Induction heating asphalt mixes to increase road durability and reduce maintenance costs and disruptions
The self-healing effect of the asphalt is supported by integrated conductive components and induction energy, to expand the technologies of maintenance.
Extending the service life of roads is of major importance to ensure the movement of goods and people throughout our transport networks. The most frequent maintenance needs are related to damage in the superficial wearing course. On the other hand, maintenance operations on other layers are carried out at greater intervals and at a larger scale. During conventional maintenance, traffic disruptions and safety issues can arise, lasting in some cases for several days.
The HEALROAD project was designed to extend the lifetime of wearing courses by using asphalt materials that can self-heal by means of electromagnetic induction. Besides conventional components that can be found in asphalt – for example filler, aggregates and bitumen – the HEALROAD mixture contains also conductive components that can be rapidly heated by induction, reducing the viscosity of the surrounding bitumen and boosting it through cracks. As a result, damage produced in the mastic that bonds the aggregate particles is healed and the service life of the road is extended.
BASt’s MLS30 accelerated pavement testing equipment will be used at duraBASt to apply realistic damage on full-scale road sections and evaluate the performance of the proposed material in real conditions.
The overall objective of the project is to overcome the technical barriers for the future industrialisation and market uptake of healable asphalt mixes via induction heating. To reach such objective, results will be analysed from both technical and feasibility perspectives.
Extensive laboratory tests have already been carried out during the project to ascertain the suitable constituents and corresponding quantities for the composition. Once the composition had been specified there were further laboratory tests which, among other things, established how often the healing using induction energy can be used and how the material may be recycled.
The next steps are therefore:
- validating the technology during practical use and
- large-scale installation and load tests.
For the EU project HEALROAD, a 1:1 scale test track was constructed on the duraBASt in mid-2017. In October and November 2017, Accelerated Pavement Tests (APT) were carried out on two load fields, each with 150,000 rollovers. The loading with the Mobile Load Simulator MLS30 was carried out with a standard truck tyre, a wheel load increased to 75 kilonewtons and with lateral misalignment. This combination made it possible to apply higher forces than usual and thus reinforce the accelerated effect on the surface course.
The first load field served as a reference for the evaluation and was only loaded by the MLS30. On the second load field, the induction machine was used at various times for the targeted self-healing of the surface course. This maintenance measure is intended to close even the smallest cracks in the bitumen film and prevent grain eruption.
Both load fields were continuously investigated with non-destructive measuring methods and the grain eruptions were documented for further evaluation.
The main objective of the project – to construct a high-quality investigation area with a self-healing asphalt – was implemented. Quality tests during the construction process showed that uniform conditions had been created. The HEALROAD surface course mixed in the Netherlands was transported to duraBASt under controlled conditions. There, the paving could be carried out within the required limits.
The Accelerated Pavement Tests with the MLS30 demonstrably led to changes in the surface course and structure of the superstructure. During the various operating times of the induction machine, it was shown that the surface course was produced very homogeneously and enabled uniform heating, which prevented the formation of harmful clusters.
Due to only minor damage in the asphalt road surface, no representative evaluations regarding the influence of induction energy could be obtained in this first APT test with self-healing asphalt. It became clear that the lateral misalignment during the running load of the MLS30 (2,000 mm/s and 75 kilonewtons) at 4 mm/s produces too little shear force and thus did not sufficiently provoke the removal of rocks. It is noticeable that the initial stone loss was detected during the tests and that only a few stones were loosened during the following rollovers.
RSAT (Rotating Surface Abrasion Test) was used to examine test specimens from a laboratory mixture, then drill cores from the finished duraBASt investigation area and then drill cores from the contaminated area. It became clear that the induction energy should be used as early as possible, because otherwise the stone loss increases significantly despite the use of the induction energy.
The results of the tests justify longer loading times and can be used well for future APT projects. In addition, a solution should be found to apply greater shear forces.