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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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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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No More Nukes

Permeable interlocking concrete pavements (PICP) require open-graded, crushed aggregate subbases and bases for water storage and infiltration into the soil subgrade. For vehicular areas, ICPI recommends using a subbase of ASTM No. 2, 3 or 4 aggregate sizes. This layer is topped with a 4 in. (100 mm) base of ASTM No. 57 or similar-sized aggregate. A 2 in. (50 mm) thick bedding layer of smaller aggregate (typically ASTM No. 8) and concrete pavers are placed on the base layer and compacted. The joints are filled with permeable aggregate and the pavers are compacted again.

Figure 1. A 13,500 pound-force plate compactor with a compaction indicator enables the operator to know when the machine is done compacting in a specific area.

Figure 1. A 13,500 pound-force plate compactor with a compaction indicator enables the operator to know when the machine is done compacting in a specific area.

As with all pavements, PICP subbase and base layers must be compacted. Lift thickness should be no thicker than 6 in. (150 mm). Maximum lift thickness can increase to 12 in. (300 mm) if a 10-ton roller compactor seats it (Figure 2). A large, reversible plate compactor can be used as well, and will be required to compact next to curbs, foundations and utility structures, as well in corners, i.e., places where roller compactors cannot reach. Large means 13,500 pound-force (60 kN) and equipped with a compaction indicator (Figure 1). These machines weigh around 900 lbs (~400 kg), so they require moving assistance with a forklift or forks attached to a Bobcat-type equipment.

A core question is determining the level of compaction. The compaction indicator on a plate compactor tells the operator when the machine is finished compacting. That is a good start. An effective way to determine compaction levels is by measuring deflection of the compacted stone. This is done with lightweight deflectometer or LWD. Figure 3 illustrates the device that uses a shaft-guided, 22 lb (10 kg) weight that when allowed to fall, hits a plate. The impact force simulates an instantaneous pressure from a car tire passing over the pavement. LWD instrumentation records the movement of the aggregate surface in millimeters, calculates the stiffness and provides GPS coordinates.

Figure 2. A 10-ton roller compactor operates in vibratory mode, then static mode until there is no visible aggregate movement.

Figure 2. A 10-ton roller compactor operates in vibratory mode, then static mode until there is no visible aggregate movement.

An LWD is useful for checking post-compaction deflection of compacted, open-graded aggregate subbase and base for PICP. An LWD can also be used to test deflection on compacted, dense-graded aggregate and subgrade soils. For these materials, LWDs are replacing nuclear density measurements. Initial acceptance by the Indiana and Minnesota Departments of Transportation likely has initiated the start of acceptance by other DOTs in the near future. Indiana and Minnesota DOTs developed quality control/quality assurance test specifications after their research and by the National Cooperative Highway Research Council in their 2014 Synthesis 456, Non-nuclear Methods for Compaction Control of Unbound Materials.

Open-graded aggregates can be difficult to test for compacted density using a nuclear density gauge. ‘Nuke’ testing must be done in backscatter mode where the gauge probe is not inserted into the material. See Figure 4. Gamma rays are shot into the material and some bounce back to the bottom of the device and measured. This test method must be used because the probe cannot penetrate compacted, open-graded aggregates. The method can produce high variability measurements. Moreover, the nuclear gauge operator and device must be certified since the latter contains radioactive material. The LWD has none of these restrictions.

Figure 3. A lightweight deflectometer (LWD) testing deflection of ASTM No. 2 aggregate subbase.

Figure 3. A lightweight deflectometer (LWD) tests deflection of No. 2 aggregate subbase.

A draft national standard, ASCE PICP design, construction and maintenance guide, is in its final stages and will likely be approved and published later this year. It includes a guide construction specification with a test method using an LWD. Testing follows ASTM E2835 Standard Test Method for Measuring Deflections Using a Portable Impulse Plate Load Test Device. The specification provides guidance on the minimum number of tests including those close to adjacent curbs, pavements and buildings. The specification includes limits for variation in measured deflection as well as a maximum allowable deflection. These are currently drafted at 0.05 mm and 0.5 mm, respectively based on in-situ tests. The test objective is to have minimum variability in stiffness across the pavement subbase and base, thereby minimizing settlement and callbacks, as well as the rate of rutting from vehicle tires.

Figure 4. A nuclear density test gauge on compacted No. 57 aggregate is operated in backscatter mode with the probe (black vertical rod) in the up position because it cannot penetrate open-graded aggregates.

Figure 4. A nuclear density test gauge on compacted No. 57 aggregate is operated in backscatter mode with the probe (black vertical rod) in the up position because it cannot penetrate open-graded aggregates.

An existing ASTM test method with acceptance criteria to help contractors, engineers and project owners verify the level of compaction on a project through measuring deflection. The LWD can measure deflection in maximum 12 in. (300 mm) thick lifts, so that provides another reason (besides time and money savings) to use equipment that can compact 12 in. (300 mm) of subbase aggregate all once. The LWD costs about $8,000, about as much as a high-end nuclear density gauge without the extra user time for certifications.

As the permeable pavement market grows, the LWD will be the means for checking if open-graded base compaction achieved consistent results. As a tool for assessing compaction of soils and dense-graded bases, it can be immediately applied to regular interlocking concrete pavement construction as well as to other pavements and subgrade soils.

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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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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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A Magnificent Mile

Located in between mountains and Roadside America off Exit 441 of Interstate 5 at Westley, California, Howard Road handles much heavy truck traffic to and from “The 5.” Faced with worn asphalt pavement in need of replacement, the Stanislaus Public Works Director, Matt Machado, PE, LS, accepted the challenge of designing a long-term, economical pavement solution for the road using interlocking concrete pavement. This resulted in the largest publicly owned stretch of concrete pavers in California, about one mile (1.6 km) long.

Built over a weak soil subgrade (R-value < 5 or California Bearing Ratio < 2%) and to a Caltrans Traffic Index of 11, or just over 5 million 18,000 lb. (80 kN) lifetime equivalent single axle loads (ESALs), the design required 14 in. (350 mm) thick Caltrans Class 2 road base over a biaxial geogrid. The Class 2 base supports one inch (25 mm) of coarse bedding sand and 170,000 sf (15,794 m²) of 3 1/8 inch (80 mm) thick, machine-installed concrete pavers placed in a herringbone pattern.

Example of worn asphalt ready for replacement along Howard Road in Westley, CA.

Example of worn asphalt ready for replacement along Howard Road in Westley, CA.

Stanislaus County received six bids from $4.50 to $6.00/sf ($48 to $64/m²) to install the concrete pavers and bedding sand. Completed in fall 2014, the pavers were manufactured by Basalite Concrete Products in Dixon, CA, and machine installed by Earth Shelter Developers from Lodi, CA. Both are Interlocking Concrete Pavement Institute (ICPI) members. With Roadside America businesses like Denny’s, Chevron, McDonald’s and Joe’s Travel Plaza open 24 hours along Howard Road (speed limit 35 mph), an extensive traffic control plan required the contractor to maintain drive lanes to accommodate truck traffic during construction.

Previous Paver Experience

Mr. Machado used interlocking concrete pavement while working in a previous position as city engineer for Ripon, CA (pop. ~15,000), a farm community in San Joaquin County. Mr. Machado developed interlocking concrete pavement as a roadway standard adopted by City Council for new roads and for some pavement rehabilitations. With the new design standard in place, developers and the City constructed more than 1.3 million sf (120,774 m²) of roads between 2005 and 2008.

Main Street in historic downtown Ripon features 50,000 sf (4,645 m2) of interlocking concrete pavement.

Main Street in historic downtown Ripon features 50,000 sf (4,645 m²) of interlocking concrete pavement.

The Ripon City Council approved interlocking concrete pavement when comparing the cost of expanding the city asphalt road network by developers and then forecasting insufficient future funds for periodic grinding and resurfacing. While the additional initial developer costs for concrete pavers were transferred to the homebuyers, the increase was marginal compared to the full price of single-family homes.

The Life-cycle Selling Point

After finding success in Ripon with lighter-load street applications (and conservative structural design assumptions), interlocking concrete pavement presented a durable pavement rehabilitation alternative for heavily trafficked Howard Road. A benefit of interlocking concrete pavements is not requiring periodic resurfacing. For Ripon’s residential streets, life-cycle costs were studied over a generous 100-year period resulting in concrete pavers having about 75% lower life-cycle costs than asphalt. Maintenance costs for concrete pavers for the same period were approximately 20% the cost of asphalt. Heavier trafficked streets such as Howard Road with interlocking concrete pavement often have even lower life-cycle costs because resurfacing costs for asphalt roads increase under such traffic.

Machine-assisted installation of interlocking concrete pavement in Westley, CA.

Machine-assisted installation of interlocking concrete pavement in Westley, CA.

According to Mr. Machado, “This (Howard Road) project was built to show the structural value of concrete pavers and their economic value for heavy truck traffic.”  Interlocking concrete pavements offer high compressive strength concrete with the flexibility of asphalt pavement. Research in the United States and overseas demonstrates that the pavers in a herringbone pattern progressively stiffen or interlock while receiving traffic loads. The resulting stiffness of the paver and bedding layers, or their resilient modulus, is equivalent to the same thickness of asphalt. In some cases, their stiffness well exceeds asphalt during hot summers as experienced in Westley, CA, where asphalt weakens under temperatures typically around 100 deg. F (38 deg. C).

In other words, the 3 1/8 in. (80 mm) thick pavers and 1 in. (25 mm) bedding sand have an AASHTO layer coefficient (an expression of stiffness) equivalent to the same thickness of asphalt. This is demonstrated in ASCE/ANSI 58-10: Structural Design of Interlocking Concrete Pavement for Municipal Streets and Roadways published by the American Society of Civil Engineers as well as in Tech Spec 4: Structural Design of Interlocking Concrete Pavements from ICPI. Pending successful performance with some years under its belt, Howard Road might mean a design standard for Stanislaus County in the future.

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Taking the Long View

When the city of North Bay, Ontario, explored the use of interlocking concrete pavers for its heavily trafficked downtown city center in the early 1980s, officials of this city of 54,000 wanted to know they’d be getting the most for their money. Not only did the resulting installation meet aesthetic and functional goals, it has since become a model of low-maintenance cost savings that has proved durable well beyond its projected lifespan of 20 years.

At the time of its completion in 1983, the $3 million, 150,000 sf (13,900 m²) Main Street project, which included roadway and sidewalks constructed on the full width of the road allowance, was hailed for its aesthetic contribution to a revitalized downtown business and retail district. When surveyed eight and 16 years later, the pavement was found to be performing exceptionally well under high traffic and extreme weather conditions, with little evidence of distress, despite minimal maintenance needed. In fact, after 12 years, a city official confirmed that there had been no maintenance at all. In addition, a 1999 life cycle cost analysis that compared the concrete paver installation with a local control section of hot-mix asphalt pavement found a difference of about $76,000/lane-km in maintenance costs favoring the concrete pavers.

Thirty-two years later, the installation is still performing, though finally ready for replacement, says Adam Lacombe, P. Eng, North Bay senior capital program engineer. The city is budgeting for a paver replacement to begin in 2017 or 2018. “Main Street has always been the centerpiece of the city, and the [pavers] set it off,” he says. “We are [considering] replacing them for their aesthetic quality and lifespan.”

Extreme Applications

The Main Street project was conceived at a time when the city of North Bay was planning to update its central business district with a more people-friendly scale and unified appearance. As part of the transformation, approximately 50 percent of the on-street parking was recommended for removal. In its place, designers envisioned wider sidewalks, boulevard areas and the addition of trees and planting areas, new benches, underground wiring and new streetlights.

Aiming to attract shoppers to a refreshed retail destination at a time when traditional Main Street businesses were losing business to shopping malls, North Bay’s Engineering and Public Works Departments gave interlocking concrete pavers first consideration in part for their potential to create an aesthetic identity for the district. But another major goal was to find a pavement that could handle an expected traffic volume of 8,000 vehicles per day (5 percent delivery trucks and buses), as well as snow removal and harsh weather conditions.

In North Bay, temperatures can range from −40 C in winter to 35 C in summer, and punishing freeze-thaw cycles occur throughout the winter months, with frost depths of up to 8 ft (2.4 m). The Main Street roadway would be subject to approximately 300 tons of salt annually, as well as the regular impact of the carbide steel blades used on snow-removing graders, slushers and plows.

At the time of the project’s conception, interlocking concrete pavers were already in use in high-load, harsh-weather projects around the world, and were just beginning to gain wider interest for heavy-use projects in North America. Just one year before the North Bay Main Street pavement was installed, 610,000 sf (56,700 m²) of interlocking concrete pavement was used in what is now called the Pier IX Terminal, in Newport News, VA. This facility handles ground storage of coal, so the pavers are subject to high loads from coal storage piles and abrasive loads from steel-tracked bulldozers. This provided an example of durability in an industrial setting.

North Bay officials had some experience with concrete pavers, which had successfully performed in an area around city hall for five winters under de-icing salts. But that area was not subject to vehicular traffic, so additional evidence was sought to prove the material and its installation could withstand projected traffic load and environmental conditions long-term.

A seminar that brought in experts from Australia, England and the Netherlands demonstrated to North Bay stakeholders how pavers had performed successfully under extreme loads and weather conditions in container ports, airports and roadways. Presenters offered compelling evidence that, when designed and executed correctly, the installation would withstand the rigors of a heavily trafficked Northern Ontario Main Street.

Best Practices Defined

The manufacturers, designers, engineers and installers involved in the Main Street installation set their sights on creating a state-of-the-art model showcase for what was recognized as a high-profile project. The pavers were manufactured to resist abrasion and freeze-thaw conditions, meet compressive strength and absorption standards, and were subject to a salt immersion test. Installation included a compacted subbase and base, edge restraints in the form of cast-in-place concrete curbs, concrete collars around utility structures such as manholes to offer a stationary restraint for the pavers, a herringbone pattern to provide the greatest degree of interlock (except in the crosswalks, which use a running bond pattern), and a slight crown in the roadway to allow for natural settling and drainage after construction. Sub-drains were utilized in some locations and surface water was designed to flow to catch basins and storm sewers.

During construction, installers performed regular density checks of the base with a nuclear density gauge to achieve the specified level of compaction that is critical to long-term performance. Nearing the end of installation, a plate compactor was used to force bedding sand into the joints and to facilitate the process of paver interlock, which in turn enables the transfer of vehicular load from paver to paver.

From today’s perspective, the North Bay Main Street project helped define best practices for interlocking concrete pavement manufacture and installation, some of which later became ASTM and CSA standards, including those for compressive strength, freeze-thaw durability and dimensional durability, and remain in use today.

Test of Durability

At eight years post-construction, an engineering consultancy performed a detailed condition survey and non-destructive deflection testing of the Main Street pavement. The survey found that about 4 percent of the approximately 57,000 sf of pavement surveyed had depressions concentrated in an area that had been subject to improper repair of the base when reinstalled after utility repairs. Another section that showed spalling resulted from incomplete joint filling and subsequently pavers losing interlock. Aside from this, the report concluded that the pavements provided “excellent performance…surface deformation occurs in less than 1.5 percent of the pavement areas surveyed,” and that the pavers were in “very good to excellent condition.”

Sixteen years after completion, in 1999, a geotechnical engineering consultant performed another condition survey that included a comparison with a local control section of asphalt pavement. It concluded that the interlocking concrete pavement showed little evidence of distress, with pavement condition indexes (PCI) for tested sections averaging 70 on a scale of 0 to 100 (with 100 showing no distress).

At 20 years, North Bay Public Works confirmed that the pavement was expected to be serviceable for another 15 to 20 years with only minimal maintenance anticipated.

A Cost-Effective Option

As part of the 1999 survey, a 40-year model was used for a life cycle cost analysis comparing the pavers and an asphalt street model that concluded rehabilitation of the pavers would be required at Year 21 in order to maintain a pavement PCI of 60. For the asphalt pavement, rehabilitation would correspond to years 18, 27 and 36 to maintain a PCI of 60.

At a 4 percent discount rate (corresponding to a secure investment of 6 percent and inflation of 2 percent), interlocking concrete pavements were shown to be more cost-effective than asphalt pavements. The study did not reflect costs to the public in downtime from routine maintenance and repairs. Interlocking concrete pavers can have a significant benefit in terms of reduction of these user delay costs because traffic can be restored very quickly after repair; also, less maintenance downtime is required over the pavement’s lifespan.

Since 1983, North Bay has continued using interlocking concrete pavers in public sidewalks, boulevards, its train station and lengthy promenades along its award-winning Lake Nipissing Waterfront Park. In 2010, it added a one-block section of pavers in a roadway that complements nearby Main Street and sets off a roadway island park. Likewise, cities across the United States and Canada have since chosen pavers for a variety of low- and high-impact projects, taking advantage of their endurance, aesthetic qualities and green attributes, more recently including permeable installations that aid in stormwater management.

The details of North Bay’s Main Street pavement rehabilitation are still to be determined as the city works on a new land use and urban design plan, says Mr. Lacombe. A rough estimate for replacing the pavement, including design and construction, is currently $2.4 million, he says.

North Bay faces the same decisions as hundreds of cities across North America: how to replace an aging downtown roadway in a way that’s economical in the short and long term, while taking into account aesthetic and environmental considerations, and the needs of stakeholders. The Main Street project offers strong evidence that interlocking concrete pavers are suitable for high-impact applications, and can be the most cost-effective pavement solution when considering total cost of ownership over the long term.

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The Smart Start Small

For those new to bituminous-set paving, there are additional variables that simply don’t exist for traditional sand-set paver installations. Knowing these variables and planning accordingly is essential for a successful and profitable installation. “There’s a lot more work to it, but the benefits make it worth the trouble, especially for commercial applications,” says Mike LaMonica, estimator and project manager for Syrstone.

WHY BITUMINOUS?

Bituminous-set applications on a rigid concrete base have a proven track record of superior performance under heavy vehicular traffic, especially in urban settings, according to ICPI’s Tech Spec 20: “Construction of Bituminous-Sand Set Interlocking Concrete Pavement.” Though more expensive (typically 30-50% higher than sand-set pavers due to additional materials and labor), long-term performance justifies the cost when compared to sand-set installations under the same wheel loads. Interlocking concrete pavement crosswalks with bituminous setting beds on concrete bases have an estimated lifespan of 7.5 million 18,000 lb equivalent single axle loads or ESALs, according to ICPI’s Tech Spec 19.

bituminouscrossssection

1.) Base thickness and reinforcing varies with traffic, climate and subgrade conditions. 2.) Concrete base minimum 2% slope from centerline to curb. 3.) Do not provide weep holes to subgrade when water table is less than 2 ft. (0.6 m) from top of soil subgrade. Provide drain holes to catch basins.

Bituminous setting beds on a rigid base have replaced mortar or sand-cement bedding materials in many pedestrian applications and in nearly all vehicular ones. Mortar-set pavers have not performed well under vehicular traffic and are susceptible to deterioration from freeze-thaw and exposure to deicing salts. Concrete bases are recommended in vehicular and pedestrian areas; asphalt bases should only be used in pedestrian areas.

MANAGING THE VARIABLES

There are two main variables with bituminous-set paver jobs that require upfront research and planning before putting a bid together. The first is availability of materials. Where is the nearest reputable asphalt batch plant that can produce a smaller quantity of the mix needed (7% asphalt to 93% concrete sand)? The plant will likely have a regional DOT-spec top mix or a performance-grade mix that is similar to ICPI guide specifications. Once a plant is located, the distance to the jobsite needs to be considered for determining trucking costs. For vehicular traffic installations, is traffic already present at the site that will require partial access? If so, the installation may need to be completed in phases which will require separate truckloads. Be sure to build the additional trucking costs of multiple deliveries into the estimate, as well as the minimum delivery load and the anticipated spoilage if the minimum is in excess of the needed quantity. These costs can add up and eat into profit margins quickly if not considered from the outset.

The second main variable is timing. “Everything with the bituminous setting bed is time-dependent,” Mr. LaMonica explains. The concrete or asphalt base is placed first and must cure. Next, an emulsified asphalt tack coat may be needed (recommended for vehicular applications, but typically not required for pedestrian applications) that will require curing time. Then, the bituminous setting bed is laid and must cure. On top of that, a neoprene-asphalt (neo-asphalt) adhesive must be applied that also requires curing. Planning around these downtimes is critical to efficiently manage labor hours. Ideally during curing downtimes, crews can work on cutting pavers or on another part of the installation that may be sand-set, or on housekeeping tasks to keep the jobsite clean and orderly. Typically, commercial jobs involve multiple other trades so keeping feet off of the installation-in-progress can be a challenge, but is very important given the messy nature of the materials. Vigilance is required and good communication will help prevent other crews from making a mess, especially if they are not familiar with the process and stages during which the surface should not receive foot traffic.

DIFFERENT WORLDS

For those looking to enter the world of commercial projects by taking on a bituminous-set installation, Mr. LaMonica advises to start small, do your research and have the proper funds.

“I almost envy the residential installer who has the design eye to incorporate multiple hardscape components into his work,” Mr. LaMonica says. “The contractor has more control in the residential world because he can see the job through from beginning to end.”

The commercial world is so different, Mr. LaMonica says. A construction manager oversees the whole project, the paperwork and record keeping are a distraction, the profit margins are generally lower, payment is slower and design changes are often problematic to get approved and paid. “I tell the residential guy looking to do commercial, if you can’t afford to fund a job for 60, 90, or 120 days, you shouldn’t be in that world,” Mr. LaMonica says.


ICPI’s Commercial Paver Technician Installer Course covers bituminous-set paver installation. For more information, visit http://www.icpi.org/installerdesignations.