Permeable interlocking concrete pavements (PICP) typically use cast-in-place concrete curbs for parking lot, alley and street applications. Some years ago, edge restraints made from metal or plastic were developed for PICP pedestrian and residential driveways. Time has demonstrated good performance of these faster-installation edge restraints that incorporate geogrids for these light-duty applications.
Start with a Firm Foundation
PICP for pedestrian and driveway applications consists of a base foundation or reservoir of crushed, washed, open-graded aggregate to store and infiltrate water. Depending on quarry sources, aggregate sizes generally close or within the ASTM No. 57 gradation are adequate. The washed ASTM No. 57 gradation follows:
Sieve Size Percent Passing
1 1/2 in. (37.5 mm) 100
1 in. (25 mm) 95 to 100
1/2 in. (12.5 mm) 25 to 60
No. 4 (4.75 mm) 0 to 10
No. 8 (2.36 mm) 0 to 5
No. 57 aggregate typically has a porosity of 35% to 40%. This porosity means that 3 inches (75 mm) of compacted aggregate thickness stores about an inch (25 mm) of water. The minimum recommended base thickness for pedestrian uses is 6 inches (150 mm) and 8 inches (200 mm) for driveways. These are often thicker over slow draining soils for structural support and for water storage.
While used in vehicular applications, a subbase (under the base) consisting of larger size, open-graded aggregate (e.g., washed ASTM No. 2, 3 or 4 stone) is omitted for pedestrian and residential driveway applications due to no or much lower vehicular loads. Caveat: When 8 inches thick or thicker, some No. 57 stone gradations move and shift during compaction in large areas due to lack of confinement. This increases compaction time and in some instances, precludes use of wheeled equipment on it. In such cases, a minimum 6-inch thick subbase of the larger size aggregate under a 4-inch (100 mm) thick No. 57 base provides a more stable and compactable foundation.
When placed, the No. 57 base does not ‘self-compact.’ In fact, no aggregate bases, dense or open-graded, self-compact. No. 57 must be compacted in all installations to maximize stability and reduce future settlement. Compaction is best achieved with vibratory plate compactor with a compaction indicator.
Prior to base placement, some applications may have geotextile placed over the subgrade and covering the walls of the excavation. Geotextile provides helps attenuate sideways and downward movement and loss of the aggregate into the soil during compaction.
Three Installation Methods
According to three edge restraint manufacturers’ literature, three installation methods are presented after compacting the No. 57 base layer in maximum 8-inch lifts. The first method applies the edge restraints with geogrid directly on the compacted No. 57 base. The resulting elevation and slope must be carefully monitored since the edge restraints rest on it. After placing these, a 2-inch (25 mm) thick layer of washed ASTM No. 8 stone (or similar gradation) is screeded smooth on the geogrid.
When the restraints themselves are not used as guides for screeding, 2-inch high screed rails may need to be placed on the No. 57 layer to enable bedding layer screeding. During rail placement, they may be adjusted slightly for elevation and slope. Obviously, after screeding the bedding layer around the rails, the voids left after the rails are removed are filled with additional bedding stone and leveled.
The second method places edge restraints and geogrid directly on the screeded bedding layer. The pavers are then installed, filling the joints with washed ASTM 8, 89 or 9 stone, and compacted. This method does not contain the bedding layer so there needs to be consideration given to its containment outside the paver field, thereby reducing the risk of spreading and settling.
The purpose of the bedding layer is to choke or lock into the geogrid and into the No. 57 base surface when compacted. If a slightly smaller gradation for bedding is used such as ASTM No. 89 or 9, it should be tested off site or in a mock-up to be sure it chokes firmly and permanently into the compacted No. 57 base surface.
Again, for all methods, the bedding layer is not compacted. This layer receives compaction when after the pavers are placed and joints are filled with washed ASTM No. 8, 89 or 9 stone, or similar gradations. Paver manufacturers often recommend the appropriate gradation based on the joint widths formed by the paver pattern. ASTM gradations are shown below (per ASTM D448 or C33) as a guide for applying to PICP joint widths.
Uncompacted pavers should be set about an inch above curbs and protrusions, as this height will be substantially reduced after compaction. The final elevation of compacted pavers should be ¼ inch above adjacent surfaces as additional settlement will occur, typical to all pavements made with aggregate bases. At a minimum, the vertical portion of the edge restraints should cover at least the bottom inch of the border pavers to help ensure
In conclusion, all the methods accelerate installation time compared to forming, pouring and finishing cast-in-place concrete. ICPI certified contractors with a PICP specialist designation likely can provide additional information on installation advantages and cost savings for each method, plus provide project examples, as well as that from edge restraint manufacturers. PICP specialist installers and PICP edge restraint suppliers can be found on www.icpi.org.