Geoscience Reference
In-Depth Information
twice, and preferably three times, the maximum size of
the coarse aggregate within the concrete. For construction
products that are supplied as units, for example precast
concrete or clay bricks, it is normally sufficient to
examine one complete representative unit. Due
consideration should be given to the health and safety
issues that arise from sampling activities, including safe
access to the sample locations.
Once obtained in the field, samples should be labelled
indicating orientation details (bedding, outer surface,
way up, and so on), placed into separate sealed sample
bags, and each bag labelled with a unique identification
reference. Concrete cores should first be wrapped in cling
film to prevent carbonation. The exact sample locations
should be recorded using a combination of written notes,
drawings, and photography. These should include
comprehensive inspection details of the location prior to
sampling and an 'as-found' record photograph. These
details should be made available to the petrographer if
he/she was not present during the sampling operations.
are soft, heat sensitive, and/or water sensitive, thin
section preparation presents considerable challenges for
the technician.
Thin section making starts with oven-drying the
sample at a temperature that will not damage heat-
sensitive materials (lower than 60ºC). Once dry the
specimen is vacuum impregnated with a low viscosity
epoxy. The resin is usually coloured with a dye to assist
in the determination of porosity and cracking when the
thin section is microscopically examined. The colour of
this dye would normally be fluorescent yellow except
when investigating the thaumasite form of sulfate attack
(TSA) in concrete, in which case blue dye is a better
choice. Following impregnation the resin is heat cured
in an oven or on a hot plate, again at <60ºC. The cured
sample block is then ground using a diamond cup wheel
to expose the examination surface.
At this stage the 'thin-sectioning machine' is first
used. The surface for examination is finely ground using
the thin-sectioning machine, cleaned, and mounted on a
glass slide using epoxy glue (either heat or ultraviolet
light curing). The excess sample is then cut off using a
special diamond saw with a vacuum chuck that holds the
slide in place (
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). All cutting and grinding must be done
in oil or alcohol rather than water to avoid damage of
water-sensitive materials. The thin section is then finely
ground to the finished sample thickness on the thin-
sectioning machine and finished by gluing a thin glass
cover slip over the sample.
T
HIN SECTION SPECIMEN PREPARATION
High-power microscopical examination requires the
preparation of petrographic 'thin section' specimens
comprising ground slices of the sample mounted on glass
slides, through which light will pass to allow
microscopical observation. The thin section making
technique was developed by geologists for the study of
rocks. When applied to other construction materials that
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Obtaining a core sample from a reinforced
concrete structure by diamond drilling.
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A thin section cut-off and trim saw. (Courtesy of
Logitech Materials Technologists and Engineers.)
