Subject matter

Cracking is one of the most prevalent deterioration modes of flexible pavements. It is caused by traffic loading, low temperature and can be accelerated by oxidation of the asphalt or when the asphalt material performs poorly. In research on asphalt pavement, these issues have often been addressed separately due to the varied reasons for the cracking. The service life of asphalt can be increased significantly if these issues could be addressed with one or a combination of solutions focussed on the prevention and healing of asphalt cracks.

Fatigue and thermal cracking are the most dominant cracking distress modes in flexible pavements resulting in structural and functional failures of pavements. The mechanisms behind the two modes are very different. Fatigue is a traffic-induced damage that results under repeated tension stresses, induced by heavy vehicles. This creates infinitesimal damage at the bottom of the layer and initiates micro-cracks. Progressively, the cracks propagate which leads to failure of the pavement. On the other hand, when the temperature of the pavement drops, thermally induced tensile stresses build up in the asphalt layer and lead to its cracking when the thermal stress exceeds the threshold of the tensile strength. This phenomenon is generally referred to as Low Temperature Cracking. In both cracking modes, the role the viscoelastic properties of the asphalt binder is crucial.

There are various mechanisms affecting the viscoelastic properties of asphalt binders. Age hardening, which is an irreversible process, is widely considered the most important. Even though asphalt binder is only one component of the asphalt mix, the overall performance of the asphalt pavement is largely dictated by the viscoelastic properties of this binder. Several factors have been cited to explain the phenomenon of age hardening in asphalt binders. The major causes of age hardening commonly cited include oxidation, volatilization, polymerization, syneresis, and separation.

Researchers are increasingly looking to nature as a source of inspiration for solving complex technological, societal, and environmental challenges. Biological systems, processes, and designs have already influenced innovation in the energy, transportation, and communication sectors, among others. Similarly, the pavement industry, which is under constant pressure to produce longer-lasting, yet high-performing materials, can take inspiration from the mechanisms that underlie the human body’s ability to heal itself. Self-Healing Materials (SHM) have actually been explored to help create asphalt mixes that can close their own cracks or even prevent cracks before they happen.

Self-healing of bituminous materials may happen naturally during rest periods (Baaj, 2002) or may be provoked through the use of self-healing materials or techniques (Schlangen et al., 2010). Self-healing has been defined as the ability of a material to heal damage automatically, without any intervention and autonomously (without external triggering). Some previously studied healing methods of bituminous and cement-based materials have included the use of microcapsules (Su et al., 2013; Yang et al., 2009), bacteria (Jonkers and Schlangen, 2008), fiber reinforcement (Schlangen and Sangadji, 2013), phase change materials among others. Other methods for healing asphalt cracks use exterior triggering such as induction (Garcia et al., 2009), microwave heating (Gallego et al., 2013).

The TC will deal with healing phenomena of asphalt, a subject which becoming more and more popular in the last decade. The focus is on the development of test methods for the induction of asphalt cracking, quantification of intrinsic self-healing properties as well as on ways to enhance the healing properties of asphalt. The work of the TC should lead to improved durability of structures with less maintenance. It is a subject which becoming more and more popular in the last decade.

The scope of the committee will be built around three primary areas of work:

1. Literature Review

• Evaluation through literature study of the pathologies and effects of cracking, as well as the effects of pavement design, aggregates, binder source and binder modification on the cracking. Particular focus will be given to low temperature and fatigue cracking;
• Determination of the most efficient ways to induce cracking in a laboratory environment, based on the real-life mechanisms of cracking;
• Determination of the analysis methods available for evaluating healing in asphalt pavement;
• Determining the most promising self-healing techniques available, which could be applied to asphalt pavement;

2. Laboratory Experimentation

• Evaluation of methods for crack induction in asphalt pavement;
• Evaluate different test methods, used by the different participant laboratories, to characterize healing in asphalt pavement;
• Preparation, with the help of industrial partners, asphalt samples with healing materials that will be used in a round-Robin campaign to evaluate their effectiveness;

3. Numerical Modelling

• Investigate simulation programs and modelling techniques to predict cracking in asphalt based on various cracking influences;
• Define the best modelling strategies for the predication of the effectiveness of crack healing techniques.

Terms of reference

The preliminary timeline for the committee will be approximately 4-5 years. The principal tasks of this committee will be divided into 3 TGs based on the literature review, laboratory experimentation and the numerical modelling. More technical groups could be foreseen if there is the possibility of interesting divergent studies during the course of the committee.

TG1 – This group will focus on the production of state of the art reports and will be subdivided into two subgroups. The reports will serve as the theoretical basis for the two subsequent groups TG2 and TG3. The members of the committee will contribute their expertise into the compilation and evaluation of these studies.

TG1a – This subgroup will review the nature of cracking in asphalt pavement, with particular focus on low temperature and fatigue cracking;

TG1b – This subgroup will review the experimental and modelling techniques that have been used or have the potential to be used with self-healing asphalt mixes with specific focus on the determination of how self-healing in asphalt will be evaluated and quantified;

TG2 – This group will be responsible for the laboratory experimentation. The experimental methods will focus on evaluating methods of crack propagation in laboratory asphalt pavement, along with the healing methods for these materials. The evaluation of the various crack propagation techniques – as well as the evaluation of their healing – will be implemented in a round-robin testing program. They will also evaluate and compare various test methods (i.e. NDT, 3 pt bending, fatigue, IDT, ductility, CT Scan, microscopy…) for evaluating self-healing techniques for asphalt pavement.

TG3 – This group will be responsible for the development and adaptation of models for pavement cracking prediction, along with the prediction of the effectiveness of various self-healing techniques. The starting point of these projects will be largely derived from the literature review of TG1. A number of modelling tools will be used in the round-Robin campaign based on the expertise available in the committee. The tools used could possibly include Finite Element modelling, constitutive modelling and others. The correctness of the models will be compared with the laboratory derived results of TG2 where possible.

The scope of the membership of this committee includes academics, PhD students, and asphalt materials researchers along with government and industry representatives around the world.

Detailed working programme

General TC: The TC will organize a kick-off meeting in September 2016 in Montreal, Canada during Cluster Meetings. The meeting will be among the committee members and other interested parties to determine the available expertise, competencies and abilities of the committee. The leading members for the TGs will be decided with one chair for each group. The TGs will also be assigned secretaries from the midst of the committee with TG1 being assigned two (responsible to each subgroup) and TG2 along with TG3 being assigned one each. These assignments will be subject to change based on change in availability or priority.

TG1 Literature Review – The completion literature review will be the first task of the committee and will be completed in a timeframe of approximately 1 year.  The reports will cover i) the cracking of asphalt pavement and its simulation in a laboratory environment and ii) self-healing in asphalt and its evaluation in laboratory conditions, performed simultaneously by TG1a and TG1b, respectively.

TG2 Laboratory Experimentation – This group will begin with a meeting to discuss a number of experimental techniques with the goal of evaluating and simulating cracking in asphalt pavement in a laboratory environment. The development of crack inducing methods will allow for work on the evaluation of the asphalt pavement self-healing. The preliminary work of this group is expected to begin several months after the first TC meeting and continue for the length of the project.

TG3 Numerical Modelling – This group will likely begin their work shortly after the completion of TG1 and the publication of its findings and continue throughout the existence of the TC. The literature reviews will determine a number of existing theories related to asphalt cracking and self-healing. These theories will be put to the test in a number of research labs focused on numerical modelling. The work produced here will be compared to the results of TG2 where possible.