Subject matter

Asphalt concrete is a mixture of aggregates and bitumen. The viscosity of bitumen is temperature-related and, when the temperature is above an interval between 30-70 oC, which is different for every type of bitumen, the binder will flow through the cracks in the material, healing them1. This has a certain impact on the lifetime of asphalt concrete pavements. If the healing process could be accelerated, thinner pavements, with longer lifetimes, could be built. This would represent a radical reduction of the carbon emissions and a disruptive change in the way we build and maintain the pavements.

In 2001, Professor Scott White, from the University of Illinois, USA, published the first sample of engineered self-healing materials. It consisted in an epoxy system where capsules containing self-healing agents had been embedded. When a crack happened, the capsules broke and the healing agents filled the crack, stopping its propagation. Since 2001, the concept of engineered self-healing has extended to other materials, such as polymers, coatings, metals, composites, cement and asphalt concrete.

An ideal self-healing asphalt concrete pavement is one with good mechanical properties, that can repair damage autonomously, completely and an infinite number of times. To reach these objectives, it is necessary that the pavement structure can detect damage and heal it locally. This cannot happen if the pavement structure does not harvest its own energy, sense the environment and react as a consequence².

The first efforts for building a fast self-healing pavement started in 2008, at the TU Delft, in the Netherlands. At the present there are three different approaches for accelerating the healing properties of asphalt concrete pavements: induction heating³,⁴, microwave heating⁵ and encapsulated healing agents⁶,⁷. Induction heating consists in mixing electrically conductive and magnetically susceptible particles in the asphalt concrete mixture. When an alternating magnetic field, with frequencies in the kilohertz range, approaches the particles, these will heat and the asphalt concrete mixture will increase its temperature. Microwave heating is the result of the energy losses that happen when the molecules of asphalt concrete increase their kinetic energy. This happens as a result of exposing asphalt concrete to an alternating electromagnetic field, in the megahertz range. Encapsulated self-healing agents consist in introducing capsules containing healing agents, such as rejuvenators, in the asphalt mixture. When a capsule is broken, the healing agents will fill the crack, dissolving the binder locally and stopping the crack propagation.

At the moment, the induction and microwave heating methods need an external trigger to work. Furthermore encapsulated healing agents would work only once and their feasibility for self-healing has not yet been fully demonstrated. On the other hand, there are different methods used for healing different materials which viability has not yet been tested in asphalt concrete. Examples of these are bacteria that can seal cracks by producing calcium carbonate, or unhydrated cementitious materials that could stop crack propagation in case of contact with water. Moreover, there is not a standardized method for testing engineered asphalt concrete self-healing.

Asphalt concrete self-healing has got an increased amount of attention and publicity in the last years. The reason for this is the need for reducing costs in the road construction and maintenance processes. As a result, many research groups around the world have started focusing their efforts on asphalt concrete engineered self-healing. Although until the present, results are not uniform and the research methods are not yet clearly defined.

¹ García A, (2011), “Self-Healing of open cracks in asphalt mastic”, FUEL. Fuel 93, 264-272.

² Sybrand van der Zwaag, (2007), “Self Healing Materials. An alternative Approach to 20 Centuries of Materials Science”, Springer Series in Materials Science 100.

³ Liu Q., “Induction healing of porous asphalt concrete”, PhD Defense, TU Delft, 2012.

⁴ Liu Q, García A, Schlangen E, van de Ven M, (2011), “Induction healing of asphalt mastic and porous asphalt concrete”, Construction and Building Materials 25, 3746-3752.

⁵ Gallego J, del Val Miguel A, Contreras V, Paez A, (2013), “Heating asphalt concrete with microwaves to promote self-healing”, Construction and Building Materials 42, 1-4.

⁶ Su Jun-Feng, Schlangen E., (2012), “Synthesis and physicochemical properties of high compact microcapsules containing rejuvenator applied in asphalt”, Chemical Engineering Journal 198-199.

⁷ García A, Schlangen E, van de Ven M, (2010), “Properties of capsules containing rejuvenators for their use in asphalt concrete”, FUEL 90, 2, 583-591.

Terms of reference

The general objective of the TC is to offer a framework for the coordinated development of engineered self-healing asphalt concrete. This committee aims to define a method for testing asphalt concrete self-healing and general criteria for characterizing the effectivity of various engineered self-healing methods.

The new TC will be active for 5 years (2014-2019). The particular objectives of the proposed committee are four: (1) Preparation and publication of a State-of-the-Art Report on asphalt concrete in order to characterize self-healing effects, (2) proposal of a harmonized test for asphalt concrete self-healing, (3) experimental evaluation of existing and alternative engineered asphalt concrete self-healing methods and (4) preparation of an international RILEM symposium.

The committee will be inaugurated during the Spring meeting, 2014 and the work will take place between 2014 and 2019 (5 years). A tentative timetable is as follows:

State-of-the-Art on asphalt concrete self-healing: This section will take place in the first year of the committee.

Evaluation of tests methods for asphalt concrete self-healing: This section will be carried out during the second and third years.

Evaluation of existing and alternative engineered asphalt concrete self-healing methods: This section will happen during the fourth and fifth years, after the test methods have been evaluated.

Preparation of an international RILEM symposium: The tentative time of organizing the symposium is end of 2017 or early 2018.

Final report of the RILEM committee: The final report is expected in 2019.

Detailed working programme

The stages and objectives of the committee’s work as follows:

1. State-of-the-Art on asphalt concrete self-healing

In this section, a bibliographical compilation of the self-healing theories and of the existing engineered self-healing methods for asphalt concrete will be done. Additionally, a chapter identifying alternative self-healing approaches, based on previous works for different materials, such as cement concrete, will be created. Finally, a section will be devoted to enumerate methods for testing self-healing asphalt concrete. All this information will be compiled and published in a report.

2. Evaluation of tests for asphalt concrete self-healing

The objective of this section is to identify the most suitable test for asphalt concrete self-healing. With this idea, various tests and damage types for asphalt concrete self-healing, such as 3 point bending, fatigue damage or IDT tests will be selected and evaluated by the different members of the committee. For this, a reference material will be fixed and round robin tests will be carried out through the different members of the committee.

3. Evaluation of existing and alternative engineered asphalt concrete self-healing methods

With the objective of evaluating the efficiency of the engineered self-healing asphalt concrete, a comparative study analysing various healing methods will be carried out. These methods will be encapsulated rejuvenators, induction and microwave heating of asphalt concrete. Parameters to be analysed are the healing capacity and the lifetime cost of the self-healing material related to the unmodified asphalt concrete. Moreover, recommendations about the use of the existing engineered self-healing approaches will be emitted. Finally, the applicability of alternative engineered self-healing methods, such as unhydrated cementitious particles, will be studied.

4. Preparation of an international RILEM symposium

In the first year of the TC activities the scope of an international symposium has to be worked out.

5. Final report of the RILEM committee

The committee should end its work with issuing a final report, which will include recommendations for testing and characterizing the effect of engineered self-healing and with the presentation of the work during the RILEM week.

These stages will be sequential, each member of the committee will participate in them according to their own scientific interests. In the stage 3, working groups on different engineered healing methods will be created. Due to the differentbackgrounds, interests and physical locations of the members of the TC, communication will be done mainly by electronic means.