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Changing the American Landscape  August 28, 2005

Field-Proven Performance of Pave Stones Segmental paving in the U.S. has primarily been used by the architectural community for aesthetic reasons only, although concrete pave stones have historically been used in Europe for their performance attributes as well as architectural characteristics. However, the vast array of available pave stone products for commercial, industrial and residential applications is fast changing the American landscape.

A key point for Landscape Contractors is to have a comfortable understanding about the role of the soils and the structural base used in the pavement system. The paving section, which consists of the paving units, the structural sub-base and the underlying soil, are all structural layers that displace the imposed load. It is important that site specific soil data is available and that the appropriate structural sub-base is selected.

For increased confidence in specifying pave stones for commercial and industrial applications, it is helpful to understand the significance of ASTM (American Society for Testing and Measurements) specifications.

"The ASTM standards are designed by mission to contribute to the reliability of materials, products, systems," explains Robert Rayburn, ASLA, Commercial Sales Manager, Pavestone Company. "By having the industry's leading participants supporting and promoting a higher standard of material guidelines, design professionals, specifiers, installers and end users can expect to see a higher level of quality. This helps protect a specifier who is confident a certain level of quality is being maintained as designed. ASTM also provides guidelines for testing of materials that laboratories adhere to strictly."

A common misconception about pave stones is that with the unlimited design options comes a hefty price tag. It is important to remember "the big picture" when comparing the cost of available paving systems. Simply put, in price comparisons, superior paving systems should not be confused with inferior surface products.

"All landscape architects, specifiers, design professionals, etc., have a responsibility to the owners and developers to guide them through the process of spending their investment money judiciously," Rayburn explains. "Evaluation of all products should be performed by weighing the real benefits of longer life cycles, maintenance and replacement costs, long-term aesthetics and then compare the different products on performance, economic benefits and aesthetics."

Because there is such a vast array of pave stone options, from a structural perspective, it is important to consider the basic criteria in determining the best product for any paving application. Richard Bodie, who is the NCMA chairman for the ACB sub-committee, in addition to serving as Pavestone Company’s National Sales Manager, provides the following general guidelines:

Paving Thickness–

Field-proven and long term peformance attributes made pave stones the choice paving solution for this taxi-way at Dallas / Fort Worth International Airport totalling more than 200,000 square feet.

Concrete pave stones can range in thickness from 1.27 cm to 12 cm. Conventional US production typically is 6 cm or 8 cm. Principally, any paving unit less than 6 cm is for pedestrian use only. 6 cm alone is generally acceptable for light vehicular use provided there are no aspect ratio limitations (discussed later in this section). 8 cm is typically used for heavy vehicular and industrial applications. It is important that the designer take into account paving traffic loading, direction and speed as these factors will have an influence on the ultimate thickness required. To assist specifiers, there are several design methods available through Pavestone Company from Uni-Group and ICPI, that will calculate appropriate thicknesses. Field-proven performance has always been a reliable source for unit thickness selection, provided similar traffic conditions are considered.

Paving Unit Shape/Pattern–

Concrete paver unit shapes' range from the traditional 4 x 8 rectangle to a myriad of proprietary and non-proprietary interlocking geometries. The structural influence of a particular unit shape relative to its adjacent units has been a subject of debate for many years. While the traditional 4 x 8 rectangular shape is the most widely used, interlocking or indented paving unit shapes have been used since the 1930's. Numerous successful projects ranging from light to heavy traffic have been accomplished utilizing indented paving unit shapes.

Most industrial projects are now utilizing indented or shaped pavers. Pattern selection in paving units is not controversial, in that interlocking patterns such as Herringbone have undoubtedly proven to be superior in pavement performance. While patterns such as running and stacked-bond are appropriate for pedestrian applications, they are not appropriate for vehicular applications.

It is recognized that geometric shapes laid in geometrically interlocking patterns have demonstrated less typical pavement failure modes than those of non-interlocking shapes in non-interlocking patterns. However, non-interlocking shapes in non-interlocking patterns have proven to be easier to repair. It will therefore be a function of these specific projects' traffic mix and utilitarian end purpose that will ultimately dictate shape and pattern selection.

Paving Unit Aspect Ratio –

Decorative pave stones add elegance and sophistication in enhancing the drive up appeal of this residential community.

There are two aspect ratio methods that need to be taken into account for the proper selection of a paving unit. One is the ratio of overall length to width, typically called "the plan ratio", which is generally between 2:1 and 3:1. The other ratio, which is the most important, is the overall length of the unit to its thickness. To determine the unit aspect ratio, simply divide the unit’s longest surface dimension by its vertical thickness. When this ratio exceeds 4:1 the unit should not be used in vehicular applications.

Units with the length thickness ratios between 3:1 and 4:1 are acceptable for light to medium vehicular applications. For example, a unit paver with a 4" x 8" x 2-3/8" thickness would yield an aspect ratio of 3.4:1, which would be appropriate for light to medium vehicular traffic. A unit paver that is 12 x 12 x 2-3/8" thickness would yield an aspect ratio of 5:1 and would not be appropriate for medium-to-heavy vehicular traffic. Aspect ratios are generally used to evaluate the paving units flexural capacity to sustain load without cracking.

With the increasing popularity of segmental pave stones, there are project applications being implemented today that previously would not have been thought possible. The innovative design features available in pave stone shapes, colors and textures are endless. These innovative tools facilitate the transformation of architectural landscapes into works of art. LCM

Alva D. Logsdon is a writer whose work has been featured in several national magazines, and is a designer of collateral material, websites and electronic media. She is the Director of Marketing for Pavestone Company.

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