A Blend of Natural Stone and Manufactured Stone Veneer Brings Out an Iconic Architectural Masterpiece
By Nader Assad
As most of today’s masonry professionals know, materials have changed in recent times. The choices have expanded, particularly in the category of manufactured stone, which can lend both performance and customized aesthetic advantages. And the manufacturing processes to achieve certain performance characteristics have greatly advanced.
Traditionally, both lightweight and full-depth stone veneers have been manufactured through a wet-cast process using latex molds. In wet-casting, natural stones are arranged in a pattern with latex sprayed on the stones to create a mold. When it came time to cast the veneers, the molds were hand painted with the colorant specified to resemble a particular variety of stone. The resulting veneers were colorized only on the outermost layer. If the veneers chipped accidentally, or were purposely cut during installation, the non-colored aggregate on the interior was exposed. In addition, special corner and trim pieces are needed to obtain finished, real-stone looks without sacrificing color consistency.
By comparison, a new mold technology known as the dry-cast production method offers significantly longer life spans and results in a dimensionally stable product. The process begins by scanning natural stone and turning those scans into patented mold technology. A low-moisture mixture of fine aggregates, cement, admixture and integrated colorant are densely compacted into the molds, then stripped from them and cured in a high‐humidity environment. The lightweight and full-depth veneers manufactured through dry-casting offer a psi strength that is two to three times stronger than traditional veneer and have color integrated throughout the unit, which eliminates the need for special corner pieces and reduces waste caused by chipping.
Additionally, dry‐cast veneers resist freeze/thaw, water and fading. Another benefit of the dry-cast veneer process is the ability to create colors, textures, widths and profile sizes similar to natural stone, which also allows the two materials to be used together.
An example of the perfect marriage of locally sourced Kansas limestone and a full-depth veneer called Cordova Stone from Oldcastle’s Echelon brand is the iconic Museum at Prairiefire in Overland Park, Kan., which rises out of the landscape in a blaze of fiery color. D&D Masonry of Kansas City, working closely with Jonathan Kharfen, AIA/LEED senior associate of Verner Johnson Inc. (Boston), created the stonework for this dramatic showpiece, which hosts exhibits from New York City’s American Museum of Natural History (AMNH).
Part of a 60-acre, mixed-use suburban development that includes shops, restaurants, entertainment venues, a wetlands park and residences, the museum now also serves as a living, growing monument to the tall-grass prairie featuring permanent native Kansas archeological specimens and interactive exhibits.
Deeply rooted in an architectural concept featuring sustainability, the $17.1-million museum project was designed to capture the beauty and agricultural importance of intentionally burning grounds to replenish and invigorate grasslands. However, the architectural materials needed to simulate flames, burnt landscape and charred remnants proved to be a major challenge. This was overcome by the use of a dichroic film laminated into an insulating glazing unit that replicates the intensity of rippling flames that change color from bottom to top depending on the viewing angle.
Since the original mix of all-natural stones was cost-prohibitive for the 40,000-sq.-ft. structure, Kharfen chose a blend of natural limestone and four standard Cordova veneer colors and fabrication of two custom colors blended by Oldcastle. The Charcoal veneer colors were used to evoke the charred landscapes, with lighter stone colors rising to the top of the building. Saw-cut limestone in near-white was affixed at the top.
To graduate from charcoal to grayish-gold on the exterior wall, a brownish-red Cordova veneer was chosen to match the natural red sandstone used on the interior, and for the low-site walls, a custom red with orange highlights. Bands were mapped by D&D Masonry so each band could be laid out with a specific number of stone courses designed in pre-determined sizes and with an exact mix of stonework — for instance, three courses of 25 percent Woodland blend and 75 percent Desert blend in 4-inch height. This also saved on labor costs from the original design, which mixed three or four stones per course.
Another issue was the original intent of mixing both the natural and the Cordova stones in one course. Kharfen and D&D determined that the different expansion and contraction rates — one natural and one manufactured — could cause the mortar to crack, so mixing within a course would not be ideal. As a compromise, they redesigned the bonding to separate the different types of stone. The bottom 15 feet of the gradient was formed with manufactured stone veneer and the colors of charcoal, brownish red and grayish beige to offset the fire elements at eye level. Veneers were also used for the site walls, which are low sloping in the front and back of the building and low enough for visitors to sit on. “At dusk, strategically placed LED lights twinkle along the low red site walls like dying embers as evening sets in,” said Kharfen.
The two types of stones were the same widths (3 5/8 inches with 3/8-inch mortar joints), so typical steel stud-framed wall construction worked for both. However, to ensure precision throughout the project, a small mockup panel of the wall was created. This was because the work veered from the traditional placement of vertical control joints with continuous gaps for expansion and contraction. The soft prairie forms kink a lot. So, rather than placing control joints at the kinks, they were located 2 feet from the kinks, consistently around the building. D&D had not initially planned on doing this, since it meant field splitting the stone and the placement of kinks at very obtuse angles. Fortunately, the manufactured veneer was split as easily as the natural stone.
“From a structural standpoint, the Cordova stone veneers were as durable as promised,” explained Charlie Adams from D&D, the project’s foreman. “They were certainly tougher than many of the products we used in the past, which eliminated the need to constantly replace veneers chipped or cracked during the process.” According to Kharfen, the Cordova veneers mixed with traditional Kansas limestone allowed this project to come in on budget and under a very tight timeframe. Kharfen sought to blend one color course into another to ensure the desired effects throughout the project’s exterior.
For the second-floor Discovery Room’s stone balcony, a specialized wall system was required. The design intent was for the cantilevered stone promontory to become a space for children, with the balcony completely wrapped in stone, even its sloping soffits. With the help of an Oldcastle subsidiary, D&D became acquainted with the IBP Fast Track Stone System, which allows kerfed stone to sit in a lipped track system that mechanically holds the stone in place, despite the gravity load of the sloping soffit. The two long sides of each stone are then secured in the track top and bottom. To make sure the sloped soffit system was safe, D&D created a mock-up in a Kansas City vocational facility. This enabled them to simulate how the soffit system would be installed and how the stone would sit securely in the track system.
“It was a great system,” explained D&D’s Adams. “It was the first time we used it, and the installation could not have gone better. It took three of our guys one week to complete the work. This included 32 separate zones of varying stone sizes meticulously blended together in random bond patterns. It turned out to be magnificent.”
Kharfen, who praised D&D Masonry’s skills for helping to bring his vision to life, said the project has won several awards and recently achieved a LEED Silver rating. “I love talking about the project because of the sense that we took on some big challenges and succeeded to create something so special.”
“I couldn’t have imagined how much attention this landmark piece of architecture would draw from all over the world,” said Fred Merrill, founder and president of Merrill Companies, who conceived and spearheaded the museum project.
The Museum at Prairiefire, which seamlessly blends natural and manufactured stone in an exquisitely crafted stone backdrop, will certainly dazzle visitors for generations to come.
About the Author
Nader Assad is the Masonry Group manager for Oldcastle Architectural since January 2014. Prior to this, Assad was engineering manager for Besser Co., a supplier of machinery equipment for the concrete product industry. Assad began his career as a mold designer and has nearly 20 years of experience in the concrete industry, engineering and assisting in new product development and deployment.