A compaction study focusing on the extent to which laboratory compaction methods (gyratory, kneading, and rolling-wheel) affect fundamental mixture properties important to pavement performance has been carried out as part of a project by the strategic highway researchprogram. The work has shown that besides compaction method, other aspects of specimen preparation significantly affect test results andtheir interpretation. Of particular importance were the method to measure air void content and the effect of surface condition (cut or as-molded) on both air void measurements and mechanical behavior. Air void content was significantly underestimated by standard procedures that leave the outer surface of the specimen unsealed during submersion. However, difficulties with the standard paraffin wax procedure, coupled with the need to compare dry specimens with partially saturated ones, led to the adoption of a new procedure. By using a surface-dry specimen (wetted and then surface dried) encased in parafilm (a stretchable, waterproof membrane), a consistent measurement of air void content was obtained. Further, it is evident that the surface of the specimen, cut or uncut, influences the measured air voids.Thus, air voids are not uniformly distributed within specimens, a factor that can significantly bias test results. The fact that surface type was found to influence the mechanics of mixture behavior under direct shear loading casts doubt on the accuracy of comparing tests results from field cores and laboratory-compacted specimens when the latter are tested in the as-molded or uncut condition. To obtain reliable and consistent results, it is recommended that laboratory testing be limited to specimens with cut surfaces and that air void measurements use the wet-with-parafilm technique. This paper appears in transportation research record no. 1317, Asphalt mixtures: design, testing, and evaluation 1991.
Abstract