The objective of this research was to develop and evaluate asphalt-rubber gap-graded (AR-Gap) mixtures produced with varying gap aggregate gradations and designate a suitable aggregate gap gradation for local conditions, which could further be implemented at global level after necessary laboratory and field evaluations. A total of fifteen gap gradations were investigated, including seven specified by various highway agencies and eight trial gradations. The gradation studies recommended clustering seven gradations into two groups of similar characteristics. Furthermore, investigations on trial gradations presented the significance of each portion of the aggregate gradation in developing a superior performing gap gradation. Based on the refined gradation studies, three gap gradations were chosen: two from agency-specified, and one from the trial gradation for basic performance evaluation. Rutting, fatigue cracking, and moisture susceptibility investigations were accomplished on ten asphalt mixtures encompassing nine AR-Gap mixtures and one conventional dense graded mix. The results depicted superior performance of the proposed gradation indigenously developed in this study in respect of all distresses. The balance of coarse and fine aggregates in the proposed gradation facilitated higher strength at higher temperatures to resist rutting. Furthermore, the customized AR binder aided in providing additional resilience to the mixture, thereby improving the fatigue cracking resistance of the proposed AR-Gap mixture. Overall, it is envisaged that this study will help understand the effect of aggregate gradation characteristics on the performance of the AR-Gap mixture and aid in the development of AR-Gap mixtures as suitable long-lasting pavements.

The mixture design and performance characteristics of crumb rubber modified asphalt concretes were investigated in this research project to meet the requirements of the Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991, which has required each State to incorporate scrap tire rubber into its asphalt paving materials. Specifically, the objectives of this research encompass the following: (i) investigation of the rheological properties of asphalt-rubber binder to determine optimum content of crumb rubber; (ii) development of optimum mix design for various applications, including both wet and dry mix processes; (iii) characterization of mechanical properties of recommended paving mixtures, including resilient modulus, fatigue cracking behavior, low-temperature thermal cracking resistance, water sensitivity test, incremental creep test and loaded wheel track test; and (iv) comparison of performance of selected paving mixes.

This study investigated the cracking mechanism of Asphalt-Rubber Gap-graded (AR-Gap) mixtures with various material properties. The research focused on using a monotonic semicircular bending (SCB) test to estimate the cracking potential of AR-Gap mixtures at 0C and 25C. A total of 28 asphalt mixtures, including 27 AR-Gap mixtures and 1 conventional dense-graded (DG) mix, covering over 220 data points were utilized for evaluation. The outcome parameter, fracture toughness,KIC, calculated from the SCB test was used to compare and contrast the mixtures resistance to cracking at low and intermediate temperatures. At both test temperatures, the results indicated that AR-Gap mixtures have higher fracture resistance (~60 %) than the DG mix. Furthermore, statistical analyses confirmed that asphalt binder type, aggregate gradation, and binder content had significant effects on cracking performance. Aggregate gradation had a more pronounced effect on cracking performance than binder type, followed by binder content. In addition, aKICpredictive model was developed that had excellent statistical goodness-of-fit measures (R2 >= 90 %), which can be conveniently utilized in case of unavailability of the test setup. Overall, the findings revealed superior performance characteristics of AR-Gap mixtures that had high potential to resist cracking. Based on the various outcomes, the study was able to recommend aggregate gradation, binder type, and binder content as the parameters that would exhibit higher resistance against cracking failure. It is envisaged that this research would further the state of the art in designing crack-resistant asphalt mixtures.

The Virginia Department of Transportation (VDOT) maintains 3,343 lane-miles of composite pavements (asphalt over jointed concrete or continuously reinforced concrete pavements). Propagation of cracks from existing pavements into new asphalt concrete overlays (reflective cracking) is a major problem with composite pavements. Treatments that are used to reduce or mitigate reflective cracking include the use of asphalt mixtures with highly modified binders. One way of modifying asphalt mixtures is by using ground tire rubber (GTR), also referred to as rubber modified asphalt. There are three ways of adding GTR to asphalt mixtures: (1) traditional wet process, (2) terminal-blend wet process, and (3) dry process. The traditional wet process blends GTR with asphalt binder or bitumen on-site at the asphalt mixture plant prior to mixing the GTR modified asphalt binder with aggregate. The traditional wet process, along with a gap-graded stone structure, is typically used for incorporating higher GTR concentrations (>15%). VDOT has limited experience with rubber modified asphalt mixtures in general and even less experience with GTR content that exceeds 10%. The purpose of this study was to establish a performance baseline for an asphalt rubber gap-graded mixture (AR-GGM 12.5) using the wet process on I-85 in the Richmond District (I-85 Southbound, Dinwiddie County). Another objective was to compare its performance with VDOT’s stone matrix asphalt (SMA) mixture, which is also a gap-graded mixture. This study found that AR-GGM mixtures can be placed with no special field accommodations (compared with SMA mixtures), and the special provision developed for AR-GGM mixtures is effective. Further, based on laboratory performance testing, both the AR-GGM and SMA control mixtures tested in this demonstration project were crack and rutting resistant, with the AR-GGM mixture exhibiting more flexibility (i.e., lower stiffness). Both sections are performing as expected after 3 years of traffic and exhibiting minor to no distresses, with a Critical Condition Index greater than 90. However, at this early stage of field service, it is too soon to quantify a performance advantage of AR-GGM mixtures in comparison with conventional SMA mixtures. This study recommends continued use of AR-GGM mixtures for suitable projects as a reflective cracking mitigation tool. Further, the study recommends continued performance monitoring of the study sections to evaluate the cost-effectiveness of AR-GGM mixtures in comparison with SMA mixtures.

A research project was conducted to identify and document current modifications to ARIZONA 815c (75-blow Marshall method) used to develop gap-graded asphalt rubber asphalt concrete (GG AR AC) mix designs, and to develop and test improvements to provide a standard mix design method for use by contractors and consultants. Based on field performance data provided by the Arizona Department of Transportation (ADOT), the existing mix design method was successful and should serve as the standard for comparison of proposed improvements. Best practices were synthesized to develop proposed improvements. Three aggregate sources and two asphalt rubber (AR) binders were used for initial testing of the existing (control) mix design method and of the proposed changes. Rebound of compacted AR AC specimens was evaluated, as were Rice test results at 6% and 7% AR binder by weight of mix. The composition of the AR binders (rubber gradation and content) had more effect on the results than which mix design method was used. Additional replicate testing was performed by MACTEC and ADOT to confirm these findings. Changes to the AR AC mix design method consist primarily of making and curing Rice specimens in the same manner as Marshall specimens, tighter temperature ranges for mixing and compaction, incorporating Asphalt Institute calculations in a "User's Guide," and improving presentation. An ADOT construction project was used as an "acid test" to pilot the proposed mix design method and provide materials for a four-laboratory round robin to evaluate the precision of testing AR AC materials. The precision of round robin testing appears very similar to that of conventional asphalt concrete mixtures based on data from Proficiency Sample Programs of the AASHTO Materials Reference Laboratory and ADOT. The results indicate that the mix design method developed can be used by qualified laboratories to provide suitable AR AC mix designs

Pavement Engineering will cover the entire range of pavement construction, from soil preparation to structural design and life-cycle costing and analysis. It will link the concepts of mix and structural design, while also placing emphasis on pavement evaluation and rehabilitation techniques. State-of-the-art content will introduce the latest concepts and techniques, including ground-penetrating radar and seismic testing. This new edition will be fully updated, and add a new chapter on systems approaches to pavement engineering, with an emphasis on sustainability, as well as all new downloadable models and simulations.

The modification of asphalt mixtures with rubber in the conventional process includes specialized equipment for mixing and storage and concurrently necessitates reheating prior to use. Reacted and Activated Rubber (RAR) is a novel rubber-based asphalt mixture modifier that can be added into the mixture matrix without the complexities associated with wet mixing. The objective of this study was to design and assess the performance of RAR-modified dense-graded asphalt mixtures against key pavement distresses. Furthermore, the performance of RAR-modified mixtures was compared to the unmodified and commercially available rubber-modified asphalt-based mixtures. Two levels of RAR dosage at 2 and 4 % by total weight of mix were included with two different base binders. Four performance characteristics were analyzed: resistance to rutting at high temperatures, failure by fatigue, fracture energy, and susceptibility to moisture damage. Increased rut resistance and higher fatigue lives were observed for higher RAR contents. Statistical analyses revealed insignificant changes in fracture energy among the different mixtures, and all the mixtures were found to be resistant to moisture damage. RAR was concluded as a compatible promising rubber modifier with a significant potential to improve resistance of asphalt pavements against distresses and capable of providing extended life cycles. Because RAR showed significant improvement in the performance of the dense-graded asphalt mixtures, this study recommended expanding the scope of utilizing the RAR materials with other aggregate gradations both at the laboratory and field levels in future.