54 Hot Mix Asphalt Overlay A hot mix asphalt (HMA) overlay is a road maintenance technique that involves the placement of a new layer of hot mix asphalt over an existing paved surface, such as a deteriorating roadway or airport runway. This new layer, which can vary in thickness, is intended to extend the life of the pavement by providing a new, smooth and durable surface. Hot mix asphalt is a mixture of aggregate (stone, sand or gravel) bound together by asphalt cement, a product of crude oil. HMA overlays are applied hot and compacted to form a solid, dense surface. The process of creating an HMA overlay typically includes the following steps: 1. Preparation of the Existing Pavement Surface: The existing surface is cleaned, and any necessary repairs are made. This might include patching potholes, fixing cracks and ensuring a proper profile through milling if required. 2. Application of Tack Coat: A tack coat, a light application of asphalt emulsion, is applied to the existing surface to promote bonding between the old and new pavements. 3. Laying the HMA Overlay: The hot mix asphalt is transported to the site in trucks, laid down by a paver and then compacted using rollers. The compaction process is essential to ensure that the overlay is fully compacted, has minimal air voids and achieves maximum density. 4. Cooling: After compaction, the new HMA overlay is allowed to cool to an ambient temperature before being opened to traffic. The thickness of an HMA overlay will depend on the existing pavement condition and the expected future traffic loads. Thin HMA overlays (typically 1.5” or less) are often used as a preventative maintenance measure or to improve ride quality, while thicker overlays may be used to add structural capacity to a pavement. HMA overlays are a popular choice for pavement rehabilitation due to their ability to be quickly constructed with minimal traffic disruption and their compatibility with a wide range of temperatures and loading conditions. Ultra Thin HMA Ultra Thin HMA is a preventive maintenance treatment used to extend a pavement’s service life, protect the pavement structure and restore pavement smoothness. It is a specialized hot mix asphalt mixture with very specific mix design requirements that allow for placement at approximately ¾” thickness.
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