.3.4). Colorimetric methods thus allow us to
evaluate the progress of this colonization. On the other hand, they allow
measurement of the efficiency of a biocide treatment [DEC 93, NUG 93].
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IR thermography
uses IR radiation emission by the stone surface (which
essentially depends on the surface temperature and emissive properties). It is able to
show
in situ
heterogeneities of the superficial layer, such as detachments preparing
the plate formation, organic deposits, wetted areas, cracks or fractures not visible
because of their location in the subsurface, and environmental impact on the stone
[ETT 93, FRI 93, JOO 96].
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Photogrammetry
represents the surface of an item such as a building wall in
three dimensions by using stereoscopic observation of photographs taken from two
different points in space. This technique, when photographs are taken at different
times, allows us to follow the state of the surface of a building and to evaluate
alteration processes [GRA 90, MAT 84].
10.1.2.
Porosimetry tests
Modification of the porous structure may be one of the consequences of the
degradation of natural stones. Various laboratory tests are available to characterize
the porous network of rocks. The most classical physical properties measured with
these test methods are the total porosity and apparent and absolute densities.
10.1.2.1
. Measurements of the porosity of stone accessible to water
We commonly measure the total porosity of a material under a vacuum. An
experimental procedure is provided in test n°I.1 of the RILEM recommendations
[RIL 80] for measuring the total porosity (
N
t
) accessible to water. It is worth noting
that the test is performed under a vacuum, and the volume accessible to water found
