Significant increases in the cost of asphalt paving and increased awareness of the need for sustainable infrastructure in recent years have in turn increased the use of recycled asphalt pavement (RAP) in the manufacture of hot-mix asphalt (HMA). The use of RAP reduces the overall cost of HMA and provides significant environmental benefits. Experience has shown, however, that the addition of RAP to HMA can have a negative effect on the low-temperature fracture characteristics of the pavement. The purpose of this study was to determine the effects of RAP amounts on the low-temperature cracking performance of asphalt mixtures. Different percentages of RAP material, ranging from 0% to 50%, were studied. The embrittlement temperature of mixtures was determined with the use of an acoustic emissions technique. The disk-shaped compact tension [DC(T)] test was used to determine the fracture energy of asphalt mixtures. DC(T) fracture tests were conducted on two control mixtures with no RAP and mixtures that contained 10%, 20%, 30%, 40%, and 50% RAP. Both control and RAP mixtures were manufactured with PG 64-22 and PG 58-28 as the virgin binders, which brought the total number of mixtures tested to 12. In addition to DC(T) fracture testing, indirect tensile testing was conducted on HMA specimens that contained 20% and 40% RAP. Test results clearly indicated the effects of the presence of RAP materials on the low-temperature performance of mixtures. This study demonstrates the benefit of performing fracture tests before RAP is added to the asphalt mixture, and it demonstrates the use of an acoustic emissions-based testing procedure to screen mixtures susceptible to cracking at low temperatures.

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.

Reflective cracking of asphalt concrete (AC) overlays is one of the most extensive pavement distress and damage mechanisms in composite pavement structures. Numerous studies have been performed to evaluate the reflective cracking potential of AC overlays under different loading scenarios. Most of these studies have focused on reflective cracking due to tyre loading. A very limited amount of work has been performed to evaluate non-load-associated thermal reflective cracking of overlays. Thermal reflective cracking mechanisms have been studied and are described in this paper using recently developed hot-mix asphalt mixture tests and fracture models. A series of finite-element-based pavement simulations were performed in an effort to better understand thermal reflective cracking mechanisms as a function of several key material and pavement structure variables. The enhanced integrated climatic model was used to estimate pavement temperature gradients as a function of position and time. A fracture mechanics-based cohesive fracture model was used for the simulation of damage and cracking, which was tailored for use with quasi-brittle materials such as AC. The pavement simulation model utilises creep and fracture properties from American Association of State Highway and Transportation Officials and American Society for Testing and Materials-specified tests and analysis procedures. Three asphalt mixtures manufactured with Superpave low-temperature performance grades of -22, -28 and -34 were studied in pavement structures with three distinct overlay thicknesses. Simulations were conducted with three Portland cement concrete (PCC) slab conditions to study the effects of joint spacing and rubblisation on thermal reflective cracking. The simulation results provide a new insight towards the mechanisms underlying the development of thermal reflective cracking. The curling of PCC slabs due to temperature differential and joint opening caused by pavement cooling was found to be critical in the initiation of thermal reflective cracking. This effect is greatly minimised or eliminated in the case of pavement rubblisation.

The use of recycled asphalt (RA) materials in pavement rehabilitation processes is continuously\ud increasing as recycling techniques, such as cold recycling (CR), are being utilised in increasing\ud magnitude and greater awareness for use of recycled materials and consideration of sustainable\ud practices is becoming common in the construction industry. The focus of this paper is on\ud developing a state of the art and state of the practice summary of processes used for classification\ud of RA as well as the curing and specimen preparation practices for cold-recycled asphalt\ud mixtures. A variety of topics were explored through an exhaustive literature search, these\ud include RA production methods, definition of RA materials, stockpiling practices, industrial\ud operations, specimen curing and preparation practices and in-field evaluation of cold-recycled\ud rehabilitation. This paper was developed through efforts of CR task group (TG6) of RILEM\ud Technical Committee on Testing and Characterization of Sustainable Innovative Bituminous\ud Materials and Systems (TC-237 SIB)

Aging can significantly affect the viscoelastic properties and cracking behavior of asphalt mixtures, causing increase in stiffness, reduction in relaxation capability, and increase in brittleness. Eleven mixtures are evaluated using different laboratory conditioning protocols to evaluate how the properties of asphalt mixtures, including viscoelastic properties, fatigue, and fracture behavior will change over time. Comparisons between different aging levels and mixtures are conducted by using complex modulus (E*) (field cores are included), simplified viscoelastic continuum damage (S-VECD) approach, semi-circular bending (SCB), and disk shaped compact tension (DCT) fracture tests. The climatic aging index developed by the NCHRP 09-54 project is utilized in this study to calculate the appropriate field aging duration corresponding to the different laboratory aging protocols. Pavement evaluation tools FlexPAVE^TM and IlliTC are also used to predict and compare the fatigue and thermal cracking performance of these mixtures. The results of E* and S-VECD tests indicate that the mixtures are more prone to fatigue cracking with aging, and the two long-term conditioning protocols induce statistically similar changes in linear viscoelastic and fatigue properties. However, prediction of fatigue performance from FlexPAVE ^TM does not show a consistent trend once pavement structure and traffic are considered. Fracture tests and IlliTC predictions show the virgin mixtures and those with soft base binders will have better capability to resist cracking after long-term aging. In this study, the two mixtures with the largest difference between high and low temperature performance grade (PG) show the largest change in fracture and fatigue properties with aging.