Overview
The emphasis in flexible pavement design in recent years has been on the development of new mechanistic-based pavement design systems. The successful development of such a system depends on the ability to measure and describe fundamental material properties and the ability to predict how these properties affect the performance of the pavement structure. This research project aimed at defining new procedures for developing micro-mechanical models and identifying fundamental mechanical properties of road materials, including unbounded mixtures used in sub-base layers and bituminous mixtures used in base course, binder and surface layers.
The benefits of an improved understanding of the mechanism of failure in road materials include improved analysis and design formulations and an improved framework for optimising the fracture resistance of flexible pavements during the material design stage. Identifying adequate road materials cracking/fatigue properties is essential to determine whether the induced pavement response is critical enough to result in failure under one or more load applications. Several testing modes can generally be used to obtain these properties, but if the property is not fundamental, then different testing modes and specimen geometries may yield different results. Fundamental properties of road materials can be obtained from multiple testing configurations when appropriate test procedures, measurement systems and analytical methods are used.
The service life of a road pavement is related to the characteristics of employed materials. In particular, its evaluation requires, during the design stage, the knowledge of the mechanical parameters for each pavement layer. At the same time, such values can be defined only if the traffic properties (e.g. speed, volume and axle loads) and climatic conditions are predicted for the service life. More precisely, a wide scientific background exists in the field of asphalt pavements that confirms the great interest regarding the evaluation of the relationships among fundamental mechanical properties, temperature and loading time.
The support provided by the development of new experimental techniques is essential, but not totally exploited. This allows for perspectives of innovation, from theoretical to methodological points of view, with the purpose of implementing and validating mechanical models. The analysis of the mechanical behaviour is traditionally based on experimental tests, conven
The objective of this project was to provide insight into key mechanisms and road materials properties that control failure in flexible pavements.
Specific objectives of the project were:
- Develop an imaged-based (non contact, full-field) surface displacement/strain measurement technique to more accurately capture localised or non-uniform stress distributions in bituminous materials and as a tool for detecting first fracture.
- Indentify fundamental road materials failure limits and verify the independence of these limits from specimen geometries and test configurations.
- Identify a suitable non-linear damage law that properly describes the failure/fatigue mechanism of road materials under generalised loading conditions.
- Provide recommendations for the development of a failure prediction model for flexible pavements.
The scientific framework of the project is represented by the theoretical-experimental inputs given so far by the research developments in both image processing and road material behaviour interpretation fields.
Three coordinated research units were focused on three different specific topics:
- Bituminous Mixtures Damage Interpretation
- Unbound Mixtures Damage Interpretation
- Image Analysis System Development.
The tasks in the project of the Research Unit n.1 - Bituminous Mixtures were as follows:
- Characterisation of materials:
- rheological characterisation of three different binders (one virgin and two modified) using the Dynamic Shear Rheometer (DSR) and the Bending Beam Rheometer (BBR) at temperatures ranging from -20°C to 80°C and at three different aging conditions (RTFOT, Pressure Aging Vessel, PAV)
- aggregate characterisation of two different kind of batches according to the Specifications of the National Research Council (CNR) and to the SUPERPAVE mix design approach
- adhesion between aggregates and binders using the Volumetric Expansion Test (VET) and an innovative procedure recently developed at the Politecnico of Torino (2002)
- Aggregates and asphalt binders were mixed to produce twelve different asphalt mixtures (3 binders x 2 aggregates x 2 levels of compaction). These mixtures were characterised according to the SUPERPAVE procedure, as well as to the Technical Specifications for Highway Works (Performance-Based Specification of the Ministry of Infrastructures and Transportation, developed by the Experimental Inter-University Road Research Centre, CIRS).
- Fatigue/Failure Characterisation of the 12 mixtures using the following test configurations: indirect tension test (IDT), semi-circular bending test (SCB), three-point bending beam test (3PB). The IDT and SCB specimens were compacted using the Superpave Gyratory Compactor. The slabs were by a proper heavy compactor made up of a cylindrical horizontally-pivoted steel cup. The tests were performed at three different temperatures (5°C, 10°C and 25°C). The respective fatigue curves in Wholer's plane (log N - log e) were plotted.
- Data Interpretation, two approaches were used:
- The first approach was based on energy principles in which d
Funding
Results
The key results can be summarised as follows:
- Fundamental road material properties can be identified for generalised loading condition and regardless of test configuration and specimen geometry when rigorous interpretation of test condition and appropriate analysis techniques are used.
- The Image Analysis System is suitable for road materials full-field strain investigations. It has been shown to overcome the shortcomings of traditional on-specimen strain measurement devices, such as strain gauges. Importantly, the new system achieves satisfactory accuracy compared to strain gauges which is important for the investigation of fracture. Moreover, the new image correlation system provides point-wise analysis, allowing for the exact determination of the location of crack initiation, and also for the calculation of strain values at the instance of crack initiation. Finally, the new DIC system is a non contact measurement tool, thus further minimising potential errors associated with on-specimen measurements.
- A failure prediction model for flexible pavements can be identified and developed when fundamental material properties are employed on the basis of a non-linear damage law that properly describes the failure/fatigue mechanism of road materials under generalised loading conditions.
Technical Implications
- The Image-based technique should be enhanced implementing the superficial information achievable using common digital cameras with X-Ray Computed Tomography internal measurements.
- For a comprehensive understanding of the key mechanisms that control failure and fracture in road materials, it should be essential to develop 3D microstructural models. In details, the development of new numerical models obtained by geometric reconstructions of the material structure for identifying fundamental physical parameters which control its mechanical behaviour.
- As a support for the mechanical interpretation of the measurements, further work should include the development of theoretical studies and numerical models based on non-continuum schematisations. Non-continuum models allow for a 'low scale modelling' of the material, enabling the estimation of local and pre-existing micro-cracks phenomena and the quantification of their effect at a meso-scale.
Policy implications
No recommendations were made for further action in a transport policy context since the research project was focused on enhancing flexible pavement design by identifying the road materials key parameters that control failure in asphalt pavements, rather than on developing methodologies directly pertaining to the transportation system.