Lessons Learned from the Failure of a Bituminous Surface Treatment in Central Oregon

During October 1996 a 10-km-long, double-lane, two-layer bituminous surface treatment (BST) was constructed near Bend, Oregon. The bottom BST layer consisted of an application of rapid setting high float emulsion (HFRS-2) followed by an application of 19- to 12.5-mm chips. The top layer consisted of various emulsions and 13- to 6.3-mm chips. Soon after construction up to 20 large commercial timber hauling trucks per day used the road during the wettest November and December in over 20 years. By the end of December 1996 most of the top layer of chips had disappeared, large numbers of potholes had developed, and the emulsion was not adhering to the bottom layer of chips. Failure resulted from interacting factors including a dust coating on the chips, an incompatibility of emulsion and chips, cold and wet weather, and a nearly impervious base course. The following lessons were learned from this project: (a) An adequate and accurate quality control and quality assurance plan should be in place before construction. (b) In wet environments, base courses under a BST should be free draining. (c) Either cool and humid construction weather or dust coatings on the aggregate may not lead to failure, but in concert they raise the potential for failure dramatically. (d) Not all emulsions are manufactured the same. An emulsion’s break and cure time may vary depending on the emulsion’s additives. (e) The asphalt emulsion and aggregate must be compatible with each other at conditions expected on the road surface.