Non-Structural Overlays

Non-structural overlays (Figure 1) do not involve extensive structural design and generally contribute little, if anything, to a pavement's structural capacity. Non-structural overlays are generally thin surface overlays on the order of 12.5 mm (0.5 in.) to 37.5 mm (1.5 in.) that are used to (NAPA, 1995¹):

• Improve ride quality.
• Correct minor surface defects.
• Improve safety characteristics such as skid resistance and drainage.
• Enhance appearance.
• Reduce road-tire noise.

Uses and Limitations

Thin non-structural overlays restore the pavement surface; they are not intended to add structural capacity. They can extend pavement life significantly, restore a smooth, quiet surface on high-traffic roads, and correct moderate distresses that originate at the surface, including longitudinal cracking outside the wheelpath, transverse cracking, weathering and raveling, and some forms of rutting. Depending on conditions, milling, crack sealing and a tack coat may be needed before the overlay is placed; where the existing elevation must be maintained, milling is combined with the overlay. More severe distresses such as fatigue cracking indicate structural problems that a thin overlay will not resolve, so a different rehabilitation strategy is needed. A careful examination of the structural, functional and drainage condition of the existing pavement should come first.

Suggested pre-overlay treatments by existing pavement condition (NAPA, n.d.²).

Advantages of thin asphalt overlays include (NAPA, n.d.²):

• Long service life with low life-cycle cost.
• Grade and slope can be maintained with little effect on drainage, particularly with mixes of small nominal maximum aggregate size.
• Ability to withstand heavy traffic and high shear stresses.
• No loose stones after construction and no binder runoff, unlike some seal treatments.
• Little or no dust during construction, and no curing time.
• Low tire-pavement noise.
• The overlay can itself be recycled.

Thin overlays cost more than seal treatments but less than structural overlays, and they can be placed quickly with little traffic disruption. The choice between a thin overlay and other treatments for a project depends on pavement condition and traffic, which determine the life-cycle cost of each option, and on agency policy. In a 1999 AASHTO survey, thin asphalt overlays were the most popular preventive maintenance treatment for asphalt and composite pavements.

Categories

Non-structural overlays can vary widely in composition depending upon local practice, traffic and general purpose. A loose classification of non-structural overlays follows (NAPA, 1995¹):

1. Light volume/residential traffic. The primary objective in light traffic areas is to retard asphalt binder aging of the underlying pavement. Since heavy traffic loads are not of great concern, overlays are generally less stiff (resulting in a more workable mix, increased durability and flexibility and a potential for the overlay to reheal under traffic) and use smaller-sized aggregates.
2. Heavy, high-speed traffic. The primary objective in heavy, high-speed traffic areas is to prevent rutting and provide good friction. Because of this, overlays typically use larger angular aggregate and more durable mixes such as SMA or OGFC.

Mix Design

Thin overlays can be designed with the same structural and mix design principles as any asphalt mix, and they can include warm mix asphalt and recycled materials such as RAP to the same degree. As surface layers they often use special-purpose mixes such as stone matrix asphalt for rut resistance or open-graded friction courses to reduce splash, spray and tire noise. The main design consideration is aggregate size: following Superpave practice, the lift thickness should be 3 to 5 times the nominal maximum aggregate size (NMAS). RAP used in a thin overlay must be sized to suit the NMAS.

Construction Notes

Non-structural overlays are generally quite thin. This results in several construction concerns (NAPA, 1995¹):

• Thin lifts require less HMA per foot of road length than thick lifts. This can result in high paver speeds (in excess of 21 m (70 ft.) per minute). Compaction may not be able to keep pace with these high speeds.
• Thin lifts will cool quicker than thick lifts. This can result in little time available for compaction before the thin overlay reaches cessation temperature (sometimes as little as 3 to 5 minutes). Therefore, roller variables should be set to account for this (e.g., enough rollers and an adequate roller pattern to compact the material before it reaches cessation temperature).
• Thin lift construction produces greater screed wear. If the lift depth is less than about twice the maximum aggregate size, the HMA may tear under the paver screed. Very thin lifts (less than 25 mm (1 inch)) can be damaged by the screed dragging large particles.
• Thin lifts are more sensitive to vibratory rolling. Incorrectly chosen amplitude, frequency or roller speed can result in aggregate degradation (i.e. breaking) and damage of the bond between the overlay and the existing pavement.
• Density control is difficult. Thin lifts provide fewer options for aggregate particles to rearrange under compaction. Thus, mat densities will tend to be less uniform than those associated with a thicker lift. This should be recognized if pay is in any way tied to mat density.

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

Because the aggregate is easily fractured, vibratory rolling should be used with care on thin overlays and avoided on overlays less than 25 mm (1 inch) thick.

@misc{pavint-non-structural-overlays,
  author = {{Pavement Interactive}},
  title = {Non-Structural Overlays},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/maintenance-and-rehabilitation/maintenance/non-structural-overlays/}},
  note = {Accessed [date]}
}