Self-healing highways

By heating up asphalt with an induction cooker, Delft researchers aim to keep the Netherlands’ highways in perfect shape.

(Photo: Erik Schlangen)
(Photo: Erik Schlangen)

Travellers on the A58 from Vlissingen to Bergen op Zoom please slow down. The right lane is closed this week to allow asphalt researcher, Dr Erik Schlangen, and his postdoc researcher, Alvaro Garcia (both of the faculty of Civil Engineering and Geoscience), to drill dozens of holes in a 400-meter long stretch of the road and take samples for subsequent experimentation.
Last December road workers resurfaced this section of the highway using a special type of asphalt developed by the Delft researchers: the blacktop contains 1-centimetre long steel fibres, an additive that is meant to make the asphalt self-healing.

“Due to the fibres, the asphalt is conductive and can thus be heated by holding a coil above it, which generates an alternating magnetic field,” Garcia explains. When the bitumen in the asphalt heats up to about 60 or 70°C, it softens and the tiny cracks in the asphalt close.

Dr Schlangen believes the life-spans of many blacktop surfaces can be doubled to about 12 years if steel fibres are added during the fabrication process and if the asphalt is subsequently heated approximately once per year, using a specially equipped vehicle that passes a large heated coil over the road surface.

The type of material Dr Schlangen is studying is called Zoab, a highly porous asphalt concrete that is much-used in the Netherlands because of its excellent water-draining properties: water seeps through the openings in the asphalt.
The main problem with this asphalt however is that, owing due to temperature cycles, wetting and drying cycles, UV lighting and traffic loads, the bitumen becomes brittle and cracks after just a couple years of use. Ravelling – the process by which the surface layer’s small stones are dislodged - then occurs. But by occasionally heating the asphalt, the micro cracks close again and the aging process slows down.

“In Spain, we don’t have to heat the asphalt with these kind of tricks,” says Garcia, a Spaniard, laughing. “During hot summers the sun heats it to 60 or 70°C.” He adds however that under such conditions heating is in fact not a good thing: “If the asphalt is heated for too long, this also damages the road. You only want to heat the asphalt for a very short time.”
The 120 drill cores taken from the A58 will be exposed to varying climate regimes. “We want to speed up the asphalt’s aging process,” Schlangen explains. “Subsequently, we want to discover how often the material must be heated to extend its lifetime, and how this depends on climatic situations.”