Thin HMA overlays are an important pavement preservation tool, with their design optimized for 3/4 to 1 1/2 inches in depth. Credit: Texas Transportation Institute
What: Plant-mixed combination of asphalt cement and aggregate placed in depths of ¾ to 1½ inches over aging asphalt or concrete pavements as a pavement preservation treatment. The liquid asphalt binds the rocks together in a strong but flexible pavement structure. Because there are hot mix plants and experienced HMA contractors almost everywhere, thin HMA overlays have been one of the most common pavement preservation treatments. Polymer modified asphalt may be used for improved performance, and reclaimed asphalt pavement (RAP) may be incorporated into the hot mix to lower cost and as a more environmentally-friendly option.
Where: Over pavements with good structure but low severity surface distresses such as rutting, raveling, polished aggregate and friction loss. Thin HMA overlays are suitable for both flexible (asphalt) and rigid (concrete) pavements. Any cracks should be sealed before application of thin hot mix overlays.
Why (advantages): Thin HMA overlay are the most effective preventive maintenance treatment for re-profiling and improving rideability. They have no loose stone to cause vehicle damage, and the construction and traffic return are quick. They seal and protect the existing pavement, correct surface deficiencies, and improve surface friction. HMA contractors and equipment are available almost everywhere.
Why not (disadvantages): Unsealed cracks will reflect through the overlay, and sealed cracks may even cause a problem. Without a good tack coat on the exiting pavement, there is danger of slippage, shoving and delamination. The cost is more than some other pavement preservation treatments.
When: Thin HMA overlays should be applied when the pavement structure is in good condition, and there are only low severity surface distresses.
Estimated life extension: 6-17 years over pavements in good condition; 5-10 years on pavements in fair condition; 2-4 years for pavements in poor condition.
For more information on successful paving, consult the Thin Hot Mix Asphalt Overlay Checklist put together by the Foundation and FHWA.