Civil Engineering Reference
In-Depth Information
Fundamental to petrographic examination is the preparation of thin
sections for examination in transmitted light with a petrological microscope.
Thin sections are samples prepared as thin slices typically 20-30 μm thick
that are mounted on to glass slides and covered with glass cover slips. A
typical thin section would cover an area of about 50 × 75 mm. The detail
of the method of preparation of thin sections is beyond the scope of this
publication and is described elsewhere (APG-SR2, 2010).
Thin sections of concrete are normally prepared using samples that
have been vacuum impregnated with low-viscosity epoxy resin containing
fluorescent dye and this facilitates the examination of microcracking and
porosity distribution (illustrated in
Figure 2.1)
and avoids the possibility of
introducing cracking during the preparation of the thin sections. It is desirable
that in making the thin sections the exposure of the samples to water be kept
to a minimum and that the samples are not heated to temperatures of >
45°C.
The thin sections are examined with a high-quality polarising petrological
photomicroscope (illustrated in
Figure 2.2)
and these are generally fitted
with dedicated digital microscope cameras. The microscopes employed for
petrographic examination have sufficient resolution to clearly distinguish
individual cement grains and the maximum resolution would be of the order
of 2 μm. Modern petrological microscopes are equipped with a fluorescence
illumination system and a range of objective lenses with magnifications
ranging from ×20 up to ×1000. Stereo binocular microscopes with much
lower magnifications are also essential for the examination of broken
surfaces and larger scale features.
Figure 2.1
Thin section photomicrograph taken using fluorescent illuminations. It
shows microcracks and areas of high porosity. The view shows a concrete surface at
the top of the field of view that is covered by a layer of resin. Abundant microcracks
are visible just below the broken surface of the concrete that result from the method
porosity and appear as dark grey particles in the lower part of the picture.
