These overlays are designed using a combination of experience and guidelines established herein. Generally, this type of overlay is used to improve ride, texture, cross-slope drainage, and weatherproofing, and is often categorized as “pavement preservation.” Often, the process is combined with milling of a like amount of existing surface, essentially keeping the same profile while improving functional characteristics (mill and fill).

Condition surveys (including deflection testing and GPR) should be undertaken to ensure that a non-structural overlay is appropriate. This would also include evaluating the existing HMA or PCC slabs for stability (and HMA stripping potential), if necessary. As with overlays designed for structural purposes, corrective action to the substructure should be accomplished prior to the overlay. The condition survey should also reveal whether minor milling, leveling, and undersealing (or grout injection beneath PCC slabs) are necessary.

The type of mix selected should complement the overall structure in terms of durability, permeability, and texture. The designer should consider appropriate lift thickness based on the desired mix type and nominal maximum aggregate size. Where permeable friction courses (PFC) are applied, it is imperative that the underlying structure is water-tight; hence, these overlays are almost always applied with an underseal or by using the thin bonded friction course (Item 348) method. Due consideration must also be given to insuring a proper cross-slope by using a level-up course before applying the PFC. Where active cracks exist (especially jointed PCC structures), thin overlays will rarely perform well without the use of geotextiles and careful consideration of the resilient properties of the mix. Some success has been realized by saw-cutting and sealing thin overlays over active joints on jointed PCC pavements.

Two statewide thin overlay options are available by standard specification: The thin bonded friction courses (Item 348) and thin overlay mixtures (Item 347 [TOM]).

Item 348 covers PFC and wearing course varieties, both of which use a warm polymer-modified asphalt emulsion membrane followed immediately by the application of a hot plant mixed paving mixture. The bonded wearing course is placed in thicknesses from 1/2 to 3/4 in. The thin bonded PFC is placed in thicknesses of 3/4 to 1.5 in., so some minor improvement in ride is possible. The PFC mixture will allow for the rapid removal of surface water, improving splash/spray characteristics; the open void structure will also reduce tire noise. These treatments should be considered on higher volume highways where average speeds exceed 45 mph, and where chip pickup and road noise from the alternate surface treatment are more objectionable to the traveling public. Item 348 allows RAP/RAS and WMA, if desired. The application of the warm polymer-modified asphalt emulsion membrane is designed to seal the existing surfaces where minor cracking (< 1/4 in.) is the most severe distress, and is seen as a potential remedy for pavements that have leaky joints and/or segregation problems. Existing rutting or more severe cracking must be addressed separately before using these options. The thin overlay mix (TOM, Item 347) uses conventional tacking procedures and allows for a WMA option. However, no RAP/RAS is allowed. The TOM-C is placed in thicknesses ranging from 0.75 to 1.25 in.; TOM-F is placed in thickness ranging from 0.50 to 0.75 in.

There are several construction-related concerns in placing these non-structural HMA overlays, including:

In general, compaction is more difficult and more variable on thin lifts.