Masonry Magazine April 2007 Page. 52
PAVERS
help create interlock among the units. Another principle influencing design and construction is the need to remove water from the base, bedding sand and surface.
The following are brief explanations of the various elements of which mason contractors providing interlocking concrete pavement services should to be mindful, specifically in sloping applications.
Soil Subgrade
IF THERE IS CUT AND FILL of the soil, it should be compacted to a minimum of 98 percent standard Proctor density for pedestrian and residential driveway applications, and a minimum of 98 percent modified Proctor for roads. Compaction should be done in lifts and density checked by a technician with a nuclear density gauge to the depth of each lift. Lift thickness will depend on the size of the compaction equipment. Establishing 100-percent Proctor (or modified) and optimum moisture content in a soil testing laboratory, and comparing it to the compacted and measured field density of the soil and moisture provides the highest degree of assurance against settlement and call backs.
Geotextile
GEOTEXTILES are recommended over clay or silty soils. Overlap at least 12 inches (30 cm) and remove all wrinkles prior to placing base material. Be sure that the fabric covers the sides of the excavated area; staples are helpful in holding the smoothed fabric in place. Be sure to place base over the geotextile so it doesn't wrinkle under moving tires from construction vehicles. Geotextile manufacturers can provide guidance on selecting a fabric for separating the base from the soil subgrade.
Drainage Mat
A KEY DESIGN consideration is draining excess water from the base and bedding sand at the concrete header, which is typically the lowest elevation of the interlocking concrete pavement. While not essential, products such as J-Drain or an equivalent drainage mat can facilitate water removal; do not use drainage mats with plastic waffles because they have an increased risk for crushing.
The drainage mat is placed vertically against the concrete header beam located at the base of the pavement. Note the placement with respect to the pavers and bedding sand in the Figure 4 cross section; the top of the mat is covered with a small strip of geotextile to keep sand out.
A drainage mat placed horizontally under the bedding sand should never be used in vehicular applications, including residential driveways.
Aggregate Base
USE MATERIAL that conforms to state or provincial Department of Transportation specifications for base under asphalt pavement - a few examples include California, Class 2; Virginia, 21A; Ontario, Granular A. Place and compact in three- to four-inch (75 to 100 mm) lifts. Compact the base to at least 98 percent of standard Proctor density at optimum moisture content.
Density and moisture information often can be obtained from the quarry supplier (eg, standard Proctor density = 145 lbs/cf [2,323 kg/m³] at 6-percent optimum moisture content).
The compacted base thickness should be at least eight inches (200 mm); however, thicker bases should be built in cold, northern climates. The compacted surface should have a surface tolerance of ±3/8 inch over a 10-foot (±10 mm over a 3 mm) straightedge. Stabilizing three feet (1 m) of base with cement next to the header beam can help prevent base rutting at the header/paver junction. Another approach is to thicken the aggregate base approximately 40 percent over normal thickness to provide extra mass for taking wheel loads. These modifications are especially important when transitioning from a rigid, concrete pavement to an interlocking concrete pavement with a flexible, compacted aggregate base.
Concrete Header Beam
THE CONCRETE header beam is poured at the same time or after pouring the curbs on the sides and top of the pavement-pre-cast concrete or stone units are not recommended. Located at the down slope end of the interlocking concrete pavement, this beam should be a minimum of six inches wide by 12 inches (150 mm x 300 mm) deep, with one #4 bar centered in the bottom third of the beam with a two-inch (50 mm) clearance from the bottom. The designer may wish to indude a second reinforcing bar along the top in street applications. The reinforcing bar should be continuous and use minimum 4,000 psi (30 MPa) concrete. Prior to forming the header, place and compact about four inches (100 mm) of base to serve as a platform for forming the bottom of the concrete header. A larger header beam may be required in more severe climates or when truck traffic is expected.
Locate the forms so that there is a 1/2-inch (13 mm) gap between the end of the curbs and the header beam. This provides space for the drainage mat to continue the full length of the header beam, allowing water to drain at each side of the driveway or road. The gaps can be covered with geotextile to contain base and bedding sand while still allowing water to drain.
The header beam should be formed, poured and the forms removed prior to placing aggregate base against it. Daylight joints should be placed a minimum of every five feet (1.5 m) to reduce cracking risks-avoid joints made by tooling the beam surface. In addition to controlling cracking, the joints will allow water to drain after any rainfall during construction. After the concrete beam has cured and the forms are removed, (if necessary, excavate) compact the soil and place the drainage mat against the upslope side of the beam.