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2.1 Semi Dry Production
The semi-dry method, as the name complexity it is possible to produce up to
implies, involves the use of a low water 80 units per day from one single mould
content or “earth-moist” mix. Semi-dry reducing mould commissioning costs and
products can either be manufactured lead times.
using a through (facing) mix or employing
a backing and facing mix. Facing mix is In general, dry cast units need no surface
used for the complete element if the shape treatment after de-moulding. If a more
is complex, or if the units are typically pronounced or “grainier” texture than the
less than 75mm thick. The facing material stone like, as-struck finish is called for,
determines the long term performance this can be achieved after de-moulding. If
of the cast stone and requires careful the designer or user requires to replicate
design. It must include carefully selected a natural stone “tooled” finish i.e. striated,
aggregates, graded such that thorough rock-faced etc. this can be achieved by
compaction is attainable thus yielding a reproducing the pattern in rubber or epoxy
strong (35-50 MPa) dense product. It must resin and then manufacturing the cast
also incorporate an integral waterproofer, stone elements from the pattern.
usually a metallic stearate to ensure low
absorption and long term performance. It is essential that the initial cure is carried
out in a controlled environment. This
Compaction of the semi-dry material environment must be protected from direct
is usually achieved using either sunlight and drying winds. An effective
pneumatic or electric sand rammers. curing regime determines the development
The efficiency of this process is very of strength and long term durability. It
important in determining the final will also reduce surface shrinkage and
quality. Standard ashlar units can be therefore crazing and improve resistance
manufactured using hand- operated to weathering and abrasion.
compression machinery or, for very
large contracts, fully automated plant. BS 1217:2008 states “Cast stone should
not be transported or installed before it
Units are generally cast face down to is 14 days old unless accelerated curing
ensure maximum compaction of the processes allow a reduction of this time”
finished surface. In two-part casting the and this period may need to be extended
facing mix is compacted to a minimum for structural units. However, some
thickness of 20mm. The layer is scratched UKCSA members employ enhanced
to form a mechanical key for the backing curing techniques to significantly reduce
concrete, which is immediately placed the timescale required from production to
and compacted in successive layers. site, and all UKCSA Members are required
Inter-diffusion is an alternative mechanism to achieve a minimum level of curing as a
for ensuring satisfactory bond between condition of their membership.
successive layers of materials and is
dependent upon successful diffusion of The semi-dry process is most suited to
two loosely filled layers under force from traditional sized products such as cills,
the compacting hammer. Both methods heads, string courses, cornice, copings
ensure that the product is effectively etc. Structural items can be produced so
homogeneous. long as the reinforcement requirement
does not inhibit compaction. The ability to
After compaction the products are often reinforce dry cast stone offers a distinct
demoulded immediately. Subject to advantage over natural stone.
