by John Sullivan III
Poured-in-place architectural concrete was considered uniquely suited to complement the plasticity of the building form and satisfy the technical resolution of the cantilevered volumes and long span of the canopy floor. We had developed considerable experience in the use of architectural concrete as a visually pleasing and economical material for the construction of designs which integrate structure with the building envelope. By this time we had built more than twenty buildings of architectural concrete in many mixes and finishes; some were in upstate New York. An extensive investigation into the sources of concrete mix materials in the Finger Lakes region was undertaken at the onset of the project, in conjunction with on-going concrete research by several of our architects. Test samples and material analyses were produced specifically for the Museum, resulting in the selection of the buff-colored mix for all exposed concrete surfaces to complement the surrounding masonry buildings. The hidden floor slabs and below-grade walls were constructed of a normal structural concrete.
The modifying term architectural defines a form of finished concrete with a more controlled mix, a more refined surface, and a more exacting tolerance than structural concrete. The construction key words are cleanliness, consistency, control, and precision—a challenge for a material composed of several components and subject to many variables. Concrete is a combination of sand and small coarse stone aggregate mixed with cement and water. Architectural concrete stresses the color and texture of the finished surface, and the stone, sand, and cement types are selected and balanced to achieve that goal. The method of mixing and pouring the batter-like concrete must be highly controlled as color and value may change in each pour sequence. Water must be pure. Temperature and humidity changes can affect the results unless considered in advance. Architectural concrete is poured into special formwork, constructed of fine boards or specifically sized panels used to create a surface pattern, and held together by steel “ties,” whose placement is visible on the concrete surface as a residual grid of small circular recesses.
A system of narrow (three-inch) tongue-and-groove boards of Douglas fir was used for the Museum formwork to create a light pattern which would not compete with the building form or details. The individual boards were assembled into units (typically in nine-foot wide segments) relating to the 4’-6” building module, and stretching from floor to floor. This systematic and practical approach allowed for the re-use of the forms. Several nine-foot units would be ganged together to form one pour; in the case of the north and south walls of the tower this meant a form almost the full width of the building and 12’-6” or 20’-5” feet in height, to make certain the full expanse of each floor would be of the same color and tone of concrete. The strong reveal line at each floor provides a visual break to minimize the variation between the consecutive vertical pours; architecturally, it is a rhythmic scaling device giving order to the wall. The board pattern helps to mitigate small imperfections in the concrete, and also provides for visually splicing adjacent pours. The location of the pour joints, determined by the building design in conjunction with a concrete pour of reasonable size and configuration, was an extensive study in itself and was worked out with the contractor.
Our concrete experts maintained their involvement during the construction phase, and were present on site as necessary for assistance. The surface of the concrete was left without any further finish after the forms were removed, and it appears today much the same as it did then. It was of benefit to the process and the result that the contractor who had built the Everson Museum in bush-hammered concrete, William C. Pahl Construction Company, was the contractor for the Johnson Museum. Harold Uris, University trustee, and builder, commented during a building tour with Sam Johnson that the concrete was the best he had ever seen.
continues with Finishes and Systems
Originally published in A Handbook of the Collection (Ithaca: Herbert F. Johnson Museum of Art, Cornell University, 1998), 29–40.