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Mechanisms of Cracking and Debonding in Asphalt and Composite Pavements
Technical Committee 241-MCD
General InformationChair: Prof. William G. BUTTLAR
Secretary: Dr. Armelle CHABOT
Activity starting in: 2011
This RILEM Technical Committee will conduct scholarly activities geared towards developing a deeper fundamental understanding of the mechanisms behind cracking and debonding in asphalt concrete and composite (e.g., asphalt overlays placed on PCC) pavement systems. Although most modern pavement design methods do not directly account for the presence of cracking and debonding in asphalt and composite pavements, many of the critical modes of pavement deterioration involve one or both of these mechanisms. These modes include: thermal cracking, reflective cracking, block cracking, top-down cracking, and fatigue cracking. Before these mechanisms can be directly accounted for in design, they must first be properly characterized, measured, modelled, etc. Some of the complexities involved in the proposed subject area include but are not limited to: nucleation, initiation, and propagation of cracks in a heterogeneous, time- and temperature- dependent material; aging and thixotropic effects during service life and the resulting spatial gradation of properties; effects of moisture and freeze-thaw cycling on fracture and interface behaviour; complexity of damage and cracking behaviour at a material interface, and; modelling and experimental complexity of damage and cracking under repeated vehicular loads and diurnal temperature variations.
Obviously, a comprehensive investigation of all of these mechanisms would be impossible to accomplish in a five-year time frame. Thus, the activities of the committee will first be focussed on the study of discrete cracks, developing and propagating under monotonic loading under constant temperature, material age, and without material property gradients. In addition, simple, yet common material interfaces will be considered: to include lift interfaces between adjacent asphalt concrete layers, and asphalt-PCC interfaces. Interfaces involving geosynthetics or reinforcing materials (grids, steel netting, etc.), are being investigated in another RILEM committee and will not be considered herein. Depending upon the progress made in the early stages of the TC, cracking under cyclic loads may be added into the scope of the investigation.
Even with the careful limitation of committee scope, the proposed areas of investigation will require a comprehensive and well-coordinated research effort. To this end, we propose three interrelated technical groups (TGs), where each TG will involve an integration of testing, modelling, and connection to related field studies.
Terms of reference
The activities of the new TC are proposed to be conducted over a 5-year time period. In order to achieve its goals, the new TC intends to perform its work within three task groups (TG): TG1 - Cracking in Asphalt Material; TG2 - Interface Debonding Behavior, and; TG3 - Advanced Measurement Systems for Crack Characterization. These topics are preliminary and subject to change upon discussion and debate amongst committee members during the first few meetings. As a starting point, procedures and data produced under TC 210 CAP, TC 206 ATB and the new TC-SIB will be reviewed and incorporated into the new TC to the extent possible.
Detailed working programme
To the extent possible, each TG will take an integrated approach, whereby testing, modeling, and the use of field data and associated materials will be simultaneously considered. It is currently proposed that TG1 will be dedicated to the development of tests and models to study the mechanisms of cracking in asphalt bulk material. TG2 will devote its efforts to understand and predict the mechanisms of debonding between two adjacent pavement layers, e.g., asphalt-asphalt or asphalt-PCC [Portland cement concrete] interfaces. The scope TG2 will be carefully developed to minimize overlap with TG4 of TC-SIB, which deals with pavement reinforcement and its characterization through interface testing. Overlap will be avoided by focusing TC-MCD efforts on the mechanisms of cracking and debonding in multi-layer pavement systems as described above. TG3 will focus on the evaluation and standardization of advanced measurement techniques that can be applied to study cracking mechanisms in asphalt concrete and composite pavement systems. Based upon this organizational structure, membership in multiple committees is expected to naturally occur. For instance, modeling experts who take a fundamental approach can actively participate in all three TGs. Researchers working on advanced measurement techniques (digital image correlation, acoustic emission, ultrasonic, x-ray CT, etc.) can join together to discuss and standardize methods in TG3, but will also find a rich, collaborative environment by applying their measurement techniques and associated data in the context of the TG1 and TG2 groups.
The three proposed TGs will now be described in more detail.
TG1: CRACKING IN ASPHALT MATERIAL (TG1 Chair, Prof. Eshan Dave - University of Minnesota-Duluth) This task group will build upon the efforts initiated under TC-CAP. The main goal of this TG will be to move more towards integration of laboratory testing, analytical and numerical modeling, and field performance for cracking prediction in asphalt materials. While in depth study of laboratory testing procedures for fracture in asphalt will be conducted by TG-3, the identification of necessary material parameters and analytical tools for extraction of such parameters will be accomplished through collaboration of this task group with TG-3. On the modeling side, a major thrust will be on defining and establishing procedures for qualification, verification, calibration and validation (QVCV) of models. These procedures will be followed for all candidate methods, and validation will be conducted using laboratory results and field studies. The final goal of this task group will be to develop integrated methods for prediction of cracking in asphalt material. These methods will utilize calibrated and validated models (TG-1), laboratory characterization methods (TG-3) and field information (TG-1).
TG2: INTERFACE DEBONDING BEHAVIOR (TG2 Chair, Prof. Christophe Petit - University of Limoges) In this task group, the effects of tack coat, pavement texture, and bimaterial system composition (AC/AC...AC/PCC) on pavement bond integrity and failure mechanisms will be studied. Similar to TG1, an integrated approach of testing coupled with modelling will be undertaken. Close collaboration with TG3 will be imperative, as the physics of cracking along pavement interfaces is poorly understood at present. A key focus of this TG will be on modelling, as very little work has been reported in the literature to date in the area of rigorous modelling approaches for cracking at pavement interfaces. Both analytical and numerical modelling efforts will be pursed. The QVCV approach, which was introduced to the pavements community during RILEM CAP, will serve as a guiding framework for the development and integration of tests, models, and the prediction of field performance.
TG3: ADVANCED MEASUREMENT SYSTEMS FOR CRACK CHARACTERIZATION (TG3 Chair, Prof. Gabriele Tebaldi - University of Parma) New measurement devices and techniques provided by other research fields (photogrammetry, biomechanical engineering, radiology and experimental physics) have created new, powerful analytical tools for the field of experimental fracture mechanics. In particular the so-called full-field measurement techniques have emerged as one of the most promising approaches in fracture mechanics research, in particular due to their capability to provide data to calibrate and/or validate numerical models. The main goal of the proposed task group is to investigate new measurement techniques that can be applied and modified to study cracking behavior in asphalt material bulk and at pavement interfaces. The following techniques are envisioned to be included: Full-field measurement, high resolution displacement, strain and temperature maps, digital image analysis, tomography, acoustic emission, and laser interferometry. Close collaboration with TG1 and TG2, as described above, will be critical for the success of all three TGs.
The new TC has links in membership and in topical area to the existing TC 210-CAP, with a significant shift of focus and organization, as described above. The proposed TC also has links to existing national and international associations (e.g. ISAP, AAPT, TRB, FHWA, FAA, etc.). There will also be links to the TG4 (Pavement Multilayer System Testing) of the newly planned RILEM TC SIB (Testing and Characterization of Sustainable Innovative Bituminous Materials and Systems Clearly, the new TC will benefit from a well established international network of collaborators.
The proposed TC fits comfortably into RILEM's technical program, and is particularly related to: "Mechanical Performance and Fracture; Materials Characterization, Properties Evaluation and Processing, and; Performance and Deterioration Mechanisms." The leadership of the new TC intends to hold its annual meeting at the same place and in series with the new TC SIB "Testing and Characterization of Sustainable Innovative Bituminous Materials and Systems."
The new TC intends to provide the following achievements:
Group of users
Specific use of the results
The findings of the proposed TC will identify rigorous testing and analysis methods that can be standardized and incorporated in performance-related specifications for asphalt and composite pavement systems. For instance, one of the tests in the previous, related TC (TC-CAP), was standardized as ASTM D7313 (Standard Test Method for the Determination of Fracture Energy of Asphalt-Aggregate Mixtures Using the Disk-Shaped Compact Tension Geometry) and has been subsequently incorporated into material specifications in several regions across the United States. It should be noted that sustainable approaches, such as the use of recycled materials and low-energy construction techniques such as warm mix asphalt, have increased the demand for rigorous, performance-based tests and associated models. The work of the proposed TC will therefore provide critically needed tools for the successful implementation of modern, sustainable practices in asphalt and composite pavement systems. For instance, the aforementioned test was used, in part, to justify the increased use of recycled asphalt pavement and asphalt shingles on the Illinois toll highway system, which spans across the city of Chicago and northern Illinois. The expected savings in the first year of implementation is excess of $5 million (USD), according to a recent communication by a Toll Highway Authority official. As similar advances are employed elsewhere, economic savings in the tens or even hundreds of millions of dollars worldwide are not out of the question. It is expected that new and improved tests and models derived from the proposed TC will similarly impact the state of practice and continue to empower the increased deployment of sustainable materials and approaches in the transportation sector.