Punyaslok Rath

With an increasing focus on improving the sustainability of asphalt mixtures, the use of recycled materials in asphalt mixtures has received significantly more attention recently. In Missouri, the incorporation of recycled materials including reclaimed asphalt pavement (RAP) and ground tire rubber (GTR) is limited to dense graded mixtures only. Missouri routinely uses stone mastic asphalt (SMA) mixtures on its interstates with high traffic for enhanced performance, and several studies have found that these recycled materials can be incorporated in SMAs without compromising their performance. This study aimed at investigating the effects of incorporating RAP and GTR in Missouri SMA mixtures. A total of 10 mixtures with varying dosages of recycled materials were designed using balanced mix design (BMD) methodology. The mechanical performance was determined by the mixture’s resistance to rutting using the Hamburg Wheel-Tracking test and cracking using the IDEAL-CT test. In addition, a disc-shaped compact tension (DC[T]) test was performed to determine thermal cracking resistance of the asphalt mixtures. Since sustainability is a key aspect of using recycled materials in asphalt mixtures, life cycle assessment (LCA) was conducted to quantify the global warming potential (GWP) of each mixture. This research showed that SMA mixture modification by 15% asphalt binder replacement with RAP proved to be the most desirable in mechanical and environmental performance. The addition of GTR did not have an adverse effect on the GWP, while the mixtures exhibited improved performance. Overall, the use of RAP and GTR in conjunction can be recommended for their implementation in SMA mixtures in Missouri.

This study employs a microstructure-based finite element modeling approach to understand the mechanical behavior of asphalt mixtures across different length scales. Specifically, this work aims to develop a multi-scale modeling approach employing representative volume elements (RVEs) of optimal size; this is a key issue in asphalt modeling for high-fidelity fracture modeling of heterogeneous asphalt mixtures. To determine the optimal RVE size, a convergence analysis of homogenized elastic properties is conducted using two types of RVEs, one made with polydisperse spherical inclusions, and another made with polydisperse truncated cylindrical inclusions, each aligned with the American Association of State Highway and Transportation Official’s maximum density gradation curve for a 12.5 mm Nominal Maximum Aggregate Size (NMAS). The minimum RVE lengths for this NMAS were found to be in the range of 32–34 mm. After the optimal RVE size for each inclusion shape is obtained, computational models of heterogeneous Indirect Tensile Asphalt Cracking Test samples are then generated. These models include the components of viscoelastic mastic, linear elastic aggregates, and cohesive zone modeling to simulate the rate-dependent failure evolution from micro- to macro-cracking. Examination of load-displacement responses at multiple loading rates shows that both heterogeneous models replicate experimentally measured data satisfactorily. Through micro- and macro-level analyses, this study enhances our understanding of the composition-performance relationships in asphalt pavement materials. The procedure proposed in this study allows us to identify the optimal RVE sizes that preserve computational efficiency without significantly compromising their ability to capture the asphalt material behavior under specific operational conditions.

Research has demonstrated that asphalt pavements can be a strategic destination for some of the major streams of waste materials around the globe, such as scrap tires and plastics. Ground tire rubber (GTR) from scrap tires has been used in asphalt pavements since the 1960s but has yet to approach its full potential in relation to market adoption. The heightened restrictions imposed by China in relation to waste stream contamination in 2018 have catalyzed research on incorporating post-consumer recycled (PCR) plastics into asphalt pavements. A field demonstration project was carried out in Columbia, MO to evaluate modern recycled plastic and GTR additives in an asphalt pavement overlay. The demonstration focused on dry-process modification, which requires minimal alterations to an existing asphalt plant and allows a higher amount of recyclates to be utilized. The project was also designed to assist in the Missouri Department of Transportation’s early roll-out of balanced mix design asphalt design and quality assurance specifications. The results of the study indicated the viability of dry-process GTR and PCR plastic additives used with neat (unmodified) asphalt binder as an alternative to binders modified with virgin polymers, chemical treatments, or both, in a wet process. The results of the study also suggest the viability of hybrid wet polymer-modified binder with dry-process PCR plastic as a greener alternative to binders modified with virgin polymers, chemical treatments, or both. Future studies will be needed to examine additional PCR plastic streams, especially mixed polymer streams.

This study examined the effects of modification of dense-grade asphalt mixtures with chemically engineered crumb rubber (ECR) via the dry process. The ECR mixtures were compared to unmodified and polymer-modified mixtures in various criteria. The study mainly had three objectives: (1) comparison of mixture performance, (2) investigation of warm mix equivalence, i.e., to determine if the chemical coating on the ECR resulted in benefits similar to warm mix additives, and (3) to determine the effects of ECR on mixture draindown. For the first objective, the disk-shaped compact tension test was conducted to determine resistance to thermal cracking, and the Hamburg wheel tracking test was performed to determine resistance to permanent deformation (rutting) and moisture damage. To fulfill the second objective, the mixtures were prepared with warm mix asphalt (WMA) additives wherever necessary, and for the third objective, a stone mastic asphalt (SMA) was employed instead of the dense-grade mixture that was used for the rest of the study. Findings showed that modification with ECR greatly improved resistance to rutting and cracking, indicating that ECR pavements will exhibit longer service life. In terms of warm mix equivalence, a comparison of volumetrics data showed that ECR without WMA additive compacted in a similar manner as a polymer (PG 76-22) mixture containing WMA additive. Additional mix cost savings may be realized with ECR modification because of the ability to avoid the cost of WMA additive for the goal of achieving mix workability and compaction at lower temperatures. Finally, an ECR-SMA mix was used for draindown assessment and was found to pass the draindown test without the need for fibers, nearly matching the draindown resistance of the polymer-WMA mix containing cellulose fibers, thereby presenting another cost-saving avenue for contractors. The study also includes details of three field projects that used ECR and reported excellent field performance.

Rubber modification of asphalt leads to improvement in certain asphalt mixture properties in lab studies, and extension of service life has been reported in a number of field studies. However, modern laboratory cracking tests often produce conflicting results in terms of the expected cracking performance of rubber-modified asphalt mixtures. To better understand cracking/toughening mechanisms in rubber modified asphalt materials, a fracture mechanics based test was devised for various unmodified and modified binder systems (unmodified control, polymer- and rubber-modified at various treatment levels). The results of this testing showed that unmodified and polymer-modified specimens failed in a brittle fashion, while the rubber-modified specimens displayed increasing ductile-type failures as the rubber modification level increased. Analysis of the fractured faces of the specimens under a Scanning Electron Microscope also revealed the presence of characteristic tails and cavities in the rubber-modified specimens associated with a crack-pinning-type toughening mechanism.

This study investigates the lab and field performance of four asphalt mixtures placed in Missouri, USA, containing reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS). Laboratory tests including disk-shaped compact tension fracture (DC(T)), indirect tension (IDT) creep and strength, Illinois flexibility index (I-FIT), indirect tensile cracking test (IDEAL-CT), and the Hamburg wheel tracking test (HWTT) were conducted on field cores and plant-produced, lab-compacted (PPLC) samples. The DC(T), I-FIT, and IDEAL-CT tests showed that all of the mixtures exhibit brittle behaviour and have high cracking potential, while the HWTT indicated that rutting does not appear to be a concern with these mixes. Statistical analysis was performed to examine the capability of the performance tests investigated to distinguish between mixes. The DC(T) exhibited the best ability to distinguish between mixtures, while IDT strength displayed the least ability. Simulations using the ILLI-TC software were conducted to further explore the low temperature cracking behaviour of the mixes. Finally, field data collected using an automatic road analyzer (ARAN) was analyzed to further assess the laboratory performance tests.