5. Surface Preparation

5.6 SUMMARY

The following key factors should be considered when monitoring surface preparation operations:

• A prime coat is generally not needed on subgrade soil unless the prepared subgrade at the proper moisture content will not be paved for several weeks. There is a difference of opinion on the benefits of using a prime coat on a granular base course, but in many cases a prime coat can be eliminated without detrimental effect on the performance of the pavement structure, especially when the pavement structure to be placed on the base course is relatively thick—maybe 6–8 inches (150–200 mm). With thinner pavements, the prime coat becomes more important.
• Before paving an existing surface, any failures in the surface of the existing pavement must be removed and replaced or repaired by patching.
• If cracks are present in an existing asphalt pavement surface, they generally should be sealed individually, or some type of surface treatment may be applied to the whole roadway area. Removing the distressed surface by milling prior to overlaying is also a very common approach. Joints in PCC pavement that are poorly sealed should be routed out and sealed. Rocking PCC slabs should be stabilized.
• A rough, uneven asphalt surface should be leveled with asphalt mix (using a paver to place the mix) to fill in the low spots on the surface, or it should be milled to create an even surface while removing surface distresses.
• Once the needed repairs have been completed, the pavement surface should be cleaned of all dust, dirt, and other debris. This should be accomplished using multiple passes of a mechanical broom. If brooming does not remove all accumulated dirt, flushing with air or water may be required.
• The application of a tack coat must be accomplished before an overlay is constructed on an existing asphalt or PCC surface.
• Proper distributor setup is critical to the successful application of tack coat. The distributor used should be checked to ensure that all the nozzles are open and set at the correct angle and that the spray bar is at the proper height above the pavement surface.
• The application rate for the tack coat should be based on the desired residual amount of asphalt cement on the road surface, which should be between 0.040 and 0.070 gal/yd² (0.181 and 0.317 L/m²) for normal surfaces. The application rate should also be based on the actual amount of asphalt cement in the emulsion—whether the emulsion is diluted or not before it is applied. For example, an undiluted SS-1 emulsion should be applied from the distributor at a rate of 0.060 gal/yd² (0.271 L/m²) to obtain 0.040 gal/yd² (0.181 L/m²) of residual asphalt on the pavement surface.
• Milled pavements may need a greater amount of residual tack coat. Too little tack coat will not provide the needed bond between the old and new layers. On the other hand, too much tack coat may promote slippage of the new overlay on the old pavement or bleeding of the tack material through a thin overlay.
• Asphalt mixture should be placed on top of an emulsion tack coat that has broken—changed color from brown to black. The tack coat should not be picked up and tracked by the haul trucks, however.
• Tack coat should not be left exposed to traffic. If doing so is necessary, proper precautions, such as reducing the posted speed limit on the roadway and sanding the surface with excess sand being broomed from the surface, should be taken.
• A tack coat should be placed between layers of new asphalt. The amount of residual asphalt on the new roadway surface should be approximately half that appropriate for an old, tight, existing pavement surface.