The Icing on Top
Residential or commercial clients can be acutely aware of how tired-looking hardscape surfaces become over time. In some cases, an upcoming or recently completed sale of the property will prompt the owner to refresh unappealing drives, courtyards or walkways. Owners who are cost-conscious may look to concrete paver overlays as a less expensive solution that delivers the visual improvement they desire.
Contractors may also view overlays as a potential high-margin, lower-effort job, but many factors must be taken into consideration to ensure a comprehensive and accurate bid, and quality installation.
Prior to submitting a bid for the job, installers need to conduct a thorough assessment of the site. Evaluating the condition of the surface to be overlaid is, of course, the first step. It is best practice to have a pavement/civil engineer assess the quality of the surface, but property owners may balk at the cost associated with this, especially for a relatively small job, such as a driveway.
Asphalt or concrete surfaces with severe cracks or chips, or ones that show heaving, rutting or pumping, are not suitable surfaces for new paver overlays. Base surfaces with this degree of damage are likely near the end of their lifespan, and pavers applied over them will reflect the underlying damage and could fail within a very short period of time. In these cases, it is best to recommend complete replacement of the surface.
Contractors should also examine the base and subbase of the existing pavement. Signs of water trapped beneath the pavement or movement of underlying soils are usually visible at the surface as heaving or deflection in the concrete slabs or sections of asphalt. Again, paver overlays are not appropriate for these kinds of conditions and a total replacement should be recommended. If the base, subbase and old surface are in good shape, an overlay job can proceed.
PREPARATION IS KEY
Once the existing surface is deemed suitable for an overlay application, site evaluation continues with an examination of the surface thickness, grade, elevation, drainage and any incorporated structures. Be sure to explain the evaluation process to the client—particularly any changes in elevation or slope—to ensure complete understanding and agreement before work begins.
The existing surface should be smooth, with a surface tolerance of ±3/8 in. (10 mm) over 10 ft (3 m) to ensure even application of the bedding sand. If there are minor deviations in the surface, higher sections can be ground down. If the milling exposes cracks within the asphalt or concrete, those sections can be patched to make them conform with the rest of the area.
Use a transit level to measure the grade of the existing surface. Because the paver overlays will follow the contour of the existing surface, any minor adjustments needed in the slope can be accomplished by selectively varying the depth of the bedding sand. Keep in mind, however, that the compacted depth of the bedding sand should not exceed 13/8 in. (35 mm) in any given area.
Drainage will be needed for the bedding sand. For a short time after the overlay installation is complete, water will continue to move through the paver joints into the bedding sand. To direct this water out, drill several 2-in. (50-mm) diameter drainage holes through the existing surface at the lowest elevation. Fill the holes with ¼- to ½-in. open-graded washed, angular aggregate.
Plan for elevation transitions to adjacent pavement so that new and old surfaces meet exactly. In most instances, this will require an adjustment in slope over 10 ft (3 m) as the overlay pavers approach the pavement, and might necessitate removal of some of the underlying concrete or asphalt. In cases where the overlays will butt against stairs, the additional height of the overlay may decrease the height of the first stair riser, which might be a code violation and may present a hazard. One solution is to adhere pavers to the stairs as well to maintain the appropriate rise and run.
If another party, such as a utility company, state or local authority, is responsible for raising or relocating structures such as manholes, catch basins, utility boxes, or gas or water valves, be sure that they receive timely notification of the work plan and construction drawings with elevation changes clearly marked.
To prevent shifting of the outermost pavers, use edge restraints made of cast concrete, steel, aluminum or plastic, fastening them directly to the existing surface with expansion bolts (on concrete) or spikes (on asphalt). Place a 12-in. (300-mm)-wide strip of geotextile against the edge restraint, turning up a 1-in. lip to hold the bedding sand. In some cases, the paver overlays will butt against an existing curb. If the curb face is perpendicular to the surface, no new edge restraint is needed, but place the geotextile strip along the curb and turn up the edge as you would for the other edging. If the curb face is angled—as is sometimes the case on street-side installations—it might be necessary to saw-cut a vertical face on the curb.
Generally speaking, an underlayment of geotextiles would not be required on existing, relatively new asphalt and where there is little to no danger of bedding sand loss. For existing concrete, place a 12-in.-wide strip of woven geotextile over seams and expansion joints to prevent bedding sand loss.
Prepare the site for bedding sand by first setting up the screed bars. To help prevent wrinkling the geotextile as sand is being poured in, shovel a small amount onto the fabric in several spots to hold it in place while the dump truck moves across the site. Have the truck driver move in a single path, preferably in a straight line, and at slow speed to minimize disruption of the textile.
The bedding sand should conform to ASTM C33 or CSA A23.1 gradations with only 1 percent throughput in a No. 200 (0.075 mm) sieve. Aim for a depth of 1 in. (25 mm) in the field overall; but to allow for variation in the underlying surface, compacted thicknesses of 5/8 in. (16 mm) at minimum or 1 3/8 in. (35 mm) are acceptable. Screed to a smooth surface and proceed with setting the pavers.