Architectural Precast Finishes
Decorative finishes for architectural precast concrete products combine the traditional skills of the concrete artisan, the design skills of the professional engineer and the practical experience of Wilco Precast.
Knowing that he has these skills at his disposal, the architect can direct his efforts towards combining shapes, textures and colours to achieve the desired effect on the structure and its surrounds, and accordingly, on the environment. Greater emphasis is now being placed on achievement of the architect’s concept, self-cleaning facades, energy conservation and maximum quality rather than minimum cost.
The following finishes for precast concrete have been selected on the basis of:
- practical precasting
- architectural appeal
- application to precast units which will provide structural solutions to project requirements
Finishes considered include:
- off-form finishes
- chemically retarded exposed aggregate
- acid-etched finishes
- honed or polished
These finishes have been selected as the most useful group of finishes that the Wilco Precast can offer to architects and clients. The list is obviously not intended to cover all finishes nor is it intended to suggest limitations in developing new finishes.
Mix design, careful selection of materials and the need for practical and innovative mould design are of vital importance in achieving pleasing results for the architect, satisfactory structural solutions and a profitable result for Wilco Precast.
A number of factors require consideration in the preparation of mix designs and trial mixes for all finishes. These include:
- finish required
- shape and size of the units
- workability required to allow the mix to be placed and consolidated in difficult areas
- the required compressive strength (usually governed by Wilco Precast’s requirement for a 24-hour casting cycle)
- durability requirements (water absorption) freeze thaw conditions, exposure to salt water spray, etc
- trial mixes should be carried out using techniques and finishing methods that follow proposed production methods as closely as possible.
The concrete produced must provide the required surface finishes and comply with the appropriate standards and project specification. All materials should be tested for compliance with the appropriate codes and for any inclusions that could cause long-term staining. Stockpiling aggregates is recommended to ensure uniformity.
Whilst a smooth off-form finish may be one of the more economical finishes, the production of off-form concrete to a consistent colour and standard of finish will require a high degree of production control if problems are to be avoided.
Retarded (Exposed Aggregate)
Retarded (Exposed Aggregate)
Retarders are painted on to the mould face to chemically delay the surface set of the concrete so that the aggregate may be exposed after stripping. This is achieved by washing and brushing the retarded faces. Above photo shows exposed aggregate cladding panel at window opening.
Retarders are available in a range of formulations to give different depths of exposure. A dense, round or crushed aggregate is useful and attractive.
Plastic, timber or rubber formliners can be used to provide a variety of surface textures and shapes. Trials to establish the mix and aggregates which provide the required appearance on the broken surface are essential. Full scale trials are essential to achieve a satisfactory result.
An almost unlimited variety of attractive patterns, shapes, and surface textures can be achieved by casting against wood, steel, plaster, elastomeric, plastic, or polystyrene-foam formliners. They can be incorporated into or attached to the surface of a mould.
Ribbed or fluted panels demand considerable attention to detailing as panel sizes and distances between openings must be a multiple of the rib spacing.
Panel joints should normally be in the bottom of a groove or valley. Panels can be produced with vertical ribs or striations in a range of sizes to suit a particular structure and the distance from which it will most often be seen.
For large wall expanses, liner size and characteristics may require that an architectural feature-in the form of a demarcation groove, recess, rib, or plain area-is detailed to hide joints between liners. Otherwise, their use should be limited to less than the available width of the liner, or the liner joints should be designed at form edges.
The cost of liners depends upon the ease of use and the number of reuses obtained. Regardless of the formliner used, draft must be considered to prevent chipping or spalling, during stripping of the unit from the mould.
Multiple Finishes on One Panel
It is not uncommon to have panels with more than one finish. Typical is any combination of polished, honed, grit-blasted, off-form and painted.
There are practical problems to be resolved with such combinations. For instance: surfaces in the same plane as and immediately adjacent to polished or honed finishes will need to be set down so that they will note be abraded by the polished process; finishes not grit-blasted will need to be protected during the blasting operations, etc.
Wilco can supply feasibility and cost advice for these finishes. Photo shows panel with two sizes of exposed aggregate separated by wide rebates.
Painted (Coated) Finish
Paint, in a variety of textures, is often used as a site-applied finish to precast elements. It is most commonly used on smooth (off-the-form) surfaces.
Gloss finishes are to be avoided due to the high cost of surface preparation necessary to provide a satisfactory appearance.
High-build paint is recommended unless very high mould standards are achieved. Although paint finishes are not as durable other finishes, they do provide the opportunity to change the appearance of a building by repainting as well as providing a limitless colour choice.
In some instances, such as access problems once installed, the panels are painted prior to being delivered.