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In the design of dense-graded bituminous pavement, some means of establishing the maximum permissible bitumen content consistent with the anticipated type and intensity of traffic is required in the laboratory. Any amount of bitumen less than this is believed to be conducive to less than optimum durability, primarily because of reduced film thicknesses and a tendency toward greater porosity. It is desirable therefore to establish as narrow a limit below this maximum as practicable for field control. This upper limit appears to be best defined in terms of plasticity, that is, it is just below the bitumen content at which the pavement will shove or rut excessively under the imposed traffic. In order to predict this condition in the laboratory, it is necessary to produce a pavement sample having the anticipated prototype density. Also, it is desirable that the laboratory sample have stress-strain properties comparable to the prototype. A means of identifying the plastic condition directly rather than through the use of voids criteria, as now used, would be desirable. The gyratory kneading-type machine, originally conceived by personnel of the Texas State Highway Dept. for laboratory compaction, has been further developed to permit mechanical compaction of specimens to any given field condition. Indications are that it may be possible to evaluate the plastic properties of these specimens directly during the compaction process and thus indicate the optimum asphalt content. A procedure is proposed for designing and controlling the construction of pavement for heavy airplane traffic.
McRae, John L.
Engineer, Chief, U. S. Army Engineer Waterways Experiment Station, Vicksburg, Miss.
Foster, Charles R.
Engineer, Assistant Chief, U. S. Army Engineer Waterways Experiment Station, Vicksburg, Miss.