Architectural Precast Economy

Architectural Precast Economy

Understanding architectural precast costs is essential to designing elaborate façades that enhance the overall building design while meeting the owner’s budget. Understanding the architectural precast manufacturing process can help achieve design goals and control costs.

Many variables need to be considered to determine what a typical architectural precast project will cost. All engineering, production, delivery and installation costs must be compiled for each specific project to derive an applicable budget price.

During a project’s conceptual stage, the designer has many items to consider. These include material selection, textures, surface geometry, cross section, unit repetition and erection methods. The custom, sculptured designs that are possible with precast concrete may be achieved within a limited budget by selecting economical aggregates and textures combined with repetitive units and effective production and erection details.

Wilco Precast can assist with preliminary design and budget estimating early in the project’s design phase.

Reasons not to “guestimate”

During a project’s preliminary design, a precast project can be budget “guesstimated” on a square-metre basis. Although this provides a good starting point, it is not recommended that designers rely on this method alone for several reasons:

A project’s square-metre quantity take-off can differ between precast manufacturers, general contractors and architects, depending on the take-off procedures used.

Square-metre prices are rough, educated guesses based on incomplete information.

Work-scope criteria (specifications, etc.) typically are non-existent in the early stages.

Erection, access and crane requirements are not defined early in a project.

The necessity of back forming and other detail manufacturing requirements cannot be predicted accurately in the early design stage. Working with Wilco on the specifics will help determine a final budget that is more accurate. A lump-sum budget price from the precast manufacturer, submitted in writing (including assumptions), will minimise surprises on tender day.

As a project evolves from preliminary sketches through working drawings, the precaster should be informed of all changes to ensure the budget prices remain valid. Pricing accuracy depends on the information provided to the precaster’s estimator.

Key Factors in Architectural Precast Economy

The key factors in designing economically with architectural precast are the repetitiveness of pieces, average piece size and erection efficiency.

A key element to cost-effective production is to minimize the number of moulds and mould changes and to maximize the number of castings from each mould, particularly if the moulds have shape. Efficiency is achieved by making it possible for similar, if not identical, shapes to be produced from the same basic (master) mould and by minimizing the time required to disassemble a mould and reassemble it for the manufacture of the next piece.

Regardless of the material used, moulds can be expensive to construct. An individual mould’s complexity determines its cost.

Simple reveals and rustications typically are considered a standard mould cost. Reveals and rustications must be placed in a repetitive pattern to minimize modification throughout a mould’s life. Reveals, like all form features, must be designed with a small draft (by creating bevels) so the panel can be stripped from the form without damaging the form feature. Adding more intricate features introduces cost premiums to a project. Only your local precast manufacturer can calculate these mould-cost premiums. The exact size, shape and locations are the designer’s options. However, repetition must be considered when applying these design features.

Considerable cost will be added if the location of these features within a mould will be changed frequently. On the other hand, these intricate features can be added at a minimal overall cost if they are used repetitively in the overall design. The point behind designing repetitive pieces is to amortise engineering and mould costs effectively. As many pieces as possible should be designed to be cast in the same mould and produced from a single shop drawing.

Repetition Mould Costs

A master mould can include numerous design elements such as bullnoses, reveal patterns or window openings. However, once in place, the design elements should be consistent (repetitive) from piece to piece. But it is important to remember that individual castings do not have to be the same size.

A master mould can be slightly modified throughout the production cycle to give the designer maximum flexibility. This strategy eliminates the need (and cost) of constructing a mould for every panel change.

When a large number of precast concrete units can be produced from a single mould the cost per square metre will be more economical. A large number of panels can be produced from a single mould, built to accommodate the largest piece, and then subdivided as needed to produce the other required sizes.

Although every project will have atypical conditions, the more cost-effective projects maximize the repetition of elements. The more often a mould is re-used, the lower the cost of the piece and thus the total project.

The premium cost for complex shapes can be controlled by adding details to specific forms only. Optimum production economy is attained if the panel can be separated from the mould without disassembling the mould. This is accomplished by providing draft or slope on the sides of all openings and perimeter sides. Designers are urged to consult with Wilco for specific draft recommendations.

All architectural precast panels are produced face down to give the maximum aggregate consolidation at the panel surface and to achieve the smoothest finish. Two-sided precast pieces (front and back) requiring identical appearances should be avoided. The most expensive forming technique is back forming. Back forming is used to create returns that give the appearance of thick, massive panels that add significant shadow features to the façade. These returns also can allow windows to be set back away from the building’s face from 150 mm to 600 mm. To achieve these shapes, special forms must be constructed and then suspended over the primary mould to create the desired panel desired panel depth per square metre. A second production method to make returns is a two-part pour. The return piece is produced on Production Day 1. On Day 2, the return piece is removed from its form and is connected to a master mould. The return is cast monolithically to the master piece. Two-part pours are preferred over the method described above because they create a more uniform texture on all sides of the panel. The required number of moulds of a given type for a project often is determined by the time allowed for completing the job. In many cases, this time factor to meet the project’s schedule is what creates the demand for duplicate moulds, trumping the desire for mould economy.

The necessity for extra moulds increases costs and partially offsets the intent of designing for high repetition.

The designer should discuss realistic precast engineering and production lead times for the project with the precast manufacturer. It is vital to include precast-scheduling information with the tender documents. This will ensure that all bidders understand the project time frames required.

Ample lead-time also will allow the manufacture of larger pieces first, followed by smaller ones, thus minimising the cost of form repairs.