Ecological Hardscape Choices for Today’s Landscapes

by Jesse Harris

Today’s landscape professionals have many choices when it comes to choosing ecological hardscapes for their designs. Proper design, installation, and maintenance are critical to their effectiveness. The pavement choices are typically grouped into three categories: rigid, flexible, and unit pavements.

Rigid Pavements

Rigid pavements are surfaces made up of poured slabs of Portland cement and remain rigid or fixed when placed under stress. Rigid pavement requires less base material and has a longer life span than flexible pavements. Concrete is one of the most widely used pavements in construction today. For the designer, concrete is very versatile because it can be integrally colored, stained, stamped, and formed into intricate shapes and patterns. It often utilizes industrial by-products, such as fly ash and slag cement in its production.

Demonstration of the porosity of a porous concrete test sample.

Demonstration of the porosity of a porous concrete test sample.

Concrete surfaces also readily reflect light; a property generally referred to as albedo. This characteristic helps to mitigate the Urban Heat Island Effect. Porous concrete pavements are comprised of specially graded coarse aggregates, cementations materials, admixtures, water, and little or no fines. Mixing these products in a carefully controlled process creates a paste that forms a thick coating around the aggregate particles, thus creating a pavement with interconnected voids which allow water to percolate. Similar to all other porous pavements, porous concrete requires a stone bed layer to infiltrate stormwater into.

Flexible Pavements

Flexible pavements are surfaces that rely on the strength of the individual component layers and will deform when placed under stress. Hot mix asphalt is the most common flexible pavement, and typically is the least expensive pavement type available. You may not think of asphalt pavement as being sustainable, but it is one of the few pavements that is regularly recycled and reused in new applications, reducing the need for additional natural resources. When trying to mitigate the Urban Heat Island Effect, designers typically have two options with asphalt pavement. Either they can apply a coating that will reflect the sun’s rays back into the atmosphere, or apply a chip sealing to the surface of the pavement. Chip sealing typically consists of two layers: a lighter color aggregate that is bound to the pavement below with a tack coat of liquid bitumen.

Traditional asphalt pavement (left) and porous asphalt pavement (right.

Traditional asphalt pavement (left) and porous asphalt pavement (right.

Porous asphalt has been around since the 1970s and it has grown in popularity in recent years, largely due to more rigorous stormwater regulations that require increased infiltration. These pavements, typically used in parking lots, allow stormwater to infiltrate through voids in the asphalt surface into a stone bed below. This pavement type promotes infiltration, improves water quality, and often reduces the need for more traditional stormwater infrastructure. However, owner maintenance is required to keep the pores clean and free from debris. Here in the Northeast maintenance can be complicated due to leaf drop in the fall and the application of sand for snow and ice during the winter. Nevertheless, with the proper design, installation, and maintenance, porous asphalt can provide a cost-effective and attractive pavement surface.

University of Connecticut, Porous Asphalt parking stalls alongside traditional asphalt drive aisle

University of Connecticut, Porous Asphalt parking stalls alongside traditional asphalt drive aisle

Unit Paving

Unit paving offers the designer an almost unlimited array of options. There are products made from concrete, asphalt, stone, and even products that incorporate recycled materials such as glass. Color and texture can vary widely amongst manufacturers and product types. Unit pavers can either be placed on a compacted gravel base or a solid material such as concrete or asphalt.

Unit paver with recycled glass.

Unit paver with recycled glass.

Concrete interlocking pavers are one of the most common types of paver used today. They come in many sizes, colors, and textures. They are typically set on a sand setting bed over a prepared base and compacted. The joints are typically filled with fine sand and compacted again to create an interlocking effect. Concrete pavers afford the designer lower initial and life cycle costs, lower maintenance, and a high end look.

Typical cross section for pervious unit pavers.

Typical cross section for pervious unit pavers.

The use of pavers with recycled material content can account towards sustainability credits, and may also create visual interest or a splash of color to your project. There is an increase in cost to using this type of unit pavement compared to other types of unit pavers.

Pervious pavers can be a great choice for the designer when challenged to meet state or local stormwater requirements. Typically made of concrete, these pavers are designed with an integral tab which creates a predetermined joint space that is filled with a larger aggregate, such as a No. 9 crushed stone. The challenge for the designer is to have the appropriate layers of open graded stone below the pavers to allow for both adequate infiltration and support for the given application.

Design Considerations

The selection of pavement types can be a formidable task for the designer. Designers are often asked to provide the best product that meets the project’s goals, while also meeting the project budget. There are many things that should be considered before arriving at a final product choice. The designer should consider such questions as “What are the goals for the project?” “Does the project have certain ‘Low Impact Development’ (LID) requirements?” “How long will the pavement last?” and “What is the desired aesthetic?” Cost can also be a major determining factor in the selection of the final product, with porous concrete and asphalt often being the highest priced. Ecological hardscapes, where feasible, can significantly enhance the appearance and performance of today’s landscapes.

About the Author

Jesse Harris, PLA, Landscape Architect Jesse has over eight years’ experience as a licensed landscape architect in the state of Connecticut where he works for BSC Group. He has been the lead designer for numerous projects throughout Connecticut with a focus on creative sustainable landscapes that enhance the user’s experience. His work encompasses public and private sector development, park and recreation facilities, as well as K-12 and higher education. Jesse holds an Associate Degree in Landscape Contracting from the Stockbridge School of Agriculture and a Bachelor of Science in Landscape Architecture from the University of Massachusetts at Amherst. He may be reached at jharris@bscgroup.com.