Geoscience Reference
In-Depth Information
These have been chosen to illustrate how petrography
can be applied to concrete products, rather than being
an exhaustive account of all product types.
material types, dimensions, manufacturing marks, colour,
and the relative hardness and coherence of the material.
The study of defects such as cracking may be enhanced
by additional low-power microscopical examination of
a finely polished slice. High-power examination of thin
section specimens is then used to determine the grading
and mineralogy of the aggregate, cement type, the
presence of mineral additions/pigments, assess the
quality of workmanship, and screen the material for
evidence of distress or deterioration.
PETROGRAPHIC EXAMINATION
Petrographic examination of concrete products is
performed in accordance with ASTM C856 (ASTM
International, 2004). Once a sample is received in the
laboratory, an initial visual and low-power microscopical
examination is conducted to determine the number of
ARCHITECTURAL CAST STONE
Cast stone units consist of concrete intended to resemble,
and be used in a similar way to, natural stone. The
desired colours and textures are obtained by blending
white and ordinary Portland cement with inert pigments,
and by the use of aggregates of appropriate colour and
texture. The aggregate used is often crushed rock derived
from the same natural stone that the cast stone is
intended to imitate. Units may be homogeneous
throughout or consist of a facing mix and a backing mix.
It is common practice only to have a 20-30 mm
thickness of the expensive decorative facing mix on a
backing mix of cheaper normal concrete.
The units are manufactured either by a 'semi-dry'
process (also known as 'vibrant dry tamp') or by a 'wet
cast' process. Semi-dry units normally comprise a facing
mix and a backing mix. However, if the shape of the
element is complex or it is less than 75 mm thick, the
facing mix is used throughout. The semi-dry process
involves the use of a low water content ('earth moist')
mix that has relatively poor workability. It has to be
mixed using a forced action mixer and compacted into
moulds using pneumatic or electric rammers. The benefit
is that the casting can be immediately removed from the
mould, giving high productivity from each mould. Other
advantages include low drying shrinkage and a relatively
open textured finish that resembles certain types of
natural stone and often requires no surface treatment to
achieve the desired finish. However, the porosity of the
open texture can be a disadvantage in terms of durability
and chemical admixtures (usually metallic stearates) are
commonly incorporated to improve the waterproofing
(United Kingdom Cast Stone Association, 2004).
In thin section, semi-dry cast stone typically exhibits an
abundance of small, interconnected entrapped air voids (up
to 30% excess voidage) and may exhibit some 'balling' of
cement caused by workability difficulties. Figures 238 and
239 show a semi-dry cast stone facing mix made from fine
quartz sand and white Portland cement that exhibits a
characteristically high entrapped air void content and
238
238 Cast stone manufactured by a 'semi-dry' process,
showing quartz fine aggregate particles (white), white
Portland cement matrix (dark brown), and air voids
(yellow). An unmixed lump of cement is seen in the
centre of view; PPT, ×35.
239
239 Close view of cast stone manufactured by a
'semi-dry' process, showing white Portland cement
matrix (brown) including an abundance of unhydrated
white Portland cement grains. Quartz fine aggregate
particles appear white and air voids are shown
yellow; PPT, ×300.
 
 
 
 
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