Geoscience Reference
In-Depth Information
minor balling of the cement. The overall visual appearance
in hand specimen is reminiscent of sandstone.
The wet cast process uses similar mix ingredients to the
semi-dry process but with higher water content, giving
good workability. Wet cast units are usually homogeneous
with one mix being used throughout. The mix is easily
compacted using conventional concreting techniques, such
as poker or table vibration. The disadvantages are lower
production rates (as each mould can only be used once per
day) and that the relatively wet mix is more prone to
shrinkage cracking and surface crazing. In addition,
significant work is required to surface finish each unit
because of the formation of cement laitance against the
mould face. The thin laitance layer can be removed by acid
etching, dry rubbing, sand blasting, or wet grinding to
reveal a uniform decorative finish.
In thin section, wet cast stone appears similar to
concrete except for the use of decorative ingredients.
Figures
240
and
241
show a wet cast unit comprising
brown limestone, bound by white Portland cement and
exhibiting a low excess voidage (<3%). The cement
matrix is seen to exhibit microcracking caused by drying
shrinkage (
242
). The overall visual appearance of the
sample in hand specimen is similar to compact light
brown limestone.
Many cast stone units, especially slender ones such as
sills, mullions, and copings, incorporate steel
reinforcement bars. This increases their tensile strength
and is primarily intended to help units withstand the
stresses of handling. Reinforcement should ideally be
embedded in wet cast units or the backing mix of semi-
dry units in order to prevent corrosion of the steel.
As cast stone is in fact concrete, it will weather and
deteriorate as concrete and can exhibit a similar range
of defects. The most common defects seen in cast stone
are mechanical damage during handling, surface crazing,
and reinforcement corrosion. Damage during handling is
usually seen as large cracks, impact marks, or lost corners
that are first visible prior to installation of the unit.
240
240
General view of 'wet cast' cast stone showing
decorative limestone aggregate (light brown/pink)
bound by uncarbonated white Portland cement matrix
(dark brown); XPT, ×35.
241
242
241
Close view of 'wet cast' cast stone showing
limestone aggregate (light brown) bound by
uncarbonated white Portland cement matrix (dark
brown). Unhydrated/partially hydrated cement grains
are sporadically seen (right of centre); XPT, ×150.
242
Close view of 'wet cast' cast stone showing a
microcrack (yellow) caused by drying shrinkage. The
fluorescence of the cement matrix (green) indicates
moderate microporosity and W/C in the normal range;
UV, ×150.