In order that considerations for fatigue distress of the asphalt concrete may be incorporated into pavement design procedures, it is necessary that definitive information on the fatigue behavior of asphalt concrete mixtures be determined. Data are presented based on a review of the literature as well as our research, which illustrate the effects of a number of mixture variables on fatigue response and include an indication of the influence of: mixture stiffness, air void content, aggregate gradation, aggregate type, asphalt content, and asphalt type. A literature review indicates the variation of air void content, aggregate gradation, and asphalt content to be expected on typical construction projects. This information together with the fatigue data presented previously allows the engineer to assess the relative importance of these mixture variables on predicting the performance of pavements. Asphalt content and test temperature appear to be more critical than the variation associated with aggregate gradation, asphalt hardness, and air void content.

The objectives of the research reported are to (a) define the material properties of central plant recycled mixes in Texas, (b) compare properties of these recycled mixes with conventional paving mixtures normally used in Texas, (c) evaluate the performance of the pavements containing central plant recycled mixes, and (d) compare performance of pavements constructed with recycled and conventional paving mixtures. Hveem, Marshall, resilient modulus, indirect tensile, direct tensile, and water sensitivity properties are reported for the recycled mixtures compacted both in the laboratory and under normal field procedures. Bituminous mixtures investigated include recycled asphalt concrete and recycled portland cement concrete. Performance is reported in terms of serviceability index as measured by a ride meter and by the use of a visual condition survey. The results of this study indicate that suitable bituminous mixtures can be produced by central plant recycling operations. Performance of pavements containing recycled mixtures appears to be suitable.

A five-year study was performed to investigate the behavior of asphalt mixtures containing plastic and latex modifiers. The plastics were functionalized polyolefins; high meltflow index acrylic acid copolymers that were used in a pelletized form. The styrene-butadiene rubber latex was supplied as an emulsion. These materials were used separately and in combinations with each other to change the characteristics of asphalt mixtures. The research included laboratory experiments to characterize the materials and field projects in five locations to ascertain their performance. Field projects of flexible pavement overlays were constructed in Alabama, Maine, Michigan, Idaho, and Texas. This paper presents some of the significant findings made during the course of the five-year effort. Mixture variables which were examined in the laboratory included asphalt grade, modifier type, and modifier concentration. Mixture preparation was investigated with respect to the order of addition of the components and compaction temperature. Laboratory results showed that the effectiveness of these modifiers was very dependent upon the source of the asphalt used in the mixtures. The order of addition of ingredients was found to be important for latex modified mixtures, and was not as significant in mixtures containing latex and polyolefins. The concentration of modifier changed the mixture behavior only up to levels of about 10 percent by weight of total binder. Compaction temperature had a profound effect on the resilient modulus of the mixtures. The field projects were built in climates ranging from hot and wet in Alabama to cold and dry in Idaho. Control sections of conventional mixtures were placed along with sections of modified mixtures. The mixtures containing latex only as a modifier showed problems during construction. These problems were confined to the mixtures sticking in the dump trucks and the workability through the paver. There were considerable differences noted between the results from behind-the-paver construction samples and test results from the inplace materials. The modifiers seemed to provide more mixture stiffness at higher temperatures which possibly indicated a greater resistance to permanent deformation, while maintaining about the same characteristics as conventional mixtures at low temperatures.

Unexpected performance of coarse-graded and crushed stone mixtures when compared to fine-graded and partially crushed gravel mixtures at WesTrack led to an examination of consensus aggregate properties for all mixtures used. These properties did not explain the differences in performance, but an analysis of mixture sensitivity to asphalt content and percent passing the 0.075 mm sieve did highlight the potential for substantial changes in volumetrics in terms of air voids for coarse-graded Superpave mixtures. The rutting performance of both coarse and fine-graded mixtures was sensitive to asphalt content, with coarse-graded mixtures highly sensitive to decreases in asphalt content and both types of mixtures sensitive to increases in asphalt content. Increased variability in aggregate gradation for the coarse-graded mixture coupled with this sensitivity may explain the unexpected performance. Based on these results, a sensitivity analysis was recommended for inclusion in the Superpave mix design process. Rutting performance models also indicated that volumetrics and aggregate properties related to gradation significantly contribute to performance.