Owing in large part to various standards and specifications developed over the last 30 years, an engineered segmental pavement system can provide superior
Owing in large part to various standards and specifications developed over the last 30 years, an engineered segmental pavement system can provide superior performance in addition to aesthetic appeal for projects ranging from small residential patios to large commercial plazas. Dedicated manufacturers, contractors and suppliers in North America have provided input and impetus for standards created by the Interlocking Concrete Pavement Institute (ICPI) and the Brick Industry Association (BIA), greatly improving the quality of concrete segmental pavements.
To date, however, no industry specification exists for the edge restraint. In addition to a proper base and durable pavers, edge restraint contributes to a pavement’s integrity: flexible paver system performance depends on a secure interlock between the units, which distributes pedestrian or vehicular traffic loads to surrounding pavers. A project is compromised, therefore, when the edge of an interlocking pavement shifts and joints begin to open up. Thus, edge restraint displacement is one of several primary causes of flexible paver system failure, including insufficient base thickness for traffic load, poor base compaction, inadequate base extension, excessively thick or uneven bedding sand layer, and improper paver aspect ratio for load demands.
As segmental pavement interlock is critical to paving performance and durability Û and edge restraint affects interlock Û Pave Tech, Inc. engaged St. Paul, Minn.-based Stork Twin City Testing Corp. to design a test that could measure and compare the performance of current plastic paver edge-restraint systems. The function of an edge restraint is to withstand horizontal loads created by inherent pavement energy, i.e., the constant pressure pavers exert against each other, as well as the momentary dynamic forces imparted by traffic. Two indications of its performance are deflection, the movement of the edge under load; and, deformation, failure to return to its original state. The ideal edge restraint withstands horizontal loads without permanent deformation. By contrast, horizontal shifting or permanent deformation under load constitutes edge-restraint failure, causing deterioration of paver interlock.
To compare the top five plastic edge restraint systems, Stork designed tests to measure deformation, i.e., permanent edge restraint shift, and load, defined as pavement energy. Test variables included material (type of plastic), design, and spiking frequency.
Several characteristics of pavement energy were noted: (1) It is created by compaction and filling of joints; (2) It is held in the field of pavement by the bond pattern and surrounding pavers; (3) It is maintained around the perimeter by structures, concrete curbs, or manufactured edge restraints; (4) Lockup does not occur at the onset of the compaction process, as all of the components act independently. The first passes of the compactor along the perimeter apply the greatest horizontal load to which the edge restraints typically will be subjected. Load sharing starts to occur once the paver system begins lockup, creating pavement energy; (5) It will continue to increase with pavement use. Measurements were taken immediately after compaction when the edge restraint was most susceptible to shifting.
Various types of plastic were represented in the study. Most commonly used for edge restraints are three materials: Polyethylene, a synthetic polymer of ethylene, is not considered a structural plastic and does not have memory. When deformed, it stays deformed. Polypropylene, a versatile thermoplastic and synthetic polymer of propylene, is a mid-price, mid-grade structural plastic. Polyvinyl chloride (PVC) is a tough, hard-wearing synthetic resin made by polymerizing vinyl chloride. PVC is a more expensive, structural plastic that will return to its original state after load is removed.
The test products were divided into four design categories: rigid, rigid dual-purpose, flexible, and flexible field-modified. Suitable for use along straight perimeters, rigid edgings are designed to resist flexing or curving. Among rigid edgings, dual-purpose products can be easily field modified by cutting to achieve flexibility. Flexible edgings are specifically designed for curved areas.
TEST AREA DESIGN
In accordance with ICPI/BIA specifications, test areas were designed to reflect perimeter configurations commonly used in actual practice. A concave (inward) curve was selected, as it is more susceptible to deflection than convex or outward curves. For each test area, a Ê-in.-minus crushed limestone base was placed to a minimum depth of 18 in. Pavers were 100-mm _ 200-mm _ 60-mm units (1.5mm spacer bars). A 90-degree herringbone pattern was installed over a 1-in. bedding layer of loose, screeded, coarse washed sand.
Edge restraints on sides 1, 2, and 3 were situated prior to placing the sand and pavers; installation of the side 4 restraint followed that of sand and pavers to accommodate full- and half-paver units. (Researchers note that no differences have been detected in the performance of plastic edge restraints on the basis of placement either before or after pavers have been laid.) All edge restraint systems were installed using 10-in.-long, ?-in.-diameter smooth steel spikes.
On the first day of testing, spikes were spaced according to each edging manufacturer’s recommendations. On the second day, using new test areas, spikes for rigid edging were spaced at 24 inches; and, those for flexible edging were spaced at 10_ in. The spiking frequency used on the second day was termed Test Recommended Spacing.
Cutting was performed on a masonry table saw, allowing gaps no larger than 3/16 in. All other areas used full and half units tightly laid by hand. Among compactor specifications were dead weight of 275 lb.; centrifugal force of 4,946 lb.; and, plate size of 384 sq. in. Stork Engineer Joel Lessard supervised the compaction, conducted testing, recorded relevant data, and compiled the final report. His raw data is available at www.pavetech.com/paveedge/report.shtm
Creating a durable segmental pavement requires constructing a surface wherein all paver components perform equally. To that end, plastic edge restraints constitute an enhancement to a proven, yet evolving hardscape technology. Stork Twin City Testing’s study on behalf of Pave Tech provided a method by which edge restraint performance can be compared across systems. The application of that design yielded the first data defining the relationship between pavement energy and edge restraint deformation.
Test results demonstrated a clear correlation between magnitude of edge restraint shift from initial compaction and amount of pavement energy restrained. Interlock maintenance and containment of pavement energy Û crucial to edge restraint performance and pavement durability Û are highly dependent on all of the test variables, including material (type of plastic), design, and spiking frequency.
This article is adapted from a report issued by Pave Tech, Inc., detailing the results of a study designed to test and compare the top five plastic paver edge restraint systems. The Stork Method employed for the research measures deformation and load of edgings used to contain interlocking segmental pavements.
SEGMENTAL PAVEMENT SYSTEM
- Bedding and Joint Sand
- Bond Pattern
- Edge Restraint
- Paver Shape
- Paver Aspect Ratio