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Cool by the Poolside

A Familiar Start: Compaction

Pools made with a flexible liner or fiberglass require much excavation and soil backfill against them. This soil is almost impossible to compact adequately because there is a high risk of compaction equipment damaging the pool wall, the metal “knee” wall/wing wall supports, and/or pipes. Pools made with gunite concrete or poured concrete can have walls of sufficient thickness and support where backfill soil against them can be compacted. However, the soil directly against the wall will be difficult to compact, again because of the risk of damage to a plate compactor or to the pool wall. Therefore, settlement can occur here as well.

Some paver installation contractors claim that installing and compacting the soil backfill and aggregate base can reduce the risk of settlement. That may be true assuming that maximum density is achieved for both materials during compaction. However, the places compaction equipment cannot reach will be at risk of settlement. If a contractor uses a compacted aggregate base, then a warranty should be included in the price to cover the cost of returning to the site a few years later to inspect and lift settled paver areas. Given unpredictable future costs and hassles for the contractor (not to mention the owner), it’s better to complete a job that doesn’t include a return visit.

To avoid returning for paid (or worse still, unpaid) callbacks due to settlement issues, use a 4 in. (100 mm) thick concrete pad as a base. Placing a 2 in. (50 mm) thick layer of free-draining compacted aggregate such as ASTM No. 57 stone under the base enables water to drain from below. Figure 2 illustrates this material placed over a compacted, crushed stone base. The concrete base surface should slope at least 1.5% to allow water to drain.

A key aspect of all pool surfaces is no settlement or undulations that present walking or tripping hazards, or slipping hazards from birdbaths. By providing a rigid, sloped foundation, a concrete deck under the concrete pavers helps maintain safety. This should be the first priority and result of all pool projects.

Wear an Apron or the Full Dress?

When a concrete base is used, should it be built as an apron covering only areas of settlement-prone compacted soil and aggregate base around the pool perimeter? Or should it extend as a full dress under the entire area of the concrete pavers? A dress is preferred. The problem with apron construction is differential settlement of the soil and base next to the concrete base under the concrete pavers will almost always occur regardless of diligent soil and base compaction. Slideshow images show installation and a completed concrete full dress surrounding the entire pool area, as well as installed coping attached to the concrete base structure. In this case, the coping is not resting on the pool wall.

Overlays on Existing Concrete Decks

Installing concrete pool decks with an overlay of concrete pavers is less expensive than removing and replacing the entire concrete deck. To qualify for an overlay on an existing concrete deck, the concrete should not be heaving or faulted, as this often indicates severe settlement of the soil beneath or expansive clay soils. In these cases, subsurface drains can remove excess water from the soil. Expansive soils can be stabilized with lime. Both should be done after demolishing the concrete deck and before pouring a new one. The advice of a professional civil engineer familiar with the local soils should be obtained in such situations.

Cracks in the existing concrete base can be filled with a cement-based patch to prevent migration of bedding sand into them. The junction of the concrete slab with the pool wall should be sealed with a neoprene or urethane sealant (often applied with a caulking gun). This keeps water from getting behind the pool wall and saturating the base and soil.

There is a growing trend toward using paving slabs, generally 12 x 12 in. (300 x 300 mm) or larger. Sometimes these are mixed with smaller units. All units should be at least 2 inches thick. They require a thin, coarse (drainable) bedding sand layer, typically no thicker than ¾ in. (20 mm), screeded smooth, ready to receive the paving slabs (or pavers).

Slabs and pavers should be compacted with a plate compactor with rollers on the bottom to reduce the risk of cracking slabs. Joints are then filled with sand, and sealer is applied. Prior to placing and screeding the bedding sand, 12 in. (300 mm) wide geotextile strips should be applied over concrete deck joints, placed and turned up at joints against structures, and placed along coping to prevent bedding sand loss.

Prior to applying the pavers, all area drains must be raised to the new finished elevation of the installed pavers. Holes must be drilled into the vertical drain pipe directly above the concrete deck. This drains excess water from under the pavers. The holes should be covered with geotextile to prevent ingress of sand.

Many overlays use thin, tile-like concrete pavers placed directly over a concrete deck. Thin pavers typically range between 1 to 1½ in. (25 and 40 mm) thick and are generally about 4 in. (100 mm) wide by 8 in. (200 mm) long. Unlike sand-set slabs or pavers, concrete pavers are directly applied to the existing concrete deck without bedding sand after cracks are patched. Edge pavers are secured with a polymer adhesive or mortar (in non-freezing areas).

Fine sand is swept and washed into the joints until they are full. Using thin pavers as new construction of course avoids bedding sand as well. In new or rehabilitative projects, washing sand in the joints enables it to flow under the pavers so no rocking or clicking of pavers occurs when walked upon.

After the surface is completely dry (usually in 24 hours), it receives sealer to hold the sand in the joints and reduce water ingress. The sealer is typically reapplied every three to five years to maintain the sand in the joints and protect the surface. Even with this maintenance cycle, overall costs are well lower than replacing the entire concrete deck. Sealers also greatly reduce the risk of mold and bacteria, thereby addressing concerns of health officials regarding public pools.

Advantages of Pavers

Tie-downs for pool covers can be installed below the pavers with high-strength grout-filled sleeves. Tie-down caps should be even with the paver surface. Should pipe or wiring repairs be needed, concrete pavers can be removed and reinstated with no ugly patches. The units resist chlorine and bromine, as well as freeze and thaw cycles. Concrete pavers offer a slip-resistant surface even when sealed. Salt-water pools can attack concrete surfaces, so be sure they are thoroughly sealed.

Besides their unmatched beauty compared to other deck surfaces, colored concrete pavers reduce the glare often associated with cast-in-place concrete pool decks. Almost every paver pool deck in warm climates consist of beige, coral, or buff colors that reduce glare from the sun. Because the units have joints, each unit has some opportunity to release heat faster than a cast-in-place concrete deck. Therefore, the units can be cooler underfoot than other surfaces. So a paver surface feels as well as looks cool.

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Research Raises the Barrier

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

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

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

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

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

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Paving Slab Progress

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

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

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

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

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

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Slabs In Demand

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

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

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

Quebec City

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

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

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

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

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

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

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Form Follows Function

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

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

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

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

For Contractors

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

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

For Designers

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

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

For Homeowners

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

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

For Members Only

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

Better Experience, Faster Access

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

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A Magnificent Mile

Located in between mountains and Roadside America off Exit 441 of Interstate 5 at Westley, California, Howard Road handles much heavy truck traffic to and from “The 5.” Faced with worn asphalt pavement in need of replacement, the Stanislaus Public Works Director, Matt Machado, PE, LS, accepted the challenge of designing a long-term, economical pavement solution for the road using interlocking concrete pavement. This resulted in the largest publicly owned stretch of concrete pavers in California, about one mile (1.6 km) long.

Built over a weak soil subgrade (R-value < 5 or California Bearing Ratio < 2%) and to a Caltrans Traffic Index of 11, or just over 5 million 18,000 lb. (80 kN) lifetime equivalent single axle loads (ESALs), the design required 14 in. (350 mm) thick Caltrans Class 2 road base over a biaxial geogrid. The Class 2 base supports one inch (25 mm) of coarse bedding sand and 170,000 sf (15,794 m²) of 3 1/8 inch (80 mm) thick, machine-installed concrete pavers placed in a herringbone pattern.

Example of worn asphalt ready for replacement along Howard Road in Westley, CA.

Example of worn asphalt ready for replacement along Howard Road in Westley, CA.

Stanislaus County received six bids from $4.50 to $6.00/sf ($48 to $64/m²) to install the concrete pavers and bedding sand. Completed in fall 2014, the pavers were manufactured by Basalite Concrete Products in Dixon, CA, and machine installed by Earth Shelter Developers from Lodi, CA. Both are Interlocking Concrete Pavement Institute (ICPI) members. With Roadside America businesses like Denny’s, Chevron, McDonald’s and Joe’s Travel Plaza open 24 hours along Howard Road (speed limit 35 mph), an extensive traffic control plan required the contractor to maintain drive lanes to accommodate truck traffic during construction.

Previous Paver Experience

Mr. Machado used interlocking concrete pavement while working in a previous position as city engineer for Ripon, CA (pop. ~15,000), a farm community in San Joaquin County. Mr. Machado developed interlocking concrete pavement as a roadway standard adopted by City Council for new roads and for some pavement rehabilitations. With the new design standard in place, developers and the City constructed more than 1.3 million sf (120,774 m²) of roads between 2005 and 2008.

Main Street in historic downtown Ripon features 50,000 sf (4,645 m2) of interlocking concrete pavement.

Main Street in historic downtown Ripon features 50,000 sf (4,645 m²) of interlocking concrete pavement.

The Ripon City Council approved interlocking concrete pavement when comparing the cost of expanding the city asphalt road network by developers and then forecasting insufficient future funds for periodic grinding and resurfacing. While the additional initial developer costs for concrete pavers were transferred to the homebuyers, the increase was marginal compared to the full price of single-family homes.

The Life-cycle Selling Point

After finding success in Ripon with lighter-load street applications (and conservative structural design assumptions), interlocking concrete pavement presented a durable pavement rehabilitation alternative for heavily trafficked Howard Road. A benefit of interlocking concrete pavements is not requiring periodic resurfacing. For Ripon’s residential streets, life-cycle costs were studied over a generous 100-year period resulting in concrete pavers having about 75% lower life-cycle costs than asphalt. Maintenance costs for concrete pavers for the same period were approximately 20% the cost of asphalt. Heavier trafficked streets such as Howard Road with interlocking concrete pavement often have even lower life-cycle costs because resurfacing costs for asphalt roads increase under such traffic.

Machine-assisted installation of interlocking concrete pavement in Westley, CA.

Machine-assisted installation of interlocking concrete pavement in Westley, CA.

According to Mr. Machado, “This (Howard Road) project was built to show the structural value of concrete pavers and their economic value for heavy truck traffic.”  Interlocking concrete pavements offer high compressive strength concrete with the flexibility of asphalt pavement. Research in the United States and overseas demonstrates that the pavers in a herringbone pattern progressively stiffen or interlock while receiving traffic loads. The resulting stiffness of the paver and bedding layers, or their resilient modulus, is equivalent to the same thickness of asphalt. In some cases, their stiffness well exceeds asphalt during hot summers as experienced in Westley, CA, where asphalt weakens under temperatures typically around 100 deg. F (38 deg. C).

In other words, the 3 1/8 in. (80 mm) thick pavers and 1 in. (25 mm) bedding sand have an AASHTO layer coefficient (an expression of stiffness) equivalent to the same thickness of asphalt. This is demonstrated in ASCE/ANSI 58-10: Structural Design of Interlocking Concrete Pavement for Municipal Streets and Roadways published by the American Society of Civil Engineers as well as in Tech Spec 4: Structural Design of Interlocking Concrete Pavements from ICPI. Pending successful performance with some years under its belt, Howard Road might mean a design standard for Stanislaus County in the future.