Fawaz Kaseer

Characterising the long-term rejuvenating effectiveness of recycling agents on asphalt blends and mixtures with high RAP and RAS contents

Although the use of high reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) contents in asphalt mixtures is desirable for environmental and economic reasons, these mixtures are prone to cracking, ravelling, and other durability-related pavement distresses mainly due to the heavily aged recycled binders. Highway agencies and the asphalt paving industry have been exploring the use of recycling agents (RA) in order to produce these mixtures with desirable performance. This study focused on characterising the long-term rejuvenating effectiveness of RA on asphalt blends and mixtures with high RAP and RAS contents. Materials from two field projects were used to prepare a number of asphalt blends and mixtures with various combinations of base binder, recycled material, and RA. These blends and mixtures were subject to various aging protocols prior to being characterised for their oxidation kinetics, rheological properties, and cracking resistance. The test results indicated that the RA evaluated in this study were effective in partially restoring the properties of recycled materials, but their rejuvenating effectiveness diminished with aging. Nevertheless, the recycled blends and mixtures with RA achieved equivalent or even better rheological properties and cracking resistance than those with an allowable amount of recycled materials per agency specifications but without RA. In addition, adding RA had no significant effect on the oxidation kinetics of the recycled blends, but increased their susceptibility to physical hardening in response to oxidation. Finally, the correlation between laboratory aging protocols for asphalt blends and mixtures were determined; the laboratory long-term oven aging protocols of 5 days at 85°C on compacted specimens and 1 day at 135°C on loose mix yielded binders with equivalent rheological properties to those subjected to rolling thin film oven (RTFO) plus approximately 10 and 40 h of pressure aging vessel (PAV), respectively.

Stiffness Characterization of Asphalt Mixtures with High Recycled Material Content and Recycling Agents

Economic and environmental considerations have prompted the use of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) in asphalt mixtures. However, given the concerns about long-term pavement performance, state departments of transportation (DOTs) tend to limit the quantities of these recycled materials unless certain mixture modifications are made [e.g., use of a softer virgin binder performance grade (PG) and warm-mix asphalt technology, the addition of a recycling agent (RA), or any combination of these modifications]. This study focused on the stiffness characterization of recycled asphalt mixtures with combinations of virgin binder PG, RAP, RAS, and RA. Materials were collected from two field projects in Texas and Indiana, and laboratory specimens were prepared and tested for resilient modulus, and dynamic modulus (|E*|) after short-term oven aging (STOA) and long-term oven aging (LTOA). An RA effectiveness parameter was proposed to quantify the rejuvenating effect of RA, which was defined as the percentage reduction in mixture stiffness for the recycled mixture with RA versus the corresponding control mixture without RA. Furthermore, the |E*| test results were analyzed with a Black Space diagram to discriminate asphalt mixtures with different stiffness and relaxation characteristics. The test results indicated that the incorporation of RA was effective in reducing the stiffness of asphalt mixtures with high recycled material content, but the effectiveness diminished with aging. Moreover, recycled mixtures with a softer and less brittle virgin binder and an RA at a higher dosage showed desirable stiffness and relaxation properties after STOA and LTOA.

Use of the Resilient Modulus Test to Characterize Asphalt Mixtures with Recycled Materials and Recycling Agents

Asphalt mixture stiffness is an important parameter used during pavement design and the evaluation of pavement performance. This study used the resilient modulus (MR) test (ASTM D7369) to measure the stiffness of several types of asphalt mixtures and various specimen types. Five types of asphalt mixtures were studied: virgin mixture without recycled materials; control mixture with recycled asphalt pavement (RAP), recycled asphalt shingles (RAS), and warm-mix asphalt additive; and three other mixtures with RAP, RAS, and different types of recycling agents. The specimen types used in the study were field cores, on-site plant mix laboratory-compacted specimens, and reheated plant mix laboratory-compacted specimens. The objectives of the study were to assess the effect on MR values of (a) smoothness of field core surfaces and (b) measurement angle in field cores and laboratory-compacted specimens. In addition, the study compared the MR results for the various mixture and specimen types. The results indicated that smoothness and measurement angle had no statistically significant effect on MR at the 95% confidence level. However, the test was able to identify statistically significant differences between mixture and specimen types. Finally, a comparison between MR and dynamic modulus (DM) test (AASHTO TP79) results showed equivalent ranking of the mixtures, confirming that MR is a practical alternative to DM testing.

The crossover temperature: significance and application towards engineering balanced recycled binder blends

Increased quantities of asphalt-based recycled materials can be incorporated in asphalt paving mixtures by using recycling agents. Previous studies demonstrated that restoring the required performance grade (PG) by the inclusion of recycling agents may not be sufficient to guarantee long-term durability. In this study the crossover temperature was used to characterise the viscoelastic properties of asphalt binders at the intermediate service temperature range for asphalt pavements. Durability thresholds for crossover temperature with aging were proposed on a trial basis from a correlation to the Glover–Rowe parameter. In combination with high PG, crossover temperature was used to evaluate rheological balance: resistance to early rutting and long-term embrittlement. Practical recommendations are provided in terms of appropriate materials selection (base binder, recycled, binders, and recycling agents) towards engineering long-lasting rejuvenated binder blends with increased quantities of recycled materials. Limitations and additional considerations for extending the rheological balance approach to evaluate polymer-modified materials are also summarised.

Performance of asphalt mixtures with high recycled materials content and recycling agents

ABSTRACT Recycled asphalt mixtures with high amounts of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) can be excessively stiff, brittle, and prone to cracking. The use of recycling agents, or rejuvenators, can significantly reduce mixture stiffness and improve performance, specifically cracking resistance. In this study, the performance of recycled and rejuvenated asphalt mixtures from several field projects, located in different environmental zones across the United States, was evaluated considering various recycling agent dosages determined by the contractors. Field core test results and the visual distress surveys of the field projects demonstrated that using the field recycling agent dosages yielded poor mixture performance. Laboratory test results demonstrated that adding the recycling agent at the dosage to match the continuous high-temperature performance grade (PGH) of the rejuvenated binder blend (virgin/base binder, recycled binder, and recycling agent) to that of the target binder PGH specified based on climate and traffic requirements yielded improved mixture performance. The rejuvenated mixtures at this recycling agent dosage showed significant reduction in stiffness and improved cracking resistance, and facilitated the use of higher quantities of recycled materials, regardless of aging level, while maintaining rutting resistance after short-term aging.