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
  • grit-blasted
  • acid-etched finishes
  • honed or polished
  • formliners

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.

Off-Form Finishes

Off-Form Finishes

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.

Cement, and sand colour to a lesser extent, will control the final colour of the unit, with the fine aggregate contributing to some extent. The colour of the coarse aggregate may not be significant unless the particular unit requires extremely heavy vibration, in which case some aggregate transparency may occur, causing a blotchy and irregular appearance. Grey cements are more likely to cause colour irregularity than are off-white or white cements.

Moulds need to be carefully designed to ensure against any leakage whatsoever. Wherever possible, tapers should be built in so that the mould can be permanently sealed. After each casting, meticulous cleaning will be necessary, followed by regular burnishing with steel wool (assuming a steel mould).

A very thin coating of mould oil, usually sprayed on and then wiped with clean rags, will ensure high quality castings from steel moulds.

Off-form units will usually have some surface imperfections. Joints in steel plates, minor variations in the surface texture of a steel casting table, voids caused by air trapped in the vertical mould surfaces, etc, will be obvious on this finish.

As with all other finishes, a visit to the Wilco factory or to a project with a finish similar to that required, should be undertaken before casting commences to ensure that the architect is aware of the surface finish that will be finally achieved.

Off-form units produced with concretes with high cement content may exhibit some surface crazing after curing. This is a surface defect which is usually only visible on careful close examination. However, if exposed to the atmosphere, accumulation of atmospheric grime will accentuate these cracks. Other than affecting the appearance, these cracks do not affect the structural properties of the unit nor its long-term durability.

Uniformity of manufacturing procedures is most important. Procedures that ensure consistent techniques for cleaning the mould, application of mould oil, uniformity of concrete quality, consistent curing processes and careful storage procedures will all contribute to produce a quality off-form product. Handling procedures and materials should be designed to minimise chipping and other damage. A smooth off-form finish is probably the most difficult of all precast finishes to repair. Despite all steps taken to ensure colour consistency, variations will occur from batch to batch. Surface figuring can vary from panel to panel. However, these are all characteristics of precast concrete.

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.

Aggregates should be selected for colour and consistent grading. Round river aggregates are ideal and generally a gap graded mix should be considered. The washing process will remove intermediate sized aggregate from a fully graded mix, thus the density of the finished surface.

The ideal maximum aggregate size for a given project will depend on a number of factors:

  • The washing process will remove a large volume of matrix. The larger the aggregate, the greater the depth of matrix to be removed.
  • The washing process should leave about two thirds of the aggregate embedded.
  • Depth of washing, ie the maximum size of aggregate, affects the unit thickness as the required cover for reinforcement may need to be increased to include the depth of aggregate exposure.

It is desirable to vary the matrix colour wherever possible to match or blend in with the colour of the aggregate. Exposed aggregate panels usually involve a two-layer casting process whereby the finished surface of the panel is cast as a separate layer shortly before the structural concrete layer is applied. This minimizes the amount of expensive “architectural” concrete and improves compaction and control of stone density on the exposed face.

The first layer is usually 20-50 mm thick. The second layer is placed and compacted before the first layer has achieved final set. Research has proven that two-layer casting does not affect durability.

Note that the “decorative” exposed layer is extra to the engineered panel thickness and is not recognised as a structural component of the panel. Eg: a 175 mm structural panel could end up at 200-225 mm thick, depending on the thickness of the exposed aggregate layer.

Formliners

Formliners

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.

These rules should be observed:

  • Limit depth of design to 10 – 20 mm
  • Keep all edges and corners rounded and chamfered
  • Maintain a 1:8 draft (taper) on all sides of rebates to prevent chipping and spalling during stripping of the panel from the mould

If the concrete is to be left as cast, without further treatment, its appearance will be determined by the surface characteristics of the liner material as well as by the chosen pattern or texture.

Variations in the absorbency of the form surface will produce corresponding variations in the color of the concrete (darker colours are associated with water loss). Sealed, textured plywood, and rough-sawn timber planks are useful in creating rugged textures.

Resultant surface texture may also be obtained by use of other liners reproducing this finish.

When preformed plastic or rubber formliners are selected, the pattern should be described and a reference to the pattern and its manufacturer specification should be included in the architectural specification.

Sculptural designs have been produced using sections of foamed polystyrene or polyurethane as formliners or inserts. Abstract patterns and deeply revealed designs with undercut edges can be shaped easily in these materials. However, these liners are typically single-use only.

Computer-controlled, hot-wire cutting devices have made custom work in polystyrene available at moderate prices. Urethane (rubber) liners are useful for finely detailed textured or profiled surfaces with some undercuts (negative drafts), because they greatly facilitate stripping.

If other materials were used for such detail, the forms would be virtually impossible to strip. Liner size and module should be coordinated with panel joints, rustication strips, and blockout size.

The application of recessed lettering in concrete is no different from that of any other incised element. Appropriate draft or taper for stripping must be established for all lettering unless characters are flexible or destructible. Thought should be given to the selection of the letter profile or cross section. Observing the principles of shades and shadows and selecting a profile will give sharp, smooth, and regular shadows.

Raised letters are fragile and subject to chipping at traffic levels and significantly increase forming costs. Plastics such as polypropylene or acrylic are ideal for lettering applications as they are easily machined to design on a CNC router and give excellent release from the concrete after casting.

Multiple Finishes

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 Finish

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.

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