The Boston College campus is known for Gothic early 20th century architecture. When designers were brought in to restore the one-hundred-year-old St. Mary’s Hall, they knew they would need to faithfully replicate every detail of the building’s cast stone facade—but in a material that would be better able to withstand the long, cold Boston winters. They chose precast concrete to do the job. “The biggest advantage in using precast over other materials is the great plasticity of it, which allowed for a wide range of details, as well as the quality of the process itself.” says Robin Larouche of precast concrete producer BPDL Béton Préfabriqué in Alma, QC.
It would not be easy. To achieve the goals of the project, the precast concrete producer had to be certain that the team could reproduce every piece and feature on the building as close to the original as possible, says Wendall Kalsow, president of McGinley Kalsow & Associates, the restoration architect on the project. “There was a tremendous number of unique pieces which needed to be measured, fabricated, and installed,” he says.
The pieces, which include angels, gargoyles, crucifixes, and other gothic elements, all feature complex shapes and details, requiring extraordinarily complex designs. “We had to make sure the replicas would look great aesthetically in terms of form, color, and quality, while also ensuring there would be no damages at the moment of demolding,” Larouche says. And they had to do it quickly. The project included 16,000 pieces, more than 50 of which were museum quality sculptures. All of them had to be reproduced in precast concrete in less than 18 months to meet the mason’s schedule.
Before the molds could be created and placed, restoration artists needed to re-sculpt each piece in clay to simulate the natural stone-tooling and to be sure the replicas exactly matched. As Kalsow explained, many different techniques were required to replicate the unweathered appearance of the original pieces. He says. “Some pieces required re-sculpting in an artist’s studio with clay, other sculptures could be replicated by repairing the original pieces with high-density liquid resins, and others required intricate birch plywood molds.”
The pieces were then manufactured with a light aggregate to prevent the cast stone from appearing dark as it weathers over the next hundred years. To improve the durability and long-term performance of the restored facade, the precast concrete producer engineered a new anchoring system, and some small units were combined to create larger units with false joints. Each unit was numbered to identify its location in the wall.
One unique advantage of using precast concrete stone was the ability to combine the 17 individual stones, forming a single unit, Kalsow says. “This large single unit dramatically reduced the time required to install each tracery section, provided a high level of dimensional control, and improved structural performance.”
To meet the tight deadline, masonry work was divided into 22 lifts and many lifts were worked simultaneously to meet the schedule. The relentless commitment to detail and precision resulted in a historic restoration that will stand the test of time, Larouche says. “The quality of the finished project speaks for itself.”