The influence of particle shape, size, and surface roughness on the strength of granular materials has been the subject of much discussion in the fields of soil mechanics and asphalt paving technology. A number of investigations have been conducted to determine the effect of variations in particle size and shape, but the results of these investigations have been somewhat varied and have led to conflicting conclusions; and although the effect of surface roughness has been the subject of much discussion, very few data are available to demonstrate its influence. This paper presents the results of a laboratory investigation to provide further evidence of the influence of these factors in the hope that the results might serve to clarify some of the controversial aspects of their effects.

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Dispersion of submicrometre-size carbon black particles in asphalt at dosages of 11 to 16 percent by weight of asphalt has been found to improve the asphalt properties of durability, wear resistance, and temperature-viscosity susceptibility. This concept of reinforcing asphalts with a carbon black filler has been made practical by pelletizing the carbon black with a high-durability maltenes binder compatible with asphalts. The pellets are packaged in 11.34-kg (25 lb) polyethylene bags which are introduced, bag and all, into hot-mix cycle of asphalt concrete batch-type plants. In continuous-type plants, the pellets are added either through the asphalt feedline in correct proportion to the hot asphalt or through the hot aggregate feed in correct proportion to the hot aggregate. Since the carbon black becomes part of and bulks the asphalt, adjustments are made in batch weights to keep the volume relationship of binder to rock constant. Other design considerations are discussed. Effects of the carbon black addition on the properties of asphalts and asphalt concrete, as measured in both laboratory and field experiments, are shown. The present status of a continuing program of field testing and evaluation is described.

The design of an asphalt concrete mix consists, essentially, of the following steps: 1. select type and gradation of aggregate, 2. select type and grade of asphalt, with and without modifier, and 3. select proportionate amount of asphalt in asphalt/aggregate blend. These steps have been incorporated into a general framework for design, which serves as the basis for the mix design procedure presented in this paper. Essentially, the system consists of a series of subsystems in which the mix components and their relative proportions are selected in a step-by-step procedure to produce a mix that can then be tested and evaluated to ensure that it will perform adequately in the specific pavement section for which it has been formulated. The latter evaluation phase includes the influence of environmental factors, effects of traffic, and the consequence of the anticipated structural cross-section design at the designated site in the following distress modes: fatigue, rutting, thermal cracking, and raveling. The paper includes a brief discussion of the important factors associated with the various steps of the design process and recommended test procedures to be followed. Information and data are presented to illustrate a procedure that might be followed to ensure that the laboratory method of specimen preparation and fabrication provides specimens with stress versus strain characteristics similar to those obtained by field compaction. The use of the creep test to assist in the design process is illustrated. Some techniques to reduce the amount of testing associated with the methodology described herein are also described.

Present-day test equipment and procedures in quality control (QC) and quality assurance (QA) programs, based on use of the Marshall and Hveem methods, are essentially limited to verification of the physical composition of the asphalt concrete mix and its compacted density to control and assure that the builder is supplying a product which falls within the constraints defined by the job-mix-formula. They do not answer the essential question as to whether or not the asphalt concrete mix will have the strength properties needed to resist the number and intensity of repeated wheel load applications for which it was designed, both at the as-constructed and the traffic-compacted stages. This paper presents a philosophical discussion of the requirements for an asphalt concrete QC/QA system, using performance-based measures as the criteria. The framework for a performance-based system is described based on use of the Repetitive Simple Shear Test at Constant Height (RSST-CH) for rutting prediction and the Flexural Bending Beam Test (FBBT) for the estimation of fatigue cracking.