Commercial asphalt overlays are generally used for one of two reasons: to increase load bearing capabilities of existing pavement surfaces, or to fix defective surface conditions. Adding load-bearing capacity is more complicated than simply fixing a surface flaw.
When applied for load-bearing capacity, structural design procedures are necessary to determine appropriate thickness of overlay. When used to correct surface issues, thickness must sufficiently correct the surface condition, but it is less complicated because you are not trying to increase load-carrying capabilities.
Types of Overlay Pavement Design
- Rigid Overlay: The use of a rigid pavement that’ll add strength to existing pavement classified as flexible or rigid.
- Flexible Overlay: The use of a flexible pavement to increase the strength of an existing flexible or rigid pavement.
Other Important Terms Related to Overlay Pavement Design:
- Rigid Base Pavement: An existing rigid pavement that requires an overlay.
- Flexible Base Pavement: The existing pavement requires an overlay that consists of bituminous concrete, base, and subbase courses.
- Composite Pavement: Existing pavement is overlaid with a rigid pavement made up of flexible overlay place on a rigid base, or an all-bituminous base pavement.
- Overlay Pavement: Pavement added to a pre-existing base in aims of increasing weight-bearing capabilities, or to correct surface issues.
The Importance of Prepping Surfaces for Overlay
Existing pavement should be carefully tested and all areas of distress must be addressed to determine the cause of wear and tear. Extensive cracks or foundation failures must be corrected before the overlay can be properly applied. In addition, existing pavement properties must be assessed to determine the right type of overlay for the job.
Using a Rigid Overlay
There are three primary conditions (fully bonded, partially bonded, and nonbonded) for forming a bond between the rigid overlay and the existing pavement.
A partial bond is secured by casting concrete directly on surfaces with no additional attempt to achieve or prohibit the bond. While the nonbonded condition entails preventing the bond by applying a superseding layer of material. Existing surfaces are cleaned before a leveling/bond-breaking course of bituminous concrete is applied to cover the area. Other materials used include sand asphalt, polyethylene, as well as other similarly stable materials.
As for a fully bonded or partially bonded rigid overlay, the pre-existing pavement must be thoroughly cleaned of all dirt and debris, as well as oil, paint, joint seal, bituminous patches, or anything else that could interfere with the bond between existing surfaces and the overlay. To further prepare surfaces, a sand-cement or epoxy grout is added after surfaces are cleaned and before adding the overlay.
Keep in mind, the bond-breaking medium should typically not exceed 1-inch in thickness unless special circumstances require greater thickness. Localized patching or cold planning is used to fix potholes or cracks that measure 1-inch or more. In addition, a leveling course made from sand-asphalt or a similar material can be applied.
Using a Flexible Overlay
When a flexible overlay is applied, there is no course of action necessary to treat the surface. The main importance is removing loose materials. Flexible overlay made from all-bituminous concrete requires cleaning existing surfaces of foreign matter, spongy joint seal material, and spalled concrete, as well as large spots in bituminous patches. A joint sealant should be used to seal off cracks or joints that measure up to 1 inch in width. Joints or cracks that measure wider than this must be properly cleaned and filled using compatible materials for existing surfaces and the overlay.
Leveling may be necessary to achieve the right grade. Before adding an all-bituminous concrete, apply a layer of tack coat to existing surfaces.
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