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Geosynthetics Part 1: Geotextiles

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

Table_1

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

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

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

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

Table_2

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

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Superb Stewardship

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

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

Contemporary Cool

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

As Above, So Below

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

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

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

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

Pixilation and Progeny

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

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

Congruent Values

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

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

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And the Survey Says…

Thanks to continued construction growth, industry survey results from the Interlocking Concrete Pavement Institute (ICPI) indicate a repeat of last year’s double-digit growth. New home construction, acceleration in sales of existing homes, a rebound in public section construction spending, and an increase in commercial construction all contributed to strong growth last year for U.S. and Canadian segmental concrete pavement. Survey respondents, including 29 companies representing 141 paver-producing machines, report shipments across all segmental concrete pavement categories up 15.4% in the U.S. and 8.9% in Canada compared to 2014 figures.

The Industry Survey encompasses interlocking concrete pavers, permeable interlocking concrete pavers, concrete grid paving units, paving slabs, and related concrete paving products. In the principal category of concrete pavers as defined by ASTM and CSA, U.S. production increased 15.2% year-over-year from 517 million to 595 million square feet, while Canadian output grew 10.6% from 85 million to 94 million square feet. At just over 78% of all sales, the residential market continues to drive segmental concrete pavement. Commercial sales, including municipal and industrial projects, claimed almost 22% of the total 2015 market.

Robust year-over-year sales activity, notes ICPI Chairman and Oldcastle APG Northeast President Matt Lynch, “demonstrates continued demand for segmental concrete pavement systems with versatile design options, low maintenance and environmental benefits. The construction industry is experiencing an economic recovery that supports the expansion of our market share in residential and commercial applications.”

Order the survey report from icpi.org. $20 for ICPI members; $99 for non-members.

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A Higher Degree of Assurance

The American Society for Testing and Materials International (ASTM) recently approved revisions to C936 Standard Specification for Solid Concrete Interlocking Paving Units that include an optional lower temperature for laboratory freeze-thaw durability testing while paver units are immersed in a 3% saline solution. For freeze-thaw durability testing, C936 references C1645 Standard Test Method for Freeze-thaw and De-icing Salt Durability of Solid Concrete Interlocking Paving Units. This test method includes a minimal freezing temperature of -5° C (23° F) though 49 freeze-thaw cycles. Mass lost is measured from the tested paver units and that loss is divided by the total surface area expressed in square meters to determine if the pavers meet the requirements in ASTM C936.

This standard now includes the optional use of -15° C (5° F) as the lowest temperature in C1645. Deciding to use the -15 ° C option is supported by a map identifying geographic cold climate zones for project locations where the specifier might want to use this colder temperature in freeze-thaw durability testing per C1645. The reason for introducing this option is that some areas of the U.S. see colder temperatures than -5° C and use deicing materials. The colder temperature option of -15° C can help provide a higher degree of assurance for winter paver durability to manufacturers and specifiers.

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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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Resilient Infrastructure

Procrastination seems endemic to human nature. However, there may be some evidence to the contrary. Most of us might recall Superstorm Sandy in October 2012. Millions in New Jersey and New York sure do. Local governments, insurance companies, businesses and homeowners also remember some $80 billion in destruction. And this event wasn’t a hurricane.

The evidence against procrastination appears to be emerging from within the highest levels of government and among business leaders. Katrina and Sandy were catalysts. Leaders are asking how to build better to reduce damage from storms and earthquakes and accelerate recovery. Most of the discussion is on making buildings stronger, i.e., more wind-, flood- and earthquake-resistant. The conversation must soon turn to how to better build things outside buildings such as parking lots, roads, utilities and communication infrastructure.

Even flooding from smaller storms, the ones with no names, are costing millions. While investments in resilient infrastructure solutions are long-term, we are seeing an emerging trend of using permeable interlocking concrete pavement (PICP) as a means to reduce flooding. Such is the case with the Southeast Atlanta Green Infrastructure Initiative that aims to capture seven million gallons under miles of PICP streets. The first six miles are already built. This exemplifies resilient infrastructure where roads also do flood control: They mitigate it rather than contribute to it.

Another little known aspect is resilience from interlocking concrete pavement. That type of segmental pavement isn’t designed to permeate due to sand joints, bedding, and a dense-graded aggregate or stabilized base. There are reports in Canada and Italy on the ability of this system to not crack when inundated, unlike monolithic asphalt or concrete. ICP doesn’t crack when flooded because it has “cracks” in it; joints between the paving units relieve the water pressure as it builds under the pavers. And the surface can be reinstated without requiring deliveries from a ready-mix concrete or asphalt plant. (They might be flooded, too.) Rapid recovery of roads from floods or earthquakes is a prerequisite to building repair.

While lots of beautiful patios are being built, the segmental concrete pavement industry is at the threshold of entirely different conversation and market opportunity. It is poised to more readily establish and institutionalize segmental pavements as part of resilient infrastructure that yields economic, environmental and social benefits to property owners, municipalities and wider society.