TRANSPORTATION RESEARCH DIGEST
ARIZONA TRANSPORTATION INSTITUTE
Thin and Ultra-Thin Whitetopping, NCHRP Synthesis 338 by Robert Otto Rasmussen and Dan K. Rozyck (Transportation Research Board, 2101 Constitution Avenue, N.W., Washington, D.C. 20418; ph. 202-334-2934; ) (2004)
Both thin whitetopping (TWT) and ultra-thin whitetopping (UTW) overlays have been used successfully on hundreds of projects worldwide.
They are a viable alternative for roads of light to-moderate traffic.
As shown from the information reviewed for this synthesis, both thin whitetopping (TWT) and ultra-thin whitetopping (UTW) overlays have been used successfully on hundreds of projects worldwide. When designed and constructed properly, these types of overlays serve as an important option for hot-mix asphalt (HMA) rehabilitation that is readily available to state departments of transportation and other agencies. Although TWT and UTW overlays cannot be used everywhere, they are a viable alternative for roads of light to-moderate traffic, which still constitute a large percentage of the nation's highways.
Since whitetopping overlays were first used more than 80 years ago, a knowledge base has been developing. Through field trials under a wide range of conditions, best practices have emerged for design and construction of these overlays. This synthesis has identified many of these best practices and it has touched on the state of the art with respect to these overlays.
Whitetopping overlays, including UTW and TWT, have proven to be successful rehabilitation methods when used properly. Although this document has identified numerous practices that can be adopted to
ensure success in design, construction, and maintenance, a few of the more important conclusions are as follows:
The performance of UTW and some TWT overlays are closely correlated with the characteristics of the support layers, especially the HMA layer. If specific and careful consideration is not made toward characterizing the existing pavement system, the white topping overlay may be designed or constructed improperly.
For UTW and TWT overlays to continue to be considered a viable rehabilitation alternative, specifiers and designers should recognize its limitations. As with other Portland cement concrete pavements, UTW and TWT overlays have their inherent benefits. However, if they are applied improperly, their reputation as an engineering solution can be tarnished. It needs to be recognized that whitetopping overlays are not a cure-all.
With respect to whitetopping design, a balance must be struck between accuracy (reliability) and simplicity. Although an overlay design of high accuracy is preferred, the designer must avoid making the product too difficult to use. That is. the designer should be cognizant of the cost and difficulty in collecting information about the existing pavement and balance those findings with the added benefits that