Geoscience Reference
In-Depth Information
PETROGRAPHIC EXAMINATION AND
COMPLEMENTARY TECHNIQUES
elemental chemical analysis either of selected points, or
an area up to 30 mm × 15 mm ('elemental mapping').
XRD requires a powdered sample and, for crystalline
minerals, actually identifies the minerals present, rather
than just giving an elemental composition. Thermal
analysis methods such as differential thermal analysis
(DTA), thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) are occasionally used for
analysing cement and concrete. Infrared spectroscopy is
used for identifying and quantifying organic materials
present in concrete.
Petrographic examination of concrete is performed in
accordance with ASTM C856 (ASTM International, 2004).
At the time of writing a European standard was being
developed but was not yet available. Useful reference
handbooks for concrete petrography have been written
by St John et al . (1998) and Walker et al . (2006).
Following arrival in the laboratory, core or lump
samples are first examined in the as-received condition,
both with the unaided eye and using a low-power
binocular microscope at magnifications of up to ×100.
Finely ground slices of concrete may be prepared to aid
the visual/low-power microscopical examination and/or
for micrometric determination of the mix proportions by
point counting. This initial examination is used to
observe macroscopic features such as coarse aggregate
type, cement matrix colour, relative hardness, mixing,
compaction, and macrocracking. It also allows selection
of the most appropriate location for thin sections to be
taken for further, more detailed high-power
microscopical examination. Owing to the presence of
coarse aggregate, the minimum size of thin sections
should be 75 mm × 50 mm and larger sizes such as
100 mm × 75 mm are desirable. For most investigations,
two thin sections are sufficient, one from the outer
surface and one from the interior at depth. It should be
noted that the outer 50 mm of concrete elements are not
considered to be representative.
Thin sections are examined in plane-polarized or
cross-polarized transmitted light using a medium- to
high-power petrological microscope at magnifications
typically up to ×600. This is used to determine the
mineralogy of the aggregate, the cement type, the
presence of mineral additions/pigments, assess the
quality of workmanship, and screen the material for
evidence of distress or deterioration. Fluorescent dye is
usually added to the consolidating resin during sample
preparation to aid the examination of cement matrix
microporosity and cracks/microcracks, when the
specimen is viewed in ultraviolet light (fluorescence
microscopy). To determine if sulfate-resisting Portland
cement is present, it is necessary to prepare a highly
polished and etched (with hydrofluoric acid [HF] vapour
or potassium hydroxide [KOH] solution) specimen for
examination under reflected light.
A range of complementary methods is available to the
concrete petrographer. The most commonly used are SEM
and XRD. SEM requires highly polished specimens to be
prepared or alternatively a thin section without a glass
cover slip. SEM allows closer microscopical examination
and the on-board EPM system provides inorganic bulk
INVESTIGATING THE COMPOSITION
AND QUALITY OF CONCRETE
O VERVIEW
Good quality of concrete mix design, ingredients used,
and workmanship are fundamental to achieving strong
and durable concrete structures. Petrographic
examination is routinely used to determine concrete
composition, either to check compliance with a
specification or to provide baseline information for asset
engineering. By petrographically examining a concrete
sample, it is possible to observe directly the geological
type and characteristics of coarse and fine aggregate, the
cement type used, and the presence of mineral additions,
fillers, and fibres. The presence of concrete ingredients
that cannot be directly observed may be suggested by
observable properties that they impart to the concrete
(e.g. air-entraining chemical admixtures). Micrometric
analysis of petrographic specimens can be used to
determine the cement content, air void content, and
water/cement ratio (W/C) of hardened concrete.
Workmanship issues that can be assessed include the
adequacy of mixing, effectiveness of compaction, and
curing (Ingham & Hamm, 2005).
A GGREGATES TYPES AND PROPERTIES
Aggregate makes up the major part of concrete (typically
75% by volume) and the aggregate properties have a
considerable affect on its engineering and aesthetic
characteristics. In Europe, concrete aggregates are specified
in accordance with EN 12620 (British Standards Institution,
2002), while in America, ASTM C33/C33M (ASTM
International, 2008a) is used. Most concrete mix designs
specify an aggregate that is continuously graded (well
graded) in order to minimize the interparticle space to be
occupied by the cement. An aggregate from a single source
with a suitable grading may be used as an 'all-in'
aggregate. However, it is more common blend a coarse
aggregate with a fine aggregate; the dividing line between
coarse and fine being set at 4 mm particle size for CEN
standards and 4.75 mm for ASTM standards.
 
 
 
 
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