Reyhaneh Rahbar-Rastegar

Comprehensive evaluation of low-temperature fracture indices for asphalt mixtures

For performance-based specifications and design processes, a number of cracking-related index parameters have been proposed for asphalt mixtures in recent years. A number of these parameters have been developed to utilise results from fracture tests. This study conducted a comprehensive evaluation of various fracture index parameters including fracture energy, Illinois flexibility index, stress intensity factor and toughness index. Over 200 tests from 61 distinct test data sets representing 21 asphalt mixtures are included. The focus of this study is on low-temperature cracking and all indices were evaluated using test results from the disk-shaped compact tension fracture tests conducted at low temperatures. The objective of this study was to determine if there is a relationship between various fracture index parameters as well as to determine typical measurement variability associated with each parameter. Comparisons were made between different indices and correlations were determined for the mix rankings provided by individual indices. The results indicate that fracture energy successfully captured the mix rankings and showed a strong correlation with other indices. In order to better capture post-peak softening behaviour of asphalt mixtures, the flexibility and toughness indices have been utilised; however, these parameters were found to have high variability. A new index called fracture strain tolerance has been proposed that was shown to provide the same level of distinction between mixtures as the flexibility and toughness indices while having considerably lower variability. Finally, several areas were identified for future extension of this research.

Laboratory versus plant production: impact of material properties and performance for RAP and RAS mixtures

ABSTRACT Agencies are moving towards performance-based design methodologies for asphalt pavements, and different methods to evaluate the asphalt performance in the laboratory have been developed. The laboratory performance can be evaluated at the mix design and/or production stages. A good understanding of differences in the behaviour of mixtures produced in the laboratory and plant is required to assess anticipated field performance at the mix design stage. The objectives of this paper are to compare the measured properties of plant-produced and laboratory-produced mixtures, to evaluate the effect of mixture variables on the differences observed, and to translate these to anticipated differences in fatigue performance through pavement evaluation using a linear viscoelastic layered analysis. In this study, 11 plant mixed, plant compacted, and their corresponding laboratory-mixed, laboratory-compacted mixtures are evaluated through binder and mixture testing. Mixture variables include aggregate gradation, binder grade and source, and recycled materials’ type and content. Performance grading on extracted and recovered binders, and complex modulus and SVECD fatigue testing on mixtures were conducted, and fatigue life was predicted using layered viscoelastic pavement design for critical distresses software. Most of the results show the laboratory mixtures are generally stiffer than the plant mixtures, but there is no constant shift for all mixtures. Larger differences are observed for the 19 mm and PG 58-28 mixtures and binder source appears to influence the differences as well. Different plants result in different effects on the properties of plant and lab-produced mixtures. This study provides a unique set of data that expands understanding of differences between laboratory and plant production of asphalt mixtures.

Evaluation of Viscoelastic and Fracture Properties of Asphalt Mixtures with Long-Term Laboratory Conditioning

Aging affects the properties of asphalt mixtures in different ways; increase of stiffness, decrease of relaxation capability, and the increase of brittleness, resulting in changes in cracking behavior of asphalt mixtures. In this study, ten plant-produced, lab-compacted mixtures with various compositions (recycled materials, binder grades, binder source, and nominal maximum aggregate size) are evaluated at different long-term aging levels (24 hours at 135°C, 5 days at 95°C, and 12 days at 95°C on loose mix and 5 days at 85°C on compacted specimens). The asphalt mixture linear viscoelastic properties (|E*| and δ) and master curve shape parameters measured from complex modulus testing and fracture properties (measured from disc-shaped compact tension and semi-circular bending fracture testing) are compared at different levels of aging. The results indicate that the mixture exposure time to aging is proportional to the dynamic modulus and phase angle changes. Generally, the fracture parameters of mixtures become worse when aging level changes from 5 to 12 days aging. In spite of the similar viscoelastic properties, the mixtures with 24 hours at 135°C and 12 days at 95°C aging do not show similar fracture parameters.

Comparison of asphalt binder and mixture cracking parameters

Cracking is one of the most prevalent types of distresses in asphalt pavements. There are different cracking index parameters that are determined from tests conducted on binders and mixtures to assess cracking potential. The objective of this study is to compare binder and mixture parameters and evaluate the similarities and differences between the rankings and values obtained. This study includes binder and mixture testing on 14 plant-produced mixtures including 3 different binder grades, 3 binder sources, 3 aggregate gradations, and mixtures containing a range of reclaimed asphalt pavement and/or recycled asphalt shingles contents. Testing included PG grading and 4 mm dynamic shear rheometer testing on the extracted and recovered binders that were long-term aged. Mixture testing included complex modulus, simplified viscoelastic continuum damage (SVECD) fatigue, and disc-shaped compact tension testing on short-term-aged mixtures. Parameters evaluated included high and low PG temperatures, ΔTcr, Glover–Rowe parameter (binder and mix-based), R value, dynamic modulus, phase angle, number of cycles to failure from SVECD and layered viscoelastic critical distresses analysis, and fracture energy. The results show that generally the binder parameters correlate well with each other but the mixture parameters do not. Good correlation was observed between binder and mixture stiffness-based parameters, but there was generally low correlation observed between binder and mixture cracking parameters for the mixtures evaluated in this study, possibly a result of differences in ageing level. Recommended future work includes non-linear statistical analysis, incorporation of field performance, and testing on long-term-aged mixtures.

Identifying Indicators for Fatigue Cracking in Hot-Mix Asphalt Pavements Using Viscoelastic Continuum Damage Principles

A critical distress in asphalt concrete pavements is fatigue cracking, which results in decreased ride quality and fuel economy, and provides an avenue for water intrusion, which causes a pavement system to deteriorate rapidly. Given the poor state of the infrastructure network, changes are needed in the current mixture design process to promote innovation and alternative approaches to production. This study addressed this need by pursuing the following objectives: (a) relate mixture stiffness, fatigue, and pavement system characteristics for performance-based mixture design; (b) identify a simplified viscoelastic continuum damage (S-VECD) output parameter that most clearly distinguishes between poor and satisfactory performance when combined with dynamic modulus information; and (c) evaluate the impact of recycled materials on performance indicators for fatigue cracking. The results show that a pavement structure selection process related to the S-VECD failure criterion produces better performance predictions than does a stiffness-based approach. Promising correlations with performance exist for the pseudostiffness at failure and storage modulus for an Interstate pavement structure, phase angle for a state highway surface and base course, and model term alpha for the same state highway base course.

Mixture and Production Parameters Affecting Cracking Performance of Mixtures with RAP and RAS

Many methods and approaches have been developed to evaluate the asphalt performance in the laboratory. The laboratory testing can be conducted on mix design (lab produced) and QA/QC (plant produced) stages. To predict the asphalt field performance, an understanding of differences between the behavior of lab and plant produced mixtures is required. There are many factors in the design and production of recycled mixtures that can potentially impact the performance of the in-place material. The project involves testing of both laboratory and plant produced mixtures containing 20–30 % recycled materials (RAP and RAS) by total weight. The mixtures have different gradations (12.5 and 19 mm NMSA), and contain different virgin binder PG grade (PG 58-28, and PG 52-34). The characteristic damage curves and failure criterion diagrams of mixtures are developed based on fatigue testing following the simplified viscoelastic continuum damage (SVECD) protocols. The results show that the most of lab produced mixtures have better fatigue properties in damage characteristics, but they do not follow a consistent trend in fatigue life.

Simulating plant produced material in the laboratory to replicate rheological and fatigue properties

As part of an effort by agencies and industry to move towards performance-based design to evaluate mixtures in the laboratory at a smaller scale before moving to full scale operation, laboratory protocols exist to simulate the aging that occurs as a material is produced. However, recent research has shown that these existing protocols may not accurately represent the changes a material experiences in a plant. Moreover, due to the focus of previous studies on the ability of the current method to replicate mixture characteristics and performance in an undamaged state, there is a lack of information as it relates to the damaged state. This paper presents a concise description of a study undertaken on a particular mixture to evaluate the differences in the behaviour of a standard asphalt concrete mixture produced in the laboratory and in the plant to assess the anticipated field performance at the mixture design stage. The results, in terms of the rheological properties of binders extracted and recovered from laboratory and plant produced mixtures as well as rheological, repeated cyclic fatigue, and cracking performance evaluation of the asphalt mixtures, have shown the ability of a short-term oven aging protocol to replicate plant produced material in the laboratory.