A study was undertaken between 1974 and 1992 to study the rates of hardening in three separate bituminous overlay projects. The report described a project in Finney County that used a penetration graded AC-5 and AC-7 asphalt cement. Cores were drilled at periodic times and the extracted asphalts were compared with each other for five and ten years. The second project in Decatur County also used an AC-7. No cores were taken and only visual observations were conducted after approximately fifteen years. The last project was in Thomas County and was conducted in conjunction with another research project. Asphalt properties of an overlay were compared at five different locations. An asphalt-rubber membrane had been placed over two locations, a slurry seal over a third location, and two locations had been directly exposed to the atmosphere.