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Ultrathin Overlays (Thinlays)

1. What

Ultra Thin Overlays are dense graded hot asphalt mixtures placed at thicknesses of 1″ or less using equipment identical to those used for standard asphalt mixes.  KYTC refers to such mixes as “ultra thins”, while the National Asphalt Pavement Association uses the term “thinlays”.  They may also be called “Number 4 mixes”, in reference to the sieve size for the nominal maximum aggregate, which is 4.75 mm. When placed on structurally sound pavements, UTA overlays can provide an extremely smooth driving surface which seals the underlying pavement from moisture, thereby extending the overall pavement life.  Ultrathin Overlays should not be confused with ultra-thin bonded wearing courses (UTBWC), which have a more open-graded surface.

2. Where

Asphalt pavements in good structural condition and low to medium severity surface distresses such as cracking and raveling.  Any cracks greater than 0.75 inches should be sealed prior to application.  Ultrathin Overlays are suitable for all traffic levels.

3. Why (advantages):

High quality materials, strict specifications and performance-based design procedures yield a high performance surface that corrects minor surface profile irregularities and can be quickly returned to traffic. It can be used to improve friction and ride, reduce ravelling and noise, and seal small non-working cracks. Ultrathin overlays seal the pavement surface and protect the pavement structure from moisture.

4. Why Not (disadvantages):

The thin depth of ultra-thin asphalt overlays makes them susceptible to quick cooling. Seasonal limitations may be necessary to ensure Ultrathin Overlays are placed when temperatures are warmer.  Ultrathin Overlays are also not suitable for correcting rutting or severely deteriorated pavements.

5. When

Ultrathin Overlays should be applied when the pavement structure is in good condition, but there are minor to moderate surface distresses.

6. Estimated life extension:

6-8 years over pavements in good condition; 5-7 years on pavements in fair condition; 2-5 years for pavements in poor condition.

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