Innovative research into 3D printed asphalt is resulting in unforeseen findings of potentially considerable significance to highways surfacing maintenance.
Development of a system that detects cracks in road surfaces autonomously and then repairs them to forestall the formation of potholes is – in essence – a principal element of the fascinating Self-Repairing Cities (S-RC) project.
Led by Leeds University, and now three years into a five-year programme, S-RC’s research into road surfacing maintenance has focused on drones and 3D printing.
The other partners on the project are University College London (UCL) and the Universities of Southampton and Birmingham.
While Leeds is concentrating hard on the robotics and autonomous control systems, UCL is developing a 3D printing system and the asphalt that will be extruded by them.
Surprisingly, the ductility and toughness of the material that is produced in the printing process is being found to be much higher than that of conventionally placed asphalt. This, combined with how 3D print delivery might be up-scaled and exploited, is beginning to attract the attention of the road maintenance sector.
“It is the unmanned aerial vehicles (UAVs) and the autonomous nature of the S-RC process that provokes public interest – the drones look fantastic,” says UCL materials scientist Richard Jackson. “However, the materials side is turning out to be every bit as interesting.”
This fact was first alluded to last autumn in a paper entitled 3-D printing of asphalt and its effect on mechanical properties; co-authored by Jackson and two UCL colleagues Adam Wojcik and Mark Miodownik
The paper firstly confirms that the S-RC developed system is able to scan a crack and carry out remedial work by 3D printing asphalt into it; and that it has the potential to be used on drones (or other means of conveyance) to autonomously repair roads and other infrastructure.
Then it goes up several notches in terms of interest. To quote the document: “Unexpectedly it was found that the 3D printed asphalt is up to nine times more ductile than cast asphalt, with commensurate fracture strengths.”
Jackson explains the mechanics. “We believe the increased ductility is due to a ‘crack bridging component’ visible on examination of the fracture surface of the printed asphalt, which we think is composed of a lighter fraction of asphalt.
“The 3D printing process seems to create a composite structure with the softer elements of asphalt coalescing in much larger concentrations than in cast asphalt, giving the printed material a higher toughness,” he continues.
“It follows that the life of repairs can be prolonged – particularly in high stress locations – because of this enhanced ductility and toughness of 3D printed asphalt.”
Jackson and colleagues spent a challenging two years refining the design of their 3D printer’s extruder – the key to the technology working but also, it seems, to much wider application of the printing process than originally planned.
“We can now print asphalt at a variety of temperatures and process conditions plus the printer’s feed and pellet system makes it relatively simple to add other materials.
“These could include micro aggregates or other nanomaterials, to vary the feedstock composition during printing, and thereby allow us to produce more complex materials with a wider range of properties.”
Jackson states that UCL’s flexible and tuneable asphalt printing system could result in a whole new approach to the maintenance of road infrastructure.
“For a start, we could go from sealing cracks to actually repairing pavement damaged by potholes, for example, using a 3D printer to extrude layer upon layer of appropriate material.”
For this, a scaled-up version of UCL’s current equipment will be required, along with a great deal of additional research and development (R&D) and – of course – funding.
Work has already started, in association with Nottingham University’s renowned pavement engineer Alvaro Garcia.
Meanwhile development of drone technology for the S-RC project plus the control systems for these continues at pace at Leeds University, which is not shy about drawing on the results of R&D carried out by others.
“We’re actively seeking to benefit from cross-over with other projects – and we’re happy at Leeds sharing our expertise in return,” says the university’s professor of materials and structures Phil Purnell.
One such beneficial cross-over is Project Rachel, among whose project team is S-RC’s drone expert Stephen Prior. Project Rachel declared a new continuous flight record in January this year, of 70 minutes with a 5kg payload. Its drone – for which Prior designed the co-axial propulsion system – was powered by a hydrogen fuel cell.
By way of comparison, conventional lithium polymer powered batteries typically keep a load carrying unmanned aerial vehicle (UAV) in the air for about 12 minutes.
Other advantages of fuel cells over batteries are listed as fast refuelling, zero vibration and emissions plus quiet operation.
One of the partners behind Project Rachel is unmanned aeriel vehicle filming specialist Batcam, which supplies aerial views of major sporting events.
Batcam’s chief operating officer Alastair Soutar says: “Flight times have always been a limiting factor for our systems….we are part of a project which aims to make short flight times a thing of the past.”
Self-Repairing Cities is a project aimed at eliminating disruption from street works in UK cities by 2050 (New Civil Engineer 29 June 2016).
The aim is to develop robots capable of identifying, diagnosing and repairing such works through minimally invasive techniques.
Several case studies were launched, including one into road surface maintenance.
Leeds University is leading the project, backed by £4.2M from the Engineering & Physical Research Council and support from commercial interests.
“Concerning roads, our specific intention is to proactively prevent potholes forming,” says Leeds University professor of materials and structures Phil Purnell.
For this, drones are being developed to recognise road surface cracks, then seal them with appropriate asphalt mixtures delivered via an on-board 3D printer.
“Leeds has long been at the cutting edge of robotics in engineering, and infrastructure maintenance is a natural progression for us.”
There are now three main strands of activity: 3D printing of asphalt for roads, unmanned autonomous vehicle bridge inspection and maintenance plus a third one to be defined with input from Highways England.
Future of Highways Maintenance | 3D printing