An apparatus for laboratory investigations of asphaltic concrete under repeated flexural deformations

Abstract

A laboratory study was performed for the evaluation of an apparatus constructed to investigate the behavior of asphaltic concrete specimens subjected to repeated flexural deformations. The apparatus, called a deflectometer, was constructed to test flat circular specimens approximately 17 1/2 inches in diameter. The specimen under test was fixed about its periphery, a uniformly distributed pressure acted on the bottom surface to give the specimen a measured amount of support, and a repeated load which varied sinusoidally in magnitude with respect to time was applied to a centrally located area on the top surface of the specimen. The following is a list of loading variables that can be achieved with the apparatus: 1. Frequency of loading 2. Maximum amplitude of load applied 3. Ratio of maximum to minimum load applied 4. Ratio of load area to specimen area 5. Load contact pressure 6. Specimen support pressure. A standard loading condition was established for the purpose of studying various specimen characteristics with respect to their contributions to the ability of specimens to resist the effects of repetitive loads. The effects of specimen characteristics were compared on the basis of number of load applications to cause failure of a specimen. The study indicated that the endurance of asphaltic concrete to repeated loads cam be correlated with the following items: 1. Asphalt content of mixture 2. Asphalt consistency 3. Surface texture of aggregate 4. Stress or strain imposed on the specimen in consideration of (a) specimen thickness (b) support given the specimen (c) load contact pressure Through the course of the study a mixture compaction method was developed to produce adequate density and particle orientation in the large test specimens. The compaction method utilized the testing machine to impart vibratory-kneading forces to the mixture being compacted. The compaction forces contained horizontal components which are believed necessary in order to tumble the particles and aid in obtaining proper densification of the specimen.