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.

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.

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.

Proceedings of RILEM TC-PRC third conference on this subject. Papers from road authorities, engineers, researchers, contractors and manufacturers discussing the implementation and the long term behaviour of overlay systems. The following topics are covered: prevention and cracking assessment, choice and design of overlay systems, practical implemen

"This synthesis will be of special interest to pavement designers, materials specialists, maintenance engineers, and others concerned with the performance of pavement overlays. Methods are presented for reducing reflection cracking in overlays."--Avant-propos.

This synthesis on the use of recycled rubber tires in highways will be of interest to administrators and policy-makers; pavement, materials, geotechnical, environmental, and traffic operations engineers; and research engineers involved with highway design and construction issues. Information is provided on the uses of rubber tires in asphalt paving materials as well as other uses, such as on fills and embankments, for erosion control and on railroad grade crossings. Specifically, information is included which identifies the agencies using or implementing applications for recycled rubber tires and defines the design parameters, technical and construction limitations, performance, costs, benefits, environmental limitations, specifications, and availability. This synthesis of information defines the use of recycled rubber tires in highways and is based on a review of nearly 500 references and on information recorded from state highway agency responses to a 1991 survey of practice. Updates are included for as much of the state practice information possible through 1993. The use of scrap tires for highway applications is dynamic with regard to policy and technical issues. Therefore, the reader should keep in mind that the information presented reflects the best available data at a particular time. The synthesis also identifies current research in the topic area, critical research needs, and legislative issues that affect application and use of recycled rubber tires.