Late last year, we introduced the Pavement Preservation & Recycling Alliance (PPRA) Treatment Toolbox, available as a free, online resource for your pavement preservation planning. Another great tool in their Treatment Resource Center is some in-depth guidance on ultra-thin lift asphalt overlay for pavements.
Traditional asphalt overlays are in the range of 1½” – 3”, depending upon project goals, existing road design and surface distresses, the nominal aggregate size, and other factors. Generally, this is done after the existing pavement has exhausted its service life and suffered distresses beyond what can be treated with pavement preservation techniques. The existing, distressed pavement is often milled (aka, rotomilled, cold planed, cold milled, etc.), before an overlay of similar compacted thickness is applied.
Conversely, an ultra-thin lift overlay is typically a compacted depth or thickness of ¾” -1” and is usually intended to extend the service life of a pavement that is only mildly distressed. The result can be an extension of the pavement life by 8-10 years.
There are many variations with ultra-thin lift materials and processes. For example, because of the importance of bonding with lower layers, some agencies require the use of a spray paver that applies the tack coat immediately ahead of the placement of the new material. In other instances, a more aggressive tack coat material is used and it is not uncommon for asphaltic binders like PG 64-22 be used as tack to better ensure consistent bond. Similarly, nominal aggregate size and degree of fines vary with ultra-thin lift designs, and other mix variants like open graded friction coarse or a bonded wearing coarse are used, depending upon the condition of the existing pavement section, traffic volume and speed, and the overall project goals.
Because an ultra-thin lift overlay may cost as high as 60% of what it would for a traditional mill and overlay, the cost-benefit ratio should be uniquely modelled for each project. One technique suggested by PPRA is to bid a project with alternates – obtain a price from each bidder for a traditional overlay (say, 2”) and a separate price for an ultra-thin lift overlay (say, ¾”) and determine if the extra cost of the traditional overlay will yield a commensurate additional life extension of the pavement.
So, take a little time, browse the PPRA site, and consider whether your pavements can benefit from some preservation techniques this spring. The Delaware T2/LTAP Center’s Municipal Engineering Circuit Rider is intended to provide technical assistance and training to local agencies and so if you have pavement management questions or other transportation issues, contact Matt Carter at matheu@udel.edu or (302) 831-7236.
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