Edith Arámbula-Mercado

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.

Long-term ageing of asphalt mixtures

Ageing of asphalt mixtures occurs during production and construction and continues throughout the service life of the pavement. Although this topic has been studied extensively, recent changes in asphalt mixture components, production parameters, and plant design have raised a need for a comprehensive evaluation that considers the impacts of climate, aggregate type, recycled materials, WMA technology, plant type, and production temperature. In this study, field cores were acquired from seven field projects at construction and several months afterwards, and raw materials were also collected for fabricating laboratory specimens that were long-term oven aged (LTOA) in accordance with selected protocols. The resilient modulus and Hamburg wheel tracking tests were conducted on both specimen types to evaluate the evolution of mixture stiffness and rutting resistance with ageing. The concepts of cumulative degree days and mixture property ratio were proposed to quantify field ageing and its effect on mixture properties. Test results indicated that the LTOA protocols of two weeks at 140°F (60°C) and five days at 185°F (85°C) produced mixtures with equivalent in-service field ageing of 7–12 months and 12–23 months, respectively, depending on climate. Finally, among the factors investigated in the study, WMA technology, recycled materials, and aggregate absorption exhibited a significant effect on the long-term ageing characteristics of asphalt mixtures, while production temperature and plant type had no effect.

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.

Numerical modelling of ravelling in porous friction courses (PFC)

Porous Friction Courses (PFCs) are asphalt mixtures located on top of pavement structures that are characterised for having high permeability properties. Although the use of PFCs provides several environmental and safety benefits, these materials typically present a short service life. This is mainly due to ravelling, a phenomenon defined as the progressive loss of aggregates from the pavement surface. This work uses a Finite Element (FE) model to investigate the potential of ravelling in PFCs. The FE model incorporates realistic geometries of PFCs that were located on top of a pavement structure and subjected to the pass of a moving wheel load. The influence of certain components of the mixture and different traffic and pavement conditions were evaluated using an energy criterion and probabilistic principles. The results suggest that ravelling is mainly a fracture Mode I process, highly influenced by the volumetric properties of the mixture and loading conditions.

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.

Evaluation of Functionality in Porous Friction Courses

Porous friction courses (PFCs) are employed as surface layers primarily because of their functionality (i.e., their high permeability and noise absorption capability). In this study, the functionality of PFCs was evaluated through permeability and noise absorption tests in pavement structures of different thicknesses (i.e., 0.75, 1.5, and 2.5 in.), each at two air void contents (i.e., design and construction). Permeability was evaluated in the laboratory and the field with the Texas Department of Transportation (DOT) permeameter, the National Center of Asphalt Technology (NCAT) permeameter, and the Florida DOT falling head apparatus. Noise absorption was evaluated in the laboratory with the impedance tube. Results showed that, on average, the coefficient of permeability obtained with the Texas DOT permeameter was 67% higher than that obtained with the NCAT permeameter and 93% higher than that obtained with the Florida DOT falling head apparatus. In general, the coefficients of permeability increased with an increase in layer thickness. The noise absorption coefficient was not sensitive to thickness, but all values represented significant noise reduction. A permeability model on the basis of the Kozeny–Carman equation also was used to predict permeability values and relate them to the laboratory values obtained at different thicknesses. The equation accurately predicted the permeability coefficients of the thick PFC layers. The results suggested that minimum and maximum functionality requirements should be set to guarantee adequate performance.

Evolution of the Surface Performance-Graded Specification for Chip Seal Binders

Over the past 15 years, a surface performance-graded (SPG) specification for chip seal binders has been developed by the Texas Department of Transportation and has been validated with laboratory measurements and the visual field performance of 120 highway sections. The SPG specification was established in an effort to extend the service life of chip seals by providing a binder grading system and an associated selection method that (a) accounted for differences in climate and (b) used existing equipment and performance-based properties that precluded bleeding and aggregate loss in the critical first year of service after construction. A multiyear implementation effort of this specification is ongoing. This paper describes the motivation and evolution of the SPG specification, including a summary of the validation effort and a round robin testing program with the Texas Department of Transportation and suppliers of chip seal binders. Binder selection guidelines that use this specification are provided.

Effect of laboratory foamer on asphalt foaming characteristics and foamed mixture properties

Abstract In the United States, mechanical foaming is the most popular method for producing warm mix asphalt, which is the latest technology implemented to reduce the production temperature and/or enhance the compactability of asphalt mixtures. Three commonly used commercially available laboratory foamers to produce asphalt foams include the Wirtgen WLB 10S (Wirtgen foamer), the InstroTek Accufoamer (InstroTek foamer) and the Pavement Technology Inc. Foamer (PTI foamer). Though these foamers have been widely used in research studies and construction practice, it is still unknown whether they produce asphalt foams with the same quality and quantity. In this study, asphalt foaming characteristics produced by these three laboratory foamers were measured using a non-contact test set-up consisting of a laser device and a digital camera, and compared in terms of instantaneous volume expansion, foam stability and surface area evolution of foam bubbles. Additionally, the workability, coatability and mechanical performance of foamed mixtures prepared using these same laboratory foamers were compared against the conventional hot mix asphalt (HMA). Test results indicated that foamed asphalts produced by the Wirtgen foamer had the largest volume expansion and greatest foam stability, followed by those produced by the InstroTek foamer and the PTI foamer. The optimum foaming water content (Wopt) was determined for each laboratory foamer based on the workability and coatability results of the corresponding foamed mixtures. In addition, the performance evaluation of the foamed mixtures produced at Wopt values indicated equivalent mixture stiffness but greater moisture susceptibility as compared to the conventional HMA.

Mix design procedure for foamed asphalt mixtures

Mechanical foaming is the most popular method for producing warm mix asphalt in the United States; yet, a standard mix design procedure for foamed asphalt mixtures has not been established. Currently, the design of foamed asphalt mixtures consists of determining the optimum binder content by following a standard hot mix asphalt (HMA) design procedure and then estimating the foaming water content based on the foamer manufacturers recommendation, engineering judgement, or previous experience. This practice fails to account for the effect of foaming on binder and mixture properties, and therefore may not ensure desirable foamed mixture performance. To overcome this shortcoming, additions to the standard mix design procedure were proposed in this study in order to accommodate foamed asphalt mixtures, which included optimisation of foaming water content and evaluation of foamed mixture performance. In addition, the proposed procedure was verified using materials procured from two field projects. The optimum foaming water content (Wopt%) was first determined by comparing the workability results of foamed mixtures at various foaming water contents. Afterwards, the foamed mixture at Wopt% was tested for coatability and performance evaluation, and all laboratory test parameters complied with established American Association of State Highway and Transportation Officials or agency specifications or were equivalent to or better than the control HMA test results. Therefore, the proposed foamed mix design procedure seemed a viable alternative for designing foamed mixtures with adequate pavement performance.