What I learned in Paver School

By Michelle DeLaria, CASFM Stormwater Quality Committee chairperson and project manager for Meza Construction Company

There are several low impact development (LID) techniques used to mimic predevelopment hydrology and reduce the negative effects of urbanization on waterways.

While vegetative LID techniques such as grass swales, buffers, green roofs and porous landscape detention areas are attractive, they have limited use and effectiveness in significantly reducing runoff volume in existing, highly impervious environments.

On the other hand, permeable pavers and porous pavement are techniques that can substantively reduce stormwater runoff volume and provide detention capacity in highly urbanized areas.  Urban Drainage & Flood Control District includes guidelines on modular block paving and other porous pavement techniques in Volume 3 of the Technical Criteria Manual.

There are several successful installations of these materials in Colorado, yet comments and concerns linger about cost, cold climate function, maintenance and plowing.  Communities along the Front Range are increasingly regulated for the effects of excess urban runoff and are collecting millions of dollars in fees annually to manage hundreds of millions of dollars in backlogged waterway stabilization and water quality needs. Permeable pavers and porous pavements can be part of the solution.

This article addresses some of the misperceptions and how paver systems and porous pavements can be used in our communities to produce a more functional, sustainable urban environment.

In September, I attended a one-day training class at the School for Advanced Segmental Paving in Franksville, Wis., taught by instructors from the Chicago area.  The school is modeled after the European apprenticeship method where students learn in a classroom setting and further develop skills in an on-site installation practice area.  The school was established by several leaders in the segmental paver industry who are dedicated to establishing standard methods and practices in this field.  The school also offers three- and four-day classes that are tailored to the level of involvement and skills desired in paver systems.  For example, there are classes to learn paver installation in variable field conditions as well as classes geared for estimators and salespeople.  The class that I attended included the history of paver systems, applications, benefits and a demonstration in the practice area. 

While the words permeable, porous and pervious are often interchanged (and Urban Drainage & Flood Control District includes pavers under the porous pavement section of best management practices), in this article they will be used as follows:  The term “permeable” will refer to water moving through openings between pavers and aggregate.  “Porous” refers to the material and how water moves through it, as is the case with porous concrete and asphalt that has voids in the material because the fines are removed.  “Pervious” refers to the ability of the surface of the material to accept water. 

Roman road in Pompeii.  Photo Courtesy John Knapton

The first thing I learned is that permeable paver installations with an open-graded aggregate system are based on Roman road construction techniques from 2000 years ago. Romans would excavate a trench and fill it with a layer of large rock on the bottom, then smaller rock, followed by a setting bed.  They would then fit large stones on top of the aggregate layers for the travel surface or a “wearing course.”  Some sections of Roman roads are still used today with a new asphalt wearing course,

although many sections have been preserved as historical remnants throughout Europe.  Roman road construction is the basis of current road construction and instead of pavers, asphalt or concrete is used as the wearing course.

Pavers with Open-Graded Aggregate

Model of aggregate layers for stability and detention capacity. Photo Courtesy Michelle DeLaria

The open-graded aggregate system consists of aggregate layers placed and compacted to provide a stable subsurface for heavy vehicles and point loads.  Pavers with the full 18” depth of open-graded aggregate also provide detention.  The layers from bottom to top are:

• Twelve inches of 1 1/2-inch size, all-fractured face aggregate.  This is called #4 aggregate and is compacted in 4” to 6” lifts.
• Four inches of 3/4-inch all-fracture face aggregate.  This is called #67 aggregate.  It is also called a choker course because this size rock remains on top of the 1 1/2-inch aggregate and “chokes” off the top of the larger aggregate, while allowing water to flow downward into the void space of the 1 1/2 inch aggregate.  The smaller rock does not sift into the voids.  This layer is compacted.
• Two inches of 3/8-inch granite “chip” material is then placed on top of the base course.  This is the setting bed.  It is important that this material be all-fracture face and not rounded “pea gravel.”  The setting bed is not compacted. 
• Pavers are placed on top of the setting bed by hand or machine.  The joints between the blocks are filled with 1⁄4-inch material and a compactor is run over the blocks to vibrate and lock in the blocks. 

Fabric, or other means of separating the aggregate layers, is not used in this system.  Current research indicates that separating aggregate layers in infiltration systems introduces a clogging layer and causes more rapid degradation of the system.   Depending on the subsoil, a geogrid may be used between the soil and the base course aggregate to increase stability.

A college in the Chicago area installed permeable pavers on an 8% slope and combined the paver installation with curb cuts directing stormwater into sumped landscape areas. Photo Courtesy Michelle DeLaria

Several million square feet of permeable pavers with the open-graded aggregate system have been installed in the Chicago area over the past 20 years. I visited several sites in various stages of completion with the paver instructor who installs pavers in the Midwest.  One site was a private road under construction in a large-lot subdivision.  The road, if built using traditional methods, would have been the majority of impervious area and the greatest source of runoff.  Since the road was constructed with permeable pavers over the aggregate layers, a detention pond and conveyance infrastructure that is typically required to handle road runoff was not needed.

Pavers can be plowed.  Photo Courtesy Chuck Taylor (Advanced Pavement Technologies)

Block pavers can be plowed and are less prone to black ice and other surface freeze/thaw problems.  In addition to the void space of the open-graded aggregate system, there is more air moving through the system to keep the surface free of ice and snow when compared to impervious surfaces such as asphalt and concrete. 

Cost Benefit

The cost benefit analysis is variable.  For example, at an installation in Florida, permeable paver systems broke even after 22 years when comparing the materials, construction and maintenance to concrete and asphalt surfaces.  On another site in the Chicago area, after 50 years an asphalt surface would have cost 10 times as much as pavers to maintain.  Additionally, comparing costs of materials and installation is not a complete and perhaps not an appropriate evaluation.   In the Denver area for example, a concrete parking lot would cost approximately 50% more than asphalt, and a permeable paver system with the full open-graded aggregate system, would cost two to three times as much as asphalt.  Based upon initial investment, asphalt or concrete appear to be more cost effective than pavers.  However, the cost of asphalt or concrete does not include the costs of inefficient use of land and associated cost if a detention structure is required.  Also not included are costs associated with managing off-site impacts that are generated such as excess stormwater runoff rate and volume, pollutants washing off of impervious area into receiving waters, and future waterway stabilization needs.  Pavers may have a larger initial investment, but the cost of detention is included and off-site impacts are reduced. 

Standing water and freeze/thaw cycles can create an auto and pedestrian hazard and shorten the life of the paving material. Photo Courtesy Michelle DeLaria	Pavers are largely free from ice and snow accumulation. Photo Courtesy Interlocking Concrete Pavement Institute

While permeable paver, porous concrete and porous asphalt systems provide infiltration and can be designed for detention capacity, there is a difference that may be important to some property owners.  These systems have similar recommended annual maintenance needs such as removing surface grit by using a sweeper or vacuum truck.  Also, it is possible that the subsurface may accumulate enough solid material resulting in decreased infiltration and detention capacity that cannot be regenerated by a power vacuum.  At this point, porous asphalt or concrete needs to be removed and discarded.  However, paver systems are completely modular. The system can be deconstructed, the aggregate layers cleaned and all the products reinstalled with minimal waste.  The reduced waste in a paver system may be preferable.

Summary

From a stormwater management perspective, porous asphalt, porous concrete or permeable pavers (all with the open-graded aggregate system) are techniques that can restore permeability and infiltration as well as provide large storm detention volume. Parking lots, alleyways, driveways, fire lanes, and parking lanes on streets are common examples of impervious flatscape areas that can instead be porous or permeable to reduce runoff.  Communities can retrofit highly urban areas to help retain the economic benefits of developed land while reducing off-site impacts.  

Additional Paver Resources

1. John Knapton: http://www.john-knapton.com/
2. Interlocking Concrete Pavement Institute: http://www.icpi.org/