THICKNESS DESIGN FOR HOT-MIX ASPHALT RAILROAD TRACKBEDS (WITH DISCUSSION)

The use of hot-mix asphalt (HMA) for railroad trackbeds has received considerable attention in recent years. Since 1968, more than thirty experimental sections of HMA trackbeds have been built in the United States. Two methods have been used to incorporate HMA in railroad trackbeds; one by placing the ties directly on the top of the HMA mat and the other by placing the HMA mat under the ballast. Due to the lack of experience and performance data, the thicknesses of these installations were selected arbitrarily on the basis of traffic and subgrade conditions. With a computer program named KENTRACK developed recently at the University of Kentucky, these installations were analyzed for the critical horizontal tensile strain at the bottom of the HMA mat and the critical vertical compressive stress on the top of the subgrade. Based on these strain and stress and the proposed failure criteria, a rational method of design was developed and is presented in this paper. This thickness design is based on the limited experience now available and should be used only as a guide.