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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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Shed the Shovel, Halt the Salt

Winter’s snowfall brings inconvenience and injury risk for many across North America. Slipping on snow or icy surfaces can cause hip dislocations, wrist fractures or even head injuries. From private residences to commercial storefronts, snow and ice removal is a responsibility not to be taken lightly. A snowmelt heating system effectively tackles that responsibility with benefits beyond merely melting snow. By significantly reducing or potentially eliminating the need for deicers and shoveling, a pavement heating system can also help preserve the beauty and longevity of pavers while reducing liability.

While the cost to install a pavement heating system may qualify it as a luxury item, the return on investment comes from saving hours of shoveling and deicer costs. When considering the savings season after season, particularly in heavy snowfall regions, the investment yields valuable returns.



HYDRONIC OR ELECTRIC?

There are two types of pavement heating systems: electric and hydronic. Electric systems conduct heat through wires or cables, whereas hydronic systems pump and recirculate a mix of glycol and water through a loop of flexible polymer or synthetic rubber tubing. Generally, an electric system is cheaper to install but costs more to operate over time because the current draws continuously while the system is on. A hydronic system is more expensive to install due to the additional components required such as a dedicated boiler, pumps and manifolds, often installed by a plumber. Hydronic systems have lower operating costs because they reheat and recirculate the fluid. With more parts, hydronic systems may require more maintenance over time than electric systems.

THE INSULATION FACTOR

Another key factor to determine at the outset is whether or not an insulation layer is required by local building codes. Places like Aspen, CO, or Sun Valley, ID, for example, require an insulation layer for pavement heating systems to maximize energy efficiency. This adds costs and can cause the pavement to fail if not correctly installed.

“I’ve seen a 70-foot driveway where the pavers slid six inches and left a gap at the top,” said Marc Larsen of Mountain West Paver Specialists. The insulation material often used is squishy, like bubble-wrap, explained Larsen, and installers mistakenly place it on top of the base. “You have to remove the flexibility of that insulation material by putting it under the rigid base of a concrete slab.”

If there is no building code requirement to use insulation, it can be presented to the customer as an efficiency option but it’s not necessary for the system to function optimally, according to Larsen. The ICPI construction guidelines in ICPI Tech Spec 12 – Snow Melting Systems for Interlocking Concrete Pavements do not recommend insulation below the bedding sand in residential driveways. However, insulation below the bedding sand is acceptable for pedestrian-only applications such as a patio or sidewalk. For roads or crosswalks, concrete or asphalt bases are recommended.



PERFORMANCE PLANNING

The design and performance of a snowmelt system depends on three environmental factors: the rate of snowfall, the temperature of the snow and wind conditions. Snowmelt rates will vary with the application. For example, melting 1 in. (25 mm) of snow per hour is usually acceptable for a residence but may be unacceptable for a sidewalk in front of a store. Most manufacturers of hydronic and electric snowmelt systems provide design guidelines and/or software to calculate the BTUs per square foot (watts/m2) required to melt a range of snowfalls for a given region.

The design methods work through a series of calculations that consider the snow temperature (density), air temperature, exposure of the pavement to wind, and unusual site conditions. The calculations indicate the size and spacing of cables or tubing required, as well as the temperature of the fluid, its flow rate, or the electricity required. The Radiant Panel Association (radiantpanelassociation.org) provides design guidelines for liquid snow melt systems.

LAYOUT AND CONSTRUCTION

With electric and hydronic systems, the best performance comes from a heat source placed as close to the pavers as possible, nestled into the bedding sand. The recommended depth for bedding sand is normally 1 inch. However, the wires or tubing need a ½ inch of sand over them for protection from abrasion and possible rupture. Therefore, the diameter of the wires or tubing may increase the bedding sand thickness to a maximum of two inches before compaction.

Once the base is installed and compacted to the proper depth and density per ICPI Tech Spec 2 – Construction of Interlocking Concrete Pavements, a galvanized wire mesh is placed over the surface of the base and secured to the base with stakes. The wires or tubing for the heating system are then fastened to the wire mesh with plastic zip ties. Installation of wires or tubing should be done by an electrician or plumbing contractor experienced with these systems. Before placing sand or pavers over the system, it should be tested for leaks.

Some contractors install the wires or tubing into the top inch of the base to forego the wire mesh and facilitate easier sand screeding. In this case, base material is added around the wires or tubing and then compacted to bring the level of the base to its final grade. The wires or tubing are exposed flush with the compacted surface of the base.

While the above guidance is suitable for pedestrian and residential driveway applications, areas subject to constant vehicular traffic such as crosswalks or roads require wires or tubing placed within a concrete slab or asphalt, rather than on top of the base. This protects the heating system from tire damage. Check with the wire or tubing manufacturer to be sure materials can withstand hot asphalt and its compaction. When an asphalt or concrete base is used, 2-inch diameter weep holes should be added at the lowest elevations for drainage, filled with washed pea gravel, and covered with geotextile to prevent bedding sand loss.

For permeable interlocking concrete pavements, wire or tube spacing will most likely be reduced to account for heat loss to the air voids within the permeable aggregate bedding layer. The manufacturer of the heating system should be consulted on durability of the wires or tubing when placed against bedding aggregate and then subjected to vehicular tire loads.

ts12INSTRUCTION AND GUIDANCE

ICPI Tech Spec 12 – Snow Melting Systems for Interlocking Concrete Pavements provides detailed installation guidance and is available for download from the resource library page of ICPI’s website: icpi.org/resource-library. ICPI also offers courses that provide instruction and certification. Visit icpi.org/education-certification to learn more and to register.

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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.

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The Permeable Future

Under development for three years, the American Society of Civil Engineers Transportation and Development Institute will likely release a national standard guide on design, construction and maintenance of permeable interlocking concrete pavement (PICP) later this year. The 100+ page draft was written by a committee of consultants, government agency personnel, stormwater advocacy groups, contractors and industry suppliers. The group is chaired by David K. Hein, P. Eng. with Applied Research Associates, Inc. Final stages include balloting by his committee, comments from the public and resolution of persuasive public comments.

Titled Design, Construction and Maintenance of Permeable Interlocking Concrete Pavement, the document is divided into five chapters. The first chapter provides a table to assist users in determining if a particular site is suited for PICP. Then the material components of PICP are covered including system infiltration options; full, partial and none.

Structural design in the new ASCE standard comes from full-scale load testing and modeling by the University of California at Davis.

Structural design in the new ASCE standard comes from full-scale load testing and modeling by the University of California at Davis.

Structural and hydrologic analyses are provided. Structural analysis includes subbase thickness tables developed from mechanistic modeling and validation using full-scale accelerated load testing at the University of California Pavement Research Center (UCPRC) in Davis. These introduce a new design approach where the designer must estimate the number of days annually water will be in the subbase. This is conservatively estimated by determining the infiltration rate of the underlying soil subgrade and then, using daily rainfall data, determining the average number of days per year with rainfall greater than the 24-hour infiltration rate of the subgrade. This approach addresses the reality that some PICP projects have water standing in the base and soil, i.e., a saturated condition, for a number of days. This weakened state of the soil subgrade is factored into tables used to determine the subbase thickness.

The other influence on subbase thickness is the required water storage. The amount of storage and resulting base thickness are determined by the infiltration rate of the soil, given requirements from the local government on how much water should be managed. The standard provides equations for estimating subbase thicknesses that infiltrate all of the water into the subgrade, and equations for infiltrating some of the water into low infiltration soils while draining the remainder into pipes. The guide encourages using computerized hydrologic models including ICPI’s Permeable Design Pro software. Model selection is left to the designer on using a single rain-event or continuous event model that simulates rainfall, infiltration and runoff for as long as a year. Whatever computational model is selected, it balances inputs from rainfall against outputs expressed as infiltration and outflows from the pavement subbase.

The emerging ASCE PICP standard guide lists ICPI’s Permeable Design Pro among several software programs used for hydrologic calculations during the design process. The advantage of this program is its ability to calculate subbase thicknesses required for water storage plus the thickness required to support vehicular traffic.

The emerging ASCE PICP standard guide lists ICPI’s Permeable Design Pro among several software programs used for hydrologic calculations during the design process. The advantage of this program is its ability to calculate subbase thicknesses required for water storage plus the thickness required to support vehicular traffic.

Construction guidelines describe the essential construction steps. The PICP installer must have a PICP Specialist, an ICPI-trained person, in charge of the construction and present on the job site. The standard includes a construction inspection checklist for use by contractors and inspectors. This aligns with ICPI’s recently released continuing education presentation on PICP inspection. The checklist also closely aligns with the one in the 2015 ASCE book, Permeable Pavements.

To address the number-one question by project owners and stormwater agencies, the standard covers maintenance surface cleaning methods and surface repairs. Specifically, the standard describes routine preventive maintenance to keep surface infiltration flowing and more concentrated remedial maintenance techniques should the surface become clogged and render very low infiltration. Like construction, the section on inspection deserves a checklist covering most aspects. A foundational inspection item is checking surface infiltration by observing ponding during or just after a rainstorm. Clogged areas should be checked using ASTM C1781 Standard Test Method for Surface Infiltration Rate of Permeable Unit Pavement Systems.

After an extensive References section, the Appendixes include a design example for structural design that leads the user through the UCPRC subbase thickness design charts. Additional hydrologic design examples quantify water volume and flow through full, partial and no-infiltration configurations. Appendixes include a guide construction specification with direction on using a lightweight deflectometer for deflection testing the compacted subbase and base. (See Engineer’s View in this issue for more information on this device.)

For comparison purposes, the Appendix also has subbase thickness design charts calculated from the flexible pavement design method (for non-permeable pavements) in the AASHTO 1993 Guide for Design of Pavement Structures. These yield conservatively thick subbase thicknesses. These are similar to those found in the UCPRC tables under the highest number days per year water stands in the subbase.

As a result of visual inspection during or right after a rainstorm, the ASCE standard points to ASTM C1781 as a means to measure PICP surface infiltration.

As a result of visual inspection during or right after a rainstorm, the ASCE standard points to ASTM C1781 as a means to measure PICP surface infiltration.

The ultimate purpose of this emerging ASCE standard is for provincial, state and local governments to reference it in their green infrastructure and low impact development manuals, as well as in stormwater management and road agency design guidelines. While the standard’s propagation and acceptance may take years, the expectation is less time will be required than an entirely new technology. PICP has been in use in parking lots, alleys and streets for over 15 years, representing well over 150 million sf (14 million m²).

In 2010, ASCE released ASCE 58-10 Structural Design of Interlocking Concrete Pavement for Municipal Streets and Roadways. This design guide is for roadways experiencing up to 10 million lifetime 18,000 lb (80 kN) single axle loads or ESALs. The emerging ASCE standard guide for PICP will complement 58-10, covering designs up to 1 million ESALs. Additional research may see an increase in lifetime ESALs with hybrid systems that include other pervious materials. Finally, as the paving slab and plank market grows, a need may arise to develop an ASCE design, construction and maintenance standard for these segmental concrete paving products and systems. Looking ahead, the vision is using such standards to further institutionalize all of these paving systems among designers, owners and government agencies.

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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.

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The Joy of Disruptive Things

Disruptive technology: One that displaces an established technology and shakes up the industry, or a groundbreaking product that creates a completely new industry. Examples: cellphones, personal computers and flat screens. From www.whatis.com.

Disruptive innovation: One that helps create a new market and value network, and eventually disrupts an existing market and value network (over a few years or decades), displacing an earlier technology. Examples: Uber, Wal-Mart and iTunes. From www.innosight.com.

Does the concrete paver industry have a disruptive technology? Maybe so, and it might be carbon curing. In very simple terms, carbon curing is using carbon dioxide to cure concrete instead of air. CO2 is captured into the concrete, holding some generated by cement production. This sounds good given the rising CO2 levels in the atmosphere and the broader implications for global warming, climate change, rising sea levels, etc. Fortunately, the segmental concrete pavement industry takes a smidgeon of comfort in knowing that 95 percent of CO2 emissions comes from burning fossil fuels to heat/cool buildings and from operating ships, trains, planes and automobiles.

A requirement fixed in concrete manufacturing is curing time. While concrete never stops curing, 28 days was established decades ago for curing time prior to testing for strength, absorption/density, and freeze-thaw deicer resistance. Concrete pavers often take less than 28 days to achieve the minimum 8,000 psi (55 MPa) unit compressive strength required in ASTM C936 or the minimum 7,200 psi (50 MPa) cube compressive strength in CSA A231.2. Nonetheless, significant sums of venture capital are being invested into carbon curing of concrete pavers because it presents a disruptive 24 hours for curing instead of 672.

What does a 24-hour cure time mean regarding substantive efficiency increases? Most of our readers haven’t experienced a concrete paver plant. It consists of millions of dollars of equipment and computers that mix concrete and quickly form it into a layer of 30 to 40 pavers within a steel mold. Paver production machines can’t go much faster to reduce cycle times for vibration and compaction of wet concrete within the mold. Perhaps this could be reduced to just a few seconds if the vibration of the concrete mix happens before it enters the production mold. Another option is placing more production machines in a plant (next to another or in line) such that daily throughput is quadrupled or taken higher. This implies a corresponding expansion of curing areas within a plant, meaning larger plants.

But let’s assume that the part of the plant that makes concrete units increases output that corresponds to the curing rate output now at one day instead of 7 to 28 days. That suggests factories won’t need much time or space next to them in “the yard” to store pavers. While a larger indoor space might be needed for higher production output, plants can make and ship paving units pretty much on order, even very large orders. Inventory management becomes just in time. The need for the yard next to the plant decreases, making inventory less important, and financing costs to create it diminish.

An innovative rearrangement of old commodities like cement and CO2 present a disruptive framework. The disruption from carbon curing extends to rearranging the plant and reprogramming computers that control mixing, batching and cycle times so equipment paces with faster curing and packaging times, and on multiple machines. This seems like the difference between using radar for airport air traffic control (linear sequencing) and more efficient GPS. The latter requires operational simultaneity in a four-dimensional space with new rules for aircraft spacing on approach, landing, take-off and hand-off.

The coming disruption within the paver industry could be CO2 curing with shorter curing times. This means rethinking the configuration of existing manufacturing equipment: its extent, layout and software programming. The joy of disruption doesn’t only come from the environmental benefits of CO2 curing. It potentially comes from disruptive pricing. All of this eventually could mean that segmental concrete pavement might have a future with a lower initial cost than asphalt. That disruption is pure joy.

For more information on companies that help reduce carbon emissions from concrete products, watch the following videos:

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Grabbing Wallets

When introduced in 2014, the U.S. Green Building Council’s LEED v4 heightened awareness of environmental product declarations or EPDs. Among significant changes to the LEED Materials & Resources credit criteria, LEED v4 now bestows a modest one point for projects with at least 20 EPDs from different construction material manufacturers. Given that buildings and sites typically contain thousands of products, this seems a small requirement by LEED v4.

One intent of this credit is to raise awareness of EPDs among construction material suppliers. This requirement has led many construction materials industries to first create product category rules (PCRs) according to ISO standards. PCRs prescribe requirements for defining the impacts of manufacturing a product, as well as outline the elements of a life-cycle assessment (LCA) of environmental impacts from manufacturing. The LCA forms the basis for creating an EPD.

EPDs list environmental impacts from manufacturing a product. They have been compared to reading a nutrition label on food packaging. Rather than fat, carbohydrates, proteins and vitamins, EPDs list the following impacts: global warming potential (carbon emissions); sulfur-dioxide, ozone and smog-type air pollutants; total energy consumed; use of renewable resources; depletion of non-renewable natural resources; nutrient emissions into waterways; and fresh-water use.

A practical yardstick for measuring these impacts is typically a unit of volume or mass of the finished construction product. For segmental concrete paving units, this is a cubic yard or meter of concrete. Most impacts per cubic yard of concrete are from carbon emissions due to producing cement and from generating electricity to run a manufacturing plant. Obviously, the energy source to make cement and electricity influence carbon emissions. EPDs favor hydroelectric, nuclear, wind and solar energy with lower carbon emissions compared to coal, gas or oil-fueled sources.

Now that ASTM issued a PCR for segmental concrete pavement products, it’s up to manufacturers to conduct LCAs, then produce and publish EPDs on their products. While the market isn’t consistently or even intermittently demanding EPDs from concrete paver manufacturers, the industry is preparing for the inevitable change. California manufacturers will likely be the first with EPDs, since that state imposed a legal mandate to trim carbon emissions. To assist the education process, the ICPI Foundation for Education & Research recently developed a guidebook for manufacturers on creating LCAs and EPDs. ICPI also developed a manufacturing material and energy-use inventory spreadsheet tool for its members.

Comparing EPDs among manufacturing segmental concrete paving products, asphalt and ready-mix concrete requires nearly equivalent PCRs. The asphalt industry will weigh in when their PCR is completed late this year or next.

While LEED and other sustainability evaluation tools have taken modest steps to raise EPD awareness in the North American construction world, where are EPDs ultimately going? They will become a critical source of data that will eventually feed into evaluating environmental impacts from a product’s construction, life and disposal/reuse. This is already happening in the building design world. It’s just starting in the pavement world.

Segmental concrete paving products are in a unique position to offer lower environmental impacts by not requiring huge paving machines and concomitant fuel consumption during construction. During their life, segmental concrete pavements offer immediate reuse in-service, a significant benefit for cities. Asphalt and cast-in-place concrete do not; those materials are removed and landfilled or later recycled.

Quantifying differences among construction, lifetime and end-of-life impacts will become increasingly important to municipal transportation agencies in the coming years.

Aggregates supplies are decreasing in some regions. Asphalt isn’t cheap. Transportation agencies are expected to build, maintain and rehabilitate pavements with less money and make them last longer.

Like Europe, agencies here will eventually move toward bidding material, construction and project maintenance life-cycle assessments. Maintenance pricing and LCA bids will spawn risk assessment/financing companies. (Maintenance price bids are already happening with some ICPI members selling permeable interlocking concrete pavements.)

All of these tools will ultimately save agencies money. LCAs will grab their wallets. That will get their attention.

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Two Worlds Together

In 2010, the Transportation and Development Institute (T&DI) of the American Society of Civil Engineers (ASCE) hosted the first national Green Streets and Highways conference. This came from a need for stormwater managers to learn more about the world of road managers and vice versa. Stormwater managers realize that roads cover about 25% of urban areas, generating significant property damage from water pollution, minor flooding and combined sewer overflows in older cities. Not surprisingly, road managers view stormwater as a lower priority compared to road user safety and efficiency. Also, road agencies generally are larger than stormwater agencies at every level of government, and that typically translates into greater financial, technical and political clout.

Most road agencies view permeable pavement as suitable for car parking lots and alleys with occasional applications in low-volume residential streets. Such projects are at the margins of road agency priorities and their budgets, and many of these applications lie in the private sector. Permeable pavements have yet to be embraced by road agencies because they are seen as new and untried under regular truck or bus traffic. This is where more structural testing and evaluation of hybrid pavements may allow for more passes from higher-weight vehicles. This can place permeable pavement more in the mainstream of the road manager’s world.

Along these lines, moving permeable pavements more into mainstream acceptance and use by road managers will require several components. As noted, first and foremost is accelerated, full-scale load testing to validate the ability to withstand truck traffic. Such testing must result in structural design methods and easy-to-use, reliable thickness charts. While there has been some full-scale load testing for pervious concrete and porous asphalt, a recent full-scale load study by UC Davis on PICP resulted in design charts. This magazine issue includes a summary of the UC Davis work, cost savings implications for designers and where the charts will be used.

The second component is specifications. Cities and county road agencies often rely on, adopt and adapt construction specifications developed by state departments of transportation (DOT). Even provisionally issued specifications by a state DOT tells local road agencies that a particular technology such as permeable pavement has been vetted by knowledgeable experts. There are currently two DOTs that have published PICP specifications; Caltrans and Washington, DC. ICPI assisted in developing these. We hope to do more of this.

The third component is training. There are two sides to the training coin: one is for contractors that results in certification of competent, experienced individuals; the other is inspection training for road agency personnel. ICPI has seen fast growth in PICP classes for contractors and in those receiving a PICP Specialist Designation. This credential is becoming a requirement in local and state agency specifications. To help address this need, an inspection presentation is now available for ICPI members to present to stormwater and road agency personnel.

The fourth component is maintenance/management procedures and costs. A critical maintenance aspect for permeable pavements is regular surface cleaning with vacuum equipment. Permeable pavement will be more readily embraced by state DOTs and especially by local road agencies when existing street cleaning equipment can be used for cleaning. Regularly maintained PICP performs for decades. However, many installations don’t see regular cleaning that results in restoration of the surface infiltration with powerful vacuum equipment and perhaps water. ICPI has funded maintenance research in the past. This includes work by North Carolina State University and the Toronto and Region Conservation Authority. ICPI and its sister organization, the ICPI Foundation for Education and Research, are reviewing more research options for the near future.

This issue’s cover story features another realm where the two worlds of stormwater and pavement are usually close together, and that’s on military bases. These mostly self-contained environments are of such a scale that one person or a small group of people down the hall from each other manage pavements and drainage. There are a growing number of them using interlocking and permeable interlocking concrete pavements. The cover story provides an example of integrating the two worlds of pavement and drainage management from a need to solve flooding problems and pavement rehabilitation.

As industry, academia and governments address the four requirements for permeable pavement that lead to it becoming mainstream road infrastructure, the two managerial worlds will work more closely together. One resource that can support this process is ASCE publishing a new book called Permeable Pavements.