Cast-on Finishes ~ these finishes include split blocks, bricks, stone,
tiles and mosaic. Cast-on finishes to the upper surface of a
horizontal casting are not recommended although such finishes
could be bedded onto the fresh concrete. Lower face treatment is
by laying the materials with sealed or grouted joints onto the base
of mould or alternatively the materials to be cast-on may be
located in a sand bed spread over the base of the mould.
Exposed Aggregate Finishes ~ attractive effects can be obtained by
removing the skin of hardened cement paste or surface matrix,
which forms on the surface of concrete, to expose the aggregate.
The methods which can be employed differ with the casting position.
Horizontal Casting † treatment to the upper face can consist of
spraying with water and brushing some two hours after casting,
trowelling aggregate into the fresh concrete surface or by using
the felt-float method. This method consists of trowelling 10 mm of
dry mix fine concrete onto the fresh concrete surface and using
the felt pad to pick up the cement and fine particles from the
surface leaving a clean exposed aggregate finish.
Treatment to the lower face can consist of applying a retarder to
the base of the mould so that the partially set surface matrix can
be removed by water and/or brushing as soon as the castings are
removed from the moulds. When special face aggregates are used
the sand bed method could be employed.
Vertical Casting † exposed aggregate finishes to the vertical faces
can be obtained by tooling the hardened concrete or they can be
cast-on by the aggregate transfer process. This consists
of sticking the selected aggregate onto the rough side of pegboard
sheets with a mixture of water soluble cellulose compounds
and sand fillers. The cream like mixture is spread evenly over the
surface of the pegboard to a depth of one third the aggregate size
and the aggregate sprinkled or placed evenly over the surface
before being lightly tamped into the adhesive. The prepared board
is then set aside for 36 hours
to set before being used as a
liner to the formwork or mould.
The liner is used in conjunction
with a loose plywood or
hardboard baffle placed against
the face of the aggregate. The
baffle board is removed as the
concrete is being placed.
Concrete Surface Finishes