Search

Cracking in asphalt pavements is the net result of fracture and healing. The ability to accurately measure and predict the appearance of cracking depends on being able to determine the material properties that govern the rate of development of these two contrary aspects of cracking. This study is devoted to identifying the datum material properties in undamaged samples. It will make use of viscoelastic formulations and of well-known mechanics concepts the way in which these properties are altered by the composition of the mixture. Also introduced in this study is a process that makes extensive use of the pseudo-strain concept in decomposing the strain components. One of the many benefits of this approach is the ability to measure the fatigue endurance limit of an asphalt mixture with a simple test that requires only half an hour. The study begins with a detailed discussion of these concepts and properties and the test methods that simply and accurately measure them. One of the great advantages of using mechanics is that it provides a rapid and efficient way to predict the rate of appearance of the two aspects of pavement cracking, fracture and healing. Mechanics requires the use of material properties. An accurate and efficient determination of undamaged material properties is fundamental and important to the prediction of the performance of asphalt mixtures. It is found that the undamaged properties of an asphalt mixture are different when they are loaded in tension or in compression and this distinction is important. This study addresses the efficiency of the laboratory testing methods, and the effects of the volumetric material components and environmental factors such as temperature and ageing on the undamaged material properties. It also introduces the non-destructive tests that must be made in order, subsequently, to measure the damaged properties of the same materials. [ABSTRACT FROM AUTHOR]

Fatigue cracking is a common form of pavement failure and is strongly tied to the properties of the asphalt binder used in the mix. The Superpave grading procedure is currently used in the United States and in other parts of the world to classify and select asphalt binders for the design of asphalt mixtures. The parameter, , in the Superpave grading system is used to control the quality of the binder at intermediate temperatures. It is also used by many as a metric to measure the inherent fatigue cracking resistance of an asphalt binder. It is widely recognised that while this parameter was a significant improvement over previous binder grading and selection methods, it does not accurately measure the inherent fatigue damage resistance of asphalt binders. This paper provides a review of several studies that have attempted to develop methods to measure the fatigue damage resistance of asphalt binders during the last two decades. The studies reviewed in this paper have been classified into four main categories based on the type of test method employed. The four categories include: time sweep performed using the Dynamic Shear Rheometer (DSR), amplitude sweep using the DSR or its variations, ductility-based test methods and strength-based tests. This paper focuses primarily on tests developed in the United States with an emphasis to improve the existing Superpave criteria along with relevant research from other parts of the world. [ABSTRACT FROM AUTHOR]

This study presents a mechanistic pavement modelling approach to predict the performance and damage characteristics of composite pavements at low-temperature conditions. To meet the research objective, laboratory tests were incorporated with mechanistic finite element modelling. A typical composite pavement structure where an asphalt overlay is placed on cement concrete layer was selected and modelled by considering environmental conditions and paving materials of individual layers. Thermally induced reflective cracking of asphalt overlay was predicted and analysed by conducting finite element simulations incorporated with cohesive zone fracture. Parametric analyses were also conducted by varying pavement geometry and material properties, which could lead to helping pavement designers and materials engineers understand the mechanical sensitivity of design variables on the overall responses and performance characteristics of pavement structures. This better understanding is expected to provide roadway engineers with more scientific insights into how to select paving materials in a more engineered way and to potentially advance the current structural pavement design practices. [ABSTRACT FROM AUTHOR]

The viscoelastic continuum damage (VECD) theory has been gaining wide acceptance throughout the world as a promising method for characterisation of fatigue cracking susceptibility of asphalt mixtures. This method is also being explored by different countries (e.g. Brazil) to be a part of pavement design and to be used as a component of performance-related specifications. While the VECD is a novel methodology, over the past 20 years, different versions of the formulations have been developed by various researchers. This paper includes history and the basic principles the most common VECD methodologies used today. A thorough review of the formulations as well as the main assumptions of different VECD models is given. In addition, a step-by-step guidance on calibration of VECD model and potential problems related to VECD modelling are presented. Finally, uniaxial tension-compression fatigue test results of six different mixtures (with varying binder PGs, recycled asphalt pavement contents and aggregate gradations) were analysed using three most common VECD formulations. The damage characteristic curves (i.e. C vs. S curves) produced by the different VECD methods as well as predicted number of cycles to failure (Nf) were compared and analysed. [ABSTRACT FROM AUTHOR]

Cracking of flexible pavements, particularly top-down cracking, is the predominant distress in Florida. However, pavement failures due to cracking have reduced by ten per cent over the last ten years. This improvement in cracking resistance is primarily due to a focused and multifaceted research effort that includes laboratory studies, field test sections, and accelerated pavement testing. Most research efforts can be grouped into three main categories: asphalt binder modification, reflection cracking mitigation, and tack coat application rates and type. This paper presents the findings of these research studies and the strategies the Florida Department of Transportation has implemented to improve cracking resistance of flexible pavements. [ABSTRACT FROM AUTHOR]

Cracking in asphalt concrete (AC) layers is among the driving modes of flexible pavement deterioration. Material properties, along with structural layer characteristics and external factors such as vehicular and environmental loading, govern cracking mechanisms in flexible pavements. Current practice of AC and pavement structural design methods does not allow for direct characterisation of AC impact on crack damage initiation and propagation. Asphalt concrete performance tests are the essential components of performance-related and performance-based specifications, for which they can be used either for screening AC to minimise deterioration risks or to directly predict specific modes of pavement deterioration. This paper evaluates commonly used performance tests intended to characterise AC fracture resistance. A multistep evaluation protocol was applied using various statistical techniques to determine the discrimination ability of each test method outcome criterion as well as strength of correlation to field performance. The results from full-scale accelerated pavement test (APT) sections were used to correlate with the results obtained from various laboratory performance tests. Mixes used in the study contained various levels of reclaimed asphalt pavement and recycled asphalt shingles with a recycled binder ratio up to 40%. It was shown that the flexibility index parameter obtained from the Illinois semi-circular bending (IL-SCB) test, also referred to as the Illinois Flexibility Index Test (I-FIT), resulted in the highest number of subsets and consistent ranking with APT field performance, indicating a high discrimination potential. Even though two of the overlay test's criteria resulted in fewer subsets, they also provided a consistent ranking with APT field performance. Composite scores indicate that the overlay test criteria and flexibility index from the I-FIT had the highest strength of correlation to field performance. [ABSTRACT FROM AUTHOR]

Asphalt binder represents the main constituent of asphalt paving mixtures. The functional performance and durability of asphalt pavements strongly depend on the rheological and mechanical properties of the binder. Asphalt binder is a highly temperature susceptible material that becomes brittle and more prone to failure at low temperature, which may ultimately result in thermal cracking at the mixture level. Significant effort was devoted over the years to reduce the occurrence of this distress. For this purpose, a number of experimental methods were developed to characterise asphalt binders at low temperature. At the same time, different models were formulated to better understand and predict the material behaviour while extrapolating more information from the testing measurements. In this paper, a comprehensive review on the behaviour of asphalt binders at low temperature is presented. First, a synthetic introduction and background on the development of the criteria for asphalt binder low-temperature behaviour are provided. Then, the current testing procedures for characterising asphalt binders are presented and the validity of the current PG specification is discussed. Finally, the most significant modelling developments are reviewed. [ABSTRACT FROM AUTHOR]

This paper addresses the measurement and modelling of the damaged properties of asphalt mixtures including the fracture, healing and viscoplastic deformation of the asphalt mixtures in both tensile and compressive loading as being affected by their composition and conditioning with ageing and exposure to temperature and moisture. An energy-based mechanics is applied to obtain the material fundamental properties such as surface energies, bond energies, anisotropy, yield functions and plastic potential functions that are valid for actual asphalt mixtures, viscoelastic crack growth criteria under both tensile and compressive loading, a simple mechanics-based method of determining the fatigue endurance limit, and the measurement and prediction of healing in restoring the damage done by fracture. Healing is antifracture and cracking is the net result of the interplay of these two complimentary mechanisms. Because fracture in asphalt mixtures is not the growth of a single crack but the simultaneous growth of multiple cracks that start out as air voids, this fact leads to the use of the growth of damage density to characterise fracture in an asphalt mixture. It was discovered that the form of Paris' law applies to the growth of damage density of asphalt mixtures in both tensile and compressive loadings. The importance of this fact lies in many developments from this discovery, e.g. compressive monotonic loading of cylindrical test samples permits a direct determination of the Paris' Law coefficient and exponent. In all cases, measured material properties are presented as they vary with mixture composition and with conditioning such as moisture and ageing, both in the lab and in the field. The measurements of these properties are made simply, quickly and accurately by the use of mechanics so that an entire characterisation of the properties of an asphalt mixture in tension and compression can be completed in the space of one day. The net effect is to reduce the efforts expended in the lab and the systematic error due to the assumptions made by the existing models and simultaneously to increase the efficiency and cost-effectiveness of materials testing and raise the reliability of the design of mixtures, pavement structures and specifications and the prediction of the life cycles in as-built pavements. [ABSTRACT FROM AUTHOR]

Crack distress causes asphalt pavement failure under repeated traffic loading. The objective of this study is to investigate the fracture properties of asphalt concrete through three dimensional (3D) numerical simulation using finite element method (FEM). The parameters and datum utilised for finite element models were captured from a grey matrix, which can be converted from digital images scanned by an X-ray computed tomography (CT) apparatus. Afterwards, the uniaxial compression test was simulated using FEM and was also conducted in laboratory to study the stress distribution and to evaluate the degree of damage during the loading process. Eighteen cylindrical asphalt concrete specimens were prepared with three different types of gradations: asphalt concrete, stone matrix asphalt and open-graded friction course. The uniaxial compression test was carried out on these specimens. Before and after each step, the specimens were scanned using X-ray CT to analyse the crack growth behaviour inside the asphalt concrete and to investigate the effect of gradation on the fracture characteristics. The degree of fracture can be evaluated by data analysis from the grey intensity histogram. The evaluation on asphalt concrete crack properties based on FEM with X-ray CT imaging was found to be practicable and reproducible. Additionally, the gradation was found to be an influential factor affecting the crack propagation of asphalt concrete. [ABSTRACT FROM AUTHOR]

Mechanism of fracture in a viscoelastic heterogeneous composite with thermo-rheological properties such as, asphalt mixture is quite involved and cannot be correctly simulated with simpler linear elastic fracture mechanics constitutive laws. Over the last decade and half, a number of researchers have adopted use of cohesive zone (CZ) fracture models for simulation of fracture in asphalt mixtures. CZ interface elements are utilised in finite element (FE) models for representation of crack path, these elements follow traction- displacement relationships that allow for gradually degrading traction capabilities along the crack path with increasing level of crack opening. This paper presents a review of CZ modelling approach for simulation of asphalt pavement and overlay cracking performances. Suitability of CZ modelling approach for capturing discrete fracture in asphalt mixtures at low temperatures is presented through simulation of lab scale test. An example is also presented to demonstrate applicability of CZ-based modelling effort in capturing the crack initiation and propagation in asphalt mixtures at low temperatures. Thereafter, an FE-based pavement simulation approach is discussed that can be utilised in design of asphalt overlays to lower the propensity of reflective cracking. A case study of designing asphalt overlay systems for four real-life pavements in Minnesota is presented to demonstrate the applicability of the CZ-based modelling approach in conducting mechanistic design of asphalt overlays. [ABSTRACT FROM AUTHOR]