TRENDING Design Professionals are taking a closer look at pavers as drainage and traffic calming problem solvers. Distance Learning Platforms are rapidly replacing traditional seminar forums for professionals.
TECH SPOTLIGHT Another Look at Interlock ICPI Foundation Lands PICP at Civil Engineering’s Front Door
HARDSCAPE SOURCEBOOK Paver Manufacturers Paver Installers Industry Suppliers
Another Look at Interlock David R. Smith ICPI Technical Director
John Knapton, Ph.D. a structural engineer, prolific researcher, writer, and one of the pioneers in the segmental paving world defined interlock as the “inability of a paver to move independently of its neighbors.” In other words, interlock spreads wheel loads across interlocking concrete pavement. This surface reduces stress and rutting on the base and soil subgrade making walking, bicycling, and driving faster and easier.
There are many technical papers measuring interlock by American, UK, European, Australian, South African and Japanese researchers. They all agree that interlock happens in the joint sand via shear transfer. A vertically applied load gets transferred or spread horizontally thanks to the shear capacity of the joint sand against the vertical paver sides. Obviously, the sand characteristics, paver thickness and joint widths all contribute to interlock. The bottom line is pavers, joint and bedding sand spread wheel loads in a manner similar to an equivalent thickness of asphalt but without its inferior material performance. Since its start, ICPI literature— especially concrete paver installer education courses—defined interlocking concrete paving units by their aspect and plan ratios. Aspect ratio is paver length divided by thickness. Plan ratio is length divided by width. The rule of thumb
is pavers should have an aspect ratio no greater than three for use in parking lots and streets. For occasional vehicular traffic, aspect ratio should be no larger than four. Plan ratios should be 2 to 3 to facilitate installation of herringbone patterns since research demonstrated these providing more interlock than running bond or stack bond patterns. The following tables recommend applications based on aspect and plan ratios.
Aspect ratio for pavers and slabs was first articulated by Lothar Pesch in 1988. He was the General Manager of the German Precast Concrete Industry Association. In those days, Germany was selling about 800 million sf of pavers. That’s the same area sold in the US and Canada last year.
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We have some catching up to do. Mr. Pesch also observed then as now that the line between paving blocks (pavers) and paving slabs was not that well defined. He did note that stouter pavers are meant for areas with regular traffic, i.e., low aspect ratios. In contrast, paving slabs with high aspect ratios such as six or more were meant for pedestrian areas only.
The pedestrian-only limitation on paving slabs has changed over time with some sand-set slabs performing without cracking or shifting in areas subject to a modest amount of vehicular traffic. Bitumen-set slabs provide significantly better performance being bound to a concrete base and not subject to risk of shifting or titling. Regardless of the setting bed material, slabs are typically 80 mm thick or thicker when subject to vehicular traffic.
A key question is how much (mostly truck) traffic? A few years ago, ICPI conducted computer finite element modeling to define how much traffic, or more specifically how many 18,000 lb. lifetime equivalent single axle loads or ESALs could be received by paving slabs set on a sand setting bed on aggregate, cement-stabilized and concrete bases. The answer was between 30,000 and 75,000 ESALs depending on the slab shape, thickness, flexural strength, and soil strength. That’s very little traffic compared to the 10 million lifetime ESALs interlocking concrete pavement can be designed to withstand according the Concrete Pavement for Municipal Streets and Roadways and ICPI Tech Spec 4 Structural Design of Interlocking Concrete Pavements. Besides aspect and plan ratios, another way to categorize paving unit size by application is to consider another ratio, the exposed surface area compared to the vertical side area. Obviously, thick paving units have more side area, so they have more interlock meaning the ability to share loads with neighboring pavers. A limiting factor for vehicular applications, however, can be this ratio, especially when viewed in relationship to aspect ratio. The following table demonstrates this point. A quick study shows that aspect ratios of three or less have surface-to-side area ratios of 0.8 down to 0.3. The lower the surface area to side area ratio, the better, because that means more interlock with neighboring pavers.
INTERLOCK DESIGN INSIGHTS
Comparing the exposed surface area to the vertical side area.
BELOW - This table also illustrates that there is lower, if any, potential for interlock among paving slabs, hence their delegation to areas that receive very little vehicular traffic, especially little truck traffic.
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A pavement crown is essential to creating interlock. Another notion in support of interlock is building a crown in a pavement. This is how interlock is created and maintained. This has been taught since the early ICPI installer classes, i.e., that the contractor creates interlock with paving units via a pavement centerline that’s a tad higher than the edges. A slight crown means that the paving units will settle a few millimeters from traffic and that tightening increases the load carrying capacity of the surface. This action is called progressive stiffening which also occurs on asphalt and concrete pavements.
In summary, small remains beautiful. Designers and especially contractors make interlock happen. This depends on three key ratios: aspect, plan and surface-to-side area ratios, as well as making roads perform structurally like kings by installing crowns. Paving slabs aren’t predisposed to sharing applied loads. Therefore, each unit shoulders its own load without cracking if the slab has sufficient thickness and base support, as well as being subject a small fraction of repeated vehicular traffic compared to interlocking concrete pavement.
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ICPI Foundation Lands PICP at Civil Engineering’s Front Door Twenty-four asphalt parking spaces at the front of the headquarters building of American Society of Civil Engineers (ASCE) in Reston, Virginia, were retrofitted in October 2020 with about 4,000 sf of permeable interlocking concrete pavement (PICP). Funded by the ICPI Foundation for Education and Research, ASCE’s visitor’s parking now features 3⅛ inch thick, nominal 4 by 8-inch pavers spaced bout ⅜ inch apart. The joints are filled with permeable aggregate that trap sediment as water flow through it.
The water flows into a 4-inch-thick ASTM No. 57 stone base and an 8inch-thick No. 3 reservoir subbase layer over geotextile. The 12 in. thick pavement base/subbase consists of open-graded aggregate with a porosity of about 40 percent so it can store about 4 inches of water while infiltrating it into the soil subgrade. The area was designed using ASCE 68-18, a national standard guide for designing PICP. The open-graded stone reservoir can store and infiltrate rainfall from a 100-year storm event.
INTERLOCK DESIGN INSIGHTS The parking spaces extend over a 3% downward slope. To contain and infiltrate water into the low infiltration, sloped clay soil, 30 mil thick 8-inch-high vertical PVC check dams are spaced about every 16 ft to detain water in the subbase reservoir, rather than allowing it to run downhill on the subgrade. The system includes a raised underdrain should there be a huge rain event that fills the reservoir. At its lowest elevation, the underdrain connects to an existing catch basin. The parking lot surrounds a a sixstory office building both owned by the ASCE Foundation. The PICP project is one of several low impact practices demonstrated on the 105,750 sf asphalt parking lot. Since the parking lot needed major renovation, BELOW - The figure illustrates the pavement cross section at ASCE.
ASCE leadership sought technologies solution that would demonstrate its commitment to the principles of sustainable design and environmental stewardship. The Foundation engaged donors, volunteers, and vendors to design and install permeable pavement, stormwater filtration systems, bioswale and other technologies that could help keep more than half the site’s potentially contaminated stormwater runoff out of the local watershed. The retrofit project reduces at least 50% of the phosphorus and nitrogen per year from flowing into the Bay, as well as reducing stream bed erosion which contributes sediment. Maintenance of all the LID practices will be outsourced to one company with experience in maintaining PICP.
The ASCE Foundation is developing outreach items to educate visitors about the various elements of this project. Signs around the headquarters building will outline the technologies, the problems they address, and how they do so. Timelapse videos will showcase the construction process of each of the project elements. This project spanned over three years and was driven by the hard work and coordination among ASCE Foundation staff, ASCE member volunteers, donors, and installation contractors. ASCE determined that to lead in sustainability, its members had to practice what they preach. This was accomplished by finding the resources to demonstrate what is required to reduce runoff and improve water quality at the association’s headquarters building.
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Segmental concrete pavement product types, shapes, sizes, colors, and finishes vary greatly. ICPI producers can supply helpful information on product characteristics, specifications, and availability.
Hardscape Industry Product Manufacturers ICPI Member Companies Acker-Stone Industries Corona, CA
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McNear Brick & Block San Rafael, CA
mcnear.com
Air Vol Block, Inc. San Luis Obispo, CA
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Mutual Materials Company Bellevue, WA
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Angelus Block Company, Inc. Rialto, CA
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Nicolock Paving Stones Lindenhurst, NY
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Barkman Concrete, Ltd. Steinbach, MB
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Oaks Concrete Products Brampton, ON
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Basalite Concrete Products Dixon, CA
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Oberfields LLC Delaware, OH
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Best Way Stone, Ltd. Woodbridge, ON
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Oldcastle Architectural, Inc. Atlanta, GA
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Bolduc Concrete Products Ste-Marie, QC
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Olsen Pavingstone, Inc. San Juan Capistrano, CA
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Borgert Products, Inc. Saint Joseph, MN
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ORCO Block & Hardscape Stanton, CA
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Brooklin Concrete Products Co Brooklin, ON
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Pavers by Ideal Westford, MA
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Brown's Concrete Products, Ltd. Sudbury, ON
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RCP Block & Brick, Inc. Lemon Grove, CA
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Building Products Corporation East Saint Louis, IL
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R.I. Lampus Company Springdale, PA
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Calstone Company Gilroy, CA
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RINOX - Canada Terrebonne, QC
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Cambridge Pavers, Inc. Lyndhurst, NJ
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Rochester Concrete Products Rochester, MN
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County Materials Corporation Marathon, WI
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Santerra Stonecraft Amherstburg, ON
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E.P. Henry Corporation Woodbury, NJ
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Shaw Brick Lantz, NS
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Fendt Builders' Supply, Inc. Ann Arbor, MI
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Southwest Block, Inc. Rio Rancho, NM
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Gagne and Son Belgrade, ME
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Techo-Bloc Saint Hubert, QC
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Genest Sanford, ME
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Wausau Tile, Inc. Wausau, WI
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Interlock Paving Systems, Inc. Hampton, VA
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Willow Creek Concrete Products Oakdale, MN
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Keystone Hardscapes Grapevine, TX
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Artistic Pavers North Miami Beach, FL
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Lee Building Products Bowling Green, KY
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FireRock Building Materials Birmingham, AL
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Lowcountry Paver Company Hardeeville, SC
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Stepstone, Inc. Gardena, CA
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ICPI provides installer training and certification programs to ensure that contractors understand product installation essentials. In both commercial and residential settings, this knowledge helps installation contractors deliver on the designer’s intent.
Hardscape Installation Contractors ICPI Voting Member Companies (1) Aqua-Paving Construction Yorkville, IL
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Fred Adams Paving Co., Inc. Morrisville, NC
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Atlas Pavers Co. Concord, CA
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Greenmark Landscaping & Development, Inc. Ottawa, ON
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Bauman Landscape & Construction San Francisco, CA
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GT Contracting Corporation Washington, DC
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B.C. Pavers, Inc. Renton, WA
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Landscape Concepts Inc. Keymar, MD
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Creative Hardscape Company Lakewood, CO
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LPS Pavement Company Oswego, IL
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Decorative Paving, Inc. Loveland, OH
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PaverBright NV LLC Reno, NV
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Designs by Stonescapes Commerce City, CO
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Precise Paving, Inc. West Palm Beach, FL
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Engledow Estates Carmel, IN
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Stowe Contracting, Inc. Marina, CA
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European Pavers Southwest, Inc Phoenix, AZ
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Structures, Inc. Shakopee, MN
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European Paving Designs, Inc. San Jose, CA
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Unit Paving, Inc. Charlotte, NC
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Exotic Landscaping Inc. Albrightsville, PA
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Urban Earth Landscaping Albuquerque, NM
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Farley Interlocking Paving Palm Desert,CA
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Westwood Contractors, Inc. Hampton, VA
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Fidus Roofing & Construction, LLC St Augustine, FL
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(1) ICPI Voting Members have a voice in developing industry standards.
Hardscape Industry Suppliers ICPI Member Companies ACM Chemistries, Inc. Norcross GA
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Advanced Concrete Technologies Greenland, NH
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Alabama Pallets Attalla, AL
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Alliance Designer Products, Inc. Laval, QC
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Automacad, Inc. Candiac, QC
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Besser Company Alpena, MI
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BP Pro Santa Ana, CA
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Cathay Industries USA Inc. Bartlett, IL
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Channel Industrial Group, Inc. Bradford, ON
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Columbia Machine, Inc. Vancouver, WA
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Dyna Concord, ON
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production, installation, and
Federated Insurance Owatonna, MN
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maintenance.
FORTRESS Engineering- Design Prior Lake, MN
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F. von Langsdorff Licensing, Ltd. Caledon, ON
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GCP Applied Technologies Cambridge, MA
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ICPI Associate Member Companies supply the industry with products and services that deliver product design,
Together with producers and installers, these members focus on system reliability.
Glow Path Pavers LLC Keego Harbor, MI
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RAMPF Molds Industries, Inc. Hagerstown, MD
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Haarup North America, Inc. Portland, OR
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Rekers (North America), Inc. Elmsford, NY
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Innovative Concrete Technology Port Murray, NJ
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Romex Canada North Vancouver, BC
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IQ Power Tools Moreno Valley, CA
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Sable Marco, Inc. Port-Rouge, QC
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IWI Concrete Equipment Group Norcross, GA
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Seal King, Inc. Brampton, ON
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J. McCoy Equipment, Ltd. Port Perry, ON
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SEK-Surebond Saint Charles, IL
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J R Automation LLC Earth City, MO
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Sika Corporation Lyndhurst, NY
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KBH Baustoffwerke Gebhart & Soehne Lachen, Germany
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Silca System LLC dba StoneDeks System Alliance, OH
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KOBRA Molds, LLC Hudson, WI
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Slab Innovation, Inc. St. Hubert, QC
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Krete Industries, Inc. Butler, WI
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Solidia Technologies, Inc. Piscataway, NJ
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KVM International A/S Kjellerup, Demark
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Solomon Colors, Inc. Springfield, IL
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LANXESS Pittsburgh, PA
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SRW Products Princeton, MN
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Lehigh White Cement Co. West Palm Beach, FL
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STAMPI Brunello San Dona di Piave VE, Italy
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MASA Green Bay, WI
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Standley Batch Systems, Inc. Cape Girardeau, MO
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Master Builders Solutions US, LLC Beachwood, OH
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St. Mary’s Cement, Inc., a VCNA Company Toronto, ON
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Op-Smart Buffalo, NY
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Techniseal Candiac, QC
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Pathfinder Systems/Tiger Machine Holland, MI
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TEKA North America, Inc. Chesapeake, VA
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PAVE TECH Hardscape Outfitters Prior Lake, MN
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The Euclid Chemical Company Cleveland, OH
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Penta US Inc. Albany, NY
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TMS International Horsham, PA
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Perma Edge Industries, LLC Vandalia, OH
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Topwerk America Ltd. Burlington, ON
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Permaloc Corporation Holland, MI
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Venator Americas LLC The Woodlands, TX
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POYATOS Export S.A. Albolote, Spain
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Weber MT, Inc. Grand Rapids, MI
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QUADRA USA, Inc. Phoenix, AZ
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Wuerschum North America, Inc. New York, NY
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Interlock Design Insights EXECUTIVE DIRECTOR: Charles McGrath, CAE PUBLISHING DIRECTOR: Steven Hawkins, AIA EDITORIAL DIRECTOR: David R. Smith CONTRIBUTING EDITOR: Robert Bowers, P.Eng. ADVERTISING SALES: Alison Stern
Interlocking Concrete Pavement Institute
14801 Murdock Street, Suite 230 Chantilly, VA 20151 Tel 703.657.6900 Fax 703.657.6901 ICPI Canada P.O. Box 1150Uxbridge, ON L9P 1N4 Canada SUBSCRIPTION AND MEMBERSHIP Qualified design professionals can receive a complimentary subscription—just click the subscribe button below. For further membership in ICPI, write to one of the addresses above or call 703.657.6900 STORY SUBMISSIONS Send submissions for consideration to Steve Hawkins at shawkins@icpi.org
AEC Daily Hosts Free Continuing Education Programs on ICP and PICP AEC Daily, the well-known continuing education provider for licensed design professionals with over 100,000 subscribers, hosts six presentations developed by the Interlocking Concrete Pavement Institute (ICPI). Three are on interlocking concrete pavement (ICP) and three are on permeable interlocking concrete pavement (PICP). Each course earns one continuing credit hour or professional development hour. All six presentations are numbered so they can be taken—at the option of the user.
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Interlock Design Insights is published bimonthly by the members of the Interlocking Concrete Pavement Institute (ICPI) for producers, suppliers, contractors, specifiers, and users of interlocking concrete pavements. The opinions expressed in Interlock Design Insights articles are those of the authors and do not necessarily represent the position of the editor or ICPI.