experiences with integrated analysis of tsd, gpr …...experiences with integrated analysis of tsd,...

Experiences with Integrated Analysis of TSD, GPR and Laser Scanner Data

Tomi HERRONEN1, Annele MATINTUPA2 and Timo SAARENKETO3

Roadscanners oy, Varastotie 2, 96190, Rovaniemi, Finland 1 Phone +358 500 917 138, email: [email protected], 2 email: [email protected],

3 email: [email protected]

ABSTRACT

This paper presents a summary of experiences of integrated analysis of Traffic Speed Deflectometer (TSD), Ground Penetrating Radar (GPR) and laser scanner data using Road Doctor 3 (RD3) software. The relatively new non-destructive techniques can be now used widely to support each other, which has already shown very promising results in road diagnostic surveys.

Integrated analysis of GPR, laser scanner and TSD data provides valuable data about asphalt quality and reason for the distress when they are viewed simultaneously and combined with results calculated from the data. With RD3 video and map can be watched at the same time.

The laser scanner data can be shown as cross sections, height and reflectivity maps; TSD provides several parameters about bearing capacity and fatigue and GPR from layer thicknesses and moisture conditions. Combining data from these three methods the result will give complete vision about road condition.

Keywords: Ground penetrating radar (GPR), Traffic Speed Deflectometer (TSD, laser scanner, road diagnostics, pavement distress

International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE)

September 15 - 17, 2015, Berlin, Germany

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1. IntroductionDue to their nature, location and size, reliable mapping of different kinds of distress in asphalt pavement, has been a major challenge over the years. Traditionally, distress mapping has been carried out visually directly from a moving car or based on a digital video of the pavement, but these methods have been expensive and not always reliable and repeatable enough. That is why numerous new automated or semi-automated technologies have been developed and tested over recent years with varying success.

Over the last year, tests with new technologies such as Traffic Speed Deflectometer (TSD), Ground Penetrating Radar (GPR) and Laser Scanners, also called Lidars, and analysis of this data in combination has presented very promising results for Roadscanners in Finland. The theory behind these tests and experiences from the integrated analysis of these methods is described in the following.

2. Pavement condition, how pavements failIn order to be able to make correct pavement diagnostics it is important to know how pavements fail. The fatigue and damage mechanisms of bituminous pavements can be classified into five main categories based on their origin:

A. Pavement fatigue

This category covers the fatigue of pavement and unbound structural layers under repeated axle loading. On a new road pavement fatigue normally requires the actions of millions of heavy axle load repetitions and for this reason it is normally only seen on streets with high traffic volumes.

Usually pavement fatigue is presumed to take place as a function of the number of heavy axles passing and it is usually evaluated using linear-elastic fatigue models. In other words, the thinner the pavement, the higher the number of load repetitions and the heavier the loading; the quicker the fatigue and damages take place. The most common damages due to pavement fatigue are a) rutting and b) cracking in the wheel paths. Net cracks are the most typical, but super single tyres, increasingly used on heavy vehicles, have created a new cracking type: “top down cracking”, which can be seen as longitudinal cracks beside the wheel path.

B. Permanent deformations

Permanent deformations take place in the structural layers of a road and are not dependent on load cycles. Permanent deformations can take place even after only a few heavy truck passes. The majority of permanent deformation damages take place over the winter period during the rainy season, and especially after periods with freeze-thaw cycles.

When discussing the diagnostics of roads and streets, it is also important to be familiar with the four types of rutting modes of permanent deformation, as defined by the ROADEX EU project (www.roadex.org). “Mode 0” rutting takes place due to the compaction of the road structure. “Mode 1” rutting happens in the bound layers or granular structural materials near the pavement surface. In City streets Mode 1 rutting in asphalt pavements can especially be seen at traffic lights



http://www.roadex.org/

where trucks and busses have to stop. “Mode 2”rutting takes place at the road structure / subgrade interface and can be seen where the subgrade soil is weak and the road structure cannot spread the load over a wide enough area. “Mode 3” rutting, otherwise known as pavement abrasion, takes place due to tyre wear on the pavement of paved streets. Figure 1 gives a diagrammatic summary of the four rutting classifications.

Figure 2_1. Rutting classifications for use in pavement diagnostics.

C. Damages related to moisture problems and insufficient drainage.

Drainage problems are often the main underlying reasons for permanent deformations in pavement structures. Pavement failures can also take place in streets with leaking water pipes, or leaks in adjacent rainwater management systems. Moisture content in the unbound pavement structure can also increase if and when a pavement begins to crack allowing water to infiltrate into the road structure through the cracked pavement.

D. Geotechnical problems.

The most typical example of geotechnical problem is settlement. Slope stability problems can also affect the pavement lifetime of city streets on side-sloping ground.

E. Design and construction errors

This category covers defects such as too thin layers, poor materials, poorly constructed culverts, damages due to transition structures, reflection cracks on widened streets, and poorly compacted structures after utility works.

3. New pavement condition monitoring techniques

3.1 GPR Methods Ground Penetrating Radar (GPR) has traditionally been used to measure thickness of the pavement and other road structures and locating objects inside or under the roadbed, such as cables and pipes. However, using GPR data it is possible to calculate and analyse other parameters that describe the condition and strength and deformation properties of the pavement structure [1].

One of the new and very promising techniques, that Roadscanners has developed, is the detection of micro and macro cracking in asphalt using a ground penetrating radar (GPR) surface reflection technique. This technique analyses electromagnetic waves in the same way that we visually see cracking in pavements. The human eye detects cracks in asphalt through a combination of diffraction and partly reflection and refraction of light. In the Roadscanners GPR method, the presence of cracking can be detected and measured by calculating the deviation of the surface



dielectric value measured using a GPR horn antenna reflection technique. Figure 31_1 shows an example of an asphalt pavement with cracking problems with a high dielectric deviation, and a sound asphalt pavement with low deviation. The advantage of this method is that, because of the longer wavelength of the GPR pulse, the GPR can also detect cracks through surface seals.

Figure 31_1. Example of GPR data from a city street showing a section of cracked asphalt (top photograph) and a section with sound asphalt (bottom photograph). The top field in each case shows the GPR horn antenna data and the interpreted asphalt thickness, base course thickness (rough depth scale on the right). The blue graph in the bottom shows the dielectric deviation value of the asphalt surface.

3.2 Laser Scanner Method

In recent years the greatest developments amongst all of the NDT techniques used in road surveys have occurred in the field of laser scanner (Lidar) techniques. Laser scanning is a technique where the distance measurement is based on the laser beam travel time from the laser scanner to the target and back. When the laser beam angle is known and beams are sent in different directions from a moving vehicle with a known position, it is possible to make a three dimensional surface image, a “point cloud”, of the road and its surroundings. [2]

As stated earlier, asphalt fatigue can appear either in the form of asphalt cracking or rutting. That is why a 2D laser scanner technique is used for quick calculation of rut depths which indicate the presence of early phase permanent deformations in the asphalt. In contrast to profilometers, common systems which were used earlier in pavement surveys, the laser scanner technique is not affected by the path taken by the vehicle during the survey. Using the laser scanner data, special rut depth maps can be made enabling accurate mapping of where and how much the road has suffered from deformations. Figure 32_2 provides an example of a map that shows the changes in road surface in Pajala Sweden from 2012 to 2013. This road was, at that time, the main route for heavy iron ore transport trucks from a nearby mine.



Figure 32_2. Changes in surface profile on Pajala Mining Road, Sweden when laser scanner surveys from 2012 and 2013 are compared. Upper map is from direction 2, and lower from direction 1 with heavy truck traffic. The darker the colour, the bigger the change in rutting. First year under traffic shows dramatic change in rut depths, reaching over 7 mm/year.

3.3 Road Doctor Survey Van Roadscanners Oy has developed a Road Doctor Survey Vehicle (RDSV) unit that collects all the data needed for evaluation of the current condition of the pavement with the exception of deflection data. The RDSV unit follows the idea of modern sensor fusion in road surveys with several sensors measuring different parameters to make a reliable “health check” of the pavement (Figure 34_1). The RDSV system consists of 1) Ground Penetrating Radar (GPR) unit equipped normally with 1-2 horn antennas and 400 MHz ground coupled antenna or 3d Radar stepped frequency radar system, 2-3) Road Doctor Cam Link system allowing pavement distress/damage analysis in real time from a moving vehicle and confirmed by a digital video collected during data collection, 4) pavement roughness measured with a 3D accelerometer mounted on the back axle of the vehicle and 5) surface survey of the pavement surface and its surroundings using a 2D laser scanner technique.

The positioning of all the collected data is ensured using RDSV’s multi-component positioning system comprising a GPS unit, optical encoders and a distance measurement unit. In addition the correct position can be confirmed from the video and laser scanners data (road crossings etc.). Road Doctor Survey Van is a turnkey package that is delivered with all the essential data collection and analysis packages together with customer training and support.



Figure 34_1. An example of Road Doctor Survey Vehicle consisting of 1. GPR system equipped with 1 or 2 2.0 GHz horn antennas, 2. /3. visual pavement distress data collection made using digital video camera and real time visual evaluation, 4. pavement roughness measurement made with a 3d accelerometer installed on the back axle of the vehicle, and 5. 2D laser scanner data collection. The vehicle also has a multi-component positioning system equipped which utilizes GPS, an optical encoder and a distance measurement instrument (DMI).

3.4 Traffic Speed Deflectometer method Deflection measurements have a key role when evaluating the structural condition of pavements and the fatigue level of the asphalt. With the aid of deflection surveys it is possible to locate road sections that still appear to be in good condition but will actually become distressed very soon if measures are not taken.

Traditionally, deflection surveys have been done with falling weight deflectometer systems but the problem with the method is that they are point like measurements, slow and traffic intrusive. The new solution to this problem is the traffic speed deflectometer (TSD) it measures continuous deflections of a pavement under a 10 tonne axle load using Doppler sensors. The normal survey speed with this truck (figure 35_1) is 80 km/h so it does not require any protection during the survey. Using the data, a deflection bowl similar to FWD can be calculated.

In summer 2015, Roadscanners has overseen TSD data collection from almost 2000 km of paved roads in Finland.



Figure 34_1. TSD truck of the Road and Bridge Research Institute (IBDiM), Poland, used in TSD data collection in Finland

3.5 Integrated Analysis of GPR, Laser scanner and TSD The greatest advantage of GPR, Laser Scanner and TSD survey results can be achieved when the results are analysed in combination in the same view as it is done with Road Doctor software. Figure 35_1 from Road 3662 in Finland provides an example of the analysis of pavement fatigue and remaining lifetime. Based on empirical data and experience it is known that pavement distress starts to increase exponentially when pavement strain exceeds 300 microstrain and, at that time, there are roughly 1 million axle loads left. Pavement loses its strength dramatically and visual cracks start to appear when strain is higher than 400 microstrain. Logically, it is very economic to repave the pavement when strains are at a level of 300 microstrain. This type of section starts in figure 35_1 from 1100 m. Laser scanner data in the figure data shows that pavement deformations have already appeared in that section.

Figure 35_2 presents an example of a section with Mode 2 rutting problems (see chapter 2) from Road 3662. In this case, Base Curvature Index (BCI) has been calculated from the TSD deflection data. Based on empirical data and experience it is known that a road starts to suffer from Mode 2 problems when BCI values are higher than 40 µm and severe damages start to appear once values surpass 60 µm. In this case, BCI values are even higher than 100 µm and a laser scanner rutting maps show severe shoulder deformation in the same place. In the same section GPR data shows that road structure layers are mixed and present signs of deformation and frost damage while in the “good” section the structures are very distinct and clear.



Figure 35_1. A Road Doctor software view of TSD, GPR data and laser scanner data. The top field presents TSD deflection bowls calculated as a 10 m average. The second field presents 2 GHz air coupled antenna GPR data. The third field presents Surface Curvature Index (SCI) and Base Curvature Index (BCI) multiplied by a factor of 10, calculated from TSD data. The lower data field presents rut depth map calculated from the right lane of the road. Red line indicates the location of the video and map.

Figure 35_2. A Road Doctor software view of TSD, GPR data and laser scanner data. The top field presents TSD deflection bowls calculated as 10 m average. The second field presents 2 GHz air coupled antenna GPR data. The third field presents Surface Curvature Index (SCI) and Base Curvature Index (BCI) multiplied by a factor of 10, calculated from TSD data. The lower data field presents a rut depth map calculated from the right lane of the road. Red line indicates the location of the video and map.



4. SummaryThe development of GPR, laser scanner, TSD and video data collection with accurate position info has been rapid in recent years. The goal has been to put together fast, good quality and versatile data collection systems to make it easy, affordable and safe to collect large amounts of data with as many parameters as possible. Combining many sensors in the same vehicle makes it easy – the data collection can be done in one survey and the data is synchronized automatically. A new advantage is that with wide angle laser scanner, the road surroundings can also be measured and evaluated. This allows engineers to tackle one of the major problems affecting pavement lifetime, drainage. The TSD method provides continuous information on bearing capacity with a high speed survey, which is a major step ahead from traditional FWD. Especially on high traffic volume roads, it makes data collection easier and safer. On local roads, where the changes in bearing capacity are sudden and local, the dense testing interval is very important.

With the combination of modern data collection methods and joint analysis of the results the diagnostics capabilities for road problems reach new levels. When all the collected data is compared and analyzed simultaneously, the actual reason behind a road’s anomalous behavior can be found. At the same time, when there is more data collected and experience has grown, it is easy to evaluate at what phase of its life-cycle the road is in at that moment and what would be the right operation to extend the road lifetime. This makes proactive maintenance possible and great savings in road life-cycle costs can be achieved.



References

1. Saarenketo, T. 2009. NDT Transportation. Chapter 13 in text book book “Ground PenetratingRadar: Theory and Applications” Ed- Harry M. Jol. Publisher Elsevier, 524 p.

2. Saarenketo, T., Matintupa, A. and Kourim, B., 2012. Experiences with New Technologies inRoad Problems Diagnostics. Proceedings of EPAM 2012 Conference, Malmo, Sweden (DVD).



experiences with integrated analysis of tsd, gpr …...experiences with integrated analysis of tsd,...

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