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.


Clearing the Air

Repeated exposure to respirable silica poses long-term health risks, according to medical studies. How much exposure is considered safe? In 1971, OSHA established 250 micrograms per cubic meter of air (µg/m³) per 8-hour workday as the permissible exposure limit (PEL) to respirable silica dust for the construction industry. In March, OSHA published a final rule lowering the PEL to 50 µg/m³ averaged per 8-hour day effective June 23, 2016, with compliance required by June 23, 2017. The construction industry in general has rejected this rule on the basis of technological infeasibility plus the untenable costs related to reducing exposure by 80%.

In an effort to educate contractors, ICPI has approved new language for inclusion in its contractor courses and manuals (see sidebar). Though OSHA might rarely visit residential jobsites, an estimated 75% of all pavers and paver products see use in residential applications. Thus, raising awareness and providing knowledge on jobsite protection are important responsibilities for ICPI. A resource webpage on respirable silica is in the works for the new website. It will include links to research and reports on the issue.

Worker health and safety are universal priorities on which everyone can agree. Workers understand why protecting their eyes, backs, knees, elbows and hands are important because there is a fairly immediate cause and effect if they do not. However, effects from repeated exposure to respirable silica can become manifest over a long period of time. When risks are not immediately apparent, prevention may seem less imperative. That is why knowledge and awareness are so important, as well as safety training. A worker’s lungs need protection too.


Couldn’t Stand the Weather

Mark Twain said, “Everybody talks about the weather, but nobody does anything about it.”

Global climate change might be altering the implications of Mr. Twain’s saying because we are likely doing something to the climate and maybe something about it. While the causes and effects of climate change receive endless debate in scientific and political spheres, regional-scale rainfall patterns are changing for certain. The result has been wetter weather in some parts of North America, drier in others.

The weather changes have been so dramatic that rainfall statistics defining storm recurrences are seeing realignment. A hypothetical example explains this shift. Say there are 80 years of storm data and some indicates that very occasionally, a city receives five inches rainfall in 24 hours. Some statistics are run and they conclude that the city has 4% probability of that rainfall depth occurring in any given year. So it’s called a 25-year storm. But data gathered over the past two decades now indicate a 10% probability. So that rain event has shifted to a 10-year storm recurrence. The old 10-year storm with maybe three inches of rain is now five inches.

This shift directly affects cities because storm sewers back up and can’t immediately drain the additional water. When that happens, it can end up in someone’s basement. Besides property damage, the city can be liable for damages. Storm sewers originally designed to manage a 10-year storm are now obsolete as confirmed by revised rainfall statistics.

Hurricanes plague the East, tornadoes the Midwest and South, and earthquakes the West. Because of natural disasters like the earthquakes in Northridge and Loma Prieta, and Hurricanes Katrina, Irene and Sandy, governments at all levels are seeking resilient designs and technologies to resist excessive wind, rain and tectonic plate movements. Resilient infrastructure resists these onslaughts from nature by designs that minimize damage to private property and society’s productivity. As urban infrastructure is rebuilt, resilient technologies and designs are increasingly included.

One city implementing resilient infrastructure is Atlanta, GA. It recently completed the Southeast Atlanta Green Infrastructure Project. Infrastructure renovation involved replacing century-old water lines, storm and sanitary sewers in two neighborhoods. The ‘green’ portion reduced stormwater runoff, a fundamental goal in most GI projects, with permeable interlocking concrete pavement. Atlanta went beyond reducing runoff. It installed around 700,000 sf (65,000 m²) of permeable interlocking concrete pavement with enormous water storage capacity to reduce increasing flood events.

According to Todd Hill, P.E., Atlanta’s Director of Watershed Management, the pavement stores about 7 million gallons. That approaches one million cubic feet of water in over 10 Olympic-sized swimming pools. As an extra bonus, maybe a fourth of that water is infiltrated back into Atlanta’s clay soils. A portion of the $66 million invested will be returned in spared litigation costs, not to mention increased property values and resulting taxes.

In their present state, many urban drainage systems simply can’t stand the weather. In response, resilient urban infrastructure is an intentional public investment goal. As they rebuild, cities and neighborhoods can resist bad weather by providing permeable pavements that control flooding while remaining un-flooded and useable during the worst storms. Atlanta certainly magnifies and affirms the role of permeable interlocking concrete pavement in resilient infrastructure.


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Research Raises the Barrier

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

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

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

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

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


Paving Slab Progress

Approximately 25 years ago, a request was made by the U.S. Army Corps of Engineers to ASTM to create a product standard for segmental concrete paving slabs. When this request was made, no industry association existed to develop a draft standard for consideration by ASTM. Moreover, there was little industry consensus on the requirements of such a standard. The closest example was one first published in 1972 by the Canadian Standards Association called CSA A231.1 Precast Concrete Paving Slabs.

Since the advent of the Interlocking Concrete Pavement Institute (ICPI) and the expansion of paving slab sales in residential, commercial, municipal at-grade and roof applications, ASTM is now considering a draft product standard called Standard Specification for Segmental Concrete Paving Slabs. While yet to be approved and published, the draft has passed through two rounds of voting by ASTM Committee C15. A final, published standard is expected during 2016 after at least one more round of voting.

The current ASTM draft includes requirements for slabs made with any manufacturing process, e.g., hydraulically pressed, as well as for dry- and wet-cast (low and high water/cement ratio) concrete. Paving unit properties are defined for dimensional tolerances, warpage, flexural strength (modulus of rupture) and freeze-thaw durability.

Since some paving slab applications are subject to vehicular traffic, flexural strength is a key contributor to slab longevity. Flexural strength is being researched through a grant from the ICPI Foundation to Applied Research Associates, Inc. They are conducting finite element modeling that assesses vehicular loads and resulting slab stresses in a range of paving slab sizes, thicknesses and flexural strengths.

While not included in the product standard, this research will inform development of pavement thickness recommendations by ICPI for various traffic loads using various bedding, base and soil types. Prior to the modeling, the project initially reviewed overseas research and paving slab structural design literature. Upon approval by ICPI, charts will be included in a new ICPI Tech Spec bulletin on paving slab structural design and referenced in another bulletin on construction.


Slabs In Demand

Each year, the Interlocking Concrete Pavement Institute (ICPI) conducts a sales survey of U.S. and Canadian manufacturers to estimate sales from the previous year. The most recent survey estimates total concrete paver sales for 2014 at around 600 million sf, about an 11% growth from 2013. (This area does not include paving slabs.) What’s interesting is where growth occurred as a percent of all concrete segmental paving units sold. For permeable interlocking concrete pavers, it was 3.6 to 4% and for paving slabs 14.6 to 16.5%. Paving slab sales were about four times higher than pavers used in permeable applications. Both products exhibit double-digit percent growth each year.

Paving slabs are units requiring two or more hands to install, whereas pavers only require one hand. Some manufacturers’ literature mislabels paving slabs “pavers.” Over the years, this has led to confusion among some designers thinking that paving slabs can withstand vehicular traffic similar to smaller interlocking concrete pavers. The absence of paving slab design and construction guidelines for vehicular areas may have contributed to this confusion.

Paving slabs are units generally 12 x 12 inches or larger used mostly in residential and commercial at-grade pedestrian and vehicular applications, and on roofs for commercial buildings. The paving slab industry came into existence about the same time as the concrete paver industry in the last quarter of the previous century. Fast-forward 40+ years to the present and we are seeing development of paving slab design and construction design guidelines by ICPI. Development of guidelines is proceeding with publication expected later in 2016.

Quebec City

ICPI addresses the expanding paving slab market for pedestrian and limited vehicular applications. This paving slab sidewalk is in Quebec City.

A parallel activity is the development of an ASTM segmental concrete paving slab product standard. Approval is expected in 2016 as well. This will complement the Canadian paving slab standard CSA A213.1 Precast Concrete Paving Slabs first issued in 1972. Like the Canadian standard, the draft ASTM standard includes a requirement for flexural strength since they primarily fail in bending and not from compressive forces.

As done previously for interlocking and permeable interlocking concrete pavements, ICPI is developing design charts for paving slabs that include inputs for soil strength, base and axle loads. There will be certain size and thickness of units applicable for axle load ranges, provided that certain base materials and strengths are designed and constructed. Maximum axle loads will be significantly lower than for interlocking concrete pavements and permeable interlocking concrete pavements.

The guidelines also will address structural design of concrete paving planks subject to vehicular loads, those thin, long (12 to 24 in.) units in vogue, especially among landscape architects. Preliminary conclusions from finite element modeling research sponsored by the ICPI Foundation points to the need to support these units under vehicular traffic with a cement-stabilized aggregate base.

The future of paving slabs might include life-cycle cost analyses and life-cycle assessments of environmental impacts compared to cast-in-place concrete and asphalt.

While the initial costs of more attractive and durable paving slabs will likely be higher, maintenance and repair costs are likely lower as paving slabs are removed and reinstated after underground repairs, i.e., no wasted discarding of the pavement surface. The possibilities from ICPI tools, commensurate with high-quality, manufactured paving units, continue to expand the product pallet for the industry, designers and users.


Form Follows Function

Returning and new site visitors alike will find a refined and inviting homepage at Contractors, designers, and homeowners are now able to access the specific information they need simply by clicking on the corresponding portal. A “Members Only” portal rounds out the quartet of initial entry points for exploration. More importantly, the new website also features a new search-by-proximity function for users to quickly find the resources and service providers closest to their location.

Ultimately, the new website design resulted from understanding the need to get ICPI members and the general public access to ICPI’s technical resources in an easier and more visually appealing manner. In such a visual industry, ICPI drew upon the stellar work of its members to showcase a range of design possibilities—new and sustainable—with easier access. Given the depth and breadth of information ICPI provides, the Institute structured the site by curating information specific to the needs of a particular site visitor— contractor, designer, or homeowner— rather than require the visitor to search through much of the available information as the old site required.

Another noteworthy change is the Paving Systems link in the green home-page navigation bar. Here, visitors can learn about the benefits, installation and maintenance requirements of concrete pavers, permeable pavers, paving slabs and grids. The Idea Gallery of old has been reimagined and renamed Project Profiles and provides more imagery and more information about each project. The familiar Resource Library tab is retained from the old site due to its usefulness for quick searches. Education & Certification remains a top-level category for visitors to access this important information. Events and Newsroom complete the offerings of the homepage navigation bar.

Not only does the new site include more photos, they are given more prominence to bring projects to life on screen. Also featured are designs for do-it-yourself ideas. Information on best practices gives guidance on homeowner topics such as how to choose the right contractor. For design professionals and contractors, Tech Specs provide a wealth of information to help design, specify, construct and maintain a project.

For Contractors

The contractor portal explains the benefits of being an ICPI contractor member. Contractors can enter and view featured projects, easily access info on how to become certified, learn how to sign up for courses, access archived webinars for self-guided education, and read tips on installation and maintenance. The contractor portal features:

  • Awards
  • Benefits
  • Certification
  • Education
  • FAQ
  • Installation
  • Maintenance
  • Resources

For Designers

The designer portal features guide specs, detail drawings, design manuals, technical papers, and more for those experienced with segmental pavement design as well as for those new to it. The designer portal features:

  • Guide Specifications
  • Detail Drawings
  • Design Manuals and Software
  • Project Profiles
  • FAQ
  • Technical Papers

For Homeowners

The homeowner portal provides information on different pavement types, design ideas, an overview of the basic considerations when selecting a contractor, and also DIY information and guidance. The homeowner portal features:

  • Pavement Types
  • Design Ideas
  • Installation Basics
  • DIY
  • Selecting a Contractor
  • Maintenance
  • Resources
  • FAQ

For Members Only

A broad and robust suite of opportunities await ICPI members through this portal. Tools to manage one’s profile and preferences, view upcoming events, a bookmark feature to collect articles, and access to the Member Directory are but a sample of the many benefits the new website offers. Additional resources include: minutes from past meetings and agendas for those upcoming; guidance for marketing one’s business; information on government relations; chapter resources; and information on promotions, incentive programs, how to sponsor an ICPI course for contractors, or become an instructor.

Better Experience, Faster Access

The redesigned website offers a better user experience and faster access to the infor-mation visitors are seeking while simultaneously presenting imagery in its proper prominence, front and center. The homeowner portal in par-ticular broadens the appeal and value of ICPI to the general public with the goal of heightening brand awareness. And the Member’s Only portal increases accessibility and usefulness for those who support the continued work of the Institute.


New Product Review – NextGel

Almost no dust, no haze and faster installation are promised by Techniseal for NextGel joint sand stabilizer. After introducing polymeric sand to the market over 15 years ago, Techniseal brings a significant improvement after embarking on upgrading its stabilizer products some five years ago. Jointing sand stabilizer was invented to accelerate interlock while preventing sand loss from runoff and wind, as well as discouraging ants and weeds.

Dust from some jointing sand stabilizers requires a lengthy cleanup, and if not done properly, results in an ugly, hard-to-remove haze on pavers. “The guys who are in a hurry to get to the next job don’t sweep exactly as they should, don’t use the leaf blower, and didn’t wait until the paver surface dried enough. Then you’ve got a missed hit,” says Al Dorais, Techniseal’s president. To minimize dust, Techniseal created a heavier, more homogeneous particle size. This reduces the smallest particles that caused dust and haze. “The smallest particles embed into a paver surface, so the trick was to create much bigger particles while still stabilizing all of them,” says Mr. Dorais.

The larger particles in NextGel also allow water to flow more easily and evenly to activate the polymer stabilizer down to the bottom of the joints. This makes the wetting process 50 percent faster, according to Techniseal. The combination of reduced dust, no haze and a speedier wetting process promises a faster installation time overall. Assuming a 1000 sf installation, Techniseal estimates that NextGel will save about 70 minutes on installation time compared to competitors’ products. Multiply this by the annual area of projects, and the efficiencies could be significant, not to mention reducing callback time and income losses to clean up surface haze.


International Conference in Fatherland of Concrete Pavers

The international concrete block paving industry affirms its technological development by conferencing every three years. The technical conferences support the continued growth of the global concrete paver industry, which manufactures one square foot (0.1 m²) of segmental concrete paving for every person on Earth every year.

With over 400 attendees and exhibitors, the 11th International Conference on Concrete Block Paving (ICCBP) held Sept. 9–11, 2015, in Dresden, Germany, was a declaration on the advanced state-of-the-art technology from the industry Fatherland. Germany earned this moniker by having the highest per capita use of segmental concrete paving. Annual sales are somewhere past 15 sf/person or a total of around 1.2 to 1.5 billion sf (120 to 150 million m²). For the record, the industry Motherland is the Netherlands, where the concrete block paving industry was born in the 1950s.

The Dresden conference included 2½ days of presentations and social events organized by Dr. Frohmut Weller, Dr. Sabine Leischner and Juliane Kraft with the Technical University of Dresden. Additional support was provided by Susanne Brachthäuser-Berg from conference sponsor FGSV, the German acronym for the Research Association for Roads and Transportation. The conference included a technical tour to a paver production mold factory hosted by Kobra Molds.

Of the 37 technical papers from 15 countries presented over 2½ days, 13 were on permeable interlocking concrete pavements (PICP). One of the most significant papers was presented by Dr. David Jones with the University of California (Davis) Pavement Research Center on full-scale PICP accelerated load testing. The research produced pavement thickness design charts adopted by the Interlocking Concrete Pavement Institute and included in a draft standard on PICP design, construction and maintenance now nearing completion by the American Society of Civil Engineers.

Among several significant papers from Germany, one covered the extent of crown required in an interlocking concrete pavement road surface to maximize progressive stiffening (interlock) and increased load-distribution across the pavement. A paper from Dr. Anne Beeldens with the Belgium Road Research Centre illustrated how stormwater runoff restrictions in Belgium have stimulated deployment of PICP there, in a manner not unlike the United States. A paper presented by Alessandra Smolek, doctoral candidate with North Carolina State University, demonstrated substantial pollutant and volume reductions by PICP over clay soils with infiltration rates at 0.01 in/hour (0.0254 cm/hr).

Another significant paper from Germany explained how paving slabs can be used to reduce road noise, as well as another by environmental consultants, PE International, comparing the environmental impacts of interlocking concrete pavements to other pavement systems, and demonstrating that impacts from the former are lowest. The Germans continued their environmental-friendliness theme with papers on solar reflectance and how it can help reduce the urban heat island, and photocatalytic surfaces on concrete pavers to reduce nitrous oxides and photochemical smog.

Slotted for spring 2018, the next conference in Seoul, South Korea, will be sponsored by the Seoul Metropolitan Government. Having a regional government as a sponsor is a first for the series of international conferences as they are typically hosted by universities and industry. An interesting aspect is that the Seoul sidewalk department is participating to affirm and expand the role of concrete paver sidewalks in the Seoul region. This magazine will keep readers informed of developments on this conference.