PREMATURE FAILURE OF ASPHALT OVERLAYS FROM STRIPPING: CASE HISTORIES

Separation and removal of asphalt binder from aggregate surface due primarily to the action of moisture and/or moisture vapor is generally termed In the identification of the cause of stripping practitioners have, historically, tended to focus their attention on the sensitivity of the aggregate and asphalt system in the presence of moisture. The authors classify this stripping as a physio-chemical incompatibility of the asphalt system, and the classical moisture sensitivity tests are relevant. The case histories in this paper document the effect of pavement saturation. The authors suggest that under saturated conditions all asphalt mixes may fail as a consequence of cyclical hydraulic stress physically scouring the asphalt binder from the aggregate. The authors classify this stripping as a mechanical failure of the asphalt pavement system, and the classical moisture sensitivity tests are irrelevant. While under saturated conditions a less moisture sensitive asphalt system may survive longer, it is probable that failure is deferred and not avoided. Stripping became a major problem in the United States in the late 1970s. Premature failures of asphalt overlays within two years of construction are not uncommon. This paper documents four such case histories from Pennsylvania, Oklahoma, and New South Wales in Australia. Case histories give the details of construction, visual observation of pavement distress, sampling and testing of pavement, and conclusions/recommendations. Moisture profile within the pavement structure was also determined by dry sampling with a jack hammer. The phenomenon of stripping was investigated from a global perspective, looking at the relative permeability of the pavement components, subsurface drainage system, and the interaction between different asphalt courses including open-graded friction courses. Hypotheses are presented to explain the mechanisms that will result in the pavement saturation observed. Recommendations have been made to minimize instances of such premature failures resulting from stripping.