An effort is made to summarize current practices for modifying paving asphalts. The types and classes of modifiers used, or considered, for paving applications are reviewed. The applicability of the Superpave binder specification (AASHTO MP1) is reviewed critically, and the modified-asphalt characteristics that are not considered in this specification are evaluated. Conventional testing results of modified binders are compared with Superpave testing results. The results indicate that a variety of additives can be classified, on the basis of their composition and their effects on asphalts, as polymers (elastomeric and plastomeric), fillers, fibers, hydrocarbons, antistripping agents, oxidants, antioxidants, crumb rubber, and extenders. These additives vary significantly in their physical and chemical characteristics and are expected to have widely variable effects on performance-related properties of asphalts. The assumptions supporting the criteria in the Superpave binder specification may not be valid for some modified binder systems. Furthermore, the testing protocols included in the specification do not consider certain important characteristics that are typical of particular modified binders. Among these characteristics are strain dependency, thixotropy, loading-rate dependency, and time-temperature equivalency. Conventional measurements used currently by state departments of transportation such as ductility, elastic recovery, and resilience, were collected. The ranking of five modified systems on the basis of conventional and Superpave measurements was analyzed. The analysis does not indicate that the conventional test methods are consistent in their rankings, nor does it indicate that these measures add much to the information deduced from the Superpave data.

The effect of different mixing times and mixing temperatures on the performance of asphalt-rubber binder was evaluated. Four different types of asphalt-rubber binders and neat asphalt were characterized using the Strategic Highway Research Program (SHRP) binder method tests. Subsequently, mix designs were carried out using both the SHRP Levels I and II mix design procedures, as well as the traditional Marshall mix design scheme. Additionally, performance testing was carried out on the mixtures using the Superpave repetitive simple shear test at constant height (RSST-CH) to evaluate the resistance to permanent deformation (rutting) of the rubberized asphalt mixtures. Also, six rectangular beams were subjected to repeated bending in the fatigue tester at different microstrain levels to establish rubberized asphalt mixtures’ resistance to fatigue cracking under repeated loadings. The results indicate that the Superpave mix design produced asphalt-rubber contents that are significantly higher than values used successfully in the field. Marshall-used gyratory compaction could not produce the same densification trends. Superpave mixture analysis testing (Level II) was used successfully for rubberized asphalt mixtures. Results clearly indicated that the mixture selected exhibited acceptable rutting and fatigue behavior for typical new construction and for overlay design. Few problems were encountered in running the Superpave models. The results of the RSST-CH indicate that rubber-modified asphalt concrete meets the criteria for a maximum rut depth of 0.5 in.; and more consistent results were measured for fatigue performance analysis using the repeated four-point bending beam testing (Superpave optional torture testing). The cycles to failure were approximately 26,000 at 600 microstrain.

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Agencies, industry, academia, and others are rapidly implementing the products of the asphalt research portion of the Strategic Highway Research Program, products collectively referred to as Superpave. One of the principal products has been the Superpave gyratory compactor (SGC). The results of an experimental program, conducted at the Asphalt Institute Research Center in Lexington, Kentucky, and aimed at assessing the effect of various compaction parameters on SGC test results, are outlined. Compaction results were most often different when different mold diameters were used. The length of the laboratory short-term aging period also affected test results. Compaction temperature was a critical factor when a modified binder was used. SGC affected moisture-susceptibility test results although specimens fabricated using SGC were successfully demonstrated to predict moisture damage using the AASHTO T283 protocol. The experiment also compared the compaction characteristics of several different units. This comparison demonstrated minor to moderate differences in test results when the same mix was evaluated.

A laboratory investigation was performed in which the influence on resistance to plastic deformation as demonstrated when rounded, smooth, sand-sized aggregate particles were replaced by rough, angular, porous particles while other aggregates and the total gradation remain the unchanged. Five asphalt concrete mixtures with varying amounts of these aggregates were tested. Resulting data were used to (1) evaluate the ability of certain test procedures to differentiate rutting potential of the mixtures, (2) develop a new rutting model for predicting pavement performance which incorporates aggregate characteristics, (3) examine fractal dimension analysis as a method of quantifying aggregate angularity and surface texture, and (4) demonstrate that octahedral shear stress theory can properly differentiate the rutting potential of asphalt concrete mixes.