The Results Are In

Two years in the making, the University of California Pavement Research Center in Davis recently released a report on full-scale load testing of permeable interlocking concrete pavement (PICP). The report proposes revised design charts that reduce the thickness of a subbase thickness chart published in ICPI’s 2011 manual, Permeable Interlocking Concrete Pavements. The revised charts from UC Davis provide more cost-effective thicknesses.

The subbase thickness in the 2011 ICPI manual are calculated using the flexible pavement design methodology in the 1993 Guide for Design with Pavement Structures by the American Association of State Highway and Transportation Officials (AASHTO). Well-known among pavement engineers, this book provides methods for calculating dense-graded base thicknesses under roads. The methods do not cover calculations for open-graded bases for permeable pavements. Conservative adjustments to AASHTO methodology were made to develop the 2011 design chart for open-graded materials.

The UC Davis research validates ICPI’s subbase thickness chart while refining it by considering the number of days per year a subbase sees standing water, i.e., 0, 10, 30, 60, 90, and 120 days. The resulting charts present thinner subbases at the lower end of this range of exposure to standing water when compared to the ICPI chart which assumes high exposures to saturated subgrades and subbases. Subbase thickness also depends on other factors such as the amount of soil support and anticipated wheel loads.

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The UC Davis Heavy Vehicle Simulator load tests PICP structures.

In pavement design, the mix of anticipated wheel loads over the pavement’s life are combined and equalized into 18,000-lb axle loads called ESALs or equivalent single axle loads. This load unit provides a consistent measurement that also relates to pavement rutting. To give an idea of an ESAL, an over-the-road tractor-trailer typically exerts 3 or 4 ESALs with one pass over a pavement. The PICP loaded at UC Davis received over 2.5 million ESALs using a machine simulating truck wheel loads. Pictures of the Heavy Vehicle Simulator (HVS) are above, as well as a video of it running dual truck tires on the test track.

In pavement research, the best way to determine how many ESALs define pavement lifespan in years is to conduct full-scale accelerated load testing. In other words, a pavement is loaded with many wheel passes until it fails or is no longer considered useful. UC Davis pavement engineers conducted load testing with an HVS that accelerates loading, accomplishing 20 years of loading in just five and a half months. For PICP, failure is defined as excessive rutting, typically over 1 in. (25 mm). Interestingly, none of the concrete pavers cracked while the pavement was loaded with truck tires at the UC Davis testing facility while rutting to over 2 in. UC Davis design charts for subbase thicknesses use 1-in. rutting as the failure criteria.

The comprehensive UC Davis study began with a literature review that found little domestic research and a paucity from overseas. The study then load tested some local existing PICP projects with an 18,000-lb truck axle to better understand deflection under it and the pavement strength. The deflection data was used to estimate the stiffness (elastic modulus) of each pavement layer by conducting computer-based mechanistic analysis modeling that correlates modeled and measured stresses, surface deflections, and permanent strains (rutting) to pavement layer strengths. This data was also used to determine subbase thicknesses for full-scale testing at a 96-ft long PICP test track over which the HVS could run truck tires and loads.

PICP Test Track Drawing

The PICP cross sections tested at UC Davis with the HVS machine.

The test track included three subbase thicknesses (approximately 18 in. or 450 mm, 27 in. or 650 mm and 37 in. or 950 mm) instrumented to provide data on stresses while loaded and rutting (see diagram below). The weak clay soil subgrade was compacted and non-woven geotextile was placed on the subgrade and sides of the excavation. Above the subbases was a 4-in. (100 mm) thick layer of ASTM No. 57 aggregate, 2 in. (50 mm) of No. 8 aggregate, 3 1/8-in. (80 mm) thick concrete pavers and permeable jointing aggregate. A concrete curb restrained the No. 57 aggregate, bedding and pavers. The figure below shows a cross section of the test track. The aggregates were granite quarried from the foothills of the Sierra Nevada Mountains.

The testing represents the first full-scale load testing on PICP in the western hemisphere and one of a few studies globally that examines the structural response of open-graded bases to wheel loads. The UC Davis design charts go to one million ESALs, the maximum loads also provided on the 2011 ICPI design chart. The revised charts will appear in an emerging ASCE national standard on PICP as well as in an updated edition of the ICPI PICP manual. Both are scheduled for release in 2016.

The project was funded by the ICPI Foundation for Education and Research, the Concrete Masonry Association of California and Nevada, the California Nevada Cement Association and the Interlocking Concrete Pavement Institute.


The Great Enheightenment

In recent surveys, readers have asked for instructional articles with practical knowledge they can use on the job. In response, Interlock Design is pleased to present this first installment of its new how-to series of feature articles that will run throughout the year. We welcome reader feedback and invite you to contact us at with any suggestions or topics you’d like us to cover in the new how-to series.

Why a Raised Patio?

As this issue’s cover story explains, residential outdoor living is a booming market for the hardscape industry. An integral component of an outdoor living space is a raised patio.

Traditionally, a raised patio allows movement from house to backyard without a change in elevation. A homeowner steps out the back door and into the outdoor living space as easily as walking from one room to the next inside the house, creating a seamless transition from interior to exterior space.

Initial Precautions

“If not done properly, a raised patio can do significant damage to the building that it’s constructed against,” says ICPI Director of Engineering Robert Bowers, P. Eng. The main factors that can cause damage are moisture accumulation and the increased lateral load placed on the foundation, and possibly on exterior above-grade walls.

Most exterior above-grade walls of a house are not designed to have moisture continuously against them, explains Mr. Bowers. Whether they’re brick, wood siding, vinyl or another material, these exterior walls are designed to resist water and shed it—to get wet and then dry out. They cannot withstand a continuously moist environment. Placing compacted soil against these types of walls can trap moisture, resulting in mold, decay and deterioration.

Regarding foundation walls, in most cases they are constructed to bear the weight of the supported structure, the lateral pressure from the soil and not much more. By constructing a raised patio, the lateral pressure against a foundation increases. This presents an increased risk of blowout and basement wall collapse, because the increased load to the wall is not counterbalanced. This is called an unbalanced fill condition. When taking on an unbalanced fill project, an engineer should be consulted to ensure the stability of the project. Additional reinforcement of the foundation wall is sometimes necessary. The cost of the engineer’s involvement will increase the cost of the construction, so it’s important for contractors to include this in the price of their proposals.

 “A homeowner should be able to appreciate it,” Mr. Bowers says, “if you say, ‘Hey, I’m concerned that we don’t damage your house in any way and I’d like to have a professional engineer tell us the best way to do this.’”

At the outset of planning, be sure to thoroughly document the existing conditions of the site. Take photos of the exterior walls, the foundation and the basement walls inside and out, carefully inspecting for cracks, bulging and any signs of dampness or water damage.

Design Considerations

The most effective way to raise a patio adjacent to a building is with a retaining wall (aka stress relief wall) that faces the building, offset from it by 3 to 4 in. This creates an air gap that prevents the patio from touching the building’s exterior cladding and also allows airflow so any moisture that gets in can dry out (See Figure 1). Additionally, the air gap prevents a raised patio from covering up weep holes. Covering weep holes compromises the exterior above-grade wall venting system, leading to deterioration and potential collapse. For this reason, covering weep holes is a building code violation. At the top of the air gap, cantilevered pavers and screens are common solutions to prevent debris from falling into the gap. A drainage system at the bottom of the air gap is also required. Another option is applying aluminum flashing against the house. This surface, however, cannot block weep holes designed to wick moisture from the walls.

The higher the patio is raised, the greater the complications and potential risks to the foundation. Most homes are constructed with 8 to 12 in. of foundation wall above grade, atop which sits another 12 in. of floor joists. That means the threshold of the back door is typically 20 to 24 in. above grade. For every foot of elevation a wall is built up, roughly 50 to 100 pounds of additional load is applied to the foundation walls. Depending on a number of conditions, it could be even more.

Coincidentally, a raised patio height of 20 to 24 in. is a gray area for determining if additional measures are required to reinforce the foundation wall. For any patio raised above 24 in., it is recommended to have an engineer review the design, test soil quality, evaluate foundation walls and make recommendations.  Heights of 20 in. or less generally carry less risk in relation to the loads. Ultimately, each contractor must decide on his or her level of comfort and corresponding liability.

“If you think there’s the slightest possibility you might need an engineer, then you need an engineer,” Mr. Bowers says. “I can’t tell you how many calls I get from contractors who say, ‘I’m not sure but I think I might be doing something that requires an engineer.’  They describe the situation and yes, they should’ve had an engineer involved weeks ago.”

Local building codes also come into play at heights around 24 in. or greater and when adjoining the raised patio to a building exit like a back door. Every building code has specific requirements for steps including tread depth, riser height and pitch, as well as for hand railings and guards. Because many aspects of raised patio construction are governed by building codes, raised patio construction often requires first obtaining a building permit.

Raised Patio Construction

Raised patios are constructed using three basic components: walls, flatwork and steps. But before building anything up, the ground must be broken.


When new home construction is completed, often the soil against the foundation wall is excavated backfill of the soil consisting of silty, clay soil unsuitable for the subgrade of a raised patio. A soil probe or test pit will confirm this and is recommended to determine soil type and quality. A common way to reduce the lateral load applied to a foundation wall is to remove poor quality soil and replace it with a higher quality dense-graded, crushed stone aggregate. As a rule of thumb, the height of the patio determines how deep to excavate and how far out from the building foundation. If a raised patio will be 48 in. (1.2 m) high, dig down 48 in. (1.2 m) and out from the building the same distance.

Subbase, Drainage, Base

Dense-graded, compacted aggregate is commonly used for the base of the wall and the raised patio. For some projects, flowable fill may be advantageous because it’s lighter and does not require compaction. However, it can be more expensive to install and may require time to cure.

Once the subgrade and base for the wall are set, install a 4 in. (100 mm) diameter perforated drainage pipe along the length of the wall that slopes to a drain. For the drainage layer above the drainage pipe, use open-graded, compacted aggregate with ¾ in. (19 mm) minus clean stone (See Figure 2).


When using segmental retaining wall (SRW) units to raise a patio, a conservative rule of thumb is that the maximum height of the wall should be approximately twice the depth of the SRW unit. For heights three times the depth of the SRW unit or greater, geogrid should be used to help stabilize the wall. Most building codes require walls over 48 in. (1.2 m) in height to be engineered, and some jurisdictions have set limits even lower.

A conservative initial design incorporating geogrid could specify continuous layers every 12 to 16 in. (300 to 400 mm) vertically with a length equal to the height of the wall, and not less than 4 ft (1.2 m). This design would only be suitable for typical conditions: dense graded aggregate backfill; pedestrian-only loading with no slope or terraced wall above; a stable, undisturbed subgrade to a maximum total height of 8 ft (2.4 m). If these typical conditions do not exist on the site, or the decision is made to optimize the design, an engineer should be consulted to develop the initial design.


Every building code has requirements for steps. For outdoor applications, a common pitch requirement is 6:12: a 6 inch (150 mm) riser and a tread depth of 12 in. (300 mm). Maximum riser heights of up to 8 in. (200 mm) may be permissible, so check local building codes. The steps must have a consistent tread depth and riser height to prevent a tripping hazard. Complete compaction of base material is extremely important. Flowable fill or a well-compacted, cement-treated aggregate can help minimize the potential for settlement.

SRW units (Figure 3) or concrete pavers (Figure 4) can be used to construct steps. Either way, choose a material that has freeze-thaw durability. Snow removal and deicers can destroy concrete materials not manufactured to freeze-thaw resistance. Some SRW systems have cap units that are not meant to support regular pedestrian traffic, so be sure to choose the proper units if using for steps. If pavers are selected for the steps, it is necessary to build the base out of concrete to prevent “roll over” that occurs if paver steps are not properly supported.


For patios with elevations greater than 24 in. (600 mm), most building codes require a guard or handrail, including minimum height requirements, as well as specifications for resistance to lateral loads. For code compliance, the railing, mount and foundation all must resist the applied load. Generally, there are four types of mounts used to connect the post to the stabilizing foundation: surface, core, side and direct.

Surface mounts are common but also typically the weakest. A plate is welded to the bottom of the post and then connected to the top of the retaining wall with lag bolts or self-tapping concrete screws. Core mounts involve drilling down 18 to 24 in. (450 to 600 mm) into the retaining wall and grouting or epoxying the post directly into the wall. Core drilling can be time-consuming and costly and risks splitting the SRW units under certain conditions in freezing environments. While core-drilled guards are potentially more stable than surface mount guards, neither should be relied upon as the only means of securing the guard.

Side mounts attach handrails to the face of a side wall. When side-mounted handrails are combined with a guard system, they contribute to the stability of the entire guard assembly. The most effective way to secure a guard system is with the fourth type, a direct mount, which attaches to a solid fixed object like a building or caisson (See Figure 5).

Site Prep

The main task in job layout is transferring the final design from paper to the site. Verify access and staging areas; identify slopes and drainage conflicts; install erosion control and containment measures; and provide protection for trees, plantings and structures. Confirm the location of all utilities and buried utility lines, making sure everything is clearly marked. Outline the extent of excavation and the patio, install string lines, and designate finished elevations with stakes, string lines and markings on adjacent structures. Make plans for equipment storage and vehicle parking. And always maintain a clean, organized site to make a favorable impression.

When defining the elevations of a project, identify the critical elevations on existing structures like a doorsill. Typically, critical elevation determines the finished elevation, so it is necessary to calculate backward from the finished elevation down to the starting elevation. Repeat this calculation in several locations on-site and double-check them.


Care must be taken when compacting adjacent to a foundation wall; excessive force may cause cracking. Less force can be used by placing soil in thinner lifts. For the first course of SRW units, dense-graded aggregate base should be compacted to a minimum of 98% standard proctor density (SPD). Although industry guidelines call for 95% SPD for the fill behind the retaining wall, ICPI recommends 98% SPD to minimize the settlement of the pavement surface above. It is important to watch the alignment of the SRW units to ensure they are not pushed out of alignment or rotated forward during compaction.

For bases and fill, in addition to the flowable fill alternative previously mentioned, geotextile or geogrid, cement-treated base (CTB) and asphalt-treated base (ATB) are also options. Installers who have limited experience with these materials and methods should receive technical support prior to selection. A geotechnical engineer’s input may also be necessary to determine the strength of the subsoil and the extent of remediation required.

Raised patios also require adhesives for retaining wall caps, treads and other materials. Adhesives that remain slightly flexible after curing are preferred. Though mortar can be a cheaper option, its use is not recommended in areas with freezing and thawing conditions.

Close Out

Equipment removal and cleanup are standard operating procedure. After running down punch-list items and performing final inspection, secure a certificate of occupancy and final payment. As a courtesy, provide the homeowner with spare pavers, sand and cleaner. Photograph the completed project for the company’s portfolio and be sure to write a thank-you note for a high-dollar job.

Continuing Education

The information provided in this article is from the ICPI Advanced Residential Paver Technician Course manual. To sign up for this course or any other offered by ICPI, visit


Tech Specs Update

The Interlocking Concrete Pavement Institute (ICPI) released three new Tech Specs and updated another. The Tech Spec titles and content summary are provided below. All publications can be downloaded at ICPI has published its Tech Spec series of technical bulletins since 1995.

Tech Spec 16: Achieving LEED Credits with Segmental Concrete Pavement consists of a complete rewrite covering how segmental concrete paving products can support earning credits under the latest version (v4) of the U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) sustainable rating system for buildings. The 20-page document presents excerpts from the LEED BD+C Reference Guide and comments on the role segmental concrete pavements can play in support of earning credits.

Tech Spec 19: Design, Construction and Maintenance of Interlocking Concrete Pavement Crosswalks provides guidance on selecting and detailing assemblies based on anticipated traffic loads. The assembly choices are based on full-scale load testing and evaluation by the University of Waterloo, Ontario, Centre for Pavement and Transportation Technology. The eight-page bulletin covers assemblies using aggregate, stabilized and concrete bases using sand and bitumen-sand setting beds for the concrete pavers. Specify interlocking concrete pavement crosswalks, bases and edge restraints with more reliable performance.

Tech Spec 20: Construction of Bituminous-Sand Set Interlocking Concrete Pavement provides a well-illustrated and thorough description on how to construct this durable system for sidewalks, crosswalks, intersections and streets. The eight-page bulletin outlines the tools, materials and steps for bitumen-set concrete pavers on a concrete base. This application is essential in high traffic, urban areas subject to buses and trucks. The text is aimed at contractors while presenting a procedure of high interest to civil engineers, landscape architects and architects. The bulletin accompanies ICPI’s guide specifications and detail drawing on bitumen-set applications.

Tech Spec 21: Capping and Compressive Strength Testing Procedures for Concrete Pavers informs paver manufacturers, paver testing laboratories and specifiers on recent sweeping changes to the compressive strength testing in ASTM C140: Standard Test Methods for Sampling and Testing Concrete Masonry Units and Related Units. The eight-page bulletin provides step-by-step cutting and refined capping procedures for crushing paver specimens using flow charts and photos to help testing labs produce consistent compressive strength test results. The bottom line on the changes to ASTM C140 is it now adjusts compressive strength results from concrete pavers with different thicknesses, thereby eliminating the confusion resulting from lower strengths from thicker pavers.


LEED v4 Review

Released in November 2013, LEED v4 continues support of segmental concrete paving products to earn credits toward certification as a sustainable project. There are, however, some significant changes under familiar credit categories. The table below lists credit categories and credits that can be supported by using segmental concrete paving products. Detailed information can be found in ICPI Tech Spec 16: Achieving LEED Credits with Segmental Concrete Pavement available at


Credit: Open Space

This credit recognizes designs that use open space to encourage interaction with the environment and other people, as well as passive recreation and physical activities. Segmental concrete pavements can be utilized in the design of such spaces.

Credit: Rainwater Management

Gone are separate points earned for stormwater management quality and quantity. Pollution and volume reduction are now combined into one credit. Credits are earned based on management of the 95th or 98th percentile rain event, or by matching pre-development conditions. Management means on-site detention and/or infiltration, which virtually requires the use of permeable pavements such as permeable interlocking concrete pavements (PICP).

Credit: Urban Heat Island

Requirements continue for roof surfaces having minimum solar reflectance indices (SRI). The minimums have been increased from 29 to 39 and a three-year (aged) minimum of 32 for low-sloped roofs. For non-roof surfaces, minimum requirements for reflectance have changed from an SRI method to a solar reflectance (SR) measurement method based on using a different ASTM test method than that for SRI. SR measures outbound (reflected) radiation divided by inbound radiation, both measured at four specific wavelengths. The initial SR required is 0.33 with a three-year aged minimum of 0.28. This means that SR (as well as SRI) requirements benefit the use of lighter paver colors. Concrete grid pavements continue receiving points so this benefit essentially remains unchanged. The grids need to be at least 50 percent “unbound,” meaning turf grass or light-colored aggregates.

Credit: Water Efficiency

Capturing and reusing runoff for irrigation with PICP or other means no longer earns points. LEED v4 makes this condition a project prerequisite to qualify for participation in their program. If the project has no landscaping, then this requirement is not applied.


The MR structure has a four-pronged approach: life-cycle assessment; toxic chemical avoidance; building reuse; and waste management. This credit category is restructured to address a more holistic, life-cycle view of creation, use and disposal of construction materials, rather than just recognizing recycling as in previous LEED versions. Here are the credits relevant to the concrete paver industry:

Credit: Building Product Disclosure and Optimization—Environmental Product Declarations (EPDs)

This new credit requires EPDs to be supplied on at least 20 different products in a project and to be cradle-to-gate impact assessments for various pollutants, including carbon emissions and energy use. EPDs can be paver-industry averages typically reported by an association or product-specific EPD reported by a paving product manufacturer.

Credit: Building Product Disclosure and Optimization—Sourcing of Raw Materials

Multiple criteria from LEED 2009 credits have been combined into this credit. Some old criteria are folded into other MR credits, such as Building Life-Cycle Impact Reduction and Building Product Disclosure and Optimization—Environmental Product Declarations. Here are the options under this credit heading:

  • MR Credit Resource Reuse: Materials reused on-site are no longer required to be repurposed.
  • MR Credit Recycled Content: The requirements for recycled content have not changed; however, this criterion is now combined with other criteria in a single option.
  • MR Credit Regional Materials: The 500-mile (800 km) radius requirement was decreased to 100 miles (160 km). The definition of regional has been expanded to include the distribution and purchase location, and now includes all points of manufacture.

Credit: Building Product Disclosure and Optimization—Material Ingredients

This is a new credit with three options for earning points:

  • Material ingredient reporting: Paver manufacturer reporting of ingredients, reporting via health product declaration or cradle-to-cradle certification.
  • Material ingredient optimization: Manufacturers undergo a “green screen” assessment, a more rigorous cradle-to-gate certification, or certify that products have no substances of “very high concern” according to the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) program Published by the European Union.
  • Product manufacturer supply chain optimization: Supply chain providers to a concrete paver manufacturer meet certain criteria for environmental, health and safety protection.

Credit: Construction and Demolition Waste Management

This has changed from LEED 2009. Reuse of segmental concrete paving products can be part of a demolition/reuse plan if the paving products are reused or recycled. This is achieved through either 50 or 75 percent diversion of project waste from a landfill. A third option sets a limit on the pounds (2.5) of waste generated per sf of building floor area.

LEED v4 continues credits and points issuance for innovation in projects and credits for participation of a LEED AP in a project who specializes in particular credit areas. In addition, credits can be earned for “regional priorities.” For example, if groundwater aquifer recharge is a regional priority, then points could be earned by using PICP.


LEED v4 Credit Category Total Available Points Maximum Points Using Segmental Concrete Pavement
Sustainable Sites

  • Open Space
  • Rainwater Management
  • Heat Island Reduction
10 1
Water Efficiency

  • Outdoor water use
11 Prerequisite (no points)
Materials & Resources
  • Building Product Disclosure and Optimization—Environmental Product Declarations
  • Building Product Disclosure and Optimization—Sourcing of Raw Materials
  • Building Product Disclosure and Optimization—Material Ingredients
  • Construction and Demolition Waste Management
  • 13 1




    Innovation 6 6
    Regional Priority 4 4
    LEED Accredited Professional 1 1
    Range of potential points 45 – 50 25 – 30


    Long-Term Payoffs

    Numerous surveys rank Minneapolis as one of America’s best places to live. Forbes magazine recently ranked the City of Lakes as the healthiest in the nation, in part because of its green spaces and walk-friendly downtown. So, when dealing with stormwater, it’s no surprise the City chose permeable interlocking concrete pavement that boasts a major benefit: extending the longevity of urban street trees. “This project was unique because the City wanted to treat rainfall where it fell while capturing nearby runoff, and it realized that tree health could benefit from that process,” says Bob Kost, Landscape Architect Director for Short Elliott Hendrickson (SEH), who worked on the project. “This was one of the first projects of its kind in the country, and I think other cities should consider this kind of system.”

    New Vision

    Named Marq2, the project extends along Minneapolis’ major downtown corridors of Marquette Avenue and Second Avenue South and covers about 15,000 sf (1,400 m2) along 48 blocks of city sidewalks. Marq2 rebuilt streetscapes, from building front to building front, widening sidewalk space and incorporating 190 new trees, public art and new transit shelters. Permeable pavers enabled a substantial reduction in stormwater and pollutant runoff on a mile-long stretch of downtown.

    The stormwater retention system consists of an underground grid of nearly 11,000 plastic-framed cells filled with about 580 cf (16 m3) of a bioinfiltration soil mix. The cell groups, which resemble milk crates stacked on top of each other, provide pavement support while preventing soil compaction in order to maintain infiltration. Perforated pipes in the cells convey excess water out of the system. Based on research by Prince George’s County, MD, the filtration by the soil inside the cells removes 80 percent of the phosphorous; 60 percent of the nitrogen; and over 90 percent of the lead, copper, zinc and iron from the stormwater.

    A grated cap on top of the cells is covered with geotextile, granite infiltration stone, followed by a layer of smaller granite bedding aggregate. Permeable pavers were placed over that, allowing runoff to enter the soil-filled cells beneath. Tree grates and guards were designed by a local artist and fit well with the natural gray-colored pavers. The grates allow water to filter down to the tree roots. The combined use of the permeable pavers and the bioinfiltration system can receive some 21,600 cf (610 m3) of stormwater from each rain event, keeping it from entering the Mississippi River.

    “This system protects the waterway,” says Kost. “Many cities face this exact issue because plenty of them have old stormwater systems connected to sewer systems. Managing each of these is important if you want to protect your water resources.”

    Role of Pavers

    Utilizing the permeable interlocking concrete pavers led to greater sustainability, believes Chris Behringer, principal at Behringer Design, who worked as senior urban designer on the Marq2 project. She notes that the permeable pavers are becoming a regular part of planning for landscape architects because they allow for better stormwater management. “There’s a higher comfort level with pavers than with other permeable surfaces like porous concrete or asphalt,” she says. “The pavers help alleviate stress on the whole system because they disperse water in a larger capacity [than other options].”

    That’s important not just for summer and fall rainstorms but also for spring thaws. Minneapolis receives an average of 45 in. (114 cm) of snowfall annually, and as the huge mounds of plowed snow melt, the permeable pavers make runoff management more effective.

    Like any well-traveled surface, the pavers require maintenance, she adds, but even then, they seemed a much stronger choice than other options. Unlike asphalt, which would have to be cut out in a chunk, for example, the pavers can be replaced on a smaller scale, limiting disruption and saving on maintenance costs. “You mainly have to replace some of the infill as well as vacuum out some of the substrate to prevent clogging,” she says. “These are very affordable, minimal maintenance tasks, though.”

    Leafy Return on Investment

    Another key factor for improved ROI for the City of Minneapolis is just above pedestrians’ heads. Typically, urban trees need to be replaced about every five to seven years, Kost notes. Trees begin to decline in health due to soil compaction and/or limited availability of suitable soil, or a city might replace them to control irrigation costs. Because of Marq2’s innovative system, the mix of hardwoods and ornamentals planted in 2009 are still growing strong and aren’t up for replacement in the near future. The permeable interlocking concrete pavers and the system beneath prevent soil from compaction while stormwater draining through the permeable pavers significantly reduces the need for additional irrigation. That means the City saves money that would have been spent for watering. Also, each tree costs about $450, so extending the lives of all 190 trees means major savings.

    Beyond those short-term savings lie longer-term benefits. As trees mature and expand their canopies, they provide more oxygen, urban island heat reduction and sidewalk shade. “You don’t get these benefits from younger trees; it’s only when trees reach a certain size,” Kost says. “Many cities are forced to replace trees just before they mature so they don’t reap these huge advantages.” By extending tree longevity and controlling stormwater management — with the help of permeable pavers — Minneapolis isn’t likely to lose its healthiest city title anytime soon. “This whole system is part of creating a healthier environment,” says Behringer. “It creates benefits for everyone.”


    Read More

    Download the following case studies to learn more:

    Willow Creek Case Study

    DeepRoot Case Study


    PICP Receives High Marks

    Before and after

    With its construction completion first reported in November 2010, the PICP drive and parking lot monitored by the University of New Hampshire Stormwater Center captured almost all rainfall without outflows from the base.

    The University of New Hampshire Stormwater Center (UNHSC) released a two-year study on the performance of a permeable interlocking concrete pavement (PICP) site built on the university campus. The 13,500 sf (1,350 m2) Hood House Drive and parking lot in Durham, NH, reduced runoff volume and pollutant mass removals some 95 percent. Water infiltrating from about half of the PICP area was monitored for pollutants such as sediments, zinc, petroleum hydrocarbons, and nutrients (i.e., phosphorous and nitrogen forms). Built over a mix of moderate and low infiltration soil, the pavement saw significant volume reductions such that no single rain event generated more than 5 gal. (20 L) of discharge to underdrains in the base. Additionally, the study confirmed that open-graded bases and soil subgrades do not heave from winter freezing and thawing.


    Surface infiltration testing was conducted using a test method similar to ASTM C1781 Standard Test Method for Surface Infiltration Rate of Permeable Unit Pavement Systems. Testing showed a decline in surface infiltration for PICP areas subject to run-on from adjacent impervious or grassed areas. Surface infiltration rates declined 69 percent over 21 months, yet retained greater than 1,000 in./hr (2,540 cm/hr). Surface maintenance included vacuuming twice annually with regenerative air equipment typically used on the UNH campus. Figure 1 illustrates infiltration rates measured from September 2010 to May 2012.

    Summertime thermal analyses compared four pavement surface types at three different times of the day. Surfaces included pervious concrete and porous asphalt sites on the campus. PICP surface temperatures were observed to be lower than that for porous asphalt, pervious concrete, and standard asphalt. Figure 2 provides data for measurements in June 2012.



    Figure 1. Surface infiltration rate monitoring over time of three separate locations representing different loading and usage characteristics.

    Thermal Performance comparison of Picp

    Thermal Performance comparison of Picp

    Figure 2. Thermal performance comparison of PICP, porous asphalt, pervious concrete and conventional asphalt with measurements taken on June 21, 2012.



    Hood House Drive and an adjoining parking lot were retrofitted from a standard asphalt surface to a PICP system in the summer of 2010. The existing condition included no stormwater control measures and conveyed surface runoff directly into the municipal storm sewer. The PICP was designed by Appledore Engineering, Inc. with input from UNHSC and the Interlocking Concrete Pavement Institute (ICPI). The lower end of the drive receives rainfall and run-on from three pedestrian walkways and an adjoining road. Pavers and the surrounding grass landscaping are separated by new granite curbing. Rainfall enters the PICP surface and passes into the reservoir consisting of an open-graded, crushed stone base and subbase. Excess stormwater not infiltrated into the soil subgrade drains through internal perforated pipes along the bottom that discharge into the municipal storm sewer system.

    Pollutant loads from conventional pavement were estimated by monitoring runoff from an adjacent road and parking lot similar in size, usage and location. The following two years consisted of monitoring the PICP, which received 26 storms with 18 water-sampling events. The PICP performance for volume reduction and pollutant load reduction was exceptional for an installation on a sandy clay soil. Some parts of the site are very rocky, with fine, sandy loam. Soil subgrade infiltration tests done prior to construction were about 3 in./hr (7.5 cm/hr) in one area, which appears to be the major reason for such little water leaving the base/subbase via perforated pipes.


    The PICP cross section consists of 3 1/8 in. (80 mm) thick concrete pavers with ASTM No. 8 stone, bedded on about 2 in. (5 cm) of the same material. The 4 in. (10 cm) thick base was compacted over a crushed stone subbase of No. 2 stone with a thickness of 24 in. (6 cm) in the parking area and 16 in. (40 cm) in the drive area. Since the soil subgrade in the drive area slopes three percent, geotextile wrapped berms about 4 in. (10 cm) high were used to delay water and allow it to infiltrate into the soil subgrade rather than continue into perforated pipes to drain into a nearby storm sewer.

    UNHSC analyses and procedures for this study comply with the Technology Acceptance and Reciprocity Partnership (TARP), and the Technology Acceptance Protocol – Ecology (TAPE) guidelines. The research project was funded by ICPI and the ICPI Foundation for Education & Research. The removal of the existing asphalt pavement and old granite curbs, new curbs, pipes and base installation were provided by the UNH Facilities Department. Pavers, bedding materials and installation were co-funded by eight ICPI members, the New England Concrete Masonry Association and the Northeastern Cement Shippers Association. Machine-assisted installation by an ICPI contractor member holding an ICPI PICP certification enabled timely installation of the bedding layer and pavers for about $4/sf ($43/m2).


    Since the early 1980s, there have been at least 18 studies lasting one year or longer on in-situ permeable interlocking and concrete grid pavements performed by stormwater agencies, universities and consultants world-wide. Of these, only three have been conducted in cold latitudes, including the UNH study. The takeaways from this study underscore past findings while presenting new ones that further demonstrate PICPs performance in cold climates:

    • PICP eliminated practically all of the stormwater runoff from the storms at UNH;
    • PICP effectively removes sediment, nutrients and metals through infiltration even during winter months;
    • The surface was vacuumed twice annually over the two-year monitoring period with regenerative air equipment;
    • Surface infiltration rates decline over time due to sediment and other debris, but rates can be increased with vacuum maintenance;
    • The PICP surface is cooler compared to porous asphalt and pervious concrete;
    • Winter snow plowing was done with no problems and there was no deicer damage;
    • PICP does not heave from winter freezing and thawing; and
    • The PICP surface provides opportunities for brining of deicing materials to prevent ice buildup, a feature not offered by other permeable pavement surfaces.

    Project cost: approximately $9.90/sf including removal of the existing asphalt pavement, excavation, drainage, aggregates, new granite curbs and machine-installed concrete pavers. Costs do not include engineering and site soil testing or monitoring.


    Geomembranes in PICP

    Permeable interlocking concrete pavement (PICP) systems can be designed and constructed to accommodate three drainage conditions:

    • Complete infiltration of water into to the high infiltration rate soil subgrade with no underdrains;
    • Partial infiltration of water into low infiltration rate soil subgrade with some outflow through underdrains;
    • No infiltration into the soil subgrade with all outflow exiting through underdrains.
    • All conditions have similar surfacing, and base/subbase reservoirconstruction. No exfiltration designs, however, use a geomembrane on the sides and bottom of the base/subbase reservoir to contain stormwater and prevent it from infiltrating into the soil subgrade. Commonly called an impermeable liner, Figure 1 illustrates a typical PICP design using such a membrane.

    A no infiltration design with a geomembrane is typically used in the following conditions:

    • The soil has very low permeability, low strength, or is expansive;
    • High depth to a water table or bedrock;
    • To protect adjacent structures and foundations from water;
    • When pollutant loads are expected to exceed the capacity of the soil subgrade to treat them.

    By storing water in the base/subbase and then slowly draining it through pipes, the design behaves like an underground detention pond with the added benefit of reducingcontaminants. A no infiltration retention design may be used for water harvesting. The water may be piped to an underground cistern for reuse on site. Harvested rainwater reduces landscaping water requirements and in some cases it can be used for gray water within buildings.

    Geomembranes are a class of geosynthetic fabricated to create a sheet barrier that is relatively impermeable and is installed to prevent the flow of liquid or gas across that barrier.

    Geomembranes can be manufactured from a range of polymers including polyvinyl chloride (PVC), chlorosulfonated polyethylene (CSPE), chlorinated polyethylene (CPE), or, more recently, polypropylene (PP), ethylene propylene diene monomer (EPDM), high-density polyethylene (HDPE) and linear lower density polyethylene (LLDPE), very flexible polyethylene (VFPE). Each of these polymers is unique and provides varying levels of resistance to acids, alkalis or petrochemicals. Some polymers can also function in extreme heat or cold. Normally, the surface of a geomembrane is smooth, but some sloped applications can benefit from a textured surface that provides greater friction with the adjacent geotextiles or soil.engineer2

    Geomembranes come in a range of thicknesses depending on the polymers and the manufacturingprocess. For example, HDPE geomembrane is typically available in 40, 60 and 80 mil (1.0, 1.5 and 2.0 mm) thicknesses and in a range of roll widths. Geomembranes have different engineering properties depending on polymer type, thickness and manufacturing process. Typically the nominal thickness, density, tensile strength, tear resistance, dimensional stability and puncture resistance are provided in manufacturers’ literature and referenced in product specifications.

    Geomembranes for PICP are typically fabricated on the job site and this requires cutting, fitting and seaming to create waterproof joints. Different seaming techniques are used depending on the polymer,environmental conditions and project requirements. Materials like EPDM and PVC are routinely seamed using adhesive or double-sided tape. Before two panels are joined, the areas to be joined are usuallycleaned and primed. HDPE and other polymers are typically welded together with extrusion welders or hot wedge welders. Seams for all materials should be field-tested to ensure their integrity, especially around underdrains penetrating the geomembrane. For smaller projects, it might be possible to have the supplier prefabricate the geomembrane to meet site requirements. Prefabricated geomembranes are typically delivered to the site folded on a pallet.

    When preparing a site for a geomembrane application, remove rocks, roots, and other sharp objects from the subgrade that may damage the geomembrane during installation, aggregate compaction,or use. Such areas should be filled with dense-graded aggregate and compacted before placing the geomembrane over them. A layer of non-woven geotextile is commonly used to protect one or both sides of the geomembrane. The thickness of the geotextile is typically selected based on the materials placed next to the geomembrane and the importance of preventing puncturesof the geomembrane. Figure 2 illustrates a green alley in Richmondwith a geomembrane that is protectedby a non-woven geotextile before placing and compacting the subbase aggregate.

    When designing a no infiltration PICP system, there are many factors that must be considered in selecting the geomembrane and protection materials. For most projects, consultation with an engineer familiar with the design of a geomembrane is recommended.


    Kortright Centre Study

    The Toronto and Region Conservation Authority (TRCA) released a two-year study on the performance of a permeable pavement parking lot built at their Kortright Centre in Vaughn, Ontario, in metropolitan Toronto. The pavements consisted of two types of permeable interlocking concrete pavement (PICP), pervious concrete and an impervious (conventional) asphalt pavement as a control surface. The contiguous parking areas each are about 3,400 sf (320 m2).

    kortright1The permeable pavements did not produce surface runoff throughout the 22-month monitoring period of this study. The permeable systems reduced the outflow volume via underdrains by 43 percent, and completely captured rainfall events up to 1/4 in. (7 mm) deep. Not surprisingly, the permeable pavements delayed and reduced peak flows existing underdrains throughout all seasons by an average of 91 percent, compared to surface runoff from the conventional asphalt pavement. The slower, controlled outflow closely mimics nature, and reduces the flooding risks and downstream erosion in receiving waters.

    Cold, hard facts

    During freezing temperatures, the permeable pavements functioned well and did not exhibit significant surface heaving or settlement. A substantial spring thaw occurred in March 2011 and the permeable pavements delayed the outflow of snowmelt by three days, greatly reducing peak flows. Increases in outflow volume happened occasionally during the winter and spring due to delayed release of stormwater stored within the aggregate reservoir.

    Monitored median and mean concentrations of several pollutants in the permeable pavement outflows were significantly lower than those from the asphalt, including suspended solids, extractable solvents (oil and grease), ammonia-ammonium nitrogen (NH3, NH4+), nitrite, total Kjeldahl nitrogen (TKN), total phosphorus, copper, iron, manganese and zinc.  The permeable pavements also generated a net reduction in total pollutant mass for all of these constituents, in addition to dissolved solids, chloride, sodium, phosphate, and nitrates.

    In the winter, deicing salt-related pollutants were considerably higher from the asphalt runoff than in the permeable pavement outflows. The reduction in concentration is from detention and diluting winter stormwater. Water quality data collected below native soils indicated that sodium and chloride migrated into them. Further investigation is needed to determine how the presence of these constituents may affect the mobility of other stormwater contaminants, such as metals, as well as impacts on groundwater.


    Surface infiltration measurements indicated substantial reductions in permeability over the monitoring period. However, reduced surface permeability still provided sufficient infiltration rates to rapidly infiltrate all rainfall from the storms. Between June 2010 and May 2012, permeability reductions of a narrow-jointed PICP, a wide-jointed PICP, and the pervious concrete were 87, 70 and 43 percent, respectively. The pervious concrete was installed about 6 months after the PICP systems, so the comparison in reductions is a bit uneven.

    kortright2The pervious concrete maintained high surface infiltration capacity even after two years, with a median rate of 422 in./hr (1,072 cm/hr) at the end of the study in 2012. The infiltration rate of the narrow jointed PICP was 8 in./hr (20 cm/hr) after two years. Vacuum sweeping provided partial restoration of surface permeability for the PICP surfaces. Interestingly, no benefit from increased infiltration rates was observed from vacuuming the pervious concrete. The researchers found that vacuuming all of the permeable pavements produced highly variable surface infiltration results, and did not provide consistent removal of embedded fines within them.

    As part of the study’s recommendations, the researchers suggested that further tests using different techniques for loosening or dislodging compacted material should be conducted in permeable pavements prior to cleaning.  Further experimentation could likely improve the effectiveness of regenerative air and vacuum sweeping trucks. Based on maintenance practices evaluated in this study, annual vacuum cleaning of PICP is recommended to increase their operational life.


    The two PICP cross sections consisted of 3 1/8 in. (80 mm thick) concrete pavers, 2 in. (50 mm) bedding (similar to ASTM No. 8 stone), 4 to 5 in. (100 to 125 mm) of material similar to No. 57 stone, and about 8 in. (200 mm) of material similar to No. 3 subbase stone. The pervious concrete was 6 in. (150 mm) thick, placed over 2 in. (50 mm) of No. 57 stone and about 14 in. (350 mm) of No. 3 subbase stone. The asphalt pavement was 3 in. (75 mm) thick over almost 15 in. (370 mm) of dense-graded aggregate base. The silt to silty-clay soil subgrade was covered with geotextile prior to placing subbase aggregates. The soil’s clay content ranged from 7 percent to 30 percent.

    The study is among several over the past three decades that underscore the ability of permeable pavements to reduce runoff and stormwater pollutants. In addition, the maintenance study confirmed the ability of highly clogged PICP surfaces to be rehabilitated by using vacuum equipment, whereas highly clogged pervious concrete presents additional challenges regarding restoration of surface infiltration. 


    Red Pill or Blue Pill

    For some, the 1999 sci- movie The Matrix accurately characterizes a world with two realities. The story is about a giant matrix of interrelated computer programs that creates a machine-based manipulated world; the other world, the real world of humans, is tormented by the machines and forced underground to survive. Their means of entering and leaving the Matrix is via airships that travel through the underground utility chambers packed with sewer pipes. At times the humans disrupt and reprogram Matrix computer codes to survive. Eventually a savior, Neo, emerges to tame out-of-control machines disguised as humans. His biggest revelation is that the Matrix is a fabricated, digital reality. He learns how to operate above and outside it, so that he can eventually defeat the wayward machines.

    This dual reality seems to exist in the stormwater management world. The Matrix exhibits itself as soulnumbing impervious pavements like sidewalks, plazas, parking lots and streets, mostly supporting petroleumfueled transportation, mostly computer generated. This reality is that big institutions support such pavement. The reality of the stormwater community, the smaller and less inuential folks who try to reduce stormwater runoff, seems to be slowly nibbling away at the programmed path of the Matrix. This begins with permeable pavement, often energized by runoff regulations, and when deployed, the Matrix programming seems to yield.

    This magazine issue demonstrates some evidence. The feature article on permeable interlocking concrete pavement sidewalks and a parking lot in downtown Raleigh, NC, could have gone conventional under the inuence of the Matrix. However, a human intervened and redened the surface, reduced runoff, and increased the possibilities for friendlier urban places. Another Raleigh space is shown in the photo, a green alley between two buildings just blocks from the Governor’s house. Again, someone intervened and reprogrammed this part of the Matrix.

    Two other things might reprogram the Matrix: research and specications. These can point it toward the humanizing reality of permeable pavement. An article in this issue notes the ICPI Foundation is supporting permeable pavement research. While research results support the stormwater community, the transportation folks (i.e., local and state DOTs) will only condently embrace permeable pavements for applications beyond lowspeed residential roads when structural testing and design charts for base thicknesses become available. The article notes that research on this is about to start at the University of California at Davis, and this effort is supported by the California concrete paver and cement industries.

    Another reprogramming of the Matrix lies with dissemination of experiencetested specications placed into state transportation agency manuals and municipal construction guidelines. The challenge is that each specication is written in the language of each agency with references to their materials and test methods. Each specication requires technical review to minimize transportation agency risks. That’s a lot of specs taking a lot of time to change.

    However, the Matrix at its core is a multiplicity of interconnected networks. And like the conclusion of The Matrix trilogy, the ultimate transformation of transportation agencies to permeable pavements will be viral. For now, the stormwater management world and the concrete paver industry are building the programming to make that happen.


    New Research Projects

    Sidewalk Smoothness

    The U.S. Access Board is currently researching smoothness criteria with the help of the University of Pittsburgh’s Human Environmental Research Laboratories. The criteria are being developed by the University from wheelchair user surveys, extensive data on wheelchair use patterns and roughness measurements. The intent is to develop smoothness criteria that eventually may be included in national Americans with Disabilities Act (ADA) Design Guidelines or in an ASTM standard. These eventual criteria rationalize wheelchair user comfort and sidewalk accessibility.

    As a parallel project, ICPI Foundation for Education and Research joined with the Brick Industry Association (BIA) to fund development of a sidewalk smoothness measurement device by the University of Pittsburgh. This University is the same one that conducted wheelchair-pavement interaction studies for ICPI and BIA in 2002 and 2004. The University is completing laboratory and in-situ pavement surface measurements, data analysis and recommended smoothness thresholds. They have begun development of a portable technology for smooth measurement capable of being operated by technician level personnel. The measurement device should be prototyped and tested by summer 2013.

    PICP Structural Testing

    As a pressing need for PICP market development, structural testing and validated design charts rank high. To address this need, the ICPI Foundation, the California Nevada Cement Association, and the Concrete Masonry Association of California and Nevada co-funded full-scale structural testing and development of design charts for PICP. Modeling, plus laboratory and full-scale load testing of PICP will be done by the University of California Pavement Research Center (UCPRC) in Davis. The ICPI Foundation support includes some of the project costs, a construction test area and project administration. Additional Foundation support includes review of the test program deliverables and review of the data for application to freeze-thaw areas by an outside consultant. About 2,800 sf (280 m2) of permeable pavers are being donated by an ICPI member for the test area. The photo below shows the specialized equipment that repeatedly applies wheel loads while measuring stresses and strains within various pavement structures. The project is expected to start in November 2012.

    PICP Cold Climate Research

    The ICPI Foundation recently co-founded a permeable interlocking concrete pavement (PICP) parking lot being monitored by the Toronto and Region Conservation Authority (TRCA). This is a continuation of monitoring porous asphalt, pervious concrete and PICP at the offices near Toronto. The funds leverage other contributors who, along with ICPI, helped launch monitoring
    over two years. The additional funds extend monitoring another two years. The two PICP parking lot installations at TRCA offices utilize pavers from ICPI Foundation donors.

    As previously reported in this magazine, TRCA published very useful PICP reports on monitoring a heavily used PICP parking lot, asphalt parking lot and bioswale at a community college between 2005 and 2008. The reports demonstrated the ability of PICP to reduce runoff and pollutants in a cold climate while confirming lower use of deicing materials than with conventional pavements. A reason for this second monitoring project at TRCA facilities is that the permeable pavement cross sections being tested are those typically recommended by each industry. This side-by-side performance evaluation of PICP, pervious concrete and porous asphalt will be important to furthering PICP in Ontario, wider Canada, and in cold climate regions in the U.S.

    PICP Nutrient Reduction Research

    The ICPI Foundation is supporting a proposal from North Carolina State University (NCSU) to monitor a small PICP parking lot on low infiltration soils owned by the City of Durham, NC. The US EPA, the City of Durham and an ICPI member are also supporting this project. These research results may likely provide data on nutrient reductions through detention/de-nitrification via a raised subbase underdrain. A raised drain means that runoff can pond within the open-graded base reservoir, and through anaerobic processes, experience greater nutrient reduction than if simply exiting a pipe along the bottom of the base. The project deliverables will help answer questions from stormwater agencies about nutrient reduction on low infiltration clay soils. The research project also has a high impervious surface to receiving PICP surface ratio (6.5:1). The research will examine the rate of sediment accumulation and potential effects on surface infiltration. In the past, ICPI supported NCSU for PICP surface infiltration and water quality/quantity research. This new project starts in fall 2012, and monitoring will continue for two years.