Coldspring teamed up with Construction Specifier to discuss the myriad of options Landscape Architects have when it comes to selecting the right materials for paving and hardscape projects. Natural stone is a suitable 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.
Natural Stone Paving Methods & Techniques
Many factors contribute to the right choice of natural stone installation for a 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.
The mortar set method is the most popular one, and 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, thereby enabling the effective setting of large and irregular-shaped pieces. In many areas of the country, mortar systems provide a long-lasting, high-performing setting system. Evacuating surface water is important in a mortar set installation, so proper drainage is important.
A sand set installation method may be best if a concrete slab is not an option for the base. This system offers 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 other methods, uniform support of the paver is critical. If the water cannot properly drain from a sand set system and gets underneath the stone pavers, the liquid will eventually move the sand and create a rocking scenario, thus compromising the pavers’ durability.
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 does not allow for paver height adjustment like mortar or sand systems. Therefore, smaller piece sizes are a common way to accommodate this.
Pedestal systems can work well in many environments because they do not 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 rooftops and plazas at the street level—anywhere draining or eliminating water is a concern.
State Street’s durability
State Street in Madison, Wisconsin, is an 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 urban 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.
Multi-Color Granite Paving
State Street’s renovation included the addition of granite pavers in shades of brown, gray, black, and green. The materials were selected based on their durability, permanence, and ease of maintenance. All pavers were installed with a non-slip finish, which provides a semi-rough surface that is textured to reveal vibrant colors in a deep, rich background.
State Street’s pavers were installed with a bituminous setting bed, which is a good solution in cold-weather regions. An oil-based bituminous setting bed includes a 25-mm (1-in.) layer of asphalt with thin mastic, and is not prone to absorbing water. Therefore, with proper drainage, water infiltration and freeze-thaw action is not an issue with these systems.
Today, nearly 20 years since the first granite pavers were installed at State Street, they look nearly the same as they did initially. With its shopping, dining, entertainment, and event options, State Street 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, the Peavey Plaza in Minneapolis 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 1/2 –ha (1-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 deteriorated and was difficult to maintain. In 2017, a revitalization of Peavey Plaza began with the Minneapolis-based landscape architecture firm Coen+Partners leading the design effort.
Since Peavey Plaza had been named to the National Register of Historic Places, preserving the original design intent was an important consideration.
Other design goals included:
- increase accessibility and safety;
- improve long-term sustainability and maintenance; and
- rehabilitate character-defining features.
The new design plan met these goals and maintained the plaza’s iconic fountain and 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.
Why choose granite as a paving material?
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 low absorption rate, it was a suitable choice for the paving materials—unlike concrete or brick, which absorb water and would disintegrate 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,” says Kamin-Lyndgaard. “The color of the granite would have a huge impact on what the water would look like.”
Since the original material for the reflecting pool was a warm-toned brown brick, the team began looking worldwide for brown shades of granite. However, when the client and design team considered the difficulties of maintaining the plaza with a foreign-sourced stone, Kamin-Lyndgaard decided to find a supplier closer to the project. Eventually, 929 m2 (10,000 sf) of locally sourced black granite pavers were used for the reflecting pool’s surface and surrounding area. The finish highlights the stone’s reflective crystals, enhancing and contrasting with the depth of the granite’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,” says 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 holds a pristine level edge and drain water to a collection trough for recirculation. With only 6 mm (¼ in.) of water, the tolerances in this system were critical. The pedestal system enabled the contractors to meet the specified 1.5-mm (1/16-in.) tolerance.
Further, the pedestal set system allows a unique functionality for the plaza. Water can quickly be drained from the reflecting pool, and the plaza could 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 space 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. Now that its revitalization is complete, the plaza has reclaimed its place as one of Minneapolis’ most dynamic and vibrant public spaces.
Unfortunately, from time to time, projects do not perform as well as intended. It is 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 because of a failure of the system used below grade. In a major urban 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 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 breakdown of the paving earlier than it should have.
This project utilized a mortar bed for its setting system. Mortar set systems can last a long time in environments with minimal freeze-thaw cycles and ample surface drainage because there is 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. Water eventually displaced the powdery mortar, leaving voids in the subgrade. The initial perception by many responsible for maintaining the surface was the granite was cracking and failing on its own. It was not until the broken pavers were removed 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.
Since the plaza area encompassed thousands of square feet, the deteriorating pavers became a maintenance obstacle over time. The plaza’s deterioration would have been slowed or avoided altogether had drainage issues not occurred. It is also possible if a more flexible and hydrophobic setting system, such as a bituminous paving bed, had been chosen, the outcome could have been 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.
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 is 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 also beautiful.
Natural Stone: The Durable, Versatile, Long-Term Building Material
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 with a lasting legacy.
Articles C/O Construction Specifier