In 2010, St. Louis-based McCarthy Building Cos. Inc. was ranked the 9th largest concrete contractor in the U.S. by CONCRETE CONSTRUCTION magazine. This year, McCarthy was chosen as the subcontractor for the St. Louis Art Museum expansion and renovation because of its expertise in concrete work. The 200,000 square-foot expansion included a 30% increase in the museum’s gallery and public space providing guests with access to new galleries, public areas, and adding more than 300 parking spaces in a below-grade garage. McCarthy’s portion of $130.5 million expansion was to add 40,000 square feet of gallery space, an underground parking garage, and to complete other infrastructure work.

Complex design

London-based architect, David Chipperfield, created a design that originally linked the expansion to the current structure by connecting to existing primary circulation axes. It featured a new central staircase to more clearly connect the main level to the lower level and a new public concourse. The design called for a dark, polished concrete facade incorporating Missouri river aggregate. Because of the complexities of the architectural design, McCarthy encountered some challenges with achieving light reflectivity, slab defects, hot weather concrete placement, and site logistics.

Adam Knoebel, vice president of operations for McCarthy says, “We cast dozens of 2X2-foot concrete samples of all different concrete mix variations to achieve a 55% LRV result.” As part of the design, ceiling coffers were used to provide a distinctive look for the new building. Creating the coffers was perhaps the most challenging part of the job, but ultimately they provide the means for a unique lighting system in the building that combines both diffused artificial and natural light.

Knoebel adds that quality control was a major factor in self-performing the architectural concrete used on the project. “Dozens of mock-ups were cast until an acceptable product was achieved, and could then be reproduced.”

McCarthy managed the process of creating the coffer ceiling system like a manufacturing plant. The overall coffer ceiling was divided into 10 areas. Each area was completed using the same means and methods, and eventually became repetitive. Tradesmen were assigned a specific activity in the coffer construction process which helped achieve productivity and quality.

The coffer forming system was composed of thousands of square feet of horizontal decking and hundreds of 4X9-foot open-top plywood boxes. Styrofoam blocks were then inserted into the plywood boxes to create a rigid structure. Each Styrofoam filled plywood box was then aligned in a straight lined pattern and fastened down creating the coffer grid pattern. “The concept was to create a form that required no ties and could resist the horizontal and vertical forces of the cast-in-place concrete,” says Knoebel.

Piecing the puzzle

The architectural concrete coffered ceiling had to be placed with the highest possible quality and exceed normal construction tolerances. McCarthy was required to produce almost flawless finished concrete, free of surface defects, with sharp, straight corners and edges, good definition of details, extremely flat, unblemished, smooth surface, and consistent in color. The concrete coffers had to meet a minimum light reflectance of 55% in accordance with IES LM-44-1990. The soffit corners of the coffer beams had to be cast with right angles with no chamfering.

Knoebel adds that the concrete forms used could only be used one time. The forms had to be straight, free of distortion and defects. They also could not have any surface variations that would reflect in the concrete. The contract specifications allowed McCarthy to patch a maximum of 2 square feet for each 1000 square feet. The patches had to be indistinguishable from the surrounding concrete surface. There were a total of 649 boxes which were constructed to complete the coffer ceiling.

“The coffered ceiling is truly a cast-in-place architectural ceiling,” says Knoebel. “The end goal of the coffer ceiling was to make cast-in-place concrete resemble painted drywall. The finished concrete surface has a consistent as-cast concrete color with minimal surface variations.” By creating a construction system that worked, McCarthy ultimately created a work of art.