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Bituminous mixes are generally designed with gradation and air void content ranges adopted by different agencies. The selected gradation and binder content are permitted to vary within specified tolerance limits. This leads to the possibility of a wide range of mixes with varying mix performance. Thus, it is essential to understand the influence of variation in these parameters on the performance of mixes. The present study aims to examine the sensitivity of the mix performance to variation within the specified gradation and air void ranges. Five aggregate gradations were considered for designing mixes for the volumetric design criterion of 4% air voids. For three of the five gradations, mixes were also designed for two additional design air void contents, 3% and 5%. Rutting resistance of the mixes was measured using a rut tester and moisture resistance was evaluated in terms of Tensile Strength Ratio (TSR). The densification characteristics represented by voids in mineral aggregate (VMA) values of the mixes prepared using the five aggregate gradations are similar to those of the corresponding aggregate packing characteristics (dry aggregate air voids, DAAV). A good relationship was observed between mix rutting and minimum voids in mineral aggregate and DAAV. Sensitivity analysis carried out with different aggregate gradations, design air void contents and tolerances, indicated that rut depth as well as TSR vary over a wide range for the combinations of mixes considered (all within the gradation, air void content and tolerance limits).Compared to the design air void content, aggregate gradation is found to have significantly larger effect on rut depth whereas both gradation and design air void content have similar effect on the TSR value. Using the results obtained from the present investigation, the Indian Marshall mix design criteria (MoRTH 2013) for stability and flow were revisited and new criteria are proposed. [ABSTRACT FROM AUTHOR]

The paper presents the details of various investigations conducted for the laboratory evaluation of the performance of crumb–rubber modified asphalt mixes. Crumb rubber (CR) obtained from discarded truck and bus tires was used to modify 80/100 penetration grade asphalt cement. Fatigue and permanent deformation characteristics, temperature and moisture susceptibility, and oxidative aging are some of the performance indicators of mixes evaluated. A static indentation test was adopted for assessing the relative permanent deformation characteristics of different mixes. Three aggregate gradations: (1) superpave; (2) gradation specified by the Ministry of Surface Transport, India for asphalt concrete; and (3) one gap gradation, were considered in the investigation. Compared to normal mixes, CR modified mixes were found to have improved fatigue and permanent deformation characteristics. They also showed lower temperature susceptibility and greater resistance to moisture damage compared to normal mixes. Superpave mixes gave better overall performance compared to the mixes having other aggregate gradations. [ABSTRACT FROM AUTHOR]