Hot Mix Asphalt Overlay

Perspectives
A Quarterly Information Source from Benchmark, Inc.
Volume 55 February 2006

Hot Mix Asphalt Overlay

by Steve Schmidt, PE – Pavement Consulting Division

In evaluating appropriate maintenance or remediation methodologies for existing asphalt parking lot or roadway pavements, there are many options available. Preventive maintenance measures such as cracksealing and surface treatments are a good means for getting started with an effective maintenance program.

However, when pavement reaches a point where structural issues are obvious, another means of preserving the existing pavement’s integrity should be considered. One possible solution is a Hot Mix Asphalt (HMA) overlay. When considering this approach, you might ask yourself the following: When is the right time to consider an HMA overlay? What types of things should I be looking for?

Structural Considerations

In the life cycle of an aging pavement, the time will come when structural deficiencies, such as the following, become more prevalent. Pavement deformation such as rutting or wheel depressions, settling, or heaving begins to occur, but are not yet severe. Pavement cracking is increased with areas of alligator or fatigue cracking. Various asphalt patches have been performed in isolated areas of previous pavement failures. Raveling or other surface wear also affect the rate at which pavement will deteriorate. These are all signs that your parking lot or roadway is in need of structural improvements.

The proactive approach is to make structural improvements to your pavement before the deterioration increases at a more rapid rate. For best long-term performance results, be sure to make necessary repairs to the existing pavement structure. This would include repairing wide cracks, removal and replacement of unstable or failed pavement sections, and leveling out deformation, or areas with poor drainage.

Pavement thickness has to be geared towards improving life expectancy goals, while factoring in the current state of existing pavement deterioration. In most situations, overlay thicknesses of 1-1/2” to 2” are common.

Use of paving fabrics is also quite common with HMA overlays. A paving fabric is installed prior to the final HMA surface of the overlay. The fabric can be an effective means of “waterproofing” the base course and subgrade materials.

Ultimately, a goal (if the subgrade is suitable) should be to build on top of the existing pavement structure by limiting milling or removal operations to the existing pavement prior to an overlay.

Transitions and Match Points

When preparing for an HMA overlay, ramps, approach slabs, roadway curbs and landscaped island curbs all need to be taken into consideration. Cold milling or removals by other methods in these areas are often the only means of accomplishing a proper transition to these match points. At certain times, removing curbs, ramps or slabs and replacing at a higher elevation can accommodate the height adjustment that results with an overlay. Vertical curbing can more readily accommodate an HMA overlay by eliminating the need for a match point at the gutter pan.

Drainage Aspects

Desirable slope for pavement performance is a minimum of 2%. However, limiting factors such as ADA requirements near buildings and general site grading practices have created roadway and parking facilities with surface slopes between 1% and 2%. For an HMA overlay, maintaining a minimum of 1% slope is the general paving rule of thumb to prevent standing or ponding water. Addressing surface drainage issues prior to paving will help make an HMA overlay project a much smoother process.