EXPOSURE BY WATERWASHING
A dense, decorative, rounded or crushed stone should be selected. These may include natural river gravels, crushed river gravel or crushed rock such as granite of which there is a wide range of colours. Aggregates should be selected for their decorative nature (colour) and shape. Rounded or cubical aggregate shapes are preferred since elongated or flaky-shaped material may not provide uniform appearance between horizontally and vertically cast surfaces.
Hanson Precast will use a mix which allows the surface to be washed to the natural depth implied by the coarse aggregate size. Using conventional mixes may lead to uneven exposure.
Aggregate size should also be compatible with the dimensional area to be exposed. The larger the aggregate, the more difficult it will become to accommodate returns, reveals etc. It is good practice to blend the matrix, using cement's and colours and, if necessary, a colouring oxide to approximate the matrix colour to that of the exposed stone. Such treatment will mask any unavoidable minor variation in aggregate distribution.
Whilst ideally suited to flat panels, cast face up, this method of finish can be applied to panels incorporating window openings. When elements incorporating a return face are required, there may be some differences between the horizontally and vertically cast faces. The vertical face may be either retarded and water washed or washed without retardation. The latter approach is achieved by sliding-off the mould face in order not to create suction at the surface, after initial set has occurred.
EXPOSURE BY CHEMICAL RETARDATION
Retarders have been notoriously unreliable and Hanson Precast advises against the exposure of whole panels by retarder. There are alternative methods not so fraught with risk. Retarders are very useful, however, for difficult returns where alternative methods cannot be used.
EXPOSURE BY GRIT/WATERBLASTING
This finish is generically referred to as sandblasting. Sandblasting is a medium cost way of achieving an excellent architectural finish. It is a technique requiring considerable skill in preparation of shop details, in mix design, in pouring technique and in the blasting operation itself.
The choice of abrasives depends on the type of architectural finish required and is also influenced by OH & S regulations and local availability. You can rely on the experience of Hanson Precast to make these decisions - the specification should concentrate on achieving the required architectural effect.
While sandblasting is used on returns and other parts of panels not poured horizontally the finish on such surfaces will often not be a perfect match for the horizontal surfaces. This is usually not a problem but should be incorporated into the approval process.
Sandblasting is always followed by a light acid wash to provide an even clean finish. Sandblasting may be combined with other finishes in the one panel. Off-form, polished and painted finishes are often used in conjunction with sandblasting (see Multiple Finishes).
EXPOSURE BY HONING AND POLISHING
The rule for economy in polishing is straightforward: maximise machine work, reduce hand-work. Hanson Precast polishes with automatic granite polishing machines with planetary heads. We also have an edge polisher for smaller returns and machines for finishing chamfers. Where machines cannot physically be used then polishing with hand held grinders must be used. This is expensive but cannot always be avoided.
Consequently polished surfaces should, for maximum efficiency, be large, flat and readily oriented beneath the polishing machine. Returns should not terminate in re-entrant corners. Reveals, false joints and awkward profiles, should ideally be left 'off-form', etched or sandblasted.
But the architectural dictates of a project will often mean that:
FIGURE 1 Some practical issues to consider when detailing honed and/or polished exposed-aggregate finishes