Construction

Surface Preparation

Repair Existing Distresses

Appropriate action should be taken where moderate to severe distresses exist. In cases where only minor distresses are observed, they may be ignored, and the whitetopping concrete may be placed without correction distress.

Milling is the most effective repair for rutting and shoving problems in the existing asphalt. This practice should be used provided there is adequate asphalt thickness remains to bond and create a composite pavement layer with the whitetopping concrete.

Potholes should be repaired prior to the placement of any whitetopping layer. Such distresses can be repaired by filling them with crushed stone, cold- or hot-mixed asphalt, and compacted to reduce further consolidation by vehicular loads propagating through the whitetopping layer after it has been constructed.

For subgrade failures, often associated with surface failures such as potholes, the entire affected area must be removed and replaced. With all material affected by the failure or prone to future failures removed, the existing asphalt concrete structure should be reconstructed to its original elevation. Existing compaction levels should be attempted to avoid future consolidation of the area after the whitetopping layer has been placed.

Milling

Milling is often specified for use with whitetopping overlays. Its uses include the following:

• Smoothing out surface distortions,
• Finishing the grade line prior to placing a whitetopping overlay,
• Establishing a cross-slope for the new pavement elevation, and
• Establishing a good surface to which the whitetopping concrete overlay can develop a good bond. From a bond development standpoint, milling is often considered the best option.

Generally, milling operations remove between 25 and 75 mm (1 - 3 inches) from the surface of the existing asphalt concrete. Figure 1.1 shows an existing asphalt pavement surface which has been milled 75 mm (3-inch).

After the milling operation has been completed, the existing asphalt surface must be cleaned to remove any particles that had been milled off. Sweeping followed by 100% coverage of compressed air or water should remove any particles left by the milling operation.

In Figure 1.2 the milled existing asphalt surface has been cleaned of all excess particles prior to concrete placement.

Final Surface Preparation

After all surface preparation activities have been completed, the existing asphalt surface must be cleaned to remove any dust, dirt, or other particles or debris that may be detrimental to the bond between the existing asphalt concrete and the new PCC surface. Sweeping followed by 100% coverage of compressed air or water should remove any particles.

Water fogging or whitewashing the existing asphalt concrete surface just prior to concrete placement can cool it enough to minimize adverse effects on the fresh concrete. Hot asphalt concrete surfaces can inhibit bonding between the existing asphalt concrete and the new PCC surface. Water fogging and whitewashing are separate activities that produce similar results.

Light water fogging can be used when the surface temperature of the existing asphalt concrete is uncomfortable to an open palm. Generally this is used for older asphalt that is not as dark as new asphalt concrete, and thus absorbs less heat from the sun.

Whitewashing may be used when the temperature of the asphalt concrete surface is expected to exceed 45 C (110° F). Whitewashing must be used with caution, however, because it may also be detrimental to the bond between the asphalt concrete and the new PCC surface.

Whitewashing can be made of lime slurry or white pigmented curing compound. See Figure 1.3. Lime slurry is made using water and hydrated lime. Salt should NOT be added to the mixture, even if the directions suggest it. Salt can be particularly harmful to reinforcing steel and to the portland cement concrete itself. Lime slurry is generally not as detrimental to the bond between the HMA and PCC layers as curing compound.

Either type of whitewashing should be applied at a rate that produces a uniform color on the asphalt concrete surface. If rain or construction traffic and activities fade the color, and if solar radiation is high, the temperature may begin to rise in the asphalt concrete surface, and a second application may be necessary.