Colored concrete reaches new heights

Jan 1, 2003 12:00 PM

Requiring in excess of 26,000 yd., the Cathedral of Our Lady of the Angels, plus adjacent bell tower and plaza, constitute the largest project to date involving integrally colored concrete. Unlike many gray concrete structures, the Cathedral, part of a five-building complex comprising the new ecclesiastical center for the Catholic Archdiocese of Los Angeles, has an earthy golden hue reminiscent of the adobe used to build the region's first mission churches. The exposed colored concrete provides a rich finish for both interior and exterior walls.

Though its “mission adobe” palette suggests a nod to the past, the Cathedral — dubbed the “Half-Millennium Cathedral” — was designed with an eye to the future: five hundred years of service was set as the performance criterion for the structure. Accordingly, integral color was selected since the iron oxide pigments that tint the concrete are permanent and fadeproof.

The concrete pigments for the Cathedral were supplied by Davis Colors. According to Davis Colors Vice President Nick Paris, “Iron oxides are the same pigments that give real adobe its buff tone and provide the color in such natural wonders as the Grand Canyon. The earth tones of the Cathedral will last as long as the concrete itself. By using integrally colored concrete, the Cathedral will be able to avoid the ongoing maintenance associated with paints and surface-applied coatings.”

DESIGN APPROACH

From early in the project, the Cathedral's designer, Spanish architect Jose Rafael Moneo, envisioned the use of cast-in-place concrete — a choice reflecting his experience in Europe and his interest in expressing the character of the material from which a building is made. When another design option was proposed, i.e., to clad a steel frame with prefabricated panels, Moneo rejected the approach, stating that he “wanted to see the hand of the craftsman” in the finished construction.

True to the architect's vision, the concrete shows subtle nuances in appearance due to the complex behavior of cementitious materials. Paris explains, “Concrete, whether colored or not, is a natural material prone to variation in appearance. Instead of being seen as defects, these variations are part of the aesthetic appeal of concrete, expressing its character and vitality.” Indeed, many visitors to the Cathedral assume that it is constructed of quarried stone because the colored concrete has such a natural look, he adds.

Among challenges facing the project team were the design of a concrete mixture and establishing construction procedures to meet both the technical and aesthetic requirements of the building. At first, large-scale mock-ups built to evaluate proposed concrete mixes cracked due to heat produced by hydrating cement. The thick walls of the building, up to 60 inches in some locations, had created a heat sink preventing adequate dissipation. Thermal cracking would not be a problem for most buildings; however, in a monument designed to last five centuries, thermal cracks could allow penetration of corrosive chemicals into the structure, thereby causing premature deterioration.

The conventional solution for this type of problem is using cement and admixtures that release less heat. As such materials were either not available from local suppliers or were too dark in color to produce the desired concrete shade, another approach had to be considered. Cooling the structure by spraying it with water as the concrete cures was also unacceptable, since concrete can pale if exposed to water before it has set sufficiently.

CONCRETE MATERIALS

Eventually, the project team developed a mix to satisfy all of the construction requirements. A white portland cement with low heat of hydration was imported from Denmark. To increase the concrete's strength and corrosion-resistance, fly ash was specially processed to assure a consistent, light-color pozzolanic material. Similarly, fine and coarse aggregates were selected for color compatibility with the desired shade.

Color additives were custom blended by Davis Colors to meet design specifications. The pigments were packaged in premeasured sacks to simplify batching at the ready mixed plant. Pouring the Cathedral walls over an entire year, a key to maintaining color consistency throughout the pour schedule was to procure all concrete mix materials at the same time. Thus, inconsistencies were avoided that might have developed if material sources changed during the course of the project. Catalina Pacific, the concrete supplier, ordered most materials in bulk quantities and stored them in silos dedicated to the project. Because a Davis Colors factory was located just a few miles from the producer's plant, color additives were warehoused for just-in-time delivery.

CONSTRUCTION TECHNIQUES

Color consistency also depended upon the high level of workmanship provided by the contractor, Morley Construction Company. As the multi-faceted project has very few 90-degree angles, formwork had to be constructed with the painstaking care typical of furniture making. A double layer of overlaid plywood form panels served to keep walls flat and resist the pressure of 12-ft.-high concrete lifts. The double layer also facilitated screw mounting face panels from the back, keeping the face of the forms smooth and free of fasteners that would have left impressions in the concrete. Joints between form panels were sealed to assure that cement paste would not leak through, leaving blemishes on exposed surfaces. Just prior to placing concrete, forms were carefully cleaned and treated with a thin, uniform application of form release. Forms were left in place several days after concrete placement until thermocouples, installed inside critical pours, indicated that the walls had cooled sufficiently to avoid thermal shock.

Special attention during concrete placement was instrumental in alleviating conditions that could have produced surface defects. For example, congested reinforcing steel — detailed for seismic conditions — could have blocked the passage of coarse aggregates, creating segregation. At sloped areas, such as the giant concrete cross above the altar, placing the concrete as close as possible to its final position prevented the formation of flow lines. Evidence of the extraordinary care and craftsmanship applied by Morley's crew to the project is obvious: almost no construction defects mar the exposed concrete.

The cured concrete was given a very light sandblasting. On the exterior, just enough sandblasting was performed to dull the sheen left by the formwork. On the interior, slightly heavier sandblasting imparted a lightly etched grain to the surface. Overall, according to Nick Paris, sandblasting helps create a more uniform appearance that “besides removing some superficial stains, produces a roughened surface that scatters reflected light to soften the impact of mottling.”

Finally, the concrete was treated with siloxane water repellent, a clear, penetrating chemical that does not alter the appearance of concrete. Siloxane will be reapplied at intervals over the life of the building to help keep the structure clean and assure continued protection against water intrusion.

Even with heightened attention given to quality assurance on the project, colored concrete proved an economical solution as it provided both structure and finish for the building. Once past an initial learning curve, construction moved swiftly, allowing the Church to meet its move-in schedule and consecrate the Cathedral in September 2002.

Prepared on behalf of Davis Colors by Michael Chusid, Chusid Associates, 818-774-0003