Landscape architects have a myriad of options when it comes to selecting the right materials for their paving and hardscape projects. Natural stone provides an ideal choice for exterior paving because of its beauty, versatility, durability and sustainability. Understanding the differences in installation methods, and the best applications for each, is essential for a successful project.
There are a variety of installation methods for natural stone paving. Many factors contribute to the right choice for the project. Methods include mortar set, sand set, bituminous set and pedestal set. Each of these methods has advantages and disadvantages, and landscape architects will benefit from knowing what makes them different.
Four common installation methods
Mortar set: The mortar set method, which is the most popular, is typically used in pedestrian or vehicular areas where a rigid system is required. The joints are grouted as part of a mortar system. Mortar also provides a degree of flexibility to the setting bed which allows for larger and irregular-shaped pieces to be effectively set. In many areas of the country, mortar systems provide a high-performing setting system that can last for a very long time. Evacuating surface water is important in a mortar set installation, so proper drainage is key.
Sand set: A sand set installation method may be best if a concrete slab is not an option for the base. It’s a system that offers both permeability and some flexibility. Polymeric sand may be used to fill the joints and reduce movement. Repair is usually easier and more economical than a mortar system. Like any system, uniform support of the paver is critical. If the water can’t properly drain from a sand set system and gets underneath the stone pavers, the water will eventually move the sand and create a rocking scenario which will compromise the pavers’ durability.
Bituminous set: The bituminous set installation method is growing in popularity. Since it is oil-based and less rigid than mortar, a bituminous set installation flexes and moves more easily with freeze-thaw cycles. Thickness tolerance of the paver is critical – a bituminous system doesn’t allow for paver height adjustment like mortar or sand systems. Therefore, smaller piece sizes are a common way to accommodate this.
Pedestal set: Pedestal systems can work well in many environments because they don’t rely on a mortar, sand or bituminous setting bed. A pedestal set system provides drainage beneath the surface of the paving.
Depending on the pavers’ size, pedestal set systems may require thicker pieces of stone than other methods, such as a mortar or bituminous set system. Pedestal set systems are often used in rooftop settings, as well as plazas at the street level – anywhere draining or eliminating water is a concern.
State Street’s durability
State Street in Madison, WI, provides a beautiful example of natural stone’s durability when installed with an appropriate setting bed for the conditions. Granite-paved State Street is a thriving pedestrian zone linking the State Capitol Square with the University of Wisconsin-Madison’s campus. The pulse of downtown Madison, State Street is closed to regular traffic and has extra-wide sidewalks for strolling and restaurant/café outdoor seating and well-marked bicycle lanes.
In the early 2000s, Saiki Design of Madison carried out the vision for State Street’s renovation, which was completed in four phases over a 10-year period. The design team worked to preserve the cherished elements of the existing streetscape and Library Mall while placing equal emphasis on restoring views to many of the city’s iconic features.
State Street’s renovation included the addition of granite pavers from Coldspring® in four colors: Carnelian®, Iridian®, Charcoal Black® and Mountain Green®. The materials were selected based on their durability, permanence and ease of maintenance. All pavers were finished with a non-slip Diamond® 100 finish, which provides a semi-rough surface that’s textured to reveal vibrant colors in a deep rich background.
State Street’s pavers were installed with a bituminous setting bed, which offers an ideal solution in cold-weather climates. An oil-based bituminous setting bed includes a 1-inch layer of asphalt with a thin mastic and is therefore not prone to absorbing water. Therefore, with proper drainage, water infiltration and freeze-thaw action is not an issue with bituminous setting beds.
Today, nearly 20 years since the first granite pavers were installed at State Street, the pavers look nearly the same as they did when first installed. State Street, with its shopping, dining, entertainment and events, is a major attraction in Madison and has become a true reflection of the city’s diversity and vibrant lifestyle.
Pedestal set at Peavey Plaza
Another example of a paving success – Peavey Plaza in Minneapolis, MN -- demonstrates the creative possibilities with granite in a hardscape. Originally designed by renowned landscape architect M. Paul Friedberg as the “living room” of Minneapolis, Peavey Plaza opened to the public in 1975. The one-acre respite in the midst of busy downtown life included signature elements such as a large central reflecting pool with a cascading concrete fountain.
Over the years, the plaza had become badly deteriorated and difficult to maintain. In 2017, a revitalization of Peavy Plaza began with landscape architecture firm Coen+Partners of Minneapolis leading the design effort.
Because Peavey Plaza had been named to the National Register of Historic Places, preserving a commitment to the original design intent was an important consideration. Other design goals included:
- Increase accessibility and safety
- Improve long-term sustainability and maintenance
- Rehabilitate character-defining features
The new design plan met these goals and maintained the plaza’s iconic cascading fountain and reflecting pool – but with a different material as the backdrop. According to Laura Kamin-Lyndgaard, senior associate at Coen+Partners, granite was the material of choice in the initial stages of design.
As a civic space with a range of programming needs, the reflecting pool’s ability to drain quickly and often was an important factor. Since granite has a very low absorption rate, it provided an ideal choice for the paving materials – unlike concrete or brick which absorb water and would disintegrate quickly in numerous cycles of drying off and rewetting.
“Very early on, we determined the granite needed to be a dark color to maintain a sense of depth and create a highly reflective water surface,” said Kamin-Lyndgaard. “The color of the granite would have a huge impact on what the water would look like.”
Because the original material for the reflecting pool was a warm-toned brown brick, the team began looking for brown shades of granite sourced from locations around the world. But when the client and design team considered the difficulties of maintaining the plaza with a foreign stone source, Kamin-Lyndgaard explained that they prioritized finding a supplier closer to the project.
Meeting the need for a local source, Coldspring provided more than 10,000 square feet of Mesabi Black® granite pavers for the reflecting pool’s surface and surrounding area. A Diamond 10 finish throughout highlights the stone’s reflective crystals, enhancing and contrasting with the depth of the Mesabi Black’s darker tones.
“One of our points of interest with the granite selection was how the granite looked when it was under water and how it made the water look,” said Kamin-Lyndgaard. “We wanted to ensure the granite supported the character of the water.”
A pedestal set system, ideal for fountains and reflecting pools, feeds water through the paver joints. Although the water is constantly circulating, the frequency of the open joints in a pedestal system enables water to flow at a rate that makes the surface appear still. Custom-milled perimeter weir stones are grouted in place, which hold a pristine level edge and drain water to a collection trough for recirculation. With only a ¼ inch of water, the tolerances in this system were critical. The pedestal system enabled the contractors to meet the specified 1/16-inch tolerance.
What’s more, the pedestal set system allows a unique functionality for the plaza. Water can quickly be completely drained from the reflecting pool so the plaza can host outdoor concerts and events. The plaza can be transformed within mere hours to become a functional paving space for concerts and public use.
Peavey Plaza re-opened on July 18, 2019 with a design that enhances sustainability and makes the plaza accessible to all, with wheelchair and stroller ramps to access all levels of the plaza. A popular gathering spot, Peavey Plaza provides an oasis of tranquility for city dwellers. And now that its major revitalization is complete, the plaza has reclaimed its place as one of Minneapolis’ most dynamic and vibrant public spaces.
Lessons learned
Unfortunately, from time to time, projects don’t perform as well as intended. It’s important to learn from these projects to avoid similar challenges on future projects. Paving can fail over time for a variety of reasons. It can be due to the material itself or due to a failure of the system used below grade.
In a major metropolitan area in the upper Midwest, a large area of granite pavers began cracking 10 to 15 years after installation. The cracking pavers are located along a 12-block portion running through the city’s downtown corridor that’s well known for its shopping, dining and pedestrian experience. Typically, an application like this should perform for 30 to 50 years with minimal maintenance. However, the climatic conditions of the area combined with insufficient drainage of water at the surface accelerated the aging and subsequent break down of the paving on this project far earlier than it should have.
This project utilized a mortar bed for its setting system. Mortar set systems can last a very long time in environments with minimal freeze-thaw cycles and ample surface drainage because there’s little impact to the sub-grade over time. However, on this project, the paving was subject to both excessive surface water and some of the most frequent freeze-thaw cycles in the country.
Over the course of several years, small voids and cracks developed in the mortar joints between the pavers enabling water to infiltrate to the mortar base. With consistent and repetitive exposure to moisture, the mortar became saturated. After the expansion and contraction of hundreds of freeze/thaw cycles the mortar broke down into a dusty powder which water eventually displaced leaving voids in the subgrade. The initial perception by many responsible for maintaining the surface was that the granite was cracking and failing on its own. It was not until the broken pavers were removed that the gaps and voids beneath the paver exposed the underlying issue. Without an adequate base, the pavers were no longer able to support the weight and pressure being placed on them from above.
Because the plaza area encompassed thousands of square feet, the deteriorating pavers sadly became a maintenance obstacle over time. The plaza’s deterioration would have been dramatically slowed or avoided all together had drainage issues not occurred. It is also possible that if a more flexible and hydrophobic setting system, such as a bituminous paving bed, been chosen, the outcome could also have been much different.
Granite paving is arguably the most durable surface material for standing the test of time. However, if environmental factors result in a subgrade base being unable to uniformly support the stone above it, the likelihood of granite paving reaching its intended performance will be compromised.
Final thoughts
When the appropriate method of installation is selected, natural stone pavers can provide a long-lasting, durable and beautiful material for public use. An experienced stone supplier can offer guidance and contribute to a successful project that will leave a lasting legacy.
Finishing the job
Once the setting bed method is selected, the best finish for the paving surface must be considered – especially when preventing slips and falls in icy or wet conditions. Just like setting bed methods, there are a number of finish options for natural stone pavers, but thermal is the most traditional. It gives the stone a nice texture and holds up very well over time. It also meets many slip-resistance testing requirements.
Depending on the stone used, the environment it’s in and the volume of foot traffic it receives, the aggressiveness of the finish may need to be adjusted. Knowing the environment both during and after installation will make for a natural stone landscape that will not only be functional, but beautiful as well.
Since 1999, Jason Kron has specialized in the natural stone industry. In his current role as director of sales at Coldspring, Kron works with the architectural and design communities to meet their natural stone needs and with cemeteries for memorial product and cemetery development needs. He can be reached at jkron@coldspringusa.com.
