The Gyratory Testing Machine as a Design Tool and as an Instrument for Bituminous Mixture Evaluation : Draft Copy

One of the main objectives of bituminous paving mixture design is to select a bitumen-aggregate combination such that the mix so obtained will be as durable as possible and yet be stable. To accomplish this objective, one of the critical aspects is to be able to produce in the laboratory a compacted specimen that is truly representative of the mixture as it will be In service on the road. Most of the present design procedures utilize a constant level of compactlve effort which is Intended to produce densities (at designed asphalt content) comparable to those occurring in the field after a period of traffic densification. This approach may be open to question because a given level of laboratory compaction cannot be considered to produce specimens representative of the density of all mixtures and service conditions after a specified period of time. It would be logical to compact specimens in the laboratory to a density which Is representative of the field compacted density at the time of construction and then to denslfy these by simulating the effects of traffic. It is desirable to measure stability continuously during this process. By this procedure it should be possible to select the maximum asphalt content that may be used under a variety of service conditions without excessive loss in stability. The gyratory testing machine can be used in this way for bituminous mixture design. Based on the above reasoning it seemed useful to undertake a laboratory study to design and evaluate bituminous mixtures using the gyratory testing machine. Accordingly, a mixture type commonly used in Indiana was selected and designed for the optimum asphalt content. The designed asphalt content and the selected gradation were subjected to permitted job-mix tolerances. Specimens covering this range of composition were prepared and tested under simulated field compaction and simulated traffic densification conditions. It was contemplated that the results obtained would help in studying the following factors: 1) Evaluation of the gyratory testing machine design method. 2) Influence of simulated traffic densification on the mixture properties. The purpose was to study the capability of the gyratory testing machine to evaluate bituminous mixes at any specified densification effort. Positive results could lead to an estimation of pavement life. 3) Job mix formula and tolerance limits. The sensitivity of the gyratory testing machine when used to study the job mix formula tolerances was investigated. Favorable results could help in modifying specifications to suit field conditions.