By Miles Chaffee
As many industries and businesses scramble to become environmentally friendly or market themselves as “Green,” new methods are being developed to meet updated environmental standards. However, an environmentally friendly way of paving has been around for more than 2,000 years.
The Romans used the natural granite stone called porphyry to pave their streets and walkways. The porphyry pavers were set using a porous material in the joints to allow water back into the ground, which was an ancient form of permeable paving. This early method remains a viable option today that is being increasingly used by landscape architects and designers that are reaching into the past to design the future.
Having an understanding of permeable paving is beneficial to all landscape contractors involved in construction and maintenance. Cities such as Chicago, Boston and Detroit have started initiatives to Green their cities by installing permeable paving for storm water management. Other cities have allowed property owners to use more of the land area by shrinking or eliminating retention ponds if permeable paving is used on the property. It is necessary for a landscape contractor to have an understanding of the different types of permeable paving options available to fit their customers’ specific projects and capitalize on the growing number of Green initiatives in their markets.
What is permeable paving?
There are many types of permeable paving: segmental paving made of concrete, brick or stone; and so-called permeable asphalt and concrete known as pervious pavement. Gravel is also another option. Each has different costs and levels of durability, which require analysis before making the right choice for a project.
In order for a surface to be permeable, it must have openings to allow water to penetrate the surface. In segmental or unit paving, like stone, brick or concrete pavers, the joints are what make the surface permeable. In the case of pervious pavement, other materials are added to the mix to make the surface “permeable.” Gravel does not have a solid surface and therefore allows water to pass through to the subsurface.
Although it is necessary for the surface to have openings to be permeable, the key is the subsurface. The subsurface must be structured to accommodate the water that is allowed through. The effect of any surface’s permeability is squandered if the substrate is not designed to accept or manage the movement of the water, thus defeating the purpose and benefits of a permeable surface. The substrate design should be based on the soil’s ability to infiltrate the water. The most common design is for full infiltration of the soil. Water drains through the paver joints and the designed sub-base made up of stone road base into the soil with overflows managed by surrounding drainage swales, retention areas or sewers. Other designs are based on a deeper sub-base with larger retention areas that hold water for as long as necessary to allow percolation into the sub-soils. Soils without the ability to drain require an impermeable liner on the bottom and sides with drainage pipes directing the water in a controlled manner into sewers and streams.
Although there are many environmental benefits of permeable paving, the primary benefit is its ability to effectively reduce and manage the quantity of surface water runoff. Water is able to penetrate the surface and naturally filter back into the aquifers in the ground, while preventing some pollutants from getting into the aquifer.
The Chicago Green Alley Handbook states, “Imagine if all of the alleys in Chicago were green alleys. Up to 80% of the rainwater falling on these surfaces throughout the year could pass through permeable paving back into the earth, thereby reducing localized flooding, recharging groundwater and saving taxpayer money that would otherwise be spent treating storm water.”
Comparing permeable paving options
Each type of permeable paving has its advantages and disadvantages. In terms of cost, gravel is the least expensive. However it requires frequent maintenance and renewal, which, over time, adds up to a higher cost for the materials. Permeable concrete and asphalt are next in terms of expense, and studies have shown that these materials are prone to clogging — negating their effectiveness. A study conducted by the Metropolitan Engineer’s Council in Denver showed a complete failure of permeable concrete under freeze/thaw conditions. Due to pervious pavements issues with clogging and vulnerability to freeze/thaw, the material is not ideal for all situations or climates. In fact, some cities have removed pervious asphalt and concrete pavement because of the problems with clogging and continued maintenance.
Another option for permeable paving is brick, concrete or stone pavers with permeable joints. The permeability and water-flow-through rate is dependent on the porosity of the material in the joints, the size of the joints, and the entire substrate design. The joint material should adequately allow the passage of water to quickly drain, possess structural capability to withstand traffic loads, and provide horizontal stability to the paving surface. Permeable pavers have the ability for rapid infiltration compared with most soils, and a conservative approach should be taken in designing the system.
Natural stone materials, such as porphyry, a type of granite, have been used for centuries throughout the world and have proven to be an excellent choice, offering beauty, longevity and low maintenance. Natural stone life cycles are measured in decades with studies showing typical minimum of 60 years. Granites such as porphyry last for centuries. Porphyry is extremely dense with a porosity of less than 0.6%. Using it in a permeable setting system requires the same pervious materials for the joints and attention to the details of the substrate discussed earlier, but offers an unparalleled surface for durability, low maintenance and overall functionality. The natural surface meets the American Disability Act (ADA) for slip resistance and smoothness and is inert to chemicals. Using ice-melting materials is not a problem, and when set correctly it can easily be snow plowed.
Specifications for installing permeable paving
The material that is typically embedded into the joints of pavers is no. 8 aggregate that allows for good water flow and excellent stabilization. This requires routine cleaning to stop clogging. Several other newer, frequently used methods are manufactured Polymeric Sands (washed sand mixed with dry polymers – adding water activates it and hardens when it dries), liquid polymers (water based polymers added to washed sand that hardens when it dries) and epoxy-based material (mixed with a combination of washed sand and water). These all offer high compressive strength, are flexible, freeze/thaw resistant and suitable for vehicular traffic.
When brick or cement paving systems have been specified, it is important to choose products that have been manufactured to specific requirements in different geographical locations, especially in freeze/thaw climates and for use in heavy traffic loads. Constant freezing and thawing is harder on these materials than climates that freeze and remain frozen for long periods. Set correctly, theses materials can be safely snow plowed. Caution should be used when choosing snow or ice melting substances, because salts can react with the pavers causing a breakdown of the material. Concrete and brick pavers offer an economical option for paving and can last 10 to 20 years with proper maintenance and replacement of some pavers when needed.
With the many different types of permeable paving options available, it is important to identify the best material for the project and the client’s needs. For some projects, a gravel path might be all that is needed, and for other projects that are more concerned with aesthetics and sustainability, a natural stone like porphyry set as permeable pavers could be a better choice. No matter the decision on the materials, it is essential for landscape architects and designers to fully understand the differences in the options available. As people become more environmentally conscious, landscape professionals have a tremendous opportunity to grow their businesses by providing more environmentally friendly options such as permeable paving.
Miles Chaffee is founder and president of Milestone Imports, the largest importer of Mexican porphyry in the United States. Seven years ago, Chaffee obtained exclusive rights to the largest porphyry quarry in Mexico, and now distributes the porphyry throughout North America and Asia. Today, Milestone Imports represents porphyry quarries from around the world, including Mexico, Argentina, Brazil and China. Working with AEC Daily.com, Chaffee has developed a course called “Porphyry as a Permeable Pavement,” offering free educational learning credits to more than 12 organizations in Canada and the United States. Chafee also presents seminars on different and historical methods of setting Porphyry pavers. For more information about Milestone Imports, including a free installation manual and project photos CD, visit www.milestoneimports.com, e-mail firstname.lastname@example.org or call 866-641-1999.