210-CAP: Cracking in asphalt pavements

Technical Committee 210-CAP

General Information

Chair: Prof. André A. A. MOLENAAR
Deputy Chair: Mr. Tom A. SCARPAS
Activity starting in: 2004
Cluster E

Subject matter

Cracking of pavements can be due to a very large number of reasons. Basically four main crack categories can be discriminated which are transverse, block, longitudinal and alligator cracking.
Transverse and block cracking are generally related to environmental effects. A well known example is the development of shrinkage cracks in cement treated bases due to hydration of the cement and subsequent seasonal thermal variations.
Another cause of these types of cracking is the thermal expansion and contraction of the cement treated bases of asphaltic pavements. Due to the horizontal movements of the base and the forces generated by the traffic loads, cracks in the base will in general propagate into the bituminous surfacing.
Although the horizontal movements perpendicular to the crack or joint are of main importance, the relative vertical movement of the crack faces is of paramount importance.
Longitudinal cracking in the wheel paths is most likely influenced by traffic. The horizontal shear forces that occur under both, free rolling and driven wheels are responsible for very high tensile stresses and strains at the pavement surface acting perpendicular to the direction of travel. These strains occur near the tire edges and produce longitudinal cracks. These cracks will propagate in the longitudinal direction of the pavement because of tearing which is commonly called mode III type cracking.

Alligator cracking is commonly related to material fatigue. According to the commonly used linear elastic multi-layer analysis, fatigue cracks redeveloped in a direction transverse to the direction of travel. These transverse cracks are only developing over a limited length given the fact that the width of the loaded area is limited to the wheel path. Propagation of these cracks is generated by the bending (mode I) and shearing (mode II) action of the traffic loads. Also fatigue cracks will propagate through layers placed on top of the cracked layer.

All in all it is quite clear that the cracking process in pavements is a complicated one. This immediately implies that a careful analysis should be made of the various influence factors in order to determine their relative importance. This is essential in order to be able to determine the most appropriate maintenance strategy.
The continuum mechanics type analysis of asphaltic concrete is made complicated by its microscopic composition. This has a strong influence on its mechanical properties.

In asphaltic concrete a tensile fracture zone starts to develop as soon as the strain corresponding to the tensile strength is exceeded. It consists of microcracks mainly perpendicular to the maximum tensile stress direction. Within this zone some tensile stress can still be transferred. With increasing tensile deformations in the transverse to the microcracks direction, the tensile strength of the fracture zone is exhausted, and the microcracks coalesce into a single crack.
Currently available experimental evidence indicates that the length of the fracture zone can be considered as a material parameter and is related to the fracture-energy of the material.

Multiaxiality is also known to influence the cracking process in asphaltic materials. In the presence of a transverse compression stress field, both the tensile strength and the fracture energy of the material diminish significantly. Simulation of the multiaxial response of the material is a main issue in current modelling attempts.

Investigation of the multiaxial response of the material is also a topic of current laboratory investigations. The inhomogeneous nature of asphaltic concrete renders the experimental tracing of crack initiation and propagation processes extremely difficult.
Additionally, the cracking response is strain-rate and temperature dependent.

From the above short discussion it becomes apparent that additional experimental evidence and computational work are necessary before thorough understanding of the cracking process in asphaltic materials can be achieved.
The objective of this TC is to advance understanding of the cracking process via laboratory and computational studies and to convey the obtained results in a form which can be utilised for the prediction of cracking in flexible pavements.

Terms of reference

The committee will consist of approximately 10-15 experts. The committee work will focus on:
1. Collection and evaluation of field data, laboratory techniques and modelling methods,
2. Calibration of selected predictive models versus laboratory tests.
3. Model refinement and improvement via case-studies simulations
4. Drafting of recommendations.

Provisional timetable
The work is scheduled for five years. The envisioned tasks are as follows:
- 2004-2005: Collection of available field data, laboratory testing techniques and predictive models.
- 2005-2006: Evaluation of available field data, laboratory testing techniques and predictive models.
- 2006-2007: Calibration/improvement of selected predictive models versus laboratory tests.
- 2007-2008: Model refinement and improvement via case-studies simulations
Organisation of the 6th RILEM Symposium on Cracking in Pavements, Virginia Tech, USA
- 2008-2009: Preparation of recommendations and final report.

Detailed working programme

The working program includes the following tasks:
Task 1 Collection of field data on crack development in asphalt concrete pavements.
Task 2 Collection of laboratory techniques representative of crack initiation and propagation in asphaltic mixes
Task 3 Collection of available predictive models for crack initiation and propagation.
Task 4 Field data evaluation.
Task 5 Evaluation of laboratory techniques.
Task 6 Evaluation of predictive models
Task 7 Calibration of selected predictive models versus laboratory tests.
Task 8 Model refinement and improvement via case-studies simulations.
Task 9 Preparation of the final report.
Task 10 Drafting of conclusions and recommendations for a follow-up TC
Task 11 Management of TC (by A.A.A. Molenaar and A.Scarpas)
Task 12 Committee visibility
-Session hosting in focused events
-Set-up and maintenance of a dedicated web-site
-Organisation of the 6th International Symposium on Cracking in Pavements

The committee will be split up in 3 Task Groups: TG1, TG2 and TG3:
- TG1 will focus on the collection, evaluation and selection of field data. (Task 1, 4, 8, 9, 10, 12)
- TG2 will focus on the collection, evaluation, selection and improvement of laboratory techniques. (Task 2, 5, 7, 9, 10, 12)
- TG 3 will focus on the collection, evaluation, selection, calibration and improvement of predictive models. (Task 3, 6, 7, 8, 9, 10, 12)
TG2 and TG3 shall work together during the calibration of the selected predictive models versus laboratory tests (Task 7).
TG1 and TG3 shall work together during the case studies simulation (Task 8)
All TG's shall participate in Tasks 9, 10 and 12.

Provisional schedule of meetings
- 1st meeting: Transportation Research Board annual meeting, Washington January 2004.
- 2nd meeting: Delft, February 2004.
- 3rd meeting: Limoges, May 2004
- 4th meeting: Paris, Oktober 2004 (in parallel with the TC on Performance and Testing of Bituminous Materials*)
- following meetings: first and third quarter each year
*In the future the TC on Cracking of Asphalt Pavment shall have their annual meeting at the same place and in series with the new TC-182 PEB on Performance and Testing of Bituminous Materials or the new TC ATB (Advanced Testing and Characterization of Bituminous Materials) which is intended to be proposed in auntumn as a successing committee of TC 182-PEB with new tasks

Technical environment

The TC will continue the work of TC97-GCR and TC 157PRC which have expired and which were set up to study prevention of cracking in pavements. The TC's organized the international RILEM Conferences on " Reflective Cracking in Pavements " that were held in 1989 and 1993 in Liège (Belgium), in 1996 in Maastricht (The Netherlands) and in 2000 in Ottawa (Canada). These were all very well attended with true international participation. The 5th Conference is entitled "Cracking in Pavements" and will be held in Limoges, France in 2004.
These events indicated the need for the establishment of the proposed RILEM TC. The TC will build on the foundations created by these and will utilise their results as a basis for its work. It will also take over the organization of this series of conferences and other related events. Recently the International Society of Asphalt Pavements (ISAP) inaugurated a new TC on Constitutive Modelling of Asphaltic Materials. The scope of this ISAP TC is the encouragement of research into constitutive modelling of asphalt and the dissemination of knowledge into the sector. The secretary of this new RILEM TC is also the chairman of the ISAP TC. Future cooperation's between the two TC's will be an option.

Expected achievements

The main achievement of the TC will be the development of a physically correct and computationally robust and efficient method for simulation of the crack initiation and crack propagation process in asphaltic concrete pavement. The methodology will be calibrated versus laboratory tests and validated with field data.
Another achievement of the TC shall be the establisment of laboratory procedures capable of objectively quantifying the process of crack initiation and propagation in laboratory specimens of asphaltic concrete.
Understanding of the crack-initiation and propagation processes will enable standards developers to draft recommendations for the design of asphaltic concrete pavements against short- and long-term cracking. This is a challenging task considering the limited level of knowledge available at present regarding both, modelling work and design standards specifications.
Understanding of these processes will also assist asphaltic-mix developers with the production of mixes capable of increased cracking resistance. Furthermore it will also facilitate the design of reinforcing systems against reflective cracking.
Another main task of the TC shall be the organisation of the International Conference on Cracking in Pavements. The TC will also present results of ongoing research to relevant international conferences and seminars and appropriate scientific journals.
Dissemination of the results of the committee will also be achieved via the set-up and maintenance of a web-site with the latest research developments and activities of the TC.

Group of users

The target group include academic and research institutions, standard organisations, designers of pavement constructions, asphaltic product manufacturers. The academics will benefit from the results as recommendations of new research topics and coordination of research work.

Specific use of the results

The new information will enable more accurate risk assessment of the service life of asphaltic pavements and also will enable the development and utilization of asphaltic products with improvement cracking resistance. Thus, it will improve the economic use of the material.