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Is Bigger Really Better?

Project owners and designers specify segmental concrete paving slabs due to their unique visual appeal and finishes. Their large format often fits a particular dimensional module for the design of the project, complements the architectural character of adjacent buildings, or enhances the landscape architecture of the site. Some designers understate segmental pavement patterns by using paving slabs with fewer joints. In other situations, designers may mix smaller and larger slab units to create strong visual effects. While most applications are for at-grade or roof deck pedestrian uses, paving slabs are seeing increased use in areas with vehicular traffic. ICPI is engaged in field-testing research to assess the performance of slabs under vehicular loads.

When properly designed and constructed, paving slabs can withstand a limited amount of automobile and truck traffic. Unlike interlocking concrete pavements, slabs offer little to no vertical, horizontal or rotational interlock. Each unit bears applied loads and does not transfer applied loads to neighboring ones. Hence, their application to areas with limited vehicular traffic.

The load-carrying capacity of paving slabs and interlocking concrete pavements is put into perspective by reading ICPI Tech Spec 4 Structural Design of Interlocking Concrete Pavements and ASCE 58-16 Structural Design of Interlocking Concrete Pavement for Municipal Streets and Roadways. Both publications provide base thickness tables for pavements receiving up to 10 million 18,000 lb (80 kN) equivalent single axle loads or ESALs. Now underway, an emerging ICPI Tech Spec on structural design of paving slabs provides designs for up to 75,000 ESALs. This suggests that the structural capacity of paving slabs is less than 1% of that offered by interlocking concrete pavement. This further suggests that paving slabs should be exposed to limited vehicular traffic, and very few trucks per day.

Paving slabs are sometimes mistakenly called pavers. This misnomer has led to applying slabs under inappropriate vehicular applications in a few instances. To reduce confusion, the segmental concrete pavement industry is following other countries where product nomenclature and product standards specifically differentiate pavers from slabs. Figure 2 illustrates the difference.

A practical construction-related difference between concrete pavers and paving slabs is the former generally requires one hand to install a unit and the latter requires at least two hands to lift and place. In reality, most slab installations use clamps or vacuum equipment shown in Figure 3. Most commercial slab applications subject to trucks will be installed on a concrete base. Asphalt is generally not used as a base because it can’t be easily formed into an even surface.

PRODUCT STANDARDS

In the U.S., ASTM C1782 Standard Specification for Utility Segmental Concrete Paving Slabs defines them as having an exposed face area greater than 101 in.2 (0.065 m2) and a length divided by thickness of greater than four. The minimum thickness is 1.2 in. (30 mm), and maximum length and width dimensions are 48 in. (1,220 mm). C1782 was issued by ASTM in 2016. Units having a length divided by thickness of 4 or smaller with a minimum 2 3/8 in. (60 mm) thickness fall under ASTM C936 Standard Specification for Solid Concrete Interlocking Paving Units.

In Canada, Canadian Standards Association or CSA A231.1 Precast Concrete Paving Slabs defines their dimensional envelope with a face area greater than 139.5 in.2 (0.09 m2) and a length divided by thickness of greater than four. The minimum thickness is 1.2 in. (30 mm), and the maximum length and width dimensions are 39.37 in. (1,000 mm). This product standard was first issued by CSA in 1972. Units having a length divided by thickness of four or smaller with a minimum 2 3/8 in. (60 mm) thickness fall under CSA A231.2 Precast Concrete Pavers.

ASTM C1782 requires an average minimum flexural strength of 725 psi (5 MPa) with no individual unit less than 650 psi (4.5 MPa). The CSA standard requires a minimum average of 650 psi (4.5 MPa) with no individual unit less than 580 psi (4.0 MPa). A noteworthy aspect of the flexural strength is doubling the thickness of a paving slab increases the flexural strength by four times. This suggests that larger units may need to increase their thickness in order to withstand vehicular traffic. Some concrete paving slabs may use fibers to increase their flexural strength as well.



Freeze-thaw durability requirements in ASTM C1782 references ASTM C1645 Standard Test Method for Freeze-thaw and De-icing Salt Durability of Solid Concrete Interlocking Paving Units. This test method involves test specimens with a specified dimensional range from the corner of paving slabs. The specimens are immersed in water or a 3% saline solution and subjected up to 49 freeze-thaw cycles. The mass lost from the coupons are measured at 28 and 49 cycles. If no more than an average of 225 grams per square meter of surface area are lost after 28 cycles, the paving slab from which the specimen was cut passes this requirement. If not, the freeze-thaw cycles continue to a maximum of 49. If no more than an average of 500 grams per square meter of surface is lost after 49 cycles, the paving slabs pass this requirement. The lowest temperature in this test is 23° F or -5° C.

In the CSA test, the top of the paving slab is enclosed with a leak-proof compartment and the interior receives a 3% saline solution. See Figure 4. After completing 28 freeze-thaw cycles, the paving slabs pass the CSA requirement if the surface yields no more than an average loss of 300 grams per square meter of the inundated surface area or 500 grams lost for specimens with an architectural finish.

An architectural finish is wearing surface amended with face mix, ground (polished) or shot blasted treatments, formed (to look like stone per Figure 5), hammered and/or flame-treated to provide a more stone-like appearance. If the architectural paving units do not meet the mass lost requirement at 28 cycles, the freeze-thaw cycles continue until 49 cycles are completed. The paving slabs meet the durability requirements in CSA A231.1 when the average loss after 49 cycles does not exceed 800 grams per square meter or 1,200 grams for units with an architectural finish. The lowest temperature in this test method is more severe than C1782, i.e., 5° F or -15° C. 

Dimensional tolerances are similar in ASTM and CSA paving slab standards. Dimensional tolerances are determined from unit dimensions provided by the manufacturer for specific products. Tolerances for length, width and height and for convex and concave warpage are as follows:

  • Length and width: -0.04 and +0.08 in. (–1.0 and +2.0 mm)
  • For units over 24 in. (610 mm), ASTM C1782 allows -0.06 and +0.12 in. (-1.5 and +3 mm)
  • Height: ±0.12 in (±3.0 mm)
  • Concave/convex warpage for units up to and including 18 in. (450 mm) in length or width: ±2.0 mm; units over 18 in. (450 mm): ±3.0 mm

Paving slabs meeting these dimensional tolerances are loosely laid, or can be installed on a sand setting bed (i.e., sand-set) if tolerances are consistent. However, these tolerances are generally not suitable for precision sand-set, bitumen-set or pedestal-set (typically roof) applications. These installation methods require length, width, thickness and warpage tolerances not exceeding 0.06 in. (1.5 mm) than the specified dimensions. In some cases, paving units may require post-production grinding to achieve these tolerances. This treatment is sometimes called gauging. For additional information of bitumen-set applications, read ICPI Tech Spec 20 – Construction of Bituminous-Sand Set Interlocking Concrete Pavement.

Soon to move through the ASTM balloting process is a second paving slab standard. This one is called Standard Specification for Architectural Segmental Concrete Paving Units. The draft has flexural strength and freeze-thaw de-icer durability requirements identical to C1272. This new standard, however, has much closer dimensional tolerances not exceeding 0.06 in. (1.5 mm), making the units suitable for tightly-fitted sand-set applications, bitumen-set applications, and roof installations supported by pedestals. When this product standard is eventually approved by ASTM, there will be two paving slab product standards; one for mostly residential applications and selected commercial applications, and almost exclusively for high-end commercial applications.

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Unconventional Intervention

In July 2012, a series of storms caused combined sanitary and storm sewer overflows in parts of southeast Atlanta, flooding homes and streets. After visiting the flood-prone neighborhoods, Mayor Kasim Reed committed to finding a long-term solution. Thus began the Southeast Atlanta Green Infrastructure Initiative, which led to the largest permeable interlocking concrete roadway project in North America, more than four miles.

The flooding occurred at the nexus of piped natural drainage systems that transfer much of the runoff from downtown Atlanta, a highly impervious area, to Peoplestown, Mechanicsville and Summerhill. Located at a natural drainage point of a 1,500-acre watershed, these downstream neighborhoods finally found relief from the city’s unconventional intervention.

IMMEDIATE RESPONSE

With a mandate from the mayor’s office to solve flooding problems, the Department of Watershed Management rose to the challenge. “We went out into the field with our contractors to do assessments and came back with several projects that could be completed quickly to start providing immediate capacity relief,” said Todd Hill, Director of Environmental Management for the Department of Watershed Management. “We developed a phased approach.”

The bottom line, comprehensive solution meant managing about 24 million gallons of runoff. Phase one, a 30-day immediate response, began with cleaning up all inlets, raising curbs and installing bioswales and rain gardens on city property. These efforts resulted in 350,000 gallons of capacity relief. “Not a lot, but a start,” Mr. Hill said.

Phase two involved constructing a 5.8 million gallon combined sewer storage vault underneath a parking lot at Turner Field during the Atlanta Braves’ four-month offseason. In March 2015, work began on the permeable interlocking concrete roadway renovations that took nearly a year and a half to complete.

Phase three is currently in development and will mitigate eight million gallons through the construction of a combined sewer vault, capacity relief ponds and a community park to be constructed in Peoplestown on the lots that saw some of the worst flooding in 2012. The city is working with homeowners to acquire these properties at fair market value plus an additional percentage to compensate for relocation.



BIGGEST BANG FOR THE BUCK

At the outset of planning their roadway renovations, Mr. Hill and his team asked, “What will get us the biggest bang for our buck?” Considering permeable interlocking concrete pavement they agreed, “If we’re going to do a paver project, we want to have the greatest impact possible,’” Mr. Hill said. Looking back, the aggregate capacity relief storage provided by the paver system was less expensive than the water storage vaults.

With a budget of $15.8 million that initially included $1.1 million in allowances for restoring utility lines, the Department of Watershed Management began excavation and installation of permeable interlocking concrete pavers on the first of many streets upstream from the flood-prone areas. The goal was to use permeable pavers and the water storage capacity of deep aggregate reservoirs beneath them to provide downpipe capacity relief. “We picked residential streets that contributed to the flooding of our combined sewer system,” said Mr. Hill. Collectively, the four miles of permeable paver roads provided four million gallons of capacity relief.

Though the original plan had six miles of roadways slated to receive permeable pavers, once crews started peeling back the streets, they unearthed some unforeseen and unfortunate complications. On some of the larger stretches of roads, crews uncovered old streetcar lines alongside utility lines encased in two feet of concrete. “The timeline to even do a few feet at a time was going to be so outrageous that it would blow our schedule, increase cost and make it impossible for residents to access their homes, so we had to make a decision to eliminate that portion,” Mr. Hill said.

According to Mr. Hill, their desire was to place as many concrete pavers as possible and not deplete the budget on extra labor costs. This pragmatic approach was applied throughout the construction phase and brought the project to completion on time and under budget. But there were still many challenges that had to be overcome during the construction phase.

WHAT LIES BENEATH

With some street sections nearly 100 years old, the first surprise encountered by construction crews was a layer of old concrete below the asphalt roads that required additional time to remove. Once the roadways were opened up, a new set of challenges emerged. “We had utilities showing up that shouldn’t have been there, and some at depths that weren’t shown on any plans,” Mr. Hill said. Brick manholes were especially difficult to work around and many were replaced. Water mains and old pipes ruptured during excavation and required repair. Of the $1.1 million originally earmarked to address utilities, adjustments brought the total closer to $3 million by project completion.

Another main concern during construction dealt with the close proximity of older homes along some streets. Crews excavated two to four feet for the permeable pavement aggregate subbase layer and installed impermeable liners along the sidewalks to prevent lateral migration of stormwater toward these homes and their basements.

Due to their layout and age, street widths varied as much as a foot from one block to the next, adding a substantial amount of cutting time for the edge pavers. Despite this challenge, machine installation maintained an average rate of about 5,000 sf per day with no time required for concrete to cure.

Managing road closures and rerouting traffic, including public transit buses, also posed a significant challenge. “During the construction phase, there was a bit of inconvenience, to put it mildly,” said Cameo Garrett, External Communications Manager for the Department of Watershed Management. “It was very important that, as things changed during construction, we continuously provided information and updates to the affected communities.”

The original construction time estimates anticipated residents would lose access to their driveways for only a few days. But with all the utility issues encountered, the average road closures stretched to one and a half weeks. “Community outreach and engagement really needs to be taken into account,” said Cory Rayburn, Construction Project Manager for the Department of Watershed Management. “It’s very important for the contractor to have a public information officer onsite at all times during construction. We wrote that into our contract documents, and that’s something we recommend on all future projects.”



SUCCESSFUL RESULTS

“Permeable pavers are a very good solution for stormwater management, especially in highly urban areas with combined sewers that need capacity relief,” Mr. Hill said. “We have been surprised by and pleased with the amount of infiltration into the ground. We were estimating much less.” Many of the sloped streets included check dam systems to encourage infiltration. The paver streets store runoff from a four-hour, 25-year storm yielding 3.68 inches of rainfall.

While achieving capacity relief was the main goal accomplished by this project, the decision to use permeable interlocking concrete pavement also contributed to increased property values for some communities and led to new development investments. “We know the houses that are on the permeable paver streets are more sought after than on other streets in these neighborhoods,” Mr. Hill said. “The residents who live in those areas really love the pavers and think they’re very beautiful,” Ms. Garrett said.

“We have councilmembers pleased, and other councilmembers asking if they can have pavers in their districts,” Mr. Hill said. And the project has drawn not only the attention of some jealous neighbors, but national attention as well. The Department has received calls from other cities including Philadelphia, Washington D.C., San Francisco and Portland, Oregon, and has presented the project at numerous industry conferences throughout the country.

AN OUNCE OF PREVENTION IS WORTH A POUND OF CURE

The Atlanta Department of Watershed Management is now focusing efforts on educating contractors who will be working on or around their permeable pavement to prevent damage before it occurs. Nonetheless, some accidents happen from uninformed workers. In one instance, a concrete truck was washed out while parked on a permeable paver street. The runoff clogged the paver joints as well as the aggregate subbase, resulting in a $6,000 repair bill. In other instances, construction sites adjacent to the permeable paver roads needed to carefully manage sediment so it didn’t run into the street.

“It’s going to take education to ensure that anyone digging into these paver roadways has either gone through training or read the maintenance manual,” Mr. Rayburn said. So far, the Department has held an in-depth ‘Train the Trainer’ course for Watershed and Public Works employees based on the maintenance manual that was developed by the contractor, and will follow up with additional guidance and resources. “As of now, the protocol is to call our construction inspectors, the ones who were onsite during the paver installations, to monitor any tie-in construction involving water or sewer lines,” Mr. Rayburn said.

The city has a three-year contract with the project’s design-build contractor to provide service and maintenance for the permeable paver streets. “But after that, we will need a coordinated effort to help ensure the permeable paver streets are maintained,” Mr. Rayburn said.

REFLECTIONS IN HINDSIGHT

For any municipality contemplating permeable interlocking concrete pavement streets, Mr. Hill advises, “Spend a lot of time planning the process, thoroughly locate all utilities and determine if they will need rehab in the near future.” Particularly with older urban streets, there may be layers upon layers of unknown mysteries beneath the surface. “Have a full-blown SUE [Subsurface Utility Exploration] performed for every road to identify some of the harder-to-locate utilities before you actually start work,” Mr. Rayburn said. The SUE helps the design-builder come up with a more comprehensive design prior to excavation or construction, saving time and minimizing surprises.

“We are very grateful that our administration was so farsighted with regard to sustainability and making this a very green city,” Mr. Hill said. “They provided the necessary support to make these things happen.”

“Green Infrastructure and Low Impact Development practices are not new. However, the regional application by municipalities to solve flooding and capacity relief is a developing industry,” Mr. Rayburn said. “The social and economic development that can occur when these practices are done right is definitely an added benefit.”

In Atlanta’s case, the green infrastructure initiative has had a direct impact on new investment. “The Historic Fourth Ward stormwater pond adjacent to the Atlanta Beltline created a miniature ecosystem within the heart of Atlanta which reconnected surrounding residents to nature. The main function of the facility is combined sewer capacity relief, but we have seen over $500 million in private redevelopment in the surrounding area,” Mr. Rayburn said.

“We always look for opportunities to utilize green infrastructure where our historical response would have been a bigger pipe or vault,” Mr. Rayburn said. “That way, you can solve the problem while creating a real benefit for the community.”

GOING GREEN

Visit www.AtlantaWatershed.org/GreenInfrastructure for more information on Atlanta’s green infrastructure initiatives.

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Relaxing Traffic

Just about every urban center in Canada and the U.S. is jammed with traffic, especially during morning and evening rush hour (or rush hours in bigger cities). Regardless of the city size, there consistently seems to be more cars and trucks than pavement to move them. It’s certainly not relaxing traffic for the drivers stuck in it.

Because they are just about the lowest density urban land use, residential areas don’t see many traffic jams. Thanks to spread out land use, residential traffic isn’t quite as hectic. While it’s not relaxing, at least it moves, even during rush hour.

Whether low or high density, residential areas are a rising source of complaints about near misses, car crashes, plus cyclist and pedestrian accidents. Vehicular traffic needs to relax, be calmed and be mindful of non-vehicular users.

There are a variety of tools and designs to calm traffic. They range from the ubiquitous (and cheap) stop sign to more visible designs that extend curbs to narrow intersections and slow traffic. Radical road remedies reduce flows and reclaim space for bus lanes, pedestrian refuge islands, bike lanes, sidewalks, bus shelters, parking or landscaping.

A main motivation for using calming remedies is creating safer streets. The benefits outweigh the costs. According to the National Safety Council, a car accident with an incapacitating injury costs the private and public sectors (medical care, loss of productivity, etc.) about $208,500. The direct and societal costs run over $4 million for each traffic death. In 2013, a motor vehicle injury occurred on average every 14 seconds according to the Rocky Mountain Insurance Information Association. Given these events and costs, an investment in traffic calming can be recovered almost immediately.

When it comes to using pavements to slow drivers, the options are limited: speed humps or the really annoying speed bumps. A forgotten form of relaxation is changing the surface to interlocking concrete pavement. A surface change means a visual and noise change that’s kinesthetically communicated to the driver via the steering wheel. Unfortunately, ICP doesn’t show up regularly in classic traffic-calming references published by the Federal Highway Administration, the American Association of State Highway and Transportation Officials, or the Institute for Traffic Engineers. Why? No experience and no hard before-and-after data.

So let’s start collecting data. The industry seeks a current condition where vehicular and pedestrian traffic conflict is a documented problem as measured by vehicle/pedestrian counts, near-miss reports, accidents and other incidents. For example, we are seeking conditions near schools where traffic calming is essential. We’d like to monitor before and after results via surveys and/or speed/traffic counters. We are seeking a partnership where other stakeholders participate with us financially as well as in the planning, execution and monitoring stages. Potential opportunities include school districts, police/fire/rescue stations, busy residential streets, libraries, parks, business districts and complete street projects. If there is traffic that needs calming, drivers that need to relax and slow down to spare injuries and deaths, we just might have a relaxing solution.

Interested in a partnership to make roads safer? Email icpi@icpi.org.

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Updates on Paving Product Standards

ASTM’s C1782 Specification for Utility Segmental Concrete Paving Slabs provides a baseline acceptance standard for slab products manufactured with dry-cast, wet-cast and hydraulically pressed processes. Available for purchase on www.astm.org, C1782 determines the minimum average flexural strength (725 psi), dimensional and warpage tolerances, and freeze-thaw durability requirements for paving slabs with dimensions ranging from 12 x 12 to 48 x 48 inches.

Due to their larger size, segmental concrete paving slabs do not conform to ASTM C936 Standard Specification for Solid Concrete Interlocking Paving Units. The only available product standard for slabs prior to C1782 was a CSA (Canadian) paving slab standard in existence since 1972. C1782 now provides requirements with familiar ASTM terms and references that producers can meet. The standard was developed by paving slab manufacturers, testing labs and other experts within the ASTM Subcommittee on Manufactured Masonry Units and Related Units (also known as C15.03).

Architects, civil engineers and landscape architects will benefit most from C1782 by using it in construction specifications. Paving slab manufacturers will use the standard to promote products that meet or exceed its requirements. The standard will also give concrete testing labs the opportunity to provide an additional service in testing paving slabs. Most importantly, C1782 clearly differentiates slabs from the pavers in C936.

ICPI indicated that another segmental concrete paving slab standard will be submitted for balloting by ASTM in the coming months. This one will likely be named Specification for Architectural Segmental Concrete Paving Slabs. The architectural designation means it will cover units with textured architectural finishes such as hammered, polished or molded surfaces. Additionally, the specification will include closer tolerances than C1782 to better accommodate precision installations that use pedestals for roof decks, bitumen-set (sand-asphalt bedding) and some sand-set bedding applications. Such units may require grinding (also known as gauging) to conform to tighter dimensional tolerances.

UPDATES TO THE CONCRETE PAVER STANDARD

ASTM C936 Standard Specification for Solid Concrete Interlocking Paving Units received an appendix with a zone map that points to optional use of -15° C (5° F) as the lowest temperature using ASTM C1645 Standard Test Method for Freeze-thaw and De-icing Salt Durability of Solid Concrete Interlocking Paving Units. This test method calls for immersing pavers or coupons cut from paving slabs into a 3% saline solution and then exposing them to a maximum of 49 freeze-thaw cycles (each 24 hours) while inside an automated freezer. The material loss from the paver is weighed and must not exceed 500 grams per square meter of surface area to meet C936. This optional ASTM test method is very similar to that in CSA A231.2 Precast Concrete Pavers. The new optional lower temperature in C936 should increase assurance of winter durability to the purchaser as well as to the manufacturer.

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Geosynthetics Part 1: Geotextiles

Geosynthetics can be grouped into several product categories; geotextiles, geogrids, geomembranes, geonets, geosynthetic clay liners, geopipes, geofoam, geocells and geocomposites. This article examines construction with geotextiles and future articles will cover construction using the other geosynthetics. The articles are excerpted from a soon-to-be released ICPI Tech Spec that provides a comprehensive view of geosynthetic materials, selection, and construction in various segmental concrete pavement assemblies.

Table_1

When placing geotextile avoid wrinkles in the fabric. Follow the overlap recommendations specified in AASHTO M-288 Geotextiles for Highway Applications as noted in Table 1. Make sure the geotextile is placed in full contact with the surrounding soils or aggregates. Voids, hollows or cavities from wrinkles created under or beside the geotextile compromises its intended function.

Figure 1 illustrates a familiar detail, i.e., separating the compacted aggregate base from the soil subgrade with geotextile. This can help maintain consolidation of the base materials over time by preventing intrusion of fines in the bottom and sides. This slows the rate of rutting in the base and on the soil subgrade.

Geotextile placed under the bedding sand next to the curb provides a ‘flashing’ function. This separates the sand from the base and prevents sand loss into joints between the concrete curb and the compacted aggregate base, as they are two structures that can move independently from each other. Table 2 provides guidelines for geotextile selection depending on the soil and fabric functions required.

Figure 2 illustrates geotextile on a concrete base in a crosswalk application. For new sidewalks, crosswalks and streets, 12 in. (300 mm) wide strips of geotextile are recommended over all joints in new concrete bases to prevent loss of bedding sand, as well as over weep holes. New asphalt generally should not require geotextile on it except at curbs, structures and pavement junctions where bedding sand might enter. For existing asphalt and concrete bases, the surface of each should be inspected for cracks, the severity and extent of which determines repairs. If cracks are few and minor (suggesting substantial remaining life in these bases), geotextile should be placed over the cracks to prevent potential future loss of bedding sand. Covering the entire asphalt or concrete surface with a loose-laid sheet of geotextile can present some risk of creating a slip plane for the bedding sand and paving units as a result of repeated vehicular traffic.

Table_2

Figure 3 illustrates a typical application of geotextile in PICP. Its application against the sides of the subbase and against the excavated soil is essential in all PICP projects that do not use full-depth concrete curbs to completely confine open-graded aggregates at the pavement perimeter. The design and selection of geotextiles for PICP is covered in detail in the ICPI manual, Permeable Interlocking Concrete Pavements – Design, Specification, Construction, and Maintenance.

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Superb Stewardship

Burbank, California, may be the famous studio location for The Tonight Show with Johnny Carson, but the EcoCampus at Burbank Water and Power (BWP) broadcasts its own show worthy of a different fame. The project debuted as the only industrial category selectee among 150 national and international projects for the Sustainable Sites pilot program in 2012. BWP and its design partner, AHBE Landscape Architects in Los Angeles transformed an aging power plant site into a regenerative green campus with aspirations toward net-zero water use.

“Being good stewards, doing the work of service, our landscape is reflective of those values we have here,” said BWP Conservation Manager Joe Flores. BWP offers regular tours of its EcoCampus to educate visitors about several stormwater management technologies there which include permeable pavers and concrete planks. At the heart of the campus, Centennial Courtyard features 4 x 16 x 3 1/8 in. thick planks in a multi-colored array of pewter, amber, caramel, mocha and charcoal. While larger and longer paving units continue growing in popularity, this three-year-old project represents early pioneering with planks.

Contemporary Cool

Two distinct trends have emerged over the past few years including a shift from warm earth-tone colors to cooler shades of gray, and growing use of larger paver units, slabs and planks. AHBE designers wanted a modern, linear appearance for the courtyard, and once BWP saw samples of the planks from an ICPI member manufacturer, BWP fell in love with them. The decision was also influenced by the salvaged and repurposed structures BWP chose to retain from the original site. Though initially the plan was to remove everything, BWP envisioned a transformation rather than a complete demolition. Old generator pads became seating areas; utility tunnels became infiltration chambers; equipment plinths became garden sculptures; and a two-story steel skeleton substation became a trellis for a shade canopy. Multicolored planks provided the desired complement. “If we just poured concrete [for the courtyard], it wouldn’t be very visually interesting,” said Mr. Flores. “The pavers add a material richness you want in that kind of environment,” he added.

As Above, So Below

From the outset, BWP wanted to develop a green campus with sustainable stormwater detention and filtration technologies. The Centennial Courtyard planks sloped to drain stormwater into a phytoextraction canal. Formerly a tunnel that carried power cables from the power plant to the electrical substation, the six-foot deep tunnel floor was perforated and then backfilled with select soils and plants that filter stormwater runoff as it permeates down into aquifers. At each side of the canal, fountains of recycled water are circulated by solar-powered pumps.

A green street development spanning three city blocks along Lake Street includes an 8 ft wide permeable paver sidewalk and filtration planter bump-outs collecting and infiltrating water into concrete bioretention cells with trees. “With permeable pavers, you’re able to do stormwater capture and then direct the water to encourage trees and plants to grow roots downward, which is healthy for the landscape, and also alleviates problems of roots uplifting sidewalks,” Mr. Flores explained.

The visual qualities of the landscape are noteworthy, but what lies beneath—a campus-wide water filtration system—is truly remarkable. Five water filtration technologies were used: infiltration, flow-through, detention, tree root cells, and stormwater capture. According to BWP, this was the first time this number of sustainable landscape technologies were integrated into a single industrial site.

Watch a video about sustainable green infrastructure projects at Burbank Water and Power.

Pixilation and Progeny

Three types of pavers were used for the courtyard and green street, according to AHBE Landscape Architects Principal Evan Mather, ASLA, RLA. “We used plank pavers for the courtyard, rectangular pavers where we didn’t want to infiltrate, for example, next to buildings, and permeable pavers where we wanted to infiltrate,” said Mr. Mather. “The overall look of the campus doesn’t present scored concrete; it’s more about the individual pixilation of the paver materials.”

Regarding maintenance, Mr. Flores said, “It’s not as much as you might think. We don’t have to do much other than blowing [leaves and debris] and cleaning up the occasional spill.” Because the pavers are multicolored, a few spots here and there aren’t nearly as noticeable as they would be on a continuous white concrete surface, Mr. Flores said.

Congruent Values

The close collaboration among the project owners, landscape designers and the paver manufacturer resulted in a creative synergy where each drove the others toward greater excellence. “They were a fantastic partner,” Mr. Mather said of BWP. AHBE has been a leader in sustainable design for 30 years, Mr. Mather said, but an industrial power plant might be the last place one would think of when it comes to sustainability. With all its green merits, the BWP EcoCampus really is for the people. Human utilization of the campus and courtyard space drove the design from the outset according to Mr. Flores. “Using the space in this way creates a healthy work environment…because it’s congruent with the values of people who want to work here.”

To anyone mulling over a similar redevelopment project, Mr. Flores offers this advice: “Consider not just the physical elements but the human and cultural aspects that define the values of your organization. How can you express that through the use of your physical space?” Addressing these human aspects resurrected this site with support from carefully selected and placed concrete paving units.

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Cyclone Sighting

Since the 1990s, municipalities and private property owners constructed millions of square feet of pervious concrete (PC), porous asphalt (PA) and permeable interlocking concrete pavements (PICP) in parking lots, alleys and streets. The number one question is about maintenance. The next questions typically are how often should the surface be cleaned and with what equipment?

All permeable pavements require regular surface cleaning to remove embedded sediment and to maintain surface infiltration. Regenerative air machines used for routine cleaning are effective in removing loose sediment and debris. Low surface infiltration into highly clogged pavements with tracked-in or settled sediments can be raised with a true vacuum machine. Equipment availability, costs, personnel time or outsourcing costs for surface cleaning suggests a need for a single machine that provides routine maintenance cleaning, as well as restoration of clogged surfaces when maintenance is neglected.

The diesel-powered Cyclone CY5500 carries approximately 1,200 liters (300 gal) of water.

The diesel-powered Cyclone CY5500 carries approximately 1,200 liters (300 gal) of water.

A machine that might qualify for this role is the Cyclone CY5500. Originally developed to clean tire rubber from runways, this machine was tried in June 2015 on porous asphalt (PA), pervious concrete (PC), and permeable interlocking concrete pavement (PICP) in a residential neighborhood in Northwest Washington, DC. All of the permeable pavements were installed by the District of Columbia Department of Transportation (DDOT) as part of a combined sewer overflow mitigation program.

The sites included PC in two nearby on-street parking lanes. One was cast-in-place PC and the other was a precast PC panel, among several. These two areas received contributing run-on from the impervious center lane of the street. The PA and PICP were situated in alleys, with some or little run-on from impervious surfaces and instead received sediment from adjacent vegetated areas. All of the permeable pavements were subject to leaves and debris from a mature urban forest canopy. None of the pavements were older than a year in service.

 

The diesel-powered Cyclone CY5500 is an off-road vehicle smaller than the truck-size equipment. This machine carries approximately 1,200 liters (300 gal) of water, much of which is drawn back into the machine, filtered and re-used.

The Cyclone machine relies on water applied under pressure in a circular motion within a surrounding chamber in close contact with the permeable pavement surface. The water pressure can be varied by the operator from 1,200 psi (8 MPa) to 4,350 psi (30 MPa). Water is blasted against the pavement surface and the speed of the rotating head applying the water provides some suction (hence the cyclone name) to pull most of the water back into the machine for reuse. The machine manufacturer claims cleaning rates as high as approximately 10,000 sf (935 m²) per hour on most permeable pavements.

Prior to conducting cleaning, ASTM C1701 Standard Test Method for Infiltration Rate of In Place Pervious Concrete and C1781 Standard Test Method for Surface Infiltration Rate of Permeable Unit Pavement Systems was applied to each surface. The former test method is applicable to PC (and PA). Both test methods produce comparable results. This is illustrated in Figures 2 through 5.

The pre-wetting initial infiltration test measurement was conducted to determine the extent of clogging. All of the pavements were clogged with little or no infiltration within the ring. Then, the four areas were cleaned in the following order: cast-in-place PC, PICP, PA, then the precast PC panel. The Cyclone machine passed twice over the same area of permeable pavement almost immediately after the initial infiltration testing (called pre-wetting). Figure 6 shows the Cyclone machine making a typical pass of about 30 ft (10 m) in length on an alley.

Figure 6

Figure 6: The Cyclone machine cleans a PICP alley.

After the second pass, the ASTM test ring was applied to the pavement surface and an additional (approximate) 8 lbs (5 kg) of water was applied (the surface infiltration rate was calculated per the ASTM standards). Both ASTM standards use the same surface infiltration calculation. If the surface infiltration rate was under 250 mm/hr (100 in./hour) the Cyclone machine made an additional two passes, the ring reapplied in the same location and an additional approximate 8 lbs (5 kg) of water applied into the ring. The table provides a summary of the surface infiltration test results.

The table indicates increased infiltration rates after the first two passes on the PC, PICP and PA. Infiltration rates doubled for the PC and PA but could be considered low. There was little if any change in the infiltration rate of the precast PC panel after the first two passes. The second two passes yielded better results with PC infiltration rate doubling again and the PA almost reaching the same level. The precast panel saw a substantial increase as well, from <20 in./hr (<508 mm/hr) to 130 in./hr (3,302 mm/hr) after the second round of two passes.

Figure 7

Figure 7: PICP after two passes of the Cyclone machine. Note aggregate and sediment removed resulting in open joints and a permeable surface.

The most notable observation is that the PICP only required two passes of the Cyclone machine rather than four to increase the infiltration rate from 20 in./hr (<508 mm/hr) to 327 in./hr (8,306 mm/hr). The joint widths in the PICP were narrow, approximately ¼ in. (6 mm) wide and many of the small aggregates were pulled out with the sediment after the first two passes. Some of the stones were left on the surface of the pavers after the second pass and these could be swept back into the joints. See Figure 7. Additional aggregate should be supplied given these results.

In this experiment, the Cyclone machine was assigned to clean highly clogged pavements. It can be set on a lower pressure setting to clean a less clogged condition, i.e., remove loose material from the pavement surface. In the case of this brief demonstration however, the PICP surrendering sediment with the jointing aggregate to the Cyclone machine explains the resulting high infiltration rate after two passes rather than four passes, as conducted on the other surfaces. This demonstrates the ability of clogged PICP to experience restored infiltration rates as compared to monolithic surfaces, even when heavily clogged.

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Why Aren’t All Streets like This?

The morning after an overnight rainstorm, Tom Sweet, AECOM senior engineer, walks two blocks from the Downtown Berkeley BART station and puts on his yellow safety vest to inspect the Allston Way permeable paver street he helped design. “How’s the ride?” he asks a passing skateboarder, who gives a thumb’s up in response. Local residents approach from the adjacent park, ask if he’s involved with the new street and tell him how beautiful it is. When he explains the street can also infiltrate stormwater, filter pollutants, reduce runoff to San Francisco Bay and improve the health of the park’s trees, they ask, “Why aren’t all streets like this?”

Facing an aging infrastructure as are many cities today, the City of Berkeley sought a durable alternative to its existing asphalt road surfaces in need of replacement. Situated on a hill that slopes down to San Francisco Bay, mitigating stormwater runoff also ranked highly among its priorities. A public works commission began extensive research into green infrastructure redevelopment options and called in David Hein, P. Eng., Vice President of Transportation with Applied Research Associates, Inc. (ARA).

“The City had wanted to do this for a long time,” said Mr. Hein, referring to the City Council’s desire to construct a green roadway using a permeable paver system as a demonstration project. A 40-year life-cycle cost analysis showed permeable interlocking concrete pavement to be almost the same cost (less than 2% difference) as an impermeable flexible pavement. However, the analysis did not take into account the benefits from permeable pavements such as reducing stormwater runoff volume, peak flows and pollutant loads. If these cost factors were taken into account in the LCCA, the permeable pavement would have the lowest total present worth cost, according to Mr. Hein.

Suitability Evaluation

With numerous sites proposed by the City for the PICP demonstration street, an evaluation matrix created by ARA for this project determined the best choice. The suitability design matrix identifies key factors that may influence design and effectiveness for a specific project, categorizes those factors as primary or secondary considerations and assigns weighted values on a scale of 0-100. If the score totals less than 65, the project is not considered a good candidate for permeable pavement. Scores between 65 and 75 are worthy of consideration, but scores over 75 indicate a well-suited site. A section of Allston Way received a high evaluation with a score of 81.

However, there were a number of initial concerns expressed about the project. The cycling and skateboarding communities weren’t sold on a segmental road surface for the heavily trafficked bike route of Allston Way. City engineers had concerns about the utilities below the street and the depth of excavation required for a properly installed PICP system as well as the suitability of Berkeley’s clay soil for infiltration. City arborists made specific requests for care and caution in excavation around tree roots. And a small time window for construction to coincide with the adjacent high school’s summer break added another challenge. To address these concerns and requirements, the initial design underwent some innovative refinement.

Refined Design

“We had a fairly deep section proposed,” explained Mr. Sweet. “Over the course of the design, as a cost-benefit, we looked into making the section thinner from a pragmatic approach.” And that’s when flexible HDPE cellular confinement for the aggregate base entered the picture. From an initially proposed depth of 41 in., the addition of an 8 in. cellular confinement layer provided enough structural stability and strength to reduce excavation to 29 in. It shaved a full foot off the excavation depth requirement, thus saving time, reducing the cost of off-haul as well as emissions and minimizing risk to the underlying utilities.

A few more innovations addressed concerns about clay soil infiltration and the street’s nearly 3% longitudinal slope. Eleven check dams were specified, but the flexible design permitted non-uniform placement at the most logical locations, i.e., where the sections had been excavated to full depth and around the existing utilities. “We didn’t want all the water to go to one end and oversaturate the subsoils, so we were very careful in the detailing to segregate the water,” Mr. Sweet said.

Additionally, the underdrain was raised up 6 in. from the subbase to take advantage of some detention and infiltration over clay soil. “It’s an opportunity that most projects miss,” explained Mr. Sweet. “A lot of folks in the profession say, ‘It’s clay soil, we can’t infiltrate.’ In fact, you can infiltrate, you just need to be more careful with how you do it and where you do it.” The sizes of the openings in the underdrain were carefully considered. “We wanted to recover but we didn’t want it to act as a conveyance,” said Mr. Sweet. The underdrains buffer the rate of flow leaving each check dam. Though the water level might reach the holes in the perforated pipe, limiting the number and the opening size allows water to go higher. Designed for a 48-hour drawdown, the underdrain system is a water recovery mechanism, not an instantaneous outflow. “Personally, I am very excited about this system. I think it’s almost the highlight of the project,” said Mr. Sweet.

Breaking New Ground

Construction of the project took place during the summer of 2014. The curb-to-curb pavement surface area totaled 29,145 sf (2,700 m²). Don Irby, P.E., Associate Civil Engineer with the City of Berkeley Public Works Department, managed the construction from beginning to end. This was Mr. Irby’s first permeable paver project. “We had to close the road for almost three months because it’s just not really economical to do this type of installation in sections,” Mr. Irby said. “That played into our location selection because we had to look at driveway access. There are a lot of things you need to take into account when you site one of these projects.”

Ghilotti Construction Co. managed the road closure as the general contractor for the project and handled the careful excavation around utilities and tree roots.

European Paving Designs Inc. (EPD), an ICPI-certified installer, installed the pavers. “As soon as we saw the tight spec for this project, we were really motivated to get the job,” said EPD CEO Randy Hays. “[The specification] referenced ICPI’s PICP manual. We knew we had the expertise and experience to make it successful.”

With a seven-man crew working in two phases, the blend of reddish orange-and-charcoal pavers was installed in a herringbone pattern before installing pavers for striping. “That seems to work the best,” Mr. Hays said. “It allows for some give and take with the location of the stripes, so if we have to shift it an inch for alignment, as long as there are no small pieces, that’s the right way to do it.” Yellow-pigmented pavers provided contrast with the darker pavers, all supplied by Pavestone Company. The EPD crew laid out the pavers for the stripes, cut the sections out from the installed field and then inserted the yellow pavers. Mr. Hays said his foremen recall people first seeing the completed installation while they were doing cleanup work. “They were looking down as they crossed the street and said, ‘Wow, that’s unique.’”

Manuals for Labor

As part of their involvement on the project, ARA created two manuals for use by the City of Berkeley. One established a maintenance plan and the other specific maintenance procedures. Mr. Hein explained, “Ultimately, the purpose of ICPI is to provide guidance to people on the use of paver products. If you handle them, here’s how you do it right. I think the Utility Cut Manual and the Maintenance Guidelines are very important.” Each manual’s appendix includes ICPI Tech Spec 6 — Reinstatement of Interlocking Concrete Pavements.

Results Exceed Expectations

In the year and a half since the Allston Way project’s completion, it has been routinely and closely monitored. “The system has exceeded expectations with regard to stormwater management,” said Mr. Irby. “The infiltration rate that we’re seeing is better than we had estimated.” He added, “We haven’t published the data yet, but what we’ve gathered to date does show that the pollutant levels have been reduced.”

“The most recent storm we monitored was 1.75 in. over 19 hours, a fairly large storm for California,” said Mr. Sweet. “And we attenuated 94% of the runoff. We did readings off the discharge pipe and it really shows the benefit of spreading the water out and letting it infiltrate at a comfortable rate, to the extent that it can.”

Additionally, the City Forestry Department is monitoring the health of trees in the adjacent park to see if there is noticeable improvement. The trees are photographed on a regular basis for analysis, but this study will require a good deal of time before results become apparent.

“There was a lot of concern about the roughness of the surface from the cycling community, but I haven’t heard a word from them since it was installed,” said Mr. Irby. “And we see hundreds of bikes on the street every day, and skateboarders too. It’s an incredibly smooth surface.” That smooth surface is due to EPD’s expert installation of the ADA-compliant pavers specified, which feature a quarter-inch joint and interlocking spacer bars. In fact, the permeable paver surface is likely safer for cyclists to traverse in wet conditions because it prevents standing water from collecting of the pavement surface. “The coefficient of friction for a permeable paver surface is better than most asphalt roads,” Mr. Irby explained. “There are lots of benefits that people aren’t really aware of.”

The Intangibles

“The arc of this project was very gratifying,” Mr. Sweet said. Given the initial concerns expressed by the City and the community, “Everyone landed in the same place saying, ‘Wow, this project is worthwhile, it’s interesting, it’s the right thing, and it turned out great.’”

“I’m happy I got to be a part of this project,” Mr. Irby said. “I have coworkers who go out of their way just to walk by that street because it’s really nice to look at.”

“The City of Berkeley is really committed to the environmental aspects of construction,” Mr. Hays said. “Sustainability in the construction industry, especially with regard to water conservation, is very important. Building streets that actually return water to an underground system is a pretty cool thing.”

As for the question “Why aren’t all streets like this?” That answer may just be a matter of time.

By

Cool by the Poolside

A Familiar Start: Compaction

Pools made with a flexible liner or fiberglass require much excavation and soil backfill against them. This soil is almost impossible to compact adequately because there is a high risk of compaction equipment damaging the pool wall, the metal “knee” wall/wing wall supports, and/or pipes. Pools made with gunite concrete or poured concrete can have walls of sufficient thickness and support where backfill soil against them can be compacted. However, the soil directly against the wall will be difficult to compact, again because of the risk of damage to a plate compactor or to the pool wall. Therefore, settlement can occur here as well.

Some paver installation contractors claim that installing and compacting the soil backfill and aggregate base can reduce the risk of settlement. That may be true assuming that maximum density is achieved for both materials during compaction. However, the places compaction equipment cannot reach will be at risk of settlement. If a contractor uses a compacted aggregate base, then a warranty should be included in the price to cover the cost of returning to the site a few years later to inspect and lift settled paver areas. Given unpredictable future costs and hassles for the contractor (not to mention the owner), it’s better to complete a job that doesn’t include a return visit.

To avoid returning for paid (or worse still, unpaid) callbacks due to settlement issues, use a 4 in. (100 mm) thick concrete pad as a base. Placing a 2 in. (50 mm) thick layer of free-draining compacted aggregate such as ASTM No. 57 stone under the base enables water to drain from below. Figure 2 illustrates this material placed over a compacted, crushed stone base. The concrete base surface should slope at least 1.5% to allow water to drain.

A key aspect of all pool surfaces is no settlement or undulations that present walking or tripping hazards, or slipping hazards from birdbaths. By providing a rigid, sloped foundation, a concrete deck under the concrete pavers helps maintain safety. This should be the first priority and result of all pool projects.

Wear an Apron or the Full Dress?

When a concrete base is used, should it be built as an apron covering only areas of settlement-prone compacted soil and aggregate base around the pool perimeter? Or should it extend as a full dress under the entire area of the concrete pavers? A dress is preferred. The problem with apron construction is differential settlement of the soil and base next to the concrete base under the concrete pavers will almost always occur regardless of diligent soil and base compaction. Slideshow images show installation and a completed concrete full dress surrounding the entire pool area, as well as installed coping attached to the concrete base structure. In this case, the coping is not resting on the pool wall.

Overlays on Existing Concrete Decks

Installing concrete pool decks with an overlay of concrete pavers is less expensive than removing and replacing the entire concrete deck. To qualify for an overlay on an existing concrete deck, the concrete should not be heaving or faulted, as this often indicates severe settlement of the soil beneath or expansive clay soils. In these cases, subsurface drains can remove excess water from the soil. Expansive soils can be stabilized with lime. Both should be done after demolishing the concrete deck and before pouring a new one. The advice of a professional civil engineer familiar with the local soils should be obtained in such situations.

Cracks in the existing concrete base can be filled with a cement-based patch to prevent migration of bedding sand into them. The junction of the concrete slab with the pool wall should be sealed with a neoprene or urethane sealant (often applied with a caulking gun). This keeps water from getting behind the pool wall and saturating the base and soil.

There is a growing trend toward using paving slabs, generally 12 x 12 in. (300 x 300 mm) or larger. Sometimes these are mixed with smaller units. All units should be at least 2 inches thick. They require a thin, coarse (drainable) bedding sand layer, typically no thicker than ¾ in. (20 mm), screeded smooth, ready to receive the paving slabs (or pavers).

Slabs and pavers should be compacted with a plate compactor with rollers on the bottom to reduce the risk of cracking slabs. Joints are then filled with sand, and sealer is applied. Prior to placing and screeding the bedding sand, 12 in. (300 mm) wide geotextile strips should be applied over concrete deck joints, placed and turned up at joints against structures, and placed along coping to prevent bedding sand loss.

Prior to applying the pavers, all area drains must be raised to the new finished elevation of the installed pavers. Holes must be drilled into the vertical drain pipe directly above the concrete deck. This drains excess water from under the pavers. The holes should be covered with geotextile to prevent ingress of sand.

Many overlays use thin, tile-like concrete pavers placed directly over a concrete deck. Thin pavers typically range between 1 to 1½ in. (25 and 40 mm) thick and are generally about 4 in. (100 mm) wide by 8 in. (200 mm) long. Unlike sand-set slabs or pavers, concrete pavers are directly applied to the existing concrete deck without bedding sand after cracks are patched. Edge pavers are secured with a polymer adhesive or mortar (in non-freezing areas).

Fine sand is swept and washed into the joints until they are full. Using thin pavers as new construction of course avoids bedding sand as well. In new or rehabilitative projects, washing sand in the joints enables it to flow under the pavers so no rocking or clicking of pavers occurs when walked upon.

After the surface is completely dry (usually in 24 hours), it receives sealer to hold the sand in the joints and reduce water ingress. The sealer is typically reapplied every three to five years to maintain the sand in the joints and protect the surface. Even with this maintenance cycle, overall costs are well lower than replacing the entire concrete deck. Sealers also greatly reduce the risk of mold and bacteria, thereby addressing concerns of health officials regarding public pools.

Advantages of Pavers

Tie-downs for pool covers can be installed below the pavers with high-strength grout-filled sleeves. Tie-down caps should be even with the paver surface. Should pipe or wiring repairs be needed, concrete pavers can be removed and reinstated with no ugly patches. The units resist chlorine and bromine, as well as freeze and thaw cycles. Concrete pavers offer a slip-resistant surface even when sealed. Salt-water pools can attack concrete surfaces, so be sure they are thoroughly sealed.

Besides their unmatched beauty compared to other deck surfaces, colored concrete pavers reduce the glare often associated with cast-in-place concrete pool decks. Almost every paver pool deck in warm climates consist of beige, coral, or buff colors that reduce glare from the sun. Because the units have joints, each unit has some opportunity to release heat faster than a cast-in-place concrete deck. Therefore, the units can be cooler underfoot than other surfaces. So a paver surface feels as well as looks cool.

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Research Raises the Barrier

A common barrier to using permeable pavements over clay soils is their lack of infiltration. A recent study released by North Carolina State University demonstrated that permeable interlocking concrete pavement (PICP) is an effective tool to improve stormwater runoff hydrology and water quality, even when sited over very low infiltration soils. Located at a city park in Durham, NC, this project researched PICP efficacy over nearly impermeable soils (approximately 0.01 in./hr or 0.254 mm/hr) from March 2014 through April 2015. Four parking stalls (540 ft² or 50 m²) were retrofitted with PICP with a very small contributing impervious area. PICP design followed design guidelines outlined in Chapter 18 of the North Carolina Department of Environment and Natural Resources (NCDENR) BMP manual.

Results through 13 months of monitoring indicated 22% volume reduction via subgrade infiltration and evaporation. Inter-event infiltration of water within the 6 in. (150 mm) thick subbase created storage to capture over 70% of the runoff volume from storm events less than 0.30 inches, and peak flows were significantly reduced by a median of 84%. The site exhibited exceptional pollutant removal efficiency with influent and effluent pollutant concentrations significantly reduced for total suspended solids (99%), total nitrogen (68%), and total phosphorous (96%). The median effluent concentrations of total nitrogen (0.52 mg/L) and total phosphorous (0.02 mg/L) were below “excellent” ambient water quality thresholds for the North Carolina Piedmont Region. The median total suspended solids effluent concentration was also very low (6.99 mg/L). Nitrogen and phosphorous are nutrients that can accelerate algae growth and damage to waterways. Many pollutants are carried with suspended solids, so their concentrations are an indirect indicator of water quality. Obviously, any reduction in runoff volumes translates to reduced pollutant loads into waterways.

Additional sampling of the various nitrogen forms at 12, 36, 60, and 84 hours post-rainfall was conducted to better understand mechanisms of nitrogen removal in permeable pavement. Results from one storm event indicated denitrification is likely occurring in the open-graded aggregate reservoir within the pavement. For the events monitored, significant reductions in average concentrations for copper (79%), lead (92%) and zinc (88%) were also observed. Typically shed by vehicles, metals in high concentrations can severely damage aquatic ecosystems.

Cumulative loading reduction for the catchment was excellent with loading removal efficiencies of 98%, 73% and 95% for total suspended solids, total nitrogen, and total phosphorous respectively. These results show permeable pavements built over low-infiltration clay soils provide considerable improvement of water quality and moderate hydrologic volume reduction benefits.

Monitored data was also used to calibrate DRAINMOD, a widely-accepted agricultural drainage model, to predict the cumulative and event-by-event hydrologic performance of the study site. DRAINMOD accurately predicted runoff volumes from the impervious drainage area with very high correlations between modeled and actual inflows to the site. Good agreement between predicted and measured drainage was also observed. Cumulative predicted drainage volume was within 6% of what was measured during the monitoring period. These results indicate DRAINMOD can be applied to predict the water balance of permeable pavements built over low-infiltration clay soils on a long-term, continuous basis. To receive a copy of the 46-page report written by Alessandra Smolek, Ph.D. student and Professor Bill Hunt, email requests to the editor at dsmith@icpi.org.