413.7 Thin Hot Mix Asphalt Overlay
To be defined as a preventive maintenance treatment a dense graded HMA mixture is limited to not more than 1-3/4” thick.
A thin dense graded hot mix asphalt overlay is the highest type alternative available in the pavement preventive maintenance program for flexible pavements. Thin overlays protect the pavement structure, reduce the rate of pavement deterioration, correct surface deficiencies, reduce permeability and improve the ride quality of the pavement, particularly when accompanied by a scratch or surface milling. A minor amount of structural enhancement is provided with this preventive maintenance treatment when the existing surface is not milled prior to the overlay.
If the pavement surface exhibit a moderate to high severity of environmental cracking or a moderate to high severity of surface wear a thin HMA overlay may be considered. The existing pavement structure should exhibit a good base condition and a uniform cross section although some minor corrections can be made to the cross section with this preventive maintenance treatment.
Consolidation rutting should not exceed 3/8 in. Where rutting exceeds this amount the surface should be milled or the ruts filled with a suitable material. Pavements that exhibit rutting to the plastic flow of one of the mixes or have with a weak base should not be considered for a thin lift preventive maintenance treatment.
Refer to Bituminous Pavement Design concerning mix criteria and recommended thickness for light, medium, and heavy duty routes.