Designers, contractors and homeowners often ask what the maximum slope is for a driveway constructed with concrete pavers. The best example of a steeply sloped project is a street in Colma, Calif., with an 18-percent grade. While there might be driveways and streets with steeper slopes, the Colma project provides the current upper limit in North America at 18 percent (see Figure 1).
|Figure 1: The highest sloping street on record in North America is this one in Colma, Calif., near San Francisco.
Of course, higher slopes have been achieved. Figure 2 shows a Costa Rican road with a maximum slope of 25 percent that leads to a mountain hotel on the Papagayo Peninsula. Likewise, moving into South America, Figure 3 shows a street and pedestrian walkway with a 14-percent slope just after construction was completed in Medellín, Colombia.
For embankment applications without vehicles, the limiting factors are the angle of repose of the bedding sand, base and soil subgrade and, most importantly, the resistance to soil and base sliding under compaction equipment. The angle of repose can be as steep as 35 degrees, or about 70 percent; however, the tendency of soil and base to slide during compaction will reduce this limit.
Adequate performance of interlocking concrete pavements on slopes greater than 7 percent depends on careful design considerations and proper execution of construction details. Experience with steep sloping streets and embankments next to bridges have demonstrated that stationary edge restraints, consistent and tight paver joints, and herringbone patterns will 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.
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.
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.
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.
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 (e.g., 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 precast 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 include 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.
Since running dump trucks and compaction equipment will damage the curbs, the following sequence should be considered: pour the curbs on the sides of the pavement, compact the soil and base up to the header beam location, pour the header beam, compact the soil, then place and compact base after removal of forms around the cured header. The soil and base along the header beam will require compaction with a hand tamper or small equipment since it is unlikely that equipment will be able to reach corners. This is where the density should be tested.
More importantly, the drainage mat cannot be damaged, soiled or be allowed to fill with base along the edges during compaction. Cover the upslope side and top of the drainage mat to prevent base material from entering. Remove the cover after the base is completely compacted, and it meets density and elevation requirements. Immediately cover the top with geotextile to prevent ingress of base or bedding sand. This is indicated in Figure 4 with a small strip over the top of the drainage mat and anchored by the pavers.
The drainage mat will extend the end of the header and direct water to one or both sides. The area outside the header and curb joints can be filled with No. 57 crushed stone or the equivalent to facilitate drainage of water out of the drainage mat and down slope.
In a few cases, the interlocking concrete pavement may abut an existing concrete slab, such as a driveway apron or an abutting street. Careful consideration should be given on whether to construct a header beam to help direct water away from this slab. Also, care should be taken not to undermine the base and soil under the existing slab during excavation, and later from water working its way through the bedding sand and base. The existing concrete slab should be free of cracks and spalls, especially along the edge that meets the header beam and/or concrete pavers.
If pavers are abutted against a concrete slab, their final surface elevation after compaction should be five to six mm higher than the concrete slab surface.
The gradation of the bedding sand should conform to ASTM C 33 or CSA A23.1 (concrete sand) with a limit of 1 percent passing the No. 200 (0.075 mm sieve). It is important that the No. 200 or fines be controlled, as an excess amount can slow the drainage of the bedding sand. Note in the detail how the bedding sand is contained by geotextile and is kept from entering the top of the drainage mat. It should be noted that coarser sand should be used on the highest slopes for drainage and its resistance to movement.
Joint sand gradation should conform to ASTM C 144 or CSA A179. This material is finer than the bedding sand and it should be completely dry to facilitate entering and filling paver joints. Concrete sand can be used for joint sand. In either case, the sand should be crushed, rather than rounded river sand, to facilitate interlock.
The pavers should be at least 2-3/8-inches (60 mm) thick for residential driveways. They should conform to requirements of the ASTM C 936 in the United States, or CSA A231.2 in Canada. Square units are not recommended on steep slopes. Pavers with spacer bars are recommended so that the sand can enter the joints.
Paving Pattern and Shapes
Herringbone patterns are recommended for steep slope applications as they resist horizontal forces from automobile braking and turning tires better than other patterns; dentated pavers may provide additional stability in steep slopes. Other patterns, such as running bond or random patterns, should be avoided.
The laying of the pavers should begin at the header beam and work up the slope, placed in a herringbone pattern. A 45-degree pattern encourages the surface water to flow to the sides of the pavement, but will likely require more cutting to install. Additional upslope header beams should not be necessary if the pavers are installed from the lowest to highest slope and within stationary curbs. Paver joint widths should be tight two to three mm and checked for consistency and alignment every six feet (2 m). Adjustments in joint widths or alignment should be made before compacting the pavers.
The pavement surface should have a minimum 2-percent crown or crossfall to direct water to its sides. A crown can increase interlock as the pavers settle slightly from traffic. The cross slope should allow for sheet drainage of runoff to the sides of the pavement. Flush curbs will allow the water to move off the pavement. Once the water is off the curb, consider how water at the sides of the pavement will be transferred down the slope it may be sent down a grass swale, a rip-rap lined ditch or a concrete gutter. Regardless of the method, the design objective is to maintain sheet flow and prevent channel flow of water over the concrete pavers.
After a second compaction of the pavers on bedding sand, the finish elevation of the pavers should be 1/4 inch (5 6 mm) higher than the header beam. This will help prevent water from being trapped against the curbs even if there is minor pavement settlement.
Joint Sand Stabilization
A joint sand stabilization material should be placed in all the joints. ICPI Tech Spec 5 "Cleaning, Sealing and Joint Sand Stabilization of Interlocking Concrete Pavement" provides guidance on joint sand stabilizers.
There are two types: liquid applied after the pavers are compacted with joint sand; and those mixed with joint sand and compacted into the joints, then activated with a water spray. Manufacturers' directions should be followed for handling and applying both types of stabilizers.
Stabilizers can reduce the amount of water infiltrating the joints and bedding sand, but they do not render the pavement completely impervious, which is why the design facilitates drainage of the bedding sand. Stabilizers also help to maintain sand in the joints even with exposure to concentrated discharges, such as downspout water, gutter-less eaves dripping water, air conditioning condensation or exterior hose faucets.
The repeated force of tires in the same locations may cause minor settlement over time, as well as minor horizontal creep of the units. This will most likely be evident at the top of the pavement or at protrusions, such as the down slope side of utility covers. If left unchecked, water can enter the opening and undermine the bedding sand. The pavement should be monitored for three to six months for this condition. If joints at the top or at protrusions open a few millimeters, they can be filled with joint sand and stabilized. If wider gaps occur, it may be necessary to relay the pavers to fill the gaps.
These guidelines will require judgment in their application to a specific project and diligent inspection on the job site. Every project will have unique conditions not addressed in this article, and the advice of a design professional experienced in high slope installations should be sought for specific project recommendations.