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Laser trucks help maintain roads

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Posted May 15, 2019

Pavement engineering company, Dynatest, has developed a truck in partnership with DTU which analyses the state of the road using laser technology.

You cannot always see whether a road is worn by looking at the surface. Roads consist of many layers, all of which must be in good condition to provide adequate load bearing capacity.

Load bearing capacity is normally investigated using a ram—a heavy rod which is used to strike the asphalt, while sensors measure how many millimetres the impact causes the road to deform. The deeper the deformation, the more worn the road section.

Image credit: Dynatest.

Image credit: Dynatest.

The problem with this method is that you can only investigate a few square metres of road at a time, and have to stop all traffic during measurement. This causes such major traffic disruption and economic costs that the condition of roads is often not investigated in time.

It has now become much easier to investigate the load bearing capacity of roads, thanks to a partnership between Dynatest and DTU. After seven years, this has resulted in a fully developed ‘Raptor’—a truck with lasers mounted underneath that can perform almost the same load bearing measurements as a ram, but at 80 km/hour.

“The aim of Raptor is to make load bearing measurements more common among road maintenance departments than is the case today. There are many places where you can only perform a visual inspection of a road’s condition, because it is too difficult to use a ram. But it is much more expensive to repair a road once all the layers have been worn out than if you make an effort to investigate which sections need maintenance in good time. It is now much easier to perform this investigation using our truck,” says Jack Larsen, Project Manager at Dynatest.

A truck equipped with laser sensors and other measuring equipment can measure the condition of a road as it is driving. The truck, which has been named Raptor, is shown here during a visit to Italy. Image credit: Dynatest.

Interest from Italy

Since the road-inspecting truck was launched in April 2018, Dynatest has performed demonstration measurements around Europe to display the Raptor technology to European road directorates. The Italian road directorate, in particular, found the technology relevant, and immediately ordered a survey of more than 1,200 kilometres of motorway.

“Italy has long been struggling with worn-out roads, and this has been regularly discussed in the public debate, because it affects traffic safety. The Italian road directorate was therefore very interested in our technology, because Raptor gives them a quick overview of which road sections need to be renovated,” says Jack Larsen.

The European Commission estimates the cost of maintaining Europe’s roads to be up to EUR 1 billion (DKK 7.5 billion) annually. Jack Larsen believes this cost can be reduced if more road directorates regularly analyse the load bearing capacity of their road networks.

Rolling analysis machine

Like a ram, the Raptor vehicle also measures road deformation. But instead of the sensors detecting how many millimetres striking the road with a heavy ram causes the road to sink, the distance sensing lasers detect how much the road deforms due to the truck’s weight.

“We have attached 12 lasers to a rigid beam, which extends in front of and behind one of the vehicle’s rear wheels. Each laser measures the distance to the asphalt at that position. Their measurements give us a very accurate impression of the load bearing capacity on the section of road we are driving on. If the weight on the rear wheel causes the asphalt to deform more than about 2 millimetres, there is probably something wrong with some of the lower layers of the road at that point,” says Jack Larsen.

Laser sensors mounted on a beam measure how much the vehicle’s weight causes the road to deform. Illustration by Dynatest

Software development

The 12 line lasers generate huge amounts of data, which must be processed by software before it can be used to say anything about the road condition. Software that DTU researchers helped Dynatest develop.

“Our software inserts the measured deflections in a model, that calculates the deflection of a computer-simulated road. We then calculate backwards to find the strength of the various layers in the road, so that the deflection of the computer simulated road matches the deflection the line lasers have measured,” says Stine Skov Madsen, postdoc at DTU Mechanical Engineering. She has helped create and implement the model for Raptor.

One of the challenges with the laser measurements is that asphalt is a coarse material with many small stone and gravel particles. This creates noise in the results, because you cannot avoid measuring the small variations in the asphalt surface, which have nothing to do with the road’s load bearing capacity. Dynatest is the first company of its kind to address this problem in a unique way.

“Instead of point lasers, we use line lasers that constantly measure a large number of distances along a broad line, perpendicular to the direction of travel. This makes it possible to take into account the road’s texture and smooth out the variations while calculating the deflection,” says Jack Larsen.

Algorithm assistance

It is not possible to perform measurements with 100 per cent precision, even though the line lasers have minimized the measurement uncertainties due to the road’s rough surface. Dynatest has therefore drawn on additional help from DTU. Asmus Skar, postdoc at DTU Civil Engineering, has developed an algorithm that can report what impacts the measurement uncertainties have on the way Dynatest can interpret its data.

“It’s important to take into account the measurement uncertainties, because variations in the roughness of the road surface structure can be greater than the deflection Raptor’s weight causes. The beam the lasers are mounted on can also move a few millimetres. These uncertainties are large in relation to the small effect we are measuring. The precision of the measurements is therefore critical in order to achieve realistic results,” says Asmus Skar.

The project is receiving two rounds of funding from Innovation Fund Denmark, with a total budget of EUR 4.7 million (DKK 35 million).

Source: DTU

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