Holistic Design

As the car pulls into the driveway at the end of a long workday, worries about schedules, traffic, politics, rising prices and other everyday concerns suddenly begin melting away. Stepping out of the car, the driver immediately is embraced by a stylish, welcoming environment promising joy, relaxation, family warmth and good company.

A rapidly growing number of stressed-out homeowners nationwide experience this transformation daily. Thanks to innovative contractors building imaginative designs with interlocking concrete pavers, dull driveways are transformed and cliché backyard patios become relaxing, resort-caliber environments.

“For both new homes and remodels, people are looking to create beautiful extensions of their homes,” said Joe Raboine, Director of Research and Development at Atlanta-based Belgard Design Studio & Elements. “They’re furnishing and putting in fireplaces, outdoor kitchens, lighting and music. They want to create a cohesive and beautiful space with a uniform look throughout their whole property.”

From Driveway to Patio

For people looking to give their property visually and functionally integrated styling and luxurious amenities of a high-end resort, pavers are the best way to go, said David Park, CEO of Landmark Pavers in Temecula, CA. “When they hear about the benefits, they’ll often contemplate, for the sake of uniformity more than anything else, doing all of their hardscapes at once, including their driveway, walkway and patio, using interlocking pavers for the whole project, front and back.”

Mr. Raboine believes that more homeowners are transforming their exterior home space into a welcoming, engaging environment as a dedicated effort to re-connect with family and friends. “It may sound a little bit sappy, but I think that as we become more connected and electronic, we’re actually becoming more disconnected than ever before in face-to-face interactions,” he said. “These types of spaces help bring people together.”

It’s not only homeowners in warm climates such as Southern California, Arizona and Florida that want to upgrade their homes’ surroundings. “In Colorado, we have four seasons,” observed Tim Lindgren, president of Lindgren Landscape & Irrigation in Fort Collins, CO. “Our winter season is not always warm enough for us to get outside, but the shoulder seasons can be extended when we build these areas and put in fireplaces and fire pits, outdoor heating elements, infrared heaters and fans.”

Mr. Raboine agreed, noting that there’s also a strong demand for visually and functionally integrated home environments in many northern areas. “In the Midwest, Northeast and Eastern Canada, their seasons are so much shorter that people want to spend every possible minute outside when it’s nice.”

Rising home prices also play an important role in motivating people to upgrade their current properties, rather than simply picking up and moving to a larger property. “In some markets, real estate sells at a premium, especially in many major cities,” Mr. Raboine says. “Land is becoming increasingly expensive and harder to find. Our market here in San Diego has really changed within the last 10 years, from pavers as a replacement of concrete, to pavers as part-and-parcel of a large, all encompassing project,” Mr. Park said. “The very successful paver installers in our area have been able to change with the market and offer a full landscape design option for the customer.”

Environmental conditions sometimes make pavers the only logical choice for a major exterior space project. “In Northern Colorado, we have very expansive soils and a freeze/thaw process unlike anywhere else that I’m aware of,” Mr. Lindgren said. In January, it’s not uncommon for daily temperatures to reach 70 degrees or higher. A few days later, lows may bottom out at sub-zero levels. “So the ground freezes, and then it thaws, then it freezes, and thaws, and it expands and it contracts,” Mr. Lindgren said. “That’s devastating on concrete, especially decorative concrete or stamped concrete—the finish doesn’t hold up.”

Designing and Planning

Not surprisingly, major, complex projects, such as a complete exterior space remodeling, require much thought and planning. “We have five designers in house, and we try to find the best designer for the scope of work that we understand the client wants to do,” Mr. Lindgren said. “Some designers have strength in hardscape, some have strength in plant material, some have strength in retrofits and others have strengths in new construction.”

The next step is an onsite consultation. “We sit down with the homeowners and listen to their wish list for the property and their goals,” Mr. Lindgren said. “We walk the site with them, give them a little insight on what we think the site is capable of and by the end of the meeting we have a proposal ready for them.”

Mr. Park said convincing homeowners to use pavers isn’t all that difficult. “People generally love the look of pavers, but when they find out the benefits—the structural integrity and the other benefits of pavers—they really are very happy with that.”

Addressing Challenges

The scope and scale of an integrated home environment project can be intimidating, even for an experienced contractor. Suddenly, the contractor finds itself responsible for an array of tasks extending far beyond paver work, including landscaping, electrical and plumbing installation and, in some cases, even long-term maintenance services. And the client always wants the job completed yesterday—or sooner.

Mr. Lindgren said contractors should be realistic about their capabilities and to outsource work they don’t feel comfortable tackling. “We, as a general contractor, will hire the electrician, framer, plumber and mason—whatever trade we need that we don’t do in house,” he said. “We handle the pavers, we do the landscaping and many other things, but we still have to coordinate with a handful of subcontractors to complete most projects.”

Finishing a project on time and on budget requires a great deal of planning and coordination with subcontractors. “The biggest challenge is trying to make sure that everything is thoroughly thought out from beginning to end,” Mr. Raboine said. “When you start talking about things like adding electrical, plumbing, gas lines, music and lighting, all of those things can take several to perhaps a half-dozen subcontractors to create.” Mr. Raboine noted that it’s important to plan tasks with as much detail as possible and to make sure that subcontractors are meeting all of their work and schedule obligations, including following local codes.

“Lighting and all other fixtures must be decided and planned before a shovel ever touches the ground,” Mr. Lindgren said. “The gas line, its size, who’s doing the installation, electrical switch and outlet locations, model numbers and so on all need to be spelled out before starting work.”

Attracted to the Flame

An increasingly popular patio centerpiece is a fireplace or fire pit, available in either kit form or custom built. “Manufacturers offer really neat kits that can be put together with predesigned segmental units,” Mr. Park said. “They’re much more cost-effective to build.”

Palletized kits can be assembled on-site with very little planning or work, Mr. Lindgren said. “I think they’re great, especially when you have an inexperienced contractor that doesn’t have the industry knowledge necessary to do custom work.” Mr. Lindgren noted that to the untrained eye, a kit is virtually indistinguishable from a conventional installation.

Kits also save a great deal of manpower. “Fire pits go together without any mortar—they’re all free stacking,” Mr. Lindgren explained. “You don’t have to build a foundation, do any mortaring or veneering on the stone or put a masonry cap on top—it’s basically a puzzle that you put together.”

However, fire pit and fireplace kits have their limits. “They have their place, and they work great, but if you have a custom site where the homeowner wants the installation to match the house’s stone or capstones, and everything needs to tie in, then kits don’t work,” Mr. Lindgren said. Fireplaces require concrete foundations due to their weight and these are generally hidden from view by the fireplace.

Another downside to kits is the relatively limited number of design options. “You kind of get what you get with a kit,” Mr. Park said. “Typically, if someone doesn’t like the look of a unit that’s prefabricated, you don’t have any alternative as a contractor other than to build something with a custom design, which, of course, costs more.”

Mr. Lindgren also recommended against mixing fireplace/fire pit kits and paver brands. “It’s nice to stick with a single manufacturer when picking products so that the colors and styles match,” he said. He suggested using the same manufacturer for pavers, fireplaces/fire pit kits, vertical elements, retaining walls, bench seating and most other key project elements.

Kit and custom-built fireplaces/fire pits generally require the same amount of maintenance. “If they’re designed to use gas, there’s very little maintenance,” Mr. Lindgren said. If they’re wood burning, there can be a lot or work.” Park says his San Diego customers rarely ask for wood-burning installations. “They’re worried about casting off embers and starting a fire in an arid climate,” he said.

Moving Forward

The growing demand for visually and functionally integrated home environments is taking paver installers into areas where many have little or no prior experience. “The trend for the traditional paver installer is going to be one of offering more all-inclusive landscape designs,” Mr. Park said. “Not only installing pavers, but also doing landscaping, synthetic turf, patio covers and offering a wide variety of other products.”

As contractors cope with new customer demands, they also face the challenge of rising prices. “The cost of materials is going up,” Mr. Lindgren said. “The cost of labor is also going up, and labor in our industry is so hard to find.” Mr. Park, however, is optimistic that most contractors will be able to adjust to the new challenges and to continue operating profitably in the years ahead. “The bottom line is that as long as the job market in a region is strong, and the housing market is strong, the hardscape market will also be strong,” he said.


Retrospective: Dayton, Ohio

As the birthplace of aviation and key manufacturing industries, Dayton, Ohio (2013 pop. 143,355) is also the birthplace of the first machine-assisted municipal street with interlocking concrete pavement. Built in November 1985, the repaving project likely never generated much cocktail chatter among Dayton’s historical society, but it was part of the city’s multimillion-dollar investment to help revitalize several historic districts. The city supported millions in private investment by urban pioneers during the 1970s and 1980s. Such funding renewed vintage late-1800s homes and brought physical and social stability to old neighborhoods.

After 20 years, this magazine issued an interim report in 2005 on the 11,000 sf Tecumseh Street in Dayton’s Oregon Historic District. When constructed in 1985, it was considered a demonstration project by the City, an expression of support for a rundown neighborhood being revitalized with a consistent infusion of restorative sweat equity by residents. While a lightly trafficked pavement in a residential neighborhood, the editor again visited the 31-year-old street last year to inspect the project. Since the editor was responsible for building the project for the City in 1985, the visit was more like listening to an old friend.

From a functional perspective, there were only a few cracked interlocking concrete pavers. Most pavement cracking occurred within the deteriorated concrete collars set around manholes and shutoff valves. By comparison, the pavers will certainly outlast the cast-in-place collars.

Tecumseh_StreetThe decades-old road base is a 7-inch thick mix of cement and aggregates, a hardened slurry that once supported a macadam surface, a thin layer of asphalt mixed with sand. This base had roughly 20% of it removed and replaced in 1985 due to deteriorated areas. As part of the renovation, the macadam, an early version of today’s asphalt surfacing, and the top of the concrete base were ground out and removed to receive an inch of bedding sand and 3 1/8-inch thick concrete pavers. These were machine-set in a 90-degree herringbone pattern in three days.

The aging base shows in a few areas where the paver surface has settled. Had the surface been asphalt, they would have been potholes. The pavers accommodate such movements while continuing to provide service to passing vehicles.

The pavement structure has at least another decade of remaining life. The pavers will almost certainly outlast the base. The strongest indicator of a need for repairs will happen when the base settles in places that eventually set off neighborhood driver complaints. That is unlikely as vehicle speeds are generally below 20 mph.

From a life-cycle cost analysis (LCCA) perspective, Tecumseh Street asks the question, what’s the long-term expense to Dayton’s taxpayers compared to asphalt or concrete? Would more street pavers ultimately save the City money? Since 1985, asphalt prices have fluctuated while interlocking concrete pavement has not. For residential streets in northern states, resurfacing (shave and pave) can be reasonably set at 17 to 20 years. An LCCA favorable to interlocking concrete pavement would compare costs for periodic asphalt pavement resurfacing. Such resurfacing during higher priced markets could likely register a savings by using concrete pavers.

The larger question asked by the street is institutional in nature. Just about every city has organized design, specifications, construction and maintenance equipment, and labor around asphalt and, to a lesser degree, concrete pavements. From a certain perspective, cities’ street maintenance departments are committed to asphalt because it’s cheap. Rather than a single street in pavers, what if Dayton had an entire neighborhood, district, or city with them? While an investment in a third pavement is an additional expense, would interlocking concrete pavement be less expensive to maintain from a city budget perspective if it was the majority pavement, replacing asphalt?

Cost comparisons could be modeled using pavement management software that most cities use to project maintenance costs. One aspect is for certain, departmental investment in construction and maintenance equipment, as well as labor and materials, would be significantly lower for concrete pavers than that required for asphalt. If deemed a cost savings, a shift by any city street maintenance department to pavers would require phasing out asphalt in low-speed streets. There would likely be a commensurate increase in property values due to enhanced neighborhood character. Whether a historic district or not, such enhancements benefit the City and property owners alike.


Industry Outlook

Gross sales for segmental concrete pavement contractors in the U.S. and Canada increased by 8.8% in 2016, according to a new report recently released by the Interlocking Concrete Pavement Institute (ICPI). In all, 218 contractors from all-sized companies participated in the 2017 ICPI Contractor Industry Survey conducted during January for ICPI by Industry Insights of Columbus, OH. Three-quarters of the companies responding to the survey pave residential projects and the remainder place commercial pavements, including housing and publicly funded works.

Most of the respondents to the survey were company owners, presidents or executives. Most companies have been in business 10 years or longer and almost half had annual sales of $1 million or greater with 10 or more employees. Most companies provide on-the-job training on installation best practices as well as on worksite safety and equipment maintenance. 69.1% of contractors required crews to review and participate in formal, documented safety programs, an increase of 4.1% from 2015.

Over half of the companies use wet saws for cutting, dust masks and/or respirators to reduce silica inhalation on job sites. Over two-thirds use wet saws for cutting with a vacuum dust collection system. Almost one-fourth use dry saws for cutting with a vacuum system.

The strengthening economy is making reliable labor harder to find, as reported by almost three-quarters of the respondents. The next most challenging aspect of running their businesses is increasing overhead costs. Almost all respondents have at least one ICPI Certified Concrete Paver Installer on their payroll. Average wages/salaries increased by 3.0% in 2016 and are forecasted to increase again in 2017 by 3.2%.

Man cutting pavers at jobsite.

Since new OSHA rules on reduced exposure to silica dust start in June 2017, the ICPI contractor survey specifically addressed how employees avoid dust exposure when cutting pavers at jobsites.

While all companies install interlocking concrete pavements, the largest companies install most of the segmental paving slabs, likely in commercial projects. The average annual total installed area of pavers is 71,000 sf. Larger companies installed as many as 300,000 sf in 2016.

About three-quarters of all projects are sand-set and 12% are permeable interlocking concrete pavements. To support sales, ICPI Tech Specs are the most often used resource, with ICPI guide specifications a close second. ICPI’s detail drawings are the third most used resource.

While about 13% of all jobs in 2016 required an ICPI certified installer, almost half of the respondents agreed or strongly agreed that ICPI certification helps increase business.

Marketing expenses constitute 4% to 5% of revenue and most leads come from referrals, jobsite and vehicle signs, dealers or general contractors.

The report also includes respondent answers about length of construction season, gross sales, salaries and wages, number of employees during the construction season and more. The detailed study provides deeper insight into an industry that installed over half-a-billion square feet of concrete pavers, slabs, and grids in 2016, as well as other paving and wall products. The complete 59-page report is available for purchase for $100 at ICPI members can purchase it at the member discounted price of $25. Shipping and handling are extra.


Enforcement Postponed

The U.S. Department of Labor’s Occupation Safety and Health Administration (OSHA) recently postponed enforcement of new, stricter rules regarding worker exposure to silica dust on job sites. Originally scheduled to begin June 23, 2017, enforcement of the new rules will now begin Sept. 23, 2017. Current rules will be changed this fall to reflect substantial reductions in exposure to airborne dust on job sites. A key rule change is a reduction in the 8-hour exposure limit from 100 to 50 micrograms per cubic meter of air averaged over an eight-hour day. Determining exposure to these small concentrations is typically done by workers wearing lightweight, portable air monitoring equipment that captures dust while on the job site.

Current rules require a written exposure control plan with specific tasks to protect workers. This is implemented by a designated, competent person who articulates housekeeping practices that reduce exposure with feasible alternatives. Employers must offer medical exams including chest x-rays and lung function tests to employees. These must be done every three years for workers who wear a respirator for 30 or more days annually. There must be ongoing worker training in saw cutting and other operations that result in silica exposure with instruction in ways to limit exposure. Finally, employers must keep records of workers’ silica exposure and medical exams.

OSHA’s new silica exposure rules are delayed to give more time for construction companies to comply. Silica dust on job sites requires control measures via wet (or dry) saw cutting with a vacuum system and worker protection equipment. The pavers shown here are among 5 million square feet in container yards at the Port of Oakland, CA.


ICPI Releases Technical Bulletin on PICP Maintenance

Available at, this 12-page bulletin with 33 figures walks the reader through a range of maintenance practices mostly to prevent sediment from collecting on the surface, or remove it should it remain.

The bulletin covers practices supporting surface infiltration, such as not using sand in the surface or within the pavement assembly, conducting effective erosion control during and after construction, and maintaining joints filled with aggregate so sediment can be more easily removed from the surface.

The text then moves on to surface infiltration inspection and testing, which includes inspection points before and after a rainstorm, as well as surface infiltration testing per ASTM C1781 Standard Test Method for Surface Infiltration Rate of Permeable Unit Pavement Systems. A tool at for calculating surface infiltration using this ASTM standard is referenced to facilitate better surface infiltration monitoring.

Tech Spec 23.

Tech Spec 23 published Feb. 2017.

The document explains routine and restorative surface cleaning. Routine means periodic preventive cleaning, i.e., keeping the surface water infiltrating. Restorative cleaning is often required when cleaning is neglected. This often results in water ponding on the surface. Sediment must be drawn out of the joints with the help of equipment to increase surface infiltration.

Advice then moves into preventive maintenance equipment options for maintaining various sized PICP applications. This section provides a range of technologies for cleaning, from a simple broom to sophisticated vacuum truck equipment. For clogged PICP with low overall surface infiltration, restorative infiltration maintenance techniques for small and large clogged surfaces are also covered.

An inspection list is provided for maintaining surface infiltration as well as a checklist for addressing distresses such as settlement or rutting. Guidance on winter maintenance is included as well as directions on how to reinstate PICP over underground utilities. This information supports cities that use PICP in highly urbanized areas.


2017 HNA Awards Entry Open

The HNA Awards recognize outstanding hardscape projects by contractors building residential and commercial walkways, patios, driveways, commercial plazas, parking lots, streets and more.

Award winners and honorable mentions will be recognized during the 2017 Hardscape North America trade show at the HNA Awards Recognition Ceremony in Louisville on October 19.

They also will be announced in a national press release, featured in Interlock Design magazine, and highlighted on the HNA website and in several major industry publications.

A distinguished panel of industry experts will select award winners and honorable mentions.
Entry Rates

Complete your online entry, submit photos and project description by August 14, 2017 for early bird rates ($100 for Members of ICPI, NCMA or BIA/$140 for Non-Members).

Entries will be accepted up to September 11, 2017 at a higher rate ($200 for Members of ICPI, NCMA or BIA/$240 for Non-Members).
2017 HNA Award Categories

Projects for consideration must have been completed between November 1, 2013 and June 30, 2017.
[insert award chart]
*Awards 12-15 can include natural stone, masonry veneer and mortared walls.


Don’t Be Fooled

Having worked with segmental and monolithic pavements for a few decades, one growing notion popularized in marketing/technical information for competing pavement systems is H-20 loading. Product literature for plastic pipes, chambers, stormwater storage crates, grids, etc., state confidently that their products can receive an H-20 load.

While that may be true, here’s a more complete picture. The H-20 load notion comes from bridge design and not from pavement design. The concept is found in the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications for Highway Bridges. Most bridges that receive trucks label such loads as H-20 or HS-20 in the bridge design process. The ‘20’ stands for a 20-ton vehicle, i.e., 4 tons on the steering axle and 16 tons on the drive (rear) axle. Adding an ‘S’ means the truck is a tractor-trailer combination. This adds 16 tons to a third (rear) axle, making a 36-ton vehicle.

The H-20 load is used in bridge structure design, a process that examines how the structure deflects under the weight of the bridge itself (dead load) and the applied truckloads (Iive). Computerized structural design models find the right size beams that limit their movement (deflection) under truckloads. The deflection in the structure is analyzed under H-20 and HS-20 loads, and likely other loading options depending on the anticipated traffic.

H-20 or HS-20 is a single truckload used in the analysis of bridge designs. These designations don’t apply to pavement design. Pavements are not designed to receive a single truck on them. In fact, my grassed front yard could easily receive an H-20 load. Obviously, that lawn is not a suitable pavement structure for repetitive loads from trucks.

Whether grass or something else, pavements do not typically fail from one H-20 or HS-20 load. Their limitations are defined by their ability to receive thousands or even millions of axle loads from trucks. Since axle loads vary with every vehicle, the AASHTO 1993 Guide for Design of Pavement Structures provides a process to equalize them to 18,000 lbs using a pavement concept developed in the 1950s called equivalent single axle loads or ESALs. Applying the ESAL concept, one H-20 load equals about 10 ESALs and one HS-20 load equals 26 ESALs.

Pavement failure (an unserviceable pavement) is typically seen as rutting in asphalt and cracking in rigid concrete pavements. In either case, the pavement surface slowly fatigues from repetitive loads over time due to tension and resulting horizontal strain at the bottom of the pavement surface. This eventually causes the pavement surface to bend or break.

Interlocking concrete pavements have a much higher compressive strength than conventional concrete (8,000 psi versus 4,000 psi). This makes the paving units especially resistant to fatigue from repeated loads compared to conventional asphalt or concrete surfaces. Most interlocking concrete pavements wear out deeper in the pavement, i.e., from repeated compressive strain within the bedding sand, base or at the top of the soil subgrade layer. This suggests that the base thickness needs to be sized and then constructed correctly, as well as testing conducted on soil subgrade compaction.

ICPI provides guidance documents to help with design on ICPI Tech Spec 4 Structural Design of Interlocking Concrete Pavements is one such document. Another is ASCE 58-16 Structural Design of Interlocking Concrete Pavement for Municipal Streets and Roadways. Both documents provide structural designs up to 10 million ESALs, a very busy major urban thoroughfare.

In the meantime, don’t be fooled. When the term H-20 appears in product literature, ask what happens when that load is repetitively applied? How long does the pavement last? When does it become unserviceable and fail from rutting or cracking? These are the core pavement design questions that require an answer for designers to create reliable pavements.


Resetting the Bar

The Transportation & Development Institute (T&DI) of the American Society of Civil Engineers (ASCE) recently updated and released ASCE 58-16 Structural Design of Interlocking Concrete Pavement for Municipal Streets and Roadways. According to the ASCE, “The standard provides preparatory information for design, key design elements, design tables for pavement equivalent (to asphalt) structural design, construction considerations, applicable standards, definitions, and best practices.” This new version, which replaces Standard ASCE/T&DI/ICPI 58-10, includes updated references to quoted ASTM standards, clarification of subgrade type and drainage characteristics, and the addition of new green infrastructure rating systems.

The 42-page design guide provides tables for base thickness using aggregate, asphalt-treated, cement-treated and asphalt bases under interlocking concrete paving units that conform to ASTM C936. The book provides thicknesses for various soil subgrade conditions under traffic up to 45 mph and exerting no more than 10 million lifetime 18,000 lb (80 kN) equivalent single axle loads or ESALs. Transportation engineers, road designers, planners, pavement manufacturers and municipal officials can rely on this comprehensive guide to interlocking concrete pavements.

ASCE Standards_August 2016_new green.inddDave Hein, P. Eng., Vice President of T&DI and chairman of the technical committee that created and updated the standard, notes that, “ASCE 58-16 continues to provide municipal and consulting engineers with this tool to design and implement interlocking concrete pavements in streets, alleys and parking lots. When properly designed and constructed, these pavements can be more cost-effective over their life than conventional asphalt and concrete. And besides, pavers upgrade the appearance of any street.”

Purchase the updated standard at The price is $60 for ASCE members and $80 for non-members.


Clarify and Confirm

Last summer, ASTM approved C1782 Standard Specification for Utility Segmental Concrete Paving Slabs. Needed for decades, the industry now provides a baseline product standard for segmental concrete paving units from 12 x 12 up to 48 x 48 in. (300 x 300 up to 1,200 x 1,200 mm). ASTM C1782, however, was written for paving units that do not require close dimensional tolerances. Such tolerances noted in Table 1 at right are from that standard.

Utility paving slabs often have architectural finishes and are used in residential and some commercial applications for at-grade and roof ballast applications. Architectural finishes include (but are not limited to) blasted, hammered, tumbled, textured and polished surfaces. The Interlocking Concrete Pavement Institute is proposing a second ASTM standard for slabs with closer (tighter) dimensional tolerances. Like products conforming to C1782, products conforming to the proposed standard typically have architectural finishes. However, the main difference between C1782 and the proposed new standard is that the latter has closer dimensional tolerances required for pedestal-set roof applications, as well as for at-grade bitumen-set and some sand-set applications. The new proposed standard formalizes these dimensional tolerances used with architectural paving units for these applications for over 20 years. This proposed new standard below is very similar to ASTM C1782 except for the closer dimensional tolerances shown in Table 2.

Rather than provide optional closer dimensional tolerances within C1782, a new standard is being proposed that differentiates itself from C1782 with higher dimensional tolerances for architectural paving slabs. Also, two distinct paving slab standards (standard and architectural grade) can help reduce confusion between these two groups of paving products among specifiers, contractors, and other users. Freeze-thaw durability and flexural strength requirements are proposed to remain the same as those in C1782. Acceptance of this new standard is anticipated in 2018, but this depends on the outcome of voting by ASTM members.


Doing the Math

Thanks to a grant from the ICPI Foundation for Education and Research, design tables were developed earlier this year that provide technically conservative base solutions for paving slabs subject to vehicles. The tables were developed using finite element modeling that simulated pressures from truck tires on paving slabs of various sizes and flexural strengths. The modeling included a 1-inch (25 mm) thick sand bed under the slabs, three base materials, and three soil subgrade conditions. This article previews the design approach for pedestrian and vehicular applications derived from that modeling.

For pedestrian applications, 12 x 12 in. (300 x 300 mm) units can be placed on a minimum 6 in. (150 mm) thickness of compacted aggregate base. Thicker bases (generally 8 to 12 in. or 200 to 300 mm thick) should be used in freezing climates and/or on weak clay soils (CBR < 3%). Designers should consider using a lean concrete or concrete base for larger paving units because achieving a very smooth base surface can be difficult with compacted aggregate base.

Design options become a bit more complex for vehicular applications. The first step is determining the maximum number of lifetime 18,000 lb equivalent single axle load or ESAL repetitions. (Caltrans Traffic Indexes are provided in parentheses.) Determining ESALs or TIs can be done using Table 1. It divides them into five categories. Higher ESAL categories generally require thicker units and concrete bases. Applications exceeding 75,000 lifetime ESALs should use interlocking concrete pavers.


The next step is determining the soil strength. The minimum values for designs is a resilient modulus of 5,100 psi (35 MPa), 3% California Bearing Ratio, or an R-value = 7. The maximum values are 11,600 psi (80 MPa), 10% and 18, respectively. Soils with higher values use the latter set for determining the unit size and thickness. after laboratory tests determine the soil strength, that points to specific slab sizes and bases that will work given the anticipated design ESALs in Table 2. Square paving units are recommended over rectangular ones for vehicular traffic with placement in a running bond pattern.

The ICPI design method offers three base options described below in ascending order of supporting stiffness. Construction should include compacting the soil subgrade and bases/subbases to at least 95% of standard Proctor density per ASTM D698 Standard Test Methods for Laboratory Compaction of Soil Standard Effort.

(a) A 12 in. (300 mm) thick compacted aggregate base with gradation conforming to provincial, state or municipal specifications for road base used under asphalt pavement. If there are no guidelines, use the gradations in ASTM D2940 Standard Specification for Graded Aggregate Material for Bases or Subbases for Highways or Airports and as described in ICPI Tech Spec 2 Construction of Interlocking Concrete Pavements.

Slabs can take a modest amount of trucks but using thicker units. The ICPI guide tells designers and contractors how thick.

Slabs can take a modest amount of trucks but using thicker units. The ICPI guide tells designers and contractors how thick.

(b) A 4 in. (100 mm) thick lean concrete base over a 6 in. (150 mm) thick compacted aggregate base. The lean concrete should have a minimum 725 psi (5 MPa) compressive strength after 7 days per ASTM D1633 Standard Test Methods for Compressive Strength of Molded Soil-Cement Cylinders. Lean concrete is typically a lower strength concrete or a cement-treated base of similar stiffness and strength where an aggregate base is charged with cement (typically 3% to 6% by weight) to bind the aggregates when the cement cures.

(c) A 4 in. (100 mm) thick concrete base over a 6 in. (150 mm) compacted aggregate base. The concrete should have a minimum compressive strength is 3,000 psi (20 MPa) per ASTM C39 Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens.

Table 2 presents a sample of the design tables using a concrete base. The designer finds the paving slab length and width and thickness that corresponds to the soil subgrade strength and slab flexural strength. Paving slab length and widths start at 12 x 12 in. and go up to 48 x 48 in. As an illustrative example, Table 2 only goes to 24 in. slabs and not up to 48 in. due to space limitations. Slab thicknesses are 2, 3, 4 and 5 in. If the exact paving slab length and width are not on the table, the designer finds the closest size paving slab, using a smaller and/or thicker unit as a conservative design measure. The design tables cover square and rectangular slabs only.


An example follows on how a design table works. The highlighted 24 x 24 x 3 in. thick slab is selected by the designer. This will be a concrete base and aggregate subbase over a 5% CBR subgrade with a minimum 750 psi flexural strength for the slabs. The intersection of the highlighted horizontal and vertical columns is marked OHV which means the maximum lifetime load is 75,000 ESALs per Table 1. If the designer wants to use a 24 x 24 x 3 in. slab on a weaker soil subgrade, then the maximum allowed ESALs would be 30,000. 

Designs using a concrete base include a 1 in. (25 mm) thick sand setting bed under the slabs. This design solution also applies to paving slabs in a bitumen-sand bed (typically 1 in. or 25 mm thick) since bitumen-set applications require a concrete base. This introduces an additional measure of conservative design since bitumen-sand materials provide a modest increase in stiffness and increased stability resisting repeated turning, accelerating, and braking tire lateral loads.

This article was intended to sample how structural design is done with paving slabs. Similar, additional design tables have been developed for planks and an ICPI Tech Spec is expected later in 2017. As partial validation, the ICPI Foundation for Education and Research is funding the construction of a full-scale load testing area at a paver manufacturing facility in Maryland. The area will receive trucks loaded with paving products where each truck pass will exert several ESALs. The condition of the slabs and planks set on aggregate and concrete bases will be monitored to see how quickly or slowly the slabs will crack, as that defines failure. The testing will likely begin in spring 2017 and run for a few years.