A number of techniques applied post-fabrication can modify the precast finish and provide various expressions of the mix design. Sandblasting yields a softer concrete surface with coarser aesthetics when properly prepared. Finishes of exposed aggregate give a distinctly rough, pebble-like look to the surface and are achieved by the application of a chemical retarder. The retarder slows the hardening of the panel’s surface, allowing the unhardened matrix to be pressure-washed away to expose the aggregate.
Polishing, as the name suggests, leads to a smooth, shining, reflective surface similar to granite. Acid wash involves using acid and high pressure water to blast and etch the surface, leading to a sugar cube-like look—a smooth, sand-textured surface resembling limestone or sandstone. A fossil finish uses a chemical reaction to produce a pattern suggesting fossils;
it can yield a beautiful finish when paired with a carefully selected pigmentation. Of course, multiple finish techniques can be incorporated into the project for creative results.
Another group of finishes originates during the casting process; namely, forms and formliners. A formliner, as the name suggests, lines the precast form. Wet concrete will be displaced by any form that is in the mold. Formliners are fabricated from polymers using computerized machining. Formliners can impart enhanced texture, such as a repeating pattern or single design element, and add depth to the panel face. The use of formliners opens up several options. However, it requires close coordination between the architect and a precaster to ensure the desired form is practical given the concrete mix design, configuration of panels and their joints, and repeatability of image.
Sandy High School in Oregon has a predominantly precast façade reflecting regional culture and natural formations (see page 48). A custom stone formliner for the base of the school and a shiplap liner for the upper portion were developed as a cost-effective and efficient alternative to solid stone. It expressed the aesthetic the architect wanted as well as elements of the Pacific Northwest region in which the building stands without the expense and labor associated with stone.
A variety of more conventional architectural forms enable architects to impart accents and relief into the precast or deliver a repeated pattern across the façade. The forms can be quite simple, such as reveals—a notch of a specific width and depth helping break up a broad expanse of wall or mimic the aesthetic of large stone. Reveals often complement the joints between the precast panels to help turn the joints themselves into aesthetic rather than merely functional elements.
Multiple reveals in the buff-colored face give the impression of cut stone to complement the geometry of the brick façade at Willow Creek Elementary School in Fleetwood, Pennsylvania.
Cornices are horizontal decorative features “crowning” the building, expressing a certain design aesthetic. Bullnoses are smooth, rounded edges that may be a simple rounded corner or a significant protrusion adding interest with light and shadow. Projections such as these are integrated into the mold and frequently repeated from panel to panel.
For the Heights towers, the cornices express the original low, wide Spanish Mission-style roof. In addition to respecting past architectural styles, the project also sought to entice commuter students to live on campus by offering attractive buildings in addition to the views of Manhattan. The panels include bay window projections, reveals, and bullnoses for added visual intrigue and design expression. Without them, the buff finish would have looked flat and sterile.
A fourth group of precast finishes are embedded materials or veneers. These include thin brick, stone, and tile cast into the panels during fabrication. By incorporating these finishes into the panel during the cast, architects can avoid the use of pins or connectors used to secure the stone to the substructure. Precast offers a faster, more durable, and sustainable method to incorporate these finishes. They usually require a formliner with inset areas to locate the brick, stone, or tile. They also have keyed backs to ensure proper placement and spacing. Any space between the embedded materials shows the concrete mix, which resembles mortar.
Piedmont Central Student Housing and Dining Hall at Georgia State University (GSU) features extensive use of thin brick in combination with other finishes (see photo at left). Approximately 2415 m2 (26,000 sf) of cast-in red thin brick complemented nearly 11,148 m2 (120,000 sf) of medium sandblast finish with a limestone color. Selected vertical runs of precast panels were painted blue to reflect GSU’s colors.
The Bloch School of Business in the University of Missouri-Kansas City, Kansas City, Missouri, with 1784 m2 (19,200 sf) of 3.6-m (12-ft) wide, fully insulated composite panels was the first building of such scale to use embedded terra cotta (see page 44). With a five-color random blend, these precast walls were lauded for their effectiveness as rain barriers compared with conventional rainscreen configurations. The precaster used color-coded instructions in the production facility to carefully place each individual piece of tile into the form as directed. Meticulous use of sealants kept the grey precast out of view, leaving the joint as a shadow. This special project required extensive cooperation between the architect, client, and precaster.